The possible use of antiprogestins for contraception

A number of compounds, antiprogestins, e.g. mifepristone, onapristone and lilopristone, have been developed which compete with progesterone at the receptor level. One of these, mifepristone, is in combination with a prostaglandin analogue currently in use for termination of early pregnancy. The possibility to use these compounds for contraceptive purposes is presently under evaluation.

The possible contraceptive effect of antiprogestins has been evaluated in both clinical and experimental studies.

Administration of antiprogestin during the follicular phase has an inhibitory effect on follicular development and ovulation, and on endometrial development and function if administered during the secretory phase of the menstrual cycle. A high dose of mifepristone, 200 mg, administered immediately following ovulation is highly effective in preventing implantation, most likely due to an effect on endometrial receptivity. It seems that the endometrium is more sensitive to antiprogestin than is the ovulatory process. Low weekly, 2.5 mg to 5 mg, and daily doses, 0.5 mg, of mifepristone did not inhibit ovulation, but a significant effect on endometrial development and especially endometrial function judged from measurement of the expression of a number of markers for endometrial receptivity could be demonstrated.

The effect of mifepristone on the endometrium may be sufficient to prevent implantation, and if so, an oral contraceptive method could be developed which has no effect on ovarian function.