Experts applauded full data released on Wednesday showing that a new drug can slow cognitive decline in Alzheimer's patients but cautioned that the improvements were comparatively small and the treatment could have serious side effects.

Preliminary data from a lecanemab trial revealed that it slowed cognitive decline by 27% over an 18-month period.

The full trial data, published in the New England Journal of Medicine, expands on those findings while also raising concerns about the frequency of "adverse effects", such as brain bleeds and swelling.

According to the findings, 17.3% of patients given the drug experienced brain bleeds, compared to 9% of those having a placebo. In addition, 12.6% of those taking the drug experienced brain swelling, compared to 1.7% in the placebo group.

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Deaths were reported at roughly the same rate in both arms of the drug trial, which was developed by Biogen and Eisai.

The findings were widely applauded by researchers and advocates for dementia patients, including Bart De Strooper, director of the UK Dementia Research Institute. "This is the first drug that provides a real treatment option for people with Alzheimer's," he said.

"While the clinical benefits appear somewhat limited, it can be expected that they will become more apparent if the drug is administered over a longer time period."

Longer trials needed 

Regarding Alzheimer's disease, two key proteins, tau and amyloid beta, build up into tangles and plaques, known together as aggregates, which cause brain cells to die and lead to brain shrinkage.

Lecanemab works by targeting amyloid, and according to De Strooper, the drug not only cleared it but also had "beneficial effects on other hallmarks of Alzheimer's, including tau."

The phase three trial lasted for 18 months and involved nearly 1,800 people who were randomly assigned to either the drug or a placebo.

They were evaluated using an Alzheimer's disease clinical scale that measures cognition and function, as well as changes in amyloid levels and other indicators. However, Tara Spires-Jones, programme lead at the UK Dementia Research Institute, noted that "there is not an accepted definition of clinically meaningful effects in the cognitive test they used."

"It is not clear yet whether the modest reduction in decline will make a big difference to people living with dementia. Longer trials will be needed to be sure that the benefits of this treatment outweigh the risks," she added.

Additionally, the drug only targets those in the early stages of the disease with a certain level of amyloid buildup, limiting the number of people who could potentially benefit from the treatment. And as Alzheimer's is not always caught quickly, some experts said an overhaul in early diagnosis would be needed to ensure more people could benefit.

Previously, Biogen and Eisai put the Alzheimer's drug Aduhelm on the market, but there was significant controversy over the evidence that it worked, and its approval resulted in the resignations of three high-level FDA officials.