FDA’s approval of Biogen’s (NSDQ:BIIB) and Eisai’s (TYO:4523) Alzheimer’s drug aducanumab has resulted in significant gains in those companies’ shares. But Lilly’s (NYSE:LLY) donanemab could be “perhaps better and cheaper,” concluded a recent investor note from Truist. Lilly recently announced that it plans to file for accelerated approval for donanemab in the second half of 2021…