By Alina Selyukh

SILVER SPRING, Maryland (Reuters) - U.S. health advisers said they were unconvinced that Teva Pharmaceutical Industries Ltd's Parkinson's drug Azilect slows the progression of the incurable disease.

Therapy, but it is the first time a drugmaker has sought approval for slowing the degenerative disease of the nervous system, instead of merely masking its symptoms.The panel of experts convened by the Food and Drug Administration were unanimous in their skepticism about the result of Teva's latest trial.

"This is really going to be the flagship... and we have to be very solid in this and set a very high standard," said Dr. Robert Clancy, one of the panel members and a neurologist at the Children's Hospital of Philadelphia. "And this is close, but it's not good enough."

The advisory panel's concerns were similar to those of FDA staff reviewers, that were published last week.

Azilect, generically known as rasagiline, is marketed by Israel-based Teva and, in several countries, with Danish partner Lundbeck.

It already has FDA approval to treat symptoms of Parkinson's Disease, such as trembling limbs, stiffness, slow movement and impaired balance. Teva is seeking an expansion of Azilect's label to include the slowed progression claim.

The FDA will make a final decision on the label but usually follows the advice of its advisory committees.

In Teva's latest trial, Parkinson's appeared to deteriorate more slowly in patients who started taking Azilect earlier than in those who began later. But while the 1 milligram (mg) dose appeared to slow the progression, the 2 mg dose did not, overshadowing the results of the 1 mg trial under review.

Much remains unknown about Parkinson's, including its causes, which has many researchers struggling to find a cure or develop ways to measure the development of the disease.

That complicated evaluation of Teva's trial of Azilect as it is hard to tell whether the drug was hiding the symptoms or affecting the disease itself.

"If you tell people in labeling that a drug has effect on disease progression, you want to be sure that it does indeed have an effect on disease progression," Dr.Russell Katz, head of the FDA's neurology products division, told the advisers.

Anywhere from 500,000 to 1.5 million Americans are estimated to have the disease, and nearly 60,000 are diagnosed each year, according to the Parkinson's Action Network.

That network, joined by five other major groups representing the Parkinson's patient community, had submitted a written statement to the FDA advisory panel ahead of the vote.

Acknowledging various concerns raised by researchers about the conclusiveness of Teva's trial, the groups said they were encouraged by the evidence but realized the data was not yet definitive and additional information was required.

"Azilect is clearly the furthest ahead, but this is an area of incredible research and need," Michael J. Fox Foundation Chief Executive Todd Sherer told Reuters ahead of the advisers' vote.

Azilect received FDA approval in 2006 for use as a single drug therapy in early Parkinson's and, in more advanced patients, in addition to levodopa, a standard treatment for the disease that alleviates the symptoms.

Teva's drug works by blocking the breakdown of dopamine, a neurotransmitter chemical.

The company declined to comment on the advisory panel's vote, which came after the regular close of the U.S. stock market. Teva's shares were little changed in afterhours trade from their $39.29 close.

(Editing by Steve Orlofsky, Gary Hill and Tim Dobbyn)