For the study, Kantoff's group randomly assigned 512 men to receive Provenge or placebo. All of patients had advanced prostate cancer that had proven resistant to standard hormonal therapy.
On average, men receiving Provenge lived 4.1 months longer than men receiving placebo, the researchers found. Average survival was 25.8 months for men in the Provenge group, compared with 21.7 months for men in the placebo group, meaning that Provenge extended survival by 22 to 25 percent, Kantoff said.
He contends that if the vaccine was used by men with less severe disease survival, it might be extended for even longer.
"Theoretically, if you take people with less diseases and you stimulate the immune system, you could have a more profound effect, but we don't really know that yet," he said.
Compared with other treatments, such as chemotherapy, radiation and hormone therapy, Provenge has been touted as having fewer and less severe side effects. In this trial, the most common side effects were chills, fever and headache, the researchers noted.
Kantoff said the vaccine is just the first step in immune system therapies for cancer. "This study opens the door for a whole new form of therapy, which is immunotherapy," Kantoff said.
Another vaccine for prostate cancer, called GVAX, had been tested in a phase 3 trial, Kantoff noted, "but that didn't work."
"There are many other things that could work and they are in development right now. The hope is there will be many things that will work. This study is really a proof of principle that the concept works and hopefully there will be many other things that will add to, or improve upon this," he said.
Prostate cancer expert Dr. Anthony D'Amico, chief of radiation oncology at Brigham and Women's Hospital in Boston, said that "the concept of immunotherapy for prostate cancer was unexpected, but important and perhaps the beginning of
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