A sustained dose of RU-486, the so-called abortion pill, prevented breast cancer tumors in mice with a genetic mutation that made them highly susceptible to the disease, researchers at UC Irvine report today.
The mutation in a gene known as BRCA1 leads to uncontrolled cell growth in the mammary glands. About one in 800 women inherits a version of BRCA1 that is damaged in some way, and the National Cancer Institute estimates that up to 80% of such women will develop breast cancer.
Cancer researcher Eva Lee and colleagues figured that RU-486 could be used to block the hormone progesterone, whose unchecked production is believed to be a primary cause of hereditary breast cancer.
RU-486 binds to receptors that normally link with progesterone. It is used as an abortion pill because progesterone is needed in the uterus to maintain a pregnancy.
Progesterone also stimulates the growth of milk-producing cells in pregnant women. Women who aren't pregnant can experience the same kind of rapid cell growth if they have the BRCA1 mutation.
The UCI team, which reported its findings in the journal Science, surgically implanted 14 of the mice that had the BRCA1 mutation with RU-486 pellets, which released the drug over a two-month period. All of the mice made it to their first birthday without developing any breast cancer tumors, according to the study.
Meanwhile, four mice implanted with a placebo pellet had all developed tumors by the time they were 5.2 months old. All of the 25 untreated control mice had tumors by the age of 8.7 months, the study found.
"It is a greater effect than I would have expected," said Eliot Rosen, a cancer researcher at Georgetown University who linked progesterone to BRCA1 but was not involved in the latest study. "It is a little surprising that it completely prevented the tumors."
Currently, the best way for women with a BRCA1 mutation to substantially reduce their risk is to surgically remove their breasts and ovaries before tumors have a chance to form.
Lee said RU-486 probably wouldn't be the best candidate for a human treatment because in addition to blocking progesterone, it binds with receptors involved in immunity and other important functions.
For short-term use to end a pregnancy, that isn't likely to be a problem. But if patients are going to take a drug for years, it should target progesterone more precisely, she said.
The study was funded by the National Cancer Institute, the Department of Defense and the Breast Cancer Research Fund, a private foundation based in New York.