A large proportion of patients at high risk for Cardiovascular Disease (CVD) continue to suffer from CVD events despite current therapies. As a consequence, additional therapies are urgently needed to lower the residual risk. Therapies directed at lowering Low Density Lipoprotein cholesterol (LDL-C) levels are deemed beneficial, since LDL-C has been shown to be causally related with CVD risk and current guidelines therefore emphasize that LDL-C lowering therapies (statins) are pivotal.¹

In 2003, Abifadel and coworkers identified a gain of function mutation in the gene encoding proprotein convertase subtilisin/kexin type 9 (PCSK9) in a family suffering from Familial Hypercholesterolemia, in which no mutation was identified in the well established FH-genes (LDLR and APOB).² FH is an autosomal dominant form of hypercholesterolemia characterized by exceedingly high LDL-C levels, specific clinical stigmata (i.e., tendon xantomas) and high risk for CVD; studies in FH patients have greatly enhanced our insight in LDL-C metabolism and CVD risk.

The molecular mechanism by which PCSK9 impacts LDL-C metabolism was further disentangled in subsequent studies, showing that PCSK9 is a crucial protein by virtue of its role in the lysosomal degradation of the LDL receptor. As a consequence, PCSK9 causes a decrease in hepatic uptake of LDL-C by increasing the endosomal and lysosomal degradation of the LDL receptor. In recent years, both in vitro and in vivo studies have greatly supplemented our understanding of the (patho)physiological role of PCSK9 in human biology.

PCSK9 in Animal Models

In animal studies it has been shown that PCDK9 plays a significant role in LDL-C metabolism. In PCSK9 knockout (KO) mice, hepatic LDLR expression was shown to significantly increase LDLR, which was found to result in increased clearance of circulating lipoproteins and decreased plasma cholesterol levels (46 mg/dl in Pcsk9(-/-) mice versus 96 mg/dl in WT mice). Of note, administering statins to Pcsk9(-/-) mice produced an exaggerated increase in hepatic LDLRs and enhanced LDL clearance from plasma.³

Accelerated atherosclerosis with larger plaque size was found in transgenic PCSK9 mice on an Apoe-knockout genetic background, when fed a regular chow diet. The opposite (protective) effect was seen in PCSK9-knockout mice on the same background, which clearly underlines the direct effect of PCSK9 on atherosclerosis. The effect has been shown to be LDLR dependent; in LDLR-KO mice lacking or overexpressing PCSK9, no significant differences were observed in plaque size.⁴

PCSK9; Genetic Variation in Humans

In 2006, Cohen and colleagues studied the effects of deleterious PCSK9 mutations identified in participants of the ARIC (Atherosclerosis Risk in Communities) study.⁵ These mutations were identified in 2.6% of the total of 3363 black participants and in 3.2% of the total of 9524 white subjects. The mutations were associated with a 28% lower LDL-C level and an impressive 88% reduction in the risk of CVD in the black subjects. In whites, these mutations resulted in a 15% reduction in LDL cholesterol accompanied by a 47% CVD risk reduction.

In addition to the “extreme genetics” described above, PCSK9 SNPs have consistently been found to be associated with both LDL-C levels and CVD risk in numerous genome wide assocation studies (GWAS). This provides additional evidence of the causal effect of PCSK9 variation in atherosclerotic disease outcome. The fact that only beneficial and no deleterious effects of a lifelong exposure to the PCSK9 mutation is found (in extreme genetics and in Mendelian randomization studies) is an important adjunct to clinical trials.⁶

Circulating PCSK9 Levels and CVD

In statin-naïve populations, PCSK9 serum concentrations have been shown to correlate with cardiovascular risk and also in a recent prospective study, serum concentrations of PCSK9 predicted CV outcomes (composite of cardiovascular death and unplanned cardiovascular hospitalization) in statin treated patients followed for 48 months (PCSK9>622ng/ml vs. <471ng/ml: HR 1.55, 95%-CI 1.11-2.16, p=0.009).⁷

Trials

The first Phase I trial addressing the role of PCSK9 inhibition was published in 2012.⁸ It was within a decade after the initial report about the role of PCSK9 in lipid metabolism. Many clinical trials have been conducted ever since in different types of patients.

Overall, one can conclude that the outcome of all of these studies are similar: administration of antibodies directed against PCSK9 appears  safe and efficacious. The largest numbers of studies have been conducted with alirocumab (SAR236553/REGN727) and evolocumab (AMG145, fully human antibody), while other monoclonal antibodies, such as RN316/PF04950615 (a humanized antibody⁹ and other methods, such as interference RNA drug¹⁰ have been studied for LDL-C reduction outcomes as well.

The efficacy of PCSK9 antibodies is similar in all of the patient populations being enrolled in the clinical trials. LDL-C levels decrease by approximately 50-70% in most of the clinical trials, irrespective of the baseline treatment. The latter is an important finding since PCSK9 directed therapy is likely to be prescribed to patients who are already receiving different lipid lowering therapies.

The effect appears to go beyond LDL-C. In the recently published PROFICIO study, a meta-analysis of 1359 randomized patients in evolocmab phase 2 trials. Evolocumab (every two or four weeks) resulted in a mean percentage reductions in LDL-C vs. placebo ranging from 40% (45%, 36%) to 59% (64%, 55%, all P < 0.001). Moreover, statistically significant reductions in apolipoprotein B (Apo B), non-high-density lipoprotein cholesterol (non-HDL-C) and triglycerides were noted, while HDL-C levels increased.¹¹ The most common side effects noted in the former two antibody-therapies are mild injection-site reactions, while no persistent or prevalent liver or skeletal muscle safety signals were found.

Unlike statins, which have not been shown to have a beneficial effect on Lp(a), PCSK9 antibody therapy has been shown to result in Lp(a) lowering. In the PROFICIO study, Lp(a) levels were significantly reduced by 20-30% in a dose dependent manner. The absolute decrease in Lp(a) levels was the largest in patients with high baseline levels.¹² It is unknown by which mechanism Lp(a) levels are lowered, but the fact that Lp(a) levels have consistently been shown to be positively associated with CVD events in large prospective epidemiological studies, combined with evidence that this is a causal relationship, suggests that PCSK9-targeted therapy might result in a larger CVD risk reduction than based on its LDL-C lowering effect only.

A number of different patient categories are likely to potentially benefit from PCSK9 inhibition, and the studies conducted so far have addressed the effect on lipid lowering in patients suffering from FH, homozygous FH, high CVD risk, and statin intolerant patients. The latter is of particular interest given that statin-associated side effects have been suggested to exceed previous estimates based on randomized trials, reaching up to 10-20%.¹³

In the recently published GAUSS-2 trial (Goal Achievement after Utilizing an anti-PCSK9 antibody in Statin Intolerant Subjects-2), a 12-week double-blind study, 307 patients were randomized (2:2:1:1) to subcutaneous administration (SC) of evolocumab 140 mg biweekly or 420 mg monthly, both with daily oral placebo (PBO); or SC PBO every two weeks or four weeks both with daily oral ezetimibe 10 mg. Evolocumab reduced LDL-C after 12 weeks by 53% to 56%, corresponding to treatment differences versus ezetimibe of almost 40%. The finding that muscle adverse events occurred in 12% of evolocumab- and 23% of ezetimibe-treated patients indicates that PCSK9 inhibition might be a suitable therapy for a large number of statin intolerant patients.

The long term (one year) effect of PCSK9 inhibition was studied in the Descartes (Durable Effect of PCSK9 Antibody Compared with Placebo Study).¹⁴ In this multicenter placebo controlled study, patients with a mean LDL-C level of 75 mg/dL (1.94 mmol per liter) were enrolled. Over 900 patients were included in the analysis and the overall reduction in LDL cholesterol from baseline in the evolocumab group was 57±2% (P<0.001) taking into account the change in the placebo group. No significant safety signals were noted.

Conclusion

PCSK9 is widely considered a target for therapy to lower LDL-C, based on data derived from genetic, epidemiological, and intervention studies. PCSK9-targeted approaches are in different stages of investigation, and thus far, results have been very promising. The question whether PCSK9 inhibition will become a viable new therapeutic option for the treatment of cardiovascular disease remains to be answered. The currently conducted end-point trials (Odyssey outcomes (alirocumab), Fourier (evolocumab) and SPIRE outcome (RN316/PF04950615) will tell us whether PCSK9 inhibition results in CVD risk reduction and these trials are anticipated to be finished by 2017.

References

1. Stone NJ, Robinson J, Lichtenstein AH, et al. 2013 ACC/AHA Guideline on the Treatment of Blood Cholesterol to Reduce Atherosclerotic Cardiovascular Risk in Adults: A Report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. J Am Coll Cardiol. 2013; [Epub Ahead of Print].
2. Abifadel M, Varret M, Rabès JP, et al. Mutations in PCSK9 cause autosomal dominant hypercholesterolemia. Nat Genet 2003;34:154-6.
3. Rashid S, Curtis DE, Garuti R, Anderson NN, Bashmakov Y, Ho YK, Hammer RE, Moon YA, Horton JD. Decreased plasma cholesterol and hypersensitivity to statins in mice lacking Pcsk9. Proc Natl Acad Sci U S A. 2005;102:5374–5379.
4. Denis M, Marcinkiewicz J, Zaid A, et al. Gene inactivation of PCSK9 reduces atherosclerosis in mice. Circulation 2012;125:894–901.
5. Cohen JC, Boerwinkle E, Mosley TH Jr, Hobbs HH. Sequence variations in PCSK9, low LDL, and protection against coronary heart disease. N Engl J Med. 2006 Mar 23;354(12):1264-72.
6. Dandona S, Roberts R. The role of genetic risk factors in coronary artery disease. Curr Cardiol Rep. 2014;16:479.
7. Werner C, Hoffmann MM, Winkler K, Böhm M, Laufs U. Risk prediction with proprotein convertase subtilisin/kexin type 9 (PCSK9) in patients with stable coronary disease on statin treatment. Vascul Pharmacol 2014; pii: S1537-1891(14)00057-3.
8. Stein EA, Mellis S, Yancopoulos GD, et al. Effect of a monoclonal antibody to PCSK9 on LDL cholesterol.N Engl J Med 2012;366:1108-18.
9. Gumbiner B, Udata T. The effects of single dose administration of RN316 (PF-04950615), a humanized IgG2a monoclonal antibody binding proprotein convertase subtilisin kexin type 9, in hypercholesterolemic subjects treated with and without atorvastatin. Circulation 2012;126:A13322, abstract.
10. Fitzgerald K, Frank-Kamenetsky M, Shulga-Morskaya S, et al. Effect of an RNA interference drug on the synthesis of proprotein convertase subtilisin/kexin type 9 (PCSK9) and the concentration of serum LDL cholesterol in healthy volunteers: a randomised, single-blind, placebo-controlled, phase 1 trial. Lancet 2014;383:60-8.
11. Stein EA, Giugliano RP, Koren MJ, for the PROFICIO Investigators, et al. Efficacy and safety of evolocumab (AMG 145), a fully human monoclonal antibody to PCSK9, in hyperlipidaemic patients on various background lipid therapies: pooled analysis of 1359 patients in four phase 2 trials. Eur Heart J 2014; [Epub ahead of print].
12. Raal FJ, Giugliano RP, Sabatine MS, Koren MJ, Langslet G, Bays H, Blom D, Eriksson M, Dent R, Wasserman SM, Huang F, Xue A, Albizem M, Scott R, Stein EA. Reduction in Lipoprotein(a) With PCSK9 Monoclonal Antibody Evolocumab (AMG 145): A Pooled Analysis of More Than 1,300 Patients in 4 Phase II Trials J Am Coll Cardiol 2014;63:1278-88.
13. Zhang H, Plutzky J, Skentzos S et al. Discontinuation of statins in routine care settings: a cohort study. Ann Intern Med 2013;158:526-34.
14. Blom DJ, Hala T, Bolognese M, Lillestol MJ, Toth PD, Burgess L, Ceska R, Roth E, Koren MJ, Ballantyne CM, Monsalvo ML, Tsirtsonis K, Kim JB, Scott R, Wasserman SM, Stein EA; the DESCARTES Investigators. A 52-Week Placebo-Controlled Trial of Evolocumab in Hyperlipidemia. N Engl J Med 2014; [Epub Ahead of Print].

Keywords: Cardiovascular Diseases, Cholesterol, LDL, Hydroxymethylglutaryl-CoA Reductase Inhibitors

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