Mifepristone (or RU-486) is a steroid hormone with a structure similar to the natural hormone progesterone.  Mifepristone is a competitive inhibitor of the progesterone receptor and is therefore classified as one of several drugs called progesterone antagonists or “antiprogestins”. RU-486 is an abortifacient or abortion-inducing chemical. This drug has other uses including treatment for fibroid tumors, ovarian cancer, endometriosis, meningioma, and some types of breast cancer.

  In the female body, the natural hormone progesterone is essential for a pregnancy to begin and to be maintained.  Even though Mifepristone has a very similar chemical structure to the natural progesterone, it functions very differently. Because of the similarity of the structures, RU-486 will be accepted by the uterus instead of progesterone.
 

   To induce an abortion using RU-486, a high dose of the drug is given.  This high dosage causes the body to stop secreting the natural hormone progesterone.  The recommended dosage results in levels of the drug that are about 1 million times the normal level of progesterone found in the female body.  Since progesterone is critical in nurturing the embryo, the absence of the hormone will cause the embryo to die.

  The structure of RU-486 enables it to competitively bind to the progesterone receptor in the uterus. RU-486 can thus be used for the termination of an early pregnancy (first nine weeks). The drug is not effective if it is used too early in pregnancy before progesterone levels are able to reach a critical threshold. RU-486 blocks end organ receptors of various hormones. When end organ receptors are blocked, the effect of a hormone and various hormone dependent processes are interrupted or altered. RU-486 blocks various end organ receptors but its principal clinically relevant action is to block progesterone receptors.  Since progesterone is necessary for the maintenance of pregnancy, blocking its effect will result in the termination of the pregnancy. Progesterone is necessary to maintain the endometrium, which provides a nourishing bed for the implantation and development of the fetus. With RU-486, progesterone is blocked and the endometrium is no longer able to nourish the developing embryo, which dies as a result.

For specific details about how this occurs see the following link:National Academies Press

    Mifepristone is also used in combination with a prostaglandin to increase efficacy.  Prostaglandins are used in combination with other abortifacients like RU-486 to induce the contraction of smooth muscles in the uterus to help expel the aborted fetus from the body.

To view mifepristone in a 3-D image, please click here.  In order to view, user must have Chem Scape Chime.

References:
www.cbctrust.com/RU486.96.html
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Progesterone.html
http://cwis.livjm.ac.uk/pac/chemistry/Project/WebProject/RU-486/RU-486intro.htm
http://nostradamus.freehomepages.com/estrogen.htm