In vitro stimulation of warfarin metabolism by quinidine: Increases in theformation of 4 '- and 10-hydroxywarfarin

It has been demonstrated that the activity of cytochrome P450 (CYP)3A4 in c ertain cases is stimulated by quinidine (positive heterotropic cooperativit y). We report herein that the 4'- and 10-hydroxylation of S- and R-warfarin are enhanced in human liver microsomal incubations containing quinidine. T hese reactions were catalyzed by CYP3A4, based on data derived from immunoi nhibitory studies, with 4'-hydroxylation being preferentially associated wi th S-warfarin and 10-hydroxylation with R-warfarin. The 4'-hydroxylation of S-warfarin and 10-hydroxylation of R-warfarin increased with increasing qu inidine concentrations and maximized at similar to3- and 5-fold the values of controls, respectively. Stimulatory effects of quinidine also were obser ved with recombinant CYP3A4, suggesting that increases in warfarin metaboli sm were due to quinidine-mediated enhancement of CYP3A4 activity, This posi tive 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 in V-max f or the 10-hydroxylation of R-warfarin, with little change in K-m values. Co nversely, V-max for the 3-hydroxylation of quinidine was not influenced by the presence of warfarin, These results are consistent with previous findin gs 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 ki netic model containing two binding domains, and a good regression fit was o btained for the experimental data, Finally, stimulation of warfarin metabol ism by quinidine was investigated in suspensions of human hepatocytes, and increases in the formation of 4'- and 10-hydroxywarfarin again were observe d in the presence of quinidine, indicating that this type of drug-drug inte raction occurs in intact cells.