The US Food and Drug Administration on Friday approved the Alzheimer's drug lecanemab developed by Eisai and Biogen for patients in the earliest stages of the disease.
The drug, to be sold under the brand Leqembi, belongs to a class of treatments that aims to slow the advance of the neurodegenerative disease by removing sticky clumps of toxic protein beta amyloid from the brain.
Nearly all previous experimental drugs using the same approach had failed.
Eisai said the drug would be launched at an annual price of $26,500.
Initial patient access will be limited by a number of factors including reimbursement decisions from Medicare, the US government insurance programme for Americans aged 65 and older who represent about 90 per cent of the people likely to be eligible for Leqembi.
The drug 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 six million Americans currently living with the memory-robbing illness.
Biogen shares were halted after rising 3.6 per cent, or $9.67, to $281.26 in earlier trading on Friday
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.
Eisai officials have said the company also plans to submit data from a recent successful clinical trial in 1,800 patients as the basis for a full standard review of Leqembi.
That trial found that Leqembi, which is given by infusion, slowed the rate of cognitive decline in patients with early Alzheimer's by 27 per cent compared to a placebo. About 13 per cent of patients treated with Leqembi in the trial had brain swelling.
Northwestern University Feinberg School of Medicine in Chicago said in correspondence published in The New England Journal of Medicine this week that the postmortem of a 65-year-old lecanemab patient who suffered a stroke showed that emergency treatment with a standard clot-busting drug led to fatal brain haemorrhaging.
At least one other patient in a follow-on study of lecanemab died from brain haemorrhaging, raising concerns that blood-thinners may increase risks of dangerous brain swelling.
