Shares of Athersys Inc. fell sharply May 20 after its partner and top investor Healios K.K. reported that Multistem (invimestrocel), the experimental cell therapy they've co-developed since 2016, missed the primary endpoint of a Japanese phase II/III ischemic stroke study called Treasure. Many Athersys investors appeared to read the results as bad news for Athersys' ongoing pivotal phase III stroke study, Masters-2.