Ex vivo response to aspirin differs in stroke patients with single or recurrent events: a pilot study

The dose of aspirin for secondary stroke prevention and the clinical meanin g of ex vivo platelet abnormalities are debated. We assessed prospectively 39 noncardioembolic stroke patients in which 300 mg/day aspirin had proved effective (n=24) or ineffective (n=15) to prevent recurrent ischemic events . We evaluated platelet aggregation induced by arachidonic acid, adenosine diphosphate and epinephrine, and the sensitivity of platelets to increasing concentrations of the synthetic thromboxane mimetic U46619. Aggregation st udies were repeated while subjects received 300 (study phase 1), and 600 (s tudy phase 2) mg/day aspirin, respectively. Overall, arachidonic acid-induc ed platelet aggregation was less effectively inhibited during study phase 1 compared to phase 2. Arachidonic acid and epinephrine promoted a stronger platelet aggregation in aspirin nonresponders than in aspirin responders wh ile taking 300 mg/day aspirin. On the other hand, 600 mg/day effectively in hibited platelet function in both clinical groups. A lower sensitivity to t hromboxane receptors was also found during phase 1 of the study, although t he response was similar between aspirin responders and nonresponders. This pilot study suggests that 300 mg/day aspirin is less effective than 600 mg/ day to block the cyclooxygenase pathway in noncardioembolic stroke and, inc omplete cyclooxygenase inhibition is associated with recurrent thromboembol ic events despite adequate aspirin compliance. It is likely that patients c ould receive a more efficacious stroke prevention if the dose of aspirin is tailored to individual needs as reflected by laboratory findings. (C) 1999 Elsevier Science B.V. All rights reserved.