The clinical trial studied 1,800 patients with mildAlzheimer's symptoms, giving some the drug lecanemab and the others a placebo.
The study found the drug resulted in moderately less cognitive decline compared to the patients who received a placebo.
The drug attacks a sticky protein called amyloid, which is considered to be a factor in Alzheimer's disease.
"We believe this provides hope to patients, caregivers and physicians, stimulates further research into Alzheimer's and neurodegenerative diseases, and provides evidence that it is possible to treat these devastating conditions," said Dr. Michael Irizarry, from the pharmaceutical company Eisai.
Six million Americans have Alzheimer's, and nearly all the available treatments only temporarily ease symptoms.
Some experts say the slowdown in progression of even just a few months is a benefit.
"I think there is no doubt that a clinically meaningful benefit when translated to a person and their family can, in fact, be translated into how much time we have with our loved ones in a stage of disease where we can still enjoy family and outings, vacations, bucket lists, weddings, you name it," said Maria Carrillo from the Alzheimer's Association.
But those benefits will be balanced against the risks of brain swelling or bleeding, the requirement for infusions, and what are expected to be major financial costs.
Two deaths were reported among the lecanemab users who were also taking blood thinners.
The deaths were not attributed to the Alzheimer's drug.
The study concluded that longer trials are warranted to determine the efficacy and safety of the drug.
"There's no doubt that this disease is complicated and there are more causes than just amyloid plaques. So, anti-tau therapies, anti-inflammation therapies, so many other things have to be explored and will absolutely be required to be used together, to really make a dent in this disease in the near future," said Carrillo.
The Food and Drug Administration is considering approving lecanemab under its fast-track program. A decision is expected in January.
