WASHINGTON - The chemical compound in the abortion pill RU-486 blocked formation in mice of breast tumors caused by a mutant gene blamed in many women's breast and ovarian cancers, researchers found.

The study, published on Thursday in the journal Science, suggests a potential new path to protect women genetically predisposed to these cancers by using drugs to interfere with a hormone called progesterone, the researchers said.

A team led by University of California at Irvine scientist Eva Lee found that the compound, mifepristone, prevented breast tumors by blocking progesterone in mouse breast tissue cells.

They said they found that progesterone, which is involved in the female reproductive cycle, stimulates the proliferation of breast cells carrying a gene linked to breast cancer, possibly feeding tumor growth.

Women with a mutated version of the gene, known as BRCA-1, face a much higher risk of breast and ovarian cancers. By age 70, more than half of them end up with one of the two types of cancer.

Hope of nonsurgical option
Some of these women opt to have their breasts or ovaries surgically removed to lower the risk. This study raises hope that they might have another option - using drugs rather than surgery to guard against cancers linked to the mutated gene.

In its normal form, BRCA-1 protects against cancer, suppressing tumor formation by assisting cells to repair DNA damage that could lead to cancer-causing mutations.

In this study, the researchers focused on how mifepristone influences the way the mutated gene functions in tissue, using mice with the rodent version of the mutated gene.

Mice given mifepristone were tumor-free by the time they were a year old, while the mice that did not get it all developed tumors by the time they were 8 months old, the researchers found.

Progesterone stimulates the development of tumors when the mutated gene is present because it accelerates cell division, but mifepristone interfered with a process needed for progesterone to spur the division of cells, the study found.

"By targeting the progesterone receptor, it's very possible that we can prevent the breast cancer, especially in the high risk BRCA-1 area," Lee said in a telephone interview.

Mifepristone is the chemical compound present in RU-486, sold as Mifeprex and made by Danco Laboratories LLC. The U.S. Food and Drug Administration approved it in 2000 for medical abortions in women pregnant as long as seven weeks.

Part of abortion-inducing combo
Mifeprex blocks production of progesterone, which keeps a fertilized egg implanted in the uterus. Mifeprex is taken with another drug called misoprostol to trigger an abortion. It is unrelated to emergency contraception sold by Barr Pharmaceuticals Inc. under the name Plan B.

Lee noted that mifepristone may not be the best treatment for long-term cancer prevention because it affects things other than progesterone and might cause unacceptable side effects like suppressing the body's immune system.

"Mifepristone may not be the best for prevention purposes. The reason is that mifepristone actually has very broad activities, and we are aware that there are other compounds that are more specific. Those kind of compounds probably have greater potential in terms of prevention," Lee said.