(CNN/WHDH) — A monoclonal antibody treatment for Alzheimer’s disease called lecanemab slows the progression of cognitive decline by 27% compared with a placebo, drugmakers Biogen and Eisai said Tuesday.

The drug, tested in a Phase 3 global clinical trial, also met all secondary endpoints, showing “target engagement” with reduced amyloid levels — a protein that is one of the hallmarks of Alzheimer’s — and positive effects on cognition and the ability to perform everyday tasks when compared with a placebo.

“We believe that helping to alleviate these burdens will positively impact society as a whole,” Eisai CEO Haruo Naito said in a statement. “Additionally, the lecanemab Clarity AD study results prove the amyloid hypothesis, in which the abnormal accumulation of Aβ in the brain is one of the main causes of Alzheimer’s disease.”

However, Dr. Richard Isaacson, director of the Alzheimer’s Prevention Clinic in the Center for Brain Health at Florida Atlantic University’s Schmidt College of Medicine, told CNN that the trial results are not proof of the amyloid hypothesis.

“It proves that, in people with a certain amount of amyloid in their brain at a certain stage of the disease, that this drug works. In terms of proving a mechanism by using a drug, no. Alzheimer’s is a very heterogeneous disease.”

But he says that does not diminish the potential significance of the trial.

“In the past, reducing amyloid in the brain has not always been tied to cognitive improvements or any meaningful clinical improvements. In this study, every endpoint was positive. That’s never happened before.”

“I’m gonna hold off on a final judgement, but I’m cautiously optimistic,” said Dr. Andrew Budson, chief of cognitive and behavioral neurology at VA Boston. “It’s actually working!”

About 2.8% of trial participants who took the drug had a symptomatic side effect called ARIA-E, swelling in the brain, but none of those taking the placebo did. The rate of symptomatic ARIA-H, brain bleeding and iron buildup in tissue, was 0.7% in the drug group and 0.2% in the placebo group.

Overall, there were ARIA side effects in 21.3% of the trial participants taking lecanemab, but Isaacson cautions that when people receive this type of treatment, they need to be monitored closely throughout the process. Symptomatic side effects are a more important measure, he says.

“The percentage slowing of clinical decline that they reported is really quite impressive,” Budson said. “If this drug or another drug like it really worked to significantly alter the slope of decline with relatively few side effects, I can foresee a time in the future that people may begin to get screening for Alzheimer’s disease, and then a treatment like this will be started right away, before there’s any clinical signs, before there’s memory loss.”

“When this drug is used correctly, the side effects are manageable, and the negative outcomes are preventable in most cases,” he said.

The Alzheimer’s Association expressed hope over the results.

“For people in the earliest stages of Alzheimer’s, this treatment has the potential to change the course of the disease in a clinically meaningful way,” the association said in a statement. “These results indicate lecanamab may give people more time at or near their full abilities to participate in daily life, remain independent and make future health care decisions. Treatments that deliver these benefits to those with mild cognitive impairment (MCI) due to Alzheimer’s disease and early Alzheimer’s dementia are just as valuable as treatments that extend the lives of those with other terminal diseases.”

Isaacson agreed: “I’m envisioning a world where, in the years to come, this will, this type of treatment will be one of the many tools in our toolbox in the management of Alzheimer’s disease.”

Although the findings come from only preliminary data, the companies say they plan to publish them in a peer-reviewed journal and plan to present the data in order to seek approval from US regulatory authorities by the end of March.

Biogen is also the company behind another Alzheimer’s drug, Aduhelm, which was controversially approved by the FDA in June 2021. Aduhelm was the first new Alzheimer’s drug approved in nearly 20 years, but there were questions about its efficacy and cost. The FDA quickly narrowed the group of people who could receive it, and it’s unclear how many doctors will prescribe it because of uncertainty surrounding its results.

Aduhelm was also the driving force behind a massive increase in Medicare Part B premiums for 2022, and Medicare restricted coverage in April.

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