Folate deficiency and in vitro palatogenesis II: Effects of methotrexate on rabbit palate fusion, folate pools, and dihydrofolate reductase activity.

Folate deficiency during pregnancy induced by dietary deprivation or folate antagonists is teratogenic. Methotrexate is a commonly used antifolate drug that produces congenital defects, with the craniofacial complex being especially vulnerable. The molecular action of this compound, therefore, was examined in vitro using fetal rabbit palates in an organ culture system. Dihydrofolate reductase activity was measured in these palates on the 4 days preceding fusion. There was no significant fluctuation in activity with gestational age. Methotrexate treatment of the palates in vitro resulted in significant reduction of enzyme activity and also decreased total folate concentration. However, the in vitro fusion ability of these palates was not altered by exposure to methotrexate. These results suggest that since methotrexate is a potent inhibitor of DNA synthesis and has its maximal effect on rapidly dividing systems, the peak in cellular proliferative activity in the fetal rabbit palate occurs very early in its development.