** Shares of drugmaker Biogen BIIB.O fall 3.9% to $196.45

** Co says experimental Alzheimer's drug diranersen missed the main goal of its mid-stage study, which tested whether higher doses provided greater benefit in patients with early Alzheimer's disease

** Says the drug, however, slowed decline in memory, thinking and daily functioning in the trial

** A 60 mg dose slowed decline by 42% and 50% on two key memory and thinking tests, with most results statistically significant

** All doses reduced levels of tau, a protein linked to Alzheimer's disease, co says

** Biogen plans a late-stage confirmatory trial; FDA granted fast track status to diranersen in 2025

** As of last close, stock up ~19% YTD