Response to the antiprogestagen RU 486 (mifepristone) during early pregnancy and the menstrual cycle in women.

RU 486 has wide potential utility as an abortifacient drug when used within the first 6 weeks of pregnancy and has the ability to induce an abortion in about 80% of subjects. Administration of low doses of prostaglandins together with RU 486 increases the success rate. It is possible that alterations in metabolism of RU 486 may explain non-responsiveness to the drug in some women. Mid-luteal phase administration of RU 486 produces bleeding within 72 h and in one-third of subjects there was luteolysis with decrease in serum FSH, oestradiol and progesterone concentrations. Administration of RU 486 in the late luteal phase does not disturb menstrual cycle length, bleeding patterns, ovulation, or hormonal parameters in treatment or posttreatment cycles. However, the drug alone cannot be used as a 'menses regulator' or 'once monthly pill' since some pregnancies do continue. Possibly the efficacy of RU 486 may be enhanced when it is combined with prostaglandins or other agents. Administration of RU 486 in the follicular phase blocks ovulation, delays the LH surge, and is associated with low concentrations of oestradiol. This is presumably the result of gonadotrophin inhibition.