TABLE B4.1 Mifepristone and the Ovarian Cycle









Early follicular




Delayed ovulation


Late follicular

± Inhibit ovulation


Early luteal



Mid to late luteal

± Luteal regression

Low: desynchronization




High: bleeding


Progesterone is a key hormone in the regulation of many reproductive processes, including the establishment and maintenance of pregnancy. In many species it is involved in the mechanism of ovulation, and both parturition and lactation occur as a result of withdrawal of progesterone secretion. In 1980 a group of chemists at Roussel-Uclaf discovered a synthetic steroid (mifepristone or RU 486) which is a potent antagonist of progesterone and cortisol but has no antiestrogen activity (Philibert et al., 1982). Very soon, it was reported in a pilot study to induce bleeding from the uterus when it was given to women in the luteal phase of the cycle or in early pregnancy (Herrman et al., 1982). This latter property has been utilized in the development of a method of inducing abortion. Mifepristone, in combination with a suitable prostaglandin (gemeprost, sulprostone, or misoprostol), has been shown to be a highly effective alternative to vacuum aspiration to induce abortion in the first nine weeks of pregnancy and has been licensed for this use in France, Great Britain, Sweden, and China (Baird, 1993).

The political controversy surrounding the "abortion pill" has obscured the fact that antigestogens have many other potential therapeutic uses. In this paper I describe the effects of mifepristone when given to women at various stages of the menstrual cycle. Although there are several other synthetic antigestogens with slightly different pharmacological properties (e.g., onapristone and lilopristone), the data in women are virtually confined to mifepristone.


The effect of mifepristone on ovarian activity is dependent on the time of the cycle and the dose used (Table B4.1). Since mifepristone binds to progesterone receptors in the hypothalamus and anterior pituitary as well as in the uterus, it is likely that it would interfere with the ovarian as well as the endometrial cycles. In the original clinical report in which mifepristone was given to three women in a dose of 50 mg/day by mouth

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