Identification of a Teratogenic Drug‐Protein Complex in Sera of Phenytoin‐Treated Monkeys

Whole rat embryos were cultured for 48 h on sera drawn from monkeys before and 10 h after phenytoin gavage (275 mg/kg body weight). Sera from treated monkeys caused exencephaly, anophthalmia, microcephaly, and incomplete ventral curvature when used as culture media, whereas sera drawn from the same monkeys before treatment supported normal embryonic development. To identify the cause of serum teratogenicity, isolated constituents of teratogenic sera were added to nonteratogenic sera for testing by embryo culture. Serum extracts containing free phenytoin and its free metabolites were not teratogenic. Teratogenicity was found associated with serum proteins. Using polyacrylamide gel electrophoresis (PAGE) and an antibody that recognized phenytoin and its metabolites, we were able to demonstrate that phenytoin was bound to a protein of 80,000 daltons. Addition of this same antibody to teratogenic sera from dosed monkeys improved the development of cultured embryos and provided additional support for this complex as the proximal teratogen. Use of the antibody to follow the uptake and distribution of phenytoin in cultured embryos suggested that only the phenytoin-protein complex (and not phenytoin itself) was able to pass through the yolk sac and reach the tissues of the embryo proper. These results suggested that a drug-protein complex may serve to transport drugs from their site of activation in the maternal liver to the developing embryo.