Alteration of embryonic folate metabolism by valproic acid during organogenesis: implications for mechanism of teratogenesis.

The antiepileptic drug valproic acid (2-propylpentanoic acid; [VPA]) is teratogenic in humans and a number of animal species. Using a murine model, we studied the mechanism of VPA-induced teratogenesis during a period of organogenesis sensitive to interference with closure of the neural tube (days 8 to 9). Teratogenic doses of valproic acid altered the pattern of folate metabolites in the embryo: Levels of 5-formyl- and 10-formyl-tetrahydrofolates decreased, and the level of tetrahydrofolate increased. These changes could be explained by VPA-mediated inhibition of transfer of the formyl group via glutamate formyltransferase. Neural-tube defects, alteration of embryonic folate metabolism, and inhibition of the specific enzyme are all produced by comparable doses and levels of the drug. A closely related structural analog of VPA (2-en-VPA, 2-propyl-2-pentenoic acid), which exhibits antiepileptic activity but not teratogenicity, did not influence the embryonic folate metabolism. Our results suggest that interference with embryonic folate metabolism might be an important aspect of the induction of neural-tube defects by VPA. The novel techniques described also should prove useful in studying the teratogenic mechanisms of other drugs.