Biogen's lead balloon

Biogen took a 29% haircut to $226.88 on Thursday following its decision to discontinue the Phase III ENGAGE and EMERGE trials of aducanumab for Alzheimer’s disease. The company's stock price has been dragged down 48% from a high of $476 in March 2015, when it announced detailed data from the Phase Ib PRIME trial. That trial formed the basis of its decision to advance the program to Phase III (see "Biogen, Eisai Discontinuing Aducanumab Studies in AD").

The ENGAGE and EMERGE trials have been viewed as the rigorous test of the amyloid hypothesis, interrogating whether aducanumab’s ability to cause reductions in plaque, seen in PRIME, would parlay into improvements in cognition...