Effect of 5-azacytidine administration during very early pregnancy.

The chemotherapeutic agent 5-azacytidine (5AZ) is cytotoxic via nucleic acid hypomethylation. Malformations and embryolethality result when rats or mice are exposed to 5AZ on any of Days 9 through 12 of pregnancy. To investigate the effect of 5AZ exposure during the pre- and early postimplantation period, we administered 0.15, 0.30, 0.60, or 1.2 mg/kg 5AZ/day to rats during Days 1-8 of pregnancy and evaluated outcome on Days 9 or 20. No adverse effects were detected on Day 9; the numbers and weights of implantation sites, the numbers of resorptions, maternal body weight gains, and hormone measures were not different from those of controls. However, when pregnancy outcome was evaluated on Day 20, dose-dependent decreases in offspring survival and fetal weight were observed and the incidences of two malformations, microphthalmia and exencephaly, were increased. In a follow-up study, 5AZ was administered during the preimplantation period (Days 1-3) or during the postimplantation period (Days 4-8) and pregnancy outcome was evaluated on Day 20. When rats were exposed to 5AZ during the preimplantation period, no adverse effects were seen. Postimplantation dosing produced an increase in resorptions and a decrease in fetal survival and fetal weight, with no gross external malformations evident. At the doses used in this study, 5AZ was embryotoxic, with limited embryonic vulnerability prior to Day 4 of pregnancy but serious consequences following postimplantation exposure. This embryotoxicity is not detectable by our measures on Day 9 but is evident on Day 20.