Termination of mid-gestation pregnancy in bitches with aglepristone, a progesterone receptor antagonist

Six pregnancies were terminated in mid-gestation with aglepristone, a proge sterone receptor antagonist, in 5 beagle bitches in order to determine the effects of aglepristone on plasma concentrations of prolactin and progester one, the duration of the luteal phase, and the interestrous interval. In ad dition, the effects of aglepristone on the condition of the uterus and fetu ses were examined by ultrasonography. After confirmation of pregnancy by ul trasonography, the dogs received 10 mg, sc aglepristone per kg body weight on 2 consecutive days at about 30 d post ovulation. Before, during and afte r treatment with aglepristone, plasma samples were collected for determinat ion of the concentrations of prolactin and progesterone. The condition of t he uterus and; fetuses was assessed by ultrasonography the day before and a t least 3 times a week for at least 2 wk after aglepristone administration. Termination of pregnancy occurred within 4 to 7 d after the start of aglep ristone treatment, which was well tolerated, with no side-effects except sl ight vaginal discharge. The results of ultrasonographic examination indicat ed that aglepristone leads to abortion but not to fetal resorption. Elevate d plasma concentrations of prolactin were observed during aglepristone trea tment, while plasma progesterone levels remained unchanged. Pregnancy termi nation with aglepristone resulted in premature cessation of luteal function . In addition, the interestrous interval was shortened. The latter effects may be the consequence of actions of the progesterone receptor antagonist a t the hypothalamus-pituitary level. In conclusion, aglepristone proved to b e a safe and effective abortifacient in mid-gestation in the bitch. The res ults of the present study also indicated that aglepristone directly or indi rectly influences pituitary function. (C) 2000 by Elsevier Science Inc.