Preserved adrenal function in fully PCSK9-deficient subject

Efficacy and Safety of an Investigational Single-Course CRISPR Base-Editing Therapy Targeting PCSK9 in Nonhuman Primate and Mouse Models

BACKGROUND

VERVE-101 is an investigational in vivo CRISPR base-editing medicine designed to alter a single DNA base in the PCSK9 gene, permanently turn off hepatic protein production, and thereby durably lower low-density lipoprotein cholesterol. We test the efficacy, durability, tolerability, and potential for germline editing of VERVE-101 in studies of nonhuman primates and a murine F1 progeny study.

METHODS

Cynomolgus monkeys were given a single intravenous infusion of a vehicle control (n=10) or VERVE-101 at a dose of 0.75 mg/kg (n=4) or 1.5 mg/kg (n=22) with subsequent follow-up up to 476 days. Two studies assessed the potential for germline editing, including sequencing sperm samples from sexually mature male nonhuman primates treated with VERVE-101 and genotyping offspring from female mice treated with the murine surrogate of VERVE-101 (VERVE-101mu).

RESULTS

Liver biopsies 14 days after dosing noted mean PCSK9 editing of 46% and 70% in monkeys treated with VERVE-101 at 0.75 and 1.5 mg/kg, respectively. This translated into mean reductions in blood PCSK9 (proprotein convertase subtilisin/kexin type 9) of 67% and 83% and reductions of low-density lipoprotein cholesterol of 49% and 69% at the 0.75 and 1.5 mg/kg doses, respectively, assessed as time-weighted average change from baseline between day 28 and up to 476 days after dosing. Liver safety monitoring noted a transient rise in alanine aminotransferase and aspartate aminotransferase concentrations after infusion that fully resolved by day 14 with no accompanying change in total bilirubin. In a subset of monkeys necropsied 1 year after dosing, no findings related to VERVE-101 were identified on macroscopic and histopathologic assessment of the liver and other organs. In the study to assess potential germline editing of male nonhuman primates, sperm samples collected after VERVE-101 dosing showed no evidence of PCSK9 editing. Among 436 offspring of female mice treated with a saturating dose of VERVE-101mu, the PCSK9 edit was transmitted in 0 of 436 animals.

CONCLUSIONS

VERVE-101 was well tolerated in nonhuman primates and led to 83% lower blood PCSK9 protein and 69% lower low-density lipoprotein cholesterol with durable effects up to 476 days after dosing. These results have supported the initiation of a first-in-human clinical trial in patients with heterozygous familial hypercholesterolemia and atherosclerotic cardiovascular disease.

Insight into the Evolving Role of PCSK9

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is the last discovered member of the family of proprotein convertases (PCs), mainly synthetized in hepatic cells. This serine protease plays a pivotal role in the reduction of the number of low-density lipoprotein receptors (LDLRs) on the surface of hepatocytes, which leads to an increase in the level of cholesterol in the blood. This mechanism and the fact that gain of function (GOF) mutations in PCSK9 are responsible for causing familial hypercholesterolemia whereas loss-of-function (LOF) mutations are associated with hypocholesterolemia, prompted the invention of drugs that block PCSK9 action. The high efficiency of PCSK9 inhibitors (e.g., alirocumab, evolocumab) in decreasing cardiovascular risk, pleiotropic effects of other lipid-lowering drugs (e.g., statins) and the multifunctional character of other proprotein convertases, were the cause for proceeding studies on functions of PCSK9 beyond cholesterol metabolism. In this article, we summarize the current knowledge on the roles that PCSK9 plays in different tissues and perspectives for its clinical use.

The consequences of PCSK9 inhibition in selected tissues

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is one of nine members of the proprotein convertase family. These serine proteases play a pivotal role in the post-translational modification of proteins and the activation of hormones, enzymes, transcription factors and growth factors. As a result, they participate in many physiological processes like embryogenesis, activity of central nervous system and lipid metabolism. Scientific studies show that the family of convertases is also involved in the pathogenesis of viral and bacterial infections, osteoporosis, hyperglycaemia, cardiovascular diseases, neurodegenerative disorders and cancer. The inhibition of PCSK9 by two currently approved for use monoclonal antibodies (alirocumab, evolocumab) slows down the degradation of low-density lipoprotein cholesterol receptors (LDLRs). This leads to increased density of LDLRs on the surface of hepatocytes, resulting in decreased level of low-density lipoprotein cholesterol (LDL-C) in the bloodstream, which is connected with the reduction of cardiovascular risk. PCSK9 inhibitors (PCSK9i) were created for the patients who could not achieve appropriate level of LDL-C using current statin and ezetimibe therapy. It seems that high therapeutic efficacy of PCSK9i will make them more common in the clinical use. The pleiotropic effects of previously mentioned lipid-lowering therapies were the reasons for literature review of possible positive and negative effects of PCSK9 inhibition beyond cholesterol metabolism.

Inhibiting PCSK9 - biology beyond LDL control

Clinical trials have unequivocally shown that inhibition of proprotein convertase subtilisin/kexin type 9 (PCSK9) efficaciously and safely prevents cardiovascular events by lowering levels of LDL cholesterol. PCSK9 in the circulation is derived mainly from the liver, but the protein is also expressed in the pancreas, the kidney, the intestine and the central nervous system. Although PCSK9 modulates cholesterol metabolism by regulating LDL receptor expression in the liver, in vitro and in vivo studies have suggested that PCSK9 is involved in various other physiological processes. Although therapeutic PCSK9 inhibition could theoretically have undesired effects by interfering with these non-cholesterol-related processes, studies of individuals with genetically determined reduced PCSK9 function and clinical trials of PCSK9 inhibitors have not revealed clinically meaningful adverse consequences of almost completely eradicating PCSK9 from the circulation. The clinical implications of PCSK9 functions beyond lipid metabolism in terms of wanted or unwanted effects of therapeutic PCSK9 inhibition therefore appear to be limited. The objective of this Review is to describe the physiological role of PCSK9 beyond the LDL receptor to provide a rational basis for monitoring the effects of PCSK9 inhibition as these drugs gain traction in the clinic.

PCSK9 Inhibition With Monoclonal Antibodies: Modern Management of Hypercholesterolemia

Current guidelines for hypercholesterolemia treatment emphasize lifestyle modification and lipid-modifying therapy to reduce the risk for cardiovascular disease. Statins are the primary class of agents used for the treatment of hypercholesterolemia. Although statins are effective for many patients, they fail to achieve optimal reduction in lipids for some patients, including those who have or are at high risk for cardiovascular disease. The PCSK9 gene was identified in the past decade as a potential therapeutic target for the management of patients with hypercholesterolemia. Pharmacologic interventions to decrease PCSK9 levels are in development, with the most promising approach using monoclonal antibodies that bind to PCSK9 in the plasma. Two monoclonal antibodies, alirocumab and evolocumab, have recently been approved for the treatment of hypercholesterolemia, and a third one, bococizumab, is in phase 3 clinical development. All 3 agents achieve significant reductions in levels of low-density lipoprotein cholesterol, as well as reductions in non-high-density lipoprotein cholesterol, apolipoprotein B, and lipoprotein(a). Long-term outcome trials are under way to determine the sustained efficacy, safety, and tolerability of PCSK9 inhibitors and whether this novel class of agents decreases the risk for major cardiovascular events in patients on lipid-modifying therapy. Available data suggest that PCSK9 inhibitors provide a robust reduction in atherogenic cholesterol levels with a good safety profile, especially for patients who fail to obtain an optimal clinical response to statin therapy, those who are statin intolerant or have contraindications to statin therapy, and those with familial hypercholesterolemia.

Proprotein convertases in atherogenesis

PURPOSE OF REVIEW

The proprotein convertases subtilisin/kexin (PCSKs) are endoproteases identified as activators of precursors from hormones and peptides. On the basis of the variety of substrates and regulation in disease, they have been recognized as mediators in atherogenesis. The discovery of PCSK9, which regulates low-density lipoprotein receptor cell membrane availability, has led to a resurgence of interest in these enzymes and their function in cardiovascular diseases.

RECENT FINDINGS

Recent data demonstrate that PCSKs are expressed in human atheroma and are regulated in animal models of atherosclerosis. In animal models, inhibition of PCSKs, such as PCSK3, affects cell proliferation and migration as well as inflammation, reducing atherosclerosis. In addition, targeting PCSK9 lowers cholesterol levels and has now been demonstrated to lessen vascular lesion formation in mice. Experimentally investigated novel anti-PCSK9 strategies include genome editing and vaccination. Furthermore, studies show that PCSKs contribute to the initiation and progression of cardiometabolic risk factors, such as insulin resistance and obesity.

SUMMARY

PCSKs affect cardiovascular diseases on multiple levels, including atherosclerotic lesion formation as well as their contribution to cardiometabolic risk factors. PCSK9 is a key regulator of plasma cholesterol levels, thereby potentially affecting atherosclerosis and has rapidly emerged as a pharmacological target.

Hypercholesterolemia, low density lipoprotein receptor and proprotein convertase subtilisin/kexin-type 9

Atherosclerotic cardiovascular disease is the main cause of mortality and morbidity in the world. Plasma levels of low density lipoprotein cholesterol (LDL-C) are positively correlated with the risk of atherosclerosis. High plasma LDL concentrations in patients with hypercholesterolemia lead to build-up of LDL in the inner walls of the arteries, which becomes oxidized and promotes the formation of foam cells, consequently initiating atherosclerosis. Plasma LDL is mainly cleared through the LDL receptor (LDLR) pathway. Mutations in the LDLR cause familiar hypercholesterolemia and increase the risk of premature coronary heart disease. The expression of LDLR is regulated at the transcriptional level via the sterol regulatory element binding protein 2 (SREBP-2) and at the posttranslational levels mainly through proprotein convertase subtilisin/kexin-type 9 (PCSK9) and inducible degrader of the LDLR (IDOL). In this review, we summarize the latest advances in the studies of PCSK9.

Role of PCSK9 beyond liver involvement

PURPOSE OF REVIEW

Proprotein convertase subtilisin kexin type 9 (PCSK9) acts as an endogenous natural inhibitor of the LDL receptor pathway, by targeting the receptor to lysosomes for degradation. Beside the liver, PCSK9 is also expressed at significant levels in other tissues, where its function remains unclear. The current review focuses on the extrahepatic actions of PCSK9.

RECENT FINDINGS

The generation of liver-specific PCSK9 knockout mice has clearly indicated that PCSK9 affects cholesterol homeostasis via its action on extrahepatic organs. PCSK9 is highly expressed in the intestine, where it controls the production of triglyceride-rich lipoproteins and the transintestinal cholesterol excretion. The role of PCSK9 in the endocrine pancreas and glucose homeostasis remains unclear because conflicting data exist concerning the metabolic phenotype of PCSK9-deficient mice. Sparse data suggest that PCSK9 might also play a role in kidneys, vascular smooth muscle cells, and neurons.

SUMMARY

Based on the virtuous combination of genetic and pharmacological approaches, the major function of PCSK9 as a key regulator of hepatic LDL receptor metabolism had quickly emerged. Accumulating evidence indicates that intestinal PCSK9 is also involved in the modulation of lipid homeostasis. Additional studies are warranted to decipher the physiological function of PCSK9 in other extrahepatic tissues and thus to better assess the safety of PCSK9 inhibitors.