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The European Medicines Agency (EMA) has for an Alzheimer’s treatment which slows cognitive decline.

The EMA said the benefits of lecanemab did not counterbalance the risk of serious side effects, especially bleeding and swelling in the brain.

The medicines regulator in the UK, the MHRA, is still considering whether to grant a licence, and a decision is expected soon. The drug was approved in the United States earlier this year.

In trials, lecanemab was shown to slow cognitive decline by about a quarter in patients in the early stages of Alzheimer’s.

Alzheimer’s researchers hailed the trial results as "historic" because no previous drug had convincingly shown that the underlying mechanism of the disease could be slowed.

Lecanemab works by clearing a rogue protein called amyloid, which builds up in the brains of Alzheimer’s patients.

Even if the drug was to be approved in the UK, it would then be up to health bodies - such as the National Institute for Health and Care Excellence (NICE) and the Scottish Medicines Consortium (SMC) - to decide whether the drug represents value for money and should be recommended to patients.

Lecanemab costs about £20,000 per patient per year in the US.

The EMA said that although patients given lecanemab, whose brand name is Leqembi, had delayed cognitive decline, the effects were small.

It said the most important safety concern was the frequent occurrence of amyloid-related imaging abnormalities (ARIA), that involve swelling and bleeding in the brain.

The EMA said: “Although most cases of ARIA in the main study were not serious and did not involve symptoms, some patients had serious events, including large bleeds in the brain which required hospitalisation.”

Overall it found the benefits of the treatment were not enough to outweigh the risks.

Prof Tara Spires-Jones, president of the British Neuroscience Association, said the EMA’s decision will come as "a disappointment to many".

But she said there were reasons to remain hopeful.

"Lecanemab has shown that it is possible to slow down disease progression, and research does work. Now we need to ramp up our efforts to discover new and safer treatments,” Prof Spires-Jones said.

Prof John Hardy, professor of neuroscience and group leader at the UK Institute for Dementia Research, University College London (UCL), said the decision could have unintended consequences.

"I am sure we will now see rich people with early Alzheimer’s disease flying to the US or other jurisdictions for treatment.”

The ����ý’s Panorama followed patients on lecanemab and another new drug donanemab.

In the programme, broadcast earlier this year, Prof Cath Mummery, consultant neurologist and head of clinical trials at the Dementia Research Centre at UCL, said although the benefits of the drugs were small, they represented a “turning point”.

She added: “I do not think that they're a false hope. For the first time, we've got drugs that show that you can alter the course of Alzheimer’s disease, and that's an extraordinary thing.”

The Alzheimer's Society said while it respected the decision on the drug, it would be "really difficult news for people in Europe who may have been eligible for this drug".

“Whatever the regulators decide, we remain at an important and exciting moment," said Mark MacDonald, from the society.

"There are currently 164 active clinical trials for Alzheimer’s disease, and we expect more treatments to be submitted for MHRA approval in the future."