* Company says rat tumors not relevant to people
* FDA staff disagree with company on tumors
* Panel vote expected Thursday afternoon
* Pill is vital to company with no approved drugs
* Analysts expect peak sales of $822 million if approved (Rewrites with FDA staff comments)
By Lisa Richwine
ADELPHI, Md., Sept 16 (Reuters) - U.S. drug reviewers criticized Arena Pharmaceuticals Inc's <ARNA.O> defense of a proposed weight-loss pill on Thursday, as an advisory panel scrutinized data on rat tumors and other safety concerns.
Arena, a small company with no approved products, said various tumors seen in rat studies would not appear in people who took the recommended doses of the drug, lorcaserin. Government scientists, however, disagreed with Arena on mammary gland and other tumors.
"The relevance of lorcaserin-related mammary tumors in rodents to the obese patient population cannot be dismissed," said Fred Alavi, a Food and Drug Administration reviewer.
Serious side effects have plagued other diet pills and industry analysts said assuring the advisory panel of lorcaserin's safety was Arena's biggest challenge as it tries to win approval for its most advanced drug.
Investors have fled the stock since FDA reviewers highlighted the cancer issue in an initial analysis released on Tuesday. The company's shares fell 45 percent from Monday's close. Trading was halted while the FDA panel met on Thursday.
Arena directly challenged concerns about the rat tumors at the start of its presentation to the panel. The company said the findings did not apply to people because the tumors resulted from high doses or biological mechanisms specific only to rodents. No increase in cancer cases was seen in people.
"Lorcaserin does not pose a cancer risk to humans at the recommended therapeutic doses," Dr. Gary Williams, a public health professor at New York Medical College who spoke for Arena, told the panel.
He also pointed to nine medicines that were approved with evidence of the same types of rat tumors seen with lorcaserin.
The FDA advisers, a group of outside experts, is set to vote Thursday afternoon on whether to recommend approval for lorcaserin. The agency usually follows panel recommendations when making final approval decisions.
Consensus forecast data from Thomson Reuters shows analysts expect Arena's drug could reach peak annual sales of $822 million if it reaches the market. Japan's Eisai Co <4523.T> holds U.S. marketing rights to lorcaserin.
RIVALS RACE TO MARKET
Drugmakers have failed for decades to produce a pill to help people shed a significant number of pounds without serious side effects. Risks of current options were highlighted on Wednesday when U.S. advisers agreed tougher action was needed on Abbott Laboratories Inc's <ABT.N> diet drug, Meridia, but split on whether the drug should be banned. [ID:nN15139776]
Two other companies, Vivus Inc <VVUS.O> and Orexigen Therapeutics Inc <OREX.O>, have weight-loss pills under FDA review this year. All three companies are trying to launch the first new prescription diet drug in a decade.
Prescription weight-loss drugs have failed to gain much traction in the United States, despite the potential for strong sales in a nation where two-thirds of the population are overweight or obese. The current U.S. diet drug market is small at about $381.5 million in 2009, according to data from IMS Health.
(For a graphic illustrating the extent of the obesity problem in the U.S. click on http://link.reuters.com/vup37m)
Arena's lorcaserin was designed to block appetite signals in the brain in a similar way to the now-withdrawn fenfluramine in the fen-phen diet-drug cocktail. The Arena drug is more selective in the receptors it affects and the company says studies have not found any of the heart-valve problems with lorcaserin that were linked to fenfluramine.
FDA reviewers said one of the doses Arena tested just met agency criteria for effectiveness. In company studies, patients who took 10 milligrams of lorcaserin twice daily for a year lost about 5.8 percent of their body weight on average. Placebo patients lost 2.5 percent.
The FDA is set to rule on lorcaserin by Oct. 22. The drug's proposed brand name is Lorqess.
Arena is developing other drugs for treatment of blood clots, sleep disorders and pulmonary arterial hypertension, but all are in early testing. (Reporting by Lisa Richwine; editing by Tim Dobbyn, Dave Zimmerman and Andre Grenon)
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