In one of the most ambitious attempts yet to thwart Alzheimer’s disease, a major study is underway to see if an experimental drug can protect healthy seniors whose brains harbor silent signs that they’re at risk.

Scientists plan to eventually scan the brains of thousands of older volunteers in the United States, Canada and Australia to find those with a sticky build-up believed to play a key role in the development of Alzheimer’s.

Having lots of that gunky protein called beta-amyloid doesn’t guarantee someone will get the disease. But the big question: Could intervening early make a difference for those who do have it?

“We have to get them at the stage when we can save their brains,” said Dr. Reisa Sperling of Boston’s Brigham and Women’s Hospital and Harvard Medical School, who is leading the huge effort to find out.

Researchers began recruiting volunteers in June, and a Rhode Island man was hooked up for an IV infusion at Butler Hospital in Providence, the first treated. Peter Bristol, 70, figured he was at risk because his mother died of Alzheimer’s and his brother has it.

“I felt I needed to be proactive in seeking whatever therapies might be available for myself in the coming years,” said Bristol, who said he was prepared when a PET scan of his brain showed he harbored enough amyloid to qualify for the research.

He won’t know until the end of the study whether he received monthly infusions of the experimental medicine by Eli Lilly & Co. or a dummy drug.

Scientists now think Alzheimer’s begins ravaging the brain at least a decade before memory problems appear. Many believe the best chance of preventing or at least slowing the disease requires intervening, somehow, when people still appear healthy. — ap