SAN ANTONIO (The New York Times News Service) -- Cancer drugs and radiation target and kill fast-growing cancer cells. But a small number of noncancer cells in the tumor often survive. These, researchers believe, are "mother" cells -- stem cells that shrug off treatment and survive to manufacture more cancer cells.

Researchers in Houston are studying the vulnerability of these mother cells, and in early studies have an experimental drug that seems to stop them in breast cancers.

"Cancer stem cells are present, we believe, in all cancers," said Dr. Jenny Chang, medical director of the breast center at Baylor College of Medicine. "So that 1 to 5 percent of the cancer contains cancer stem cells. And while chemotherapy kills 99 to 95 percent of the tumor, what's left behind is the 5 percent of cancer stem cells. Those will then regrow and regenerate."

Chang spoke at a press briefing on experimental new cancer drugs Friday at the 32nd annual San Antonio Breast Cancer Symposium , which continues through Sunday.

Chang first identified a vulnerable target on the breast stem cells called the Notch receptor and used an experimental drug, gamma-secretase inhibitor, along with a common anticancer drug, docetaxel -- first on mice grafted with human tumors, and later in a small number of women with advanced breast cancer.

In one dramatic set of photos, Chang showed an inoperable egg-sized mass on one patient's breast that eventually shrank to the point where surgery was possible.

Still, proving the effectiveness of these types of anti-stem-cell drugs will be challenging, researchers said. Given alone, they still leave the large mass of cancer cells intact.

An experimental vaccine against chronic lymphocytic leukemia also being studied at Baylor killed cancer stem cells, but the patients didn't improve for several months, said Dr. C. Kent Osborne, director of the Baylor Cancer Center.

"If you get rid of the mother so that she can't have any more offspring, as the daughter cells that were there die off, they can't be replaced," Osborne said. "It's going to be much more difficult to study these because we don't see an immediate tumor shrinkage. But the hope is that over time -- and it may take a long time in a tumor like breast cancer that grows more slowly -- to see the tumor actually go away under the way we usually study drugs."

"As oncologists, we like to see things shrink," added Dr.

William Gradishar, professor of medicine at Northwestern University in Chicago, who wasn't involved in the research. "The problem is that things regrow. And the current available therapies aren't really affecting the population of cells that Jenny is talking about, that are often resistant" to traditional cancer treatment.

The researchers are planning on larger human studies of the drug, made by Merck & Co., which funded the research. They're also looking at other targets on the stem cells.

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