AIMWe aimed to establish a method to assess systemic and pre-systemic cytochrome P450 (CYP) 3A activity using ineffective microgram doses of midazolam. METHODSIn an open, one sequence, crossover study, 16 healthy participants received intravenous and oral midazolam at microgram (0.001 mg intravenous and 0.003 mg oral) and regular milligram (1 mg intravenous and 3 mg oral) doses to assess the linearity of plasma and urine pharmacokinetics. RESULTS Dose CONCLUSIONThe pharmacokinetics of an intravenous midazolam microdose is linear to the applied regular doses and can be used to assess safely systemic CYP3A activity and, in combination with oral microdoses, pre-systemic CYP3A activity. WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT• Midazolam pharmacokinetics of oral doses are linear over a 30 000-fold range.• An oral microdose of midazolam is suitable to measure total CYP3A activity.• CYP3A inhibition with strong inhibitors can be evaluated with a microdose in healthy volunteers and patients. WHAT THIS STUDY ADDS• The pharmacokinetics of intravenous midazolam microdoses are linear to milligram doses.• The bioavailability and metabolic clearance of midazolam is similar after administration of microdoses and milligram doses.• Midazolam microdoses are a suitable tool to assess both systemic and pre-systemic CYP3A activity.

Voriconazole is both a substrate and a potent inhibitor of cytochrome P450 (CYP) 3A. It has a high bioavailability and non-linear pharmacokinetics. We investigated the pharmacokinetics and metabolism of 50 mg and 400 mg doses of intravenous and oral voriconazole in 14 healthy volunteers. Concurrently, we determined systemic and presystemic CYP3A activity with microdosed midazolam. Bioavailability of voriconazole 50 mg was 39 % compared with 86 % of the 400 mg dose. Voriconazole area under the concentration-time curve extrapolated to infinity (AUC) was 416 and 16,700 h·ng/mL for the 50 and 400 mg oral doses, respectively, and 1110 and 19,760 h·ng/mL for the 50 and 400 mg intravenous doses, respectively. Midazolam metabolism was dose-dependently inhibited by voriconazole. Dose-dependent autoinhibition of CYP3A-dependent first-pass metabolism and systemic metabolism is a possible explanation for the dose-dependent bioavailability and elimination of voriconazole, either as additional mechanism to, or instead of, saturation of presystemic metabolism. Higher bioavailability and non-linear pharmacokinetics are expected to be a common property of drugs that are substrates and inhibitors of CYP3A, e.g. clarithromycin.