Antifertility effects of pyrimethamine.

The antifertility effect of pyrimethamine was studied both in vitro and in vivo in 5-6 month old virgin rabbits. Some inhibition of blastocyst growth in culture for 12 hours was observed. When blastocysts cultured for 4 hours were transplanted into receptor animals implantation occurred but the number of live fetuses obtained at 28 days was significantly (p less than .025) lower compared with controls. However there was no evidence of congenital abnormalities. 1 mg/100 gm body weight of pyrimethamine significantly (p less than .01) reduced the fertility index of animals treated on Day 1 of pregnancy and sacrificed on Day 11. Treatment on Day 4 of pregnancy resulted in reduced implantations on Days 11 and 21 of pregnancy and a decrease in the number of live fetuses on Day 21. It is concluded that doses required to obtain antifertility effects would be too close to toxic levels for clinical application.