Progesterone Inhibitor Prevents Breast Cancer in Mice

Cancer researchers have found that inhibition of progesterone by the drug mifepristone (RU 4860), called the "morning-after pill,” prevented the growth of mammary tumors in a mouse model of human breast cancer. Mifepristone was designed to abort pregnancy in the first trimester by blocking progesterone, thereby ending the viability of the fetus.

Investigators at the University of California, Irvine (USA), worked with a mouse line genetically engineered to lack the BRCA-1 gene. In humans, women with a mutation that inactivates BRCA-1 have significantly greater likelihood of developing breast and ovarian cancers than do women with normal BRCA-1.

The mice were divided into two groups, one of which was treated with mifepristone. Results published in the December 1, 2006, issue of Science revealed that the treated mice not develop mammary tumors by the time they reached one year of age. All of the untreated mice, however, developed tumors by eight months of age.

"We found that progesterone plays a role in the development of breast cancer by encouraging the proliferation of mammary cells that carry a breast cancer gene, said senior author Dr. Eva Lee, professor of developmental and cell biology and biological chemistry at the University of California, Irvine. "Mifepristone can block that response. We are excited about this discovery and hope it leads to new options for women with a high risk for developing breast cancer.”



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