Fentanyl inhibits metabolism of midazolam: competitive inhibition of CYP3A4 in vitro

Fentanyl decreases clearance of midazolam administered i.v., but the mechanism remains unclear. To elucidate this mechanism, we have investigated the effect of fentanyl on metabolism of midazolam using human hepatic microsomes and recombinant cytochrome P450 isoforms (n = 6). Midazolam was metabolized to l'-hydroxymidazolam (l'-OH MDZ) by human hepatic microsomes, with a Michaelis-Menten constant (K(m)) of 5.0 (SD 2.7) microgramsmol litre-1. Fentanyl competitively inhibited metabolism of midazolam in human hepatic microsomes, with an inhibition constant (Ki) of 26.8 (12.4) microgramsmol litre-1. Of the seven representative human hepatic P450 isoforms, CYP1A2, 2A6, 2C9, 2C19, 2D6, 2E1 and 3A4, only CYP3A4 catalysed hydroxylation of midazolam, with a K(m) of 3.6 (0.8) microgramsmol liter-1. Fentanyl competitively inhibited metabolism of midazolam to l'-OH MDZ by CYP3A4, with a Ki of 24.2 (6.8) microgramsmol litre-1, comparable with the Ki obtained in human hepatic microsomes. These findings indicate that fentanyl competitively inhibits metabolism of midazolam by CYP3A4.