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Feb. 1, 2008 -- A better prostate cancer test -- and a vaccine to keep early prostate tumors at bay -- may be just over the horizon, new studies suggest.
The current PSA screening test for prostate cancer isn't as sensitive or specific as anyone would like. The test sends many men to doctors for needless biopsies. It fails to identify many men who have dangerous cancers. And even when it detects cancer, it can't tell whether the cancer is aggressive.
Experimental cancer vaccines haven't worked well at all. They've been explored as last-ditch treatments when standard treatments fail, and they've been used to mop up cancer cells left behind after surgery. Results of such studies have been disappointing.
Now researchers at the University of Michigan report incremental progress in an improved urine test for prostate cancer. And exciting mouse studies at the University of Southern California suggest that when tests identify men with early prostate cancer, vaccination can keep the cancer from ever becoming deadly.
Both studies appear in the Feb. 1 issue of the journal Cancer Research.
"This represents a paradigm shift in how we treat prostate cancer," W. Martin Kast, PhD, USC professor of microbiology and immunology, tells WebMD.
Kast hasn't treated any humans. But his team has done something nobody else has ever done. Their new vaccine keeps a strain of mice alive indefinitely, even though they are genetically programmed to die of prostate cancer.
The state-of-the-art vaccine is a two-step process in which mice are inoculated against a protein that becomes more and more abundant in prostate cancers as they morph fromprecancerous "prostate intraepithelial neoplasia" (PIN) into aggressive cancers.
So-called TRAMP mice develop prostate cancer as soon as they reach puberty. Kast and colleagues vaccinated the animals just as they developed the early PIN. While all unvaccinated mice died, 18 of 20 vaccinated mice lived to be 1 year old -- the oldest TRAMP mice ever seen.
A close look at the surviving animal's tumors showed that they did not go away. The animal's genetic programming kept trying to turn the early cancers into deadly tumors. But immune cells swarmed around the cancers, keeping them at bay.
"The equivalent in a man is very clear. In the U.S., for instance, some 90,000 to 100,000 men every year are diagnosed with high-grade PIN," Kast says. "That is the exact equivalent of the disease stage in mice. So there is a group of men that could benefit very much from this approach."
A big worry is that the vaccine's target is a protein found on normal cells in the esophagus, bladder, and stomach. But at least in mice, the vaccine did not harm these organs.
"I think the explanation is that the expression levels of this protein, PSCA, is so low in normal tissue that the immune system makes a differentiation between prostate cancer cells that make this antigen and normal cells," Kast says.