Genetically altered immune cells wiped out tumors in two men with a deadly form of skin cancer and kept the patients disease-free for at least 18 months, U.S. scientists said on Thursday.

Fifteen patients did not respond to the treatment, however, and the researchers and other experts said more work was needed to make it more effective.

Still, the findings were welcomed as evidence that cancer patients can be successfully treated using gene therapy, a troubled field that has been hindered by safety concerns. Scientists voiced hope the approach could work for other cancers.

In the new study, researchers at the National Cancer Institute (NCI) infused 17 advanced melanoma patients with their own white blood cells that had been removed and genetically engineered to fight tumors. The cancer was eliminated in two male patients, the researchers said.

“The tumors went away and both of the patients are now completely disease-free over a year and a half later,” Dr. Steven Rosenberg, chief of surgery at the NCI, said in an interview.

Before the experiment, the patients had advanced skin cancer that was not helped by standard therapies and they were expected to live just three to six months, he said.

Writing in the journal Science, the researchers said the white blood cells had been armed with genes to spark production of proteins called T-cell receptors. Those receptors recognized molecules on the melanoma cells and directed the white blood cells to destroy the cancer, they said.

DISEASE-FREE 18 MONTHS LATER

In a 52-year-old man, a tumor in his armpit disappeared and another on the liver shrunk enough to be surgically removed. He remained disease-free 19 months after treatment, the study said.

Another patient, a 30-year-old man, had a lung tumor recede and showed no signs of disease 18 months later.

Other gene therapy researchers said the results were a major step forward, but the technique needed to be improved so more patients would benefit.

“This certainly is a significant technical advance that is going to fuel more interest ... and more enthusiasm, I hope, among researchers,” said Dr. Michel Sadelain, director of Memorial Sloan-Kettering Cancer Center’s somatic cell engineering laboratory.

But he added it was “undeniable the response here is rather disappointing.”

Savio Woo, a professor at Mt. Sinai School of Medicine and a past president of the American Society of Gene Therapy said the researchers “need to do it in more patients and get better response rates, and when that happens we can all pop the champagne.”

Scientists have touted gene therapy as holding great promise for a range of ailments, but safety problems have set the field back. One experiment cured two French “bubble boys” with a rare immune disorder, but later gave them leukemia, and an 18-year-old died in a 1999 gene therapy experiment.

Rosenberg said there were no side effects from the melanoma gene therapy. It was administered with the drug interleukin-2, which can cause fluid retention, he said.

The researchers said they hope the same approach can fight breast, lung and other cancers. They are seeking regulatory approval to test the technology in patients with other cancers, Rosenberg said.