Alzheimer's disease plaque. (University of Pennsylvania School of Medicine Photo/James Eberwine)

WASHINGTON (AP) — Scientists will be testing therapies that could protect people who are susceptible to Alzheimer’s disease but are not yet showing symptoms of dementia.

One international study announced Tuesday will track whether an experimental drug can stall the disease in people who appear healthy but are genetically destined to get a type of Alzheimer’s that runs in the family. If so, the results could be evidence that regular Alzheimer’s is also preventable.

A second study will test whether a nasal spray that sends insulin to the brain helps people with very early memory problems, based on separate research linking diabetes to an increased risk of Alzheimer’s.

Researchers note that Alzheimer’s starts ravaging the brain at least a decade before memory problems appear.

“Once the train leaves the station of degeneration, it might be too late to stop it,” Dr. Reisa Sperling of Harvard Medical School said. “We need to define the critical window for intervention.”

Already, 5.4 million Americans have Alzheimer’s or related dementias. Barring a research breakthrough, those numbers will jump by 2050, when up to 16 million Americans are projected to have Alzheimer’s.

There is no cure, and the five medications available today only temporarily ease some symptoms. Finding better ones has been a disappointing slog: Over the last decade, 10 drugs that initially seemed promising failed in late-stage testing, Sperling said.

Moreover, scientists still don’t know exactly what causes Alzheimer’s. The chief suspects are a sticky gunk called beta-amyloid, which makes up the disease’s hallmark brain plaques, and tangles of a protein named tau that clogs dying brain cells. One theory: Amyloid may kick off the disease while tau speeds up the brain destruction.

Previous studies of anti-amyloid drugs have failed, but that new international study will test a different one, in a different way: About 300 people from a huge extended family in Colombia who share a gene mutation that triggers Alzheimer’s in their 40s will test an experimental drug, Genentech’s crenezumab, to see if it delays onset of symptoms. The study also will include some Americans who inherit Alzheimer’s causing gene mutations.

-ab