Evaluation of STS-557 as a postcoital contraceptive in the baboon.

A postcoital interceptive action of STS-557, a synthetic progestin, has been reported in the mouse, rat, and baboon. In the present study we reinvestigated the effects in the baboon with a modification of the original protocol: prolongation of the mating period from 6 to 48 hours. The time of mating was determined by observation of the perineal sex skin and the pattern of plasma estradiol in the periovulatory period. STS-557 (0.4 mg) was administered 6 hours after the initiation of mating, which was continued for an additional 42 hours. Controls received an empty gelatin capsule in the same mating protocol. Conception rate in the control baboons (n = 20) was 25%; in the STS-557-treated baboons (n = 18) it was 22%. There was also no difference in the pattern of plasma estradiol and progesterone in control versus treated baboons, either in pregnant or in nonpregnant animals. All pregnancies were normal. In light of the earlier report of effective postcoital action in the baboon from a single administration of STS-557 after a breeding period limited to 6 hours, our results indicate that the effect of a single dose is brief, and may not be clinically useful.