It may be too late to stop Alzheimer’s in people who already have some mental decline. But what if a treatment could target the very earliest brain changes while memory and thinking skills are still intact, in hope of preventing the disease? Two big studies are going all out to try.

Clinics throughout the United States and some other countries are signing up participants — the only studies of this type enrolling healthy older people.

“The excitement in the Alzheimer’s field right now is prevention,” said Dr. Eric Reiman, executive director of the Banner Alzheimer’s Institute in Phoenix, which is leading the work.

Science so far has failed to find a drug that can alter the progression of Alzheimer’s, the most common form of dementia; 146 attempts have failed over the last decade, a recent industry report found. Even drugs that help remove the sticky plaques that clog the brains of people with the disease have not yet proved able to stave off mental decline.

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It may be that they were tried too late, like lowering cholesterol after someone has suffered a heart attack whose damage can’t be undone, Reiman said.

His agency is funding the prevention studies with the Alzheimer’s Association, several foundations, and Novartis and Amgen, makers of two experimental drugs being tested.

They are checked for the APOE4 gene, which doesn’t destine someone to develop Alzheimer’s but raises that risk. About one in four people have one copy of the gene and about 2 percent have two copies, one from each parent.

More than 70,000 people have signed up since the registry began three years ago, said Jessica Langbaum, one of the Banner study leaders.

“Most of them have been touched by the disease personally,” either by having a family member or close friend with it, she said.

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Langbaum’s 67-year-old mother, Ivy Segal, gave a DNA sample through a cheek swab and joined the registry in August. Her father was a patient at Banner and died of Alzheimer’s in 2011 at age 87. Watching him go from a mild-mannered man whose smile could light up a room to what he was like when he died was devastating, she said.

Being in GeneMatch doesn’t necessarily mean you’ll find out if you have the gene — folks with and without it may be contacted about various studies. But to be in one of the two prevention studies, people must agree to learn their APOE4 status and have at least one copy of the gene.