Clopidogrel is a potent antithrombotic drug that inhibits ADP-induced platelet aggregation. The results of large clinical trials have demonstrated an overall benefit of clopidogrel over aspirin in the prevention of vascular ischemic events (myocardial infarction, stroke, vascular death) in patients with a history of symptomatic atherosclerotic disease. The antiaggregating effect of clopidogrel is attributed to an irreversible inhibition of ADP binding to a purinergic receptor present at the platelet surface. Clopidogrel is not active in vitro and can be considered a precursor of an active metabolite formed in the liver. The chemical structure of this active metabolite and its biological activity have been described recently. Several purinergic receptors have been described on platelets; P2X (1), a calcium channel, and P2Y1 a Gq-coupled seven-transmembrane domain receptor, have been found not to be antagonized by clopidogrel. Another Gi (2)-coupled receptor (named P2Y12) has been recently cloned and stably expressed in CHO cells. These cells displayed a strong affinity for (33)P-2MeS-ADP, a stable analogue of ADP, the binding characteristics of which corresponded in all points to those observed on platelets. The binding of (33)P-2MeS-ADP to these cells was strongly inhibited by the active metabolite of clopidogrel with a potency that was consistent with that observed for this compound on platelets. In these transfected CHO cells, as in platelets, ADP and 2MeS-ADP induced adenylyl cyclase downregulation, an effect that was inhibited by the active metabolite of clopidogrel. These results demonstrate that this receptor corresponds to the previously called "P2t" platelet receptor and show that the active metabolite of clopidogrel binds in a covalent manner to this receptor, thus explaining how it blocks the aggregating effect of ADP on platelets.