Early Potent Antithrombotic Effect With Combined Aspirin and a Loading Dose of Clopidogrel on Experimental Arterial Thrombogenesis in Humans

BACKGROUND: We conducted a double-blind, randomized, crossover study to assess the antithrombotic effects of the combination of aspirin (acetylsalicylic acid, ASA) and clopidogrel, with or without a loading dose, versus ASA alone in a model of arterial thrombosis in humans. METHODS AND RESULTS: Eighteen male volunteers received the following 3 regimens for 10 days separated by a 1-month period: (1) 325 mg ASA daily, (2) 325 mg ASA+75 mg clopidogrel daily, (3) 325 mg ASA daily+300-mg clopidogrel loading dose on day 1 and +75 mg clopidogrel per day on days 2 to 10. The antithrombotic effect was measured 1.5, 6, and 24 hours after drug intake on day 1 and 6 hours after drug intake on day 10. Arterial thrombus formation was induced ex vivo by exposing a collagen-coated coverslip in a parallel-plate perfusion chamber to native blood for 3 minutes at an arterial wall shear rate. Without a loading dose, clopidogrel+ASA developed an antithrombotic effect within 6 hours after the first intake. It was superior to that produced by ASA, but it was moderate (P</=0.03). However, with the loading dose, the antithrombotic effect of clopidogrel+ASA appeared within 90 minutes, and after 6 hours it was comparable to that on day 10. On day 10, clopidogrel+ASA decreased platelet thrombus formation by approximately 70%, and the effect was significantly more potent than that produced by ASA alone (P<0.001). CONCLUSIONS: This study confirms the synergistic antithrombotic effects of a combined ASA and clopidogrel therapy and shows the early benefit obtained with a loading dose of clopidogrel.