Feb. 27, 2009 (Orlando, Fla.) -- An experimental pill that blocks the production of male hormones that fuel prostate cancer may help slow the growth of hard-to-treat tumors.
The novel agent, called abiraterone, shrank tumors by 30% or more in one-fourth of 31 patients whose prostate cancer continued to spread despite standard hormone therapy. In an additional 35% of men, tumors stopped growing.
"A few patients are still alive, without any signs of progression, more than a year after therapy began," says researcher Charles J. Ryan, MD, a cancer specialist at the University of California, San Francisco.
Without the new drug, they would have only been expected to live three or four months, he tells WebMD.
The researchers also used PSA levels to evaluate abiraterone's effectiveness. PSA levels are a measure of a protein called prostate-specific antigen, which is produced by cells in the prostate. High PSA levels can signal cancer. The National Cancer Institute views a response to treatment as being seen when there is at least a 50% decline in PSA blood level.
After 12 weeks of treatment, abiraterone reduced PSA levels by 50% or more in 71% of the men. In two men, PSA fell to undetectable levels.
Ryan presented the results here at the 2009 Genitourinary Cancers Symposium. The meeting is co-sponsored by the American Society of Clinical Oncology (ASCO) and two other cancer care organizations.
How Abiraterone Works
"This is currently the most promising prostate cancer drug on the horizon," says ASCO spokesman Howard Sandler, MD, chairman of radiation oncology at the Samuel Oschin Cancer Institute at Cedars-Sinai Medical Center in Los Angeles.
"Here's a novel therapy hormone that seems to be active in men thought to be unresponsive to hormone therapy," he tells WebMD.
That may be because abiraterone works differently than other hormone treatments.
Prostate cancer grows when exposed to the male hormone testosterone and its related hormones, called androgens. Hormone treatment is given to halt the production of testosterone and androgens.
Current treatments -- surgical removal of the testes or medication -- prevent the production of male hormones in the testes. But these approaches do not prevent other parts of the body from making male hormones. Abiraterone targets an enzyme called CYP17 that is necessary for the production of male hormones throughout the body.
The new study involved men who had been surgically or medically treated to prevent testosterone production in the testes. None had been treated with chemotherapy, which is sometimes given when cancer continues to grow and spread despite treatment with hormone therapy.
The men took abiraterone orally once a day. The drug was generally well tolerated, with no patient stopping treatment due to side effects.
Ryan notes that another study that did include men who were given chemotherapy after hormone treatment stopped working had similar results.
The new study was funded by Cougar Biosciences, which makes abiraterone.
Researchers are now enrolling men in a larger, longer study in which they will be randomly assigned to abiraterone or a placebo. If the promising results hold up, the company will apply for FDA approval of the drug. But that's at least a few years away, doctors say.
Prostate cancer is the second most common cause of cancer death among men in the U.S. It was diagnosed in more than 180,000 men and claimed the lives of more than 28,000 men in the U.S. last year.