Effects of metronidazole on midazolam metabolism in vitro and in vivo.

OBJECTIVE: Case reports have described elevated concentrations of CYP3A4 substrates (e.g. cyclosporin) during metronidazole treatment. Therefore, we wanted to study whether metronidazole affects CYP3A4 activity, using midazolam as a model substrate in vitro and in vivo.

METHODS: In the in vitro part of the study, the effects of various concentrations of metronidazole (0-500 microM) on the formation of 1'-hydroxymidazolam from midazolam were studied using human liver microsomal preparations (n = 4). In the in vivo part, the effects of metronidazole on the pharmacokinetics and pharmacodynamics of oral midazolam were evaluated in a randomised, placebo-controlled cross-over study in ten healthy subjects. The subjects took either 400 mg metronidazole or matched placebo orally twice daily for 3 days. On day 3, 15 mg midazolam was administered orally. Plasma concentrations of midazolam and 1'-hydroxymidazolam were determined up to 24 h. The effects of midazolam were measured up to 10 h.

RESULTS: Metronidazole (10-500 microM) showed no inhibitory effect on 1'-hydroxymidazolam formation by human liver microsomes. In healthy volunteers, metronidazole had no statistically significant effects on the pharmacokinetics of midazolam and 1'-hydroxymidazolam, and also the ratio of 1'-hydroxymidazolam to midazolam in plasma remained unchanged by metronidazole. The four employed psychomotor tests did not show significant differences between the metronidazole and placebo phases.

CONCLUSION: Metronidazole had no effects on the 1'-hydroxylation of midazolam in vitro or on the pharmacokinetics and pharmacodynamics of midazolam in vivo. These findings indicate that metronidazole is not an inhibitor of CYP3A4.