OBJECTIVE: Agonism of protease-activated receptor (PAR) 1 by activated protein C (APC) provides neuro- and vasculoprotection in experimental neuroinjury models. The pleiotropic PAR1 agonist, 3K3A-APC, reduces neurological injury and promotes vascular integrity; 3K3A-APC proved safe in human volunteers. We performed a randomized, controlled, blinded trial to determine the maximally tolerated dose (MTD) of 3K3A-APC in ischemic stroke patients. METHODS: The NeuroNEXT trial, RHAPSODY, used a novel continual reassessment method to determine the MTD using tiers of 120, 240, 360, and 540 µg/kg of 3K3A-APC. After intravenous tissue plasminogen activator, intra-arterial mechanical thrombectomy, or both, patients were randomized to 1 of the 4 doses or placebo. Vasculoprotection was assessed as microbleed and intracranial hemorrhage (ICH) rates. RESULTS: Between January 2015 and July 2017, we treated 110 patients. Demographics resembled a typical stroke population. The MTD was the highest-dose 3K3A-APC tested, 540 µg/kg, with an estimated toxicity rate of 7%. There was no difference in prespecified ICH rates. In exploratory analyses, 3K3A-APC reduced ICH rates compared to placebo from 86.5% to 67.4% in the combined treatment arms (p = 0.046) and total hemorrhage volume from an average of 2.1 ± 5.8 ml in placebo to 0.8 ± 2.1 ml in the combined treatment arms (p = 0.066). INTERPRETATION: RHAPSODY is the first trial of a neuroprotectant for acute ischemic stroke in a trial design allowing thrombectomy, thrombolysis, or both. The MTD was 540 µg/kg for the PAR1 active cytoprotectant, 3K3A-APC. A trend toward lower hemorrhage rate in an exploratory analysis requires confirmation. CLINICAL TRIAL REGISTRATION: Clinical Trial Registration-URL: http://www.clinicaltrials.gov. Unique identifier: NCT02222714. ANN NEUROL 2019;85:125-136.