NSAIDs anti-thrombotic action
NSAIDs bind to the cyclo-oxygenase enzyme in platelets inhibiting their production of thromboxane A2 (TXA2). Low doses of aspirin can preferentially inhibit the production of TXA2 relative to PGI2 by irreversibly acetylating the platelet cyclo-oxygenase and can thus be used therapeutically to decrease the likelihood of development or growth of a thrombus e.g. cats with ileal thrombosis, dogs with heartworm disease. Low dose aspirin is used therapeutically for this purpose since it covalently acetylates cyclo-oxygenase. Platelets have no nucleus and thus no protein synthesis machinery so thromboxane production is inhibited for the life of the platelet (about 7 days), whereas cyclo-oxygenase in other cell types can be replaced.
Although TXA2 generation is only part of the clotting process, the increase in blood clotting times may have implications in aspirin's use in post trauma situations where internal haemorrhage is a consideration. The other NSAIDs are less effective anti-thrombotics and do not prolong bleeding time significantly.
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Massey University
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