Abstract 349: Measurement of Platelet Cyclooxygenase-1 Acetylation Quantifies Interaction of Nonsteroidal Anti-Inflammatory Drugs with Aspirin in vivo

Abstract

Aspirin prevents heart attack and stroke by irreversibly acetylating serine-529 in human platelet cyclooxygenase-1 (COX-1) and thereby inhibiting the biosynthesis of thromboxane (Tx). We hypothesized that precise quantification of COX-1 acetylation would allow the direct detection of drug-drug interactions between reversible COX inhibitors, nonsteroidal anti-inflammatory drugs (NSAIDs), and aspirin, which may abrogate its cardio-protective effect. We developed a stable isotope dilution liquid chromatography multiple reaction monitoring/mass spectrometry (LC-MRM/MS) method for measurement of platelet COX-1 acetylation. Serine-529 acetylation reflected faithfully aspirin pharmacology determined with conventional markers (serum TxB2 formation, urinary excretion of 11-dehydro-TxB2, and arachidonic acid induced platelet aggregation), but showed lower technical (CV% 3.7±2.7) and inter-individual variability (CV% 3.7). Oral administration of 325 mg of plain aspirin resulted in complete acetylation of platelet COX-1 within 2 hours and sustained maximal acetylation (71 ± 3% of total COX-1 amount) for up to two days in health volunteers (n=8). Recovery from acetylation, which reflects platelet turn-over, occurred at a rate of 7.2 ± 0.2 % per day. We compared the potential of five NSAIDs to block aspirin from acetylating platelet COX-1 in healthy volunteers (n=7 for each NSAID). Single doses of naproxen (500 mg), ibuprofen (600 mg), and diclofenac (100 mg) but not celecoxib (200 mg) and acetaminophen (1000 mg) prevented complete COX-1 acetylation by 325 mg aspirin administered 2 hours after the NSAID. The competition of NSAIDs with aspirin was conditioned by NSAID pharmacokinetics, potency, and COX isoform selectivity, suggesting variability in the likelihood of the drug-drug interaction between compounds and between patients. Monitoring platelet COX-1 acetylation may have utility in the identification of drug interactions that may limit the cardio-protective effect of low dose aspirin.