When an experimental Alzheimer’s drug recently showed impressive early-stage results, the company immediately planned large-scale trials, and the scientists behind it responded like weary optimists.

They’d been down this path before, only to see their hopes dashed when a drug aimed at the neurological disease failed in larger groups of patients.

This time they sense things could be different.

The new drug created a stir in March when its maker, Biogen in Cambridge, Mass., announced the results of a small human trial that exceeded expectations.

The treatment, an engineered antibody tested in a first-phase study of 166 patients, substantially reduced the amount of abnormal protein deposits in the brain that have long been considered the primary cause of dementia in Alzheimer’s patients. Patients who received the therapy also showed significantly less cognitive decline than those who had been given a placebo.

“I’m very encouraged — the most I’ve ever been,” said Dr. Allan Bernstein, a neurologist who plans to begin a clinical trial of the drug in July for Santa Rosa’s Annadel Medical Group, which is affiliated with St. Joseph Health.

Other Alzheimer’s experts also expressed tempered hope for the drug.

“It’s fantastic news, but that’s a really small study so we’re going to wait and see,” said Elizabeth Edgerly, chief program officer with the Alzheimer’s Association of Northern California and Northern Nevada. “We haven’t succeeded with a disease-altering drug or even one that improves symptoms for many, many years.”

Advancements in imaging

Biogen’s antibody is not the only antiamyloid antibody being tested, but not all have done as well in human clinical trials. Still, Biogen’s strong results have spurred hope that similar treatments may fare better if patients are more carefully screened and are in the early stages of the disease when treatment can be most effective.

“Older drugs tried and failed because they were started in people who already had serious loss of function and couldn’t really recover anything,” said Bernstein, an associate professor of medicine at UCSF, who is also testing other antiamyloid antibodies.

Advancements in imaging have spurred the latest Alzheimer’s research, experts said. High-tech scans have allowed scientists to confirm the diagnosis by seeing the buildup of amyloid and detect the disease in its earliest stages.

Researchers at UCSF and Stanford are testing Eli Lilly’s solanezumab and crenezumab, from South San Francisco’s Genentech. Both drugs failed early-stage trial goals, but the researchers think they may have potential value for early-stage Alzheimer’s patients and continue to test them.

“Even though the trials were negative, there were hints there were still some beneficial effect of the antibodies,” said Dr. Adam Boxer, director of UCSF’s Alzheimer’s Disease and Frontotemporal Dementia Clinical Trials program. “We’re pretty optimistic about these drugs and hopefully they’ll have a big impact.”

High-stakes battle

All these antibodies operate on the same principle of binding to and removing amyloid from the brain, but they bind differently and have different risks of side effects. Brain swelling has been the most concerning.

The stakes are high, considering more than 5 million Americans have Alzheimer’s, and the number is expected to rise as the Baby Boomers age. The fatal disease ultimately impairs the body’s ability to swallow and breathe.

Carolyn McLeese, 79, a retired elementary school teacher who lives in Bodega Bay, volunteered for a trial because, like others, she wants to help find potential treatments both for themselves and for millions of others with the disease.

McLeese, a patient of Bernstein’s, had been detecting memory problems that she knew weren’t just age-related for about three years. She had lost two aunts from Alzheimer’s, so she wasn’t entirely surprised when she, too, was diagnosed.

Convinced she had to do something, she enrolled in St. Joseph’s trial for Lilly’s antibody drug as soon as she was diagnosed.

“This is more than about me. This is very much about all the people — my two aunts, people from my church and from all over the world — who all of a sudden find out they have Alzheimer’s disease,” said McLeese, who is in the very early stages of the disease. “I hope the little bit I do comes to something that will help people going through the disease.”

No declines apparent

As part of the trial rules, McLeese does not know if she’s taking the placebo or solanezumab. But she and her husband say they haven’t noticed a decline in her mental function, as would be typical of the disease. Nor has she had any side effects.

“I tell myself I’m on the real the stuff,” McLeese said. “Until something happens differently, I’m just telling myself I’m getting whatever will keep me right here. If I stay right where I am right now, that would make me satisfied.”

Victoria Colliver is a San Francisco Chronicle staff writer. E-mail: vcolliver@sfchronicle.com Twitter: @vcolliver