CHICAGO (Reuters) - Adding the common diabetes drug metformin to chemotherapy helped shrink breast cancer tumours faster in mice and keep them away longer than chemotherapy alone, raising hope for a more effective way to treat cancer, US researchers said last Monday.

They said metformin appeared to target breast cancer stem cells — a kind of master cancer cell that resists conventional treatment and may be the source of many tumors that grow back.

"What's exciting here is we now have something that is mechanistically a different kind of killer of cancer that can synergize with chemotherapy," Kevin Struhl of Harvard Medical School, whose study appears in the journal Cancer Research, said in a telephone briefing.

Many teams have been looking for ways to destroy the master cancer cells in the hope of making cancer easier to cure.Last month, a team at the Broad Institute of Harvard and the Massachusetts Institute of Technology reported that a chemical called salinomycin could kill breast cancer stem cells.

What is different with his study, Struhl said, is that metformin is a widely used drug with a long safety track record. "There are tens of millions of people who take this drug," he said.