Shares of Biogen Inc. fell 1.6% in premarket trading Friday, after the the biotechnology company and Tokyo-based Eisai Co. Ltd. /zigman2/quotes/202565502/delayed ESALY +1.08% /zigman2/quotes/203064480/delayed JP:4523 +13.74% said they will discontinue the phase 3 trial of an Alzheimer's disease (AD) treatment, citing data showing an unfavorable risk-benefit ratio. The decision was made after the Data Safety Monitoring Board's recommendation to discontinue the trials. The trial on the investigational oral beta amyloid cleaving enyzme (BACE) inhibitor elenbecestat in patients with early AD. The companies said as part of the decision to discontinue the phase 3 trial, the long-term extension of the phase 2 trial of elenbecestat will also be discontinued. Biogen's stock has dropped 22.8% year to date through Thursday and the U.S.-listed shares of Eisai have tumbled 34.6%, while the S&P 500 /zigman2/quotes/210599714/realtime SPX +0.27% has gained 20.1%.