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Combination antiplatelet agents, particularly aspirin and ticlopidine, have found increased use in the prevention of arterial thrombosis. Clopidogrel, a thienopyridine derivative, like ticlopidine was recently approved by the U.S. Food and Drug Administration (FDA) for the reduction of ischemic events in patients with myocardial infarction, stroke, or peripheral arterial disease and appears to have much less hematologic toxicity than ticlopidine has. Thrombosis of hemodialysis access grafts is a major cause of morbidity in this patient population. Combination antiplatelet agents may be particularly useful in the prevention of hemodialysis access graft thrombosis. In preparation for such a study, we have performed a pharmacodynamic study of the platelet inhibitory effects of clopidogrel in patients on maintenance hemodialysis. Nine chronic hemodialysis patients were studied. Baseline platelet aggregation studies were performed, after which the subjects were begun on clopidogrel 75 mg daily. Platelet aggregation studies were repeated after 14 days of therapy. Drug was stopped and a final set of platelet aggregation studies were performed 7 days later. Because clopidogrel acts by inhibiting adenosine diphosphate (ADP)-induced platelet aggregation, we used ADP as the agonist in the platelet aggregation studies. We also measured the time required to achieve hemostasis after removing the dialysis needles at the termination of a dialysis session. Patients were carefully monitored for any adverse reaction to clopidogrel. Fourteen days' treatment with clopidogrel inhibited ADP-induced platelet aggregation from 48 to 23% with ADP 2 microM (P=0.0113), from 59 to 38% with ADP 5 microM (P=0. 0166), and from 66 to 44% with ADP 10 microM (P=0. 0172). This inhibition of platelet aggregation was reversed 7 days after stopping clopidogrel. Clopidogrel administration did not affect the time required to achieve hemostasis after removal of the dialysis needles. No adverse reactions were noted. No patient had evidence of bleeding, rash or gastro-intestinal (GI) upset. Clopidogrel inhibits ADP-induced platelet aggregation in subjects receiving chronic maintenance hemodialysis. The magnitude of inhibition is similar to that reported in nonuremic subjects with atherosclerosis. This inhibition is reversible within 7 days of discontinuing the drug. No adverse reactions to the drug were noted in this short-term (14-day) trial.