The first drug that slows down Alzheimer's disease could be available within three years after trials showed it prevented mental decline by a third.

Pharmaceutical company Eli Lilly said Solanezumab had been shown to put the brakes on memory loss and cognitive decline in those with mild symptoms of dementia. It is the first time that a drug has been shown to tackle the underlying disease process, rather than just the symptoms.

Although trials are continuing the treatment could be available for use by 2018 if approved. Health experts said the research demonstrated a "huge step forward from the current treatment options".

Alzheimer's is caused when sticky amyloid plaques form in the brain, preventing neurons from communicating with each other. Solanezumab is an antibody which binds to amyloid in its early soluble form, allowing it to be cleared by the body before it can form dangerous plaques. The drug was originally developed for those with late-stage dementia but was found to be ineffective. However, researchers noticed that it was having an impact on people with mild symptoms. Tests are in development that could pick up Alzheimer's 10 years before the first symptoms emerge, meaning that treatment could start very early and perhaps prevent the plaques forming.

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Richard Morris, a professor of neuroscience at the University of Edinburgh, said the latest research proved that amyloid plaques were driving dementia and showed that preventing them forming was the key to slowing or stopping the disease.

"This is not a mouse study, it's a people study. And that matters," he added.

The trial followed 1322 people with mild Alzheimer's for three and a half years. Cognitive tests showed that the mental decline of those taking the drug was a third less over the period than those on placebo.

Results released yesterday also showed that two other drugs, Gantanerumab and Aducanumab, were effective at reducing biomarkers for Alzheimer's disease and offered early hope of new treatments.

The research was presented at the Alzheimer's Association International Conference in Washington DC.

Is Solanezumab a cure for Alzheimer's disease?

Solanezumab is not a cure, but it does slow down the progress of the disease which could mean extra years of lucid life. Over three and a half years it was found to slow the disease by 30 per cent.

What are the current treatments for Alzheimer's?

While there are no treatments that can cure or slow the disease, a number of medicines can temporarily alleviate symptoms. Some boost a brain chemical called acetylcholine which allows increased communication between brain cells.

How is Solanezumab different?

Solanezumab is the first drug to target the disease itself rather than the symptoms. In Alzheimer's disease, sticky beta-amyloid plaques build up between brain cells, like a blockage in a pipe. Solanezumab is an antibody which seeks out the early form of beta-amyloid before it becomes sticky and latches on. The body's own cleaning process then removes it before it can clump together.

Will it be available?
Only interim trial results have been announced so far. The full findings will be released in August next year. It usually takes between 18 months and two years for new drugs to be regulated by the health watchdogs.