PCSK9 inhibitors

PCSK9 (proprotein convertase subtilisin/kexin type 9) is a protein secreted by hepatocytes that binds to LDL receptors on the cell surface, promoting their degradation in lysosomes rather than recycling back to the membrane. Normally, LDL receptors bind circulating LDL particles and are internalized and recycled to clear LDL from blood; PCSK9 disrupts this recycling, reducing the number of LDL receptors available and thus increasing serum LDL levels. PCSK9 inhibitors are fully human monoclonal antibodies (evolocumab, alirocumab) that bind circulating PCSK9, preventing it from binding LDL receptors. This preserves LDL receptor recycling, increases hepatic LDL receptor density, and enhances clearance of LDL-C from the blood, leading to significant reductions in LDL cholesterol (up to 50-60%), independent of statin mechanism.

Clinical use: Indicated for patients with heterozygous or homozygous familial hypercholesterolemia, established atherosclerotic cardiovascular disease (ASCVD) requiring additional LDL lowering, or statin intolerance. They are typically added to maximally tolerated statin therapy and/or ezetimibe. Outcome trials (FOURIER for evolocumab, ODYSSEY OUTCOMES for alirocumab) demonstrated reduction in major adverse cardiovascular events (MACE) when added to statin therapy in high-risk patients.

Mechanistic significance: Loss-of-function PCSK9 mutations are associated with lower LDL levels and reduced cardiovascular risk, providing genetic validation for this drug class. Conversely, gain-of-function mutations cause autosomal dominant hypercholesterolemia.

Adverse effects: Generally well tolerated; most common are injection site reactions, nasopharyngitis, and myalgias. Unlike statins, they do not significantly affect liver enzymes or cause myopathy directly, though they may be used in statin-intolerant patients.

Administration: Subcutaneous injection every 2 weeks (evolocumab, alirocumab) or every 4 weeks depending on dosing regimen. A newer siRNA-based agent, inclisiran, works via a different mechanism (reducing PCSK9 protein synthesis) and is dosed twice yearly.

High-yield board points: Understand mechanism of PCSK9 in LDL receptor degradation, contrast with statins (which inhibit HMG-CoA reductase, upregulating LDL receptor expression via SREBP), and recognize PCSK9 inhibitors as an add-on therapy for LDL lowering in high-risk cardiovascular patients.