PCSK9 Inhibition

PCSK9 (proprotein convertase subtilisin/kexin type 9) is a serine protease secreted primarily by hepatocytes that regulates LDL receptor (LDL-R) density on the hepatocyte surface. Following LDL-C uptake, the LDL-R normally recycles to the cell surface for repeated ligand binding. PCSK9 binds the LDL-R in the endosome, directing the receptor-PCSK9 complex to lysosomal degradation rather than recycling, thereby reducing the number of functional LDL receptors available for LDL-C clearance. Loss-of-function PCSK9 mutations are associated with lifelong low LDL-C levels and markedly reduced cardiovascular event rates, providing the genetic proof of concept for therapeutic inhibition.

In familial hypercholesterolaemia (FH), gain-of-function PCSK9 mutations or LDL-R mutations result in severely impaired LDL-C clearance and premature atherosclerotic cardiovascular disease. Even on maximally tolerated statin therapy, many FH patients fail to achieve LDL-C targets recommended by ESC/EAS guidelines (below 1.4 mmol/L for very high risk), providing the primary indication for PCSK9 inhibition. Cardiovascular outcome trials (FOURIER for evolocumab, ODYSSEY OUTCOMES for alirocumab) demonstrated significant reductions in MACE in patients with established ASCVD already on statin therapy.

Inclisiran introduces a fundamentally different mechanism of action — small interfering RNA (siRNA) targeting hepatocyte PCSK9 mRNA, administered subcutaneously twice yearly after an initial dose at three months. This RNA interference approach degrades PCSK9 messenger RNA within hepatocytes via the RISC complex, producing sustained reductions in PCSK9 protein synthesis and consequent LDL-C lowering without the need for frequent injections. The twice-yearly dosing schedule has significant implications for adherence in patients requiring long-term lipid-lowering therapy.