A Study to Evaluate the Efficacy and Safety of BIIB131 for Participants With Ischemic Stroke Between 4.5 and 24 Hours After Last Known Well

The primary objective of the study is to evaluate the effects of BIIB131 on arterial revascularization (Part 1) and to determine if BIIB131 improves functional outcome as measured by the Modified Rankin Scale (mRS) when compared with placebo following acute ischemic stroke (AIS) (Part 2). The secondary objectives are to evaluate the effects of BIIB131 on angiographic reperfusion and infarct evolution, to determine if BIIB131 improves functional outcome, pharmacokinetic profile of BIIB131 (Part 1); to evaluate the effects of BIIB131 on acute and 90-day clinical outcomes (Part 2).