RU 486, discovered more than 10 years ago, is an antiprogestin. Antiprogestins attach to progesterone receptors on the cell membrane in competition with the steroid hormone progesterone. When progesterone binds with a receptor, a cascade of biochemical events leads to DNA transcription. An antiprogestin, once bound to the progesterone receptor, suppresses transcription. In the uterus, this leads to hormonal and biochemical changes that can prevent implantation of a fertilized ovum or, if implantation has occurred, interrupt the pregnancy. Because the uterus is not the only site in the body that has progesterone receptors on its cells, other medical uses of antiprogestins are evident. The most prominent possible uses are in breast or ovarian cancers that may respond by growing in the presence of estrogens and progesterones.

RU 486's other potential clinical uses result from its ability to block glucocorticoid receptors in cells throughout the body. They are particularly important in...

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