TY - JOUR T1 - Diclofenac-Induced Inactivation of CYP3A4 and Its Stimulation by Quinidine JF - Drug Metabolism and Disposition JO - Drug Metab Dispos SP - 1143 LP - 1148 DO - 10.1124/dmd.30.10.1143 VL - 30 IS - 10 AU - Yasuhiro Masubuchi AU - Atsushi Ose AU - Toshiharu Horie Y1 - 2002/10/01 UR - http://dmd.aspetjournals.org/content/30/10/1143.abstract N2 - Incubation of human liver microsomes with diclofenac in the presence of NADPH resulted in a decrease in testosterone 6β-hydroxylation activity. The decrease in the activity followed time- and concentration-dependent kinetics, required oxidative metabolism, and was resistant to reduced glutathione, suggesting that diclofenac causes a mechanism-based inactivation of cytochrome P450 (P450) 3A4 (CYP3A4). The inactivation was reproduced by using microsomes from B-lymphoblastoid cell lines expressing CYP3A4 instead of human liver microsomes. No other monooxygenase activities measured as indexes of P450 enzymes; CYP2C8, CYP2C9, or CYP2C19 was inactivated by the same incubation procedure. Quinidine, a stimulant of CYP3A4-mediated diclofenac 5-hydroxylation, did not affect the inactivation of CYP3A4 assessed by testosterone 6β-hydroxylation activity but accelerated the inactivation assessed by diazepam 3-hydroxylation activity. These results supported the idea that diclofenac 5-hydroxylation is involved in the inactivation of CYP3A4 and described for the first time a stimulation of mechanism-based inactivation attributable to CYP3A4 heterotropic cooperativity. Preincubation of human liver microsomes with 5-hydroxydiclofenac instead of diclofenac did not cause the inactivation of CYP3A4, suggesting that 5-hydroxydiclofenac is not a precursor of a postulated reactive metabolite that inactivates CYP3A4, and thus 5-hydroxylation step is critical to inactivation of CYP3A4. The American Society for Pharmacology and Experimental Therapeutics ER -