An experimental treatment that harnesses the immune system to attack tumors can extend the lives of men fighting advanced prostate cancer, according to a study released yesterday, marking the first time a cancer vaccine has been shown to improve survival for any malignancy.
In addition to providing badly needed hope for victims of one of the most common cancer killers, the findings mark a watershed for the long-sought goal of using vaccines to fight malignancies, experts said.
"We are very excited," said Eric J. Small of the University of California at San Francisco, who led the study. "We think this is going to open up this whole field."
Other researchers agreed, saying that although the results need to be confirmed, they should reinvigorate a strategy for treating cancer that had been the focus of intense interest over the past decade but failed to fulfill initial expectations.
"This is a significant development," said James L. Gulley of the National Cancer Institute. "It is both meaningful for patients who have prostate cancer but also for the field. It provides a proof of concept that vaccines can in fact work, and that has generally been lacking."
Unlike traditional vaccines, cancer vaccines are designed to treat rather than prevent disease by stimulating the immune system to attack cancer cells as it would invading bacteria or viruses. Although hope had dimmed for the approach, some scientists have quietly continued to pursue it.
"It seemed really good, and then the pendulum swung back because we didn't see promising results, but now this is clearly some of the strongest evidence we've seen of a clinical benefit," Gulley said.
For this vaccine treatment, doctors removed certain immune system cells from patients with advanced prostate cancer, processed them in the laboratory with a protein on prostate cancer cells (called prostatic acid phosphatase, or PAP), and then injected the modified cells back into the patients in three infusions over the course of a month. The idea is to provoke other immune system entities known as T cells to seek out and destroy prostate cancer cells throughout the body.
"The theory is that these [modified immune system] cells then communicate with the T cells, which go out and do their thing and kill the prostate cancer cells," Small said.
In the new study, Small and his colleagues gave the vaccine to 82 men whose cancer had progressed after surgery and radiation treatment, and a placebo to 45 similar men. Those receiving the vaccine survived a median of 25.9 months, compared with 21.4 months for those receiving the dummy vaccine -- a 4 1/2-month difference that exceeds the benefit produced by chemotherapy. After 36 months, 34 percent of the men receiving the treatment were alive, compared with 11 percent of those who received the placebo.