Aducanumab targets the underlying cause of the disease in the brain
The first new treatment for Alzheimer's disease for nearly 20 years has been approved by regulators in the United States, paving the way for its use in the UK.
Aducanumab targets the underlying cause of Alzheimer's, the most common form of dementia, rather than its symptoms.
At least 100,000 people in the UK with a mild form of the disease could be suitable for the drug.
But approval from the UK regulator could take more than a year.
Charities have welcomed the news.
The US Food and Drug Administration (FDA) said there was "substantial evidence that aducanumab reduces amyloid beta plaques in the brain" and that this "is reasonably likely to predict important benefits to patients".
Aducanumab targets amyloid, a protein that forms abnormal clumps in the brains of people with Alzheimer's that can damage cells and trigger dementia, including:
memory and thinking problems
In March 2019, late-stage international trials of aducanumab, involving about 3,000 patients, were halted when analysis showed it was no better, given as a monthly infusion, at slowing the deterioration of memory and thinking problems than a dummy drug.
But later that year, the US manufacturer Biogen analysed more data and concluded higher doses of aducanumab significantly slowed cognitive decline.
'Heading in right direction'
Aldo is given the drug treatment by a nurse
image captionAldo was one of just a handful of UK patients in the trial
Aldo Ceresa first noticed problems differentiating between left and right 10 years ago.
After his diagnosis, the 68-year-old, who is originally from Glasgow and now lives in Oxfordshire, close to his family, had to give up his job, as a surgeon.
Mr Ceresa took aducanumab for two years before the trial was halted - and then had to wait almost as long for another trial, at the National Hospital for Neurology and Neurosurgery, in London, to begin.
"I'm quite happy to volunteer," he says.
"I really, really enjoy this journey that I'm going through - and obviously the benefits I'm getting from it, which I'm very, very grateful for.
"I feel like I'm not quite as confused.
"Although it's still there, it's not quite as bad.
"And I'm just getting that bit more confident now.
"Before, if I was going to get something, I couldn't remember, you know, where to find things in the kitchen.
"That has become less of a problem.
"Now, I'm beginning to find more things out for myself.
"And so, you know, it's not 100% there - but I'm getting better at it.
"I haven't caught up to the level that I was before.
"But I'm heading in the right direction."
More than 30 million people around the world are thought to have Alzheimer's, with most aged over 65.
For the about 500,000 in the UK, those eligible for aducanumab will:
probably require a definitive diagnosis, involving a detailed 3D brain scan
be mostly in their in their 60s or 70s and at an early stage
In the past decade, more than 100 potential Alzheimer's treatments have flopped.
And although aducanumab is not a miracle drug, and many doctors are doubtful of its benefits, its US approval will be a huge boost to dementia research, traditionally underfunded compared with cancer or heart disease.