Zytiga is one of several new prostate cancer treatments that may significantly prolong the life of patients by zeroing in more closely on certain mechanisms that help tumors proliferate. New data on the medications are being presented at the American Society of Clinical Oncology (ASCO) annual meeting in Chicago.

The trial of 1,088 prostate cancer patients showed those given a placebo and the steroid prednisone went a median of 8.3 months before their disease worsened. Patients treated with the steroid and Zytiga were still faring better at the time the data were analyzed, so a comparable statistic was not yet available.

The study's lead investigator, Dr Charles Ryan of the University of California, San Francisco's Helen Diller Family Comprehensive Cancer Center, estimated that the Zytiga patients would enjoy at least 16 months of progression free survival (PFS).

The trial also found that prednisone-only patients lived for a median of 27.2 months. Overall survival for patients treated with Zytiga had also not yet been reached, but J&J estimated that it improved their survival by 33 percent, or 9 months.

That would clearly surpass the 4.1 month survival benefit demonstrated by Provenge, the cancer vaccine sold by Dendreon Corp that was tested in a similar patient population.

Prostate cancer is the second most common form of cancer in men, with some 30,000 dying from the disease each year in the United States alone and 250,000 globally. Around one-third of patients need no treatment, because their disease does not metastasize, or spread, while another third are treated and cured.

But for the remaining patients, the cancer will recur following treatment or spread to the bones, lymph nodes or other parts of the body. Prostate cancer can turn lethal when it spreads and when it resists standard hormonal therapy.

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Experimental prostate cancer drugs due to be featured at ASCO include Medivation Inc's eznalutamide and alpharadin, which is being developed by Bayer AG and Algeta ASA.

Zytiga is already approved to treat advanced prostate cancer in patients who previously received chemotherapy. J&J expects to file in the second half of this year for U.S. regulatory approval of the drug as a treatment for men with metastatic prostate cancer who have not yet received chemotherapy.

"Clearly, if we're showing greater benefit in the pre-chemotherapy population ... I don't think it's unreasonable that they (clinicians) would move it up," said Michael Meyers, head of the drug's development at J&J.

The study also showed that Zytiga significantly delayed the need for pain drugs taken to reduce side effects and eventual chemotherapy.

In March, the pre-chemotherapy trial was stopped ahead of schedule after it became clear patients were benefiting from Zytiga, a member of a new drug class designed to work inside cancer cells to block the production of testosterone, the male hormone that fuels prostate cancer cell growth.

Zytiga, also known as abiraterone, is currently given for eight months at a cost of around $44,000.

Barclays estimates peak U.S. Zytiga sales of $1.5 billion by 2015, with $700 million from cases where it is used after chemotherapy and $800 million from patients who have yet to try chemo.

Side effects associated with the drug included cardiac disorders, high blood pressure and increased liver enzyme levels.

J&J's Meyers said the safety profile seen with Zytiga in the latest trial was similar to earlier studies, even though patients were exposed to the drug for nearly twice as long.

He said the side effects "for the most part did not disrupt treatment and were easily managed by routine medical interventions."

(Editing by Michele Gershberg and Andre Grenon)

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