PCSK9: an emerging target for treatment of hypercholesterolemia

Unusual effects of PCSK9 E670G (rs505151) variation in patients with in-stent restenosis: Variable effects on restenosis risk according to concomitant chronic conditions

Recent reports showing that neo-atherosclerosis formation in stented coronary artery is characterized by the accumulation of lipid-laden macrophages within the neointima has strengthened the possibility that elevated low-density lipoprotein (LDL)-cholesterol may be a risk factor for in-stent restenosis (ISR). Protein Convertase Subtilisin/Kexin-9 (PCSK9) protein plays an important role in cholesterol metabolism by degrading of LDL receptors. The gain-of-function E670G (rs505151) mutation of the PCSK9 gene is a well-known genetic risk factor for hypercholesterolemia. This study evaluated for the first time the association of the E670G variation with the serum lipids, PCSK9 levels and concomitant diseases on the ISR risk. The study included 109 ISR, and 82 Non-ISR patients, based on the results of coronary angiography. Genotypes were determined using the real-time PCR and serum PCSK9 levels were measured by ELISA technique. The rare G allele of PCSK9 E670G (p < 0.05), hyperlipidemia (HL) (p < 0.001), and type 2 diabetes (T2DM) (p < 0.01) were associated with increased risk for ISR. In hyperlipidemic conditions, the E670G-G allele was associated with hypercholesterolemia and a higher risk of ISR (p < 0.001), while the E670G-AA genotype has been associated with a high prevalence of T2DM and hypertension. In addition, diabetic ISRs had higher serum PCSK9 levels (p < 0.05) and the E670G-AA genotype was associated with increased levels of diabetes markers. Our results indicated that the unusual effects of both G allele and AA genotype of the PCSK9 E670G variation may be involved in the risk of ISR in association with concomitant metabolic diseases.

PCSK9-D374Y Suppresses Hepatocyte Migration through Downregulating Free Cholesterol Efflux Rate and Activity of Extracellular Signal-Regulated Kinase

Proprotein convertase subtilisin/kexin type 9 can mediate the intracellular lysosomal degradation of the low-density lipoprotein receptor protein in hepatocytes and decrease the liver's ability to scavenge low-density lipoprotein cholesterol from circulation, resulting in high levels of cholesterol in the circulatory system. Current studies have primarily focused on the relationship between PCSK9 and blood lipid metabolism; however, the biological function of PCSK9 in hepatocytes is rarely addressed. In this study, we evaluate its effects in the human hepatoma carcinoma cell line HepG2, including proliferation, migration, and free cholesterol transport. PCSK9-D374Y is a gain-of-function mutation that does not affect proliferation but significantly suppresses the migration and cholesterol efflux capacity of these cells. The suppression of the transmembrane outflow of intracellular-free cholesterol regulates small G proteins and the suppression of extracellular signal-regulated kinase. In summary, PCSK9-D374Y affects hepatocyte features, including their migration and free cholesterol transport capabilities.

PCSK9 Gene Participates in the Development of Primary Dyslipidemias

Dyslipidemias are a group of diseases, which are characterized by abnormal blood concentrations of cholesterol, triglycerides and/or low-density lipoprotein-cholesterol (LDL-c). Dyslipidemia is a determinant condition for the progress of an atherosclerotic plaque formation. The resulting atherogenicity is due to at least two mechanisms: first, to the accumulation in the plasma of lipid particles that have the capacity to alter the function of the endothelium and deposit at the atheromatous plaque, and second, at an insufficient concentration of multifactorial type of high density lipoprotein-cholesterol (HDL-c), whose function is to protect against the development of atherosclerosis. Its highest prevalence is encountered among individuals with diabetes, hypertension or overweight. Hyperlipidemia is one of the main predisposing factors for the development of cardiovascular disease. Hyperlipidemia can be the result of a genetic condition, the secondary expression of a primary process or the consequence of exogenous factors (food, cultural, socio-economic, etc.), all of which lead to the elevation of plasma lipid levels. The objective of this study was to carry out an analysis of the genes involved in the development of dyslipidemias that lead to cardiovascular disease with special emphasis on the proprotein convertase subtilin/kexin type 9 (PCSK9) gene. The PCSK9 gene participates in the development of primary dyslipidemias, mainly familial hypercholesterolemia, currently the pharmacological treatment of choice to reduce LDL-c are statins, however, it has been observed that these have been insufficient to eliminate cardiovascular risk, especially in subjects with primary forms of hypercholesterolemia related to genetic mutations, or statin intolerance.

Fermented Rhus Verniciflua Stokes Extract Alleviates Nonalcoholic Fatty Liver through the AMPK/SREBP1/PCSK9 Pathway in HFD-Induced Nonalcoholic Fatty Liver Animal Model

Background: We have previously reported the anti-hepatic lipogenic effect of fermented Rhus verniciflua stokes extract (FRVE) in an oleic-acid-treated HepG2 cell model. Methods: Herein, we advanced our understanding and evaluated the impact of FRVE in HFD-fed C57BL/6 mice using an animal model of nonalcoholic fatty liver disease (NAFLD). Milk thistle extract was used as a positive control to compare the effects of FRVE. Results: FRVE decreased body weight, intra-abdominal fat weight, and liver weight. Furthermore, FRVE decreased HFD-induced elevated serum levels of ALT, AST, TC, and TG, and downregulated the increase in hepatic lipid accumulation and TG levels. FRVE reduced hepatic SREBP-1, PCSK-9, SREBP-2, and ApoB mRNA levels. IHC data showed that FRVE reduced the levels of nucleic SREBP-1, increased the levels of LDLR, and upregulated the expression of p-AMPK. Conclusion: Overall, these results demonstrate the anti-hepatic lipidemic effect of FRVE in an animal model. These findings are consistent with our previous study and strongly suggest that FRVE exerts anti-hepatic lipogenic effects by activating AMPK.

The influence of rare variants in circulating metabolic biomarkers

Circulating metabolite levels are biomarkers for cardiovascular disease (CVD). Here we studied, association of rare variants and 226 serum lipoproteins, lipids and amino acids in 7,142 (discovery plus follow-up) healthy participants. We leveraged the information from multiple metabolite measurements on the same participants to improve discovery in rare variant association analyses for gene-based and gene-set tests by incorporating correlated metabolites as covariates in the validation stage. Gene-based analysis corrected for the effective number of tests performed, confirmed established associations at APOB, APOC3, PAH, HAL and PCSK (p<1.32x10-7) and identified novel gene-trait associations at a lower stringency threshold with ACSL1, MYCN, FBXO36 and B4GALNT3 (p<2.5x10-6). Regulation of the pyruvate dehydrogenase (PDH) complex was associated for the first time, in gene-set analyses also corrected for effective number of tests, with IDL and LDL parameters, as well as circulating cholesterol (pMETASKAT<2.41x10-6). In conclusion, using an approach that leverages metabolite measurements obtained in the same participants, we identified novel loci and pathways involved in the regulation of these important metabolic biomarkers. As large-scale biobanks continue to amass sequencing and phenotypic information, analytical approaches such as ours will be useful to fully exploit the copious amounts of biological data generated in these efforts.

In vivo genome and base editing of a human PCSK9 knock-in hypercholesterolemic mouse model

Background

Plasma concentration of low-density lipoprotein (LDL) cholesterol is a well-established risk factor for cardiovascular disease. Inhibition of proprotein convertase subtilisin/kexin type 9 (PCSK9), which regulates cholesterol homeostasis, has recently emerged as an approach to reduce cholesterol levels. The development of humanized animal models is an important step to validate and study human drug targets, and use of genome and base editing has been proposed as a mean to target disease alleles.

Results

To address the lack of validated models to test the safety and efficacy of techniques to target human PCSK9, we generated a liver-specific human PCSK9 knock-in mouse model (hPCSK9-KI). We showed that plasma concentrations of total cholesterol were higher in hPCSK9-KI than in wildtype mice and increased with age. Treatment with evolocumab, a monoclonal antibody that targets human PCSK9, reduced cholesterol levels in hPCSK9-KI but not in wildtype mice, showing that the hypercholesterolemic phenotype was driven by overexpression of human PCSK9. CRISPR-Cas9-mediated genome editing of human PCSK9 reduced plasma levels of human and not mouse PCSK9, and in parallel reduced plasma concentrations of total cholesterol; genome editing of mouse Pcsk9 did not reduce cholesterol levels. Base editing using a guide RNA that targeted human and mouse PCSK9 reduced plasma levels of human and mouse PCSK9 and total cholesterol. In our mouse model, base editing was more precise than genome editing, and no off-target editing nor chromosomal translocations were identified.

Conclusions

Here, we describe a humanized mouse model with liver-specific expression of human PCSK9 and a human-like hypercholesterolemia phenotype, and demonstrate that this mouse can be used to evaluate antibody and gene editing-based (genome and base editing) therapies to modulate the expression of human PCSK9 and reduce cholesterol levels. We predict that this mouse model will be used in the future to understand the efficacy and safety of novel therapeutic approaches for hypercholesterolemia.

Electronic supplementary material

The online version of this article (10.1186/s12915-018-0624-2) contains supplementary material, which is available to authorized users.

In vivo genome and base editing of a human PCSK9 knock-in hypercholesterolemic mouse model

BACKGROUND

Plasma concentration of low-density lipoprotein (LDL) cholesterol is a well-established risk factor for cardiovascular disease. Inhibition of proprotein convertase subtilisin/kexin type 9 (PCSK9), which regulates cholesterol homeostasis, has recently emerged as an approach to reduce cholesterol levels. The development of humanized animal models is an important step to validate and study human drug targets, and use of genome and base editing has been proposed as a mean to target disease alleles.

RESULTS

To address the lack of validated models to test the safety and efficacy of techniques to target human PCSK9, we generated a liver-specific human PCSK9 knock-in mouse model (hPCSK9-KI). We showed that plasma concentrations of total cholesterol were higher in hPCSK9-KI than in wildtype mice and increased with age. Treatment with evolocumab, a monoclonal antibody that targets human PCSK9, reduced cholesterol levels in hPCSK9-KI but not in wildtype mice, showing that the hypercholesterolemic phenotype was driven by overexpression of human PCSK9. CRISPR-Cas9-mediated genome editing of human PCSK9 reduced plasma levels of human and not mouse PCSK9, and in parallel reduced plasma concentrations of total cholesterol; genome editing of mouse Pcsk9 did not reduce cholesterol levels. Base editing using a guide RNA that targeted human and mouse PCSK9 reduced plasma levels of human and mouse PCSK9 and total cholesterol. In our mouse model, base editing was more precise than genome editing, and no off-target editing nor chromosomal translocations were identified.

CONCLUSIONS

Here, we describe a humanized mouse model with liver-specific expression of human PCSK9 and a human-like hypercholesterolemia phenotype, and demonstrate that this mouse can be used to evaluate antibody and gene editing-based (genome and base editing) therapies to modulate the expression of human PCSK9 and reduce cholesterol levels. We predict that this mouse model will be used in the future to understand the efficacy and safety of novel therapeutic approaches for hypercholesterolemia.

Engineering of a GLP-1 analogue peptide/anti-PCSK9 antibody fusion for type 2 diabetes treatment

Type 2 diabetes (T2D) is a complex and progressive disease requiring polypharmacy to manage hyperglycaemia and cardiovascular risk factors. However, most patients do not achieve combined treatment goals. To address this therapeutic gap, we have developed MEDI4166, a novel glucagon-like peptide-1 (GLP-1) receptor agonist peptide fused to a proprotein convertase subtilisin/kexin type 9 (PCSK9) neutralising antibody that allows for glycaemic control and low-density lipoprotein cholesterol (LDL-C) lowering in a single molecule. The fusion has been engineered to deliver sustained peptide activity in vivo in combination with reduced potency, to manage GLP-1 driven adverse effects at high dose, and a favourable manufacturability profile. MEDI4166 showed robust and sustained LDL-C lowering in cynomolgus monkeys and exhibited the anticipated GLP-1 effects in T2D mouse models. We believe MEDI4166 is a novel molecule combining long acting agonist peptide and neutralising antibody activities to deliver a unique pharmacology profile for the management of T2D.

Naringin Activates AMPK Resulting in Altered Expression of SREBPs, PCSK9, and LDLR To Reduce Body Weight in Obese C57BL/6J Mice

Previous investigations have shown molecular cross-talk among activated adenosine monophosphate-activated protein kinase (AMPK), proprotein convertase subtilisin/kexin type 9 (PCSK9), sterol regulatory element-binding proteins (SREBPs), and low-density lipoprotein receptor (LDLR) and that it may be an innovative pharmacologic objective for treating obesity. We scrutinized the beneficial effect of naringin, a flavanone-7- O-glycoside, on obesity and the mechanisms in the present study. We arbitrarily divided 50 mice into five groups ( n = 10): 25 or 50 or 100 mg/kg/day naringin-treated obese mice (gavage for 8 weeks), untreated obese mice, and C57BL/6J control. After 8 weeks, body weight was 51.8 ± 4.4 in the untreated obese mice group, while the weights were 41.4 ± 4.1, 34.6 ± 2.2, and 28.0 ± 2.3 in 25, 50,100 mg/kg naringin groups, respectively. Moreover, naringin treatment significantly decreased plasma 8-isoprostane (an indicator of the oxidative stress) level, fat weight, liver weight, hepatic total cholesterol concentration, hepatic triglyceride concentration, plasma leptin level, plasma insulin content, plasma low-density lipoprotein cholesterol level, and plasma PCSK9 production concomitantly with down-regulated expression of SREBP-2, PCSK9, and SREBP-1, and up-regulated expression of p-AMPKα and LDLR. The present results suggest that naringin activates AMPK resulting in altered expression of SREBPs, PCSK9, and LDLR to reduce the body weight of obese C57BL/6J mice.

Clinical Application of Genetic Testing in Heart Failure

PURPOSE OF REVIEW

The purpose of this review is to present our current understanding of the genetic etiologies that may cause or predispose to heart failure. We highlight known phenotypes for which a genetic evaluation has clinical utility.

RECENT FINDINGS

The literature continues to demonstrate and confirm a genetic basis for conditions that cause heart failure. Evidence suggests a genetic model involving rare and common variants of strong or weak effect, in combination with environmental factors that may manifest as familial or simplex disease. Clinical genetic testing is available for several phenotypes, which can aid in the diagnosis and identification of at-risk family members. The evaluation of heart failure should include investigating etiologies with a genetic basis. Conducting a genetic evaluation in patients with heart failure requires the ability to identify possible genetic etiologies in an individual's phenotype, obtain relevant family history, and clinically interpret genetic testing results.

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.

Proprotein convertase subtilisin kexin type 9 promotes intestinal overproduction of triglyceride-rich apolipoprotein B lipoproteins through both low-density lipoprotein receptor-dependent and -independent mechanisms

BACKGROUND

Proprotein convertase subtilisin kexin type 9 (PCSK9) promotes the degradation of the low-density lipoprotein (LDL) receptor (LDLR), and its deficiency in humans results in low plasma LDL cholesterol and protection against coronary heart disease. Recent evidence indicates that PCSK9 also modulates the metabolism of triglyceride-rich apolipoprotein B (apoB) lipoproteins, another important coronary heart disease risk factor. Here, we studied the effects of physiological levels of PCSK9 on intestinal triglyceride-rich apoB lipoprotein production and elucidated for the first time the cellular and molecular mechanisms involved.

METHODS AND RESULTS

Treatment of human enterocytes (CaCo-2 cells) with recombinant human PCSK9 (10 μg/mL for 24 hours) increased cellular and secreted apoB48 and apoB100 by 40% to 55% each (P<0.01 versus untreated cells), whereas short-term deletion of PCSK9 expression reversed this effect. PCSK9 stimulation of apoB was due to a 1.5-fold increase in apoB mRNA (P<0.01) and to enhanced apoB protein stability through both LDLR-dependent and LDLR-independent mechanisms. PCSK9 decreased LDLR protein (P<0.01) and increased cellular apoB stability via activation of microsomal triglyceride transfer protein. PCSK9 also increased levels of the lipid-generating enzymes FAS, SCD, and DGAT2 (P<0.05). In mice, human PCSK9 at physiological levels increased intestinal microsomal triglyceride transfer protein levels and activity regardless of LDLR expression.

CONCLUSIONS

PCSK9 markedly increases intestinal triglyceride-rich apoB production through mechanisms mediated in part by transcriptional effects on apoB, microsomal triglyceride transfer protein, and lipogenic genes and in part by posttranscriptional effects on the LDLR and microsomal triglyceride transfer protein. These findings indicate that targeted PCSK9-based therapies may also be effective in the management of postprandial hypertriglyceridemia.

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.

Berberine inhibits dyslipidemia in C57BL/6 mice with lipopolysaccharide induced inflammation

BACKGROUND

Inhibiting the action of proprotein convertase subtilisin/kexin type 9 (PCSK9) on the low-density lipoprotein receptor (LDLR) has emerged as a novel therapeutic target for hypercholesterolemia. Here we investigated the effect of berberine, natural plant extracts, on PCSK9-LDLR pathway in C57BL/6 mice with lipopolysaccharide (LPS) induced inflammation.

METHODS

Forty female mice were divided into four groups (n =10): control, LPS (5 mg/kg), LPS + berberine 10 (5 mg/kg LPS plus 10 mg/kg berberine), and LPS + berberine 30 (5 mg/kg LPS plus 30 mg/kg berberine). Changes in the levels of blood lipids [total cholesterol (TC), triglyceride (TG), low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C)]; pro-inflammatory cytokines [interferon-γ (IFNγ), tumor necrosis factor α (TNFα), and interleukin-1α (IL-1α)], 8-isoprostane, hepatic expressions of PCSK9 and LDLR were determined.

RESULTS

Berberine pretreatment reduced the expression of hepatic PCSK9, decreased the plasma TC, TG, LDL-C, IFNγ, TNFα, IL-1α, and 8-isoprostane concentrations; increased HDL-C level and LDLR expression in mice.

CONCLUSION

The present results suggest that berberine inhibits dyslipidemia in C57BL/6 mice with LPS induced inflammation through regulating PCSK9-LDLR pathway.

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.

Novel pharmacotherapies of familial hyperlipidemia

Familial hyperlipidemia is an inherited metabolic disorder characterized by elevated lipid and/or lipoprotein levels in the blood. Despite improvements in lipid-lowering therapy during the last decades, it still remains a substantial contributor to the incidence of cardiovascular disease since patients on current conventional therapies do not achieve their target LDL-cholesterol levels. With a view to lower LDL-cholesterol levels, a number of new therapeutic strategies have been developed over recent years. In this review, we provide an overview of these treatment options that are currently in clinical development and may offer alternative or adjunctive therapies for this high-risk population.

The next generation of novel low-density lipoprotein cholesterol-lowering agents: proprotein convertase subtilisin/kexin 9 inhibitors

Proprotein convertase subtilisin/kexin 9 (PCSK9) has been shown to degrade hepatic low-density lipoprotein receptors (LDLR). Gain-of-function mutations promote the development of familial hypercholesterolemia, whereas loss-of-function mutations are associated with lower levels of circulating low-density lipoprotein cholesterol (LDL-C) and significant protection against coronary heart disease. The major classes of commonly prescribed lipid-lowering medications, such as statins, increase serum PCSK9 levels, thus PCSK9 inhibition would increase the efficacy of statins on LDL-C lowering. Therefore, PCSK9 is an attractive therapeutic target for the new generation of cholesterol-lowering drugs. Here, we present a brief overview of the development of PCSK9 inhibitors and highlight the effect of currently prescribed LDL-C-lowering drugs on PCSK9, and the strategies that are being explored for its therapeutic inhibition. Current research and clinical trial results indicate that a PCSK9 inhibitor may be an exciting new therapeutic drug for the treatment of dyslipidemia and relevant cardiovascular diseases.

The safety of therapeutic monoclonal antibodies: implications for cardiovascular disease and targeting the PCSK9 pathway

Monoclonal antibodies (mAbs) are established therapies for many conditions, including cancers, autoimmune conditions and infectious diseases. mAbs can offer benefits over conventional pharmacotherapy in terms of potency, dosing frequency and specificity for their target antigen. Mouse-derived antibodies were initially used in humans; however, patients often developed human anti-mouse antibodies, resulting in rapid antibody clearance (and a resulting loss of efficacy) and hypersensitivity reactions. Chimeric, humanized, and fully human antibodies were thus developed, with increasing amounts of human sequence, to reduce immunogenicity. Although generally well tolerated, mAbs may be associated with adverse events (AEs). Many AEs are target-related, and will be specific to the antibody target and the therapeutic area of use. However, off-target AEs, such as hypersensitivity reactions, are observed with many antibodies. Within the realm of cardiovascular medicine, new antibody-based therapies are under investigation to reduce low-density lipoprotein cholesterol (LDL-C) levels. Proprotein convertase subtilisin/kexin type 9 (PCSK9) regulates plasma LDL-C levels by increasing degradation of the LDL receptor (LDLR). Therefore, inhibition of the interaction between PCSK9 and the LDLR with mAbs targeting PCSK9 has great potential for patients with hypercholesterolaemia. Early clinical phase studies suggest these mAbs are effective and well tolerated; however, further studies are required to assess their long-term safety.

Anti-PCSK9 therapies for the treatment of hypercholesterolemia

INTRODUCTION

Proprotein convertase subtilisin kexin type 9 (PCSK9), a serine protease that binds to the low density lipoprotein (LDL) receptor promoting its degradation, is an important regulator of LDL metabolism. PCSK9 'gain-of-function' mutations are rare and cause high plasma LDL-cholesterol and increase atherosclerotic cardiovascular disease, whereas more common 'loss-of-function' mutations cause low LDL-cholesterol and atheroprotection. PCSK9 is a novel, attractive and viable therapeutic target for the treatment of hypercholesterolemia, with human studies using a variety of anti-PCSK9 therapies underway.

AREAS COVERED

This review summarizes the latest findings in clinical trials of PCSK9 inhibitors, including antibodies, gene silencing and small peptides.

EXPERT OPINION

PCSK9 inhibition would appear to be an effective strategy for lowering plasma LDL-cholesterol and enhancing the LDL-cholesterol lowering ability of statins in patients with familial hypercholesterolemia, patients with refractory hypercholesterolemia at high risk of cardiovascular disease and patients with severe hypercholesterolemia who are not at target or are intolerant of statins, with a variety of anti-PCSK9 therapies in clinical trials.

Furin-cleaved proprotein convertase subtilisin/kexin type 9 (PCSK9) is active and modulates low density lipoprotein receptor and serum cholesterol levels

Proprotein convertase subtilisin/kexin 9 (PCSK9) regulates plasma LDL cholesterol levels by regulating the degradation of LDL receptors. Another proprotein convertase, furin, cleaves PCSK9 at Arg²¹⁸-Gln²¹⁹ in the surface-exposed “218 loop.” This cleaved form circulates in blood along with the intact form, albeit at lower concentrations. To gain a better understanding of how cleavage affects PCSK9 function, we produced recombinant furin-cleaved PCSK9 using antibody Ab-3D5, which binds the intact but not the cleaved 218 loop. Using Ab-3D5, we also produced highly purified hepsin-cleaved PCSK9. Hepsin cleaves PCSK9 at Arg²¹⁸-Gln²¹⁹ more efficiently than furin but also cleaves at Arg²¹⁵-Phe²¹⁶. Further analysis by size exclusion chromatography and mass spectrometry indicated that furin and hepsin produced an internal cleavage in the 218 loop without the loss of the N-terminal segment (Ser¹⁵³–Arg²¹⁸), which remained attached to the catalytic domain. Both furin- and hepsin-cleaved PCSK9 bound to LDL receptor with only 2-fold reduced affinity compared with intact PCSK9. Moreover, they reduced LDL receptor levels in HepG2 cells and in mouse liver with only moderately lower activity than intact PCSK9, consistent with the binding data. Single injection into mice of furin-cleaved PCSK9 resulted in significantly increased serum cholesterol levels, approaching the increase by intact PCSK9. These findings indicate that circulating furin-cleaved PCSK9 is able to regulate LDL receptor and serum cholesterol levels, although somewhat less efficiently than intact PCSK9. Therapeutic anti-PCSK9 approaches that neutralize both forms should be the most effective in preserving LDL receptors and in lowering plasma LDL cholesterol.

PCSK9 siRNA suppresses the inflammatory response induced by oxLDL through inhibition of NF-κB activation in THP-1-derived macrophages

Proprotein convertase subtilisin/kexin 9 (PCSK9), a member of the protein-converting enzyme family, is highly expressed in adult hepatocytes and small intestinal enterocytes. To our knowledge, in this study, we demonstrate for the first time that PCSK9 is upregulated in a dose-dependent manner via oxidized low-density lipoprotein (oxLDL) stimulation in THP-1-derived macrophages. PCSK9 small interfering RNA (siRNA) suppresses the oxLDL-induced inflammatory cytokine expression in THP-1-derived macrophages. The exposure of macrophages to oxLDL markedly increased the expression of NF-κB protein in the nucleus. However, this effect was significantly attenuated by PCSK9 siRNA. These findings indicate that PCSK9 expression is induced by oxLDL, and that PCSK9 siRNA protects against inflammation via the inhibition of NF-κB activation in oxLDL-stimulated THP-1-derived macrophages. Our results suggest that PCSK9 may be used as a therapeutic target for the treatment of atherosclerosis since PCSK9 siRNA suppresses oxLDL-induced IκB-α degradation and NF-κB nuclear translocation into THP-1-derived macrophages.

The therapeutic potential of PCSK9 inhibition in primary dyslipidemia, the example from SAR236553/REGN727

The proprotein convertase subtilisin kexin 9 (PCSK9) is a serine protease that has recently emerged as a potential target for the treatment of hypercholesterolemia. SAR236553/REGN727 is a highly specific monoclonal antibody to PCSK9 that significantly reduced LDL cholesterol levels in healthy volunteers and in subjects with hypercholesterolemia in three Phase-I studies. It was subsequently tested in a recent Phase-II study in patients with primary hypercholesterolemia with impressive results. In this report the trial is discussed along with the significance of the results in guiding further research in PCSK9 inhibition.

The biology and therapeutic targeting of the proprotein convertases

The mammalian proprotein convertases constitute a family of nine secretory serine proteases that are related to bacterial subtilisin and yeast kexin. Seven of these (proprotein convertase 1 (PC1), PC2, furin, PC4, PC5, paired basic amino acid cleaving enzyme 4 (PACE4) and PC7) activate cellular and pathogenic precursor proteins by cleavage at single or paired basic residues, whereas subtilisin kexin isozyme 1 (SKI-1) and proprotein convertase subtilisin kexin 9 (PCSK9) regulate cholesterol and/or lipid homeostasis via cleavage at non-basic residues or through induced degradation of receptors. Proprotein convertases are now considered to be attractive targets for the development of powerful novel therapeutics. In this Review, we summarize the physiological functions and pathological implications of the proprotein convertases, and discuss proposed strategies to control some of their activities, including their therapeutic application and validation in selected disease states.

Treatment of familial hypercholesterolemia: is there a need beyond statin therapy?

Familial hypercholesterolemia (FH) is a genetic lipid disorder that is characterized by severely elevated cholesterol levels and premature cardiovascular disease. Both the heterozygous and homozygous forms of FH require aggressive cholesterol-lowering therapy. Statins alone frequently do not lower these patients' cholesterol to therapeutic levels, and some patients are intolerant to statins. Combination or monotherapy with other current pharmacotherapies are options, but even with these some FH patients do not meet their low-density lipoprotein (LDL) cholesterol goals. In the cases of statin intolerance, LDL apheresis may be another treatment option. There are currently several novel therapies in development for LDL lowering that target either production or catabolism of LDL, plaque regression, and potentially gene transfer. We conclude that there is a need beyond statins for patients with FH, especially in cases of statin intolerance, and when even the highest doses of statin do not get patients to goal cholesterol levels.

Targeted In Situ Gene Correction of Dysfunctional APOE Alleles to Produce Atheroprotective Plasma ApoE3 Protein

Cardiovascular disease is the leading worldwide cause of death. Apolipoprotein E (ApoE) is a 34-kDa circulating glycoprotein, secreted by the liver and macrophages with pleiotropic antiatherogenic functions and hence a candidate to treat hypercholesterolaemia and atherosclerosis. Here, we describe atheroprotective properties of ApoE, though also potential proatherogenic actions, and the prevalence of dysfunctional isoforms, outline conventional gene transfer strategies, and then focus on gene correction therapeutics that can repair defective APOE alleles. In particular, we discuss the possibility and potential benefit of applying in combination two technical advances to repair aberrant APOE genes: (i) an engineered endonuclease to introduce a double-strand break (DSB) in exon 4, which contains the common, but dysfunctional, ε2 and ε4 alleles; (ii) an efficient and selectable template for homologous recombination (HR) repair, namely, an adeno-associated viral (AAV) vector, which harbours wild-type APOE sequence. This technology is applicable ex vivo, for example to target haematopoietic or induced pluripotent stem cells, and also for in vivo hepatic gene targeting. It is to be hoped that such emerging technology will eventually translate to patient therapy to reduce CVD risk.

Novel LDL-oriented pharmacotherapeutical strategies

Elevated levels of low-density cholesterol (LDL-C) are highly correlated with increased risk of cardiovascular diseases (CVD). Thus, current guidelines have recommended progressively lower LDL-C for cholesterol treatment and CVD prevention as the primary goal of therapy. Even so, some patients in the high risk category fail to achieve recommended LDL-C targets with currently available medications. Thereby, additional pharmaceutical strategies are urgently required. In the review, we aim to provide an overview of both current and emerging LDL-C lowering drugs. As for current available LDL-C lowering agents, attentions are mainly focused on statins, niacin, bile acid sequestrants, ezetimibe, fibrates and omega-3 fatty acids. On the other hand, the emerging drugs differ from mechanisms are including: intervention of cholesterol biosynthesis downstream enzyme (squalene synthase inhibitors), inhibition of lipoprotein assembly (antisense mRNA inhibitors of apolipoprotein B and microsomal transfer protein inhibitors), enhanced lipoprotein clearance (proprotein convertase subtilisin kexin type 9, thyroid hormone analogues), inhibition of intestinal cholesterol absorption (Niemann-Pick C1-like 1 protein and acyl coenzyme A:cholesterol acyltransferase inhibitors) and interrupting enterohepatic circulation (apical sodium-dependent bile acid transporter inhibitors). Several ongoing agents are in their different stages of clinical trials, in expectation of promising antihyperlipidemic drugs. Therefore, alternative drugs monotherapy or in combination with statins will be sufficient to reduce LDL-C concentrations to optimal levels, and a new era for better LDL-C managements is plausible.

The facts behind niacin

Although low-density lipoprotein cholesterol (LDL-C) lowering represents the mainstay of current lipid treatment, high-density lipoprotein cholesterol (HDL-C) has generated increasing interest as a secondary therapeutic target because of strong evidence that serum HDL-C concentration is inversely associated with coronary heart disease risk. Niacin is a lipid-altering drug that has been used to lower cholesterol since the 1950s. In addition to its LDL-C-lowering effects, niacin is the most effective agent currently available for raising HDL-C. Despite its long history as a lipid-altering drug, only limited data are available regarding its clinical benefit alone and in combination with other agents, and the majority of studies investigating its impact on clinical outcomes are from the pre-statin area. Several studies have demonstrated a beneficial effect of treatment with niacin in combination with statin therapy on surrogate cardiovascular markers (e.g. carotid intima-media thickness). However, the clinical significance of these surrogate markers has been questioned. Two large randomized trials will address whether niacin–statin combination therapy is an appropriate therapeutic alternative to statin monotherapy.

Cardiovascular complications of diabetes: recent insights in pathophysiology and therapeutics

Cardiovascular complications represent the principal cause of death in patients with Type 2 diabetes. It is therefore of great importance to dissect the genetic determinants and molecular mechanisms responsible for diabetic cardiovascular complications. New research is of particular importance since, somewhat unexpectedly, large-scale clinical trials have indicated that glycemic control does not appear to have the anticipated major influence as a factor dictating cardiovascular outcome in diabetics. Hence, additional pathophysiological factors such as dyslipidemia, as well as proinflammatory and proatherosclerotic mechanisms, need to be more carefully examined. In this article, we will focus on recent studies in both animal models and humans as well as cellular mechanistic studies that advance our knowledge on the role of dyslipidemia, inflammation and atherosclerotic events in the cardiovascular complications of diabetes. We also translate our focus on research insights to related therapeutic opportunities.

Mechanisms and genetic determinants regulating sterol absorption, circulating LDL levels, and sterol elimination: implications for classification and disease risk

This review integrates historical biochemical and modern genetic findings that underpin our understanding of the low-density lipoprotein (LDL) dyslipidemias that bear on human disease. These range from life-threatening conditions of infancy through severe coronary heart disease of young adulthood, to indolent disorders of middle- and old-age. We particularly focus on the biological aspects of those gene mutations and variants that impact on sterol absorption and hepatobiliary excretion via specific membrane transporter systems (NPC1L1, ABCG5/8); the incorporation of dietary sterols (MTP) and of de novo synthesized lipids (HMGCR, TRIB1) into apoB-containing lipoproteins (APOB) and their release into the circulation (ANGPTL3, SARA2, SORT1); and receptor-mediated uptake of LDL and of intestinal and hepatic-derived lipoprotein remnants (LDLR, APOB, APOE, LDLRAP1, PCSK9, IDOL). The insights gained from integrating the wealth of genetic data with biological processes have important implications for the classification of clinical and presymptomatic diagnoses of traditional LDL dyslipidemias, sitosterolemia, and newly emerging phenotypes, as well as their management through both nutritional and pharmaceutical means.

Familial hypercholesterolemia: the lipids or the genes?

Familial Hypercholesterolemia (FH) is a common cause of premature cardiovascular disease and is often undiagnosed in young people. Although the disease is diagnosed clinically by high LDL cholesterol levels and family history, to date there are no single internationally accepted criteria for the diagnosis of FH. Several genes have been shown to be involved in FH; yet determining the implications of the different mutations on the phenotype remains a hard task. The polygenetic nature of FH is being enhanced by the discovery of new genes that serve as modifiers. Nevertheless, the picture is still unclear and many unknown genes contributing to the phenotype are most likely involved. Because of this evolving polygenetic nature, the diagnosis of FH by genetic testing is hampered by its cost and effectiveness.

In this review, we reconsider the clinical versus genetic nomenclature of FH in the literature. After we describe each of the genetic causes of FH, we summarize the known correlation with phenotypic measures so far for each genetic defect. We then discuss studies from different populations on the genetic and clinical diagnoses of FH to draw helpful conclusions on cost-effectiveness and suggestions for diagnosis.

Making sense of therapeutics using antisense technology

INTRODUCTION

Antisense oligonucleotides (ASOs) are short synthetic single-stranded DNA sequences that bind to and induce the cleavage of homologous stretches of mRNA sequences. These result in targeted destruction of mRNA and correction of genetic aberrations. ASOs thus can act as drug molecules and potentially rectify many disease conditions. The broad range of applications reported in the literature highlights the advances in the field.

AREAS COVERED

This review covers different areas in which use of ASOs has been shown to have therapeutic effects. Some drugs in different stages of preclinical and clinical trials are discussed in detail. The problems faced and the strategies to surmount them are also described. The readers will gain an understanding of the recent developments in the field of ASOs with emphasis on their therapeutic applications. They will also become aware of the different strategies used for targeted delivery of ASOs and their stabilization, which may be useful for their work in this field, or in the area of nucleic acid therapeutics in general.

EXPERT OPINION

The design and application of ASOs for recognition of target mRNA sequences have become a fairly straightforward protocol. The main problem lies in designing ASOs which are stable in in vivo milieu. The delivery and bioavailability of the oligonucleotide to the site of action continue to be hurdles in the development of ASOs and therapeutic molecules.