Researchers at the University of Pennsylvania transformed the white blood cells of patients suffering from late-stage chronic lymphocytic leukemia into "serial killer" cells capable of annihilating cancer cells within the body.

While only three patients were enrolled in the study, published Wednesday in The New England Journal of Medicine and Science Translational Medicine, the results were startling, as two of the patients experienced full recovery and are still in remission more than a year later.

The two patients achieved 100 percent remission - no cancer cells remaining - for up to a year, while the third patient achieved a 70 percent reduction in cancer cells, a strong anti-tumor response, researchers noted.

The third patient improved but still had cancer.

"We put a key onto the surface of the T-cells that fits into a lock that only the cancer cells have," said Dr. Michael Kalos, director of translational and correlative studies at the University of Pennsylvania's Perelman School of Medicine and an investigator on the study.

The results provide "a tumor-attack road map for the treatment of other cancers," including those of the lung and ovaries as well as "myeloma" and "melanoma", researchers added.

Kalos said past efforts to use the technique, known as "adoptive T-cell transfer," failed either because the T-cell response was too weak or proved too toxic for normal tissue.

"[The serial killers] can kill one tumor cell and then go and kill another, and we found in all three of our patients that the T-cells killed at least a thousand tumor cells, and that's the first time that has ever been shown anywhere near that kind of efficiency," said Dr. Carl June, the lead author of the study, in a video released with the research.

Scientists for the first time used gene therapy to successfully destroy cancer tumors in patients with advanced disease - a goal that has taken 20 years to achieve.

"We knew [the therapy] could be very potent," Dr. David Porter, director of the blood and marrow transplantation program at the Hospital of the University of Pennsylvania in Philadelphia and a coauthor of both studies, told The Los Angeles Times.

Although in 2006, Rosenberg published the first study in which T-cell receptors were used for gene therapy, combined with chemotherapy, in 17 people who had advanced melanoma.

The research group plans to treat four more patients with Leukemia before moving into a larger Phase II trial of the study. This type of Leukemia affects the blood and bone marrow and is the most common form of the cancer.