Mechanism-based inhibition of rat liver microsomal diazepam C-3-hydroxylase by mifepristone associated with loss of spectrally detectable cytochrome P450

Since initial studies with the steroids norethindrone and ethynylestradiol, reported by White and Muller-Eberhard in 1977 (Biochem. J. 166, 57-64), th ere has been continuing interest in xenobiotics that bear terminal or sub-t erminal acetylenic groups which can cause catalysis-dependent inhibition of CYP monooxygenases associated either with loss of prosthetic group heme or protein adduct formation. Mifepristone is a synthetic steroid bearing a pr opyne substitution on carbon 17 and this suggested to us that it may act as a mechanism-based inhibitor of the CYP isoforms responsible for its metabo lism. In human and rat liver, CYP3A isoforms have been implicated in mifepr istone clearance and mifepristone administration to rats has also been show n to induce CYP3A enzymes and the associated diazepam C-3-hydroxylase activ ity (Cheesman, Mason and Reilly, J. Steroid Biochem. Mel. Biol. 58, 1996, 4 47-454). With microsomes prepared from the livers of untreated female rats and others in which diazepam C-3-hydroxylase has been induced, we show here that mifepristone can cause catalysis-dependent inhibition of this monooxy genase. In addition, incubation of microsomes with mifepristone in the pres ence, but not in the absence, of NADPH caused loss of spectrally detectable cytochrome P450. These results suggest that heme adduct formation may resu lt from mifepristone metabolism by CYP3A monooxygenases which undergo self- catalysed irreversible inactivation with this drug as substrate. Since mife pristone administration in vivo is able also to cause induction of the synt hesis of hepatic CYP3A apoprotein, mifepristone may have the potential in h uman medicine for complex interactions with other co-administered drugs whi ch are also substrates for CYP3A monooxygenases. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.