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Home , Abetalipoproteinemia, Bezafibrate, Clofibrate, Dalcetrapib, Dyslipidemia, Evacetrapib

Diabetes Volume 65, July 2016 1767

Changting Xiao,1 Satya Dash,1 Cecilia Morgantini,1 Robert A. Hegele,2 and Gary F. Lewis1

Pharmacological Targeting of the Atherogenic Complex: The Next Frontier in CVD Prevention Beyond Lowering LDL

Diabetes 2016;65:1767–1778 | DOI: 10.2337/db16-0046

Notwithstanding the effectiveness of lowering LDL cho- has been the primary goal of dyslipidemia management, lesterol, residual CVD risk remains in high-risk popula- with as the treatment of choice for CVD prevention. tions, including patients with diabetes, likely contributed

Large-scale, randomized, clinical trials of LDL-lowering DIABETES IN PERSPECTIVES to by non-LDL abnormalities. In this Perspectives therapies have demonstrated significant reduction in CVD in Diabetes article, we emphasize that changing demo- events over a wide range of baseline LDL-C levels (2,3). graphics and lifestyles over the past few decades have However, even with LDL-C levels lowered substantially or “ resulted in an epidemic of the atherogenic dyslipidemia at treatment goals with therapy, CVD risks are not ” complex, the main features of which include hypertrigly- eliminated and there remains significant “residual risk.” In- ceridemia, low HDL cholesterol levels, qualitative changes tensifying statin therapy may provide additional benefits in LDL particles, accumulation of remnant , (4,5); this approach, however, has limited potential, owing and postprandial . We brieflyreviewthe to tolerability, side effects, and finite efficacy. Further LDL-C underlying pathophysiology of this form of dyslipidemia, lowering may also be achieved with the use of nonstatin in particular its association with resistance, obe- sity, and type 2 diabetes, and the marked atherogenicity agents, such as cholesterol absorption inhibitors and PCSK9 of this condition. We explain the failure of existing classes inhibitors, added to statin therapy. In patients with acute of therapeutic agents such as fibrates, , and cho- coronary syndrome, , when added to statin ther- fi lesteryl ester transfer protein inhibitors that are known to apy, signi cantly improved CVD outcomes, in which im- modify components of the atherogenic dyslipidemia com- provement was proportional to LDL-C reduction (6). plex. Finally, we discuss targeted repurposing of existing Additional benefits of PCSK9 inhibitors on CVD outcomes therapies and review promising new therapeutic strate- are anticipated, considering their efficacy in further low- gies to modify the atherogenic dyslipidemia complex. We ering LDL-C levels when added to statin therapy (7). How- postulate that targeting the central abnormality of the ath- ever, it remains to be established to what extent LDL-C can erogenic dyslipidemia complex, the elevation of - be safely reduced and whether earlier treatment or extreme rich particles, represents a new frontier in CVD LDL-C lowering alone is sufficient for CVD risk reduction. prevention and is likely to prove the most effective strategy The intent of this Perspective is to highlight an additional in correcting most aspects of the atherogenic dyslipidemia form of highly prevalent dyslipidemia, the atherogenic complex, thereby preventing CVD events. dyslipidemia complex, which represents a promising therapeu- tic target for further prevention of atherosclerotic CVD beyond LDL-C lowering. We fully acknowledge that lifestyle modifi- Epidemiological, genetic, and animal studies and random- cation (e.g., body weight reduction in overweight or obese ized, controlled clinical trials support a central role for LDL individuals and an increase in physical activity), if successfully cholesterol (LDL-C) in the development of atherosclerotic implemented, remains the cornerstone of therapy for the (CVD) events (1). LDL-C lowering atherogenic dyslipidemia complex. Other nonlipid CVD risk

1Departments of Medicine and Physiology and the Banting & Best Diabetes Received 10 January 2016 and accepted 23 March 2016. Centre, University of Toronto, Toronto, Ontario, Canada © 2016 by the American Diabetes Association. Readers may use this article as 2 Robarts Research Institute, Schulich School of Medicine and Dentistry, Western long as the work is properly cited, the use is educational and not for profit, and University, London, Ontario, Canada the work is not altered. Corresponding author: Gary F. Lewis, [email protected]. 1768 Atherogenic Dyslipidemia Complex and CVD Diabetes Volume 65, July 2016 factor modifications, such as smoking cessation and antihy- world. The presence of high TG and low HDL-C levels, pertensive, antiplatelet, and anti-inflammatory therapies, have concurrent with high LDL-C levels, increased CVD risks in also been shown to effectively reduce the risk of CVD. A the Scandinavian Survival Study (16) and in review of nonlipid CVD risk factor modifications is beyond the the Helsinki Heart Study (17). Likewise, TG levels were scope of this Perspective, which focuses instead on pharma- strongly and independently associated with coronary cological modulation of the atherogenic dyslipidemia complex. heart disease risk in a meta-analysis of 29 prospective studies (18). Nonfasting TG level, which may reflect the THE ATHEROGENIC DYSLIPIDEMIA COMPLEX atherogenic capacity of TRL remnants, is a stronger pre- AND ITS RELATION TO ATHEROSCLEROTIC dictor of CVD events than fasting TG levels (19,20). Be- DISEASE cause most people are in the postprandial state for many The fourth quarter of the 20th century heralded the onset hours each day, changes in postprandial lipid levels may of a worldwide epidemic of , , have an important impact on development. and type 2 diabetes (T2D). These conditions are associ- Atherogenic properties have been assigned to athero- ated with a cluster of lipid and lipoprotein abnormalities, genic dyslipidemia complex components, as has been the presence of which has increased in parallel with these reviewed for TRL levels (21), remnant lipoprotein levels conditions (8). Such lipid and lipoprotein abnormalities (22), postprandial lipemia (23), nonfasting TG levels (24), have been referred to as the “atherogenic lipoprotein phe- sdLDL particles (25), and the antiatherosclerotic proper- notype” (9) or “atherogenic dyslipidemia” (10), which im- ties of HDL (26). Epidemiological or observational ap- ply a causal relationship between the phenotype and proaches are not suited to unravel causality, hence the atherosclerosis. As discussed below, atherogenicity can ongoing debate as to which individual component or com- only be attributed to the collective lipid/lipoprotein ab- ponents play a direct role in atherosclerosis. One should normality as a whole complex, but not to a specific fea- also bear in mind that the associated chronic inflamma- ture. Also, as discussed below, the features of this cluster tion, increased levels of prothrombotic factors, perturbed of lipid/lipoprotein abnormalities intertwine in their me- tissue factors, , and hyperglycemia play tabolism converging on triglyceride (TG)-rich lipoproteins important roles in promoting atherosclerosis. The inabil- (TRLs). This atherogenic dyslipidemia complex is charac- ity to disentangle individual components of the athero- terized by the presence of the following cluster of abnor- genic dyslipidemia complex and clearly implicate direct malities: (HTG; indicative of an roles of each in atherogenesis suggests that the phenotype elevation of TRLs), low HDL cholesterol (HDL-C) levels, is likely multidimensional and that continued efforts to high small dense LDL (sdLDL) levels, elevated levels of reduce causality to a single component might be mis- remnant lipoproteins, and postprandial hyperlipidemia. guided. Indeed, individual components of the atherogenic There is currently no uniformly accepted definition of dyslipidemia complex seldom exist in isolation; instead, the atherogenic dyslipidemia complex or its component they often present as a cluster because of their interrelated parts, nor would a strict definition necessarily add clarity, as metabolism and etiology (see below). In addition, most ther- was evidenced by a much criticized attempt to propose a apies aimed primarily at correcting a certain lipid moiety, as working definition of the metabolic syndrome (11). Further- well as lifestyle modification, improve multiple components more, certain characteristic abnormalities of the atherogenic of the atherogenic dyslipidemia complex simultaneously. dyslipidemia complex, such as postprandial hyperlipid- In contrast with elevated LDL-C or apoB levels, simple emia, sdLDL particles, and remnant lipoproteins, have biomarkers to assess the atherogenicity of the atherogenic not typically been measured in large-scale population stud- dyslipidemia complex are not well defined. Aside from plasma ies.Asaresult,thetrueprevalenceofthecomplexiscur- levels of TG, HDL-C, non-HDL-C, and ratios of these lipid rently unknown. Nevertheless, because several features of parameters, various methodologies besides standard assays of the atherogenic dyslipidemia complex are integral compo- plasma are available for the assessment of atherogenic nents of the metabolic syndrome (12), and metabolic syn- dyslipidemia complex components. Specialized analyses, such drome affects nearly 35% of all adults and 50% of those .60 as gradient gel electrophoresis, vertical auto profile, nuclear years of age in the U.S. (13), the prevalence of the athero- magnetic resonance, or ion mobility, may yield information genic dyslipidemia complex is expected to be alarmingly high. on cholesterol levels in lipoprotein subfractions to gain deeper The atherogenicity of the atherogenic dyslipidemia insight into atherosclerotic risk (27). ApoB-48, the apolipo- complex is supported by epidemiological studies. For protein present in and remnants, may serve as instance, high TG levels and low HDL-C levels in the a surrogate marker of atherosclerosis in patients at risk (28). NHANES (National Health and Nutrition Examination In addition, the postprandial lipid response to a high- meal Survey) III study (14) were each strongly and independently may reveal useful information with regard to TG and TRL correlated with major CVD events. In the INTERHEART metabolism. In the atherogenic dyslipidemia complex, LDL study (15), elevated (apo)B/apoA-I ratio particles are often smaller in size; therefore, the number of (reflective of apoB-containing lipoproteins and HDL, re- LDL particles may not be accurately reflected by LDL-C levels, spectively) was found to be a major predicator of myo- especially in many patients with diabetic dyslipidemia in cardialinfarctioninthegeneralpopulationaroundthe whom LDL-C levels are near normal. Non-HDL-C (29) and diabetes.diabetesjournals.org Xiao and Associates 1769 total apoB (30) levels are often increased along with the atherogenic dyslipidemia complex. Non-HDL-C includes cho- lesterol in all apoB-containing lipoproteins, including TRL (VLDL and ) and their remnants, IDL, LDL, and lipoprotein(a), thus reflecting the total atherogenic cho- lesterol burden. Consequently, non-HDL-C is strongly associ- ated with coronary heart disease risk (29,31). Because much of the predictive risk of non-HDL-C is likely to be attrib- uted to LDL-C, which is its major component, nonfast- ing, non-HDL-C may be an even stronger predictor of coronary heart disease, but this requires further analysis. Plasma total apoB level reflects the number of all apoB- containing lipoproteins. Both non-HDL-C and apoB may serve as alternate treatment targets (32). From a practi- cal standpoint, non-HDL-C and apoB are easily measur- able clinical parameters. Non-HDL-C has additional appeal because it is calculated from the two commonly measured lipid entities, as total cholesterol subtracted by HDL-C. Finally, it has been suggested that the choles- terol rather than the TG content of remnant lipopro- teins, which can be estimated from nonfasting lipids, drives the atherogenicity of these particles (33). Figure 1—Central role of abnormal metabolism of TRL in the ath- erogenic dyslipidemia complex. The cumulative effects of the In summary, there is convincing evidence that the ath- polygenic susceptibility and multiple environmental burdens det- erogenic dyslipidemia complex as a whole is highly ath- rimentally affect TRL kinetics by increasing production (from the erogenic, although it is not known which component/s of and intestine) and/or impairing clearance. Common variants the complex are directly implicated in atherogenesis. with small effect size create a background of susceptibility. Rare fi variants with large effect size in LPL and in proteins enhancing There is currently no widely accepted clinical de nition (e.g., APOC2, GPIHBP1, LMF1, APOA5) or suppressing (e.g., of the atherogenic dyslipidemia complex, and, hence, its ANGPTL3, ANGPTL4, APOC3) LPL functionality increase the sever- true prevalence is not known, although it is predicted to ity of the phenotypes by modulating TRL clearance. Certain genetic be highly prevalent in populations that also have a high variants (e.g., MTTP, APOB, DGAT, APOC3, GCKR) may contribute to altered TRL production. Nongenetic factors, including lifestyle prevalence of the metabolic syndrome. There are a number (e.g., central obesity, physical inactivity, diets of high or high of biomarkers of the atherogenic dyslipidemia complex, but refined sugars), medical conditions (e.g., insulin resistance, T2D, met- there is no consensus as to which biomarkers best predict abolic syndrome), pregnancy, and certain further atherogenicity in this condition. exacerbate the phenotypes. The consequent expanded TRL pool or the hypertriglyceridemic condition promotes the generation of sdLDL and the catabolism of HDL. Collectively, HTG, elevated lev- THE ATHEROGENIC DYSLIPIDEMIA COMPLEX: els of remnant lipoproteins (RLPs), postprandial hyperlipidemia PATHOPHYSIOLOGY AND ETIOLOGY (PPHL), increased sdLDL levels, and decreased HDL-C levels con- stitute the atherogenic dyslipidemia complex, which increases the In individuals with the atherogenic dyslipidemia com- risks of atherosclerotic CVDs (ASCVDs). Targeting the central as- plex, the lipid phenotype primarily reflects the abnormal pect of the atherogenic dyslipidemia complex, TRL metabolism, metabolism of TRL, as affected by both genetic and may offer novel strategies to reduce CVD risk beyond the lowering environmental factors, with secondary effects on HDL of LDL-C levels. and LDL metabolism (Fig. 1). Net plasma TRL concen- tration reflects the balance between the production and clearanceofthewholeparticleorvariouscomponentsof the particle. Elevated plasma TRL levels can therefore be Impaired Clearance of TRL a result of impaired clearance, increased production, or a The atherogenic dyslipidemia complex is frequently asso- combination of both (34). Increased TRL concentration ciated with reduced clearance capacity, impairing catabo- in the circulation enhances lipid and apolipoprotein ex- lism of VLDL, chylomicrons, and their remnants, including change with HDL, resulting in TG enrichment and sub- TG hydrolysis and particle removal (apoB catabolism) sequent accelerated catabolism of HDL particles (35). (reviewed in Lewis et al. [34] and Watts and Chan [38]). The formation of sdLDL particles is also closely related Such clearance defects may arise due to several abnormal- to increased TRL levels. In HTG, there is increased se- ities, including deficiency of (LPL) or its cretion of TG-enriched, larger VLDL particles, which are key cofactors, such as apoC2; defects in cell surface recep- precursors of sdLDL, and increased delipidation of larger tors; and changes in lipoprotein composition (particularly, buoyant LDL to form sdLDL; furthermore, the liver di- the apolipoprotein composition of TRL). Indeed, genetic rectly secretes more LDL particles of smaller size in HTG variants in association with TG elevation that have been (25,36,37). identified in genome-wide association studies (GWASs) 1770 Atherogenic Dyslipidemia Complex and CVD Diabetes Volume 65, July 2016 mostly reflect defects in TG clearance, at least among iden- numerous loss-of-function mutations in several genes that tified genes encoding proteins with known functions (see encode proteins that affect the functionality of LPL have below). been identified. Mutations in APOC2, GPIHBP1, LMF1,and APOA5 TRL Overproduction are associated with elevated TG, whereas mutations in ANGPTL3, ANGPTL4,andAPOC3 are associated with reduced TRL overproduction is a prominent feature that coexists TG levels (53). Furthermore, noncoding RNAs (e.g., micro- with clearance defects and exacerbates the severity of the RNAs) may regulate the expression of proteins involved in atherogenic dyslipidemia complex. Insulin resistance is a lipoprotein-cholesterol and TG metabolism (56), which adds major cause and is frequently associated with TRL over- production by the liver. Under such metabolic conditions, to the expanded genetic contribution to dyslipidemia. Whereas such genetic evidence points to defects in- adipose lipolysis is increased along with less effective volving TRL clearance mostly related to LPL, it is likely that trapping of fatty acids released into the capillary micro- TRL overproduction is also genetically determined; many of circulation by intravascular lipolysis of circulating TRLs the novel GWAS loci associated with TG levels that encode (39). The resultant increase in free fatty acid flux to the products with unknown function may prove to have a role liver provides excessive substrate for VLDL TG synthesis in the synthesis and secretion of TRL (51). Mutations in and drives increased VLDL secretion. In addition, insulin GPD1, which encodes glycerol-3-phosphate dehydrogenase resistance leads to increased hepatic de novo lipogenesis and decreased fatty acid oxidation, further stimulating 1, are associated with transient infantile HTG, likely through increased TG secretion (57). Besides modulating VLDL TG secretion (39–41). Diminished post-translational the clearance of TRL by LPL, apoC3 may be involved in apoB-100 degradation in hepatocytes with increased bio- TRL production, as suggested by the findings that apoC3 genesis, stability, and secretion of nascent VLDL particles antisense reduced TG levels and TRL particle numbers in has been demonstrated in insulin resistance, resulting in patients with null LPL mutation (58). Variants in GCKR increased production of VLDL by the liver (42). Although encoding glucokinase regulatory protein are associated there are some differences in the regulation of VLDL and with HTG, possibly through affecting TG biosynthesis chylomicron particle production, similar mechanisms are also likely to be involved in enhancing chylomicron pro- (59). In addition, some insulin resistance risk alleles are also associated with high TG and low HDL-C levels and duction in the intestine (43,44). Thus, the production of increased risk of coronary heart disease (60). It is well intestinally derived TRL apoB-48 is also increased in insulin established that insulin resistance underlies many features resistance (45), obesity (46), and T2D (47,48), and chylo- of metabolic defects in metabolic syndrome and T2D micron TG secretion is increased in metabolic syndrome (49). and that TRL production is known to be increased in in- Genetic and Environmental Factors in TG Elevation sulin resistance. Both genetic and environmental contributions are impli- At the opposite end of the TG spectrum is hypolipidemia. cated in the etiology of HTG. Although rare monogenic Loss-of-function mutations in APOB impair apoB traffick- forms of severe HTG manifesting as chylomicronemia exist ing and secretion or enhance its catabolism, causing hypo- (50), the majority of cases of HTG are typically mild to betalipoproteinemia with low plasma TG, apoB, and LDL-C moderate and have a polygenic basis. Common genetic levels (61) and reduced risk of atherosclerotic CVD (62). variants with small effect sizes, single nucleotide polymor- Loss-of-function mutations in ANGPTL3 also cause hypo- phisms, such as those identified by large GWASs in .45 lipoproteinemia with low–apoB-containing lipoproteins loci associated with variation in plasma TG concentrations and HDL-C (63), and loss-of-function mutations in other (51), create a genetic background of susceptibility. Many of angiopoietin-like protein (ANGPTL) family members also these loci have joint associations with other lipid pheno- are associated with low plasma TG levels (64,65). The prev- types. In particular, the TG-raising allele at a GWAS locus is alence of atherosclerotic CVD with ANGPTL mutations has more often than not associated with lower HDL-C levels not been clearly established. Loss-of-function mutations in (51). Indeed, huge sample sizes and complicated statistical MTTP cause , a condition too rare to genetic models cannot fully dissociate such interrelation- assess atherosclerotic CVD prevalence. ships (52). These genetic correlations confirm the connec- Finally, many environmental (nongenetic) factors can tivity of the biological pathways governing both TG and exacerbate HTG and, in many cases, cause clinical HTG in HDL levels, and help to explain the overall difficulty in otherwise dormant phenotypes in individuals with pre- dissecting out the effects of one lipid variable over the disposing genetic backgrounds (53). These include lifestyle other, as discussed above. and acquired factors (e.g., adiposity, especially visceral obe- On top of these common predisposing variants, rare sity; physical inactivity; increased intake of saturated fats; heterozygous variants with large effect sizes may further and high carbohydrate levels, especially refined sugar or exacerbate the phenotype and increase the severity of the alcohol intake), medical conditions (e.g., insulin resistance, clinical presentation (53). A few rare and many common metabolic syndrome, untreated or poorly controlled diabe- genetic variants in the LPL gene have been identified and tes), pregnancy, and certain medications (54). HTG, there- linked to the lipid abnormalities that characterize the ath- fore, may be better depicted as a continuum of interactions erogenic dyslipidemia complex (54,55). Among rare variants, between genetic and environmental contributions, where diabetes.diabetesjournals.org Xiao and Associates 1771 the cumulative effects of the polygenic susceptibility and in the whole study population were very modest. How- multiple environmental burdens may each increase the ever, in a meta-regression analysis of the fibrate trials, risks for the development of HTG (34). in subjects with baseline values of TGs .2 mmol/L, major CVD events were inversely associated with the magnitude of WHY CURRENTLY AVAILABLE THERAPIES TG lowering (75). A recently published Cochrane Database TARGETING COMPONENTS OF THE systematic review (76) of 13 fibrate randomized con- ATHEROGENIC DYSLIPIDEMIA COMPLEX HAVE trolled trials showed evidence for a significant 12% pro- FAILED OR HAVE BEEN RELATIVELY INEFFECTIVE tective effect of fibrates compared with placebo for CVD IN PREVENTING CVD but no reduction in all-cause mortality or death from vascular disease. In our opinion, these studies prove the Fibrates are peroxisome proliferator–activated receptor futility of the use of fibrates in the prevention of CVD in a (PPARa) agonists that increase fatty acid oxidation in the those with normotriglyceridemia, but show small benefit liver and muscle and decrease hepatic lipogenesis. In ad- in the population with the atherogenic dyslipidemia com- dition, fibrates increase LPL activity and decrease the he- plex,evenwithbackgroundstatinuse.Theefficacy of patic secretion and VLDL concentration of apoC3 (66). fibrates, alone or in combination with statins, on CVD Kinetic studies (67) have indicated that fibrates lower outcomes needs to be further evaluated in trials with VLDL apoB-100 levels through enhanced clearance appropriate patient selection. Currently, fibrates are being and decreased production. Fibrates increase apoA-I and used as adjunctive therapy to statins in high-risk patients apoA-II transcription, enhance reverse cholesterol trans- with baseline HTG. More potent and selective PPARa port, and indirectly, through altered VLDL metabolism, agonists and PPARa/g dual-receptor agonists are being de- increase HDL levels (66). Fibrates may also increase veloped (77). Such compounds may provide more effective LDL particle removal, although this is a relatively minor TG lowering and may have additional insulin-sensitizing effect. Collectively, fibrates are very effective TG-lowering properties, although major questions remain regarding therapies with modest HDL-raising effects. Fibrates are their overall safety. less effective at lowering LDL-C or apoB concentrations but may shift LDL particles from small to large size (68). Niacin Despite these potentially beneficial effects in modify- Earlier studies, dating back to the 1970s, showed that ing the typical dyslipidemia that characterizes the ath- niacin (nicotinic acid) reduced CVD (78). High-dose niacin erogenic dyslipidemia complex, fibrate clinical trials have increases HDL-C levels and lowers the levels of LDL-C, been generally disappointing with respect to CVD pre- lipoprotein(a), TRL, and sdLDL particles (78). Immediate- vention. Early studies with clofibrate (the Coronary releaseniacinisassociatedwiththesideeffectofflush- Project and the WHO Clofibrate trial) conducted in the ing, and its use for lipid control in T2D patients is 1970s yielded conflicting results on CVD outcomes, pos- discouraged because it may worsen insulin resistance sibly due to aspects of the population studied and in- and glycemic control. Slow-release forms of niacin reduce clusion criteria. More recent trials, including FIELD flushing but have increased risks of hepatotoxicity and (Fenofibrate Intervention and Event Lowering in Dia- reduced efficacy on HDL-C. Extended-release niacin also betes) (fenofibrate in T2D patients) (69,70), BIP (Bezafi- improves lipid profiles, without causing flushing or rais- brate Infarction Prevention) (bezafibrate) (71), HHS ing fasting glucose levels. The amide of niacin, nicotin- (Helsinki Heart Study) (gemfibrozil) (17), ACCORD-Lipid amide, despite providing biochemical functions of niacin, (fenofibrate in statin-treated T2D patients) (72), and VA- does not provide benefits on lipids. The putative antia- HIT (Veterans Affairs HDL Intervention Trial) (gemfibro- therosclerotic effects of niacin were frequently attributed zil) (73) trials, have investigated the efficacy of several to its HDL-raising effect, despite the fact that it had mul- fibrates on CVD outcomes. Other than in the VA-HIT tiple effects on plasma lipids and possible nonlipid anti- and HHS trials, the primary composite CVD end point atherosclerotic effects. Kinetic studies have indicated that was not significantly reduced by fibrate treatment. These extended-release niacin decreases the production of apo(a) results are mostly attributed to the failure of these trials and apoB-100–containing lipoproteins (79), increases the to enroll the population with the typical dyslipidemia that production of apoA-I, enhances the clearance of TRL apoB- is most likely to benefit from fibrate therapy (i.e., the 100 and apoB-48 (80), and reduces the fractional catabolic atherogenic dyslipidemia complex). In contrast, the VA- rate of apoA-I (81). HIT trial targeted individuals with low HDL-C levels; Only until recently has the efficacy of niacin, when therefore, the study population was enriched with those added to statin therapy, been examined in atherosclerosis who have features of the atherogenic dyslipidemia com- and CVD. In the ARBITER 6-HALTS (Arterial Biology for plex. Meta-analyses (74) of these fibrate outcomes trials the Investigation of the Treatment Effects of Reducing have demonstrated significant CVD risk reduction in Cholesterol 6-HDL and LDL Treatment Strategies in subgroups with HTG, but not in those with normotri- Atherosclerosis) trial (82), extended-release niacin in- glyceridemia. In a meta-analysis (75) of 18 prospective duced the regression of carotid intima-media thickness randomized controlled trials with fibrates, CVD benefits in patients receiving statin. Large, well-powered clinical 1772 Atherogenic Dyslipidemia Complex and CVD Diabetes Volume 65, July 2016 trials have failed to demonstrate the CVD benefits of The aggregate of results thus far does not support extended-release niacin when added to statin therapy. In CETP inhibition as an effective approach for CVD pre- the AIM-HIGH (Atherothrombosis Intervention in Meta- vention. There may be several reasons for this. First, bolic Syndrome with low HDL/High : Impact CETP inhibition per se may not be antiatherogenic (88). It on Global Health Outcomes) trial, extended-release niacin has been increasingly suggested that HDL function in- was added to therapy with simvastatin 40–80 mg/day, plus stead of concentration may underlie its link to CVD ben- ezetimibe 10 mg/day if needed, to maintain LDL-C at levels efits (26). Furthermore, these negative trials with CETP between 40 and 80 mg/dL (between 1 and 2 mmol/L). inhibitors studied patients whose HDL-C levels were on Despite improvement in plasma HDL-C, TG, and LDL-C average in the normal range, and thus could not completely levels, there was no incremental clinical benefitderived rule out the possibility that the raising of HDL-C levels from the addition of niacin to statin therapy (83). The provides CVD benefits. In our opinion, exclusively targeting HPS2-THRIVE trial, which used extended-release niacin this aspect of the atherogenic dyslipidemia complex (i.e., and (to prevent niacin-induced flushing), did HDL alone) without ameliorating the accompanying abnor- not reduce the number of CVD events, but increased the malities of elevated TRL and apoB-containing lipoprotein risk of adverse events in patients with vascular disease and levels may not be an effective therapeutic strategy. More well-controlled LDL-C levels during the 3.9-year follow-up potent and highly selective CETP inhibitors are being period (84). Both trials were prematurely terminated. We developed. For instance, increases levels of speculate that the effect of niacin on reducing apoB-contain- HDL-C and decreases levels of both LDL-C and apoB-100 ing lipoprotein levels is beneficial but is quantitatively small to (89,90), and, when added to statin treatment, results in negligible with background statin therapy, masking any puta- further reductions in LDL-C (91,92). Anacetrapib treat- tive antiatherosclerotic effect. Besides, if its HDL-raising effect ment also increases the LDL TG/cholesterol ratio and is not accompanied by improved HDL function, its possible LDL size and increases LDL-apoB-100 clearance (93). protection against atherosclerosis is questionable. Niacin is far CETP inhibitors that more potently lower atherogenic, less effective at lowering TRL levels than are the fibrates and apoB-containing lipoproteins, and that lack off-target ad- is therefore less likely to be effective for treating the verse effects on pressure, may reduce the number of atherogenic dyslipidemia complex. CVD events. Results from the ongoing CVD outcomes trial with anacetrapib ( reg. no. NCT01252953, Cholesteryl Ester Transfer Protein Inhibitors clinicaltrials.gov) may help to address this issue. Cholesteryl ester transfer protein (CETP) mediates the Insummary,wehaveprovidedsomeexplanationsand heteroexchange of neutral lipids between the denser lipo- perspective with respect to thefailureoftheabovethree proteins (HDL and LDL) and TRL (i.e., the transfer of TGs modalities of therapy, which all modify certain aspects of the fromTRLtoHDLandLDLinexchangeforcholesterylester atherogenic dyslipidemia complex. In our opinion, these fromHDLandLDLtoTRL).CETP-mediatedexchangeof apparent failures point to specific limitations of each partic- neutrallipidsbetweenHDLandTRLisoneoftwomajor ular agent or, in the case of fibrates,topitfallsinthedesign pathways in which cholesterol is transported from tissues of the clinical trials (i.e., patient selection). Their failure back to the liver for excretion in bile and feces, namely, should not be interpreted as overall futility in attempting to reverse cholesterol transport. The inhibition of CETP in- modify the atherogenic dyslipidemia complex and should creases HDL-C concentration; however, this potentially ad- not prevent ongoing and future exploration of therapies to vantageous effect may be offset by the impairment of prevent atherosclerosis by non-LDL–lowering modification reverse cholesterol transport due to HDL particle dysfunc- of dyslipidemia. tion.Thusfar,clinicaldevelopment of CETP inhibitors has been plagued by disappointing results. A clinical trial of WHERE TO GO FROM HERE? added to (ILLUMINATE) was termi- The atherogenic dyslipidemia complex presents a plethora nated prematurely because of an increased risk of mortality of potential therapeutic targets to lower fasting and post- and morbidity in patients with high CVD risk, despite a 72% prandial TRL levels, raise HDL-C levels, and shift LDL increase in HDL-C and a 25% reduction in LDL-C levels (85). particle size to the larger, more buoyant, and potentially less The adverse effects of torcetrapib may have been due to atherogenic species. We will briefly discuss the potential of off-target effects (86). In the Dal-OUTCOMES study (87), new therapeutic strategies that target what we believe to be did not reduce the risk of recurrent cardiovascu- the primary pathophysiological aspects of the atherogenic lar events in patients who had experienced a recent acute dyslipidemia complex: the clearance and secretion of hepatic coronary syndrome event, despite increased HDL-C levels. It and intestinally derived TRL particles, which will result in should be noted that dalcetrapib therapy resulted in a very secondary beneficial effects on HDL particle concentration modest reduction in TG levels and no effect on apoB. A trial and LDL size. with was recently discontinued because of futil- ity in preventing CVD outcomes; although top line results Novel Therapies Targeting Enhanced TRL Clearance were made public, a detailed peer review manuscript has not One method of lowering circulating TRL concentration is been published to date. to enhance LPL-mediated TG clearance. Among patients diabetes.diabetesjournals.org Xiao and Associates 1773 with LPL deficiency and chylomicronemia with severe HTG, Novel Therapies Targeting TRL Synthesis ongoing evaluation of LPL gene therapy shows some and Secretion promise (94). Although this highlights the important role Microsomal Triglyceride Transfer Protein Inhibitors of LPL, gene therapy is limited to cases of marked HTG due and ApoB Antisense Therapy to homozygous LPL loss-of-function mutations. Several re- Despite targeting the biogenesis of TRL, these therapies cent studies of nongene replacement therapies have pro- have major therapeutic effects in lowering LDL rather vided some promise in improving LPL activity, as follows. than TRL levels and therefore are not ideal agents for the treatment of the atherogenic dyslipidemia complex, APOC3 Inhibition although they could be considered for the treatment of ApoC3 is an LPL inhibitor. It has also been shown to inhibit very severe cases of genetic HTG not responding to other hepatic lipase activity, enhance VLDL secretion, and sup- therapies (106,107). These therapies are currently approved press TRL remnant clearance. Carriers of rare loss-of- in some countries for the treatment of homozygous familial APOC3 function mutations in the gene have low TG levels . Both of these costly therapies are and reduced CVD risk (95,96). Antisense inhibition of apoC3 associated with significant side effects. The apoB antisense in preclinical models and in a phase I clinical trial of healthy drug () is administered by subcutaneous injec- subjects decreased plasma concentrations of apoC3 and TG tion, and injection site reactions and flusymptomsfre- (97). In three patients with familial chylomicronemia syn- quently develop in treated patients. Both mipomersen and APOC3 drome, antisense RNA dramatically reduced TG lev- the oral microsomal triglyceride transfer protein inhibitor els and decreased the numbers of chylomicron and VLDL () can also cause hepatotoxicity (106,107). These particles (58). Because familial chylomicronemia is caused by therapies are currently reserved for the treatment of a very mutations in the gene encoding LPL or in the genes encod- rare life-threatening genetic condition. ing proteins affecting LPL function, and the patients in the above study had homozygous or compound heterozygous- Diacylglycerol Acyltransferase and Monoacylglycerol null LPL mutation, the reduction in TG concentration with Acyltransferase Inhibition APOC3 antisense treatment suggests that apoC3 may affect Pharmacological inhibition of enzymes involved in TG bio- TRL metabolism through LPL-independent pathways. The synthesis, namely, diacylglycerol acyltransferase (DGAT) and findings on the efficacy of anti–APOC3 RNA-based strategies monoacylglycerol acyltransferase (MGAT), has shown were expanded in a larger cohort of patients with fasting promise for the treatment of obesity, diabetes, and HTG, where APOC3 antisense as monotherapy or as an add- dyslipidemia in animal models (108). MGAT and DGAT on to fibrates dose-dependently reduced plasma TG concen- catalyze the final consecutive steps of TG biosynthesis. trations by up to 70% from baseline (98). MGAT is mainly expressed in the , whereas DGAT is expressed in multiple tissues, including ANGPTL3 and ANGPTL4 Inhibition the intestine and the liver. Isozymes with different func- Several members of the ANGPTL family of proteins are tions and expression have been identified for both MGAT found to regulate LPL. Rare loss-of-function mutations in and DGAT. MGAT2 is exclusively expressed in the intestine, ANGPTL3 are associated with lower levels of all lipids and where the MGAT pathway accounts for ;75% of TG syn- lipoproteins, including TG, LDL-C, and HDL-C (63). The thesis (109), and MGAT plays an important role in dietary blockade of ANGPTL3 with antibody reduced plasma lipids, lipid absorption. DGAT1 is abundantly expressed in the small including TG, non-HDL-C, and HDL-C in mice and monkeys intestine (110). DGAT1 inhibition reduced postprandial TG (99), which is likely associated with reduced VLDL TG pro- plasma excursion in humans; however, this was accompanied duction and increased clearance of apoB-containing lipopro- by gastrointestinal side effects (111). Other DGAT1 inhibitor teins (100). ANGPTL4 inhibits LPL and is a major regulator compounds are being developed, with the hope of satisfac- of LPL activity during fasting and exercise (101). Loss-of- tory efficacy, safety, and tolerability (112). function variants in ANGTPL4 are associated with reduced plasma TG levels and a lower risk of Acetyl-CoA Carboxylase Inhibitors (102–105). The inhibition of ANGPTL4 with antibody re- Acetyl-CoA carboxylases (ACCs) catalyze the rate-limiting step duced serum TG levels in mice and monkeys (105). The of fatty acid biosynthesis (i.e., carboxylation of acetyl-CoA to hypolipidemic effects of the pharmacological inhibition of malonylCo-A).Twoisozymes,ACC1andACC2,areexpressed ANGPTL3 and ANGPTL4 in humans await further studies. in different tissues. ACC1 is expressed in lipogenic tissues (the As discussed above, genetic studies have identified muta- liver and adipose tissue) primarily catalyzing long-chain fatty tions in several genes encoding proteins that affect LPL acid biosynthesis, whereas ACC2 is expressed in oxidative function and activity. Therapeutic approaches targeting these tissues (the liver, skeletal muscle, and heart). Isozyme- genes or related proteins may yield effective TG reduction and nonselective ACC inhibitors have the potential of inhibiting CVD risk benefits in the future. In addition, the identification lipogenesis and increasing fatty acid oxidation (113). A phase of noncoding microRNAs in the regulation of lipoprotein I clinical study (114) of one such compound demonstrated metabolism opens more opportunities for the development of the inhibition of de novo lipogenesis and increased whole- novel therapeutics against atherosclerotic CVD. body fatty acid oxidation. ACC inhibition may be useful in the 1774 Atherogenic Dyslipidemia Complex and CVD Diabetes Volume 65, July 2016 treatment of T2D, HTG, and nonalcoholic steatohepatitis, but For example, an FGF21 analog, originally developed as a hasnotyetbeenexaminedinclinicaltrials. glucose-lowering agent for the treatment of T2D, has demonstrated greater lipid-modifying effects in obese ETC-1002 () subjects with T2D, including lowering TG, total cholesterol, The investigational agent ETC-1002 is a dual modulator and LDL-C levels, and increasing HDL-C levels (127). The of two hepatic enzymes; it inhibits ATP-citrate lyase and mechanism of FGF21 modulating lipid metabolism likely activates AMPK in the liver to inhibit sterol and fatty acid involves multiple organs, but its effects on TRL metabolism synthesis and to promote fatty acid oxidation (115). In have not been examined. Recently, it has been shown patients with elevated LDL-C levels, ETC-1002 lowers LDL-C (128,129) in rodents that the activation of brown adipose levels regardless of baseline TG levels (116). In addition, tissue enhances fatty acid uptake by brown adipose tissue it lowers apoB-100, non-HDL-C, and LDL particle number, and the clearance of remnant cholesterol by liver, leading to and improves factors related to cardiovascular health (e.g., the lowering of TG and cholesterol levels, translation of inflammatory markers, , and body weight). which into therapeutic means in humans awaits further The lowering of LDL-C levels, as well as non-HDL-C and studies. total cholesterol levels, was also observed in patients with It is important to point out that lifestyle management, T2D (117). In statin-intolerant patients with hypercholes- such as diet, exercise, and achievement of a healthy body terolemia, ETC-1002 decreased non-HDL-C, total choles- weight, is an integral and key component of lipid man- terol, and apoB levels without significantly affecting TG agement. Such approaches are difficult to implement and and HDL levels (118). Long-term studies are needed to adhere to, but are highly effective in modulating, the ath- assess its safety, tolerability, and efficacy as combination erogenic dyslipidemia complex. The above lifestyle changes therapy compared with those of other lipid-lowering . are accompanied by improvement in insulin sensitivity and general metabolic status, which is expected to contribute to Gut Hormones in the Regulation of Postprandial improved lipid profiles. Weight loss improves many com- Lipemia ponents of the atherogenic dyslipidemia complex (130,131). Recent advances in the understanding of intestinal lipo- Sustained weight loss also delaystheonsetofT2D,inwhich protein production offer novel therapeutic approaches in the atherogenic dyslipidemia complex is often a prevalent attenuating postprandial lipemia (reviewed in Dash et al. feature. Bariatric surgery, the most effective intervention in [44]). Among those, the incretin-based antidiabetic drugs inducing sustained weight loss, also reduces TRL production exenatide (a glucagon-like peptide-1 receptor agonist) and (132,133). Nonpharmacological therapies thus far have not sitagliptin (a dipeptidyl peptidase-4 inhibitor) suppress in- shownreductionsinCVDoutcomes. testinal lipoprotein production in the short term in humans (119,120). Incretin-based antidiabetic drugs also attenuated CONCLUSIONS postprandial TG excursion in clinical trials (reviewed in Xiao Although much success has been achieved in LDL-C et al. [121]). Several recent CVD outcomes trials (122–124) have demonstrated the noninferiority of several such agents lowering and CVD reduction with statin-based therapies, residual CVD risk in statin-treated patients remains a in T2D patients with established CVD or with increased significant challenge, especially with the ever increasing prev- CVD risks, and more trials with other agents are underway. alence of obesity, diabetes, and metabolic syndrome. Recent n-3 Fatty Acids of Marine Source advances in the management of dyslipidemia have pro- These nutraceuticals have been shown to provide cardiovas- vided promise in several aspects, including further lower- fi cular health benefits and are recommended in treating ing of LDL-C levels and CVD risk with intensi ed statin and marked HTG. Their mechanisms of action are multifaceted, nonstatin drugs, such as ezetimibe, and effective LDL-C including an antiarrhythmic effect, plasma TG reduction, lowering with PCSK9 inhibition. The atherogenic dyslipide- blood pressure lowering, a decrease in platelet aggregation, mia complex contributes to the residual CVD risk and re- improvement of vascular reactivity, and an anti-inflammation presents an expanded panel of therapeutic targets, especially effect (125). It has been postulated that the hypolipidemic in at-risk patients with near-normal LDL-C levels who are effect of n-3 fatty acids may be due to reduced hepatic lipo- less responsive to statins or are statin intolerant. Among fi genesis, increased fatty acid oxidation, and suppressed VLDL existing and currently approved therapies, brates should secretion, although the exact mechanisms remain to be elu- be reevaluated in those with the atherogenic dyslipidemia cidated (126). The efficacy of n-3 fatty acids on reducing CVD complex. Other emerging therapies, such as those directly outcomes is currently being evaluated in HTG patients re- targeting TRL metabolism, may provide novel opportuni- ceiving statin therapy (clinical trial reg. nos. NCT01492361 ties for reducing CVD risk through the management of and NCT02104817, clinicaltrials.gov). dyslipidemia. Targeting the atherogenic dyslipidemia com- plex is the next frontier in antiatherosclerotic therapy. Other Therapeutic Approaches and the Effectiveness of Nonpharmacological Therapy Several developments in the field have also indicated Funding. R.A.H. is supported by the Jacob J. Wolfe Distinguished Medical potentially novel mechanisms of lipid modification. Research Chair, the Edith Schulich Vinet Canada Research Chair (Tier I) in diabetes.diabetesjournals.org Xiao and Associates 1775

Human Genetics, the Martha Blackburn Chair in Cardiovascular Research, and coronary heart disease events and response to simvastatin therapy in 4S. Cir- operating grants from the Canadian Institutes of Health Research (MOP-13430 culation 2001;104:3046–3051 and MOP-79533), the Heart and Foundation of Ontario (T6066 and 17. Manninen V, Tenkanen L, Koskinen P, et al. Joint effects of serum tri- 000353), and Genome Canada through Genome Quebec. G.F.L. is supported by glyceride and LDL cholesterol and HDL cholesterol concentrations on coronary the Sun Life Financial Chair in Diabetes, the Drucker Family Chair in Diabetes heart disease risk in the Helsinki Heart Study. Implications for treatment. Cir- Research, an operating grants from the Canadian Institutes of Health Research culation 1992;85:37–45 (MOP-43839) and the Heart and Stroke Foundation of Ontario (000032). 18. Sarwar N, Danesh J, Eiriksdottir G, et al. Triglycerides and the risk of fl Duality of Interest. 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