In the presence of strong P2Y12 receptor blockade, aspirin provides little additional inhibition of platelet aggregation

Abstract

Background:Aspirin and antagonists of platelet ADP P2Y12 receptors are often coprescribed for protection against thrombotic events. However, blockade of platelet P2Y12 receptors can inhibit thromboxane A2 (TXA2)-dependent pathways of platelet activation independently of aspirin. Objectives:To assess in vitro whether aspirin adds additional antiaggregatory effects to strong P2Y12 receptor blockade. Methods:With the use of platelet-rich plasma from healthy volunteers, determinations were made in 96-well plates of platelet aggregation, TXA2 production and ADP/ATP release caused by ADP, arachidonic acid, collagen, epinephrine, TRAP-6 amide and U46619 (six concentrations of each) in the presence of prasugrel active metabolite (PAM; 0.1-10μmolL-1), aspirin (30μmolL-1), PAM+aspirin or vehicle. Results:PAM concentration-dependently inhibited aggregation; for example, aggregation in response to all concentrations of ADP and U46619 was inhibited by ≥95% by PAM at >3μmolL-1. In further tests of PAM (3μmolL-1), aspirin (30μmolL-1) and PAM+aspirin, aspirin generally failed to produce more inhibition than PAM or additional inhibition to that caused by PAM. The antiaggregatory effects of PAM were associated with reductions in the platelet release of both TXA2 and ATP+ADP. Similar effects were found when either citrate or lepirudin were used as anticoagulants, and when traditional light transmission aggregometry was conducted at low stirring speeds. Conclusions:P2Y12 receptors are critical to the generation of irreversible aggregation through the TXA2-dependent pathway. As a result, strong P2Y12 receptor blockade alone causes inhibition of platelet aggregation that is little enhanced by aspirin. The clinical relevance of these observations remains to be determined. © 2011 International Society on Thrombosis and Haemostasis.