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Experimental Vaccine Halts Prostate Cancer in Mice

FRIDAY Feb. 1, 2008 -- An experimental prostate cancer vaccine has stopped the progress of the disease in 90 percent of the mice who got it, California researchers report.

"The vaccine turned the cancer into a chronic, manageable disease," said W. Martin Kast, lead author of a report published in the Feb. 1 issue of Cancer Research.

Twenty mice, genetically bred to develop prostate cancer, were given the vaccine in a two-step process, said Kast, a professor of molecular microbiology and immunology at the University of Southern California, Los Angeles.

When the mice were 8 weeks old, they got one injection consisting of a fragment of DNA that coded for prostate stem cell antigen (PSCA), a protein that is overproduced as prostate cancers grow. That injection alerted the immune system, Kast said.

A second shot, given two weeks later, used a modified horse virus to deliver the gene for PSCA, throwing the immune system into full action against the tumor.

Only two of the 20 vaccinated mice developed full-blown prostate cancer at the end of one year. Twenty other similarly bred mice who did not get the vaccine died of their cancers.

A vaccine for prostate cancer already exists, but it is only designed to extend survival for men in advanced stages of the disease and it has not been approved for use in the United States.

The new vaccine is designed to be used much earlier, Kast said. "Our vaccine approach would be to give it before you actually develop the disease," he said. Candidates for vaccination would be men who have high levels of prostate-specific antigen (PSA), a protein associated with the cancer.

Physicians now perform biopsies in most such cases, looking for evidence of cancer, which is usually inconclusive. The vaccine could be given instead of the "watchful waiting" now common for these men, Kast said.

The series of animal tests needed to prepare for a first human trial will take perhaps two years, Kast said. As for marketing the vaccine, he added, "I have already started to talk to some companies about this."

A precedent for such use already exists in the form of the vaccine against cervical cancer that is in medical use in the United States, Kast noted.

A new urine test that could help narrow the number of men diagnosed with possible prostate cancer was also described in the same issue of the journal.

"It is an extension of a current test," said Dr. Arul Chinnaiyan, director of the University of Michigan Center for Translational Pathology and lead author of the report.

The current test, developed at Michigan and marketed by the biotechnology company Gen-Probe, screens for one molecular marker of prostate cancer. The new test screens for four more key markers.

"It is for early screening to supplement PSA testing," Chinnaiyan said. "The benefit of this is that it could spare some people from some unnecessary biopsies. Eighty percent of men with high PSA levels don't have cancer. This could pick up 80 percent of them."

Both reports brought guarded reaction from Dr. Durado Brooks, director of prostate and colorectal cancers for the American Cancer Society.

The California work, Brook said, "is an interesting direction in terms of a way to develop a vaccine, but it is a long way from clinical use."

As for the new test, he added, while it could possibly reduce the number of biopsies, it does not fulfill the big hope for prostate cancer tests: distinguishing between slow-growing cancers that are best left alone and fast-growing malignancies that require quick and aggressive treatment.

"We really need a better handle on which prostate cancers will be more aggressive," he said.

Meanwhile, researchers at the U.S. National Cancer Institute report in the same journal that they have discovered genetic factors that drive the development of prostate cancer in black men, who are more likely to die of the disease than whites.

An analysis shows numerous differences in genes governing the immune response to cancer, they reported.

More information

Learn about prostate cancer from the U.S. National Cancer Institute.

Posted: February 2008


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