Ticlopidine in unstable angina. A more expensive aspirin?

U nstable angina is a syndrome in which the occlusion of coronary arteries by plateletdependent thrombus formation may result in myocardial infarction and death.' Angiographic2 and angioscopic3 evidence of thrombus formation is complemented by the measurement of increased concentrations of thromboxane (TX) metabolites in plasma and urine during ischemic episodes in patients with this disease.4 TXA2 is the major cyclooxygenase product of arachidonic acid metabolism in human platelets and is a potent platelet activator and vasoconstrictor. Aspirin, which irreversibly inhibits this enzyme by acetylation of a serine residue close to the active site,5 has been shown to reduce significantly myocardial infarction and death in four placebocontrolled, double-blind trials.6-9 Although it is possible that other pharmacological properties of aspirin are relevant, its efficacy has been explicable in terms of its action as an inhibitor of platelet cyclooxygenase.