Anti-PCSK9 therapies for the treatment of hypercholesterolemia

Targeting PCSK9 to tackle cardiovascular disease

Lowering blood cholesterol levels efficiently reduces the risk of developing atherosclerotic cardiovascular disease (ASCVD), including coronary artery disease (CAD), which is the main cause of death worldwide. CAD is caused by plaque formation, comprising cholesterol deposits in the coronary arteries. Proprotein convertase subtilisin kexin/type 9 (PCSK9) was discovered in the early 2000s and later identified as a key regulator of cholesterol metabolism. PCSK9 induces lysosomal degradation of the low-density lipoprotein (LDL) receptor in the liver, which is responsible for clearing LDL-cholesterol (LDL-C) from the circulation. Accordingly, gain-of-function PCSK9 mutations are causative of familial hypercholesterolemia, a severe condition with extremely high plasma cholesterol levels and increased ASCVD risk, whereas loss-of-function PCSK9 mutations are associated with very low LDL-C levels and protection against CAD. Since the discovery of PCSK9, extensive investigations in developing PCSK9 targeting therapies have been performed. The combined delineation of clear biology, genetic risk variants, and PCSK9 crystal structures have been major drivers in developing antagonistic molecules. Today, two antibody-based PCSK9 inhibitors have successfully progressed to clinical application and shown to be effective in reducing cholesterol levels and mitigating the risk of ASCVD events, including myocardial infarction, stroke, and death, without any major adverse effects. A third siRNA-based inhibitor has been FDA-approved but awaits cardiovascular outcome data. In this review, we outline the PCSK9 biology, focusing on the structure and nonsynonymous mutations reported in the PCSK9 gene and elaborate on PCSK9-lowering strategies under development. Finally, we discuss future perspectives with PCSK9 inhibition in other severe disorders beyond cardiovascular disease.

[New Lipid-lowering Agents]

Atherosclerotic cardiovascular disease (ASCVD) remains a leading cause of morbidity and mortality. The fact that elevated levels of low-density lipoprotein-cholesterol (LDL-C) play a causal role in the development of ASCVD is now well accepted, given the results of numerous epidemiological and genetic studies, as well as randomized controlled clinical trials. Statins have become a primary therapeutic cornerstone in ASCVD prevention since they have been shown to reduce CV events by reducing levels of LDL-C. But despite the proven efficacy and safety of statin therapy, several aspects indicate a substantial need for additional or alternative LDL-C lowering therapies. These aspects include not only a high variability in individual response to therapy, but also possible side effects, potentially reducing adherence to treatment. Most importantly, an elevated risk for cardiovascular (CV) events remains in a large proportion of high-risk patients, especially in those with persistent elevation of LDL-C levels despite a maximum tolerated dose of statin therapy. Also, large clinical trials currently investigate a potential CV benefit of drug therapies targeting elevated levels of triglycerides and lipoprotein (a), respectively.

Low-density Lipoprotein-Cholesterol Lowering Strategies for Prevention of Atherosclerotic Cardiovascular Disease: Focus on siRNA Treatment Targeting PCSK9 (Inclisiran)

Purpose of Review

The aim of low-density lipoprotein-cholesterol (LDL-C) lowering therapies is to safely achieve a consistent and long-term reduction in exposure of the vasculature to atherogenic lipoproteins in order to reduce the risk of atherosclerotic cardiovascular (CV) disease and the associated CV events, such as myocardial infarctions and ischemic strokes. This review summarizes the concept and clinical development of a novel molecular approach to efficiently lower LDL-C, a synthetic small interfering ribonucleic acid (siRNA)—inclisiran—directed against proprotein convertase subtilisin-kexin type 9 (PCSK9).

Recent Findings

The understanding of genes regulating atherogenic lipoproteins and their causal role in the development of atherosclerotic CV disease has substantially advanced over the past years. This has opened the possibility for development of molecular therapies targeting these atherogenic lipoproteins, in particular by RNA-targeted treatment approaches. The most advanced clinical development program is the siRNA-treatment targeting PCSK9 (inclisiran), involving more than 4000 patients in clinical studies. Phase 1 and 2 studies have identified the dose of 300 mg inclisiran for efficient LDL-C lowering. Most recently, three phase 3 studies demonstrated that a regimen of inclisiran every 6 months was feasible and reduced LDL-C by approximately 50% in patients at high or very high CV risk or with familial hypercholesterolemia. Adverse events were similar in the inclisiran and the placebo groups, except for more frequent transient injection site reactions with inclisiran than with placebo.

Summary

siRNA therapy targeting PCSK9 (inclisiran) applied twice a year efficiently reduced LDL-C by approximately 50% and was safe in recent phase 3 studies. The effects of this treatment on CV outcome are currently further assessed in a large ongoing CV outcome trial.

Advances in oligonucleotide drug delivery

Oligonucleotides can be used to modulate gene expression via a range of processes including RNAi, target degradation by RNase H-mediated cleavage, splicing modulation, non-coding RNA inhibition, gene activation and programmed gene editing. As such, these molecules have potential therapeutic applications for myriad indications, with several oligonucleotide drugs recently gaining approval. However, despite recent technological advances, achieving efficient oligonucleotide delivery, particularly to extrahepatic tissues, remains a major translational limitation. Here, we provide an overview of oligonucleotide-based drug platforms, focusing on key approaches - including chemical modification, bioconjugation and the use of nanocarriers - which aim to address the delivery challenge.

PCSK9: A Key Target for the Treatment of Cardiovascular Disease (CVD)

Proprotein convertase subtilisin/kexin type 9 (PCSK9), as a vital modulator of low-density lipoprotein cholesterol (LDL-C) , is raised in hepatocytes and released into plasma where it binds to LDL receptors (LDLR), leading to their cleavage. PCSK9 adheres to the epidermal growth factor-like repeat A (EGF-A) domain of the LDLR which is confirmed by crystallography. LDLR expression is adjusted at the transcriptional level through sterol regulatory element binding protein 2 (SREBP-2) and at the post translational stages, specifically through PCSK9, and the inducible degrader of the LDLR PCSK9 inhibition is an appealing new method for reducing the concentration of LDL-C. In this review the role of PCSK9 in lipid homeostasis was elucidated, the effect of PCSK9 on atherosclerosis was highlighted, and contemporary therapeutic techniques that focused on PCSK9 were summarized. Several restoration methods to inhibit PCSK9 have been proposed which concentrate on both extracellular and intracellular PCSK9, and they include blockage of PCSK9 production by using gene silencing agents and blockage of it's binding to LDLR through antibodies and inhibition of PCSK9 autocatalytic processes by tiny molecule inhibitors.

RNA Therapy: Current Status and Future Potential

Recent studies identified diverse RNAs including noncoding RNAs and their various action mechanisms in the cells. These RNAs regulate a variety of cellular pathways and are therefore expected to be important targets for the treatment of human diseases. Along with their extensive functional studies, RNA-based therapeutic techniques have developed considerably in recent years. After years of research and various trial and error, antisense RNAs and small interfering RNAs-based drugs have been developed and are now being used in the clinic. In addition, active research is ongoing to develop drugs based on RNA aptamer and messenger RNA. Along with the development of these RNA-based drugs, diverse strategies have been developed to transport RNA drugs into the cells efficiently. RNA therapy has many advantages over existing small molecule or monoclonal antibody-based therapies, including its potential to target all genes in the cells. This review will introduce the history of RNA therapy, and explain the basic concepts of RNA therapy and RNA-based drugs on the market or clinical trials. In addition, the future potential of RNA therapy will be discussed.

Comparative quantitative systems pharmacology modeling of anti-PCSK9 therapeutic modalities in hypercholesterolemia

Since the discovery of proprotein convertase subtilisin/kexin type 9 (PCSK9) as an attractive target in the treatment of hypercholesterolemia, multiple anti-PCSK9 therapeutic modalities have been pursued in drug development. The objective of this research is to set the stage for the quantitative benchmarking of two anti-PCSK9 pharmacological modality classes, monoclonal antibodies (mAbs) and small interfering RNA (siRNA). To this end, we developed an integrative mathematical model of lipoprotein homeostasis describing the dynamic interplay between PCSK9, LDL-cholesterol (LDL-C), VLDL-cholesterol, HDL-cholesterol (HDL-C), apoB, lipoprotein a [Lp(a)], and triglycerides (TGs). We demonstrate that LDL-C decreased proportionally to PCSK9 reduction for both mAb and siRNA modalities. At marketed doses, however, treatment with mAbs resulted in an additional ∼20% LDL-C reduction compared with siRNA. We further used the model as an evaluation tool and determined that no quantitative differences were observed in HDL-C, Lp(a), TG, or apoB responses, suggesting that the disruption of PCSK9 synthesis would provide no additional effects on lipoprotein-related biomarkers in the patient segment investigated. Predictive model simulations further indicate that siRNA therapies may reach reductions in LDL-C levels comparable to those achieved with mAbs if the current threshold of 80% PCSK9 inhibition via siRNA could be overcome.

Effect of disease-associated SLC9A9 mutations on protein-protein interaction networks: implications for molecular mechanisms for ADHD and autism

Na⁺/H⁺ Exchanger 9 (NHE9) is an endosomal membrane protein encoded by the Solute Carrier 9A, member 9 gene (SLC9A9). SLC9A9 has been implicated in attention deficit hyperactivity disorder (ADHD), autism spectrum disorders (ASDs), epilepsy, multiple sclerosis and cancers. To better understand the function of NHE9 and the effects of disease-associated variants on protein-protein interactions, we conducted a quantitative analysis of the NHE9 interactome using co-immunoprecipitation and isobaric labeling-based quantitative mass spectrometry. We identified 100 proteins that interact with NHE9. These proteins were enriched in known functional pathways for NHE9: the endocytosis, protein ubiquitination and phagosome pathways, as well as some novel pathways including oxidative stress, mitochondrial dysfunction, mTOR signaling, cell death and RNA processing pathways. An ADHD-associated mutation (A409P) significantly altered NHE9's interactions with a subset of proteins involved in caveolae-mediated endocytosis and MAP2K2-mediated downstream signaling. An ASD nonsense mutation in SLC9A9, R423X, produced no-detectable amount of NHE9, suggesting the overall loss of NHE9 functional networks. In addition, seven of the NHE9 interactors are products of known autism candidate genes (Simons Foundation Autism Research Initiative, SFARI Gene) and 90% of the NHE9 interactome overlap with SFARI protein interaction network PIN (p < 0.0001), supporting the role of NHE9 interactome in ASDs molecular mechanisms. Our results provide a detailed understanding of the functions of protein NHE9 and its disrupted interactions, possibly underlying ADHD and ASDs. Furthermore, our methodological framework proved useful for functional characterization of disease-associated genetic variants and suggestion of druggable targets.

Novel fatty acyl apoE mimetic peptides have increased potency to reduce plasma cholesterol in mice and macaques

Ac-hE18A-NH₂ is a dual-domain apoE mimetic peptide that possesses the putative receptor binding domain from apoE (LRKLRKRLLR, denoted hE; residues 141–150) covalently attached to lipid-associating peptide 18A. Like apoE, Ac-hE18A-NH₂ reduces plasma cholesterol in animal models and exhibits anti-inflammatory properties independent of its cholesterol-reducing effect. Ac-hE18A-NH₂ has already undergone phase I clinical trials as a lipid-lowering agent. To explore the therapeutic potential more, we designed and synthesized new analogues by linking ɑ-aminohexanoic acid, octanoic acid, or myristic acid to LRRLRRRLLR-18A-NH₂ ([R]hE18A-NH₂) and examined the cholesterol-lowering potency in animals. The modified peptides effectively reduced plasma cholesterol in apoE-null mice fed standard chow or a Western diet; the myristyl analogue was the most effective. A single administration of the myristyl analogue reduced plasma total and LDL cholesterol in a dose-dependent manner in hypercholesterolemic cynomolgus macaques for up to 1 week despite the continuation of a cholesterol-supplemented diet. The myristyl peptide (7.4 mg/kg) reduced total and LDL cholesterol at 24 h by 64% and 74%, respectively; plasma HDL levels were modestly reduced and returned to baseline by day 7. These new analogues should exhibit enhanced potency at lower doses than Ac-hE18A-NH₂, which may make them attractive therapeutic candidates for clinical trials.

Methylomic profiling and replication implicates deregulation of PCSK9 in alcohol use disorder

Alcohol use disorder (AUD) is a common and chronic disorder with substantial effects on personal and public health. The underlying pathophysiology is poorly understood but strong evidence suggests significant roles of both genetic and epigenetic components. Given that alcohol affects many organ systems, we performed a cross-tissue and cross-phenotypic analysis of genome-wide methylomic variation in AUD using samples from 3 discovery, 4 replication, and 2 translational cohorts. We identified a differentially methylated region in the promoter of the proprotein convertase subtilisin/kexin 9 (PCSK9) gene that was associated with disease phenotypes. Biological validation showed that PCSK9 promoter methylation is conserved across tissues and positively correlated with expression. Replication in AUD datasets confirmed PCSK9 hypomethylation and a translational mouse model of AUD showed that alcohol exposure leads to PCSK9 downregulation. PCSK9 is primarily expressed in the liver and regulates low-density lipoprotein cholesterol (LDL-C). Our finding of alcohol-induced epigenetic regulation of PCSK9 represents one of the underlying mechanisms between the well-known effects of alcohol on lipid metabolism and cardiovascular risk, with light alcohol use generally being protective while chronic heavy use has detrimental health outcomes.

MicroRNAs: New Therapeutic Targets for Familial Hypercholesterolemia?

Familial hypercholesterolemia (FH) is the most common inherited form of dyslipidemia and a major cause of premature cardiovascular disease. Management of FH mainly relies on the efficiency of treatments that reduce plasma low-density lipoprotein (LDL) cholesterol (LDL-C) concentrations. MicroRNAs (miRs) have been suggested as emerging regulators of plasma LDL-C concentrations. Notably, there is evidence showing that miRs can regulate the post-transcriptional expression of genes involved in the pathogenesis of FH, including LDLR, APOB, PCSK9, and LDLRAP1. In addition, many miRs are located in genomic loci associated with abnormal levels of circulating lipids and lipoproteins in human plasma. The strong regulatory effects of miRs on the expression of FH-associated genes support of the notion that manipulation of miRs might serve as a potential novel therapeutic approach. The present review describes miRs-targeting FH-associated genes that could be used as potential therapeutic targets in patients with FH or other severe dyslipidemias.

Loss-of-function PCSK9 mutants evade the unfolded protein response sensor GRP78 and fail to induce endoplasmic reticulum stress when retained

The proprotein convertase subtilisin/kexin type-9 (PCSK9) plays a central role in cardiovascular disease (CVD) by degrading hepatic low-density lipoprotein receptor (LDLR). As such, loss-of-function (LOF) PCSK9 variants that fail to exit the endoplasmic reticulum (ER) increase hepatic LDLR levels and lower the risk of developing CVD. The retention of misfolded protein in the ER can cause ER stress and activate the unfolded protein response (UPR). In this study, we investigated whether a variety of LOF PCSK9 variants that are retained in the ER can cause ER stress and hepatic cytotoxicity. Although overexpression of these PCSK9 variants caused an accumulation in the ER of hepatocytes, UPR activation or apoptosis was not observed. Furthermore, ER retention of endogenous PCSK9 via splice switching also failed to induce the UPR. Consistent with these in vitro studies, overexpression of PCSK9 in the livers of mice had no impact on UPR activation. To elucidate the cellular mechanism to explain these surprising findings, we observed that the 94-kDa glucose-regulated protein (GRP94) sequesters PCSK9 away from the 78-kDa glucose-regulated protein (GRP78), the major activator of the UPR. As a result, GRP94 knockdown increased the stability of GRP78-PCSK9 complex and resulted in UPR activation following overexpression of ER-retained PCSK9 variants relative to WT secreted controls. Given that overexpression of these LOF PCSK9 variants does not cause UPR activation under normal homeostatic conditions, therapeutic strategies aimed at blocking the autocatalytic cleavage of PCSK9 in the ER represent a viable strategy for reducing circulating PCSK9.

Cell-based, bioluminescent assay for monitoring the interaction between PCSK9 and the LDL receptor

Monitoring the expression of cell-surface receptors, their interaction with extracellular ligands, and their fate upon ligand binding is important for understanding receptor function and developing new therapies. We describe a cell-based method that utilizes bioluminescent protein complementation technology to interrogate binding of a cellular receptor with its extracellular protein ligand, specifically LDL receptor (LDLR) and proprotein convertase subtilisin/kexin type 9 (PCSK9). Purified, full-length tagged PCSK9 is added to assay wells containing cells that stably express LDLR with an extracellular complementary tag. When the tagged PCSK9 binds the receptor, a bright luminescence signal is generated. The interaction is detected at the cell membrane with add-and-read simplicity, no wash steps, and flexibility, allowing data to be collected in endpoint format, kinetically, or with bioluminescent imaging. The assay is flexible, is rapid, and reports accurate biology. It is amenable to 96-well and 384-well formats, and the robustness allows for screening of new drug candidates (Z′ = 0.83). The assay reports correct potencies for antibody titrations across a 50%–150% potency range and detects potency changes due to heat stress, suggesting that it may be useful during drug development. This assay technology can be broadly applied when studying other receptors with their extracellular ligands, whether protein or small-molecule binding partners.

Selection of DNA aptamers with two modified bases

Aptamers are now used ubiquitously as binding agents for a broad range of applications. Natural (unmodified) DNA and RNA aptamers have considerably less chemical diversity than protein-based ligands such as antibodies, limiting their utility. Aptamers possessing a single chemical modification have helped bridge this diversity gap. We report the selection and identification of aptamers with two diversity-enhancing chemical modifications that bind and inhibit proprotein convertase subtilisin/kexin type 9 (PCSK9), a representative human therapeutic protein target. The addition of a second modification, especially in certain pairwise combinations, resulted in significant improvements in affinity, ligand efficiency, epitope coverage, metabolic stability, and inhibitory activity. Extensively chemically functionalized aptamers have the potential to become the next generation of nucleic-acid–based ligands.

The nucleobases comprising DNA and RNA aptamers provide considerably less chemical diversity than protein-based ligands, limiting their versatility. The introduction of novel functional groups at just one of the four bases in modified aptamers has recently led to dramatic improvement in the success rate of identifying nucleic acid ligands to protein targets. Here we explore the benefits of additional enhancement in physicochemical diversity by selecting modified DNA aptamers that contain amino-acid–like modifications on both pyrimidine bases. Using proprotein convertase subtilisin/kexin type 9 as a representative protein target, we identify specific pairwise combinations of modifications that result in higher affinity, metabolic stability, and inhibitory potency compared with aptamers with single modifications. Such doubly modified aptamers are also more likely to be encoded in shorter sequences and occupy nonoverlapping epitopes more frequently than aptamers with single modifications. These highly modified DNA aptamers have broad utility in research, diagnostic, and therapeutic applications.

PCSK9 inhibitors: A new era of lipid lowering therapy

Hyperlipidemia is a well-established risk factor for developing cardiovascular disease (CVD). The recent American College of Cardiology and American Heart Association guidelines on lipid management emphasize treatment of individuals at increased risk for developing CVD events with 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) at doses proven to reduce CVD events. However, there are limited options for patients who are either intolerant to statin therapy, develop CVD despite being on maximally tolerated statin therapy, or have severe hypercholesterolemia. Recently the Food and Drug Administration approved two novel medications for low-density lipoprotein (LDL)-cholesterol reduction: Evolocumab and Alirocumab. These agents target and inactivate proprotein convertase subtilsin-kexin type 9 (PCSK9), a hepatic protease that attaches and internalizes LDL receptors into lysosomes hence promoting their destruction. By preventing LDL receptor destruction, LDL-C levels can be lowered 50%-60% above that achieved by statin therapy alone. This review explores PCSK-9 biology and the mechanisms available to alter it; clinical trials targeting PCSK9 activity, and the current state of clinically available inhibitors of PCSK9.

A Highly Durable RNAi Therapeutic Inhibitor of PCSK9

BACKGROUND

Inclisiran (ALN-PCSsc) is a long-acting RNA interference (RNAi) therapeutic agent that inhibits the synthesis of proprotein convertase subtilisin-kexin type 9 (PCSK9), a target for the lowering of low-density lipoprotein (LDL) cholesterol.

METHODS

In this phase 1 trial, we randomly assigned healthy volunteers with an LDL cholesterol level of at least 100 mg per deciliter in a 3:1 ratio to receive a subcutaneous injection of inclisiran or placebo in either a single-ascending-dose phase (at a dose of 25, 100, 300, 500, or 800 mg) or a multiple-dose phase (125 mg weekly for four doses, 250 mg every other week for two doses, or 300 or 500 mg monthly for two doses, with or without concurrent statin therapy); each dose cohort included four to eight participants. Safety, the side-effect profile, and pharmacodynamic measures (PCSK9 level, LDL cholesterol level, and exploratory lipid variables) were evaluated.

RESULTS

The most common adverse events were cough, musculoskeletal pain, nasopharyngitis, headache, back pain, and diarrhea. All the adverse events were mild or moderate in severity. There were no serious adverse events or discontinuations due to adverse events. There was one grade 3 elevation in the γ-glutamyltransferase level, which was considered by the investigator to be related to statin therapy. In the single-dose phase, inclisiran doses of 300 mg or more reduced the PCSK9 level (up to a least-squares mean reduction of 74.5% from baseline to day 84), and doses of 100 mg or more reduced the LDL cholesterol level (up to a least-squares mean reduction of 50.6% from baseline). Reductions in the levels of PCSK9 and LDL cholesterol were maintained at day 180 for doses of 300 mg or more. All multiple-dose regimens reduced the levels of PCSK9 (up to a least-squares mean reduction of 83.8% from baseline to day 84) and LDL cholesterol (up to a least-squares mean reduction of 59.7% from baseline to day 84).

CONCLUSIONS

In this phase 1 trial, no serious adverse events were observed with inclisiran. Doses of 300 mg or more (in single or multiple doses) significantly reduced levels of PCSK9 and LDL cholesterol for at least 6 months. (Funded by Alnylam Pharmaceuticals and the Medicines Company; ClinicalTrials.gov number, NCT02314442 .).

Genetic architecture for human aggression: A study of gene-phenotype relationship in OMIM

Genetic studies of human aggression have mainly focused on known candidate genes and pathways regulating serotonin and dopamine signaling and hormonal functions. These studies have taught us much about the genetics of human aggression, but no genetic locus has yet achieved genome-significance. We here present a review based on a paradoxical hypothesis that studies of rare, functional genetic variations can lead to a better understanding of the molecular mechanisms underlying complex multifactorial disorders such as aggression. We examined all aggression phenotypes catalogued in Online Mendelian Inheritance in Man (OMIM), an Online Catalog of Human Genes and Genetic Disorders. We identified 95 human disorders that have documented aggressive symptoms in at least one individual with a well-defined genetic variant. Altogether, we retrieved 86 causal genes. Although most of these genes had not been implicated in human aggression by previous studies, the most significantly enriched canonical pathways had been previously implicated in aggression (e.g., serotonin and dopamine signaling). Our findings provide strong evidence to support the causal role of these pathways in the pathogenesis of aggression. In addition, the novel genes and pathways we identified suggest additional mechanisms underlying the origins of human aggression. Genome-wide association studies with very large samples will be needed to determine if common variants in these genes are risk factors for aggression. © 2015 Wiley Periodicals, Inc.

[An updated overview of the high intensity lipid lowering therapy in high cardiovascular risk patients]

Statins are highly effective drugs to decrease the plasma concentrations of atherogenic lipoproteins and prevent cardiovascular disease. The clinical practice guidelines recommend the use of high-intensity statins to lower LDL-cholesterol by at least 50% in patients with CVD and those at high cardiovascular risk. The recommendations for the treatment of hypercholesterolaemia by the ACC/AHA have led to a paradigm shift in cardiovascular prevention. These recommendations have abandoned the therapeutic goals of LDL-cholesterol, and recommend the treatment with statins of high or moderate intensity in four high cardiovascular risk groups. These recommendations are different from the European guidelines on cardiovascular disease prevention, in which their objectives are still towards LDL-cholesterol. This paper reviews this controversy from different angles and from the perspective of the Spanish Interdisciplinary Committee for Cardiovascular Disease Prevention. Intervention studies with high intensity statins in primary prevention, in patients with acute coronary syndrome, and with stable ischaemic heart disease are also described. Likewise, treatment with statins of high intensity is addressed in terms of their effectiveness in cardiovascular prevention and in terms of their safety, with particular attention to muscle effects, as well as taking into account the pharmacological characteristics of the different statins and the increased safety of those with less potential for interactions. Finally, new agents are described for the treatment of hypercholesterolaemia, with special emphasis on anti-PCSK9 monoclonal antibodies, a new therapeutic group for the treatment of hypercholesterolaemia that will offer a huge progress in the prevention of cardiovascular diseases.

Evaluation of HDL-modulating interventions for cardiovascular risk reduction using a systems pharmacology approach

The recent failures of cholesteryl ester transport protein inhibitor drugs to decrease CVD risk, despite raising HDL cholesterol (HDL-C) levels, suggest that pharmacologic increases in HDL-C may not always reflect elevations in reverse cholesterol transport (RCT), the process by which HDL is believed to exert its beneficial effects. HDL-modulating therapies can affect HDL properties beyond total HDL-C, including particle numbers, size, and composition, and may contribute differently to RCT and CVD risk. The lack of validated easily measurable pharmacodynamic markers to link drug effects to RCT, and ultimately to CVD risk, complicates target and compound selection and evaluation. In this work, we use a systems pharmacology model to contextualize the roles of different HDL targets in cholesterol metabolism and provide quantitative links between HDL-related measurements and the associated changes in RCT rate to support target and compound evaluation in drug development. By quantifying the amount of cholesterol removed from the periphery over the short-term, our simulations show the potential for infused HDL to treat acute CVD. For the primary prevention of CVD, our analysis suggests that the induction of ApoA-I synthesis may be a more viable approach, due to the long-term increase in RCT rate.

Challenges and opportunities for non-antibody scaffold drugs

The first candidates from the promising class of small non-antibody protein scaffolds are now moving into clinical development and practice. Challenges remain, and scaffolds will need to be further tailored toward applications where they provide real advantages over established therapeutics to succeed in a rapidly evolving drug development landscape.

Recent advances in the understanding and care of familial hypercholesterolaemia: significance of the biology and therapeutic regulation of proprotein convertase subtilisin/kexin type 9

Familial hypercholesterolaemia (FH) is an autosomal co-dominant disorder that markedly raises plasma low-density lipoprotein-cholesterol (LDL-C) concentration, causing premature atherosclerotic coronary artery disease (CAD). FH has recently come under intense focus and, although there is general consensus in recent international guidelines regarding diagnosis and treatment, there is debate about the value of genetic studies. Genetic testing can be cost-effective as part of cascade screening in dedicated centres, but the full mutation spectrum responsible for FH has not been established in many populations, and its use in primary care is not at present logistically feasible. Whether using genetic testing or not, cholesterol screening of family members of index patients with an abnormally raised LDL-C must be used to determine the need for early treatment to prevent the development of CAD. The metabolic defects in FH extend beyond LDL, and may affect triacylglycerol-rich and high-density lipoproteins, lipoprotein(a) and oxidative stress. Achievement of the recommended targets for LDL-C with current treatments is difficult, but this may be resolved by new drug therapies. Lipoprotein apheresis remains an effective treatment for severe FH and, although expensive, it costs less than the two recently introduced orphan drugs (lomitapide and mipomersen) for homozygous FH. Recent advances in understanding of the biology of proprotein convertase subtilisin/kexin type 9 (PCSK9) have further elucidated the regulation of lipoprotein metabolism and led to new drugs for effectively treating hypercholesterolaemia in FH and related conditions, as well as for treating many patients with statin intolerance. The mechanisms of action of PCSK9 inhibitors on lipoprotein metabolism and atherosclerosis, as well as their impact on cardiovascular outcomes and cost-effectiveness, remain to be established.

High-fructose feeding promotes accelerated degradation of hepatic LDL receptor and hypercholesterolemia in hamsters via elevated circulating PCSK9 levels

BACKGROUND

High fructose diet (HFD) induces dyslipidemia and insulin resistance in experimental animals and humans with incomplete mechanistic understanding. By utilizing mice and hamsters as in vivo models, we investigated whether high fructose consumption affects serum PCSK9 and liver LDL receptor (LDLR) protein levels.

RESULTS

Feeding mice with an HFD increased serum cholesterol and reduced serum PCSK9 levels as compared with the mice fed a normal chow diet (NCD). In contrast to the inverse relationship in mice, serum PCSK9 and cholesterol levels were co-elevated in HFD-fed hamsters. Liver tissue analysis revealed that PCSK9 mRNA and protein levels were both reduced in mice and hamsters by HFD feeding, however, liver LDLR protein levels were markedly reduced by HFD in hamsters but not in mice. We further showed that circulating PCSK9 clearance rates were significantly lower in hamsters fed an HFD as compared with the hamsters fed NCD, providing additional evidence for the reduced hepatic LDLR function by HFD consumption. The majority of PCSK9 in hamster serum was detected as a 53 kDa N-terminus cleaved protein. By conducting in vitro studies, we demonstrate that this 53 kDa truncated hamster PCSK9 is functionally active in promoting hepatic LDLR degradation.

CONCLUSION

Our studies for the first time demonstrate that high fructose consumption increases serum PCSK9 concentrations and reduces liver LDLR protein levels in hyperlipidemic hamsters. The positive correlation between circulating cholesterol and PCSK9 and the reduction of liver LDLR protein in HFD-fed hamsters suggest that hamster is a better animal model than mouse to study the modulation of PCSK9/LDLR pathway by atherogenic diets.

[New agents for hypercholesterolemia]

An elevated proportion of high cardiovascular risk patients do not achieve the therapeutic c-LDL goals. This owes to physicians' inappropriate or insufficient use of cholesterol lowering medications or to patients' bad tolerance or therapeutic compliance. Another cause is an insufficient efficacy of current cholesterol lowering drugs including statins and ezetimibe. In addition, proprotein convertase subtilisin kexin type 9 inhibitors are a new cholesterol lowering medications showing safety and high efficacy to reduce c-LDL in numerous already performed or underway clinical trials, potentially allowing an optimal control of hypercholesterolemia in most patients. Agents inhibiting apolipoprotein B synthesis and microsomal transfer protein are also providing a new potential to decrease cholesterol in patients with severe hypercholesterolemia and in particular in homozygote familial hypercholesterolemia. Last, cholesteryl ester transfer protein inhibitors have shown powerful effects on c-HDL and c-LDL, although their efficacy in cardiovascular prevention and safety has not been demonstrated yet. We provide in this article an overview of the main characteristics of therapeutic agents for hypercholesterolemia, which have been recently approved or in an advanced research stage.

Anti-PCSK9 antibody pharmacokinetics and low-density lipoprotein-cholesterol pharmacodynamics in nonhuman primates are antigen affinity-dependent and exhibit limited sensitivity to neonatal Fc receptor-binding enhancement

Proprotein convertase subtilisin/kexin type 9 (PCSK9) has emerged as an attractive therapeutic target for cardiovascular disease. Monoclonal antibodies (mAbs) that bind PCSK9 and prevent PCSK9:low-density lipoprotein receptor complex formation reduce serum low-density lipoprotein-cholesterol (LDL-C) in vivo. PCSK9-mediated lysosomal degradation of bound mAb, however, dramatically reduces mAb exposure and limits duration of effect. Administration of high-affinity mAb1:PCSK9 complex (1:2) to mice resulted in significantly lower mAb1 exposure compared with mAb1 dosed alone in normal mice or in PCSK9 knockout mice lacking antigen. To identify mAb-binding characteristics that minimize lysosomal disposition, the pharmacokinetic behavior of four mAbs representing a diverse range of PCSK9-binding affinities at neutral (serum) and acidic (endosomal) pH was evaluated in cynomolgus monkeys. Results revealed an inverse correlation between affinity and both mAb exposure and duration of LDL-C lowering. High-affinity mAb1 exhibited the lowest exposure and shortest duration of action (6 days), whereas mAb2 displayed prolonged exposure and LDL-C reduction (51 days) as a consequence of lower affinity and pH-sensitive PCSK9 binding. mAbs with shorter endosomal PCSK9:mAb complex dissociation half-lives (<20 seconds) produced optimal exposure-response profiles. Interestingly, incorporation of previously reported Fc-region amino acid substitutions or novel loop-insertion peptides that enhance in vitro neonatal Fc receptor binding, led to only modest pharmacokinetic improvements for mAbs with pH-dependent PCSK9 binding, with only limited augmentation of pharmacodynamic activity relative to native mAbs. A pivotal role for PCSK9 in mAb clearance was demonstrated, more broadly suggesting that therapeutic mAb-binding characteristics require optimization based on target pharmacology.

A Mechanistic Systems Pharmacology Model for Prediction of LDL Cholesterol Lowering by PCSK9 Antagonism in Human Dyslipidemic Populations

PCSK9 is a promising target for the treatment of hyperlipidemia and cardiovascular disease. A Quantitative Systems Pharmacology model of the mechanisms of action of statin and anti-PCSK9 therapies was developed to predict low density lipoprotein (LDL) changes in response to anti-PCSK9 mAb for different treatment protocols and patient subpopulations. Mechanistic interactions and cross-regulation of LDL, LDL receptor, and PCSK9 were modeled, and numerous virtual subjects were developed and validated against clinical data. Simulations predict a slightly greater maximum percent reduction in LDL cholesterol (LDLc) when anti-PCSK9 is administered on statin background therapy compared to as a monotherapy. The difference results primarily from higher PCSK9 levels in patients on statin background. However, higher PCSK9 levels are also predicted to increase clearance of anti-PCSK9, resulting in a faster rebound of LDLc. Simulations of subjects with impaired LDL receptor (LDLR) function predict compromised anti-PCSK9 responses in patients such as homozygous familial hypercholesterolemics, whose functional LDLR is below 10% of normal.

PCSK9 inhibition fails to alter hepatic LDLR, circulating cholesterol, and atherosclerosis in the absence of ApoE

LDL cholesterol (LDL-C) contributes to coronary heart disease. Proprotein convertase subtilisin/kexin type 9 (PCSK9) increases LDL-C by inhibiting LDL-C clearance. The therapeutic potential for PCSK9 inhibitors is highlighted by the fact that PCSK9 loss-of-function carriers exhibit 15-30% lower circulating LDL-C and a disproportionately lower risk (47-88%) of experiencing a cardiovascular event. Here, we utilized pcsk9(-/-) mice and an anti-PCSK9 antibody to study the role of the LDL receptor (LDLR) and ApoE in PCSK9-mediated regulation of plasma cholesterol and atherosclerotic lesion development. We found that circulating cholesterol and atherosclerotic lesions were minimally modified in pcsk9(-/-) mice on either an LDLR- or ApoE-deficient background. Acute administration of an anti-PCSK9 antibody did not reduce circulating cholesterol in an ApoE-deficient background, but did reduce circulating cholesterol (-45%) and TGs (-36%) in APOE*3Leiden.cholesteryl ester transfer protein (CETP) mice, which contain mouse ApoE, human mutant APOE3*Leiden, and a functional LDLR. Chronic anti-PCSK9 antibody treatment in APOE*3Leiden.CETP mice resulted in a significant reduction in atherosclerotic lesion area (-91%) and reduced lesion complexity. Taken together, these results indicate that both LDLR and ApoE are required for PCSK9 inhibitor-mediated reductions in atherosclerosis, as both are needed to increase hepatic LDLR expression.

Quercetin-3-glucoside increases low-density lipoprotein receptor (LDLR) expression, attenuates proprotein convertase subtilisin/kexin 9 (PCSK9) secretion, and stimulates LDL uptake by Huh7 human hepatocytes in culture

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Quercetin-3-glucoside can increase LDLR expression by hepatocytes.

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Quercetin-3-glucoside can reduce PCSK9 secretion by hepatocytes.

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Quercetin-3-glucoside can down regulate sortilin expression.

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Quercetin-3-glucoside can increase LDL uptake by hepatocytes.

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Quercetin-3-glucoside is a potential anti-cholesterolemic agent.

Low-density lipoprotein receptor (LDLR) mediates hepatic clearance of plasma cholesterol; proprotein convertase subtilisin/kexin 9 (PCSK9) opposes this clearance by promoting LDLR degradation. The plant flavonoid quercetin-3-β-d-glucoside (Q3G) has been shown to reduce hypercholesterolemia in experimental animals. Here, we examined how it affects LDLR and PCSK9 expression as well as LDL uptake by human Huh7 hepatocytes. At low micromolar concentrations, Q3G increased LDLR expression, reduced PCSK9 secretion, and stimulated LDL uptake. It also diminished intracellular sortilin, a sorting receptor known to facilitate PCSK9 secretion. Thus, as an LDLR inducer and a PCSK9 anti-secretagogue, Q3G may represent an effective anti-cholesterolemic agent.

Lipoprotein apheresis and new therapies for severe familial hypercholesterolemia in adults and children

Familial hypercholesterolemia (FH), the most common and severe monogenic form of hypercholesterolemia, is an autosomal co-dominant disease characterized by an increased plasma low density lipoprotein (LDL)-cholesterol concentration and premature coronary heart disease (CHD). The clinical phenotype depends on the gene involved and severity of mutation (or mutations) present. Patients with homozygous or compound heterozygous FH have severe hypercholesterolemia (LDL-cholesterol >13 mmol/L) due to a gene dosing effect and without treatment have accelerated atherosclerotic CHD from birth, and frequently die of CHD before age 30. Cholesterol-lowering therapies have been shown to reduce both mortality and major adverse cardiovascular events in individuals with FH. Lipoprotein apheresis concomitant with lipid-lowering therapy is the treatment of choice for homozygous FH. This article describes the rationale and role of lipoprotein apheresis in the treatment of severe FH and outlines the recent advances in new pharmacotherapies for this condition.

Hypobetalipoproteinemia and abetalipoproteinemia

PURPOSE OF REVIEW

Several mutations in the apoB, proprotein convertase subtilisin/kexin type 9 (PCSK9), and MTP genes result in low or absent levels of apoB and LDL-cholesterol in plasma, which cause familial hypobetalipoproteinemia and abetalipoproteinemia. Mutations in the ANGPTL3 gene cause familial combined hypolipidemia. Clinical manifestations range from none to severe, debilitating, and life-threatening disorders. This review summarizes recent genetic, metabolic, and clinical findings and presents an update on management strategies.

RECENT FINDINGS

Cases of cirrhosis and hepatocellular carcinoma have now been identified in heterozygous familial hypobetalipoproteinemia probably because of decreased triglyceride transport capacity from the liver. ANGPTL3 mutations cause low levels of LDL-cholesterol and low HDL-cholesterol in compound heterozygotes and homozygous individuals, decrease reverse cholesterol transport, and lower glucose levels. The effect on atherosclerosis is unknown; however, severe fatty liver has been identified. Loss-of-function mutations in PCSK9 cause familial hypobetalipoproteinemia, which appears to lower risk for coronary artery disease and has no adverse sequelae. Phase III clinical trials are now underway examining the effect of PCSK9 inhibitors on cardiovascular events in combination with statin drugs.

SUMMARY

Mutations causing low LDL-cholesterol and apoB have provided insight into lipid metabolism, disease associations, and the basis for drug development to lower LDL-cholesterol in disorders causing high levels of cholesterol. Early diagnosis and treatment are necessary to prevent adverse sequelae from familial hypobetalipoproteinemia and abetalipoproteinemia.

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

BACKGROUND

Proprotein convertase subtilisin/kexin type 9 (PCSK9) binds to LDL receptors, leading to their degradation. Genetics studies have shown that loss-of-function mutations in PCSK9 result in reduced plasma LDL cholesterol and decreased risk of coronary heart disease. We aimed to investigate the safety and efficacy of ALN-PCS, a small interfering RNA that inhibits PCSK9 synthesis, in healthy volunteers with raised cholesterol who were not on lipid-lowering treatment.

METHODS

We did a randomised, single-blind, placebo-controlled, phase 1 dose-escalation study in healthy adult volunteers with serum LDL cholesterol of 3·00 mmol/L or higher. Participants were randomly assigned in a 3:1 ratio by computer algorithm to receive one dose of intravenous ALN-PCS (with doses ranging from 0·015 to 0·400 mg/kg) or placebo. The primary endpoint was safety and tolerability of ALN-PCS. Secondary endpoints were the pharmacokinetic characteristics of ALN-PCS and its pharmacodynamic effects on PCSK9 and LDL cholesterol. Study participants were masked to treatment assignment. Analysis was per protocol and we used ANCOVA to analyse pharmacodynamic endpoint data. This trial is registered with ClinicalTrials.gov, number NCT01437059.

FINDINGS

Of 32 participants, 24 were randomly allocated to receive a single dose of ALN-PCS (0·015 mg/kg [n=3], 0·045 mg/kg [n=3], 0·090 mg/kg [n=3], 0·150 mg/kg [n=3], 0·250 mg/kg [n=6], or 0·400 mg/kg [n=6]) and eight to placebo. The proportions of patients affected by treatment-emergent adverse events were similar in the ALN-PCS and placebo groups (19 [79%] vs seven [88%]). ALN-PCS was rapidly distributed, with peak concentration and area under the curve (0 to last measurement) increasing in a roughly dose-proportional way across the dose range tested. In the group given 0·400 mg/kg of ALN-PCS, treatment resulted in a mean 70% reduction in circulating PCSK9 plasma protein (p<0·0001) and a mean 40% reduction in LDL cholesterol from baseline relative to placebo (p<0·0001).

INTERPRETATION

Our results suggest that inhibition of PCSK9 synthesis by RNA interference (RNAi) provides a potentially safe mechanism to reduce LDL cholesterol concentration in healthy individuals with raised cholesterol. These results support the further assessment of ALN-PCS in patients with hypercholesterolaemia, including those being treated with statins. This study is the first to show an RNAi drug being used to affect a clinically validated endpoint (ie, LDL cholesterol) in human beings.

FUNDING

Alnylam Pharmaceuticals.

PCSK9: From discovery to therapeutic applications

The proprotein convertase subtilisin/kexin type 9 (PCSK9) regulates cholesterol metabolism mainly by targeting the low-density lipoprotein receptor (LDLR) for degradation in the liver. Gain-of-function mutations in PCSK9 are one of the genetic causes of autosomal dominant hypercholesterolaemia. Conversely, loss-of-function mutations are associated with lower concentrations of LDL cholesterol (LDL-C) and reduced coronary heart disease. As these loss-of-function mutations are not associated with apparent deleterious effects, PCSK9 inhibition is an attractive new strategy for lowering LDL-C concentration. Among the various approaches to PCSK9 inhibition, human data are only available for inhibition of PCSK9 binding to LDLR by monoclonal antibodies. In phase II studies, the two most advanced monoclonal antibodies in development (alirocumab and evolocumab) decreased atherogenic lipoproteins very effectively and were well tolerated. A dramatic decrease in LDL-C up to 70% can be obtained with the most efficacious doses. Efficacy has been evaluated so far in addition to statins in hypercholesterolaemic patients with or without familial hypercholesterolaemia, in patients with intolerance to statin therapy and in monotherapy. Large phase III programmes are ongoing to evaluate the long-term efficacy and safety of these very promising new agents.

Proprotein convertases subtilisin/kexin type 9, an enzyme turned escort protein: hepatic and extra hepatic functions

Proprotein Convertases Subtilisin/Kexin Type 9 (PCSK9) is a serine endoproteinase. Biosynthesized as a zymogen, it cleaves itself once, and then turns into an escort protein for transmembrane proteins, leading them into lysosomes for degradation. It is primarily produced and secreted by the liver. It attaches to the low-density lipoprotein receptor (LDLR) at the surface of hepatocytes and, after co-endocytosis, directs it into lysosomes where it is degraded. By downregulating LDLR, PCSK9 reduces hepatic clearance of LDL-cholesterol. Inborn or induced increase of this function causes hypercholesterolemia; its decrease causes hypocholesterolemia. This has been experimentally demonstrated ex vivo and in vivo, and corroborated by epidemiological studies associating PCSK9 genetic variations with plasma cholesterol levels. PCSK9 is now a proven target for inactivation in the treatment of hypercholesterolemia and associated atherosclerosis. However, it is still uncertain whether its severe or complete inactivation, combined with other predispositions, will be without undesirable side-effects. Some experimental data suggest that PCSK9 could contribute positively to the physiology of non-hepatic cells such as pancreatic islets β cells, adipocytes and macrophages, protecting them from excessive lipid uptake, in an endocrine, autocrine, or paracrine manner. Genetic variations that attenuate PCSK9 anti-LDLR activity are common in human populations. Their evolutionary significance still needs to be evaluated on the background of environmental pressures, such as infectious diseases, cold weather and famine, which have threatened survival and reproduction in the course of human prehistory and history.

PCSK9 and resistin at the crossroads of the atherogenic dyslipidemia

The atherogenic dyslipidemia is a pathophysiological lipid triad, composed of high triglycerides and low-density lipoprotein and low high-density lipoprotein. The dyslipidemia is highly prevalent in individuals who are obese, insulin resistant and those with Type 2 diabetes and is the major contributing factor to the high atherosclerotic cardiovascular disease risk in these subjects. The primary initiating event in atherogenic dyslipidemia development is the hepatic overproduction of very-low-density lipoprotein (VLDL). The intracellular and extracellular protein triggers of hepatic VLDL production were not known until the recent identification of the causal roles of PCSK9 and resistin. Both PCSK9 and resistin act in large part by targeting and reducing the hepatic degradation of VLDL apoB through distinctly different mechanisms. In the current review, we discuss both the individual roles and the interaction of these proteins in driving atherogenic dyslipidemia, and thus, atherosclerotic cardiovascular disease progression in humans. We further explore the important therapeutic implications of these findings.

Current Phase II proprotein convertase subtilisin/kexin 9 inhibitor therapies for dyslipidemia

INTRODUCTION

Reduction of plasma low-density lipoprotein (LDL) cholesterol concentration with statins reduces adverse cardiovascular outcomes. However, lack of efficacy and intolerance of statins in many patients requires alternative treatments. Currently available non-statin alternatives include bile acid sequestrants, the cholesterol absorption inhibitor ezetimibe, niacin-based preparations and fibrates; however, each of these has limitations. Newer agents for LDL cholesterol reduction include the cholesterol ester transfer protein inhibitors, the microsomal triglyceride transfer protein inhibitor lomitapide, the apolipoprotein B antisense oligonucleotide mipomersen and several molecules that inhibit or interfere with proprotein convertase subtilisin/kexin 9 (PCSK9).

AREAS COVERED

Among the various PCSK9 inhibitors, human data are available for monoclonal antibodies against PCSK9 of which the two most advanced are alirocumab (SAR236553/REGN727) and AMG 145. Phase II studies of these agents as monotherapy or in combination with statins have shown reductions of LDL cholesterol by > 70%, with acceptable safety and tolerability so far.

EXPERT OPINION

Despite their biochemical efficacy, clinical efficacy, reflected by reduction of cardiovascular end points, remains to be shown for two leading monoclonal antibodies against PSCK9. Other issues to be evaluated with these agents over the longer term include development of rare adverse effects and potential attenuation of efficacy.

PCSK9 inhibitors

PURPOSE OF REVIEW

To summarize the therapeutic strategies to inhibit PCSK9 and to describe the main results obtained in phase I and II trials with monoclonal antibodies targeting PCSK9.

RECENT FINDINGS

Among the various approaches for PCSK9 inhibition, human data are only available for inhibition of PCSK9 binding to LDL receptor by monoclonal antibodies. Promising preclinical studies have also been reported with other strategies, including inhibition of PCSK9 synthesis by gene silencing agents. The two most advanced monoclonal antibodies in development are SAR236553/REGN727 and AMG145. In phase II, these two monoclonal antibodies administered subcutaneously are well tolerated and effective to decrease atherogenic lipoproteins. A dramatic decrease in LDL cholesterol up to 70% can be obtained. The efficacy has been evaluated so far in addition to statins in hypercholesterolemic patients with or without familial hypercholesterolemia, in patients with intolerance to statin therapy and in monotherapy.

SUMMARY

The short-term efficacy, safety and tolerability of two monoclonal antibodies to PSCK9 have been demonstrated in several phase II trials. These PCSK9 inhibitors are now tested in larger phase III studies to provide insights into the long-term safety and clinical efficacy of this very promising approach.