Novel strategies to target proprotein convertase subtilisin kexin 9: beyond monoclonal antibodies

Abstract

Since the discovery of the role of proprotein convertase subtilisin kexin 9 (PCSK9) in the regulation of low-density lipoprotein cholesterol (LDL-C) in 2003, a paradigm shift in the treatment of hypercholesterolaemia has occurred. The PCSK9 secreted into the circulation is a major downregulator of the low-density lipoprotein receptor (LDLR) protein, as it chaperones it to endosomes/lysosomes for degradation. Humans with loss-of-function of PCSK9 exhibit exceedingly low levels of LDL-C and are protected from atherosclerosis. As a consequence, innovative strategies to modulate the levels of PCSK9 have been developed. Since 2015 inhibitory monoclonal antibodies (evolocumab and alirocumab) are commercially available. When subcutaneously injected every 2-4 weeks, they trigger a ∼60% LDL-C lowering and a 15% reduction in the risk of cardiovascular events. Another promising approach consists of a liver-targetable specific PCSK9 siRNA which results in ∼50-60% LDL-C lowering that lasts up to 6 months (Phases II-III clinical trials). Other strategies under consideration include: (i) antibodies targeting the C-terminal domain of PCSK9, thereby inhibiting the trafficking of PCSK9-LDLR to lysosomes; (ii) small molecules that either prevent PCSK9 binding to the LDLR, its trafficking to lysosomes or its secretion from cells; (iii) complete silencing of PCSK9 by CRISPR-Cas9 strategies; (iv) PCSK9 vaccines that inhibit the activity of circulating PCSK9. Time will tell whether other strategies can be as potent and safe as monoclonal antibodies to lower LDL-C levels.

Keywords: Gene silencing; LDL-C; Monoclonal antibodies; PCSK9.

Figure 1

Strategies targeting PCSK9. PCSK9 activity can be inhibited at several levels. To date, only the PCSK9-LDLR interaction and PCSK9 mRNA stability are successfully targeted in humans with injectable mAbs (alirocumab, evolocumab) and siRNA (inclisiran), respectively.

Figure 2

PCSK9 function and potential targets for inhibition. Following transcription and translation, PCSK9 is processed in the endoplasmic reticulum into the mature form and then secreted. In the absence of PCSK9, LDLR binds to circulating low-density lipoprotein (LDL) particle and the complex LDLR/LDL is internalized in the endosomes; LDL are shuttled in the lysosome for degradation while LDLR is recycled to the cell surface. When PCSK9 is present, it binds and escorts the LDLR/LDL complex for degradation in the lysosomes, with the net effect of reducing the number of LDLR on cell surface. PCSK9 can be inhibited at different levels including DNA gene editing (1); mRNA gene silencing (2); mRNA translational inhibition (3). In addition molecules targeting circulating PCSK9 are available or under development, these include adnectins (4), ABD-fused Anticalin (5), or selective antibodies (6–9) which, by binding PCSK9 prevent its interaction with LDLR (6–8) or the interaction of a hypothetical ‘Px’ protein to the complex LDLR-PCSK9 (9).