The lack of augmentation by aspirin of inhibition of platelet reactivity by ticlopidine

A decreased threshold for platelet activation apparently contributes to the risk of cardiovascular events, such as acute myocardial infarction. To evaluate the impact of specific agents, we characterized platelet reactivity in 9 healthy subjects before and after 5 days of ingestion of 4 commonly prescribed regimens, 81 mg of aspirin daily, 325 mg of aspirin daily, ticlopidine 250 mg twice daily, and ticlopidine plus 325 mg of aspirin daily. Platelet reactivity was assessed with (1) aggregometry induced by 4 μM adenosine diphospate (ADP) and collagen (0.19 mg/ml) and performed in platelet-rich plasma; and (2) flow cytometric determination of ADP-induced (0.2, 0.8, and 1.5 μM) P-selectin expression in whole blood. Because anticoagulants alter platelet reactivity, results were obtained with 3 anticoagulants, citrate, enoxaparin, or corn trypsin inhibitor (CTI, a specific inhibitor of factor XIIa without effect on other coagulation factors). Ingestion of aspirin did not alter platelet activation as assessed with flow cytometry. Inhibition of the second phase of aggregation was seen with ADP-induced aggregation in platelet-rich plasma anticoagulated with citrate but not enoxaparin or CTI. Ingestion of ticlopidine led to inhibition of ADP-induced aggregation and P-selectin expression. Inhibition of platelet reactivity after the combination of aspirin and ticlopidine did not differ from ticlopidine alone. Marked interindividual variability in platelet reactivity was seen after ingestion of ticlopidine. The results indicate that assessment of effects of specific pharmacologic regimens with accurate and readily available assays of platelet reactivity may facilitate effective prophylaxis and treatment of high-risk subjects with antiplatelet regimens designed to optimally diminish platelet reactivity.