The trial results released on late Tuesday could mark a rare victory in the search for a treatment for the memory-robbing disease after years of clinical failures. Biogen's stock surged 46.6% to $290.99 in premarket trading, putting it on track to add over $13.5 billion in market capitalization. BMO analyst Evan Seigerman called the data an "unequivocal win" for Biogen and Alzheimer's patients. The latest trial boost for Biogen and Eisai's lecanemab has raised hope for success in trials testing Roche's gantenerumab and Lilly's donanemab. Shares of smaller Alzheimer's drug developers also rose in premarket U.S. trading.

---