A Biogen facility in Cambridge, Massachusetts. Brian Snyder | Reuters Biogen plans to advance an experimental drug for Alzheimer's disease to late-stage testing despite disappointing mid-stage trial data, the company said Thursday. Biogen said its experimental drug that targets tau, a protein associated with the memory-robbing disease, failed to show better responses at higher doses. Nonetheless, Biogen plans to move it into Phase 3 testing because of signals suggesting the treatment decreases levels of tau and slows cognitive decline, particularly at the lowest dose. Dr. Priya Singhal, Biogen's head of development, said the results are compelling. "We're really excited that we've been able to demonstrate an unprecedented combination of tau reduction in pathology and the cognitive benefit and have been really getting close to isolating a dose," she said.…