Chicago: US researchers have discovered a compound 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, they reported on Thursday.

The discovery came using a new method of screening for drugs that specifically target and kill cancer stem cells, and it could be used to find drugs targeting other cancer stem cells as well, they said.

Many teams have been looking for ways to destroy these master cancer cells in hopes of making cancer 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.

To study the cells, Gupta's team first devised a method for stabilising 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 Bristol-Myers Squibb Co's cancer drug Taxol, or paclitaxel.