Different Reactive Metabolites of Nevirapine Require Distinct Glutathione S-Transferase Isoforms for Bioinactivation

Nevirapine (NVP) is a non-nucleoside reverse transcriptase-inhibitor, which is associated with severe idiosyncratic skin rash and hepatotoxicity. These adverse drug reactions are believed to be mediated by the formation of epoxides and/or quinone methide formed by oxidative metabolism by P450s and 12-sulfoxyl-NVP formed by sequential 12-hydroxylation and O-sulfonation. Although different GSH-conjugates and corresponding mercapturic acids have been demonstrated previously in vitro and in vivo, the role of the glutathione S-transferases in the inactivation of the different reactive metabolites has not been studied so far. In the present study the activity of 10 recombinant human glutathione S-transferases (GSTs) in the detoxification of the different reactive metabolites of NVP was studied. The results show that GSTP1-1 is a highly active catalyst of GSH-conjugation of the oxidative metabolites of NVP, even at high GSH-concentration. Experiments with trideuterated NVP suggest involvement of a reactive epoxide rather than quinone methide in the formation of the GSH-conjugate formed after oxidative bioactivation. GSH-conjugation of 12-sulfoxyl-NVP forming NVP-12-GSH was only catalyzed by GSTM1-1, GSTA1-1, and GSTA3-3. Although the exact expression levels of these enzymes in the skin is unknown, the relatively low activity of this catalysis makes it unlikely that GSTs can provide significant protection against this metabolite. However, since NVP-12-GSH is specifically formed via the 12-sulfoxyl-NVP, its corresponding urinary mercapturic acid can be considered as a biomarker for recent internal exposure to this protein-reactive sulfate. However, it has to be taken into account that 12-sulfoxyl-NVP is not completely trapped by GSH and that rates of bioinactivation will differ between patients due to variability in expression of GSTM1, GSTA1, and GSTA3.