The goal of this clinical trial is to learn if AB126, an investigational treatment made from neural exosomes, is safe and well tolerated when given to adults with moderate to severe acute ischemic stroke (AIS) who have undergone endovascular thrombectomy (EVT) but have had limited neurological improvement after the procedure. The study will also evaluate different doses of AB126 to help determine the highest dose that can be given safely. The main questions it aims to answer are: 1. Is AB126 safe and well tolerated when given to participants with AIS following EVT? 2. What dose of AB126 can be given safely? 3. Does treatment with AB126 provide preliminary evidence of improved neurological recovery following stroke? Researchers will compare participants who receive AB126 with participants who receive placebo to evaluate the safety and tolerability of AB126 and to collect preliminary information about recovery following stroke. Participants will: 1. Be randomly assigned to receive either AB126 or placebo. Neither participants nor the study team will know which treatment is assigned. 2. Receive 3 intravenous (IV) doses of AB126 or placebo. The first dose will be given as soon as possible after EVT, but no later than 6 hours after blood flow has been restored. The second and third doses will be given approximately 24 and 48 hours after EVT. 3. Undergo medical examinations, laboratory tests, neurological assessments, and brain imaging during the study. 4. Remain under observation at a comprehensive stroke center until hospital discharge. 5. Complete follow-up assessments by telephone and in person for approximately 1 year after treatment, including assessments at approximately 10, 30, 90, 180, 270, and 360 days after treatment.