The mechanism of action of clopidogrel is similar to that of ticlopidine but different from that of aspirin.[1] Both clopidogrel and ticlopidine require biotransformation for their pharmacologic activity.
	Clopidogrel is a potent, noncompetitive inhibitor of ADP-induced platelet aggregation. Clopidogrel inhibits the binding of ADP to platelet membrane receptors. The effect of clopidogrel on ADP binding is irreversible[2] and lasts for the duration of platelet life, about 7 to 10 days. The inhibition is also specific and does not significantly affect cyclooxygenase or arachidonic acid metabolism.[1]
	Both low- and high-affinity ADP receptors are present on platelets, and the active metabolite of clopidogrel binds to the low-affinity receptors.[1] ADP binding to this site is necessary for activation of the GP IIb/IIIa receptor, which is the binding site for fibrinogen. Fibrinogen links different platelets together to form the platelet aggregate.[3] Clopidogrel thus ultimately inhibits the activation of the GP IIb/IIIa receptor and its binding with fibrinogen.[3]
	Aspirin inhibits the cyclooxygenase enzyme, preventing the production of prostaglandin and thromboxane A2 (TXA2) from arachidonic acid.[3] TXA2 activates the GP IIb/IIIa binding site on the platelet, allowing fibrinogen to bind. Aspirin also exerts its effects on other parts of the body system.[3] Paradoxically, aspirin blocks synthesis of prostacyclin by endothelial cells, resulting in an effect that promotes platelet aggregation.[3]
	Dipyridamole has been suggested to act as an antiplatelet drug by several possible mechanisms. It directly stimulates prostacyclin synthesis, potentiates the platelet inhibitory actions of prostacyclin, and inhibits phosphodiesterase to raise platelet cyclic AMP (cAMP) levels. However, these effects may not occur at therapeutic levels of the drug; hence the mechanism of action of dipyridamole remains to be elucidated.[3]
		Schrör K. The basic pharmacology of ticlopidine and clopidogrel. Platelets. 1993;4:252-261.
		Plavix® (clopidogrel bisulfate) Prescribing Information.
		Schafer AI. Antiplatelet therapy. Am J Med. 1996;101:199-209.