Shares of Biogen Inc. /zigman2/quotes/201531540/composite BIIB +3.38% gained 5.6% in trading on Monday after another drugmaker shared positive clinical data about an experimental Alzheimer's disease treatment that is similar to a Biogen therapy under review by the Food and Drug Administration. Eli Lilly & Co. /zigman2/quotes/200106384/composite LLY +2.72% on Monday said its investigational drug, donanemab, slowed cognitive decline in patients with early forms of Alzheimer's in a mid-stage clinical study. The Lilly and Biogen drug candidates are in the same class of therapies. Both therapies focus on beta amyloid, which has been a somewhat controversial target for drug developers trying to figure out how best to treat people with Alzheimer's disease. The FDA is expected to decide whether to approve Biogen's drug, aducanumab, by March 7. "Though reported results are limited, today's topline is likely to reignite the bull debate on the amyloid hypothesis," RBC Capital Markets analyst Brian Abrahams told investors on Monday. "However, we believe key differences between donanemab and aducanumab and questions remain." Biogen's stock is down 9.8% over the last year. The S&P 500 /zigman2/quotes/210599714/realtime SPX +1.78% is up 17.5%.