Researchers from the Royal Marsden NHS Foundation Trust and the Institute of Cancer Research in the United Kingdom recruited 21 men with late-stage prostate cancer who were no longer responding to hormone therapy and prescribed a once-a-day regimen of the pill abiraterone.

After taking the drug, 70% to 80% of the men showed lower prostate-specific antigen (PSA) levels and tumor shrinkage. Disease progression was delayed by a median of 400 days. Side effects, which included high blood pressure, weight gain and fatigue, were minimal.

While these are impressive results for an early clinical trial, abiraterone is not yet approved for use by the US Food and Drug Administration, so it isn't available outside of clinical trials. And experts caution that the drug's approval isn't a foregone conclusion.

"All of us in cancer care have seen reports of promising results showing a small number of patients surviving a long time, and once the drug has entered into a randomized trial against a placebo or the current standard of care, the results show no real difference," said Otis Brawley, MD, Chief Medical Officer of the American Cancer Society.

And although the PSA test is often an indicator of tumor activity, it doesn't give the whole picture. Prostate-specific antigen is a protein produced by prostate cells. When the prostate enlarges or when there's a tumor in the gland, PSA levels often increase.

"A PSA decline or tumor shrinkage are only evidence of activity, and activity only means there's a need for a good randomized trial. The bottom line for patients and the public is we need a randomized trial showing longer survival time or improved quality of life as the endpoint," said Brawley.

The drug, which is manufactured by Cougar Biotechnology, Inc, works by blocking the enzyme CYP17, which plays a role in hormone production and tumor growth. Researchers say they will soon be reporting results of a phase II study and are planning phase III studies. They hope the drug can be on the market in 2011.

More than 186,320 new cases of prostate cancer will be diagnosed, and 28,660 men in the United States will die of prostate cancer this year, according to Cancer Facts and Figures 2007-2008. For more information on prostate cancer and its treatment, see Detailed Guide: Prostate Cancer.