** 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
(Reporting by Christy Santhosh)
((Christy.Santhosh@thomsonreuters.com))