SUNDAY, May 18 (HealthDay News) -- A therapeutic vaccine to treat prostate cancer appears safe and may be effective, according to the results of an early trial.

The vaccine could give hope to men with metastatic prostate cancer by activating their immune systems to fight the disease. The vaccine was developed to enable a patient's immune system to produce anti-antigens and attack cancer cells, which can improve quality of life and extend survival.

"The primary objective of the study was to determine whether or not the vaccine was safe or whether it induced any serious adverse events," said lead researcher Dr. David Lubaroff, director of urology research at the University of Iowa. "The vaccine was quite safe."

In addition, the researchers wanted to see if the vaccine produced an immune response to prostate specific antigen (PSA).

"We found that 68 to 70 percent of the patients in the trial demonstrated immune responses to PSA," Lubaroff said. "This was their last resort, and we were encouraged by the fact that we could detect any immune response in these patients."

Results of the phase 1 trial were expected to be presented Sunday at the American Urological Association annual meeting, in Orlando, Fla.

In the trial, Lubaroff's team tested the adenovirus/PSA vaccine in 32 men with metastatic prostate cancer. The men were treated with one of three different doses of the vaccine and followed for 12 months.

In addition to developing immune responses to PSA, 57 percent of the patients survived longer than predicted. Forty-eight percent actually doubled their expected life span. The longest survival was almost six years, the researchers reported.

Based on these results, Lubaroff's group has started a phase II trial, which will determine whether the immune response and survival seen in this trial is really therapeutically meaningful in a larger number of patients.

"If this vaccine proves to induce a strong anti-PSA immune response, and if there is a correlation between this PSA response and an effect on the disease, then we could use this vaccine as another therapy," Lubaroff said.

One expert is skeptical that this vaccine will ever prove to be a viable treatment.

"I think about all you can prove from this kind of study is safety," said Dr. Bruce Roth, a professor of medicine and urology at Vanderbilt University. "But that's a world of difference from saying that there is evidence of efficacy."

Roth doesn't think changes in PSA in this kind of trial are enough to prove the vaccine works.

"I would not take away from this that this is the breakthrough we have been waiting for, for 35 years," Roth said. "It's an OK theory, but we have been disappointed a lot in the past couple of decades with immune therapies that look great but never produce the results we had hoped for."

Another study expected to be presented Sunday showed that a PSA reading taken between the ages of 45 and 50 actually helps predict prostate cancer up to 30 years later. These findings suggest that prostate cancer may start to develop very early and that PSA levels affect the development of prostate cancer.

A third study found that black men who have a family history of prostate cancer could benefit from a PSA reading, which could determine their probability of developing the disease.

Black men who have known risks for prostate cancer and higher levels of PSA are more likely to develop the disease, compared with people in the general population.

However, black men with a family history of prostate cancer are unlikely to develop the disease if their baseline PSA was below what is normal for their age. The effect of family history and PSA level actually overrode other prostate cancer risk factors, the researchers said.

More information

For more on prostate cancer, visit the American Cancer Society.