Aspirin and Antiplatelet Medications (cont.)

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What is the relative potency of the antiplatelet agents?

  • Since aspirin blocks only one of the several pathways by which platelet aggregation can occur, aspirin is a weak antiplatelet agent because platelet aggregation can be stimulated via another pathway.

  • Since glycoprotein IIb/IIIa inhibitors block the final common pathway for platelet aggregation (platelet aggregation is blocked regardless of the nature of the initial stimuli), glycoprotein IIb/IIIa inhibitors are the most potent antiplatelet agents. The maximal antiplatelet effect of glycoprotein IIb/IIIa inhibitors is approximately nine times that of aspirin.

  • The maximal antiplatelet effect of thienopyridines is in between that of aspirin and the glycoprotein IIb/IIIa inhibitors.

How quickly do antiplatelet agents work?

Aspirin

When aspirin is given in low doses (75 mg/day), the complete inhibition of the COX-1 enzyme and hence maximal antiplatelet effect may take several days. At a dose of 160-325 mg/day, the maximal antiplatelet effect of aspirin occurs within 30 minutes. Thus, aspirin at low doses (75-150 mg/day) is used for the long term prevention of heart attacks and strokes, whereas moderate doses (160-325 mg/day) of aspirin are given in situations where an immediate anti-clotting effects are needed (such as in the treatment of acute heart attacks and unstable angina).

Thienopyridines

Like aspirin, the onset of action of clopidogrel (Plavix) is dose related. Maximal antiplatelet effect occurs several days after initiation of clopidogrel (75 mg/daily), but can occur within hours after larger doses of 300 or 600 mg. Therefore, larger doses of clopidogrel are used initially when immediate antiplatelet actions are needed (such as after placement of intracoronary stents) while the lower doses are used as maintenance.

Glycoprotein IIb/IIIa inhibitors

The glycoprotein IIb/IIIa inhibitors have a rapid onset of action. Their maximal antiplatelet effect occur within minutes after an intravenous infusion, and are used mainly in patients with unstable angina or acute heart attack (myocardial infarction).

What is dipyridamole?

Dipyridamole (Persantine IV, Persantine) is another medication that decreases platelet aggregation, though the exact mechanism of its antiplatelet action is not well understood. Dipyridamole is not commonly used in heart attack prevention, although it is sometimes used with aspirin to lessen the chance of stroke.

When is aspirin used for preventing and treating heart attacks and strokes?



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