A turning point in the fight against Alzheimer's may have been reached with the first trial evidence that progression of the disease can be held back.
The experimental drug solanezumab reduced mental decline by 34 per cent in a group of patients taking a standard battery of memory and thinking tests.
It is the first time an Alzheimer's drug has been shown to have a "disease modifying" effect rather than merely alleviating symptoms.
The findings offer a glimmer of hope to victims of the devastating disease suffered by an estimated 500,000 people in the UK.
But only patients with mild Alzheimer's who started treatment early experienced the beneficial effect over a period of three-and-a-half-years.
A follow-up trial targeting patients with mild disease is expected to provide more conclusive data next year. Experts have called the new findings "encouraging" and "exciting" while at the same time urging caution.
The results were presented at the Alzheimer's Association International Conference in Washington DC, US, where another drug was shown to shrink protein deposits in the brain linked to Alzheimer's.
Again, the effect was seen in people at an early stage of the disease. An interim safety study found that the drug aducanumab reduced the size of beta amyloid plaques increasingly as the dose was raised.
Both drugs are laboratory-made antibodies that target specific proteins, in this case sticky clumps of beta amyloid.
Dr Doug Brown, head of research at Alzheimer's Society, said: "Today's findings strongly suggest that targeting people in the earliest stages of Alzheimer's disease with these antibody treatments is the best way to slow or stop Alzheimer's disease.
"These drugs are able to reduce the sticky plaques of amyloid that build up in the brain, and now we have seen the first hints that doing this early enough may slow disease progression.
"After a decade of no new therapies for dementia, today's news is an exciting step forward.
"We will have to wait for the ongoing trials to finish to know the full risks and benefits of these drugs. If they are positive, these drugs will be the first identified to directly interfere with the disease process and slow the progression of Alzheimer's."
Commenting on the solanezumab trial, Dr Eric Karran, director of research at Alzheimer's Research UK, said: "The results provide encouraging evidence that solanezumab could indeed be acting on the disease processes that drive Alzheimer's.
"Although this effect represents a small improvement for people experiencing mild symptoms, it will be important for longer trials to explore whether this treatment could produce greater benefits in the long term.
"While this could be evidence of the first disease-modifying treatment for Alzheimer's, the ultimate test will be whether these promising effects repeat again in the third, more targeted, phase III trial in people with mild Alzheimer's due to finish late next year. We await the results of that trial with great interest."
A total of 1,024 patients with an average age of 73 took part in the solanezumab investigation. Researchers pooled data from two earlier trials, Expedition and Expedition 2, and a later extension study involving the same participants.
Initial results from the Expedition trials, published in 2012, suggested that the drug – like a number of other experimental medicines showing early promise – was a failure. Treated patients fared no better than those given a dummy placebo.
But the picture changed when researchers picked out a subset of patients with mild symptoms who had started treatment early. These patients experienced a benefit that was sustained over time, so that those on the drug for three-and-a-half years were better off than others who had only taken it for two years.
This was evidence that the drug was treating the disease itself, not just its symptoms.
Peter Roberts, emeritus professor of pharmacology at the University of Bristol, warned against over-hyping the results and reaching premature conclusions.
He said: "I would very cautiously go along with this announcement possibly being significant, though unfortunately in the media this morning we have the usual 'medical breakthrough' spin.
"Whether or not solanezumab proves to be disease-modifying will take some time to establish. The published data so far show only a small statistically-significant effect in the subset of mild cases of AD [Alzheimer's disease]. The cognitive benefits are not astounding."
He added that Alzheimer's was a "very complicated" disease that would probably require multi-target therapies.
Neuroscientist professor Richard Morris, from the University of Edinburgh, said: "I am cautiously optimistic. This is not a mouse study, it's a people study. And that matters."
Solanezumab is made by the pharmaceutical company Eli Lilly. Lead researcher Dr Hong Liu-Seifert, a full time employee at the company, said: "We are particularly excited about these data because this is the first time the delayed-start methodology has been implemented for an Alzheimer's disease clinical trial.
"These results support the trial design and delayed-start analysis plan of Expedition 3, which is expected to have the last patient visit in October 2016."
Professor John Hardy, a neuroscientist at University College London, said: "These reports are good news in the same way that a forecast of sunny weather at the weekend is good news. It raises hopes for good weather, but it does not mean good weather is a certainty."
