Study findings showed the vaccine group lived up to an average of four-and-a-half months longer and had a greater than three-fold increase in survival at 36 months when compared to patients in the placebo group.
The study is reported in the July 1, 2006 issue of the Journal of Clinical Oncology.
The double-blind, placebo-controlled phase III clinical trial was conducted to test the efficacy of the vaccine, called sipuleucel-T, in delaying disease progression and prolonging survival in patients with asymptomatic metastatic hormone refractory prostate cancer (HRPC).
Study results showed that the vaccine was well-tolerated by participants. The most common reported adverse effects such as fever and chills were typically mild.
Led by Eric J. Small, MD, UCSF professor of medicine and urology, the study was conducted in collaboration with 19 institutions in the United States and funded by the Dendreon Corporation, a biotechnology company that developed the vaccine.
"This trial is an important milestone in the development of new treatments for prostate cancer patients," said Small. "The potential survival benefit that was observed may offer important benefits to patients and would represent the first time that immunotherapy has provided a survival advantage in prostate cancer."
Sipuleucel-T, known by its product name Provenge, is an investigational immunotherapy vaccine designed to stimulate T-cell immunity to prostatic acid phosphatase, an antigen found in about 95 percent of prostate cancers but not in non-prostate tissue.
A total of 127 patients with asymptomatic metastatic HRPC received three transfusions of sipuleucel-T or placebo every two weeks. Of this group, 115 patients had progressive disease at the time of data analysis and all patients were followed for survival for 36 months.
Prostate cancer is the most common non-skin cancer in the U.S. with more than 200,000 new cases each year. It is the third leading cause of cancer deaths in men after lung and colorectal cancer. Unlike prostate cancer that is detected early, asymptomatic metastatic HRPC is resistant to traditional hormonal therapy, and treatment options have been limited.
The study showed that the median overall survival was 25.9 months for sipuleucel-T-treated patients and 21.4 months for placebo-treated patients. After three years, survival was 34 percent for those treated with the vaccine compared to 11 percent for those taking the placebo.
The clinical trial did not meet its primary endpoint of demonstrating a statistically significant difference in progression of the disease from diagnosis, according to Small.
"We found that the time to disease progression for sipuleucel-T was 11.7 weeks compared to 10.0 weeks for placebo," he said. "This shows the difficulties in using the worsening of the disease as an intermediate marker for overall survival of patients treated with immunotherapy. The study however, suggests that sipueucel-T may provide a survival advantage to asymptomatic HRPC patients."
Many of the phase I and II clinical trials of the vaccine were also undertaken at UCSF and led by Small. He first presented results from the phase III trial at the 2005 meeting of the American Society of Clinical Oncology.
Dendreon Corporation, based in Seattle, Washington, hopes to market the Provenge product commercially in the coming year.
Co-authors of the study are Paul F. Schelhammer, Eastern Virginia Medical School, Norfolk, VA; Celestia S. Higano, University of Washington; Charles H. Redfern, Sharp Healthcare, San Diego; John J. Nemunaitis, Mary Crowley Medical Research Center, Dallas; and Frank H. Valone, Suleman S. Verjee, Lori A. Jones and Robert M. Hershberg, Dendreon Corporation.
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