Hepatic Metabolic Clearance of Midazolam, a Cytochrome P450 3A Substrate, in the Presence of Ketoconazole in Rats

Abstract

Evaluation of the unbound concentration of an inhibitor (Iu) at a target site is the key to predicting in-vivo drug-drug interactions in metabolism, based on in-vitro data. In this study, the in-vivo hepatic extraction ratio (EH) and intrinsic clearance (CLint) of midazolam, a cytochrome P450 (CYP) 3A substrate, in the presence of ketoconazole, a CYP inhibitor, was predicted. The plasma concentration of unbound ketoconazole (Iu) in the portal vein or hepatic vein was evaluated in untreated and dexamethasone-pretreated rats. The in-vivo EH of midazolam in the presence and absence of ketoconazole was, 0.70 and 0.86 in untreated rats, and 0.81 and 0.98 in dexamethasone-pretreated rats, respectively. The in-vivo CLint of midazolam under different conditions was estimated using well-stirred, parallel-tube and dispersion models. The in-vivo EH of ketoconazole alone was approximately 0.59. The invitro CLint of midazolam was 0.95 and 3.44 mL min−1 (mg microsomal protein)−1 in untreated and dexamethasone-pretreated rats, respectively. Ki values of ketoconazole, which showed non-competitive inhibition, were 0.126 and 0.063 μM in untreated and dexamethasone-pretreated rats, respectively. The predicted in-vivo CLint and EH values of midazolam in the presence of ketoconazole based on in-vitro data using the portal Iu as the Iu of ketoconazole in the liver were in good agreement with corresponding in-vivo values, determined using parallel-tube and dispersion models. The in-vivo hepatic metabolic clearance of midazolam in the presence of ketoconazole was reasonably predicted from in-vitro data using the portal rather than the hepatic Iu, as the Iu of ketoconazole in the liver.