The U.S. Food and Drug Administration on Friday approved the Alzheimer's drug lecanemab developed by Eisai Co Ltd and Biogen Inc for patients in the earliest stages of the mind-wasting disease.

The drug, to be sold under the brand Leqembi, belongs to a class of treatments that aims to slow advance of the neurodegenerative disease by removing sticky clumps of the toxic protein beta amyloid from the brain.

Nearly all previous experimental drugs using the same approach had failed.

“Today’s news is incredibly important," said Dr. Howard Fillit, chief science officer of the Alzheimer’s Drug Discovery Foundation. "Our years of research into what is arguably the most complex disease humans face is paying off and it gives us hope that we can make Alzheimer’s not just treatable, but preventable."

Eisai said the drug would launch at an annual price of $26,500. Biogen shares, which had been halted, were up 5% at $285.19.

Initial patient access will be limited by a number of factors including reimbursement decisions from Medicare, the U.S. government insurance program for Americans aged 65 and older who represent some 90% of individuals likely to be eligible for Leqembi.

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.

The U.S. Centers for Medicare & Medicaid Services (CMS) said on Friday that current coverage restrictions for drugs approved under the accelerated pathway could be reconsidered based on its ongoing review of available information. If the drug receives traditional FDA approval, CMS said it would provide broader coverage. Eisai officials have said the company plans to submit data from a recent successful clinical trial in 1,800 patients as the basis for a full standard review of Leqembi.

Lecanemab is intended for patients with mild cognitive impairment or early Alzheimer's dementia, a population that doctors believe represents a small segment of the estimated 6 million Americans currently living with the memory-robbing illness.

“This treatment option is the latest therapy to target and affect the underlying disease process of Alzheimer's instead of only treating the symptoms of the disease,” FDA neuroscience official Billy Dunn said in a statement.

In the large trial of lecanemab, which is given by infusion, the drug slowed the rate of cognitive decline in patients with early Alzheimer's by 27% compared to a placebo. Nearly 13% of patients treated with Leqembi in the trial had brain swelling.

An autopsy analysis of a lecanemab patient who had a stroke and later died that was published this week suggests the drug should not be used with blood clot preventers.

Dr. Babak Tousi, a neuro-geriatrician at the Cleveland Clinic, said the approval will make a "big difference" in the field because it is based on biomarkers rather than just symptoms.

"It's going to change how we make a diagnosis for Alzheimer's disease, with more accuracy," he said.

Tousi acknowledged that the benefit of the drug will likely be modest. "Still, it is a benefit that we were not able to achieve" before this approval.