** Shares of Biogen BIIB.O rise 10% to $227.28 premarket

** Co says it plans to advance its experimental Alzheimer’s treatment diranersen to late-stage development

** Says the therapy showed robust reduction in buildup of tau, an Alzheimer's-related protein, and demonstrated cognitive benefit in patients during a mid-stage trial

** BIIB says the therapy failed to show that higher doses worked better than lower ones on a standard scale used to track dementia severity

** Shares of Denali Therapeutics DNLI.O, which also has an Alzheimer's treatment in development, rose 8.6% premarket

** Up to last close, BIIB up 16.2% YTD