Currently used antiplatelet agents include aspirin, clo-pidogrel (ticlopidine still has a role, see Section III.B), dipyridamole plus aspirin, and platelet glycoprotein IIb/IIIa receptor blockers.
A. Aspirin
Acetylsalicylic acid irreversibly acetylates the enzyme cyclooxygenase. This enzyme is necessary for the conver¬sion of platelet arachidonic acid to thromboxane A2, a powerful platelet-aggregating agent. Cyclooxygenase is inhibited by all nonsteroidal anti-inflammatory drugs (NSAIDs), aspirin transfers the acetyl group to the enzyme that is irreversibly inactivated. Other NSAIDs such as ibuprofen act as reversible inhibitors of cyclooxy-genase (see the chapter Nonsteroidal Anti-Inflammatory Drugs).
Table 1 gives the clinically useful indications for aspirin. Most important, both the use of aspirin and the adminis¬tration of nitroglycerin sublingually also contribute to the prevention of fatal and nonfatal heart attacks. It is important for individuals to realize that a rapid-acting aspirin formulation such as two 80- to 81-mg chewable aspirins taken soon after the onset of chest pain has been shown to cause a 25% reduction in fatal and nonfatal myocardial infarction, whereas nitroglycerin has no effect on prevention. Nitroglycerin ameliorates the pain of stable, mild angina and its use has been overvalued.

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