A large clinical trial of a promising Alzheimer's drug has failed, leaving many in the field and families of those with early signs of dementia disappointed.

An experimental drug that many hoped would treat Alzheimer's disease has failed in a major clinical trial.

Pharmaceutical company Eli Lilly announced they would not be seeking regulatory approval to market their drug solanezumab, for mild dementia, because it didn't significantly slow down cognitive decline in patients.

Australian expert Professor Bryce Vissell, from the Neuroscience and Regenerative Medicine at UTS, says the results of the trial had been highly anticipated.

If the drug had succeeded it would have demonstrated that beta amyloid, clumps of a "toxic" protein that build up in the brains of Alzheimer's patients, contribute to the causes of the disease.

The pursuit of a treatment of Alzheimer's disease has long focused on clearing these clumps of protein.

Many scientists "controversially" think that treating the disease before the start of symptoms - like memory loss - is key.

"This is disappointing news for families who are watching Alzheimer's rob their loved ones of their memories and emotions," said Prof Vissell.

There are more than 353,800 Australians living with dementia, and the costs of managing the disease are expected to rise to $83 billion by the 2060s.

Despite the failed drug trial, Prof Vissell says there's still good reason to have hope and optimism that a treatment will become available.

"This isn't the end for beta amyloid theory. There's a trial of another important drug for Alzheimer's, aducanumab, being run by Biogen (another pharmaceutical company) which has shown interesting and encouraging signs," he said