U.S. researchers have found a chemical that can kill breast cancer stem cells — a kind of master cancer cell that resists conventional treatment and may explain why many cancers grow back.

Finding ways to destroy these cells could make cancer far easier to cure.

"There is a lot of evidence to suggest now that these cells are responsible for many of the recurrences that are observed after treatment has stopped," Piyush Gupta of the Massachusetts Institute of Technology and the Broad Institute, whose study appears in the journal Cell, said in a telephone interview on Thursday.

The problem is that cancer stem cells are rare and difficult to study in the lab because they quickly change into other types of cells. And they are hard to kill.

"It wasn't clear it would be possible to find compounds that selectively kill cancer stem cells," Gupta said in a statement. "That's what we did."

To study the cells, Gupta's team first devised a method for stabilizing cancer stem cells in the lab and getting them to multiply. They then tested them against 16,000 natural and commercial chemical compounds to see which ones were able to kill the cancer stem cells specifically.

That turned up 32 contenders.

They narrowed down this list to a handful of chemicals, and tested these in the lab and in mice.

A chemical called salinomycin hit the target. It was 100 times more potent at killing breast cancer stem cells than the common chemotherapy drug called paclitaxel or Taxol.

Cancer stem cells treated with salinomycin were far less able to start breast cancers when injected into mice than cancer stem cells treated by paclitaxel. And the treatment also appeared to slow the growth of tumors in the mice.

Gupta said it is not clear if salinomycin will emerge as the best drug compound for killing breast cancer stem cells — or that it will be safe to use in people with cancer.

But the study offers a new roadmap for drug companies to isolate and test compounds capable of killing the cells.

"We now have an approach that can be used very systematically to find such compounds," he said.

Ultimately, he said it may be possible to treat cancers with dual therapies that wipe out the bulk of tumor cells and the tumor-cell making machinery many conventional treatments leave behind.

Cancer is the No. 2 killer of Americans, with about 560,000 deaths annually, topped only by heart disease, according to the American Cancer Society.