People aged 55 to 90 are currently being recruited to a final-stage study in the US, which will examine if the drug solanezumab slows the cognitive and functional decline of people with mild Alzheimer’s.
Previous trials of the drug — made by pharmaceutical giant Eli Lilly — have shown no effect in people with later stages of the disease but there are signs it could be effective at an earlier stage.
Researchers found the drug had an effect on daily behaviour and functioning of brain and memory in people with earlier-stage disease.
In the new trial, people will be given solanezumab as a monthly 400mg injection for 18 months.
The trial is expected to end in Dec 2016.
Dr Eric Karran, director of research at Alzheimer’s Research UK, said scientists were “full of hope” that a breakthrough could come within five years.
He said the results may suggest monthly injections should be given early to ward off the disease, in the same way people take statins to prevent heart disease.
Dr Karran said the drug had been shown to have a “reasonable” effect in people with mild Alzheimer’s and results would come within five years.
“I think the pathway now is, if solanezumab is shown to work in mild Alzheimer’s disease, then the pathway would be to give it earlier and earlier and earlier . . . and you could have confidence you will see an effect.”
Dr Karran said changes in the brain associated with dementia can happen as much as a decade before first symptoms arise. “I am full of hope that we are going to have a breakthrough in five years.”
He added that should trials on sufferers with mild dementia succeed, “there is a logic” to use the drug therapies at least a decade earlier, to prevent the onset of dementia.
“That’s exactly the path that blood pressure-lowering agents have taken — people taking them before they have a stroke,” he said. “It’s the path that’s been taken with statins which first showed efficacy against the disease and then you go earlier. That has to be the pathway we take.”
Prof Nick Fox, from the Institute of Neurology, at University College London, said that preventing disease before symptoms were present offered the best “window of opportunity” to halt the impact of disease.