Nearly all drugs tested to treat Alzheimer's - a fatal brain disease that affects an estimated 55 million globally - have stumbled in clinical trials.

Against the odds, Biogen and Eisai on Tuesday said their experimental drug, lecanemab, slowed progress of the brain-wasting disease by 27% compared with a placebo, in a large trial of patients in the early stages of Alzheimer's.

Biogen's stock surged 50% in early premarket trading. Shares of Eisai jumped 17% to the daily limit in Tokyo.

Shares of Roche, which is expected to report results on a rival Alzheimer's drug gantenerumab later this year, jumped as much as 6% to a two-month high in early trading.

Data on Eli Lilly's competing drug, donanemab, is expected by mid-2023. The U.S. drugmaker's stock was up 7.8% before the bell on Wednesday.

(Reporting by Natalie Grover in London)