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Nevirapine, a non-nucleoside reverse transcriptase inhibitor used for the treatment of AIDS, can cause serious skin rashes and hepatotoxicity. Previous studies have indicated that the benzylic sulfate 12-sulfoxynevirapine, the formation of which is catalyzed by human sulfotransferases (SULTs), may play a causative role in these toxicities. To characterize better the role of 12-sulfoxynevirapine in nevirapine-induced cytotoxicity, the ability of 12 expressed human SULT isoforms to conjugate 12-hydroxynevirapine was assessed. Of the 12 human SULTs, no detectable 12-sulfoxynevirapine was observed with SULT1A3, SULT1C2, SULT1C3, SULT2B1, SULT4A1, or SULT6B1. As determined by the Vmax/Km ratio, SULT2A1 had the highest overall 12-hydoxynevirapine sulfonation activity; lower activities were observed with SULT1A1, SULT1A2, SULT1B1, SULT1C4, and SULT1E1. Incubation of 12-sulfoxynevirapine with glutathione and cysteine led to adduct formation; lower yields were obtained with deoxynucleosides. 12-Hydroxynevirapine was more cytotoxic than nevirapine to TK6, TK6/SULT vector, and TK6/SULT2A1 cells. With nevirapine, there was no difference in cytotoxicity among the three cell lines, whereas with 12-hydroxynevirapine, TK6/SULT2A1 cells were more resistant than TK6 and TK6/SULT vector cells. Co-incubation of 12-hydroxynevirapine with the competitive SULT2A1 substrate dehydroepiandrosterone decreased the level of 12-sulfoxynevirapine and increased the cytotoxicity in TK6/SULT2A1 cells. These data demonstrate that although 12-sulfoxynevirapine reacts with nucleophiles to form adducts, sulfonation of 12-hydroxynevirapine decreases the cytotoxicity of 12-hydroxynevirapine in TK6 cells.