The European Commission on Tuesday authorized the use of Eisai and Biogen's drug to treat some patients with mild cognitive impairment in the early stages of Alzheimer's disease, ending a more than two-year-long review process.

This makes Leqembi the first approved drug in the European Union that targets an underlying cause of the fatal mind-wasting disease. It had been under regulatory review since January 2023.

Biogen's head of development, Priya Singhal, said the company and its partner Eisai were "moving with urgency" to make the therapy available to patients in Europe.

Rival Eli Lilly's Alzheimer's drug was rejected last month as its benefits were not significant enough to outweigh serious safety risks.

The authorization allows the use of Leqembi to treat people with only one or no copy of the ApoE4 gene and who exhibit sticky clumps of a protein called amyloid beta in the brain, which is believed to be a hallmark of Alzheimer's.

The decision to exclude those with two copies of the gene is the regulator leaning on the side of safety, said William Blair analyst Myles Minter. He estimates over $900 million peak sales for the drug in the 2030s.

The EC's decision is in line with that of the European Medicines Agency, which recently reiterated that Leqembi would be approved for a narrower set of patients than those in which it was tested.

The regulator had backed Leqembi for patients with only one copy of the ApoE4 gene, but the EC had requested another safety review. Initially, the European medicines regulator had refused to back the drug's approval due to serious safety risks.

Leqembi is approved in the United States for patients with two copies of the gene, but patients must undergo regular brain scans to monitor for any brain swelling.

Leqembi is also approved in Japan, China, Great Britain and several other markets.