Shares of Biogen Inc. /zigman2/quotes/201531540/composite BIIB -1.60% plummeted 26% toward a near 3-year low in premarket trade Thursday, after the biotechnology company and Japan's Eisai Co. Ltd. /zigman2/quotes/203064480/delayed JP:4523 +2.74% /zigman2/quotes/202565502/composite ESALY +0.71% said Thursday they have decided to discontinue late-stage trials of a treatment for Alzheimer's disease, after an independent data monitoring committee said they were unlikely to meet its primary goal. The phase 3 trials sought to evaluate the efficacy and saftey of aducanumab in patients with mild cognitive impairment due to Alzheimer's disease and mild Alzheimer's disease dementia. "This disappointing news confirms the complexity of treating Alzheimer's disease and the need to further advance knowledge in neuroscience," Biogen Chief Executive Michael Vounatsos said in a statement. The companies are planning to present detailed data from the trials at future medical meetings to help with ongoing research. A phase 2 safety study and a phase 1b study will also be discontinued. Aducanumab is an investigational compound being studied for treatment in early Alzheimer's. Biogen shares were on track to open at the lowest level seen during regular-session hours since July 2016. The stock has gained 15.6% over the past 12 months through Wednesday, while the S&P 500 /zigman2/quotes/210599714/realtime SPX -0.20% has advanced 4.1%.