TY - JOUR T1 - In Vitro Stimulation of Warfarin Metabolism by Quinidine: Increases in the Formation of 4′- and 10-Hydroxywarfarin JF - Drug Metabolism and Disposition JO - Drug Metab Dispos SP - 877 LP - 886 VL - 29 IS - 6 AU - Jason S. Ngui AU - Qing Chen AU - Magang Shou AU - Regina W. Wang AU - Ralph A. Stearns AU - Thomas A. Baillie AU - Wei Tang Y1 - 2001/06/01 UR - http://dmd.aspetjournals.org/content/29/6/877.abstract N2 - It has been demonstrated that the activity of cytochrome P450 (CYP)3A4 in certain cases is stimulated by quinidine (positive heterotropic cooperativity). We report herein that the 4′- and 10-hydroxylation of S- and R-warfarin are enhanced in human liver microsomal incubations containing quinidine. These reactions were catalyzed by CYP3A4, based on data derived from immunoinhibitory studies, with 4′-hydroxylation being preferentially associated with S-warfarin and 10-hydroxylation withR-warfarin. The 4′-hydroxylation ofS-warfarin and 10-hydroxylation ofR-warfarin increased with increasing quinidine concentrations and maximized at ∼3- and 5-fold the values of controls, respectively. Stimulatory effects of quinidine also were observed with recombinant CYP3A4, suggesting that increases in warfarin metabolism were due to quinidine-mediated enhancement of CYP3A4 activity. This positive cooperativity of CYP3A4 was characterized by a 2.5-fold increase in V max for the 4′-hydroxylation of S-warfarin and a 5-fold increase inV max for the 10-hydroxylation ofR-warfarin, with little change inK m values. Conversely,V max for the 3-hydroxylation of quinidine was not influenced by the presence of warfarin. These results are consistent with previous findings suggesting the existence of more than one binding site in CYP3A4 through which interactions may occur between substrate and effector at the active site of the enzyme. Such interactions were subsequently illustrated by a kinetic model containing two binding domains, and a good regression fit was obtained for the experimental data. Finally, stimulation of warfarin metabolism by quinidine was investigated in suspensions of human hepatocytes, and increases in the formation of 4′- and 10-hydroxywarfarin again were observed in the presence of quinidine, indicating that this type of drug-drug interaction occurs in intact cells. The American Society for Pharmacology and Experimental Therapeutics ER -