Antiprogestins are characterized by substitutions at the 11 beta and 17 alpha positions of the steroid ring system and bind strongly to both progesterone and glucocorticoid receptors. Although they function predominantly as antiprogestins and antiglucocorticoids, on occasion they display progestin agonistic and even antiestrogenic properties. The most common clinical use of the antiprogestin mifepristone is to induce a medical abortion in the early stages of pregnancy. Progesterone maintains the endometrium, transforming it from a proliferative to a secretory state. It also facilitates the luteinizing hormone surge, which initiates ovulation. As a consequence, antiprogestins may also have contraceptive potential. Although antiprogestins do delay ovulation, this effect is inconsistent unless high doses are given, and under these circumstances, the antiprogestin effect is associated with unopposed estrogen action on the endometrium. Very low doses of antiprogestins do not affect hormonal secretion or ovulation or alter bleeding patterns, but they do have contraceptive potential by inducing profound alterations in endometrial morphology. Mifepristone is also a very effective and safe postcoital agent. This new class of pharmacological agents has numerous other gynecological and obstetrical indications, such as endometriosis, uterine myoma, and expulsion of the fetus in the case of fetal death in utero. Antiprogestins may also be used in the treatment of steroid-dependent tumors. There are also therapeutic implications consequent to their antiglucocorticoid properties.
PIP: Antiprogestins have a 17-alpha substitution, which promotes higher binding affinity to both progesterone and glucocorticoid receptors, and an 11-beta substitution, which appears to be responsible for the antagonistic action. Antiprogestins also exhibit progestin agonistic and even antiestrogenic properties. Induction of a medical abortion in very early pregnancy is the most frequent clinical use of the antiprogestin mifepristone (RU-486). Antiprogestins may someday be used as a contraceptive since very low doses of antiprogestins cause considerable changes in endometrial morphology. At these low doses, they do not affect hormonal secretion or ovulation or bleeding patterns. At high doses, they delay ovulation but are associated with unopposed estrogen action on the endometrium. At low doses, antiprogestins do not always delay ovulation. RU-486 is a very effective and safe postcoital contraceptive. Antiprogestins may be beneficial in treating endometriosis, uterine myoma, and steroid-dependent tumors (e.g., breast cancer). They may be used to induce labor in cases of intrauterine fetal death. Antiprogestins may prove useful in treating Cushing's syndrome due to ectopic adrenocorticotropic hormone secretion, in lowering intraocular pressure in glaucoma, and in preventing the progression of viral diseases in humans. They also appear to have even more potential benefits, such as treatment of burns.