Modification of the mutagenicity and teratogenicity of cyclophosphamide in rats with inducers of the cytochromes P-450.

Cyclophosphamide must be enzymatically activated to be either mutagenic or teratogenic. This activation is thought to be catalyzed by the cytochrome P-450 monooxygenase system. To study the relationship between the mutagenic and teratogenic metabolites of cyclophosphamide, the mutagenicity and teratogenicity of this drug were compared after activation by rats pretreated with chemicals (phenobarbital and beta-naphthoflavone) inducing different cytochromes P-450. Activation of cyclophosphamide to mutagenic metabolites by enzyme fractions from rats on day 13 of gestation was measured with the Ames test using Salmonella typhimurium TA1535. Teratogenicity was assessed in vivo by treatment of rats with cyclophosphamide on day 13 of gestation. Cyclophosphamide was activated to mutagenic metabolites to the same extent (on a tissue wet weight basis) by enzyme fractions from maternal liver, kidney and placenta, despite differences in cytochrome P-450 content. Fetal homogenates did not activate cyclophosphamide. Phenobarbital pretreatment increased the activation of cyclophosphamide to mutagenic metabolites by maternal liver microsomes 10-fold and liver cytochrome P-450 content 1.8 fold; however, this drug did not alter the activation of cyclophosphamide by maternal kidney, by placenta or by the fetus. Phenobarbital pretreatment increased the teratogenicity of cyclophosphamide in rats on day 13 of gestation (increased incidence of malformed embryos, decreased fetal weight). Pretreatment with beta-naphthoflavone did not induce liver cytochrome P-450 in the pregnant rat and did not change the activation of cyclophosphamide to mutagenic metabolites by liver, kidney, placenta or the fetus. Pretreatment with this polycyclic aromatic hydrocarbon had no effect or decreased the teratogenicity of cyclophosphamide. Thus, these experiments suggest that the mother, rather than the fetus, is the site of activation of cyclophosphamide; after phenobarbital pretreatment the predominant site of cyclophosphamide activation is the maternal liver. There appears to be a correlation between the teratogenicity and mutagenicity of cyclophosphamide after induction of the cytochromes P-450. We can speculate that the "proximate teratogen" of cyclophosphamide may also be the "proximate mutagen".