Genetically engineered stem cells from bone marrow showed promise as a potential new way to deliver a cancer-killing protein to tumors, British researchers said on Tuesday.

Experiments in cell cultures and in mice showed the adult stem cells — a type known as mesenchymal stem cells — could home in on cancer cells and deliver a lethal protein that attacked only the cancer while sparing normal healthy tissue.

"We've developed cells which specifically target cancer through the body and deliver an anti-cancer protein to where it is needed in a seek-and-destroy approach," said Dr. Michael Loebinger of University College London, who presented his findings at the American Thoracic Society conference in San Diego.

"Essentially, we've combined two pieces of research. The first is that mesenchymal stem cells have an innate ability to seek out tumors throughout the body," Loebinger said in a telephone interview.

Loebinger, Dr. S. M. Janes and colleagues altered the cells to express or make the cancer-killing protein called TNF-related apoptosis-inducing ligand or TRAIL.

"This protein has the ability to cause the death only of cancer cells. By combining these two approaches, we have a cell which has the ability to go around the body and find and destroy tumors," Loebinger said.

Studies in cell cultures showed the cells were able to find and kill cells from lung, squamous, breast and cervical cancer. "Lots of cancer have sensitivity to this TRAIL protein," Loebinger said.

They also injected the cells into mice with breast tumors and showed they were able to safely kill the tumors but leave healthy tissue intact.

"When we delivered this therapy, 38 percent of the tumors were completely eliminated."

He said the goal would be to develop a cell-based cancer treatment for humans that specifically targets cancer cells.

An attractive property of these cells is that they are "immunoprivileged," meaning the body will not reject them as foreign invaders. That means they can be made in batches instead of having to make custom stem cell treatments for each patient, Loebinger said.

He said a number of safety studies would be needed, but the team hopes human trials could begin in two or three years.