Abstract

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_1/2β 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). TheK_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 vitroK_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 thatK_i values corrected by unbound fraction of KTZ and unbound concentrations of the drug in plasma enable precise in vitro-in vivo scaling.