Abstract

PIP: Animal and human volunteer research involving the hormone antagonist Mifegyne (mifepristone) is reviewed. Studies in animals and humans show that the potent antiprogesterone, Mifegyne, causes pregnancy interruption by acting directly at the level of the endometrium. Pharmacokinetic studies indicate that gastrointestinal absorption is low (25%), but subcutaneous and intramuscular routes do not fare better than does oral administration. Experimental studies in women testing for antiprogesterone effects indicate that mifegyne does not affect menstrual cycle length in women with regular ovulatory cycles, except when using the highest doses (600 mg). Mifegyne inhibits gonadotrophin secretion in a dose-dependent way. In humans, mifegyne has some progestomimetic activity in the endometrium in the absence of progesterone. Researchers know that larger doses of mifegyne than those sufficient to induce uterine bleeding are required to cause antiglucocorticosteroid effects. Studies demonstrate that in 18% of patients studied the only significant side effect is prolonged uterine bleeding. In addition, mifegyne fails to cause an abortion in 15% of the cases. The success rate is 85% when mifegyne is given prior to the 5th week of amenorrhea. Due to the occurrence of failed abortions and prolonged uterine bleeding in some women, researchers advise close medical supervision. An added effect of mifegyne is that in both animal and human studies it is effective in inducing labor. Preliminary studies suggest that mifegyne taken once a month only on the expected date of individual menses could be used as a safe and effective form of fertility control.