The panel of outside medical experts unanimously recommended that the company undertake a large study, possibly after the basal insulin is approved, to verify heart safety of the once-daily drug.

Panel members said during an all-day meeting that they were concerned about a trend toward higher incidence of cardiovascular events with degludec than other drugs in 16 large clinical trials, even though the difference was not statistically significant.

But they expressed enthusiasm for degludec's 24-hour duration of action, saying it was perhaps unmatched by other drugs and would allow patients to take the insulin at a different time of the day if they missed taking it at their usual time.

"Currently available basal insulins are imperfect and don't last 24 hours," said Dr. David Cooke, a panel member who is an associate professor of pediatrics at Johns Hopkins University School of Medicine.

"A true basal that gives constant coverage for 24 hours would make a difference," Cooke said.

Dr. Kenneth Burman, chief of endocrinology at Washington Hospital Center in Washington, D.C., said the drug's duration of action "seems unique. Instead of a 12-hour half life, it's probably 24 hours."

But Sanford Bernstein analyst Tim Anderson said degludec's sales potential was "meaningfully impaired" by the panel's concern about the drug's cardiovascular risks and the likelihood they will show up in its package insert label.

"An approved label in the United States is highly likely to call out the unknowns about the cadiovascular signal seen," Anderson said in a research note.

The panel began weighing the benefits and risks of the medicine two days after FDA staff members said combined data from the 16 studies suggest degludec may increase the risk of cardiovascular death, non-fatal heart attacks and strokes and unstable angina, compared to standard insulins.

Moreover, FDA staff reviewers had suggested degludec may offer no strong advantage over other drugs in avoiding hypoglycemia -- dangerously low blood sugar levels that are a common side effect of insulin. Some members of the FDA advisory panel echoed those concerns on Thursday.

But the panel voted 8 to 4 to recommend degludec's approval, despite concerns about heart safety, saying its benefits appear to outweigh its risks. The FDA usually follows the recommendations of its advisory panels.

The stakes are high for Novo, the world's largest insulin maker, because Wall Street deems the medicine capable of generating annual sales of $1.5 billion by 2016 if it is approved in the United States.

It would compete with Lantus, Sanofi's dominant long-acting insulin, which had sales last year of about $5 billion. U.S. drugmaker Eli Lilly is developing a similar medicine that is a few years behind in development.

The company's many completed studies of degludec did not enroll enough patients, or last long enough, to ascertain heart risks. A large trial with thousands of patients could reliably assess its safety, but could take a number of years to complete.

The panel did not vote on whether the trial should be conducted before, or after, degludec is approved.

Novo officials on Thursday, speaking at the advisory-panel meeting in Silver Spring, Maryland, said they were committed to working with the FDA on a post-approval cardiovascular outcomes trial.

The negative commentary from FDA staff members on Tuesday sent shares of the Danish drugmaker sharply lower. It plans to sell degludec under the brand name Tresiba.

The European Medicines Agency last month recommended degludec's approval, and it has already been approved in Japan.

(Reporting By Ransdell Pierson; editing by Jim Marshall)

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