Researchers from the Dana-Farber Cancer Institute in Boston gave three consecutive monthly doses of a recombinant vaccinia virus encoding PSA (rV-PSA) to 33 men with rising PSA levels after radical prostatectomy (removal of the prostate), radiation therapy or both. Some of the patients had cancer that had already spread beyond the prostate.

Fourteen of the 33 men were stable for at least six months after the first immunization, according to the researchers. Nine patients remained stable for 11 to 25 months, and six of those had stable PSA levels and no disease progress at the time the study was completed.

PSA is a protein produced by epithelial cells in the prostate. It is present in healthy males at low levels. In men with prostate cancer, PSA levels are increased, making it a target for immunotherapy, according to the study authors.

Among prostate cancer patients, 30 percent have metastatic disease, or cancer that has spread beyond the prostate, at the time they are diagnosed. Of those treated with radical prostatectomy or radiation therapy, a significant proportion will relapse, according to the researchers.

Inevitably, the disease spreads in patients who have rising PSA levels after primary treatment, the researchers say. Currently, options for these patients are to wait for the occurrence of metastatic disease, enroll in investigational therapies or begin hormonal therapy. Hormone therapy requires surgical or medical castration, which results in impotence, loss of libido, loss of muscle mass, weight gain and other side effects.

The rV-PSA vaccine may represent a new option for these patients, according to the researchers. The vaccine had been tested in primates and humans without serious side effects, so researchers conducted their Phase I trial in men with advanced prostate cancer. They found the vaccine, which is made by inserting the PSA gene into a virus that serves as a carrier, was safe and could produce responses in patients.

"This study demonstrates for the first time that PSA can serve as a target for cell-mediated immunotherapy approaches," the researchers write. Their findings, combined with those of earlier studies, have led to a Phase II clinical trial of the vaccine.

T. J. Koerner, Ph.D., director of research information management for the American Cancer Society (ACS), says the study was limited to only 33 men with advanced prostate cancer, so it is unknown at this time how useful the therapy will be for all prostate cancer patients. However, he says it is heartening to learn of one more potential treatment for this cancer that affects so many Americans. "It is a promising discovery that yet another attack on this disease has shown positive results at this early stage of development."

Prostate cancer patients are advised to talk with their health care provider about which treatments are appropriate for their individual case.

The ACS estimates 180,400 new cases of prostate cancer will be diagnosed in the United States this year, and 31,900 men are expected to die of the disease, making it the second leading cause of cancer death in men.