A sign for the Food And Drug Administration is seen outside of the headquarters on July 20, 2020 in White Oak, Maryland.

A sign for the Food And Drug Administration is seen outside of the headquarters on July 20, 2020 in White Oak, Md.

Biogen Inc. 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 U.S. Food and Drug Administration to slow the course of the mind-wasting disease that afflicts 6 million Americans.

The antibody therapy, which will be sold under the brand name Aduhelm, works by removing amyloid, a harmful protein that clogs the brains of Alzheimer’s patients. The last new drug treatment for the disease was first approved in 2003; existing therapies help with symptoms but don’t alleviate the underlying damage.

The FDA announced the decision in a statement on Monday.

“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 most consequential FDA decisions in recent years, and welcome news for millions of Americans with Alzheimer’s disease and their families. Patient advocacy groups have supported the drug’s clearance despite skepticism from some scientists about its mixed record in clinical trials.

The approval is also a significant victory for Cambridge, Mass.-based Biogen and its Tokyo-based partner Eisai Co. Biogen has seen wild swings in its share price with nearly every twist and turn of aducanumab’s development.

Now, it’s on the cusp of rolling out a product widely expected to be a blockbuster, even amid questions about the health-care system’s capacity to cope with demand.

Biogen shares were halted ahead of the news in New York trading and hadn’t yet resumed as of 11:26 a.m. in New York. U.S.-traded shares of Eisai leapt 42 percent to $105.21.

“This historic moment is the culmination of more than a decade of groundbreaking research in the complex field of Alzheimer’s disease,” said Biogen Chief Executive Officer Michel Vounatsos in an emailed statement.

The long-awaited verdict is certain to be scrutinized. 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 percent over 18 months.

The FDA approved the drug under its accelerated approval program, which means the companies will need to conduct further studies, and the agency could in theory remove it from the market if those studies don’t show a benefit.

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. Skeptics have said it’s 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 $5 billion, analysts have said, providing a needed financial jolt to Biogen. Beyond the drug itself, health-care system expenses will include the eligibility tests, infusions and expensive scans to watch for side effects such as brain swelling. It’s unclear whether insurers will readily pay for the drug and all its associated costs.

Shares of Biogen competitors soared on the FDA approval.

Eli Lilly & Co. jumped as much as 13 percent to a record high. Lilly is working on its own medicine for the brain-wasting disease.

Small-cap companies also working on treatments also climbed. Cassava Sciences Inc. surged 19 percent, while Annovis Bio Inc. gained 31 percent. The Nasdaq Biotech Index traded to a session high led by other Alzheimer’s stock plays like AC Immune and Prothena Corop.

Two years ago, aducanumab appeared destined to take its place on the heap of failed Alzheimer’s disease drugs. In early 2019, Biogen and Tokyo-based Eisai discontinued the two big trials after an initial analysis indicated they were unlikely to work. The drug appeared to be all but dead.

But later that year, the companies stunned outside researchers when they reversed course, and declared that one of the trials had worked after all.

In November 2020, the FDA issued an unusual joint report with Biogen ahead of a meeting of an agency advisory panel, calling the data backing the drug “exceptionally persuasive.” That seemed to put it on a glide path for approval.

Days later, the FDA’s outside panel of medical experts showed itself to be far more skeptical. It voted 8-1, with two undecided, that the single successful trial was insufficient to prove the drug’s efficacy.

In January, Biogen and Eisai said the FDA pushed back its deadline to decide on the drug by three months to June 7. The companies submitted an analysis and clinical data in response to an FDA request for information that would take additional time to review, they said at the time.

In the months leading up to the decision, patients and advocates pushed hard for approval. The Alzheimer’s Association helped organize a session for patients to talk to FDA staff in January. In May, the group followed up with a five-day advertising and social media blitz. Critics of the drug have also been outspoken in recent months, publishing journal articles and opinion pieces highlighting flaws in the drug’s data.

The approval is likely to put new strains on health-care systems. A 2017 Rand Corp. analysis estimated that 88 million Americans 55 and older might be eligible for initial screening for a drug like aducanumab, but only 2.4 million would ultimately be candidates for treatment.

Each step between screening and treatment poses a potential bottleneck. A limited number of dementia specialists are available to evaluate patients. Infusion treatment centers were preparing for a rush of demand before the decision, with some weighing extending their hours or adding capacity.

“One thing we know is there’s probably not enough infusion chairs in America to support the potential onslaught of people,” Charlie Schadewald, CEO of Wasatch Infusion in Utah, said in an interview in May.

Payers including private insurers and Medicare will have to determine how to reimburse for the drug and associated care. Medicare doesn’t cover most scans for amyloid. The agency said last month that it was evaluating coverage and payment rules for aducanumab ahead of the FDA decision.

Doctors worry that there may be pressure to approve therapy for additional Alzheimer’s patients. That decision is complicated by wiggle room in interpreting results of cognitive assessments.

“There are specific criteria, but they’re a little bit fuzzy,” said Erik Musiek, a neurologist at Washington University in St. Louis in a May interview.

He said he’s concerned that physicians might feel pressure from patients whose impairment is either too severe or not sufficient to meet the criteria for aducanumab.