Aspirin and Antiplatelet Medications (cont.)

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Blood clots are important because they stop bleeding (for example, a cut or laceration on the skin). When bleeding occurs from a cut, platelets become activated and form a network by attaching to the blood vessel wall at the site of bleeding, and by also attracting other clotting factors in the blood (such as fibrin) to stop ongoing bleeding rapidly.

However, if a blood clot forms inside an artery, it blocks the flow of blood to the tissue that the artery supplies, which can damage the tissue. For example, a blood clot that forms in a coronary artery supplying blood to the muscle of the heart causes a heart attack, and a blood clot that forms in an artery supplying blood to the brain causes a stroke.

How do antiplatelet agents work?

Aspirin

Aspirin prevents blood from clotting by blocking the production by platelets of thromboxane A-2, the chemical that causes platelets to clump. Aspirin accomplishes this by inhibiting the enzyme cyclo-oxygenase-1 (COX-1) that produces thromboxane A-2. While other NSAIDs also inhibit the COX-1 enzyme, aspirin is the preferred NSAID for use as an antiplatelet agent because its inhibition of the COX-1 enzyme lasts much longer than the other NSAIDs (aspirin's antiplatelet effect lasts days while the other NSAIDs' antiplatelet effects last only hours).

Thienopyridines

In addition to thromboxane A-2, platelets also produce adenosine diphosphate (ADP). When ADP attaches to receptors on the surface of platelets, the platelets clump. The thienopyridines, for example, ticlopidine (Ticlid) and clopidogrel (Plavix), block the ADP receptor. Blocking the ADP receptor prevents ADP from attaching to the receptor and the platelets from clumping.

Glycoprotein IIb/IIIa inhibitors

The glycoprotein IIb/IIIa inhibitors, such as abciximab (Reopro) and eptifibatide (Integrilin), prevent clumping by inhibiting a different receptor on the surface of platelets, the receptor for glycoprotein IIb/IIIa. The glycoprotein IIb/IIIa inhibitors that are approved by the FDA must be given intravenously (in the veins).



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