UPDATE 2-U.S. FDA panel urges approval of Savient gout drug

* FDA advisers back Savient drug in 14-1 vote

* Panel: drug clearly works, safety concerns remain

* Best-suited patients, long term use unclear -panel (Adds panel, company comments, background, byline)

By Susan Heavey

SILVER SPRING, Md., June 16 (Reuters) - Savient Pharmaceuticals Inc's (SVNT.O) experimental gout drug is safe and effective for certain patients with the debilitating joint disorder and should be approved, a U.S. advisory panel recommended on Tuesday.

Savient shares soared 27 percent to $11.80 in extended trading after the panel's recommendation.

The company is seeking Food and Drug Administration approval for the infused drug, called Krystexxa, for those who have the painful type of arthritis, but do not improve with other treatments or cannot take the alternatives for various reasons.

FDA's panel of outside experts, in a 14-1 vote, said the dramatic results in nearly half of the patients studied were encouraging, despite risks that could include serious heart problems, including sudden death, and allergic reactions.

"There's a definite unmet need for this drug," said panelist Dr. Michael Weisman of Cedars-Sinai Medical Center in Los Angeles.

The agency will weigh the recommendation before making its final decision. Savient President Paul Hamelin said a ruling was expected by August 1.

Krystexxa, known generically as pegloticase, could be a key product for the specialty biotech firm, which has just one other product on the U.S. market.

Investors have been eyeing the drug's progress, and shares of Savient soared more than 50 percent on Friday when related agency documents were released ahead of the panel meeting. Its shares were halted ahead of the committee's vote after closing at $9.27 on Nasdaq on Monday.

Still, the potential market for the drug is small.

Roughly 5 million Americans have gout, in which uric acid build-up can cause swollen joints. Of those, about 40,000 to 60,000 see no improvement with other therapies, Savient said.

Doctors can prescribe a variety of medications for gout, including corticosteroids painkillers known as non-steroidal anti-inflammatory drugs. A number of other drugs are also available, including allopurinol, probenecid and Takeda Pharmaceutical Co Ltd's (4502.T) recently approved Uloric. Allopurinol and probenecid are made by a variety of companies.

Panelist Dr. Lenore Buckley, a professor at Virginia Commonwealth University's School of Medicine, said the number of patients who would need a new treatment may even smaller than Savient's estimates given Uloric's February approval.