Effect of maximum OTC doses of naproxen sodium or acetaminophen on low-dose aspirin inhibition of serum thromboxane B2

Objectives: This study evaluated the platelet inhibitory effects of low-dose enteric-coated aspirin (EC-ASA) when used concomitantly with maximum over-the-counter (OTC) doses of naproxen sodium (NAPSO) or acetaminophen to determine whether NAPSO and acetaminophen interfere with the anti-platelet effect of aspirin. Research design and methods: Phase I, randomized, open-label, multi-dose, three-period, parallel group, pharmacodynamic trial conducted in healthy male and female volunteers (n=47 randomized subjects and n=37 evaluable subjects), mean age 40.2 years. All subjects received 5 days of EC-ASA 81 mg once daily followed by 5 days of EC-ASA 81 mg once daily alone or co-administered with either NAPSO 220 mg three times daily or acetaminophen 1 g four times daily. Primary outcome measure: Inhibition of serum thromboxane B-2 (TXB2), as a marker of platelet cyclooxygenase-1 (COX-1) inhibition, measured on Day 11. Results: Mean inhibition of TXB2 on Day 11 was >99% for subjects taking EC-ASA alone as well as for those who received EC-ASA co-administered with NAPSO or acetaminophen. For subjects taking EC-ASA monotherapy, mean serum TXB2 inhibition was 99.7% (range 99.0-100%), for those taking EC-ASA with acetaminophen it was 99.6% (range 98.3-99.9%), and for those taking EC-ASA with NAPSO, mean serum TXB2 inhibition was 99.7% (range 99.2-100%). Study limitation: Small sample size and open-label trial design. Conclusions: The anti-platelet effect of EC-ASA 81 mg once daily was maintained following its co-administration with maximum OTC doses of NAPSO or acetaminophen.