biogen stock is slumping after the biotech giant disclosed the failure of a trial of its experimental drug tofersen as a treatment for amyotrophic lateral sclerosis or als on sunday though said that there were some hopeful signs in the data tofersen drug failed to slow disease progression in als patients though the company said that “the totality of evidence” showed that participants who started on tofersen earlier had “better clinical outcomes” biogen also said that “trends favoring tofersen” were observed in the trial