* Zytiga study of patients who had not received chemotherapy
* Safety monitors said placebo patients should get J&J drug
* Data likely to significantly expand Zytiga sales
* J&J shares up 1 pct; rival drugmaker Dendreon down 8.5 pct (Adds analyst comment, additional approval plan, background, updates shares)
By Bill Berkrot
March 8 (Reuters) - Johnson & Johnson's drug for advanced prostate cancer, Zytiga, was deemed so beneficial in patients who had not received chemotherapy that safety advisors said those in the placebo group of a late stage trial should be offered the treatment, the company said
The unanimous recommendation by independent safety monitors was based on improvements in overall survival data, progression free survival -- or how long it takes the disease to worsen -- and other secondary goals of the 1,088-patient trial, as well as a favorable safety profile, the company said.
The actual data will be presented at a major medical meeting later this year, most likely the American Society of Clinical Oncology (ASCO) meeting in June, J&J said on Thursday.
"Since the trial was stopped earlier than expectations, we are operating under the assumption that the data were truly spectacular," said ISI Group analyst Mark Schoenebaum.
In its interim analysis, the committee found a statistically significant improvement in progression free survival and a trend toward an overall survival benefit that had not yet reached statistical significance, J&J said.
Virtually all blinded clinical trials use an independent monitoring committee to view results at determined intervals to make sure no serious safety problems arise.
These committees can interrupt a trial due to safety concerns, or if it becomes clear that a study drug is likely to fail. In best case scenarios, such as this one, the committee can determine that an experimental drug is so effective that it would be unethical not to offer it to patients in the trial who were not getting the medicine.
Zytiga is already approved to treat advanced prostate cancer in patients who previously received chemotherapy. Late stage trials in that patient population were also stopped early due to the clear benefit of the drug known chemically as abiraterone.
The positive news from the Zytiga trial sent J&J shares up 1 percent and also boosted Medivation Inc, which is developing a prostate cancer pill that works in a similar fashion to Zytiga, by 16 percent. The drugs are expected to eventually become direct rivals.
The success of Zytiga is likely to hurt the prospects for Dendreon Corp's Provenge prostate cancer vaccine, which is already approved to treat pre-chemo patients. Dendreon shares fell 7 percent.
DOUBLE THE SALES POTENTIAL
The latest results should significantly expand sales of Zytiga as there are far more men with advanced prostate cancer who have not been treated with chemotherapy.
"An approval in the pre-chemo population should more than double the market opportunity for Zytiga," said Sanford Bernstein analyst Derrick Sung in a research note.
"We would expect physicians to increase their use of Zytiga off-label in this population once the data is released even before official FDA approval is achieved," said Sung, who estimates Zytiga annual sales reaching $1.3 billion as early as 2013.
J&J expects to seek the additional approval in the United States and elsewhere in the second half of this year.
There are an estimated 218,000 new cases of advanced prostate cancer in the United States each year, with the worldwide market estimated to be about 899,000, J&J said. An estimated 33,000 American men die from the disease each year.
Once a disease with few pharmaceutical options, prostate cancer has been an increasingly hot area of drug company research, making for what should become a very competitive field over the next few years.
The Zytiga data suggests that the Medivation drug "is likely to succeed and show both the progression free survival and overall survival benefit," Sanford Bernstein analyst Geoffrey Porges said in a research note.
Britain's BTG Plc will also benefit from expanded use of Zytiga as it is entitled to royalty payments on sales of the drug. BTG, originally licensed the drug to Cougar Biotechnology, which was acquired by J&J.
Zytiga, a pill given once daily, works by inhibiting production of hormones called androgens that can fuel tumor growth in prostate cancer patients.
Provenge, which works by boosting a patient's immune system to fight the cancer, is far more complicated to manufacture and administer, involving taking cells from a patient, sending them off for processing and then being infused back into that specific patient. (Reporting By Bill Berkrot in New York, Debra Sherman in Chicago and Kavyanjali Kaushik and Esha Dey in Bangalore; Editing by Michele Gershberg)