* Goal is dementia cure or disease-modifying therapy by 2025
* Leaders promise significant increase in research funding
* World "empty-handed" in treating Alzheimer's - WHO head
* More than 100 drug failures in past 15 years
By Ben Hirschler
LONDON, Dec 11 (Reuters) - Leading countries set a goal of finding a cure or effective treatment for dementia by 2025 on Wednesday and ministers said the world needed to fight the spread of the memory-robbing condition just as it fought AIDS.
The move by the Group of Eight (G8) nations matches the date set by the United States last year for beating Alzheimer's - but the target is ambitious, considering there is no obvious cure on the horizon.
Global cases of dementia are expected to treble by 2050, yet scientists are still struggling to understand its basic biology, and the current medicine cupboard is bare.
"In terms of a cure, or even a treatment that can modify the disease, we are empty-handed," World Health Organisation (WHO) Director-General Margaret Chan told ministers, campaigners, scientists and drug industry executives from the Group of Eight leading economies at the summit.
Ending that drug drought will require more investment by governments and the private sector. The G8 ministers pledged to increase spending "significantly" - with Britain promising to double its expenditure - but officials stopped short of giving an overall funding figure.
The London meeting - the first G8 summit on a specific illness since HIV and AIDS - was hosted by British Prime Minister David Cameron, who said it was vital to show that dementia was not a normal part of ageing.
Health Minister Jeremy Hunt said there were lessons to be learnt from the fight against AIDS, where a 2005 G8 summit played a key role in pushing for better and more widely available drugs.
"We have turned the global tide in the battle against AIDS. Now we need to do it again. We will bankrupt our healthcare systems if we don't," he said.
The health ministers also agreed to appoint a global envoy for dementia innovation, following a template used for HIV and climate change.
Dementia - of which Alzheimer's disease is the most common form - already affects 44 million people worldwide and this is set to reach 135 million by 2050, according to new estimates this month from Alzheimer's Disease International, a non-profit campaign group.
More than 70 percent of them will be living in poorer countries with scant access to health care.
Experts say many people could avoid dementia by adopting healthier diets, exercising more, and giving up smoking, but that what the world needs urgently is effective drugs.
It is a decade since the last drug was approved to treat Alzheimer's, and there is still no treatment that can slow the progression of the disease. Current drugs can do no more than ease some of the symptoms of the disorder.
Over the past 15 years more than 100 experimental Alzheimer's drugs have failed in development, according to the Pharmaceutical Research and Manufacturers of America.
Some companies are still trying to crack the problem, since the potential prize would be sales running into many billions of dollars a year.
Merck & Co said on Tuesday it would begin late-stage trials of a drug designed to block the buildup of brain plaques that are a central feature of the disease, after safety problems with other similar drugs.
Eli Lilly is starting a new trial of an antibody treatment that failed in earlier testing but is now being tried out in patients with mild disease.
Others in the Alzheimer's drug race include Roche, Johnson & Johnson and Eisai.
Much of the current research is focused on the idea that early intervention is likely to be a key to success, since once dementia has developed enough to show serious symptoms, it may be too late for medicines to work.
Cameron said Britain aimed to double its annual spending by 2025 to more than 130 million pounds ($214 million), up from a 2015 target of 66 million pounds, but dementia campaigners say spending will still be only a fraction of that spent on cancer research.
The global cost of dementia is already more than $600 billion, or around 1 percent of global gross domestic product - and that figure will only increase, according to the WHO's Chan.
"Dementia is costly," she told the summit bringing together ministers from the United States, Canada, Britain, France, Germany, Italy, Russia and Japan. "And not only is it costly, it is a heartbreaking disease."