Pharmacodynamics of cytochrome P450 2B induction by phenobarbital, 5-ethyl-5-phenylhydantoin, and 5-ethyl-5-phenyloxazolidinedione in the male rat liver or in cultured rat hepatocytes.

The pharmacodynamics of rat hepatic cytochrome P450 2B (P450 2B) induction by phenobarbital (PB) and two structural congeners, dl-5-ethyl-5-phenylhydantoin (EPH) and dl-5-ethyl-5-phenyloxazolidinedione (EPO), were investigated. The in vivo induction of P450 2B was probed in F344/NCr rats by measuring immunoreactive hepatic P450 2B1 protein and by assaying the hepatic 16 beta-hydroxylation of testosterone and O-dealkylation of (benzyloxy)- and pentoxyresorufin. The induction of (benzyloxy)resorufin O-dealkylation activity was also measured in adult rat hepatocyte cultures exposed to the three xenobiotics. The concentration of xenobiotic at the putative active site in the in vivo studies was approximated by measuring serum total xenobiotic levels, while in the hepatocyte culture studies, the nominal xenobiotic concentration in the culture medium was used. Concentration-dependent induction of P450 2B activities was observed in the in vivo and hepatocyte culture studies. The in vivo ED50 values for P450 2B induction were approximately 110, approximately 100, and approximately 3000 dietary ppm (14 days administration) for PB, EPH, and EPO, respectively. The in vivo EC50 values for P450 2B induction were approximately 9, approximately 6, and approximately 130 microM (total serum) for PB, EPH, and EPO, respectively. In cultured rat hepatocytes, the ED50 values for induction of (benzyloxy)resorufin O-dealkylation activity were 14.5, 14.2, and 108 microM for PB, EPH, and EPO, respectively. These data indicate that pharmacodynamic results obtained with cultured hepatocytes represent a good qualitative and quantitative approximation of the in vivo hepatic responses in male rats caused by PB-type inducers.(ABSTRACT TRUNCATED AT 250 WORDS)