Mifepristone: a novel estrogen-free daily contraceptive pill

When the first synthetic progesterone antagonist (mifepristone) was synthesized over 20 years ago, it was clear that it had a potential as an antifertility agent. Research into the use of antiprogestogens for contraception have concentrated on three general approaches: (1) inhibition of ovulation, (2) inhibition of implantation and (3) disruption of implantation or "menstrual induction". The effect of mifepristone on the ovarian and endometrial cycle depends on dose, timing and frequency of administration. Doses of 10 mg per day or more suppress follicular development and estradiol levels. Ovulatory cycles are maintained in the dose of less than 2 mg although there is increased variability in cycle length. The endometrium shows some minor asynchronous changes, although these are not sufficient to prevent pregnancy. We have chosen to investigate daily doses between 2 and 5 mg which inhibit ovulation and menstruation in over 90% of cycles while still maintaining follicular development and levels of estradiol within the range found during the follicular phase. The endometrium shows proliferative or cystic changes lined by a layer of inactive glandular epithelium set in densely packed stroma. There is, however, an absence of proliferative activity as reflected by a reduced mitotic index and Ki67 staining. These unusual histological appearances are associated with downregulation of PR but a massive upregulation of AR in particularly glandular epithelium. The antiproliferative effect of mifepristone is reassuring suggesting that the risk of atypical hyperplasia due to the effect of prolonged exposure to estrogen unopposed by progesterone is low. In a pilot study, there were no pregnancies in 200 months of exposure in 50 women who used this method as their sole method of contraception. Daily mifepristone could provide a novel contraceptive method which should be devoid of the risks associated with estrogen containing combined oral contraceptive (COC), e.g. venous thromboembolism. The health benefits of avoiding the morbidity associated with menstruation are considerable. Recent surveys suggest that amenorrhoea would be popular with many women.