PCSK9

PCSK9 is secreted by hepatocytes and binds to LDL receptors on the cell surface, targeting them for lysosomal degradation instead of recycling back to the membrane. Normally, LDL receptors bind circulating LDL particles, internalize them via endocytosis, and recycle back to the surface after releasing LDL for degradation. When PCSK9 binds the LDL receptor-LDL complex, it prevents this recycling, leading to fewer LDL receptors on the hepatocyte surface, decreased LDL clearance, and increased plasma LDL-cholesterol.

Clinical relevance: Gain-of-function mutations in PCSK9 lead to autosomal dominant familial hypercholesterolemia (similar phenotype to LDL receptor mutations), with markedly elevated LDL and early atherosclerotic disease. Conversely, loss-of-function mutations result in low LDL levels and are associated with reduced risk of coronary artery disease, providing the rationale for pharmacologic inhibition of PCSK9.

PCSK9 inhibitors (evolocumab, alirocumab) are injectable monoclonal antibodies that bind circulating PCSK9, preventing it from binding LDL receptors. This preserves LDL receptor recycling, increases hepatic LDL uptake, and substantially lowers LDL-C (by 50-60%), often used as add-on therapy to statins or in statin-intolerant patients, particularly those with familial hypercholesterolemia or established atherosclerotic cardiovascular disease requiring additional LDL lowering. Newer therapies include inclisiran, a small interfering RNA (siRNA) that inhibits hepatic PCSK9 synthesis, given as an injection every 6 months.

Side effects of PCSK9 inhibitors are generally mild (injection site reactions, rare neurocognitive effects reported but not definitively causal). They are notably expensive, limiting widespread first-line use despite strong LDL-lowering efficacy and demonstrated reduction in cardiovascular events in trials (e.g., FOURIER, ODYSSEY OUTCOMES).