Although sirolimus inhibits human liver microsomal cytochrome P450 CYP2C9, CYP2C19, CYP2D6, and CYP3A4/ 5 in vitro, the active substance is not expected to inhibit the activity of these isozymes in vivo since the sirolimus concentrations necessary to produce inhibition are much higher than those observed in patients receiving therapeutic doses of Rapamune.