Biogen shares were up nearly 5% at $281.34.

The number of people in the United States living with Alzheimer's, the most common form of dementia, is expected to triple to nearly 14 million by 2050 without effective treatments, according to the US Centers for Disease Control and Prevention. If approved, Biogen's aducanumab would be the first treatment designed to delay progression of the fatal, memory-robbing disease.

The drug has had a tumultuous journey, with promising early data raising hopes and Biogen's share price, only to be dashed by disappointment in later trials.

Biogen in October revived plans to seek approval for aducanumab based on a detailed analysis of what had seemed like less than compelling data, months after scrapping its development.

The biotech company said its analysis showed patient improvements in memory and other measures of cognition and ability to function. Some experts have expressed skepticism over Biogen's interpretation of the data and called for another large trial that could take several years to complete.

Wall Street analysts have been betting on aducanumab to restore Biogen's growth prospects following a successful patent challenge to its top-selling multiple sclerosis drug Tecfidera and with looming competition for its high-priced gene therapy Spinraza weighing on its shares.

However, the drug's approval is far from guaranteed despite the desperate need for treatments. It has been at least 15 years since the Food and Drug Administration has reviewed an application for a new Alzheimer's treatment.

Prior to the focus on COVID-19 vaccines and treatments, the wait for aducanumab's filing and approval was probably the most-watched saga to play out in biopharma this year, SVB Leerink analyst Marc Goodman said.

The FDA now has 60 days to decide whether to accept the application for review, Biogen and partner Eisai Co Ltd said in a joint statement.

"It is unclear to us if the totality of the data support FDA approval. Just as there are signs of activity, there are inconsistencies in the data that raise questions about aducanumab's biologic effect," Cowen analyst Phil Nadeau said.

Biogen said it has asked for a "priority review" for the drug that could cut the time for an approval decision to six months, if granted.

Any successful Alzheimer's treatment is expected to eventually be a multibillion-dollar seller. Analysts are forecasting aducanumab sales of $673.7 million in 2022 if approved, according to Refinitiv IBES estimates.

Biogen said it was also working toward submitting marketing applications for the drug in Europe, Japan and elsewhere.

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