Involvement of human cytochrome P450 2D6 in the bioactivation of acetaminophen

Acetaminophen (APAP), a widely used analgesic and antipyretic agent, can ca use acute hepatic necrosis in both humans and experimental animals when con sumed in large doses. It is generally accepted that N-acetyl-p-benzoquinone imine (NAPQI) is the toxic, reactive intermediate whose formation from APA P is mediated by cytochrome P450. Several forms of P450 in humans, includin g 2E1, 1A2, 2A6, 3A4, have been shown to catalyze the oxidation of APAP to NAPQI. We now present evidence which demonstrates that human cytochrome P45 0 2D6 (CYP2D6) is also involved in the bioactivation of APAP. The formation of NAPQI from APAP by cDNA-expressed CYP2D6 was examined. K-m and V-max va lues were 1.76 mM and 3.02 nmol/min/nmol of P450, respectively, such that t he efficiency of CYP2D6 in the conversion of APAP to NAPQI is approximately one-third of that of CYP2E1. The contribution of CYP2D6 to the total forma tion of NAPQI from APAP (1 mM) in human liver was investigated using quinid ine (1 mm) as a CYP2D6-specific inhibitor, and varied from 4.5 to 22.4% amo ng 10 livers, with an average at 12.6%. The correlation between the contrib ution of CYP2D6 to NAPQI formation in human liver microsomes and the CYP2D6 activity probed by the O-demethylation of dextromethorphan was studied, an d found to be strong (r(2) = 0.85), and significant (P < .0001). Our findin gs indicate that CYP2D6, one of the major P450 isoforms in humans and also one of the pharmacogenetically important isoforms, may contribute significa ntly to the formation of the cytotoxic metabolite NAPQI, especially in CYP2 D6 ultra-rapid and extensive metabolizers and at toxic doses of APAP when p lasma APAP concentrations reach 2 mM or more.