U.S. researchers say they've been successful in genetically engineering one-of-a-kind stem cells capable of producing and secreting a toxin that can kill brain cancer cells.

Scientists at Harvard University report experiments in which the toxin-carrying stem cells were used to kill cancer cells left behind in the brains of mice after the main tumor had been removed.

Previous clinical trials which attempted to introduce cancer-destroying toxins into patients' brains failed because of toxin-delivery issues, leading the Harvard scientists to turn to stem cells as possible delivery "vehicles."

"Cancer-killing toxins have been used with great success in a variety of blood cancers, but they don't work as well in solid tumors because the cancers aren't as accessible and the toxins have a short half-life," says research leader Khalid Shah of the Harvard Stem Cell Institute.

While it has long been assumed that stem cells could be utilized to continuously deliver therapeutic toxins to brain tumors, the challenge has been to create stem cells that would not themselves be killed by the toxins, the researchers said.

The solution, the researchers report in the journal Stem Cells, was to create stem cells possessing a mutation that keeps the toxins from acting inside the cell.

"Now, we have toxin-resistant stem cells that can make and release cancer-killing drugs," Shah says.

Once in the brain, the specially-tagged toxins will only enter cancer cells displaying certain specific surface molecules, so the cancer can be attacked with any risk to normal cells, the researchers reported.

"After doing all of the molecular analysis and imaging to track the inhibition of protein synthesis within brain tumors, we do see the toxins kill the cancer cells," Shah says.

Other cancer scientists said the Harvard work could signal the introduction of a new regimen of cancer therapies.

Such therapies could help increase survival rates and drive progress in the treatment of brain cancers such as glioblastoma, the most common brain tumor in adult human, they said.

"It shows you can attack solid tumors by putting mini pharmacies inside the patient which deliver the toxic payload direct to the tumor," said Chris Mason, professor of regenerative medicine at University College London.

Shah said therapies using genetically modified stem cells to deliver cancer drugs directly to the brain could see human clinical trials in the next 5 years, and that he and his team are currently seeking FDA approval for such trials.