The pharmacokinetics of ketoconazole and its effects on the pharmacokinetics of midazolam and fentanyl in dogs.

Ketoconazole inhibits the Cytochrome P450 3A12 (CYP3A12) metabolizing enzyme as well as the p-glycoprotein efflux pump. The extent and clinical consequence of these effects are poorly understood in dogs. The objective was to assess the pharmacokinetics of ketoconazole after single and multiple doses and the effect of multiple doses of ketoconazole on midazolam (a known CYP3A12 substrate) and the opioid fentanyl. Six greyhound dogs were studied. The study consisted of three phases. Phase 1 consisted of i.v. midazolam (0.23 mg/kg base) and fentanyl (15.71 microg/kg base). Phase 2 consisted of a single oral dose of ketoconazole (mean dose 12.34 mg/kg). Phase 3 consisted of i.v. midazolam (0.23 mg/kg) and fentanyl (10 microg/kg) after 5 days of oral ketoconazole (12.25 mg/kg/day). Ketoconazole significantly inhibited its own elimination with the mean residence time (MRT) increasing from 6.24 h in Phase 1 to 12.54 h in Phase 3. Ketoconazole significantly decreased the elimination of midazolam, as expected, with the MRT increasing from 0.81 to 1.49 h. The elimination of fentanyl was not significantly altered by co-administration of ketoconazole with the MRT being 3.90 and 6.35 h. The MRT was the most robust estimate of decreased drug elimination.