SACRAMENTO, Calif. — For decades, Alzheimer’s disease has been silently ravaging brains, stealing memories and shortening the lives of millions of Americans. Now, researchers say they may be on the brink of tantalizing treatment breakthroughs that could for the first time at least slow the disease’s deadly progression.

It could help patients such as David Johnson, a 59-year-old former truck driver in Sacramento, who wasn’t surprised when he was diagnosed with early-onset Alzheimer’s in 2012. The debilitating disease had already taken his father, six aunts and uncles, and a cousin.

Four years ago, “I resigned myself to dying. I knew I had three to five years left,” said Johnson, a trim, goateed grandfather.

Instead, he was enrolled in a clinical trial at Sacramento’s Sutter Neuroscience Institute that Johnson believes has slowed, if not halted, the disease.

Four years into the five-year clinical trial, it’s still too soon for Sutter doctors to confirm how well Johnson’s treatment, involving infusions of special antibodies, is working. So far, his brain scans have showed the disease has not progressed.

Johnson’s treatment is one of hundreds of clinical trials under way nationwide. Amyloid, the sticky protein that attaches to brain cells and causes Alzheimer’s, is at the forefront of new therapies. Although none of the clinical therapies are yet FDA-approved, some are in the final phases with promising results, say researchers.

If so, it could mean the arrival of disease-disrupting treatments that patients, caregivers and researchers have been anticipating for decades.

“We’re entering a new era where we are very close to having the first proven disease-modifying therapy. It’s taken an awful lot of work for the last decade, but we think it’s slowing down the progression of the disease,” said Dr. John Olichney, neurology professor and director of clinical trials for the University of California, Davis Alzheimer’s disease Center.

More than 5 million Americans are living with Alzheimer’s, a progressive form of dementia that destroys brain cells. It’s the sixth leading cause of death among U.S. adults.