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, 59, a former truck driver in Sacramento, who wasn't surprised when his early-onset Alzheimer's was diagnosed 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," he said.

Instead, he was enrolled in a clinical trial at Sacramento's Sutter Neuroscience Institute.

Four years into the five-year clinical trial, it's still too soon for 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 underway nationwide focused on Alzheimer's and dementia. 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, researchers say. If so, it could mean the arrival of disease-disrupting treatments that more than 5 million American patients have been eagerly 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, director of clinical trials for the University of California, Davis Alzheimer's Disease Center.

An estimated 5,000 researchers across the country are conducting trials into Alzheimer's treatments including looking at the impact of Vitamin D on cognitive function and examining ethnic disparities in treatments.

"Although these [new therapies] may not be a home run and don't cure the disease, if we slow it down by 25 to 33 percent, that'd be a whole extra year of function. That would be better than all the current therapies," Olichney said.

During the past three decades, "remarkable progress" has been made in understanding the neurobiology of chronic brain diseases such as Alzheimer's, notes a 2012 report by a national group of Alzheimer's researchers, part of the U.S. plan to defeat the disease.

One research effort identifies the disease earlier, even before symptoms appear — partly motivated by previous research showing some treatments were more effective in the disease's early stages.

"We used to say you couldn't accurately diagnose Alzheimer's until an autopsy, not with 100 percent certainty," said Dr. Shawn Kile, who heads a clinical trial on the effectiveness of PET scans for diagnosis. "Now, we can diagnose it 10 to 15 years before symptoms begin to show. It's a breakthrough."