WASHINGTON (BLOOMBERG) - Biogen received approval for its controversial Alzheimer's disease therapy, a landmark decision that stands to dramatically change treatment for the debilitating brain condition.

After decades of research failures by numerous companies, Biogen's aducanumab became the first drug cleared by the United States Food and Drug Administration (FDA) to slow the course of the mind-wasting disease that afflicts 6 million Americans.

The antibody therapy, which will be sold under the name Aduhelm, works by removing amyloid, a harmful protein that clogs the brains of Alzheimer's patients.

The latest new drug treatment for the disease was first approved in 2003, and existing therapies help with symptoms but do not alleviate the underlying damage.

The FDA announced the decision in a statement on Monday (June 7).

"In determining that the application met the requirements for accelerated approval, the agency concluded that the benefits of Aduhelm for patients with Alzheimer's disease outweighed the risks of the therapy," the agency said.

The approval is one of the more consequential FDA decisions in recent years and likely to be welcome news for millions of Americans with Alzheimer's disease and their families.

Patient advocacy groups have supported the drug's clearance despite scepticism from some scientists about its mixed record in clinical trials.

It is also a significant victory for Cambridge, Massachusetts-based Biogen and its Tokyo-based partner Eisai.

Biogen has seen wild swings in its share price with nearly every twist and turn of aducanumab's development.

Now, it is on the cusp of rolling out a product that is widely expected to be a blockbuster, even amid questions about the healthcare system's capacity to cope with demand.

Biogen shares were halted ahead of the news in New York trading and had not yet resumed.

US-traded shares of Eisai leapt 56 per cent to US$116 at 11.01am in New York.

Scientific controversy

The long-awaited verdict is certain to be scrutinised. Scientists are still furiously debating amyloid's role in the disease, and aducanumab's clinical trial results were decidedly mixed.

In one giant trial conducted by Biogen, the drug was ineffective overall.

A second, similar study found that high doses of the drug slowed progression of the disease by a modest 22 per cent over 18 months.

For months, scientists and doctors have been bickering about which of the results to believe.

Biogen and proponents of the medicine have argued that one trial succeeded because patients received higher doses than in the study that failed.

Sceptics have said it is just as likely that the successful trial was a fluke and argued that a third round is needed as a tiebreaker.

Annual sales could peak at US$5 billion, analysts have said, providing a needed financial jolt to Biogen.

Beyond the drug itself, healthcare system expenses will include the eligibility tests, infusions and expensive scans to watch for side effects such as brain swelling.

Itis unclear whether insurers will readily pay for the drug and all its associated costs.