Derivation of retinoic acid and metabolites from a teratogenic dose of retinol (vitamin A) in mice.

Megadose supplements of vitamin A are under suspicion as hazards to the developing embryo after the discovery that two vitamin A-related drugs, Accutane and Tigason, are human teratogens. Retinoic acid (all-trans-RA) is a natural metabolite of vitamin A which participates in many of the known functions of vitamin A and may be the active agent in teratogenesis. In this investigation we gave a single, high oral dose of retinol (vitamin A) to pregnant mice to assess its transplacental pharmacokinetics as well as to measure the formation and distribution of its metabolites in the embryo. Retinol was estimated to be 4-fold less active than retinoic acid in the whole animal teratogenesis and 20-fold less active in the in vitro bioassay. A fully teratogenic dose, 200 mg/kg, yielded considerable quantities of retinoic acid which were transferred to the embryo with kinetics similar to that of retinol. During the first 8 hr after administration of retinol, the metabolites (including all-trans-RA, 13-cis-RA, and 4-oxo-RA) constituted almost 50% of the quantity of all retinol derivatives found in the embryo. A comparison of combined peak concentrations of the metabolites (or their AUC values) with the extent of teratogenesis associated with them individually provided sufficient evidence to implicate the metabolites themselves as mediators of retinol-induced teratogenesis. However, since both retinol and retinoic acid were present in sufficient concentrations in the embryo to act as teratogens we cannot at present rule out the possibility that they may act independently. Further experimentation will be necessary to address whether retinoic acid detected in the embryo is the product of the embryo's own metabolic capability or is transferred from the maternal circulation.