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Familial Isolated Pituitary Adenoma

ABSTRACT

 

Familial Isolated Pituitary Adenoma (FIPA) is a term used to identify a genetic condition with pituitary tumors without other endocrine or other associated abnormalities. FIPA families contribute around 2% to the overall incidence of pituitary tumors. FIPA is a heterogeneous disease both in terms of the clinical phenotype as well as from the genetic background point of view. Some FIPA families have been identified to have germline mutations in the aryl hydrocarbon receptor interacting protein (AIP) gene leading to incomplete penetrance of young-onset, mostly growth hormone, mixed growth hormone/prolactin-secreting, or prolactin-secreting pituitary adenomas. Due to the low penetrance, almost half of the AIPmutation-positive patients do not have a positive family history. Duplication of the orphan G protein coupled receptor GPR101 gene, located on Xq26.3, leads to high penetrance pituitary hyperplasia or adenoma resulting in infant-onset GH excess, usually with concomitant hyperprolactinemia, named X-linked acrogigantism (XLAG). The majority of the FIPA families, however, have no known genetic mutation. Their clinical picture includes various types of pituitary adenomas, either homogeneous (all affected family members have the same adenoma type) or heterogeneous (different adenoma types within the same family), presenting with low penetrance and an age of onset not significantly different from patients with sporadic pituitary adenomas. Here we review the clinical features, genetics and screening aspects of FIPA.

 

INTRODUCTION

 

Familial Isolated Pituitary Adenoma (FIPA) is a relatively new term. Introduced by Professor Beckers in 1999, FIPA describes families with pituitary adenoma and no other associated symptoms (1, 2). As opposed to occurring in isolation, familial pituitary adenomas have been recognized in several syndromic diseases, such as the classical MEN1 syndrome or Carney complex or the most recently described, such as hereditary paraganglioma syndromes (3-5), MEN4, and DICER1 syndrome (6) (Figure 1).  For additional information we refer the reader to other chapters within ENDOTEXT on syndromic familial pituitary adenomas.

 

Figure 1. Germline or Mosaic Mutations Causing Pituitary Tumors. Details for the syndromic forms can be found, among others, in the following sections https://www.endotext.org/chapter/multiple-endocrine-neoplasia-type-i/, https://www.endotext.org/chapter/carney-complex/, https://www.endotext.org/chapter/pituitary-adenomas-in-childhood/ and in these references (6-10).

 

Descriptions of familial pituitary adenoma families have been around for several hundreds of years, but only over the last decade has the clinical phenotype and, in some cases, the genetic abnormality been described. Interestingly, some of the patients with germline mutations present as simplex patients without any known family history, either due to low penetrance or due to de novo mutations.

 

Figure 2. Family Trees Demonstrating Examples of the Various Types of FIPA Families. In some AIP mutation-negative FIPA families unaffected obligate carriers can be identified by their position in the family tree, while in other family’s possible carriers of the unidentified gene cannot be identified. AIP mutation-positive kindreds can be ‘families’ or simplex cases. Most XLAG kindreds are simplex cases with females having de novo germline mutations while males have somatic mosaic mutations.

 

Previous data suggest that FIPA families contribute around 2% of the overall incidence of pituitary tumors, but this number may increase with increasing recognition of this clinical entity.

 

Around 10-20% of all FIPA families and 50% of familial isolated GH-producing Tumor families (11, 12) have been identified to have mutations within the aryl hydrocarbon receptor interacting protein (AIP) gene, located at 11q13. Germline mutations in AIP have also been identified in patients with young-onset pituitary adenomas, mostly GH-secreting or prolactin-secreting or silent GH/prolactin-producing adenomas with no apparent family history. These are called ‘simplex’ cases. Until recently, no somatic mutations had been described in the AIP gene in pituitary or other tumors (1). Duplication of the orphan G protein-coupled receptor GPR101 causes X-linked acrogigantism (XLAG) (13).While most of the XLAG cases are due to de novo mutations (germline or somatic mosaicism (14, 15)), to date three families have also been described. The causative gene for the rest and therefore the vast majority (90% only considering kindreds with 2 or more affected subjects) of FIPA families is currently unknown (16). Recently, a microdeletion upstream the GHRH gene, on chromosome 20, has been identified as another possible cause of severe infant-onset gigantism (17). New candidate genes are under active investigation in somatic and familial cases of pituitary adenomas (18), but some need further validation. Representative examples of FIPA family trees are shown in Figure 2.

 

CLINICAL FEATURES OF FIPA

 

Families with AIP mutations usually have a characteristic phenotype, which is usually substantially different from that ofAIP mutation-negative phenotype. In this section, we compare characteristics of AIP-mutated and non-AIP-mutated FIPA. Germline chromosomal defects leading to gigantism, including XLAG and a recently described microdeletion in chromosome 20 that leads to GHRH overexpression, have a drastically different phenotype and are discussed separately below.

 

Tumor Types

 

FIPA families can be homologous (i.e. all affected family members have the same type of tumor) or heterologous (i.e. family members can have different type of tumor) (Figure 2). Therefore, pure acromegaly, pure prolactinoma, and pure non-functioning pituitary adenoma (NFPA) families have been identified, while also mixed families such as acromegaly-prolactinoma, acromegaly-NFPA, prolactinoma-NFPA, prolactin-corticotrophinoma or even acromegaly-prolactinoma-NFPA families have been described. Somato-mammotrophinomas occur commonly, but are not consistently reported, probably as a result of variations in the reporting of tumor histology type. Figure 3a, b and c demonstrate the distribution of histological tumor types in FIPA families.

 

Figure 3a. Proportion of histological tumor types in the AIP positive FIPA population in the International FIPA Consortium cohort (n=911) (19).

Figure 3b. Proportion of tumor types in AIP mutation-positive FIPA families (12).

Figure 3c. Proportion of tumor types in AIP mutation-negative FIPA families (12).

 

In a study including familial as well as simplex (apparently sporadic) patients with germline AIP mutations, 78% of 96 patients developed GH-secreting adenomas (20) (half of the GH-secreting adenomas were somato-mammotrophinomas), 13.5% of patients developed prolactinomas, 7% developed non-functioning pituitary adenomas (NFPAs), and 1 patient developed a TSH-secreting adenoma. In another study, comprising 171 patients carrying AIPmutations, based on clinical diagnosis 70% had somatotrophinomas, 11% mixed GH/PRLomas, 12% had prolactinomas, and 8% had clinically non-functioning tumors (12). On histological testing some tumors show plurihormonal profile (Figure 3b). It is important to note that some non-functioning tumors are found to be somatotroph/lactotroph upon histological examination (21) – these are therefore ‘silent adenomas’. The distribution of tumors amongst 318 non-AIPmutated FIPA families (1310 patients) is represented in Figure 3c (12). Somatotrophinomas are the most common tumor type in both AIP mutation-positive and negative FIPA families (12, 19).

 

Gender Distribution

 

While higher numbers of males are identified with AIP mutations both in familial and simplex setting (12, 20), ascertainment bias due to physiological later puberty of boys and their normally taller stature cannot be ruled out (19), as in a carefully-studied large AIP mutation family equal number of affected males and females are present (22). There is a greater prevalence of females within AIP mutation-negative families, probably due to a higher number of prolactinomas (19).

 

Age of Onset

 

AIP gene mutation-positive FIPA patients have an earlier age of onset of diagnosis compared to those with AIP mutation-negative familial (23) or sporadic (20) pituitary adenomas. The age of onset of pituitary adenoma symptoms is 8 years earlier in the AIP mutation-positive group (mean age 19 years, SD ± 9.5, p<0.001), with diagnosis being made 6 years earlier (mean age of diagnosis 24.3, SD ± 11.9 vs 30, SD ± 13.5, p<0.001) than in the AIP mutation-negative population (12). In our international FIPA cohort, the familial cohort with AIP mutation-positive tumors had a peak age of onset during the 2nd and 3rd decades of life, with 65% of these patients’ developing symptoms aged ≤18 years (28.8% in the AIP mutation-negative group) and 87% by the age of 30 years (12). Previous work has shown that those families with AIP mutation-negative tumors demonstrate a more even spread of occurrence between the ages of 20 and 50, with a peak incidence around the age of 30 years old (19); the latest data suggests that the modal age group (42%) is 20-29 years (12). 

 

Young (<30 years) onset simplex patients, the AIP mutation-positive group, also developed tumors at a younger age than the mutation-negative group, with median ages of 16 years (IQR 14.8-22.3) and 22 years (IQR 16-26) respectively (19).

 

In the Bart’s international cohort, over 80% of the families with AIP mutations have at least one affected patient with gigantism or disease onset before the age of 18 years, while only 3 out of 46 AIP mutation-negative families have an onset of pituitary adenoma before the age of 18 years (23). Interestingly, probably due to earlier recognition of symptoms in affected FIPA families, the age of tumor onset appeared to be earlier in the second generation than in the first (mean age 29 ±10.2 years vs. 50.5± 14.2 years p<0.0001) (24).

 

Disease Penetrance

 

Disease penetrance in FIPA is incomplete. As there is a clear natural bias of affected patient referral and the clinical and genetic data in the individual families are incomplete, the calculation of disease penetrance is difficult. Additionally, it is important that penetrance always be considered in the context of the subject’s age.

 

In AIP positive mutation families, current data suggests 12.5-30% penetrance, but ranges between 10-90%, also depending on available data (19, 20, 23). It seems that the nature of the AIP mutation (truncating or non-truncating) does not have any effect on penetrance (19).

In AIP mutation-negative families, penetrance calculations are even more difficult as carrier unaffected family members (other than obligate carriers) cannot be distinguished from non-carrier unaffected subjects. The current calculation based on affected subjects, obligate carriers and 50% of potential carriers suggest 38±16% (23), but this is obviously a very significant overestimate.

 

Another way to compare penetrance between AIP positive and negative families is to count the known affected subjects within families. Penetrance in AIP mutation-negative families is probably lower than in AIP mutation-positive families, as the mean number of patients with disease in AIP mutation-positive families is 3.2±1.8 and in AIP mutation-negative families 2.2±0.5, P<0.001 (23).

 

De novo AIP mutation has been described in two cases so far: in a child with prolactinoma (c.721A>T; p.Lys241*) where the AIP mutation was not found in the parents (paternity confirmed) or his sister (19, 25). A second case was with identical twin girls, where both of them carry a mutation in the leukocyte derived DNA (p.R304*), while their parents (paternity confirmed) were negative (26).

 

Phenocopies (patients who show manifestations of a disease that are usually associated with mutations of a particular gene but instead are, in this case, due to another etiology) (27) have been described in families with AIP mutations (16, 23) and are probably present in AIP negative families as well, therefore careful and cautious genetic studies and counselling need to be conducted in every family.

 

Tumor Behavior

 

SIZE

 

FIPA patients in general have larger, more aggressive tumors and earlier onset of disease compared to sporadic pituitary adenomas (11, 20, 23, 28).

 

Macroadenomas predominate amongst AIP mutation-negative and positive FIPA groups. However, when compared to sporadic pituitary adenomas, AIP gene mutation-positive FIPA patients were more likely to have larger tumors (1, 11, 19, 28) and macroadenomas (19), and these tumors were more likely to invade the extrasellar region (19, 20).

 

There was no statistical difference between the AIP mutation-positive and negative groups in the occurrence of giant (>40mm) adenomas (19), nor in the incidence of macroadenomas (mutation-positive 83.2% vs 79.2% p=0.259) or cavernous sinus invasion (mutation-positive 36.7% vs 28.3%, p=0.122) (12). Suprasellar extension was more frequent in the pituitary adenomas of AIP mutation-positive FIPA patients (mutation-positive 54.3% vs 42.4%, p=0.043).

 

No correlation was observed between the presence of truncating and non-truncating AIP mutations and the size of the pituitary adenoma, the incidence of macroadenoma or the propensity to invade extrasellar structures (19).

 

APOPLEXY PROPENSITY   

 

Pituitary apoplexy is a relatively rare event; incidence is variously estimated to be as high as 6.8% (in 560 adenoma cases) (29) to as low as 0.6% (in 664 adenoma cases) (30). In a previous study, it was shown that apoplexy occurred more commonly in individuals with AIP mutation-positive tumors than those with mutation-negative tumors (7.6% vs 1.3% of cases respectively) (19). No size difference was observed between tumors that did and those that did not undergo apoplexy in the AIP mutation-positive tumor group (19). Excluding simplex cases from these analyses (i.e. just considering patients with a family history of pituitary adenomas) demonstrated an even bigger disparity in apoplexy incidence with AIP mutation-positive tumors having an apoplexy rate of 10.6% vs 2.3% in mutation-negative families (19). The latest data from the international FIPA consortium has shown similar rates of apoplexy (8.2% vs 3.6% respectively, p=0.009) (12). Familial apoplexy has also been described in AIP mutation-positive families (19, 31). It was previously observed that GH-secreting tumors with AIP mutations were significantly more likely than their mutation-negative counterparts to undergo apoplexy (19) and this has been demonstrated once again (8.3% vs 2.8% p=0.005) (12). The mechanism for this observation is unclear.

 

Treatment Resistance

 

Many of the somatotrophinomas described in FIPA families have been described as sparsely granulated adenomas (1), a subtype which has been previously suggested to respond less well to somatostatin analogues and to be more aggressive (32, 33). Sparsely granulated adenomas occur more commonly in AIP mutation-positive GH-secreting adenomas than in their mutation-negative GH secreting counterparts (19). In one study (12), all of the AIP mutation-positive somatotrophinomas were sparsely granulated, compared to 68% in the AIP mutation-negative group (p<0.001).

 

There is speculation that somatostatin analogues mediate their anti-proliferative effects through AIP up-regulation, which in turn increases the expression of ZAC1, a tumor suppressor gene known to be upregulated by somatostatin analogues  (34, 35), therefore, dysfunction at the AIP step would reduce the expression of ZAC1 and so the usefulness of this class of drug (36).  Another potential mechanism for this treatment resistance involving defective Gαi signaling has been postulated and is discussed in detail below.

 

It has previously been observed that AIP mutation-positive tumors are more difficult to treat - mutation-positive somatotrophinomas undergo less shrinkage and there is a smaller reduction in GH production with first generation somatostatin analogues than in the mutation-negative sporadic patients (1, 20, 28, 37). This may be accounted for by a relative paucity of expression of SSTR2 in the former (38); however, in human samples rather, a higher level of SSTR2 was found (36), and this is also seen in a pituitary Aip-knockout mouse model (39, 40). A greater need for re-operation after initial surgery and a greater use of multiple therapies and >2 types of therapy, including radiotherapy (12) and the failure of pegvisomant to control IGF-1 (20) have also been described. However, some studies (19) failed to demonstrate any difference in the number of therapeutic interventions between AIP positive and negative mutation tumors. Where primary surgery has failed to control the tumor’s GH production, there is some evidence that pegvisomant (37, 41), or pasireotide in patients whose tumor expresses the type 5 somatostatin receptor (38, 42), may reduce the IGF-1 burden.  In some cases, drastic treatment is necessary: for example, in the youngest known case, who presented at the age of 4 years-old, surgery followed by first generation somatostatin analogue, temozolomide, bevacizumab, radiotherapy, pegvisomant, gamma knife therapy and somatostatin analogue combined with increasing dose of pegvisomant, was necessary (43).

 

No correlation was observed between the presence of truncating and non-truncating AIP mutation tumors and the number of treatment modalities required by these patients (19).

 

In addition to sparsely granulated histopathology, other well-known predictive factors of resistance to first generation somatostatin analogues are younger age at diagnosis, hyperintense T2 image on MRI, and low tumor expression of somatostatin receptor subtype 2 (44). Recently, a machine-learning based model accounting for age at diagnosis, sex, pretreatment GH and IGF-1 levels, tumor granulation pattern and expression of somatostatin receptor subtypes 2 and 5 was shown to predict therapeutic response to first generation somatostatin analogues with high negative and positive predictive values (45).

 

Currently, some experts already suggest that the first-line medical treatment for patients that show one or more of these features could be pegvisomant or pasireotide; and that pegvisomant could be preferred in patients with diabetes or low somatostatin receptor subtype 5, whilst pasireotide could be preferred in the presence of significant tumor volume (44). Therefore, in select cases, these two drugs could be considered early in postsurgical medical therapy in patients with persistent disease, especially in younger patients with ongoing uncontrolled height gain, as seen in patients with AIPmutations.

 

Hormone Secretion

 

When matched with acromegaly mutation-negative controls, AIP mutation-positive somatotrophinomas produce more growth hormone (GH) (20) but there was no difference in the levels of IGF-1 (12, 20). Prolactin co-secretion was more common in AIP mutation-positive GH secreting tumors than their non-AIP mutated counterparts (19).

 

Gigantism was observed to be more common among AIP mutation-positive patients (55.9% vs 18.2%, p=0.005) and was the most common clinical diagnosis (12) – which is predicted by their earlier onset of disease, with cases in males predominating in both AIP positive and negative patients (19): 60% of FIPA families in one study had at least one case of gigantism and instances of two cases of gigantism within the same family only occurred in AIP mutation-positive families (19).

 

No correlation was observed between the presence of truncating and non-truncating AIP mutations and the incidence of GH secreting tumors (19); however, there was a significantly greater prevalence of gigantism amongst the GH secreting tumor patients in those with truncating as opposed to non-truncating AIP mutations (54.7% vs 30%). There is also a suggestion that patients with GH-secreting adenomas and the truncating R304* mutation present more commonly at a very young age then rest of the described AIP mutation-positive population with GH secreting adenomas.

 

A previous case report described the co-existence of pituitary hyperplasia and pituitary adenoma in two AIP mutation-positive adenomas from a family. Loss of heterozygosity was seen in the adenoma tissue but not in the surrounding hyperplastic tissue and loss of AIP protein expression was seen in the adenoma tissue with preservation of AIPexpression in the hyperplastic tissue (46). Villa and colleagues hypothesize that this may demonstrate that tumorigenesis is a multi-stage event starting with hyperplasia in haploinsufficient tissue and then the development of further genetic events (including loss of the one remaining wild-type AIP allele) leading to true adenoma formation. They suggest that this could explain the incomplete penetrance seen in pituitary disease in AIP mutation-positive subjects (46).

 

In GH-secreting non-AIP mutated sporadic pituitary tumors, an association was noted between the levels of AIP staining on histology and the aggressiveness of the adenoma. Low levels of AIP staining were associated with a more aggressive phenotype (higher Ki-67 index and a greater likelihood of suprasellar tumor extension) when compared to tumors with higher levels of AIP staining. In the same tumors, none of those with low AIP staining showed significant shrinkage despite pre-operative treatment with a somatostatin analogue. Tumors treated pre-operatively with somatostatin analogues that did shrink showed a higher level of AIP on immunohistochemistry (47).

 

No difference in rates of hypopituitarism was seen between AIP mutation-positive and negative patients with pituitary adenomas at diagnosis (12).

 

Other Tumors in Individuals with an AIP Mutation

 

In one study (19) involving 290 AIP mutation-positive individuals (some with pituitary adenomas), there were 10 cases of tumors occurring outside of the pituitary gland in 9 individuals. These included a gastrointestinal stromal tumor, glioma, meningioma, non-Hodgkin’s lymphoma, and spinal ependymoma. Parathyroid adenomas were excluded from this analysis due to the rare finding of AIP mutations in parathyroid adenomas (48), as were colonic polyps and thyroid nodules due to their frequent occurrence in patients with acromegaly (19). Four of the 9 individuals with extra-pituitary tumors had GH-secreting pituitary tumors, the other 5 were AIP mutation carriers without pituitary tumors.

 

While AIP acts as a tumor suppressor gene in the pituitary gland, and patients with pituitary tumors show heterozygous loss-of-function mutations of AIP, a possible role for AIP as an oncogene has been described in other tumor types. To date, increased expression of AIP was found in association with increased tumorigenic and metastatic properties of colorectal cancer cells (49), with increased survival of primary diffuse large B cell lymphoma (DLBCL) cells (50), and with a bad prognosis in cholangiocarcinoma (51). In colorectal cancer, increased AIP expression was associated with increased cell migration and epithelial-to-mesenchymal transition, possibly by the facilitation of N-cadherin expression and suppression of functional E-cadherin on the cell surface (49). On the other hand, for DLBCL, AIP promoted tumor survival by reducing ubiquitin-mediated proteasomal degradation of BCL6, a protein that reduces the transcription of pro-apoptotic genes such as TP53 and that is frequently overexpressed in DLBCL (50).

 

Therefore, AIP behaves as a double agent, either as a tumor suppressor or as an oncogene, and further studies on AIP regulation mechanisms will be essential for a better understanding of AIP derived tumorigenesis and for unravelling new possible therapeutic targets (52).

 

THE GENETICS OF FIPA

 

The currently known genes causing FIPA are AIP and GPR101 and we will discuss the diseases associated with these genes in detail. Furthermore, there are some pituitary adenoma cases described with other germline mutations, that will be more briefly addressed, as they are still under investigation and require additional validation.

 

AIP

 

There are over 100 heterozygous mutations identified in AIP, showing an autosomal dominant inheritance pattern with incomplete penetrance (53). Mutations that affect the AIP gene commonly lead to truncated or missing protein due to nonsense mutations, small deletions or large deletions, insertions, splicing or promoter mutations, while 21% result in full length mutated protein due to missense mutations or in-frame deletions or insertions (Figure 4). Large deletions cannot be identified with Sanger sequencing and other technologies, such as MLPA, or next generation sequencing methods are required to identify them.

 

Figure 4. Distribution of mutation types found within the AIP gene in the International FIPA consortium (12).

Figure 5. The three-dimensional structure of the AIP protein. Three characteristic tetratricopeptide (TPR) domains, the A and B helices of the first TPR domain, orange, TPR2 blue. TPR3 green and the 7th C-terminal alpha helix with light blue (54, 55).

 

The AIP protein is a well-conserved molecular chaperone, with multiple binding partners. It has three tetratricopeptide (TPR) repeats, conserved anti-parallel pair of alpha helices and a final 7th alpha helix at its carboxyl terminal end (Figure 5). This C-terminal section is known to be important for interaction with other proteins and therefore, it is postulated, that in the case of FIPA it loses its ability to bind its binding partners, such as the aryl hydrocarbon receptor (AHR) or phosphodiesterase (PDE) subtype 4A5, and therefore loses its activity as a tumor suppressor (56).

 

There are a few mutational hotspots, the majority affecting CpG sites, where a mutation has been identified in several independent patients or families (Table 1).

 

Table 1. A Few Examples of AIP Mutation ‘Hotspots’

Variant

References (examples)

c.910C>T; p.R304*

 

 

Cazabat et al. 2007 (57)

Daly et al. 2007 (11)

Georgitsi et al. 2007 (58)

Igreja et al. 2010 (23)

Leontinou et al. 2008 (28)

Variglou et al. 2009 (59)

Vierimaa et al. 2006 (16)

Chahal et al. 2011 (60)

Hernandez-Ramirez et al. 2015 (19)

Ramirez Rentaria et al. 2016 (26)

Marques et al. 2020 (12)

c.811C>T; p.R271W

Daly et al. 2007 (11)

Jennings et al. 2009 (61)

Hernandez-Ramirez et al. 2015 (19)

c.721A>T; p.R81*

Leontiou et al. 2008 (28)

Toledo et al. 2010 (62)

Hernandez-Ramirez et al. 2015 (19)

Marques et al. 2020 (12)

 

AIP Mouse Models

 

AIP knockout in mice is lethal in utero and is associated with ventricular septal defects, double outlet right ventricle and pericardial edema (63). The embryonic mice are also unable to undergo a crucial step in initiating adult erythropoiesis at E11-14, a step which is vital for embryonic survival beyond E13.5 (64). This suggests that AIP may have an important role to play in fetal growth signaling in utero.

 

Heterozygote AIP knockout mice invariably develop mostly GH-secreting pituitary tumors, with 100% penetrance by the age of 18 months, compared to wild-type mice where around 1/3 of mice spontaneously developed prolactin-secreting adenomas, but no GH adenomas are observed (65). AIP expression was lost in these GH-secreting tumors and this corresponded to higher tumor proliferation rates (65), compared to spontaneous pituitary adenomas in the wild-type littermates, with normal AIP expression. These data mirror the increased aggressiveness of tumors seen in mutation-positive FIPA families (11, 20, 23, 28). ARNT expression was also lost in the mouse tumors (65), reflecting a pattern observed in human mutation-positive tumors (66) and therefore suggesting a possible role for loss of ARNT in the development of pituitary tumors (65). Somatotroph-specific AIP deficient mice (sAipKO) have also been created, using Cre/Lox and Flp/Frt technology (67). In keeping with the heterozygote AIP knockout mice described above, >80% of the sAipKO mice developed GH secreting adenomas by 40 weeks of age, by 18 weeks they also displayed elevated IGF-1 and GH levels, increased body and organ size (compared to control animals) and glucose intolerance. Pituitary hyperplasia was consistently observed in the sAipKO mice (on histology and on MRI imaging), suggesting (but not absolutely proving) a progression from hyperplasia to adenoma. The investigators point out that 40 weeks of age for a mouse represents ‘middle adulthood’ and so hypothesize that, in common with other tumors, additional somatic mutations are required on top of the AIP loss of function for somatotroph tumors to occur (67). A pituitary-specific Aipknockout using the Hesx1/Cre model has also developed gigantism with elevated IGF-1 levels (40).

 

ARNT knockout mice die in utero in early gestation (68, 69): the reasons for this are disputed, in one study it appeared that there was faulty angiogenesis in the yolk sac (69), whilst in another the embryos survived slightly longer and had a normally developed yolk sac vasculature but the placental vasculature failed to develop correctly. The embryos in the latter study also displayed a range or anomalies, including neural tube closure defects, brain hypoplasia and placental hemorrhage (68). It has been hypothesized, therefore, that ARNT plays a role in angiogenesis in response to hypoxia secondary to the increasing tissue mass in embryonic development (69).

 

Ahr knockout mice are viable, though they too suffer physiologic dysfunction, including cardiac hypertrophy (with cardiac myocyte enlargement but without the molecular signatures that would indicate cardiac overload) and subsequent cardiomyopathy (70). These mice also have hypertension (71), reduced body weight, reduced reproductive capabilities, smaller livers as a result of a patent ductus venosus, persistence of fetal vascular and liver parenchymal structures and aberrant vasculature in the kidneys. This underlines the importance of AhR signaling mechanisms in the development of a normal, mature vasculature (72).  AhR protein-protein interactions were further characterized, with one of the most interesting interactions being with the mitochondrial protein MRPL40 (73), which codes for a mitochondrial ribosomal 39S subunit. Deletions in this gene have been associated with the 22q11.2 deletion syndromes Velo-cardial facial syndrome and Di George syndrome (OMIM #188400), both of which involve congenital cardiac malformations, further suggesting the importance of AhR in normal cardiac development.

 

It has been suggested that interplay between AhR and ARNT/HIF1α may govern normal vascular development (72).

 

MECHANISM OF TUMORIGENESIS IN PITUITARY ADENOMAS WITH AIP MUTATIONS

 

In the pituitary, AIP is a tumor suppressor, and truncating mutations presumably lead to loss of function mutations. However, for missense mutations change in protein folding or loss of partner protein binding sites could explain the lack of function. Based on data from half-life studies, (74) it seems that a significant proportion of the missense mutations lead to unstable proteins and rapid degradation explaining the loss of function. Furthermore, in vitro measured half-life of missense proteins correlated well with age of onset of disease. (74)

AIP interacts with numerous other molecules (see Table 2), full details of each of these interactions has recently been summarized (56).

 

Table 2. A List of Factors that Have Been Demonstrated to Interact with the AIP Protein (56)

Viral Proteins

Hepatitis B Virus X protein (HBV X)

Epstein Barr Virus Nuclear Antigen 3 (EBNA3)

AIP-AHR-Hsp90 Complex

Aryl Hydrocarbon Receptor (AHR)

Heat Shock Protein 90 (Hsp90)

Heat Shock Cognate 70 (Hsc70)

Aryl Hydrocarbon Receptor Nuclear Translocator (ARNT)

p23

AIP self-association

Cytoskeletal Proteins

Possible interaction with actin

Tubulin (75)

Phosphodiesterases

PDE4A5

PDE2A3

Nuclear Receptors

Estrogen Receptor α (ERα)

Glucocorticoid Receptor (GR)

Peroxisome Proliferator-Activated Receptor α (PPARα)

Thyroid Hormone Receptor β1 (TRβ1)

Transmembrane Receptors

RET

EGFR

G Proteins

Translocase of the Outer Membrane of Mitochondria (TOMM20) Proteins (64)

Survivin (64)

Cardiac Troponin Interacting Kinase 3 (TNNI3K)

Protein Kinase A (76)

 

The exact mechanism by which AIP mutations lead to pituitary tumor formation is unclear; however, several theories have been put forward. AHR is widely expressed in the body and binds numerous compounds, both endogenous and exogenous (77, 78). It is a nuclear transcription factor and prior to ligand binding it is found in the cellular cytoplasm, bound to AIP (77, 78). It is known that AHR is a receptor for environmental pollutants, such as dioxin – a known carcinogen. The binding of dioxin leads to increased AHR nuclear translocation, with activation of detoxification mechanisms (79), including increased expression of the enzyme CYP1A1, which has also been shown to bio-activate polycyclic aromatic hydrocarbon carcinogens (80, 81). Interestingly, an increase of acromegaly incidence (82) has been described in a heavily polluted industrial area. Pituitary adenoma incidence was also studied in an area heavily polluted with dioxin after a chemical factory accident, but data were not sufficient to draw appropriate conclusions (83). A recent follow-up study (84) examined links between the characteristics of patients with GH-secreting pituitary adenomas, residing in an area of high pollution and AHR/AIP variants. It was found that pituitary tumors were significantly larger and IGF-1 burden significantly greater in patients with AHR/AIP gene variants who lived in polluted areas compared to either those who had no gene variants and lived in the same highly polluted areas or those who had gene variants but lived in cleaner areas. Further, the use of somatostatin analogues in patients with GH-secreting pituitary adenomas, who also had AHR/AIP gene variants and lived in highly polluted areas, seemed to be less effective (IGF-1 only normalized in 14%). Overall, the reduction in GH/IGF-1 levels did not reach statistical significance. GH secreting pituitary patients with no AHR/AIP variants had a statistically significant reduction in GH/IGF-1, as did those without gene variants living in polluted areas (IGF-1 normalized in 54-56% of cases).  These data need confirmation.

 

Fibroblasts with heterozygous AIP mutations taken from patients have lower AIP protein levels (probably through nonsense-mediated decay of truncated proteins (74)) compared to wild-type fibroblast controls, but AHR expression is unaffected. However, AIP mutation did result in altered regulation of the AHR transcriptional target CYP1B1, both with and without AHR ligand stimulation (85). The mechanism by which this happens and therefore the role of AHR in signaling in pituitary tumorigenesis is still to be elucidated.

 

It has been noted that the loss of function of the AIP gene allows dysregulated ERα mediated gene transcription by its disinhibition (86). Cumulatively, high levels of estrogen and therefore estrogen mediated gene transcription products have been associated with an increased risk of developing various tumors, including pituitary tumors (86, 87) and so this work provides a novel avenue for investigation into pituitary tumorigenesis.

 

In the previous years, the role of cAMP elevation in pituitary tumors has been further investigated - It had previously been noted that cAMP levels were elevated in a subset of pituitary tumors (88). cAMP is a mitogenic factor in somatotroph cells, this therefore suggests a link between its dysregulation and tumor growth (89, 90). AIP is known to be a binding partner of some of the phosphodiesterases. AIP binding to PDE4A appears to inhibit its phosphodiesterase activity; however, this did not appear to prevent the cell’s in vitro ability to reduce forskolin-induced cAMP driven transcription. Therefore, it was felt unlikely that AIP-phosphodiesterase was the mechanism for cAMP elevation in pituitary tumors (91).  The same study also hypothesized that  AIP’s interactions with other binding partners is vital in its role of reducing cAMP, as R304* mutant AIP transfected cells (which produces a truncated AIP protein, losing its protein-interacting C-terminal) were not able to reduce  cAMP signaling in the same way that  wild-type AIP transfected cells could (91).  This correlated with reduced GH secretion after forskolin stimulation in the wild-type AIP cells, but not in the AIP mutant cells (91).

 

Disordered cAMP regulation is also seen in McCune Albright syndrome – where there is a mutation of the GNAS1 gene which results in a constitutionally active Gαs and raised cAMP (92), and Carney complex (93) – where there is an inactivating mutation in the PRKAR1A gene, a subunit of Protein Kinase A (PKA), a cAMP dependent kinase (94).There is evidence that AIP interacts with some of the subfamily protein of Gα (95), providing a possible way through which AIP can influence intracellular cAMP levels. To investigate this further, Tuominen et al. (96) developed an immortalized fibroblast cell line from the embryos of an AIP knockout mouse. AIP knockout in the mouse embryonic fibroblasts (MEFs) cell line resulted in higher cAMP level with a 2-3 times increase the AIP knockout cells. This result was concordant with AIP knockdown in a rat pituitary tumor cell line, with an observed 20-30% rise in cAMP levels.

 

Figure 6. Role of G alpha Inhibitory Protein. (A) - cells with normally functioning G alpha inhibitory protein (Gai-2) respond to stimulation of the somatostatin receptor (SSTR) by somatostatin (SST) by inhibiting the action of adenylyl cyclase, reducing the cell's secretory and proliferative capabilities. The role of AIP in this process is unknown, but cells with defective/absent AIP (B) also have a reduction in Gai-2 and so a lack of response from SST binding to SSTR with resulting disinhibition of adenylyl cyclase and increased GH secretion and cell proliferation.

 

Sequential knockdown of the Ga subfamily of proteins (Ga12, Ga13, Ga11, Gaq, Ga14, Ga15, and Gas,) produced only a significant reduction in cAMP levels in AIP knockout mouse cells when Gas and Ga13 were knocked down, although this effect was not sufficient to explain the observed difference in cAMP levels between AIP knockout and wild-type cells (96). Sequential knockdowns of the Ga inhibitory subfamilies (Gai-1, Gai-2, and Gai-3) was also performed. Gai-2 and Gai-3 knockdown caused a rise in cAMP levels by 77% and 115% respectively in wild-type MEFs, but minimal changes in the cAMP levels in AIP knockout cells. This was interpreted as evidence of a pre-existing defect in the Gai system of the AIPknockout cells (96) (Figure 6).

 

Immunohistochemical staining was subsequently performed on human somatotrophinomas which showed a reduction in the Gai-2 expression in AIP mutation-positive tumors compared to mutation-negative tumors (96, 97). No difference was observed in the expression of Gai-3 between the two types of tumors (96).

 

These findings may also explain the observed phenomenon whereby AIP mutation-positive tumors appear to respond poorly to somatostatin analogue treatment, as somatostatin receptors mediate reduction in cAMP levels through the Gaisystem (98), particularly through Gai-2 and therefore defective Gai signaling in AIP mutation-positive tumors maybe abrogate the effect of these drugs (96).

 

There is also in vitro evidence that AIP may play a role in reducing PKA activity through binding to its subunits (catalytic Cα and regulatory R1α). It was shown that AIP is able to interact with these two subunits, either as part of the PKA complex or separately. Cα stabilizes AIP and also R1α.  Overexpression of AIP lowered PKA activity, perhaps through inhibition of Cα or through the stabilization of the inactivating Cα-R1α complex. AIP overexpression also led to lower levels of Cα in the nucleus. Conversely, AIP silencing led to an increase in PKA activity. AIP’s interaction with these subunits is partly mediated by its c-terminal and so this may explain why common AIP truncation mutations (such as R304*), which affect this region, have a shorter protein half-life. It is hypothesized that this would then lead to lower intracellular AIP levels and may contribute to tumorigenesis through increased PKA activity (76).

 

The most recent and plausible mechanism relates to an interaction between AIP and the tyrosine kinase receptor RET. Although the first report on this interaction was over a decade ago (59), only recently there has been new insight about how this interaction affects tumorigenesis in the pituitary gland (99).  RET is a dependent receptor in somatotroph cells: in the absence of its ligand GDNF, the monomeric RET receptor is processed intracellularly by caspase-3, leading to PIT1 accumulation and upregulation of the RET/PIT1/ARF/p53-apoptotic pathway (99).  AIP was shown to be a key factor in the initial steps of this pathway, by forming a complex with RET/caspase-3/PKCδ, that allows for the intracellular processing of RET. In the absence of AIP or in the presence of pathological mutations in AIP, there is an inhibition of RET-induced apoptosis, that may be a key feature in somatotroph hyperplasia and adenoma formation (99).  However, PIT1 is a transcription factor that is present in somatotroph, lactotroph and thyrotroph cells; therefore, despite previous studies focusing mostly in somatotroph tumors, the same pathway is probably involved in other tumor types, such as prolactinomas (99), and this seems to be the explanation for the tissue specificity of AIP mutations. In line with this finding, the reported pituitary tumors in patients with AIP mutations are mostly GH and/or prolactin secreting tumors, but also clinically non-functioning adenomas with positive GH and/or prolactin immunostaining and, in one case, thyrotropinoma (12, 20, 100). There have been no unequivocal cases of corticotrophinomas or gonadotroph adenomas in patients with pathological AIP mutations. This extraordinary finding may pave the way for new therapeutic options in sporadic and familial cases of pituitary tumors with AIP mutations.

 

The increased tendency of AIP mutation-positive tumors to invade locally may be a result of an altered tumor microenvironment. One study (40) observed markedly more infiltration of tumors by macrophages in human AIP mutation-positive adenomas compared to sporadic somatotroph tumors. There was also an upregulation in the tumor-derived cytokine, CCL5, which is chemotactic for leukocytes. The macrophages themselves may play an important role in breaching local structures with their secretion of matrix metalloproteinases (MMP2 & 9) (101). Gene expression profiling experiments comparing AIP mutation-positive human pituitary adenomas to sporadic human pituitary adenomas showed a partial epithelial to mesenchymal transition pattern in keeping with a tumor that invades locally but exceedingly rarely metastasizes (40). In recent years, intensive research on pituitary tumor microenvironment has expanded our knowledge on pituitary tumor behavior and tumorigenesis mechanisms and raised the possibility for immunotherapy in aggressive and refractory pituitary tumors (102).

 

In contrast, few studies have focused on the mechanisms of AIP regulation. miR-34, a microRNA that binds to the 3-UTR region of AIP, was shown to be overexpressed and to downregulate AIP at the protein level in sporadic somatotrophinomas with low AIP expression (103) and in somatotrophinomas due to germline AIP mutations (104). Additionally, the high expression of miR-34 is one of the mechanisms driving the increased intracellular cAMP levels seen in AIP mutation-positive tumors (104). Thus, overexpression of miR-34 promotes cell proliferation and migration and may be responsible for the invasive phenotype and typical resistance to first generation somatostatin analogues seen in these tumors (103, 104). Recently, a regulation of AIP at the transcription level was also proposed. GTF2B, a transcription factor that binds the 5-UTR region of AIP, was shown to promote AIP expression and inhibit somatotroph cell proliferation and invasion (105).

 

AIP Mutations and Associations with Other Tumors

 

Germline AIP variants (R304Q, this variant is controversial, likely to be benign) were noted in sporadic parathyroid adenomas in 2 (unrelated) out of 136 patients in one study. One of these patients had a co-existent MEN1 mutation; both had reduced AIP staining in their tumors at histology (48). Concomitant AIP and MEN1 deletions through chromosomal translocations with a variety of partners are also associated with hibernomas (benign brown fat tumors). AIP transcription is down-regulated in these tumors (106) and its loss results in the upregulation of the brown fat marker UCP1 (107). Two patients from different FIPA kindreds, carriers of germline pathogenic mutations in AIP (Leu115Trpfs*41 and p.Q285*) with unaffected pituitary, were described to have follicular thyroid carcinomas showing loss of heterozygosity in the AIPlocus in the tumor tissue (42, 108), raising the possibility for a role of AIP mutation as an initiating event in both pituitary and thyroid. However, differentiated thyroid carcinoma (DTC) is rare in acromegaly, and the most frequent tumor mutations found in patients with known pathogenic AIP mutations are very similar to the ones found in sporadic cases, mostly comprising mutations of BRAF and NRAS (108). Therefore, the potential role of AIP mutations as a possible rare initiating event on the pathogenesis of DTC, although unlikely, requires further investigation.

 

OTHER POSSIBLE CANDIDATE GENES

 

Currently, only two well-characterized genes have been implicated in the pathogenesis of FIPA: AIP, the most common one, and GPR101. However, they only account for a minority of patients with FIPA, while other genes remain largely unknown.

 

At present, the genetics of familial and apparently sporadic pituitary tumors is under active investigation and some new candidate genes have been identified, but additional data is required to convincingly support them as a possible cause of FIPA.

 

Recently, germline loss of function mutations in the peptidylglycine α-amidating monooxygenase (PAM) gene were described in one family with pituitary gigantism and in multiple sporadic cases of several types of pituitary adenomas (18). PAM plays an important role in post-translational processing and secretion of hormones and is highly expressed in all pituitary cells, but the mechanisms linking its altered function with hormone hypersecretion still require clarification. Also, the fact that some of the identified PAM variants were relatively common, and that no deleterious variants were identified in other familial cases from 17 FIPA kindreds in the validation cohort raises some reasonable doubts. Therefore, additional studies in FIPA kindreds are required to further explore and validate this new candidate gene.

 

Another gene, described in sporadic corticotrophinomas, is CABLES1. Heterozygous germline mutations in CABLES1appear to decrease the negative feedback response from glucocorticoids, resulting in increased corticotroph cell growth. They were identified in two young adults, two children with Cushing’s Disease, and in one unaffected parent (109); but, to date, there have been no reports of possible familial cases with this mutation. Cushing disease is only rarely described in FIPA families, mostly in kindreds with heterogeneous tumor types (19). In homogenous corticotroph adenoma families no CABLES1 mutation has been identified (Korbonits unpublished observation). Corticotrophinomas have not been reported in kindreds with AIP mutation (19), and this is also in line with the recently described RET-derived AIP tissue specificity for PIT1 expressing cells (99).

 

A gain of function mutation in PRLR has been described in association with sporadic and familial prolactinomas (110), but additional data is needed to convincingly reinforce that association. Other germline mutations have also been associated with familial pituitary tumors (RXRG, TH, CDH23)(53, 111, 112), but lack functional validation studies as well as independent confirmation to support them as possible candidates involved in the pathogenesis of FIPA (113).

 

Additional conditions with excess GH in the absence of pituitary tumors have been described, and include germline mutations in genes such as IGSF1 and NF1.

 

IGSF1 is a transmembrane glycoprotein that is highly expressed in the anterior pituitary and hypothalamus, and that is considered essential for normal hormone production (114-116). Loss-of-function mutations in IGSF1 have been associated with an X-linked syndrome of central hypothyroidism and a variable prevalence of other endocrinopathies, including disharmonious pubertal development with delayed testosterone rise but normal or advanced testicular growth and postpubertal macroorchidism, hyperprolactinemia and GH dysregulation (114, 117). A minority of male children with such mutations show partial and transient GH deficiency, while adults more often show high IGF-1 levels, a 2- to 3-fold increase in GH pulsatile and basal secretion and mild acromegaloid features (117, 118). Similar features of GH excess were observed in mice (117). A potentially pathogenic variant in IGSF1 was described in three individuals from the same family showing somato-mammotroph hyperplasia or tumor and gigantism (115), but, to date, most case series of patients with IGSF1 mutations have consistently showed normal height and no evidence of pituitary tumors (116, 117, 119). It has been proposed that IGSF1 acts as a regulator of pituitary hormone synthesis, but the mechanism behind this is still poorly understood (114, 117).

 

Pathogenic mutations in the NF1 gene lead to neurofibromin deficiency and neurofibromatosis type 1 (NF-1). NF-1 is an autosomal dominant condition with increased risk of several benign and malignant tumors, including optic pathway gliomas (OPG), that are frequently diagnosed at a young age. An association between NF-1 and increased growth velocity or tall stature due to GH excess has been described in several case series, with a prevalence ranging from 4.5% (120) to 46% (in large deletions of NF1) (121). Excess GH is diagnosed in children with NF-1 and OPG, with a prevalence of 10.9% in this patient group according to the largest series published (122). The most plausible and widely accepted mechanism to explain this association is an induced hypothalamic dysfunction from infiltrative OPG, with reduced somatostatinergic inhibition of GH secretion, corresponding to the fact that there is absence of other pituitary abnormalities in the majority of cases (123). Another suggestion is that GPR101 dysregulation may occur. However, there are some case reports of NF-1 with concomitant pituitary hyperplasia or tumor, with or without OPG, which leads to the hypothesis that GHRH overexpression may be another possible mechanism leading to excess GH (123). Nevertheless, the pathophysiology of GH excess in NF-1 remains to be clarified.

 

GERMLINE CHROMOSOMAL DEFECTS PRESENTING WITH PITUITARY HYPERSECRETION/GIGANTISM- XLAG

 

This is a unique condition described in 2014 caused by a microduplication at Xq26.3 area containing the GPR101 gene, resulting in the overexpression of the orphan G protein coupled receptor GPR101 (13). It may be familial or sporadic, and can be due to a germline or a mosaic somatic mutation (14, 15). It shows an X-linked dominant inheritance with complete penetrance. Most cases are de novo germline (female) or mosaic (males) cases, with, to date, only three kindreds described where affected mothers passed on the mutation to male offspring (124-126). It constitutes 8-10% of the cases with gigantism (125, 127), and practically all the non-syndromic infant-onset gigantism.

 

XLAG Characteristics

 

In addition to the most prominent symptom of very early-onset gigantism with significantly elevated growth velocity, acral enlargement and coarse facial features are also observed (37). Fasting hyperinsulinemia was noted in 1/3 of patients and around 20% had acanthosis nigricans (125). Elevated BMI is often observed, and up to 1/3 of patient with XLAG have increased appetite, something not noted previously in gigantism. Hyperprolactinemia accompanies the GH excess in over 80% of the cases. Three quarters of the patients are females. GHRH levels can be normal or slightly elevated, and in some patients a paradoxical response was seen to the TRH test (127).

 

Tumor Types

 

All GPR101 duplication-related pituitary tumors described so far are GH producing, with the majority also secreting prolactin. There are a few cases of pure GH excess patients, some of these with hyperplasia rather than tumor (128). A rare GPR101 germline variant (p.E308D) does not play a role in somatotrophinoma tumorigenesis based on human (127, 129, 130) and in vitro data (131).

 

Age of Onset

 

Accelerated growth has been reported as early as 2-3 months of age (125), and abnormal hormone levels started to develop soon after birth in a prenatally diagnosed case (126). The median age of onset of rapid growth is at 1 year (range 0.5-2) with a median age at diagnosis being 3 years old (range 1-22) (13, 132).

 

Somatic Mosaicism

 

It seems that male patients, except the few familial cases, in which a germline duplication is inherited from an affected mother (124-126), have mosaic GPR101 duplication with pituitary tissue (and other tissues) showing the microduplication, while blood-derived DNA is negative or has a low level of mutation burden (14, 15, 127). The phenotype of somatic and germline GPR101 duplication patients is the same (132).

 

Tumor Behavior

 

SIZE

 

The size of the pituitary is variable in XLAG cases ranges from large tumors (133) to pituitary hyperplasia (13, 14, 127). It is currently unclear why some patients develop tumors while others have hyperplasia, both have been described in males and females. While Ki-67 is low in the tumor samples in most cases and such tumors do not show any tendency to invasion or apoplexy (127), invasive growth and a high Ki-67 has also been described (126, 133).

 

HORMONE SECRETION

 

Xq26.3 microduplication tumors invariably secrete GH and frequently also prolactin (13, 125). Random levels of GH were markedly raised in one study of 18 XLAG patients with a median of 52.5 times the upper limit of normal (range 6-300 times upper limit of normal) (125).

 

TREATMENT

 

Treatment of XLAG is complex and the tumors may grow rapidly, producing not only local effects due to their size but also causing worsening systemic manifestations of gigantism through their hormone production if not treated promptly (133). Despite widespread expression of type 2 somatostatin receptors, it has proved difficult to control GH levels in XLAG with somatostatin analogues or prolactin with dopamine agonists, even at relatively high doses. Extensive neurosurgery is often needed and effective, but the rates of post-operative hypopituitarism are high (125). In contrast, radiation therapy typically does not lead to disease control (125, 133). First generation somatostatin analogues are also usually ineffective in controlling GH hypersecretion, even in the presence of high tumor expression of somatostatin receptor 2 (125). In patients not controlled by surgery, the GH antagonist pegvisomant has proven effective in controlling IGF-1 levels (14, 41, 125, 128), but radiotherapy may be used as an alternative for tumor control if radical surgery is not possible. Patients with pituitary hyperplasia have previously been treated with hypophysectomy (134), while now combined treatment with somatostatin analogue, cabergoline and pegvisomant provides appropriate control (14). If lesion control and prolactin is not an issue, then patients can be treated just with pegvisomant (135).

 

Mechanism of Tumorigenesis in XLAG

 

It is unclear what role the hypothalamus plays and what is the role of the pituitary tissue in this disease. As some patients do not have a tumor, but produce very high level of GH, abnormal hypothalamic regulation could play a key role. Indeed, some patients have elevated circulating GHRH levels and mutated cells respond strongly to GHRH (136). GPR101 is strongly expressed in the normal pituitary during fetal development, from 19 weeks of gestation onwards, with levels declining through to ‘very low’ in adult life, suggesting a role in pituitary maturation (137). It is strongly over-expressed (both mRNA and protein) in the pituitary lesions of XLAG patients (131, 138). A recent paper has identified the mechanism for this. The duplication disrupts the regulatory region borders around the GPR101 gene (the so-called topologically associated domain or TAD) and this leads to overexpression of GPR101 by regulatory elements that normally do not regulate the expression of this gene (139). Therefore, XLAG is the first endocrine TADopathy. GPR101 has been shown to strongly activate the cAMP pathway. This therefore suggests a mechanism by which its overexpression may lead to tumorigenesis. The transient overexpression of GPR101 in GH3 rat pituitary tumor cells produced increased cellular proliferation and an increase in GH secretion, supporting this hypothesis (13).

 

MICRODELETION CAUSING GHRH OVEREXPRESSION

 

This novel condition, described for the first time in 2023 (17), is another genetic cause of severe non-syndromic infant-onset gigantism. It is caused by a heterozygous microdeletion upstream of the GHRH gene, in chromosome 20, that leads to aberrant splicing and produces a chimeric mRNA consisting of exon 1 of the TTI1 gene followed by all the coding exons of the GHRH gene. Since TTI1 is ubiquitously expressed and exon 1 has features of an active promotor, this fusion gene leads to constitutive GHRH overexpression and ectopic production of GHRH. There is only one case described so far, in a Japanese woman, that unfortunately already passed away. Her clinical phenotype was very similar to X-LAG, with significant weight gain starting a few months after birth and rapid growth diagnosed in the first years of life. She had marked GH elevation, prolactin elevation and no evidence of pituitary tumor in the MRI. She had no familial history of tall stature. Treatment with radiotherapy and bromocriptine did not ensure a complete biochemical response and the patient reached an adult height of 197.4 cm. Genome-edited mice with this mutation exhibited the same phenotype of prominent growth starting in the first weeks of life, pituitary hyperplasia and GHRH expression in several tissues besides the hypothalamus, validating the hypothesis that pituitary gigantism was driven by constitutive GHRH overexpression due to an acquired promoter.

 

CLINICAL MANAGEMENT IN FIPA

 

Pituitary adenoma patients with family members also with pituitary adenoma need to be studied for signs and symptoms of MEN1 and Carney complex (Figure 7). If MEN1 and Carney complex are ruled out by the family history and biochemical and clinical assessment of the index patient and family members, the diagnosis of FIPA needs to be considered. These patients would benefit from referral to genetic counselling. Currently, patients can be offered screening for AIP mutations. Childhood-onset pituitary adenoma cases, even without family history, should also been offered genetic counselling and screening for AIP mutation, as a high percentage of young-onset GH-secreting adenomas show mutations in the AIP gene (20, 60, 140, 141). Around 12% of patients diagnosed with a pituitary tumor before the age of 30 years (and 20% of pediatric patients) were found to have a germline AIP mutation in one study (142) and so it has been recommended that AIP mutation screening be conducted in anyone diagnosed with a somatotroph or lactotroph adenoma or a macroadenoma (diameter >10mm) before the age of 30 years (143), and also in any cases of gigantism. One study which examined the incidence in apparently sporadic young-onset pituitary adenoma patients found 6.8% to have an AIP mutation, with a slightly lower incidence of 10.5% in those sporadic patients with somatotrophinomas. Reassuringly, the incidence of mutation in sporadic prolactinoma was only 1.5% (12).

 

Those diagnosed with a pituitary tumor after the age of 40 years are unlikely to have a germline mutation (none were found in a sample of 443 patient with pituitary adenomas of all histiotypes) (57) and so screening in this latter population is likely to be unrewarding.

The phenomenon of phenocopy needs to be kept in mind both in AIP mutation-positive and AIP mutation-negative families (16, 23).

 

Figure 7. Proposed strategy for evaluating the patient with pituitary adenoma with (A) – negative family history and (B) – positive family history (*rare case report).

It is suggested that family members of an AIP mutation-positive proband should undergo genetic testing (Figure 8 suggests a strategy for this process), though this testing may involve significant numbers of people from the affected family and is probably best carried out in genetic centers that are able to arrange testing and counselling of many people, have experience of discussing results of screening, and can maintain family registers (22). Salivary DNA testing is available for those that are needle-phobic.

 

Figure 8. A proposed strategy for family screening in a family with an AIP mutation-positive proband. *Family member are first degree relatives of those with AIP mutations, or of obligate carriers. Further screening targets are then identified through genetic testing.

 

AIP mutation carriers should be referred to an endocrine service (pediatric or adult) for baseline assessment (clinical examination, biochemical testing, and MRI) (141). MRI can be delayed for young children if clinical and biochemical results are normal (143).  Children aged 4 years and older should be evaluated annually, with height and weight measurements, height velocity, and pituitary function testing (143). The frequency of imaging surveillance if biochemical and clinical findings are normal is difficult to judge with the available data: every 5 years was suggested until the age of 30 (143), with annual clinical assessment and basal hormone profiling (19). More recently, the emergence of an inverted-U shape pattern to the age of onset has led to the suggestion that if there is no evidence of disease at the age of 20 years, then surveillance protocols can be relaxed slightly (12).

 

The youngest case identified of AIP mutation-positive patient with a large macroadenoma with apoplexy was 4 years old with significant symptoms and rapid growth velocity already from age 3 years (43). Although only 15% of the AIP cases present symptoms before the age of 10 years (19), and the above mentioned patient is the single case known presenting before the age of 5 years, these data need to be taken into account when counselling AIP mutation-positive families for the timing of genetic screening and starting clinical follow-up (141, 143).

 

If AIP screening, which includes exons, exon-intron junction and promoter area sequencing as well as MLPA is negative, then currently no further genetic screening is possible. In AIP mutation-negative family’s potential carriers with a 50% chance inheriting the disease-causing mutation should be offered clinical assessment. The age of first clinical assessment of family members in AIP-negative families should be around early teenage years as the current youngest case was found at the age of 12 years (143).

 

We have already prospectively diagnosed several pituitary adenomas (both functioning macroadenomas and non-functioning microadenomas) in our cohort in both AIP-positive and AIP mutation-negative families (12, 60).  Screening allows the early detection and treatment of those with adenomas, perhaps before the endocrine effects become apparent or before the local effects of tumor bulk are problematic. It is important to draw the attention of the family to the possible symptoms of pituitary disease, as awareness of symptoms results in earlier diagnosis of the disease in subsequent generations (1, 11). Data on long-term follow-up of asymptomatic carriers is currently being collected. In our clinic, we see asymptomatic young (<30 years old) carriers once a year and after a normal baseline MRI we will consider a repeat MRI in 5 years. We consider relaxing follow-up at 30 years and stopping follow-up at 50 years for AIP mutation-positive family members if no tumor has been detected by this time.

 

The relatively high frequency of pituitary incidentalomas in the general population (144) also needs to be carefully considered both in AIP positive and negative cases. One paper (22) has suggested repeating an MRI pituitary and hormone testing at 6 months after the discovery of a pituitary incidentaloma in AIP mutation-positive individual with normal biochemistry, with annual hormone testing thereafter if the MRI was unchanged.

 

Those with apparently cured AIP mutation-positive tumors (but without external beam radiotherapy) should be followed up carefully as any residual pituitary tissue will be heterozygous for the AIP mutation and so there is a risk of the occurrence of further pituitary adenomas (22).

 

SUMMARY

 

FIPA is a condition where there is an inherited propensity to the development of pituitary adenomas. The causative gene for the vast majority (76%) of kindreds is unknown: 21% of these have a mutation in the AIP gene, 3% have a duplication on the X chromosome (X-linked acrogigantism, XLAG).

 

There are significant phenotypic differences between these groups, with XLAG presenting with infant-onset gigantism (range 0.5-2 years) most often with prolactin co-secretion, AIP cases presenting with childhood-onset GH or prolactin-secreting tumors, while the spectrum of AIP-negative FIPA kindred represent the full spectrum of pituitary adenoma subtypes with age of onset between the ages of 20 and 50 years with a peak incidence around the age of 30 years.

 

FIPA patients are more likely to have larger (macroadenomas), more aggressive tumors, and an earlier onset of disease compared to sporadic pituitary adenomas. AIP mutation-positive tumors are more likely to be larger and invade the extrasellar region than sporadic adenomas. It has also been observed that the AIP mutated adenomas are more prone to undergoing apoplexy than AIP mutation-negative adenomas. All XLAG tumors described so far are GH producing, with a majority also secreting prolactin. XLAG can result in a spectrum of pituitary gland appearances, ranging from large adenomas to pituitary hyperplasia. The tumors tend not to invade or undergo apoplexy.

 

AIP mutated adenomas are more difficult to treat than their non-mutated counterparts, they are more likely to be resistant to somatostatin analogue therapy, more likely to require radiotherapy, and have higher rates of failure to gain control of IGF-1 with pegvisomant treatment.

 

Treatment of XLAG is also challenging. Tumors can grow rapidly and are difficult to control even with high doses of somatostatin analogue or dopamine agonists. Pegvisomant is effective in normalizing IGF-1, while tumor control may need radical surgery or radiotherapy.

 

FIPA Diagnosis and Screening

 

The first step in trying to establish a diagnosis in patients with pituitary adenomas and with a family history of pituitary adenoma should be to exclude MEN1 and Carney complex. This can be achieved through the taking of a thorough family history and through the clinical and biochemical assessment of the index patient, and if possible other affected family members. If these conditions are excluded then the diagnosis of FIPA should be considered, and these patients should be referred for genetic counselling. Additionally, any childhood onset pituitary adenoma case (irrespective of family history), any somatotroph or lactotroph adenoma, or any macroadenoma diagnosed before the age of 30 and any cases of gigantism should all be referred for genetic counselling. No cases of AIP germline mutation were found in a large study of patients diagnosed with a pituitary tumor after the age of 40 years – and for this reason, genetic screening in this population is unlikely to be rewarding.

 

AIP mutation carriers should be referred to an endocrine service (pediatric or adult) for baseline assessment (clinical examination, biochemical testing, and MRI). MRI can be delayed for young children if clinical and biochemical results are normal. Children aged 4 years and older should be evaluated annually, with height and weight measurements, height velocity, and pituitary function testing. If biochemical and clinical findings are normal then 5-yearly MRIs until the age of 30, with annual clinical assessment and basal hormone profiling, is the suggested follow-up protocol.

 

For AIP positive families we suggest starting genetic screening as soon as the family agrees as the youngest case identified was at the age of 4 years with 1-year history of symptoms, presenting with a large macroadenoma.

 

If AIP screening, which includes exons, exon-intron junction and promoter area sequencing as well as multiple ligation probe amplification (MLPA), is negative, then currently no further genetic screening is possible. In AIP mutation-negative families, potential carriers with a 50% chance of inheriting the disease-causing mutation should be offered clinical assessment. The age of first clinical assessment of family members in AIP negative families should be around early teenage years as the current youngest case was found at the age of 12 years.

 

Prospectively-diagnosed pituitary adenomas have been shown to have a better outcome. Screening allows the early detection and treatment of those with adenomas, perhaps before the endocrine effects become apparent or before the local effects of tumor bulk become problematic. It is important to draw the attention of the family to the possible symptoms of pituitary disease, as awareness of symptoms results in earlier diagnosis of the disease in subsequent generations. In unaffected AIP mutation carriers, follow-up can be relaxed at the age of 30 years if no tumor has been detected by this time, and follow-up can cease at 50 years, based on the available data. The relatively high frequency of pituitary incidentalomas in the general population also needs to be carefully considered both in AIP positive and negative family members. One strategy involves repeating an MRI pituitary and hormone testing at 6-12 months after the discovery of a pituitary incidentaloma in AIP mutation-positive individuals with normal biochemistry, with annual hormone testing thereafter if the MRI is unchanged.

 

ACKNOWLEDGEMENT

 

We are grateful for Dr Craig Stiles (Barts Health NHS Trust, London), who contributed to the previous version of this Endotext chapter.

 

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Gastrointestinal Disorders in Diabetes

ABSTRACT

 

Gastrointestinal manifestations of type 1 and 2 diabetes are common and represent a substantial cause of morbidity and health care costs, as well as a diagnostic and therapeutic challenge. Predominant among them, and most extensively studied, is abnormally delayed gastric emptying or diabetic gastroparesis. Abnormally increased retention of gastric contents may be associated with symptoms, including nausea, vomiting, postprandial fullness, bloating, and early satiety, which may be debilitating. However, the relationship of upper gastrointestinal symptoms with the rate of gastric emptying is relatively weak. Moreover, gastrointestinal symptoms also occur frequently in people without diabetes, which may compromise the capacity to discriminate gastrointestinal dysfunction resulting from diabetes from common gastrointestinal disorders such as functional dyspepsia. A definitive diagnosis of gastroparesis thus necessitates measurement of gastric emptying by a sensitive technique, such as scintigraphy or a stable-isotope breath test. There is an inter-dependent relationship of gastric emptying with postprandial glycemia. Elevated blood glucose (hyperglycemia) slows gastric emptying while, conversely, the rate of emptying is a major determinant of the glycemic response to a meal. The latter recognition has stimulated the development of dietary and pharmacological (e.g. short-acting GLP-1 receptor agonists) approaches to improve postprandial glycemic control in type 2 diabetes by slowing gastric emptying. The outcome of current management of symptomatic diabetic gastroparesis is often sub-optimal - optimizing glycemic control, the correction of nutritional deficiencies, and use of pharmacotherapy, are important. A number of promising and novel pharmacotherapeutic agents are in development. This chapter focusses on gastric motor function, but also provides an overview of the manifestations of esophageal, gall bladder, small and large intestinal function, in diabetes.

 

INTRODUCTION

 

The gastrointestinal tract extends from the mouth to the anus and performs functions vital to sustaining life including ingestion, breakdown and digestion of nutrients, facilitating nutrient absorption and preparation and expulsion of the waste product. Gastrointestinal symptoms occur commonly in people with diabetes, and include gastro-esophageal reflux, bloating, nausea, constipation, diarrhea, and fecal incontinence. It has been suggested that more than 50% of individuals attending outpatient diabetic clinics will at some stage experience a distressing gastrointestinal symptom. Gastrointestinal motor dysfunction is also common in diabetes and may have an impact on glycemic control. Of the motor dysfunctions, gastroparesis, or delayed gastric emptying, is the most important and will be discussed in relatively greater detail. This chapter is limited to the gastrointestinal manifestations of type 1 and 2 diabetes and does not address other causes of diabetes, such as that related to cystic fibrosis. 

 

GASTROINTESTINAL SYMPTOMS

 

Gastrointestinal symptoms are exhibited frequently in type 1 and 2 diabetes and most, but not all, studies suggest that they are significantly more common in diabetes than in controls without diabetes (1); reported inconsistencies likely reflect discrepancies in the methodology used and the patient populations studied. It should be appreciated that gastrointestinal symptoms are often not volunteered, particularly those considered embarrassing (such as fecal incontinence) and it would not be surprising if current estimates are less than is really the case. Symptoms, unfortunately, continue to be evaluated in clinical trials solely using participant ‘self-report’ despite its appreciated unreliability, rather than simple, validated measures, which are used widely in the assessment of ‘functional’ gastrointestinal disorders (e.g.  irritable bowel syndrome and functional depression) (2)  (1)  Symptoms appear to be more common in women with diabetes, as is the case with functional gastrointestinal disorders (3). While it is unclear whether symptom prevalence varies between type 1 and type 2 diabetes there is no doubt that gastrointestinal symptoms have a substantial negative impact on quality of life in people with diabetes (4). There is, however, a poor correlation between gastrointestinal symptoms and measures of function, such as the rate of gastric emptying. The natural history of gastrointestinal symptoms remains poorly defined, although it is known that onset and disappearance of symptoms is common i.e. there is considerable ‘symptom turnover’ - approximately 15-25 % over a 2-year period has been observed in type 2 patients (1). This symptom turnover has been reported to be associated with the onset of depression, but not with autonomic neuropathy or glycemic control (5).  

 

GASTROINTESTINAL MANIFESTATIONS IN DIABETES

 

Esophagus

 

The esophagus, a muscular tube connecting the pharynx to the stomach, enables propulsion of swallowed food, with a sphincter at either end (the upper and lower esophageal sphincters) to prevent esophago-pharyngeal and gastro-esophageal reflux, respectively.

 

Two common esophageal symptoms are heartburn (as part of gastro-esophageal reflux disease) and dysphagia (potentially indicating esophageal motor dysfunction). Techniques to evaluate esophageal motility include conventional and high-resolution manometry (HRM). Scintigraphy can measure esophageal transit but has not been standardized and is not commonly employed in clinical settings.

 

The relationship between esophageal transit and gastric emptying in diabetes is poor (6). Acute hyperglycemia inhibits esophageal motility (7), and reduces the basal lower esophageal sphincter pressure (8). While the esophagus has been less well studied than the stomach, it is clear that disordered esophageal function occurs frequently and that disordered motility in both the esophagus and stomach may share a similar pathogenesis. It has been postulated that the major mechanism underlying esophageal dysmotility is a reduction of cholinergic activity and vagal parasympathetic dysfunction (9). The pathological abnormalities associated with gastroparesis, such as a reduction in interstitial cells of Cajal and inhibitory intrinsic neurons, have also been postulated to be relevant to esophageal dysmotility (10). Diffuse esophageal muscular hypertrophy was reported in two-thirds of people with diabetes in one case series (11).

 

There are limited evidence-based options for the management of esophageal disorders in diabetes. General measures include lifestyle modifications (improved glycemic control, weight loss, dietary modifications, and physical exercise). Prokinetic agents have been used, albeit with limited evidence to support efficacy. The latter include dopaminergic agents (metoclopramide, domperidone), serotonin receptor agonists (cisapride), and motilin agonists (erythromycin). Botulinum toxin was trialed in a pilot study in patients with achalasia (including those with diabetes) and peripheral neuropathy and improvements in effective peristalsis induction and contraction amplitude were reported (12).

 

Gastro-esophageal reflux disease (GERD) is extremely common in the general population and also frequently seen in diabetes. In non-erosive GERD, treatment involves lifestyle measures (bed elevation of 30 degrees at head- end) and use of proton-pump inhibitors. In a community study, a reduced rate of heartburn was found in type 1 patients when compared with a control population (13), although this observation remains to be confirmed and the implications are unclear.

 

Disordered esophageal motility, especially the elderly, increases the risk of ‘pill-induced esophagitis’, with mucosal injury due to prolonged exposure to impacted medications (14). Diabetes is an independent risk factor (14), and the condition usually presents as chest pain with or without odynophagia. Treatment involves withdrawal of the offending agent and use of proton pump inhibitors (15).

 

Stomach - Diabetic Gastroparesis

 

INTRODUCTION

 

Delayed gastric emptying in diabetes was first reported almost a century ago, but it was Kassander who, in 1958, documented asymptomatic increased gastric retention of barium in diabetes and coined the descriptive term ‘gastroparesis diabeticorum’ (16). Interestingly, Kassander also suggested in their paper that gastroparesis could adversely impact glycemic control. Some sixty years on, diabetic gastroparesis, traditionally defined as abnormally delayed gastric emptying of solid food in the absence of mechanical obstruction, remains a diagnostic and management challenge (17). Gastroparesis occurs in both type 1 and 2 diabetes and may not, necessarily, be indicative of a poor prognosis (18,19).

 

The rate of gastric emptying is now appreciated as a major determinant of postprandial glycemia in both health and diabetes (20), and novel anti-diabetic medications, such as short acting GLP-1 receptor agonists, diminish postprandial glycemic excursions predominantly by slowing gastric emptying, are used widely.

 

EPIDEMIOLOGY OF DIABETIC GASTROPARESIS

 

The ‘true’ incidence and prevalence of diabetic gastroparesis globally remain uncertain largely due to inconsistencies in the definition of gastroparesis, study populations, and methodology. It is, however, clear that diabetes is a leading cause of gastroparesis, accounting for about 30% of cases in tertiary referral studies (17). A recent analysis of data from the follow-up arm of the landmark prospective study in type 1 diabetes, called DCCT-EDIC (Diabetes Control and Complications –Epidemiology of Diabetes Interventions and Complications) found that delayed gastric emptying of a solid meal occurred in 47% of this population, consistent with the prevalence reported in other cross-sectional studies (21). Previously believed to be essentially a complication of advanced type 1 diabetes (T1D), it is now apparent that gastroparesis also occurs frequently in type 2 diabetes (T2D) (16,22). Risk factors for gastroparesis include a long duration of diabetes, the presence of other microvascular complications, female gender, obesity. and smoking (17).  In a recent report from the NIH Gastroparesis Consortium, the proportion of T1D and T2D was comparable (although many more people with T2D have gastroparesis as its prevalence is much higher), although a US-based community study based on symptomatic cases, reported an incidence of approximately 5% in T1D and 1% in T2D (compared with 0.01% in controls) (23). Data from the US indicate that hospitalizations due to diabetic gastroparesis rose 158% between 1995-2004, which may reflect a true increase in incidence and / or greater clinical awareness of the condition (24). Not surprisingly, health care costs related to diabetic gastroparesis have also increased substantially in recent years. It should, however, also be noted that the awareness of the central importance of glycemic control to the development and progression of microvascular complications, and the consequent increased priority in management to improve it, may have led to a reduction in the incidence of gastroparesis. Consistent with this, it has recently been shown that in well-controlled T2D, even when longstanding, the prevalence of gastroparesis is low and, not infrequently, gastric emptying is modestly accelerated (19,25).

 

DIAGNOSIS OF DIABETIC GASTROPARESIS

 

As alluded to, the presence of gastrointestinal symptoms is poorly predictive of delayed gastric emptying. It is well established that patients with debilitating upper GI symptoms may have normal, or even rapid emptying, while others with unequivocally markedly delayed emptying may report few, or no symptoms. Measurement of gastric emptying, after exclusion of mechanical obstruction at the gastric outlet or proximal small intestine is, accordingly, mandatory for a formal diagnosis of gastroparesis, for which scintigraphy, developed in the 1970s, remains the ‘gold standard’ technique. An attempt has been made to standardize the methodology, with the American Neurogastroenterology and Motility Society and the Society of Nuclear Medicine defining gastroparesis by the intra-gastric retention of >60% of a standardized meal at 2 hours and/or >10% at 4 hours (26). The test meal advocated in the consensus statement comprises two egg-whites, two slices of bread and jam (30 g) with water (120 ml), providing 255 kcal with little fat (72% carbohydrate, 24% protein, 2% fat and 2% fiber) (26). While a useful exercise, the probability of universal adoption of a specific meal, especially outside Western cultures, is intuitively low. The advantages of scintigraphy are its capacity for precise, concurrent measurement of both solid and liquid meal components (the ‘consensus’ test meal only labels the solid component); however, it involves radiation exposure and requires sophisticated equipment and technical expertise. Acceptable alternatives include 13C based breath tests and ultrasonography, neither of which involve radiation exposure, although the latter is operator-dependent (22). Newer techniques, such as the wireless motility capsule, MRI, and SPECT imaging have emerged, but at present these should be considered less accurate than scintigraphy and / or only relevant to a research setting (17).

 

PATHOGENESIS OF DIABETIC GASTROPARESIS

 

Gastric emptying is a complex, coordinated process by which chyme is delivered to the small intestine at a tightly regulated rate and involves the gastro-intestinal musculature, nervous systems (intrinsic and extrinsic), gastric ‘pacemaker’ (so-called ‘Interstitial cells of Cajal or ICC), immune cells, and fibroblast-like cells that stain positive for platelet derived growth factor receptor alpha.  In the fasting state, a cyclical pattern of contractile activity known as the ‘migrating motor complex’ (MMC) sweeps from the stomach through to the small intestine, which serves a “housekeeping’ role i.e. facilitating the movement of ingestible food particles and bacteria from the stomach through the intestine (27). There are distinct phases of the MMC: phase I consists of motor quiescence lasting approximately 40 min, phase II, approximately 50 min, is comprised of irregular contractions, and phase III is characterized by regular contractions (at approximately 3 per min in the stomach and about 10-12 per small intestine) for 10 min during which the bulk of indigestible solids are emptied (28). Following meal ingestion, the MMC is replaced by a ‘postprandial’ motor pattern. Solids are then mixed with gastric acid and ground into small particles (usually < 1-2 mm) in the distal stomach. Gastric accommodation is mediated by vagal and nitrergic mechanisms, antral contractions by vagal and intrinsic cholinergic mediation, and pyloric relaxation by nitrergic mechanisms (17). The resultant chyme is delivered through the pylorus to the proximal duodenum predominantly in a pulsatile manner (22,27). It is now appreciated that the rate of emptying is regulated primarily by nutrient-induced inhibitory feedback arising from the small intestine, rather than by ‘intragastric’ mechanisms (29). Digestible solids and high nutrient liquids empty from the stomach in an overall linear fashion as a result of this feedback (6); solid emptying is preceded by an initial so-called ‘lag-phase’ of 20-40 min during which solids are ground into small particles. In contrast to solids, low or non-nutrient liquids empty in an overall, volume-dependent, monoexponential pattern because small intestinal feedback is less (27). A number of gut peptides play a key role in providing intestinal feedback, including GLP-1, CCK, and peptide YY. In contrast, ghrelin and motilin, which accelerate gastric emptying, are suppressed following food intake (22,27). Both the length and region of small intestine exposed to nutrients modulate feedback to slow gastric emptying (30).

 

Disordered gastric emptying represents the outcome of impairments of variable combinations of these diverse components. Advances in understanding the underlying pathophysiology have been made over the past decade, particularly through the efforts of the NIH-funded Gastroparesis Clinical Research Consortium. Histological studies from this group and others have shown a reduction in the number of interstitial cells of Cajal in diabetic gastroparesis, which correlates with the magnitude of delay in emptying (31). Interstitial cells of Cajal loss appears to be driven by an immune infiltrate involving a shift from protective M2, to classically activated, M1 macrophages, with defective regulation of heme oxygenase-1 and resultant oxidative stress.  Altered expression of the Ano-1 gene which influences conduction in the Interstitial cells of Cajal has also been reported (32). A reduction in inhibitory neurons expressing nitric oxide synthase also appears to contribute (31).

 

GASTRIC EMPTYING AND GLYCEMIA (FIGURE 1)

 

Figure 1. Bidirectional relationship between gastric emptying and glycemia. Abbreviations: CCK, cholecystokinin; GIP, gastric inhibitory polypeptide; GLP-1, glucagon-like peptide 1; PYY, peptide YY. Reproduced with permission from Philips et al (22)

 

Gastric emptying exhibits a wide inter-individual variation (ranging between 1-4kcal /min in health and even wider in diabetes because of the high prevalence of gastroparesis and, less often, abnormally rapid emptying) (Figure 2).  Gastric emptying is a major determinant of postprandial glycemia across glucose-tolerant states and these relationships are time- dependent. In individuals with normal glucose tolerance, following a 75g oral glucose drink, the early (approximately 30 min) rise in glucose is directly proportional to the rate of emptying, while the 120 min value (the standard endpoint in an OGTT) is inversely related. This relationship shifts to the right as glucose tolerance worsens, such that both 30- and 120-min glucose values are directly proportional to the gastric emptying rate in type 2 diabetes(33-35).Epidemiological studies indicate that about 50% of people with impaired glucose tolerance or IGT will develop frank type 2 diabetes and, hence, factors affecting progression are of considerable interest. We have shown that the disposition index – a predictor of progression to type 2 diabetes – is inversely related to the gastric emptying rate (36), suggesting that the rate of emptying may influence the progression. There is evidence that the 1-hour plasma glucose level in a 75g oral glucose tolerance test is strongly associated with the risk of future type 2 diabetes (37)  and this is known to be dependent on the rate of gastric emptying (34,35).

 

In type 2 diabetes, slowing of gastric emptying (such as by morphine) reduces the postprandial glycemic profile, while accelerating emptying by pro-kinetics (such as erythromycin) increases it (38). Bypassing the stomach and delivering glucose directly into the small intestine at specified rates (within the physiological span of gastric emptying) via naso- duodenal catheters has been used as a model to characterize the impact of gastric emptying on glycemia. These 'surrogate' studies indicate that gastric emptying is a major determinant of postprandial insulin secretion and the magnitude of the so-called 'incretin' effect (the augmented insulin secretory response to oral or enteral, compared with intravenous, glucose). Moreover, the relative contribution of the two 'incretin' hormones (GIP and GLP-1) to the incretin effect in health varies such that GIP is the predominant contributor when glucose enters the small intestine at 2kcal/ min or less, with GLP-1 contributing only at higher rates of duodenal glucose delivery (3 or 4 kcal per min) (39). It is, therefore, likely that the relative contributions of GIP and GLP-1 to the postprandial insulin response and glycemia depend on an individual’s intrinsic rate of emptying. Variations in blood glucose also affect gastric emptying. Through ‘glucose clamp’ studies, we have shown that abrupt elevations in blood glucose slows gastric emptying in a ‘dose- dependent’ manner, i.e. the slowing is dependent on the magnitude of the elevation in blood glucose (7).  Moreover, when blood glucose is ‘clamped’ at about 8 mmol/L or 144 mg/dL (i.e. physiological hyperglycemia), gastric emptying is modestly slower in both health and well-controlled type 1 diabetes (40). This may, however, not apply to spontaneous elevations in blood glucose (41)  and further clarification is required. On the other hand, acute hypoglycemia (blood glucose about 2.6 mmol/L or 46.8 mg/dL) accelerates gastric emptying markedly in both groups (42), and is likely to represent an important counter-regulatory mechanism. It follows that the acute glycemic environment, by altering gastric emptying, is likely to influence intestinal absorption of nutrients, as well as oral medications, which has hitherto been poorly appreciated in clinical practice. The impact of chronic glycemic control on gastric emptying remains uncertain.

 

Figure 2. Gastric emptying of solids (minced beef) (A), shown as the retention at 100 min (percent); and the gastric emptying of liquids (10% dextrose) (B), shown as the 50% emptying time (minutes) in 101 outpatients with diabetes. The normal range is indicated by the shaded area. Reproduced with permission from Jones et al (43).

 

In people with insulin-treated diabetes (type 1 or type 2), it is important to match exogenous insulin delivery with the availability of carbohydrate to minimize the risk of postprandial hypoglycemia. It is, therefore, intuitively likely that delayed gastric emptying predisposes to lower blood glucose concentrations in the early postprandial period (so-called ‘gastric hypoglycemia’) (44) and subsequent hyperglycemia. A study in type 1 patients reported that insulin requirements were lower in those with gastroparesis during the first 120 min post-meal, but greater during 180-240 min, compared to patients with normal gastric emptying (45). It is increasingly appreciated that greater glycemic variability is associated with worse outcomes (46). Knowledge of the rate of emptying may potentially assist the clinician in developing strategies to reduce postprandial glycemic variability in individual patients, although this needs to be evaluated formally.

 

MANAGEMENT OF SYMPTOMATIC GASTROPARESIS (FIGURE 3)

 

Figure 3. Treatment Algorithm for Diabetic Gastroparesis. PRN, as needed. Reproduced with permission from Du et al (1)

 

General Measures

 

Management of gastroparesis should be individualized. In clinical practice, patients are generally advised to consume small, frequent meals that are low in fat and fiber, with more calories as liquids than solids; ingested solids should be those that fragment readily into small particles (47). It should, however, be noted that this advice has not been rigorously evaluated and may be difficult to adhere to, so that the involvement of a dietitian is recommended (48). While optimizing glycemic control is intuitively important, given the inhibitory effect of acute hyperglycemia on gastric emptying, this has not been clearly established to be the case in the chronic setting, although the use of continuous subcutaneous glucose infusion and continuous glucose monitoring has recently been advocated (49).

 

Concurrent medications should be reviewed and, if possible, those which may slow gastric emptying (e.g. opiates, anticholinergics) ceased. In this regard, it should be appreciated that short-acting GLP-1 receptor agonists (e.g. exenatide BD and lixisenatide) and the amylin analogue, pramlintide (50), improve chronic glycemic control primarily by slowing gastric emptying.

 

Medications

 

Although studies involving pro-kinetic medications for treatment of gastroparesis have nearly all been of short duration and involved a modest number of participants, these drugs are used widely and form the mainstay of therapy. Major limitations are their adverse effect profile and tachyphylaxis i.e. diminution in pharmacological effect over time. Tachyphylaxis is thought to particularly affect motilin agonists, although this has not been well studied. Cisapride (a 5HT4 agonist) was used widely for symptomatic management, but shown subsequently to be associated with cardiac adverse effects (prolonged QT interval and ‘torsades de pointes’) and taken off the market. The most commonly used medications are discussed below. Some prokinetic drugs also have antiemetic properties.

 

Metoclopramide, a dopamine D2 receptor antagonist, improves gastric emptying (48), and can be administered via oral, intranasal, and subcutaneous routes, but is associated with central nervous system adverse events (including tardive dyskinesia), which may be irreversible. Accordingly, the US Food and Drug Administration (FDA) recommends short duration (12 weeks) use only. An intranasal formulation of metoclopramide under development was reported to be efficacious in women, but not men, implying the potential importance of gender in selecting the route of delivery (51). Metoclopramide can also be injected subcutaneously in an attempt to abort attacks of vomiting. It is the only medication that is approved currently by the FDA for the management of gastroparesis.

 

Domperidone is another D2 receptor antagonist, but unlike metoclopramide, does not cross the blood-brain barrier and is associated with fewer adverse events, with apparently comparable improvements in gastric emptying and upper gastrointestinal symptoms (48,52). Domperidone may prolong the QT interval and affect metabolism of other medications through the CYP2D6 pathway (48).

 

The antibiotic, erythromycin, is a motilin receptor agonist and is effective acutely, and inexpensive, but needs to be administered frequently and may also prolong the QT interval and interact with other medications, in this case through the CYP3A4 pathway (48). Acute, intravenous, administration of erythromycin markedly accelerates delayed gastric emptying (53) and may assist in the placement of neuroenteric tubes (54). The gastrokinetic effect of erythromycin is, however, subject to tachyphylaxis (55).

 

A number of novel agents are in Phase 2-3 trials, including ghrelin and 5HT4 receptor agonists. Ghrelin (sometimes referred to as the 'hunger' hormone) is secreted from the fundus of the stomach and has important roles in nutrient sensing and appetite regulation. Administration of ghrelin accelerates gastric emptying in both animals and humans (56). The outcome of phase 2 trials of the ghrelin agonist, relamorelin, have been promising, with a reduction in upper gastrointestinal symptoms in type 1- and 2 patients with gastroparesis as well as an acceleration of gastric emptying (57). An international phase 3 trial is in progress. Similarly, the oral highly selective 5 HT4 agonists, velusetrag (which was marketed for constipation) and prucalopride, accelerate gastric emptying (58,59). A recent study reported that 4 weeks’ of treatment with prucalopride in 32 people with gastroparesis (including 6 with diabetes) improved both symptoms and accelerated gastric emptying, although sub-group analysis of the diabetic cohort was not performed due to small numbers (59).

 

Treatment-Refractory Gastroparesis

 

Gastroparesis refractory to dietary and pharmacological intervention is debilitating for the patient and management represents a substantial challenge. Bypassing the stomach using jejunal or parenteral feeding, may be required to sustain nutrition. Gastric electrical stimulation (GES) using the ‘Enterra’ device) appeared to be a promising therapeutic option when initial unblinded studies were indicative of symptom improvement (22,48) and is currently approved by the FDA for ‘humanitarian exemption’; however, a subsequent blinded study failed to show a difference between periods where the stimulator was switched ‘on’ or ‘off’ (22,48,60,61).  A recently reported randomized cross-over trial reported a reduction in frequency of refractory vomiting following GES for a 4-month period in gastroparesis with or without diabetes but improvement in symptom control did not accelerate gastric emptying or benefit quality of life (62). Similarly, pyloric botulinum toxin injections have fared much better in uncontrolled, than in sham-controlled trials (22).  Surgical and endoscopic interventions, such as pyloroplasty and pyloromyotomy, and acupuncture have been described in literature, but lack controlled outcome data (22,48).

 

Gall Bladder

 

Gall stones are encountered more frequently in people with diabetes, which is not surprising given that risk factors for the development of stones, such as intestinal dysmotility, obesity, and hypertriglyceridemia, are more common in this group (particularly type 2 diabetes) (63). In addition, impairment of gall bladder motility and autonomic neuropathy, as well as factors such as cholesterol supersaturation and crystal nucleation promoting factors, are considered important. Common techniques used to measure gall bladder motor function include ultrasound and scintigraphy. Some studies have found increased fasting gall bladder volume, while in others, there was no difference, or even a reduction, in people with diabetes. It is possible that differences in the techniques employed (ultrasound or scintigraphy), and the presence of autonomic neuropathy may account for these discrepancies. Many studies, however, have reported impairment in postprandial gall bladder emptying in diabetes, sometimes termed ‘diabetic cholecystoparesis’ (63). It is also possible that delayed gastric emptying contributes to delayed emptying from the gall bladder. In health, acute hyperglycemia inhibits gall bladder motility in a dose-dependent manner (64). An increased prevalence of gall bladder-related disorders (including cholecystitis and cholelithiasis) is associated with the use of GLP-1 receptor agonists (65) and may potentially relate to a drug-induced prolongation of gall bladder refilling time (66). Similarly, an increase in gall-bladder disease has been reported post-bariatric surgery in obese individuals (including those with diabetes) with the implication that dramatic weight loss may predispose (67).

 

Small Intestine

 

While diabetic enteropathy is common, it has been studied much less comprehensively than diabetic gastroparesis (68). Symptoms of constipation and diarrhea are discussed in the section on large intestinal disorders in diabetes, which follows.

 

Traditionally, vagal dysfunction has been regarded as the major impairment in diabetic enteropathy. However, as is the case with gastroparesis, recent evidence has suggested a critical role for both interstitial cells of Cajal and nNOS (31). Acute hyperglycemia also has a major effect on postprandial small intestinal motility in health (and, presumably, diabetes) by reducing the amplitude of duodenal and jejunal pressure waves, as well as retarding duodenal-cecal transit (69).

 

Small intestinal bacterial overgrowth (SIBO), probably secondary to altered small intestinal motility, is commonly encountered in diabetes; estimates range between 15-40% in type 1 diabetic cohorts. A major limitation of these studies is lack of a ‘gold standard’ method for diagnosis.

 

There is limited information about small intestinal glucose absorptive function in diabetes but, based on animal models, it has been suggested that carbohydrate digestion is disordered. For example, streptozotocin-induced diabetes in rats, is associated with an increase in mucosal absorption of glucose (70). We have demonstrated that small intestinal glucose absorption is comparable in uncomplicated type 1 patients and healthy controls, but probably affected by both duodenal motility and the prevailing glycemic environment (71) - when blood glucose was elevated, intestinal glucose absorption was increased, while absorption was comparable to that in healthy controls during euglycemia. A fundamental limitation in interpreting the outcome of the numerous studies which have reported the potent modulatory effect of the rate of gastric emptying on postprandial glycemia is their failure to discriminate between effects mediated by changes in gastric emptying from those potentially secondary to changes in small intestinal transit  (72).

 

DIAGNOSIS OF ENTEROPATHY

 

Diabetic enteropathy is often a diagnosis of exclusion. It is essential to exclude underlying non-diabetes related etiologies where relevant – for example, testing for celiac disease in type 1 patients is recommended. It should be appreciated that gastrointestinal adverse effects occur frequently with commonly used anti-diabetic medications. Metformin, GLP-1RAs, SGLT2 inhibitors, and particularly alpha-glucosidase inhibitors (e.g. acarbose), which are used widely, are commonly associated with intestinal symptoms.

 

Small intestinal manometry (measurement of contractile activity) may provide mechanistic insights, but its use is limited to specialized centers. Scintigraphy can quantify small intestinal transit, but the diagnostic significance is uncertain. More recently, technologies, including ingestible wireless capsules (such as the SmartPill) and continuous tracking of capsules (3D-Transit system), have been employed; these are promising, but require further validation before clinical exploitation. Small intestinal bacterial overgrowth can be diagnosed by aspiration and culture of intestinal fluid or breath tests, but both have substantial limitations and neither technique can be regarded as a “gold standard”.

 

MANAGEMENT OF ENTEROPATHY

 

Symptom management with medications is common. Prokinetic agents used for gastroparesis are commonly employed for management of disordered intestinal motility, but much less well evaluated. Small intestinal bacterial overgrowth can be treated with antibiotics, such as rifamixin (most common but expensive), amoxicillin-clavulanic acid, or metronidazole. Not surprisingly, small intestinal bacterial overgrowth frequently relapses.

 

Large Intestine

 

The major function of the colon is to re-absorb water and electrolytes from the intraluminal contents, to concentrate and solidify the waste product, and prepare for its elimination. The most common lower gastrointestinal symptoms are constipation, diarrhea, abdominal pain, and distention. It is difficult to estimate a ‘true’ incidence and prevalence. Cohort studies have reported the presence of chronic constipation in up to 25% of people with type 1 and 2 diabetes, while that of chronic diarrhea is up to 5% (73).  Bytzer et al reported a higher prevalence of constipation and diarrhea in people with type 2 diabetes (15.6% compared with 10% in those without diabetes) (3). A recent report analyzing data from the large-scale US public survey, NHANES, found that chronic diarrhea was more common in people with type 1 and 2 diabetes compared with non-diabetic controls (~ 11% vs 6%) (74).

 

CONSTIPATION

 

The etiology of constipation in diabetes is likely to be multifactorial. A study involving only 10 patients found prolonged colonic transit time in those with constipation (13). Autonomic neuropathy is thought to be important; constipation is more common in those with diabetes and autonomic impairment (75). Validated techniques for evaluation include colonic transit scintigraphy and the use of radio-opaque markers and wireless motility capsules (76), but their utility in routine clinical practice has not been fully established.

 

Management of diabetic constipation must include a medication history review and those that may cause constipation should be ceased, if feasible (figure 4). For mild constipation, the American Diabetes Association recommends lifestyle modification such as increased physical exercise and dietary fiber. Over-the-counter laxatives (bulk, osmotic or stimulatory) such as Senna, Bisacodyl and water- soluble fiber supplements are commonly prescribed. Other medications like lactulose, linaclotide, and lubiprostone (the latter two available by prescription in the United States) have been used. There are no head-to-head trials to determine which agent is superior. However, it has been suggested that lactulose may potentiate glucose- lowering (77). Lubiprostone, which acts by direct activation of CIC-2 chloride channels on enterocytes, has been reported to improve both spontaneous bowel movements and accelerate colon transit in a randomized controlled trial in a cohort with diabetes (78). In a randomized trial cholinesterase inhibition with pyridostigmine in 30 people with diabetes (12 T1D, 18 T2D) and chronic constipation, there were superior improvements in both bowel function and colonic transit compared with placebo (79).

 

Figure 4. Algorithm for Management of Chronic Constipation in Patients with Diabetes.

 

CHRONIC DIARRHEA

 

“Diabetic diarrhea” has been traditionally considered a manifestation of autonomic neuropathy (80). The typical symptom is large volume, painless, nocturnal, diarrhea with or without fecal incontinence. Again, the diagnosis essentially represents one of exclusion and it is important to distinguish diarrhea from fecal incontinence. It should be remembered that widely used glucose-lowering therapies, including metformin (malabsorptive), acarbose (osmotic), and GLP-1 receptor agonists, not infrequently cause diarrhea. It is likely that optimizing glycemic control is important in the management of diabetic diarrhea (81), but again, this has not been rigorously evaluated. Dietary strategies include a low FODMAP (Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols) diet under guidance of a qualified dietitian, although this has not been evaluated specifically for the diabetes population in clinical trials. Loperamide, an over-the-counter mu opioid receptor agonist, is used widely. Bile acid sequestrants, such as cholestyramine and colesevelam, are used when bile salt malabsorption is suspected, and have the added advantage of reducing LDL cholesterol and glycated hemoglobin. Other agents include clonidine, diphenoxylate, octreotide, and ondansetron (figure 5).

 

It has been reported that people with diabetes, especially type 1 diabetes, are more likely to have inflammatory bowel disease (IBD) such as ulcerative colitis. Diabetes also appears to be an independent risk factor for Clostridium difficile infection where metformin appears to be protective, probably via its action on the gut microbiota (82). It has also been suggested that there is a link between diabetes and colorectal malignancy (83), and diabetes is associated with worse outcomes and response to colorectal surgery. Interestingly, some observational studies suggest that metformin may have chemo-preventative properties against colorectal malignancy (84).

 

Figure 5. Algorithm for Management of Chronic Diarrhea in Patients with Diabetes.

 

Rectum and Anus

 

Fecal incontinence occurs more frequently in people with diabetes and is associated with the duration of disease, and the presence of microvascular complications, including autonomic and peripheral neuropathy (85). Both internal anal sphincter tone and anal squeeze pressures are reduced in diabetes compared with healthy controls (86,87). A key step in management is to exclude important differential diagnoses, such as colorectal malignancy and irritable bowel disease (88). No single test can be regarded as ‘gold standard’, but anorectal manometry (conventional, 3D or high resolution) is very useful in clinical practice to estimate ano-rectal motor abnormalities, while barium defecography is useful to detect rectal motory, sensory and structural abnormalities (89).

 

Treatment of fecal incontinence is rarely curative, and the focus of management is to improve symptoms and quality of life. Fecal impaction with overflow can be managed by initial manual removal of stool from the rectum and enemas (promoting evacuation) and the subsequent prescription of bulk laxatives, increasing fiber intake, and toilet training. Operant reconditioning of rectosphincteric responses, called ‘biofeedback’ training, was first described by Engel et al in 1974 (90) and can be useful in treating fecal, as well as urinary, incontinence. The technique involves visual demonstration of voluntary contraction of external anal sphincter (EAS) contraction to the patient and training to improve the quality of the response (both strength and duration). Biofeedback training is effective in the longer term in only about 60% of patients in clinical trials; those with a low bowel satisfaction score and having digital evacuations fare better (91).

 

GASTROINTESTINAL EFFECTS OF ANTI-DIABETIC MEDICATIONS AND THEIR IMPLICATIONS FOR CLINICAL PRACTICE

 

Gastrointestinal adverse effects are extremely common in people treated with glucose-lowering medications for type 2 diabetes. In the case of alpha glucosidase inhibitors such as acarbose and miglitol, these effects (e.g., diarrhea and abdominal distention) are predictable sequelae of the malabsorption of carbohydrate (92) . There is new information in relation to two classes of medications (biguanides and GLP-1 receptor agonists).

 

Metformin, a biguanide of herbal origin, remains a first line pharmacological agent of choice for type 2 diabetes. The precise mechanisms of action remain uncertain, although it clearly has multiple effects, including in the liver (block gluconeogenesis), as an insulin sensitizer, and direct actions through the gut, including slowing of gastric emptying (93) . Up to 25% of people using metformin report gastrointestinal adverse events, particularly diarrhea and nausea. Common outpatient clinic strategies to minimize these include initiating treatment at a low dose (i.e., 500mg/day) and gradually up-titrating to usually ~2000 mg/day, use of extended-release formulations and avoiding ingestion on an empty stomach, although evidence to support these approaches is not robust (94).

 

Similarly, GLP-1 receptor agonists (but not DPP-IV inhibitors which lead to only a modest rise in plasma GLP-1 levels), commonly cause gastrointestinal adverse effects. As mentioned, GLP-1 is a gut-based peptide with a profound, but variable, action to slow gastric emptying. This slowing is more marked when baseline gastric emptying is relatively more rapid and is predictive of the reduction in blood glucose following a meal (95) . GLP-1 plays a physiological role to slow gastric emptying - gastric emptying is accelerated by the specific GLP-1 antagonist, exendin 9-39 (96)  and delayed by exogenous administration of GLP-1 in modestly supra-physiological plasma levels (97). Upper gastrointestinal events induced by GLP-1 are, likely to reflect, in part, delayed emptying. As effects are also observed in the fasting state, a direct action on CNS GLP-1 receptors (most notably, area postrema in the brain stem) has also been postulated. A direct effect on the gut is likely to contribute to lower gastrointestinal adverse events such as diarrhea. GLP-1 secreting cells (specialized entero-endocrine ‘L’ cells) are found throughout the gastrointestinal tract, and GLP-1 may exert a local excitatory action in smooth muscle or through the intramural autonomic plexus to increase motility and induce diarrhea (98,99) . A fundamental limitation of the vast majority of clinical trials involving GLP-1 receptor agonists is that gastrointestinal adverse effects have been assessed using participant recall and not validated questionnaires. Nevertheless, results from large cardiovascular outcome trials relating to the use of GLP-1 agonists indicate that the proportion of participants discontinuing GLP-1 receptor agonists due to adverse gastrointestinal events ranges between 4.5 to 13% (100) . Nausea appears to be the most common symptom (up to 25%), with vomiting and diarrhea reported by about 10% (101). A retrospective analysis of 32 phase-3 trials involving ‘long’ and ‘short’ acting GLP-1 receptor agonists reported that gastrointestinal adverse effects are also dose-dependent, and that ‘long’ acting GLP-1 receptor agonists are associated with less nausea and vomiting, but more diarrhea when compared to short-acting GLP-1 receptor agonists (101). Symptoms are reported most frequently at the time of initiation of a GLP-1 receptor agent and may persist for several hours or days probably dependent on the Tmax of the drug (100). Gradual titration of dose is recommended, although evidence to support this approach is uncontrolled.

 

We, and others, have demonstrated employing the gold standard technique of scintigraphy to quantify gastric emptying and both ‘long’ and ‘short’ acting GLP-1 receptor agonists delay gastric emptying, although the magnitude of this deceleration appears to be greater with ‘short’ acting GLP-1 receptor agonists (95,102-104) . Moreover, it is appreciated that GLP-1 receptor agonists may slow gastric emptying profoundly in doses much lower than those used in the management of type 2 diabetes (2). It had been suggested, incorrectly, that long acting GLP-1 receptor agonists, which are now the most widely used form, have no effect on gastric emptying with sustained use (2). A further limitation of clinical trials of GLP-1 receptor agonists is that gastric emptying has either not been measured or a sub-optimal technique used (105).

 

Instances of apparently GLP-1 receptor agonist-induced gastroparesis are increasingly appearing in the medical literature as case reports (106). The prevalence of marked delay in gastric emptying induced by GLP-1 receptor agonists remains uncertain but has stimulated guidelines in relation to their use prior to elective surgery or endoscopy. For example, the American Society of Anesthesiologists (ASA), has recently published consensus guidelines on pre-operative management of people using GLP-1 agonists and have advised withholding a long-acting agent for at least one week prior to the procedure/surgery (107). Such recommendations lack a strong evidence base. It is uncertain whether these recommendations from the ASA will be universally adopted but it appears intuitively unlikely. Recently a UK-based expert group comprising endocrinologists, anesthetists, and pharmacists have recommended against this generic advice  (107) primarily on the lack of robust data demonstrating an increased risk of aspiration under anesthesia, while being on a GLP-1 receptor agonist, that the recommended duration of avoidance may be inadequate (for example, in people taking 1mg semaglutide, avoidance of one week is likely to reduce the plasma drug concentration by about half, which is still likely to slow gastric emptying), at least in some people and reintroduction of GLP-1 receptor agonists once normal food intake has been established has not clearly defined and there is intuitively the high potential for a deterioration in glycemic control, postoperatively including an increase in glycemic variability. They instead recommend that preoperative assessment for risk of aspiration be individualized.

 

In people co-prescribed with insulin and GLP-1 receptor agonists, there is likely to be an increased risk of a mismatch between insulin delivery and availability and intestinal glucose absorption due to prolonged gastric retention to predispose to hypoglycemia. Clinicians should be circumspect in prescribing a GLP-1 agonist and insulin combination in those who have impaired awareness of hypoglycemia or suspicion of delayed gastric emptying.

 

PANCREATIC EXOCRINE SUFFICIENCY IN DIABETES

 

There is an intricate anatomical association of endocrine and exocrine components of the pancreas which appears to translate to a reciprocal relationship between endocrine and exocrine dysfunction (108). However, a wide variation in the prevalence of pancreatic exocrine insufficiency in diabetes has been reported, with evidence that it is greater in type 1 (approx. 25-75%). compared with type 2 (approx. 25-50%) diabetes (109). The majority of these studies are in hospitalized populations; it is likely that prevalence in the community is lower. Our recent study of community type 2 patients reported a lower prevalence of 9% (110). The etiology in type 1 is thought to be a combination of lack of insulin (+/- glucagon and somatostatin), autoimmunity, autonomic neuropathy, and microvascular damage while the latter two contribute to pancreatic exocrine insufficiency in type 2 diabetes – it has been suggested that this may explain why pancreatic exocrine insufficiency is more common in patients with type 1 diabetes (109). Common symptoms are variable and include diarrhea (steatorrhea), abdominal pain, and failure to thrive in children. It is important to discriminate pancreatic and non-pancreatic causes of malabsorption. The relatively deep-seated location of the pancreas hinders easy assessment of its exocrine function. Diagnostic tests can be direct or indirect (111). Direct tests involve stimulation with exogenous hormones or nutrients while simultaneously collecting pancreatic secretions via duodenal intubation. This technique has many logistical issues (high costs, requirement of expertise, invasive nature) which limits its clinical utility despite being the most sensitive and specific. Examples of indirect tests include the 3-day fecal fat, fecal elastase-1 measurement, and breath tests (14C-triolein). Of these, the most common indirect and non-invasive (as well as relatively inexpensive) test in clinical practice is measurement of fecal elastase 1. It has been suggested that a fecal elastase-1 level less than 200 ug/g stool is indicative of mild pancreatic exocrine insufficiency, and a level of 100 ug/g stool of severe pancreatic exocrine insufficiency (108). It should be appreciated that sensitivity (55%) and specificity (60%) of fecal elastase-1 in diagnosing pancreatic exocrine insufficiency are modest. Measurement of fat-soluble vitamins may be indicated.

 

The principles of general management of pancreatic exocrine insufficiency include consumption of smaller, frequent meals, abstinence from alcohol, and involvement of an experienced dietitian. Pancreatic enzyme replacement therapy is regarded as the cornerstone of treatment (108). It is uncertain whether supplementation with pancreatic enzyme replacement therapy in those with type 2 diabetes and pancreatic exocrine insufficiency reduces postprandial glycemic excursions (110). Adjunctive therapies such as acid-suppressing agents are reserved for those with symptoms despite high-dose pancreatic enzyme replacement therapy.

 

CONCLUSIONS

 

Both gastrointestinal symptoms and dysmotility are common in diabetes and represent an important component of management. Gastric emptying is also a major determinant of postprandial glycemic control and may be modulated therapeutically to improve it. Current management of disordered gastrointestinal function, particularly gastroparesis, is primarily empirical, although a number of novel agents are in development; results of these clinical trials are eagerly anticipated.

 

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Understanding Ethical Dilemmas in Pediatric Lipidology- Genetic Testing in Youth

ABSTRACT

 

Over the past 25 years there has been an increasing focus on early identification of individuals at-risk of premature cardiovascular disease (CVD), with the goal of improving outcomes and reducing premature CVD-related events such as myocardial infarction and stroke. In 2011, a National Heart, Lung and Blood Institute (NHLBI) Expert Panel recommended universal cholesterol screening of all children, irrespective of health status and family history, beginning at 10 years-of-age (range 9-11) and, if normal, repeated once between 17 and 20 years-of-age (1). Children found to have significant hypercholesterolemia are encouraged to adopt a heart-healthy lifestyle and, when appropriate, offered treatment with lipid-lowering medication, starting at 8 years-of-age and older. Research studies have convincingly demonstrated the safety and effectiveness of lipid-lowering medications in reducing risk and improving outcomes in adults, providing indirect support for universally cholesterol screening of children. Data from individuals with familial hypercholesterolemia (FH), treated for 20 years with pravastatin starting at a young age, have shown no adverse effects of growth, development, or reproductive function during adulthood. Shared decision-making in this population, however, is complex. Unlike most adults who are capable of making informed healthcare decisions, children have a wide range of developmentally-related intellectual and cognitive function, creating unique challenges in their ability to 1) understand long-term risk and benefit; and 2) make informed decisions regarding testing and medical management. In addition, some children have mental health and developmental disabilities that limit their cognitive abilities and judgement. Furthermore, legal guardians have the moral responsibility and legal right to make decisions on behalf of a minor.  In this article, we will discuss 1) privacy, discrimination, and the legal rights of children; 2) ethical considerations and concerns and 3) recommendations for clinicians when providing medical care of children with disorders of lipid and lipoprotein metabolism.

 

OVERVIEW OF LIPID AND LIPOPROTEIN DISORDERS IN YOUTH

 

Children with abnormal levels of lipids and lipoproteins are generally identified as result of targeted, universal or occasionally, coincidental testing.  Current recommendations for lipid screening of children are listed below.

 

  1. Targeted screening in all children ≥2 years of age in whom:
    1. One or both biologic parents are known to have hypercholesterolemia or are receiving lipid-lowering medications
    2. Who have a family history of premature cardiovascular disease (men <55 years of age and women <65 years of age)
    3. Whose family history is unknown (e.g., children who were adopted)
  2. Universal screening of all children 10 years of age (range 9-11), regardless of general health or the presence/absence of CVD risk factors. If normal, repeat screening is recommended at 17-20 years-of-age.

 

Since hypercholesterolemia is often caused by an underlying genetic mutation, such as in FH, cascade screening of biologic relatives is also recommended.  Cascade screening involves systematic testing of all first-degree relatives (parents and siblings) of a child with FH, followed by testing of second- and third-degree relatives if any of the first-degree relatives are affected. The most practical approach to cascade screening is biochemical testing of cholesterol, which is inexpensive, readily available and can be performed without the need for fasting. However, up to 25% of family members may be misdiagnosed as being either affected or unaffected when screening is based on cholesterol levels alone. Testing for a known genetic mutation in the family combined with fasting or non-fasting LDL-C levels will yield the most definitive information. While helpful if known, the child’s family history is often unknown, incomplete, or inaccurate. Reliance upon family history alone fails to identify as many as 30-60% of children with significant hypercholesterolemia. For additional information see the Endotext chapters entitled “Guidelines for Screening, Prevention, Diagnosis, and Treatment of Dyslipidemia in Children and Adolescents” and “Principles of Genetic Testing for Dyslipidemia in Children”.

 

Abnormalities of lipids and lipoproteins in youth may be caused by genetic mutations, acquired conditions, or both.  Those with acquired conditions, such as obesity and insulin resistance, are encouraged to adopt a heart-healthy lifestyle, which includes a low-fat,   calorically appropriate carbohydrate diet, weight loss if overweight or obese, participation in 30-60 minutes of moderate-to-vigorous physical activity per day and smoking avoidance or cessation.  Those suspected of having a genetic mutation are generally diagnosed based upon clinical criteria with or without genetic testing.

 

Genetic mutations that cause lipid and lipoprotein abnormalities vary depending upon the mode of inheritance (autosomal co-dominant vs autosomal recessive), the type of mutation present (slice vs missense), the number of genes involved (monogenic vs polygenic) and their phenotypic expression. When a genetic mutation is present, its expression may potentially be modified by other gene abnormalities (often small effect mutations) and environmental factors (e.g., obesity, insulin resistance, medications). For additional information see the Endotext chapter entitled “Genetics and Dyslipidemia”.

 

Early identification and treatment of children with clinically suspected or genetically confirmed FH has become increasingly common.  However long-term outcome studies demonstrating the safety and efficacy of this approach are lacking. Since lifestyles and therapeutic options are likely to change over the extended period of time that would be necessary to reach “hard” end points in children with FH, such as myocardial infarction and stroke, outcome studies are unlikely to be forthcoming.  Given the significant benefit statins have shown in reducing CVD-related mortality in adults, it has been suggested that withholding effective treatment in moderate-to-high risk children would be unethical (2). For additional information see the Endotext chapter entitled “Familial Hypercholesterolemia”.

 

A novel approach has been suggested to potentially lower costs and avoid prolonged exposure of at-risk children to lipid-lowering medication, while offering timely and presumably effective intervention.  Rather than continuous treatment implemented at an early age, Robinson and Gidding proposed intermittent lipid-lowering medication guided by noninvasive measures of atherosclerosis, such as carotid intima-media thickness (3). As with conventional approaches, the goal of such therapy would be regression of atherosclerotic lesions, with retreatment periodically throughout adulthood as needed.  While intriguing, the benefits of this recommendation have not been proven.

 

To date recommendations for early identification and treatment of children with hypercholesterolemia have focused primary on the potential benefits.  Fortunately, no significant physical or psychological harms have been shown in children who have undergone early screening and treatment. However, healthcare providers who advocate screening, genetic testing and treatment of children should carefully consider potential ethical issues, including the rights of the child to participate in clinical decision-making, the presumed benefits to the child and the family, as well as potential harms.

 

 

Over the last 50 years, in the U.S. Congress has passed a variety of laws to assure the privacy of an individual’s health information and eliminate discrimination based upon an individual’s health status. While most clinicians have an awareness of these laws, it is unclear how clinicians use this information in clinical decision-making, particularly as it relates to the current or future interests of the child.

 

Privacy

 

In 1996, Congress passed the Health Insurance Portability and Accountability Act or HIPAA. This law mandates the protection and confidential handling of protected health information, including genetic information. Furthermore, HIPAA states that genetic information in the absence of a diagnosis (e.g., predictive genetic test results) cannot be considered a pre-existing condition. Since children with heterozygous FH are rarely affected by their hypercholesterolemia during childhood, genetic testing would be considered “predictive” of adult-onset disease.  Children found to have a pathogenic or presumed pathogenic mutation, therefore, are afforded privacy under HIPPA and are not consider to have a pre-existing condition.

 

The Genetic Information Nondiscrimination Act (GINA), passed in 2008, adds to HIPPA by prohibiting health insurers and employers from asking or requiring a person to take a genetic test and using genetic information in 1) setting insurance rates and 2) making employment decisions.

 

Discrimination and Pre-existing Medical Conditions

 

Prior to 2014, insurance companies based eligibility for and the cost of health insurance on the presence or absence of pre-existing medical conditions.  A pre-existing condition is typically one for which an individual has received treatment or a diagnosis before being enrolled in a health plan.  Because they were determined by insurance providers, criteria defining pre-existing conditions varied widely. This meant that when applying for health insurance individuals, including children, previously diagnosed with and/or treated for hypercholesterolemia were considered to have a pre-existing condition.

 

Since 2014, with the passage of the Affordable Care Act, insurance companies can no longer deny coverage or discriminate against individuals due to a pre-existing condition. Nor can individuals be charged significantly higher premiums, subjected to an extended waiting period, or have their benefits curtailed or coverage withdrawn because of a pre-existing condition.  However, this protection does not extend to an individual’s ability to obtain nor the rates charged for life, disability, and long-term care insurance.

 

Despite these reassurances, in some cases exemptions may apply, particularly for members of the United States military, veterans obtaining healthcare through the Veterans Administration (VA), and individuals who receive services through the Indian Health Service.

 

Children’s Rights

 

A child’s rights can be considered in two parts 1) nurturance rights, i.e., the right to care and protection and 2) self-determination rights, i.e., the right to have some measure of control over their own lives.  Historically, society has focused on the former. Increasingly there is a growing emphasis on shared decision-making in medicine that recognizes children have the right to take an active part in many of the decisions regarding their own lives. While such efforts are commendable, the ability of children to become actively and willfully involved in the decision process is complicated by normal, and sometimes abnormal, growth and development. This raises an important question about a child’s ability to understand their rights in a reasonable and meaningful way (4). It also assumes that healthcare providers are trained, capable of and willing to provide developmentally-appropriate information to children in a comprehendible and non-threatening way.

 

In the 1980s, Melton (5, 6) suggested that children progress through three distinct stage-like levels of reasoning about rights: Level 1, children exhibit an egocentric orientation where they perceive rights in terms of privileges that are bestowed or withdrawn on the whims of an authority figure. Level 2 children see rights as being based on fairness, maintaining social order and obeying rules. Finally, in Level 3 rights are seen in terms of abstract universal principles. Subsequent models favored the gradual acquisition of context specific knowledge (7-9).  When and how well a child progresses from limited to abstract reasoning presents challenges for physicians who strive to involve children in decisions regarding early screening and intervention for CVD risk prevention.

 

MIGHT EARLY DIAGNOSIS AND TREATMENT OF HYPERCHOLESTEROLEMIA COMPROMISE A CHILD’S FUTURE RIGHTS?

 

Laws such as HIPPA, the Affordable Care Act, and GINA protect privacy and prohibit health insurance companies from denying coverage or discriminating against individuals due to a pre-existing condition, including hypercholesterolemia. Nonetheless, current laws do not preclude an individual being denied other forms of coverage, such as life, disability, or long-term care insurance. Furthermore, laws governing privacy, healthcare, and insurance coverage are subject to change over the course of the child's lifetime. This potential vulnerability needs to be considered by clinicians who provide care to children and fully disclosed to the family prior to diagnostic evaluation and treatment of children with hypercholesterolemia. To the extent that they can participate in such conversations, children should be included in the clinical decision-making. The accelerated risk of atherosclerosis beginning in young adults notwithstanding, the urgency of screening and early treatment of children needs to be considered in the context of the child’s overall best interest and, ideally, with their approval.  

 

ETHICAL CONSIDERATIONS AND CONCERNS

 

Since 1953, there has been an impressive increase in new technology and expanded uses of genetic testing and screening. Application of these diagnostic tools in minors has increasingly become commonplace, raising concerns about ethical issues. While pediatric screening and genetic testing are much less common outside of newborn screening, universal screening and increased use of genetic testing has been advocated by many national professional organizations and societies. Justification for such recommendations cite early identification of a child with an underlying genetic abnormality as an opportunity to initiate treatment that may prevent or reduce morbidity or mortality.

 

Over the past 50 years, genetic testing has increasingly played an important role in helping to understand the basis of many disorders of lipid and lipoprotein metabolism, identifying those who are affected and aiding our understanding of an individual’s risk. While only a minority of individuals with hypercholesterolemia who undergo genetic testing are found to have a pathogenic mutation, epidemiologic and Medallion randomization studies suggest these individuals are at significantly higher risk of premature ASCVD-related morbidity and mortality than the general population. 

 

Genetic testing of an asymptomatic child based upon an abnormal blood test and/or positive family history for a specific genetic condition, such as FH, has also been proposed, particularly if early treatment may affect future morbidity or mortality.  Some genetic tests can reasonability predict disease which only manifest in adults. 

 

Ultimately, decisions about whether to offer genetic testing and screening should be driven by the best interest of the child. This, perhaps, is best determined by a thoughtful discussion between the child’s healthcare provider, the parents, and, when appropriate, the child.  Current recommendations and guidelines suggest early intervention to achieve the best outcomes. Yet, there is no clear definition as to the optimum age at which intervention should be recommended, nor clear understanding about a child’s ability to understand and make a rational decision regarding testing and/or treatment.

 

The genetic testing of children raises specific considerations. Because of the need to respect a children's rights, caution has been advised in performing genetic tests during childhood. Newborn genetic testing is now ubiquitous, yet it is not always seen as routine for older children despite specific indications. Testing for drug responsiveness or disease susceptibility is supported by the ethical principle of beneficence when the benefit/risk ratio is in favor of discovering these results during childhood. Possible harms are seen when such knowledge may impact a child negatively, or foreclose future autonomy about the decision to accept the consequences of such testing. Therefore, there is a difference between genetic confirmation in symptomatic children, and that of pre-symptomatic children in which the benefit may accrue later, but the risks may occur in childhood. Such immediate risks potentially include stigmatization by the disease, depression, or decreased self-esteem. Conversely, altered family dynamics may result in parental favoritism, and survivor's guilt in siblings who test negative. This limitation on future autonomy is not confined to just refusing or allowing an adult decision for testing, but also dealing with the impact on future employment, education, and social relationships when the diagnosis is made at an early age.

 

Tests which help diagnose an ongoing, treatable condition that could affect current and future manifestations and complications clearly can be in the child's best interest. However, when a child is asymptomatic and the disorder is late-onset, it is no longer obvious that such a diagnosis during childhood is in the child's best interest. Therefore, it is advised the children only undergo genetic testing when there is immediate medical benefit in childhood, either through diagnosis and treatment of a disease manifesting in the pediatric age range, or a disease whose prevention is possible and should not be delayed. Under these circumstances, informed decision-making is essential, with parental permission being linked to the child's assent whenever possible.

 

CHOLESTEROL SCREENING AND TREATMENT

 

Currently, universally cholesterol testing is recommended for all children in the U.S., starting at 10 years-of-age (range 9-11). The primary purpose of cholesterol screening is to identify individuals with familial hypercholesterolemia.  For those found to have a significant elevation of cholesterol a low-fat diet is recommended. Lipid-lowering medications, such as a statin, are recommended for children with a persistently elevated LDL-C, starting at approximately 8-10 years-of-age. 

 

Genetic Testing

 

Genetic testing of all children suspected of having FH has been recommended (10). The purported benefits of genetic testing are 1) to assist in clinical decision-making regarding the need for lipid-lowering medication, 2) to help determine the appropriate on-treatment goal of LCL cholesterol; and 3) facilitate cascade screening of biologic relatives.

 

To help better understand the complexities of genetic testing and provide guidance, in 2013 both the American Academy Pediatrics (AAP) and the American College of Medical Genetics (ACMG) published recommendations for genetic testing of children. These guidelines are particularly relevant for those providing care for children with lipid and lipoprotein disorders since, with the exception of homozygous disease, children with heterozygous FH are asymptomatic. Hence, genetic testing in this unique population would be considered “predictive” of adult disease.

 

However, although there is much emphasis on early screening and genetic testing of children for FH, children have a variety of genetic conditions that affect other lipids and lipoproteins as well, such as triglycerides. The infantile form of lysosomal acid lipase deficiency, for example, is generally fatal in the absence of early diagnosis and enzyme replacement therapy. Thus, biochemical screening and genetic testing in this condition becomes imperative in order to reduce early morbidity and prevent premature mortality. Examples of other conditions in which there is a sense of urgency include familial chylomicronemia syndrome (FCS), cerebrotendinous xanthomatosis (CTX), and homozygous mutations of MTTP (abetalipoproteinemia), APOB (familial hypobetalipoproteinemia), and SAR1 (chylomicron retention disease).  When considering screening and genetic testing of children with lipid and lipoprotein disorders, therefore, “one size” clearly does not fit all circumstances. Clinicians must consider each child and condition as unique, carefully weighing the presumed benefits and potential harms individually, before making diagnostic and therapeutic recommendations.  

 

In deciding whether a child should undergo predictive genetic testing, the AAP and ACMG emphasize that the focus must be on the child’s medical best interest. Both organizations concluded that unless ameliorative interventions are available during childhood, children should not undergo testing for predispositions to adult-onset conditions and clinicians should generally decline to order testing. With the exception of those with homozygous FH, this suggests that children with heterozygous disease could defer treatment until adulthood. There is convincing evidence using noninvasive techniques, however, that early initiation of lipid-lowering medication can significantly reduce subclinical atherosclerosis. It is presumed that as a consequence of early and persistent LDL-cholesterol lowering that ASCVD-related events will be prevented or delayed. Yet proof of improved outcomes is currently limited and generally inferred from adult data.  

 

The AAP and ACMG did recognize that the potential psychosocial benefits and harms to the child and the extended family also need to be carefully considered. Extending consideration beyond the child’s medical best interest not only acknowledges the traditional deference given to parents about how they raise their children, but also recognizes that the interest of a child is embedded in and dependent on the interests of the family unit.

 

Predictive genetic testing for adult-onset conditions generally should be deferred unless an intervention initiated in childhood may reduce morbidity or mortality. In some families, the psychosocial burden of ambiguity may be so great as to justify testing during childhood, particularly when parents and mature adolescents jointly express interest in doing so.

 

AAP AND ACMG RECOMMENDATIONS

 

Genetic testing performed in children can be considered either as diagnostic or predictive (11).

 

  1. Diagnostic Genetic Testing - Is performed on a child with physical, developmental, or behavioral features consistent with a potential genetic syndrome or for pharmacogenetic drug selection and dosing decisions. Medical benefits include the possibility of preventive or therapeutic interventions, decisions about surveillance, the clarification of diagnosis and prognosis, and recurrence risks. If the medical benefits of a test are uncertain, will not be realized until a later time, or do not clearly outweigh the medical risks, the justification for testing is less compelling.

 

  1. Predictive Genetic Testing - Is performed on an asymptomatic child with a positive family history for a specific genetic condition, particularly if early surveillance or treatment may affect morbidity or mortality. When there is uncertainty that the presence of a genetic mutation will give rise to clinical manifestations, testing is referred to as “pre-dispositional” testing. Most predictive genetic testing for adult-onset conditions is pre-dispositional.

 

Recommendations for Genetic Testing of Children

 

  1. General
    1. Decisions about whether to offer genetic testing and screening should be driven by the best interest of the child.
    2. Genetic testing is best offered in the context of genetic counseling.
  2. Diagnostic Testing
    1. In a child with symptoms of a genetic condition:
      1. Parents or guardians should be informed about the risks and benefits of testing, and their permission should be obtained.
      2. Ideally and when appropriate, the assent of the child should be obtained.
    2. When performed for therapeutic purposes:
      1. Pharmacogenetic testing of children is acceptable, with permission of parents or guardians and, when appropriate, the child’s assent.
      2. If a pharmacogenetic test result carries implications beyond drug targeting or dose-responsiveness, the broader implications should be discussed before testing.
    3. Newborn Screening
      1. The AAP and ACMG support the mandatory offering of newborn screening for all children. Parents should have the option of refusing the procedure, and an informed refusal should be respected.
    4. Carrier Testing
      1. The AAP and ACMG do not support routine carrier testing in minors when such testing does not provide health benefits in childhood. This recommendation accords with previous statements supporting the future decisional autonomy of the minor, who will be able to make an informed choice about testing once he or she reaches the age of majority.
      2. For pregnant adolescents or for adolescents considering reproduction, genetic testing and screening should be offered as clinically indicated, and the risks and benefits should be clearly explained.
    5. Predictive Genetic Testing
      1. Parents or guardians may authorize predictive genetic testing for asymptomatic children at risk of childhood onset conditions.
      2. Ideally, the assent of the child should be obtained.
      3. Predictive genetic testing for adult-onset conditions generally should be deferred unless an intervention initiated in childhood may reduce morbidity or mortality.
      4. An exception might be made for families in whom diagnostic uncertainty poses a significant psychosocial burden, particularly when an adolescent and his or her parents concur in their interest in predictive testing.
      5. For ethical and legal reasons, health care providers should be cautious about providing predictive genetic testing to minors without the involvement of their parents or guardians, even if a minor is mature. Results of such tests may have significant medical, psychological, and social implications, not only for the minor, but also for other family members.

 

Potential Benefits and Harms of Predictive Genetic Testing of Children. Adapted from (11)

Medical

 

Benefits

Possibility of evolving therapeutic interventions, targeted surveillance, refinement of prognosis and clarification of diagnosis

Harms

Misdiagnosis to the extent that genotype does not correlate with phenotype, ambiguous results in which a specific phenotype cannot be predicted and use of ineffective or harmful preventive or therapeutic interventions.

Psychosocial

 

Benefits

Reduction of uncertainty and anxiety, the opportunity for psychological adjustment, the ability to make realistic life plans and sharing the information with family members.

Harms

Alteration of self-image, distortion of parental perception of the child, increased anxiety and guilt, altered expectation by self and others, familial stress related to identification of other at-risk family members, difficulty obtaining life and/or disability insurance, and the detection of misattributed parentage.

Reproductive

 

Benefits

Avoiding the birth of a child with genetic disease or having time to prepare for the birth of a child with genetic disease.

Harms

Changing family-planning decisions on the basis of social pressures.

 

It is essential that parents, guardians and maturing minors receive genetic counseling before undergoing predictive testing, which includes a discussion of the limits of genetic knowledge and treatment capabilities as well as the potential for psychological harm, stigmatization, and discrimination (12).

 

If an adolescent declines genetic testing, and the benefits of knowing will not be relevant for years to decades, the adolescent’s decision should be final. If a minor is young or immature, genetic testing should be delayed until the minor can actively participate. 

 

If predictive testing of conditions for which childhood interventions will ameliorate future harm, this may favor early testing. In such cases, parental authority to determine medical treatment supersedes the minor’s preferences with regard to liberty and privacy.

 

CONCLUSION

 

Although recommended for all individuals, including children, with clinically suspected familial hypercholesterolemia, genetic testing should be approached with caution. Parents and, when appropriate, children should be provided with a comprehensive discussion of the pros and cons of genetic testing, and informed about out-of-pocket costs prior to testing.

 

REFERENCES

 

  1. Expert Panel on Integrated Guidelines for Cardiovascular Health and Risk Reduction in Children and Adolescents, National Heart, Lung, and Blood Institute. Expert Panel on Integrated Guidelines for Cardiovascular Health and Risk Reduction in Children and Adolescents: summary report. Pediatrics. 2011;128(Suppl 5):S213-S256. doi:10.1542/peds.2009-2107C
  2. Wilson DP, Gidding SS. Atherosclerosis: Is a cure in sight? J Clin Lipidol. 2015;9(5 Suppl):1. doi:S1933-2874(15)00266-4
  3. Robinson JG, Gidding SS. Curing atherosclerosis should be the next major cardiovascular prevention goal. J Am Coll Cardiol. 2014;63(25 Pt A):2779-2785. doi:S0735-1097(14)02138-X
  4. Ruck MD, Abramovitch R, Keating DP. Children's and adolescents' understanding of rights: balancing nurturance and self-determination. Child Dev. 1998;69(2):404-417.
  5. Melton GB. Children's concepts of their rights. J Clin Child Psychol. 1980;9(3):186-190. doi:10.1080/15374418009532985
  6. Melton GB. Child advocacy: Psychological issues and interventions. Plenum; 1983.
  7. Peterson-Badali M, Abramovitch R. Grade related changes in young people's reasoning about plea decisions. Law Hum Behav. 1993;17(5):537-552.
  8. Saywitz KJ. Children’s conceptions of the legal system: “Court is a place to play basketball”. Ceci J, Ross DF, Toglia MP, eds. Perspectives on children’s testimony. Springer-Verlag; 1989:131-157.
  9. Scott ES, Reppucci D, Woolard JL. Evaluating adolescent decision making in legal contexts. Law Hum Behav. 1995;19(3):221-244.
  10. Sturm AC, Knowles JW, Gidding SS, Ahmad ZS, Ahmed CD, Ballantyne CM, Baum SJ, Bourbon M, Carrié A, Cuchel M, de Ferranti SD, Defesche JC, Freiberger T, Hershberger RE, Hovingh GK, Karayan L, Kastelein JJP, Kindt I, Lane SR, Leigh SE, Linton MF, Mata P, Neal WA, Nordestgaard BG, Santos RD, Harada-Shiba M, Sijbrands EJ, Stitziel NO, Yamashita S, Wilemon KA, Ledbetter DH, Rader DJ; the Familial Hypercholesterolemia Foundation. Clinical genetic testing for familial hypercholesterolemia: JACC Scientific Expert Panel. J Am Coll Cardiol. 2018;72(6):662-680. doi:S0735-1097(18)35065-4
  11. American Academy of Pediatrics Committee on Bioethics, Committee on Genetics, and American College of Medical Genetics and Genomics Social, Ethical, Legal Issues Committee. Ethical and policy issues in genetic testing and screening of children. Pediatrics. 2013;131(3):620-622. doi:10.1542/peds.2012-3680
  12. American Academy of Pediatrics Committee on Genetics. Molecular genetic testing in pediatric practice: A subject review. Pediatrics. 2000;106(6):1494-1497. doi:10.1542/peds.106.6.1494

Cholesterol Lowering Drugs

ABSTRACT

 

There are currently several different classes of drugs available for lowering cholesterol levels. There are currently seven HMG-CoA reductase inhibitors (statins) approved for lowering cholesterol levels and they are the first line drugs for treating cholesterol disorders and can lower LDL-C levels by as much as 60%. Statins also are effective in reducing triglyceride levels in patients with hypertriglyceridemia. Statins lower LDL levels by inhibiting HMG-CoA reductase activity leading to decreases in hepatic cholesterol content resulting in an up-regulation of hepatic LDL receptors, which increases the clearance of LDL. The major side effects are muscle complications and an increased risk of diabetes. The different statins have varying drug interactions. Ezetimibe lowers LDL-C levels by approximately 20% by inhibiting cholesterol absorption by the intestines leading to the decreased delivery of cholesterol to the liver, a decrease in hepatic cholesterol content, and an up-regulation of hepatic LDL receptors. Ezetimibe is very useful as add on therapy when statin therapy is not sufficient or in statin intolerant patients. Ezetimibe has few side effects. Bile acid sequestrants lower LDL-C by10-30% by decreasing the absorption of bile acids in the intestine which decreases the bile acid pool consequently stimulating the synthesis of bile acids from cholesterol leading to a decrease in hepatic cholesterol content and an up-regulation of hepatic LDL receptors. Bile acid sequestrants can be difficult to use as they decrease the absorption of multiple drugs, may increase triglyceride levels, and cause constipation and other GI side effects. They do improve glycemic control in patients with diabetes, which is an additional benefit. PCSK9 inhibitors, either monoclonal antibodies or small interfering RNA, lower LDL-C by 50-60% by decreasing PCSK9, which decreases the degradation of LDL receptors. PCSK9 inhibitors also decrease Lp(a) levels. PCSK9 inhibitors are very useful when maximally tolerated statin therapy do not reduce LDL sufficiently and in statin intolerant patients. PCSK9 inhibitors have very few side effects. Bempedoic acid lowers LDL-C by 15-25% by inhibiting hepatic ATP citrate lyase activity resulting in a decrease in cholesterol synthesis in the liver, a decrease in hepatic cholesterol content, and an up-regulation of LDL receptors. Bempedoic acid is employed in patients who do not reach their LDL-C goals on maximally tolerated statin therapy or in patients who do not tolerate statins. Bempedoic acid is associated with elevations in uric acid levels and gouty attacks. Lomitapide and evinacumab are approved for lowering LDL levels in patients with homozygous familiar hypercholesterolemia, as they are not dependent on LDL receptors for decreasing LDL levels. Lomitapide inhibits microsomal triglyceride transfer protein decreasing the formation of chylomicrons in the intestine and VLDL in the liver. Lomitapide has the potential to cause liver toxicity and therefore they were approved with a risk evaluation and mitigation strategy (REMS) to reduce risk. Evinacumab is a monoclonal antibody that inhibits the activity of angiopoietin-like protein 3 resulting in the increased activity of lipoprotein lipase and endothelial cell lipase resulting in a decrease in LDL-C, HDL-C, and triglyceride levels. Mipomersen, which is no longer available, is a second-generation apolipoprotein anti-sense oligonucleotide that decreases apolipoprotein B synthesis resulting in a reduction in the formation and synthesis of VLDL and was approved for the treatment of homozygous familial hypercholesterolemia.

 

INTRODUCTION

 

This chapter will discuss the currently available drugs for lowering total cholesterol levels, especially LDL-C: statins, ezetimibe, bile acid sequestrants, PCSK9 inhibitors, bempedoic acid, lomitapide, mipomersen, and evinacumab. We will not discuss the effect of lifestyle changes or food additives, such as phytosterols, on LDL-C as this is addressed in the chapter entitled “The Effect of Diet on Cardiovascular Disease and Lipid and Lipoprotein Levels” (1). Additionally, we will not discuss guidelines for determining who to treat, how aggressively to treat, or targets of treatment as these issues are discussed in detail in the chapters entitled “Guidelines for the Management of High Blood Cholesterol” and “Approach to the Patient with Dyslipidemia” (2,3).

 

STATINS

 

Introduction

 

In the 1970s Dr. Akira Endo, working at Sankyo, discovered that compounds isolated from fungi inhibited the activity of HMG-CoA reductase, a key enzyme in the synthesis of cholesterol (4). Further studies at Merck led to the development of the first HMG-CoA reductase inhibitor, lovastatin, approved in 1987 for the treatment of hypercholesterolemia (5). There are currently seven HMG-CoA reductase inhibitors (statins) approved in the United States for lowering cholesterol levels. Three statins are derived from fungi (lovastatin, simvastatin, and pravastatin) and four statins are synthesized (atorvastatin, rosuvastatin, fluvastatin, and pitavastatin). Most of these statins are now generic drugs and therefore they are relatively inexpensive. Which particular statin one elects to use may depend on the degree of cholesterol lowering needed and the potential of drug-drug interactions. Statins are the first line drugs for treating elevated cholesterol levels and therefore one of the most widely utilized class of drugs. Statins have revolutionized the field of preventive cardiology and made an important contribution to the reduction in atherosclerotic cardiovascular events.

 

Effect on Statins on Lipid and Lipoprotein Levels

 

The major effect of statins is lowering LDL-C levels. The effect of the various statins at different doses on LDL-C levels is shown in Table 1. As can be seen in Table 1 different statins have varying abilities to lower LDL-C with maximal reductions of approximately 60% seen with rosuvastatin 40mg. Doubling the dose of a statin results in an approximate 6% further decrease in LDL-C levels. The percent reduction in LDL-C levels is similar in patients with high and low starting LDL-C levels but the absolute decrease is greater if the starting LDL-C is high. Because of this profound ability of statins to lower LDL-C levels, treatment with these drugs as monotherapy is often sufficient to lower LDL-C below target levels.

 

Table 1. Approximate Effect of Different Doses of Statins on LDL-C Levels

% LDL Reduction

Simvastatin (Zocor)

Atorvastatin (Lipitor)

Lovastatin (Mevacor)

Pravastatin (Pravachol)

Fluvastatin (Lescol)

Rosuvastatin (Crestor)

Pitavastatin (Livalo)

27

10mg

-

20mg

20mg

40mg

-

-

34

20mg

10mg

40mg

40mg

80mg

-

1mg

41

40mg

20mg

80mg

80mg

-

-

2mg

48

80mg

40mg

-

-

-

10mg

4mg

54

-

80mg

-

-

-

20mg

-

60

-

-

-

-

-

40mg

-

Data modified from package inserts

 

As would be predicted from the effect of statins on LDL-C levels, statins are also very effective in lowering non-HDL-C levels (LDL-C is the major contributor to non-HDL-C levels) (6,7). In addition, statins also lower plasma triglyceride levels (8,9). The ability of statins to lower triglyceride levels correlates with the reduction in LDL-C (9). Statins that are most efficacious in lowering LDL-C are also most efficacious in lowering plasma triglyceride and VLDL-C levels. Notably the percent reduction in plasma triglyceride levels is dependent on the baseline triglyceride levels (9). For example, in patients with normal triglyceride levels (<150mg/dL), simvastatin 80mg per day lowered plasma triglyceride levels by 11%. In contrast, if plasma triglyceride levels were elevated (> 250mg/dL), simvastatin 80mg per day lowered triglyceride levels by 40% (9). In patients with both elevated LDL-C and triglyceride levels statin therapy can be very effective in improving the lipid profile and are therefore the first line class of drugs to treat patients with mixed hyperlipidemia unless the triglyceride levels are markedly elevated (>500-1000mg/dL). As expected, given the ability of statins to lower LDL-C and triglyceride/VLDL levels, statin therapy is very effective in lowering apolipoprotein B levels (6,7).

 

Of note despite the ability of statins to lower LDL-C, non-HDL-C, and apolipoprotein B levels, statins do not lower Lp(a) levels and may even increase levels (10,11). Finally, statins modestly increase HDL-C levels (8,12,13). In most studies HDL-C levels increase between 5-10% with statin therapy. Interestingly, while low dose atorvastatin increases HDL levels similar to other statins at high doses the effect of atorvastatin is blunted with either very modest increases or no change observed (12).

 

Table 2. Effect of Statins on Lipid/Lipoprotein Levels

LDL-C

Decrease

Non-HDL-C

Decrease

Apolipoprotein B

Decrease

Triglycerides

Variable. If TG levels increased will decrease

HDL-C

Small Increase

Lp(a)

No change or small increase

 

Non-Lipid Effects of Statins

 

In addition to effects on lipid metabolism statins also have pleiotropic effects that may not be directly related to alterations in lipid metabolism (14). For example, statins are anti-inflammatory and consistently decrease CRP levels (15). Other pleiotropic effects of statins include anti-proliferative effects, antioxidant properties, anti-thrombosis, improving endothelial dysfunction, and attenuating vascular remodeling (14). Whether these pleiotropic effects contribute to the beneficial effects of statins in preventing cardiovascular disease is uncertain and much of the beneficial effect of statins on cardiovascular disease can be attributed to reductions in lipid levels.

 

Mechanism Accounting for the Statin Induced Lipid Effects

 

Statins are competitive inhibitors of HMG-CoA reductase, which leads to a decrease in cholesterol synthesis in the liver. This inhibition of hepatic cholesterol synthesis results in a decrease in cholesterol in the endoplasmic reticulum resulting in the movement of sterol regulatory element binding proteins (SREBPs) from the endoplasmic reticulum to the golgi where they are cleaved by proteases into active transcription factors (16). The SREBPs then translocate to the nucleus where they increase the expression of a number of genes including HMG-CoA reductase and, most importantly, the LDL receptor (16). The increased expression of HMG-CoA reductase restores hepatic cholesterol synthesis towards normal while the increased expression of the LDL receptor results in an increase in the number of LDL receptors on the plasma membrane of hepatocytes leading to the accelerated clearance of LDL (Figure 1) (16). The increased clearance of LDL accounts for the reduction in plasma LDL-C levels. In patients with a total absence of LDL receptors (Homozygous Familiar Hypercholesterolemia) statin therapy is not very effective in lowering LDL-C levels.

 

Figure 1. Mechanism for the Decrease in LDL Levels

 

In addition to lowering LDL and VLDL levels by accelerating the clearance of lipoproteins some studies have also shown that statins reduce the production and secretion of VLDL particles by the liver (17). This could contribute to the decrease in triglyceride levels. The mechanism by which statins increase HDL-C levels is not clear. The small increase in Lp(a) may be due to increased production as studies have shown that incubating HepG2 hepatocytes with a statin increased the expression of LPA mRNA and apolipoprotein(a) protein (18).

 

Pharmacokinetics and Drug Interactions

 

Statins have different pharmacokinetic properties which can explain clinically important differences in safety and drug interactions (19-22). Most statins are lipophilic except for pravastatin and rosuvastatin, which are hydrophilic. Lipophilic statins can enter cells more easily but the clinical significance of this difference is not clear. Most of the clearance of statins is via the liver and GI tract (19-21). Renal clearance of statins in general is low with atorvastatin having a very low renal clearance making this particular drug the statin of choice in patients with significant renal disease. The half-life of statins varies greatly with lovastatin, pravastatin, simvastatin, and fluvastatin having a short half-life (1-3 hours) while atorvastatin, rosuvastatin, and pitavastatin having a long half-life (19-22). In patient’s intolerant of statins, the use of a long-acting statin every other day or 2 times per week has been employed. Short acting statins are most effective when administered in the evening when HMG-CoA reductase activity is maximal while the efficacy of long-acting statins is equivalent whether given in the AM or PM (23). In patients who prefer to take their statin in the morning one should use a long-acting statin.

 

A key difference between statins is their pathway of metabolism. Simvastatin, lovastatin, and atorvastatin are metabolized by the CYP3A4 enzymes and drugs that affect the CYP3A4 pathway can alter the metabolism of these statins (19-22,24). Fluvastatin is metabolized mainly by CYP2C9 with a small contribution by CYP2C8 (19-21,24). Pitavastatin and rosuvastatin are minimally metabolized by the CYP2C9 pathway (19-21,24). Pravastatin is not metabolized at all via the CYP enzyme system (19-21).

 

Drugs that inhibit CYP3A4 can impede the metabolism of simvastatin, lovastatin, and to a smaller extent atorvastatin resulting in high serum levels of these drugs (19-22,24). These higher levels are associated with adverse effects particularly muscle toxicity. Drugs that inhibit CYP3A4 include intraconazole, ketoconazole, erythromycin, clarithromycin, HIV protease inhibitors (amprenavir, darunavir, fosamprenavir, indinavir, nelfinavir, ritonavir, and saquinavir), amiodarone, diltiazem, verapamil, and cyclosporine (19-22,24). It should be noted that grapefruit juice contains compounds that inhibit CYP3A4 and the consumption of grapefruit juice can significantly increase statin blood levels (25). The inhibition of CYP3A4 by grapefruit juice is dose dependent and increases with the concentration and volume of grapefruit juice ingested. One glass of grapefruit juice everyday can influence the metabolism of statins that are metabolized by the CYP3A4 pathway (25). If a patient requires treatment with a drug that inhibits CYP3A4 the clinician has a number of options to avoid potential drug interactions. One could use a statin that is not metabolized via the CYP3A4 system such as pravastatin or rosuvastatin, one could use an alternative drug to the CYP3A4 inhibitor (for example instead of using erythromycin use azithromycin), or one could temporarily suspend for a short period of time the use of the statin that is metabolized by the CYP3A4 pathway (this is particularly useful when a short course of treatment with an antifungal, antiviral, or antibiotic is required). Drugs that inhibit CYP2C9 do not seem to increase the toxicity of fluvastatin, pitavastatin, or rosuvastatin probably because metabolism via the CYP2C9 pathway is not a dominant pathway.

 

Most statins are transported into the liver and other tissues by organic anion transporting polypeptides, particularly OATP1B1 (19-21,24). Drugs, such as clarithromycin, ritonavir, indinavir, saquinavir, and cyclosporine that inhibit OATP1B1 can increase serum statin levels thereby increasing the risk of statin muscle toxicity (19-21,24). Fluvastatin is the statin that is least affected by OATP1B1 inhibitors. In fact, fluvastatin 40mg per day has been studied in patients receiving renal transplants concomitantly treated with cyclosporine and over a five year study period the risk of myopathy or rhabdomyolysis was not increased in the fluvastatin treated patients compared to those treated with placebo (26).

 

Gemfibrozil inhibits the glucuronidation of statins, which accounts for a significant portion of the metabolism of most statins (24). This can lead to the reduced clearance of statins and elevated blood levels increasing the risk of muscle toxicity (24). The only statin whose metabolism is not altered by gemfibrozil is fluvastatin (24). Notably, fenofibrate, another fibrate that has very similar effects on lipid and lipoprotein levels as gemfibrozil, does not inhibit statin glucuronidation (24). Therefore, in patients on statin therapy who also need treatment with a fibrate one should use fenofibrate and not gemfibrozil in combination with statin therapy. Studies have shown that fenofibrate combined with statins does not significantly increase toxicity (27).

 

There are other drug interactions with statins whose mechanisms are unknown. For example, the lopinavir/ritonavir combination used to treat HIV increases rosuvastatin levels by 2-5-fold and atazanavir/ritonavir increases rosuvastatin levels by 2-6-fold (28-32). Similarly, the tipranavir/ritonavir combination increases rosuvastatin levels 2-fold and atorvastatin levels 8-fold (31). When HIV patients are on these drugs other statins should be used to lower LDL-C levels. The use of statins in patients with HIV is discussed in detail in the Endotext chapter entitled “Lipid Disorders in People with HIV” (33).

 

Thus, despite the excellent safety record of statins, careful attention must be paid to the potential drug-drug interactions. For additional information see Kellick et al (22,24).

 

Effect of Statin Therapy on Clinical Outcomes

 

A large number of studies using a variety of statins in diverse patient populations have shown that statin therapy reduces atherosclerotic cardiovascular disease. The Cholesterol Treatment Trialists have published meta-analyses derived from individual subject data. Their first publication included data from 14 trials with over 90,000 subjects (34). There was a 12% reduction in all-cause mortality in the statin treated subjects, which was mainly due to a 19% reduction in coronary heart disease deaths. Non-vascular causes of death were similar in the statin and placebo groups indicating that statin therapy and lowering LDL-C did not increase the risk of death from other causes such as cancer, respiratory disease, etc. Of particular note there was a 23% decrease in major coronary events per 1 mmol/L (39mg/dL) reduction in LDL-C. Decreases in other vascular outcomes including non-fatal MI, coronary heart disease death, vascular surgery, and stroke were also reduced by 20-25% per 1 mmol/L (39mg/dL) reduction in LDL-C. Additionally, analysis of these studies demonstrated that the greater the reduction in absolute LDL-C levels the greater the decrease in cardiovascular events.  For example, while a 40mg/dL decrease in LDL-C will reduce coronary events by approximately 20%, an 80mg/dL decrease in LDL-C will reduce events by approximately 40%. These results support aggressive lipid lowering with statin therapy.

 

Of note the decrease in the number of events begins to be seen in the first year of therapy indicating that the ability of statins to reduce events occurs relatively quickly and increases over time. The ability of statins to reduce cardiovascular events was seen in a wide diversity of patients including those with and without a history of prior cardiovascular disease, patients over age 65 and younger than age 65, males and females, and patients with and without a history of diabetes or hypertension. Additionally, the beneficial effects of statins were seen regardless of the baseline lipid levels. Subjects with elevated or low LDL-C, HDL-C, or triglyceride levels all had similar decreases in the relative risk of cardiovascular events.

 

A subsequent publication by the Cholesterol Treatment Trialists has focused on five studies with over 39,000 subjects that have compared usual vs. intensive statin therapy (35). It was noted that there was a 0.51mmol/L (20mg/dL) further reduction in LDL-C in the intensively treated subjects. This further decrease in LDL-C resulted in a15% reduction in cardiovascular events. The strong numerical relationship between decreases in LDL-C levels and the reduction in cardiovascular events provides evidence indicating that much of the beneficial effect of statins is accounted for by lipid lowering.

 

In addition, the authors added 7 additional trials to their original comparison of statin treatment vs. placebo for a total of 21 trials with over 129,000 subjects. In this larger cohort a 1mmol/L (39mg/dL) decrease in LDL was associated with a 21% reduction in major cardiovascular events. As seen previously the benefits of statin therapy were seen in a wide variety of subjects including patients older than age 75, obese patients, cigarette smokers, patients with decreased renal function, and patients with low and high HDL-C levels. Additionally, a reduction of cardiovascular events with statin therapy was seen regardless of baseline LDL-C levels.

 

A more recent meta-analysis by the Cholesterol Treatment Trialists examined the effect of statins in 27 trials that included 46,675 women and 127,474 men (36). They found that statin therapy was similarly effective in reducing cardiovascular events in both men and women. Thus, there is an overwhelming database of randomized clinical outcome trials showing the benefits of statin therapy in reducing cardiovascular disease, which, coupled with their excellent safety profile, has resulted in statins becoming a very widely used class of drugs and first line therapy for the prevention of cardiovascular disease.  

 

Effect of Statins Therapy on Clinical Outcomes in Specific Patient Groups

 

PRIMARY PREVENTION

 

While there is no doubt that individuals with pre-existing cardiovascular disease require statin therapy, the use of statins for primary prevention was initially debated. There are now a large number of statin primary prevention studies. The Cholesterol Treatment Trialists reported that statin therapy was very effective in reducing cardiovascular events in subjects without a history of vascular disease and the relative risk reduction was similar to subjects with a history of cardiovascular events (35). Additionally, vascular deaths were reduced by statin treatment even in subjects without a history of vascular disease. As expected, non-vascular deaths were not altered in these subjects without a history of pre-existing vascular disease. Additionally, the Cholesterol Treatment Trialists compared the benefits of statin therapy based on baseline risk of developing cardiovascular disease (<5%, ≥5% to <10%, ≥10% to <20%, ≥20% to <30%, ≥30%) (37). The proportional reduction in major vascular events was at least as big in the two lowest risk categories as in the higher risk categories indicating that subjects at low-risk benefit from statin therapy. Similar to the Cholesterol Treatment Trialists analysis, a Cochrane review published in 2013 on the effect of statins in primary prevention patients reached the following conclusion: “Reductions in all-cause mortality, major vascular events, and revascularizations were found with no excess adverse events among people without evidence of CVD treated with statins” (38). An additional study (HOPE-3 trial), not included in the above analyses, has been completed that focused on intermediate risk patients without cardiovascular disease. In this trial 12,705 men and women who had at least one risk factor for cardiovascular disease were randomized to 10mg rosuvastatin vs. placebo (39). Rosuvastatin treatment resulted in a 27% reduction in LDL-C levels and a 24% decrease in cardiovascular events providing additional evidence that statin treatment can reduce events in primary prevention patients. It is therefore clear that statins are effective in safely reducing events in primary prevention patients.

 

The key issue is “which primary prevention patients should be treated” and this is still controversial. It should be noted that the higher the baseline risk the greater the absolute reduction in events with statin therapy. For example, in a high-risk patient with a 20% risk of developing a vascular event, a 25% risk reduction will result in a 15% risk of an event (absolute decrease of 5%). In contrast in a low-risk patient with a 4% risk of developing a vascular event, a 25% risk reduction will result in a 3% risk (absolute decrease of only 1%). Thus, the absolute benefit of statin therapy over the short term will depend on the risk of developing cardiovascular disease.

 

Additionally, based on the Cholesterol Treatment Trialists results the reduction in cardiovascular events is dependent on the absolute decrease in LDL-C levels. Thus, the effect of statin treatment will be influenced by baseline LDL-C levels. For example, a 50% decrease in LDL-C is 80mg/dL if the starting LDL is 160mg/dL and only 40mg/dL if the starting LDL-C is 80mg/dL. Based on studies showing that a decrease in LDL-C of 1 mmol/L (40mg/dL) reduces cardiovascular events by ~20% the relative benefit of statin therapy will be greater in the patient with the starting LDL-C of 160mg/dL (40% decrease in events) than in the patient with the starting LDL-C of 80mg/dL (20% decrease in events). Thus, decisions on treatment need to factor in both relative risk and baseline LDL levels.

 

Finally, it should be recognized that clinical trials represent short term reductions in LDL-C levels (typically 2-5 years) in a disorder that begins early in life and progresses over decades. Life-long decreases in LDL-C levels due to genetic polymorphisms are associated with a much greater reduction in cardiovascular events than would be expected based on the clinical trial results (40). These results suggest that earlier and longer lasting therapy that decreases LDL-C levels will result in a greater reduction in cardiovascular events (41). An in depth discussion of the benefits of early therapy is discussed in the following reference (42).

 

ELDERLY

 

Few studies have focused on lowering LDL-C in elderly patients, which we define as individuals greater than 75 years of age (this is based on the ACC/AHA guidelines using age 75 in their decision algorithms) (3). The Prosper Trial determined the effect of pravastatin 40mg/day (n= 2891) vs. placebo (n= 2913) on cardiovascular events in older subjects (70-82) with pre-existing vascular disease or who were at high risk for vascular disease (43). The average age in this trial was 75 years of age and approximately 45% had cardiovascular disease. As expected, pravastatin treatment lowered LDL-C by 34% compared to the placebo group. The primary end point was coronary death, non-fatal myocardial infarction, and fatal or non-fatal stroke which was reduced by 15% (HR 0.85, 95% CI 0.74-0.97, p=0.014). However, in the individuals without pre-existing cardiovascular disease pravastatin did not significantly reduce cardiovascular events (HR- 0.94; CI- 0.77–1.15). In contrast, in individuals with cardiovascular disease pravastatin therapy reduced cardiovascular events (HR- 0.78, CI- 0.66–0.93). Thus, this study demonstrated benefits of statin therapy in the elderly with cardiovascular disease but failed to demonstrate benefit in the elderly without cardiovascular disease.

 

A meta-analysis by the Cholesterol Treatment Trialists of 28 trials with 14,483 of 186,854 participants older than 75 years of age found a decrease in cardiovascular events in all age groups including participants older than 75 years of age (Figure 2) (44). Similar to the Prosper Trial a decrease in cardiovascular events was clearly demonstrated in individuals with pre-existing cardiovascular disease (secondary prevention) but in individuals without pre-existing cardiovascular disease (primary prevention) the decrease in cardiovascular events was not statistically significant (Figure 3). Thus, in older patients with cardiovascular disease lowering LDL-C levels with statins clearly reduces cardiovascular events but in older patients without cardiovascular disease the data demonstrating that statins reduce cardiovascular events is less robust but suggests a reduction in cardiovascular events.

 

Figure 2. Effect of Statin Treatment on Major Vascular Events. Modified from (44).

Figure 3. Effect of Statin Treatment on Major Vascular Events in Individuals With and Without Pre-Existing Cardiovascular Disease. Modified from (44).

 

Studies are currently underway to provide definitive information on whether statin therapy is beneficial as primary prevention in the elderly. STAREE (NCT02099123) is a multicenter randomized trial in Australia of atorvastatin 40mg vs. placebo in adults ≥ 70 years of age without cardiovascular disease and PREVENTABLE (NCT04262206) is a multicenter randomized trial in the USA of atorvastatin vs. placebo in adults ≥ 75 years of age without cardiovascular disease (45,46).

 

WOMEN

 

As noted above a meta-analysis by the Cholesterol Treatment Trialists examined the effect of statins in 27 trials that included 46,675 women and 127,474 men (36). They found that statin therapy was similarly effective in reducing cardiovascular events in both men and women.

 

ASIANS

 

Pharmacokinetic data have shown that the serum levels of statins are higher in Asians than in Caucasians (47). Moreover, Asians achieve similar LDL lowering at lower statin doses than Caucasians (47). Therefore, the statin dose used should be lower in Asians. For example, the starting dose of rosuvastatin is 5mg in Asians as compared to 10mg in Caucasians. Additionally, the maximum recommended dose of statin is lower in Japan vs. the United States (Table 3). In contrast, studies suggest that South Asian patients may be treated with atorvastatin and simvastatin at doses typically applied to white patients (48). Studies have demonstrated that statins reduce cardiovascular events in Asians (49,50)

 

Table 3. Maximum Statin Dose in Japan and United States

Statin

Japan

United States

Atorvastatin

40

80

Fluvastatin

60

80

Pravastatin

20

80

Rosuvastatin

20

40

Simvastatin

20

40

 

DIABETES

 

Statin trials, including both primary and secondary prevention trials, have consistently shown the beneficial effect of statins on cardiovascular disease in patients with diabetes (51). The Cholesterol Treatment Trialists analyzed data from 18,686 subjects with diabetes (mostly type 2 diabetes) from 14 randomized trials (52). In the statin treated group there was a 9% decrease in all-cause mortality, a 13% decrease in vascular mortality, and a 21% decrease in major vascular events per 1mmol/L (39mg/dL) reduction in LDL-C. The beneficial effect of statin therapy was seen in both primary and secondary prevention patients. The effect of statin treatment on cardiovascular events in patients with diabetes was similar to that seen in non-diabetic subjects. It should be noted that while the data for patients with type 2 diabetes is robust, the number of patients with type 1 diabetes in these trials is relatively small and the results less definitive. Also, of note is that information on young patients with diabetes (< age 40) is very limited. Thus, these studies indicate that statins are beneficial in reducing cardiovascular disease in patients with diabetes. For addition details on the treatment of dyslipidemia in patients with diabetes see the chapter entitled “Dyslipidemia in Patients with Diabetes” (51).

 

RENAL DISEASE

 

The Cholesterol Treatment Trialists examined the effect of renal function on statin effectiveness. They reported that the relative risk reduction for cardiovascular events was similar if the eGFR was < 60ml/min as compared to > 90 or 60-90 (35). In a follow-up analysis it was reported that the relative risk reduction per 1mMol/l (~39mg/dL) decrease in LDL-C levels with statin therapy was 0·78 for an eGFR ≥60 mL/min, 0·76 for an eGFR 45 to <60 mL/min, 0·85 for an eGFR 30 to <45 mL/min, and 0·85 for an eGFR <30 mL/min in patients not on dialysis (53). In patients on dialysis the relative risk reduction was 0·94 (99% CI 0·79-1·11). Similarly, a meta-analysis of 57 studies with >143,000 participants with renal disease not on dialysis reported a 31% reduction in major cardiovascular events in statin treated subjects compared to placebo groups (54). Thus, in patients with renal disease not on dialysis, treatment with statins is beneficial and should be utilized in this population at high risk for vascular disease.

 

In contrast to the above results, studies examining the role of statins in dialysis patients have not found a benefit from statin therapy. The Deutsche Diabetes Dialyse Studie (4D) randomized 1,255 type 2 diabetic subjects on hemodialysis to either 20 mg atorvastatin or placebo (55). The LDL-cholesterol reduction was similar to that seen in non-dialysis patients but there was no significant reduction in cardiovascular death, nonfatal myocardial infarction, or stroke in the atorvastatin treated compared to the placebo group. Similarly, A Study to Evaluate the Use of Rosuvastatin in Subjects on Regular Hemodialysis (AURORA) randomized 2,776 subjects on hemodialysis to rosuvastatin 10 mg or placebo (56). Again, the LDL-cholesterol lowering in dialysis patients was similar to that seen in other studies but there was no significant effect on the primary endpoint of cardiovascular death, nonfatal myocardial infarction, or stroke. A meta-analysis of 25 studies involving 8,289 dialysis patients found no benefit of statin therapy on major cardiovascular events, cardiovascular mortality, all-cause mortality, or myocardial infarction, despite efficacious lipid lowering. The reason for the failure of statins in patients on maintenance dialysis is unclear but could be due to a number of factors including the possibility that the marked severity of atherosclerosis in end stage renal disease may limit reversal, that different mechanisms of atherosclerosis progression occur in dialysis patients (for example an increased role for inflammation, oxidation, or thrombosis), or that cardiovascular events in this patient population may not be due to atherosclerosis. We would recommend continuing statin therapy in patients on dialysis who have been previously treated with statins but not initiating therapy in the rare statin naïve patient beginning dialysis.

 

Statins are primarily metabolized in the liver and therefore the need to adjust the statin dose is not usually needed in patients with renal disease until the eGFR is < 30ml/min. The effect of renal dysfunction on statin clearance varies from statin to statin (57). For some statins such as atorvastatin, there is no need to adjust the dose in renal disease because there is limited renal clearance (57). However, for other statins it is recommended to adjust the dose in patients when the eGFR is < 30ml/min. In patients with an eGFR < 30ml/min the maximum dose of rosuvastatin is 10mg, simvastatin 40mg, pitavastatin 2mg, pravastatin 20mg, lovastatin 20mg, and fluvastatin 40mg per day (57).

 

For additional information on the treatment of dyslipidemia in patients with renal disease see the chapter entitled “Dyslipidemia in Chronic Kidney Disease” (57).

 

CONGESTIVE HEART FAILURE

 

In the Corona study 5,011 patients with New York Heart Association class II, III, or IV ischemic, systolic heart failure (most were class III) were randomly assigned to receive 10 mg of rosuvastatin or placebo per day (58). While rosuvastatin treatment reduced LDL-C levels by 45% compared to placebo, rosuvastatin did not decrease death from cardiovascular causes, nonfatal myocardial infarction, or nonfatal stroke. Similarly, the GISSI-HF trial randomized 4,574 patients with class II, III, of IV congestive heart failure (most were class II) to 10mg of rosuvastatin or placebo (59). The primary endpoints were time to death, and time to death or admission to hospital for cardiovascular reasons and these were similar in the statin and placebo groups. Why statin treatment was not beneficial in patients with congestive heart failure is unknown.

 

LIVER DISEASE

 

Many patients with liver disease, particularly those with nonalcoholic fatty liver disease (NAFLD), are at high risk for cardiovascular disease and therefore require statin therapy (60). There have been concerns that these patients would not tolerate statin therapy and that statin therapy would worsen their underlying liver disease. Fortunately, there are now studies of statin therapy in patients with abnormal liver function tests and underlying liver disease at baseline (60-62). With a variety of statins, studies have demonstrated no significant worsening of liver disease and in fact several studies have suggested improvement in liver function tests with statin therapy (62). This is true for patients with hepatitis C, NAFLD/NASH, and primary biliary cirrhosis. Additionally, in the GREACE trial, statin treatment reduced cardiovascular events in patients with moderately abnormal liver function tests (transaminases < 3x the upper limit of normal) (63). Thus, in patients with mild liver disease without elevations in bilirubin or abnormalities in synthetic function, statins are safe and reduce the risk of cardiovascular disease. 

 

For additional information on the treatment of dyslipidemia in patients with liver disease see the chapter entitled “Lipid and Lipoprotein Metabolism in Liver Disease” (64).

 

HIV

 

Patients living with HIV have an increased risk of cardiovascular disease (33). A trial randomized 7,769 participants with HIV infection with a low-to-moderate risk of cardiovascular disease to either pitavastatin 4 mg or placebo (65). The primary outcome was the occurrence of cardiovascular death, myocardial infarction, hospitalization for unstable angina, stroke, transient ischemic attack, peripheral arterial ischemia, revascularization, or death from an undetermined cause. In the pitavastatin group cardiovascular events were decreased by 35% (HR, 0.65; 95% CI, 0.48 to 0.90; P=0.002). For additional information on the use of statins in HIV patients see the Endotext chapter “Lipid Disorders in People with HIV” (33).  

 

Statin Side Effects

 

An umbrella review of meta-analyses of observational studies and randomized controlled trials examined 278 unique non-CVD outcomes from 112 meta-analyses of observational studies and 144 meta-analyses of RCTs and found that the only adverse effects associated with statin therapy were the development of diabetes and muscle disorders (66). For a detailed discussion of the side effects of statin therapy a scientific statement from the American Heart Association provides a comprehensive review (67).

 

DIABETES

 

After many years of statin use it was recognized that statins increase the risk of developing diabetes. In a meta-analysis of 13 trials with over 90,000 subjects, there was a 9% increase in the incidence of diabetes during follow-up among subjects receiving statin therapy (68). All statins appear to increase the risk of developing diabetes. In comparisons of intensive vs. moderate statin therapy, Preiss et al observed that patients treated with intensive statin therapy had a 12% greater risk of developing diabetes compared to subjects treated with moderate dose statin therapy (69). Older subjects, obese subjects, and subjects with high glucose levels were at a higher risk of developing diabetes while on statin therapy (70). Thus, statins may be unmasking and accelerating the development of diabetes that would have occurred naturally in these subjects at some point in time. In patients without risk factors for developing diabetes, treatment with statins does not appear to increase the risk of developing diabetes.

 

In patients with diabetes, an analysis of 9 studies with over 9,000 patients with diabetes reported that the patients randomized to statin therapy had a 0.12% higher A1c than the placebo group indicating that statin therapy is associated with only a very small increase in A1c levels in patients with diabetes that is unlikely to be clinically significant (71). Individual studies, such as CARDS and the Heart Protection Study, have also shown only a very modest effect of statins on A1c levels in patients with diabetes (72,73).

 

The mechanism by which statins increase the risk of developing diabetes is unknown (74). A study has demonstrated that a polymorphism in the gene for HMG-CoA reductase that results in a decrease in HMG-CoA reductase activity and a small decrease in LDL levels is also associated with an increase in body weight and plasma glucose and insulin levels (75). Additionally, a cross sectional study that compared the change in BMI in individuals on statins to individuals not on statins observed an increased BMI in the subjects taking statins (+1.3 in stain users vs. + 0.4 in non-users over a 10 year period; p=0.02) (76). These observations suggest that the inhibition of HMG-CoA reductase per se may be contributing to the statin induced increased risk of diabetes via weight gain. However, studies have now shown that polymorphisms in different genes (NPC1L1 and PCSK9) that lead to a decrease in LDL-C levels are also associated with an increase in diabetes suggesting that decreases in LDL-C levels per se alter glucose metabolism and increase the risk of diabetes (74,77). How a decrease in LDL-C levels might affect glucose metabolism is unknown. Clearly further studies are required to understand the mechanisms by which statins increase the risk of developing diabetes.

 

In balancing the benefits and risks of statin therapy it is important to recognize that an increase in plasma glucose levels is a surrogate marker for an increased risk of developing micro and macrovascular disease (i.e., an increase in plasma glucose per se is not an event but rather increases the risk of future events). In contrast, statin therapy is preventing actual clinical events that cause morbidity and mortality. Furthermore, it may take many years for an elevated blood glucose to induce diabetic complications while the reduction in cardiovascular events with statin therapy occurs relatively quickly. Finally, the number of patients needed to treat with statins to avoid one cardiovascular event is much lower (10-20 depending on the type of patient) than the number of patients needed to treat to cause one patient to develop diabetes (100–200 for one extra case of diabetes) (74). Patients on statin therapy, particularly those with risk factors for the development of diabetes, should be periodically screened for the development of diabetes with measurement of fasting glucose or A1c levels.

 

CANCER

 

Analysis of 14 trials with over 90,000 subjects by the Cholesterol Treatment Trialists did not demonstrate an increased risk of cancer or any specific cancer with statin therapy (34). An update with an analysis of 27 trials with over 174,000 participants also did not observe an increase in cancer incidence or death (36). Additionally, no differences in cancer rates were observed with any particular statin.

 

COGNITIVE DYSFUNCTION

 

Several randomized clinical trials have examined the effect of statin therapy on cognitive function and have not indicated any increased risk (78-80). The Prosper Trial was designed to determine whether statin therapy will reduce cardiovascular disease in older subjects (age 70-82) (43). In this trial cognitive function was assessed repeatedly and no difference in cognitive decline was found in subjects treated with pravastatin compared to placebo (43,81). In the Heart Protection Study over 20,000 patients were randomized to simvastatin 40mg or placebo and again no significant differences in cognitive function was observed between the statin vs. placebo group (82). Additionally, a Cochrane review examined the effect of statin therapy in patients with established dementia and identified 4 studies with 1154 participants (83). In this analysis no benefit or harm of statin therapy on cognitive function could be demonstrated in this high-risk group of patients. Thus, randomized clinical trials do not indicate a significant association.

 

HEMORRAGIC STROKE

 

In a scientific statement from the American Heart Association on statin safety reached the following conclusions; “The available data in aggregate show no increased risk of brain hemorrhage with statin use in primary stroke prevention populations. An increased risk in secondary stroke prevention populations is possible, but the absolute risk is very small, and the benefit in reducing overall stroke and other vascular events generally outweighs that risk” (67).

 

LIVER DISEASE

 

It was in initially thought that statins induced liver dysfunction and it was recommended that liver function tests be routinely obtained while patients were taking statins. However, studies have now shown that the risk of liver function test abnormalities in patients taking statins is very small (61). For example, in a survey of 35 randomized studies involving > 74,000 subjects, elevations in transaminases were seen in 1.4% of statin treated subjects and 1.1% of controls (84). Similarly, in a meta-analysis of > 49,000 patients from 13 placebo controlled studies, the incidence of transaminase elevations greater than three times the upper limit of normal was 1.14% in the statin group and 1.05% in the placebo group (85). Moreover, even when the transaminase levels are elevated, repeat testing often demonstrates a return towards normal levels (86). The increases in transaminase levels with statin therapy are dose related with high doses of statins leading to more frequent elevations (87). At this time, routine monitoring of liver function tests in patients taking statins is no longer recommended. However, obtaining baseline liver function tests prior to starting statin therapy is indicated (61). If liver function tests are obtained during statin treatment, one should not be overly concerned with modestly elevated transaminase levels (less than 3x the upper limit of normal) (61). If the transaminase is greater than 3x the upper limit of normal the test should be repeated and if it remains > 3x the upper limit of normal, statin therapy should be stopped and the patient evaluated (61).

 

A more clinically important issue is whether statins lead to an increased risk of liver failure. Studies have suggested that the incidence of liver failure in patients taking statins is very similar to the rate observed in the general population (approx. 1 case per 1 million patient years) (88,89). Thus, statin therapy causing serious liver injury is a very rare event.

 

Non-alcoholic fatty liver disease (NAFLD) is very common and is associates with obesity, metabolic syndrome, diabetes, and cardiovascular disease. In patients with NAFLD studies have shown that statins decrease liver enzymes and reduce steatosis (90).

 

MUSCLE

 

The most common side effect of statin therapy is muscle symptoms. These can range from life threatening rhabdomyolysis to myalgias (Table 4) (91).

 

Table 4. Spectrum of Statin Induced Muscle Disorders (Adapted from J. Clinical Lipidology 8: S58-71, 2014)

Myalgia- aches, soreness, stiffness, tenderness, cramps with normal CK levels

Myopathy- muscle weakness with or without increased CK

Myositis- muscle inflammation

Myonecrosis- mild (CK >3x ULN); moderate (CK> 10x ULN); severe (CK> 50x ULN)

Rhabdomyolysis- myonecrosis with myoglobinuria or acute renal failure

 

Many patients will discontinue the use of statins due to muscle symptoms. Risk factors associated with an increased incidence of statin associated muscle symptoms are listed in Table 5 (92,93).

 

Table 5. Risk Factors for Statin Myopathy

Medications that alter statin metabolism

Older age

Female

Hypothyroidism

Excess alcohol

Vitamin D deficiency

History of muscle disorders

Renal disease

Liver disease

Personal or family history of statin intolerance

Low BMI

Polymorphism in SLCO1B1 gene

High dose statin

Drug-drug interactions

 

 The Cholesterol Treatment Trialists analyzed individual participant data on the development of muscle symptoms from 19 double-blind trials of statin versus placebo with 123,940 participants and four double-blind trials of a more intensive vs. a less intensive statin regimen with 30,724 participants (94). After a median follow-up of 4.3 years 27.1% of the individuals taking a statin vs. 26.6% on placebo reported muscle pain or weakness representing a 3% increase greater than placebo (risk ratio- 1.03; 95% CI 1.01-1.06) (Table 6). The specific muscle symptoms caused by statin therapy, myalgia, muscle cramps or spasm, limb pain, other musculoskeletal pain, or muscle fatigue or weakness were similar to those caused by placebo. The increase in muscle symptoms in the statin treated individuals was manifest in the first year of therapy but in the later years muscle symptoms were similar in the statin treated and placebo groups. The relative risk of statin induced muscle symptoms was greater in women than men. Intensive statin treatment with 40-80 mg atorvastatin or 20-40 mg rosuvastatin resulted in a higher risk of muscle symptoms than less intensive or moderate-intensity regimens but different statins at equivalent LDL-C lowering doses had similar effects on muscle symptoms. This study demonstrates that there is a small increase in muscle symptoms that primarily manifests in the first year of therapy. Statin therapy caused approximately 11 additional complaints of muscle pain or weakness per 1000 patients during the first year, but little excess in later years. Of particularly note is that 26.6% of patients taking a placebo had muscle symptoms demonstrating a very high frequency of this clinical complaint. Given the high prevalence of muscle complaints and the small increase attributed to statins it is very difficult to determine if a muscle complaint is actually due to the statin, which presents great clinical difficulties in patient management.                                                       

 

Table 6. Effect of Statin vs. Placebo on Muscle Symptoms

Symptom

Statin Events

Placebo Events

RR (95% CI)

Myalgia

12.0%

11.7%

1·03 (0·99–1·08)

Other musculoskeletal pain

13.3

13.0

1·03 (0·99–1·08)

Any muscle pain

26.9%

26.3%

1·03 (1·01–1·06)

Any muscle pain or weakness

27.1%

26.6%

1·03 (1·01–1·06)

Modified from (94).

 

While the results of the randomized trials suggest that muscle symptoms are not frequently induced by statin therapy, in typical clinical settings a significant percentage of patients are unable to tolerate statins due to muscle symptoms (in many studies as high as 5-25% of patients) (95-97). Recently there was a randomized trial that explored the issue of myopathy with statin therapy in great detail (98). In this trial the effect of atorvastatin 80mg a day vs. placebo for 6 months on creatine kinase (CK), exercise capacity, and muscle strength was studied in 420 healthy, statin-naive subjects. Atorvastatin treatment led to a modest increase in CK levels (20.8U/L) with no change observed in the placebo group. None of the subjects had an elevation of CK > 10x the upper limits of normal. There were no changes in muscle strength or exercise capacity with atorvastatin treatment. However, myalgia was reported in 19 subjects (9.4%) in the atorvastatin group compared to 10 subjects (4.6%) in the placebo group (p=0.05).  In this study “myalgia” was considered to be present if all of the following occurred: (1) subjects reported new or increased muscle pain, cramps, or aching not associated with ex­ercise; (2) symptoms persisted for at least 2 weeks; (3) symptoms resolved within 2 weeks of stopping the study drug; and (4) symp­toms reoccurred within 4 weeks of restarting the study medication. Notably these myalgias were not associated with elevated CK levels. In the atorvastatin group the myalgias tended to occur soon after therapy (average 35 days) whereas in the placebo group myalgias occur later (average 61 days). In the atorvastatin group the symptoms were predominantly localized to the legs and included aches, cramps, and fatigue, whereas in the placebo group they were more diverse including whole body fatigue, foot cramps, worsening of pain in previous injuries, and groin pain. A number of conclusions can be reached from this study. First, statin treatment does in fact increase the incidence of myalgias. Second, a substantial number of patients treated with placebo will also develop myalgias. Third, clinically differentiating statin induced myalgias from placebo induced myalgias is difficult, as there are no specific symptoms, signs, or biomarkers that clearly distinguish between the two. It should be recognized that the patient population typically treated with statins (patients 50-80 years of age) often have muscle symptoms in the absence of statin therapy and it is therefore difficult to be certain that the muscle symptoms described by the patient are actually due to statin therapy.

 

Additionally, when patients know that they are taking a statin they are more likely to have muscle symptoms (i.e. the nocebo effect). This was nicely demonstrated in the ASCOT-LLA extension trial (99). In the initial phase of the study the patients were randomly assigned to atorvastatin 10 mg (n= 5101) or matching placebo (n= 5079) in a double-blind fashion. During the 3.3 years of the double blinded phase adverse muscle symptoms were very similar in the atorvastatin and placebo groups (HR 1.03; p=0.72). This double-blind phase was followed by a non-blinded non-randomized extension where 6409 patients were treated with atorvastatin 10mg and 3490 were untreated. During the 2.3 years of this extension study muscle symptoms were significantly increased in the atorvastatin group (HR 1·41; p=0.006).    

 

In a very small study in the Annals of Internal Medicine eight patients with “statin related myalgia” were re-challenged with statin or placebo and there were no statistically significant differences in the recurrence of myalgias on the statin or placebo (100). This approach has been expanded upon in other studies. In 120 patients with “statin induced myalgia” patients were randomized in a double blinded crossover trial to either simvastatin 20mg per day or placebo and the occurrence of muscle symptoms was determined (101). Only 36% of these patients were confirmed to actually have statin induced myalgia (presence of symptoms on simvastatin without symptoms on placebo). In a similar study, Nissen and colleagues studied 491 patients with “statin induced myalgia” treating with either atorvastatin 20mg per day or placebo in a double-blind crossover trial (102). In this trial 42.6% of patients were confirmed to have statin induced muscle symptoms. In a trial of 156 patients with prior statin induced muscle symptoms patients were treated with alternating periods of atorvastatin 20mg or placebo (103). In this trial no difference in muscle symptoms was found between the statin and placebo treatment periods. A smaller crossover trial in 49 patients who had stopped statin therapy also found no difference in muscle symptoms when patients were taking atorvastatin 20mg or placebo (104)

 

Thus, while statin induced myalgias are a real entity careful studies have shown that in the majority of patients with “statin induced muscle symptoms” the symptoms are not actually due to statin therapy. In the clinic it is difficult to be certain whether the muscle symptoms are actually due to true statin intolerance or to other factors. The approach to treating these patients will be discussed later in this chapter (Treatment of Stain Intolerant Patients). While some patients will not tolerate statin therapy due to myalgias, this side effect does not appear to result in serious morbidity or long-term consequences. In contrast, studies have found that discontinuing statins increases the risk of myocardial infarctions and death from cardiovascular disease (105,106).

 

Fortunately, the more serious muscle related side effects of statin therapy are rare. In a meta-analysis of 21 statin vs. placebo trials there was an excess risk of rhabdomyolysis of 1.6 patients per 100,000 patient years or a standardized rate of 0.016/patient years (86). Other studies report a rate of rhabdomyolysis between 0.03- 0.16 per 1,000 patient years (107). Similarly, the risk of statin induced myositis (muscle symptoms with an increase in CK 10 times the upper limits of normal) is also very low. In an analysis of 21 randomized trials myositis occurred in only 5 patients per 100,000 person years or 0.05/1000 patient years (86). The higher the dose of statin used the greater the risk of myositis and rhabdomyolysis. In a comparison of five trials that compared high dose statin vs. low dose statin there was an excess risk of rhabdomyolysis of 4 per 10,000 people treated (35). The likely basis for an increased risk of myositis or rhabdomyolysis is elevated statin blood levels, which are more likely to occur with high doses of statins. In the development of statins, manufacturers have studied higher doses that are not approved for clinical use. For example, simvastatin and pravastatin at 160mg per day were studied but discontinued due to an increased incidence of muscle side effects (108,109). The use of simvastatin 80mg per day, a previously approved dose, was discontinued due to an increased risk of muscle side effects. Similarly, pitavastatin at doses greater than 4mg per day was investigated, but development was abandoned when an increased risk of rhabdomyolysis was observed. Along similar lines, in many of the patients that develop rhabdomyolysis, the etiology can be linked to the use of other drugs that alter statin metabolism thereby increasing statin blood levels (93). For example, prior to drug interactions being recognized the use of cyclosporine, gemfibrozil, HIV protease inhibitors, and erythromycin in conjunction with certain statins was linked with the development of rhabdomyolysis (93). Finally, common variants in SLCO1B1, which encodes the organic anion-transporting polypeptide OATP1B1, are strongly associated with an increased risk of statin-induced myopathy (110). OATP1B1 facilitates the transport of statins into the liver and certain polymorphisms are associated with an increased risk of developing statin induced muscle disorders, due to the decreased transport of statins into the liver resulting in increased blood levels (111). The exact mechanism by which elevated blood levels induce muscle toxicity remains to be elucidated.

 

Recently it has been recognized that a very small number of patients taking statins develop a progressive autoimmune necrotizing myopathy, which is characterized by progressive symmetric proximal muscle weakness, elevated CK levels (typically >10x the ULN), and antibodies against HMG-CoA reductase (112). It is estimated that this occurs in 2 or 3 per 100,000 patients treated with a statin (112). This myopathy may begin soon after initiating statin therapy or develop after a patient has been on statins for many years (112). Muscle biopsy reveals necrotizing myopathy without severe inflammation (112). In contrast to the typical muscle disorders induced by statin therapy, the autoimmune myopathy progresses despite discontinuing therapy. Spontaneous improvement is not typical and most patients will need to be treated with immunosuppressive therapy (glucocorticoids plus methotrexate, azathioprine, or mycophenolate mofetil) (112). It should be recognized that this disorder can occur in individuals that have not been exposed to statin therapy (113). Statins likely potentiate the development of this disorder in susceptible individuals, perhaps by increasing HMG-CoA reductase levels.

 

From the above certain conclusions can be reached. First, the risk of serious muscle disorders due to statin therapy is very small, particularly if one is aware of the potential drug interactions that increase the risk. Second, the muscle toxicity is usually linked to elevated statin blood levels and the higher the dose of the statin the more likely the chance of developing toxicity. Third, myalgias in patients taking statins are very common and can be due to statin treatment. However, in the individual patient, it is very difficult to know if the myalgia is actually secondary to statin therapy and in many, if not most patients, the myalgias are not due to statin therapy. Fourth, the muscle symptoms that occur in association with statin treatment are a major reason why patients discontinue statin use and therefore better diagnostic algorithms and treatments are required to allow patients to better comply with these highly effective treatments to reduce cardiovascular disease. 

 

Contraindications

 

Previously statins were contraindicated in pregnant women or lactating women. However, in July 2021 the FDA requested the removal of the strongest recommendation against using statins during pregnancy. They continue to advise against the use of statins in pregnancy given the limited data and quality of information available. The decision of whether to continue a statin during pregnancy requires shared decision-making between the patient and clinician, and healthcare professionals need to discuss the risks versus the benefits in high-risk women, such as those with homozygous FH or prior ASCVD events, that may benefit from statin therapy. For a detailed discussion of the use of statins during pregnancy see the Endotext chapter entitled “Effect of Pregnancy on Lipid Metabolism and Lipoprotein Levels” (114).

 

In addition, liver function tests should be obtained prior to initiating statin treatment and moderate to severe liver disease is a contraindication to statin therapy (61). 

 

Summary

 

An enormous data base has accumulated which demonstrates that statins are very effective at reducing the risk of cardiovascular disease and that statins have an excellent safety profile. The risk benefit ratio of treating patients with statins is very favorable and has resulted in this class of drugs being widely utilized to lower serum lipid levels and to reduce the risk of cardiovascular disease and death.

 

EZETIMBE (ZETIA)

 

Introduction

 

Ezetimibe (Zetia) inhibits the absorption of cholesterol by the intestine thereby resulting in modest decreases in LDL-C levels (115). Ezetimibe is primarily used in combination with statin therapy when statin treatment alone does not lower LDL-C levels sufficiently or when patients only tolerate a low statin dose. It may also be used as monotherapy or in combination with other lipid lowering drugs to lower LDL-C levels in patients with statin intolerance. Finally, it is the drug of choice in patients with the rare genetic disorder sitosterolemia, which is discussed in detail in the chapter “Sitosterolemia” (116). Ezetimibe is relatively inexpensive as it is now a generic drug.

 

Effect of Ezetimibe on Lipid and Lipoprotein Levels

 

Pandor and colleagues have published a meta-analysis of ezetimibe monotherapy that included 8 studies with 2,722 patients (117). They reported that ezetimibe decreased LDL-C levels by 18.6%, decreased triglyceride levels by 8.1%, and increased HDL-C levels by 3% compared to placebo. In a pooled analysis by Morrone and colleagues of 27 studies with 11, 714 subjects treated with ezetimibe in combination with statin therapy similar results were observed (118). Specifically, LDL-C levels were decreased by 15.1%, non-HDL-C levels by 13.5%, triglycerides by 4.7%, apolipoprotein B levels by 10.8%, and HDL-C levels were increased by 1.6%. The combination of a high dose potent statin plus ezetimibe can lower LDL-C levels by 70% (119). A meta-analysis of the effect of ezetimibe on Lp(a) revealed that with either monotherapy or combination with statin there was no change in Lp(a) levels (120). The effect of ezetimibe on lipid parameters occurs quickly and can be seen after 2 weeks of treatment. In patients with Heterozygous Familial Hypercholesterolemia who have marked elevations in LDL-C levels, the addition of ezetimibe to statin therapy resulted in a further 16.5% decrease in LDL-C levels (121). Thus, in comparison with statins, ezetimibe treatment produces modest decreases in LDL-C levels (15-20%). In addition to these changes in lipid parameters, ezetimibe in combination with a statin decreased hs-CRP by 10-19% compared to statin monotherapy (122,123). However, ezetimibe alone does not decrease hs-CRP levels (123).

 

Table 7. Effect of Ezetimibe on Lipid/Lipoprotein Levels

LDL-C

Decrease

Non-HDL-C

Decrease

Apolipoprotein B

Decrease

Triglycerides

Small decrease

HDL-C

Small increase

Lp(a)

No change

 

Mechanisms Accounting for the Ezetimibe Induced Lipid Effects

 

NPC1L1 (Niemann-Pick C1-like 1 protein) is highly expressed in the intestine with the greatest expression in the proximal jejunum, which is the major site of intestinal cholesterol absorption (124,125). Knock out animals deficient in NPC1L1 have been shown to have a decrease in intestinal cholesterol absorption (124). Ezetimibe binds to NPC1L1 and inhibits cholesterol absorption (115,124,125). In animals lacking NPC1L1, ezetimibe has no effect on intestinal cholesterol absorption, demonstrating that ezetimibe’s effect on cholesterol absorption is mediated via NPC1L1 (115,125). Thus, a major site of action of ezetimibe is to block the absorption of cholesterol by the intestine (115,125). Cholesterol in the intestinal lumen is derived from both dietary cholesterol (approximately 25%) and biliary cholesterol (approximately 75%); thus the majority is derived from the bile (125). As a consequence, even in patients that have very little cholesterol in their diet, ezetimibe will decrease cholesterol absorption. While ezetimibe is very effective in blocking intestinal cholesterol absorption it does not interfere with the absorption of triglycerides, fatty acids, bile acids, or fat-soluble vitamins including vitamin D and K.

 

When intestinal cholesterol absorption is decreased the chylomicrons formed by the intestine contain less cholesterol and thus the delivery of cholesterol from the intestine to the liver is diminished (126). This results in a decrease in the cholesterol content of the liver, leading to the activation of SREBPs, which enhance the expression of LDL receptors resulting in an increase in LDL receptors on the plasma membrane of hepatocytes (Figure 1) (126). Thus, similar to statins the major mechanism of action of ezetimibe is to decrease the levels of cholesterol in the liver resulting in an increase in the number of LDL receptors leading to the increased clearance of circulating LDL (126). In addition, the decreased cholesterol delivery to the liver may also decrease the formation and secretion of VLDL (126).

 

In addition to NPC1L1 expression in the intestine this protein is also expressed in the liver where it mediates the transport of cholesterol from the bile back into the liver (127). The inhibition of NPC1L1 in the liver will result in the increased secretion of cholesterol in bile and thereby could also contribute to a decrease in the cholesterol content of the liver and an increase in LDL receptor expression and a decrease in VLDL production.

 

Pharmacokinetics and Drug Interactions

 

Following absorption by intestinal cells ezetimibe is rapidly glucuronidated. The glucuronidated ezetimibe is then secreted into the portal circulation and rapidly taken up by the liver where it is secreted into the bile and transported back to the intestine (115). This enterohepatic circulation repeatedly returns ezetimibe to its site of action (note glucuronidated ezetimibe is a very effective inhibitor of NPC1L1) (115). Additionally, this enterohepatic circulation accounts for the long duration of action of ezetimibe and limits peripheral tissue exposure (115). Ezetimibe is not significantly excreted by the kidneys and thus the dose does not need to be adjusted in patients with renal disease.

 

Ezetimibe is not metabolized by the P450 system and does not have many drug interactions (115). It should be noted that cyclosporine does increase ezetimibe levels.

 

Effect of Ezetimibe Therapy on Clinical Outcomes

 

There have been a limited number of ezetimibe clinical outcome trials. Two have studied the effect of ezetimibe in combination with a statin vs. placebo making it virtually impossible to determine if ezetimibe per se has beneficial effects. However, one study has compared ezetimibe plus a statin vs. a statin alone and one study compared ezetimibe vs. placebo. Finally, a study compared moderate-intensity statin with ezetimibe vs. high-intensity statin monotherapy.

 

SEAS TRIAL

 

The SEAS Trial was a randomized trial of 1,873 patients with mild-to-moderate, asymptomatic aortic stenosis (128). The patients received either simvastatin 40mg per day in combination with ezetimibe 10mg per day vs. placebo daily. The primary outcome was a composite of major cardiovascular events, including death from cardiovascular causes, aortic-valve replacement, non-fatal myocardial infarction, hospitalization for unstable angina pectoris, heart failure, coronary-artery bypass grafting, percutaneous coronary intervention, and non-hemorrhagic stroke. Secondary outcomes were events related to aortic-valve stenosis and ischemic cardiovascular events. Simvastatin plus ezetimibe lowered LDL-C levels by 61% compared to placebo. There were no significant differences in the primary outcome between the treated vs. placebo groups. Similarly, the need for aortic valve replacement was also not different between the treated and placebo groups. However, fewer patients had ischemic cardiovascular events in the simvastatin plus ezetimibe treated group than in the placebo group (hazard ratio, 0.78; 95% CI, 0.63 to 0.97; P=0.02), which was primarily accounted for by a decrease in the number of patients who underwent coronary-artery bypass grafting. The design of this study does not allow for one to determine if the beneficial effect on ischemic cardiovascular events typically produced by statin therapy was enhanced by the addition of ezetimibe.

 

SHARP TRIAL

 

The SHARP Trial was a randomized trial of 9,270 patients with chronic kidney disease (3,023 on dialysis and 6,247 not on dialysis) with no known history of myocardial infarction or coronary revascularization (129). Patients were randomly assigned to simvastatin 20 mg plus ezetimibe 10 mg daily vs. placebo. The primary outcome was first major atherosclerotic event (non-fatal myocardial infarction or coronary death, non-hemorrhagic stroke, or any arterial revascularization procedure). Treatment with simvastatin plus ezetimibe resulted in a decrease in LDL-C of 0.85 mmol/L (~34mg/dL). This decrease in LDL-C was associated with a 17% reduction in major atherosclerotic events. In patients on hemodialysis there was a 5% decrease in cardiovascular events that was not statistically significant. Unfortunately, similar to the SEAS Trial, it is impossible to determine whether the addition of ezetimibe improved outcomes above and beyond what would have occurred with statin treatment alone.

 

IMPROVE-IT TRIAL

 

The IMPROVE-IT Trial tested whether the addition of ezetimibe to statin therapy would provide an additional beneficial effect in patients with the acute coronary syndrome (130). The IMPROVE-IT Trial was a large trial with over 18,000 patients randomized to simvastatin 40mg vs. simvastatin 40mg + ezetimibe 10mg per day. On treatment LDL-C levels were 70mg/dL in the statin alone group vs. 54mg/dL in the statin + ezetimibe group. There was a small but significant 6.4% decrease in major cardiovascular events (cardiovascular death, MI, documented unstable angina requiring rehospitalization, coronary revascularization, or stroke) in the statin + ezetimibe group (HR 0.936 CI (0.887, 0.988) p=0.016). Cardiovascular death, non-fatal MI, or non-fatal stroke were reduced by 10% (HR 0.90 CI (0.84, 0.97) p=0.003). There was a significant 21% reduction in ischemic stroke (HR, 0.79; 95% CI, 0.67-0.94; P=0.008) and a nonsignificant increase in hemorrhagic stroke (HR, 1.38; 95% CI, 0.93-2.04; P=0.11) (131). Patients with a prior stroke were at a higher risk of stroke recurrence and the risk of a subsequent stroke was reduced by 40% (HR, 0.60; 95% CI, 0.38-0.95; P=0.030) with ezetimibe added to simvastatin therapy (131). In patients with diabetes or other high risk factors the benefits of adding ezetimibe to statin therapy was enhanced (132). In fact, patients without DM and at low or moderate risk demonstrated no benefit with the addition of ezetimibe to simvastatin (132). Similarly, patients who also had peripheral arterial disease or a history of cerebral vascular disease also had the greatest absolute benefits from the addition of ezetimibe (133). Thus, the addition of ezetimibe to statin therapy is of greatest benefit in patients at high risk (for example patients with diabetes, peripheral vascular disease, cerebrovascular disease, etc.).

 

The results of this study have a number of important implications. First, it demonstrates that combination therapy has benefits above and beyond statin therapy alone. Second, it provides further support for the hypothesis that lowering LDL per se will reduce cardiovascular events. The reduction in cardiovascular events was similar to what one would predict based on the Cholesterol Treatment Trialists results. Third, it suggests that lowering LDL levels into the 50s will have benefits above and beyond lowering LDL levels to the 70mg/dL range in patients with diabetes or other factors that result in a high risk for cardiovascular events. These results have implications for determining goals of therapy and provide support for combination therapy.

 

EWTOPIA 75

 

This was a multicenter, randomized trial in Japan that examined the preventive efficacy of ezetimibe for patients aged ≥75 years (mean age 80.6 years), with elevated LDL-C (≥140 mg/dL) without a history of coronary artery disease who were not taking lipid lowering drugs (134). Patients were randomized to ezetimibe 10mg (n=1,716) or usual care (n=1,695) and followed for 4.1 years. The primary outcome was a composite of sudden cardiac death, myocardial infarction, coronary revascularization, or stroke. In the ezetimibe group LDL-C was decreased by 25.9% and non-HDL-C by 23.1% while in the usual care group LDL-C was decreased by 18.5% and non-HDL-C by 16.5% (p<0.001 for both lipid parameters). By the end of the trial 9.6% of the patients in the usual care group and 2.1% of the ezetimibe group were taking statins. Ezetimibe reduced the incidence of the primary outcome by 34% (HR 0.66; P=0.002). Additionally, composite cardiac events were reduced by 60% (HR 0.60; P=0.039) and coronary revascularization by 62% (HR 0.38; P=0.007) in the ezetimibe group vs. the control group. There was no difference in the incidence of stroke or all-cause mortality between the groups. It should be noted that the reduction in cardiovascular events was much greater than one would expect based on the absolute difference in LDL-C levels (121mg/dL in ezetimibe group vs. 132mg/dL). As stated by the authors “Given the open-label nature of the trial, its premature termination, and issues with follow-up, the magnitude of benefit observed should be interpreted with caution.” Nevertheless, this study provides additional support that ezetimibe can reduce cardiovascular events.

 

RACING TRIAL

 

The RACING trial was a randomized, open-label trial in patients with atherosclerotic cardiovascular disease carried out in South Korea (135). Patients were randomly assigned to either rosuvastatin 10 mg with ezetimibe 10 mg (n= 1894) or rosuvastatin 20 mg (n= 1886). The primary endpoint was cardiovascular death, major cardiovascular events, or non-fatal stroke. The median LDL-C level during the study was 58mg/dL in the combination therapy group and 66mg/dL in the statin monotherapy group (p<0·0001). The primary endpoint occurred in 9.1% of the patients in the combination therapy group and 9·9% of the patients in the high-intensity statin monotherapy group (non-inferior). Non-inferiority was observed in patients with LDL-C levels < 100mg/dL and >100mg/dL and in patients greater than 75 years of age (136,137).

 

This study demonstrates that moderate intensity statin with ezetimibe was non-inferior to high-intensity statin therapy with regards to cardiovascular death, major cardiovascular events, or non-fatal stroke. Interestingly a lower prevalence of discontinuation or dose reduction caused by intolerance to the study drug was seen with combination therapy. This indicates that using a moderate intensity dose of a statin with ezetimibe is a useful strategy in patients that do not tolerate high intensity statin therapy.

 

Side Effects

 

Ezetimibe has not demonstrated significant side effects. In monotherapy trials, the effect on liver function tests was similar to placebo. In a meta-analysis by Toth et al. of 27 randomized trials in > 20,000 participants evaluating statin plus ezetimibe vs. statin alone the incidence of liver function test abnormalities was slightly greater in the combination therapy group (statin alone- 0.35% vs. statin plus ezetimibe 0.56%) (138). In contrast, Luo and colleagues in a meta-analysis of 20 randomized with > 14,000 subjects did not observe a difference in liver function tests in the ezetimibe plus statin vs. statin alone group (139). With regards to muscle side effects, a meta-analysis of seven randomized trials by Kashani and colleagues found that monotherapy with ezetimibe or ezetimibe in combination with a statin did not increase the risk of myositis compared to placebo or monotherapy with a statin (140). Similarly, Luo et al also did not observe that combination therapy with ezetimibe and a statin increased the risk of myositis (139). In a meta-analysis by Savarese et al. of 7 randomized long-term studies including SEAS, SHARP, and IMPROVE-IT, the incidence of cancer was similar in patients treated with ezetimibe vs. patients not treated with ezetimibe (141). This confirms a previous study that also did not demonstrate an increased cancer risk in the three largest ezetimibe trials (142). Ezetimibe does not appear to have adverse effects on fasting glucose levels or A1c levels (143).

 

Thus, over many years of use ezetimibe has been shown to be a very safe drug without major side effects.

 

Contraindications

 

Ezetimibe is contraindicated in patients with active liver disease. The use of ezetimibe during pregnancy and lactation has not been studied.  

 

Summary

 

Ezetimibe has a modest ability to lower LDL-C levels and can be a very useful adjunct to statin therapy. When added to statin therapy it will lower the LDL-C by an additional 15-20% which is equivalent to three titrations of the statin dose (for example adding ezetimibe is equivalent to increasing atorvastatin from 10mg to 80mg per day). Additionally, the combination of a high dose of a potent statin (rosuvastatin 40mg per day) with ezetimibe was able to lower the LDL by approximately 70%, which will allow many patients to reach their LDL goal (123). In patient’s intolerant of statins who either cannot take a statin or can only take low doses of a statin, ezetimibe is extremely useful in further lowering LDL-C. The ease of taking ezetimibe, the lack of serious side effects, and that it is inexpensive as it is now a generic drug make it an obvious second choice drug after statins to lower LDL-C levels.   

 

BILE ACID SEQUESTRANTS 

 

Introduction

 

There are three bile acid sequestrants approved for use in the United States. The first bile acid sequestrant, cholestyramine (Questran), was developed in the 1950s and was the second drug available to lower cholesterol levels (niacin was the first drug). Colestipol (Colestid) was developed in the 1970s and is very similar to cholestyramine. In 2000, Colesevelam (Welchol) was approved. Colesevelam has enhanced binding and affinity for bile acids compared to cholestyramine and colestipol and therefore can be given in much lower doses reducing some side effects (144).

 

​Cholestyramine is available as a powder and the dose ranges from 8-24 grams per day given with meals. Colestipol is available as a tablet and the dose ranges from 2-16 grams per day given with meals or granules and the dose ranges from 5-30 grams per day given with meals. The dose of colesevelam is 3.75 grams per day and can be given as tablets (​take 6 tablets once daily or 3 tablets twice daily), oral suspension (​take one packet once daily), or chewable bars (take one bar once daily). Because bile acid sequestrants mechanism of action starts with the binding of bile acids in the intestine (see below) these drugs are most effective when administered with meals.

 

Effect of Bile Acid Sequestrants on Lipid and Lipoprotein Levels

 

The major effect of bile acid sequestrants is to lower LDL-C levels in a dose dependent fashion. Depending upon the specific drug and dose the decrease in LDL-C ranges from approximately 5 to 30% (144-146). The effect of monotherapy with bile acid sequestrants on LDL-C levels observed in various studies is shown in table 8.

 

Table 8. Effect of Bile Acid Sequestrants on LDL-C

Drug

LDL lowering

Cholestyramine 4g/day

7% decrease

Cholestyramine 24g/day

28% decrease

Colestipol 4g/day

12% decrease

Colestipol 16g/day

24% decrease

Colesevelam 3.8g/day

15% decrease

Colesevelam 4.3g/day

18% decrease

 

Bile acid sequestrants are typically used in combination with statins and the addition of bile acid sequestrants to statin therapy will result in a further 10% to 25% decrease in LDL-C levels (144-146). Combination therapy can result in a 60% reduction in LDL-C levels when high doses of potent statins are combined with high doses of bile acid sequestrants. Bile acid sequestrants will also further lower LDL-C levels by as much as 18% when added to statins and ezetimibe (147). This is particularly useful in patients with Heterozygous Familial Hypercholesterolemia who can have very high LDL-C levels at baseline. Additionally, in patients who are statin intolerant, the combination of a bile acid sequestrant and ezetimibe resulted in an additional 10-20% decrease in LDL-C compared to either drug alone  (148,149). Thus, both in monotherapy and in combination with other drugs that lower LDL-C levels, bile acid sequestrants are effective in lowering LDL-C levels

 

Bile acid sequestrants have a very modest effect on HDL-C levels, typically resulting in a 3-9% increase (144-146). The effect of bile acid sequestrants on triglyceride levels varies (144-146). In patients with normal triglyceride levels, bile acid sequestrants increase triglyceride levels by a small amount. However, as baseline triglyceride levels increase, the effect of bile acid sequestrants on plasma triglyceride levels becomes greater, and can result in substantial increases in triglyceride levels. In patients with triglycerides > 400mg/dL the use of bile acid sequestrants is contraindicated.

 

Table 9. Effect of Bile Acid Sequestrants on Lipid/Lipoprotein Levels

LDL-C

Decrease

Non-HDL-C

Decrease

Apolipoprotein B

Decrease

Triglycerides

Variable. If TG levels elevated will increase significantly

HDL-C

Small Increase

Lp(a)

No change

 

Non-Lipid Effects of Bile Acid Sequestrants

 

Bile acid sequestrants have been shown to reduce fasting glucose and hemoglobin A1c levels (150). Colesevelam has been most intensively studied and in a number of different studies colesevelam has decreased A1c levels by approximately 0.5-1.0% in patients also treated with a variety of glucose lowering drugs including metformin, sulfonylureas, and insulin. The Food and Drug Administration (FDA) has approved colesevelam for improving glycemic control in patients with type 2 diabetes.

 

Bile acid sequestrants decrease CRP. For example, Devaraj et al have shown that colesevelam decreases hs-CRP by 18% compared to placebo (151). In combination with a statin, colesevelam reduced hs-CRP levels by 23% compared to statin alone (152). 

 

Mechanisms Accounting for Bile Acid Sequestrants Induced Lipid Effects

 

Bile acid sequestrants bind bile acids in the intestine, preventing their reabsorption in the terminal ileum leading to the increased fecal excretion of bile acids (153). This decrease in bile acid reabsorption reduces the size of the bile acid pool, which stimulates the conversion of cholesterol into bile acids in the liver (153). This increase in bile acid synthesis decreases hepatic cholesterol levels leading to the activation of SREBPs that up-regulate the expression of the enzymes required for the synthesis of cholesterol and the expression of LDL receptors (153). The increase in hepatic LDL receptors results in the increased clearance of LDL from the circulation leading to a decrease in serum LDL-C levels (Figure 1). Thus, similar to statins and ezetimibe, bile acids lower plasma LDL-C levels by decreasing hepatic cholesterol levels, which stimulates LDL receptor production and thereby accelerates the clearance of LDL from the blood.

 

The key regulator of bile acid synthesis is FXR (farnesoid X receptor), a nuclear hormone receptor that forms a heterodimer with RXR to regulate gene transcription (154,155). Bile acids down-regulate cholesterol 7α hydroxylase, the first enzyme in the bile acid synthetic pathway by several FXR mediated mechanisms. In the ileum, bile acids via FXR stimulate the production of FGF19, which is secreted into the portal vein and inhibits cholesterol 7α hydroxylase expression in the liver (154). Additionally, in the liver, bile acids activate FXR leading to the increased expression of SHP (small heterodimer partner), which inhibits the transcription of cholesterol 7α hydroxylase (155). Thus, a decrease in bile acids will lead to the decreased activation of FXR in the liver and intestines and thereby result in an increase in cholesterol 7α hydroxylase expression and the increased conversion of cholesterol to bile acids resulting in a decrease in hepatic cholesterol content.

 

Decreased activation of FXR can also explain the adverse effects of bile acid sequestrants on triglyceride levels (156,157). Activation of FXR increases the expression of apolipoprotein C-II, apolipoprotein A-V, and the VLDL receptor, proteins that decrease plasma triglyceride levels while decreasing the expression of apolipoprotein C-III, a protein that is associated with increases in plasma triglycerides (156,157). Thus, activation of FXR would be expected to decrease triglyceride levels as increases in apolipoprotein C-II, apolipoprotein A-V, and the VLDL receptor and decreases in apolipoprotein C-III would reduce plasma triglyceride levels. With bile acid sequestrants the activation of FXR would be reduced and decreases in the expression of apolipoprotein C-II, apolipoprotein A-V, and the VLDL receptor and increased expression of apolipoprotein C-III would increase plasma triglyceride levels.

 

The mechanism by which treatment with bile acid sequestrants improves glycemic control is unclear (158). 

 

Pharmacokinetics and Drug Interactions

 

Bile acid sequestrants are not absorbed and not altered by digestive enzymes and thus their primary effects are localized to the intestine (144-146). It should be noted that bile acid sequestrants can indirectly have systemic effects by decreasing the reabsorption of bile acids and thereby reducing the exposure of cells to bile acids, which are biologically active compounds.

 

Unfortunately, in the intestine bile acid sequestrants can impede the absorption of many other drugs (144-146). This is particularly true for cholestyramine and colestipol which are used in large quantities (maximum doses- cholestyramine 24 grams per day; colestipol 30 grams per day). In contrast, colesevelam, which requires a much lower quantity of drug because of its high affinity and binding capacity for bile salts, has less of an effect on the absorption of other drugs (recommended dose of colesevelam 3.75 grams/day). Of particular note colesevelam does not interfere with absorption of statins, fenofibrate, or ezetimibe. A list of some of the drugs whose absorption is affected by cholestyramine or colestipol is shown in table 10 and a list of drugs whose absorption is affected by colesevelam is shown in table 11.

 

Table 10.  Some of the Drugs Affected by Cholestyramine/Colestipol

Statins

Ezetimibe

Gemfibrozil

Fenofibrate

Thiazides

Furosemide

Spironolactone

Digoxin

Warfarin

L-thyroxine

Corticosteroids

Vitamin K

Cyclosporine

Raloxifine

NSAIDs

Sulfonylureas

Aspirin

Beta blockers

Tricyclic

 

 

Table 11. Some of the Drugs Affected by Colesevelam

L-thyroxine

Cyclosporine

Glimepiride

Glipizide

Glyburide

Phenytoin

Olmesartan

Warfarin

Oral contraceptives

 

 

 

 

It is currently recommended that medications should be taken either 4 hours before or 4 hours after taking bile acid sequestrants. This is particularly important with drugs that have a narrow toxic/therapeutic window, such as thyroid hormone, digoxin, or warfarin. It can be very difficult for many patients, particularly those on multiple medications, to take bile acid sequestrants given the need to separate pill ingestion.

 

Cholestyramine and colestipol may also interfere with the absorption of fat-soluble vitamins. Taking a multivitamin 4 hours before or after these drugs can reduce the likelihood of a vitamin deficiency.

 

Effect of Bile Acid Sequestrants on Clinical Outcomes

 

The Lipid Research Clinics Coronary Primary Prevention Trial (LRC-CPPT) of cholestyramine vs. placebo was the first large drug study to explore the effect of specifically lowering LDL-C on cardiovascular outcomes (159). LRC-CPPT was a multicenter, randomized, double-blind study in 3,806 asymptomatic middle-aged men with primary hypercholesterolemia. The treatment group received cholestyramine 24 grams per day and the control group received a placebo for an average of 7.4 years. In the cholestyramine group total and LDL-C was decreased by 8.5% and 12.6% as compared to the placebo group. In the cholestyramine group there was a 19% reduction in risk (p < 0.05) of the primary end point accounted for by a 24% reduction in definite CHD death and a 19% reduction in nonfatal myocardial infarction. In addition, the incidence rates for new positive exercise tests, angina, and coronary bypass surgery were reduced by 25%, 20%, and 21%, respectively, in the cholestyramine group. The reduction in events correlated with the decrease in LDL-C levels (160). Of note, compliance with cholestyramine 24 grams per day was limited with many patients taking much less than the prescribed doses. These results indicate that lowering LDL-C with bile acid sequestrant monotherapy reduces cardiovascular disease.

 

In addition to the LRC-CPPT clinical outcome study, two studies have examined the effect of cholestyramine monotherapy on angiographic changes in the coronary arteries. The National Heart, Lung, and Blood Institute Type II Coronary Intervention Study and the St Thomas Atherosclerosis Regression Study reported that cholestyramine decreased the progression of atherosclerosis (161,162). There are a number of studies that have employed bile acid sequestrants in combination with other drugs and have shown a reduction in the progression of atherosclerosis or an increase in the regression of atherosclerosis but given the use of multiple drugs it is difficult to attribute the beneficial effects to the bile acid sequestrants (163-165). Unfortunately, there are no clinical outcome studies comparing statins alone vs. statins plus bile acid sequestrants.

 

Side Effects

 

Bile acid sequestrants do not have major systemic side effects as they are not absorbed and remain in the intestinal tract. However, they do cause gastrointestinal (GI) side effects (144-146). Constipation is a very common side effect and can be severe. In addition, patients will often complain of bloating, abdominal discomfort, and aggravation of hemorrhoids. Because of GI distress, a significant number of patients will discontinue therapy with bile acid sequestrants. These GI side effects are much more common with cholestyramine and colestipol compared to colesevelam, which is much better tolerated. One can reduce or ameliorate these GI side effects by increasing hydration, adding fiber to the diet (psyllium), and using stool softeners. Notably, bile acid sequestrants do not cause liver or muscle problems.

 

One should also be aware that bile acid sequestrants can be difficult for many patients to take. Colestipol and colesevelam pills are large and can be difficult for some patients to swallow. Additionally, patients need to take a large number of these pills (colesevelam- 6 pills per day; colestipol- as many as 16 pills per day). The granular forms of cholestyramine and colestipol do not dissolve and are ingested as a suspension in liquid. Many patients find mixing with water leads to an unpalatable mixture that is difficult to take. Sometimes mixing with fruit juice, apple sauce, mash potatoes, etc. make the mixture more palatable. The suspension form of colesevelam with either 1.875 or 3.75 grams is preferred by many patients.

 

As noted, earlier bile acid sequestrants can increase triglyceride levels, particularly in patients with elevated baseline triglyceride levels.

 

Contraindications

 

Bile acid sequestrants usually should be avoided in patients with pre-existing GI disorders. Bile acid sequestrants are contraindicated in patients with recent or repeated intestinal obstruction and patients with plasma triglyceride levels > 400mg/dL. In contradistinction from other lipid lowering drugs, bile acid sequestrants are not contraindicated during pregnancy or lactation (category B) (166). In women of child bearing age who are planning to become pregnant bile acid sequestrants can be a good choice to lower LDL levels.

 

Summary

 

Bile acid sequestrants are useful secondary drugs for the treatment of elevated LDL-C levels. They are typically used in combination with statin therapy as a second line drug or as an addition to statin plus ezetimibe therapy as a third line drug. In statin intolerant patients the combination of ezetimibe and a bile acid sequestrant is frequently employed. Bile acid sequestrants can be difficult drugs for patients to take due to GI side effects, difficulty taking the medication, and the need to avoid taking these drugs with other medications. To improve compliance with these drugs the clinician needs to spend time educating the patient on how to take these drugs and how to avoid side effects. Because of these difficulties other cholesterol lowering drugs are used more commonly than bile acid sequestrants. In patients with type 2 diabetes who need an improvement in glycemic control and LDL-C lowering colesevelam can be used to target both abnormalities.

 

PCSK9 MONOCLONAL ANTIBODIES

 

Introduction

 

In 2015 two monoclonal antibodies that inhibit PCSK9 (proprotein convertase subtilisin kexin type 9) were approved for the lowering of LDL-C levels. Alirocumab (Praluent) is produced by Regeneron/Sanofi and evolocumab (Repatha) is produced by Amgen (167,168). Alirocumab is administered as either 75mg or 150mg subcutaneously every 2 weeks or 300mg once a month while evolocumab is administered as either 70mg subcutaneously every 2 weeks or 420mg subcutaneously once a month.

 

Effect of PCSK inhibitors on Lipid and Lipoprotein Levels

 

There are a large number of studies that have examined the effect of PCSK9 inhibitors on lipid and lipoprotein levels. A meta-analysis of 24 studies comprising 10,159 patients reported a reduction in LDL-C levels of approximately 50% and in an increase in HDL of 5-8% (169). Notably, in 12 RCTs with 6,566 patients, Lp(a) levels were reduced by 25-30% (169). The higher the baseline Lp(a) the greater the reduction with treatment (170). It should be recognized that most LDL-C lowering drugs (statins, ezetimibe, bempedoic acid, and bile acid sequestrants) do not lower Lp(a) levels. PCSK9 inhibitors have not been shown to decrease hs-CRP levels (171).

 

MONOTHERAPY

 

Both alirocumab and evolocumab have been studied as monotherapy vs. ezetimibe. In the Mendel-2 study patients were randomly assigned to evolocumab, placebo, or ezetimibe (172). In the evolocumab group, LDL-C levels decreased by 57% while in the ezetimibe group LDL-C levels decreased by 18% compared to placebo. Additionally, non-HDL-C was decreased by 49%, apolipoprotein B by 47%, triglycerides by 5.3% (NS), and Lp(a) by 18.5% while HDL levels increased by 5.5% in the evolocumab treated subjects. In a study of alirocumab vs. ezetimibe, LDL-C levels were reduced by 47% in the alirocumab group and 16% in the ezetimibe group (173). In addition, alirocumab decreased non-HDL-C by 41%, apolipoprotein B by 37%, triglycerides by 12%, and Lp(a) by 17% and increased HDL by 6%. Thus, PCSK9 monoclonal antibodies are very effective in lowering pro-atherogenic lipoproteins when used in monotherapy and have a more robust effect than ezetimibe.

 

IN COMBINATION WITH STATINS

 

In the Odyssey Combo I study, patients on maximally tolerated statin therapy were randomized to alirocumab or placebo (174). Similar to monotherapy results, when alirocumab was added to statin therapy there was a further decrease in LDL-C levels by 46%, non-HDL-C by 38%, apolipoprotein B by 36%, and Lp(a) by 15% with an increase in HDL of 7% and no change in triglyceride levels. In the Odyssey Combo II study, patients on maximally tolerated statin therapy were randomized to alirocumab vs. ezetimibe (175). Alirocumab reduced LDL levels by 51% while ezetimibe reduced LDL by 21%, demonstrating that even when added to statin therapy, alirocumab has a significantly greater ability to reduce LDL-C levels than ezetimibe. In Odyssey Combo II, non-HDL-C levels were decreased by 42%, apolipoprotein B by 41%, triglycerides by 13%, and Lp(a) by 28% while HDL increased by 9% in the alirocumab treated group. In the Laplace-2 study, evolocumab was added to various statins used at different doses (176). It didn’t make any difference which statin was being used (atorvastatin, rosuvastatin, or simvastatin) or what dose (atorvastatin 10mg or 80mg; rosuvastatin 5mg or 40mg); the addition of evolocumab resulted in an approximately 60% further decrease in LDL-C levels beyond statin alone. Additionally, the Laplace-2 trial also showed that evolocumab was much more potent than ezetimibe when added to statin therapy (evolocumab resulted in an approximately 60% decrease in LDL vs. while ezetimibe resulted in an approximately 20-25% reduction).

 

IN COMBINATION WITH STATINS AND EZETIMIBE  

 

When evolocumab was added to patients receiving atorvastatin 80mg and ezetimibe 10mg there was 48% further reduction in LDL-C levels indicating that even in patients on very aggressive lipid lowering therapy the addition of a PCSK9 inhibitor can still result in a marked reduction in LDL-C (177). In addition to decreasing LDL-C there was also a 41% decrease in non-HDL-C, a 38% decrease in apolipoprotein B, and a 19% decrease in Lp(a) when evolocumab was added to statin plus ezetimibe therapy.

 

PATIENTS WITH HETEROZYGOUS FAMILIAL HYPERCHOLESTEROLEMIA  

 

Both alirocumab and evolocumab have been tested in patients with Heterozygous Familial Hypercholesterolemia (178,179). In the Rutherford-2 trial, evolocumab lowered LDL-C by 60%, non-HDL-C by 56%, apolipoprotein B by 49%, Lp(a) by 31%, and triglycerides by 22% while increasing HDL by 8% (178). In the Odyssey FH I and FH II studies, alirocumab lowered LDL-C by approximately 55%, non-HDL-C by ~50%, apolipoprotein B by ~43%, Lp(a) by ~19% and triglycerides by ~14% while increasing HDL by ~7% (179). Thus, in these difficult to treat patients PCSK9 monoclonal antibodies were still very effective at lowering pro-atherogenic lipoproteins.

 

PATIENTS WITH HOMOZYGOUS FAMILIAL HYPERCHOLESTEROLEMIA  

 

Evolocumab resulted in a 21-31% decrease in LDL-C levels compared to placebo in patients with Homozygous Familial Hypercholesterolemia (180,181). The response to therapy appears to be dependent on the underlying genetic cause. Patients with mutations in the LDL receptor leading to the expression of defective receptors respond to therapy whereas patients with mutations leading to negative receptors (null variants) have a poor response (180-182). Given the mechanism by which PCSK9 inhibitors lower LDL-C levels it is not surprising that patients that do not have any functional LDL receptors will not respond to therapy (see section on Mechanism of Lipid Lowering). Alirocumab decreased LDL-C by 35.6%, non-HDL-C by 32.9%, apolipoprotein B by 29.8%, and lipoprotein (a) by 28.4% (183). Given that PCSK9 monoclonal antibodies decrease LDL-C levels in some patients with Familial Hypercholesterolemia these drugs can be useful in this very difficult to treat patient population.

 

STATIN INTOLERANT PATIENTS  

 

A number of studies have examined the effect of PCSK9 monoclonal antibodies in statin intolerant patients (myalgias) and compared the response to ezetimibe treatment (102,184,185). As expected, treatment with a PCSK9 inhibitor was more effective in lowering LDL-C levels than ezetimibe. Importantly, muscle symptoms were less frequent in the PCSK9 treated patients than those treated with ezetimibe, indicating that PCSK9 monoclonal antibodies will be an effective treatment choice in statin intolerant patients with myalgias.

 

PATIENTS WITH DIABETES  

 

A meta-analysis of three trials with 413 patients with type 2 diabetes found that in patients with type 2 diabetes evolocumab caused a 60% decrease in LDL-C compared to placebo and a 39% decrease in LDL-C compared to ezetimibe treatment (186). In addition, in patients with type 2 diabetes, evolocumab decreased non-HDL-C 55% vs. placebo and 34% vs. ezetimibe) and Lp(a) (31% vs. placebo and 26% vs. ezetimibe). These beneficial effects were not affected by glycemic control, insulin use, renal function, and cardiovascular disease status. Thus, PCSK9 inhibitors are effective therapy in patients with type 2 diabetes and the beneficial effects on pro-atherogenic lipoproteins is similar to what is observed in non-diabetic patients.

 

PATIENTS WITH HYPERTRIGLYCERIDEMIA  

 

There are no studies that have examined the effect of PCSK9 monoclonal antibodies in patients with marked elevations in triglyceride levels (>400mg/dL).

 

Table 12. Effect of PCSK9 Inhibitors on Lipid/Lipoprotein Levels

LDL-C

Decrease

Non-HDL-C

Decrease

Apolipoprotein B

Decrease

Triglycerides

No change or small decrease

HDL-C

Small Increase

Lp(a)

Decrease

 

Mechanism Accounting for the PCSK9 Inhibitor Induced Lipid Effects

 

The linkage of PCSK9 with lipoprotein metabolism was first identified by Abifadel and colleagues in 2003, when they demonstrated that certain mutations in PCSK9 could result in the phenotypic appearance of Familiar Hypercholesterolemia (187). Subsequent studies demonstrated that gain of function mutations in PCSK9 are an uncommon cause of Familiar Hypercholesterolemia (167,168,188). In 2005 it was shown that loss of function mutations in PCSK9 resulted in lower LDL-C levels and this decrease in LDL-C levels was associated with a reduction in the risk of cardiovascular events (189,190).

 

The main route of clearance of clearance of plasma LDL is via LDL receptors in the liver (191). When the LDL particle binds to the LDL receptor the LDL particle- LDL receptor complex is taken into the liver by endocytosis (191). The LDL particle and the LDL receptor then disassociate and the LDL lipoprotein particle is delivered to lysosomes where it is degraded and the LDL receptor returns to the plasma membrane (Figure 2) (191). After endocytosis LDL receptors recirculate back to the plasma membrane over 100 times.

 

PCSK9 is predominantly expressed in the liver and secreted into the circulation. Once extracellular, PCSK9 can bind to the LDL receptor and alter the metabolism of the LDL receptor (192,193). Instead of the LDL receptor recycling to the plasma membrane the LDL receptor bound to PCSK9 remains associated with the LDL particle and is delivered to the lysosomes where it is also degraded (Figure 4) (192,193). This results in a decrease in the number of plasma membrane LDL receptors resulting in the decreased clearance of circulating LDL leading to elevations in plasma LDL-C levels.

 

The PCSK9 monoclonal antibodies bind PCSK9 preventing the PCSK9 from interacting with LDL receptors and thereby preventing PCSK9 from inducing LDL receptor degradation (192,193). The decreased LDL receptor degradation results in an increase in hepatic LDL receptors on the plasma membrane leading to the increased clearance of LDL and decreases in plasma LDL-C levels (194,195). Thus, similar to statins, ezetimibe, bempedoic acid, and bile acid sequestrants, PCSK9 inhibitors are reducing plasma LDL-C levels by up-regulating hepatic LDL receptors. The difference is that PCSK9 inhibitors are decreasing the degradation of LDL receptors while statins, ezetimibe, bempedoic acid, and bile acid sequestrants stimulate the production of LDL receptors.

 

Figure 4. PCSK9 Directs LDL Receptor to Degradation in Lysosome.

 

The expression of PCSK9 is stimulated by SREBP-2 (192,193). Statins and other drugs that lower hepatic cholesterol levels lead to the activation of SREBP-2 and thereby increase plasma PCSK9 levels (192,193). Inhibition of PCSK9 with monoclonal antibodies is more effective in lowering plasma LDL-C levels in patients on statin therapy due to the higher levels of plasma PCSK9 in these individuals.

 

The mechanism by which PCSK9 inhibitors reduce Lp(a) levels is unclear. Studies have shown that PCSK9 inhibitors increase the catabolism of lipoprotein(a) particles (196,197). In some circumstances PCSK9 inhibitors may also decrease the production rate (197). It has been postulated that increasing hepatic LDL receptor levels in the setting of marked reductions in circulating LDL levels will result in the clearance of Lp(a) by liver LDL receptors (198).

 

Pharmacokinetics and Drug Interactions

 

PCSK9 monoclonal antibodies are eliminated primarily by cellular endocytosis, phagocytosis, and target-mediated clearance. They are not metabolized or cleared by the liver or kidneys and therefore there is no need to adjust the dose in patients with either liver or kidney disease. There are no interactions with the cytochrome P450 system or transport proteins and thus the risk of drug-drug interactions is minimal. Currently there are no reported drug-drug interactions with PCSK9 monoclonal antibodies.

 

Effect of PCSK9 Inhibitors on Clinical Outcomes

 

FOURIER TRIAL

 

The FOURIER trial was a randomized, double-blind, placebo-controlled trial of evolocumab vs. placebo in 27,564 patients with atherosclerotic cardiovascular disease and an LDL-C level of 70 mg/dL or higher who were on statin therapy (199). The primary end point was cardiovascular death, myocardial infarction, stroke, hospitalization for unstable angina, or coronary revascularization and the key secondary end point was cardiovascular death, myocardial infarction, or stroke. The median duration of follow-up was 2.2 years. Baseline LDL-C levels were 92mg/dL and evolocumab resulted in a 59% decrease in LDL levels (LDL-C level on treatment approximately 30mg/dL). Evolocumab treatment significantly reduced the risk of the primary end point (hazard ratio, 0.85; 95% confidence interval (CI), 0.79 to 0.92; P<0.001) and the key secondary end point (hazard ratio, 0.80; 95% CI, 0.73 to 0.88; P<0.001). The results were consistent across key subgroups, including the subgroup of patients in the lowest quartile for baseline LDL-C levels (median, 74 mg/dL). Of note, a similar decrease in cardiovascular events occurred in patients with diabetes treated with evolocumab and glycemic control was not altered (200). Additionally, in patients with peripheral arterial disease evolocumab also reduced cardiovascular events (201). Further analysis has shown that in the small number of patients with a baseline LDL-C level less than 70mg/dL, evolocumab reduced cardiovascular events to a similar degree as in the patients with an LDL-C greater than 70mg/dL (202). The lower the on-treatment LDL-C levels (down to levels below 20mg/dL), the lower the cardiovascular event rate, suggesting that greater reductions in LDL-C levels will result in greater reductions in cardiovascular disease (203). Finally, the relative risk reductions with evolocumab for the cardiovascular events tended to be greater in high-risk subgroups (20% for those with a more recent MI, 18% with multiple prior MI, and 21% with residual multivessel coronary artery disease), whereas the relative risk reduction was 5% to 8% in patients without these risk factors (204). This observation suggests that certain groups of patients will derive greater benefit from the addition of a PCSK9 inhibitor.

 

It should be noted that that the duration of the FOURIER trial was very short and it is well recognized from previous statin trials that the beneficial effects of lowering LDL-C levels take time with only modest effects observed during the first year of treatment. In the FOURIER trial the reduction of cardiovascular death, myocardial infarction, or stroke was 16% during the first year but was 25% beyond 12 months.

 

ODYSSEY TRIAL

 

The ODYSSEY trial was a multicenter, randomized, double-blind, placebo-controlled trial involving 18,924 patients who had an acute coronary syndrome 1 to 12 months earlier, an LDL-C level of at least 70 mg/dL, a non-HDL-C level of at least 100 mg/dL, or an apolipoprotein B level of at least 80 mg/dL while on high intensity statin therapy or the maximum tolerated statin dose (205). Patients were randomly assigned to receive alirocumab 75 mg every 2 weeks or matching placebo. The dose of alirocumab was adjusted to target an LDL-C level of 25 to 50 mg/dL. The primary end point was a composite of death from coronary heart disease, nonfatal myocardial infarction, fatal or nonfatal ischemic stroke, or unstable angina requiring hospitalization. During the trial LDL-C levels in the placebo group was 93-103mg/dL while in the alirocumab group LDL-C levels were 40mg/dL at 4 months, 48mg/dL at 12 months, and 66mg/dL at 48 months (the increase with time was due to discontinuation of alirocumab or a decrease in dose). The primary endpoint was reduced by 15% in the alirocumab group (HR 0.85; 95% CI 0.78 to 0.93; P<0.001). In addition, total mortality was reduced by 15% in the alirocumab group (HR 0.85; 95% CI 0.73 to 0.98). The absolute benefit of alirocumab was greatest in patients with a baseline LDL-C level greater than 100mg/dL. In patients with an LDL-C level > than 100mg/dL the number needed to treat with alirocumab to prevent an event was only 16. It should be noted that the duration of this trial was very short (median follow-up 2.8 years) which may have minimized the beneficial effects. Additionally, because alirocumab 75mg every 2 weeks was stopped if the LDL-C level was < 15mg/dL on two consecutive measurements the beneficial effects may have been blunted (7.7% of patients randomized to alirocumab were switched to placebo).

 

SUMMARY OF OUTCOME TRIALS

 

It should be noted that that the duration of the PCSK9 outcome trials were relatively short and it is well recognized from previous statin trials that the beneficial effects of lowering LDL-C levels take time with only modest effects observed during the first year of treatment. In the FOURIER trial the reduction of cardiovascular death, myocardial infarction, or stroke was 16% during the first year but was 25% beyond 12 months. In the ODYSSEY trial the occurrence of cardiovascular events was similar in the alirocumab and placebo group during the first year of the study with benefits of alirocumab appearing after year one. Thus, the long-term benefits of treatment with a PCSK9 inhibitor may be greater than that observed during these relatively short-term studies.

 

GLAGOV TRIAL

 

While not an outcome trial the GLAGOV trial provides further support for the benefits of further lowering of LDL-C levels with a PCSK9 inhibitor added to statin therapy (206). This trial was a double-blind, placebo-controlled, randomized trial of evolocumab vs. placebo in 968 patients presenting for coronary angiography. The primary efficacy measure was the change in percent atheroma volume (PAV) from baseline to week 78, measured by serial intravascular ultrasonography (IVUS) imaging. Secondary efficacy measures included change in normalized total atheroma volume (TAV) and percentage of patients demonstrating plaque regression. As expected, there was a marked decrease in LDL-C levels in the evolocumab group (Placebo 93mg/dL vs. evolocumab 37mg/dL; p<0.001). PAV increased 0.05% with placebo and decreased 0.95% with evolocumab (P < .001) while TAV decreased 0.9 mm3 with placebo and 5.8 mm3 with evolocumab (P < .001). There was a linear relationship between achieved LDL-C and change in PAV (i.e., the lower the LDL-C the greater the regression in atheroma volume down to an LDL-C of 20mg/dL). Additionally, evolocumab induced plaque regression in a greater percentage of patients than placebo (64.3% vs 47.3%; P < .001 for PAV and 61.5% vs 48.9%; P < .001 for TAV). These results demonstrate the anti-atherogenic effects of PCSK9 inhibitors. Other trials in different patient populations have also shown that treatment with PCSK9 inhibitors are anti-atherogenic (207,208). 

 

VENOUS THROMBOEMBOLISM

 

In the FOURIER trial treatment with evolocumab resulted in a reduction in venous thromboembolism (VTE) (HR 0.71; 95% CI, 0.50-1.00; P=0.05) (209). Interestingly no effect was observed in the 1st year (HR, 0.96; 95% CI, 0.57-1.62) but a 46% reduction in VTE (HR, 0.54; 95% CI, 0.33-0.88; P=0.014) beyond 1 year occurred. In patients with low baseline Lp(a) levels, evolocumab reduced Lp(a) by only 7 nmol/L and had no effect on VTE risk but in patients with high baseline Lp(a) levels, evolocumab reduced Lp(a) by 33 nmol/L and risk of VTE by 48% (HR, 0.52; 95% CI, 0.30-0.89; P=0.017). In the ODYSSEY OUTCOMES trial, the risk of VTE was reduced but just missed being statistically significant (HR, 0.67; 95% CI, 0.44-1.01; P=0.06) (210). A meta-analysis of FOURIER and ODYSSEY OUTCOMES demonstrated a 31% relative risk reduction in VTE with PCSK9 inhibition (HR, 0.69; 95% CI, 0.53-0.90; P=0.007) (209).

 

Side Effects

 

The major side effect of PCSK9 monoclonal antibodies has been injection site reactions including erythema, itching, swelling, pain, and tenderness. Allergic reactions have been reported and as with any protein there is potential immunogenicity. In general side effects have been minimal, which is not surprising, as monoclonal antibodies do not typically have off target side effects. Since PCSK9 does not appear to have important functions other than regulating LDL receptor degradation, it is not surprising that inhibiting PCSK9 function has not resulted in major side effects.

 

A meta-analysis of 20 randomized controlled trials with 68,123 subjects found a very modest effect on fasting glucose (mean difference 1.88 mg/dL) and A1c levels (mean difference 0.032%) and did not observe an increased risk of developing diabetes (211). It should be recognized that the duration of these trials was relatively short (median follow-up 78 weeks) and therefore further long-term studies are required.

 

In the large outcome trials (ODYSSEY and FOURIER) there was no significant difference between the PCSK9 treated group vs. the placebo group with regard to adverse events (including new-onset diabetes and neurocognitive events). The only exception was the expected increase in injection-site reactions in the patients treated with a PCSK9 inhibitor. Additionally, in a subgroup of patients from the FOURIER trial a prospective study of cognitive function (EBBINGHAUS Study) was carried out and no significant differences in cognitive function was observed over a median of 19 months in the PCSK9 treated vs. placebo group (212). It should be recognized that while short-term treatment with PCSK9 inhibitors have not demonstrated any significant side effects it is possible that long-term use could lead to unexpected side-effects.

 

An issue of concern is whether lowering LDL-C to very low levels has the potential to cause toxicity. In a number of the PCSK9 studies a significant number of patients had LDL-C levels < 25mg/dL. For example, in the Odyssey long term study 37% of patients on alirocumab had two consecutive LDL-C levels below 25mg/dL and in the Osler long term study in patients treated with evolocumab 13% had values below 25mg/dL (213,214). In these short term PCSK9 studies, toxicity from very low LDL-C levels has not been observed. Additionally, in patients with Familial Hypobetalipoproteinemia LDL levels can be very low and these patients do not have any major disorders other than hepatic steatosis, which is not mechanistically due to low LDL-C levels (215). Similarly, there are rare individuals who are homozygous for loss of function mutations in the PCSK9 gene and they also do not appear to have major medical issues (168). Finally, in a number of statin trials there have been patients with very low LDL-C levels and an increased risk of side effects has not been consistently observed in those patients (216-218). Thus, with the limited data available there does not appear to be a major risk of markedly lowering LDL-C levels.   

 

Contraindications

 

Other than a history of a hypersensitivity to these drugs there are currently no contraindications. There are no studies during pregnancy or lactation.

 

Summary

 

PCSK9 monoclonal antibodies robustly reduce LDL-C levels when used as monotherapy, in combination with statins, or when added to the combination of statins + ezetimibe. In distinction to most other cholesterol lowering drugs the PCSK9 inhibitors also decrease Lp(a) levels. Outcome studies have clearly demonstrated that decreasing LDL-C levels with PCSK9 inhibitors reduces cardiovascular events. The side effect profile appears to be very favorable and there are no drug-drug interactions. The major limitation is the high expense of these drugs, which has limited their widespread use.

 

INCLISIRAN (LEQVIO)

 

Introduction

 

Inclisiran (Leqvio) is a double-stranded, siRNA (small interfering RNA) conjugated on the sense strand with triantennary N-acetylgalactosamine (GalNAc) to facilitate uptake into hepatocytes (219). In hepatocytes, inclisiran stimulates the catalytic breakdown of PCSK9 mRNA thereby reducing the hepatic synthesis of PCSK9 and markedly decreasing plasma PCSK9 levels (219,220). The recommended dose of inclisiran is 284 mg by subcutaneous injection, followed with a repeat injection at 3 months, and then every 6 months (package insert). If a dose is missed by more than 3 months it is recommended to repeat the dosage schedule described above (package insert). It is recommended that inclisiran be administered by a healthcare professional.

 

Effect on Inclisiran on Lipid and Lipoprotein Levels

 

There have been several large trials examining the efficacy of inclisiran. The ORION-10 trial was conducted in the United States and included adults with atherosclerotic cardiovascular disease on a maximally tolerated statin with an LDL-C > 70mg/dL (220). Patients were randomized to inclisiran 284mg (n=781) at initial visit, 3 months, 9 months, and 15 months or placebo (n=780) and followed for 540 days. After 3 months the LDL-C was reduced by approximately 50% and this reduction was sustained throughout the duration of the trial (at 540 days the LDL-c was reduced by 52.3% (P<0.001)). As expected, total cholesterol (-33%), non-HDL-C (-47%), and apolipoprotein B (-43%) were also decreased. Additionally, triglyceride (-13%) and Lp(a) (-26%) levels were decreased while HDL-C levels (+5.1%) and hsCRP (+8.8%) were slightly increased. ORION-11 was a very similar trial with an identical protocol conducted in Europe and South Africa and included adults with ASCVD or an ASCVD risk equivalent on maximally tolerated statin therapy (inclisiran n=810 and placebo n=807) (220). At 540 days LDL-C was reduced by 49.9% (P<0.001). Changes in other lipid parameters were similar to those observed in ORION 10. Subgroup analysis revealed that in both the ORION 10 and 11 trials that all subgroups had a similar reduction in LDL-C levels with inclisiran therapy including subjects with diabetes, moderate renal impairment, and greater than 75 years of age (220). Statin therapy and whether statin therapy was moderate intensity or high intensity also did not affect the reduction in LDL-C (220). Additionally, in patients with renal disease, including individuals with an estimated creatinine clearance between 15-29 mL/min, the reduction in LDL-C levels with inclisiran administration were similar to individuals with normal renal function (221). The decrease in LDL-C with inclisiran treatment has been shown to persist for 4 years (222).

 

HETEROZYGOUS FAMILIAL HYPERCHOLESTEROLEMIA

 

The effect of inclisiran on LDL-C levels was determined in patients with heterozygous familial hypercholesterolemia who were randomized to receive subcutaneous injections of inclisiran 284mg (n= 242) or placebo (n=240) on days 1, 90, 270, and 450 (223). The mean baseline LDL-C level was 153±54mg/dL and 90% of the patients were receiving statins with most on high intensity statins (75%). At day 510 LDL-C levels were reduced by 47.9% compared to placebo (P<0.001). The reduction in LDL-C was similar in all genotypes of familial hypercholesterolemia. Total cholesterol was reduced by 33%, non-HDL-C by 44%, Lp(a) by 17.2%, and triglycerides by 12%. HDL-C and hsCRP were not markedly altered.

 

HOMOZYGOUS FAMILIAL HYPERCHOLESTEROLEMIA

 

A small study reported that inclisiran treatment lowered LDL-C levels in 3 of 4 patients with homozygous familiar hypercholesterolemia  (17.5% to 37% decrease) but less than that seen in individuals with fully functioning LDL receptors (224). A larger more recent trial failed to demonstrate a decrease in LDL-C levels with inclisiran treatment (225). Of note there was considerable variation in the LDL-C response, which could be due to differences in genetic variants. Individuals with null-null LDL receptor variants (i.e. no functioning LDL receptors) are unlikely to respond to inclisiran due to the absence of LDL receptors and the group treated with inclisiran in this study was enriched in patients with this genotype, which could explain the absence of a significant reduction in LDL-C.     

 

Mechanisms Accounting for Inclisiran Induced Lipid Effects

 

The mechanism of action of inclisiran is the same as for PCSK9 monoclonal antibodies (219). Briefly, decreasing the production of PCSK9 in the liver, the primary source of circulating PCSK9, leads to a decrease in plasma PCSK9 levels resulting in a decrease in LDL receptor degradation (219). An increase in the number of hepatic LDL receptors increases the clearance of LDL leading to a decrease in LDL-C levels (219).  

 

Pharmacokinetics and Drug Interactions

 

There are no drug interactions. The reduction in LDL-C occurs within 14 days after drug administration and persists for an extended period of time allowing for administration every 6 months.

 

Effect of Inclisiran on Clinical Outcomes

 

No outcome studies are currently available. A cardiovascular outcome study (ORION-4) is ongoing and includes 15,000 patients with established ASCVD. The trial duration is five years and completion is expected in 2024 (NCT03705234) (ClinicalTrials.gov, 2020a).

 

Side Effects

 

The only adverse reactions associated with inclisiran were injection site reactions including rash, pain, and erythema (220). In an analysis of 7 studies with 3,576 patients treated with inclisiran for up to 6 years and 1,968 patients treated with placebo for up to 1.5 years, hepatic, muscle, and kidney events; incident diabetes; and elevations of creatine kinase or creatinine were not increased in patients treated with inclisiran (226).

 

Contraindications

 

In patients with severe hepatic or renal impairment inclisiran should be used with caution as there is limited data and experience in these patients. There are no studies during pregnancy or lactation.

 

Summary

 

Inclisiran very effectively lowers LDL-C levels. The major advantage of this drug compared to PCSK9 monoclonal antibodies is the ability to administer inclisiran every 6 months, which may improve compliance.

 

BEMPEDOIC ACID (NEXLETOL)

 

Introduction

 

Bempedoic acid was approved in the US in February 2020 and is an adenosine triphosphate-citrate lyase (ACL) inhibitor. It is administered orally once daily with or without food at a dose of 180mg (Nexletol). It is also available as a combination tablet containing 180 mg of bempedoic acid and 10 mg of ezetimibe (Nexlizet).

 

Effect on Bempedoic on Lipid and Lipoprotein Levels

 

EFFECT WITHOUT STATINS

 

In a study that randomized 345 patients with hypercholesterolemia (LDL-C 158mg/dL) and a history of intolerance to statin to either bempedoic acid or placebo (2:1), bempedoic acid decreased LDL-C by 21.4%, non-HDL-C by 17.9%, and apolipoprotein B by 15% (227). One third of patients were on background non-statin therapy most commonly ezetimibe and fish oil. Triglyceride levels were not altered but there was a small decrease in HDL-C levels that was statistically significant (-4.5%).

 

IN COMBINATION WITH STATINS

 

There have been two large trials that determined the effect of adding bempedoic acid to statin therapy. In a study that randomized 779 patients on maximally tolerated statin therapy +/- ezetimibe (only a small number on ezetimibe) with an LDL-C level greater than 70mg/dL (baseline LDL-C 120mg/dL) to either bempedoic acid or placebo it was observed that bempedoic acid decreased LDL-C levels by 17.4% compared to placebo (p<0.001) (228). In addition, non-HDL-C and apolipoprotein B levels were decreased by 13% compared to placebo while there was no significant change in triglyceride levels. Bempedoic acid decreased HDL-C levels by approximately 6%. In a similar study, patients with atherosclerotic cardiovascular disease, heterozygous familial hypercholesterolemia, or both with an LDL-C level greater than 70 mg/dL (baseline LDL-C 103mg/dL) while on maximally tolerated statin therapy with or without additional lipid-lowering therapy (only a small number on ezetimibe) were randomized to bempedoic acid (n= 1,488) or placebo (n= 742) (229). Compared to placebo, treatment with bempedoic acid decreased LDL-C by 18.1%, non-HDL-C by 13.5%, and apolipoprotein B by 11.9%. Triglyceride levels were unchanged but HDL-C decreased by 5.92%. Of note in both of the above studies the decrease in LDL-C was maintained over 52 weeks.

 

Notably, the addition of bempedoic acid to atorvastatin 80mg per day was still capable of significantly decreasing LDL-C (22%), non-HDL-C (13%), and apolipoprotein B (-15%) compared to placebo (230). The addition of bempedoic acid to high dose atorvastatin therapy did not cause meaningful changes in atorvastatin pharmacokinetics.   

 

IN COMBINATION WITH EZETIMIBE

 

Patients on maximally tolerated statin therapy with LDL-C levels greater 100 mg/dL if they had cardiovascular disease and/or Familiar Hypercholesterolemia or greater than 130 mg/dL if they had multiple CVD risk factors were randomized to bempedoic acid + ezetimibe, bempedoic acid alone, ezetimibe alone, or placebo (231). The key results of this study are shown in Table 14. Changes from baseline in HDL-C and triglyceride level were modest (<10%) in all treatment groups. In another study patients with a history of statin intolerance on ezetimibe therapy were randomized to bempedoic acid (n=181) or placebo (n= 88) (232). Compared to placebo, bempedoic acid decreased LDL-C by 28.5%, non-HDL-C by -23.6%, and apolipoprotein B by -19.3%. As seen in other studies bempedoic acid did not alter triglyceride levels but slightly decreased HDL-C levels (approximately 6% decrease compared to placebo).

 

Table 14. Effect of Bempedoic Acid and Ezetimibe on Lipid Parameters (231)

 

LDL-C

Non-HDL-C

Apo B

hsCRP

Bempedoic acid + ezetimibe

-38%

-33.7%

-30.1

-35.1

Bempedoic acid

-19%

-15.9%

-17.3

-31.9

Ezetimibe

-25%

-21.7

-20.8

-8.2

 Results are percent decrease compared to the placebo group.

 

Summary

 

Bempedoic acid typically lowers LDL-C by 15-25%, non-HDL-C by 10-20%, and apolipoprotein B levels by 10-20% with no significant effects on triglyceride levels. HDL-C levels decrease by 5-8% and Lp(a) are unchanged (233).

 

Table 15. Effect of Bempedoic Acid on Lipid/Lipoprotein Levels

LDL-C

Decrease

Non-HDL-C

Decrease

Apolipoprotein B

Decrease

Triglycerides

No change

HDL-C

Small decrease

Lp(a)

No change

 

Non-Lipid Effects of Bempedoic Acid

 

Bempedoic acid decreases hsCRP levels (see table 14, 16).

 

Table 16. Effect of Bempedoic Acid on hsCRP Levels

Reference

Percent decrease in hsCRP

(227)

-24.3

(228)

-8.7

(229)

-21.5

(230)

-44

(232)

-31

 

In the CLEAR Outcome study with a median follow-up of 3.4 years there was no difference in the development of new onset diabetes in the bempedoic acid and placebo groups (429 of 3848, -11·1% with bempedoic acid vs 433 of 3749, 11·5% with placebo; HR 0.95; 95% CI 0.83-1.09) (234). Additionally, during the study HbA1c concentrations and fasting glucose levels were similar between the bempedoic acid and placebo groups in patients who had either prediabetes or normoglycemia. In the CLEAR Outcome study in patients with diabetes the prevalence of worsening diabetes was similar in the bempedoic acid and placebo group (235,236)   

 

Mechanisms Accounting for Bempedoic Acid Induced Lipid Effects

 

Bempedoic acid is a potent inhibitor of ATP-citrate lyase, which catalyzes the formation of acetyl-CoA in the cytoplasm (237). Acetyl-CoA is a precursor for the synthesis of cholesterol (figure 5). The inhibition of ATP-citrate lyase by bempedoic acid decreases cholesterol synthesis in liver reducing hepatic intracellular cholesterol levels (237). Of note, bempedoic acid is a pro-drug and conversion to its CoA-derivative by very-long-chain acyl-CoA synthetase-1 is required for inhibition of cholesterol synthesis (237). Very-long-chain acyl-CoA synthetase-1 is highly expressed in the liver but is not expressed in adipose tissue, kidney, intestine or skeletal muscle (237). The inability of bempedoic acid to be activated in muscle and inhibit cholesterol synthesis suggests that bempedoic acid is unlikely result in muscle toxicity.

 

Figure 5. Inhibition of Cholesterol Synthesis by Bempedoic Acid.

 

The decrease in plasma LDL-C levels in patients treated with bempedoic acid is primarily due to an increase in hepatic LDL receptors secondary to the inhibition of cholesterol synthesis resulting in a reduction in hepatic cholesterol levels (237). It should be noted that bempedoic acid also decreases circulating LDL-C levels in LDL receptor deficient mice and LDL receptor deficient miniature pigs indicating that mechanisms in addition to up-regulation of hepatic LDL receptors may contribute to the decrease in LDL-C levels (237). The inhibition of hepatic cholesterol synthesis may decrease the production and secretion of VLDL, which could contribute to a decrease in LDL-C.

 

Pharmacokinetics and Drug Interactions

 

No dose adjustments are required in patients with mild or moderate renal or hepatic impairment or in the elderly (package insert). Concomitant use of bempedoic acid with simvastatin or pravastatin causes an increase in the concentrations of these drugs and therefore may increase the risk of myopathy (package insert). This drug interaction may be secondary to bempedoic acid inhibiting organic anion-transporting polypeptide OATP1B1. It is recommended to avoid concomitant use of bempedoic acid with simvastatin greater than 20 mg/day or pravastatin 40mg/day. While concomitant administration of bempedoic acid with atorvastatin or rosuvastatin elevated the area under the curve by 1.7-fold these elevations were generally within the individual statin exposures and do not impact dosing recommendations (package insert).

 

Effect of Bempedoic Acid on Clinical Outcomes

 

In animal models of atherosclerosis, treatment with bempedoic acid had favorable effects on atherosclerosis (237). Moreover, genetic variants of ATP citrate lyase that lower LDL-C levels are associated with a decrease in cardiovascular disease suggesting that bempedoic acid will have favorable effects on reducing the risk of cardiovascular disease (238).

 

The CLEAR Outcome trial was a double-blind, randomized, placebo-controlled trial involving patients with cardiovascular disease or at high risk of cardiovascular disease who were unable or unwilling to take statins ("statin-intolerant" patients) (239). The patients were randomized to bempedoic acid 180 mg (n= 6992) or placebo (n= 6978) and the median duration of follow-up was 40.6 months. As expected, LDL-C levels were decreased by 21% in the bempedoic group compared to placebo (29mg/dL difference). The primary endpoint, death from cardiovascular causes, nonfatal myocardial infarction, nonfatal stroke, or coronary revascularization, was reduced by 13% in the bempedoic acid group (HR 0.87; 95% CI 0.79 to 0.96; P = 0.004). Bempedoic acid also decreased fatal and non-fatal myocardial infarctions and coronary revascularization but had no significant effects on fatal or nonfatal stroke, death from cardiovascular causes, and death from any cause. In the patients who were at high risk for cardiovascular disease (primary prevention), 66% had diabetes, and the primary endpoint was reduced by 30% in the bempedoic acid group (HR 0.70; 95% CI, 0.55-0.89; P = .002) (235). In patients with diabetes with or without cardiovascular disease the primary endpoint was reduced by 17% in the bempedoic acid group (HR 0.83; 95% CI 0.72-0.95) (234). This study clearly demonstrates that treatment with bempedoic acid reduces the risk cardiovascular events.     

 

Side Effects

 

HYPERURICEMIA

 

In clinical trials, 26% of bempedoic acid-treated patients with normal baseline uric acid values experienced hyperuricemia one or more times versus 9.5% in the placebo group (package insert). In the CLEAR Outcomes trial elevated uric acid levels occurred in 10.9% of the patients on bempedoic acid compared to 5.6% taking the placebo (239). The increase in uric acid is due to bempedoic acid inhibiting renal tubular OAT2. The Increase in uric acid levels typically occurred within the first 4 weeks of treatment and persisted throughout treatment. After 12 weeks of treatment, the mean placebo-adjusted increase in uric acid compared to baseline was 0.8 mg/dL for patients treated with bempedoic acid (package insert). Elevations in blood uric acid levels may lead to the development of gout. Gout was reported in 1.5% of patients treated with bempedoic acid vs. 0.4% of patients treated with placebo. The risk for gout attacks were higher in patients with a prior history of gout (11.2% for bempedoic acid treatment vs. 1.7% in the placebo group) (package insert). In patients with no prior history of gout only 1% of patients treated with bempedoic acid and 0.3% of the placebo group had a gouty attack (package insert). In the CLEAR Outcomes trial gout was increased in the bempedoic acid group (3.1% vs. 2.1%) (239).

 

TENDON RUPTURE

 

In clinical trials tendon rupture occurred in 0.5% of patients treated with bempedoic acid vs. 0% of placebo treated patients and involved the rotator cuff (the shoulder), biceps tendon, or Achilles tendon (package insert). Tendon rupture occurred within weeks to months of starting bempedoic acid and occurred more frequently in patients over 60 years of age, in those taking corticosteroid or fluoroquinolone drugs, in patients with renal failure, and in patients with previous tendon disorders. In the CLEAR Outcomes trial tendon rupture was similar in the bempedoic acid and placebo group (bempedoic acid 1.2% and placebo 0.9%) (239).

 

RENAL FUNCTION

 

Bempedoic acid treatment resulted in a mean increase in serum creatinine of 0.05 mg/dL compared to baseline. Approximately 3.8% of patients treated with bempedoic acid had BUN levels that doubled vs. 1.5% in the placebo group and about 2.2% of patients treated with bempedoic acid had creatinine values that increased by 0.5 mg/dL vs. 1.1% in the placebo group (package insert). Renal function returned to baseline when bempedoic acid was discontinued. In the CLEAR Outcomes trial renal impairment was increased in the bempedoic acid group (11.5% vs.8.6%) as was the change from baseline creatinine (0.05±0.2 mg/dL vs. 0.01±0.2 mg/dL)  (239).

 

CHOLELITHIASIS

 

In the CLEAR Outcomes trial cholelithiasis was increased in the bempedoic acid group (2.2 vs 1.2) (239).

 

BENIGN PROSTATIC HYPERPLASIA

 

Bempedoic acid was associated with an increased risk of benign prostatic hyperplasia (BPH) in men with no reported history of BPH, occurring in 1.3% of NEXLETOL-treated patients versus 0.1% of placebo-treated patients (package insert).

 

MISCELLANEOUS LABORATORY ABNORMALITIES

 

Approximately 5.1% of patients on bempedoic acid vs. 2.3% on placebo had decreases in hemoglobin levels of 2 or more g/dL and below the lower limit of normal on one or more occasion. Anemia was reported in 2.8% of patients treated with bempedoic acid and 1.9% of patients treated with placebo. Hemoglobin decrease was generally asymptomatic and did not require medical intervention (package insert).

 

Approximately 9.0% of bempedoic acid treated patients with a normal baseline leukocyte count decreased leukocyte count to less than the lower limit of normal on one or more occasions vs. 6.7% in the placebo group. The leukocyte decrease was generally asymptomatic and did not require medical intervention (package insert).

 

Approximately 10.1% of bempedoic acid treated patients vs. 4.7% in the placebo group had

increases in platelet counts of 100× 109/L or more on one or more occasion. The platelet count increase was asymptomatic, did not result in an increased risk for thromboembolic events, and did not require medical intervention (package insert).

 

Increases to more than 3× the upper limit of normal (ULN) in AST occurred in 1.4% of patients treated with bempedoic acid vs. 0.4% of placebo patients, and increases to more than 5× ULN occurred in 0.4% of bempedoic acid treated patients vs. 0.2% of placebo-treated patients. Increases in ALT were similar in bempedoic acid treated patients and placebo-treated patients. Elevations in transaminases were generally asymptomatic and not associated with elevations ≥2× ULN in bilirubin or with cholestasis. In most cases, the elevations were transient and resolved or improved with continued therapy or after discontinuation of therapy (package insert).

 

Contraindications

 

The use of bempedoic acid during pregnancy and lactation has not been studied (package insert).

 

Summary

 

In patients on statins and ezetimibe with an LDL-C that is not at goal the addition of bempedoic acid is a reasonable third drug. In addition, in patients that cannot tolerate statin therapy the combination of ezetimibe and bempedoic acid may allow for the lowering of LDL-C to goal. One can expect a reduction in LDL-C of approximately 15-25% with bempedoic acid monotherapy therapy or when used in combination with other LDL-C lowering drugs.

 

LOMITAPID (JUXTAPID)

 

Introduction

 

Lomitapide (Juxtapid), a selective microsomal triglyceride transfer protein inhibitor, was approved in December 2012 for lowering LDL-C levels in adults with Homozygous Familial Hypercholesterolemia (240-242). As will be discussed below it lowers LDL-C levels by an LDL receptor independent mechanism.

 

Effect on Lomitapide on Lipid and Lipoprotein Levels

 

The effect of lomitapide on lipid and lipoprotein levels has been studied in patients with Homozygous Familial Hypercholesterolemia. The pivotal study was a 78-week single arm open label study in 29 patients receiving treatment for Homozygous Familial Hypercholesterolemia (243). Lomitapide was initiated at 5mg per day and was up-titrated to 60mg per day based on tolerability and liver function tests. On an intention to treat basis, LDL-C was decreased by 40% and apolipoprotein B by 39%. In patients who were actually taking lomitapide, LDL-C levels were reduced by 50%. In addition to decreasing LDL-C levels, non-HDL-C levels were decreased by 50%, Lp(a) by 15%, and triglycerides by 45%. Interestingly HDL and apolipoprotein A-I levels were decreased by 12% and 14% respectively in this study. Follow-up revealed that the decrease in LDL-C could be sustained for a prolonged period of time (294 weeks) (244).

 

The effect of lomitapide has also been studied in patients without Homozygous Familial Hypercholesterolemia. A study by Samaha and colleagues compared the effect of ezetimibe and lomitapide in patients with elevated cholesterol levels(245). Patients were treated with ezetimibe alone, lomitapide alone, or the combination of ezetimibe and lomitapide. Ezetimibe monotherapy led to a 20–22% decrease in LDL-C levels, lomitapide monotherapy led to a dose dependent decrease in LDL-cholesterol levels (19% at 5.0 mg, 26% at 7.5 mg and 30% at 10 mg). Combined therapy produced a larger dose-dependent decrease in LDL-C levels (35%, 38% and 46%, respectively).  Additionally, lomitapide decreased triglycerides by 10%, non-HDL-C by 27%, apolipoprotein B by 24%, and Lp(a) by 17%.

 

The above studies demonstrate that lomitapide decreases LDL-C, non-HDL-C, triglycerides, and Lp(a) levels.

 

Mechanism Accounting for the Lomitapide Induced Lipid Effects

 

Lomitapide is a selective inhibitor of microsomal triglyceride transfer protein (MTTP) (240-242). MTTP is located in the endoplasmic reticulum of hepatocytes and enterocytes where it plays a key role in transferring triglycerides onto newly synthesized apolipoprotein B leading to the formation of VLDL and chylomicrons (246). Loss of function mutations in both alleles of MTTP results in abetalipoproteinemia, which is characterized by the virtual absence of apolipoprotein B, VLDL, chylomicrons, and LDL in the plasma due to the failure of the liver and intestine to produce VLDL and chylomicrons (215). Lomitapide by inhibiting MTTP activity reduces the secretion of chylomicrons by the intestine and VLDL by the liver leading to a decrease in LDL, apolipoprotein B, triglycerides, non-HDL-C, and Lp(a) (240-242). 

 

Pharmacokinetics and Drug Interactions

 

Lomitapide is extensively metabolized in the liver by the CYP3A4 pathway (240,241). Therefore, lomitapide is contraindicated in patients on strong CYP3A4 inhibitors and lower doses should be used in patients on weak inhibitors. Of particular note, in patients on atorvastatin the maximal dose of lomitapide is 30mg per day and lomitapide should not be used in patients taking more than 20mg of simvastatin (240,241). Lomitapide can increase warfarin levels and therefore close monitoring is required. Finally, given the risk of liver abnormalities (see side effect section) the avoidance of alcohol or a reduction in alcohol intake is prudent.

 

Effect of Lomitapide on Clinical Outcomes

 

There are no clinical outcome trials but it is presumed that lowering LDL-C levels in patients with Homozygous Familial Hypercholesterolemia will reduce cardiovascular events. After initiating lomitapide therapy 1.7 cardiovascular events per 1000 patient months on treatment was observed vs. 26.1 cardiovascular events per 1000 patient months in a comparison cohort (247).

 

Side Effects

 

As expected from its mechanism of action lomitapide causes side effects in the GI tract and liver. In the GI tract diarrhea, nausea, vomiting, and dyspepsia occur very commonly (240-242). In the pivotal study in patients with Homozygous Familial Hypercholesterolemia, 90% of the patients developed GI symptoms during drug titration (243). GI side effects are potentiated by high fat meals and it is therefore recommended that dietary fat be limited. Approximately 10% of patients will discontinue lomitapide, mostly from diarrhea. Lomitapide also reduces the absorption of fat soluble vitamins and therefore patients need to take vitamin supplements (240,241). Additionally, it may also block the absorption of essential fatty acids and it is therefore recommended that supplements of essential fatty acids also be provided (at least 200 mg linoleic acid, 210 mg alpha-linolenic acid (ALA), 110 mg eicosapentaenoic acid (EPA), and 80 mg docosahexaenoic acid (DHA) (240,241).

 

Blocking the formation of VLDL in the liver can lead to fatty liver with elevated liver enzymes (240-242). Approximately 30% of patients will develop increased transaminase levels but in the small number of patients studied this has not resulted in liver failure. After stopping the drug, the transaminases have returned to normal. Whether long term treatment with lomitapide will lead to an increase in liver disease is unknown. There is a single case of a patient with lipoprotein lipase deficiency who was treated for 13 years with lomitapide who developed steatohepatitis and fibrosis (248). In an observational study of a small number of patients on lomitapide for > 5 years liver failure or cirrhosis was not noted (249). In another study in Italy, 34 patients were treated with lomitapide for more than 9 years and elevations in hepatic fat were mild-to-moderate, hepatic stiffness remained normal, and the mean FIB-4 score remained below the fibrosis threshold (250). The studies suggest that in most patients’ severe liver disease will not develop. To reduce the risk of liver dysfunction it is important that patients avoid or limit alcohol intake and avoid drugs that inhibit Cyp3A4 activity.

 

Because of the high potential risk of serious complications the FDA has mandated several measures to ensure that patients are closely followed and monitored for liver toxicity ((Risk Evaluation and Mitigation Strategy (REMS) Program) (240,241). ALT, AST, alkaline phosphatase, and total bilirubin should be measured before initiating treatment. During the first year, liver function tests should be measured prior to each increase in dose or monthly, whichever occurs first. After the first year, liver function tests should be measured at least every 3 months and before any increase in dose.

 

Contraindications

 

Lomitapide should not be used during pregnancy and in patients with moderate or severe liver disease. In addition, it should not be used in patients on strong CYP3A4 inhibitors.

 

Summary

 

Lomitapide is approved only for the treatment of lipid disorders in patients with Homozygous Familiar Hypercholesterolemia. The frequent GI side effects and the potential risk of serious liver disease greatly limit the use of this drug and it should be reserved for the patients in which more benign therapies are not sufficient in lowering LDL-C into a reasonable range. It is used as an adjunct to other lipid lowering therapies and lipoprotein apheresis in patients with Homozygous Familiar Hypercholesterolemia.

 

MIPOMERSEN (KYNAMRO)

 

Introduction

 

Mipomersen (Kynamro) is a second generation apolipoprotein B antisense oligonucleotide that was approved in January 2013 for the treatment of patients with Homozygous Familiar Hypercholesterolemia (241,242,251). It is administered as a 200mg subcutaneous injection once a week (241,242,251). As will be discussed below, it lowers LDL-C levels by an LDL receptor independent mechanism. In May 2018 sales were discontinued due to safety concerns related to increased liver transaminases and fatty liver.

 

Effect on Mipomersen on Lipid and Lipoprotein Levels

 

In the pivotal trial, 51 patients with Homozygote Familial Hypercholesterolemia on treatment were randomized to additional treatment with mipomersen (n= 34) or placebo (n=17) and followed for 26 weeks (252). Mipomersen lowered LDL-C levels by 21% and apolipoprotein B levels by 24% compared to placebo. In addition, non-HDL-C was decreased by 21.6%, triglycerides by 17%, and Lp(a) by 23% while HDL and apolipoprotein A-I were increased by 11.2% and 3.9% respectively.

 

Mipomersen has also been studied in patients with Heterozygous Familial Hypercholesterolemia. In a double-blind, placebo-controlled, randomized trial, patients on maximally tolerated statin therapy were treated weekly with subcutaneous mipomersen 200 mg or placebo for 26 weeks (253). LDL-C levels decreased by 33% in the mipomersen group compared to placebo. Additionally, mipomersen significantly reduced apolipoprotein B by 26%, triglycerides by 14%, and Lp(a) by 21% compared to placebo with no significant changes in HDL-C levels. In an extension follow-up study the beneficial effects of mipomersen were maintained for at least 2 years (254). 

 

In a meta-analysis of 8 randomized studies with 462 subjects with either non-specified hypercholesterolemia or Heterozygous Familial Hypercholesterolemia, Panta and colleagues reported that mipomersen decreased LDL-C levels by 32% compared to placebo (255). Additionally, non-HDL-C was decreased by 31%, apolipoprotein B by 33%, triglycerides by 36%, and Lp(a) by 26% with no effect on HDL-C levels.

 

Mechanism Accounting for the Mipomersen Induced Lipid Effects

 

Apolipoprotein B 100 is the main structural protein of VLDL and LDL and is required for the formation of VLDL and LDL (191). Familiar Hypobetalipoproteinemia is a genetic disorder due to a mutation of one apolipoprotein B allele that is characterized by very low concentrations of LDL and apolipoprotein B due to the decreased production of lipoproteins by the liver (215). Mipomersen, an apolipoprotein B antisense oligonucleotide, mimics Familiar Hypobetalipoproteinemia by inhibiting apolipoprotein B 100 production in the liver by pairing with apolipoprotein B mRNA preventing its translation (241,242,251). This decrease in apolipoprotein B synthesis results in a decrease in hepatic VLDL production leading to a decrease in LDL levels.

 

Pharmacokinetics and Drug Interactions

 

No significant drug interactions have been reported. Given the risk of liver abnormalities (see side effect section) the avoidance of alcohol or a reduction in alcohol intake would be prudent.

 

Effect of Mipomersen on Clinical Outcomes

 

There are no clinical outcome trials but it is presumed that lowering LDL-C levels in patients with Homozygous Familial Hypercholesterolemia will reduce cardiovascular events. In a study comparing cardiovascular events in patients with Homozygous Familial Hypercholesterolemia in the 24 months prior to initiating mipomersen therapy and after initiating mipomersen revealed a decrease in events (prior to treatment 61.5% of patients had an event vs. 9.6% after initiating mipomersen; P < .0001) (256). In this trial mipomersen resulted in a mean absolute reduction in LDL-C of 70 mg/dL (-28%), non-HDL cholesterol of 74 mg/dL (-26%), and Lp(a) of 11 mg/dL (-17%).

 

Side Effects

 

The most common side effect is injection site reactions, which occur in 75-98% of patients and typically consist of one or more of the following: erythema, pain, tenderness, pruritus, and local swelling (241,242,251).  Additional, influenza like symptoms, which typically occur within 2 days after an injection, occur in 30-50% of patients and include one or more of the following: influenza-like illness, pyrexia, chills, myalgia, arthralgia, malaise or fatigue which result in a substantial percentage of patients discontinuing therapy (241,242,251).

 

A major safety concern is liver toxicity (241,242,251). By inhibiting VLDL formation and secretion the risk of fatty liver is increased. Fatty liver has been observed in 5-20% of patients treated with mipomersen (241,242,251). In 10-15% of patients treated with mipomersen increases in transaminases occur (241,242,251). Additionally, liver biopsies from 7 patients after a minimum of 6 months of mipomersen therapy have demonstrated the presence of fatty liver although there was no inflammation despite elevations in liver enzymes (257). Liver function should be measured prior to initiating therapy and monthly during the first year and every 3 months after the first year. Fortunately, when treatment is discontinued liver function tests and fatty liver return to normal.

 

Because of the potential for liver toxicity this drug is no longer available.

 

Contraindications

 

Mipomersen is contraindicated in patients in patients with liver disease or severe renal disease. Mipomersen is not recommended for use during pregnancy or lactation. In animal studies mipomersen has not resulted in fetal abnormalities.

 

Summary

 

Mipomersen was approved only for the treatment of lipid disorders in patients with Homozygous Familiar Hypercholesterolemia. The potential risk of serious liver disease greatly limits the use of this drug and therefore it was reserved for patients in which more benign therapies were not sufficient in lowering LDL-C into a reasonable range. It was used as an adjunct to other lipid lowering therapies in patients with Homozygous Familiar Hypercholesterolemia but because of safety concerns is no longer available.

 

EVINACUMAB (EVKEEZA)

 

Introduction

 

Evinacumab is a human monoclonal antibody against angiopoietin-like protein 3 (ANGPTL3). It is approved for the treatment of Homozygous Familial Hypercholesterolemia. Evinacumab decreases LDL-C levels by mechanisms independent of LDL receptor activity. The recommended dose of evinacumab is 15 mg/kg administered by intravenous infusion over 60 minutes every 4 weeks.

 

Effect on Evinacumab on Lipid and Lipoprotein Levels

 

HOMOZYGOUS FAMILIAL HYPERCHOLESTEROLEMIA

 

A double-blind, placebo-controlled trial randomly treated patients with Homozygous Familial Hypercholesterolemia with an intravenous infusion of evinacumab 15 mg/Kg every 4 weeks (n= 43) or placebo (n= 22) (258). The individuals in this trial were on lipid lowering therapy (94% were on a statin with 77% on a high-intensity statin, 77% on a PCSK9 inhibitor, 75% on ezetimibe, 25% on lomitapide, and 34% undergoing apheresis) and the mean baseline LDL-C level was approximately 250-260mg/dL. After 24 weeks of treatment patients in the evinacumab group had a 47% reduction in LDL-C levels vs. a 1.9% increase in the placebo group (table 17). This decrease in LDL-C levels was observed after 2 weeks of therapy and was observed regardless of concomitant use of other lipid lowering drugs or apheresis. Notably, in individuals with null-null LDL receptor variants evinacumab resulted in a 43% decrease in LDL-C levels indicating that evinacumab therapy was effective in the absence of functional LDL receptors. As expected, total cholesterol, non-HDL-C cholesterol, and apo B levels were also decreased. Moreover, triglyceride levels decreased 55% and HDL-C levels decreased 30% with evinacumab administration while Lp(a) levels were unchanged.

 

Table 17. Effect of Evinacumab on Lipid Levels in Homozygous Familial Hypercholesterolemia

 

LDL-C

Apo B

Non-HDL-C

TG

HDL-C

Baseline mg/dL

255

171

278

124

44

Evinacumab % Change

−47%

−41%

−50%

−55%

−30%

Placebo  % Change

+2%

−5%

+2%

−5%

+1%

 

REFRACTORY HYPERCHOLESTEROLEMIA

 

In a double-blind, placebo-controlled trial, patients with refractory hypercholesterolemia with a screening LDL-C level > 70 mg/dL with atherosclerosis or LDL-C > 100 mg/dL without atherosclerosis were randomized to receive subcutaneous or intravenous evinacumab or placebo (259). The hypercholesterolemia was refractory to treatment with a PCSK9 inhibitor and a statin at a maximum tolerated dose, with or without ezetimibe. In this trial a number of different treatment regimens of evinacumab were employed (intravenous or subcutaneous; different doses) and in this summary only the results of intravenous evinacumab 15 mg/kg every 4 weeks (39 patients) vs. placebo (34 patients) will be presented. Baseline LDL-C levels were approximately 145mg/dL. After 16 weeks of treatment the LDL-C level was decreased by 50% with evinacumab administration vs. a 0.6% decrease with placebo. An extension of this trial for 72 weeks found that the reduction in LDL-C were sustained (260). The decrease in LDL-C was observed after 2 weeks of treatment. As expected, total cholesterol, non-HDL-C, and apo B levels also decreased in the evinacumab group. Evinacumab administration decreased triglyceride levels by 53% and HDL-C levels by 31%. In contrast to the results in the homozygous familiar hypercholesterolemia study described above in this study evinacumab decreased Lp(a) levels by 16%. The effect of the subcutaneous administration of evinacumab on lipid levels was similar to that observed with intravenous administration.

 

The effect of evinacumab on triglyceride levels in patients with marked hypertriglyceridemia is described in the Endotext chapter “Triglyceride Lowering Drugs” (261).

 

Mechanism Accounting for the Evinacumab Induced Lipid Effects

 

ANGPTL3 inhibits lipoprotein lipase (LPL) activity thereby slowing the clearance of VLDL and chylomicrons resulting in an increase in plasma triglyceride levels (262,263). Mice deficient in ANGPTL3 have lower plasma triglyceride levels while mice overexpressing ANGPTL3 have elevated plasma triglyceride levels (263). Evinacumab by inhibiting the ability of ANGPTL3 to inhibit LPL activity will accelerate the clearance of TG rich lipoproteins decreasing plasma triglyceride levels (263). Furthermore, ANGPTL3 has also been shown to reduce endothelial lipase activity (263). Endothelial lipase is a phospholipase that catabolizes phospholipids on HDL and accelerates HDL clearance (264,265). Evinacumab by inhibiting the ability of ANGPTL3 to inhibit endothelial lipase activity will lead to a decrease in HDL levels (266).

 

The mechanism(s) that explain the decrease in LDL-C levels with evinacumab administration is not completely understood. A study has demonstrated that the increase in endothelial lipase activity induced by evinacumab leads to VLDL remodeling and lipid depletion that increases VLDL clearance when the LDL receptor is absent (267). This decrease in VLDL, the precursor of LDL, limits LDL particle production resulting in a reduction in plasma LDL-C levels (267). Kinetic studies in four patients with homozygous familial hypercholesterolemia observed that evinacumab markedly increased the fractional catabolic rate of IDL (intermediate-density lipoprotein) and LDL apoB (268). Whether decreases in VLDL production also plays a role in the decrease in LDL-C levels with evinacumab treatment requires additional studies. It should be noted that inhibition of ANGPTL3 decreases LDL-C levels independent of LDL receptor activity (269).

 

Pharmacokinetics and Drug Interactions

 

There are no significant drug interactions.

 

Effect of Evinacumab on Clinical Outcomes

 

There are no cardiovascular outcome studies. In two patients with homozygous Familial Hypercholesterolemia evinacumab therapy markedly reduced LDL-C levels with a concomitant decrease in plaque volume determined by coronary computed tomography angiography (268).  

 

Homozygosity for loss-of-function mutations in ANGPTL3 is associated with significantly lower plasma levels of LDL-C, HDL-C, and triglycerides (familial combined hypolipidemia) (215,263,270). Heterozygous carriers of loss-of-function mutations in ANGPTL3, which occur at a frequency of about 1:300, have significantly lower total cholesterol, LDL-C, and triglyceride levels than noncarriers (263). Moreover, patients carrying loss-of-function variants in ANGPTL3 have a significantly lower risk of coronary artery disease (271,272). Additionally, in an animal model of atherosclerosis treatment with evinacumab decreased atherosclerotic lesion area and necrotic content (271). Taken together these observations suggest that inhibiting ANGPTL3 with evinacumab will reduce cardiovascular disease.

 

Side Effects

 

Serious hypersensitivity reactions have occurred with evinacumab. In clinical trials, 1 (1%) of evinacumab treated patients experienced anaphylaxis vs. 0% of patients who received placebo (package insert).

 

Contraindications

 

Based on animal studies, evinacumab may cause fetal harm when administered to pregnant patients (package insert). Patients should be advised of the potential risks to the fetus of pregnancy. Patients who may become pregnant should be advised to use effective contraception during treatment with evinacumab and for at least 5 months following the last dose.

 

Summary

 

In patients with Homozygous Familiar Hypercholesterolemia the ability of evinacumab to lower LDL-C levels independent of LDL receptor activity makes this agent very useful in these patients. Most patients with Homozygous Familial Hypercholesterolemia do not achieve goal LDL-C levels with triple drug therapy with maximally tolerated statin therapy, ezetimibe, and a PCSK9 inhibitor and therefore the addition of evinacumab will be needed in many of these patients. Evinacumab is also effective in patients with refractory hypercholesterolemia but the drug is not yet FDA approved in this situation. Nevertheless, one can foresee in patients with refractory hypercholesterolemia at high risk for cardiovascular events the use of evinacumab. In addition to lowering LDL-C levels evinacumab also lowers triglyceride levels and could be useful in selected patients with very severe hypertriglyceridemia (261,273).  

 

APPROACH TO TREATING PATIENTS WITH HYPERCHOLESTEROLEMIA

 

Introduction

 

The issues of deciding who to treat, how aggressive to treat, and the goals of therapy are discussed in detail in the chapter “Guidelines for the Management of High Blood Cholesterol” and therefore will not be addressed in this chapter (3). Additionally, the role of life style changes to lower LDL-C is discussed in great depth in chapter “The Effect of Diet on Cardiovascular Disease and Lipid and Lipoprotein Levels” and therefore will also not be addressed here (1). Rather we will focus on how to use the drugs discussed in this chapter to treat various categories of patients. The factors to consider when deciding which drugs are appropriate to use for lowering plasma LDL-C levels are; the efficacy in lowering LDL-C levels, the effect on other lipid and lipoprotein levels, the ability to reduce cardiovascular events, the side effects of drug therapy, the ease of complying with the drug regimen, and the cost of the drugs. Many statins and ezetimibe are generic drugs and therefore they are relatively inexpensive.

 

Isolated Hypercholesterolemia with Cardiovascular Disease

 

In patients with isolated hypercholesterolemia and cardiovascular disease, initial drug therapy should be high intensity statin therapy (atorvastatin 40-80mg or rosuvastatin 20-40mg). In patients with cardiovascular disease, one should aim to lower the LDL-C to below 70mg/dL. Many experts, based on studies comparing statin alone vs. statin + ezetimibe or statin + a PCSK9 inhibitor, would recommend a more aggressive LDL-C goal in high-risk patients (LDL-C <55mg/dL). If statin therapy alone is not sufficient adding ezetimibe, is a reasonable next step. Because a considerable amount of data indicates that the lower the LDL-C the greater the reduction in cardiovascular events many experts would use a combination of high intensity statin therapy plus ezetimibe in all high-risk patients to maximize LDL-C reduction. Ezetimibe is inexpensive, easy to take, has few side effects, will modestly lower LDL-C, and has been shown in combination with statins to further reduce cardiovascular events. High dose statin and ezetimibe will lower LDL-C by as much as 70%, which will lower LDL-C to goal in a large number of patients who do not have a genetic basis for their elevated LDL-C levels. If the combination of statin plus ezetimibe does not lower the LDL to goal one can add a third drug. If the LDL is close to goal, one could add a bile acid sequestrant such as colesevelam or bempedoic acid. If the LDL is not very close to goal one could instead use a statin +/- ezetimibe plus a PCSK9 inhibitor, which will result in marked reductions in LDL-C levels. If the patient has diabetes with a moderately elevated A1c level using a bile acid sequestrant such as colesevelam instead of ezetimibe or in combination with ezetimibe could improve both glycemic control and further lower LDL levels. If the cost of PCSK9 inhibitors decrease the earlier use of these drugs will become feasible.

 

Isolated Hypercholesterolemia in Primary Prevention

 

In patients with isolated hypercholesterolemia (LDL-C < 190mg/dL) without cardiovascular disease initial drug therapy is with a statin. The statin dose should be chosen based on the percent reduction in LDL-C required to lower the LDL-C level to below the target goal (typically < 100mg/dL but if multiple risk factors with a high risk for cardiovascular events is present many experts would aim for <70mg/dL). As discussed earlier, the side effects of statin therapy increase with higher doses so one should not automatically start with high doses, but instead should choose a dose balancing the benefits and risks. Generic statins are inexpensive drugs and are very effective in both lowering LDL-C levels and reducing cardiovascular events. Additionally, they have an excellent safety profile. If the initial statin dose does not lower LCL-C sufficiently, one can then increase the dose or add ezetimibe. If the maximal statin dose does not lower LDL-C sufficiently adding ezetimibe is a reasonable next step if the LDL-C level is in a reasonable range and an additional 20-25% reduction in LDL will be sufficient. High dose statin and ezetimibe will lower LDL-C by as much as 70%, which will lower LDL-C to goal in the majority of patients who do not have a genetic basis for their elevated LDL-C levels. If the combination of statin plus ezetimibe does not lower the LDL-C to goal one can add a third drug, such as bempedoic acid or colesevelam. If the patient has diabetes with a moderately elevated A1c level using colesevelam instead of ezetimibe or in combination with ezetimibe could improve both glycemic control and further lower LDL-C levels.

 

Mixed Hyperlipidemia

 

In patients with mixed hyperlipidemia (elevated LDL-C and triglyceride levels) Initial drug therapy should also be a generic statin unless triglyceride levels are greater than 500-1000mg/dL. If triglycerides are > 500-1000mg/dL initial therapy is directed at lowering triglyceride levels (261). In addition to lowering LDL-C levels, statins are also effective in lowering triglyceride levels particularly when the triglycerides are elevated. If LDL-C is not lowered sufficiently ezetimibe is a reasonable next step. Bile acid sequestrants are not appropriate drugs in patients with hypertriglyceridemia. The approach to the patient whose LDL-C levels are at goal but the triglycerides and non-HDL-C are still elevated is discussed in the chapter on triglyceride lowering drugs (261).

 

Heterozygous Familial Hypercholesterolemia

 

In patients with Heterozygous Familial Hypercholesterolemia or other disorders with very elevated LDL-C levels (>190mg/dL), high doses of a potent statin such as atorvastatin 40-80mg or rosuvastatin 20-40mg are the first step to lower LDL-C levels. In many patients this will not be sufficient. If the LDL-C levels are above goal then adding ezetimibe is a reasonable next step. If after ezetimibe the LDL-C is still slightly above goal triple drug therapy with bempedoic acid or a bile acid sequestrant can be employed. If on statin alone or with the combination of statin and ezetimibe the LDL-C still needs to be markedly reduced a PCSK9 inhibitor may be a better choice as these drugs can markedly lower LDL-C levels.

 

Homozygous Familiar Hypercholesterolemia

 

In patients with Homozygous Familiar Hypercholesterolemia initial therapy with a maximally tolerated statin and ezetimibe can be instituted. This will likely not result in an acceptable LDL-C level and then one can add a PCSK9 inhibitor. Because these therapies depend on LDL receptor activity to lower LDL-C a high percentage of patients will not reach goal and then one can add lomitapide and/or evinacumab, drugs that lower LDL-C levels independent of LDL receptor activity. Because side effects are fewer with evinacumab this is the preferred initial drug in most patients. Studies have shown that with the addition of evinacumab many patients will reach acceptable LDL-C levels. If LDL-C levels are still not acceptable one could then initiate lipoprotein apheresis (274).  

 

Statin Intolerance

 

Statin intolerance is frequently due to myalgias but on occasion can be due other issues, such as increased liver or muscle enzymes, cognitive dysfunction, or other neurological disorders. The percentage of patients who are “statin intolerant” varies greatly but in clinical practice a significant number of patients have difficulty taking statins.

 

As discussed earlier it can be difficult to determine if the muscle symptoms that occur when a patient is taking a statin are actually due to the statin or are unrelated to statin use. The first step in a “statin intolerant patient” is to take a careful history of the nature and location of the muscle symptoms and the timing of onset in relation to statin use to determine whether the presentation fits the typical picture for statin induced myalgias. The characteristic findings with a statin induced myalgia are shown in table 18 and findings that are not typical for statin induced myalgia are shown in table 19. The disappearance of symptoms within a few weeks of stopping statins and the reappearance after restarting statins is very suggestive of the symptoms being due to true statin intolerance. An on-line tool (htpp://tools.acc.org/statinintolerance/#!/) and an app produced by the ACC/AHA are available. This tool characterizes patients based on 8 criteria into possible vs. unlikely to have statin induced muscle symptoms (table 20)

 

Table 18. Characteristic Findings with Statin Induced Myalgia

Symmetric

Proximal muscles

Muscle pain, tenderness, weakness, cramps

Symptom onset < 4 weeks after starting statin or dose increase

Improves within 2-4 weeks of stopping statin

Cramping is unilateral and involves small muscles of hands and feet

Same symptoms occur with re-challenge within 4 weeks

 

Table 19. Symptoms Atypical in Statin Induced Myalgia

Unilateral

Asymmetric

Small muscles

Joint or tendon pain

Shooting pain, muscle twitching or tingling

Symptom onset > 12 weeks

No improvement after discontinuing statin

 

Table 20. Diagnosis of Statin Associated Muscle Symptoms

Symptom timing

Symptom type

Symptom location

Sex

Age

Race/ethnicity

CK elevation > 5 times the upper limit of normal

Known risk factors for statin induced muscle symptoms and non-statin causes of muscle symptoms

 

One should also check a CK level but this is almost always in the normal range. If the CK is not elevated and the symptoms do not suggest a statin induced myalgia one can often reassure the patient and continue statin therapy. This is often successful and studies have shown that many patients that stop taking statins due to “statin induced myalgia” can be successfully treated with a statin. If the CK is elevated it should be repeated after instructing the patient to avoid exercise for 48 hours. Also, the CK levels should be compared to CK levels prior to starting therapy. If the CK remains elevated (3x upper limit of normal) the statin should be discontinued. Similarly, if the CK is normal but the symptoms are suggestive of a statin induced myalgia the statin should also be discontinued. The next step is to determine if one can identify reversible factors that could be increasing statin toxicity (hypothyroidism, drug interactions).  If none are identified the next step after the myalgias have resolved is to try a low dose of a different statin that is metabolized by a different pathway (for example instead of atorvastatin, which is metabolized by the CYP3A4 pathway, rosuvastatin, which has a different pathway of metabolism). Because statin side effects are dose related, a low dose of a statin may often be tolerated. One can also try several different statins as sometimes a patient may tolerate one statin and not others. A meta-analysis has shown that every other day administration of statins is as effective as daily administration in lowering lipid levels and therefore is a very reasonable strategy (275). In some instances, using a long-acting statin (rosuvastatin or atorvastatin) 1-3 times per week can work (we usually start with once per week and then slowly increase frequency as tolerated) (276). In these circumstances (low doses or 1-3 times per week) the reduction in LDL-C may not be sufficient but one can use combination therapy with other drugs such as ezetimibe, bempedoic acid, bile acid sequestrants, or PCSK9 inhibitors to achieve LDL target goals.

 

Many providers have combined Coenzyme Q10 with statins to prevent statin induced myalgias. However, randomized trials with Coenzyme Q10 supplementation have not consistently shown benefit (277-282). A trial, which carefully screened patients to make sure they actually had statin induced myalgias, failed to show a benefit from Coenzyme Q10 supplementation (101). It has also been recommended that vitamin D supplementation be used to prevent statin induced myalgias but a large randomized trial failed to show a reduction in muscle symptoms with vitamin D therapy (283).

 

If after trying various approaches a patient still has myalgias and is unable to tolerate statin therapy one needs to utilize other approaches to lower LDL levels. Similarly, if there are other reasons why a patient cannot take a statin, such as developing muscle pathology, one will also need to utilize other approaches to lower LDL levels. These patients can be treated with ezetimibe, bempedoic acid, bile acid sequestrants, or PCSK 9 inhibitors either as monotherapy or in combination to achieve LDL goals.

 

There are patients who will refuse statins and other drug therapy because they do not believe in taking pharmaceuticals but will take natural products. In these patients we have employed red yeast rice, which decreases LDL-C because it contains a form of lovastatin (284,285). It is effective but one should recognize that the quality control is not similar to the standards of pharmaceutical products and that there can be batch to batch variations. Furthermore, there is a risk of drug-drug interactions if used with inhibitors of CYP3A4. However, in this particular patient population, who refuses to take statins or other drugs, this can be a reasonable alternative. If a patient just refuses statins (usually based on a belief that statins are toxic) we will employ other cholesterol lowering drugs.

 

CONCLUSIONS

 

With currently available drugs to lower LDL-C levels we are now able to markedly reduce LDL-C levels and achieve our LDL-C goals in the vast majority of patients and thereby reduce the risk of cardiovascular disease. Patients with Homozygous Familial Hypercholesterolemia and some patients with Heterozygous Familial Hypercholesterolemia still present major clinical challenges and it can be very difficult in these patients to achieve LDL-C goals.

 

ACKNOWLEDGEMENTS

 

This work was supported by grants from the Northern California Institute for Research and Education.

 

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Endocrinology of Pregnancy

ABSTRACT

A coordinated sequence of events must occur in order to establish and successfully maintain a healthy pregnancy. Synchrony between the development of the early embryo and establishment of a receptive endometrium is necessary to allow implantation and subsequent progression of pregnancy. The endocrinology of human pregnancy involves endocrine and metabolic changes that result from physiological alterations at the boundary between mother and fetus. Known as the feto-placental unit (FPU), this interface is a major site of protein and steroid hormone production and secretion. Many of the endocrine and metabolic changes that occur during pregnancy can be directly attributed to hormonal signals originating from the FPU. The initiation and maintenance of pregnancy depends primarily on the interactions of neuronal and hormonal factors. Proper timing of these neuro-endocrine events within and between the placental, fetal, and maternal compartments is critical in directing fetal growth and development and in coordinating the timing of parturition. Maternal adaptations to hormonal changes that occur during pregnancy directly affect the development of the fetus and placenta. Gestational adaptations that take place in pregnancy include establishment of a receptive endometrium; implantation and the maintenance of early pregnancy; modification of the maternal system in order to provide adequate nutritional support for the developing fetus; and preparation for parturition and subsequent lactation.

INTRODUCTION

A coordinated sequence of events must occur in order to establish and successfully maintain a healthy pregnancy.  The endocrinology of human pregnancy involves endocrine and metabolic changes that result from physiological alterations at the boundary between mother and fetus. Known as the feto-placental unit (FPU), this interface is a major site of protein and steroid hormone production and secretion (Figure 1). Additionally, it serves as an endocrine, respiratory, alimentary, and excretory organ, facilitating the exchange of nutrients and metabolic products between the mother and fetus. The fetus is dependent on this effective exchange with the mother for its proper intrauterine growth and development. Thus, it is not surprising that the fetus initiates and influences maternal adaptations to optimize this exchange via complex hormonal mechanisms. Many of the endocrine and metabolic changes that occur during pregnancy can be directly attributed to hormonal signals originating from the FPU. The initiation and maintenance of pregnancy depends primarily on the interactions of neuronal and hormonal factors. Proper timing of these neuro-endocrine events within and between the placental, fetal, and maternal compartments is critical in directing fetal growth and development and in coordinating the timing of parturition. Maternal adaptations to hormonal changes that occur during pregnancy directly affect the development of the fetus and placenta. Gestational adaptations that take place in pregnancy include establishment of a receptive endometrium; implantation and the maintenance of early pregnancy; modification of the maternal system in order to provide adequate nutritional support for the developing fetus; and preparation for parturition and subsequent lactation.

 Figure 1. The interface between mother and fetus, known as the feto-placental unit (FPU), is a major site of protein and steroid hormone production and secretion. 

ENDOMETRIAL RECEPTIVITY 

The menstrual cycle, involves a synchronous production of ovarian steroid hormones, estrogen and progesterone, which induces structural and functional changes within the endometrium in anticipation for embryo implantation and the establishment of a pregnancy.  During the luteal phase, under the primary influence of progesterone, the proliferative endometrium changes into secretory endometrium, which is well vascularized and composed of spiral arteries. A favorable environment for implantation is established via chemokines, growth factors, and cell adhesion molecules (CAMs) produced by the glandular secretory endometrium (1). The chemokines and CAMs serve to attract the blastocyst to the specific sites of implantation where the endometrium is strategically prepared for invasion and placentation (1). When implantation does not occur, a timely regression and destruction of the fully developed endometrium leads to menstruation. However, if implantation occurs, the endometrium continues to grow and undergoes further morphological and molecular changes to provide supportive environment for the growing embryo (2).

Endometrial “receptivity” refers to this physiological state when the endometrium allows a blastocyst to attach, firmly adhere, penetrate, and induce localized changes in the endometrial stroma resulting in decidualization (2). The specific period, known as the “implantation window” opens 4-5 days after endogenous or exogenous progesterone stimulation and closes approximately 9-10 days later (3, 4). Implantation has three stages: apposition, adhesion and penetration. Apposition is an initial unstable adhesion of the blastocyst to the endometrial surface.  This stage is characterized histologically by the appearance of microprotrusions from the apical surface of the epithelium, termed pinopodes, occurring six days after ovulation and retained for 24 hours during the implantation window. The pinopods express chemokines and CAMs, which attract the blastocyst floating within the endometrial cavity to appose.  Additionally, the smooth surface of the pinopodes facilitates the apposition of the blastocyst to the endometrium.  Further encouraging the blastocyst to appose to the pinopods is the removal of adhesion inhibiting mucin, while the areas between pinopods have been shown to express MUC-1, which prevents embryo adhesion (5).  Once the blastocyst is apposed, a stronger attachment is achieved through local paracrine signaling between the embryo and the endometrium. At this stage, the blastocyst is sufficiently adherent to the endometrium as to resist dislocation of the blastocyst by flushing the uterine lumen. The first sign of the attachment reaction coincides with a localized increase in stromal vascular permeability which is manifested as stromal edema at the site of blastocyst attachment (6).  Thus, vascular changes also appear to be an important factor in establishing endometrial receptivity. Following adhesion, the embryo invades through the luminal epithelium into the stroma to establish a relationship with the maternal vasculature. In response to this invasion and the presence of progesterone stimulation, the endometrial stromal cells undergo a process termed decidualization by which they differentiate and become specialized decidual stromal cells. Decidualization is essential for the survival and continued development of the pregnancy. In humans, decidual changes occur throughout the entire endometrium during the luteal phase even in the absence of an embryo, but become widespread in early gestation. These decidual stromal cells are very metabolically active and support the implanting embryo by secreting a wide array of hormones and growth factors including prolactin, relaxin, insulin-like growth factors (IGFs) and insulin growth factor binding proteins (IGFBPs). The endometrial stromal cells are the precursors of decidual stromal cells and appear to originate from both resident uterine mesenchymal stem cells as well as adult bone marrow-derived stem cells (7, 8). Interestingly, bone marrow-derived progenitors have been shown to give rise to functional prolactin-producing decidual stromal cells in decidua of pregnant mice, and appear to play an important role in implantation and pregnancy maintenance (9). The bone marrow is also the source of many leukocytes that infiltrate the endometrium during the secretory phase. In humans, a large influx of leukocytes to the uterus occurs in response to ovulation and rising ovarian P4 production, elevating them to 40% of all endometrial cells in the mid-late secretory phase of the menstrual cycle (10). This gain in leukocyte numbers is primarily due to the accumulation of uterine natural killer (uNK) cells. Studies in mice additionally show that the selected entry of uNK cells into early decidua optimizes angiogenesis and promotes decidual spiral artery vascular remodeling. This influences the timing of uterine lumen closure and thereby the appropriate rate of early fetal development including initiation of trophoblast invasion (11). Macrophages are the second most abundant leukocyte population in the luteal phase endometrium. In addition to uterine NK cells and macrophages, the endometrium contains T cells with no apparent cyclic changes, and rare populations of dendritic cells in luteal phase endometrium, both of which become more abundant in the pregnant decidua. The composition and function of these immune cells at the implantation site and the maternal-fetal interface are highly specialized to foster embryo and placental development and to minimize the chance of immune rejection (12).   

Progesterone is essential in mediating the changes that the endometrium undergoes in the luteal phase in preparation for embryo implantation (13). The effects of progesterone on the uterus have been elucidated through elegant experiments in knockout mice as well as studies using progesterone receptor (PR) antagonists. Mice with global PR knockout are infertile due to defects in ovulation and implantation (14). Their endometrium displays hypertrophy and inflammation of the glandular epithelium associated with failure to undergo decidualization. Mice with a specific knockout in PR-B isoform, however, have normal ovarian function, implantation and reproductive capacity (15, 16). In contrast, mice with a specific knockout in PR-A exhibit lack of decidualization in the endometrial stroma along with endometrial epithelial hyperplasia and inflammation (15, 16), indicating that PR-A is critical for embryo implantation and the normal function of the endometrial epithelium and stroma, while PR-B promotes epithelial hyperplasia of the endometrium. Moreover, administration of the progesterone antagonist mifepristone (RU486) in humans during pregnancy induces abortion, fetal loss or parturition, depending on the gestational age (17, 18). If administered at low doses at the mid- or late follicular phase, it prevents pregnancy by delaying endometrial maturation, while at high doses it delays the LH surge and inhibits ovulation (19, 20).   

The key to endometrial receptivity is the dynamic and precisely controlled molecular and cellular events that involve coordinated effects of autocrine, paracrine, and endocrine factors. Analysis of the transcriptosome of the endometrium during the implantation window using microarray technology has revealed numerous genes that are up- and down-regulated during the “window of implantation” when compared with late proliferative phase endometrium (4, 21). In particular, transcription factors such as the homeobox (HOX) genes are essential for endometrial receptivity by mediating some functions of the sex steroids. HOXA10- and HOXA11-deficient mice have uterine factor infertility due to an implantation defect (22, 23). Both HOXA10 and HOXA11 mRNAs are expressed in human endometrial epithelial and stromal cells; their expression is upregulated by estrogen and progesterone, and is significantly higher in the mid- and late-secretory phases, coinciding with time of embryo implantation (24, 25). As transcription factors, HOX genes regulate other downstream target genes specific to the implantation window, including pinopodes, β3 integrin and insulin-like growth factor-binding protein-1 (IGFBP-1), leading to the proper development of the endometrium and receptivity to implantation (26). Other growth factors, cytokines, and transcription factors produced by the endometrium also assist in the establishment of endometrial receptivity (26, 27).  Impaired endometrial receptivity is considered to be a major limiting factor for the establishment of a pregnancy. Implantation during this time of uterine receptivity is associated with high (85%) success rate for continuing a pregnancy, whereas implantation after cycle day 25 has a much lower success rate (11%) (28).

IMPLANTATION

Pregnancy-related proteins can be found in maternal circulation shortly after fertilization. For example, platelet activating factor (PAF)-like substance, which is produced by the fertilized ovum, is present almost immediately (29-32). After ovulation and fertilization, the embryo remains in the ampullary portion of the fallopian tube for up to 3 days. The embryo undergoes a sequence of cell divisions and differentiation that is not dependent on the hormonal milieu of the fallopian tube or the uterus, as fertilization and early embryonic development occur successfully in vitro. The developing conceptus travels toward the uterus, through the isthmic portion of the tube, for approximately 10 hours, and then enters the uterus as an embryo at the 2- to 8-cell stage (33, 34). With further development, between 3-6 days after fertilization, the embryo becomes a blastocyst floating unattached in the endometrial cavity (34). A schematic representation of the pre-implantation phase of pregnancy is shown in Figure 2. Before implantation, the blastocyst also secretes specific substances that enhance endometrial receptivity. Successful implantation requires precise synchronization between blastocyst development and endometrial maturation. Indeed, there appears to be a cross-talk between the embryo and the endometrium with the endometrium acting as a biosensor that is able to respond favorably to competent embryos but less favorably to incompetent poorly viable embryos destined to fail (35). Ultimately, implantation failure is the result of impaired embryo developmental competence or impaired endometrial receptivity, both having negative effects on the embryo-endometrium cross-talk. It is estimated that embryos account for one third of implantation failures, while suboptimal endometrial receptivity and aberrant embryo-endometrial cross-talk are responsible for the remaining two-thirds (36).

Figure 2. A diagrammatic summary of the ovarian cycle leading to embryo development as it occurs during the first week after fertilization. (Adapted from (37), with permission)

To date, little information exists regarding regulation of steroid production in the embryo. The early embryo and its surrounding cumulus cells secrete detectable estradiol and progesterone well before the time of implantation (38, 39). Mechanical removal of these cells results in the cessation of steroid secretion, while return of the removed cells through co-culture results in restoration of steroid secretion (38). Given this finding, steroid production by the conceptus is thought to be negligible by the time it has reached the endometrial cavity, since it is gradually denuded of cumulus cells as it travels through the fallopian tube.

Conceptus-secreted progesterone may itself affect tubal motility as the conceptus is carried to the uterus (40). Progesterone, by action mediated through catecholamines and prostaglandins (PG), is believed to relax utero-tubal musculature. Moreover, progesterone is thought to be important in tubal-uterine transport of the embryo to the uterine cavity, since receptors for progesterone are found in highest concentrations in the mucosa of the distal one third of the fallopian tube. Estradiol, also secreted by these structures, may balance the progesterone effect so as to maintain the desired level of tubal motility and tone (40). Progesterone antagonizes estrogen-augmented uterine blood flow through depletion of estrogen receptors in the cytoplasm (41). Likewise, estrogen and progesterone also appear to balance one another in the maintenance of blood flow at the implantation site. Both estrogen and progesterone are known to upregulate the expression of multiple angiogenic factors in the uterus, including VEGF, bFGF, PDGF, and TGF-β (42). It is well known that estrogen stimulates an increase in uterine angiogenesis, blood flow and vasodilation by acting both directly on endothelial cells, and/or indirectly on other endometrial cell types via numerous potential promoters (43). In pregnant baboons and sheep, estrogen stimulated uterine and placental blood flow (44). Estrogen treatment significantly increased the paracellular cleft width between endometrial endothelial cells within 6 h considered to result in the increased vascular permeability associated with estrogen administration (45). Unlike estrogen, the angiogenic effects of progesterone in the uterus are believed to occur without concurrent vasodilation (46), as there was no change in endometrial endothelial paracellular cleft width 6 h after progesterone treatment in baboons (45). However, much is still unknown regarding uterine blood flow regulation in pregnancy and how the implanting embryo may influence this process. Human chorionic gonadotropin (hCG) messenger ribonucleic acid (mRNA) is detectable in the blastomeres of 6- to 8-cell embryos; however, it is not detectable in blastocyst culture media until the 6th day (47-49). After implantation is initiated, the embryo is actively secreting hCG, which can be detected in maternal serum as early as the 8th day after ovulation. However, due to the absence of direct vascular communication, secretion of hCG into the maternal circulation is initially limited (50). The primary role of hCG is to prolong the biosynthetic activity of the corpus luteum, which allows continued progesterone production and maintenance of the gestational endometrium. As implantation progresses, the conceptus continues to secrete hCG and other pregnancy-related proteins, and resumes detectable steroid production (38, 39, 51).

Termed trophectoderm (aka outer cell mass), blastomeres lining the periphery of the blastocyst are destined to form the placenta and can be identified at 5 days post-fertilization. The main structural and functional units of the placenta are the chorionic villi, which increase significantly in number during the first trimester of pregnancy. The structure of the chorionic villi is pictured in Figure 3. The villous structure provides a tremendous absorptive surface area to facilitate exchange between the maternal and fetal circulation. The maternal blood arrives from the spiral arteries and circulates through the intervillous space. Fetal blood moves in the core of the chorionic villi within the villous vessels; thus, fetal and maternal blood is never mixed in this system. The key cells inside the chorionic villi are the cytotrophoblasts. They have the ability to proliferate, invade and migrate or to differentiate, through aggregation and fusion, to form a syncytial layer of multi-nucleate cells lining the placental villi, known as the syncytiotrophoblasts.

By 10 days post-fertilization, 2 distinct layers of invading trophoblasts have formed. The inner layer, the cytotrophoblasts, is composed of individual, well-defined and rapidly dividing cells. The outer layer, the syncytiotrophoblasts, is a thicker layer comprised of a continuous cell mass lacking distinct cell borders. Syncytiotrophoblasts line the fetal side of the intervillous space opposite the decidualized endometrium of the maternal side. Immunohistochemically, cytotrophoblasts stain for hypothalamic-like protein hormones: gonadotropin releasing hormone (GnRH), corticotrophin releasing hormone (CRH), and thyrotropin releasing hormone (TRH) (52-64). Juxtaposed syncytotrophoblasts stain immunohistochemically for the corresponding pituitary-like peptide hormones: human chorionic gonadotropin (hCG; analogous to pituitary luteinizing hormone, LH), adrenocorticotropic hormone (ACTH) and human chorionic thyrotropin (hCT). Anatomically, this arrangement suggests that these 2 layers mirror the paracrine relationship of the hypothalamic-pituitary axis (52-64).

Syncytiotrophoblasts, the principal site of placental steroid and protein hormone biosynthesis, have a large surface area and line the intervillous space which exposes them directly to maternal bloodstream without the vascular endothelium and basement membrane which separates them from the fetal circulation (Figure 3-5). This anatomic arrangement explains why placental proteins are secreted almost exclusively into the maternal circulation in concentrations much higher than those in the fetus (65). The syncytiotrophoblast layer contains the abundant subcellular machinery characteristic of cells primarily responsible for hormone synthesis. Amino acids of maternal origin are assembled into pro-hormones. Pro-hormones are then packaged into early secretory granules and transferred across the trophoblastic cell membranes as mature granules. Mature granules become soluble as circulating hormones in maternal blood as they pass through the intervillous space (65).

Figure 3. A. A depiction of a blastocyst implanting in the uterus. B. A longitudinal section of a chorionic villus at the feto-maternal interface at about 10 weeks' gestation. The villous serves as a bridge between maternal and fetal compartments. C. Human placental ultra-structure seen in cross section. Syncytiotrophoblasts line the fetal surface of the intervillous space and interact with the maternal blood supply to secrete placental hormones directly into the circulation. Decidua lines the maternal surface of the intervillous space and secretes protein hormones. (From (66), with permission)

DECIDUA AND DECIDUAL HORMONES

The decidua is the endometrium of pregnancy. Decidualized endometrium is a site of maternal steroid and protein biosynthesis that relates directly to the maintenance and protection of the pregnancy from immunologic rejection. For instance, decidual tissue secretes cortisol, and in combination with hCG and progesterone secreted by the conceptus, cortisol produced by the decidua acts to suppress the maternal immune response conferring the immunologic privilege required by the implanting conceptus (67, 68).

Decidual Prolactin

Decidual prolactin is a peptide hormone having chemical and biological properties identical to pituitary prolactin (69). Prolactin, derived from decidualized endometrium, is first detectable in the endometrium at a time corresponding to implantation-cycle day 23. Progesterone is known to induce decidual prolactin secretion (70). Scant decidual prolactin enters the fetal or maternal circulation after it is transported across the fetal membranes from the adherent decidua and is released into the amniotic fluid (71). Unaffected by bromocriptine administration, decidual production of prolactin takes place independent of dopaminergic control (69).

Decidual prolactin secretion rises in parallel with the gradual rise in maternal serum prolactin seen until 10 weeks’ gestation, then it rises rapidly until 20 weeks, and falls as term approaches (72). Decidua-derived prolactin serves to regulate fluid and electrolyte flux through fetal membranes by reducing permeability of the amnion in the fetal-to-maternal direction (69-71, 73-77). Circulating prolactin in the fetus is secreted by the fetal pituitary gland, while prolactin found in the maternal circulation is secreted by the maternal pituitary gland under the influence of estrogens.  Unlike decidual prolactin, these circulating levels are both suppressed by maternal ingestion of bromocriptine.

Decidual Insulin-like Growth Factor Binding Protein-1 (IGFBP-1)

IGF binding protein-1 (IGFBP-1) is a peptide hormone that originates from decidual stromal cells. In non-pregnant women, circulating IGFBP-1 does not change during cycling of the endometrium, while IGFBP-3 is the main circulating IGFBP. During pregnancy, however, there is a several-fold increase in serum IGFBP-1 levels that begins during the first trimester, peaks during the second trimester, and falls briefly only to peak a second time before term (78). IGFBP-1 inhibits the binding of insulin-like growth factor (IGF) to receptors in the decidua and inhibits fetal growth. Newborn birth weight correlates directly with maternal IGF-1 levels, and inversely with circulating IGFBP-1 levels (79).  

Progesterone-Associated Endometrial Protein (PAEP)

Previously known as pregnancy protein-14, PAEP is a glycoprotein hormone synthesized by secretory and decidualized endometrium that is detectable around cycle day 24 (80). In serum, it rises sharply around cycle day 22 to 24, reaching its peak value at the onset of menstruation; if pregnancy occurs, levels remain high (81). In pregnancy, PAEP rises in parallel with hCG (78). Like hCG, PAEP is thought to have immunosuppressant properties in pregnancy (80). PAEP levels are often low in those patients with conditions, such as ectopic pregnancy, in which there is little decidual tissue produced (82).

PROLONGATION OF CORPUS LUTEUM FUNCTION

Primary steroid products of the corpus luteum are progesterone, 17β-progesterone, estradiol and androstenedione. Low-density lipoprotein (LDL) cholesterol is the main precursor responsible for corpus luteum progesterone production (83). Between 6- and 7-weeks’ gestation, corpus luteum function naturally begins to decline. During this luteal-placental transition period, production of progesterone shifts to the developing placenta (Figure 4).

Pulsatile pituitary LH secretion in the early luteal phase followed by hCG secreted from the implanting conceptus act to stimulate progesterone production from the corpus luteum. Removal of the corpus luteum before 6 weeks of gestation increases the risk of abortion (67a). Thus, regarding early pregnancy, progesterone is considered the most important steroid product in this group because progesterone alone can maintain a pregnancy that would otherwise abort in a lutectomized woman (84). For example, exogenous progesterone, given to an agonadal woman pregnant through egg-donor in vitro fertilization (IVF), maintains the pregnancy through the first trimester until placental progesterone secretion is established (85). For this reason, in patients with corpus luteum dysfunction or in whom the corpus luteum has been removed surgically, supplementation with exogenous progesterone is frequently initiated and extended beyond approximately 10 weeks of gestation, the critical period of the luteal-placental shift.

Figure 4. A shift in progesterone production from the corpus luteum to the placenta occurs at approximately the 7th to 9th week of gestation. The small, shaded area represents the estimated duration of this functional transition. (From (86), with permission)

In women with first-trimester threatened abortion, progesterone concentrations at the time of initial evaluation are often predictive of ultimate outcome (87). Abortion will occur in approximately 80% of those with progesterone concentrations under 10 ng/mL; viable pregnancies are virtually never observed at concentrations of <5.0 ng/mL (88).

Corpus Luteum Relaxin

Relaxin is a peptide hormone produced by the corpus luteum, and not detected in non-pregnant women or men.  Although it is argued that the corpus luteum is the sole source of relaxin in pregnancy, it has also been identified in the placenta, decidua and chorion (89-91). The maternal serum concentrations of relaxin rise during the first trimester, when the corpus luteum is dominant, and decline in the second trimester. Interestingly, when women with a normal pregnancy were compared with pregnant women using egg donor (therefore, no corpus luteum), relaxin was only identified in the women with a pregnancy derived from her own eggs.  However, the duration of pregnancy and labor outcomes were not different between the two groups (92).  The presence of relaxin suggests that it may play a role in early pregnancy, but its function is still unclear. 

In animals, relaxin softens (ripens) the cervix, inhibits uterine contractions, and relaxes the pubic symphysis (93).  These changes are similar to those seen during human labor.  Additionally, in vitro studies of human cervical stromal cells have shown that relaxin induces changes consistent with cervical ripening (94, 95).  Human relaxin primarily binds to relaxin receptors in the decidua and chorionic cytotrophoblasts (96).  Relaxin, originating in the decidua and binding to its receptors in the fetal membranes, increases cytokine levels that can activate matrix metalloproteinases and lead to rupture of fetal membranes and labor (97). Thus, relaxin may play a facilitatory role in labor, however its role is still not clearly defined.  

PLACENTAL COMPARTMENT

Unique to mammals, the placenta plays a major role in balancing fetal growth and development with maternal homeostasis. The fetus develops in an environment where respiration, alimentation and excretory functions are provided by the placenta. The human placenta is hemochorial, which means the chorion is in direct contact with maternal blood. Cyto- and syncytiotrophoblast cells of the placenta have direct access to the maternal circulation.  In contrast, the trophoblast layer prevents most maternal hormones from entering the fetal compartment, and consequently the fetal/placental endocrine system generally develops and functions independently of that of the mother.  Over time, the placenta has evolved as a system through which viviparity or livebirth could take place with dependable success.

The placenta functions, to some extent, as a hypothalamic-pituitary-end organ-like entity owing to the inherent ability of this type of system, with its stimulatory and inhibitory feedback mechanisms, to dynamically regulate factors that affect fetal growth and development under a variety of conditions. In the fully developed hypothalamic-pituitary-end organ schema of humans, neural inputs to the hypothalamus serve to regulate the secretion of hypothalamic releasing hormone peptides. However, in the placenta there are no such direct neural inputs, and the exact mechanism(s) responsible for regulation of the secretion of hypothalamic-like placental peptides is unknown.

Changes in maternal hormone concentrations play a critical role in modulating the metabolic and immunologic changes required for successful outcome in pregnancy. The fetus and placenta produce and secrete steroids and peptides into the maternal circulation as well as stimulate maternal hormone production. The origins and amounts of the fetal and placental hormones secreted during pregnancy changes dramatically over the course of the gestational period. Some of the pregnancy-related protein hormones previously discussed are, in part, responsible for the altered steroid concentrations typical of pregnancy.

Placental Steroid Hormones

The placenta is a site of active steroidogenesis which depends on highly integrated and active interactions with both mother and fetus. This is consequent to an elegant complementary of enzymatic deficiencies between placental and fetal compartments (Table 1). The placenta is characterized by significant aromatase, sulfatase, and 11b-hydroxysteroid dehydrogenase type 2 activities juxtaposed with a lack of P450C17 (17a-hydroxylase and 17/20 lyase) activity.

Table 1. Enzymatic Limitations by Compartment

Fetal

Placental

3b-hydroxysteroid dehydrogenase

17a-hydroxylase

 

StAR protein

17/20 lyase

16α-hydroxylase

PLACENTAL PROGESTERONE

The placenta is the main source of progesterone during pregnancy. From the luteal phase to term, maternal progesterone levels rise six- to eight-fold. (Figures 5 and 8) Although, progesterone originates almost entirely from the corpus luteum before 6 weeks' gestational age, its production shifts more to the placenta after the 7th week. Beyond 10 weeks, the placenta is the major definitive source of progesterone (51, 98).

While the placenta produces large amounts of progesterone, it has a limited capacity to synthesize cholesterol de novo (Figure 7). Maternal cholesterol enters the trophoblasts in the form of low-density lipoprotein (LDL) cholesterol which serves as the principal precursor for the biosynthesis of progesterone by the placenta (51, 83, 99). The fetal contribution of progesterone is negligible. This is evident as progesterone levels remain high even after fetal demise.  In the non-human primate estrogen regulates placental progesterone production (100). Progesterone concentrations are less than 1 ng/mL during the follicular phase of the normal menstrual cycle (101, 102). However, in the luteal phase of cycles in which fertilization occurs, progesterone concentrations rise from about 1-2 ng/mL on the day of the LH surge to a plateau of approximately 10-35 ng/mL over the subsequent 7 days. Concentrations remain within this luteal-phase range from the 10th week from the last menstrual flow, and then show a sustained rise that continues until term (Figure 5). At term, progesterone concentrations can range from 100-300 ng/mL (51). Most of the progesterone produced in the placenta enters the maternal circulation.

Figure 5. Relative values of circulating concentrations (mean ±SEM) of progesterone and 17α-progesterone during the course of human pregnancy from fertilization to term. The data displayed demonstrates values before and after the luteinizing hormone (LH) surge. Gestational ages are calculated from last menstrual flow. (Adapted from (103), with permission)

The human deciduas and fetal membranes also synthesize and metabolize progesterone (104).  In this case, neither cholesterol nor LDL-cholesterol are significant substrates; pregnenolone sulfate may be the most important precursor.  Progesterone has been shown to exert important functions in implantation and parturition to include promotion of endometrial decidualization; inhibition of smooth muscle contractility; decrease in prostaglandin (PG) formation, which helps maintain myometrial quiescence and prevent the onset of uterine contractions; and inhibition of immune responses like those involved in graft rejection. It is believed to work in concert with hCG and decidual cortisol to inhibit T-lymphocyte-mediated tissue rejection and confer immunologic privilege to the implanted conceptus and developing placenta (105, 106). In animal models, progesterone extends the survival of transplanted human trophoblasts, and high intervillous concentrations of progesterone are of major importance in blocking the cellular immune rejection of the foreign antigens originating from the pregnancy (106).

In addition to its roles in endometrial and myometrial function, progesterone also serves as a substrate for fetal adrenal gland production of glucocorticoids (cortisol) and mineralocorticoids (aldosterone) (107). This important function is consequent to the deficiency of 3b-hydroxysteroid dehydrogenase (3b-HSD) activity in the active fetal zone of the fetal adrenal gland.

PLACENTAL 17α-HYDROXYPROGESTERONE

Like progesterone, during the first several weeks of gestation and through the time of the luteal-placental shift, 17α-hydroxyprogesterone concentrations primarily reflect the steroidogenic status of the corpus luteum (108). However, by the tenth week of gestation, 17 α-hydroxyprogesterone has returned to baseline levels, indicating that the placenta has little 17 α-hydroxylase activity.  During the third trimester the placenta uses fetal D5-sulfoconjugated precursors to secrete increasing amounts of 17α-hydroxyprogesterone, and at this point the placenta becomes the major source of this hormone at term (108).

Concentrations of 17α-hydroxyprogesterone are less than 0.5 ng/mL during the follicular phase of normal menstrual cycles. In cycles leading to pregnancy, 17α-hydroxyprogesterone concentrations rise to about 1 ng/ml on the day of the LH surge, decline slightly for about 1 day, and rise again over the subsequent 4-5 days reaching a level of 1-2 ng/ml. Concentrations then increase slightly to a mean of approximately 2 ng/ml (luteal phase levels) by the end of the 12th week. This level remains stable until a gestational age of about 32 weeks at which time it begins an abrupt, sustained rise at about 37 weeks to approximately 7 ng/ml, a level that persists until term (108) (Figures 5 and 8). The rise in 17α-hydroxyprogesterone that begins at 32 weeks strongly correlates with the fetal maturational processes known to begin at this time. Hence, 17α-hydroxyprogesterone concentration exhibits a bimodal pattern in normal pregnancy.

PLACENTAL 17β-ESTRADIOL

The corpus luteum is the exclusive source of 17β-estradiol during the first 5-6 weeks of gestation. After the first trimester, the placenta is the major source of circulating 17β-estradiol (51). The rate of estrogen production and the level of circulating estrogens increase markedly during pregnancy. Concentrations of 17β-estradiol are less than 0.1 ng/mL during the follicular phase of the cycle and reach about 0.4 ng/mL during the luteal phase of normal menstrual cycles (101). Following fertilization, 17β-estradiol increases gradually to a range of 6-30 ng/mL at term (102) (Figures 6 and 8). Because it is deficient in 17-hydroxylase enzyme activity and 17-20 desmolase (lyase) activity, the placenta is unable to convert progestogens to estrogens. Thus, the placenta relies on 19-carbon androgen precursors produced by the fetal and maternal adrenal glands. Sources of estrogen biosynthesis by the maternal-fetal-placental unit are depicted in Figure 8. The major source of fetal adrenal dihydroepiandrostenediene sulfate (DHEAS) is LDL-cholesterol circulating in the fetal blood. A minor source of fetal adrenal DHEAS is derived from pregnenolone secreted by the placenta. Twenty percent of fetal cholesterol is derived from the maternal compartment. Since amniotic fluid cholesterol levels are negligible, the main source of cholesterol is the fetal liver. As gestation advances, increasing quantities of 17β-estradiol are synthesized from the conversion of circulating maternal and fetal DHEAS by the placenta. At term, approximately equal amounts of estrogens are produced from circulating maternal DHEAS and fetal DHEAS (51, 109). The fetal endocrine system is notable for extensive conjugation of steroids with sulfate. Consequently, the placenta relies on sulfatase activity to cleave sulfate conjugates in the fetal compartment. Naturally occurring placental sulfatase deficiency results in a low estrogen state in pregnancy (110).

The cytochrome P450 aromatase enzyme is responsible for converting 19-carbon precursors to estrogen (111). The efficiency of this enzyme affords the fetus protection from virilization even in the presence of large amounts of aromatizable androgens.

Figure 6. Relative values of circulating concentrations (mean ±SEM) of 17β-estradiol, estriol and estrone during the course of human pregnancy from fertilization to term. Data displayed demonstrate values before and after the luteinizing hormone (LH) surge. Gestational ages are calculated from last menstrual flow. (Adapted from (112), with permission)

The vasodilatory function of estrogens in pregnancy are well described. In animal models, direct estrogen injection into the uterine arteries produces striking increases in blood flow. Without question, 17β-estradiol is the most potent estrogen in this role. Estriol and estrone, though less active, also produce this effect (113). Because the exposure of the utero-placental bed to direct estriol secretion is enormous, estriol may be the principal up-regulator of uterine blood flow. This may be the dominant role of estriol in human pregnancy (113). Estrogen regulated mechanisms may also allow the fetus to govern production and secretion of progesterone during the third trimester. In primates, estrogen regulates the biosynthesis of placental progesterone by regulating the availability of LDL-cholesterol for conversion to pregnenolone and its downstream steroid products (114). Estrogens are also thought to contribute to mammary gland development and fetal adrenal gland function.

PLACENTAL ESTRADIOL

Estriol is first detectable in maternal serum at 9 weeks of gestation (51, 109, 115, 116). This temporal relationship closely corresponds to the early stages of steroidogenic maturation in the fetal adrenal cortex (51). Hence, the continued production of estriol is dependent upon the presence of a living fetus. Concentrations of estriol are less than 0.01 ng/ml in non-pregnant women. First detectable at approximately 0.05 ng/ml by 9 weeks, estriol increases gradually to a range of approximately 10-30 ng/ml at term (51, 98, 115, 117). Between 35- and 40-weeks gestational age, estriol concentrations increase sharply in a pattern that reflects a final surge of intrauterine steroidogenesis just prior to term (Figures 6 and 8).

Figure 7. Synthesis of estrogen and progesterone within and between the maternal, placental and fetal compartments. (Adapted from (118), with permission)

The placenta lacks 16a-hydroxylase activity and consequently, estriol with its 16a-hydroxyl group, must be synthesized from an immediate fetal precursor. The fetal liver provides 16a-hydroxylation of DHEAS for placental estriol synthesis. Interestingly, hepatic 16a-hydroxylation activity disappears postnatally.

Figure 8. Circulating maternal steroid hormone levels throughout early pregnancy. The first-trimester relationship of these steroid hormones to human chorionic gonadotropin (hCG) is shown.

Progestogens
Progesterone o--o--o-
17-a-hydroxyprogesterone -Δ-Δ-Δ-

Estrogens
17-β-estradiol ---
Estriol -o-o-o-
Estrone -x-x-x-

Human chorionic gonadotropin (hCG)
-Δ-Δ-Δ- 
(From ref. 89, with permission)

PLACENTAL ESTRONE

For the first 4-6 weeks of pregnancy, estrone originates primarily from maternal sources such as the ovaries, adrenals, or peripheral conversion (102). Later, the placenta secretes increasing quantities of estrone from the conversion of circulating maternal and fetal DHEAS. The placenta continues to be the major source of circulating estrone for the remainder of the pregnancy (51). Estrone concentrations are less than 0.1 ng/mL during the follicular phase and may reach a maximum of 0.3 ng/mL during the luteal phase of a normal menstrual cycle. Following fertilization, estrone concentrations remain within the luteal phase range through weeks 6-10 of gestation (98). Subsequently there is a gradual increase to a wide range of 2-30 ng/ml at term (51, 98, 102) (Figures 6 and 8). In the absence of fetal adrenal gland function, as in the case of anencephaly, maternal estrogen levels are extremely low, suggesting that the maternal contribution of DHEAS to total estrogen synthesis is negligible.

Placental Protein Hormones

As detailed previously, the placental cytotrophoblast-syncytiotrophoblast relationship mirrors the hypothalamic-pituitary system. The surface of the syncytiotrophoblast is in direct contact with maternal blood within the intervillous space, and consequently, placental proteins are preferentially secreted into the maternal compartment. Table 2 outlines the various peptides associated with the endocrinology of human pregnancy.

Table 2. Pregnancy Specific Protein Hormones by Compartment

Fetal

Placental

Maternal

Alpha-fetoprotein

Hypothalamic-like (cytotrophoblast)         

- GnRH                                                

- CRH                                     

- TRH                                     

- GHRH                                  

- Somatostatin           

Pituitary-like (syncytiotrophoblast)           

- hCG

- hGH

- ACTH

- hPL

- hCT                                      

- Oxytocin

Growth factors

- Inhibin

- Activin                                              

- IGF-I/IGF-II

Other proteins

- Pregnancy specific β1-glycoprotein

- PAPP-A

Decidual derived

-Prolactin

-IGFBP-1

-PP14

Corpus luteum derived

-Relaxin

PLACENTAL PROTEINS: HYPOTHALAMIC-LIKE PROTEINS

Placental Gonadotropin Releasing Hormone (GnRH)

Gonadotropin releasing hormone derived from the placenta is biologically and immunologically similar to the hypothalamic decapeptide GnRH (54). Gonadotropin releasing hormone activity has been localized to the cytotrophoblast cells along the outer surface of the syncytiocytotrophoblast layer. Human chorionic gonadotropin (hCG) has been localized to the adjacent syncytiocytotrophoblast layer. GnRH production peaks at about 8 weeks’ gestation and then decreases as the pregnancy advances in gestational age (54-57). Furthermore, GnRH levels parallel those of hCG in both the placenta and maternal circulation (57).

Placental GnRH stimulates hCG release through a dose-dependent, paracrine mechanism (119). There is little augmentation of hCG secretion by GnRH in first trimester placental culture, because hCG production is already close to maximum (57). In contrast, at mid-trimester there is a marked dose-dependent GnRH augmentation of hCG release in vitro, with this effect diminishing in the term placenta. Likely due to the low affinity of placental GnRH receptors and dilution effect of the maternal circulation, intravenous administration of GnRH during pregnancy does not increase serum hCG. Thus, it seems most likely that locally produced placental GnRH is responsible for stimulation of placental hCG production via paracrine mechanisms (119). GnRH release is increased by estrogen, activin-A, insulin and prostaglandins, and inhibited by progesterone, inhibin, follistatin and endogenous opiates (120).

Placental Corticotrophin Releasing Hormone (CRH)


Placental CRH is structurally similar to the hypothalamic peptide, CRH (121, 122). Due to this similarity, it is easily measured in amniotic fluid as well as fetal and maternal plasma. Pro-CRH mRNA is present in cytotrophoblasts (123). CRH is also intensely immunoreactive in the decidua (53). CRH is found in maternal serum at low levels during the first and second trimesters of uncomplicated pregnancies, but rises dramatically in the third trimester of normal gestations or earlier if there are pregnancy complications resulting from such factors as prematurity, diabetes, or hypertension.(124). The highest levels of CRH are found at labor and delivery. Although concentrations of CRH in fetal plasma are lower than those found in maternal plasma, there exists a significant correlation between maternal and fetal plasma CRH (124). There is a 3-fold rise, in amniotic fluid CRH between the second and third trimester (124, 125). Placenta-derived CRH stimulates placental ACTH release in a dose-dependent manner in vitro (126, 127). Corticotrophin releasing hormone and ACTH are both released into fetal and maternal circulation; their activity is moderated by maternal CRH binding proteins (124).

Placental CRH participates in the surge of fetal glucocorticoids associated with late third trimester fetal maturation (124, 126, 128). When uterine blood flow is restricted, secretion of both CRH and ACTH is increased. Corticotrophin releasing hormone is a potent utero-placental vasodilator (129, 130). Corticotrophin releasing hormone is released into the fetal circulation in response to fetal stress and in conditions leading to fetal growth restriction (131-133). High circulating maternal CRH is believed to be responsible for the elevated plasma ACTH and cortisol found in pregnancy, which renders them unresponsive to feedback suppression of plasma cortisol (124-126, 128, 134). Corticotrophin releasing hormone stimulates prostaglandin synthesis in fetal membranes and placenta. In pre-eclampsia, fetal asphyxia, premature labor, and other conditions leading to fetal growth restriction CRH is frequently elevated (131-133).

Placental Thyrotropin Releasing Hormone

Thyrotropin releasing hormone is found in the cytotrophoblast layer; however, this molecule is different from the tripeptide produced by the hypothalamus (135). It is localized primarily in the syncytiotrophoblast but also in the fetal and maternal blood vessels as well as in the extravillous trophoblast. The concentration of TRH is higher in the fetal circulation, which is likely due rapid protease degradation on the maternal side (136). Since hCG is regarded as the principal placenta-derived thyroid stimulator, a significant role for TRH is uncertain, although it may be involved in thyroid function regulation during fetal life (137).

Placental Growth Hormone Releasing Hormone (GHRH)

GHRH has also been identified in the human placenta, but its cellular localization and function are unknown (126). Its structure is identical to that of the hypothalamic GHRH peptide. The levels of placental GHRH do not contribute to maternal circulating levels of the extra villous the presence of GHRH receptor in the placenta GHRH does not regulate placental growth hormone production. 

Somatostatin (SRIF)  

Somatostatin (SRIF) is a peptide that exerts a variety of regulatory actions interacting with G protein-coupled receptors. Placental somatostatin has been found in early pregnancy villi, cytotrophoblast and in the decidua; while its binding sites have been identified in term placental membranes and cytotrophoblast (64, 138, 139). The amount of placental somatostatin decreases with increasing gestation and it does not contribute to maternal circulating levels of the peptide.  The role of placental somatostatin remains unclear.

PLACENTAL PROTEINS: PITUITARY-LIKE HORMONES

Placental Human Chrorionic Gonadotropin (hCG)

Human chorionic gonadotropin is a glycoprotein structurally similar to follicle stimulating hormone (FSH), luteinizing hormone (LH), and thyroid stimulating hormone (TSH). It is similar to luteinizing hormone (LH) in action. As is true of the other glycoprotein hormones, hCG is composed of 2 non-identical subunits that associate non-covalently (52, 140). The α subunit consists of an amino acid sequence essentially identical to and shared with the other pituitary glycoprotein hormones. On the other hand, the β subunit is structurally similar to the α subunit yet it differs enough to confer specific biologic activity on the intact dimeric hormone. The subunits differ primarily at the carboxyl terminus where the β subunit of hCG has a 30-amino-acid tailpiece that is not present in the human LH β subunit. Glycosylation in this region of HCG accounts for the longer half-life (32-37 hours) of hCG relative to LH (24h vs. 2h, respectively). The molecular weight of the hCG dimer is estimated at 36.7 kDa with the α subunit contributing 14.5 kDa and the β subunit 22.2 kDa (140). The hCG α subunit is found in the cytotrophoblast layer only (57, 60).

As mentioned previously, hCG mRNA is detectable in embryos as early as the 6- to 8-cell stage (47). After implantation of the conceptus, hCG is detectable in the syncytiotrophoblast layer (outer trophectoderm layer) (57, 60-62). Human chorionic gonadotropin is secreted by the syncytiotrophoblasts of the placenta into both the fetal and maternal circulation. Plasma levels increase, doubling in concentration every 2-3 days between 60 and 90 days of gestation. At 3-4 weeks' gestation, the mean doubling time of dimeric hCG is 2.0 ±1.0 days and increases to about 3.5 ±1.5 days at 9-10 weeks (57). The average peak hCG level is approximately 110,000 mIU/mL and occurs at 10 weeks’ gestation (57). Between 12 and 16 weeks, average hCG decreases rapidly with the concentration halving every 2.5 ±1.0 days before reaching 25% of first trimester peak values. Levels continue to fall from 16 to 22 weeks at a slower rate (mean halving rate of 4.0 ±2.0 days) to become approximately 10% of peak first trimester values (57). During the third trimester mean hCG levels rise in gradual, yet significant, manner from 22 weeks until term (57). Interestingly, hCG levels are comparatively higher in women bearing female fetuses.

Human chorionic gonadotropin secretion is related directly to the mass of hCG-secreting trophoblastic tissues. In vivo, the release of hCG has been correlated with the widths of trophoblast tissue from 4 to 20 weeks and with placental weight from 20 to 38 weeks, respectively (57). The rapidly rising hCG seen between 3-4 and 9-10 weeks’ gestation coincides with the proliferation of immature trophoblastic villi and the extent of the syncytial layer (57). As expected, declining hCG levels are associated with a relative reduction in the mass of the syncytiotrophoblast and cytotrophoblast tissue. From 20-22 weeks until term a gradual increase in dimeric hCG corresponds with a similar increase in placental weight and villus volume (57).

Thus, in early gestation rising hCG levels reflect the histological finding of a rapidly proliferating and increasingly invasive placenta. Later in pregnancy, declining hCG levels are associated with a relative reduction in the number and mass of trophoblasts; therefore, hCG levels mirror the placenta's morphologic transformation from an organ of invasion to an organ of transfer (57).

Levels of the β subunit of hCG mirror those of dimeric hCG. The α subunit, undetectable until around 6 weeks' gestation, rises in a sigmoid fashion to reach peak levels at 36 weeks. Levels of the individual subunits are very low relative to dimeric hCG; they are approximately 2,000-fold to 150-fold less than dimeric forms at 6 and 35 weeks, respectively) (57).

With respect to the regulation of hCG production and secretion, hCG secretion appears to be related to placental GnRH release (119). In vitro, hCG is released in pulses at a frequency and amplitude that correlate with the release of placental GnRH (119). In addition, hCG production is stimulated by glucocorticoids and suppressed by DHEAS (126). In vitro, cyclic AMP (cAMP) analogues augment hCG secretion. In humans, similar to pituitary secretion of gonadotropins, decidual inhibin and prolactin inhibit hCG production by term trophoblasts whereas decidua-derived activin augments it (140, 141), with stimulation by estrogen and a negative feedback by progesterone.

Human chorionic gonadotropin, the primary luteotropic factor involved in supporting and maintaining the corpus luteum, ensures the continuous secretion of progesterone until the placenta can perform this function (142). It has immunosuppressive properties, likely involving maternal T-lymphocyte function and it possesses thyrotropic activity (143). Human chorionic gonadotropin may stimulate steroidogenesis in the early fetal testes resulting in virilization and sexual differentiation in males (144, 145). The functions of hCG are summarized in Figure 9.

Figure 9. The physiological roles of human chorionic gonadotropin (hCG) during the course of human pregnancy from fertilization to term. (Adapted from (146), with permission)

Placental Growth Hormone (GH)

Growth hormone is a single-chain peptide hormone structurally related to prolactin and human chorionic somatomammotropin (hCS). Up to the first 15-20 weeks of pregnancy, pituitary growth hormone (GH) is the main form present in the maternal circulation. From 15-20 weeks to term, placental GH gradually replaces pituitary GH, which eventually becomes undetectable (147-151). In contrast to the pulsatile output of pituitary GH, the daily profile of placental GH release is non-pulsatile (150). Syncytiotrophoblasts directly bathing in maternal blood are the site of placental GH synthesis. This cell layer is the placental site of the major glucose transporter, Glut1, and responds to rapid variations in maternal blood glucose levels by modifying placental GH secretion (152, 153).

The rate of secretion of pituitary GH is known to change rapidly, depending on the net result of multiple stimulatory and inhibitory input. The regulation of placental GH is quite different. The rate of synthesis of placental GH, and thus the maternal circulating levels, increases with the growth of the placenta (154). Growth hormone releasing hormone (GHRH) does not modulate placental GH expression in vitro, in vivo, or in the presence of glucose (155, 156). Figure 10 shows both the stimulatory and inhibitory mediators of maternal pituitary GH output, including the influence of placental growth hormone.

Production of maternal insulin-like growth factor-1 (IGF-I) is regulated by placental growth hormone. IGF-1 concentrations in the maternal plasma, studied in a large number of pregnancies, correlate with the corresponding placental GH. The IGF-1 levels do not vary significantly during the first weeks of gestation, but then increase gradually from 165 ±44.5 mg/L at about 24-25 weeks' gestation, and reach levels of 330.5 ±63.5 mg/L in a manner similar to the increases seen in placental GH. It should be noted that circulating maternal IGF-I levels also reflect placental IGF-I secretion. This growth factor, however, does not appear to be strongly expressed in human placenta; in particular; it is not expressed in the syncytiotrophoblast cell layer (157).

The biologic activities of GH and related peptide hormones can be classified into two general categories: somatogenic and lactogenic. Somatogenic activities are related to linear bone growth and alterations in carbohydrate metabolism (158, 159). The primary function of GH is to protect nutrient availability for the fetus. Via local and hepatic IGF-1, placental GH stimulates gluconeogenesis and lipolysis in the maternal compartment.

Figure 10. Shown is a representation of the hypothalamic-growth hormone-IGF-I axis, with details of its modification during pregnancy. A. In the non-pregnant state, pituitary GH secretion is regulated through hypothalamic control. Pituitary GH regulates the secretion of IGF-I, which, in turn, exerts negative feedback action on GH at the hypothalamic-pituitary level. B. During the latter half of pregnancy, the GH-IGF axis is inhibited by large amounts of estrogen. The large increase in placental GH exerts an inhibitory effect on GH secretion mediated by placental GH on the hypothalamus and pituitary. (From (160), with permission)

Placental Human Placental Lactogen (hPL), [Human Chorionic Somatomammotropin (hCS)]

Human placental lactogen is a single-chain polypeptide with two intramolecular disulfide bridges. The structures of hPL, prolactin, and growth hormone are very similar. Eighty-five percent of its amino acids are identical to human pituitary growth hormone and human pituitary prolactin (69, 161). Furthermore, hPL shares biologic properties with both growth hormone and prolactin (69, 161). Thus, it has primarily lactogenic activity but also exhibits some growth hormone-like activity; therefore, it is also referred to as chorionic growth hormone (hCGH) or human chorionic somatomammotropin (hCS). Human placental lactogen is secreted from the syncytiotrophoblast cell layer. Unlike hCG concentrations, levels of hPL rise with advancing gestational age and plateau at term. Human placental lactogen is first detectable during the fifth week of gestation, and rises throughout pregnancy maintaining a constant hormone weight to placenta weight relationship (162). Concentrations reach their highest levels during the third trimester, rising from approximately 3.5 µg/mL to 25 µg/mL at term (162). Although the level of hPL in serum at term is the highest of all placenta-derived protein hormones, its clearance from the circulation is so rapid that it cannot be detected after the first post-partum day.

Since hPL is secreted primarily into the maternal circulation, most of its functions occur at sites of action in maternal tissues. Human placental lactogen is thought to be responsible for the marked rise in maternal plasma IGF-1 concentrations as the pregnancy approaches term (162-164). Human placental lactogen exerts metabolic effects during pregnancy via IGF-I. It is associated with insulin resistance, enhances insulin secretion which stimulates lipolysis, increases circulating free fatty acids, and inhibits gluconeogenesis; in effect, it antagonizes insulin action, induces glucose intolerance, as well as lipolysis and proteolysis in the maternal system (69). In response to fasting and glucose loading, hPL levels rise and fall (162). These metabolic effects favor the transport of ketones and glucose to the fetus in the fasting and fed states, respectively.

Circulating levels of glucose and amino acids are reduced, while levels of free fatty acids, ketones, and triglycerides are increased. The secretion of insulin is augmented in response to a glucose load. The fuel requirements of the developing fetus are met primarily by glucose. It provides the energy needed for protein synthesis and serves as a precursor for the fat synthesis and glycogen formation. Fetal blood glucose levels are generally 10-20 mg/100 ml below those of the maternal circulation; thus, diffusion and facilitated transport favor the net movement of glucose from mother to fetus.

Pregnancy is associated with profound alterations in maternal metabolism. The fetal-maternal relationship favors glucose use by the fetus and forces the maternal tissues to increase their use of alternative energy sources. The endocrine hallmark of this hormonal environment is insulin resistance. Several hormones prevalent during pregnancy are believed to responsible for this altered milieu: estrogens, progesterone, glucocorticoids, human placental lactogen (hPL) and placental GH. Additionally, placental cytokines such as tumor necrosis factor-alpha (TNF-α) contribute to this metabolic state (165).

Placental Adrenocorticotropic Hormone (ACTH)

Placental ACTH is structurally similar to pituitary ACTH (166-178). Under the paracrine influence of placental CRH released from proximal cytotrophoblasts, placental ACTH is secreted by syncytiotrophoblasts into the maternal circulation (179-181). Circulating maternal ACTH is increased above non-pregnancy levels, but still remains within the normal range (182, 183).

Placental ACTH stimulates an increase in circulating maternal free cortisol that is resistant to dexamethasone suppression (179, 182). Thus, relative hypercortisolism in pregnancy occurs despite high-normal ACTH concentrations. This situation is possible due to two main differences in endocrine relationships during pregnancy. First, the maternal response to exogenous CRH is blunted (182). Second, a paradoxical relationship exists between placental CRH, ACTH, and their end-organ product, cortisol; glucocorticoids augment placental CRH and ACTH secretion, not suppress it (127, 180). This positive feedback mechanism allows an increase in glucocorticoid secretion in times of stress in excess of the amount necessary if the mother were not pregnant (127).

Placental Human Chorionic Thyrotropin (hCT)

Human chorionic thyrotropin is structurally similar to pituitary TSH, but it does not possess the common α subunit (135). The placental content of hCT is very small (58). Human chorionic gonadotropin possesses 1/4000th of the thyrotropic activity of TSH, and is thought to exert a more significant effect on the maternal thyroid than does hCT (137), particularly in conditions with high hCG levels such as trophoblastic disease.

PLACENTAL PROTEINS: GROWTH FACTORS  

Placental Inhibin/Activin/Follistatin  

Inhibin and activin are heterodimeric glycoproteins with the former comprised of an α and β subunit and the latter composed of two β subunits.  Inhibin is secreted by the corpus luteum and is present in decidualized endometrium (184, 185). Inhibin and activin dimers have been localized to the syncytiotrophoblast layer, but their individual subunits have been localized to both cytotrophoblasts and syncytiotrophoblasts (186).

Inhibin begins to increase in the maternal circulation above non-pregnant levels by 12 days post-fertilization, dramatically increasing at about 5 weeks' gestation to peak at 8-10 weeks. Subsequently, levels decrease at 12-13 weeks and stabilize until around 30 weeks before they rise again as term approaches (185). The early fluctuations in inhibin levels reflect release from the corpus luteum, whereas the increase seen in the third trimester originates from the placenta and decidua. After delivery, inhibin is undetectable. The inhibin A dimer is the principal bioactive inhibin secreted during pregnancy. Quantification of inhibin A is part of the prenatal quad screen that can be administered during pregnancy at a gestational age of 16–18 weeks. An elevated inhibin A (along with an increased beta-hCG, decreased AFP, and a decreased estriol) is suggestive of the presence of a fetus with Down syndrome.

Activin-A is the major trophoblastic activin product, which similarly increases in maternal circulation throughout pregnancy and peaks at term (187). Interestingly, higher levels of activin-A are found in mid-gestation in women with preeclampsia (188, 189).  Similar to their action in the ovarian follicle, inhibin and activin are regulators within the placenta for the production of GnRH, HCG, and steroids; as expected, activin is stimulatory and inhibin is inhibitory. 

Follistatin is the activin-binding protein expressed in placenta, membranes, and decidua (190). Since follistatin binds activin, it antagonizes the stimulatory effects of activin on placental steroid and peptide production. 

Placental Insulin-Like Growth Factors-I and II (IGF I and II)

Without question, the most important site of IGF-I and IGF-II production is the placenta (191). IGF-I and IGF-II are involved in prenatal growth and development. These growth factors do not cross the placenta into the fetal circulation; however, they may be involved in placental growth (192, 193).  An increase in maternal IGF-I levels during pregnancy with a rapid decrease after delivery indicates a significant placental influence.  There is however, no change in IGF-II levels throughout pregnancy.  In animal studies, the IGF-I produced in the placenta regulates the transfer of nutrients across the placenta to the fetus and thus enhances growth.  Interestingly, neonates with intrauterine growth restriction have reduced levels of IGF-I. IGF-II secreted by the placenta is also important in influencing β cell sensitivity to glucose and modulation of maternal insulin and glucose concentrations during pregnancy (194).

Placental Soluble FMS-Like Tyrosine Kinase (SFLT-1) and Souble Endoglin (sENG)

Soluble Flt-1 is a circulating splice variant of Flt-1, the receptor for VEGF and placental growth factor (PLGF), while sENG is the circulating receptor for transforming growth factor-β (TGF-β). VEGF, PLGF, TGF-β as well as other pro-angiogenic proteins are known to be essential for normal placental and fetal vascular development. Soluble Flt-1 and sENG are almost undetectable in the circulation of non-pregnant individuals, and are produced in large quantities by the placenta leading to marked elevation in their circulating levels during pregnancy which steadily rise until term (195, 196). These two soluble receptors are increased in serum and placentas of preeclamptic women compared to normal pregnancies and their abnormal elevation presages the development of preeclampsia. Experimental evidence indicates that sENG cooperates with sFlt-1 to induce endothelial dysfunction in vitro and preeclampsia in vivo (197). It is thought that sFlt-1 and sENG neutralize their ligands, reducing the concentration of VEGF, PLGF, and TGF-b in maternal circulation, which results in a shift in the angiogenic balance towards anti-angiogenesis, which in turn leads to endothelial damage and the clinical onset of the syndrome. However, large prospective studies have failed to show sufficient accuracy of these biomarkers for clinical utility in preeclampsia prediction (198, 199).

PLACENTAL PEPTIDE HORMONES: OTHER PLACENTAL PEPTIDES

In addition to the pregnancy-related proteins produced analogous to hypothalamic and pituitary glycoproteins, the placenta also produces several other proteins that have no known analogues in the non-pregnant state. These proteins have been isolated and identified from serum drawn during pregnancy or purified from placental tissue. Figure 11 shows the changes in concentration of each of these pregnancy-related proteins throughout gestation.

Placental Pregnancy-Specific b1-Glycoprotein (SP1)

Pregnancy-specific b1-glycoprotein is a glycoprotein hormone that can be detected about 18-23 days after ovulation. It is secreted from trophoblast cells (200, 201). Initially, it exhibits a 2- to 3-day doubling time, reaching peak concentrations between 100-200 ng/mL at term. Pregnancy-specific b1-glycoprotein has immunosuppressive effects on T-lymphocyte proliferation, and is thought to be involved in preventing rejection of the implanting conceptus (202).

Placental Pregnancy-Associated Plasma Protein-A (PAPP-A)

Pregnancy-associated plasma protein-A is the largest of the pregnancy-related glycoproteins. It originates, mainly, from placental syncytiotrophoblasts (203, 204). Pregnancy-associated plasma protein-A can first be detected at approximately 32-33 days after ovulation. With a 3-day doubling time, its levels initially rise rapidly, and then continue to rise more slowly until term (203). Like SP-1 and hCG, PAPP-A is believed to play an immunosuppressive role in pregnancy (204). It has recently gained favor as a clinically useful, first-trimester screening marker for Down syndrome (trisomy 21). Authors have confirmed decreased PAPP-A levels in association with early pregnancy failure (205). However, when compared with serum hCG and progesterone measurements to evaluate the clinical usefulness of PAPP-A values in predicting the outcome of early pregnancy, hCG and progesterone remained the best clinical tools (206).

Placental Protein-5 (PP5)

This glycoprotein is produced by the syncytiotrophoblasts. It is detected beginning at 42 days after ovulation, and steadily rises until term (207). Placental protein-5 has anti-thrombin and anti-plasmin activities, and is believed to be a naturally occurring blood coagulation inhibitor active at the implantation site (208).

Figure 11. Maternal serum concentrations of human chorionic gonadotropin (hCG) and some other pregnancy-associated protein hormones (SP-1, PAPP-A, PP-5) throughout pregnancy. The timing of implantation, missed menses and parturition is shown to demonstrate the temporal relationships. (Modified from (209), with permission)

PLACENTAL METABOLIC PROTEINS  

Placental Leptin  

Leptin is a key regulator of satiety and body mass index (BMI), and its levels are thought to reflect the amount of energy stores and nutritional state (210).  The placenta is the principal source of leptin during pregnancy (211). Most of the leptin produced by the placenta is secreted into the maternal circulation, and as a consequence leptin levels are elevated during pregnancy.  In the first trimester, maternal plasma leptin levels are double nonpregnant values and continue to increase during the second and third trimesters (212-214).  In the second and third trimesters leptin is also expressed in the chorion and amnion (215).  The amount of leptin directed to the fetus is uncertain, and its role in fetal development is also unclear.  Leptin levels decline to normal nonpregnant levels within 24 hours after delivery (216). Interestingly, leptin levels during pregnancy do not correlate with BMI as they do in the nonpregnant state (217).  Although not clear, it is thought that leptin may be utilized by the placenta to modulate maternal metabolism and partition energy supplies to the fetus (218). There is evidence that placental leptin is anti-apoptotic and promotes proliferation, protein synthesis and the expression of tolerogenic maternal response molecules such as HLA-G (219). Placental leptin expression is regulated by hCG, insulin, steroids, hypoxia and many other growth hormones, suggesting that it may have an important endocrine function in trophoblast cells (219). Additionally, the human placenta also expresses leptin receptors, and therefore can act in a paracrine manner to modulate placental function (220, 221).

Placental Ghrelin   

Ghrelin, is a gastric peptide isolated primarily from the stomach which is thought to stimulate GH release and participates in the regulation of energy homeostasis, increasing food intake, decreasing energy output, as well as exert a lipogenetic effect (222).  Ghrelin and its receptors have been isolated in the placenta, clearly indicating a role for ghrelin in reproduction. Circulating ghrelin levels peak at mid-gestation, then with advancing gestational age declining ghrelin levels are observed.  After delivery, near prepregnancy levels of ghrelin are seen (223).  It is thought that ghrelin may well be involved in regulation of energy intake during pregnancy (224), however its exact role is still unknown.

PLACENTAL MATURATION

As pregnancy advances, the relative numbers of trophoblasts increase as feto-maternal exchange begins to dominate the placenta's secretory functions. Later, throughout the second and third trimester, the placenta adapts its structure to reflect its function such that near term, the villi consist mainly of fetal capillaries with sparse supporting stroma beyond that which is required to maintain its anatomic integrity. In contrast to the early placental villus where trophoblasts are abundant as part of a continuous layer of basal cytotrophoblasts, the term placenta's membranous interface between the fetal and maternal circulation is extremely thin (65). Thus, as the gestation progresses toward term, the number of cytotrophoblasts declines and the remaining syncytial layer becomes thin and barely visible. This structural arrangement facilitates transport of compounds across the feto-maternal interface. Consistent with the cytologic changes that occur in the maternal fetal interface from mid-gestation to term, striking changes in the global gene expression profile of this tissue has been demonstrated over this interval (225).

FETAL COMPARTMENT

The endocrine system, a system that is functional from the time of intrauterine existence through old age, is one of the first systems to develop during fetal life. As in the placenta, the regulation of the fetal endocrine system relies, to some extent, on precursors secreted by the other compartments. As the fetus develops, its endocrine system matures and eventually becomes more independent, preparing the fetus for extrauterine life.

Fetal Hypothalamus and Pituitary

By the end of the fifth week of gestation, the primitive hypothalamus can be identified as a swelling on the inner surface of the diencephalic neural canal (226).  By the 9th to 10th week, the median eminence of the hypothalamus is evident.  By week 14 to 16 the hypophysiotropic hormones GnRH, TRH, CRH, GHRH and somatostatin appear in the fetal hypothalamus (227) .  The portal-vessel system that delivers the releasing hormones to the anterior pituitary is fully developed by 18 weeks of gestation (227).

The anterior pituitary cells that develop from those cells lining Rathke's pouch are capable of secreting growth hormone (GH), follicle-stimulating hormone (FSH), luteinizing hormone (LH) and adrenocorticotropic hormone (ACTH), in vitro, as early as 7 weeks of fetal life (Figure 12).

Figure 12. Fetal serum pituitary hormone levels. PrL indicates prolactin; TSH, thyroid-stimulating hormone; ACTH, corticotropin; GH, growth hormone; LH/FSH, luteinizing hormone/follicle stimulating hormone. (Modified from (228), with permission)

Fetal Thyroid Gland

The fetal thyroid gland develops initially in the absence of detectable TSH. By 12 weeks’ gestation, the thyroid is capable of iodine-concentrating activity and thyroid hormone synthesis (226) .  Prior to that time, the maternal thyroid appears to be the primary source for T4.  The levels of TSH and T4 are relatively low in fetal blood until mid-gestation. At 24-28 weeks' gestation, serum T4 and reverse tri-iodothyronine (rT3) concentrations begin to rise progressively until term while the TSH concentration peaks. At birth, there is an abrupt release of TSH, T4, and T3. The relative hyperthyroid state of the newborn is believed to facilitate thermoregulatory adjustments for extrauterine life.  The function of the fetal thyroid hormones is crucial for somatic growth and neonatal adaptation. 

Fetal Gonads

The internal genitalia in the embryo have the inherent tendency to feminize. The Wolffian (mesonephric) and Mullerian (paramesonephric) ducts are discrete primordia that temporarily coexist in all embryos during the ambisexual undifferentiated development period (up to 8 weeks). The critical factors in determining which of the duct structures stabilize or regress are the hormones secreted by the testes: Anti-Mullerian hormone (AMH) and testosterone. The testis is histologically identifiable at 6 weeks’ gestation. Primary testis differentiation begins with development of the Sertoli cells at 8 weeks’ gestation. SRY, the sex-determining region on the Y chromosome, determines male gonadal sex and directs the differentiation of the Sertoli cell (229).  Sertoli cells secrete AMH which triggers the resorption of the Mullerian ducts in males and prevents development of female internal structures (230). At approximately 8 weeks’ gestation Leydig cells differentiate and testosterone secretion commences.  Maximum levels of fetal testosterone are observed at about 15 – 18 weeks and decrease thereafter.

Differentiation of the ovaries occurs several weeks later than that of the testis.  If the primordial germ cells lack the SRY region on the Y chromosome, ovaries develop from the indifferent gonads.  Fetal ovarian function becomes apparent by 7 to 8 weeks gestation; the time when the ovary becomes morphologically recognizable. During this time ovarian differentiation is occurring with mitotic multiplication of germ cells, reaching 6-7 million oogonia, their maximal number, by 16-20 weeks’ gestation (231, 232). 

The pattern of luteinizing hormone (LH) levels in fetal plasma parallels that of follicle-stimulating hormone (FSH). The decline in pituitary gonadotropin content, and plasma concentration of gonadotropins after mid-gestation is believed to result from the maturation of the hypothalamic-pituitary-gonadal axis. The hypothalamus becomes progressively more sensitive to sex steroids originating from the placenta and circulating in fetal blood. Early secretion of fetal testosterone is important in initiating sexual differentiation in males. In the absence of testosterone, the Wolffian system regresses. Human chorionic gonadotropin (hCG), supplemented by fetal LH, is believed to be the primary stimulus effecting the early development and growth of Leydig cells as well as stimulating the subsequent peak of testosterone production. In females, the fetal ovary is involved primarily in the formation of follicles and germ cells and less involved in hormone secretion.

Fetal Adrenal Glands

The human fetal adrenal gland is a remarkable organ due to its incredible capacity for steroid biosynthesis in utero, and because of its unique morphologic features. The human fetal adrenals are disproportionately large, and at mid-pregnancy their size exceeds that of the fetal kidneys. At term, the adrenals are as large as those of adults, weighing 10 grams or more. The region that ultimately develops into the adult adrenal cortex, the outer or definitive zone, accounts for only about 15% of the fetal gland (Figure 13). The unique inner or fetal zone comprises 80-85% of the volume of the adrenal in utero, and is largely responsible for the tremendous secretory capacity of this organ. The fetal zone rapidly undergoes involution at parturition and by one year it has completely disappeared (233). Changes in the fetal adrenal volume throughout fetal life and into young adulthood are graphically depicted in Figure 14.

The adrenal function of 10 preterm infants of gestational age 27-34 weeks was assessed for up to 80 days after delivery. The changes in steroid excretion with time in preterm infants of gestation over 28 weeks reflect involution of the fetal adrenal zone at a similar rate to term infants. These findings are consistent with the removal at birth of the inhibitory effects of estrogen on the 3 beta-hydroxysteroid dehydrogenase enzyme. The continued function of the adrenal fetal zone beyond the first month in preterm infants of less than 28 weeks’ gestation may however be due to persistence of some other fetal regulatory adrenal mechanism. This suggests that it is gestation that determines fetal zone activity rather than birth (234).

The fetal adrenal gland secretes large quantities of steroid hormones (up to 200-mg daily) near term. The rate of steroidogenesis is 5-times that observed in the adrenal glands of adults at rest. The principal steroids secreted are C-19 steroids (mainly DHEAS), which serve as substrates for estrogen biosynthesis by the placenta (Figure 13).

The fetal adrenal gland contains a zone, unique to in-utero fetal life that accounts for the rapid growth of the adrenal gland; this zone regresses during the first few weeks after birth. In addition to the fetal zone, an outer layer of cells forms the adrenal cortex (definitive zone). The fetal zone differs not only histologically, but also biochemically from the cortex (i.e., the fetal zone is deficient in 3b-hydroxysteroid dehydrogenase enzyme activity and, therefore, secretes C-19 steroids (mainly DHEAS); the cortex secretes primarily cortisol).

Figure 14. An illustration demonstrating generalized pathways for steroid hormone formation in the fetal adrenal gland. DHA: dehydroepiandrosterone. DHAS: dehydroepiandrosterone sulfate. LDL: low-density lipoprotein cholesterol. (Modified from (235), with permission)

Figure 15. Changes in the fetal adrenal volume throughout fetal life and into young adulthood. (Modified from (236), with permission)

Research involving the fetal adrenal gland has attempted to determine the factors that stimulate and regulate fetal adrenal growth and steroidogenesis. Other work has focused on the mechanisms responsible for fetal zone atrophy after delivery. All investigations have shown that, in vitro, adrenocorticotropic (ACTH) stimulates steroidogenesis. Furthermore, there is clinical evidence that, in vivo, ACTH is the major trophic hormone of the fetal adrenal gland. For example, in anencephalic fetuses, the plasma levels of ACTH are very low and the fetal zone is markedly atrophic. Maternal glucocorticoid therapy suppresses fetal adrenal steroidogenesis by suppressing fetal ACTH secretion. Despite these observations, ACTH -related peptides, growth factors and other hormones have been proposed as possible trophic hormones for the fetal zone. After birth, the adrenal gland shrinks in size by more than 50% because of the regression of fetal zone cells.

Fetal Parathyroid Glands and Calcium Homeostasis

In the fetus, calcium concentrations are regulated by the movement of calcium across the placenta from the maternal compartment. In order to maintain fetal bone growth, the maternal compartment undergoes adjustments that provide a net transfer of sufficient calcium to the fetus. Maternal compartment changes that permit calcium accumulation include increases in maternal dietary intake, increases in maternal 1, 25-dihydroxyvitamin D3 levels, and increases in parathyroid hormone (PTH) levels.  The levels of total calcium and phosphorus decline in maternal serum, but ionized calcium levels remain unchanged. During pregnancy, the placenta forms a calcium pump in which a gradient of calcium and phosphorus is established which favors the fetus.  Thus, circulating fetal calcium and phosphorus levels increase steadily throughout gestation. Furthermore, fetal levels of total and ionized calcium, as well as phosphorus, exceed maternal levels at term.

By 10-12 weeks' gestation, the fetal parathyroid glands secrete PTH. Fetal plasma levels of PTH are low during gestation, but increase after delivery. In contrast to the unchanged maternal calcitonin levels, the fetal thyroid gland produces increasing levels of calcitonin. Since there is no transfer of parathyroid hormone across the placenta, changes noted in fetal calcium levels are related to fetal changes in these hormones (PTH and calcitonin). These adaptations are consistent with the need to conserve calcium and stimulate bone growth within the fetus. After birth, neonatal serum calcium and phosphorus levels fall. Parathyroid hormone levels start to rise within 48 hours after birth. Calcium and phosphorus levels steadily increase over the following several days, with some dependence on dietary intake of milk.

Fetal Endocrine Pancreas

The pancreas’ exocrine function begins after birth, while the endocrine function (hormone release) can be measured from 10 to 15 weeks onward.  The α-cells which contain glucagon, and the β-cells which contain somatostatin, can be recognized by 8 weeks’ gestation (234). Alpha cells are more numerous than β-cells in the early fetal pancreas and reach a peak at midgestaion; β-cells increase through the second half of gestation so that by term the ratio of α-cells to beta cells is approximately 1:1 (237, 238).  Human pancreatic insulin and glucagon concentrations increase with advancing fetal age, and are higher than concentrations found in the adult pancreas. In vivo studies of umbilical cord blood obtained at delivery and fetal scalp blood samples obtained at term show that fetal insulin secretion is low and tends to be relatively unresponsive to acute changes in glucose. In contrast, fetal insulin secretion in vitro is responsive to amino acids and glucagon as early as 14 weeks' gestation. In maternal diabetes mellitus, fetal islet cells undergo hypertrophy such that the rate of insulin secretion increases.

Fetal Alpha-Fetoprotein (AFP)

Alpha-fetoprotein is a glycoprotein synthesized first by the yolk sac, then the gastrointestinal tract, and lastly by the fetal liver (239, 240). After entering the fetal urine, it is readily detected in amniotic fluid. Amniotic fluid AFP (afAFP) peaks between 10-13 weeks’ gestation, and then declines from 14-32 weeks. In the fetus, AFP peaks at 12-14 weeks and steadily decreases until term (241). The fall in fetal plasma AFP (fpAFP) is most likely due to the combination of increasing fetal blood volume and a decline in fetal production. The concentration gradient between fpAFP and maternal serum AFP (msAFP) is approximately 150- to 200-fold. Detectable as early as 7 weeks' gestation, msAFP reaches peak concentrations between 28-32 weeks (241). The seemingly paradoxical rise in msAFP in association with decreasing afAFP and fetal serum levels can be accounted for by the increasing placental permeability to fetal plasma proteins that occurs with advancing gestational age (241). Alpha-fetoprotein acts as an osmoregulator to help adjust fetal intravascular volume (241). It may also be involved in certain immunoregulatory functions (242). Amniotic fluid AFP and maternal serum AFP are clinically important because they are elevated in association conditions such as neural tube defects (243). Additionally, msAFP is decreased in pregnancies in which the fetus has Down syndrome (trisomy 21) (244).

MATERNAL COMPARTMENT

Maternal Hypothalamus and Pituitary

Little information is definitively known about the endocrine alterations of the maternal hypothalamus during pregnancy. Thought to result from estrogen stimulation, the anterior pituitary undergoes a 2- to 3-fold enlargement during pregnancy, primarily because of hyperplasia and hypertrophy of lactotroph cells. Thus, plasma prolactin levels parallel the increase in pituitary size throughout gestation. In contrast to the lactotrophs, the size of the other pituitary cells decreases or remains unaltered during pregnancy. In line with these findings, maternal levels of somatotrophs and gonadotrophs are lower and the level of thyrotrophs and corticotrophs remains unchanged.  In contrast, adrenocorticotrophic hormone (ACTH) levels do increase with advancing gestation. Corticotrophin-releasing hormone (CRH) in the maternal plasma increases during pregnancy due to increased placental secretion, but alterations in binding-protein concentrations prevent increased biologic activity of this releasing hormone.

The size of the posterior pituitary gland diminishes during pregnancy (245).  Additionally, maternal plasma arginine vasopressin (AVP) levels remain low throughout gestation and are not believed to play a pivotal role in human pregnancy.  In contrast, maternal oxytocin levels progressively increase in the maternal blood and parallel the increase in maternal serum levels of estradiol and progesterone (246). Uterine oxytocin receptors also increase throughout pregnancy, resulting in a 100 fold increase in oxytocin binding at term in the myometrium (247).

Maternal Thyroid Gland

As a result of increased vascularity and glandular hyperplasia, the thyroid gland increases in size by 18% during pregnancy; however, true goiter is not usually present (248).  Enlargement is associated with an increase in the size of the follicles with increased amounts of colloid and enhanced blood volume.  This enlargement may be a response to the thyrotropic effect of hCG, which may account for some of the increase in serum thyroglobulin concentrations observed during pregnancy.  During gestation the mother remains in a euthyroid state. Total thyroxine (T4) and tri-iodothyronine (T3) levels increase but do not result in hyperthyroidism because there is a parallel increase in T4-binding globulin that results from estrogen exposure (Figure 15). The increase seen in binding-protein concentrations is similar to that observed in women who use oral contraceptives (OC). A modest increase in the basal metabolic rate (BMR) rate occurs during pregnancy secondary to increasing fetal requirements. Some T4 and T3, but no TSH, are transferred across the placenta.

Figure 15. Relative changes in maternal thyroid function during the course of human pregnancy from fertilization to term. (Modified from (249), with permission)

Maternal Adrenal Glands

The maternal adrenal gland does not change morphologically during pregnancy.  However, plasma adrenal steroid levels increase with advancing gestation. Total plasma cortisol concentrations increase to three times nonpregnant levels by the third trimester.  The hypoestrogenic state of pregnancy results in increased hepatic production of cortisol-binding globulin. This increase in cortisol-binding globulin results in decreased metabolic clearance of cortisol, resulting in an increase in plasma free cortisol and total free cortisol.  Additionally, cortisol production increases due to an increase in maternal plasma ACTH concentration and the hyperresponsiveness of the adrenal cortex to the ACTH stimulation (250).  Despite the elevated free cortisol levels, pregnant women do not exhibit any overt signs of hypercortisolism, likely due to the anti-glucocorticoid activities of the elevated levels of progesterone.

Plasma renin substrate levels are increased as a consequence of the effects of estrogen on the liver.  The higher levels of renin and angiotensin during pregnancy, lead to elevated angiotensin II levels and markedly elevated levels of aldosterone.  Similar to cortisol, the elevated aldosterone levels do not have a detrimental effect on maternal health.  The high level of progesterone is thought to displace aldosterone from its renal receptors.

Androgen levels are elevated during pregnancy, but the free androgen levels remain normal to low secondary to the estrogen-induced increase in hepatic synthesis of sex hormone-binding globulin. Dehydroepiandrosterone (DHEA) and DHEAS production is increased twofold during pregnancy. However, serum concentrations of DHEAS are reduced to less than nonpregnant levels secondary to enhanced 16 –hydroxylation and placental use of DHEAS in estrogen production (251).

Maternal Endocrine Pancreas

A dual-hormone secretion mechanism is partially responsible for the metabolic adaptation of pregnancy in which glucose is spared for the fetus by the maternal endocrine pancreas. Compared to the non-pregnant state, in response to a glucose load, there is a greater release of insulin from the β-cells and a greater suppression of glucagon release from the α-cells. Associated with the increased release of insulin, the maternal pancreas undergoes β-cell hyperplasia and islet-cell hypertrophy, with an accompanying increase in blood flow to the endocrine pancreas. During pregnancy, when fasting blood glucose levels fall, they rise to a greater extent in response to a glucose load than do levels in non-pregnant women. The increased release of insulin is related to insulin resistance due to hPL, which spares transfer of glucose to the fetus. Glucagon levels are also suppressed in response to a glucose load, with the greatest suppression occurring near term.

REGULATION OF FETO-MATERNAL STEROIDOGENESIS

Using in vitro investigations utilizing placental tissue explants as well as in vivo, catheterized primate models to study steroidogenic regulation in pregnancy, researchers have determined LDL-cholesterol, fetal pituitary hormones, intra-placental regulators, and intra-adrenal regulators act as the primary modulators of feto-placental steroid production (252-254).

Regulation by Low Density Lipoprotein Cholesterol (LDL)

A limiting factor in adrenal steroid output is the availability of LDL-cholesterol, the primary lipoprotein used in fetal adrenal steroid synthesis (Figure 16). Circulating LDL-cholesterol accounts for 50-70% of the cholesterol utilized for fetal adrenal steroidogenesis (255-257). The fetal adrenal is known to contain high affinity, low-capacity LDL binding sites. The presence of ACTH increases this binding capacity (256, 258, 259). Within the adrenal gland, hydrolysis of LDL makes cholesterol available for conversion to steroids. The majority of fetal LDL-cholesterol is made, de novo, in the fetal liver (260). In addition, cortisol from the fetal adrenal cortex and estradiol (aromatized from fetal DHEAS) augment this de novo synthesis within the fetal liver. These systems interact in a manner that is linked, self-perpetuating, and serves to increase steroid production to meet the needs of the maturing fetus (260).

Figure 16. Shown are the maternal, placental and fetal compartments for estrogen and progesterone synthesis in human pregnancy. The fetal adrenal gland lacks 3β-hydroxysteroid dehydrogenase, but has sulfation and 16α-hydroxylase capabilities. Likewise, the placenta lacks 17α-hydroxylase activity but contains sulfatase in order to cleave the sulfated fetal products. Modified from (261), with permission)

Regulation by Fetal Pituitary Hormones  

Fetal ACTH regulates steroidogenesis in both adrenal zones. Adrenocorticotropic hormone receptor activity is diminished in the fetal zone of the cortex during the early second trimester when other factors, such as hCG, are more important in the maintenance of this zone (260). In vitro studies in human fetal adrenal tissue, demonstrate that ACTH stimulates the release of D5 pregnenolone sulfate and DHEAS, whereas in adult adrenal cortex secretes only cortisol when stimulated by ACTH (260). Moreover, ACTH can act on its own adrenal-cell membrane receptor to express a direct stimulatory effect on steroidogenic enzymes (260).

Adrenocorticotropic hormone extracted from the human fetal pituitary gland has been shown, in vitro, to stimulate the production of DHEAS and cortisol (262, 263). Interestingly, concentrations of ACTH throughout gestation do not correlate with the increasing mass of the fetal adrenal cortex or the increasing steroidogenic function that are hallmarks of the third trimester (259). Fetal pituitary ACTH is detectable by 9 weeks’ gestation (263, 264). Thereafter, levels of ACTH increase steadily until 20 weeks’ gestation. The levels remain stable until approximately 34 weeks, when a significant decline is initiated and persists until term (259).

Prolactin may act as a co-regulator, along with ACTH, hCG and certain growth factors, in fetal adrenal steroid production (265, 266). Both in vitro and in vivo, prolactin augments ACTH-stimulated adrenal androgen production (253). Fetal pituitary prolactin is detectable at 10 weeks’ gestation (264). Umbilical cord prolactin levels increase with advancing gestational age and rise in parallel with increased fetal adrenal mass (267).

Regulation by Intra-Placental Mechanisms  

The placenta is an important co-regulator of the fetal adrenal zone due its ability to secrete hCG, placental CRH, progesterone and estradiol (233). In vitro and in vivo, hCG receptor activity is present in the fetal zone, and hCG stimulates fetal adrenal production of DHEAS (233, 268). However, after the 20th week of gestation ACTH primarily influences the fetal zone of the adrenal, and at this time hCG plays only a minor role. Placental CRH, acts in a paracrine relationship with placental ACTH, to complement the actions of the fetal hypothalamus and pituitary in producing the surge in fetal glucocorticoids notable in the late third trimester as fetal growth and maturity become increasingly important (125, 269).

Placental progesterone inhibits D5 to D4 steroid transformations in the fetal zone of the adrenal (101, 270). This effect is another explanation for fetal adrenal 3β-HSD deficiency. Placental estradiol modifies the production and metabolism of corticosteroids and progesterone. In vivo, the placenta regulates the inter-conversion of maternal cortisol to cortisone, and the fetal pituitary production of ACTH (264, 269). Modulation of the transfer of maternal cortisol across the placenta, into the fetus, is the primary mechanism through which this effect occurs.

Regulation by Intra-Adrenal Mechanisms  

With advancing gestational age, the fetal adrenal becomes more sensitive to circulating ACTH (253). Between 32 and 36 weeks’ gestation, the fetal adrenal mass increases (271-273). Blood flow to the fetal adrenal is affected by many factors that, in turn, affect the exposure of the fetal adrenal receptors of the different trophic stimuli. Growth factors modulate adrenal steroid pathways just as they do in the adult adrenal cortex. The fetal adrenal produces IGF-I and IGF-II; ACTH originating from either the fetal pituitary or the placenta can stimulate production of their respective mRNAs (274, 275).

PARTURITION

Parturition is a coordinated process of transition from a quiescent myometrium to an active rhythmically contractile state requiring elegant interplay between placental, fetal and maternal compartments. Though fetal maturity does not always predate the onset of labor, the two processes are related in primates. The timing of birth is a crucial determinant of perinatal outcome. Both preterm birth (<37wk) and post-term pregnancy (>42 wk) are associated with greater risk of adverse perinatal outcomes. The traditional dogma, supported by robust evidence from animal studies, has the fetoplacental unit as being in charge of the timing of parturition (276). While this is certainly true in some species, the presence of such a “placental clock” is not established in humans. Rather, it has become clear that the maternal endometrium/decidua also plays an important role in triggering the cascade of event leading to parturition (277).  

The precise mechanisms involved in human parturition are thought to involve CRH, functional progesterone withdrawal, increased estrogen bioavailability, and finally, increased responsiveness of the myometrium to prostaglandins and oxytocin. There is no simple chain of events as there are in other species.

Numerous lines of evidence support a role for CRH in human parturition. Studies have demonstrated increased CRH and decreased CRH-binding protein levels prior to the onset of both term and preterm labor (278, 279). CRH directly stimulates release of prostaglandins in decidua and myometrium (280). Interestingly, a paradoxical augmentation of placental CRH release by serum cortisol is maximal in the last ten weeks of pregnancy. This may be a function of cortisol competition with progesterone for placental glucocorticoid receptors, thereby blocking the inhibitory action of progesterone on CRH synthesis (281).

The ratios of estradiol and progesterone in various animal models are closely related to the stimulation of myometrial gap-junction formation (282). With decreasing progesterone relative to estradiol, gap junctions permit cell-cell communication for the synchronized myometrial smooth muscle contractions required for labor. Progesterone and the estrogens are antagonistic in the parturition process. Progesterone produces uterine relaxation, stabilizing lysosomal membranes and inhibiting prostaglandin synthesis and release. By contrast, estrogens destabilize lysosomal membranes and augment the synthesis of prostaglandin and their release (283). Although gradual increase in umbilical cord DHEAS and maternal estriol occurs toward term, there is no corresponding drop in either fetal or maternal progesterone concentrations (284).

Though a reduction in maternal or fetal progesterone levels during spontaneous labor has not been documented, functional progesterone withdrawal at the receptor level is believed to be involved in the process of parturition. This may occur via altered progesterone receptor isoform PR-A/PR-B levels in myometrium (285). Undoubtedly, progesterone is important in uterine quiescence because in the first trimester removal of the corpus luteum leads rapidly to myometrial contractions (84). Likewise, labor ensues following the administration of progesterone receptor antagonists in the third trimester (286). The anti-progesterone agents occupy progesterone receptors and inhibit the action of progesterone, which is clearly essential for maintenance of uterine quiescence. Consistent with these findings, pharmacologic treatment of women at risk for preterm labor with progesterone or synthetic progestational agents has demonstrated efficacy in the prevention of preterm labor (287-289).

A role for estrogen in the process of parturition is supported by the finding that pregnancies are often prolonged when estrogen levels in maternal blood and urine are low, as in placental sulfatase deficiency or when associated with anencephaly (290). In human studies, there is a correlation in uterine activity with circulating maternal estrogens and progesterone as labor approaches (291-293). Feto-placental estrogens are closely linked to myometrial irritability, contractility, and labor. In primates, estrogens ripen the cervix, initiate uterine activity, and established labor (294). Estrogens also increase the sensitivity of the myometrium to oxytocin by augmenting prostaglandin biosynthesis (283, 295). Because placental release of estrogens is linked to the fetal hypothalamus, pituitary, adrenals, and placenta the fetal pituitary adrenal axis appears to fine-tune parturition timing in part through its effect on estrogen production.

Prostaglandins (PG) are thought to play a central role in human parturition. For years, it has been known that rupture, stripping, or infection of the fetal membranes, as well as instillation of hypertonic solutions into the amniotic fluid results in the onset of labor. These facts have led to the hypothesis that a fetal-amniotic fluid-fetal membrane complex is a metabolically active unit that triggers the onset of labor. Evidence supporting a causative role of prostaglandins in the labor process is present since PGs induce myometrial contractions in all stages of gestation. While there is still no direct evidence relating endogenous PGs to labor,  there are several lines of evidence implicating PGs in this process; PG levels increase in maternal circulation and amniotic fluid in association with labor; indomethacin prevents the onset of labor in nonhuman primates and stops preterm labor in humans; stimuli that are known to induce labor (e.g. cervical ripening, rupture of membranes) are associated with PG release; the process of cervical ripening is mediated by PGs. Important to this hypothesis is the understanding that at least one mechanism in the onset of parturition is the release of stored precursors of PGs from the fetal membranes.

The major precursor for PGs is arachidonic acid, which is stored in glycerophospholipids. The fetal membranes are enriched with two major glycerophospholipids, phosphatidylinositol and phosphatidylethanolamine. As gestation advances, the progressively increasing levels of estrogen stimulate the storage, in fetal membranes, of these glycerophospholipids containing arachidonic acid.

A series of fetal membrane lipases, including phospholipase A2 and Phospholipase C control the release of arachidonic acid from storage in fetal membrane phospholipids. Once in a free state, arachidonic acid is available for conversion to PG. Additional factors that augment and accentuate the normal process of labor include the liberation of corticosteroid by the mother and fetus, resulting in a decrease in the production of myometrial prostacyclin, a smooth muscle relaxant.

Active labor is characterized by a dramatic increase in the number of oxytocin receptors in the myometrium. Once begun, the process appears to be self-perpetuating. The level of maternal catecholamines increases, resulting in the liberation of free fatty acids, including arachidonic acid; there is also an increase in the level of maternal or fetal cortisol, which decreases the production of uterine smooth muscle prostacyclin. It is unlikely that oxytocin is the initiator of labor despite the fact that oxytocin receptors are present in the myometrium and increase before labor, and it stimulates decidual prostaglandin E2 and prostaglandin F2a production. There is firm evidence of increasing, rhythmical fetal adrenal and placental steroid output over the 5 weeks just before term that is important in preparing human pregnancy for the final cascade of oxytocin and prostaglandins that stimulate labor (283, 291-293, 295, 296).

KEY POINTS

  • Synchrony between the development of the early embryo and establishment of a receptive endometrium is necessary to allow implantation and subsequent progression of pregnancy.
  • The placenta is a unique, dynamic organ with the inherent ability to produce, regulate, and inhibit factors that directly affect fetal growth and development.
  • During the luteal-placental transition period, between 6-10 weeks of gestation, corpus luteal function and progesterone production naturally declines and shifts to the developing placenta.
  • Steroidogenesis in pregnancy is characterized by enzymatic deficiencies within the placental and fetal compartments which foster interdependent transfer of precursors among compartments for the synthesis of steroid hormones. This process is modulated by LDL-cholesterol, fetal pituitary hormones, intra-placental regulators, and intra-adrenal regulators.
  • Redundancy in protein hormone – receptor interactions such as hPL and hPGH serve to ensure that adequate nutrition is supplied to the developing fetus.
  • A relatively insulin resistant state is generated within the maternal compartment to supply glucose and free fatty acids for fetal nutrition.
  • Human parturition exemplifies the interplay between placental, fetal, and maternal compartments, characterized by increased estrogen bioavailability, functional progesterone withdrawal, increased CRH synthesis and release, culminating in increased responsiveness of the myometrium to prostaglandins and oxytocin.

ACKNOWLEDGMENT

In addition to the journal and text references listed above, the following sources were used in the preparation of this chapter:

Taylor HS, Pal L, Seli E (eds.). Speroff’s Clinical Gynecologic Endocrinolofy & Infertility. Ninth edition, 2020. Wolters-Kluwer, Philadelphia.

Gabbe SG, Niebyl JR, Simpson JL [eds.]. Obstetrics: normal and problems pregnancies. Fifth edition, 2007. Churchill-Livingstone, New York.
Benirschke K, Kaufmann P, Baergen RN [eds.]. Pathology of the human placenta. Fifth edition, 2006. Springer, New York.Strauss JF,
Barbieri RL [eds.]. Yen and Jaffe’s Reproductive endocrinology: physiology, pathophysiology and clinical management. Fifth edition, 2004. Elsevier Saunders, Philadelphia.
Reece EA, Hobbins JC [eds.]. Clinical obstetrics: the fetus and mother. Third edition, 2007. Wiley-Blackwell, Malden, MA.

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Non-Diabetic Hypoglycemia

ABSTRACT

 

Objective: To review the diagnosis, evaluation, and management of non-diabetic hypoglycemia in adults. Methods: A literature review using PubMed and Google Scholar was performed. In absence of data, clinical expert opinion was provided. Results: Hypoglycemia in an individual without diabetes is uncommon mainly because of a tightly regulated counterregulatory physiological response. A detailed medical history, review of medications and physical exam findings are critical first steps in providing guidance for further investigation in a non-diabetic person with documented hypoglycemia based on Whipple’s triad (presence of symptoms when plasma glucose concentrations are low and absence of symptoms with normalized glycemia). In this review, we highlight strategies to diagnose and treat hypoglycemic disorders in non-diabetic individuals based on underlying mechanisms. Conclusion: Evaluation and management of non-diabetic hypoglycemia should be individualized based on clinical presentation and suspected diagnoses.

 

INTRODUCTION

 

In healthy humans, glucose concentrations are efficiently maintained within a narrow range by the physiological mechanisms that respond to intermittent exogenous nutrient ingestion by enhancing glucose utilization and respond to intervals of nutrient deprivation by enhancing glucose production. Deviation of glucose from the normal range in both hyper or hypoglycemia only occurs when physiological mechanisms involved in maintaining the balance between the glucose utilization and the glucose production fail. As such, hypoglycemia is a manifestation of a heterogeneous group of underlying disorders that increase glucose utilization or reduce glucose production or a combination of both.

 

While hypoglycemia in persons without diabetes is relatively rare (1), the clinical relevance of this condition regarding patient safety, cognitive function, and quality of life is undeniable. Depending on severity and duration, hypoglycemia also can be fatal (2). Furthermore, hypoglycemia blunts defense against subsequent hypoglycemia leading to a vicious cycle of recurrent hypoglycemia (3), which in turn not only exaggerates related morbidities but also makes the diagnosis more complex.

 

Here, we review the current recommendations regarding diagnosis, pathophysiology, and management of hypoglycemia in non-diabetic individuals. Hypoglycemia in the pediatric population, hypoglycemia caused by anti-diabetic medications, and a comprehensive review of insulinomas can be found in the Endotext chapters entitled “Hypoglycemia in Neonates, Infants, and Children” (4), “Hypoglycemia During Therapy of Diabetes” (5) and “Insulinoma” (6), respectively.

 

PHYSIOLOGY / PATHOPHYSIOLOGY

 

Glucose is the main fuel for the brain since it cannot store glycogen or synthesize glucose (7,8). To minimize any disturbance in glucose supplies to the brain and cerebral function, therefore, redundant but very efficient physiological counter-regulatory responses are in place to prevent or correct hypoglycemia (8-10).

 

Prandial

 

In the prandial condition, the extent of glycemic excursion (difference between glycemic peak and nadir concentrations) is determined by the pace of food transition from the stomach into the gut (gastric emptying) as well as the net hepatic and extrahepatic glucose uptake (11). In healthy subjects, during the first 30-60 min of an oral glucose or mixed tolerance meal test, plasma glucose and insulin concentrations rise, shifting the hepatic net glucose output during fasting condition to net glucose uptake during the prandial state (12). As a result of changes in glucose kinetics in the early absorptive phase of glucose/mixed meal ingestion, plasma glucose concentrations start to decline in the latter absorptive phase falling below premeal levels in parallel with reduction in ingested glucose delivery to the gut. The regulatory mechanisms that are responsible for preventing hypoglycemia and restoring euglycemia during the transition of glucose flux from the ingested glucose delivery to the hepatic glucose production are not fully characterized, but both hormonal and non-hormonal factors play a role (13).

 

Fasting

 

In the fasting or postabsorptive state (4-6 hours after nutrient ingestion), plasma glucose  concentrations range from 80-90 mg/dl and rates of glucose utilization and production are equal (2mg/kg/min) (14). Glucose homeostasis during fasting is tightly regulated by a reciprocal bihormonal response, in which reduction in glucose concentrations below baseline reduces β-cell insulin secretion and stimulates α-cell glucagon release (15). While the full range of glucoregulatory effects of glucagon in the prandial state is unclear, the main function of glucagon in the fasting state is to counterbalance the action of insulin on hepatic glucose production (15-17). Glucose production is mainly (~80%) attributed to hepatic glycogenolysis with a smaller contribution (~20%) from hepatic gluconeogenesis primarily from amino acids and lactate (11). After an overnight fast, the liver contains ~ 50 g of glycogen storage, which can supply glucose for 24 hours after complete depletion. With prolonged fasting, hyperglucagonemia enhances gluconeogenesis and hypoinsulinemia promotes lipolysis.  Lipolysis releases glycerol, a gluconeogenic substrate, and free fatty acids that are converted to ketones, mainly beta hydroxybutyrate (BOHB) and acetoacetate, in the liver to be used as an alternate fuel by the brain.

 

Using a hyperinsulinemic hypoglycemic clamp in the fasting condition in normal humans, a decrement in plasma glucose concentration from the physiological range, in a hierarchical manner: (1) suppresses endogenous insulin secretion to lower glucose utilization, (2) increases glucagon response to increase glucose production, and (3) enhances epinephrine secretion (more relevant in absence of glucagon secretion) as well as cortisol and growth hormone release (more relevant during prolonged hypoglycemia) (8). Under physiological conditions during the fasting state, this counterregulatory response can reestablish euglycemia and prevent symptoms. A greater decline in plasma glucose concentration, though, would result in a symptomatic autonomic response to warn of low glucose and prompt the person to correct it by eating. However, if the individual did not (or could not) intervene, such as in the presence of overwhelming hyperinsulinemia) or blunted glucose counterregulatory responses, plasma glucose concentrations would drop further and neuroglycopenic symptoms and cognitive dysfunction would occur (Table 1) (18).

 

Table 1. Symptoms of Hypoglycemia

Autonomic (neurogenic)

Neuroglycopenic

Sweating

Anxiety

Tremor

Palpitation

Hunger

Tingling

Ill-defined symptoms

Warmth

Behavioral changes

Blurred vision

Confusion/difficulty speaking

Dizziness/lightheadedness

Lethargy and weakness

Seizure

Loss of consciousness/coma

 

It is unclear whether this hierarchy in hormonal responses or glycemic thresholds described during the fasting state would also apply to the prandial state.  Here, we classify the hypoglycemic disorders based on timing from meal ingestion (fasting versus prandial) given the differences in the regulatory factors involved in glucose metabolism between the two conditions.

 

DIAGNOSIS AND EVALUATION

 

Diagnosis of hypoglycemia should be made when symptoms accompany low plasma glucose concentrations (chemical hypoglycemia) but symptoms are absent when plasma glucose levels are normalized (Whipple’s triad (19)).

 

Chemical hypoglycemia has been defined based on a glucose threshold that can evoke a counterregulatory response. In the fasting state, a decline in glucose below 55 mg/dl causes neurogenic symptoms while insulin secretion is maximally suppressed (insulin < 3 uU/ml and C-peptide < 0.6 ng/ml) and glucagon response is maximized (9).

 

Awareness of hypoglycemia alerting individuals to correct hypoglycemia is mainly because of increased autonomic nervous system activity triggered by hypoglycemia. The autonomic (neurogenic) hypoglycemic symptoms can be adrenergic (such as palpitation, tremor, and anxiety) or cholinergic (such as sweating, hunger, and paresthesia) (18). As glucose concentrations drop below 48-50 mg/dl during the fasting state, the neuroglycopenic symptoms (caused by brain glucose deprivation) manifest; these symptoms range from behavioral changes, fatigue, and confusion to loss of consciousness or seizure (18,20-22).

 

Diagnosis of hypoglycemia in the prandial state is much more complex since the glycemic threshold to define hypoglycemia in the prandial state has not been well characterized. Using an oral glucose challenge in 650 healthy individuals in a previous study (23), 10% of subjects developed postprandial nadir glucose concentrations below 47 mg/dl without associated symptoms, suggesting that asymptomatic low glucose events are relatively common following an oral glucose load in normal humans. Hence, the mixed meal test (described below) is the preferred provocative test to diagnose prandial hypoglycemia. Furthermore, recurrent postprandial symptoms suggestive of hypoglycemia but not associated with low glucose concentrations have also been observed in normal individuals (24), indicating that other factors beyond hypoglycemia play a role in provoking autonomic symptoms.

 

Therefore, after obtaining a detailed medical history and physical exam, diagnosis of hypoglycemia should be confirmed by verification of low glucose concentration associated with symptoms or signs that are relieved by raising glucose values (Whipple’s triad).

 

A careful history of nutritional status, current medication use, and concurrent multisystem illnesses such as liver, heart, kidney failure, or sepsis, as well as a thorough physical exam and laboratory data, can point to existing primary conditions that predispose to hypoglycemia. This is especially crucial in patients who are often too ill to be subjected to extensive evaluation. For healthy subjects who lack any background predisposing illnesses, the details about timing (relationship to food ingestion, physical activities, day versus nocturnal time), severity (frequency, presence of neuroglycopenia, and requiring assistance to treat), and time of onset of hypoglycemic episodes are critical in differential diagnosis. To understand the pattern of hypoglycemic episodes, reviewing the records of symptoms, activity, food intake along with capillary blood or interstitial glucose levels measured by glucometer or continuous glucose monitoring (CGM), respectively, may be helpful. However, the accuracy of glucometer and CGM is low in the hypoglycemic range, and they should not be used for diagnostic purposes.  On the other hand, masked (blinded) monitoring by CGM can provide insights into patterns of hypoglycemic episodes and triggering factors during patients’ daily routine (25).

 

The flowchart in Figure 1 demonstrates the suggested approach for evaluation in healthy appearing patients after a careful medical history, physical exam, and laboratory data excludes an underlying illness that can predispose to hypoglycemia. Disorders that may cause hypoglycemia are listed in Table 2.

 

To confirm the diagnosis and explore etiology, it is necessary to collect blood samples during hypoglycemia, whether it occurs spontaneously or by provoked testing that can be selected based on clues from the medical history.

 

In asymptomatic patients with documented chemical hypoglycemia, artifactual hypoglycemia due to conditions such as reticulocytosis (polycythemia, sickle cell anemia), leukocytosis (leukemia), and thrombocytosis that increase in vitro glycolysis in the blood sample while awaiting laboratory analysis should be considered (26).  Also, nadir glucose levels in the prandial state can be low without any associated symptoms, particularly in persons with a history of upper gastrointestinal (GI) surgery (25). A potential diagnostic challenge in using clinical criteria remains in patients who are adapted to recurrent hypoglycemia by blunted autonomic response, so called hypoglycemic unawareness (27).  It has been well recognized that antecedent insulin-induced hypoglycemia impairs counterregulatory glucose responses and blunts hypoglycemia symptoms (mainly autonomic symptoms) in normal humans (28). Therefore, in patients with a high index of clinical suspicion, monitoring of symptoms and signs of neuroglycopenia, which is less likely to be affected by recurrent hypoglycemia, and reevaluation over time should be considered.

 

Figure 1. Evaluation of non-diabetic hypoglycemia in healthy appearing adults.

 

 

Table 2. Causes of Hypoglycemia

Artifactual Hypoglycemia (without symptoms)

Reticulocytosis (polycythemia, sickle cell anemia)

Leukocytosis (leukemia)

Thrombocytosis

Fasting Hypoglycemia (> 5 hour from the last meal)

High Insulin, Low beta-hydroxy butyrate, High glucagon response

Insulinoma

Auto immune syndrome (antibodies to insulin or the insulin receptor)

Factitial due to exogenous insulin 

Factitial due to insulin secretagogues

Induced by non-diabetic medications

Low Insulin, High beta-hydroxy butyrate, Low glucagon response

Ketotic hypoglycemia

Prolonged exercise

Alcohol induced

Glycogen storage diseases

Post Prandial Hypoglycemia (within 5 hours from the last meal)

Bariatric surgery

Nesidioblastosis

Hereditary fructose intolerance

Associated with Other Disorder

Critical illness (liver failure, congestive heart failure, sepsis, renal failure, etc.)

Malnutrition

Adrenal insufficiency

Non-islet cell tumors

 

Fasting Hypoglycemia

 

In patients with concern for fasting hypoglycemia, confirmation of Whipple’s triad and exploration of the cause is recommended during an episode of spontaneous hypoglycemia or with a supervised fast of up to 72 hours (9)(Figure.1).  During the fast, patients can consume non-caloric caffeine-free beverages with all non-essential medications discontinued.  Plasma glucose, insulin, c-peptide, and BOHB are collected every 6 hours until plasma glucose is < 60 mg/dL; at that time, frequency of blood collection should be increased to every 1-2 hours. The fast is terminated after collecting the last blood sample when the plasma glucose is < 45 mg/dL and the patient has signs and/or symptoms of hypoglycemia or if the patient has not exhibited symptoms after 72 hours have elapsed.  Alternatively, the fast can be terminated when plasma glucose  is < 55 mg/dL in men and < 35 mg/dl in women, given the sex differences in abnormal fasting glycemic concentrations (29), without signs/symptoms if Whipple’s triad was documented previously but blood samples were not collected (9). At the end of the fast, glucose response to 1 mg of glucagon IV bolus injection will be measured every 10 minutes for a 30-minute period and then the patient is fed. Insulin antibodies from baseline blood samples as well as hypoglycemic anti-diabetic medications (sulfonylureas and meglitinides) screening from baseline blood and urine samples are also collected. After confirmation of diagnosis, the results of the fasting test will help to differentiate hypoglycemia mediated by insulin- versus non-insulin factors.  

 

INSULIN-DEPENDENT HYPOGLYCEMIA (HIGH PLASMA INSULIN CONCENTRATION)

 

C-peptide is secreted from β-cells at an equimolar ratio to insulin (30). Approximately half of the insulin which is secreted into the portal vein is removed by the liver (31). Therefore, plasma insulin concentration reflects not only insulin secretion or exogenous insulin administration, but also hepatic insulin degradation. In contrast to insulin, c-peptide undergoes minimal extraction by the liver and other organs (32), therefore c-peptide concentration represents endogenous insulin secretion (32,33). It has been well documented that exogenous insulin administration during euglycemia or hypoglycemia inhibits endogenous insulin secretion (34,35). In fact, this physiological phenomenon has been used to support the diagnosis of insulinoma using hyperinsulinemic hypoglycemic or euglycemic clamp (36,37).

 

Therefore, low plasma concentration of c-peptide in presence of elevated insulin values during hypoglycemia indicates factitial hypoglycemia due to exogenous insulin administration, whereas elevated c-peptide and insulin represents inappropriately greater endogenous insulin secretion due to insulin secretagogues (sulfonylurea or meglitinides), autoimmune syndromes (insulin antibody syndrome or type B insulin resistance) or an insulin-producing tumor (insulinoma).

 

Further, hypoglycemia induced by hyperinsulinemia is associated with low BOHB and glycemic response >25 mg/dl to glucagon injection. Insulin increases fatty acid synthesis and esterification and decreases fatty acid oxidation and ketogenesis in the liver, leading to lower plasma concentrations of BOHB. Enhanced insulin signaling in the liver activated by hyperinsulinemia or any non-insulin ligands, such as insulin-like growth factor—2 (IGF-2), also results in higher glycogen storage, hence a larger glycemic response to glucagon injection.

 

Factitial Hypoglycemia

 

Factitial hypoglycemia due to exogenous insulin (high insulin and low c-peptide) or insulin secretagogues medications (high insulin and c-peptide) remains a diagnostic challenge and often leads to extensive and costly investigation to rule out other causes because of limitations in biochemical assays (38), as well as patients’ denial of medication misuse. Factitial hypoglycemia is observed more often in patients who work in the medical health care system, have relatives with diabetes living in the same household, and those with underlying mental illness such as major depression. Therefore, obtaining detailed information regarding the patient’s medication list including herbal preparations that can be contaminated with sulfonylurea as well as family history is essential (39).

 

Hypoglycemia due to exogenous insulin is characterized by elevated plasma insulin, suppressed c-peptide and low BOHB, as well as an increase in plasma glucose > 25mg/dL after glucagon challenge (9).  Insulin antibodies may also be positive (40). 

 

Insulin-induced hypoglycemia due to insulin secretagogues (sulfonylurea or meglitinide) has a similar biochemical profile except that the c-peptide is elevated.  Therefore, the only way to differentiate anti-diabetic factitial hypoglycemia from insulinoma is by detecting the drug in blood or urine.

 

In a single-center retrospective study, factitious hypoglycemia accounted for 11 of 70 (16%) of admissions for evaluation of hypoglycemia. (41).  Prognosis is poor based on a small-size study, in which only 30% of affected patients during several years of follow-up recovered (42). Treatment requires a multisystemic treatment team led by a psychiatrist (42).

 

Autoimmune Syndromes

 

Autoimmune syndromes are a rare cause of hypoglycemia characterized by high concentrations of insulin autoantibodies (insulin autoimmune syndrome [IAS]) or anti-insulin receptor antibodies (type B insulin resistance) (43,44). While IAS is the third leading cause of hypoglycemia in Japan, it is very uncommon in the non-Asian population; type B insulin resistance is even less common (43). 

 

Insulin autoantibodies (IAAs) are mainly immunoglobulins (Ig) directed against endogenously released insulin in response to nutrient ingestion with a high binding capacity but low affinity to insulin. As a result, patients may manifest hyperglycemia in the early absorptive phase of meal or oral glucose intake, when exogenous glucose appearance into circulation is maximal, followed by hypoglycemia in a few hours during the late prandial condition or postabsorptive state. Hypoglycemia, in IAS, is caused by the binding and release of insulin from the antigen-antibody complex independently of changes in glucose concentrations. Therefore, insulin and c-peptide are both elevated at the time of low plasma glucose concentrations.  IAAs are different from insulin antibodies produced against exogenous insulin that are generally low binding and high affinity, thus, unable to cause hypoglycemia.

 

In non-Asian patients, IAS is mainly reported in individuals with autoimmune (lupus, rheumatoid arthritis) /hematological diseases (multiple myeloma, benign monoclonal gammopathy), who are exposed to triggering factors, such as medications (captopril, propylthiouracil, penicillin G) and viral infections (measles, mumps, rubella, varicella zoster, coxsackie B, and hepatitis C) (43,44).  In Japanese patients, IAS is commonly associated with exposure to medications with a sulphydryl group (methimazole) (43). The prognosis is relatively good with self-remission reported in 82% of patients (44). Treatment is often dietary modification (small, frequent low-carbohydrate meals and uncooked cornstarch) and occasionally requires medications to decrease insulin secretion (somatostatin analogues, diazoxide) or immunosuppressants (high -dose corticosteroids, azathioprine, rituximab) (43,44).

 

Type B insulin resistance is caused by anti-insulin receptor antibodies. Affected patients tend to be middle-aged women of Black race with obesity, acanthosis nigricans, and hyperandrogenism.  Co-occurrence of systemic autoimmune disease (i.e., lupus) is common (43). Patients typically present with hyperglycemia; however, a subset of patients (8 of 34 patients in an NIH cohort) experience fasting or postprandial hypoglycemia after period of hyperglycemia or without a history of hyperglycemia (43). The autoantibodies are believed to be partial agonists for the insulin receptor. Hyperglycemia or hypoglycemia ensues depending on the antibody titer: high titers antagonize the receptor, resulting in hyperglycemia and high insulin and c-peptide levels to compensate for the resistance; low titers activate the receptor, leading to hypoglycemia. There are diagnostic challenges as immunoprecipitation, the gold standard method to detect insulin receptor autoantibodies, is generally not commercially available (45).  Unlike IAS, prognosis of type B insulin resistance is poor with high mortality especially in patients that transition from a hyperglycemic to hypoglycemic phase.  Deaths are related to hypoglycemia and other causes (lupus, renal failure, cancer, cardiovascular events).  Therapy can include immunosuppressants, but response is variable or poor (43).

 

Insulinoma

 

Neuroendocrine insulin-producing tumors (insulinoma) are relatively rare with an estimated incidence of ~ 1 – 4 new cases per million people/year (46). Less than 10% of insulinomas are malignant, 10% multiple, and 4% associated with multiple endocrine neoplasia type 1 (MEN-1) syndrome (47).  They primarily manifest in the 5th decade of life and are slightly more common in females (48).  Typically, patients experience episodes of hypoglycemia fasting or after exercise, but some individuals may experience fasting and prandial hypoglycemia (49).  Less frequently (6% of 237 patients in a Mayo Clinic cohort), patients present solely with prandial hypoglycemia (50).  Diagnosis relies on biochemical testing. The 72-hour fast is usually successful in capturing hypoglycemia in patients with insulinoma, as 65% of patients will experience hypoglycemia within 24 hours, 93% within 48 hours, and 99% within 72-hours (51).

 

Individuals with confirmed fasting (or postprandial) hyperinsulinemic hypoglycemia, negative screening for oral hypoglycemic medications, and negative insulin autoantibody testing should undergo diagnostic tests to locate the insulinoma prior to surgery (9).  Non-invasive imaging can include transabdominal and endoscopic ultrasonography, abdominal computed tomography (CT), and magnetic resonance imaging (MRI) (52). CT has been shown to detect ~70-80% of tumors and MRI 85% (47). Given that insulinomas tend to be small in size (< 1 cm in diameter in 40% of diagnosed cases) (47), negative imaging does not rule out the diagnosis (9). If non-invasive imaging cannot determine preoperative localization, selective arterial calcium stimulation with hepatic venous sampling can been utilized (53). Calcium is injected into arteries supplying the pancreas, which stimulates insulin secretion from insulinomas; a ≥ 2-fold increase in insulin concentrations from baseline localizes the site of an insulinoma with a > 90% sensitivity (54,55).

 

Treatment is surgical enucleation of the tumor, performed more commonly via open surgery than laparoscopic (48). If not localized prior to surgery, intraoperative palpation by an experienced surgeon coupled with intraoperative ultrasound detects > 80% of tumors (48). Pancreatectomy (distal or central depending on location of tumor) is preferred over enucleation if the insulinoma is large or there is concern for malignancy or metastases. Pancreatic fistula is the most common complication in both open and laparoscopic surgeries.  As noted earlier, the hyperinsulinemic hypoglycemic or euglycemic clamp has also be utilized to differentiate insulinoma from other causes when conventional evaluation (prolonged fasting, imaging) were equivocal (37) or when surgery failed to detect an insulinoma but clinical suspicion was high (56).

 

Benign insulinomas have a high 5yr survival rate of 95-100% post-resection with relapse frequency 6% at 10yr and 8% at 20 yrs (48). Malignant insulinomas carry a poor prognosis with median life expectancy of 2 years (48). Inoperable cases may benefit from medical management such as diazoxide, streptozocin, verapamil, and phenytoin by reducing insulin secretion and corticosteroids by diminishing insulin action (48,57-59). Non-surgical procedures such as CT guided radiofrequency ablation, US-guided ablation with ethanol, peptide receptor radionuclide therapy, and robotic radiosurgery have also been utilized (48).

 

Non-Diabetic Medications

 

Non-diabetic medications such as quinolones, non-steroidal anti-inflammatory drugs, antipsychotics, and α and β blockers have been implicated in inducing hypoglycemia by stimulating insulin secretion (60).  Other medications, such as pentamidine, damage the β cells, resulting in transient hypoglycemia induced by cytolytic insulin release (60).  Risk factors related to hypoglycemia include higher medication doses, concomitant renal failure, older age, and poor nutrition  (60).  Treatment may include discontinuation of the medication and supportive care (60).  

 

INSULIN-INDEPENDENT HYPOGLYCEMIA (LOW PLASMA INSULIN CONCENTRATION)

 

In this group, plasma BOHB is elevated and glucose response to glucagon is small. In patients who appear healthy the following conditions should be considered and ruled out: 

 

Ketotic Hypoglycemia

 

Ketotic hypoglycemia is a relatively rare condition that can occur during extended periods of carbohydrate deprivation, as during fasting or starvation. Prolonged adherence to a ketogenic diet, which severely restricts carbohydrate intake to 20-50g/day to promote weight loss and increase plasma ketone bodies (61), combined with other factors interfering with counterregulatory response, such as alcohol intake, also can result in ketotic hypoglycemia (62). Hypoglycemia in this condition is mainly caused by reduction in hepatic glucose production by hyperketonemia, which outweighs the diminished glucose utilization (63).  Management is largely supportive depending on the severity of the presentation.

 

Prolonged Exercise

 

Prolonged exercise can result in plasma glucose to decline to the hypoglycemic range in 30-40% of healthy subjects, but these events are not associated with symptoms (Whipple’s triad) nor incompatible with continued exercise (64). While the mechanisms for lack of symptoms is not completely understood, studies in non-diabetic dogs using a hypoglycemic clamp with and without exercise have shown that (a) counterregulatory hormonal responses to exercise and hypoglycemia combined are greater than the response to either conditions alone, and (b) larger insulin action during exercise are negated by the counterregulatory response during hypoglycemia (both enhanced endogenous glucose production and reduced glucose utilization) (65). Thus, disturbed counterregulatory response in prolonged exercise combined with inadequate carbohydrate ingestion may result in clinical hypoglycemia (66).  Management is supportive.

 

Alcohol-Induced Hypoglycemia

 

Hypoglycemia due to alcohol has been attributed to inhibition of gluconeogenesis and blunting of growth hormone response to hypoglycemia (67). Management is mainly supportive depending on the severity and length of hypoglycemia. 

 

Glycogen Storage Diseases

 

Glycogen storage diseases (GSD) are rare genetic disorders that impair the breakdown of glycogen.  Although these diseases are commonly diagnosed in infancy, GSDs Type I (deficiency in glucose-6-phosphatase), III (deficiency in amylo-1,6-glucosidase), and 0 (lack of glycogen synthase) can present in adulthood or continue to persist in adulthood (68).  Adults with Type 1 GSD (most common form, annual incidence 1/100,000) may present with hypoglycemia, lactic acidosis, hyperuricemia, hypertriglyceridemia, and hepatomegaly (68).  Patients with glycogen storage disease are generally managed by frequent feeding with complex carbohydrates and cornstarch to prevent hypoglycemia.  Patients need to be followed long-term by a metabolic specialist.

 

Fatty Acid Oxidation (FAO) Disorders

 

FAO disorders are a rare group of autosomal recessive conditions characterized by impaired breakdown of fatty acids, leading to hypoketotic hypoglycemia and myopathy. FAO disorders also typically manifest in childhood but can continue through adulthood. Prognosis depends on the specific condition and severity. Treatment typically includes avoidance of fasting and high carbohydrate/low fat diets (68). Patients are managed by metabolic specialists. 

 

ASSOCIATED WITH OTHER DISORDERS

 

In ill-appearing patients with hypoglycemia but low insulin concentrations the following conditions should be considered:

 

Critical Illness

 

Critical illness including organ failure such as acute liver failure and congestive heart failure with hepatic congestion have been associated with hypoglycemia, likely due to impaired gluconeogenesis and depletion of hepatic glycogen stores (69,70).  Sepsis-induced hypoglycemia has been appreciated in humans and animal models with depleted glycogen stores, impaired gluconeogenesis, and increased peripheral glucose utilization implicated as contributing factors (71,72).  Hypoglycemia in non-diabetic people with end stage renal disease is attributed to concomitant adrenal insufficiency, certain medications, malnutrition, and infection (73,74).  Management of Ill-appearing individuals due to sepsis or organ failure is mainly treatment of underlying disorders and treatment of severe hypoglycemia by intravenous glucose administration.

 

Addison’s Disease

 

Addison’s disease is a rare disease that results in primary adrenal insufficiency characterized by glucocorticoid deficiency with or without mineralocorticoid deficiency.  Most cases are caused by autoimmune damage to the adrenal cortex by 21-hydroxylase antibodies (75).  Hypoglycemia, although rare, is likely due to cortisol deficiency that interferes with counterregulatory response during times of stress (76).  Addison’s disease should be considered in a hypoglycemic individual with hyperpigmentation, hyponatremia, hyperkalemia and acidosis (75). Further testing, such as ACTH stimulation test, may be warranted to confirm diagnosis (77).  Treatment for patients with primary adrenal insufficiency in adrenal crisis should be initiated by volume replacement and immediate treatment with intravenous or intramuscular hydrocortisone 100 mg followed by 100 mg every 6-8 hours until clinically stable.  Etiology of the precipitating adrenal crisis should be identified and treated (i.e., infection, hemorrhage, etc.) (75).

 

Non-Islet Cell Tumors

 

Nonislet cell tumors are rare mesenchymal and epithelial tumors that can be benign or malignant, are often large (> 10 cm), and clinically apparent.  Hypoglycemia results when the tumor overproduces incompletely processed IGF-2 (9,78-80). Tumors can secrete IGF-2 or its posttranslational precursor “big IGF” (81).  IGF-2 structurally is similar to insulin; high levels of IGF-2 can bind to the insulin receptor and mimic the action of insulin, resulting in hypoglycemia (82). Insulin and c-peptide levels are appropriately suppressed in response to the hypoglycemia.  Management of non-islet cell tumors may include surgery, radiotherapy, chemotherapy and medical therapy with glucocorticoids, GH, or octreotide (9).

 

Prandial Hypoglycemia

 

The glycemic threshold to define hypoglycemia after meal ingestion is unknown. However, symptoms associated with plasma glucose less than 50-55 mg/dL during mixed meal test that is relieved by normalization of glucose has been used to confirm meal-induced hypoglycemia (11,25).  In the prandial state, provocative testing should use a mixed meal containing protein, carbohydrates, and fat and not oral glucose. This is mainly because the oral glucose challenge has low specificity for detecting clinical hypoglycemia by causing asymptomatic low glucose nadirs as well as hypoglycemia symptoms (mainly autonomic) that do not correlate with low glucose concentrations (23,24).

 

Currently meal tests are not standardized as both solid and liquid mixed meals as well as variable carbohydrate content from 45 to 105 grams have been used (83). Regardless of approach, meal studies can increase the risk of inducing hypoglycemia, thus, these tests need to be done under supervision by personnel trained in a safe environment.

 

HYPOGLYCEMIA AFTER BARIATRIC SURGERY

 

Meal-induced hypoglycemia after upper GI tract (gastrectomy and pyloroplasty) (84) or bariatric surgery (85-87) (Roux-en y gastric bypass surgery [RYGB] and sleeve gastrectomy [SG]) are well documented. One in 10 bariatric subjects develop a late-complication of hypoglycemia (88,89), and one in 150 suffer from severe hypoglycemia requiring an emergency room visit or hospitalization (88). Hypoglycemia in this population is postprandial, progressive, often associated with cognitive impairment and occasionally with loss of consciousness or seizures and is only partially responsive to diet modification or available therapeutic options (25,86,90-92). Despite sporadic case reports of postprandial hypoglycemia after SG (85), this condition, in our experience, is less prevalent, and likely to be of lesser severity than RYGB. Severe hypoglycemia after bariatric surgery is debilitating as it compromises patient safety, cognition, and quality of life (both professional and personal). The long-term health outcomes of this debilitating complication are largely unknown. A recent study (93) using a driver simulator has demonstrated that driving performance and cognitive function is impaired following RYGB during prandial hypoglycemia without any changes in perception of symptoms (94,95).

 

Differentiating true hypoglycemia from those with prandial asymptomatic low glucose concentration or prandial symptoms without low glucose levels is more challenging in patients after GI surgery than non-operated individuals because of higher frequency of both conditions after bariatric surgery. Using CGM for 5 days has demonstrated that 70% of non-diabetic subjects after RYGB (n=40) had at least one episode of low interstitial glucose concentration (<55 mg/dl) (94). However, 80% of these low glucose events have been shown to be asymptomatic. Furthermore, it is well documented that a large proportion of bariatric patients experience dumping symptoms (91), which are almost identical with autonomic symptoms of hypoglycemia, but not associated with low glucose concentrations. Therefore, it is critical to document Whipple’s triad (neuroglycopenic rather than autonomic symptoms associated with low glucose) during free-living conditions or using mixed meal test to confirm hypoglycemia in this population.

 

Additional testing should be considered in ill-appearing patients after bariatric surgery to exclude adrenal insufficiency, other critical illnesses, and malnutrition (25). Post bariatric patients who experience fasting hypoglycemia (beyond 5 hours from previous meal ingestion) or hypoglycemia within 6-12 months from surgery should be evaluated for other causes of hypoglycemia such as insulinoma (25).

 

Underlying mechanisms by which rerouted gut after GI surgeries cause hypoglycemia is not completely understood. However, it is well documented that following RYGB, and to a smaller extent after SG, meal ingestion enhances glucose excursion leading to higher glucose peaks and lower nadir glucose concentrations mainly due to faster nutrient emptying from the stomach pouch/stomach to the gut (96) (Figure 2). Increased glucose delivery from the stomach pouch/ tube-like stomach to the gut after bariatric surgery is associated with hyperinsulinemia, which is exaggerated in RYGB patients with hypoglycemia compared to asymptomatic RYGB subjects (92) (Figure 2). Enhanced meal-induced beta-cell secretion in patients with hypoglycemia after RYGB has been attributed to not only a greater beta-cell sensitivity to increasing glycemia in the first absorptive phase, but also a lower insulin suppression during glycemic decline from peak to glucose nadir (87).

 

Prandial hyperinsulinemia after RYGB, particularly in patients with hypoglycemia, has been shown to be associated with greater prandial plasma concentration of glucagon-like peptide 1 (GLP-1), an insulinotropic gut hormone (87,90,97)(Figure 2). These observations hinted towards a key role for GLP-1 signaling beyond glycemic stimuli in meal-stimulated hyperinsulinemia after RYGB. In fact, we and others have shown that blocking the GLP-1 receptor (GLP-1R) corrects post-RYGB hypoglycemia (87,98).

 

Figure 2. The prandial glycemic effects of RYGB are exaggerated in patients with late-complication of hypoglycemia. RYGB enhances prandial glycemic excursion and increases insulin secretion rate (ISR) along with plasma GLP-1 concentrations. Patients with documented hypoglycemia after RYGB have greater insulin and GLP-1 secretin compared to those without. Following RYGB, glucagon response to meal ingestion is enhanced but there is no further increase in response to hypoglycemia. Adapted with permission from Salehi, JCEM, 2014.

 

Despite a larger meal-induced glucagon response after RYGB compared to non-operated individuals, there is no further increase in plasma glucagon concentration during prandial hyperglycemia (Figure 2), suggestive of dysregulated pancreatic α-cell response. In fact, we have shown that patients with RYGB and SG, glucagon response to insulin-induced hypoglycemia is smaller than healthy individuals without GI surgery (99,100). This data is aligned with a report demonstrating that counterregulatory hormonal response (glucagon, cortisol, and catecholamines) to hypoglycemia is significantly reduced after RYGB compared with before surgery (101). In prandial state using tracer technique, we also have shown that despite a larger prandial plasma glucagon concentration, endogenous glucose production response to hypoglycemia is smaller after RYGB compared to non-operated controls (102), suggestive of diminished liver sensitivity to glucagon. However, we have demonstrated that blocking the GLP-1R increases the prandial hepatic glucose production response to insulin-induced hypoglycemia in RYGB subjects but not in non-operated controls, suggesting that enhanced GLP-1 signal due to rerouted gut can potentially contribute to the impaired counterregulatory response to hypoglycemia (103).

 

Based on current pathophysiologic understanding treatment strategies that selectively reduce the pace of nutrient delivery to the gut and prandial insulin secretion or improve counterregulatory response are the most effective options. Because of limited therapeutic options at this point, dietary modification remains the cornerstone of management. The goal of dietary modification is to lower prandial glucose spikes while increasing glucose nadirs by (a) lowering the amount of carbohydrates for every meal (<30g) or snacks (<15 g), (b) avoiding simple carbohydrates with high-glycemic index, (c) adding fats and proteins to every meal and snack, and (d) changing the composition of carbohydrate from glucose to fructose (25). Uncooked starch has also been used in this population based on effectiveness in reducing hypoglycemic episodes in patients with diabetes mellitus (104).

 

The current medical interventions rely on drugs that had previously been used for treatment of other hypoglycemic conditions.  Acarbose, an antidiabetic medication, has been utilized as the first drug started with dietary modification. The effect of this intestinal alpha-glucosidase blocker is to block carbohydrate absorption and reduce prandial glycemic excursion after RYGB {Valderas, 2012 #6617}. Adverse effects include flatulence and bloating, especially if the dose is not gradually titrated. Other medications such as somatostatin analogues, diazoxide, and GLP-1R agonists also are used based on sporadic case reports (25). Unblinded CGM in a small size study of patients with post-RYGB hypoglycemia has also been shown to reduce hypoglycemic episodes, likely due to better self-assessment of glycemic excursion and the need for treatments (105). The investigational drugs that are in various phases of development include exendin-(9-30) (Eiger Biopharmaceutical, Paol Alto, CA, USA), a potent GLP-1R antagonist (87,106), glucagon-based drugs (107,108), somatostatin analogues (109) and sodium-glucose cotransporter-1 inhibitor (110).

 

OTHER PRANDIAL HYPOGLYCEMIC CONDITIONS (RARE)

 

Nesidioblastosis

 

Non-insulinoma pancreatogenous hypoglycemia is a rare condition that typically causes hypoglycemia in the postprandial state due to diffuse nesidioblastosis (β cell hypertrophy, islet hyperplasia, increase in β cell mass) (111). The pathogenesis in adults is largely unknown, but likely differs from nesidioblastosis that occurs in congenital hyperinsulinism that is caused by a genetic mutation (112,113). This etiology should be considered in a hypoglycemia patient with a negative 72 hour fast, positive mixed meal test concerning for endogenous hyperinsulinism without a history of GI surgery, and negative imaging for insulinoma. Selective arterial calcium stimulation demonstrates diffuse insulin secretion. Histopathology reveals nesidioblastosis (114-116). Ideal management is difficult to determine as the condition is rare; a majority of the publications are from case reports or case-series, and there is a lack of long-term follow up (82,117).  Management can include dietary interventions (low-carbohydrate frequent meals) or medical interventions with diazoxide, acarbose, verapamil or octreotide (82).  When these interventions fail, partial pancreatectomy can be performed in patients with severe neuroglycopenic symptoms (118).

 

Hereditary Fructose Intolerance

 

Hereditary fructose intolerance is a rare autosomal recessive disorder (<1-9/100,000 annual incidence) caused by fructose-1-phoshate deficiency that results in postprandial hypoglycemia after ingestion of fructose (fruits) or sucrose (sweet foods) that is usually diagnosed in childhood. The diagnosis should be suspected in ill-appearing adults with hypoglycemia associated with GI symptoms (nausea, vomiting, diffuse abdominal pain) after eating fruits or sugar. Clinical symptom resolution within days of elimination suggests hereditary fructose intolerance and can be confirmed by molecular diagnosis on DNA obtained from peripheral leukocytes (68).

 

CONCLUSION

 

Non-diabetic hypoglycemia is a rare phenomenon since in healthy individuals counterregulatory mechanisms prevent and correct hypoglycemia by reducing glucose uptake and by enhancing hepatic glucose production. These mechanisms are less well characterized in the prandial state compared to the fasting state. Nonetheless, hypoglycemia only occurs when impaired physiological responses offset the balance between glucose utilization and production. Evaluation of hypoglycemia starts with a detailed history, comprehensive review of medications and clinical presentation, and a thorough physical exam that guides the diagnostic approach in patients with documented hypoglycemia based on Whipple’s triad. In absence of confirmation of hypoglycemia during free living condition provoked testing, prolonged fast or mixed meal test for hypoglycemic conditions reported during fasting or prandial state, respectively, are indicated. Diagnostic and management strategies for non-diabetic hypoglycemia is individualized depending on specific pathophysiology and can include interventions that are dietary, medical, or surgical.

 

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Triglyceride Lowering Drugs

ABSTRACT

 

The two major goals of the treatment of hypertriglyceridemia are the prevention of cardiovascular disease and pancreatitis. Here we discuss the drugs used for the treatment of hypertriglyceridemia: (niacin, fibrates, omega-3-fatty acids, volanesorsen (available in Europe) and lipoprotein lipase gene therapy (alipogene tiparvovec- no longer available). Niacin decreases total cholesterol, TGs (20-50% decrease), LDL-C, and Lp(a). Additionally, niacin decreases small dense LDL resulting in a shift to large, buoyant LDL particles. Moreover, niacin increases HDL-C. Skin flushing, insulin resistance, and other side effects have limited the use of niacin. The enthusiasm for niacin has greatly decreased with the failure of AIM-HIGH and HPS-2 Thrive to decrease cardiovascular events when niacin was added to statin therapy. The omega-3-fatty acids eicosapentaenoic acid (C20:5n-3) (EPA) and docosahexaenoic acid (C22:6n-3) (DHA) lower TGs by 10-50% but do not affect total cholesterol, HDL-C, or Lp(a). LDL-C may increase with EPA + DHA when the TG levels are markedly elevated (>500mg/dL). EPA alone does not increase LDL-C. Omega-3-fatty acids have few side effects, drug interactions, or contraindications. Numerous studies of low dose omega-3-fatty acids on cardiovascular outcomes have failed to demonstrate a benefit. However, in the JELIS trial and REDUCE-IT trial high doses of EPA alone reduced cardiovascular events while in the STRENGTH trial high dose EPA+DHA did not reduce cardiovascular events. Fibrates reduce TG levels by 25-50% and increase HDL-C by 5-20%. The effect on LDL-C is variable. If the TG levels are very high (>500mg/dL), fibrate therapy may result in an increase in LDL-C, whereas if TGs are not markedly elevated fibrates decrease LDL-C by 10-30%. Fibrates also reduce apolipoprotein B, LDL particle number, and non-HDL-C and there may be a shift from small dense LDL towards large LDL particles. Fibrates do not have any major effects on Lp(a). Monotherapy with fibrates appears to reduce cardiovascular events in patients with high TG and low HDL-C levels. Whether the addition of fibrates to statin therapy will reduce cardiovascular disease is uncertain. In patients with diabetes fibrates appear to slow the progression of microvascular disease. Volanesorsen is an antisense oligonucleotide that inhibits the production of apolipoprotein C-III. In patients with the familial chylomicronemia syndrome (FCS) volanesorsen decreases TG by 77% (mean decrease of 1712 mg/dL) with 77% of the patients having TG levels less than 750 mg/dL. In addition, volanesorsen treatment resulted in decreases in non–HDL-C by 46%, and VLDL-C by 58% and increases in HDL-C by 46%, LDL-C by 136%, (LDL-C increased from 28 to 61 mg/dL), and total apolipoprotein B by 20%. Studies have suggested that volanesorsen may reduce episodes of pancreatitis. Patients with FCS have also reported that volanesorsen improved symptoms and reduced interference of FCS with work/school responsibilities. Of concern has been decreases in platelet levels with 47% of patients treated with volanesorsen developing platelet counts below100 x 109/L. Thus, a number of drugs are available for the treatment of hypertriglyceridemia and may be employed when lifestyle changes are not sufficient. 

 

INTRODUCTION

 

The two primary goals of the treatment of hypertriglyceridemia are the prevention of cardiovascular disease and the prevention of pancreatitis. The evaluation and guidelines for the management of hypertriglyceridemia are discussed in detail in the Endotext chapter “Risk of Fasting and Non-Fasting Hypertriglyceridemia in Coronary Vascular Disease and Pancreatitis” (1) and the approach to evaluating a patient with hypertriglyceridemia is discussed in the Endotext chapter “Approach to the Patient with Dyslipidemia” (2). The treatment of hypertriglyceridemia by diet and weight loss are discussed in detail in the Endotext chapter “The Effect of Diet on Cardiovascular Disease and Lipid and Lipoprotein Levels” and “Obesity and Dyslipidemia” (3,4). Lifestyle changes are recommended as the first line for therapy of hypertriglyceridemia, but drug therapy is often required. In this chapter we will discuss the drugs used for the treatment of elevated plasma TG levels. Statins, ezetimibe, PCSK9 inhibitors, bempedoic acid, lomitapide, mipomersen, and evinacumab, which are primarily used to lower LDL-C, are discussed in the chapter “Cholesterol Lowering Drugs” (5).  

 

NIACIN

 

Introduction

 

Niacin was the first drug approved to treat dyslipidemia. In 1955, Altschul et al showed that pharmacologic doses of niacin decreased plasma cholesterol levels (6). Several forms of niacin are available for clinical use. Immediate release niacin has a short duration of action and is typically given two or three times per day with meals, whereas sustained release niacin and extended-release niacin are once a day drugs usually given at bedtime. The extended release form of niacin exhibits release rates that are intermediate between immediate release niacin and sustained release niacin (7). While the effects of the various forms of niacin on plasma lipid levels are similar, the side effect profiles are different. Because of an increased risk of serious liver toxicity with sustained release niacin this preparation is no longer widely used to treat dyslipidemia. Over-the- counter “No flush” niacin is also available but is generally ineffective as a lipid-modifying agent because most of these preparations do not contain active nicotinic acid.    

 

Effect of Niacin on Lipid and Lipoprotein Levels

 

Table 1. Effect of Niacin on Lipid and Lipoproteins

Decreases Total Cholesterol

Decreases LDL-C

Decreases TGs

Decreases Non-HDL-C

Decreases Lp(a)

Increases HDL-C

Decreases Apolipoprotein B

Shifts Small Dense LDL to Large Buoyant LDL

 

Niacin decreases all the pro-atherogenic lipid and lipoprotein particles including total cholesterol, TG, LDL-C, and Lp(a) levels (Table 1) (8,9). Additionally, niacin has been shown to decrease small dense LDL resulting in a shift to large, buoyant LDL particles (10). Moreover, niacin increases HDL-C levels (8,9).

 

In a meta-analysis of 30 trials with 4,749 subjects treatment with immediate release, sustained release, or extended release niacin decreased total cholesterol by 10%, decreased TGs by 20%, decreased LDL-C by 14%, and increased HDL-C by 16% (11). All three niacin preparations were effective in decreasing total cholesterol, TG, and LDL-C levels and increasing HDL-C levels (11). At a dose of 1.5 grams per day, immediate release niacin and extended release niacin produced similar decreases in total cholesterol, TGs, and LDL-C and a similar increase in HDL-C (12). A meta-analysis of 14 studies with 9,013 subjects reported a 23% decrease in Lp(a) with extended release niacin treatment (13).

 

A small meta-analysis of 5 trials in 432 subjects compared the response to extended release niacin in men and women (14). The effect of niacin on LDL-C was greater in women than men at all niacin doses (1,000mg 6.8% decrease in women vs 0.2% in men, p = 0.006; 1,500mg 11.3% decrease vs 5.6% decrease, p = 0.013; 2,000 mg 14.8% decrease vs 6.9% decrease, p = 0.010; 3,000mg 28.7% decrease vs 17.7% decrease, p = 0.006). The effect of niacin on plasma TG levels also tended to be greater in women but the difference only reached statistical significance at the 1,500mg dose (28.6% vs 20.4%, p = 0.040). The mechanism for the more robust decrease in LDL-C and TGs in women is unknown but might be due to a smaller body mass in women leading to increased circulating niacin levels and hence a greater response. However, the effect of niacin on HDL-C and Lp(a) levels were similar in males and females. Not unexpectedly the effect of niacin is dose dependent with higher doses having a greater effect on plasma lipid and lipoprotein levels (Table 2) (14).

 

Table 2. Effect of Niacin Dose on Lipid and Lipoprotein Response in Women (percent change)

Niacin Dose

LDL-C

TG

HDL-C

Lp(a)

500mg

-5.2

-9.5

7.7

-2.6

1000mg

-6.8

-14.5

17.6

-11.5

1500mg

-11.3

-28.6

21.1

-4.0

2000mg

-14.8

-37.3

25.2

-24.7

2500mg

-28.7

-45.6

34.5

-28.6

3000mg

-28.7

-51.0

28.7

-29.9

 

Numerous studies have examined the effect of the addition of niacin to statin therapy. Combination therapy typically results in further reductions in atherogenic lipoprotein particles and an increase in HDL-C levels. An example of such a study is shown in Table 3 (15).

 

Table 3. Effect of the Addition of Niacin to Statin Therapy on Lipid and Lipoprotein Levels

 

 Change in Lipids with Addition of Extended-Release Niacin 2000mg/day to Simvastatin 20mg/day

LDL-C

7.1% Decrease

HDL-C

18.2% Increase

TG

22.7% Decrease

Non-HDL-C

15.1% Decrease

Lp(a)

17.4% Decrease

 

While a literature search did not find any studies comparing the combination of ezetimibe + niacin vs. monotherapy there is a large trial that has examined the effect of adding 2 grams niacin to ezetimibe/simvastatin 10/20 (16). In this study the addition of niacin improved the lipid profile with a marked decrease in TGs and an increase in HDL-C levels (table 4).

 

Table 4. Effect of the Addition of Niacin to Ezetimibe/Statin Therapy on Lipid and Lipoprotein Levels

 

 Change in Lipids with Addition of Niacin 2000mg/day to Ezetimibe/Simvastatin 10/20mg/day

LDL-C

4.8% Decrease

HDL-C

21.5% Increase

TG

17.6% Decrease

Non-HDL-C

7.3% Decrease

 

In patients with marked hypertriglyceridemia combining niacin with other drugs that also lower plasma TGs can be considered. Sixty patients with the metabolic syndrome were randomized to 16 weeks of treatment with placebo, omega-3-fatty acids (Lovaza 4 g/day), extended release niacin (2 g/day), or both drugs in combination (17). In the niacin group TGs were decreased by 30%, in the omega-3-fatty acids group by 22%, and in the combination group by 42% compared to the placebo group. Of note the beneficial effects of niacin on decreasing LDL and non-HDL-C levels were blunted by omega-3-fatty acids, which are known to raise LDL-C levels in patients with marked hypertriglyceridemia (see below). These results show that the combination of niacin and fish oil will lower TG levels more than either drug individually but at the expense of diminishing the effect of niacin on LDL and non-HDL-C levels.   

 

Surprisingly there are few large randomized trials examining the effect of combination therapy with niacin + fibrate vs. monotherapy. One very small trial reported that while both niacin monotherapy and bezafibrate monotherapy were effective in lowering serum TGs there was no statistically significant added benefit of combination therapy in reducing serum TG levels (18). However, a larger trial in HIV+ patients reported that the combination of niacin and fenofibrate was better at lowering TGs and non-HDL-C and increasing HDL-C levels than monotherapy with either niacin or fenofibrate (19). It would be informative if additional trials of combination therapy were carried out in patients with marked hypertriglyceridemia that can often be difficult to control with lifestyle changes and monotherapy.

 

Mechanisms Accounting for the Niacin Induced Lipid Effects

 

TRIGLYCERIDES

 

Early studies demonstrated that niacin inhibited the release of free fatty acids from cultured adipocytes and decreased circulating free fatty acid levels (20-22). The ability of niacin to inhibit adipose tissue lipolysis is mediated by the activation of GPR109A (hydroxycarboxylic acid 2 receptor), a G protein-coupled receptor that is highly expressed in adipose tissue (22-24). It was initially thought that the decrease in plasma TGs induced by niacin therapy was due to niacin inhibiting lipolysis in adipose tissue resulting in a decrease in the transport of fatty acids to the liver leading to the decreased availability of fatty acids for hepatic TG synthesis. However, studies have shown that while niacin acutely decreases plasma free fatty acid levels this inhibition is not sustained (25). Additionally, studies in mice lacking GPR109A have shown that niacin does not inhibit lipolysis but still decreases plasma TG and LDL-C levels (26). Moreover, studies in humans using GPR109A agonists lowered plasma free fatty acid levels but did not cause the expected effects on plasma TGs and LDL-C (26). Thus, the effects of niacin on adipose tissue lipolysis are no longer thought to mediate the niacin induced decrease in plasma TG levels.

 

Niacin has been shown to inhibit diglycerol acyltransferase 2 (DGAT2) activity in the liver (22,27). DGAT2 is the key enzyme that catalyzes the final step in TG synthesis. Inhibition of DGAT2 will reduce hepatic TG synthesis and the availability of TG for VLDL assembly and secretion (22). A decrease in TG will result in an increase in apolipoprotein B degradation in the liver. Kinetic studies in humans have shown that treatment with niacin decreases VLDL TG production (28,29).

 

In addition, in animal models, niacin reduces the hepatic expression of apolipoprotein C-III, which could result in the accelerated clearance of TG rich lipoproteins (30). Whether this plays a significant role in mediating the decrease in plasma TG levels induced by niacin therapy remains to be determined.

 

LOW DENSITY LIPOPROTEIN

 

The decrease in plasma LDL-C with niacin therapy is thought to be secondary to a reduction in VLDL and LDL formation and secretion by the liver (22).

 

HIGH DENSITY LIPOPROTEIN

 

There are multiple potential mechanisms by which niacin may increase HDL-C levels. Some of these changes may be anti-atherogenic while others may be pro-atherogenic. One hypothesis for the increase in HDL induced by niacin therapy is a decrease in the surface expression of hepatocyte beta chain ATP synthase, a receptor that has been proposed to be involved in the uptake of HDL particles by the liver (31). Studies have further shown that niacin inhibits HDL protein degradation by cultured hepatocytes but does not inhibit the selective uptake of cholesterol esters carried in HDL (22,32).

 

Some kinetic studies have shown that niacin decreases HDL and apolipoprotein A1 fractional catabolic rate (33,34). In contrast, other kinetic studies have shown that niacin increase apolipoprotein AI production (35).

 

In addition, in monocytes, niacin also increased the expression of ABCA1 and CD36 resulting in an increase in cholesterol efflux to HDL, which would increase HDL-C levels and likely have anti-atherogenic effects (36). Similarly, in vitro studies suggest that niacin may increase the transport of cholesterol and phospholipids via ABCA1 from the liver to lipid poor apolipoprotein A1 particles thereby decreasing the clearance of apolipoprotein A1, which might not be anti-atherogenic (22,37).

 

Finally, decreasing plasma TG levels may result in a reduction in CETP mediated exchange of TGs on VLDL for cholesterol on HDL leading to an increase in HDL-C levels. Additionally, studies have shown that niacin decreases the expression of CETP (38).

   

LIPOPROTEIN(a)

 

Niacin decreases the synthetic rate of Lp(a) but does not increase Lp(a) catabolism (39,40). In cell culture and animal studies niacin has been shown to decrease the expression of apo (a) (41).

 

Pharmacokinetics

 

Oral niacin is well absorbed with immediate release niacin resulting in a rapid increase in plasma levels while extended release and sustained release niacin result in a delayed peak in plasma levels. Niacin undergoes metabolism in the liver by two primary pathways; conjugation or amidation (7,42). The conjugative pathway is low affinity and high capacity that metabolizes niacin to nicotinuric acid while the amidation pathway is high affinity and low capacity that converts niacin into several oxidative-reductive intermediates, which can induce hepatic toxicity (7,42) (Figure 1). The clinical importance is that immediate release niacin results in high levels of niacin and therefore is primarily metabolized by the conjugative pathway (low affinity, high capacity), which does not result in toxic intermediates that can cause liver damage. In contrast, sustained release niacin results in lower levels of niacin for a longer period and therefore metabolism via the amidation pathway (high affinity, low capacity) is dominant leading to an increase in the formation of toxic intermediates that can induce hepatic injury (7,42). Extended-release niacin would be metabolized midway between immediate release and sustained release niacin (42).

 

Figure 1. Pathways of Niacin Metabolism.

 

Effect of Niacin on Cardiovascular Outcomes

 

MONOTHERAPY

 

The Coronary Drug Project, conducted between 1966 and 1975, was the first large randomized, double-blind clinical trial to show that lowering lipids reduced cardiovascular disease (43). This trial determined the effect of clofibrate (1.8g/day), dextrothyroxine (6mg/day), two doses of oral estrogen (2.5 or 5mg per day), or immediate release niacin (3 grams/day) vs. placebo in 8,341 men 30 to 64 years of age with an electrocardiogram documented myocardial infarction. The mean baseline total cholesterol level was 251mg/dL and the TG level was 183mg/dL. The two estrogen regimens and dextrothyroxine treatment groups were discontinued early because of increased adverse effects. Clofibrate treatment did not demonstrate clinical benefit. In the niacin treated patients there was an average 10% decrease in serum cholesterol and 26% decrease in serum TGs despite modest compliance with the study medication. Moreover, niacin treatment (n=1,119) decreased recurrent myocardial infarctions by 26%, stroke by 24%, and revascularization by 67% compared to placebo (n=2,789) but did not decrease total mortality, which was the primary endpoint. Long term follow-up (6.2 years during the study and 8.8 years post study after niacin was discontinued in most participants) demonstrated an 11% decrease in mortality in the niacin group vs. the placebo group (52.0 versus 58.2%; p = 0.0004) (44). The majority of this difference in mortality was accounted for by a decrease in coronary heart disease mortality (36.5% vs. 41.3%; p=0.005). Further analysis revealed that niacin reduced the risk of 6-year recurrent myocardial infarction and coronary heart disease death and 15-year total mortality similarly in patients at all levels of baseline fasting plasma glucose, including those with glucose levels ≥126mg/dL (i.e. patients with diabetes) (45). Additionally, the beneficial effect of niacin on cardiovascular events and total mortality was not diminished, even among those with one hour plasma glucose levels > 220mg/dL (45). Moreover, the beneficial effects of niacin on recurrent myocardial infarction and total mortality were similar in patients with or without the metabolic syndrome at baseline (46). These results demonstrate that immediate release niacin monotherapy decreases recurrent atherosclerotic cardiovascular events in a broad spectrum of patients with pre-existing cardiovascular disease (secondary prevention).

 

COMBINATION WITH FIBRATES

 

In the Stockholm Ischemic Heart Disease Secondary Prevention Study survivors of a myocardial infarction below 70 years of age were randomized to a control group (n = 276) (no placebo) and a group treated with clofibrate (2 grams) and immediate release nicotinic acid (up to 3 grams) (n = 279) (47). Serum cholesterol and TG was lowered by 13% and 19%, respectively, in the treatment group compared to the control group. Recurrent myocardial infarction was reduced by 50% within one year (48). Total mortality was decreased by 26% in the group treated with clofibrate + niacin (p< 0.05) while ischemic heart disease mortality was decreased by 36% (p< 0.01). Notably, the benefit of clofibrate + niacin was only observed in patients with a baseline TG level > 143mg/dL. In the age of statins, the clinical implications of this early study are unclear. 

 

COMBINATION WITH STATINS

 

The AIM-HIGH trial was designed to determine if the addition of Niaspan, an extended-release form of niacin, to aggressive statin therapy would result in a further reduction in cardiovascular events in patients with pre-existing cardiovascular disease (49). In this trial 3,314 patients were randomized to extended-release Niaspan (1500-2000mg/day) vs. placebo that contained 100-150mg of immediate release niacin. On trial, LDL-C levels were in the 60-70mg/dL range in both groups. As expected, HDL-C levels were increased in the Niaspan treated group (approximately 44mg/dL vs. 38mg/dL), while TGs were decreased (approximately 121mg/dL vs. 155mg/dL). However, there were no differences in the primary endpoint between the control and Niaspan treated groups (Primary endpoint consisted of the first event of death from coronary heart disease, nonfatal myocardial infarction, ischemic stroke, hospitalization for an acute coronary syndrome, or symptom-driven coronary or cerebral revascularization). There were also no differences in secondary endpoints except for a possible increase in strokes in the Niaspan treated group. The addition of Niaspan to statin therapy did not result in a significant increase in either muscle or liver toxicity. Thus, this study does not provide support for the addition of niacin to statins. However, most of the patients included in this study did not have a lipid profile that one would typically consider treating with niacin therapy. In the subset of patients with TG > 198mg/dL and HDL-C < 33mg/dL Niaspan treatment showed a trend towards benefit (hazard ratio 0.74; p=0.073), suggesting that if the appropriate patient population was studied the results may have been different (50).

 

HPS 2 Thrive also studied the effect of niacin added to statin therapy (51). This trial utilized extended-release niacin (2000mg/day) combined with laropiprant, a prostaglandin D2 receptor antagonist, which reduces the flushing side effect of niacin treatment. HPS 2 Thrive was a very large trial with over 25,000 patients randomized to either niacin therapy or placebo. As in the AIM HIGH study, the baseline LDL-C levels were low at 63mg/dL, the HDL-C levels were 44mg/dL, and the TGs were 125mg/dL at baseline. As expected, niacin therapy resulted in a modest reduction in LDL-C (10mg/dL), a modest increase in HDL-C (6mg/dL), and a marked reduction in TGs (33mg/dL) compared to placebo. However, despite these lipid changes there were no significant differences in major cardiovascular events between the niacin and control group (risk ratio 0.96 CI 0.90- 1.03). It is unknown whether laropiprant, the prostaglandin D2 receptor antagonist, might have effects that worsen atherosclerosis and increase event rates. Mice deficient in the prostaglandin D2 receptor have been noted to have an increase in atherogenesis in response to angiotensin II (52). Similar to the AIM-HIGH study, the group of patients included in the HPS 2 Thrive trial may not have been the ideal patient population to study for the beneficial effects of niacin treatment added to statin therapy. Ideally, patients with high TGs and high non-HDL-C levels coupled with low HDL-C levels should be studied.

 

Thus, these two studies have failed to demonstrate that adding niacin to statin therapy results in a decrease in cardiovascular events. It should be recognized that both the AIM-HIGH study and the HPS-2 Thrive study had limitations. First, the patient populations that were included in these studies were not ideal as the TG and non-HDL-C levels were not elevated in a range that one would usually consider adding niacin therapy. Second, in both trials a significant percentage of patients stopped niacin therapy (AIM-HIGH 25.4% discontinued niacin; HPS-2 Thrive 25.4% discontinued niacin). Third, the duration of these studies was relatively short and it is possible that the beneficial effects of niacin take longer to occur (AIM-HIGH 3 years; HPS-2 Thrive 3.9 years). Fourth, in the HPS-2 Thrive it is possible, as noted earlier, that laropiprant had adverse effects that increased the risk of cardiovascular events. Fifth, in the AIM-HIGH study the placebo contained a low dose of niacin, which may have resulted in beneficial effects. Finally, both of these trials used extended-release niacin, whereas the Coronary Drug Project and the Stockholm Ischemic Heart Disease Secondary Prevention Study used immediate release niacin. It is possible that these different formulations of niacin have different effects on cardiovascular events. Additional studies are required to definitively determine the effect of niacin added to a statin therapy on cardiovascular events.

 

Effect of Niacin on Atherosclerosis

 

Many of the initial niacin therapy imaging studies combined niacin with other drugs and compared these combinations vs. placebo. These studies showed that niacin in combination with other drugs reduced the progression and/or increased the regression of atherosclerosis. However, because of the use of other drugs it is impossible to determine if niacin therapy per se was beneficial (Table 5).

 

Table 5. Niacin Angiography Imaging Studies

Combination Studies

Drugs

Cholesterol Lowering Atherosclerosis Study (CLAS) (53)

Niacin + colestipol vs. placebo

Familial Atherosclerosis Treatment Study (FATS) (54)

Niacin + colestipol or lovastatin + colestipol vs. placebo

UCSF-SCORE (55)

Niacin + colestipol +/- lovastatin vs. placebo +/- low dose colestipol

HDL Atherosclerosis Study (HATS) (56)

Niacin + simvastatin vs. placebo

Armed Forces Regression Study (57)

Niacin + gemfibrozil + cholestyramine vs. placebo

Harvard Atherosclerosis Reversibility Project (HARP)  (58)

Niacin + pravastatin + cholestyramine + gemfibrozil as needed vs. placebo

 

However, there are studies that compared niacin to placebo or other drugs added to standard statin therapy that do provide useful insights (Table 6).

 

Table 6. Effect of Niacin Added to Statin Therapy on Atherosclerosis

ARBITER 2/3

(59,60)

ER niacin vs. placebo

Decrease in CIMT vs. placebo

ARBITER 6 (61)

ER niacin vs. ezetimibe

Decrease in CIMT vs. ezetimibe

Thoenes (62)

ER niacin vs. placebo

Decrease in CIMT vs. placebo

Lee (63)

Modified release niacin vs. placebo

Decrease in carotid wall area on MRI vs. placebo

 

The ARBITER 2 Trial was a double-blind randomized study of extended-release niacin (1000mg) vs. placebo added to background statin therapy in 167 patients with coronary heart disease and low HDL-C levels (<45mg/dL) (60). At the initiation of the study mean LDL-C levels were < 100mg/dL. The primary end point was the change in common carotid intima-media thickness (CIMT). As expected, plasma TGs decreased and HDL-C levels increased with niacin therapy. LDL-C levels were unchanged. After 12 months, mean CIMT increased significantly in the placebo group (P<0.001) and was unchanged in the niacin group (P=0.23). The overall difference in CIMT progression between the niacin and placebo groups was almost statistically significant (P=0.08). Cardiovascular events occurred in 3 patients treated with niacin (3.8%) and 7 patients treated with placebo (9.6%; P=0.20). ARBITER 3 was a 12-month extension and in the 57 patients that continued on niacin therapy there was an additional regression of CIMT (p = 0.001 vs. placebo) (59).

 

In ARBITER 6, patients with coronary heart disease or a coronary heart disease risk equivalent on long-term statin therapy with LDL-C level < 100mg/dL and an HDL-C level < 50mg/dL for men or 55mg/dL for women were randomly assigned to receive either extended-release niacin (target dose, 2000mg per day) or ezetimibe (10mg per day) (61). The primary end point was the change from baseline in the mean CIMT. LDL-C levels decreased in the ezetimibe group by −18mg/dL (~ 20%) and by −10.0mgdl (~ 12%) in the niacin group (P=0.01) while HDL-C levels were slightly decreased in the ezetimibe group −2.8mg/dL and increased by 7.5mg/dL (~18%) in the niacin group (P<0.001). TG levels were not markedly altered in the ezetimibe group but decreased by ~ 15-20% in the niacin group.  Notably niacin therapy resulted in a significant reduction of both mean (P = 0.001) and maximal CIMT (P < 0.001) while ezetimibe therapy significantly increased CIMT (P < 0.001). The incidence of major cardiovascular events was lower in the niacin group than in the ezetimibe group (1% vs. 5%, P = 0.04).

 

In a trial by Thoenes and colleagues fifty patients with the metabolic syndrome not on statin therapy were randomized to either extended-release niacin (1000mg/day) or placebo (62). Treatment with niacin decreased LDL-C by 17% and TGs by 23% and increased HDL-C levels by 24% without significant changes in the placebo group. After 52 weeks of treatment, there was an increase in CIMT of +0.009 +/- 0.003 mm in the placebo group and a decrease in CIMT of -0.005 +/- 0.002 mm in the niacin group (p = 0.021 between groups).

 

Finally, Lee and colleagues performed a double-blind, randomized study of 2 g daily modified-release niacin or placebo added to statin therapy in 71 patients with low HDL-C (<40mg/dL) and either: 1) type 2 diabetes with coronary heart disease; or 2) carotid/peripheral atherosclerosis (63). The primary end point was the change in carotid artery wall area, quantified by magnetic resonance imaging, after 1 year. Treatment with niacin increased HDL-C by 23% and decreased LDL-C by 19% and TGs by 11%. At 12 months, niacin significantly reduced carotid wall area compared with placebo (Mean change in carotid wall area was -1.1 +/- 2.6 mm2 for niacin vs +1.2 +/- 3.0 mm2 for placebo).

 

While these imaging studies provide data suggesting that niacin therapy when added to statin therapy may reduce atherosclerotic cardiovascular disease, one must recognize that the studies described above were relatively small studies and that decreases or the lack of progression in CIMT or carotid wall area are surrogate markers, which may not necessarily indicate that cardiovascular events will be decreased.  

 

Side Effects

 

Treatment with niacin frequently results in side effects and these side effects are a major limitation of niacin therapy.

 

SKIN FLUSHING

 

This is a very common side effect and is characterized by redness and warmth due to vasodilation of the blood vessels in the skin (8,64). It is often most apparent in the head and neck region. Itching can occur and a tingling and burning sensation may also be noted. Niacin induced flushing is usually not accompanied by diaphoresis. The cutaneous flushing usually lasts for approximately one hour and in some patients is extremely annoying. In a review of 30 studies, it was noted that flushing occurred in 85% of participants treated with immediate release niacin, 66% of participants treated with extended release niacin, and 26% of participants treated with slow release niacin (11).  The occurrence of flushing is related to a rapid increase in plasma nicotinic acid levels, which differs depending upon the niacin preparation. Flushing was the primary reason that subjects discontinued niacin therapy during studies and with either immediate release or extended release niacin approximately 20% of study participants discontinue niacin, which is twice the rate of discontinuation observed in the placebo groups (11). Continuous administration of niacin for approximately one- week results in tachyphylaxis and the flushing decreases. Unfortunately, if a patient skips taking niacin for a few days this tachyphylaxis is lost and the flushing returns.

 

The mechanism for the niacin induced skin flushing has been partially elucidated (8,64). Niacin activates GPR109A in dermal Langerhan cells (macrophages in the skin), which leads to the increased production of prostaglandin D2.  Additionally, niacin activates GPR 109A in keratinocytes, which leads to the production of prostaglandin E2.  The prostaglandins then interact with prostaglandin receptors on blood vessels resulting in vasodilation and the flushing phenomena. Aspirin and nonsteroidal anti-inflammatory drugs (NSAIDS) taken prior to niacin administration can decrease flushing by inhibiting the synthesis of prostaglandins (8,65). Laropiprant decreases flushing by blocking the D prostanoid receptor (8). Since flushing is related to rapid increases in plasma nicotinamide levels taking immediate release niacin with food slows absorption and thereby reduces flushing. Extended-release niacin is typically taken at bedtime so that the flushing will occur when the patient is asleep. Conditions that predispose to cutaneous vasodilatation such as alcohol intake, hot liquids, spicy foods, or hot showers should be avoided. One should increase the dose of niacin slowly to reduce the severity of flushing reactions and allow tolerance to develop.

 

HEPATIC TOXICITY

 

Sustained release niacin has a much greater propensity to induce hepatic toxicity than other niacin preparations and therefore is no longer widely used (7,42,66). The explanation for this difference is due to the increased metabolism of sustained release niacin by the amidation pathway described in the pharmacokinetics section, which results in toxic compounds that injure the liver (7,42). Patients who have developed signs of liver toxicity on sustained release niacin can often be treated with immediate release niacin without developing liver problems (67). Extended-release niacin can induce liver dysfunction but the rate is much lower than sustained release niacin. Because of the potential for liver disease, serum transaminase levels (SGOT and SGPT) should be monitored before treatment begins, every 6 to 12 weeks for the first year, and periodically thereafter (e.g., at approximately 6-month intervals).

 

It should be noted that there is some evidence that niacin may be beneficial for non-alcoholic fatty liver disease (NAFLD) but further studies are required (68).

 

MUSCLE SYMPTOMS

 

Myalgias and myopathy have not been a significant adverse effect with niacin monotherapy (11). In combination with statins, an increased risk of muscle symptoms has been observed in some studies. In the HPS-2 Thrive study the combination of simvastatin and extended-release niacin increased the risk of myopathy four-fold (1.2% of patients on combined therapy) (51). Of note, this increase occurred predominantly in Chinese participants. In contrast, in the AIM-HIGH trial muscle related symptoms were not increased with the simvastatin + niacin combination (49,69).

 

HYPERGLYCEMIA

 

It has been recognized for many years that niacin induces insulin resistance (70). The mechanisms by which niacin induces insulin resistance are unknown but possible mechanisms include a rebound increase in free fatty acids with niacin therapy or the accumulation of diacylglycerol (29,71). A recent analysis of the AIM-HIGH trial demonstrated that in subjects with normal glucose metabolism, subjects with impaired fasting glucose, and subjects with diabetes, treatment with extended release niacin resulted in only small increases in fasting glucose levels but increased serum insulin levels due to an increase in insulin resistance (72). Additionally, there was an increased risk of progressing from normal to impaired fasting glucose in subjects treated with niacin in the AIM-HIGH trial (niacin 58.6% vs placebo 41.5%; P < .001) (72).

 

A meta-analysis examined the effect of niacin therapy on the development of new onset diabetes (73). In 11 trials with 26,340 non-diabetic participants, niacin therapy was associated with a 34% increased risk of developing diabetes (RR of 1.34; 95% CIs 1.21 to 1.49). This increased risk results in one additional case of diabetes per 43 initially non-diabetic individuals who are treated with niacin for 5 years (0.47% ten-year risk or 4.7 per 1000 patient years). Results were similar in patients who were receiving niacin therapy in combination with statin therapy.

 

Studies have shown that niacin is usually well tolerated in diabetic subjects who are in good glycemic control (74,75). In patients with poor glycemic control, niacin is more likely to adversely impact glucose levels. A meta-analysis of 7 studies with 838 patients with diabetes found that niacin therapy did not result in a significant increase in fasting glucose levels in short term studies but in long term studies there was a very small increase in fasting glucose levels (1.5mg/dL) that was not clinically significant (76). An important caveat is that in most of these trials adjustments in diabetes therapy was permitted, which could blunt worsening of glycemic control. In contrast to these findings, the HPS-2 Thrive Trial reported that in the 8,299 participants who had diabetes at the time of randomization, treatment with niacin–laropiprant was associated with a 55% increase in serious disturbances in diabetes control, most of which led to hospitalization (11.1% vs. 7.5%, P<0.001) (51). The extent to which the latter was due to laropiprant is unknown. Thus, care must be used in treating patients with diabetes with niacin. In patients in whom adjustments in diabetic therapy can easily be carried out the risk of adverse effects will likely be limited whereas in patients in whom adjustments in diabetic therapy will be difficult the risks of niacin therapy are likely to be increased. Careful patient selection and education are important steps to reduce the risks of niacin therapy in patients with diabetes.

 

Thus, while niacin therapy may adversely affect glucose homeostasis one needs to balance these adverse effects with the potential benefits of niacin therapy. One should note that in the Coronary Drug Project participants with abnormal glucose metabolism also demonstrated a decrease in cardiovascular events with niacin therapy (45).  

 

URIC ACID  

 

Niacin may increase uric acid levels by inhibiting the secretion of uric acid (8,77). In susceptible patients niacin therapy can precipitate gouty attacks (8).   

 

GASTROINTESTINAL SYMPTOMS  

 

Niacin therapy can induce heartburn, indigestion, nausea, diarrhea, and abdominal discomfort (8). High dose niacin is more likely to cause these gastrointestinal disturbances. The mechanism for these symptoms is not clear. 

 

MISCELLANEOUS  

 

Recent trials have reported an increased incidence of infections with niacin therapy (51,69). A trial of niacin in combination with laropiprant found increased bleeding (51). The increased bleeding could be due to the approximately 10% decrease in platelet levels that can occur with niacin (see Niaspan Package Insert). However, a very large observational study that compared rates of major gastrointestinal bleeding and intracranial hemorrhage in patients treated with niacin (>200,000 subjects) to propensity matched subjects on fenofibrate did not observe an increase in bleeding (78). Niacin has been reported to induce cystoid macular edema, which resolves when the drug is stopped (79).

 

Contraindications

 

There are a number of contraindications to niacin therapy (Table 7).

 

Table 7. Contraindications for Niacin Therapy

Active gastritis or peptic ulcer disease

Impaired liver function (elevated transaminases 2-3X the upper limit or cholestasis)

Uncontrolled gout

Pregnancy

Lactation

Poorly controlled diabetes

Active bleeding

 

Summary

 

The enthusiasm for the use of niacin has greatly decreased with the failure of AIM-HIGH and HPS-2 Thrive to show a decrease in cardiovascular events when niacin was added to statin therapy. In the absence of definitive data showing benefits from niacin therapy when added to a statin it is hard to justify the use of this drug given the frequent side effects. The availability of ezetimibe, bempedoic acid, and PCSK9 inhibitors has greatly reduced the need to use niacin to lower LDL-C levels. Additionally, in patients with markedly elevated TG levels (>500mg/dL), niacin can be employed in combination with other drugs to reduce the risk of pancreatitis but fibrates and omega-3-fatty acids are the initial choices.

 

OMEGA-3-FATTY ACIDS (FISH OIL)

 

Introduction

 

The lipid lowering effects of fish oil are mediated by two omega-3-fatty acids; eicosapentaenoic acid (C20:5n-3) (EPA) and docosahexaenoic acid (C22:6n-3) (DHA). There are four prescription products approved by the FDA which contain various amounts of EPA and DHA (Table 8). Lovaza and Omacor contain a mixture of EPA and DHA fatty acid esters (ethyl esters), Vascepa contains only EPA fatty acid esters (ethyl esters), and Epanova contains a mixture of EPA and DHA free fatty acids (Epanova is currently not available in the US).

 

Table 8. Prescription Omega-3-fatty acid products (data from package inserts)

Generic Name

Omega-3-ethyl esters

Icosapent ethyl

Omega-3-carboxylic acid

Brand Name

Lovaza or Omacor

Vascepa

Epanova

EPA/capsule

0.465g

1.0g

See below

DHA/capsule

0.375g

---

See below

Daily Dose

4 capsules/day

4 capsules/day

2-4 capsules/day

1-gram capsules of Epanova contain at least 850mg of fish oil derived fatty acids including multiple omega-3-fatty acids with EPA and DHA being the most abundant

 

Fish oil is also sold as a food supplement. It should be recognized that dietary fish oil supplements are not approved by the FDA and quality control will not meet the same rigorous standards as prescription or over the counter drugs. The amount of EPA and DHA can vary greatly in these supplements and one needs to read the labels carefully, as products can contain less than 100mg of EPA/DHA per 1 gram capsule (80). It is helpful to have the patient bring their fish oil supplements to the clinic for verification of the actual amount of EPA and DHA in the product. Moreover, the amount of EPA and DHA indicated on the label may not be accurate (81). One needs to take a sufficient number of capsules to provide 2-4 grams of EPA/DHA per day to effectively lower plasma TG levels. Depending upon the fish oil supplement, the patient may be required to take a large number of capsules to obtain 2-4 grams of EPA/DHA per day. Furthermore, these supplements may contain other compounds in addition to omega-3-fatty acids, such as cholesterol, oxidized lipids, and saturated fatty acids. The major advantage of fish oil supplements is that they are much less expensive than prescription omega-3-fatty acid drugs. If one elects to use fish oil supplements, one should have the patient use a single brand to try to ensure as much consistency as possible.

 

Some omega-3 supplements contain alpha linolenic acid (C18:3n-3) (ALA), a plant omega-3-fatty acid rather than EPA/DHA. ALA can be converted to EPA and DHA but the conversion is limited and hence it is ineffective in lowering plasma TG levels or altering other lipid or lipoprotein levels (82).

 

Effect of Omega-3-Fatty Acids on Lipid and Lipoprotein Levels

 

Table 9. Effect of Fish Oil Supplements on Lipids and Lipoproteins

Decreases TGs

No Change in Total Cholesterol

No Change in LDL-C; if TGs are very high may increase LDL-C

No Change in HDL-C

No Change in Lp(a); modest decrease in some studies

Shift from Small Dense LDL to Large Buoyant LDL

 

Several meta-analyses have examined the effect of fish oil supplements on lipid and lipoprotein levels. A meta-analysis by Eslick and colleagues of 47 studies with 16,511 participants found that fish oil supplements significantly decreased plasma TG levels by approximately 14% without resulting in clinically significant changes in total, LDL-C, or HDL-C levels (83). These authors also reported that the reduction in plasma TG levels was directly related to baseline plasma TG levels (i.e., the higher the baseline TG level the greater the reduction in TGs with fish oil). Additionally, the higher the dose of EPA/DHA, the greater the reduction in plasma TGs, with clinically significant reductions occurring with approximately 3.25 grams per day. A meta-analysis by Balk and colleagues of 21 studies also found minimal effects of fish oil supplements on total, LDL-C, and HDL-C levels (< 5% change) with significant decreases in plasma TG levels (most of the studies in this meta-analysis had at least a 15% decrease) (84). Similar to the meta-analysis by Eslick et al, the higher the baseline TG levels the greater the reduction in TG levels. 

 

Several meta-analyses have focused on specific patient populations. In a meta-analysis of patients with diabetes, twenty three trials with1075 participants were analyzed and similar to patients without diabetes the major effect of fish oil supplements was a reduction in plasma TG levels with no change in total cholesterol or HDL-C (85). A small increase in LDL-C was observed (4.3mg/dL). Of note, fish oil supplementation did not alter fasting glucose or glycated hemoglobin levels indicating that fish oil supplementation does not adversely affect glycemic control. In a meta-analysis that included patients with type 2 diabetes or impaired glucose metabolism a decrease in TGs was observed without significant changes in total cholesterol, LDL-C, or HDL-C levels (86). Again, no adverse effects on glycemic control were observed.

 

In patients with end stage renal disease several meta-analyses have consistently shown a decrease in plasma TGs with fish oil administration but the effect on total, LDL-C, and HDL-C has been variable (87-89). This variability was likely due to the small changes that were observed. In patients with nephrotic syndrome a study has shown a reduction in plasma TGs and an increase in LDL-C levels without a change in total cholesterol or HDL-C levels (90). In patients with non-alcoholic fatty liver disease, omega-3-fatty acids have also been shown to decrease plasma TG levels (91). Finally, In HIV infected subjects, fish oil supplementation was also effective in lowering plasma TG levels (92,93).

 

Thus, fish oil supplementation in a variety of different patient populations lowers plasma TG levels. In patients with elevated TG levels treated with 3-4 grams of EPA/DHA one can expect an approximate 25% decrease. Total plasma cholesterol levels are usually not altered by fish oil supplementation. The exceptions are patients with high chylomicron and/or VLDL levels where a substantial portion of the plasma cholesterol is carried on these TG rich lipoproteins. Fish oil supplementation will decrease the levels of these TG rich lipoproteins and thereby result in a decrease in total plasma cholesterol levels. LDL-C levels are not markedly affected by fish oil supplementation except in patients with very high TG levels (>500mg/dL) where increases in LDL-C levels have been observed (94-96). If there are sufficient reductions in plasma TG levels a shift from small dense LDL to large buoyant LDL may be observed (97,98). The effect of fish oil supplements on HDL-C levels is minimal except if the patient has very high TG levels where significant elevations (>10%) have been reported (94-96). Finally, some but not all studies have shown that the administration of fish oil modestly lowers Lp(a) levels (99-103)

 

During the development of pharmacological omega-3-fatty acid drugs for approval by the FDA, extensive clinical trials were carried out and will be reviewed below (Tables 10 and 11). It should be noted that these studies are not directly comparable as they studied different patient populations at different times.

 

EPA + DHA FATTY ACID ESTERS (LOVAZA)  

 

In patients with marked elevations in plasma TG levels (500-2000mg/dL) a 6 week trial of EPA + DHA esters resulted in a 31% decrease in plasma TGs, a 21% increase in LDL-C levels, and a 12% increase in HDL-C levels compared to the placebo group (96). In a 16 week trial TG concentrations were decreased by 45% and LDL-C and HDL-C were increased by 31% and 13%, respectively (94). Studies have also been carried out in patients with moderate hypertriglyceridemia (200-500mg/dL) who were on statin therapy (104). EPA + DHA esters resulted in a 23% decrease in plasma TGs and a 7% decrease in non-HDL-C levels, and a 4.6% increase in HDL-C levels (104).

 

EPA FATTY ACID ESTER ALONE (VASCEPA)  

 

In patients with marked elevations in plasma TGs (500-2000mg/dL), 4 grams of EPA ester alone significantly decreased TG levels by 33.1% and non-HDL-C levels by 17.7% (105). In contrast to EPA and DHA fatty acid esters, LDL-C and HDL-C levels were not significantly altered by EPA fatty acid esters alone (105). Studies have also been carried out in patients with moderate hypertriglyceridemia (200-500mg/dL) who were on statin therapy. EPA esters resulted in a 21.5% decrease in plasma TGs, 13.6% decrease in non-HDL-C, 6.2% decrease in LDL-C, and a 4.5% decrease in HDL-C levels (106)

 

EPA + DHA FATTY ACIDS (EPANOVA)  

 

In patients with marked elevations in plasma TGs (500-2000mg/dL), 4 grams of EPA + DHA fatty acids decreased plasma TGs by 31% and non-HDL-C by 9.6% and increased LDL-C by 19% and HDL-C by 5.8% (107). Studies have also been carried out in patients with moderate hypertriglyceridemia (200-500mg/dL) who were on statin therapy. EPA + DHA fatty acids resulted in a 20.6% decrease in plasma TGs, 6.9% decrease in non-HDL-C with no significant changes in LDL-C or HDL-C levels (95).

 

These studies demonstrate that in patients on statin therapy with moderate elevations in plasma TG levels the effects of these three pharmaceutical products on lipids and lipoprotein levels are similar (table 11). However, in patients with marked elevations in plasma TG levels EPA ethyl esters alone do not increase LDL-C levels whereas products containing EPA and DHA result in a substantial increase in LDL-C levels (table 10). It should also be noted that the ability of omega-3-fatty acids to reduce plasma TGs and increase HDL-C levels is enhanced if baseline TG levels are markedly elevated.

 

Table 10: Effect of Omega-3-Fatty Acids on Lipids and Lipoprotein in Patients with Marked Hypertriglyceridemia (500-2000mg/dL)

 

TGs

Non-HDL-C

LDL-C

HDL-C

EPA+DHA ethyl esters-

6 weeks

31% decrease

ND

21% increase

12% increase

EPA+DHA ethyl esters

12 weeks

45% decrease

ND

31% increase

13% increase

EPA ethyl esters

33% decrease

18% decrease

NS

NS

EPA+DHA fatty acids

31% decrease

9.6% decrease

19% increase

5.8% increase

ND- not determined; NS- no significant change

 

Table 11: Effect of Omega-3-Fatty Acids on Lipids and Lipoprotein in Patients with Moderate Hypertriglyceridemia (200-500mg/dL) on Statin Therapy

 

TGs

Non-HDL-C

LDL-C

HDL-C

EPA+DHA ethyl esters

23% decrease

7% decrease

__

4.6% increase

EPA ethyl esters

22% decrease

14% decrease

6.2% increase

4.5% decrease

EPA+DHA fatty acids

21% decrease

6.9% decrease

NS

NS

NS- no significant change

 

HEAD-TO-HEAD COMPARISONS  

 

A meta-analysis of six studies has compared the effect of EPA alone vs. DHA alone on plasma lipids and lipoproteins (108). Administration of DHA increased LDL-C by 4.6mg/dL compared to EPA (95% CI 2.2- 7.1). In contrast, DHA reduced plasma TG levels to a greater extent than EPA (6.1mg/dL; 95% CI 2.5- 9.8). Finally, DHA increased HDL-C levels more than EPA (3.7mg/dL; 95% CI: 2.4- 5.1). Whether these very modest differences are clinically significant is unknown.

 

Tatsuno et al compared the effect of DHA + EPA ethyl esters vs. EPA ethyl esters alone on lipid and lipoprotein levels in patients with mean baseline plasma TG of 250-270mg/dL and mean LDL-C levels of 125-135mg/dL (109,110). These authors found that at equivalent doses there were no differences in effect on plasma TG, LDL-C, or HDL-C levels between DHA + EPA ethyl ester or EPA ethyl ester treatment.

 

These head-to-head studies indicate that in subjects with moderate hypertriglyceridemia the effects of EPA and DHA on lipid and lipoprotein levels are similar. Perhaps if the baseline TGs were markedly elevated differences in response might have been observed.

 

IN COMBINATION WITH FENOFIBRATE  

 

In patients with marked hypertriglyceridemia a single drug is often not sufficient to lower TGs into the desired range. In patients with TG levels > 500mg/dL the combination of fenofibrate 130mg per day and 4 grams of Lovaza reduced TG levels to a greater extent than monotherapy with fenofibrate alone (7% decrease; P = 0.059) (111). Not unexpectedly, LDL-C levels were increased to a greater extent with combination therapy (9% increase; p= 0.03). When subjects who had received 8 weeks of fenofibrate monotherapy were treated with Lovaza during the 8-week, open-label extension study, TG levels were reduced by an additional 17.5% (P = 0.003). These results indicate that the addition of omega-3-fatty acids to fenofibrate will further decrease TG levels.

 

IN COMBINATION WITH NIACIN

 

Sixty patients with the metabolic syndrome were randomized to 16 weeks of treatment with placebo, Lovaza (4 g/day), extended release niacin (2 g/day), or both drugs in combination (17). In the niacin group TGs were decreased by 30%, in the omega-3-fatty acids group by 22%, and in the combination group by 42% compared to the placebo group. Of note, the beneficial effects of niacin on decreasing LDL and non-HDL-C were blunted by omega-3-fatty acids. These results show that the combination of niacin and fish oil will lower TG levels more than either drug individually but at the expense of diminishing the effect of niacin on LDL and non-HDL-C levels.

 

Mechanism Accounting for the Omega-3-Fatty Acid Induced Lipid Effects

 

As noted above, the major effect of fish oil is to lower plasma TG levels. The predominant cause of the reduction in plasma TG levels is a decrease in the hepatic production and secretion of TG rich lipoproteins (112-115). In cultured hepatocytes, omega-3-fatty acids inhibit the assembly and secretion of VLDL and apolipoprotein B 100 (113,115-117).  The incorporation of TGs into VLDL is a key regulatory step in determining the rate of formation and secretion of VLDL and there are a number of mechanisms by which omega-3 fatty acids reduce the level of hepatic TGs available for VLDL formation (112,113,115). Studies in animal models have demonstrated that omega-3-fatty acids inhibit fatty acid synthesis and stimulate fatty acid oxidation in the liver, which would reduce the availability of fatty acids for TG synthesis (112-115). The increase in fatty acid oxidation is due to omega-3-fatty acids activating PPAR alpha, which stimulates fatty acid oxidation in the liver and other tissues (112,114,115,118). The decrease in fatty acid synthesis is due to omega-3-fatty acids inhibiting the expression of SREBP-1c, a key transcription factor that regulates fatty acid synthesis (114,115,118). In addition, omega-3-fatty acids decrease TG synthesis, which may be due to the decreased availability of fatty acids and an inhibition of the activity of DGAT, a key enzyme required for TG synthesis (112,114,115). Finally, omega-3-fatty acids also decrease the flux of free fatty acids from adipose tissue to the liver, which will lead to a decreased quantity of fatty acids available for TG synthesis in the liver (112). This decrease in flux of free fatty acids is due to omega-3-fatty acids reducing hormone sensitive lipase mediated intracellular lipolysis in adipose tissue (112). It is likely that these and perhaps other factors lead to the decreased availability of TGs resulting in a reduction in VLDL formation and secretion. In addition, the peroxidation of omega-3-fatty acids may stimulate the degradation of apolipoprotein B-100, which would provide another pathway that could contribute to a decrease in VLDL formation and secretion (115).

 

While not the primary mechanism for the decrease in plasma TGs, studies have shown that omega-3-fatty acids may increase the clearance of TG rich lipoproteins (112,119). Post heparin lipoprotein lipase activity is not increased by omega-3-fatty acid administration but the lipolytic activity of non-stimulated plasma is enhanced (112,119).  Additionally, apolipoprotein C-III levels are decreased with omega-3-fatty acid administration which could also contribute to an increase in the clearance of TG rich lipoproteins (120-123).

 

The increase in LDL-C levels that occurs in patients with marked hypertriglyceridemia treated with omega-3-fatty acids is thought to be due to the enhanced conversion of VLDL to LDL (114). The increase in HDL-C observed in studies in patients with very high TG levels may be due to the increased clearance of TG rich lipoproteins.    

 

Pharmacokinetics and Drug Interactions

 

Omega-3 ethyl esters and fatty acids are absorbed by the GI tract similar to other dietary lipids. It is worth noting that omega-3-free fatty acids (Epanova) are directly absorbed by the small intestine and are not dependent on pancreatic lipases for absorption. Thus, absorption of omega-3-fatty acids is not decreased in patients with pancreatic insufficiency and therefore may be preferred in patients with pancreatic disease. Additionally, the bioavailability of omega-3-fatty acids with a low fat diet was greater than omega-3-ethyl esters while there was little difference between these different formulations with a high fat diet (124,125).

 

Drug interactions have not been seen with omega-3-fatty acids (Package Inserts for Lovaza, Vascepa, and Epanova).

 

Effect of Low Dose Omega-3-Fatty Acids on Clinical Outcomes

 

Initial studies of the effect of low dose fish oil administration on cardiovascular outcomes were favorable, demonstrating a reduction in events including all-cause mortality. However, more recent studies have failed to confirm these favorable results. In these more recent studies the use of other drugs, such as statins, that reduce cardiovascular disease were more intensively utilized. The outcomes studies that will be described below were carried out with doses of EPA and DHA that are lower than the doses used to lower plasma TGs. We will limit our discussion to the administration of fish oil as a drug and not discuss diet studies, such as DART, which had patients increase fatty fish intake (126,127).

 

  • GISSI-Prevenzione trial was a randomized trial of 850-882mg of EPA and DHA ethyl esters per day in 11,323 participants with a recent myocardial infarction (< 3 months) for 3.5 years (128). The primary endpoint was death, non-fatal myocardial infarction, and stroke. No change in total cholesterol, LDL-C, or HDL-C was observed but plasma TG levels were decreased by 5%. Patients treated with EPA/DHA had a significant decreased risk of major cardiovascular events (RR 0.90), cardiac death (RR 0.78), and sudden death (RR 0.74). The decrease in sudden death occurred very quickly and was noted as early as 4 months after initiation of therapy. Interestingly, non-fatal cardiovascular events were not affected by EPA/DHA treatment (RR 0.98). The decrease in total mortality was driven by a reduction in sudden death suggesting an anti-arrhythmic effect of EPA/DHA.

 

  • GISSI-Heart Failure (GISSI-HF) trial was a randomized, double-blind, placebo-controlled trial in patients with chronic heart failure who were randomly assigned to 850-882mg of EPA and DHA ethyl esters per day (n=3,494) or placebo (n=3,481) (129). Patients were followed for a median of 3.9 years. Primary endpoints were time to death, and time to death or admission to the hospital for cardiovascular reasons. Omega-3-fatty acid treatment at these low doses resulted in a slight decrease in plasma TG levels with no change in total, LDL-C or HDL-C levels. In the omega-3-fatty acid group 27% patients died from any cause vs. 29% in the placebo group (HR 0.91; p=0.041). In the omega-3-fatty acid group 57% of patients died or were admitted to hospital for cardiovascular reasons vs. 59% in the placebo group (HR 0.92; p=0.009). No significant differences were observed in fatal or non-fatal myocardial infarctions or strokes. In this trial, similar to the GISSI-Prevenzione trial, the benefit was primarily due to a reduction in arrhythmic events and little benefit on atherothrombotic events was noted.

 

  • OMEGA was a randomized, placebo-controlled, double-blind, trial in 3,851 survivors of an acute myocardial infarction (130). Patients were randomized 3 to 14 days after an acute myocardial infarction to omega-3-acid ethyl esters, 1 gram/day (460mg EPA and 380mg DHA) or placebo capsules containing 1 gram of olive oil and followed for one year. The primary endpoint was rate of sudden death and secondary end points were total mortality and nonfatal clinical events. No significant differences were seen in the primary or secondary endpoints.

 

  • Alpha Omega was a double-blind, placebo-controlled trial in 4,837 patients between 60 and 80 years of age (78% men) who had had a myocardial infarction (131). Patients were randomized to receive for 40 months one of four trial margarines: a margarine supplemented with a combination of EPA and DHA (with a targeted additional daily intake of 400mg of EPA-DHA; actual intake 226mg EPA and 150mg DHA), a margarine supplemented with alpha-linolenic acid (ALA) (with a targeted additional daily intake of 2g of ALA), a margarine supplemented with EPA-DHA and ALA, or a placebo margarine. The primary end point was the rate of major cardiovascular events, which comprised fatal and nonfatal cardiovascular events and cardiac interventions. Neither low dose EPA-DHA, ALA, nor the combination of EPA/DHA and ALA significantly reduced the rate of major cardiovascular events or cardiac interventions.

 

  • FOL.OM3 Study was a double blind, randomized, placebo-controlled trial in 2,501 patients with a history of a myocardial infarction, unstable angina, or ischemic stroke in the past 12 months (132). Patients were randomized to a daily dietary supplement containing 5-methyltetrahydrofolate (560μg), vitamin B-6 (3mg), and vitamin B-12 (20μg) or placebo; and a dietary supplement containing omega 3 fatty acids (600mg of EPA and DHA) or placebo. Median duration of treatment was 4.7 years. The primary outcome was a composite of non-fatal myocardial infarction, stroke, or death from cardiovascular disease. Treatment with B vitamins or omega 3 fatty acids had no significant effect on major vascular events.

 

  • Origin was a double-blind study in 12,536 patients at high risk for cardiovascular disease who had impaired fasting glucose, impaired glucose tolerance, or diabetes (133). Patients were randomized to receive a 1-gram capsule containing at least 900mg of ethyl esters of omega-3 fatty acids (EPA 465mg and DHA 375mg) or placebo for approximately 6 years. The primary outcome was death from cardiovascular causes. TG levels were reduced by 14.5mg/dL in the group receiving omega-3-fatty acids compared to the placebo group (P<0.001), without a significant effect on other lipids. The incidence of the primary outcome was not significantly decreased among patients receiving omega-3-fatty acids as compared with those receiving placebo. The use of omega-3-fatty acids also had no significant effect on the rates of major vascular events, death from any cause, or death from arrhythmia.

 

  • Risk and Prevention Study was a double-blind, placebo-controlled trial in 12,513 men and women with multiple cardiovascular risk factors or atherosclerotic vascular disease but not myocardial infarctions (134). Patients were randomly assigned to 1-gram daily omega-3 fatty acids (EPA and DHA content not <85 %,) or placebo (olive oil) for 5 years. The initially specified primary end point was the rate of death, nonfatal myocardial infarction, and nonfatal stroke. At 1 year, after the event rate was found to be lower than anticipated, the primary end point was revised as time to death from cardiovascular causes or admission to the hospital for cardiovascular causes. Plasma TG levels decreased slightly more in the omega−3-fatty acid group than in those who received placebo (−28.2±1.3mg/dL vs. −20.1±1.3mg/dL; P<0.001). Total, LDL, and HDL-C levels were similar in the omega-3-fatty acid and placebo groups. No significant differences were observed between the omega-3-fatty acid group and placebo group for the primary endpoint or any of the secondary endpoints.

 

  • A Study of Cardiovascular Events in Diabetes (ASCEND) was a randomized, placebo controlled, double blind trial of 1-gram omega-3-fattys acids (400mg EPA and 300mg DHA ethyl esters) vs. olive oil placebo in 15,480 patients with diabetes without a history of cardiovascular disease (primary prevention trial) (135). The primary end point was serious vascular events (non-fatal myocardial infarction, non-fatal stroke, transient ischemic attack, or vascular death). Total cholesterol, HDL-C, and non-HDL-C levels were not significantly altered by omega-3-fatty acid treatment (changes in TG levels were not reported). After a mean follow-up of 7.4 years the composite outcome of a serious vascular event or revascularization occurred in 882 patients (11.4%) on omega-3-fatty acids and 887 patients (11.5%) on placebo (rate ratio, 1.00; 95% CI, 0.91 to 1.09). Serious adverse events were similar in placebo and omega-3-fatty acid treated groups.

 

  • The Vitamin D and Omega-3 Trial (Vital) was a randomized, double blind, placebo-controlled trial of 1-gram omega-3 fatty acids (465mg EPA and 375mg DHA ethyl esters) vs. placebo in 25,875 men (>50 years of age) and women (>55 years of age) that were not selected on the basis of an elevated risk (primary prevention) (136). Changes in lipid levels were not reported. The primary end point was major cardiovascular events, a composite of myocardial infarction, stroke, or death from cardiovascular causes. After a median follow-up of 5.3 years, major cardiovascular event occurred in 386 participants in the omega-3 fatty acid group and in 419 in the placebo group (hazard ratio, 0.92; 95% confidence interval (CI), 0.80 to 1.06; P=0.24). Serious adverse events were similar in placebo and omega-3-fatty acid treated groups.

 

  • Summary: The above results indicate that low dose fish oil (doses that do not greatly affect lipid levels) do not consistently reduce the risk of cardiovascular disease.

 

Effect of High Dose Omega-3-Fatty Acids on Clinical Outcomes

 

  • Japan EPA Lipid Intervention Study (JELIS) was an open label study without a placebo in patients with total cholesterol levels > 254mg/dL with (n= 3,664) or without cardiovascular disease (n=14,981) who were randomly assigned to be treated with 1800 mg of EPA (Vascepa) + statin (n=9,326) or statin alone (n= 9,319) with a 5-year follow-up (130). The primary endpoint was any major coronary event, including sudden cardiac death, fatal and non-fatal myocardial infarction, and other non-fatal events including unstable angina pectoris, angioplasty, stenting, or coronary artery bypass grafting. Total, LDL-C, and HDL-C levels were similar in the two groups but plasma TGs were modestly decreased in the EPA treated group (5% decrease in EPA group compared to controls; p = 0.0001). In the EPA group the primary endpoint occurred in 2.8% of the patients vs. 3.5% of the patients in the statin alone group (19% decrease; p = 0.011). Unstable angina and non-fatal coronary events were also significantly reduced in the EPA group but in this study sudden cardiac death and coronary death did not differ between groups. Unstable angina was the main component contributing to the primary endpoint and this is a more subjective endpoint than other endpoints such as a myocardial infarction, stroke, or cardiovascular death. In patients with high TG levels (>150 mg/dL) and low HDL-C levels (<40 mg/dL EPA treatment decreased the risk of CAD by 53% (HR: 0.47; P=0.043) (137). A subjective endpoint has the potential to be an unreliable endpoint in an open label study and is a limitation of the JELIS Study.

 

  • The Reduction of Cardiovascular Events with EPA – Intervention Trial (REDUCE-IT) was a randomized, double blind trial of 2 grams twice per day of EPA ethyl ester (icosapent ethyl) (Vascepa) vs. mineral oil placebo in 8,179 patients with hypertriglyceridemia (135mg/dL to 499mg/dL) and established cardiovascular disease or high cardiovascular disease risk (diabetes plus one risk factor) who were on stable statin therapy (138). The primary end point was a composite of cardiovascular death, nonfatal myocardial infarction, nonfatal stroke, coronary revascularization, or unstable angina. The key secondary end point was a composite of cardiovascular death, nonfatal myocardial infarction, or nonfatal stroke. At baseline, the median LDL-C level was 75.0 mg/dL, HDL-C level was 40.0 mg/dL, and TG level was 216.0 mg/dL. The median change in TG level from baseline to 1 year was a decrease of 18.3% (−39.0 mg/dL) in the EPA group and an increase of 2.2% (4.5 mg/dL) in the placebo group. After a median of 4.9 years the primary end-point occurred in 17.2% of the patients in the EPA group vs. 22.0% of the patients in the placebo group (hazard ratio, 0.75; P<0.001), indicating a 25% decrease in events. The number needed to treat to avoid one primary end-point event was 21. The reduction in cardiovascular events was noted after approximately 2 years of EPA treatment. Additionally, the rate of cardiovascular death was decreased by 20% in the EPA group (4.3% vs. 5.2%; hazard ratio, 0.80; P=0.03). The cardiovascular benefits of EPA were similar across baseline levels of TGs (<150, ≥150 to <200, and ≥200 mg per deciliter). Moreover, the cardiovascular benefits of EPA appeared to occur irrespective of the attained TG level at 1 year (≥150 or <150 mg/dL), suggesting that the cardiovascular risk reduction was not associated with attainment of a normal TG level. An increase in hospitalization for atrial fibrillation or flutter (3.1% vs. 2.1%, P=0.004) occurred in the EPA group. In addition, serious bleeding events occurred in 2.7% of the patients in the EPA group and in 2.1% in the placebo group (P=0.06). There were no fatal bleeding events in either group and the rates of hemorrhagic stroke, serious central nervous system bleeding, and serious gastrointestinal bleeding were not significantly higher in the EPA group than in the placebo group.

 

It should be noted that in this trial mineral oil was used as the placebo. In the placebo group the LDL-C, non-HDL-C, and CRP levels were increased compared to the EPA group during the trial (LDL-C 96mg/dL vs 85mg/dL; non-HDL-C 130mg/dL vs. 113mg/dL; hsCRP 2.8mg/L vs. 1.8mg/L). The impact of these adverse changes on clinical outcomes is uncertain and whether they contributed to the apparent beneficial effects observed in the individuals treated with EPA is unknown.

 

  • The STRENGTH Trial was a double-blind, randomized, trial comparing 4 grams per day of a carboxylic acid formulation of omega-3 fatty acids (EPA and DHA; Epanova) (n = 6,539)) vs. corn oil placebo (n = 6539) in statin-treated participants with high cardiovascular risk, hypertriglyceridemia, and low levels of HDL-C (139). Approximately 55% of patients had established cardiovascular disease and approximately 70% had diabetes. Median LDL-C level was 75.0 mg/dL, median TG level was 240 mg/dL and median HDL-C level was 36 mg/dL. There were minimal differences in the change in LDL-C and HDL-C levels between the treated and placebo groups after treatment for 12 months but as expected there was a greater reduction in TG levels in the group treated with omega-3-fatty acids (−19.0% vs −0.9%). The primary endpoint was a composite of cardiovascular death, nonfatal myocardial infarction, nonfatal stroke, coronary revascularization, or unstable angina requiring hospitalization which occurred in 12.0% of individuals treated with omega-3 CA vs. 12.2% treated with corn oil (hazard ratio, 0.99; P = .84). There were no significant differences between the treatment groups with regard to the risk of the individual components of the primary end point over the 3-4 years of the study. Similar to the REDUCE-IT trial atrial fibrillation was increased with EPA + DHA treatment (HR 1.69 CI 1.29- 2.21). Thus, in contrast the JELIS and REDUCE-IT trials the STRENGTH trial did not demonstrate a benefit of treatment with a mixture of omega-3-fatty acids (EPA + DHA).

 

  • The OMEMI trial was a randomized trial of 1.8 grams per day of omega-3-fatty acids (930 mg EPA and 660 mg DHA) (n= 505) vs. corn oil placebo (509) in patients aged 70 to 82 years with a recent myocardial infarction (2-8 weeks) (140). Baseline LDL-C was approximately 76mg/dL, HDL-C was 49mg/dL, and TGs 110mg/dL. The primary endpoint was a composite of nonfatal myocardial infarction, unscheduled revascularization, stroke, all-cause death, and heart failure hospitalization after 2 years of follow-up. The primary endpoint occurred in 21.4% of patients on omega-3-fatty acids vs. 20.0% on placebo (hazard ratio, 1.08; P=0.60). TGs levels decreased 8.1% in the omega-3-fatty acid group and increased 5.1% in the placebo group (between group difference 13.2%; P<0.001) while changes in LDL-C were minimal in both groups. Thus, similar to the STRENGTH trial no benefits on cardiovascular disease were observed with EPA + DHA treatment.

 

Summary of Omega-3-Fatty Acid Clinical Outcome Trials

 

  • Low dose omega-3-fatty acids are not effective at decreasing cardiovascular outcomes.
  • High dose EPA (JELIS and REDUCE-IT) reduced cardiovascular outcomes while high dose EPA+DHA (STENGTH and OMEMI) did not decrease cardiovascular outcomes.
  • The decrease in TG levels is not a major contributor to the beneficial effect of high dose EPA as the combination of high EPA+DHA lowers TG levels to the same degree as EPA alone without benefit. Additionally, the JELIS trial only lowered TG levels by 5% but nevertheless reduced cardiovascular events. It is likely that the beneficial effects of EPA seen in the JELIS and REDUCE-IT trials are multifactorial with TG lowering making only a small contribution to the decrease in cardiovascular disease. Other actions of EPA, such as decreasing platelet function, anti-inflammation, decreasing lipid oxidation, stabilizing membranes, etc. could account for or contribute to the reduction in cardiovascular events (141). A large meta-analysis, excluding the REDUCE-IT trial, demonstrated that a 40mg/dL decrease in triglyceride levels resulted in a relative risk reduction of only 0.96 (4% decrease) indicating that one needs to markedly lower triglyceride levels to reduce cardiovascular events (142).
  • Whether EPA has special properties that resulted in the reduction in cardiovascular events in the REDUCE-IT trial or there were flaws in the trial design (the use of mineral oil as the placebo) is uncertain and debated. It should be noted that in the REDUCE-IT trial LDL-C and non-HDL-C levels were increased by approximately 10% in the mineral oil placebo group (138). Additionally, Apo B levels were increased by 7% (6mg/dL) by mineral oil (138). Finally, an increase in hsCRP (20-30%) and other biomarkers of atherosclerosis (oxidized LDL-C, IL-6, IL-1 beta, and lipoprotein-associated phospholipase A2) were noted in the mineral oil group (138,143). In the STRENGTH trial there were no differences in LDL-C, Non-HDL-C, HDL-C, Apo B, or hsCRP levels between the treated vs. placebo groups (139). Whether EPA has special properties compared to DHA leading to a reduction in cardiovascular events or the mineral oil placebo resulted in adverse changes increasing ASCVD in the placebo resulting in an artifactual decrease in the EPA group is debated (144,145). Ideally, another large randomized cardiovascular trial with EPA ethyl ester (icosapent ethyl) (Vascepa) using a placebo other than mineral oil would help resolve this controversy. In the meantime, clinicians will need to use their clinical judgement on whether to treat patients with modest elevations in TG levels with EPA (icosapent ethyl; Vascepa) balancing the potential benefits of treatment vs. the potential side effects.

 

Side Effects

 

Gastrointestinal side effects such as diarrhea, nausea, dyspepsia, abdominal discomfort, and eructation have been observed with fish oil therapy (Package Inserts for Lovaza, Vascepa, and Epanova).

 

At very high doses, omega-3-fatty acids can inhibit platelets and prolong bleeding time. However, at the recommended doses this has not been a major clinical problem but nevertheless when patients are on anti-platelet drugs one should be alert for the possibility of bleeding problems (Package Inserts for Lovaza, Vascepa, and Epanova). Increased bleeding was noted in the REDUCE-IT trial in the patients treated with icosapent ethyl 4 grams/day (EPA) (see above discussion of this trial). A recent review found no evidence for discontinuing the use of omega-3 fatty acid treatment before invasive procedures or when given in combination with other agents that affect bleeding (146).

 

As noted above an increase in atrial fibrillation was observed in the REDUCE-IT trial in the patients treated with icosapent ethyl 4 grams/day (EPA) and in the STRENGTH trial in the patients treated with EPA + DHA.

 

Contraindications

 

There are no contraindications to the use of omega-3-fatty acids. Lovaza, Omacor, and Vascepa are pregnancy category C drugs and they should only be used if the benefits to the mother outweigh the potential risks to the fetus.

 

Conclusions

 

Omega-3-fatty acids are effective drugs in reducing TG levels with few significant side effects, drug interactions, or contraindications.  High dose EPA (4 grams/day) reduced cardiovascular disease events in the REDUCE-IT trial and a moderate dose of EPA (1.8 grams/day) reduced cardiovascular events in the JELIS trial but trials of EPA and DHA have not produced cardiovascular benefits. The basis for these differences is debated and discussed in the “Summary of Omega-3-Fatty Acid Clinical Outcome Trials” section above. Finally, omega-3-fatty acids are effective in lowering TGs in patients with marked hypertriglyceridemia and while not proven will likely reduce the risk of development of pancreatitis.

 

FIBRATES

 

Introduction

 

The fibrate drug class includes clofibrate, gemfibrozil, fenofibrate, bezafibrate, and ciprofibrate. Clofibrate was developed in the 1960s and was the first member of this class. Clofibrate is no longer available because of an increased risk of adverse effects. Gemfibrozil and fenofibrate are available in the United States while gemfibrozil, fenofibrate, bezafibrate, and ciprofibrate are available in Europe. All of the fibrates work via activation of the nuclear hormone receptor PPAR alpha.

 

Effect of Fibrates on Lipid and Lipoprotein Levels

 

Table 12. Effect of Fibrates on Lipids and Lipoproteins

Decreases TG

Increases HDL-C

Decreases LDL-C; if TGs Very High can Increase LDL-C

Decreases Non-HDL-C

Decreases Apolipoprotein B

Decreases LDL Particle Number

Shift Small Dense LDL to Large Buoyant LDL

No Effect on Lp(a)

 

Fibrates reduce fasting TG levels by 25-50% (147-149). The magnitude of the reduction in TGs is dependent on the baseline TG levels. Patients with marked elevations in TGs have a greater reduction in TG levels (147,149,150). In addition, fibrates increase HDL-C levels by 5-20% (148,149). The increase in HDL-C levels is more robust if the TG levels are elevated and/or if the HDL-C levels are low (150). The effect on LDL-C is more variable (149). If the TG levels are very high (>400-500mg/dL), fibrate therapy may result in an increase in LDL-C levels whereas if TGs are not elevated fibrates decrease LDL-C by 10-30% (147). Given the decrease in plasma TGs and LDL-C levels, fibrates also reduce apolipoprotein B, LDL particle number, and non-HDL-C levels (149). Depending upon the TG level there may be a shift from small dense LDL towards large LDL particles (149). Fibrates do not have any major or consistent effects on Lp(a) levels (151). Table 13 below shows the effect of fenofibrate on lipid and lipoprotein levels in patients with different lipid profiles and illustrates some of the principles outlined above.

 

Table 13. Effect of Fenofibrate on Lipid and Lipoprotein Levels

 

TGs

LDL-C

HDL-C

Elevated TG Levels

 

 

 

Baseline Levels

~404mg/dL

~125mg/dL

~35mg/dL

Change with Fenofibrate

45% Decrease

2.5% Increase

16% Increase

Elevated LDL-C and TG Levels

 

 

 

Baseline Levels

232mg/dL

220mg/dL

46.7mg/dL

Change with Fenofibrate

37% Decrease

13% Decrease

12% Increase

Elevated LDL-C and Normal TG Levels

 

 

 

Baseline Levels

102mg/dL

228mg/dL

58.1mg/dL

Change with Fenofibrate

35% Decrease

29% Decrease

7% Increase

The values are adjusted for changes in the placebo group. Data modified from Tricor Package Insert.

 

In large, randomized, fibrate outcome trials similar changes in lipid and lipoprotein levels were noted (Table 14). These trials are discussed in detail in the effect of fibrates on cardiovascular outcomes section presented below.

 

Table 14. Effect of Fibrates on Lipid and Lipoprotein Levels in Large Outcome Studies*

 

TGs

LDL-C

HDL-C

Helsinki Heart Study- Gemfibrozil (152)

35% Decrease

11% Decrease

10% Increase

VA-HIT Study

Gemfibrozil (153)

31% Decrease

No Change

6% Increase

BIP Study

Bezafibrate (154)

21% Decrease

7% Decrease

18% Increase

Leader Study

Bezafibrate (155)

23% Decrease

8% Decrease

8% Increase

Field Study

Fenofibrate (156)

29% Decrease

12% Decrease

5% Increase

*The values are adjusted for changes in the placebo group.

 

The different fibrates in general cause similar changes in lipid and lipoprotein levels. There are only a few comparative trials of fibrates comparing their effects on lipid and lipoprotein levels and these trials have been very small. Comparisons of ciprofibrate and gemfibrozil have not shown any major differences between these two fibrates (157,158). In contrast, two very small trials have compared gemfibrozil vs. fenofibrate and reported that fenofibrate was more efficacious in lowering LDL levels than gemfibrozil (159,160).

 

In very rare instances fibrates can cause a paradoxical marked decrease in HDL-C levels (161-164). This rare paradoxical decrease in HDL-C typically occurs when fibrates are used in combination with a thiazolidinedione (rosiglitazone and pioglitazone) but can occur when fibrates are used alone or with ezetimibe (161-165). The decrease in HDL-C can be extreme with decreases of 50% to 88% reported and recovery to normal can take weeks after the fibrate is discontinued (162). The mechanism for this paradoxical effect is unknown.

 

Effect of Fibrates in Combination with Other Lipid Lowering Drugs on Lipid and Lipoprotein Levels

 

STATINS

 

Statins are the primary drugs used to treat most patients with dyslipidemia. Statins are very effective in lowering LDL-C levels but have only modest effects on TG and HDL-C levels. Therefore, it is appealing to add a fibrate to patients who on statin therapy have LDL-C levels at goal but still have elevated non-HDL-C and TG levels and decreased HDL-C levels. Therefore, there have been numerous studies examining the effect of the combination of statins and fibrates on lipid and lipoprotein levels. An example is the Safari Trial which compared the effect of simvastatin only (n=207) vs. simvastatin + fenofibrate (n=411) in patients with combined hyperlipidemia (166). The results of this trial are shown in table 15. As anticipated, adding a fibrate results in a further lowering of LDL-C, non-HDL-C, and TG levels with a further increase in HDL-C.

 

Table 15. Effect of Simvastatin Alone vs. Simvastatin + Fenofibrate on Lipid and Lipoprotein Levels

 

LDL

TG

Non-HDLC

HDL

Simvastatin

-26%

-20%

-26%

+10%

Simvastatin + Fenofibrate

-31%

-43%

-35%

+19%

 

A meta-analysis of 9 studies with over 1,200 participants compared the effect of statin alone vs. statin + fibrate on lipid and lipoprotein levels (167). The combination of statins and fibrates provided significantly greater reductions in total cholesterol, LDL-C, and TGs, and a significantly greater increase in HDL-C than treatment with statins alone. A larger meta-analysis of 13 randomized controlled trials, involving 7,712 patients, similarly demonstrated significant decreases in LDL-C (8.8mg/dL), TGs (58mg/dL), and total cholesterol (11.2mg/dL), and increases in HDL-C (4.65mg/dL) in patients receiving the combination of statins + fibrates compared with statin therapy alone (168). The combination of statins + fibrates also result in a shift of LDL particles from small dense particles to large buoyant particles whereas no change in LDL particle size was observed with statin monotherapy (169).  

 

A recent meta-analysis of 6 studies with over 400 participants compared the effect of adding a statin to fibrate therapy (fibrate alone vs. fibrate + statin) and showed similar changes (170).  The fibrate-statin combination produced significantly greater reductions in the levels of total cholesterol, LDL-C, and TGs compared to fibrate alone. In contrast there was no significant difference in HDL-C levels in the fibrate vs. fibrate + statins group.

 

EZETIMIBE

 

In patients unable to tolerate statin therapy one needs to use other drugs to treat dyslipidemia. In a study comparing the effect of ezetimibe 10mg alone, fenofibrate 145mg alone, or ezetimibe + fenofibrate the combination had a better effect on the lipid profile resulting in a greater decrease in LDL-C levels and increase in HDL-C levels than either drug alone (Table 16) (171).

 

Table 16. Effect of the Combination of Ezetimibe and Fenofibrate on Lipid and Lipoprotein Levels

 

LDL-C

HDL-C

TG

Ezetimibe

23% Decrease

2.2% Increase

10% Decrease

Fenofibrate

22% Decrease

7.5% Increase

38% Decrease

Ezetimibe + Fenofibrate

34% Decrease

11.5% Increase

38% Decrease

 

Similar results were observed in another randomized trial of ezetimibe 10mg and fenofibrate 160mg (172). Moreover, both fibrate therapy and the combination of ezetimibe and fenofibrate results in a shift of LDL particles from small dense LDL particles to large buoyant particles (172).

 

EZETIMIBE + STATIN

 

A large randomized trial has compared the effect of ezetimibe /simvastatin 10mg/20mg, fenofibrate 160mg, or ezetimibe/simvastatin + fenofibrate on lipid and lipoprotein levels. As one would expect triple drug therapy had a better effect on the lipid profile (Table 17) (173). While ezetimibe/simvastatin was very effective in lowering LDL-C levels and fenofibrate was very effective in lowering TGs and raising HDL-C levels the combination resulted in more favorable changes in TGs. In a similar study the addition of fenofibrate 135mg to atorvastatin 40 mg + ezetimibe 10 mg resulted in a greater reduction in TGs (-57% vs. -40%; p<0.001) and a greater increase in HDL (13% vs. 4.2%; p<0.001) than placebo (174).  Fibrate therapy and ezetimibe/simvastatin + fenofibrate also resulted in a shift of LDL particles from small dense LDL particles to large buoyant particles (173).

 

Table 17. Effect of the Combination of Ezetimibe/Simvastatin and Fenofibrate on Lipid and Lipoprotein Levels

 

LDL-C

HDL-C

TG

Placebo

-3.5%

+1.1

-3.1%

Ezetimibe/Simvastatin

-47%

+9.3%

-29%

Fenofibrate

-16%

+18.2

-41

Eze/Simva + Fenofibrate

-46%

+18.7

-50%

 

BILE ACID SEQUESTRANT  

 

Studies have also examined the effect of fibrates in combination with bile acid sequestrants. Participants receiving fenofibrate 160 mg/day were randomized to receive either colesevelam HCl 3.75 g/day or placebo (175). No significant differences in TG or HDL-C levels were observed between the two groups. However, LDL-C levels were decreased in the fenofibrate + colesevelam group compared to the fenofibrate + placebo group (12.4% greater decrease: p<0.001). A study of the combination of fenofibrate and colestipol also demonstrated a more marked decrease in LDL-C with that combination compared to either drug alone (colestipol -18%; fenofibrate -17%, colestipol + fenofibrate 37%) (176). The combination of both drugs did not blunt the effects of fenofibrate on VLDL and HDL. Other studies of the combination of a fibrate with a bile acid sequestrant have also demonstrated an enhanced effect in lowering LDL-C levels (177-179).

 

NIACIN

 

Surprisingly there are few large randomized trials examining the effect of combination therapy with niacin + fibrate vs. monotherapy. One very small trial did reported that while both niacin monotherapy and bezafibrate monotherapy were effective in lowering serum TGs there was no added benefit of combination therapy in reducing serum TG level although a large variance may have reduced the ability to detect statistically significant results (16). A larger trial in HIV+ patients reported that the combination of niacin and fenofibrate was better at lowering TGs and non-HDL-C and increasing HDL-C levels than monotherapy with either niacin or fenofibrate (17). It would be informative if additional trials of fibrate + niacin combination therapy were carried out in patients with marked hypertriglyceridemia that can often be difficult to control with lifestyle changes and monotherapy.

 

FISH OIL  

 

In patients with TG levels > 500mg/dL the combination of fenofibrate 130mg per day and 4 grams of Lovaza (DHA and EPA) reduced TG levels to a greater extent than monotherapy with fenofibrate alone (7% decrease; P = 0.059) (103). Not unexpectedly, LDL levels were increased to a greater extent with combination therapy (9% increase; p= 0.03). When subjects who had received 8 weeks of fenofibrate monotherapy were treated with Lovaza (DHA and EPA) during the 8-week, open-label extension study TG levels were reduced by an additional 17.5% (P = 0.003). These results indicate that the addition of omega-3-fatty acids to fenofibrate will further decrease TG levels.

 

Mechanisms Accounting for the Fibrate Induced Lipid Effects

 

Fibrates are ligands that bind and activate PPAR alpha, a member of the family of nuclear hormone receptors that are activated by lipids (180,181). PPAR alpha is highly expressed in the liver and other tissues important in fatty acid metabolism. PPAR alpha forms a heterodimer with RXR and together the PPAR alpha:RXR complex when activated binds to the PPAR response elements in a large number of genes and regulates the expression of these genes (180,181). The natural ligands of PPAR alpha are fatty acid derivatives formed during lipolysis, lipogenesis, or fatty acid catabolism (180,181).

 

TRIGLYCERIDES  

 

Fibrates lower plasma TG levels by decreasing VLDL production and by increasing the clearance of TG rich lipoproteins (182,183). The decrease in VLDL production is primarily due to PPAR alpha activation of the beta oxidation of fatty acids, which reduces the substrate available for the synthesis of TGs and the formation of VLDL (180,183). Additionally, a decrease in hepatic fatty acid synthesis may also contribute to the decrease in fatty acids (180,183). The increased clearance of TG rich lipoproteins is due to PPAR alpha stimulating the transcription of lipoprotein lipase, the key enzyme that catabolizes the TGs carried by VLDL and chylomicrons (180,183). In addition, activation of PPAR alpha also inhibits the transcription of APO C-III, which inhibits lipoprotein lipase activity (180,183). A decrease in Apo C-III enhances the clearance of TG rich lipoproteins by increasing lipoprotein lipase activity. Notably, a decrease in Apo C-III also decreases TG levels in patients deficient in lipoprotein lipase indicating that there are multiple mechanisms for its effects on TG metabolism (184). Recent studies suggest that Apo C-III inhibits the uptake of TG rich lipoproteins into the liver by the LDL receptors/ LDLR-related protein 1 axis (185). PPAR alpha activation also increases the transcription of Apo A-V, which would also facilitate the activity of lipoprotein lipase (180).

 

HIGH DENSITY LIPOPROTEINS

 

The increase in HDL induced by fibrates is due to PPAR alpha activation stimulating Apo A-I and A-II transcription (180,183). This leads to the increased production of HDL (182). In addition, a decrease in TG rich lipoproteins may result in a reduction in CETP mediated transfer of cholesterol from HDL to VLDL and of TG from VLDL to HDL (183). This would lead to less TG enrichment of HDL and a decrease in the opportunity of hepatic lipase to remove TG leading to small HDL particles that may be rapidly catabolized.

 

LOW DENSITY LIPOPROTEINS

 

As noted above the effect of fibrates on LDL-C levels is variable with increases in LDL seen in patients with high TG levels (>400mg/dL) and decreases in LDL-C levels in patients with lower TG levels. In patients with modest elevations in plasma TG levels the clearance of LDL is enhanced (182). The mechanism for this enhanced clearance could be due to a decrease in Apo C-III, as increased levels of this protein inhibits LDL receptor activity (185,186). Additionally, the shift from small dense LDL to large buoyant LDL would enhance the uptake of LDL by the LDL receptor (187). In patients with TG levels > 400mg/dL fibrate therapy decreases LDL clearance (182). Prior to treatment, patients with marked hypertriglyceridemia have hypercatabolism of LDL, which is likely due to increased uptake by the reticuloendothelial system (182). This increased clearance is LDL receptor independent. Treatment with fibrates lowers the plasma TGs leading to normalization of reticuloendothelial cell function and a decrease in LDL clearance resulting in an increase in LDL-C levels with fibrate therapy (182). In addition, the metabolism of VLDL to LDL may be enhanced by fibrates when the TG levels are markedly elevated.

 

Effect of Monotherapy with Fibrates on Cardiovascular Outcomes

 

There have been a number of studies that have examined the effect of monotherapy with a variety of different fibrates on cardiovascular disease. We will describe the major studies below.

 

  • Coronary Drug Project (CDP): CDP conducted between 1966 and 1975, was a randomized, double-blind clinical trial that determined the effect of clofibrate (1.8g/day), dextrothyroxine (6mg/day), two doses of oral estrogen (2.5 or 5mg per day), or immediate release niacin (3 grams/day) vs. placebo in 8,341 men aged 30 to 64 years of age with an electrocardiogram documented myocardial infarction on cardiovascular events and mortality (43). The mean baseline total cholesterol level was 251mg/dL and TG level was 183mg/dL. The two estrogen regimens and dextrothyroxine treatment groups were discontinued early because of increased adverse effects. Clofibrate treatment (n= 1,051) compared to placebo (n= 2,680) also did not demonstrate clinical benefit. The five-year mortality in subjects treated with clofibrate was 20.0% as compared with 20.9% in subjects on placebo therapy (P = 0.55). The results with niacin are discussed above in the section on niacin and cardiovascular outcomes.

 

  • WHO: WHO was a double-blind trial in middle-aged men, age 30-59 years of age, without evidence of heart or other major disease, who were treated with 1.6 grams/day clofibrate (n=5,000) or placebo (n=5,000) for an average of 5.3 years (188). Average serum cholesterol levels were approximately 248mg/dL and a mean reduction of approximately 9 per cent occurred in the clofibrate group. The incidence of ischemic heart disease was decreased by 20% in the clofibrate group compared to the control group (P <0.05). This decrease was confined to non-fatal myocardial infarcts which were reduced by 25% while the incidence of fatal heart attacks and angina was similar in the clofibrate and placebo groups. Importantly, the numbers of deaths and crude mortality rates from all causes were increased in the clofibrate-treated group compared to the control group (P < 0.05). The excess deaths were partially accounted for by increased deaths due to liver, biliary tract, and intestinal disease. There was also an increase in cholecystectomies in subjects treated with clofibrate. Because of increased toxicity clofibrate is no longer available.

 

  • Helsinki Heart Study (HHS): HSS was a randomized double-blind trial in middle aged men age 40-55 years of age without cardiovascular who had non-HDL-C levels greater than or equal to 200mg/dL (152). Subjects were randomized to receive 600mg gemfibrozil twice a day (n=2,051) or placebo (n=2,030) for five years. At initiation of the study total cholesterol was 289mg/dL, HDL-C 47mg/dL, non-HDL-C 242mg/dL, and TGs 176mg/dL. Gemfibrozil caused an increase in HDL-C (approximately 10%) and reductions in total (~10%), LDL-C (~11%), non-HDL-C (~14%), and TG levels (~35%). There were minimal changes in serum lipid levels in the placebo group. Fatal and non-fatal myocardial infarctions and cardiac death were the principal end points and the cumulative rate of these cardiac end points were reduced 34% in the gemfibrozil group (27.3 per 1,000 in the gemfibrozil group vs. 41.4 per 1,000 in the placebo group; P< 0.02). The decrease in cardiovascular disease in the gemfibrozil group became evident in the second year and continued throughout the remainder of the study. There was no difference in mortality between the gemfibrozil and placebo groups. The benefit of gemfibrozil therapy was greatest in participants with elevated TGs and decreased HDL-C levels (189,190). Risk reduction with gemfibrozil was 78% (P = .002) among those with BMI > 26 kg/m2 and dyslipidemia (TGs > ~200mg/dL and HDL-C < 42mg/dL) suggesting that certain types of patients are likely to derive greater benefit from fibrate treatment (191).

 

  • Veterans Affairs High-Density Lipoprotein Cholesterol Intervention Trial (VA-HIT): VA-HIT was a double-blind trial in men with coronary heart disease who had an HDL-C level <40mg/dL and LDL-C level <140mg/dL (153). Subjects were randomized to gemfibrozil 1200mg per day (n=1,264) or placebo (n=1,267) for 5.1 years. Mean lipid levels at study initiation were HDL-C 32mg/dL, LDL-C 111mg/dL, total cholesterol 175mg/dL, and TGs 160mg/dL. At one year, the mean HDL-C level was 6 percent higher, the mean TG level was 31 percent lower, and the mean total cholesterol level was 4 percent lower in the gemfibrozil group than in the placebo group. LDL-C levels did not differ significantly between the groups. The primary study outcome was nonfatal myocardial infarction or death from coronary causes. The primary outcome occurred in 21.7% of patients in the placebo group and 17.3% of patients in the gemfibrozil group (22 percent decrease; P=0.006). A 24% reduction in the combined outcome of death from coronary heart disease, nonfatal myocardial infarction, and stroke was observed in the gemfibrozil group (P< 0.001). There were no significant differences in the rates of coronary revascularization, hospitalization for unstable angina, death from any cause, and cancer. Similar to HHS the beneficial effect of gemfibrozil did not become apparent until approximately two years after treatment. A low HDL-C (<33.5mg/dL) and high TGs (>180mg/dL) at baseline predicted a beneficial response to gemfibrozil therapy (192).

 

  • Bezafibrate Infarction Prevention Study (BIP): BIP was a double-blind study in male and female patients aged 45-74 with a previous myocardial infarction or stable angina (154). Patients were randomized to receive either 400 mg of bezafibrate per day (n=1,548) or a placebo (n=1,542) and were followed for 6.2 years. At the initiation of the study total cholesterol was 212mg/dL, LDL-C was 148mg/dL, HDL-C was 34.6mg/dL, and TGs were145mg/dL. Bezafibrate increased HDL-C by 18% and reduced TGs by 21%. There was a small 7% decrease in LDL-C. The primary end point was fatal or nonfatal myocardial infarction or sudden death. The primary end point occurred in 13. 6% of the bezafibrate group vs. 15.0% of the placebo (9.4% reduction; P=0.26). Total and non-cardiac mortality rates were similar. In a post hoc analysis in the subgroup with high baseline TGs (> or =200 mg/dL), the reduction in the primary end point in the bezafibrate group was 39.5% (P=0.02). Additionally, bezafibrate reduced cardiovascular events in patients with the metabolic syndrome (193). These results again suggest that patients with high TGs are likely to derive benefit from fibrate therapy.

 

  • Leader Trial: The Leader trial was a double blind placebo controlled randomized trial in men age 35 to 92 with lower extremity arterial disease (194,195). Subjects were randomized to bezafibrate 400mg per day (n=783) or placebo (n=785). At baseline total cholesterol levels were 218mg/dL, LDL-C levels 132mg/dL, HDL-C levels 44mg/dL, and TGs 187mg/dL. Bezafibrate therapy reduced total cholesterol levels by 7.6%, LDL-C by 8.1%, and TGs by 23% and increased HDL-C levels by 8%. The primary endpoint of coronary heart disease and strokes was not reduced by bezafibrate treatment. Neither major coronary events nor strokes were significantly reduced.

 .

  • Fenofibrate Intervention and Event Lowering in Diabetes Trial (FIELD): In the FIELD Trial patients with Type 2 diabetes between the ages of 50 and 75 with or without pre-existing cardiovascular disease not taking statin therapy were randomized to fenofibrate 200 mg daily (n=4,895) or placebo (n=4,900) and followed for approximately 5 years (156). At initiation of the study total cholesterol was 196mg/dL, LDL-C was 120mg/dL, HDL-C was 43mg/dL, and TGs were 152mg/dL. Fenofibrate therapy resulted in an 11% decrease in total cholesterol, a 12% decrease in LDL-C, a 29% decrease in TGs, and a 5% increase in HDL-C levels. The primary outcome was coronary events (coronary heart disease death and non-fatal MI), which were reduced by 11% in the fenofibrate group but this difference did not reach statistical significance (p= 0.16). However, there was a 24% decrease in non-fatal MI in the fenofibrate treated group (p=0.01) and a non-significant increase in coronary heart disease mortality. Total cardiovascular disease events (coronary events plus stroke and coronary or carotid revascularization) were reduced 11% (p=0.035). These beneficial effects of fenofibrate therapy on cardiovascular disease were observed in patients without a previous history of cardiovascular disease. In patients with a previous history of cardiovascular disease no benefits were observed. Additionally, the beneficial effect of fenofibrate therapy was seen only in those subjects less than 65 years of age. The beneficial effects of fenofibrate in this study may have been blunted by the increased use of statins in the placebo group, which reduced the differences in lipid levels between the placebo and fenofibrate groups. If one adjusted for the addition of lipid-lowering therapy, fenofibrate reduced the risk of coronary heart disease events by 19% (p=0.01) and of total cardiovascular disease events by 15% (p=0.004). Additionally, many patients in the Field trial did not have elevations in TGs and decreased HDL-C levels. In a post hoc analysis, patients with high TGs 200mg/dL) and low HDL levels (<40mg for men and <50mg/dL for women) derived greater benefit from fenofibrate therapy (196).

 

  • Summary: While the above monotherapy fibrate studies suggest that fibrates reduce cardiovascular event, particularly in patients with high TG and low HDL levels, the results are not as robust or consistent as the beneficial effects of statins on cardiovascular outcomes (5).

 

Effect of Combination Therapy of Fibrates and Statins on Cardiovascular Outcomes

 

Given the marked benefits of statin therapy it is essential to know if adding fibrates to statin therapy further reduces cardiovascular events. Two large trials described below have addressed this key question.

 

  • ACCORD LIPID Trial: The ACCORD-LIPID Trial was designed to determine if the addition of fenofibrate to aggressive statin therapy would result in a further reduction in cardiovascular disease in patients with Type 2 diabetes (197). In this trial, 5,518 patients on statin therapy were randomized to placebo or fenofibrate therapy. The patients had diabetes for approximately 10 years and either had pre-existing cardiovascular disease or were at high risk for developing cardiovascular disease. During the trial, LDL-C levels were approximately 80mg/dL. There was only a small difference in HDL-C with the fenofibrate groups having a mean HDL-C of 41.2mg/dL while the control group had an HDL-C of 40.5mg/dL. Differences in TG levels were somewhat more impressive with the fenofibrate group having a mean TG level of 122mg/dL while the control group had a TG level of 144mg/dL. First occurrence of nonfatal myocardial infarction, nonfatal stroke, or death from cardiovascular causes was the primary outcome and there was no statistical difference between the fenofibrate treated group and the placebo group. Additionally, there were also no statistically significant differences between the groups with regards to any of the secondary outcome measures of cardiovascular disease. Of note, the addition of fenofibrate to statin therapy did not result in an increase in either muscle or liver side effects. On further analysis there was a suggestion of benefit with fenofibrate therapy in the patients in whom the baseline TG levels were elevated (>204mg/dL) and HDL-C levels decreased (<34mg/dL). While this was a negative study, it must be recognized that most of the patients included in this study did not have the lipid profile that would typically lead to treatment with fibrates.

 

  • PROMINENT Trial: The PROMINENT trial studied the effect of pemafibrate, a new selective PPAR-alpha activator, in reducing cardiovascular outcomes in 10,497 patients (66.9% with previous ASCVD) with diabetes (198). This was a double-blind, randomized, controlled trial, in patients with Type 2 diabetes, with mild-to-moderate hypertriglyceridemia (TG level, 200 to 499 mg/dL), LDL-C < 100mg/dL, and HDL-C levels < 40 mg/dL who received either pemafibrate (0.2-mg tablets twice daily) or placebo in addition to statin therapy (96% on statins). The primary end point was a composite of nonfatal MI, ischemic stroke, coronary revascularization, or death from cardiovascular causes. Baseline fasting TG was 271 mg/dL, HDL-C 33 mg/dL, and LDL-C 78 mg/dL. Compared with placebo, pemafibrate decreased TG by 26.2%, while HDL-C increased 5.1% and LDL-C increased 12.3%. Notably non-HDL-C levels were unchanged and Apo B levels increased 4.8%. The primary endpoint was similar in the pemafibrate and placebo group (HR 1.03; 95% CI 0.91 to 1.15). The increase in LDL-C and Apo B levels may have accounted for the failure to reduce cardiovascular events.

 

  • Summary: The results of the ACCORD and PROMINENT trials were disappointing and have greatly reduced the enthusiasm for adding fibrates to statin therapy to cardiovascular events.

 

Effect of Fibrates on Non-Cardiovascular Outcomes

 

DIABETIC RETINOPATHY

 

Small studies in the 1960’s presented suggestive evidence that treatment with clofibrate improved diabetic retinopathy (199,200). Randomized trials have confirmed these observations.

 

The Fenofibrate Intervention and Event Lowering in Diabetes (FIELD) study was a randomized trial in patients with Type 2 diabetes mellitus. Patients were randomly assigned to receive either fenofibrate 200 mg/day (n=4,895) or placebo (n=4,900). Laser treatment for retinopathy was significantly lower in the fenofibrate group than in the placebo group (3.4% patients on fenofibrate vs 4.9% on placebo; p=0.0002) (201). Fenofibrate therapy reduced the need for laser therapy to a similar extent for maculopathy (31% decrease) and for proliferative retinopathy (30% decrease). In the ophthalmology sub-study (n=1,012), the primary endpoint of 2-step progression of retinopathy grade did not differ significantly between the fenofibrate and control groups (9.6% patients on fenofibrate vs 12.3% on placebo; p=0.19). In patients without pre-existing retinopathy there was no difference in progression (11.4% vs 11.7%; p=0.87). However, in patients with pre-existing retinopathy, significantly fewer patients on fenofibrate had a 2-step progression than did those on placebo (3.1% patients vs 14.6%; p=0.004). A composite endpoint of 2-step progression of retinopathy grade, macular edema, or laser treatments was significantly reduced in the fenofibrate group (HR 0.66, 95% CI 0.47-0.94; p=0.022).

 

In the ACCORD Lipids Study a subgroup of participants were evaluated for the progression of diabetic retinopathy by 3 or more steps on the Early Treatment Diabetic Retinopathy Study Severity Scale or the development of diabetic retinopathy necessitating laser photocoagulation or vitrectomy over a four-year period (202). At 4 years, the rates of progression of diabetic retinopathy were 6.5% with fenofibrate therapy (n=806) vs. 10.2% with placebo (n=787) (adjusted odds ratio, 0.60; 95% CI, 0.42 to 0.87; P = 0.006). Of note, this reduction in the progression of diabetic retinopathy was of a similar magnitude as intensive glycemic treatment vs. standard therapy.

 

A double-blind, randomized, placebo-controlled study in 296 patients with type 2 diabetes mellitus evaluated the effect of placebo or etofibrate on diabetic retinopathy (203). After 12 months an improvement in ocular pathology was more frequent in the etofibrate group vs the placebo group ((46% versus 32%; p< 0.001).

 

The MacuFen study was a small double-blind, randomized, placebo-controlled study in 110 subjects with diabetic macular edema who did not require immediate photocoagulation or intraocular treatment (204). Patients were randomized to fenofibric acid or placebo for 1 year. Patients treated with fenofibric acid had a modest improvement in total macular volume that was not statistically significant compared to the placebo group.

 

Taken together these results indicate that fibrates have beneficial effects on the progression of diabetic retinopathy (205). The mechanisms by which fibrates decrease diabetic retinopathy are unknown, and whether decreases in serum TG levels plays an important role is uncertain. Fibrates activate PPAR alpha, which is expressed in the retina (206). Diabetic PPARα KO mice developed more severe DR while overexpression of PPARα in the retina of diabetic rats significantly alleviated diabetes-induced retinal vascular leakage and retinal inflammation, suggesting that fibrates could have direct effects on the retina to reduce diabetic retinopathy (206).

 

DIABETIC KIDNEY DISEASE

 

The Diabetes Atherosclerosis Intervention Study (DAIS) evaluated the effect of fenofibrate therapy (n= 155) vs. placebo (n=159) on changes in urinary albumin excretion in patients with Type 2 diabetes (207). Fenofibrate significantly reduced the worsening of albumin excretion (fenofibrate 8% vs. placebo 18%; P < 0.05). This effect was primarily due to reduced progression from normal albumin excretion to microalbuminuria (fenofibrate 3% vs. 18% placebo; P < 0.001).

 

 In the Fenofibrate Intervention and Event Lowering in Diabetes (FIELD) trial, Type 2 diabetic patients aged 50 to 75 years were randomly assigned to fenofibrate (n = 4,895) or placebo (n = 4,900) for 5 years (208). Fenofibrate reduced urine albumin/creatinine ratio by 24% vs 11% in placebo group (p < 0.001), with 14% less progression and 18% more albuminuria regression (p < 0.001) in the fenofibrate group than in participants on placebo. As expected, fenofibrate therapy acutely increased plasma creatinine levels and decreased eGFR (209). However, over the long-term, the increase in plasma creatinine was lower in the fenofibrate group compared to the placebo group (14% decrease; p=0.01). Similarly, there was a slower annual decrease in eGFR in the fenofibrate group (1.19 vs 2.03 ml/min/1.73 m2annually, p < 0.001). End-stage renal disease, dialysis, renal transplant, and renal death were similar in the fenofibrate and placebo groups, likely due to the small number of events.

 

In the ACCORD-LIPID trial, 5,518 patients on statin therapy were randomized to placebo or fenofibrate therapy (197). The patients had diabetes for approximately 10 years and either had pre-existing cardiovascular disease or were at high risk for developing cardiovascular disease. The post-randomization incidence of microalbuminuria was 38.2% in the fenofibrate group and 41.6% in the placebo group (p=0.01) and post-randomization incidence of macroalbumuria was 10.5% in the fibrate group and 12.3% in the placebo group (p=0.04) indicating a modest reduction in the development of proteinuria in patients treated with fenofibrate (197). There was no significant difference in the incidence of end-stage renal disease or need for dialysis between the fenofibrate group and the placebo group, likely due to the small number of events.

 

A small randomized study in patients with Type 2 diabetes and hypertriglyceridemia compared the effect of fenofibrate (200mg/day) (n=28) vs. no treatment (n=28) on urinary albumin excretion (210). After 180 days urinary albumin/creatine ratio was decreased in the fenofibrate group vs. controls (control -8.15 vs fenofibrate -44.05 mg/g; P<0.05).

 

These studies suggest that fibrates may have a beneficial effect on diabetic kidney disease (211). One should recognize that reducing proteinuria is a surrogate marker and may not indicate a reduction in the development of end stage renal disease. The mechanisms accounting for the decrease in proteinuria are unknown.

 

AMPUTATIONS

 

In the Fenofibrate Intervention and Event Lowering in Diabetes (FIELD) study, patients aged 50-75 years with Type 2 diabetes were randomly assigned to receive fenofibrate 200 mg per day (n=4,895) or matching placebo (n=4,900) for 5 years' duration (212). The risk of first amputation was decreased by 36% (p=0.02) and minor amputation events without known large-vessel disease by 47% (p=0.027) in the fenofibrate treated group (212). The reduction in amputations was independent of glucose control or dyslipidemia. No difference between the risks of major amputations was seen in the placebo and fenofibrate groups. The basis for this reduction in amputations is unknown.

 

GOUT

 

In the Field trial treatment, fenofibrate reduced uric acid levels by 20% and reduced episodes of gout by approximately 50% compared to placebo (HR 0·48, 95% CI 0·37-0·60; p<0·0001) (213). Interestingly, a meta-analysis of fibrate trials found that fenofibrate but not bezafibrate reduced serum uric acid levels suggesting that the reduction in uric acid levels is not a class effect (214).

 

SUMMARY

 

The above studies provide substantial evidence that fibrates have a favorable effect on diabetic microvascular disease (155). While fibrates are not approved specifically for the prevention or treatment of diabetic microvascular disease one should consider these potential beneficial effects when deciding on treatment choices. For example, in a patient with diabetes and microvascular disease and hypertriglyceridemia needing therapy one might elect to use fibrates to lower plasma TGs given their potential beneficial effects on slowing the progression of microvascular disease. 

 

Side Effects

 

RENAL

 

Fibrate therapy leads to an increase in serum creatinine and cystatin C levels (215-217). For example, in the Field Trial serum creatinine levels increased from 0.88mg/dL to 0.99mg/dL, a 12% increase (156). This increase in creatinine has been seen with all fibrates but appears to be less profound with gemfibrozil (215). The increase in cystatin C occurs with fenofibrate but not with other fibrates (216). It must be recognized that this increase in creatinine is reversible on stopping fibrate therapy and does not reflect kidney damage (215). In fact, careful measurements of renal function have not demonstrated a decrease in glomerular filtration rate despite the increase in serum creatinine (209,218,219). As discussed above, studies of renal function in patients with diabetes actually suggests that treatment with fibrates may be protective. The precise mechanism by which fibrates increase serum creatinine levels is unknown.

 

In patients with chronic renal disease fibrates should be used with caution and at lower doses (215). Fibrates are all excreted by the kidneys and thus the excretion of fibrates is decreased in patients with renal dysfunction (215). Therefore, one needs to adjust the fibrate dose depending upon renal function. The National Kidney Foundation recommends the dose adjustments shown in Table 18 (220).

 

Table 18. Fibrate Dose Adjustments in Renal Disease

 

No Kidney Disease

GFR 30-60

GFR < 30

Kidney Transplant

Bezafibrate

400-600mg

200mg

Avoid

Avoid

Ciprofibrate

1000-2000mg

?

Avoid

Avoid

Fenofibrate

150-200mg

40-60mg

Avoid

Avoid

Gemfibrozil

1200mg

1200mg

600mg

600mg

 

GALLBLADDER DISEASE

 

It is clear that clofibrate increases the risk of gallbladder disease. In both the WHO trial and the Coronary Drug Project, cholecystectomies occurred two to three times more often in the patients treated with clofibrate compared to placebo (43,188,221). Whether gemfibrozil, fenofibrate, or other fibrates increases the risk of gallbladder disease is uncertain. In the large randomized outcome studies presented earlier (Effect of fibrates on cardiovascular outcomes section) a statistically significant increase in either gallbladder disease or cholecystectomies were not observed. However, in a sub-study of 450 Helsinki Heart Study participants a trend toward a greater prevalence of gallstones during the study in the gemfibrozil group was observed (7.5% versus 4.9% for the placebo group, a 55% excess for the gemfibrozil group) (Lopid Package Insert). A trend toward a greater incidence of gallbladder surgery was also observed in the gemfibrozil group (17 versus 11 subjects, a 54% excess) (Lopid Package Insert). In a single epidemiological trial fibrate treatment independently correlated with the presence of gallstones with a relative risk of 1.7 (p=0.04) (222).

 

All fibrates alter the composition of bile resulting in an increase in the concentration of cholesterol, which will predispose to the formation of cholesterol gallstones (215). In a comparison of clofibrate and gemfibrozil it was observed that clofibrate resulted in changes in bile composition that would be more lithogenic than gemfibrozil (223).

 

The effect of combining fibrates with statins on the risk of gallbladder disease is unknown.  An increased risk of gallbladder disease or cholecystectomies was not reported in the ACCORD-LIPID trial where fenofibrate was added to statin therapy or the PROMINENT trial where pemafibrate was added to statin therapy (197,198).

 

While it is clear that clofibrate increases the risk of gallbladder disease the effect of other fibrates either as monotherapy or in combination with other drugs is less well defined.

 

PANCREATITIS  

 

In a meta-analysis of 7 fibrate trials involving 40,162 participants conducted over 5.3 years, 144 participants developed pancreatitis (84 assigned to fibrate therapy, 60 assigned to placebo) (RR, 1.39 (95% CI, 1.00-1.95; P = .053) (224). These observations raise the possibility that fibrates may increase the risk of pancreatitis.

 

CANCER

 

A large meta-analysis of 17 randomized controlled trials, involving 44,929 participants, with an average follow-up of 5.2 years has examined if fibrates lead to an increased risk of cancer. No increase in either cancer incidence (RR = 1.02, 95% CI 0.92-1.12) or cancer death (RR = 1.06, 95% CI: 0.92-1.22) was noted with fibrate treatment (225).

 

LIVER DISEASE

 

Fenofibrate has rarely been associated with idiosyncratic hepatotoxicity manifesting as hepatocellular to cholestatic disorders (226). The hepatitis may be acute self-limited or persistent chronic hepatitis. Liver abnormalities are very rare and in large trials such as the FIELD trial described above liver function test abnormalities were similar in the fenofibrate and placebo groups (156).   

 

GLYCEMIC PARAMETERS

 

A meta-analysis of 22 randomized placebo-controlled trials involving a total of 11,402 subjects demonstrated that fibrate therapy significantly decreased fasting plasma glucose, insulin levels, and insulin resistance measured by HOMA-IR, but did not effect HbA1c levels (227).

 

MUSCLE DISORDERS

 

Fibrate monotherapy has been reported to cause myopathy (215). In a large epidemiological study the incidence of hospitalization for rhabdomyolysis per 10,000 person-years for monotherapy with a fibrate was 2.82 (95% CI, 0.58-8.24) while in patients not exposed to lipid lowering drugs the incidence was 0 (95% CI, 0-0.48) (228). The risk of rhabdomyolysis was greater with gemfibrozil therapy than with fenofibrate. Interestingly the incidence of rhabdomyolysis was greater for patients treated with fibrate monotherapy than for patients treated with statin monotherapy (incidence for atorvastatin, pravastatin, or simvastatin was only 0.44 per 10,000 person-years). In an epidemiological study focusing on myopathy similar results were observed (229). The relative risks of myopathy in current users of fibrates and statins compared with nonusers were 42.4 (95% CI = 11.6-170.5) and 7.6 (95% CI = 1.4-41.3), respectively. It should be recognized though that in large randomized clinical trials the risk of muscle symptoms was low in patients treated with fibrates and not dissimilar to that seen in the patients treated with placebo (215). For example, in the Helsinki Heart Study over 2,000 patients were treated and in the VA-HIT over 1,000 patients were treated with gemfibrozil for five years and no cases of  myopathy were reported in either trial (152,153). In the Bezafibrate Infarction Prevention Study, seven patients in the placebo group and five patients in the bezafibrate group reported muscle pain, while CPK levels greater than 2x the upper range of normal was seen in four patients in the bezafibrate group and one patient in the placebo group (154). Finally, in the Field Trial, patients with diabetes were treated with fenofibrate (n=4,895) or placebo (n=4,900) (156). Myositis was observed in one patient treated with placebo and two patients treated with fenofibrate while rhabdomyolysis was observed in one patient treated with placebo and three patients treated with fenofibrate. Elevations in CPK levels values > 10x the upper range of normal were seen in three patients on placebo and 4 patients treated with fenofibrate. Thus, while fibrates can lead to significant muscle dysfunction this is a rare event and appears to occur only slightly more often in patients treated with a fibrate than in patients treated with a placebo. The risk of serious muscle disease appears to be increased in patients with renal failure, hypothyroidism, and in the elderly (215). The mechanism by which fibrates predispose to muscle disorders is unknown.

 

The effect of fibrates in combination with statins on muscle disorders will be discussed in detail in the section on drug interactions below.

 

Drug Interactions

 

STATINS

 

The combination a fibrate and a statin may increase the risk of developing muscle symptoms (215). The degree of risk is dependent on both the specific statin and the specific fibrate that is being used in combination (215). For example, the average incidence per 10,000 person-years for hospitalization for rhabdomyolysis with monotherapy with atorvastatin, pravastatin, or simvastatin was 0.44 (95 % CI, 0.20-0.84); with fibrate alone was 2.82 (95% CI, 0.58-8.24); and with combined therapy of atorvastatin, pravastatin, or simvastatin with a fibrate was 5.98 (95% CI, 0.72-216.0) (228). Of note, the average incidence per 10,000 person-years for hospitalization for rhabdomyolysis with the combination of cerivastatin with a fibrate was 1035 (95% CI, 389-2117), clearly demonstrating an increased risk of the cerivastatin-fibrate combination compared to other statin-fibrate combinations (228). A study by Alsheikh-Ali and colleagues looking at cases of rhabdomyolysis reported to the FDA relative to the total number of prescriptions reached the conclusion that the combination of cerivastatin with a fibrate markedly increased the risk of this complication (230). Additionally, it was noted that the risk of rhabdomyolysis was greater with gemfibrozil compared to fenofibrate and that the combination of cerivastatin and gemfibrozil was particularly toxic (230). Other studies have also noted a marked risk with the combination of cerivastatin and gemfibrozil (231). Cerivastatin is no longer available.

 

Studies comparing the risk of rhabdomyolysis with gemfibrozil-statin combination therapy compared to fenofibrate-statin combination therapy have shown an increased risk with gemfibrozil (215). For example, the number of cases of rhabdomyolysis reported with fenofibrate and statins other than cerivastatin was 0.58 per million prescriptions whereas with gemfibrozil and statins other than cerivastatin was 8.6 per million prescriptions (232). Reviews of the FDA’s adverse events reporting system database have estimated that the risk of myopathy for the combination of gemfibrozil with a statin was much greater than the risk with the combination of fenofibrate with a statin (230,232).  Additionally, studies that employed the combination of gemfibrozil and statins have reported a significant occurrence of muscle related symptoms whereas studies of fenofibrate in combination with statins have not shown an increase in muscle related symptoms (215). For example, the rate of myopathy in over 4,000 patients taking lovastatin was only 0.4% but in patients on the combination of lovastatin and gemfibrozil the frequency increased to 5% (233). In contrast, in the ACCORD-LIPID Trial over 5,000 patients on statin therapy were randomized to fenofibrate or placebo for 4.7 years and no increase in the incidence of muscle related symptoms was observed with fenofibrate therapy (197). Similarly, in the Field Trial approximately 1,000 patients were taking fenofibrate and a statin and with 5 years of follow-up no cases of rhabdomyolysis were reported (156). Finally, a meta-analysis by Geng and colleagues identified 13 randomized trials with 7,712 patients receiving combination fenofibrate-statin therapy compared with statin therapy alone (168). The incidence of elevated creatine kinase levels, muscle-associated adverse events, or withdrawals attributed to muscle dysfunction did not differ significantly between the fenofibrate + statin patients vs. the statin alone patients (168). The American College of Cardiology and American Heart Association Guidelines recommend against using the combination of a statin and gemfibrozil but recognize that the use of a statin and fenofibrate is appropriate under certain circumstances (234).

 

The increased risk of combining gemfibrozil with statins is due to alterations in statin metabolism leading to increases in the serum levels of statins and hence an increased risk of myopathy (215,235). In contrast, fenofibrate does not alter statin metabolism and therefore can be safely combined with statins (Table 19) (235).   

 

Table 19. Effect of Fibrates on Statin Pharmacokinetics (215,235,236)

Statin

Gemfibrozil

Fenofibrate

Atorvastatin

Increase in C-Max by 1.5-Fold

No Change

Simvastatin

Increase in C-Max by 2-Fold

No Change

Pravastatin

Increase in C-Max by 2-Fold

No Change

Rosuvastatin

Increase in C-Max by 2-Fold

No Change

Lovastatin

Increase in C-Max by 2.8-Fold

No Change

Pitavastatin

Increase in C-Max by 41%

Unknown

Fluvastatin

No Change

No Change

  

The explanation for the difference between gemfibrozil and fenofibrate is that gemfibrozil uses the same family of glucuronidation enzymes as the statins thereby inhibiting statin metabolism (215,237). In contrast, fenofibrate uses a different family of glucuronidation enzymes and does not inhibit statin metabolism (215).

 

COUMADIN ANTI-COAGULANTS

 

Gemfibrozil and fenofibrate can potentiate the effect of coumadin anti-coagulants leading to a prolongation of prothrombin time and an increased risk of bleeding. When starting a fibrate in patients on coumadin therapy the dose of coumadin should be decreased and prothrombin times should be closely monitored (Lopid and Tricor Package Inserts).    

 

REPAGLINIDE

 

Gemfibrozil in combination with rapaglinide increases blood levels of rapaglinide and therefore this combination should not be used because of the increased risk of hypoglycemia (Lopid Package Insert).

 

Contraindications

 

Fibrates are contraindicated in patients with severe hepatic dysfunction. Additionally, patients with pre-existing gallstones should not be treated with fibrates. Fenofibrate and gemfibrozil are pregnancy category C drugs and should only be used if the potential benefit justifies the potential risk to the fetus. The combination of gemfibrozil and a statin should be avoided.

 

Conclusions

 

Fibrates are effective drugs in reducing TG levels and modestly increase HDL-C levels. Additionally, they also reduce LDL-C and non-HDL-C levels. Fibrates have a number of side effects and one should avoid using gemfibrozil in combination with statins. In contrast, fenofibrate can be used in combination with statins. Studies have not consistently demonstrated that fibrate monotherapy therapy reduces cardiovascular events and the combination of fibrates and statins in two studies has not been shown to be beneficial. Therefore enthusiasm to use fibrates to reduce cardiovascular events has markedly diminished. In patients with diabetes fibrates appear to slow the progression of microvascular disease. Finally, fibrates are effective in lowering TGs in patients with marked hypertriglyceridemia and while not proven will likely reduce the risk of the development of pancreatitis.

 

VOLANESORSEN

 

Introduction 

 

Volanesorsen (Waylivra) is an antisense oligonucleotide inhibitor of apolipoprotein C-III (apo C-III) mRNA that is approved in Europe for the treatment of familial chylomicronemia syndrome (FCS). This drug has not been approved by the FDA for use in the United States. FCS is a rare metabolic disorder involving the impaired function of lipoprotein lipase (LPL) due to mutations in LPL, Apo C-II, Apo A-V, lipase maturation factor 1, and glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1) (238,239). For a detailed discussion of the diagnosis and treatment of FCS see the following references (238-240).

 

Effect of Volanesorsen on Lipid and Lipoprotein Levels

 

FAMILIAL CHYLOMICRONEMIA SYNDROME (FCS)

 

A double-blind, randomized 52-week trial (APPROACH study) evaluated the ability of volanesorsen (300 mg subcutaneously once weekly) vs. placebo to decrease TG levels in 66 patients with FCS (baseline TGs 2,209mg/dL) (241). The primary end point was the percentage change in fasting TG levels at 3 months. As expected, there was a marked reduction in Apo C-III levels (84% decrease) in the volanesorsen group and a small increase (6%) in the placebo group. Most importantly patients treated with volanesorsen had a 77% decrease at 3 months in TG levels (mean decrease of 1,712 mg/dL) whereas patients receiving placebo had an 18% increase in TG levels. The decrease in TGs in patients treated with volanesorsen persisted for 24 months (242). Significantly, 77% of the patients in the volanesorsen group vs. only 10% of patients in the placebo group had TG levels of less than 750 mg/dL, a level that would greatly reduce the risk of pancreatitis. In addition, patients who received volanesorsen had decreases in levels of chylomicron TG by 83%, apolipoprotein B-48 by 76%, non–HDL-C by 46%, and VLDL-C by 58% and increases in levels of HDL-C by 46%, apolipoprotein A1 by 14%, LDL-C by 136% (note LDL-C increased from 28 to 61 mg/dL), and total apolipoprotein B by 20%.

 

While the APPROACH study was not powered to examine the effect of volanesorsen on pancreatitis, during the study three patients in the placebo group had four episodes of acute pancreatitis, whereas one patient in the volanesorsen group had one episode. In patients with a history of recurrent pancreatitis events (≥ 2 events in the 5 years prior to study, n = 11), a reduction in pancreatitis attacks was seen in patients treated with volanesorsen compared with placebo (none of the 7 patients in the volanesorsen group and 3 of the 4 patients in the placebo group experienced a pancreatitis attack over the 52-week study period).

 

In a retrospective global web-based survey open to all patients with the FCS who received volanesorsen for ≥3 months, 22 patients responded and reported reductions in steatorrhea, pancreatic pain, and constant worry about an attack of pain/ acute pancreatitis (243). The patients also reported that volanesorsen improved overall management of symptoms and reduced interference of FCS with work/school responsibilities. Decreases in the negative impact of FCS on personal, social, and professional life were also reported.

 

HYPERTRIGLYCERIDEMIA

 

A randomized, double-blind, placebo-controlled, study evaluated volanesorsen in patients with hypertriglyceridemia (244). Patients who were not receiving TG-lowering therapy (n=57) were eligible if they had fasting TG level between 350 mg/dL and 2000 mg/dL and were assigned to volanesorsen 100, 200, or 300 mg or placebo. Patients who were receiving a fibrate (n=28) were eligible if they had a fasting TG level between 225 mg/dL and 2000 mg/dL and were randomly assigned to volanesorsen 200 or 300 mg or placebo. The study drug was administered as a single subcutaneous injection once a week for 13 weeks. Baseline TG levels were 581±291 mg/dL in patients not on fibrates and 376±188 mg/dL in patients on fibrates. In patients not on fibrates volanesorsen 300 mg decreased Apo C-III levels by 79.6% vs. an increase of 4.2% in the placebo group (P<0.001) and decreased TG levels by 70.9% compared with an increase of 20.1% in the placebo group (P<0.001). Additionally, HDL-C levels increased by 45.7% from baseline in the 300 mg group, as compared with an increase of 0.7% in the placebo group (P<0.001). LDL-C levels increased from 79.5±29.9 mg/dL to 127.8±44.9 mg/dL with 300 mg of volanesorsen and was associated with an increase in LDL particle size. However, non-HDL-C and total apo B levels remained relatively unchanged and similar to those in the placebo group. Similar changes in Apo C-III, TGs, HDL-C, non-HDL-C, VLDL-C, and total apo B levels were observed in the patients on fibrates treated with volanesorsen. Of note, LDL-C levels did not increase in the patients on fibrates treated with volanesorsen perhaps due to the lower baseline TG levels. 

 

The COMPASS study randomized 113 patients with fasting TGs ≥500 mg/dL (mean TG 1,261mg/dL) to receive either volanesorsen 300 mg or placebo subcutaneously once weekly for 26 weeks (245). Most of these patients had the multifactorial chylomicronemia syndrome but a small number had FCS. A 71% reduction in TGs from baseline after 3 months was observed in patients treated with volanesorsen vs. a 0.9% reduction in placebo-treated patients (P<0.0001). LDL-C levels increased 96% (64 to 111mg/dL), HDL-C increased 61% (25 to 39mg/dL) and non-HDL-C decreased 27% (232 to 158mg/dL) Notably pancreatitis episodes were reduced with 5 events in 3 patients occurring in the placebo group vs. none with volanesorsen treatment (P=0.036). 

 

DIABETES

 

A randomized, double-blind, placebo-controlled trial of volanesorsen 300 mg weekly or placebo was performed in 15 adult patients with type 2 diabetes (HbA1c >7.5%) and hypertriglyceridemia (TG >200 and <500 mg/dL) (246). Treatment with volanesorsen significantly reduced plasma apo C-III (-88%, P = 0.02) and TG (-69%, P = 0.02) levels and raised HDL-C (42%, P = 0.03) without altering LDL-C levels compared with placebo. These changes were accompanied by a 57% improvement in whole-body insulin sensitivity (P < 0.001) and decreases in HbA1c (-0.44%, P = 0.025) 3 months postdosing. The improvement in insulin sensitivity was strongly related to the decrease in plasma apo C-III and TGs.

 

FAMILIAL PARTIAL LIPODYSTROPY (FPL)

 

Patients with FPL were randomized to volanesorsen 300mg weekly (n=21) or placebo (n=19) (247). Median TG level was 781mg/dL in the placebo group and 749mg/dL in the volanesorsen group. Volanesorsen treatment at 3 months resulted in an 88% decrease in TG levels while in the placebo group TG levels decreased by 22% (net difference of −67%; P=0.0009). Non-HDL-HDL-C levels decreased while LDL-C and HDL-C levels increased.

 

Mechanisms Accounting for the Volanesorsen Induced Lipid Effects

 

Volanesorsen binds to apo C-III mRNA leading to increased degradation and thereby inhibits the hepatic synthesis of apo C-III protein resulting in a reduction in plasma apo C-III levels (248,249). Apo C-III has a number of important effects on the metabolism of TG rich lipoproteins (250). Apo C-III is an inhibitor of LPL and therefore decreasing apo C-III levels will enhance LPL activity. In patients with FCS this will not be important because patients with this disorder have defects in components of the LPL complex that result in the inability to increase LPL activity. However, in patients with increased TG levels not due FCS this would accelerate the clearance of TG rich lipoproteins. Studies have also shown that apo C-III stimulates the production and secretion of VLDL by the liver. This effect is also not likely to be of primary importance in patients with FCS as the very high TG levels are primarily due to chylomicrons and not VLDL. However, in other situations increased hepatic secretion of VLDL may be an important contributor to the hypertriglyceridemia. Whether apo C-III regulates chylomicron secretion by the intestine is unknown. Finally, Apo C-III inhibits the binding of TG rich lipoproteins to hepatic LDL receptors and LDL receptor–related protein 1 decreasing the clearance of TG rich lipoprotein particles. A decrease in apo C-III will accelerate the clearance of TG rich lipoproteins, which likely accounts for the ability of volanesorsen to decrease TG levels in patients with FCS.

 

Drug Administration and Pharmacokinetics

 

The recommended starting dose is 285 mg injected subcutaneously once weekly for 3 months after which the dose should be reduced to 285 mg every 2 weeks. If serum TGs decrease by less than 25% or are not below 2000 mg/dL (22.6 mmol/L) after 3 months on volanesorsen 285 mg weekly, treatment should be discontinued (package insert;https://www.ema.europa.eu/en/documents/product-information/waylivra-epar-product-information_en.pdf).

 

After 6 months of treatment one can consider increasing the dose frequency back to 285 mg weekly if the serum TG response has been inadequate and the platelet counts are in the normal range. Patients should return to 285 mg every 2 weeks if the higher 285 mg once weekly dose does not provide a significant additional TG reduction after 9 months (package insert).

 

Effect on Clinical Outcomes

 

As described above in the description of the effect of volanesorsen on lipid/lipoprotein levels in patients with FCS and marked hypertriglyceridemia there is suggestive evidence that lowering the very high TG levels with volanesorsen treatment will reduce the risk of pancreatitis and improve the quality of life.

 

Volanesorsen treatment reduced hepatic fat assessed by MRI in patients with FCS, severe hypertriglyceridemia, and familial partial lipodystrophy (251). The greater the hepatic fat the greater the decrease induced by volanesorsen.

 

The effect of volanesorsen on cardiovascular disease has not been determined. However, epidemiologic studies have demonstrated that increased Apo C-III levels are associated with an increased risk of cardiovascular events (252-254)and coronary artery calcification (255). Moreover, carriers of rare heterozygous loss-of-function mutations in Apo C-III have reduced TG levels and reduced cardiovascular disease risk (256-258). One can speculate that lowering Apo C-III and TG levels with volanesorsen will have beneficial effects on the development of cardiovascular disease.

 

Side Effects

 

Treatment with volanesorsen is very commonly associated with reductions in platelet count in patients with the FCS and may result in thrombocytopenia (package insert; https://www.ema.europa.eu/en/documents/product-information/waylivra-epar-product-information_en.pdf). Platelet counts below 140 x 109/L were observed in 75% of patients treated with volanesorsen vs. 24% of placebo patients. Reductions to below 100 x 109/L were observed in 47% of patients treated with volanesorsen compared with none of the patients in the placebo group. Bleeding secondary to low platelets may occur. Careful monitoring for thrombocytopenia is important during treatment and recommendations for adjustments to monitoring frequency and dosing are shown in table 20 (package insert). Platelet counts recover following drug discontinuation and administration of glucocorticoids where medically indicated.

 

Table 20.  Volanesorsen Monitoring and Treatment Recommendations

Platelet Count (x109/L)

Dose

Monitoring Frequency

Normal (≥140)

Starting dose: Weekly

After 3 months: Every 2 weeks

Every 2 weeks

100-139

Every 2 weeks

Weekly

75-99

Pause treatment for ≥4 weeks and resume treatment after platelet levels ≥ 100 x 109/L

Weekly

50-74

Pause treatment for ≥4 weeks and resume treatment after platelet levels ≥ 100 x 109/L

Every 2-3 days

Less than 50

Discontinue treatment

Glucocorticoids recommended

Daily

 

Renal toxicity has been observed after administration of volanesorsen. Monitoring for evidence of nephrotoxicity by routine urine dipstick is recommended on a quarterly basis. In the case of a positive assessment, one should measure serum creatinine and collect a 24-hour urine collection to quantify the proteinuria and assess creatinine clearance. Treatment should be discontinued if proteinuria ≥ 500 mg/24 hour is present, or an increase in serum creatinine ≥ 0.3 mg/dL that is >ULN occurs, or the creatinine clearance estimated by the CKD-EPI equation is ≤ 30 mL/min/1.73m2(package insert).

 

Elevations of liver enzymes have been observed after administration of volanesorsen. Serum liver enzymes and bilirubin should be monitored every 3 months. Treatment should be discontinued if there is a single increase in ALT or AST > 8 x ULN, or an increase > 5 x ULN, which persists for ≥ 2 weeks, or lesser increases in ALT or AST that are associated with total bilirubin > 2 x ULN or INR > 1.5 (package insert).

 

As expected, injection site reactions are frequently observed and were reported in 82% of patients (erythema, pain, pruritus, or local swelling) (package insert).

 

Contraindications

 

Treatment should not be initiated in patients with thrombocytopenia (platelet count <140 x 109/L). Safety and efficacy have not been established in patients with severe renal disease or patients with hepatic impairment (package insert). There are no data on the use of volanesorsen in pregnant women and it is preferable to avoid the use of volanesorsen during pregnancy (package insert).

 

Drug Interactions

 

Discontinuation of antiplatelet drugs/NSAIDs/anticoagulants should be considered for

platelet levels < 75 x 109/L. Treatment with these products must be discontinued at platelet levels < 50 x 109/L. No other drug interactions have been described (package insert)

 

Conclusions

 

Volanesorsen is a useful drug in patients with the FCS, particularly in patients who have repeated episodes of acute pancreatitis. Whether volanesorsen will be useful for the treatment of less severe hypertriglyceridemia remains to be determined, particularly given its potential side effects. Drugs similar to volanesorsen (Olezarsen) that do not adversely affect platelets are underdevelopment (259).  

 

ALIPOGENE TIPARVOVEC (GLYBERA)

 

Introduction

 

Alipogene tiparvovec is a gene therapy that was approved in Europe for adult patients with Familial Lipoprotein Lipase deficiency and a history of multiple or severe episodes of pancreatitis who have failed dietary therapy (260). The diagnosis of Familial Lipoprotein Lipase with loss of function mutations must be confirmed by genetic testing but patients need to have detectable levels of lipoprotein lipase protein (to avoid immunological reactions) (260). Alipogene tiparvovec is an adeno-associated virus gene therapy that results in the expression of the naturally occurring S447X variant of the human lipoprotein lipase gene that has increased lipoprotein lipase activity compared to “normal” lipoprotein lipase (260). Approximately 20% of Caucasians express this gene variant and these individuals have lower plasma TG levels and an increase in HDL-C levels (261,262). Because of the lack of long-term efficacy alipogene tiparvovec is no longer clinically available.

 

Effect of Alipogene Tiparvovec on Lipid and Lipoprotein Levels

 

In patients with plasma TG levels > 880mg/d, treatment with alipogene tiparvovec resulted in an approximately 40% decrease in fasting plasma TGs with half of the patients having > 40% decrease in fasting plasma TG levels at 3-12 weeks post treatment (263). By week 16-26, fasting TG levels returned to baseline values but chylomicron levels were reduced (263). While fasting TG levels returned to baseline, postprandial TG levels were reduced by approximately 60% suggesting that there are long term effects that are not reflected by fasting TG levels (264). In fact, in some patients treated with alipogene tiparvovec, lipoprotein lipase expression was demonstrated in muscle biopsies at 26 weeks (263).

 

Mechanisms Accounting for the Alipogene Tiparvovec Induced Lipid Effects

 

Gene therapy with alipogene tiparvovec results in the expression of lipoprotein lipase in muscle, which accelerates the clearance of chylomicrons (260,263). Studies have demonstrated a reduced peak level and a reduced area under the curve for postprandial chylomicrons (264).

 

Drug Administration and Pharmacokinetics

 

Alipogene tiparvovec is administered by multiple intramuscularly injections in the legs given at a single visit (260). The number of injections is > 40 and therefore the injections are given under spinal anesthesia (263). From 3 days before administration until 12 weeks after administration patients may be treated with cyclosporine (3mg/kg/day) and mycophenolate (2g/day) and on the day of administration methylprednisolone 1mg/kg) may be administered IV (260,263).

 

Effect on Clinical Outcomes

 

In patients with Familial Lipoprotein Lipase Deficiency the outcome of interest is pancreatitis. In a retrospective study of 19 patients treated with alipogene tiparvovec an approximate 50% decrease in pancreatitis was observed (265). In addition, patients treated with alipogene tiparvovec have reported benefits including discontinuing lipoprotein apheresis, increased energy, and the ability to liberalize their diet, which is difficult to comply with due to the marked limitation in dietary fat (263,266).

 

Conclusions

 

Alipogene tiparvovec may be a useful treatment for the rare patient with Familial Lipoprotein Lipase deficiency but the lack of long-term efficacy and the difficulty of giving the required injections led to this drug being removed from the market.  Because of the rarity of this disorder the information on patients treated with this drug is limited and randomized trials are impossible.

 

EVINACUMAB (EVKEEZA)

 

Introduction

 

Evinacumab is a human monoclonal antibody against angiopoietin-like protein 3 (ANGPTL3). It is approved for the treatment of Homozygous Familial Hypercholesterolemia. Evinacumab decreases LDL-C levels by mechanisms independent of LDL receptor activity. The recommended dose of evinacumab is 15 mg/kg administered by intravenous infusion over 60 minutes every 4 weeks. While it is not approved for TG lowering it is effective in lowering TG levels.

 

Effect on Evinacumab on TG Levels

 

For information on the effect of evinacumab on LDL-C levels see the Endotext chapter on “Cholesterol Lowering Drugs (5). Because of the difficulty in treating severe hypertriglyceridemia, I have focused on evinacumab in this group of patients. Phase 1 studies have shown that various doses of evinacumab lower TG levels in individuals with TG levels between 150-450mg/dL with maximal effects of approximately 80% reductions (267). As one would expect LDL-C and HDL-C levels also decreased in these individuals with modest hypertriglyceridemia.

 

A phase 2 study evaluated evinacumab in three groups of patients with severe hypertriglyceridemia; FCS patients with bi-allelic loss-of-function mutations in the lipoprotein lipase (LPL) pathway (n = 17), multifactorial chylomicronemia syndrome (MFCS) with heterozygous loss-of-function LPL pathway mutations (n = 15), and MFCS without LPL pathway mutations (n = 19) (268). Patients were randomized to evinacumab 15 mg/kg IV or placebo every 4 weeks over 12-weeks. The effect on TG and non-HDL-C levels are shown in table 21. Despite the very small number of patients the results suggest that evinacumab can lower TG levels in patients with MFCS but not in patients with FCS. This result Is not surprising based on the proposed mechanism of action of inhibiting ANGPTL3 (see below).

 

Table 21. Change in Lipid/Lipoprotein Parameters

 

FCS

MFCS/heterozygous LPL pathway mutations

MFCS/ without LPL pathway mutations

 

Placebo (n=5)

Evinacumab (n=12)

Placebo (n=8)

Evinacumab ((n=9)

Placebo (n=5)

Evinacumab (n=14)

Fasting TG

Baseline

3,918mg/dL

3,140mg/dL

1,351mg/dL

1,238mg/dL

1,030mg/dL

1,917mg/dL

% change

−22.9

−27.7

9.4

−64.8*

80.9

−81.7**

Non-HDL-C

Baseline

356mg/dL

345mg/dL

202mg/dL

220mg/dL

209mg/dL

296mg/dL

% change

−15.2

−34.2^

8.0

−31.0^^

48.4

−38.5^^^

*p= 0.0076, **p= 0.0418, ^p= 0.0074, ^^p= 0.0677, ^^^p= 0.1016.

FCS= familial chylomicronemia syndrome, MFCS= multifactorial chylomicronemia syndrome.

 

Mechanism Accounting for the Evinacumab Induced Decrease in TG

 

ANGPTL3 inhibits lipoprotein lipase (LPL) activity thereby slowing the clearance of VLDL and chylomicrons resulting in an increase in plasma triglyceride levels (269,270). Mice deficient in ANGPTL3 have lower plasma triglyceride levels while mice overexpressing ANGPTL3 have elevated plasma triglyceride levels (270). Evinacumab by inhibiting the ability of ANGPTL3 to decrease LPL activity results in an increases in LPL activity, which accelerates the clearance of TG rich lipoproteins decreasing plasma triglyceride levels (270). In patients with FCS who lack a functioning lipoprotein lipase clearance system evinacumab will not accelerate the clearance of TG rich lipoproteins. For information on the mechanism by which evinacumab lowers LDL-C and HDL-C see the Endotext chapter on “Cholesterol Lowering Drugs” (5).

 

Pharmacokinetics and Drug Interactions

 

There are no significant drug interactions.

 

Effect of Evinacumab on Clinical Outcomes

 

There are no cardiovascular outcome studies.

 

Homozygosity for loss-of-function mutations in ANGPTL3 is associated with significantly lower plasma levels of LDL-C, HDL-C, and triglycerides (familial combined hypolipidemia) (270,271). Heterozygous carriers of loss-of-function mutations in ANGPTL3, which occur at a frequency of about 1:300, have significantly lower total cholesterol, LDL-C, and triglyceride levels than noncarriers (270). Moreover, patients carrying loss-of-function variants in ANGPTL3 have a significantly lower risk of coronary artery disease (272,273). Additionally, in an animal model of atherosclerosis treatment with evinacumab decreased atherosclerotic lesion area and necrotic content (272). Taken together these observations suggest that inhibiting ANGPTL3 with evinacumab will reduce cardiovascular disease.

 

Side Effects

 

Serious hypersensitivity reactions have occurred with evinacumab. In clinical trials, 1 (1%) of evinacumab treated patients experienced anaphylaxis vs. 0% of patients who received placebo (package insert).

 

Contraindications

 

Based on animal studies, evinacumab may cause fetal harm when administered to pregnant patients (package insert). Patients should be advised of the potential risks to the fetus of pregnancy. Patients who may become pregnant should be advised to use effective contraception during treatment with evinacumab and for at least 5 months following the last dose.

 

Summary

 

Evinacumab lowers triglyceride levels in patients with severe hypertriglyceridemia due to multifactorial chylomicronemia syndrome and could be useful in selected patients with hypertriglyceridemia. Note it is not approved to treat severe hypertriglyceridemia and administration intravenously every 4 weeks will limit its use to special circumstances.

 

CLINICAL USE OF TRIGLYCERIDE LOWERING DRUGS

 

Marked Hypertriglyceridemia (>500mg/dL); Prevention of Pancreatitis

 

In patients with marked elevations in TG levels (>500-1000mg/dL) the major concern is an increased risk of pancreatitis (274,275). Because of this increased risk it is imperative to lower TG levels. The initial steps are to 1) treat any disease states that could be leading to an elevation in plasma TG levels, 2) if possible, discontinue any drugs that could be leading to an elevation in plasma TGs, and 3) initiate lifestyle changes (Table 22) (2,276).

 

Table 22. Causes of Secondary Hypertriglyceridemia

Lifestyle

Diseases

Medications

Excess calories

Poorly controlled diabetes

Corticosteroids

Excess dietary fat intake

Hypothyroidism

Oral estrogen

Excess simple sugars

Renal disease

Retinoic acid derivatives

Overweight/Obesity

HIV infection

Beta adrenergic blockers

Alcohol intake

Cushing’s syndrome

Thiazide diuretics

Pregnancy

Acromegaly

Protease inhibitors

 

Growth hormone deficiency

Bile acid sequestrants

 

Lipodystrophy

Anti-psychotic drugs

 

Paraproteinemia

Cyclosporine/tacrolimus

 

Nephrotic Syndrome

L-asparaginase

 

Inflammatory Disorders

Interferon alpha 2b

 

 

Cyclophosphamide

 

These initial steps are often sufficient to result in marked reductions in plasma TG levels eliminating the need for TG lowering medications. For example, in patients with diabetes in very poor glycemic control, treatment that results in good glycemic control can markedly lower TG levels (277). Similarly, the restoration of euthyroidism in a hypothyroid patient can also markedly lower lipid levels (278). If these initial steps do not result in a lowering of TGs into an acceptable range, then the use of drugs to lower plasma TG levels is indicated. There have been no randomized controlled trials demonstrating that treatment diminishes pancreatitis but most experienced clinicians believe that lowering TG levels to below 500-1000mg/dL reduces the risk of developing pancreatitis (274,275). The addition of either fibrates or fish oil to lifestyle changes are commonly used to lower markedly elevated TG levels. In some patients, combination therapy is required to lower plasma TGs to an acceptable range. In patients with Familial Chylomicronemia syndrome volanesorsen is a promising therapeutic tool.

 

Moderate Hypertriglyceridemia (150-500mg/dL); Prevention of Cardiovascular Disease

 

In the era of statin therapy, it is uncertain whether lowering TG levels in patients on statin therapy will further reduce cardiovascular events. As discussed in detail in the sections on individual drugs, the studies carried out so far have not shown that adding niacin or fibrates to statin therapy is beneficial with regards to cardiovascular disease. As also discussed, some of the available studies have major limitations because many of the patients in these outcome studies did not have substantial elevations in TGs. Nevertheless, at this time there is little enthusiasm for adding either fibrates or niacin to statins to lower the risk of cardiovascular event.

 

Notably, the REDUCE-IT trial, which tested the effect of high dose EPA (4 grams per day) in patients with elevated TG levels on statin therapy demonstrated a 25% reduction in cardiovascular events. However, the decrease in cardiovascular events was considerably greater than one would expect based on the reduction in TG levels suggesting that the decrease in cardiovascular events was not solely due to lowering TG levels and that other effects of EPA likely played a role. Additionally, as discussed in detail in the section discussing cardiovascular trials in the omega-3-fatty acid section there are concerns that the use of mineral oil as the placebo in the REDUCE-IT trial may have caused harmful effects leading to increased events. Thus, the role of EPA in reducing cardiovascular events is debated with some experts feeling that it is beneficial while others feeling that the evidence for benefit is very weak. Clearly additional studies are required to resolve this controversy. In the meantime, clinicians will need to use their clinical judgement on whether to treat patients with modest elevations in TG levels with EPA (icosapent ethyl; Vascepa) balancing the potential benefits of treatment vs. the potential side effects.

 

Some guidelines use non-HDL-C as a therapeutic goal and thus the use of omega-3-fatty acids and fibrates will often be required to lower TG levels to achieve these non-HDL-C goals. In contrast, other guidelines focus on LDL-C levels and the use of statins and thus de-emphasize the use of omega-3-fatty acids and fibrates. Given the absence of definitive data one needs to use clinical judgement. Consideration should also be given to the use of fenofibrate in hypertriglyceridemic patients with diabetes at high risk for microvascular disease given the studies that have shown that fibrates reduce the microvascular complications of diabetes. Because of the side effects of niacin, the use of niacin to lower TG levels has markedly diminished. In the past we used to use niacin to lower both LDL-C levels and TGs but with the availability of ezetimibe, bempedoic acid, and PCSK9 inhibitors the need to use niacin to lower LDL-C levels has markedly decreased.

 

ACKNOWLEDGEMENTS

 

This work was supported by grants from the Northern California Institute for Research and Education.

 

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Introduction to Lipids and Lipoproteins

ABSTRACT

 

Cholesterol and triglycerides are insoluble in water and therefore these lipids must be transported in association with proteins. Lipoproteins are complex particles with a central core containing cholesterol esters and triglycerides surrounded by free cholesterol, phospholipids, and apolipoproteins, which facilitate lipoprotein formation and function. Plasma lipoproteins can be divided into seven classes based on size, lipid composition, and apolipoproteins (chylomicrons, chylomicron remnants, VLDL, VLDL remnants (IDL), LDL, HDL, and Lp (a)).  Chylomicron remnants, VLDL, IDL, LDL, and Lp (a) are all pro-atherogenic while HDL is anti-atherogenic. Apolipoproteins have four major functions including 1) serving a structural role, 2) acting as ligands for lipoprotein receptors, 3) guiding the formation of lipoproteins, and 4) serving as activators or inhibitors of enzymes involved in the metabolism of lipoproteins. The exogenous lipoprotein pathway starts with the incorporation of dietary lipids into chylomicrons in the intestine. In the circulation, the triglycerides carried in chylomicrons are metabolized in muscle and adipose tissue by lipoprotein lipase releasing free fatty acids, which are subsequently metabolized by muscle and adipose tissue, and chylomicron remnants are formed. Chylomicron remnants are then taken up by the liver. The endogenous lipoprotein pathway begins in the liver with the formation of VLDL. The triglycerides carried in VLDL are metabolized in muscle and adipose tissue by lipoprotein lipase releasing free fatty acids and IDL are formed. The IDL are further metabolized to LDL, which are taken up by the LDL receptor in numerous tissues including the liver, the predominant site of uptake. Reverse cholesterol transport begins with the formation of nascent HDL by the liver and intestine. These small HDL particles can then acquire cholesterol and phospholipids that are effluxed from cells, a process mediated by ABCA1 resulting in the formation of mature HDL.  Mature HDL can acquire addition cholesterol from cells via ABCG1, SR-B1, or passive diffusion. The HDL then transports the cholesterol to the liver either directly by interacting with hepatic SR-B1 or indirectly by transferring the cholesterol to VLDL or LDL, a process facilitated by CETP. Cholesterol efflux from macrophages to HDL plays an important role in protecting from the development of atherosclerosis.

 

INTRODUCTION

 

Because lipids, such as cholesterol and triglycerides, are insoluble in water these lipids must be transported in association with proteins (lipoproteins) in the circulation. Large quantities of fatty acids from meals must be transported as triglycerides to avoid toxicity. These lipoproteins play a key role in the absorption and transport of dietary lipids by the small intestine, in the transport of lipids from the liver to peripheral tissues, and the transport of lipids from peripheral tissues to the liver and intestine (reverse cholesterol transport). A secondary function is to transport toxic foreign hydrophobic and amphipathic compounds, such as bacterial toxins, from areas of invasion and infection (1). For example, lipoproteins bind endotoxin (LPS) from gram negative bacteria and lipoteichoic acid from gram positive bacteria thereby reducing their toxic effects (1). In addition, apolipoprotein L1, associated with HDL particles, has lytic activity against the parasite Trypanosoma brucei brucei and lipoproteins can neutralize viruses (2,3). Thus, while this article will focus on the transport properties of lipoproteins the reader should recognize that lipoprotein may have other important roles.

 

STRUCTURE OF LIPOPROTEINS (4)

 

Lipoproteins are complex particles that have a central hydrophobic core of non-polar lipids, primarily cholesterol esters and triglycerides. This hydrophobic core is surrounded by a hydrophilic membrane consisting of phospholipids, free cholesterol, and apolipoproteins (Figure 1). Plasma lipoproteins are divided into seven classes based on size, lipid composition, and apolipoproteins (Table 1 and Figure 2).

 

Figure 1. Lipoprotein Structure (figure modified from Biochemistry 39: 9763, 2000)

 

 

Table 1. Lipoprotein Classes

Lipoprotein

Density (g/ml)

Size (nm)

Major Lipids

Major Apoproteins

Chylomicrons

<0.930

75-1200

Triglycerides

Apo B-48, Apo C, Apo E, Apo A-I, A-II, A-IV

Chylomicron Remnants

0.930- 1.006

30-80

Triglycerides Cholesterol

Apo B-48, Apo E

VLDL

0.930- 1.006

30-80

Triglycerides

Apo B-100, Apo E, Apo C

IDL

1.006- 1.019

25-35

Triglycerides Cholesterol

Apo B-100, Apo E, Apo C

LDL

1.019- 1.063

18- 25

Cholesterol

Apo B-100

HDL

1.063- 1.210

5- 12

Cholesterol Phospholipids

Apo A-I, Apo A-II, Apo C, Apo E

Lp (a)

1.055- 1.085

~30

Cholesterol

Apo B-100, Apo (a)

 

Figure 2: Classes of Lipoproteins (figure modified from Advances Protein Chemistry 45:303, 1994)

 

Chylomicrons (5)

 

These are large triglyceride rich particles made by the intestine, which are involved in the transport of dietary triglycerides and cholesterol to peripheral tissues and liver. These particles contain apolipoproteins A-I, A-II, A-IV, A-V, B-48, C-II, C-III, and E. Apo B-48 is the core structural protein and each chylomicron particle contains one Apo B-48 molecule. The size of chylomicrons varies depending on the amount of fat ingested. A high fat meal leads to the formation of large chylomicron particles due to the increased amount of triglyceride being transported whereas in the fasting state the chylomicron particles are small carrying decreased quantities of triglyceride. The quantity of cholesterol carried by chylomicrons also can vary depending upon dietary intake.

 

Chylomicron Remnants (5-7)

 

The removal of triglyceride from chylomicrons by lipoprotein lipase in peripheral tissues results in smaller particles called chylomicron remnants. Compared to chylomicrons these particles are enriched in cholesterol and are pro-atherogenic.

 

Very Low-Density Lipoproteins (VLDL)

 

These particles are produced by the liver and are triglyceride rich. They contain apolipoprotein B-100, C-I, C-II, C-III, and E. Apo B-100 is the core structural protein and each VLDL particle contains one Apo B-100 molecule. Similar to chylomicrons the size of the VLDL particles can vary depending on the quantity of triglyceride carried in the particle. When triglyceride production in the liver is increased, the secreted VLDL particles are large. However, VLDL particles are smaller than chylomicrons.

 

Intermediate-Density Lipoproteins (IDL; VLDL Remnants) (6,7)

 

The removal of triglycerides from VLDL by muscle and adipose tissue results in the formation of IDL particles which are enriched in cholesterol. These particles contain apolipoprotein B-100 and E. These IDL particles are pro-atherogenic.

 

Low-Density Lipoproteins (LDL)

 

These particles are derived from VLDL and IDL particles and they are even further enriched in cholesterol. LDL carries the majority of the cholesterol that is in the circulation. The predominant apolipoprotein is B-100 and each LDL particle contains one Apo B-100 molecule. LDL consists of a spectrum of particles varying in size and density. An abundance of small dense LDL particles is seen in association with hypertriglyceridemia, low HDL levels, obesity, type 2 diabetes (i.e. patients with the metabolic syndrome) and infectious and inflammatory states. These small dense LDL particles are considered to be more pro-atherogenic than large LDL particles for a number of reasons (8). Small dense LDL particles have a decreased affinity for the LDL receptor resulting in a prolonged retention time in the circulation. Additionally, they more easily enter the arterial wall and bind more avidly to intra-arterial proteoglycans, which traps them in the arterial wall. Finally, small dense LDL particles are more susceptible to oxidation, which could result in an enhanced uptake by macrophages. 

 

High-Density Lipoproteins (HDL) (9-11)

 

These particles play an important role in reverse cholesterol transport from peripheral tissues to the liver, which is one potential mechanism by which HDL may be anti-atherogenic. In addition, HDL particles have anti-oxidant, anti-inflammatory, anti-thrombotic, and anti-apoptotic properties, which may also contribute to their ability to inhibit atherosclerosis. HDL particles are enriched in cholesterol and phospholipids. Apolipoproteins A-I, A-II, A-IV, C-I, C-II, C-III, and E are associated with these particles. Apo A-I is the core structural protein and each HDL particle may contain multiple Apo A-I molecules. In addition, using mass spectrometry proteins involved in proteinase inhibition, complement activation, and the acute-phase response have been found associated with HDL particles (12). HDL particles are very heterogeneous and can be classified based on density, size, charge, or apolipoprotein composition (Table 2).

 

Table 2. Classification of HDL

Method of classification

Types of HDL

Density gradient ultracentrifugation

HDL2, HDL3, very high-density HDL

Nuclear magnetic resonance

large, medium, and small

Gradient gel electrophoresis

HDL 2a, 2b, 3a, 3b, 3c

2-dimensional gel electrophoresis

pre-beta 1 and 2, alpha 1, 2, 3, 4

Apolipoprotein composition

A-I particles, A-I: A-II particles, A-I: E particles

 

Lipoprotein (a) (Lp (a)) (13-16)

 

Lp (a) is an LDL particle that has apolipoprotein (a) attached to Apo B-100 via a disulfide bond. Lp (a) contain Apo (a) and Apo B-100 in a 1:1 molar ratio. The size of Lp(a) particles can vary greatly based on the size of apolipoprotein (a). This particle is pro-atherogenic.

 

APOLIPOPROTEINS (17,18)

 

Apolipoproteins have four major functions including 1) serving a structural role, 2) acting as ligands for lipoprotein receptors, 3) guiding the formation of lipoproteins, and 4) serving as activators or inhibitors of enzymes involved in the metabolism of lipoproteins (Table 3). Apolipoproteins thus play a crucial role in lipoprotein metabolism.

 

Apolipoprotein A-I (19)

 

Apo A-I is synthesized in the liver and intestine and is the major structural protein of HDL accounting for approximately 70% of HDL protein. It also plays a role in the interaction of HDL with ATP-binding cassette protein A1 (ABCA1), ABCG1, and class B, type I scavenger receptor (SR-B1). Apo A-I is an activator of lecithin: cholesterol acyltransferase (LCAT), an enzyme that converts free cholesterol into cholesteryl ester. High levels of Apo A-I are associated with a decreased risk of atherosclerosis.

 

Apolipoprotein A-II (20)

 

Apo A-II is synthesized in the liver and is the second most abundant protein on HDL accounting for approximately 20% of HDL protein. The role of Apo A-II in lipid metabolism is unclear. Apo A-II is a strong predictor of risk for CVD.

 

Apolipoprotein A-IV (21)

 

Apo A-IV is synthesized in the intestine during fat absorption. Apo A-IV is associated with chylomicrons and high-density lipoproteins, but is also found in the lipoprotein-free fraction.  Its precise role in lipoprotein metabolism remains to be determined but studies have suggested a role for Apo A-IV in regulating food intake.

 

Apolipoprotein A-V (22,23)

 

Apo A-V is synthesized in the liver and associates with triglyceride rich lipoproteins. It is an activator of LPL mediated lipolysis and thereby plays an important role in the metabolism of triglyceride rich lipoproteins.

 

Apolipoprotein B-48 (24)

 

Apo B-48 is synthesized in the intestine and is the major structural protein of chylomicrons and chylomicron remnants. There is a single molecule of apo B-48 per chylomicron particle. There is a single apolipoprotein B gene that is expressed in both the liver and intestine. The intestine expresses a protein that is approximately ½ the size of the liver due to mRNA editing. The apobec-1 editing complex is expressed in the intestine and edits a specific cytidine to an uracil in the apo B mRNA in the intestine creating a stop codon that results in the cessation of protein translation and a shorter Apo B (Apo B-48). The portion of Apo-B that is recognized by the LDL receptor is not contained in Apo-B48 and therefore Apo B-48 is not recognized by the LDL receptor.

 

Apolipoprotein B-100

 

Apo B-100 is synthesized in the liver and is the major structural component of VLDL, IDL, and LDL. There is a single molecule of Apo B-100 per VLDL, IDL, LDL and Lp(a) particle. Apo B-100 is a ligand for the LDL receptor and therefore plays an important role in the clearance of lipoprotein particles. Certain mutations in Apo B-100 result in decreased binding to the LDL receptor and familial hypercholesterolemia (25). High levels of Apo B-100 are associated with an increased risk of atherosclerosis.

 

Apolipoprotein C (26-29)

 

The C apolipoproteins are synthesized primarily in the liver and freely exchange between lipoprotein particles and therefore are found in association with chylomicrons, VLDL, and HDL.

 

Apo C-II is a co-factor for lipoprotein lipase (LPL) and thus stimulates triglyceride hydrolysis and the clearance of triglyceride rich lipoproteins (26,29). Loss of function mutations in Apo C-II result in marked hypertriglyceridemia due to a failure to metabolize triglyceride rich lipoproteins (30).

 

Apo C-III is an inhibitor of LPL (31). Additionally, Apo C-III inhibits the interaction of triglyceride rich lipoproteins with their receptors (31). Recent studies have shown that loss of function mutations in Apo C-III lead to decreases in serum triglyceride levels and a reduced risk of cardiovascular disease. Interestingly, inhibition of Apo C-III expression results in a decrease in serum triglyceride levels even in patients deficient in lipoprotein lipase indicating that the ability of Apo C-III to modulate serum triglyceride levels is not dependent solely on regulating lipoprotein lipase activity (32).

 

Apolipoprotein E (33)

 

Apolipoprotein E is synthesized in many tissues but the liver and intestine are the primary source of circulating Apo E. Apo E exchanges between lipoprotein particles and is associated with chylomicrons, chylomicron remnants, VLDL, IDL, and a subgroup of HDL particles. There are three common genetic variants of Apo E (Apo E2, E3, and E4). ApoE2 differs from the most common isoform, Apo E3, by a single amino acid substitution where cysteine substitutes for arginine at residue 158. Apo E4 differs from Apo E3 at residue 112, where arginine substitutes for cysteine. Apo E3 and E4 are ligands for the LDL receptor while Apo E2 is poorly recognized by the LDL receptor. Patients who are homozygous for Apo E2 can develop familial dysbetalipoproteinemia (30). Apo E4 is associated with an increased risk of Alzheimer’s disease and an increased risk of atherosclerosis.

 

Apolipoprotein (a) (14,16)

 

Apo (a) is synthesized in the liver. This protein is a homolog of plasminogen and its molecular weight varies from 300,000 to 800,000. It is attached to Apo B-100 via a disulfide bond. High levels of Apo (a) are associated with an increased risk of atherosclerosis. Apo (a) is an inhibitor of fibrinolysis and can also enhance the uptake of lipoproteins by macrophages, both of which could increase the risk of atherosclerosis. The physiologic function of Apo (a) is unknown. Interestingly this apolipoprotein is found in primates but not in other species.

 

Table 3. Apolipoproteins

Apolipoprotein

MW

Primary Source

Lipoprotein Association

Function

Apo A-I

28,000

Liver, Intestine

HDL, chylomicrons

Structural protein for HDL, Activates LCAT

Apo A-II

17,000

Liver

HDL, chylomicrons

Structural protein for HDL, Activates hepatic lipase

Apo A-IV

45,000

Intestine

HDL, chylomicrons

Unknown

Apo A-V

39,000

Liver

VLDL, chylomicrons, HDL               

Promotes LPL mediated TG lipolysis

Apo B-48

241,000

Intestine

Chylomicrons

Structural protein for chylomicrons

Apo B-100

512,000

Liver

VLDL, IDL, LDL, Lp (a)

Structural protein, Ligand for LDL receptor

Apo C-I

6,600

Liver

Chylomicrons, VLDL, HDL

Activates LCAT

Apo C-II

8,800

Liver

Chylomicrons, VLDL, HDL

Co-factor for LPL

Apo C-III

8,800

Liver

Chylomicrons, VLDL, HDL

Inhibits LPL and uptake of lipoproteins

Apo E

34,000

Liver

Chylomicron remnants, IDL, HDL

Ligand for LDL receptor

Apo (a)

250,000- 800,00

Liver

Lp (a)

Inhibits plasminogen activation

                                

LIPOPROTEIN RECEPTORS AND LIPID TRANSPORTERS

 

There are several receptors and transporters that play a crucial role in lipoprotein metabolism.

 

LDL Receptor (34)

 

The LDL receptor is present in the liver and most other tissues. It recognizes Apo B-100 and Apo E and hence mediates the uptake of LDL, chylomicron remnants, and IDL, which occurs via endocytosis (Figure 3). After internalization, the lipoprotein particle is degraded in lysosomes and the cholesterol is released. The delivery of cholesterol to the cell decreases the activity of HMGCoA reductase and other enzymes required for the biosynthesis of cholesterol, and the expression of LDL receptors. LDL receptors in the liver play a major role in determining plasma LDL levels (a low number of receptors is associated with high plasma LDL levels while a high number of hepatic LDL receptors is associated with low plasma LDL levels). The number of LDL receptors is regulated by the cholesterol content of the cell (35). When cellular cholesterol levels are decreased the transcription factor SREBP is transported from the endoplasmic reticulum to the Golgi where proteases cleave and activate SREBP, which then migrates to the nucleus and stimulates the expression of LDL receptors (Figure 4). Conversely, when cellular cholesterol levels are high SREBP remains in the endoplasmic reticulum in an inactive form and the expression of LDL receptors is low. As discussed later PCSK9 regulates the rate of degradation of LDL receptors.

 

Figure 3. LDL Receptor Pathway (figure modified from Annual Review of Biochemistry 46: 897, 1977)

Figure 4. SREBP Pathway (figure modified from Journal of Lipid Research 50: Supp S15, 2009)

 

 

LRP-1 is a member of the LDL receptor family. It is expressed in multiple tissues including the liver. LRP-1 recognizes Apo E and mediates the uptake of chylomicron remnants and IDL (VLDL remnants).

 

VLDL Receptor (37)

 

The VLDL receptor is a member of the LDL receptor family. The VLDLR is expressed in the heart, skeletal muscle, adipose tissue, endothelium, brain, macrophages, and other tissues. Interestingly it is not usually expressed in the liver but hepatic expression can be induced by endoplasmic reticulum stress and PPAR alpha activation. Apo E but not Apo B bind to the VLDL receptor thereby allowing for the uptake of triglyceride rich lipoprotein particles (VLDL and chylomicrons).

 

Class B Scavenger Receptor B1 (SR-B1) (38)

 

SR-B1 is expressed in the liver, adrenal glands, ovaries, testes, macrophages, and other cells. In the liver and steroid producing cells, it mediates the selective uptake of cholesterol esters from HDL particles. In macrophages and other cells, it facilitates the efflux of cholesterol from the cell to HDL particles.

 

ATP-Binding Cassette Transporter A1 (ABCA1) (39)

 

ABCA1 is expressed in many cells including hepatocytes, enterocytes, and macrophages. It mediates the transport of cholesterol and phospholipids from the cell to lipid poor HDL particles (pre-beta-HDL).

 

ATP-Binding Cassette Transporter G1 (ABCG1) (40)

 

ABCG1 is expressed in many different cell types and mediates the efflux of cholesterol from the cell to HDL particles.

 

ATP-Binding Cassette Transporter G5 and G8 (ABCG5/ABCG8) (41,42)

 

ABCG5 and ABCG8 are expressed in the liver and intestine and form a heterodimer. In the intestine, these transporters mediate the movement of plant sterols and cholesterol from inside the enterocyte into the intestinal lumen thereby decreasing the absorption of cholesterol and limiting the uptake of dietary plant sterols. In the liver, these transporters play a role in the movement of cholesterol and plant sterols into the bile facilitating the excretion of sterols.

 

Niemann-Pick C1-Like 1 (NPC1L1) (41)

 

NPC1L1 is expressed in the intestine and mediates the uptake of cholesterol and plant sterols from the intestinal lumen into the enterocyte. NPC1L1 is also expressed in the liver where it mediates the movement of cholesterol from hepatocytes into the bile.

 

ENZYMES AND TRANSFER PROTEINS INVOLVED IN LIPOPROTEIN METABOLISM

 

There are several enzymes and transfer proteins that play a key role in lipoprotein metabolism.

 

Lipoprotein Lipase (LPL) (43)

 

LPL is synthesized in muscle, heart, and adipose tissue, then secreted and attached to the endothelium of the adjacent blood capillaries. This enzyme hydrolyzes the triglycerides carried in chylomicrons and VLDL to fatty acids, which can be taken up by cells. The catabolism of triglycerides results in the conversion of chylomicrons into chylomicron remnants and VLDL into IDL (VLDL remnants). This enzyme requires Apo C-II as a cofactor. Apo A-V also plays a key role in the activation of this enzyme. In contrast Apo C-III and Apo A-II inhibit the activity of LPL. Insulin stimulates LPL expression and LPL activity is reduced in patients with poorly controlled diabetes, which can impair the metabolism of triglyceride rich lipoproteins leading to hypertriglyceridemia (44).

 

Hepatic Lipase (45)

 

Hepatic lipase is localized to the sinusoidal surface of liver cells. It mediates the hydrolysis of triglycerides and phospholipids in IDL and LDL leading to smaller particles (IDL is converted to LDL; LDL is converted from large LDL to small LDL). It also mediates the hydrolysis of triglycerides and phospholipids in HDL resulting in smaller HDL particles.

 

Endothelial Lipase (46)

 

Endothelial lipase plays a major role in hydrolyzing the phospholipids in HDL.

 

Lecithin: Cholesterol Acyltransferase (LCAT) (47)

 

LCAT is made in the liver. In the plasma, it catalyzes the synthesis of cholesterol esters in HDL by facilitating the transfer of a fatty acid from position 2 of lecithin to cholesterol. This allows for the transfer of the cholesterol from the surface of the HDL particle (free cholesterol) to the core of the HDL particle (cholesterol ester), which facilitates the continued uptake of free cholesterol by HDL particles by reducing the concentration of cholesterol on the surface of HDL.

 

Cholesteryl Ester Transfer Protein (CETP) (48,49)

 

This protein is synthesized in the liver and in the plasma mediates the transfer of cholesterol esters from HDL to VLDL, chylomicrons, and LDL and the transfer of triglycerides from VLDL and chylomicrons to HDL. Inhibition of CETP activity leads to an increase in HDL cholesterol and a decrease in LDL cholesterol.

 

Microsomal Triglyceride Transfer Protein (MTTP) (50)

 

MTTP is expressed primarily in the liver and small intestine and plays a crucial role in the synthesis of lipoproteins in these tissues. MTTP mediates the transfer of triglycerides to apolipoprotein B-100 in the liver to form VLDL and to apolipoprotein B-48 in the intestine to form chylomicrons.

 

EXOGENOUS LIPOPROTEIN PATHWAY (CHYLOMICRONS)

 

Figure 5. Exogenous Lipoprotein Pathway

 

Fat Absorption (51-54)

 

The exogenous lipoprotein pathway starts in the intestine. Dietary triglycerides (approximately 100 grams per day) are hydrolyzed to free fatty acids and monoacylglycerol by intestinal lipases and emulsified with bile acids, cholesterol, plant sterols, and fat-soluble vitamins to form micelles. While the fatty acids in the intestine are overwhelmingly accounted for by dietary intake the cholesterol in the intestinal lumen is primarily derived from bile (approximately 800-1200mg of cholesterol from bile vs. 200-500mg from diet). Plant sterols account for approximately 25% of dietary sterol intake (approximately 100-150mg/day). The cholesterol, plant sterols, fatty acids, monoacylglycerol, and fat-soluble vitamins contained in the micelles are then transported into the intestinal cells. The uptake of cholesterol and plant sterols from the intestinal lumen into intestinal cells is facilitated by a sterol transporter, Niemann-Pick C1- like 1 protein (NPC1L1) (Figure 6). Ezetimibe, a drug which inhibits intestinal cholesterol and plant sterol uptake, binds to NPC1L1 and inhibits its activity. Once in the intestinal cell the cholesterol and plant sterols may be transported back into the intestinal lumen, a process mediated by ABCG5 and ABCG8, or converted to sterol esters by acyl-CoA cholesterol acyl transferase (ACAT), which attaches a fatty acid to the sterol. Compared to cholesterol, plant sterols are poor substrates for ACAT and therefore the formation of plant sterol esters does not occur as efficiently as the formation of cholesterol esters. In humans, <5% of dietary plant sterols are absorbed and the vast majority are transported out of the intestine cell, a process mediated by ABCG5 and ABCG8, which are very efficient at effluxing plant sterols from the intestinal cell into the intestinal lumen. Patients with sitosterolemia have mutations in either ABCG5 or ABCG8 and net absorption of dietary plant sterols is increased (20-30% absorbed vs. < 5% in normal subjects) (55). Thus, ABCG5 and ABCG8 along with ACAT serve as gate keepers and block the uptake of plant sterols and likely also play an important role in determining the efficiency of cholesterol absorption (humans typically absorb only approximately 50% of dietary cholesterol with a range of 25-75%).

 

Figure 6. Intestinal Cell and Sterol Metabolism. C= cholesterol, CE= cholesterol ester.

 

The pathway of absorption of free fatty acids is not well understood but it is likely that both passive diffusion and specific transporters play a role. The fatty acid transporter CD36 is strongly expressed in the proximal third of the intestine and is localized to the villi. While this transporter likely plays a role in fatty acid uptake by intestinal cells, this transporter is not essential as humans and mice deficient in this protein do not have fat malabsorption. However, in mice deficient in CD36 there is a shift in the absorption of lipid to the distal intestine, suggesting pathways that can compensate for the absence of CD36. Fatty acid transport protein 4 (FATP4) is also highly expressed in the intestine. However, mice deficient in FATP4 do not have abnormalities in fat absorption. It is likely that there are multiple pathways for the absorption of fatty acids into intestinal cells. The pathways by which monoacylglycerols are absorbed by intestinal cells remain to be defined.

 

Formation of Chylomicrons (51,54)

 

The absorbed fatty acids and monoacylglycerols are utilized to synthesize triglycerides. The key enzymes required for triglyceride synthesis are monoacylglycerol acyltransferase (MGAT) and diacylglycerol transferase (DGAT). MGAT catalyzes the addition of a fatty acid to monoacylglycerol while DGAT catalyzes the addition of a fatty acid to diacylglycerol resulting in triglyceride formation.  As noted above, the majority of the cholesterol absorbed by the intestine is esterified to cholesterol esters by ACAT. The triglycerides and cholesterol esters are packaged into chylomicrons in the endoplasmic reticulum. The size and composition of the chylomicrons formed in the intestine are dependent on the amount of fat ingested and absorbed by the intestine and the type of fat absorbed. Increased fat absorption results in larger chylomicrons. The formation of chylomicrons in the endoplasmic reticulum requires the synthesis of Apo B-48 by the intestinal cell (Figure 6).  Microsomal triglyceride transfer protein (MTTP) is required for the movement of lipid from the endoplasmic reticulum to the Apo B-48. The absence of MTTP results in the inability to form chylomicrons (Abetalipoproteinemia) (56). Lomitapide inhibits MTTP function and is used to treat patients with homozygous Familial Hypercholesterolemia (57).

 

Chylomicron Metabolism (26,31,43,58-62)

 

Chylomicrons are secreted into the lymph and delivered via the thoracic duct to the circulation. It should be noted that this results in the newly formed chylomicrons being delivered to the systemic circulation and not delivered directly to the liver via the portal circulation. This facilitates the delivery of the nutrients contained in the chylomicrons to muscle and adipose tissue. In muscle and adipose tissue lipoprotein lipase (LPL) is expressed at high levels. LPL is synthesized in muscle and adipocytes and transported to the luminal surface of capillaries. Lipase maturation factor 1 plays a key role in the stabilization and movement of LPL from muscle cells and adipocytes to the capillary endothelial cell surface. Glycosylphosphatidylinositol anchored high density lipoprotein binding protein 1 (GPIHBP1) binds LPL and transports it to the capillary lumen and anchors LPL to the capillary endothelium. Activation of LPL by Apo C-II, carried on the chylomicrons, leads to the hydrolysis of the triglycerides that are carried in the chylomicrons resulting in the formation of free fatty acids, which can be taken up by the adjacent muscle cells and adipocytes for either energy production or storage.  Fatty acid transport proteins (FATPs) and CD36 facilitate the uptake of fatty acids into adipocytes and muscle cells. Some of the free fatty acids released from chylomicrons bind to albumin and can be transported to other tissues. Apo A-V also plays an important role in activating LPL activity. Loss of function mutations in LPL, Apo C-II, GPIHPB1, lipase maturation factor 1, and Apo A-V can result in marked hypertriglyceridemia (familial chylomicronemia syndrome) (30). In addition, there are proteins that inhibit LPL activity. Apo C-III inhibits LPL activity and loss of function mutations in this gene are associated with increases in LPL activity and decreases in plasma triglyceride levels. Similarly, angiopoietin like protein 3 and 4, which target LPL for inactivation, regulate LPL activity. Loss of function mutations in these proteins also are associated with decreases in plasma triglyceride levels. Finally, the expression of LPL by muscle cells and adipocytes is regulated by hormones (particularly insulin), nutritional status, and inflammation.

 

The metabolism of the triglycerides carried in the chylomicrons results in a marked decrease in the size of these particles leading to the formation of chylomicron remnants, which are enriched in cholesterol esters and acquire Apo E. As these particles decrease in size phospholipids and apolipoproteins (Apo A and C) on the surface of the chylomicrons are transferred to other lipoproteins, mainly HDL. The transfer of Apo C-II from chylomicrons to HDL decreases the ability of LPL to further breakdown triglycerides. These chylomicron remnants are cleared from the circulation by the liver. The Apo E on the chylomicron remnants binds to the LDL receptor and other hepatic receptors such as LRP and syndecan-4 and the entire particle is taken up by the hepatocytes. Apo E is crucial for this process and mutations in Apo E (for example homozygosity for the Apo E2 isoform) can result in decreased chylomicron clearance and elevations in plasma cholesterol and triglyceride levels (familial dysbetalipoproteinemia) (30).

 

The exogenous lipoprotein pathway results in the efficient transfer of dietary fatty acids to muscle and adipose tissue for energy utilization and storage. The cholesterol is delivered to the liver where it can be utilized for the formation of VLDL, bile acids, or secreted back to the intestine via secretion into the bile. In normal individuals, this pathway can handle large amounts of fat (100 grams or more per day) without resulting in marked increases in plasma triglyceride levels. In fact, in a normal individual, a meal containing 75 grams of fat results in only a very modest increase in postprandial triglyceride levels. 

 

ENDOGENOUS LIPOPROTEIN PATHWAY (VLDL AND LDL)

 

Figure 7. Endogenous Lipoprotein Pathway

 

Formation of VLDL (50,63,64) 

 

In the liver triglycerides and cholesterol esters are transferred in the endoplasmic reticulum to newly synthesized Apo B-100. Similar to the intestine this transfer is mediated by MTTP. The availability of triglycerides is the primary determinant of the rate of VLDL synthesis. If the supply of triglyceride is limited the newly synthesized Apo B is rapidly degraded. Thus, in contrast to many proteins the rate of synthesis of the Apo B-100 is not the major determinant of the rate of secretion. Rather the amount of lipid available determines whether Apo B-100 is degraded or secreted. MTTP is required for the early addition of lipid to Apo B-100 particles but additional lipid is added via pathways that do not require MTTP. Additionally, the size of the VLDL particles is determined by the availability of triglycerides. When triglycerides are abundant the VLDL particles are large.

 

The quantity of fatty acids available for the synthesis of triglycerides is the main determinant of   triglyceride synthesis in the liver. The major sources of fatty acids are a) de novo fatty acid synthesis, b) the hepatic uptake of triglyceride rich lipoproteins, and c) the flux of fatty acids from adipose tissue to the liver. Diabetes, obesity, and the metabolic syndrome are common causes of an increase in hepatic triglyceride levels and the increased secretion of VLDL (44,65).

 

Loss of function mutations in either Apo B-100 or MTTP result in the failure to produce VLDL and marked decreases in plasma triglyceride and cholesterol levels (familial hypobetalipoproteinemia or abetalipoproteinemia) (56). The precise pathway by which the newly synthesized VLDL particles are secreted from the hepatocyte into the circulation is not resolved.

 

VLDL Metabolism (6,58)

 

VLDL particles are transported to peripheral tissues where the triglycerides are hydrolyzed by LPL and fatty acids are released. This process is very similar to that described above for chylomicrons and there is competition between the metabolism of chylomicrons and VLDL. High levels of chylomicrons can inhibit the clearance of VLDL. The removal of triglycerides from VLDL results in the formation of VLDL remnants (Intermediate density lipoproteins (IDL)). These IDL particles are relatively enriched in cholesterol esters and acquire Apo E from HDL particles. In a pathway analogous to the removal of chylomicron remnants these IDL particles can be removed from the circulation by the liver via binding of Apo E to LDL and LRP receptors. However, while the vast majority of chylomicron remnants are rapidly cleared from the circulation by the liver, only a fraction of IDL particles are cleared (approximately 50% but varies). The remaining triglycerides in the IDL particles are hydrolyzed by hepatic lipase leading to a further decrease in triglyceride content and the exchangeable apolipoproteins are transferred from the IDL particles to other lipoproteins leading to the formation of LDL. These LDL particles predominantly contain cholesterol esters and Apo B-100.  Thus, LDL is a product of VLDL metabolism.

 

LDL Metabolism (34,66-69)

 

The levels of plasma LDL are determined by the rate of LDL production and the rate of LDL clearance, both of which are regulated by the number of LDL receptors in the liver. The production rate of LDL from VLDL is partially determined by hepatic LDL receptor activity with a high LDL receptor activity resulting in a decrease in LDL production due to an increase in IDL uptake. Conversely, low LDL receptor activity results in an increase in LDL production formation due to a decrease in IDL uptake. With regards to LDL clearance, approximately 70% of circulating LDL is cleared via hepatocyte LDL receptor mediated endocytosis with the remainder taken up by extrahepatic tissues. An increase in the number of hepatic LDL receptors therefore increases LDL clearance leading to a decrease in plasma LDL levels. Conversely, a decrease in hepatic LDL receptors slows LDL clearance leading to an increase in plasma LDL levels. Thus, the level of hepatic LDL receptors plays a key role in regulating plasma LDL levels. Many of the drugs used to lower plasma LDL levels, such as the statins, ezetimibe, PCSK9 inhibitors, bile acid sequestrants and bempedoic acid lower plasma LDL levels by increasing the number of hepatic LDL receptors (57).

 

The levels of LDL receptors in the liver are mainly regulated by the cholesterol content of the hepatocyte. As cholesterol levels in the cell decrease, inactive sterol regulatory element binding proteins (SREBPs), which are transcription factors that mediate the expression of LDL receptors and key genes involved in cholesterol and fatty acid metabolism, are transported from the endoplasmic reticulum to the Golgi where proteases cleave the SREBPs into active transcription factors (Figure 4). These active SREBPs move to the nucleus where they stimulate the transcription of the LDL receptor and enzymes required for cholesterol synthesis, including HMG-CoA reductase, the rate limiting enzyme in cholesterol synthesis. If cholesterol levels in the cell are high, then the SREBPs remain in the endoplasmic reticulum in an inactive form and do not stimulate LDL receptor synthesis. In addition, cholesterol in the cell is oxidized and oxidized sterols activate LXR, a nuclear hormone receptor that is a transcription factor, which stimulates the transcription of E3 ubiquitin ligase that mediates the ubiquitination and degradation of the low-density lipoprotein receptor (Inducible degrader of the low-density lipoprotein receptor (IDOL)). Thus, the cell can sense the availability of cholesterol and regulate LDL receptor activity. If the cholesterol content of the cell is decreased LDL receptor activity is increased to allow for the increased uptake of cholesterol. Conversely, if the cholesterol content of the cell is increased LDL receptor activity is decreased and the uptake of LDL by the cell is diminished. Statins, ezetimibe, bile acid sequestrants, and bempedoic acid decrease hepatic cholesterol levels thereby increasing LDL receptor levels and decreasing plasma LDL levels (57). Finally, the LDL receptor is targeted for degradation by PCSK9, a secreted protein that binds to the LDL receptor and enhances LDL receptor degradation in the lysosomes. Loss of function mutations in PCSK9 and drugs that inhibit PCSK9 result in increased LDL receptor activity and decreased LDL levels while gain of function mutations in PCSK9 lead to decreased LDL receptor activity and elevations in LDL levels.

 

Thus, the endogenous lipoprotein pathway facilitates the movement of triglycerides synthesized in the liver to muscle and adipose tissue. Additionally, it also provides a pathway for the transport of cholesterol from the liver to peripheral tissues.

 

HDL METABOLISM AND REVERSE CHOLESTEROL TRANSPORT (38-40,47,48,70,71)

 

Figure 8. HDL Metabolism

 

HDL Formation

 

Several steps are required to generate mature HDL particles. The first step involves the synthesis of the main structural protein contained in HDL, Apo A-I. Apo A-I is synthesized predominantly by the liver and intestine. After Apo A-I is secreted, it acquires cholesterol and phospholipids that are effluxed from hepatocytes and enterocytes. The efflux of cholesterol and phospholipids to the newly synthesized lipid poor Apo A-I (pre-beta HDL) is facilitated by ABCA1. Patients with loss of function mutations in ABCA1 (Tangiers disease) fail to lipidate the newly secreted Apo A-I leading to the rapid catabolism of Apo A-I and very low HDL levels (72). Using mice with targeted knock-out of ABCA1 it has been shown that HDL cholesterol levels are reduced by 80% in mice lacking ABCA1 in the liver and 30% in mice lacking ABCA1 in the intestine. While initially cholesterol and phospholipids are obtained from the liver and intestine, HDL also acquires lipid from other tissues and from other lipoproteins. Muscle cells, adipocytes, and other tissues express ABCA1 and ABCG1 and are able to transfer cholesterol and phospholipids to Apo A-I particles. Additionally, as noted above, newly formed HDL can also obtain cholesterol and phospholipids from chylomicrons and VLDL during their lipolysis by LPL. This accounts for the observation that patients with high plasma triglyceride levels due to decreased clearance frequently have low HDL cholesterol levels. Additionally, phospholipid transfer protein (PLTP) facilitates the movement of phospholipids between lipoproteins; mice lacking PLTP have a marked reduction in HDL cholesterol and Apo A-I levels. Finally, the lipolysis of triglyceride rich lipoproteins also results in the transfer of apolipoproteins from these particles to HDL.

 

HDL Cholesterol Esterification

 

As noted earlier the cholesterol in the core of HDL is esterified (cholesterol esters). The cholesterol that is effluxed from cells to HDL is free cholesterol and is localized on the surface of HDL particles. In order to form mature large spherical HDL particles with a core of cholesterol esters the free cholesterol transferred from cells to the surface of HDL particles must be esterified. LCAT, an HDL associated enzyme catalyzes the transfer of a fatty acid from phospholipids to free cholesterol resulting in the formation of cholesterol esters. The cholesterol ester formed is then able to move from the surface of the HDL particle to the core allowing additional free cholesterol to be transferred from cells to HDL particles. Apo A-I is an activator of LCAT and facilitates this esterification process. LCAT activity is required for the formation of large HDL particles. LCAT deficiency in humans results in decreased HDL cholesterol and Apo A-I levels and a higher percentage of small HDL particles (72).

 

HDL Metabolism

 

Lipases and transfer proteins play an important role in determining the size and composition of HDL particles. The cholesterol ester carried in the core of HDL particles may be transferred to Apo B containing particles in exchange for triglyceride. This transfer is mediated by CETP and results in HDL enriched in triglycerides which may then be metabolized by lipases. Humans deficient in CETP activity have very high HDL cholesterol levels and large HDL particles (72). CETP also impacts LDL cholesterol levels and the absence of CETP results in a decrease in LDL cholesterol. Mice do not have CETP and have relatively high HDL cholesterol levels and low LDL cholesterol levels. Hepatic lipase hydrolyzes both triglycerides and phospholipids in HDL. The triglycerides that are transferred to HDL by CETP activity are catabolized by hepatic lipase resulting in the formation of small HDL particles and Apo A-I more easily disassociates from small HDL resulting in the release of Apo A-I and increased Apo A-I degradation. Genetic deficiency of hepatic lipase results in a modest elevation in HDL cholesterol levels and larger HDL particles (72). Hepatic lipase activity is increased in insulin resistant states and this is associated with reduced HDL cholesterol levels. Endothelial cell lipase is a phospholipase that hydrolyzes the phospholipids carried in HDL particles. In mice increased endothelial lipase activity results in decreased HDL cholesterol levels while decreased endothelial lipase activity increases HDL cholesterol levels.

 

The cholesterol carried on HDL is primarily delivered to the liver. The uptake of HDL cholesterol by the liver is mediated by SR-BI, which promotes the selective uptake of HDL cholesterol. The HDL particle binds to SR-BI and the cholesterol in HDL is transported into the liver without internalization of the HDL particle. A smaller cholesterol depleted HDL particle is formed, which is then released back into the circulation. In SR-BI deficient mice there is a marked increase in HDL cholesterol levels. Interestingly the risk of atherosclerosis is increased in these SR-BI deficient mice despite an increase in HDL cholesterol levels. Notably, while HDL cholesterol levels are increased in SR-B1 deficient mice the reverse cholesterol transport pathway is actually reduced. While in mice the physiological importance of the hepatic SR-BI pathway is clear, the role in humans is uncertain. In mice, the movement of cholesterol from peripheral tissues to the liver is dependent solely on SR-BI while in humans CETP can facilitate the transport of cholesterol from HDL to Apo B containing lipoproteins, which serves as an alternative pathway for the transport cholesterol to the liver. 

 

Apo A-I is metabolized independently of HDL cholesterol. Most of the Apo A-I is catabolized by the kidneys with the remainder catabolized by the liver. Lipid free or lipid poor Apo A-I is filtered by the kidneys and then taken up by the renal tubules. The size of the Apo A-I particle determines whether it can be filtered by the kidneys and hence the degree of lipidation of Apo A-I determines the rate of catabolism. Conditions or disease states (for example Tangiers disease, which is due to a mutation in ABCA1, or LCAT deficiency) that result in lipid poor HDL led to the accelerated catabolism of Apo A-I by the kidney. Apo A-I binds to cubilin, which in conjunction with megalin, a member of the LDL receptor gene family, leads to the uptake and degradation of filtered Apo A-I by renal tubular cells. While the liver is also involved in the catabolism of Apo A-I, the mechanisms are poorly understood. HDL particles may contain Apo E and it is therefore possible that Apo E containing HDL particles are taken up via the LDL receptor and other Apo E receptors in the liver and degraded.

 

Reverse Cholesterol Transport (73-78)

 

Peripheral cells accumulate cholesterol through the uptake of circulating lipoproteins and de novo cholesterol synthesis. Most cells do not have a mechanism for catabolizing cholesterol. Cells that synthesize steroid hormones can convert cholesterol to glucocorticoids, estrogen, testosterone, etc. Intestinal cells, sebocytes, and keratinocytes can secrete cholesterol into the intestinal lumen or onto the skin surface thereby eliminating cholesterol. However, in order for most cells to decrease their cholesterol content reverse cholesterol transport is required. From a clinical point of view, the ability of macrophages in the arterial wall to efficiently efflux cholesterol into the reverse cholesterol transport pathway may play an important role in the prevention of atherosclerosis.

 

As noted earlier ABCA1 plays an important role in the efflux of cholesterol to lipid poor pre-beta Apo A-I particles (Figure 9). ABCG1 plays an important role in the efflux of cholesterol from cells to mature HDL particles. In some studies, SR-B1 also plays a role in the efflux of cholesterol to mature HDL particles. Additionally, passive diffusion of cholesterol from the plasma membrane to HDL may also contribute to cholesterol efflux. The levels of both ABCA1 and ABCG1 are increased by LXR activation. LXR is a nuclear hormone transcription factor that is activated by oxysterols. As the cholesterol levels in a cell increase the formation of oxysterols increases leading to the activation of LXR resulting in an increase in ABCA1 and ABCG1 expression, which will result in the enhanced efflux of cholesterol from the cell to HDL.  Additionally, ABCA1 and ABCG1 mRNAs are targeted for degradation by miR-33, a microRNA that is embedded within the SREBP2 gene. An increase in cellular cholesterol decreases the expression of SREBP2 leading to a decrease in miR-33 resulting in enhanced LXR expression. Thus, the decrease in SREBP2 transcription will lead to a decrease in LDL receptor activity and a reduction in cholesterol uptake, while simultaneously, a decrease in miR-33 will lead to an increase in LXR activity stimulating the expression of ABCA1 and ABCG1 resulting in increased cholesterol efflux. Conversely a decrease in cellular cholesterol levels will increase SREBP2 expression resulting in an increase in LDL receptor activity and an increase in miR-33, which will result in a decrease in LXR activity, decreased expression of ABCA1 and ABCG1, and a reduction in cholesterol efflux. Together changes in cholesterol uptake mediated by the LDL receptor and cholesterol efflux mediated by ABCA1 and ABCG1 will maintain cellular cholesterol homeostasis.

 

Figure 9. Cholesterol Efflux from Macrophages (modified from J. Clinical Investigation 116: 3090, 2006)

 

Once cholesterol is transferred from cells to HDL there are two pathways for the cholesterol to be transported and taken up by the liver. As discussed earlier, HDL can interact with hepatic SR-BI receptors resulting in the selective uptake of cholesterol from HDL particles. Alternatively, CETP can transfer cholesterol from HDL particles to Apo B containing particles with the subsequent uptake of Apo B containing lipoproteins by the liver. After the delivery of cholesterol to the liver there are several pathways by which the cholesterol can be eliminated. Cholesterol can be converted to bile acids and secreted in the bile. Alternatively, cholesterol can be directly secreted into the bile. ABCG5 and ABCG8 promote the transport of cholesterol into the bile and the expression of these genes is enhanced by LXR activation. Thus, an increase in hepatic cholesterol levels leading to increased oxysterol production will activate LXR resulting in the increased expression of ABCG5 and ABCG8 facilitating the secretion of cholesterol in the bile.

 

Evidence suggests that reverse cholesterol transport plays an important role in protecting from the development of atherosclerosis. It should be noted that HDL cholesterol levels may not be indicative of the rate of reverse cholesterol transport. As described above reverse cholesterol transport involves several steps and the level of HDL cholesterol may not accurately reflect these steps. For example, studies have shown that the ability of HDL to promote cholesterol efflux from macrophages can vary. Thus, the same level of HDL cholesterol may not have equivalent abilities to mediate the initial step of reverse cholesterol transport.  

 

LIPOPROTEIN (a) (14-16,79)

 

Figure 10. Lp (a)

 

Lp (a) consists of an LDL molecule and a unique apolipoprotein (a), which is attached to the Apo B-100 of the LDL via a single disulfide bound. Lp (a) contain Apo (a) and Apo B-100 in a 1:1 molar ratio. Like Apo B-100, apo (a) is also made by hepatocytes. Apo (a) contains multiple kringle motifs that are similar to the kringle repeats in plasminogen. The number of kringle repeats can vary and thus the molecular weight of apo (a) can range from 250,000 to 800,000.  The levels of Lp (a) in plasma can vary more than a 1000-fold ranging from undetectable to greater than 100mg/dl. Lp (a) levels largely reflect Lp (a) production rates, which are primarily genetically regulated and not greatly affected by environmental factors. Individuals with high molecular weight Apo (a) proteins tend to have lower levels of Lp (a) while individuals with low molecular weight Apo (a) tend to have higher levels. It is hypothesized that the liver is less efficient in secreting high molecular weight Apo (a). The mechanism of Lp (a) clearance is uncertain but does not appear to primarily involve LDL receptors. Therapies that accelerate LDL clearance and lower LDL levels do not lower Lp (a) levels (for example statin therapy). The kidney appears to play an important role in Lp (a) clearance as kidney disease is associated with delayed clearance and elevations in Lp (a) levels.

 

 Elevated plasma Lp(a) levels are associated with an increased risk of atherosclerosis. Apo (a) is an inhibitor of fibrinolysis and enhances the uptake of lipoproteins by macrophages, both of which could account for the increased the risk of atherosclerosis in individuals with elevated Apo (a) levels. Additionally, Lp (a) is the major lipoprotein carrier of oxidized phospholipids, which are inflammatory and could also increase the risk of atherosclerosis. The physiologic function of Apo (a) is unknown. Apo (a) is found in primates but not in other species.

 

ACKNOWLEDGEMENTS

 

This work was supported by grants from the Northern California Institute for Research and Education.

 

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Monogenic Disorders Causing Hypobetalipoproteinemia

ABSTRACT

 

Monogenic mutations leading to hypobetalipoproteinemia are rare. The monogenic causes of hypobetalipoproteinemia include familial hypobetalipoproteinemia, abetalipoproteinemia, chylomicron retention disease, loss of function mutations in PCSK9, and loss of function mutations in angiopoietin-like protein 3 (ANGPTL3) (Familiar Combined Hypolipidemia). This chapter describes the etiology, pathogenesis, clinical and laboratory findings, and the treatment of these rare monogenic disorders.

 

INTRODUCTION

 

Monogenic mutations leading to hypobetalipoproteinemia are rare. The monogenic causes of hypobetalipoproteinemia include familial hypobetalipoproteinemia (FHBL), abetalipoproteinemia (ABL), chylomicron retention disease (CMRD), loss of function mutations in PCSK9, and loss of function mutations in angiopoietin-like protein 3 (ANGPTL3) (Familial Combined Hypolipidemia, FCH) (1). Increased understanding of the genetic and the molecular underpinnings of these disorders has allowed a focused prioritization of therapeutic targets for drug development. Table 1 summarizes genetic, lipid, and clinical features of the major hypobetalipoproteinemia syndromes and table 2 provides a new classification of these disorders. Of note the parental lipid profile is normal in abetalipoproteinemia and chylomicron retention disease.

 

It should be recognized that secondary, non-familial, forms of hypobetalipoproteinemia occur and include strict vegan diet, malnutrition, malabsorption, hyperthyroidism, malignancy, and chronic liver disease. In addition, hypobetalipoproteinemia can also be due to polymorphisms in multiple genes that together result in hypobetalipoproteinemia (polygenic etiology) (2-4). In a study of 111 patients with LDL-C levels below the fifth percentile 36% had monogenic hypobetalipoproteinemia, 34% had polygenic hypobetalipoproteinemia, and 30% had hypobetalipoproteinemia from an unknown cause (2). In a study of women with an LDL-C ≤1st percentile (≤50 mg/dL) 15.7% carried mutations causing monogenic hypocholesterolemia and 49.6% were genetically predisposed to a low LDL-C on the basis of an extremely low weighted polygenetic risk score (4). Of note individuals with monogenic hypobetalipoproteinemia are more likely to have liver steatosis than individuals without a monogenic disorder (2).

 

Table 1. Characteristics of the Hypobetalipoproteinemia Syndromes

 

Inheritance

Effected gene

Prevalence

Lipids

Clinical features

FHBL

ACD

Truncation mutations in Apo B

1:1000 – 1:3000

Apo B <5th percentile,

LDL-C 20- 50 mg/dL

Hepatic steatosis

Mild elevation of transaminases. Lower prevalence of ASCVD

ABL

 

FHBL

AR

 

AR

MTTP

 

Apo B

<1:1,000,000

Triglycerides < 30 mg/dL,

Cholesterol < 30 mg/dL),

LDL and Apo B undetectable

Hepatic steatosis

Malabsorption, steatorrhea, diarrhea, and failure to thrive.

Deficiency of fat-soluble vitamins.

PCSK9

ACD

Loss of function mutations in PCSK9

 

Heterozygous – mild to moderate reduction in LDL-C

Homozygous – LDL-C ~15 mg/dL

Normal health; significantly lower prevalence of ASCVD

FCH

ACD

Loss of function mutations in ANGPTL3

Very rare

Panhypolipidemia

Normal health; significantly lower prevalence of ASCVD

CMRD

AR

SAR1B

Very rare

LDL-C and HDL-C -decreased by 50%,

Triglycerides - normal

hypocholesterolemia associated with failure to thrive, diarrhea, steatorrhea, and abdominal distension

ACD- autosomal co-dominant; AR- autosomal recessive; FHBL- familial hypobetalipoproteinemia; ABL- abetalipoproteinemia; FCH- Familiar Combined Hypolipidemia; CMRD- chylomicron retention disease, MTTP- microsomal triglyceride transfer protein; ANGPTL3- angiopoietin-like protein 3; ASCVD- atherosclerotic cardiovascular disease.

 

Table 2. Classification of Disorders Causing Familial Hypocholesterolemia

New Name

Common Name

Gene Defect

Class I: Familial hypobetalipoproteinemia due to lipoprotein assembly and secretion defects

FHBL-SD1

Abetalipoproteinemia

Microsomal Triglyceride Transfer Protein

FHBL-SD2

Familial Hypobetalipoproteinemia

Apolipoprotein B

FHBL-SD3

Chylomicron retention disease

SAR1B

Class II: Familial hypobetalipoproteinemia due to enhanced lipoprotein catabolism

FHBL-EC1

Familial Combined Hypolipidemia

ANGPTL3

FHBL-EC2

 

PCSK9

Modified from (5).

 

FAMILIAL HYPOBETALIPOPROTEINEMIA  

 

Familial Hypobetalipoproteinemia (FHBL) is a relatively common autosomal semi-dominant disorder most commonly due to truncation mutations in the gene coding for Apo B (1,6-8). The prevalence of heterozygous FHBL is estimated to be 1 in 700 to 3000 (1). Variants that lead to truncated proteins that are 30% in length or shorter have more severe signs and symptoms than those with longer truncated proteins (6,7). The truncated forms of Apo B found in FHBL are generally non-functional (truncation decreases lipidation and secretion) and are catabolized quickly, resulting in markedly reduced levels in the plasma (Apo B <5th percentile and LDL-C typically between 20- 50 mg/dL) (7,8). Although there is one normal allele in heterozygous FHBL, plasma Apo B levels are approximately 25% of normal rather than the predicted 50% (8). These lower-than-expected levels result from a lower secretion rate of VLDL Apo B from the liver, decreased production of LDL Apo B, increased catabolism of VLDL, and extremely low secretion of the truncated Apo B (6-8). Given the reduced substrate (Apo B) for lipid (predominantly triglyceride) loading, fatty liver develops in these patients (6,9). Hepatic steatosis and mild elevation of liver enzymes are common in heterozygous FHBL (6,9). Interestingly, individuals with monogenic hypobetalipoproteinemia had a much greater prevalence of hepatic dysfunction than individuals with polygenic hypobetalipoproteinemia (2). In contrast to non-alcoholic fatty liver disease, FHBL is not associated with hepatic or peripheral insulin resistance (9). This observation, however, does not imply that hepatic steatosis associated with FHBL is benign. There are several reports of steatohepatitis, cirrhosis, and hepatocellular carcinoma in patients with FHBL and it is estimated that 5-10% of individuals with FHBL develop relatively more severe nonalcoholic steatohepatitis (6). Because of the risk of developing liver disease liver function tests should be checked every 1-2 years and a hepatic ultrasound in those with elevated liver transaminases (6). While hepatic fat accumulation is the rule, there is generally sufficient chylomicron production to handle dietary fat. However, oral fat intolerance and intestinal fat malabsorption have been reported (6). On the positive side the decrease in proatherogenic lipoproteins has been associated with a reduced risk of cardiovascular disease (10).

 

Given the association of FHBL and low LDL-C, Apo B has been an attractive target for drug development. Indeed, unraveling the genetic and molecular mechanisms of FHBL provided the motivation to pharmacologically antagonize Apo B synthesis for therapeutic gains. This culminated in the production of mipomersen, a synthetic single strand anti-sense oligonucleotide to Apo B (11,12). Essentially, anti-sense oligonucleotides contain approximately ~20 deoxyribonucleic acid (DNA) base pairs complementary to a unique messenger ribonucleic acid (mRNA) sequence. The hybridization of the anti-sense oligonucleotide to the mRNA of interest leads to its catabolism via RNase H1, with markedly reduced mRNA levels and ultimately reduced target protein levels. In this case, mipomersen binds to Apo B mRNA leading to reduced production of the protein, and mimicking (albeit to a lesser extent) FHBL. Mipomersen is the first anti-sense oligonucleotide approved by the United States Food and Drug Administration (FDA) and was commercialized in 2013 with a limited indication for adjunctive LDL-C lowering in patients with homozygous familial hypercholesterolemia (HoFH) (12). It is an injectable agent administered subcutaneously once a week. In the clinical trials, mipomersen was associated with a reduction of LDL-C by 21% in subjects with HoFH and 33% in subjects with heterozygous familial hypercholesterolemia (HeFH) (12). Interestingly, it was also found to lower Lp(a) by 21- 23% (12). While it is highly efficacious in LDL-C lowering, it has side effects, many of which can be predicted based on the experience with FHBL (e.g., hepatic steatosis, elevated liver enzymes) (12). It is also associated with injection site reactions in a considerable number of subjects (12). In May 2018 sales were discontinued due to safety concerns related to increased liver transaminases and fatty liver.

 

Homozygous hypobetalipoproteinemia (HHBL) is extremely rare (6). These patients are homozygous or compound heterozygous for mutations in the Apo B gene. The clinical manifestations mimic ABL (see below) (6).

 

ABETALIPOPROTEINEMIA  

 

Abetalipoproteinemia (ABL) is a rare autosomal recessive disorder characterized by very low plasma concentrations of triglyceride and cholesterol (under 30 mg/dL) and undetectable levels of LDL and Apo B (1,7,13,14). The incidence of ABL is < 1 in 1,000,000. HDL-C levels are usually normal or modestly reduced. It is due to mutations in the gene that codes for microsomal triglyceride transfer protein (MTTP) (7,13-15). MTTP lipidates nascent Apo B in the endoplasmic reticulum to produce VLDL and chylomicrons in the liver and small intestine, respectively (15,16). Unlipidated Apo B is targeted for proteasomal degradation leading to the absence of Apo B containing lipoproteins in the plasma (and thus markedly reduced levels of LDL-C and triglycerides) (15,16). Similar to FHBL, VLDL production is inhibited (14). The reduced triglyceride export from the liver leads to hepatic steatosis, which rarely may progress to steatohepatitis, fibrosis, and cirrhosis (1,9,13). Additionally, lack of MTTP facilitated lipidation of chylomicrons in the small intestine results in lipid accumulation in enterocytes with associated malabsorption, steatorrhea, and diarrhea (1,7,13). The malabsorption and diarrhea lead to failure to thrive during infancy (1,7,13). A decrease in dietary fat can reduce the gastrointestinal symptoms. Acanthocytosis may encompass 50% of circulating red blood cells (red blood cells with spiked cell membranes, due to thorny projections) due to alterations in the lipid composition and fluidity of red cell membranes (1,13,14). An additional issue of importance related to ABL is deficiency of fat-soluble vitamins (1,13). Early diagnosis of ABL and homozygous hypobetalipoproteinemia is extremely important as vitamin E deficiency culminates in atypical retinitis pigmentosa, spinocerebellar degeneration with ataxia, vitamin K deficiency can lead to a significant bleeding diathesis, vitamin A deficiency can contribute to eye disorders, and vitamin D deficiency can lead to defects in bone formation (1,13). High dose supplementation with fat soluble vitamins early in life can prevent or delay these devastating complications (Table 3) (7,13). Additional treatment measures include a low-fat diet and supplementation with essential fatty acids (Table 3) (7,13).

 

 Table 3. Dietary Recommendations for Abetalipoproteinemia

Fat calories

Less than 10-15% (<15 g/day) of total daily caloric requirement. Increase as tolerated.

Essential fatty acids

Ensure 2-4% daily caloric intake of EFAs (alpha-linolenic acid/linoleic acid)

Medium chain triglycerides

May prevent or treat malnutrition

Vitamin E

100-300 IU/kg/day

Vitamin A

100-400 IU/kg/day

Vitamin D

800-1200 IU/day

Vitamin K

5-35 mg/week

Derived from (1)

 

Given the very low level of atherogenic lipoproteins and lipids associated with ABL, there was interest in inhibiting MTTP therapeutically. Lomitapide is an oral MTP inhibitor that has been developed over the course of many years (12,17). In early trials, it was tested at a relatively high dose and the side effect profile was prohibitive (nausea, flatulence, and diarrhea). The more recent clinical trial program tested lower doses with drug titration in subjects with Homozygote Familial Hypercholesterolemia (HoFH) (12,17). On an intention to treat basis, LDL-C was decreased by 40% and apolipoprotein B by 39% (12). In patients who were actually taking lomitapide, LDL-C levels were reduced by 50% (12). In addition to decreasing LDL-C levels, non-HDL-C levels were decreased by 50%, Lp(a) by 15%, and triglycerides by 45% (12). Lomitapide received the same limited indication as mipomersen for adjunctive treatment of patients with HoFH (12). Besides the gastrointestinal issues already alluded to, its side effect profile includes hepatic steatosis (12). Its long-term safety has not been established.

 

PROPROTEIN CONVERTASE SUBTILISIN/KEXIN TYPE 9 (PCSK9)

 

Proprotein convertase subtilisin/ kexin type 9 (PCSK9) belongs to the proprotein convertase class of serine proteases (18-20). After synthesis, PCSK9 undergoes autocatalytic cleavage. This step is required for secretion, most likely because the prodomain functions as a chaperone and facilitates folding (18,19). PCSK9 is associated with LDL particles and the LDL-receptor (LDLR) (20). In 2003, Abifadel reported the seminal work that mapped PCSK9 as the third locus for autosomal dominant hypercholesterolemia (Familial Hypercholesterolemia- FH) (21). This finding revealed a previously unknown actor involved in cholesterol homeostasis and served to launch a series of investigations into PCSK9 biology. As it turns out, PCSK9 functions as a central regulator of plasma LDL-C concentration (18-20). It binds to the LDLR and targets it for destruction in the lysosome (18-20). Overactivity of PCSK9 results in a decrease in LDLR and an increase in LDL-C levels while decreased activity of PCSK9 results in an increase in LDLR and a decrease in LDL-C.

 

Since the discovery of gain-of-function mutations in PCSK9 as a cause of FH, investigators have also uncovered loss of function mutations of PCSK9. Loss-of-function mutations in PCSK9 are associated with low LDL-C levels and markedly reduced ASCVD (18,19). In African Americans 2.6 percent had nonsense mutations in PCSK9 that resulted in a 28 percent reduction in LDL-C and an 88 percent reduction in the risk of coronary heart disease (22). The hypolipidemia is not associated with liver abnormalities or other disorders. Interestingly, rare individuals homozygous or compound heterozygotes for loss of function mutations in PCSK9 have been reported with extremely low levels of LDL-C (~15 mg/dL), normal health and reproductive capacity, and no evidence of neurologic or cognitive dysfunction (20,23,24). Collectively, these observations served as further motivation to pursue antagonism of PCSK9 as a therapeutic target. Antagonizing PCSK9 would prolong the lifespan of LDLR, leading to significant reductions in plasma LDL-C levels. Two fully human monoclonal antibodies (alirocumab and evolocumab) targeting PCSK9 became commercially available in 2015 and inclisiran, a small interfering RNA that inhibits translation of PCSK9 is also available. Other approaches to inhibit PCSK9 are under investigation.  

 

FAMILIAL COMBINED HYPOLIPIDEMIA   

 

Familial combined hypolipidemia (FCH) is due to loss of function mutations in the gene encoding angiopoietin-like protein 3 (ANGPTL3) (25,26). ANGPTL3 inhibits various lipases, such as lipoprotein lipase and endothelial lipase (25,26). Therefore, loss of function mutations in ANGPTL3 relinquishes this inhibition increasing the activity of lipases resulting in more efficient metabolism of VLDL and HDL particles (25,26). In addition, to increasing VLDL clearance the secretion of VLDL is also decreased due to a decrease in free fatty acid flux to the liver (25). LDL clearance is increased but the mechanism remains to be fully elucidated (25). Studies have suggested that ANGPTL3 inhibition lowers LDL-C by limiting LDL particle production due to ANGPTL3 inhibition and increased endothelial lipase activity reducing VLDL-lipid content and size, generating remnant particles that are efficiently removed from the circulation rather than being further metabolized to LDL (27).

 

Clinically, FCH manifests as panhypolipidemia (decreased triglycerides, LDL-C, HDL-C, apo B, and apo A-I) (25,26,28). Interestingly, heterozygotes for certain nonsense mutations in the first exon of ANGPTL3 have moderately reduced LDL-C and triglyceride levels while compound heterozygotes have significant reductions in HDL-C as well (25,26).  Homozygosity or compound heterozygosity for other loss-of-function mutations in exon 1 of ANGPTL3 have no detectable ANGPTL3 in plasma and striking reductions of atherogenic lipoproteins with HDL particles containing only apo A-I and preß-HDL. Individuals who are heterozygous for the loss of function mutations in ANGPTL3 have significantly reduced LDL-C and triglyceride levels and a reduced risk of atherosclerosis (25,26,28).

 

A pooled analysis of cases of familial combined hypolipidemia was published 2013 (29). One hundred fifteen individuals carrying 13 different mutations in the ANGPTL3 gene (14 homozygotes, 8 compound heterozygotes, and 93 heterozygotes) and 402 controls were evaluated. Homozygotes and compound heterozygotes (two mutant alleles) had no measurable ANGPTL3 protein. In heterozygotes, ANGPTL3 was reduced by 34-88%, according to genotype. All cases (homozygotes and heterozygotes) demonstrated significantly lower concentrations of all plasma lipoproteins (except for Lp(a)) as compared to controls. Familial combined hypolipidemia is not associated with any comorbidity. In fact, the prevalence of fatty liver was the same as controls. However, ANGPTL3 deficiency is associated with a reduced risk of cardiovascular disease (25,30).

 

Recently, evinacumab, a human monoclonal antibody against ANGPTL3, was approved for the treatment of Homozygous Familial Hypercholesterolemia (12). Evinacumab decreases LDL-C levels by mechanisms independent of LDL receptor activity (12).

 

CHYLOMICRON RETENTION DISEASE

 

Chylomicron retention disease (CMRD), known also as Anderson’s disease for the individual who first described the condition in 1961, is a rare inherited lipid malabsorption syndrome (31,32). It is due to mutations in the SAR1B gene which codes for the protein SAR1b, a small GTPase, involved in intracellular protein trafficking (31). Mutations in SAR1b result in the failure of pre-chylomicrons to move from the endoplasmic reticulum to the golgi (31). This disorder usually presents in young infants with diarrhea, steatorrhea, abdominal distention, and failure to thrive, which can improve with a low-fat diet (1,31,32). Patients with CMRD demonstrate a specific autosomal recessive hypocholesterolemia that differs from other familial hypocholesterolemias. CMRD is associated with a 50% reduction in both plasma LDL-C and HDL-C with normal fasting triglyceride levels (31,32). Mutations in SAR1B do not affect VLDL secretion by the liver. The decrease in HDL-C is postulated to be due to a decrease in Apo A-I secretion and cholesterol efflux by the small intestine (31). The mechanism accounting for the decrease in LDL-C is not clear. The usual increase in triglycerides and chylomicron levels following a fat meal is blocked (31). The duodenal mucosa is white on endoscopy and intestinal biopsy reveals cytosolic lipid droplets and lipoprotein-sized particles in enterocytes (31). As one would expect the absorption of fat-soluble vitamins (A, D, K, and E) and essential fatty acids is impaired (31,32). Neurological and eye manifestations are milder and occur at an older age compared to abetalipoproteinemia (1). Red blood cell acanthosis is rare (1). Heterozygotes with mutations in SAR1B are unaffected.

 

Treatment for individuals with CMRD is similar to that described above for individuals with ABL (32).

 

ACKNOWLEDGEMENTS

 

This work was supported by grants from the Northern California Institute for Research and Education.

 

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Hypertriglyceridemia: Pathophysiology, Role of Genetics, Consequences, and Treatment

ABSTRACT

 

Hypertriglyceridemia (HTG) can result from a variety of causes. Mild to moderate HTG tracks along with the metabolic syndrome, obesity and diabetes. HTG can be the result of multiple small gene variants or secondary to several diseases and drugs. Severe HTG with plasma triglyceride (TG) levels >1000-1500 mg/dL typically results from: (1) rare variants in the lipoprotein lipase (LPL) complex, where it is termed the familial chylomicronemia syndrome (FCS), and (2) the co-existence of genetic and secondary forms of HTG, termed the multifactorial chylomicronemia syndrome (MFCS), which is a much more common cause of severe HTG.  Mild to moderate HTG is associated with an increased risk of premature cardiovascular disease (CVD), while severe HTG can lead to pancreatitis as well as an increased risk of premature CVD. Appropriate management of the patient with HTG requires knowledge of the likely cause of the HTG, to prevent itscomplications.

 

PHYSIOLOGY

 

A detailed overview of lipoprotein physiology is provided in the Endotext chapter on Lipoprotein Metabolism (1).  Here we will briefly review some aspects the metabolism of the triglyceride (TG)-rich lipoproteins, very low-density lipoproteins (VLDL) and chylomicrons (CM) of particular relevance to this chapter.

 

Secretion of TG-rich Lipoproteins Into Plasma

 

TGs are transported through plasma as VLDL), which transport TGs primarily made in the liver, and as CM, which transport dietary (exogenous) fat.  VLDL secretion by the liver is regulated in several ways.  Each VLDL particle has one ApoB100 molecule, making ApoB100 availability a key determinant of the number of VLDL particles, and hence, TG secretion by the liver.  In addition to one molecule of ApoB-100, each VLDL particle contains multiple copies of other apolipoproteins, together with varied amounts of TGs, cholesteryl esters, and phospholipids.  The extent of TG synthesis is in part determined by the flux of free fatty acids (FFA) to the liver.  The addition of TG to the developing VLDL particle in the endoplasmic reticulum is mediated by the enzyme microsomal triglyceride transfer protein (MTTP).  The pool of ApoB100 in the liver is not typically regulated by its level of synthesis, which is relatively constant, but by its level of degradation, which can occur in several proteolytic pathways (2). Insulin also plays a role in the regulation of VLDL secretion -  it decreases hepatic VLDL production by limiting fatty acid influx into the liver and decreases the stability of, and promotes the posttranslational degradation of ApoB100 (3).  Recent studies have shown that ApoC-III, an Apolipoprotein thought to primarily play a role in inhibiting TG removal (see below), also is involved in the assembly and secretion of VLDL (4).  VLDL particles (containing ApoB100) also increase in plasma in the postprandial state as well as CM that contain ApoB48 (5).

 

Consumption of dietary fat results in the formation of CM by enterocytes.  Fatty acids and monoacylglycerols that result from digestion of dietary TGs by acid and pancreatic lipases are transported into enterocytes by mechanisms that are not completely understood.  In the enterocyte, monoacylglycerol and fatty acids are resynthesized into TGs by the action of the enzymes acyl-coenzyme A: monoacylglycerol acyltransferase and acyl-coenzyme A: diacylglycerol acyltransferase 1 and 2 (DGAT 1 and 2).  The resulting TGs are packaged with ApoB48 to form CM, a process also mediated by MTTP (6).   CM then pass into the thoracic duct from where they enter plasma and acquire additional apolipoproteins.  Of particular relevance to their clearance from plasma is the acquisition of ApoC-II and ApoC-III. 

 

Catabolism of the TG-rich Lipoproteins

 

TGs in both VLDL and CM are hydrolyzed by the lipoprotein lipase (LPL) complex.  LPL is synthesized by several tissues, including adipose tissue, skeletal muscle, and cardiac myocytes.  After secretion by adipocytes, the enzyme is transported by glycosylphosphatidylinositol-anchored high-density lipoprotein–binding protein 1 (GPIHBP1) to the luminal side of the capillary endothelium, where it becomes tethered to glycosaminoglycans (GAGs). This pool of LPL is referred to as “functional LPL”, since it is available to hydrolyze TGs in both VLDL and chylomicrons. LPL can be liberated from these GAG binding sites by heparin injection. Several other proteins, reviewed in (7), regulate LPL activity. These include ApoC-II, which activates LPL, and ApoC-III, which inhibits LPL in addition to its effect on VLDL secretion alluded to earlier. Both are produced by the liver and are present on TG-rich lipoproteins.  ApoC-III also inhibits the turnover of TG-rich lipoproteins through a hepatic clearance mechanism involving the LDL receptor/LDL receptor-related protein 1 (LDLR/LRP1) axis (8).  ApoE also is present on the TG-rich lipoproteins and plays an important role in the uptake and clearance of the remnants of the TG-rich lipoproteins that result from hydrolysis of TGs in these lipoproteins. Other activators of LPL include ApoA-IV (9), ApoA-V (10-12) and lipase maturation factor 1 (LMF1) (13, 14). In addition, several members of the angiopoietin-like (ANGPTL) protein family play a role in regulating LPL activity.  ANGPTL3 is produced by the liver and is an endocrine regulator by inhibiting LPL in peripheral tissues (7, 15, 16).  ANGPTL4 is produced in several tissues (7), where it inhibits LPL in a paracrine fashion (7, 17). Both ANPGTL3 and ANGPTL4 delay the clearance of the TG-rich lipoproteins (7).

 

The core TGs in VLDL and chylomicrons are hydrolyzed by ApoC-II activated LPL; FFA thus formed are taken up by adipocytes and re-incorporated into TGs for storage, or in skeletal and cardiac muscle, utilized for energy. Hydrolysis of chylomicron- and VLDL-TG results in TG-poor, cholesteryl ester and ApoE-enriched particles called chylomicron and VLDL remnants, respectively, which under physiological conditions are removed by the liver by binding to LDL receptors, LDL receptor related protein, and cell surface proteoglycans (12, 18). Hepatic TG lipase and ApoA-V also are involved in the remnant clearance process (10-12, 19, 20).

 

The clearance of TGs from plasma is saturable when plasma TGs exceed ~500-700 mg/dL (21).  When removal mechanisms are saturated, additional chylomicrons and VLDL entering plasma cannot readily be removed and hence accumulate in the plasma. As a result, plasma TGs can increase dramatically, resulting in very high levels and the accumulation of chylomicrons in plasma obtained after an overnight fast. 

 

NORMAL RANGE FOR PLASMA TRIGLYCERIDES AND DEFINITION OF HYPERTRIGLYCERIDEMIA

 

Plasma TG levels reflect the TG content of multiple lipoprotein particles, primarily chylomicrons and VLDL. Fasting TG levels less than 150 mg/dL has been generally accepted as “normal” (22, 23). A non-fasting TG of 175mg/dL represents ~75th percentile of the normal range, and levels of ~400mg/dL represent the 97th percentile (24). Plasma TGs are heavily skewed to the right in the general population with a tail towards highest levels and vary depending upon the population mix (25). The full range of TG extends from 30mg/dL to 10,000mg/dL (22). TG levels are different between sexes, being higher in males than in females, and increase with age and development of other coexisting conditions such as central adiposity, metabolic syndrome, and diabetes (24). TG levels also vary between geographic areas, among people of different ethnic backgrounds, with higher levels observed in certain populations such as Mexicans and South Asians. Lower TG levels have been observed in people of African descent and African-Americans but this may be changing due to adoption of urban lifestyles (26). Because of this skewed distribution, logarithmic transformation is required to establish statistical normal ranges of TG levels.  There is no current widely accepted definition of elevated non-fasting TG levels, but some groups have utilized 175mg/dL as a cut point (27, 28). Due to high variability of TG levels, precise definitions for non-fasting levels are difficult to establish. It is worthwhile noting that post-prandial TG levels rarely exceed 400mg/dL even after a high fat challenge.

 

Normal Range Based on Risk of Complications of Hypertriglyceridemia

 

The major complications of hypertriglyceridemia (HTG) are (1) acute pancreatitis and (2) increased risk of atherosclerotic cardiovascular disease (ASCVD). These two complications occur at different levels of TGs, the risk of pancreatitis occurring at much higher TG levels than the risk of premature ASCVD and are discussed in detail later in this chapter. 

 

Normal Range According to Guidelines

 

Despite concerns regarding establishment of an upper limit of normal for TGs, most guidelines define values for HTG, often without a strong biological rationale. Definitions for the diagnosis of HTG provided in several guidelines are shown in Table 1.   

 

Cut points for HTG were first defined by the National Cholesterol Education Program Adult Treatment Panel (NCEP-ATP). The terms mild, moderate and severe have been used based on degree of TG elevation (table 1). In general, mild to moderate HTG reflects TG levels under 500mg/dL. Severe hypertriglyceridemia (sHTG) has been arbitrarily defined by different national guidelines as either TG levels ≥500 mg/dL by the American Heart Association (AHA)/American College of Cardiology (ACC), Multispecialty Cholesterol and Canadian Cardiovascular Society Guidelines (29, 30) or TG levels ≥880 mg/dL according to the European Society of Cardiology guidelines (31). The Endocrine Society has used severe HTG for 1000 to 1999 mg/dL and very severe HTG for values >2000 mg/dL (23).  

 

Table 1. Definition of Hypertriglyceridemia According to Various Clinical Guidelines

Guideline

Classification

Triglyceride Levels

NCEP/ ATP III (32)

American Heart Association (33)

National Lipid Association (34)

Normal

Borderline-high TGs

High TGs

Very high TGs

<150 mg/dL (< 1.7 mmol/L)

150-199 mg/dL (1.7-2.3 mmol/L)

200-499 mg/dL (2.3-5.6 mmol/L)

≥500 mg/dL (≥5.6 mmol/L)

The Endocrine Society (35)

Normal

Mild HTG

Moderate HTG

Severe HTG

Very severe HTG

<150 mg/dL (< 1.7 mmol/L)

150-199 mg/dL (1.7-2.3 mmol/L)

200-999 mg/dL (2.3-11.2 mmol/L)

1000-1999 mg/dL (11.2-22.4 mmol/L)

≥2000 mg/dL (≥22.4 mmol/L)

European Society of Cardiology/European Atherosclerosis Society (36)

Normal

Mild-moderate HTG

Severe HTG

<1.7 mmol/L (<150mg/dL)

>1.7-< 10mmol/L (150-880 mg/dL)

> 10 mmol/L (> 880mg/dL)

Hegele (22)

Normal

Mild to moderate

Severe

<2.0 mmol/L (<175 mg/dL)

2.0-10 mmol/L (175- 885 mg/dL)

>10 mmol/dL (>885 mg/dL)

 

In summary, establishing a precise definition of what constitutes abnormal TG values is fraught with difficulty.  An acceptable level for the prevention of pancreatitis is likely to be quite different from that at which CVD risk might be increased. The impact of HTG on CVD risk needs to be evaluated in the context of the family history of premature CVD, associated abnormalities of lipids and lipoproteins, and other CVD risk factors, particularly those associated with the metabolic syndrome (see below).

 

CAUSES AND CLASSIFICATION OF HYPERTRIGLYCERIDEMIA

 

In general, HTG has been classified as primary, when a genetic or familial basis is suspected, or secondary, where other conditions that raise TG levels can be identified. However, this classification is likely overly simplistic. It has become clear in the past decade that the spectrum of plasma TG levels, ranging from mild elevation to very severe HTG, is modulated by a multitude of genes working in concert with non-genetic secondary and environmental contributors. Thus, in the vast majority of individuals, mutations in multiple genes with interaction from non-genetic factors result in altered TG-rich lipoprotein synthesis and catabolism and subsequent HTG.

 

Historical Perspective

 

Phenotypic heterogeneity among patients with HTG has been historically defined by qualitative and quantitative differences in plasma lipoproteins. In the pre-genomic era, the Fredrickson classification of hyperlipoproteinemia was based on electrophoretic patterns of lipoprotein fractions (37). The phenotypes are distinguished based on the specific class or classes of accumulated TG-rich lipoprotein particles, including chylomicrons, VLDL and VLDL-remnants. This classification included 6 phenotypes, five of which included HTG in their definition (except for Frederickson type 2 A hyperlipoproteinemia, which equates with genetic primary hypercholesterolemia). It has now become apparent that except for type 1 hyperlipoproteinemia (FCS), the HTG phenotypes, particularly Frederickson type 4 and type 5 hyperlipoproteinemia, are due to the accumulation of polygenic traits predisposing to HTG. However, this classification system is dated, has neither improved clinical or scientific insight, and therefore does not find wide use at this time (22).

 

In 1973, Goldstein and colleagues characterized a variable pattern of lipid abnormalities in families of survivors of myocardial infarction that they termed familial combined hyperlipidemia (FCHL) (38).  At the same time, this phenotype of mixed or combined hyperlipidemia was observed in another cohort, where it was called multiple-type familial hyperlipoproteinemia (39).  Affected family members can present with hypercholesterolemia alone, HTG alone, or with elevations in both TGs and LDL. This pattern was estimated to have a population prevalence of 1-2% (40), making it the most common inherited form of dyslipidemia.

 

In the aforementioned study, a pattern of isolated HTG, historically called familial HTG (FHTG) also was described (38). This condition was characterized by increased TG synthesis, with secretion of normal numbers of large TG-enriched VLDL particles (41), elevated VLDL levels, but normal levels of LDL and HDL cholesterol (42).  FHTG did not appear to be associated with an increased risk of premature CVD in an early study (43), but baseline TG levels predicted subsequent CVD mortality after 20 years of follow up among relatives in families classified as having FHTG (44, 45).  

 

FCHL and FHTG were initially believed to be monogenic disorders (38). However, more recent genetic characterization of individuals with familial forms of HTG indicates that these are not disorders associated with variation within a single gene, but rather polymorphisms in multiple genes associated with HTG, as detailed below. Therefore, classification of FCHL and FHTG is potentially misleading. Nevertheless, it is important to note that FCHL as originally described is associated with a very high prevalence of premature CVD (43, 44, 46).   

 

Genetic Forms of Hypertriglyceridemia

 

It is now evident that clinically relevant abnormalities of plasma TG levels appear to require a polygenic foundation of common or rare genetic variants (22).  Common small-effect gene variants confer a background predisposition that interact with rare large-effect heterozygous variants in genes that govern synthesis or catabolism of TG-rich lipoproteins, or nongenetic secondary factors, leading to the expression of a more severe TG phenotype (47). Recently, the most prevalent genetic feature underlying severe HTG was shown to be the polygenic accumulation of common (rather than rare) variants—more specifically, the accumulation of TG-raising alleles across multiple SNP loci (48).

 

Thus, mild to moderate hypertriglyceridemic states are complex, genetically heterogeneous disorders. Mild-to-moderate HTG is typically polygenic and results from the cumulative burden of common and rare variants in more than 30 genes, as quantified by genetic risk scores. All genetic forms can be exacerbated by non-genetic factors. Because they are a consequence of interaction between multiple susceptibility genes and lifestyle factors, individuals with moderate HTG should be considered as a single group without distinction, irrespective of concomitant lipoprotein disturbances (22). Because of the complexity of these disorders, routine genetic testing is not recommended.

 

Pathogenesis of Genetic Forms of Hypertriglyceridemia

           

Genetic forms of HTG without other lipoprotein disturbances (i.e., pure HTG) are characterized by increased TG synthesis, where normal numbers of large TG-enriched VLDL particles are secreted (41, 49-51). Reduced TG clearance also has been observed in some individuals (50-52).  Affected people have elevated VLDL levels, but normal levels of LDL, and are generally asymptomatic unless clinical CVD or severe HTG develops. 

 

A variety of metabolic defects that differ among families are associated with the combined hyperlipidemia phenotype. The characteristic lipoprotein abnormalities are increased ApoB levels and increased number of small dense LDL particles (42), a phenotype similar to that seen in the metabolic syndrome and type 2 diabetes (53). These primary defects occur due to 1) hepatic overproduction of VLDL particles (41) due to increased ApoB synthesis in the setting of disordered adipose metabolism (54, 55), insulin resistance (41, 56-58), and liver fat accumulation, and, 2) impaired clearance of ApoB containing particles (59, 60). Increased VLDL secretion results in an elevated plasma ApoB and HTG (56).  Long residence time of VLDL particles favors the formation of small dense LDL (59). An abundance of small dense LDL particles traditionally is associated with the presence of HTG; however, these LDL characteristics remain even after correction of the HTG by treatment with fibrates (61, 62).

 

In addition to Apo B abnormalities, other lipoprotein disturbances include abnormal expression of ApoA-II, ApoC-III, and PCSK9.  VLDL-TG levels in combined hyperlipidemia are modulated by ApoA-II and ApoC-III (63).  Plasma PCSK9 levels are higher in these patients, and levels correlate with TG and Apo B levels (64).

 

Visceral adiposity appears to be an important determinant of insulin resistance, which occurs commonly in subjects with both isolated HTG (65) and combined hyperlipidemia (65-69). Other abnormalities that have been reported in clinical FCHL include impaired lipolysis due to decreased cyclic AMP dependent signaling (54, 69), abnormal adipocyte TG turnover (70), fatty liver (71), increased arterial stiffness (72), and increased carotid intimal-medial thickness (73). 

 

In all of the phenotypes described above, severe HTG can occur when secondary causes of HTG such as untreated diabetes, marked weight gain, or use of TG-raising drugs are present concurrently, leading to the Multifactorial Chylomicronemia Syndrome (MFCS), described later (74).

 

Secondary Forms of Hypertriglyceridemia

 

These are described in greater detail in the chapters on Secondary Disorders of Lipid and Lipoprotein Metabolism (75-78).  However, in the section where we describe MFCS we will briefly touch on some aspects of secondary forms of HTG, since they assume importance in the pathogenesis of the severe HTG seen in the MFCS, where they often co-exist in individuals with genetic forms of HTG. In our experience, the commonest secondary forms of HTG that interact with genetic forms of HTG are type 2 diabetes (usually as part of the metabolic syndrome), obesity, recent weight gain, excessive alcohol consumption, the use of drugs that can raise TGs, and chronic kidney disease (CKD)(74, 79, 80). (Table 3)

           

Severe Hypertriglyceridemia and the Chylomicronemia Syndrome

 

In the late 1960s Fredrickson, Levy and Lees (37) classified HTG into types dependent on the pattern of lipoproteins on paper electrophoresis and the presence or absence of chylomicrons in fasting plasma. They recognized that acute pancreatitis and eruptive xanthomata occurred in the presence of chylomicronemia that accumulate in what they termed Type I and Type V hyperlipoproteinemia. Chylomicrons are present in the post-prandial state, and usually are present in fasting plasma when TG levels exceed 800 mg/dL, but absent in fasting plasma below that value (81). The term chylomicronemia syndrome was first used to describe a constellation of clinical findings such as abdominal pain, acute pancreatitis, eruptive xanthoma and lipemia retinalis that occurred in association with very high TG levels (82). Two groups of conditions can lead to severe HTG and clinical manifestations of the chylomicronemia syndrome; (1) familial chylomicronemia syndrome (FCS) due to variants in the LPL complex, and (2) multifactorial chylomicronemia syndrome (MFCS), in which genetic predisposition and secondary forms of HTG co-exist.

 

FAMILIAL CHYLOMICRONEMIA SYNDROME (FCS)

 

FCS is a monogenic disorder due to variants in one or more genes of the LPL complex that affect chylomicron catabolism. FCS incidence is very rare, with an estimated prevalence ranging from 1 in 20,000 to 300,000 (83).    

 

Genetics: Biallelic loss of function variants in five canonical genes lead to impaired hydrolysis of TG-rich lipoproteins, with subsequent increases in chylomicron particle numbers and markedly increased TG concentrations. The most common gene affected in FCS is LPL itself, in which patients are homozygous or compound heterozygous for two defective LPL alleles. Over 180 variants that result in LPL deficiency have been described with some clustered mutations due to founder effects (84-87). Loss of function variants account for over 90% of cases (83). Many are missense variants, some in catalytically important sites and some in regions that predispose to instability of the homodimeric structure of LPL required for enzyme activity (88). However, many common LPL gene variants have been described that have no clinical phenotype (89). Variants in the APOC2 gene, encoding ApoC-II, an activator of LPL, is another cause of FCS.  Variants have been described in several families (90, 91). In FCS thus far there is no known gene variant that affects synthesis or production of TG-rich lipoproteins.

 

FCS can also occur from biallelic loss of function variants in other components of the LPL complex, namely APOA5, LMF1, and GPIHBP1 genes (Table 2), each of which plays an important role in determining LPL function (92). The lipoprotein phenotype in these mimics that seen in classical LPL deficiency. Loss of function variants in GPIHBP1, which directs transendothelial LPL transport and helps anchor chylomicrons to the endothelial surface near LPL, thereby providing a platform for lipolysis, has been described in several families (83).  Autoantibodies to GPIHBP1 also can lead to chylomicronemia (93). A small number of individuals with homozygous variants in Apo A-V, which stabilizes  the lipoprotein–enzyme complex thereby enhancing lipolysis (10), have been described (94). Variants in LMF1, an endoplasmic reticulum chaperone protein required for post-translational activation of LPL, have also been identified in a few individuals (95).

 

Clinical presentation: FCS usually manifests in childhood or early adolescence with nausea, vomiting, failure to thrive and recurrent abdominal pain in infancy and childhood. Occasionally it can present in adulthood (87) but this is often due to delayed diagnosis with median age at diagnosis being due to unfamiliarity in most healthcare providers (96). Adults may report “brain fog” or transient confusion.

 

Classical clinical findings include eruptive xanthomas (often seen on buttocks, back, extensor surfaces of upper limbs), lipemia retinalis, and hepatosplenomegaly. Less common symptoms of FCS can include intestinal bleeding, anemia, and neurological features such as irritability and seizures. Patients present with TG levels ≥1000 mg/dL and often much higher, due to abnormal accumulation of chylomicrons, which can be detected by the appearance of lipemic/milky plasma.  Despite prolonged overnight fasting, plasma TG levels are >1000mg/dL due to the presence of chylomicrons in the circulation as a result of impaired clearance. The most serious concern, however, is the development of acute pancreatitis, which can lead to systemic inflammatory response syndrome, multi-organ failure, and death.

 

Table 2. Genetic Disorders Resulting in Familial Chylomicronemia Syndromes (FCS)

Disorder

Inheritance

Incidence

Lipid Phenotype

Underlying Defect

Clinical Features

LPL deficiency

Autosomal Recessive

1 in 1,000,000

Marked HTG/ chylomicronemia in infancy or childhood

Very low or absent LPL activity; circulating inhibitor of LPL

Hepato-splenomegaly; severe chylomicronemia

Apo C-II deficiency

Autosomal Recessive

Rare

Marked HTG/ chylomicronemia in infancy or childhood

Absent Apo C-II

Hepato-splenomegaly; severe chylomicronemia

Apo A-V mutation

Autosomal Recessive

Rare

Marked HTG/ chylomicronemia in adulthood

Defective or absent Apo A-V

Chylomicronemia

GPIHBP1 mutation

Autosomal Recessive

Rare

Marked HTG/ chylomicronemia in adulthood

Defective or absent GPIHBP1

Chylomicronemia

LFM1 mutation

Autosomal Recessive

Rare

Marked HTG/ chylomicronemia in adulthood

Defective or absent LFM1

Chylomicronemia

 Adapted from Ref (83)

 

MULTIFACTORIAL CHYLOMICRONEMIA SYNDROME (MFCS)

 

In contrast to FCS, MFCS is more common and complex. The prevalence of MFCS is much higher than FCS and estimated to be ~1:600-1000 (84).   

 

Genetics: MFCS has a genetic basis, but unlike FCS (where recessive or biallelic variants in the affected genes are causative), the genetic alteration does not always result in the phenotypic expression of the trait but only increases the possibility of the risk of developing the condition. Other factors, including non-genomic effects (epigenetics, methylation), gene-gene, or gene-environment interactions can also contribute. MFCS develops due to two main types of genetic factors that increase the odds that a patient will develop very high TG levels. First, heterozygous rare large-effect variants in one of the five canonical TG metabolism genes (LPL, Apo C-II, Apo A5, LMF-1 and GPIHBP-1) can contribute to TG elevations. These variants have variable penetrance, i.e.- clinical presentation can vary from normal to severe hypertriglyceridemia.

 

Secondly, the presence of a high burden of common small-effect TG-raising SNPs; cumulatively, these common SNP alleles increase susceptibility for developing hypertriglyceridemia. These SNPs may have an indirect impact of the metabolism of TG-rich

Lipoproteins. There is incomplete understanding of how an excess burden of SNPs contributes to TG levels, but their prevalence in patients with severe hypertriglyceridemia has been consistently demonstrated.

 

Several polygenic risk scores (PRS) for TG levels have been published (97). A recent study found that 32.0% of patients had a high polygenic score of TG-raising alleles across 16 loci compared to only 9.5% of normolipidemic controls (25). When the PRS is high, there is a significantly increased risk of developing HTG but this is not diagnostic or definitive.

 

Secondary Causes Contributing to Severe Hypertriglyceridemia in MFCS: The most common secondary cause in the past was undiagnosed or untreated diabetes (74), although earlier detection of diabetes may be making the association of marked hyperglycemia of untreated diabetes with very severe HTG less common. In addition, individuals with the metabolic syndrome and obesity have mild to moderate HTG which can become severe HTG; weight regain following successful weight loss can lead to marked HTG (23, 84). These patients almost always have relatives with genetic forms of HTG, whose TG levels are considerably lower than the index patient with severe HTG, in whom secondary forms of HTG also are present (74).  MFCS can result from the addition of specific drugs in patients with a genetic predisposition (23). These drugs include beta-adrenergic blocking agents (selective and non-selective) and/or diuretics (thiazides and loop-diuretics such as furosemide) used for hypertension, retinoid therapy for acne, oral estrogen therapy for menopause or birth control, selective estrogen receptor modulators (particularly raloxifene) for osteoporosis or breast cancer, protease inhibitors for HIV/AIDS, atypical anti-psychotic drugs, alcohol, and possibly sertraline (84).  Rarer causes of very severe HTG include autoimmune disease (sometimes with LPL- or GPIHBP1- specific antibodies), asparaginase therapy for acute lymphoblastic leukemia (98), (99) and bexarotene, a RXR agonist used in the treatment of cutaneous T cell lymphoma (100).  

 

Table 3. Secondary Causes That Can Contribute to Severe HTG

Conditions

Hypothyroidism

Suboptimally managed or new onset diabetes

Obesity

Sudden weight gain, weight regain after weight loss

Chronic kidney disease

Nephrotic syndrome

Pregnancy

Acute hepatitis

Sepsis

Inflammatory disorders

Cushing syndrome

Autoimmune chylomicronemia

            Systemic lupus erythematosis

            Anti-LPL antibodies

GPIHBP-1 antibodies

Rare Genetic Causes

Glycogen storage disorders

Lipodystrophies

            Congenital- generalized or partial

            Acquired- HIV, autoimmune

Drugs

Alcohol ingestion

Beta blockers

Thiazide diuretics

Oral estrogens

Selective estrogen reuptake modulators - tamoxifen, raloxifene, clomiphene

Androgens

Glucocorticoids

Atypical anti-psychotics

Sertraline

Bile acid resins

Sirolimus, tacrolimus

Cyclosporine

RXR agonists -bexarotene, isotretinoin, acetretin

HIV Protease inhibitors

L- asparaginase

Alpha-interferon

Propofol

Lipid emulsions

 

Following correction of treatable secondary forms of HTG in the MFCS, TG levels usually decrease to the moderately elevated levels seen in their affected relatives (101, 102).  

 

OTHER CONDITIONS RESULTING IN HYPERTRIGLYCERIDEMIA

 

Familial Dysbetalipoproteinemia (FDB or Remnant Removal Disease)

 

Familial dysbetalipoproteinemia, also referred to as remnant removal disease or type III hyperlipoproteinemia, is a rare autosomal recessive disorder that can present with elevated TG levels. This disorder is characterized by the accumulation of remnant lipoproteins.

 

PATHOGENESIS AND GENETICS

 

Remnant removal disease requires homozygosity for the ApoE2 genotype or a rare heterozygosity for a variant in the ApoE gene, which results in pathologic accumulation of remnant lipoproteins in the circulation due to impaired hepatic uptake of ApoE-containing lipoproteins (103). ApoE is a glycoprotein synthesized in the liver, brain and tissue macrophages and present on chylomicrons, VLDL and HDL. Apo E through interaction with the LDLR and heparan sulphate proteoglycans promotes the hepatic clearance of remnants of chylomicrons and VLDL (104); it also facilitates cholesterol efflux from macrophages to HDL (105). In humans, there are 3 common isoforms of ApoE , ApoE2, ApoE3, and ApoE4 (106).  Each differs in isoelectric point by one charge unit, ApoE4 being the most basic isoform and ApoE2 the most acidic.  ApoE3 (Cys112Arg158) is the commonest isoform.  ApoE2 (Arg158Cys) and ApoE4 (Cys112Arg) differ from ApoE3 by single amino acid substitutions at positions 158 and 112, respectively (107).  In the majority of cases (90%), remnant removal disease is associated with the E2/E2 genotype and results from impaired binding to the Apo E receptor. It is  an autosomal recessive disorder with the prevalence of ApoE2 homozygosity in Caucasian populations estimated to be about 1% (108).  Rarer ApoE variants such as ApoE3-Leiden (109) and ApoE2 (Lys1463Gln) that also can cause remnant accumulation are dominantly inherited (110) and account for 10% of cases (111, 112).  Rare APOE variants in the population, other than the APOE2 and APOE4 alleles, play an important role in the development of isolated hypercholesterolemia (113) and mixed hyperlipidemia, with and without familial dysbetalipoproteinemia (114). Thus it is becoming apparent that two different DBL phenotypes may exist- a genetic Apo E dysfunction and a multifactorial form (115). Modern prevalence of FDB is estimated at 1-2% (116).

 

In the absence of additional genetic, hormonal, or environmental factors, remnants do not accumulate to a degree sufficient to cause hyperlipidemia in ApoE2 homozygotes; in fact, lipid levels are commonly low. Remnant accumulation results when the E2/2 genotype is accompanied by a second genetic or acquired defect that causes overproduction of VLDL such as obesity or diabetes (117) (111, 118) , a decrease in remnant clearance, or a reduction in LDL receptor activity (e.g., hypothyroidism (119)). Thus, full phenotypic expression requires the presence of other environmental or genetic factors (120). In these circumstances, the reduced uptake of remnant lipoproteins by the liver results in reduced conversion of VLDL and intermediate density lipoproteins to LDL, with subsequent accumulation of remnant lipoproteins (121, 122), hence the term remnant removal disease.

 

DIAGNOSIS

 

Patients with remnant removal disease have roughly equivalent elevations in plasma cholesterol and TGs. The disease rarely manifests before adulthood, and in some individuals never manifests clinically. It is more common in men than in women, where expression seldom occurs before menopause, since estrogen has a protective effect in women who are ApoE2 homozygotes (108). Palmar xanthomas (Figure 1), orange lipid deposits in the palmar or plantar creases, are pathognomonic of remnant removal disease but are not always present (123). Tuberoeruptive xanthomas can be found at pressure sites on the elbows, knees and buttocks. The presence of remnant removal disease should be suspected when total cholesterol and TG levels range from 300 to 1000 mg/dL and are roughly equal in magnitude. Special diagnostic tests such as beta-quantification or lipoprotein electrophoresis are often required and are time consuming and not widely available. VLDL particles are cholesterol- enriched, which can be determined by isolation of VLDL by ultracentrifugation and by the demonstration of beta migrating VLDL on lipoprotein electrophoresis.  A VLDL-cholesterol/plasma TG ratio of <0.30 is usually observed (124).  A low ApoB/total cholesterol ratio of <0.33 also can be helpful in making the diagnosis (125). Simplified criteria for the diagnosis of DBL using a 3-step process has been proposed (126). The diagnosis of remnant removal disease should be confirmed by demonstrating the presence of the E2/E2 genotype. If the genotype result is not E2/E2, an autosomal dominant variant of APOE should be suspected. There is a high prevalence of premature coronary artery disease (127-129) and peripheral arterial disease (130-132).  Occasionally severe HTG and an increased risk of pancreatitis can develop in the presence of a concomitant secondary form of HTG or TG-raising drugs.

Figure 1. Palmar Xanthomas: Orange-yellow discoloration confined to the palmar creases.

 

Familial Partial Lipodystrophy (FPLD) Syndromes

 

A distinct entity that results in moderate and severe hypertriglyceridemia include partial lipodystrophy syndromes. Inherited lipodystrophies are a heterogeneous group of disorders considered to be rare, that manifest as complete or partial loss of white adipose tissue with accompanying severe metabolic dysregulation(133) and are reviewed elsewhere in the Endotext chapter on Lipodystrophies (134).  Loss of fat can be either localized to small discrete areas, in some cases partial with loss from extremities, or generalized with fat loss from nearly the entire body. Inherited lipodystrophies, while rare, can be autosomal dominant or recessive.  Some forms manifest at birth, while others become evident later in life.

 

Partial or generalized lipodystrophic disorders frequently are associated with significant metabolic derangements associated with severe insulin resistance, including HTG. The extent of fat loss sometimes determines the severity of metabolic complications (135).  HTG is a common accompaniment of many lipodystrophies, often in conjunction with low HDL-C levels.    The pathophysiology of hypertriglyceridemia in these subjects is possibly related to the reduced ability to deposit free fatty acids in adipose tissue due to its maldevelopment, and accelerated lipolysis with increased hepatic VLDL synthesis and delayed clearance (135).

 

Genetics: Several genes have been implicated in the manifestation of various forms including LMNA, PPARG, LIPE, CIDEC (136). In the Dunnigan variety, the most commonly identified genetic variant of FPLD, the commonest variants are in the LMNA gene and less frequently PPARG (133).  No specific genetic defect has been identified in Köbberling’s FPLD, although recent evidence suggests a heavy polygenic burden in these individuals (137, 138).

 

Diagnosis: Congenital generalized lipodystrophy (CGL) is a rare autosomal recessive disorder in which near total absence of subcutaneous adipose tissue is evident from birth.  HTG and hepatic steatosis are evident at a young age and are often difficult to control. Severe HTG, often associated with eruptive xanthoma and recurrent pancreatitis, can occur in patients with CGL. The prevalence of HTG in case series of CGL patients is over 70% (135, 139).  Plasma TGs are normal or slightly increased during early childhood, with severe HTG manifesting at puberty along with onset of diabetes mellitus. 

 

Familial partial lipodystrophies (FPLD) are complex metabolic disorders that are often not recognized clinically (140).  Partial lipodystrophies are characterized by partial loss of adipose tissue and significant metabolic derangements.  The Dunnigan variety of FPLD (FPLD type 2) is a rare autosomal dominant disorder in which fat loss mostly involves the extremities and the trunk.  Onset of fat loss in the buttocks and extremities occurs at puberty or late adolescence, with gain of fat to the face and neck. Acanthosis nigricans, calf muscle hypertrophy, and phlebomegaly (prominent veins) due to lack of subcutaneous fat, can be observed. Significant metabolic dysfunction including diabetes, which is often very insulin resistant, resistant hypertension, and HTG often severe and difficult to treat, can occur.  Myopathy, cardiomyopathy, and/or conduction system abnormalities can occur (141).  ASCVD risk also is increased (142, 143).

 

Some lipodystrophies, where fat loss appears to be proportionate to loss of total and lean body mass, do not result in dyslipidemia. Elevated TG levels have been reported in patients with atypical progeroid syndrome due to LMNA mutations (144, 145).  Of the acquired lipodystrophies, the HIV-associated form usually is characterized by more moderate HTG.  HIV-associated lipodystrophy occurs in patients receiving protease inhibitor containing highly active anti-retroviral therapy regimens (146).  Fat loss occurs in the face, buttocks, and extremities.

 

CONSEQUENCES OF HYPERTRIGLYCERIDEMIA

 

Atherosclerotic Cardiovascular Disease

 

EPIDEMIOLOGY

 

HTG has long been known to be a risk factor for ASCVD (33, 147-150), which has been confirmed in meta-analyses (45).  However, HTG also is frequently associated with low levels of HDL-cholesterol and an accumulation of remnants of the TG-rich lipoproteins, both known risk factors for ASCVD.  When adjusted for both HDL-C and non-HDL-C, which contains both remnants of the TG-rich lipoproteins and LDL, the association of TGs with ASCVD risk remained significant, although somewhat attenuated (151).  Postprandial TGs are elevated throughout the day in subjects with HTG, and postprandial TG-rich lipoproteins and their remnants also have been hypothesized to be important in the pathogenesis of atherosclerosis (150). It is therefore of interest that non-fasting TGs have been associated with ASCVD risk (150, 152, 153), despite non-fasting TGs being quite variable. However, unlike the situation with elevated LDL-C levels, the magnitude of the TG elevation does not appear to correlate with the extent of ASCVD risk. In particular, very severe HTG per se does not always appear to confer increased ASCVD risk, possibly because the chylomicrons that accumulate are too large to enter the arterial intima (154, 155). 

 

TRIGLYCERIDES IN THE PATHOGENESIS OF ASCVD

 

Although chylomicrons may be too large to enter the arterial intima, ApoE-and cholesterol-enriched remnants of the TG-rich lipoproteins can enter with ease (153) where they can bind to vascular proteoglycans, similar to LDL (156, 157).  Modification of these retained lipoproteins by either oxidative damage or enzyme digestion of some of the lipid components can liberate toxic by-products, which have been hypothesized to play a role in atherogenesis by facilitating local injury, generation of adhesion molecule, and cytokine expression and inflammation (157).  Remnants of the TG-rich lipoproteins also can be taken up by macrophages leading to the formation of foam cells, an important component of atherosclerotic plaques.  HTG also is associated with a preponderance of small, dense LDL, particles, reduced levels of HDL-C, and in the metabolic syndrome, with abnormalities of HDL composition (see earlier). Small, dense LDL can traverse the endothelial barrier more easily than large, buoyant LDL particles (158), are retained more avidly than large, buoyant LDL (159), and also are more readily oxidized (160, 161), all of which may facilitate atherogenesis. HDL particles in some hypertriglyceridemic states, e.g., in association with the metabolic syndrome, might be dysfunctional with respect to their cholesterol efflux, anti-inflammatory, and anti-oxidant properties. Moreover, a hypercoagulable state has been reported in association with both HTG and the metabolic syndrome (162). Thus, HTG might accelerate atherosclerosis by several mechanisms, all of which could increase CVD risk.

 

GENETIC EVIDENCE OF HYPERTRIGLYCERIDEMIA AND ATHEROSCLEROSIS

 

Recent human genetic studies have provided important insight into the contribution of TGs to ASCVD. Several genetic approaches, including candidate gene sequencing, GWAS of common DNA sequence variants, and genetic analysis of TG phenotypes have unraveled new proteins and gene variants involved in plasma TG regulation (163). Some genetic variants that influence TG levels appear to be associated with increased CVD risk even after adjusting for their effects on other lipid traits (164).  GWAS have identified common noncoding variants of the LPL gene locus associated with TG and CVD risk (165, 166).  A common gain-of-function mutation in the LPL gene, S447X (10% allele frequency), is associated with reduced TG levels and reduced risk of CVD (167) and an LPL variant associated with reduced TG and ApoB levels was associated with reduced CVD similar to LDL-C lowering variants, suggesting that the clinical benefit of lowering triglyceride and LDL-C levels may be proportional to the absolute change in ApoB (168).  Conversely, several loss-of-function LPL variants linked with elevated TG levels are associated with increased CVD risk (169). Variants in the TRIB1 locus have been associated with LDL, HDL-C and TG levels (166), hepatic steatosis (170) and coronary artery disease (171). Mutations that disrupt APOC3 gene function and reduce plasma ApoC-III concentration are associated with lower TG levels and decreased risk of clinical CVD (172, 173).  In contrast, carriers of rare mutations in APOA5, encoding ApoA-V, an activator of LPL, are associated with elevated TGs and with increased risk of myocardial infarction (174, 175).  Loss of function variants in ANGPTL4 that had lower TG levels also were associated with reduced CVD risk (176, 177).  Thus, exciting new human genetics findings have causally implicated TG and TG-rich lipoproteins in the development of CVD risk. In particular, the LPL pathway and its reciprocal regulators ApoC-III and ApoA-V appear to have an important influence on atherosclerotic CVD risk. However, despite this mountain of evidence demonstrating a causal relationship of TG with atherosclerosis, the possible involvement of a correlated trait, usually low HDL-C levels, or other unmeasured traits, cannot be ruled out.

 

CLINICAL TRIAL EVIDENCE OF TRIGLYCERIDE LOWERING AND ASCVD

 

In the pre-statin era, use of gemfibrozil monotherapy demonstrated cardiovascular benefit in men with coronary heart disease. However, since the advent of statins, drugs that specifically lower plasma triglyceride levels have not clearly been shown to have a benefit with cardiovascular risk reduction in clinical trials when added to background statin therapy. Reasons for this are unclear. In genetic studies to get a comparable reduction in Apo B and coronary heart disease (CHD) risk in clinical trials, a TG reduction of ~70mg/dL is required compared to a decrease in LDL-C of only 14mg/dL. Additionally, lipid alterations due to genetics are lifelong and result in much bigger reductions in CHD than a 5-year drug study. Based on genetic studies, the magnitude of TG reduction required to demonstrate cardiovascular benefit is quite large.

 

CARDIOVASCULAR DISEASE IN THE CHYLOMICRONEMIA SYNDROME

 

As described earlier, chylomicrons have been considered to be too large to penetrate the vascular endothelium and play a role in atherogenesis (152), although  remnants of the TG-rich lipoproteins may be atherogenic (152, 178-181). The incidence of CVD is low in individuals with FCS (182), although premature atherosclerosis has been documented in well characterized subjects with this disorder (183).  However, CVD risk clearly is increased in many patients with MFCS, although the exact frequency remains unclear. The frequency of CVD outcomes does not appear to relate to the magnitude of the TG elevations (184). It is not surprising that CVD is increased in MFCS considering the association between TGs and CVD that has been documented in many studies (reviewed in (150, 185, 186)).  Many subjects develop severe HTG due to the co-existence of polygenic mutations that result in mild to moderate HTG (22) with secondary causes of HTG.  Residual HTG due to these genetic disorders persists even after severely elevated TG levels have been reduced by treatment of the secondary forms of HTG and treatment of the HTG per se.  Moreover, many patients with the MFCS have other CVD risk factors such as diabetes, reduced levels of HDL-C, and hypertension, the latter resulting in use of diuretics and beta-blockers, which play a role in raising their TGs to levels at which chylomicrons accumulate due to saturation of clearance mechanisms. Therefore, strategies to prevent CVD need to be undertaken once the TGs have been lowered to a level where pancreatitis is unlikely to recur. 

 

Pancreatitis

 

Severe hypertriglyceridemia is the third most common cause of acute pancreatitis after alcohol and gallstones. The chylomicronemia syndrome describes a constellation of findings that occur with severe elevations of plasma TG levels. There is some lack of consensus as to what constitutes severe HTG, values >1000-1500 mg/dL are generally classified as severe, although some groups consider values in the 500-1000 mg/dL range also severe hypertriglyceridemia (187). 

 

Individuals with both FCS and MFCS often present with hypertriglyceridemia induced acute pancreatitis, which can be recurrent if triglyceride levels remain elevated persistently. Women with genetic HTG can develop severe HTG and pancreatitis during pregnancy particularly during the third trimester (188).

 

The pancreatitis that occurs with severe HTG can be recurrent.  In a prospective study of patients admitted with acute pancreatitis, the distribution of plasma TGs was bimodal when measured at the peak of the pain (101, 102).  TG levels <880 mg/dL were associated with gall bladder disease and chronic alcoholism, while those above 2000 mg/dL were associated with the simultaneous presence of familial and secondary forms of HTG.  It has been suggested that individuals become prone to the development of TG-induced pancreatitis at TG values between 1500-2000 mg/dL (189).  TG-induced pancreatitis has been reported with TG levels lower than 500 mg/dL(190, 191),  although in our experience this usually occurs when patients with severe HTG stopped eating some time prior to the blood draw. The frequency of severe HTG leading to acute pancreatitis varies widely from about 6-20% of subjects, possibly related to the type of patient presenting to different type of medical centers (192, 193).  Pancreatitis often is recurrent if HTG is not appreciated to be the cause and if TG levels are not adequately controlled (87). With long term multiple episodes of acute, recurrent pancreatitis, exocrine pancreatic insufficiency or insulin deficient secondary diabetes may occur. A meta-analysis of observational studies suggests that TG-induced pancreatitis has worse outcomes than pancreatitis from other causes, with an approximate doubling of renal and respiratory failure, a nearly 4-fold increase of shock and a near doubling of mortality (194). Pancreatitis due to very severe HTG also may occur during infusion of lipid emulsions for parenteral feeding (195) or with use of the anesthetic agent propofol, which is infused in a 10% fat emulsion (196). 

 

MECHANISM OF SEVERE HYPERTRIGLYCERIDEMIC PANCREATITIS

 

The mechanism by which very severe HTG leads to pancreatitis remains speculative. Suggested mechanisms include the local liberation of FFA from TGs and lysophosphatidylcholine from phosphatidycholine when pancreatic lipase encounters very high levels of TG-rich lipoproteins in the pancreatic capillaries (197). High local concentrations of FFA overwhelm the binding capacity of albumin with resultant aggregation into micellar structures with detergent properties.  Both FFA and lysophosphatidylcholine have been shown to cause chemical pancreatitis when infused into pancreatic arteries in animal models (198-200). This leads to local liberation of more lipases from the damaged pancreatic acini, resulting in a vicious cycle (198, 201).  It also has been hypothesized that increased plasma viscosity due to the presence of increased numbers of chylomicrons in the pancreatic microcirculation contributes to the development of pancreatitis (202). There also is recent evidence of gene associations in TG-induced pancreatitis; in a Chinese cohort with HTG, a CFTR variant and TNF alpha promoter polymorphism were found to be independent risk factors for developing pancreatitis (203), while another study found an increased frequency of ApoE4 (204).

 

DIAGNOSIS OF SEVERE HYPERTRIGLYCERIDEMIC INDUCED PANCREATITIS

 

The diagnosis of HTG-associated pancreatitis can be made by the presence of severely elevated TG levels in a patient with acute pancreatitis. Falsely low serum amylase levels can be encountered due to assay interference by the TG-rich lipoproteins (205). Pseudohyponatremia due to the presence of large numbers of TG-rich lipoproteins in plasma can be seen with very high TG levels. Interference with liver transaminase assays may also occur, giving spuriously high values making it difficult to exclude alcoholic liver disease (205).

 

With chronic chylomicronemia, patients may develop eruptive xanthomata (Figure 2). These xanthomas represent an inflammatory response to the deposition of chylomicron-associated lipids in tissues and are yellow-red papules that usually appear on the buttocks, back and extensor surfaces of the upper limbs. Histologically, these lesions contain lipid laden foamy macrophages (206).  

 

Figure 2. Eruptive Xanthomas. The commonest site is on the buttocks. The lesions are papular with an erythematous base. They often are itchy.

 

Lipemia retinalis, where the retinal vessels take on a whitish hue with pallor of the optic fundus and retina can be observed with very high TG levels (Figure 3).  There is no associated visual impairment.  

Figure 3. Lipemia retinalis. Note the pale color of the retinal vessels.

 

Acute recent memory loss and mental fogginess (82) can also occasionally be seen, but has not been extensively studied. Symptoms such as fatigue, blurred vision, dysesthesias, and transient ischemic attacks have been suggested to be related to hyperviscosity resulting from high TG levels (207, 208).  Hepatosplenomegaly is frequently present in FCS due to macrophage infiltration in response to the chylomicron accumulation. Fatty liver is a common finding on imaging in both FCS and MFCS.

 

MANAGEMENT OF SEVERE HYPERTRIGLYCERIDEMIA

 

Management of HTG by lifestyle and pharmacological means is discussed in detail in the Endotext chapters on The Effect of Diet on Cardiovascular Disease and Lipid and Lipoprotein Levels and Triglyceride Lowering Drugs (209, 210).  However, in this section we will make a few points specifically relevant to this chapter. 

 

Before initiating lifelong therapy for hypertriglyceridemia, evaluation for and treatment of reversible secondary disorders that can elevate plasma triglyceride levels is crucial. This includes appropriate management of diabetes and hypothyroidism and substituting drugs that can elevate triglyceride levels with lipid-neutral agents. Management of hypertension should include calcium channel blockers, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, and alpha-adrenergic blockers rather than beta-adrenergic blocking agents and diuretics.

 

Cardiovascular Disease Prevention

 

ASCVD risk in HTG is modulated by the presence of several other factors, including other lipoprotein abnormalities, other CVD risk factors, and family history of CVD, with some families with HTG appearing to have a greater risk of CVD than others (44). The role of TG lowering by pharmacological means remains controversial, but there is consensus that the presence of HTG imparts residual risk after LDL has been adequately lowered with statins.

 

Statins: The best clinical trial data currently available for the prevention of ASCVD in patients with HTG demonstrate that statins are likely to confer the most benefit, even though their primary mode of action is not to reduce plasma TGs, nor are they very effective in so doing (211).  In patients with elevated TG levels statins will result in a significant decrease in TG levels. Based on the results of the IMPROVE-IT trial (212), the addition of ezetimibe may be of additional benefit.

 

Fibrates: Fibrates such as gemfibrozil and fenofibrate, are PPAR-α agonists, and very effective in lowering plasma TG levels (by up to 50%).  Several studies have failed to demonstrate a benefit of fibrates on ASCVD events, either alone or in combination with statins.  However, participants in these studies were not confined to individuals with HTG.  Nonetheless, post-hoc analysis showed that subgroups of subjects who had mild HTG >200mg/dL and LDL-C <34mg/dL had a significant reduction of ASCVD events (213-216).  In addition, the Action to Control Cardiovascular Risk in Diabetes (ACCORD)-LIPID trial, which was confined to subjects with diabetes, showed a similar outcome in the subgroups with HTG, although the trial was negative for all subjects (214).  Recently a novel selective peroxisome proliferator-activated receptor α modulator, pemafibrate, that possesses unique PPARα activity and selectivity (217), was evaluated in individuals with HTG and diabetes in the Pemafibrate to Reduce Cardiovascular OutcoMes by Reducing Triglycerides IN patiENts With diabetes (PROMINENT) trial. Patients with type 2 diabetes, triglyceride level 200 to 499 mg/dL and HDL-C of </=40 mg/dL were assigned to pemafibrate or placebo. Unfortunately, pemafibrate failed to demonstrate cardiovascular benefit in patients with type 2 diabetes and mild to moderate HTG despite significantly lowering triglyceride levels (218).  Thus, addition of a fibrate to a statin for cardiovascular risk reduction cannot be recommended at this time.

 

Omega-3 fish oil: Omega-3 (n-3) fatty acids are polyunsaturated fatty acids that lower TGs. The two main n-3 fatty acids are eicosapentanoic acid (EPA) and docosahexanoic acid (DHA) which can lower VLDL secretion and are agonists of PPARa. However, their role in ASCVD prevention also has been controversial as several RCTs of various dosages of n-3 mixtures failed to demonstrate CV benefit in mild to moderate HTG (219). The Reduction of Cardiovascular Events with Icosapent Ethyl–Intervention Trial (REDUCE-IT), evaluated the addition of high dose icosapent ethyl (highly purified eicosapentanoic acid or EPA) compared to placebo in high-risk patients with mild to moderate HTG on statin therapy, demonstrated a surprising 25% lower CV risk in subjects. Notably this effect was independent of baseline TG levels and TG reduction.  (220).  Subsequently the STRENGTH (Statin Residual Risk Reduction With Epanova in High Cardiovascular Risk Patients with Hypertriglyceridemia) trial of n–3 fatty acid mixtures (EPA + DHA) failed to demonstrate cardiovascular benefit and was terminated early for futility (221). Similarly, use of an EPA/DHA mixture (1.8 g/d) in patients with a recent myocardial infarction in the OMega-3 fatty acids in Elderly with Myocardial Infarction (OMEMI) trial (222) also failed to meet its primary endpoint. Tissue EPA levels may be a contributor to the positive results in the REDUCE-IT study, as there is evidence that EPA inhibits inflammation, causes membrane stabilization, and decreases plaque volume. The REDUCE-IT trial has generated controversy due to use of mineral oil in the control group which resulted in an increase in both LDL-C and C-reactive protein (223, 224). Nonetheless, several current guidelines suggest addition of icosapent ethyl in addition to a statin for residual hypertriglyceridemia in high-risk individuals (those with known ASCVD or diabetes with additional risk factors).

.

Niacin: Niacin effectively lowers triglycerides and LDL-C while raising HDL-C. Niacin inhibits lipolysis in adipocytes, therefore decreasing the available fatty acids for VLDL synthesis. However, it has fallen out of favor for ASCVD risk reduction. Two RCTs, Atherothrombosis Intervention in Metabolic Syndrome with Low HDL/High Triglycerides: Impact on Global Health Outcomes (AIM-HIGH) and Treatment of HDL to Reduce the Incidence of Vascular Events (HPS2-THRIVE) demonstrated no benefit to the addition of niacin to statins for decreasing cardiovascular risk. However, it is important to note that neither of these trials were confined to subjects with high TG levels. Therefore, due to the lack of efficacy and potential for increasing insulin resistance, niacin is not recommended for treatment of HTG.

 

Newer Therapies for HTG

 

Apo C-III Inhibitors: ApoC-III is an endogenous inhibitor of LPL, by displacing ApoC-II an activator of LPL. This leads to inhibition of lipolysis and elevations in TG levels. Apo C-III can also increase TGs by LPL independent mechanisms. Currently, ApoC-III inhibitors include volanesorsen (a second-generation antisense oligonucleotide (ASO)), olezarsen (a third generation ASO), and ARO-APOC3 (a small interfering RNA), all of which are directed at APOC3 gene (225). Volanesorsen has been studied in individuals with FCS and MFCS and has been found to decrease TG levels by up to 70% from baseline and potentially prevent pancreatitis. However, thrombocytopenia and injection site reactions are common. This drug is therefore not approved for use by the FDA due to concerns of bleeding but is approved for use by the European Medicines Agency.  

Olezarsen is a third generation ASO for which early studies have been completed.

 

ANGPTL-3 inhibitors: ANGPTL3 is a key regulator of lipoprotein metabolism and is able to repress LPL and endothelial lipase activity, with resultant increase in TGs and TG rich lipoprotein levels (226). Homozygous loss of function variants in ANGPTL3 result in combined hypolipidemia with low TG and LDL-C levels. ANGPTL3 inhibition results in decreased LDL-C, and TGs. ANGPTL3 inhibitors include evinacumab, a monoclonal antibody, vupanorsen, an antisense oligonucleotide (ASO), and ARO-ANG3, a small-interfering ribonucleic acid (siRNA). Evinacumab has been studied in patients with sHTG. A phase 2 study of evinacumab in patients with sHTG, demonstrated significant reductions in TGs only in patients with MFCS with and without LPL mutations, but not in patients with FCS, suggesting that ANGPTL3 inhibition is dependent on presence of some LPL activity (227). In the cohort with polygenic sHTG and MCS, evinacumab 15 mg/kg IV every 4 weeks resulted in an 81.7% reduction in TGs. It should be noted that evinacumab is currently only approved for homozygous familial hypercholesterolemia.

 

Management of Severe HTG-Induced Pancreatitis

 

Because of the low frequency of severe HTG in the general population, and because only some patients with severe HTG develop pancreatitis, large random controlled clinical trials are difficult to perform and unlikely to be undertaken in the foreseeable future. Therefore, therapeutic decisions need to be based on less stringent criteria than might otherwise be desirable.  However, keeping TG levels <500 mg/dL should prevent the onset of TG-induced pancreatitis (187, 228, 229). 

 

ACUTE MANAGEMENT

 

The clinical presentation of HTG-induced pancreatitis is similar to that from other causes of acute pancreatitis and can be preceded by episodic nausea, epigastric pain radiating through to the back, and increasing heart-burn. Individuals with recurrent acute pancreatitis may present without severe elevations in pancreatic enzymes (230). The immediate goal is to lower TGs in hospitalized patients.  Management is similar to the management of non-TG induced pancreatitis, which includes cessation of all oral intake for pancreatic rest, fluid resuscitation, pain management, and management of metabolic abnormalities. TGs fall rapidly with discontinuation of oral intake, often to under 1000mg/dL with cessation of oral intake. With clinical improvement, oral diet advancement should be done slowly and cautiously in the hospital as this can result in rebound TG elevations. Supportive care as needed should be instituted for organ failure. Lipid emulsions for parenteral feeding should be avoided since their use will further delay clearance and exacerbate the HTG. If long term nutrition is required for very ill individuals who cannot eat, total parental nutrition without lipids should be utilized.   

 

Heparin: Heparin will liberate LPL into plasma from its endothelial binding sites and hence rapidly lowers TGs (231). However, it also can cause rebound HTG due to rapid degradation of released LPL (232) and increases the risk of hemorrhagic pancreatitis. Therefore, the use of heparin is not recommended (233). 

 

Insulin: The rationale for the use of an IV insulin infusion of regular insulin (in conjunction with IV glucose administration as needed) is that it can activate LPL and enhance clearance of TG-rich lipoproteins (234). Intravenous insulin can be beneficial in individuals with diabetes needing glycemic control. Its use in TG--induced pancreatitis without diabetes has been reported in several case reports (235-239), and has become widespread but it is unclear whether similar changes would have occurred simply by restricting oral intake without the use of insulin.  Regular insulin at 0.1-0.2 units/kg/hour with a separate iv dextrose infusion to prevent hypoglycemia in individuals without diabetes is often used. IV insulin can be stopped when TG drops to below 1000mg/dL. However, TGs will increase when the individual consumes an oral diet; therefore, caution should be exercised with slow advancement of the diet. In a study of chylomicronemia with uncontrolled diabetes, insulin infusion lowered TGs more rapidly than plasmapheresis (240).

 

Plasmapheresis: The use of plasmapheresis to acutely lower TGs is controversial. Plasmapheresis is extracorporeal therapy where plasma is removed and replaced; plasma is separated from the blood and discarded removing chylomicrons. Substitute fluid is replaced to maintain blood volume. The procedure is highly effective in rapidly decreasing TG levels by 85% after 1 session. However, the effect is not persistent, without evidence of long-term efficacy or mortality and morbidity benefit demonstrated. Although recommended by some (241, 242), the current evidence for the benefit of use of plasmapheresis is limited to small uncontrolled anecdotal series (243) from which no firm conclusion can be made regarding its use in acute TG-induced pancreatitis (244). A recent retrospective analysis demonstrated no benefit on length of hospital stay or mortality when therapeutic plasma exchange was added to medical management of severely elevated plasma TGs (245). TG levels fall rapidly with cessation of oral intake and use of non-lipid-containing intravenous fluids.  Plasmapheresis requires a specialized center, needs central venous access, and transient anticoagulation; it only temporarily improves TG levels without addressing the underlying cause (83). Risks include line sepsis, deep vein thrombosis, and bleeding. Therefore, we do not recommend its routine use in this situation unless clinical circumstances necessitate plasmapheresis such as severe acute necrotizing pancreatitis (246), shock, or pregnancy (247). 

 

LONG-TERM MANAGEMENT TO PREVENT PANCREATITIS

 

After TG lowering in the setting of acute pancreatitis, it is essential to determine both the primary and secondary causes of the severe HTG that precipitated the acute pancreatitis.   Continued management of any secondary form of HTG, as well as lifestyle and drug therapy to maintain low TG levels is required to prevent recurrent pancreatitis. If fasting plasma TG levels remain above 1000 mg/dL after treating or removing the precipitating causes of the severe HTG, life-long therapy with fibrates or n-3 fatty acids, as described earlier, might be considered for these patients. Limited evidence suggests that orlistat, a gastrointestinal lipase inhibitor that decreases absorption of ingested fat, thereby reducing intestinal chylomicron synthesis, may be of benefit in reducing TG levels when used in conjunction with fibrate therapy (248, 249). TG and glucose control can be particularly challenging in individuals with familial partial lipodystrophy.

 

Management of Specific Syndromes that Accompany Severe HTG

 

FAMILIAL CHYLOMICRONEMIA SYNDROME (FCS)

 

Treatment of FCS includes management of an acute crisis (pancreatitis) and long-term management of HTG. Management of acute HTG-induced pancreatitis is described in the previous section. Long-term management of individuals with FCS involves patient education and maintaining a very low-fat diet. Consumption of even small amounts of fat can lead to severe HTG in FCS due to the absence of functional LPL. Infants with FCS presenting with abdominal pain or failure to thrive require discontinuation of breast feeding with replacement by very low-fat formula feeding to decrease TG levels and symptoms. In children and adults with FCS, dietary fat calories should be severely restricted to control the severe HTG and abdominal pain. This translates to about 5% to 10% of total daily calories from fat, which is a major burden for these patients (250).  Medium-chain TGs, which are taken up directly by the liver after absorption and do not enter plasma as chylomicrons via the thoracic duct, are a potential alternate fat source for these patients. n-3 fatty acids can aggravate the severe HTG of FCS and therefore are contraindicated in these individuals (251, 252).  Fibrates are not efficacious in FCS (253).  There are limited studies showing that orlistat might be beneficial in patients with FCS (254, 255).  Alcohol, oral contraceptives, and other TG-elevating drugs (see Table 3) can exacerbate severe HTG and precipitate acute pancreatitis in FCS.  Successful pregnancies in patients with FCS have become more common of late (256, 257).

 

Alipogene tiparvovec, an adeno-associated virus LPL gene therapy that was developed and resulted in significant improvement in postprandial chylomicron metabolism in patients with FCS has been abandoned and no longer available (258), Antisense oligonucleotide inhibitors of ApoC-III (volanesorsen is approved in Europe but no in the US) and of ANGPLT3 are in development for FCS (96).

 

MULTIFACTORIAL CHYLOMICRONEMIA SYNDROME (MFCS)

 

Management of acute HTG-induced pancreatitis is described in the previous section.

 

Long term management: To prevent acute HTG-induced pancreatitis in MFCS, the goal is to maintain TG levels below the threshold for pancreatitis, preferably <500 mg/dL. This requires instituting lifestyle adjustments, reversal of any secondary causes of HTG, such as treatment of suboptimally managed or undiagnosed diabetes, treating hypertension with lipid neutral agents such as ACE inhibitors, ARBs, calcium channel inhibitors, or alpha blockers rather than beta-adrenergic blockers and diuretics, and discontinuing other TG-raising drugs (table 3) where possible.  Alcohol intake should be limited or eliminated, since even small amounts of alcohol can substantially raise TG levels in individuals with baseline HTG.  Attention should be paid to avoid rebound weight gain that commonly occurs after successful weight loss. Oral estrogens should be substituted by transdermal or vaginal preparations, which raise plasma TGs to a lesser extent than oral estrogens (259, 260). Residual HTG should be treated with fibrates (229), which together with management of the secondary disorder or disorders, can reduce TG levels to below the threshold for developing pancreatitis. Other agents that can be used to lower TGs alone or in combination with fibrates, include n-3 fatty acids, and high-dose statins. We do not recommend using niacin due to risk of worsening insulin resistance and lack of clinical trial data for benefit. Lifestyle measures and weight loss are important, but patients should be educated on risks of rapid weight regain after successful weight loss can be associated with rebound severe HTG. Bariatric surgery also has been used to reduce severe HTG in refractory HTG (261). Inhibition of ApoC-III or ANGPTL3, which have been shown to lower TGs in patients with severe HTG (262), may have a role to play in the future the management of severe HTG in patients with MFCS. 

 

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