In a large late-stage clinical trial, a drug being developed for Alzheimer's disease slowed the rate of cognitive decline.

The strong results boost the drug's chances of winning approval and offer renewed hope for a class of Alzheimer's drugs that have failed before.

The bad news for Biogen is that it has a second chance after the debacle with Aduhelm. Despite little evidence that it could slow cognitive decline, the medication was given regulatory approval last year and received only limited coverage by Medicare.

The results seem better for lecanemab. The group of volunteers who received lecanemab had a reduction in cognitive decline compared to the placebo group.

The trial of lecanemab was the largest to date to see if it could slow the progression of Alzheimer's disease. The design of Aduhelm is similar.

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Some patients taking lecanemab experienced brain swelling or brain bleeding, but the prevalence of these side effects was lower than with Aduhelm.

Aduhelm had been approved for accelerated approval. The FDA can greenlight drugs if they have uncertain evidence of benefit but affect a diseases biological pathway in a way that is likely to benefit patients. An F.D.A. decision is expected by early January and the company will use the newer data to seek full approval. Further trials and proof that the drug works are required for accelerated approval.

According to analysts, lecanemab is likely to be a billion dollar blockbuster.

Brian Skorney said that it puts Biogen back in the Alzheimer's game.

Ivan Cheung, the chairman and chief executive of Eisai, told reporters that the results of the trial showed that it could slow down Alzheimer's disease at its earliest stages.

He said that the drug started to show a benefit to patients about six months after they started taking it and that the benefit increased until the trial ended 18 months after patients started on the drug.

More detailed results will be presented in November.

The drug's ability to slow cognitive decline was modest at best, and may not be a difference that patients in the early stages of the disease would notice.

The effect is small and would not be considered a clinically important difference according to the director of the California Alzheimer's Disease Center at the University of Southern California. He said that other people would disagree and say it is clinically meaningful.

Lecanemab is easier to use than Aduhelm, according to Dr. Schneider.

Although taking a press release at face value, which is often a chancy thing to do without having real data or reports, it seems that lecanemab would most likely get regular marketing approval based on this one study alone.

The company considered the results to be very clinically meaningful, but Mr. Cheung said there were differing opinions on what clinical meaningfulness was for this stage of disease.

Aduhelm saw a novel treatment for Alzheimer's for the first time in two decades. The data from the two studies of gantenerumab will be reported by the end of the year. The F.D.A. is expected to make a decision on whether to grant accelerated approval to an Alzheimer's drug from Eli Lilly known as donanemab by early January, with results from a bigger study of that drug due in the middle of next year.

The results of the study on lecanemab set a high bar that will be hard for the other drugs to beat.