Previous animal research has suggested that the phenytoin arene oxide metabolite is teratogenic in acute studies and that the fetal effects were increased after injecting an inhibitor of microsomal epoxide hydrolase (mEH) (Martz et al., Pharmacol Exp Ther 203:231-239, 1977, Barcellona et al., Teratog Carcinog Mutagen 7:159-168, 1987). We have studied the effects of chronic oral phenytoin exposure in utero and the mEH inhibitor trichloropropene oxide (TCPO) on the prenatal growth and development of an inbred mouse strain with a low incidence of spontaneous oral clefting (C57BL/6J). Chronic daily gastric gavage of phenytoin produced a plasma level (mean 10.7 micrograms/ml on gestation Day 8) within the range recommended to prevent epilepsy in humans; this did not produce an increase in oral clefting or ventricular septal defects in the exposed C57BL/6J pups. It did produce a significant delay in prenatal growth and development, including phalangeal ossification. However, except for percentage resorptions/implantation, there was no synergism between phenytoin and TCPO in contrast to the finding reported by Martz et al. in Swiss mice. This issue was also assessed in a test of the fetal effect of phenytoin injected with TCPO, as had been done by Martz et al. There were no oral clefts or ventricular septal defects or a difference (P > 0.05) in prenatal growth and development in these C57BL/6J pups compared to the chronic gastric phenytoin plus TCPO group. This suggests either that differences in the genotypes of Swiss and C57BL/6J mice may be a contributing factor or that other teratogenic mechanisms were involved.
