Contranidatory activity of early luteal phase administration of mifepristone in the rhesus monkey.

The contranidatory action of mifepristone (RU 486) given as a single application at different dosages to mated rhesus monkeys (Macaca mulatta) on second day after ovulation has been examined in the present study. In group 1, monkeys (n = 3) received only vehicle (benzyl benzoate: olive oil, 1:4, v/v) and were treated as controls. In group 2 monkeys (n = 4), RU 486 was given by gavage at 10 mg/kg in group 4 (n = 5). The patterns of cycles and profiles of serum estrogen and progesterone were monitored for assessing the occurrence of implantation and pregnancy. At a single dose of 10 mg/kg, RU 486 was found to be ineffective in preventing nidation, resulting pregnancy in three females out of four treated monkeys. Similarly, an s.c. administration of 1 mg/kg could provide pregnancy protection in two of the four treated monkeys. In these monkeys, however, the menstrual cycle characteristics were not affected as compared to pretreatment cycles. Interestingly, the administration of 2 mg/kg, s.c., RU 486 could provide a hundred percent pregnancy protection in mated monkeys, and there was no significant changes in the pattern of menstrual cycle characteristics. It appears that an early post-ovulatory administration of RU 486 may be successfully used in human as an effective once-a-month, early luteal phase contranidatory agent.