The treatment offers new hope to patients in the early stages of the most common form of dementia, which affects about 850,000 people living in the UK.
Biogen says its drug, aducanumab, would be the first treatment to “reduce the clinical decline” of patients in the early stages of Alzheimer’s disease if approved.
The company is now seeking regulatory approval for the drug in the US after new analysis of trial data revealed some patients experienced benefits in memory, orientation, language and everyday living skills such as shopping and doing laundry.
Reacting to the company's announcement, Hilary Evans, chief executive of Alzheimer's Research UK, said: "People affected by Alzheimer's have waited a long time for a life-changing new treatment and this exciting announcement offers new hope that one could be in sight."
Dementia causes an ongoing decline in brain function, which can affect memory, thinking speed, speech, mood and movement.
The decision to seek regulatory approval of aducanumab comes after clinical studies ended in March this year.
Biogen said the findings of an initial analysis were "incorrect" with an examination of a larger dataset showing aducanumab reduced clinical decline in patients in the early stages of Alzheimer's.
The company said the positive results of the new analysis were driven by patients having a greater exposure to a high dose of aducanumab, a drug designed to help the body clear harmful plaques from the brain.
Ms Evans said: "As more data emerges, we hope it will spark global discussions about the next steps for delivering much-needed treatments into people's hands."
Biogen said it would submit a licence application for aducanumab in the US in early 2020.
It said it was continuing to speak to regulatory authorities in other markets, including Europe.
Michel Vounatsos, chief executive at Biogen, said: "With such a devastating disease that affects tens of millions worldwide, today's announcement is truly heartening in the fight against Alzheimer's.
"This is the result of groundbreaking research and is a testament to Biogen's steadfast determination to follow the science and do the right thing for patients."
Additional reporting by Press Association.
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