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) -- U.S. regulators approved

Arena Pharmaceuticals'

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weight-loss drug Belviq on Wednesday -- the first new prescription diet pill cleared for sale in the U.S. in 13 years.

Arena shares are up 26% to $11.13 in afternoon trading.

In approving Belviq (formerly known as lorcaserin), the U.S. Food and Drug Administration followed the recommendation of an outside panel of experts who voted 18-4 with one abstention last month to support the weight-loss drug while still voicing concerns about lackluster efficacy and lingering safety questions.

Approximately one-third of adult Americans are obese today, according to the Centers for Disease Control. Obesity is growing public health problem in the U.S., in part because people who carry unhealthy weight are at greater risk for heart disease, diabetes and other diseases.

"Obesity threatens the overall well being of patients and is a major public health concern," said Janet Woodcock, director of the FDA's Center for Drug Evaluation and Research. "The approval of this drug, used responsibly in combination with a healthy diet and lifestyle, provides a treatment option for Americans who are obese or are overweight and have at least one weight-related comorbid condition."

Belviq's FDA-approved label states that patients who don't lose 5% of their body weight in the first 12 weeks of treatment should stop taking the drug.

On a conference call, Arena said 42% and 32% of patients achieved 5% weight loss at 12 weeks in the company's phase III trials of Belviq, depending on whether they also had Type 2 diabetes or not.

"If patients aren't losing weight by 12 weeks, there is no reason to continue," said Arena CEO Jack Lief, referring to Belviq therapy.

Arena and its marketing partner


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plan to launch Belviq commercially once the drug receives DEA scheduling as a controlled substance, which could take 4-6 months. Previously, Eisai had said Belviq would likely hit the U.S. market in the first quarter 2013.

Belviq pricing was not disclosed.

Belviq's arrival comes after other prescription weight-loss drugs, including


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Meridia and


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Accomplia, were removed from U.S. and European pharmacy shelves because of unacceptable safety problems.

Arena won FDA approval for Belviq on its second attempt by assuaging the FDA's safety concerns. Regulators rejected Belviq in 2010 because of unsettled concerns about the drug causing tumors in rats and the risk that prolonged use could lead to heart valve damage.

Belviq's safety issues were magnified because the drug's benefits are modest. In clinical trials, patients only lost about 3% of their body weight after one year when adjusted for placebo. In another measure, 47% of patients using Lorqess lost at least 5% of their body weight after one year compared to 23% of patients treated with a placebo. The latter measure of efficacy only met the FDA's approval standard for weight-loss drugs.

Eisai will market Belviq under an existing partnership with Arena, which will manufacture the drug. Arena will be required to conduct six postmarketing studies, including a long-term cardiovascular outcomes trial to assess the effect of Belviq on the risk for major adverse cardiac events such as heart attack and stroke.

FDA is expected to make its next approval decision for a weight-loss drug on July 17 when it weighs in on


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Orexigen Therapeutics


will seek approval for its weight-loss drug Contrave pending positive results from an ongoing heart-safety study.

--Written by Adam Feuerstein in Boston.

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