Aspirin irreversibly acetylates COX-1 and COX-2. Its antiplatelet effect lasts ~7–10 days (platelet lifespan) despite its short plasma half-life of 15–20 minutes. Why is the antiplatelet effect of low-dose aspirin preserved while its anti-inflammatory effect requires higher doses?

• A At low doses, aspirin preferentially binds COX-1 because its Km for COX-1 is lower than for COX-2; anti-inflammatory action requires COX-2 inhibition which needs higher concentrations
• B Antiplatelet effect requires only COX-1 inhibition in megakaryocytes; nucleated endothelial cells continuously resynthesise COX-1, maintaining prostacyclin production at any dose
• C Low-dose aspirin deacetylates platelet membrane phospholipids, preventing arachidonic acid release; anti-inflammatory action requires systemic salicylate levels which only occur at higher doses
• D Low-dose aspirin selectively acetylates platelet COX-1 in the portal blood before hepatic metabolism reduces plasma concentrations; nucleated cells regenerate COX-2 through de novo protein synthesis but anucleate platelets cannot regenerate any COX ✓

Explanation

Aspirin is extensively hydrolysed by portal first-pass esterases, so only low portal blood levels reach systemic circulation at small doses. However, platelets passing through the portal blood are irreversibly acetylated on their COX-1 before hepatic metabolism. Since platelets are anucleate, they cannot regenerate new COX protein throughout their 7–10 day lifespan. Nucleated vascular endothelial cells can resynthesise prostacyclin-producing COX-2 within hours. Anti-inflammatory doses must achieve systemic salicylate concentrations sufficient to inhibit COX in inflamed tissues — nucleated cells that continually replenish their COX supply — requiring >1g/day. This acetylation selectivity and the pre-systemic mechanism is the pharmacokinetic basis for the 'aspirin paradox' at low doses.

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

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Written and medically reviewed by the StethoPrep medical team.