Background: A recent study showed that clopidogrel reduces thrombo-occlusive complications in patients with symptomatic atherosclerosis more effectively than aspirin.
Methods and results: The effects of clopidogrel and aspirin have been compared, singly and in combination, for measurements of 111In-labeled platelets and 125I-labeled fibrin deposition in baboon models of arterial thrombosis and related to platelet aggregation and expression of activation epitopes induced by ADP, collagen, and thrombin receptor agonist peptide (TRAP) and to template bleeding times (BTs). Low-dose oral clopidogrel (0.2 mg. kg-1. d-1) produced cumulative (1) intermediate decreases in 111In-platelet and 125I-fibrin deposition for segments of prosthetic vascular graft, deployed endovascular metallic stents, and endarterectomized aorta (P<0.009 in all cases); (2) elimination of ADP-induced platelet aggregation (P<0.001); (3) modest inhibition of collagen-induced platelet aggregation (P<0.01); (4) no reduction in TRAP-induced platelet aggregation; and (5) minimal prolongation of BTs (P=0.03). High-dose oral clopidogrel (>/=2 mg/kg) produced the same effects within 3 hours. The effects of clopidogrel dissipated over 5 to 6 days. Aspirin 10 mg. kg-1. d-1 alone did not decrease 111In-platelet and 125I-fibrin deposition on segments of vascular graft but detectably decreased 111In-platelet and 125I-fibrin accumulation on stents (P<0.01), minimally inhibited ADP- and collagen-induced platelet aggregation (P<0.05 in both cases), and minimally prolonged BTs (P=0.004). Within 3 hours of aspirin administration, the antithrombotic effects of acute high-dose or chronic low-dose clopidogrel were substantially enhanced, and BTs were modestly prolonged without inhibiting platelet aggregation induced by TRAP (P<0.001 in all cases compared with clopidogrel alone).
Conclusions: Clopidogrel produces irreversible, dose-dependent, intermediate reduction in thrombosis that is substantially enhanced by the addition of aspirin. The effects of combining aspirin and clopidogrel need to be evaluated in patients at risk of vascular thrombosis.