Background: The cytochrome P450 3A (CYP3A) isoforms are responsible for the metabolism of a majority of therapeutic compounds, and they are abundant in the intestine and liver. CYP3A activity is highly variable, causing difficulty in the therapeutic use of CYP3A substrates. A practical in vivo probe method that characterizes both intestinal and hepatic CYP3A activity would be useful.
Objectives: To determine the intestinal and hepatic contribution to the bioavailability of midazolam with use of the CYP3A inhibitor ketoconazole.
Methods: The pharmacokinetics of midazolam was assessed in nine (six men and three women) healthy individuals after single doses of 2 mg intravenous and 6 mg oral midazolam (phase I). These pharmacokinetic values were compared with those obtained after single doses of 2 mg intravenous and 6 mg oral midazolam and three doses of 200 mg oral ketoconazole (phase II).
Results: After ketoconazole therapy, area under the concentration versus time curve of midazolam increased 5-fold after intravenous midazolam administration (P < or = .001) and 16-fold after oral midazolam administration (P < or = .001). Intrinsic clearance decreased by 84% (P = .003). Total bioavailability increased from 25% to 80% (P < .001). The intestinal component of midazolam bioavailability increased to a greater extent than the hepatic component (2.3-fold [P = .003] and 1.5-fold [P < or = .001], respectively). In the control phase, female subjects had greater midazolam clearance values than the male subjects.
Conclusions: Ketoconazole caused marked inhibition of CYP3A activity that was greater in the intestine than the liver. Administration of single doses of oral and intravenous midazolam with and without oral ketoconazole exemplifies a practical method for differentiating intestinal and hepatic CYP3A activity.