The pharmaceutical industry is eagerly awaiting the US regulator’s decision on a potential blockbuster Alzheimer’s drug. The drug is controversial but, if approved, would be the first new treatment for the debilitating disease in years.

The US Food and Drug Administration is expected to announce on Monday whether or not it approves Biogen’s drug aducanumab, which claims to slow the progression of Alzheimer’s disease.

If aducanumab is approved, it would be one of the most significant decisions the FDA has made in years. For many Alzheimer’s sufferers and their families it would be hailed as revolutionary in a field where few treatments exist and no new drugs have been approved since 2003.

But the drug, which Biogen made alongside Japanese company Eisai, has been mired in controversy, with many arguing that the treatment simply does not work.

In January, the FDA postponed its decision after months of delays. Pharma investors, as well as the 6m people living with Alzheimer’s in the US, are nervously waiting for the regulator’s conclusion.

“Without question this is the biggest binary event in biopharma in 2021,” said Colin Bristow, an analyst at UBS, referring to the drug’s approval or rejection.

Aducanumab is a monoclonal antibody treatment that is administered intravenously to patients with moderate Alzheimer’s disease.

Biogen says that the drug binds to beta amyloid molecules that create plaques in the brain and breaks up the clumps, thereby reducing the progression of Alzheimer’s so people can continue to perform daily tasks such as cleaning and shopping.

The drug tests the “amyloid hypothesis”, proponents of which believe that plaque build-up in the brain causes Alzheimer’s.

Howard Fillit, founding executive director of the Alzheimer’s Drug Discovery Foundation, called it “the first rigorous test of the amyloid hypothesis”.

Many scientists dispute that aducanumab works at all following a series of issues with Biogen’s clinical trials.

In March 2019, Biogen halted its two trials after an independent committee said the drug was not going to be effective. But in October 2020, the company said new analysis of a larger trial data set showed the drug worked when given at a higher dose.

Biogen has also been criticised for supplying analysis after its trials have concluded and some scientists say its data is insufficient to merit regulatory approval. In November, an FDA advisory panel hit back at the agency’s findings of “substantial evidence” that aducanumab worked.

“There’s a clear and well delineated bar that a product has to be able to get over in order to access the market and this product didn’t,” said Caleb Alexander, professor at the Johns Hopkins Bloomberg School of Public Health, and one of the panel members.

“I’ll fall out of my chair if they approve it,” he said, adding that a green light “would mark one of the most consequential and poor decisions that the FDA has made in recent history”.

A string of pharmaceutical companies have tried and failed to prove the amyloid hypothesis and create a drug that slows the advance of Alzheimer’s. Little evidence exists that clearing amyloids reduces progression of the disease.

The aborted trials include Pfizer and Johnson & Johnson’s scrapped study in 2012 and Merck’s failed trial in 2017. Last year, Eli Lilly and Roche’s two drugs also both failed to slow the rate of cognitive decline.

Approval will be hailed by those living with Alzheimer’s as a way forward, as well as a vindication for Biogen, which has battled criticism of its handling of the drug’s development for years.

The Alzheimer’s Association has urged the FDA to endorse aducanumab, saying that any treatment that gives patients more time is worthwhile.

Analysts expect the drug to become a blockbuster seller for Biogen, although revenue estimates vary.

Matthew Harrison, head of biotech research at Morgan Stanley, said patient demand was likely to be vast, but some doctors have refused to administer aducanumab even if it gets approval. “Investors aren’t sure to understand what that dynamic will ultimately mean.”

He expects Biogen to reap $7bn worth of sales by 2030 while Bristow of UBS anticipates peak annual sales worth $18bn worldwide.

“It would be one of the biggest drugs in the industry,” said Bristow.

Rejection of aducanumab will be a knock for people suffering from Alzheimer’s who have pinned their hopes for a slower mental decline on the drug.

More than 6m people in the US are living with the disease, according to the Alzheimer’s Association, and many conditions have worsened during the coronavirus pandemic as contact between loved ones was reduced in order to slow the spread of the virus.

Rejection will also be a heavy blow for Biogen, which has spent millions on the drug’s development and fallen behind in other areas. The company’s shares have fluctuated wildly following previous announcements about the drug.

“Aducanumab really hurt Biogen for a while because it was such an overwhelming focus of the company, they got behind in gene therapy [and] new neurodegenerative treatments,” said Tom Shrader, managing director of BTIG.

A green light from the FDA is likely to instil confidence in the amyloid hypothesis and boost investment in further studies. Eli Lilly is trialling donanemab, another drug that removes amyloids, and positive news for Biogen is likely to boost sentiment towards Lilly’s study.

Geoff Meacham, analyst at Bank of America, said a rejection “is not necessarily going to put a death nail in the coffin of the amyloid hypothesis” because Biogen’s controversial trials are difficult to compare to others’. Meanwhile, imaging technologies have progressed so patients can be more effectively screened to see whether they have amyloids in their brains in the first place.