Platelet Reactivity With Prolonged Aspirin Treatment

Abstract

Circulation Journal Official Journal of the Japanese Circulation Society http://www. j-circ.or.jp latelets play a critical role in ischemic heart disease, the cause of 10% of all deaths in Japan. 1 Aspirin is the most commonly used antiplatelet agent for secondary prevention of ischemic heart disease, providing an estimated 20-25% reduction in the risk for serious vascular events in high-risk patients. 2,3 Recurrent thrombotic events, however, have been reported in 10-20% of aspirin-treated patients 3 and several studies have found an association between less-than-expected inhibition of in vitro aspirin-sensitive platelet function tests and the occurrence of adverse clinical outcomes. 4-7 Adding complexity to the problem, conflicting results have been obtained with respect to whether aspirin's antiplatelet effect is maintained 8 or attenuated 9 over time. Therefore, the extent to which platelets in Japanese patients are inhibited by aspirin, and the consistency of this inhibition over time, are of great interest. Article p 1227In this issue of the Journal, Ikeda et al have reported on the antiplatelet effect of aspirin and its 2-year change in high cardiovascular risk Japanese patients. 10 Important strengths of that study include the relatively large group of subjects studied (239 at enrollment) over this time interval and that the study addressed the issue of platelet reactivity under reallife conditions in a Japanese population. As was the case with the aspirin-sensitive tests that were found to be associated with poor clinical outcomes, 4-7 the aspirin-sensitive tests evaluated by Ikeda et al, that is, collagen-stimulated light transmission aggregometry and whole blood aggregation measured by screen filtration pressure, measure endpoints many steps removed from aspirin inhibition of platelet cyclooxygenase-1 (COX-1) and must be interpreted in that context. 10 For example, the authors reported that at baseline 27% of aspirin-treated patients produced a threshold platelet aggregation response to 1 mg/L collagen, while only 10% of healthy aspirin-treated donors responded to this dose of collagen. Such patients with high on-treatment platelet reactivity may be at increased risk for poor clinical outcomes. But because many factors other than aspirin inhibition of platelet COX-1 contribute to collagen-stimulated platelet aggregation, not the least of which is the platelet collagen receptor Ia -IIa, 11 it would be more correct to refer to this as platelet hyperreactivity rather than poor response to aspirin. In studies in which platelet COX-1 activity was evaluated by measuring serum thromboxane B2 (TxB2), the most direct measure of platelet COX-1 activity, poor aspirin inhibition of platelet COX-1 was infrequent (<2% of aspirin-treated patients undergoing cardiac catheterization). 12 Nevertheless, high residual serum TxB2 has been associated with adverse clinical outcomes. 13 In the same study, platelet hyperreactivity as measured using a COX-1-independent platelet function assay was also associated with subsequen...