Washington, United States --- The United States on Monday approved a drug called Aduhelm to treat patients with Alzheimer's, the first new medicine against the disease in almost two decades and the first to address cognitive decline linked to the condition.

The decision was highly anticipated but also contentious because an independent expert panel convened by the regulatory Food and Drug Administration last November found insufficient evidence of Aduhelm's benefit and recommended against approval.

"Aduhelm is the first treatment directed at the underlying pathophysiology of Alzheimer's disease, the presence of amyloid beta plaques in the brain," said the FDA's Patrizia Cavazzoni.

Nevertheless, the decision falls under the FDA's "Accelerated Approval" pathway which it uses when it believes a medicine may provide meaningful benefit over existing treatments but there is still some uncertainty.

"As is often the case when it comes to interpreting scientific data, the expert community has offered differing perspectives," said Cavazzoni in a statement acknowledging the controversy.

Aduhelm, a monoclonal antibody also known by its generic name aducanumab that is administered intravenously, was tested in two late-stage human trials known as Phase 3 trials.

It showed a reduction in cognitive decline in one, but not the other.

But in all studies, it convincingly showed a reduction in 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 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 buildup.

"We feel a great sense of purpose and responsibility to turn the hope of today's FDA approval of Aduhelm into a reality for people living with Alzheimer's disease and their families," said Alisha Alaimo, president of Biogen US in a statement.

She added the company was committed to "health equity" and helping lower-income patients get access to the treatment, delivered once a month.

The yearly cost of the treatment would be $56,000, but the cost to US patients would depend on their type of insurance.

Experts mixed

"On behalf of those impacted by Alzheimer's & all other dementia, we celebrate today's historic decision," tweeted the Alzheimer's Association, a US nonprofit, in response to the news.

Expert reaction was more mixed, with scientists hoping the approval might serve as a catalyst for better drugs to be developed in the future.

"While I am pleased that aducanumab has received approval, we have to be clear that, at best, this is a drug with marginal benefit which will help only very carefully selected patients," said John Hardy, a professor of neuroscience at University College London.

"We will need better amyloid drugs down the line," he added.

The last Alzheimer's drug was approved in 2003, and all previous medicines have targeted symptoms associated with the disease, not its underlying cause.

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

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

It is also associated with dramatic mood swings and trouble communicating.