The poor cyclooxygenase (COX) inhibitor and major aspirin metabolite salicylic acid is known to exert analgesic and anti-inflammatory effects by still unidentified mechanisms. In RAW 264.7 macrophages, lipopolysaccharide (LPS)-induced COX-2-dependent synthesis of prostaglandin E-2 (PGE(2)) was suppressed by aspirin (IC50 of 5.35 mu M), whereas no significant inhibition was observed in the presence of sodium salicylate and the salicylate metabolite salicyluric acid at concentrations up to 100 mu M. However, the salicylate metabolite gentisic acid (2,5-dihydroxybenzoic acid; 10-100 mu M) and salicyl-coenzyme A (100 mu M), the intermediate product in the formation of salicyluric acid from salicylic acid, significantly suppressed LPS-induced PGE(2) production. In contrast, gamma-resorcylic acid (2,6-dihydroxybenzoic acid) as well as unconjugated coenzyme A failed to affect prostanoid synthesis, implying that the para-substitution of hydroxy groups and the activated coenzyme A thioester are important for COX-2 inhibition. Using realtime RT-PCR, none of the salicylate derivatives tested were found to interfere with COX-2 expression. Overall, our results suggest that certain metabolites of salicylic acid may contribute to the pharmacological action of its parent compound by inhibiting COX-2-dependent PGE(2) formation at sites of inflammation.