The report, published Monday in the online journal Cancer Research, argued that the drug metformin may improve breast cancer outcomes in people.
"We have found a compound selective for cancer stem cells," said senior author Kevin Struhl, a professor of biological chemistry and molecular pharmacology at Harvard Medical School. "What's different is that ours is a first-line diabetes drug."
In this study, the diabetes drug seemed to work independently of its ability to improve insulin sensitivity and lower blood sugar and insulin levels, all of which are also associated with better breast cancer outcomes.
The combination of metformin and the cancer drug doxorubicin killed human cancer stem cells and non-stem cancer cells in culture, the report said. The researchers used four genetically distinct breast cancer cell lines.
In mice, pretreatment with the diabetes drug prevented the otherwise dramatic ability of human breast cancer stem cells to form tumors.
In other mice where tumors were allowed to take hold for 10 days, the dual therapy also reduced tumor mass more quickly and prevented relapse for longer than doxorubicin alone, according to the study.
In the two months between the end of treatment and the end of the experiment, tumors regrew in mice treated with chemotherapy alone, but not in mice that had received both drugs.
By itself, metformin was ineffective in treating tumors.
"There is a big desire to find drugs specific to cancer stem cells," Struhl explained.
"The cancer stem cell hypothesis says you cannot cure cancer unless you also get rid of the cancer stem cells. From a purely practical point of view, this could be tested in humans. It's already used as a first-line diabetes drug."