STS 557 as an interceptive in baboons.

STS 557 (17 alpha-cyanomethyl-17 beta-hydroxy-estra-4, 9-dien-3-one) or levonorgestrel was given orally as a single dose of 0.4 mg per animal. Treatment was performed immediately after the 18 h or 6 h mating period. Regarding the 18 h mating period, in 30 control cycles (20 animals) 11 pregnancies (36.7%) and in 20 cycles (16 animals) with levonorgestrel treatment 2 pregnancies (10.0%) were observed. In 80 cycles (31 animals) with STS 557 treatment 7 pregnancies occurred (8.7%). With regard to the 6 h mating period, in 30 control cycles (18 animals) 9 pregnancies (30%) and in 26 cycles (17 animals) with levonorgestrel administration 3 pregnancies were recorded (11.5%). In 50 cycles (21 animals) with postcoital administration of STS 557 only two pregnancies occurred (4%). Therefore, after the 6 h mating period STS 557 showed high interceptive activities in this non-human primate model. In comparison with controls, STS 557 did not decrease the postovulatory rise of plasma progesterone. This finding demonstrates that postcoital STS 557 in the chosen dosage does not suppress ovulation. Repeated STS 557 administration starting early in the follicular phase was without marked influence on cycle characteristics. On the other hand, treatment start in the middle or at the end of the cycle caused cycle lengthening and reduced progesterone levels in some animals.