Shares of biotech AC Immune SA /zigman2/quotes/202776583/composite ACIU +5.43% slid 15% in premarket trade Thursday, after the company said a a trial of a treatment for Alzheimer's disease failed to slow or prevent cognitive decline in patients with a specific genetic mutation that causes early-onset disease. The trial involved 252 people who are members of the world's biggest extended family with Autosomal Dominant Alzheimer's Disease, or ADAD, in Colombia. Some two thirds of those enrolled had the Presenilin 1 E280A mutation which typically causes cognitive impairment due to Alzheimer's disease around age 44. The participants were randomized to receive the treatment called crenezumab or placebo over a period of five to eight years. Crenezumab was well tolerated and no new safety issues were identified. The company will present initial data at the Alzheimer's Association International Conference on August 2. "While we are disappointed that the primary endpoints were not met, crenezumab's safety profile and the favorable numerical differences observed across primary, secondary and exploratory endpoints, warrant further analyses of the data," AC Immune CEO Dr. Andrea Pfeifer said in a statement. AC Immune shares are down 42% in the year to date, while the SPDR S&P Biotech ETF /zigman2/quotes/205950134/composite XBI +1.16% has fallen 41% and the S&P 500 /zigman2/quotes/210599714/realtime SPX +0.37% has fallen 20%.