Developed by Eisai and Biogen, the drug is called lecanemab and will be marketed under the brand Leqembi.

It's intended for patients in the early stages of Alzheimers, in the hopes it can remove sticky clumps of what's called beta amyloid from the brain.

Thomas Wisniewski is the director of the Alzheimer's Research Center at NYU Langone.

"Amyloid beta is a protein that accumulates in Alzheimer's disease, and it specifically forms aggregates. And those aggregates are toxic to neurons, that is brain cells, and it causes neuronal dysfunction and death that correlates with the cognitive symptoms of Alzheimer's disease, dementia."

Nearly all previous experimental drugs following the same approach have so far failed.

An Eisai trial found that the drug slowed the rate of cognitive decline in patients with early Alzheimer's by 27% compared to a placebo. But nearly 13% of patients treated with Leqembi in the trial had brain swelling.

"This form of therapy is not without risks, but in the present data, it looks like the risks are greatly outweighed by the potential benefits. But nevertheless, the treatment would need to be closely monitored with the clinician who's familiar with this type of therapeutic approach."

Eisai said the drug would launch at an annual price of $26,500.

Leqembi was approved under the FDA's accelerated review process, an expedited pathway that speeds access to a drug based on its impact on underlying disease-related biomarkers believed to predict a clinical benefit.