The evolution of non-steroidal anti-inflammatory drugs and their mechanisms of action.

The pro-inflammatory effects of prostaglandins have been clearly demonstrated with the use of various animal models of inflammation. Furthermore, the anti-inflammatory effects and some of the side effects of aspirin and other non-steroidal anti-inflammatory agents have been shown to depend on their ability to inhibit cyclo-oxygenase. These drugs, therefore, reduce the synthesis of prostaglandins, prostacyclin and thromboxane. They do not affect leukotriene production and there is no firm evidence to suggest that they alleviate inflammation through any other mechanism. In contrast, the corticosteroids facilitate the release of lipocortin which, through inhibition of phospholipase A2 reduces arachidonic acid release. These drugs possess potent anti-inflammatory properties and attempts have been made to develop non-steroidal drugs, such as BW755C, that display similar anti-inflammatory activity through inhibition of the 2 main pathways of the arachidonic acid cascade. Administration of low dose aspirin 40 mg/day selectively inhibits production of thromboxane A2 without affecting prostacyclin. This may be because, firstly, about 60% of an administered dose of aspirin is deacylated to salicylate during first-pass metabolism and, secondly, platelets cannot regenerate cyclo-oxygenase. Thus, absorbed aspirin irreversibly affects platelet thromboxane production in the pre-systemic circulation, but the systemic plasma aspirin concentration is likely to be too low to affect prostacyclin synthesis. Studies in experimental inflammation have shown that after the administration of aspirin, the concentration of salicylate in inflammatory exudate is considerably higher than that of aspirin. In addition, a comparison of prostaglandin synthesis inhibitory potencies shows that the concentration of salicylate, but not of aspirin, at the inflammatory site is high enough to substantially inhibit prostaglandin synthesis.