Objectives: To study the effect of antiprogestin on ovarian function and endometrial development during the menstrual cycle and the possibility of using these compounds for contraceptive purposes.
Methods: Administration of different doses of the antiprogestin mifepristone during the menstrual cycle; intermittent measurements of luteinizing hormone, progestin and estrogen in blood and/or urine; endometrial morphology and concentration of markers for endometrial receptivity; efficacy trials of the contraceptive effect of mifepristone.
Results: A high dose of mifepristone administered in the follicular phase will inhibit follicular development. If mifepristone is given immediately after ovulation, the secretory development of the endometrium and the expression of, for instance, leukemia inhibitory factor and integrins will be inhibited. Similar effects on the endometrium are obtained with small weekly doses (2.5 or 5.0 mg) or small daily doses (0.5 mg) of mifepristone, which do not inhibit ovulation. Once-monthly administration of 200 mg mifepristone on the day after ovulation, and emergency postcoital treatment, are highly effective methods for preventing pregnancy. Even 5 mg once weekly has a significant contraceptive effect.
Conclusions: The antiprogestin mifepristone has a number of effects during the menstrual cycle which makes the compound suitable for contraceptive use. Treatment after a single act of unprotected intercourse, and once-a-month treatment immediately after ovulation, have shown high contraceptive efficacy. A low-dose regimen which does not influence ovulation also has a contraceptive effect, but the efficacy needs to be improved before routine clinical use.