Acetylsalicylic acid, also known as ASA or aspirin, belongs to a class of drugs known as non-steroidal anti-inflammatory drugs (NSAIDs). In addition to its anti-inflammatory properties, aspirin also acts as an analgesic, antipyretic and antithrombotic agent. Like most other NSAIDs, aspirin exerts its therapeutic effects by inhibiting prostaglandin G/H synthase 1 and 2, better known as cyclooxygenase-1 and -2 or simply COX-1 and -2. COX-1 and -2 catalyze the conversion of arachidonic acid to prostaglandin G2 and prostaglandin G2 to prostaglandin H2. Prostaglandin H2 is the precursor to a number of other prostaglandins, such as prostaglandin E2, involved in pain, fever and inflammation. The antipyretic properties of aspirin arise from inhibition of prostaglandin E2 synthesis in the preoptic region of the hypothalamus. Interference with adhesion and migration of granulocytes, polymorphonuclear leukocytes and macrophages at sites of inflammation account for its anti-inflammatory effects. The analgesic effects of aspirin likely occur due to peripheral action at the site of injury and possibly within the CNS.
Aspirin is unique from other NSAIDs in that it is an irreversible COX inhibitor. Aspirin irreversibly acetylates a serine side chain of COX rendering the enzyme inactive. Enzyme activity can only be regained by production of more cyclooxygenase. This unique property of aspirin and its higher selectivity for COX-1 over COX-2 makes it an effective antiplatelet agent. Platelets contain COX-1, a key enzyme in the production thromboxane A2 (TXA2), which is a potent inducer of platelet aggregation. Since platelets lack the ability to make more enzyme, TXA2 production is inhibited for the lifetime of the platelet (approximately 8 – 12 days). Aspirin is commonly used at low doses to prevent cardiovascular events such as strokes and heart attacks. At higher doses, aspirin may be used as an analgesic, anti-inflammatory and antipyretic. Aspirin may cause gastric irritation and bleeding by inhibiting the synthesis of prostaglandins that enhance and maintain the protective gastric mucous layer.