Vol. 30, Issue 1, 63-68, January 2002

Effect of Multiple Dosing of Ketoconazole on Pharmacokinetics of Midazolam, a Cytochrome P-450 3A Substrate in Beagle Dogs

Masanori Kuroha, Akinori Azumano, Yoji Kuze, Minoru Shimoda, and Eiichi Kokue

Department of Veterinary Medicine, Tokyo University of Agriculture and Technology, Tokyo, Japan

To evaluate effects of multiple dosing of ketoconazole (KTZ) on hepatic CYP3A, the pharmacokinetics of intravenous midazolam (MDZ, 0.5 mg/kg) before and during multiple dosing of KTZ were investigated in beagle dogs. KTZ tablets were given orally to dogs (n = 4) for 30 days (200 mg b.i.d.). With coadministration of KTZ, t_1/2 of MDZ were significantly increased both on day 1 (2-fold) and on day 30 (3-fold). Total body clearance (CL_tot) of MDZ declined gradually during the first 5 days after the start of KTZ treatment, and thereafter CL_tot appeared to reach a plateau phase (one-fourth), depending on plasma KTZ concentrations. The effects of KTZ on the biotransformation of MDZ were also investigated using dog liver microsomes (n = 5). The K_i values of KTZ for MDZ 1'-hydroxylation and 4-hydroxylation were 0.0237 and 0.111 µM, respectively, indicating that KTZ extensively inhibits hepatic CYP3A activity in dogs. CL_tot values estimated from in vitro K_i values corrected by unbound fraction of KTZ and unbound concentrations of the drug in plasma were consistent with in vivo CL_tot of MDZ. The results in this study suggest that KTZ treatment is necessary until plasma concentrations of the drug reach a steady state to evaluate the effect of multiple dosing of the drug on hepatic CYP3A in vivo. In addition, it is suggested that K_i values corrected by unbound fraction of KTZ and unbound concentrations of the drug in plasma enable precise in vitro-in vivo scaling.