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Early but intriguing research suggests the first vaccine for lung cancer may substantially prolong victims' lives and perhaps even cure some of them.

Unlike vaccines for flu and measles, which keep people from catching disease, cancer vaccines are intended to fight malignancies once they occur. The idea is to harness the body's own cancer-killing machinery to seek out tumors and destroy them.Several different vaccines are under study, but researchers are especially enthusiastic about one being tested for melanoma, the deadly form of skin cancer, and a type of lung tumor called small cell cancer.

About 40,000 Americans are diagnosed annually with small cell lung cancer, and virtually all of them die from it. While chemotherapy can lengthen these people's lives, it does not remove all of the microscopic bits of the lung cancer, and these eventually prove fatal.

No new chemotherapy approaches have made any important difference in treating this kind of lung cancer in the past 20 years.

The cancer vaccine is being tested by doctors at Memorial Sloan-Kettering Cancer Center in New York. They described their latest results Tuesday at a meeting of the American Society for Clinical Oncology.

Of the first six patients treated, four are still alive at least three years after their diagnosis. The typical survival is 16 months.

"It may have a very substantial impact on their disease," Dr. Stefan Grant said. "These people are living much longer."

He said those who live for three years with this disease "are likely to live much longer and may in fact be cured."

Dr. Paul Chapman, a colleague who developed the vaccine, is using it to treat melanoma. Of 14 patients getting the vaccine, nine are still alive after three years.

"They are potentially cured," Chapman said.

"Cure" is a word doctors use sparingly when talking about cancer, a disease that yields only grudgingly to progress. At the same cancer conference, other researchers spoke encouragingly about new treatments that appeared to increase patients' lives by weeks, not years.

However, the Memorial Sloan-Kettering doctors said the results of their initial studies were so striking, they believe they are having a positive effect, even though they have treated only a handful of patients.

The next step will be large comparison studies involving hundreds of patients with both small cell lung cancer and melanoma. These experiments are still being designed and will be financed largely by Imclone Systems of New York City, which has licensed rights to the cancer vaccine.

Dr. Mark S. Kaminski of the University of Michigan, who is testing antibodies as a cancer therapy, called the vaccine results encouraging.

"It's a neat idea," he said. "I'm intrigued by what I've heard."

Creating the vaccine involved an act of immunologic trickery.

Cells of small cell lung cancer and melanoma carry molecules known in medical shorthand as GD3. Unfortunately, the body does not recognize GD3 as foreign and so doesn't mount an immune response against it.

Nonetheless, GD3 is an inviting target for therapy, since it is displayed by cancer cells but not most healthy ones.

It turns out that a protein called R24 normally attaches itself to GD3. The researchers took advantage of this by crafting an antibody to R24. The human immune system recognizes this newly created antibody as foreign. And the antibody looks enough like GD3 that the immune system now considers GD3 to be foreign as well.

Doctors inject the new antibody, called BEC2, along with a harmless kind of bacteria called BCG, which helps stimulate the immune system. The treatment has virtually no side effects.