Species differences in the hepatic effects of inducers of CYP2B and CYP4A subfamily forms: relationship to rodent liver tumour formation.

Phenobarbitone and related compounds induce hepatic microsomal cytochrome P450 (CYP) 2B forms (mediated by the constitutive androstane receptor), whereas peroxisome proliferators induce CYP4A forms (mediated by the peroxisome proliferator-activated receptor alpha) in rats and mice. A number of non-genotoxic CYP2B and CYP4A inducers have been shown to produce liver tumours in rats and mice. The hepatic effects of CYP2B and CYP4A inducers are reviewed and evaluated with respect to their established modes of action for rodent liver tumour formation and species differences in response. While CYP2B and CYP4A inducers stimulate replicative DNA synthesis in rodent liver, they do not appear to be mitogenic agents in human hepatocytes. Epidemiological studies have demonstrated that phenobarbitone and rodent peroxisome proliferators do not increase the incidence of liver tumours in humans. It is concluded that rodent CYP2B and CYP4A inducers do not pose a hepatocarcinogenic hazard for humans.