Shares of Biogen Inc. /zigman2/quotes/201531540/composite BIIB -0.32% bounced 1.7% in premarket trade Monday, after the drugmaker disclosed that it set a new $5.0 billion stock repurchase program, without an expiration date. The new program is in addition to the $1.7 billion remaining under the program set in August 2018. The new buyback announcement comes after the stock plummeted 32.4% to close Friday at the lowest level since June 2016, after the company said it would discontinue its phase 3 trials of its Alzheimer's treatment, citing results of an interim analysis of trial data. Based on Friday's stock closing price of $216.71, the total buyback authorization would represent about 30.92 million shares, or 15.7% of the shares outstanding. Biogen's stock has shed 16.7% over the past 12 months, while the iShares Nasdaq Biotechnology ETF /zigman2/quotes/206189322/composite IBB +0.51% has gained 2.7% and the S&P 500 /zigman2/quotes/210599714/realtime SPX -1.12% has advanced 8.2%.