Abstract

Cardiovascular disease (CVD) is the leading cause of mortality worldwide and lowering low-density lipoprotein cholesterol (LDL-C) is an established therapeutic approach for reducing cardiovascular risk. Statins are widely regarded as the front-line therapy for lowering LDL-C but approximately 25% of patients fail to reach the desired LDL-C levels with the maximal statin dose or are intolerant to statins.

Proprotein convertase subtilisin/kexin 9 (PCSK9) plays an important role in regulating lipoprotein metabolism by binding to low density lipoprotein receptors, leading to their degradation. Consequently, LDL-C-lowering drugs that operate through inhibition of PCSK9 are being pursued for the management of hypercholesterolemia and reducing its associated CVD risk.

In 2015, the PCSK9-blocking monoclonal antibodies alirocumab and evolocumab were approved for hypercholesterolemia. However, the identification of small molecule, orally bioavailable PCSK9 inhibitors has proven to be a significant challenge for drug discovery. This has been explained by the expansive, ‘undruggable’ flat binding interface targeted by the antibodies.

Following a library screening campaign, a fragment-like hit compound was identified that bound with micromolar affinity to a novel, cryptic binding site of the PCSK9 protein. Optimisation of that screening hit afforded a small molecule PCSK9 inhibitor with a K_D value of 200 nM, albeit with little in vitro stability. Following a structure-based drug design approach this modestly potent PCSK9 inhibitor was transformed into a series of picomolar and low nanomolar PCSK9 inhibitors. Moreover, when orally administered to dyslipidemic non-human primates, multiple analogues elicit significant and robust lowering of LDL-C following multiple weeks of administration.