Structure-activity relationships for hepatic CYP2B induction in the rat

The structure-hepatic CYP2B-inducing activity relationships within structural congeners of the prototype CYP2B-type inducing agent, phenobarbital, were systematically evaluated in the rat. A number of congeners were fed to rats for two weeks at dietary concentrations equimolar to 500 ppm phenobarbital. At the conclusion of these experiments, liver post-mitochondrial supernatants (S-9s) were prepared and examined for catalytic activity toward the CYP2B-specific substrates benzyloxy- and pentoxyresorufin. In addition, the relative levels of hepatocellular CYP2B1 and CYP2B1/2B2 protein were determined by immunodetection with monoclonal antibodies. Based upon the results obtained, certain of the congeners were selected for pharmacodynamic studies. These experiments involved dose-response studies in which the measured increases in in vitro catalytic activities, immunoreactive protein, and, for selected congeners, RNAs coding for CYP2B1 or CYP2B2, were related to serum concentrations of the congeners. Sigmoid serum concentration-CYP2B induction response curves were obtained for many of the congeners, which is suggestive of receptor mediation. Profound differences in potency and efficacy for this effect among the series of congeners were also identified. In addition, sex differences in potency of CYP2B induction were demonstrated for phenobarbital. Such differences in potency may relate to differences in the efficacy or affinity of these congeners at a specific receptor. However, no enantioselectivity for CYP2B induction in the F344/NCr rat was identified in examination of the CYP2B inducing abilities of 5-ethyl-5-phenylhydantoin and its two enantiomers. Although to date, no receptor for the induction of CYP2B1 activity has been demonstrated, the present studies have the potential of providing evidence for or against receptor-mediation in this phenomenon.