A panel of outside experts to the U.S. Food and Drug Administration on Tuesday voted 11-0 against approving the injectable drug, called Arcalyst, because its clinical trials were too short to prove the treatment was safe, or that it would help patients over time.

Gout is a painful condition that occurs when the bodily waste product, uric acid, is deposited in the joints and soft tissues, causing inflammation and sometimes making it difficult to walk. It affects about 5 million to 6 million Americans.

"A lot of the patients where we envision using this drug are going to take it for longer than 16 weeks. It's hard for me to allow patients to be exposed (without longer safety data)," said panel member Dr. David Blumenthal, an assistant professor at Case Western Reserve University School of Medicine.

The FDA usually follows panel recommendations, although it is not required to, and will make a final decision on the drug by July 30.

Regeneron's drug, known generically as rilonacept, is already approved to treat a group of rare genetic auto-inflammatory diseases. But the company is hoping to expand its use to prevent gout flares in people starting uric acid-lowering therapy.

To test its drug, Regeneron gave it to patients for 16 weeks, the length of time it said most people experience gout flare-ups. But panel members said in the most severe cases, people would need to take the drug for longer periods.

Shares of Regeneron, the maker of eye drug Eylea, closed up 0.5 percent on Nasdaq at $130.93. If approved, most analysts said the drug would contribute about $2 to $3 to the small biotech company's share price.

(Reporting by Anna Yukhananov; editing by Matthew Lewis)

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