Tests show just a six-week course of commonly available tablets leads to a marked improvement in mental function.
Montelukast appears to work by reducing inflammation in a part of the brain associated with learning and memory.
It is the latest drug to be “repurposed” – used to treat a different condition than it was originally intended for.
Experts said the exciting discovery gives hope to hundreds of thousands of people who will be struck down with dementia.
They believe the pill could hold the key to providing the first “safe and durable” treatment for old age.
The breakthrough came after researchers at the Paracelsus Medical University in Salzburg, Austria, found it boosted the memories of older rats.
“Counteracting some, or ideally all, of such age-related changes might rejuvenate the brain and lead to preservation or even improvement of cognitive function in the elderly,” the researchers claimed in a landmark study published in the respected journal Nature.
The gradual loss of memory and cognitive skills is a key sign of ageing.
Growing old is also associated with a slowdown in the growth of new brain cells and an chronic increase in toxic inflammation.
Neurologist Ludwig Aigner tested montelukast because it blocks inflammation in asthma and could do the same in the brain.
Researchers gave 10mg daily doses to four-month-old and 20-month-old rats, the older rodents being the equivalent in age to 65 to 70-year-old humans.
The older animals did not have a rodent form of dementia, but showed the same decline in brain agility that humans experience as they age.
Groups of rats were trained to find a submerged platform in a water pool.
After five days, older rodents were still unclear about its location but those given the drug could find it almost as well as younger animals.
After two days the rats were again lowered into the water pool and went in search of the platform.
Again, older rodents were bad at remembering where it was, but those given the drug appeared to have better memories, and found the platform nearly as quickly as younger animals.
Further experiments examined how animals behaved when familiar objects were moved.
Young rodents spent more time checking out where they had gone, which researchers believe is because they have better memories.
When older rodents were given montelukast they behaved in an identical way.
Studies of their brains revealed older animals given the drug had more freshly-grown neutrons than those given a dummy pill.
Crucially, they showed less obvious inflammation.
Professor Christian Hölscher, a leading Alzheimer’s researcher at Lancaster University, said: “These new research results show very promising effects. As more and more inflammation builds up during ageing, it is a good approach to test currently available drugs that reduce inflammation such as in asthma to see if they can also be helpful in protecting against the long term effects of chronic inflammation.
“Since this drug is already on the market, we know exactly how people respond to it and what the potential side effects are.
“Also, it is much easier to test this drug in clinical trials as it is already licensed for use in people.”
One in three people born this year are expected to develop the harrowing condition.
The scale of “dementia dread” is such that eight in ten over 55s admit to being terrified of the condition with 76 per cent of 18-24 year olds saying the same.
It now affects 850,000 people in the UK and costs £26bn a year to treat.
Experts estimate more than 2m will be struck down by 2050.
Dementia is caused by diseases, most commonly Alzheimer’s, resulting in the loss of brain cells, which impair mental function.
Research shows a third of cases could be prevented by adopting healthier lifestyles like not smoking, taking more exercise and eating a diet rich in vitamins and fish.
Telltale symptoms include memory loss, confused thinking, speech and difficulty problem-solving. It is progressive, meaning it worsens over time.
Dr Laura Phipps, of Alzheimer’s Research UK, said: “We now know that inflammation could play a harmful role in many brain diseases, including those that cause dementia.
“Researchers are investigating a wide range of anti-inflammatory approaches in the search for new treatments for Alzheimer’s and this study in rats uses an existing anti-asthma drug to dampen brain inflammation.”
Doug Brown, of Alzheimer’s Society, said: “We know that inflammation in the brain may contribute to the development of Alzheimer’s disease, and so finding ways to treat this is a potential avenue for researchers.
“The approach of repurposing existing drugs is a promising one as it could mean new treatments for dementia become available in half of the time of a standard drug – bringing hope to hundreds of thousands.”
Source The Express