Abstract
Pregnancy termination was obtained in both Beagle and mongrel bitches after a single s.c. or i.m. injection of 2-(3-ethoxy-phenyl)-5,6-dihydro-s-triazole[5,1-a]isoquinoline (DL 204-IT) dissolved or suspended in an oily vehicle. The activity of the compound was dependent on the dose and time of pregnancy. The optimal time of treatment was found to be day 20 of gestation, at which time the smallest effective dose was 6.25 ng/kg. Pregnancy arrest normally occurs during the first stage of embryonic development (between day 25 and 30) due to the degeneration and subsequent resorption of the products of fertilization. Pregnancy termination is never accompanied by intrauterine or placental hemorrhage. After pregnancy arrest the animals return to estrus within a normal interval of time; they exhibit normal mating behaviour and their ability to conceive is not impaired. Later, pregnancies can be interrupted again by the same treatment. After subeffective doses normal parturition occurs at the expected time and the pups do not have any external malformations. High doses given every 15 days for more than one year are well tolerated and do not inhabit either the return of estrous cycle or fertility. Studies of the mechanism of action suggest that the primary site of action is the uteroplacental complex.