Inhibitory effects of nilutamide, a new androgen receptor antagonist, on mouse and human liver cytochrome P-450

Abstract The effects of nilutamide were studied first with human liver microsomes. At concentrations expected in the human liver (110 μM), nilutamide inhibited hexobarbital hydroxylase, benzphetamine N -demethylase, benzo( a )pyrene hydroxylase and 7-ethoxycoumarin O -deethylase activities by 85, 40, 35 and 25%, respectively. There was no in vitro inhibition of NADPH-cytochrome c reductase activity, no in vitro loss of CO-binding cytochrome P-450, and no spectral evidence for the in vitro formation of a possible cytochrome P-450Fe(II)-nitroso metabolite complex. Other studies were performed with mouse liver microsomes. Nilutamide (550 μM) did not significantly increase the consumption of NADPH by aerobic microsomes, and did not modify the kinetics for the reduction of cytochrome P-450 by NADPH-cytochrome P-450 reductase in an anaerobic system. Nilutamide (22 μM) produced either a type I or a type II binding spectrum. Kinetics for the inhibition of hexobarbital hydroxylase were consistent with competitive inhibition. A last series of experiments was performed after administration of nilutamide in mice. Thirty minutes after administration of doses (15 or 30 ,umol. kg~ ' i.p.) similar to those used in humans, the hexobarbital sleeping time was increased by 40 and 60%, respectively. There was no evidence, however, for the irreversible inactivation of microsomal enzymes since CO-binding cytochrome P-450 and monooxygenase activities remained unchanged in liver microsomes from mice killed 1 or 6 hr after administration of nilutamide (30 μmol. kg −1 .p.). These results show that nilutamide inhibits hepatic cytochrome P-450 activity, and suggest that inhibition may actually occur after therapeutic doses of nilutamide in humans.