A critical review of the reproductive safety of Leflunomide

Leflunomide, an inhibitor of pyrimidine synthesis, is used for the treatment of rheumatic diseases, which are prevalent in women of childbearing age. Due to the very long half-life of the active metabolite, its mechanism of action and the teratogenicity observed in animal studies at doses similar to or lower than human therapeutic doses on a weight basis, it is recommended that women stop the treatment before conception and a drug elimination procedure be performed. However, unintended gestational exposures may occur, posing challenges in risk assessment. In order to address the safety of leflunomide in unintended exposures in pregnancy, we performed a critical review of human studies. We located 13 publications in Medline and Embase, which reported on 222 pregnancies with known outcomes exposed to leflunomide preconception and/or during pregnancy. Among the 169 live births, there were eight congenital malformations with no consistent pattern of anomalies. These studies collectively showed no significant difference in the rates of malformations between exposed and unexposed pregnancies. At present, accumulating human data do not point toward leflunomide as a potent human teratogen, which may inform risk assessment of unintended gestational exposure to leflunomide.