THURSDAY, Aug. 31 (HealthDay News) -- U.S. scientists are pointing to the full, 18-month-long remission of two melanoma patients as the first evidence that gene therapy can work to beat back cancer.

"I'm not aware of any other gene therapy that's gotten this far in cancer therapy," said Dr. Len Lichtenfeld, deputy chief medical officer at the American Cancer Society.

Lichtenfeld said the study, which was conducted by a team at the National Cancer Institute, is the "first real 'proof of concept' that this type of approach could really work."

The findings are reported in the Sept. 1 issue of Science.

The notion that the targeted manipulation of genetic material might help fight cancer has long held a special allure for researchers. However, despite decades of dedicated research, real success -- in terms of effective, durable treatments -- has remained elusive.

That era may now be over, experts say.

In their research, the NCI team focused on boosting the immune system's ability to recognize and destroy cancer cells.

To do so, the researchers focused on healthy white blood cells called T-lymphocytes. These cells are naturally designed to identify and destroy "foreign" cells, including those found in tumors. But for many patients with advanced cancer, T-lymphocytes simply aren't up to the job.

The group at the NCI sought to give these T-cells an extra "boost" to help get the job done.

"In our work, we genetically modify normal lymphocytes to give them the capacity to fight a cancer -- then return them to the patient," explained lead researcher Dr. Steven Rosenberg, chief of surgery at the NCI.

Working with 17 patients with advanced, metastatic melanoma skin cancers, Rosenberg's team first removed a small sample of normal T-cells from each patient. Next, the researchers genetically engineered the cells to carry a specific cell-surface receptor, one that would help them recognize melanoma cells.

These "re-armed" cells were then introduced back into the patient's bloodstream, where it was hoped they would gradually replace less-able lymphocytes and mount a fierce, sustained attack against the cancer.

That's exactly what happened for two of the 17 patients in the study, who quickly went into a sustained remission. "These two patients were treated over a year ago and are now disease-free," Rosenberg said.

That's a real breakthrough when it comes to gene therapy, Lichtenfeld said.

"This has real meaning in terms of cancer treatment," he said. "It isn't as if the tumor just shrugged a bit and then a couple weeks later came back. These responses have been ongoing over a period of months, and that's significant."

Although most of the other patients in the study didn't fare as well, tests showed that, for most of them, the engineered T-cells did "persist" in their immune systems. That means the new cells survived long enough to make up at least 10 percent of circulating lymphocytes for at least two months after treatment, the researchers said.

So, why didn't the therapy beat back cancer in those patients? According to Rosenberg, the strategy is still in its rough form, and no one expected that every patient would experience even partial remission. "This is the first time we have ever done this," he said, "so the kinds of receptors that we are using, the way we are putting them into cells, the way we are changing them -- all of that is not optimal."

Improvements on all of those fronts are underway, however. "We also have better receptors that can recognize many common cancers -- lung, breast and colon. Those trials haven't started yet, but we plan on starting them within the next several months," Rosenberg said.

Lichtenfeld cautioned that this research is still in its early stages. "It's not something that's going to be available tomorrow," he said. "It's going to take a substantial amount of work to refine this, find out how effective it truly is, and determine how many people it could help. It's clearly a first step."

Still, the achievement is heartening after decades of struggle, he said.

"This is the culmination of a long process of research that's being going on for many years," Lichtenfeld said. "And it's just the beginning of new research that could really hold out hope for patients."

Moe information

For more on gene therapy and cancer, visit the American Cancer Society.