Shares of Biogen and other drugmakers researching Alzheimer’s disease have soared after Japan’s Eisai Co. said its potential treatment appeared to slow the fatal disease in a late-stage study. Eisai announced results late Tuesday from a global study of nearly 1,800 people with early-stage Alzheimer’s. The drugmaker said early results showed that its treatment, lecanemab, reduced patient clinical decline by 27% when compared to a placebo or fake drug after 18 months of the infused treatment. Patients were monitored using a scale that measures how they do in areas like memory, judgement, problem solving and personal care.