Uterine contractility and interaction between prostaglandins and antiprogestins. Clinical implications.

It is generally believed that progesterone and PGF2 alpha are of major importance in the regulation of uterine contractility. The results summarized herein indicate that progesterone withdrawal is essential for the changes in uterine contractility normally observed during the secretory phase of the menstrual cycle and that the inactivity of the early pregnant uterus is progesterone dependent. Treatment with the antiprogestin RU486 will convert the inactive early pregnant uterus to an active organ and will increase the sensitivity of the myometrium to prostaglandin. These latter effects of antiprogestin have resulted in the development of highly effective, nonsurgical procedures to terminate early pregnancy based on a combined treatment with RU486 and different PG analogues administered orally, vaginally, or intramuscularly. RU486 also has a softening effect on the cervix as demonstrated in late first trimester of pregnancy. This effect may be useful as pretreatment to vacuum aspiration in late first and early second trimester abortion performed by vacuum aspiration or dilatation and curettage. In prostaglandin-induced second trimester abortions, pretreatment with RU486 will significantly reduce the induction-to-abortion interval and the dose of prostaglandin needed.