Propofol decreases the clearance of midazolam by inhibiting CYP3A4: An in vivo and in vitro study

Objective To examine the effect of propofol on the pharmacokinetics of midazolam in vivo and to elucidate the mechanism of the pharmacokinetic changes of midazolam by propofol with the use of human liver microsomes and recombinant CYP3A4. Methods In an in vivo, double-blind randomized study, 24 patients received 0.2 mg/kg midazolam and either 2 mg/kg propofol (propofol group) or placebo (placebo group) for induction of anesthesia. In the propofol group, continuous infusion of propofol at 9 mg/kg/h was started immediately after the bolus infusion of propofol and was maintained for an hour. In the placebo group the same dose of soybean emulsion as a placebo was given and infused intravenously for an hour instead of propofol. In an in vitro study the effect of propofol on the metabolism of midazolam was studied with human liver microsomes and recombinant CYP3A4. Results In the propofol group the mean clearance of midazolam was decreased by 37% (P = .005) and the mean elimination half-life was prolonged by 61% (P = .04) compared with the placebo group. The mean plasma concentrations of 1′-hydroxymidazolam were lower in the propofol group than in the placebo group at 5, 10, 15, 20, and 30 minutes after midazolam was administered (P < .05). The mean (±SD) Michaelis-Menten constant for midazolam 1′-hydroxylation by human liver microsomes was 5.6 ± 3.3 μmol/L. The formation of 1′-hydroxymidazolam was competitively inhibited by propofol, and the mean inhibition constant was 56.7 ± 16.6 μmol/L. The mean Michaelis-Menten constant and mean inhibition constant values for midazolam 1′-hydroxylation by recombinant CYP3A4 were 4.0 μmol/L and 61.0 μmol/L, respectively, consistent with the mean values obtained from human liver microsomes. Conclusion Propofol decreases the clearance of midazolam, and the possible mechanism is the competitive inhibition of hepatic CYP3A4. Clinical Pharmacology & Therapeutics (1999) 66, 110–117; doi: 10.1053/cp.1999.v66.100038001