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Another promising drug treatment for Alzheimer's disease has tanked.

Scientists and analysts alike had high hopes for solanezumab. There were predictions that it would be a blockbuster that could rack up sales of $10-billion a year.

After all, in laboratory tests, the drug was able to bind to a protein called amyloid beta that builds up in the brain of Alzheimer's patients. Theoretically, if you can keep the protein from clumping together to form sticky plaque that clogs up the brain, you should be able to stop the disease from progressing.

But, in the real world, it didn't work.

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In two earlier studies, solanezumab failed to slow the progress of Alzheimer's symptoms in patients with moderate to severe illness and, last week, new research showed that it was not effective in those with mild disease either.

It's an all-too-familiar tale. Between 1998 and 2014, there were 123 potential Alzheimer's drugs in clinical trials, but only four were approved, according to PhRMA (the Pharmaceutical Research and Manufacturers of America).

The drugs that are currently used to treat Alzheimer's, like donepezil (brand name Aricept) and memantine (Namenda) have modest benefits, such as easing symptoms like memory loss, confusion and agitation only temporarily.

We're a long way from being able to slow progression of the disease, let alone talk about a cure, despite the bold promise from G8 leaders to do so by 2025. We don't even know the basic mechanisms of what causes Alzheimer's disease and other forms of dementia.

While the amyloid hypothesis – the notion that sticky plaque clogs up the brain – is widely accepted, it may have to be rethought given the latest drug failure. Another theory is that the key protein is tau, which damages nerve cells in the brain, and there are tau-lowering drugs in the pipeline. In fact, there are at least 70 dementia drugs in development. Investments are being made because the market is so potentially lucrative.

Worldwide, an estimated 47 million people are living with dementia, a number that is expected to soar to 131 million by 2050, according to Alzheimer's Disease International. In Canada alone, there are an estimated 564,000 people living with dementia, according to the Alzheimer Society of Canada.

(There are actually about 100 forms of dementia – an umbrella term for conditions in which brain cells die on a large scale – but 70 to 80 per cent of cases are Alzheimer's.)

The sliver of good news emerging is that the proportion of seniors developing dementia seems to be dropping.

Research published last week in the medical journal JAMA Internal Medicine showed the rate of dementia in people over the age of 65 in the United States fell to 8.8 per cent in 2012 from 11.6 per cent in 2000. A study published last year in Lancet Neurology showed similarly big drops in Europe.

"Our results add to a growing body of evidence that this decline in dementia is a real phenomenon, and that the expected growth in the burden of dementia may not be as extensive as once thought," said Dr. Kenneth Langa of the University of Michigan and the lead author of the U.S. study.

There is also no simple explanation for the drop in dementia rates, though researchers posit that education is a big factor. In the 2000 to 2012 study period, the average time people spent in school increased to 12.7 years from 11.8 years on average. Education matters because it is fairly well-established that the more active your brain, the later in life you are likely to develop dementia.

Still, we have to be cautious in interpreting the numbers. While the proportion of those afflicted by dementia may be falling, the number of seniors continues to soar, so the actual number of people with Alzheimer's and related conditions is going to continue to rise.

There are two main risk factors for dementia: Genetics and aging.

We can't treat Alzheimer's or other forms of dementia effectively, and we can't cure them. But, in many cases, we can defer dementia – delay its onset – by years, maybe even decades. As we search for the miracle drug, let's not forget that fairly simple lifestyle choices, such as not smoking and being socially, mentally and physically active are by far the most effective tools we have to keep the scourge of dementia at bay.

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