A US government-convened expert committee on Friday voted against recommending the approval of the first new Alzheimer's drug in almost two decades after finding insufficient evidence of its benefit. 

The decisions reached by such panels, while nonbinding, are often accepted by the Food and Drug Administration. 

Maker Biogen was hopeful its drug aducanumab would be the first ever approved to treat the cognitive decline associated with Alzheimer's -- a major medical milestone.

It would also have been the first drug approved against the disease since 2003.

But an FDA advisory committee concluded that the data in its favor fell well short of the mark.

"I think Alzheimer's treatment is a huge, urgent, unmet need -- I also think that if we approve something where the data is not strong, we have a risk of delaying good treatments and effective treatment for many years," said Joel Perlmutter, a professor of neurology at Washington University in St. Louis, one of the experts who participated.

Ahead of the meeting, the FDA released a set of documents that signaled a positive tone and raised speculation the panel might vote in favor of the drug.

There have been two major late-stage clinical trials to test aducanumab, with one demonstrating benefits from taking the intravenous medicine -- and the other failing to produce the same level of evidence.

FDA staff wrote that they found the results of one of the trials "highly persuasive" and believed the other also showed benefit once a subset of very sick patients were removed from the analysis.

That view was not shared by the expert panel's 11 members, who after discussing and reviewing the data for several hours voted 10 against, none for, and one "uncertain" on the question of whether, considering the totality of evidence, the drug provided enough evidence it works.

The result is a blow to the millions of people around the world who suffer from the neurodegenerative disease and their loved ones, but may not be the end of the line for aducanumab.

Biogen could conduct more studies in order to better make its case. 

- Personal accounts -

The panel also heard moving accounts from the carers of several patients who said the drug was highly effective and had an overwhelmingly positive impact on their loved ones -- but these types of testimonies are considered anecdotal only and of little scientific value. 

Aducanumab is a monoclonal antibody that targets the build-up of a protein called beta-amyloid in the brain tissue of Alzheimer's patients.

One theory holds that Alzheimer's disease comes about from an excessive accumulation of these proteins in some people's brains as they age and their immune systems decline.

Providing such patients antibodies could therefore be a means to restore some of their capacity to clear the plaque build-up.

While the experts at Friday's meeting agreed that aducanumab was effective at clearing the protein, as seen in brain imaging, they said the cognitive benefits were less clear and it's therefore possible that future drugs might need to target different molecules.

Alzheimer's, the most common form of dementia, is thought to affect 50 million people worldwide and usually starts over the age of 65.

It progressively destroys brain tissue, robbing people of their memory, leaving them disoriented and at times unable to carry out everyday tasks.

It's also associated with dramatic mood swings and trouble communicating.