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Carboxylic acid-containing nonsteroidal anti-inflammatory drugs (NSAIDs) can be metabolized to chemically reactive acyl glucuronide and/or S-acyl-CoA thioester metabolites capable of transacylating GSH. We investigated the metabolism of the NSAID mefenamic acid (MFA) to metabolites that transacylate GSH, leading to MFA-S-acyl-GSH thioester (MFA-SG) formation in incubations with rat and human hepatocytes and in vivo in rat bile. Thus, incubation of MFA (1-500 μM) with rat hepatocytes led to the detection of MFA-1-β-O-acyl glucuronide (MFA-1-β-O-G), MFA-S-acyl-CoA (MFA-SCoA), and MFA-SG by liquid chromatography-tandem mass spectrometric analysis. The C(max) of MFA-SG (330 nM; 10-min incubation with 100 μM MFA) was 120- to 1400-fold higher than the C(max) of drug S-acyl-GSH adducts detected from studies with other carboxylic acid drugs to date. MFA-SG was also detected in incubations with human hepatocytes, but at much lower concentrations. Inhibition of MFA acyl glucuronidation in rat hepatocytes had no effect on MFA-SG formation, whereas a 58 ± 1.7% inhibition of MFA-SCoA formation led to a corresponding 66 ± 3.5% inhibition of MFA-SG production. Reactivity comparisons with GSH in buffer showed MFA-SCoA to be 80-fold more reactive than MFA-1-β-O-G forming MFA-SG. MFA-SG was detected in MFA-dosed (100 mg/kg) rat bile, where 17.4 μg was excreted after administration. In summary, MFA exhibited bioactivation in rat and human hepatocytes and in vivo in rat, leading to reactive acylating derivatives that transacylate GSH. The formation of MFA-SG in hepatocytes was shown not to be mediated by reaction with MFA-1-β-O-G, and not solely by MFA-SCoA, but perhaps also by intermediary MFA-acyl-adenylate formation, which is currently under investigation.

The 2-methyl substituted indole, 2MI [2-(4-(4-(2,4-dichlorophenylsulfonamido)-2-methyl-1H-indol-5-yloxy)-3-methoxyphenyl)acetic acid] is a potent dual inhibitor of 1) chemoattractant receptor-homologous molecule expressed on T-helper type-2 cells and 2) d-prostanoid receptor. During evaluation as a potential treatment for asthma and allergic rhinitis, 2MI was identified as a mechanism-based inactivator of CYP3A4 in vitro. The inactivation was shown to be irreversible by dialysis and accompanied by an NADPH-dependent increase in 2MI covalent binding to a 55- to 60-kDa microsomal protein, consistent with irreversible binding to CYP3A4. Two glutathione (GSH) adducts, G1 and G2, were identified in vitro, and the more abundant adduct (G1) was unambiguously determined via NMR to be GSH adducted to the 3-position of the 2-methylindole moiety. The potential for a clinical drug-drug interaction arising from mechanism-based inactivation of CYP3A4 by 2MI was predicted using a steady-state model, and a 4.3- to 7.5-fold increase in the exposure of midazolam was predicted at anticipated therapeutic concentrations. To better assess the potential for in vivo drug-drug interactions, the Sprague-Dawley rat was used as an in vivo model. An excellent in vitro-in vivo correlation was observed for the reduction in enzyme steady-state concentration (E'(ss/Ess)) as well as the change in the exposure of a prototypical CYP3A substrate, indinavir (area under the curve (AUC) for indinavir/AUC). In summary, 2MI was identified as a potent mechanism-based inactivator of CYP3A and was predicted to elicit a clinically relevant drug-drug interaction in humans at an anticipated therapeutic concentration.

Flunoxaprofen (FLX) is a chiral nonsteroidal anti-inflammatory drug that was withdrawn from clinical use because of concerns of potential hepatotoxicity. FLX undergoes highly stereoselective chiral inversion mediated through the FLX-S-acyl-CoA thioester (FLX-CoA) in favor of the (R)-(-)-isomer. Acyl-CoA thioester derivatives of acidic drugs are chemically reactive species that are known to transacylate protein nucleophiles and glutathione (GSH). In this study, we investigated the relationship between the stereoselective metabolism of (R)-(-)- and (S)-(+)-FLX to FLX-CoA and the subsequent transacylation of GSH forming FLX-S-acyl-glutathione (FLX-SG) in incubations with rat hepatocytes in suspension. Thus, when hepatocytes (2 million cells/ml) were treated with (R)-(-)- or (S)-(+)-FLX (100 microM), both FLX-CoA and FLX-SG were detected by sensitive liquid chromatography-tandem mass spectrometry techniques. However, these derivatives were observed primarily from (R)-(-)-FLX incubation extracts, for which the formation rates of FLX-CoA and FLX-SG were rapid, reaching maximum concentrations of 42 and 2.8 nM, respectively, after 6 min of incubation. Incubations with (S)-(+)-FLX over 60 min displayed 8.1 and 2.7% as much FLX-CoA and FLX-SG area under the concentration versus time curves, respectively, compared with corresponding incubations with (R)-(-)-FLX. Coincubation of lauric acid (1000 microM) with (R)-(-)-FLX (10 microM) led to the complete inhibition of FLX-CoA formation and a 98% inhibition of FLX-SG formation. Reaction of authentic (R,S)-FLX-CoA (2 microM) with GSH (10 mM) in buffer (pH 7.4, 37 degrees C) showed the quantitative formation of FLX-SG after 3 h of incubation. Together, these results demonstrate the stereoselective transacylation of GSH in hepatocyte incubations containing (R)-(-)-FLX, which is consistent with bioactivation by stereoselective (R)-FLX-CoA formation.