Antiprogestogen

Antiprogestogens, or antiprogestins, also known as progesterone antagonists or progesterone blockers, are a class of drugs which prevent progestogens like progesterone from mediating their biological effects in the body. These drugs competitively inhibit progestin at progesterone receptors. They act by blocking the progesterone receptor (PR)and/or inhibiting or suppressing progestogen production. Antiprogestogens are one of three types of sex hormone antagonists, the others being antiestrogens and antiandrogens.

Antiprogestogens are used as abortifacients and emergency contraceptives and in the treatment of uterine fibroids. They are also being studied in the treatment of breast cancer. Examples of antiprogestogens include the progesterone receptor weak partial agonist mifepristone, the selective progesterone receptor modulator (SPRM) ulipristal acetate, and the silent antagonist aglepristone. For medical abortion, mifepristone is combined with a prostaglandin (e.g., gemeprost). Ulipristal is used as a form of emergency contraception.

Several hundred antiprogestogens have been developed, but only three, mifepristone, lilopristone, and onapristone, have been given to humans, and of these, only mifepristone has been approved and introduced for clinical use.

Progestins, including progesterone are important for the body to prepare the uterus for implantation of fertilized egg during the luteal phase of the menstrual cycle and, if fertilization, implantation, and other events necessary for pregnancy occur, increased progestin (i.e. progesterone) levels are heavily involved in maintaining the pregnancy. Blocking the effects of progestins, such as progesterone, is associated with anti-gestational events.