The abortion pill (RU486 or mifepristone) might ward off breast cancer in women at high risk for the disease. New research found that the chemical compound in RU486 prevented tumors from growing in mice that were genetically engineered to carry the BRCA1 breast cancer gene.

RU486 blocks the production of the hormone progesterone, and this antiprogesterone effect could have prevented the growth of tumors in these mice, the authors speculated. RU486 aborts a pregnancy via the same mechanism.

Still, experts are far from recommending RU486 as breast cancer therapy in people.

The Study

Researchers studied mice that carried the mutated form of BRCA1, which caused them to be highly susceptible to breast cancer.

The mice's mammary cells accumulated high levels of progesterone receptors and then divided and proliferated at an abnormally rapid rate.

However, mice treated with RU486 did not develop breast tumors by the time they reached one year of age. On the other hand, untreated mice developed tumors by eight months.

Progesterone may encourage the proliferation of mammary cells that carry a breast cancer gene, the researchers said.


Although the study was done in mice, the same mechanism occurs in human cells, said study author Eva Lee, a professor of developmental and cell biology and biological chemistry at the University of California, Irvine.

She speculated that clinicians might one day be able to use progesterone-blocking compounds to prevent breast cancer in women with a genetic predisposition.

But RU 186 may not be the best candidate, however.

"It is the most widely available antiprogesterone drug," Lee said. "We are currently testing a more specific anti-progesterone drug to see whether it has the same effect and if that's proven, we'll go to a small clinical trial to see if that antiprogesterone drug is effective in a high-risk population."

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