The first leukemia patient to receive a breakthrough gene therapy treatment at the University of Pennsylvania has been cancer-free for more than two years.

The technique, which used altered T-cells to kill cancer cells, was used on select patients only after a battery of traditional drugs and procedures failed. Though only twelve patients have gotten the treatment, the results are raising hope.

"Twelve is still a small number, but I think that more and more physicians and scientists are recognizing that there's really something here," said Dr. Michel Sadelain, head of the Center for Cell Engineering & Gene Transfer at Memorial Sloan-Kettering Cancer Center in New York. Sadelain was not involved in the research.

Of the dozen patients treated, five are in complete remission, and four have had their cancer cells greatly reduced, according to the University of Pennsylvania's Dr. David Porter. Porter treated the 10 adult patients, and presented the findings at a conference this week.

"There is a great, great deal of interest," Porter said. "I feel a lot of energy and a lot of excitement about this approach to cancer therapy."

Diabled form of HIV transports genes

In the new treatment, doctors remove millions of T-cells from patient's bodies. They then use a disabled form of  HIV to carry new genes into those cells. When the altered T-cells are put back into patients, the new genetic material programs them to kill B-cells -- the immune system cells that proliferate to cause certain types of leukemia.

The initial results of the transfusion experiments were reported in the summer of 2011, but longer-term survival data and a larger sample size is garnering new attention.

Unlike chemotherapy, Porter said the T-cells kill cancer while leaving bone marrow cells intact.

"The fact that these T-cells are targeted specifically to the patient's leukemia, and specifically to the B-cells, makes it much more targeted, much more specific, without the chance of having toxicity to other parts of the body," Porter said.

Patients who undergo the treatment still need immune-boosting drugs afterward.

The transfusions of altered T-cells can cause severe side effects, such as high temperatures and breathing problems. This year, in treating a 6-year-old Pennsylvania girl, doctors found a common arthritis drug can resolve those side effects.

Porter said he hopes this therapy will eventually be used as a front-line leukemia treatment.

Researchers around the country are investigating whether similar methods can be used to treat other types of cancer.