The progesterone antagonist RU 486. A potential new contraceptive agent.

Since progesterone supports endometrial nidation of the fertilized ovum, a progesterone antagonist would theoretically block this process and thus have contraceptive potential. We have explored the ability of RU 486, a newly developed competitive progesterone antagonist, to function as a contraceptive agent. A single oral dose of 10 mg per kilogram of body weight given in the midluteal phase consistently induced menses within 72 hours in women with normal cycles and no risk of pregnancy. Bleeding was not prevented by administration of human chorionic gonadotropin in the midluteal phase. This suggested that giving a single dose of RU 486 late in the menstrual cycle might be an effective contraceptive strategy. This concept was tested in monkeys. When given to rhesus females on day 25 of the cycle, a single intramuscular dose of RU 486 (5 mg per kilogram) prevented pregnancy. The vehicle-treated control animals had a 28 percent pregnancy rate (P less than 0.05 by chi-square analysis). No side effects were noted in women or monkeys. These data suggest that a progesterone antagonist such as RU 486 has the potential to be an effective, safe, and convenient contraceptive agent. Further work will be necessary to assess the safety of long-term monthly administration and to define the optimal dose and time of administration in women.