Sept. 14, 2009 - The next breakthrough breast cancer treatment may be a diabetes drug already on the shelves of nearly every pharmacy.

The drug is metformin, available generically and under brand names such as Glucophage and Fortamet. A growing body of evidence suggests that diabetes patients taking metformin are less likely to get cancer, and have better outcomes if they do get cancer, than those not taking the drug.

Now Harvard researcher Kevin Struhl, PhD, and colleagues find that metformin can kill breast cancer stem cells, thought to be the cells responsible for breast cancer spread and recurrence.

And in mice carrying human breast cancers, metformin made standard chemotherapy vastly more effective. Mice treated with the combination remain cancer-free for four months, unlike mice treated with either drug alone.

"We have discovered new properties of metformin that can be of some use in cancer treatment and even prevention," Struhl said at a news conference held to announce the findings.

While his current study looked at metformin's effects on breast cancer, Struhl says the drug may affect other types of cancer as well.

"Although our studies were pretty much done on breast cancer cells, a lot of the principles are not specific just to breast cancer," Struhl said. "A lot of data shows lower cancer risk -- not just breast cancer -- in people taking metformin for diabetes."