Nimesulide, a widely used nonsteroidal anti-inflammatory drug (NSAID), has been associated with rare idiosyncratic hepatotoxicity. The chemical mechanisms underlying the liver injury remain unknown. We have undertaken the detailed study of the metabolic pathways of nimesulide in an effort to identify potential reactive metabolites. A previous report from this laboratory has demonstrated that one of the known nimesulide metabolites, termed reduced nimesulide (M1), is further bioactivated by human liver microsomes (HLMs) to form a reactive diiminoquinone species M2. The formation of M2 was confirmed indirectly by trapping with N-acetylcysteine (NAC). The aim of this study was to explore the fate of M1 in an inflammatory environment created by the recruitment of leukocytes. Leukocytes upon activation produce hydrogen peroxide (H(2)O(2)) and other myeloperoxidase (MPO) products, such as hypochlorous acid (HOCl), that are capable of metabolite oxidation. We demonstrate here that the reduced nimesulide, M1, undergoes a facile oxidation with activated neutrophils or with MPO in the presence of H(2)O(2) or HOCl to produce a variety of reactive as well as stable metabolites. One major metabolite, M3, was also produced by HLM as determined by trapping with NAC. Other metabolites, for example, M6, M8, and M9, were unique to the myeloperoxidase, because of their mode of formation from activation of the amino group of reduced nimesulide. The structures of some of these reactive metabolites were proposed on the basis of liquid chromatography-tandem mass spectrometry analyses and established by their comparison with synthetic standards. Metabolite M6 is interesting because it provides clear evidence of amine activation and indicates the potential of the reactive intermediate of M1 to conjugate with protein nucleophiles. In summary, our results demonstrate that a known nimesulide metabolite could be bioactivated by MPO through a pathway distinct from HLM-mediated pathways and that the generation of reactive species by the MPO-mediated bioactivation pathway at the site of inflammation may contribute to the toxicity associated with nimesulide.