The closely watched "A4" prevention study will select 1,000 participants aged 70 to 85 who have varying levels in their brains of amyloid protein - believed to be a main cause of the memory-robbing disease.

"This is the first time investigators will test an amyloid-clearing drug in older individuals thought to be in the pre-symptomatic stage of Alzheimer's disease," Brigham and Women's Hospital said on Friday.

The affiliate of Harvard Medical School is helping coordinate the three-year study, which could cost up to $100 million.

Solanezumab, a monoclonal antibody given by infusion, failed in two earlier Lilly-sponsored trials to slow the progression of the disease in patients who already had mild and moderate symptoms.

But when data from the two large Phase III trials was combined and analyzed last summer, cognitive declines were slowed by 34 percent among patients who started out with only mild symptoms. It was the first time any drug ever arrested the progression of Alzheimer's.

In the new prevention trial, patients will be selected by using a radioactive Lilly imaging agent called Amyvid (florbetapir) that can detect amyloid plaques in the brain. The agent is approved in the United States and was also approved in Europe this week.

Dr. Reisa Sperling, a Harvard neurology professor who will lead the A4 trial, said she and her colleagues are hoping solanezumab will reduce memory decline by 30 percent. That would be similar to the benefit seen among already mildly symptomatic patients in Lilly's earlier pair of large studies.

"But in asymptomatic people, a 30 percent slowing might prevent symptoms from ever developing at all," Sperling said. "In others, they might develop symptoms, but not get to the stage where they need help with daily activities," she said in an interview.

Sperling said final data from the trial will not be seen until 2018. Meanwhile, other Alzheimer's drugs will be tested beginning next year as part of her project, including a promising class of drugs called beta secretase inhibitors.

Solanezumab and an anti-amyloid drug being developed by Roche Holding AG were selected in October for another high-profile prevention trial, but it involves younger patients with a genetic high risk of developing Alzheimer's disease.

The study will begin early this year at Washington University School of Medicine in St. Louis. It is being supported by DIAN, a U.S.-funded collaboration of leading Alzheimer's disease centers worldwide.

Lilly shares closed 0.8 percent higher on the New York Stock Exchange.

(Corrects seventh paragraph to say Amyvid was approved in Europe this week, instead of awaiting approval in Europe)

(Reporting By Ransdell Pierson; Editing by Gerald E. McCormick, Steve Orlofsky and Andre Grenon)

• A Permanent Economic Emergency