quinone oxidoreductase 1 (NQO1) is an enzyme capable of reducing a broad range of chemically reactive quinones and quinoneimines (QIs) and can be strongly upregulated by Nrf2/Keap1-mediated stress responses. Several commonly used drugs implicated in adverse drug reactions (ADRs) are known to form reactive QI metabolites upon bioactivation by P450, such as acetaminophen (APAP), diclofenac (DF), and mefenamic acid (MFA). In the present study, the reductive activity of human NQO1 toward the QI metabolites derived from APAP and hydroxy-metabolites of DF and MFA was studied, using purified bacterial P450 BM3 (CYP102A1) mutant M11 as a bioactivation system. The NQO1-catalyzed reduction of the QI metabolites was quantified relative to spontaneous glutathione (GSH) conjugation. Addition of NQO1 to the incubations strongly reduced the formation of all corresponding GSH conjugates, and this activity could be prevented by dicoumarol, a selective NQO1 inhibitor. The GSH conjugation was strongly increased by adding human GSTP1-1 in a wide range of GSH concentrations. Still, NQO1 could effectively compete with the GST catalyzed GSH conjugation by reducing the QIs. In conclusion, we identified the QI metabolites of the 4'- and 5-hydroxy-metabolites of DF and MFA as novel substrates for human NQO1. NQO1-mediated reduction proves to be an effective pathway to detoxify these QI metabolites in addition to GSH conjugation. Genetically determined deficiency of NQO1 therefore might be a risk factor for ADRs induced by reactive QI drug metabolites.

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Paracetamol (acetaminophen, APAP) overdose is a leading cause of acute drug-induced liver failure. APAP hepatotoxicity is mediated by the reactive metabolite N-acetyl-p-benzoquinone imine (NAPQI). NAPQI is inactivated by conjugation with glutathione (GSH) to APAP-GSH, which is further converted into its cysteine derivative APAP-CYS. Before necrosis of hepatocytes occurs, APAP-CYS is measurable in plasma of the affected patient and it has been proposed as an early biomarker of acetaminophen toxicity. APAP-GSH and APAP-CYS can be extruded by hepatocytes, but the transporters involved are unknown. In this study we examined whether ATP-binding cassette (ABC) transporters play a role in the cellular efflux of APAP, APAP-GSH, and APAP-CYS. The ABC transport proteins P-gp/ABCB1, BSEP/ABCB11, BCRP/ABCG2, and MRP/ABCC1-5 were overexpressed in HEK293 cells and membrane vesicles were produced. Whereas P-gp, BSEP, MRP3, MRP5, and BCRP did not transport any of the compounds, uptake of APAP-GSH was found for MRP1, MRP2 and MRP4. APAP-CYS appeared to be a substrate of MRP4 and none of the ABC proteins transported APAP. The results suggest that the NAPQI metabolite APAP-CYS can be excreted into plasma by MRP4, where it could be a useful biomarker for APAP exposure and toxicity. Characterization of the cellular efflux of APAP-CYS is important for its development as a biomarker, because plasma concentrations might be influenced by drug-transporter interactions and upregulation of MRP4.

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