Effect of atorvastatin and fluvastatin on the metabolism of midazolam by cytochrome P450 in vitro

We have investigated the effects of the statins atorvastatin and fluvastatin on the cytochrome P450 3A4 enzyme (CYP 3A4)-mediated metabolism of midazolam in vitro, using pooled human liver microsomes. Midazolam was metabolised by human hepatic microsomes with a Michaelis-Menten constant (K-m) of 5.25 (SD 1.2) mu mol.l(-1). Atorvastatin was a moderate competitive inhibitor of CYP 3A4 with an inhibitory constant (K-i) of 12.4 (95% CI 4.65-20.06) mu mol.l(-1). Fluvastatin was a weak non-competitive inhibitor of CYP 3A4 with a K-i of 94.3 (95% CI 55.01-133.5) mu mol.l(-1). Both atorvastatin and fluvastatin inhibit the CYP 3A4-mediated metabolism of midazolam in vitro.