"Our hope is that this can become a new option" for up to 30,000 men each year in the United States, said study leader Dr. Charles Ryan of the University of California, San Francisco.
He gave results Saturday at a meeting in Chicago of the American Society of Clinical Oncology.
Prostate cancer is the most common cancer in American men. In the United States alone, more than 240,000 new cases and 28,000 deaths from the disease are expected this year. Many cases don't need treatment because the cancer grows so slowly it isn't a threat. For men who choose treatment when the disease is still confined to the prostate, surgery and radiation are common options.
Once it spreads, though, it often is treated with drugs that curb testosterone. They block about 90 per cent of the hormone, "but that remaining 10 per cent can still stimulate the cancer and cause death from the disease," Ryan said.
Zytiga blocks virtually all of it - "it's really complete chemical castration," said another study leader, Duke University's Dr. Daniel George.
The drug won federal approval last year for men with advanced prostate cancer whose disease had worsened despite treatment with standard hormones and chemotherapy. The new study tested it earlier in the course of illness - in men with advanced prostate cancer who have not yet received chemotherapy.
Researchers gave more than 1,000 patients in the U.S., Canada, Europe and Australia either Zytiga or dummy pills plus a steroid that also is widely used to treat such cases.
When the study was stopped, the median time until cancer worsened in the group getting dummy pills was about eight months. Those on Zytiga were faring much better, so doctors can only estimate the time it is taking for their cancers to worsen - at least 16 months, Ryan said.
There were more cases of high blood pressure, fluid retention and heart problems among men on Zytiga. Like other hormone-blockers, it can cause hot flashes and sexual problems. And Zytiga is expensive - $5,500 a month in the United States.
The study was sponsored by the drug's maker, Johnson & Johnson's Janssen division, and some study leaders have financial ties to the company. The company plans to seek approval later this year to sell the drug for men like those in the new study.
Michael Wells, 65, a retired construction worker from Oakville in California's Napa Valley, was diagnosed nearly 12 years ago with prostate cancer that had spread to his bones. Standard hormone and radiation treatments kept it in check for many years, but when it started growing, he enrolled in the study.
Zytiga gave him high blood pressure but other than that, "I have not noticed any difference taking the pills - I've always had hot flashes" on prostate medicines, he said.
He has now been on Zytiga for nearly three years, and scans show his cancer is in check.
"It's worked for me, and it's still working," Wells said.
Marilynn Marchione can be followed at http://twitter.com/MMarchioneAP