Of 39 patients given a single injection of CP-675,206 (known as CP-675), tumors disappeared in three patients, shrunk in a fourth patient, and cancer stopped growing in five other patients. These responses have remained since their initial treatment, which ranged from 13 to 28 months ago.
Most of the patients in the trial had advanced melanoma, which has a median survival of less than a year, says the study's principal investigator, Luis Camacho, M.D., MPH, assistant professor in the Department of Melanoma Medical Oncology.
"We were very pleasantly surprised to find such objective antitumor responses in a Phase I clinical trial, which is designed to find the ideal dose and to look for side effects," says Camacho. "These results are very early, but they are encouraging to us because there are no good agents available to treat melanoma once it has spread."
The researchers gradually increased the amount of the initially tested dose by 1,500 fold, evaluating seven different dose levels, before they found higher doses that both produced an effect and had tolerable side effects. Most of the patients who did not respond to the drug were those treated with the lower doses, the investigators say.
The study was conducted at M. D. Anderson Cancer Center and at the University of California, Los Angeles. A collaborating researcher is Jesus Gomez Navarro, M.D., clinical director of the monoclonal antibody program at Pfizer, Inc., which developed the antibody and is sponsoring the clinical trial.
The researchers say the antibody seems to act as a "nonspecific immune booster" which enlists the immune system to fight cancer. It acts by blocking a key negative regulator of the activity of the immune system. This regulator, cytotoxic T lymphocyte-associated antigen 4 (CTLA4), stops activated immune cells from attacking the body's own tissues. The antibody, in turn, stops the function of CTLA4, a receptor that works as "the brakes" of the immune system.
Like a vaccine, CP-675 seems to continue to work long after patients receive the single two- to four-hour injection, Camacho says. "We believe the monoclonal antibody enlists the immune system to fight any new cancer cells trying to grow," he says.
The antibody may work particularly well in melanoma, he adds, because previous research has shown the immune system, if activated, can recognize this cancer.
Because the antibody allowed the immune system to attack cells that "looked" similar to the body's own, researchers worried that it could produce autoimmune disorders such as rheumatoid arthritis. But the only side effects that were observed, including rashes and diarrhea, occurred at the highest doses and were resolved without long-term problems, Camacho says.
Based on the results, Pfizer has launched a Phase II study, which is enrolling 100 patients at seven institutions nationwide. Camacho will serve as the principal investigator for this trial as well.
Additional contact information:
Laura Sussman
ASCO
Cell: 832-264-8893
Julie Penne
Tel: 713-792-0655
Cell: 281-460-1788
Abstract #2505