Figure 3.

Pharmacokinetics of aspirin. Aspirin is absorbed in the stomach and small intestine, exerts its pharmacodynamic effect, for instance, the acetylation of a Serine (Ser) 529 residue of COX-1 already in the portal blood, and is biotransformed to inactive salicylic acid by intestine, plasma, and liver esterases. On average, its systemic bioavailability is approx. 50% of the administered dose. Once in the systemic circulation, aspirin reaches bone-marrow megakaryocytes (MKs) and inhibits COX-1 and -2 of MKs and developing platelets. HCE: human carboxylesterase.