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An experimental Alzheimers drug moderately slowed the effects of the disease but was linked to patient safety risks that warrant longer clinical trials, according to a study published late Tuesday.

The study, in the New England Journal of Medicine, found that a drug developed by Tokyo-based Eisai and Cambridge, Mass.-based Biogen reduced a key marker of Alzheimers disease, the amyloid beta protein, and that patients who received the drug performed better on cognitive and physical measures than a placebo group.

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But the detailed results also concluded that the drug, lecanemab, was associated with adverse events and warranted more study.

Marwan Sabbagh, a neurologist at the Barrow Neurological Institute and a co-author of the study, described two patient deaths that had raised concern about the safety of the drug ahead of Tuesdays presentation. Causality with lecanemab is a little difficult, he said, noting that both patients, a 65-year-old woman and an 87-year-old man, had underlying health issues. Though the rate of brain bleeding was low, he said, the risk increases with medications to prevent blood clotting.

That might be a relative risk that needs to be managed, he said at the Clinical Trials in Alzheimers Disease conference late Tuesday.

Eisai confirmed the two deaths Tuesday night and denied they were related to the drug.

The newdetails have been the subject of intense anticipation by doctors and Wall Street since Eisai and Biogen announced in September that lecanemab had slowed cognitive decline by 27 percent compared with a placebo.

Lecanemab has emerged as the front-runner among aclass of drugs that seeks to remove clumps of a protein in the brain called amyloid beta, which researchers have long suspected plays a role in Alzheimers. The Food and Drug Administration is set to make a decision on approving the drug as early as January. That could translate into a multibillion-dollar prize for treating a progressively debilitating disease that affects 6million Americans and hasfew approved therapies.

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Its the best data weve seen in Alzheimers in a pivotal trial, Myles Minter, a biotechnology analyst at William Blair, said of the initial results announced in September.

Lecanemabs positive results followed a controversy over a different drug developed by Eisai and Biogen, aducanumab. That drug, known by the brand name Aduhelm, was approved by the FDA last year despite conflicting data on its effectiveness. Aduhelmfizzled commercially after Medicare declined to broadly reimburse for it.

Lecanemab is designedto work by removing clumps of tiny proteins known as amyloids from the brain. Yet, despite the drugs early success, some experts remain skeptical that targeting these amyloids is the key to treating Alzheimers. This month, Roche announced disappointing results from its anti-amyloid drug.

Matthew Schrag, a neurology professor at Vanderbilt University Medical School, said the lecanemab trial was well-designed and showed strong statistical results on a cognitive measure. But he doubted that the drug would cause a noticeable improvement for many sufferers, and noted that the medication can cause significant side effects.

I worry any minor benefit may be washed out by the practical difficulties of living with the drug and the substantial risks associated with taking the drug, he said in an interview.

Eisai and Biogen reported that patients in the trial experienced brain swelling and bleeding, which are known to be complications of anti-amyloid drugs, but the companies said the rates were within expectations. Concerns about the drugs safety, however, were heightened by the two patient deaths.

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On Sunday, Science magazine reported that a patient taking lecanemab died after suffering a stroke and receiving a medication to bust blood clots. That followed a report last month by Stat news that another patient in the trial had died while on a blood-thinner. Both deaths reportedly stemmed from a condition where amyloid binds to blood vessels in the brain and makes them more susceptible to rupture.

Research analysts at investment bank UBS questioned whether the deaths would lead to FDA restrictions for patients taking blood-thinners, which it estimated could make up as many as 20 percent of Alzheimers patients.

Libby Holman, a spokeswoman for Eisai, said the patients who died had underlying medical conditions and risks, including being on medications that prevent blood clots, that contributed to their deaths. It is Eisais assessment that the deaths cannot be attributed to lecanemab, she said, adding that the rates of deaths were 0.1 percent for patients in both the placebo and treatment groups.

Despite unknowns about lecanemabs risk and benefits, it remains a draw for patients suffering from Alzheimers eager for any option to slow its degenerative effects and contribute to finding a cure.

Hugh Courtney, a 59-year-old economist diagnosed with early Alzheimers, said he feels lucky to have participated in the clinical trial even if he isnt sure how much hes benefited.

Its hard to tell how much has changed, quite frankly, he said of the lecanemab infusions he gets about twice a month. Still, he said, its given me a sense of purpose, a concrete way to help.