The specific thromboxane receptor antagonist, S18886, was evaluated for prevention of coronary arterial thrombosis and myocardial ischemia-reperfusion in anesthetized canines. For the primary thrombosis study in left circumflex (LCX) coronary artery, 26 dogs were randomized to receive either vehicle (n = 7) or intravenous S18886 (0.3 mg/kg, n = 6; 1.0 mg/kg, n = 6; and 3.0 mg/kg, n = 7). The respective times to occlusion after S18886 were as follows: 56.8 +/- 9.3, 83.5 +/- 14.9, and 92.4 +/- 15.7 minutes compared to 43.3 +/- 8.2 minutes after vehicle. S18886 caused a minimal increase in tongue bleeding time and a significant decrease in ex vivo platelet aggregation to arachidonic acid or U46619. Another 37 dogs were randomized to receive placebo (n = 12), clopidogrel 1.0 mg/kg p.o. QDX3 (n = 9), clopidogrel + S18886 0.3 (n = 9) or 1.0 (n = 7) mg/kg intravenous. Clopidogrel produced a 50% reduction in adenosine diphosphate-induced platelet aggregation and a slight increase in the time to occlusion. However, clopidogrel + S18886 1.0 mg/kg prevented occlusive thrombus formation in most of the coronary vessels over 6 hours. S18886 did not alter myocardial infarct size in the ischemia-reperfusion model. In conclusion, S18886 alone caused a dose-dependent prolongation in the time to primary occlusive coronary artery thrombosis, whereas S18886 + clopidogrel displayed effective in preventing occlusive thrombus formation with only a moderate increase of tongue-bleeding time.