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By Deena Beasley

CHICAGO, June 3 (Reuters) - Adding Johnson & Johnson's Zytiga to standard hormone therapy reduced by nearly 40 percent the chance of death for men newly diagnosed with high-risk prostate cancer that had spread to other parts of the body, according to new trial results.

The data, which supports expanding the drug's use to earlier stages of prostate cancer, is "likely to change clinical practice overnight," said Dr Richard Shilsky, chief medical officer at the American Society of Clinical Oncology, which featured the studies on Saturday at its annual meeting in Chicago.

J&J is already seeking European Union approval in the earlier treatment setting, but is still considering whether to pursue U.S. approval.

Zytiga, or abiraterone acetate, is a pill that decreases production of testosterone, the hormone that stimulates growth of prostate tumors. It is approved for men whose cancer has worsened after treatment with docetaxel chemotherapy and for use before chemo for disease resistant to traditional hormone drugs.

At a U.S. list price of $9,400 a month, Zytiga is expensive. J&J's domestic Zytiga sales totaled just over $1 billion last year, and global sales were $2.26 billion.

"The difference here is ... patients who have not been on hormonal therapy," said Craig Tendler, head of global medical affairs at J&J's Janssen division. "This is much less common in the U.S. where we have very aggressive screening guidelines." Those guidelines, however, are changing and more men are declining to undergo testing.

In late 2011, the U.S. Preventive Services Task Force, a government-backed panel of independent physicians, recommended against routine prostate cancer testing, citing concerns that it often caught tumors that did not need treatment and led to unnecessary procedures with side effects such as impotence and incontinence.

The latest studies, following 2015 trials showing an advantage with chemotherapy, offer the first new options for metastatic prostate cancer patients in nearly 70 years, said Dr Karim Fizazi, head of the department of cancer medicine at France's Gustave Roussy Cancer Center, who led one study.

He estimated that just 3 to 5 percent of men in developed countries have metastatic cancer by the time the disease is diagnosed, but the number rises to 60 percent in countries like China and India where screening is less common.

The Gustave Roussy trial showed that at a median of 30 months after beginning treatment, men taking Zytiga and the steroid prednisone plus standard hormone therapy had a 38 percent lower risk of death compared with men given hormone therapy and a placebo.

Zytiga was also associated with a 53 percent lower risk of the cancer worsening.

Severe side effects from the drug included high blood pressure in 20 percent of patients.

Fizazi said a trial to see if Zytiga has a benefit when added to hormone therapy and chemotherapy is still under way.

A second study conducted by the University of Birmingham in the United Kingdom found that adding Zytiga to a standard initial regimen of hormone therapy for high-risk, advanced prostate cancer lowered the relative risk of death by 37 percent.

"We think these will be game-changers," said Dr Daniel Hayes, ASCO President from the University of Michigan Comprehensive Cancer Center, who was not involved in the studies. (Editing by Matthew Lewis)