Shares of Biogen Inc. (NASDAQ: BIIB) were up 15.2% as of 11:35 a.m. EDT on Friday after the biotech, along with partner Eisai, reported encouraging results from a phase 2 clinical study of BAN2401 in treating Alzheimer's disease. Patients taking BAN2401 achieved statistically significant improvement compared to patients on placebo after 18 months in slowing progression of Alzheimer's disease and in the reduction of amyloid accumulations in the brain.The two drugmakers enrolled 856 patients with early Alzheimer's disease in the phase 2 clinical study.