Abstract

The ability of the benzodiazepines, as a chemical class, to cause the induction and/or inhibition of cytochromes P450 has not been well characterized. In the present study, the induction of the cytochrome P450 2B subfamily (CYP2B) in vivo and the inhibition of CYP2B activity in vitro by selected benzodiazepines was examined in hepatic tissues derived from male F344/NCr rats. Initial studies of the in vivo induction or in vitroinhibition of benzyloxyresorufin O-dealkylation activity revealed that both clonazepam and diazepam were relatively effective in vivo inducers of CYP2B when administered in the diet at 500 ppm for 5 days and also were fairly potent inhibitors of the activity of these hemoproteins in vitro. Oxazepam, in contrast, was ineffective as an inducer or an inhibitor of this activity. Further studies were performed to characterize the subfamily selectivity of the P450 induction and inhibition displayed by clonazepam. Specifically, microsomes from rats treated with clonazepam (1000 or 1800 ppm in the diet for 5 days) were found to be highly induced with respect to catalytic activities mediated by CYP2B, including benzyloxyresorufin and pentoxyresorufin O-dealkylation or testosterone 16β-hydroxylation, but other CYP proteins were minimally induced. In addition to inducing the CYP2B subfamily, clonazepam also induced the RNA encoding other drug metabolizing enzymes (e.g., epoxide hydrolase and the glutathione S-transferase α-subfamily) that are typically induced by phenobarbital-type inducers. Finally, clonazepam proved to be a potent noncompetitive or “mixed-type” competitive inhibitor of catalytic activities mediated by CYP2B, but not by other CYP proteins (e.g. CYP2A, CYP3A) in microsomes derived from phenobarbital-pretreated rats.