Introduction

Consistent findings from clinical endpoint trials and from genetic studies have reaffirmed the causal role of low-density lipoprotein cholesterol (LDL-C) concentration in the development of atherosclerotic cardiovascular disease (ASCVD) [1]. Lowering LDL-C with a statin reduces the risk of ASCVD and all-cause mortality [2, 3], and statin therapy is endorsed by international guidelines as first-line therapy for cholesterol-lowering management in patients with ASCVD risk [4, 5].

Despite an increased use of statin therapy, a large proportion of patients with high ASCVD risk fail to achieve optimal LDL-C lowering on statin monotherapy, including those on high-dose statins [6]. Recently, ezetimibe added to statin therapy was found to reduce cardiovascular (CV) events in acute coronary syndrome patients more than statins alone [7]; further reduction of CV events was related to the magnitude of LDL-C lowering as demonstrated for statin therapy. However, there are patient populations either unable to achieve optimal LDL-C levels despite statin and ezetimibe combination therapy, or intolerant to statins, particularly at a high dose [8].

In clinical practice, a substantial number of patients at high and very high risk of ASCVD require LDL-C lowering larger than currently achievable with statins alone or in combination with other available lipid-lowering drugs such as ezetimibe. Moreover, other lipoprotein abnormalities particularly elevated triglyceride-rich lipoprotein levels, with/without low levels of high-density lipoprotein cholesterol (HDL-C), have been linked to the residual cardiovascular risk on statin therapy. Even if the residual risk can also be reduced by a better control of associated CV risk factors such as blood pressure, smoking, or lack of physical activity, it remains an important need for efficacious agents to decrease LDL-C and triglyceride-rich lipoproteins, and to modulate HDL metabolism.

New Drugs Targeting LDL Metabolism

Despite the availability of statins, ezetimibe, and other classes of drugs lowering LDL such as bile acid sequestrants, there remains an intense interest in the development of new LDL-lowering agents (Table 1); some agents have been already approved and others are still in clinical development. Among all the drugs listed in Table 1, the most exciting and promising class is certainly proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors.

Table 1 Lipid-altering therapies: new drugs or drugs in development
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PCSK9 Inhibitors

Recent comprehensive reviews have described the discovery, genetics, structure, and function of PCSK9 in regulating LDL-C levels [9–12, 13•]. In summary, since the discovery of PCSK9 in 2003 [14, 15], it has been well established that PCSK9 regulates cholesterol metabolism mainly by targeting the LDL receptor for degradation in the liver: Gain-of-function mutations in PCSK9 are one of the genetic causes of autosomal dominant hypercholesterolemia [15]. Conversely, low-of-function mutations are associated with lower concentrations of LDL-C and reduced coronary heart disease [16, 17]. PCSK9, secreted into the plasma by the liver, binds the LDL receptor of the surface of hepatocyte and induces modification of LDL receptor conformation, avoiding normal recycling of LDL receptor, enhancing its degradation in endosomes/lysosomes and resulting in reduced clearance of LDL particles. By consequence, PCSK9 inhibition is a very attractive strategy for lowering LDL-C and enhancing the efficacy of statin treatment.

Several therapeutic approaches to the inhibition of PCSK9 have been proposed [18, 19•], targeting either extracellular PCSK9 by monoclonal antibodies (mAbs) or adnectins, or intracellular PCSK9 by antisense oligonucleotides (ASOs) or small interfering RNA (siRNA). The development of two ASOs was stopped in phase I due to safety concerns. In a phase I trial of ALN-PCS, a siRNA inhibitor of PCSK9 developed by Alnylan Pharmaceuticals, a dose-dependent reduction in LDL-C was observed, with a 40 % reduction with the highest dose, associated with a 70 % reduction in plasma PCSK9 levels [20]. However, in this trial, ALN-PCS was administered intravenously and healthy subjects required premedication with dexamethasone and antihistamines. A new formulation has been developed for subcutaneous (SC) administration, and in recent phase I trial, this new formulation was generally well tolerated, with similar LDL-C reduction to data reported for mAbs in subjects with and without statin coadministration [21]. The durability of the effect on LDL-C and PCSK9 levels supporting a biannual dose regimen needs to be confirmed in larger studies, and this agent will enter in phase II. Inhibition of PCSK9 binding to LDL receptor by adnectins such as BMS-962476 are in phase I [19•]. An alternative approach for PCSK9 inhibition could be a peptide-based anti-PCSK9 vaccine to provide a long-term LDL-C management [22]. Finally, mAbs are the most studied and advanced approach with two mAbs approved in 2015 alirocumab [23•] and evolocumab [24•], and two other with published phase II data, bococizumab [25] and LY3015014 [26].

Efficacy of PCSK9 Inhibition with mAbs

Globally, in combination with a statin or in monotherapy, mAbs induced dramatic significant decreases in LDL-C (from 45 to 70 %) and in all the other atherogenic parameters non-HDL-C, apolipoprotein(apo)B, and also lipoprotein(a) (Lp(a)). Three large phase III programs have been developed with alirocumab, evolocumab, and bococizumab administered as SC injections every 2 weeks or every 4 weeks. So far, in the phase III trials published with alirocumab and evolocumab, the efficacy has been demonstrated:

  • In patients with heterozygous familial hypercholesterolemia (FH) [27••, 28] and with homozygous FH [29••]

  • In high-risk patients not controlled by maximally tolerated statin and other lipid lowering therapies [30••, 31, 32]

  • In combination with statins in patients with high LDL-C [33–36]

  • In patients as monotherapy [37, 38]

  • In patients who could not tolerate statins due to muscle-related side effects [39, 40]

Encouraging findings suggesting that PCSK9 mAbs can decrease CV events have been observed from exploratory or post hoc analysis as part of the longer term safety trials ODYSSEY LONG TERM with alirocumab [30••] and OSLER with evolocumab [41••]. Moreover, meta-analyses of phase II and III trials found reduced total mortality with alirocumab and evolocumab, in trials ranging from 12 to 78 weeks [42, 43], with several limitations in these analyses, particularly due to the limited number of either CV events or deaths. Whether PCSK9 mAbs definitively reduce the incidence of CV events in patients on statin therapy shall be demonstrated with the four ongoing CV outcome trials [44–47] including more than 70,000 high-risk patients (Table 2).

Table 2 Ongoing cardiovascular outcomes phase III trials with PCSK9 monoclonal antibodies
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To date, alirocumab and evolocumab SC injections appeared well-tolerated in trials up to 78 weeks in duration. Injection site reactions were relatively rare and mild. Neurocognitive events were reported more frequently with both alirocumab in ODYSSEY LONG TERM (1.2 vs 0.5 %) and evolocumab in OSLER-1 and OSLER-2 (0.9 vs 0.3 %) [30••, 41••]. The possibility that PCSK9 inhibitors may induce neurocognitive disorders is evaluated by the ongoing EBBINGHAUS trial [48], realized in a substudy of FOURIER trial [44]. Concerns regarding the risk of very low LDL-C levels have been addressed in complementary analysis of long-term safety trials over 78 weeks of treatment [30••, 41••]: No excess of adverse events has emerged in patients with LDL-C <25 mg/dL.

PCSK9 inhibition may become a major breakthrough in prevention of ASCVD [49]. Results of ongoing outcomes studies shall be crucial to evaluate the cost-effectiveness of PCSK9 inhibitor treatment [50].

Mipomersen

Mipomersen is an ASO that specifically blocks the messenger RNA (mRNA) translation into apoB and thereby the synthesis of apoB-containing lipoproteins. Mipomersen 200 mg in SC injections once weekly decreases LDL-C, apoB, and Lp(a) by 25–35 % in patients with severe hypercholesterolemia, including heterozygous and homozygous FH [51]. However, side effects are frequently related to either injection site reactions and flu-like symptoms, or hepatic fat accumulation and transaminases elevations. In a long-term evaluation of efficacy and safety, 55 % of the patients discontinued treatment within the first 2 years of treatment [52]. By consequence, the drug was approved by the FDA with a box warning only for homozygous FH. The approval has been refused by EMA due to side-effects. In the specific population of homozygous FH, mipomersen decreased LDL-C, apoB, and Lp(a) by ≈25 % [53].

Lomitapide

Microsomal triglyceride transfer protein (MTP) is essential for assembly and secretion of very-low-density lipoproteins (VLDL) in the liver and of chylomicrons in the intestine. Several MTP inhibitors have been tested in humans, most abandoned due to poor gastrointestinal tolerability, elevations in hepatic transaminases and increase of hepatic steatosis [54]. However, lomitapide, an oral MTP inhibitor, has been approved by both FDA and EMA for patients with homozygous FH. This approval has been based on the results of an open-label study in 29 homozygous FH patients [55]. In this trial, lomitapide started at a dose of 5 mg/day and titrated at 4-week intervals to a maximum of 60 mg daily has reduced LDL-C by 50 % at week 26 and 38 % at week 78. The efficacy was similar for patients with or without apheresis [56]. However, despite the LDL-C lowering effect, the poor tolerability and side effects (increases in liver enzymes and hepatic steatosis) should limit the use of lomitapide to negative-negative LDL receptor homozygous FH patients.

Bempedoic Acid (ETC-1002)

Bempedoic acid (ETC-1002) is an oral agent with a novel dual mechanism of action: By inhibiting ATP citrate lyase, the drug can decrease LDL-C, and by activation of AMP kinase, beneficial effects on glucose, lipids, inflammation, and weight gain could be expected, suggesting a development for patient with hypercholesterolemia and type 2 diabetes [57]. In patients with hypercholesterolemia, ETC-1002 reduced LDL-C levels up to 27 % in a phase II trial [58]. In patients with type 2 diabetes, ETC-1002 120 mg/day significantly lowered LDL-C by 43 % (vs 4 % with placebo) and high-sensitivity C-reactive protein (hsCRP) by 41 % (vs 11 % with placebo) [57]. In patients with or without statin intolerance, ETC-1002 lowered LDL-C and hs-CRP more than ezetimibe and the combination of ETC-1002 and ezetimibe induced an additive effect in terms of LDL-C reduction [59]. Larger phase III trials are required to assess long-term efficacy and safety.

Gemcabene

Gemcabene is an oral drug initially evaluated for a broad use in hypercholesterolemia, but with moderate LDL-C lowering effect (up to 25 %) [60]. The mechanisms of action include an inhibition of acetyl CoA carboxylase and an enhancement of VLDL clearance by decreasing apoCIII. The drug is now planned to be developed for use in homozygous FH based on mechanism of action and preclinical animal data.

CAT-2054

CAT-2054, a conjugate of eicosapentaenoic acid and niacin, is a novel inhibitor of the sterol response element-binding protein (SREBP) transcription factor. CAT-2054 does not activate the GPR109A receptor that causes flushing. In preclinical studies, CAT-2054 reduced plasma LDL-C and PCSK9 in nonhuman primates. A phase I trial has been recently presented [61], and the drug is planned to enter phase II.

New Drugs Targeting Triglyceride-Rich Lipoproteins

The causal relevance of high fasting and non-fasting triglyceride (TG) levels for an increased risk of ASCVD has been largely debated [62]. Indeed in observational studies, the relationship between TG and ASCVD risk is attenuated after adjustment for associated variables, including HDL-C [63]. However, post hoc analyses of statin trials [64, 65] suggest that higher levels of TG-rich lipoproteins correlate with residual risk. The action of lipases on VLDL and chylomicrons results in the formation of remnant particles with increased cholesterol content and potential role for atherogenicity [62, 66]. Moreover, recent genetic studies highly support a causal role for TG-rich lipoproteins in ASCVD risk [67–70] and identify potential targets for TG reduction. Several new drugs specifically aimed at reducing TG are being developed.

New Omega-3 Fatty Acid Preparations

Two new omega-3 fatty acid (FA) formulations have been developed: Vascepa® containing eicosapentaenoic acid (EPA) ethyl esters and Epanova® containing free FA formulations of EPA and docosahexaenoic acid (DHA). These two formulations reduced plasma TG by around 30 % at a 4 g daily dose [71, 72]. These formulations are currently evaluated in long-term outcome trials, REDUCE-IT for Vascepa® [73] and STRENGTH for Epanova® [74] in high cardiovascular risk patients with hypertriglyceridemia on optimal statin therapy.

ApoC3 Antisense Oligonucleotide

Loss-of-function variants in the gene of ApoC3 encoding the protein apoC-III known to inhibit lipoprotein lipase and to enhance intrahepatic production of VLDL are associated with reduced TG levels and decreased ASCVD risk [69, 70], suggesting that ApoC3 inhibitors should be an adequate target.

An ASO targeted to the ApoC3 mRNA in the liver, volanesorsen (formerly ISIS 304801), has been shown to reduce TG levels by 56–86 % in three patients with familial chylomicronemia syndrome [75•]. This effect was confirmed in a phase II trial conducted in hypertriglyceridemic patients with or without fibrate therapy [76•]. Treatment with volanesorsen resulted in dose-dependent decreases in both plasma ApoC3 and triglyceride levels up to 70–80 %. Phase III studies are ongoing.

Angiopoietin-Like Protein Inhibitors

Angiopoietin-like proteins (ANGPTLs) 3 and 4 are other inhibitors of the activity of lipoprotein lipase. Inhibition of ANGPTLs 3 and 4 using neutralizing mAbs (such as REGN 1500) has been reported to reduce TG levels in animal models [77, 78]. Early human clinical trials are ongoing.

New PPAR-α Agonists

Peroxisome proliferator-activated receptor (PPAR)-α agonists are an old class of drugs known to decrease TG and ApoC3 levels. However, clinical outcome trials have not yielded consistent results [72], even if a prespecified subgroup of ACCORD trial highly suggests a beneficial effect of fenofibrate/simvastatin therapy for diabetic patients with high TG and low HDL-C on simvastatin. The development of new PPAR-α agonists more potent and selective such as Pemafibrate (K-877) [79] remains of interest. The clinical efficacy of Pemafibrate shall be evaluated in a landmark trial named PROMINENT recently decided.

Lipoprotein Lipase Gene Therapy

Alipogene tiparvovec gene therapy has been approved by EMA in 2012. Alipogene tiparvovec is administered through intramuscular injections of an adeno-associated virus vector [80]. This strategy is intended to treat rare patients with LPL deficiency and severe or multiple pancreatitis attacks, despite dietary fat restrictions.

New Drugs Targeting HDL Metabolism

Although HDL-C levels are strongly inversely associated with ASCVD risk, the causal role of HDL-C is uncertain and is complicated by the inverse relationship between HDL-C and TG levels. Moreover, human genetic studies have led to major doubts about the causality of HDL-C levels in atherosclerosis [81, 82]. However, HDL-targeted therapies need to be explored as strategies that aim to normalize HDL function [83]. Indeed, recent epidemiological studies have shown that HDL cholesterol efflux capacity predicts ASCVD risk, independently of HDL-C levels [84], and it remains of interest to evaluate new strategies modulating HDL metabolism.

CETP Inhibitors

Cholesteryl ester transfer protein (CETP) transfers cholesteryl esters from HDL to proatherogenic lipoproteins. Even if some human genetic studies have concluded that CETP gene polymorphisms associated with decreased CETP activity are accompanied by a lower ASCVD risk, all the genetic variations at the CETP gene locus are not conclusive in terms of CV risk [85•] and three different CETP inhibitors—torcetrapib, dalcetrapib, and evacetrapib—have failed in large clinical outcome trials. Two other CETP inhibitors are still in development (anacetrapib and TA-8995). These two drugs induce large significant increases in HDL-C and ApoA-I and decreases in LDL-C (up to 45 % with TA-8995) [86, 87]. However, the future of CETP inhibition will depend on the results of the ongoing REVEAL trial in which >30,000 high-risk patients have been randomized to receive anacetrapib or placebo [88].

HDL Infusion Agents

Approaches to promote cholesterol efflux could be the infusion of reconstituted HDL (rHDL) containing ApoA-I and phospholipids. Indeed, in 2003, a small clinical trial has demonstrated that infusions of ApoA-I Milano (ETC-216) induced a modest reduction in atheroma volume [89]. Three rHDL are currently in clinical development MDCO-216 (originally ETC-216), CSL-112, and CER-001 with some published data reporting a positive role on reverse cholesterol transport and carotid atherosclerosis [90–92] and supporting further clinical investigations.

ApoA-I Upregulators

Another approach to promote cholesterol efflux from tissues is an increase of the endogenous production of ApoA-I by stimulating its gene transcription. The oral drug RVX-208 increased Apo-AI and HDL-C levels, and enhanced serum cholesterol efflux capacity [93]. However, in the ASSURE trial, RVX-208 treatment has not been associated with regression of coronary atherosclerosis evaluated by serial intravascular ultrasound imaging [94].

Strategies Targeting Lipoprotein(a)

The consistent association between elevated Lp(a) levels and increased ASCVD risk, together with genetic findings, indicates that elevated Lp(a) is a causal risk factor for ASCVD [95, 96]. By consequence, lowering Lp(a) is a new target in CV prevention [97]. Some new drugs previously described such as PCSK9 inhibitors, mipomersen, lomitapide, and CETP inhibitors also decreased Lp(a) usually by around 25–30 %, but the clinical significance of this effect is unknown. Highly specific and potent Lp(a) lowering strategy would provide the opportunity to demonstrate that lowering Lp(a) leads to a reduction in ASCVD.

Antisense Therapy Targeting Lp(a)

ASO ISIS-Apo(a)RX was designed to reduce the synthesis of apo(a) in the liver and consequently to decrease the plasma levels of Lp(a). In a phase I study, SC injections of ISIS-Apo(a)RX (100–300 mg) resulted in dose-dependent, selective reductions of plasma Lp(a) up to 78 % in the 300-mg group [98••].

Conclusion

Clinical trials and genetic studies have demonstrated the cause role of LDL in the development of ASCVD. Although statins represent the cornerstone of dyslipidemia management, ezetimibe is also effective to reduce CV risk. However, the need for complementary agents reducting LDL-C remains important, especially for patients unable to achieve optimal LDL-C on current available therapies and for patients unable to tolerate efficacious statins. Among the new LDL-lowering drugs, PCSK9 inhibitors are certainly the most promising class. The ongoing CV endpoint trials using PCSK9 monoclonal antibodies shall be crucial not only in terms of CV prevention but also to appreciate the cost-effectiveness of this approach.

Recent genetic studies also support a causal role of TG-rich lipoproteins in the risk of ASCVD. Consequently, new clinical endpoint trials with either newer generation of omega-3 FA or PPAR-α agonist have been designed to evaluate the CV benefit in patients with elevated triglycerides on maximally tolerated statin therapy. Moreover, genetic studies have identified new strategies targeting TG such as apoC3 inhibitors. The development of new drugs increasing HDL-C has been disappointing, and the future of CETP inhibitors is linked to the results of the ongoing REVEAL trial. Other strategies to modulate HDL metabolism are at the early stage of development. Finally, promising preliminary data to reduce Lp(a) have been published with specific oligonucleotides blocking the synthesis of apo(a).