Aspirin at low dose (75–150 mg/day) selectively inhibits platelet COX-1. At analgesic doses (325–650 mg), it additionally inhibits endothelial COX-1 and COX-2. Why does low-dose aspirin selectively and irreversibly inhibit platelet COX-1 without prolonged inhibition of endothelial prostacyclin synthesis?

• A Low-dose aspirin only crosses platelet membranes due to its lipid solubility; endothelial cells are shielded by the blood-brain barrier equivalent
• B Platelets lack nuclei and cannot synthesise new COX-1 protein once irreversibly acetylated; endothelial cells have nuclei and regenerate new COX-2 (and some COX-1) within hours ✓
• C Platelets express a unique COX-1 isoform (COX-1p) with higher aspirin affinity than the endothelial isoform
• D Low-dose aspirin undergoes complete first-pass metabolism; the portal blood has higher aspirin concentrations reaching the platelet-rich hepatic sinusoids compared to arterial endothelium

Explanation

The explanation for low-dose aspirin's selectivity is pharmacokinetic-cellular biology rather than enzyme selectivity. Platelets are anucleate cells with no DNA or protein synthesis machinery — once aspirin irreversibly acetylates their COX-1, that platelet cannot regenerate the enzyme for its entire 7–10 day lifespan. In contrast, nucleated endothelial cells (which express COX-2 constitutively in vessels) rapidly synthesise new COX-2 enzyme within a few hours. Furthermore, the presystemic first-pass effect of aspirin in portal blood exposes portal vein platelets to high aspirin concentrations before systemic distribution, allowing selective irreversible platelet COX-1 acetylation at doses that spare systemic endothelial prostacyclin significantly.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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