WASHINGTON (AP) -- In one of the most ambitious attempts yet to thwart Alzheimer's disease, a major study got underway Monday to see if an experimental drug can protect healthy seniors whose brains harbor silent signs that they're at risk.

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Scientists plan to eventually scan the brains of thousands of older volunteers in the U.S., Canada and Australia to find those with a sticky build-up believed to play a key role in development of Alzheimer's -- the first time so many people without memory problems get the chance to learn the potentially troubling news.

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

"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 are just beginning to recruit volunteers, and on Monday, a Rhode Island man was hooked up for an IV infusion at Butler Hospital in Providence, the first treated.

Peter Bristol, 70, of Wakefield, Rhode Island, 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.

"Just because I have it doesn't mean I'm going to get Alzheimer's," he stressed. But Bristol and his wife are "going into the situation with our eyes wide open."

He won't know until the end of the so-called A4 Study -- it stands for Anti-Amyloid Treatment in Asymptomatic Alzheimer's -- whether he received monthly infusions of the experimental medicine, Eli Lilly & Co.'s solanezumab, or a dummy drug.

Solanezumab is designed to help catch amyloid before it builds into the brain plaques that are a hallmark of Alzheimer's. It failed in earlier studies to treat full-blown Alzheimer's -- but it did appear to help slow mental decline in patients with mild disease, raising interest in testing it even earlier.

Scientists now think Alzheimer's begins ravaging the brain at least a decade before memory problems appear, much like heart disease is triggered by quiet cholesterol build-up. Many believe the best chance of preventing or at least slowing the disease requires intervening, somehow, when people still appear healthy.

The $140 million study, funded by the National Institutes of Health, Lilly and others, will track if participants' memory and amyloid levels change over three years.

Whether this particular drug works or not, the Alzheimer's study is being watched closely as a chance to learn more about how amyloid works and how people handle the uncertainty of knowing it's there.

"Amyloid we know is a huge risk factor, but someone can have a head full of amyloid and not decline" mentally, Sperling said. "We need to understand more about why some brains are resilient and some are not."

Before any brain scans, interested 65- to 85-year-olds will undergo cognitive tests to be sure their memory is normal. Volunteers also must be willing to learn their amyloid levels, and researchers can turn away those whose psychological assessments suggest they may not cope well with the news. Sperling expects to screen more than 5,000 healthy seniors to find the needed 1,000 participants, who will be monitored for anxiety or distress.

"It is breaking new ground," said Dr. Laurie Ryan of the NIH's National Institute on Aging. "We really do have to understand how that affects people."

More than 35 million people worldwide have Alzheimer's or similar dementia, including about 5 million in the U.S., numbers expected to rise rapidly as the baby boomers age.

Alzheimer's affects 1 in 9 people over age 65, and about a third of those 85 and older, according to the Alzheimer's Association.

Today's medications only temporarily ease some symptoms, and scientists don't even know exactly how the disease forms. A leading theory is that amyloid plaques kick off the disease but tangles of a second protein, named tau, speed up the brain destruction.

As scientists shift their attention to the still healthy, a few studies are under way to try blocking Alzheimer's in people genetically at risk to get a form of the disease that runs in their families.

The A4 study widens the focus beyond a genetic link.

Like Bristol, the first participant, some people do want to know if they're at risk, said Dr. Jason Karlawish, a bioethicist at the University of Pennsylvania who helped design the study's psychological precautions. After all, many already get tested for Alzheimer's-related genes.

He calls the research "an opportunity to study the future of the way we're going to think about, talk about and live with the risks of Alzheimer's disease."

