Aspirin's selectivity for irreversible COX-1 inhibition in platelets vs. transient COX-2 inhibition in endothelial cells at low doses depends on:
- A Platelets lack nuclei and cannot synthesize new COX, so irreversible acetylation permanently abolishes TXA2 synthesis; endothelial cells have nuclei and regenerate COX-2, restoring PGI2 production between doses ✓
- B Platelet COX-1 has a larger arachidonic acid binding channel, making irreversible acetylation by aspirin stoichiometrically more complete
- C Low-dose aspirin reaches higher portal vein concentrations that preferentially acetylate mesenteric venous endothelial COX-2
- D Low-dose aspirin is deacetylated to salicylate before reaching systemic circulation; salicylate is a selective COX-2 inhibitor
Explanation
This is the pharmacokinetic/pharmacodynamic basis of aspirin's selectivity. Platelets are anucleate and cannot synthesize new proteins; once COX-1 is irreversibly acetylated, TXA2 production is abolished for the platelet's lifespan (~7–10 days). Endothelial cells, being nucleated, continuously synthesize fresh COX-2, restoring PGI2 (prostacyclin, the antithrombotic counterpart to TXA2) production. This pharmacological selectivity is the rationale for low-dose aspirin (75–100 mg/day) in cardiovascular prophylaxis — net antithrombotic effect from selective, durable COX-1/TXA2 inhibition while preserving PGI2.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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