Abstract only Background: Early infarct growth after endovascular therapy (EVT) may lead to unfavorable outcomes despite successful reperfusion. The REPERFUSE-NA1 study replicated the preclinical NA1 experiment by investigating the effect of NA1 on early DWI infarct growth in acute ischemic stroke patients receiving EVT. Methods: REPERFUSE-NA1 was a sub-study of the randomized controlled trial ESCAPE-NA1 (ClinicalTrialGov NCT02930018). Patients received MRI within 5 hours and 24 hours of EVT. The primary outcome was early diffusion weighted (DWI) infarct growth. The secondary outcome was region-specific DWI infarct growth in the white matter, cortical grey matter and basal ganglia. Results: A total of 71 patients was included, of whom 67 had sufficient MR imaging at 5h and 24h post-EVT. For patients who received NA1 compared to placebo, the median age (68.8 v 67.5), baseline NIHSS (15.5 v 16), time from symptom onset to reperfusion (161 v 167 minutes) and mTICI 2b-3 (94.4% v 94.3%) were statistically not different. Median DWI volumes post-EVT (5h) were 13.0 mL (IQR, 5.9-28.1) in NA1 and 13.3 mL (IQR, 3.1-27.0) in placebo. At 24h median DWI volumes increased to 22.6 mL (IQR, 11.2-63.4) in the NA1 group and 22.4 mL (IQR, 7.4-52.3) in the placebo group, equating to a 48.4% DWI volume growth in the NA1 group and a 66.0% growth in the placebo group. Median DWI volume growth was 55.1% for NA1 patients who received alteplase compared to 41.3% for NA1 patients who did not receive alteplase (p=0.65). In terms of region-specific infarct volumes, absolute growth was 2.27mL (IQR, 0.70-6.19) in the white matter, 3.93 mL (IQR, 0.18-12.40) in cortical grey matter, and 1.20 mL (IQR, 0.31-2.62) in basal ganglia in the overall cohort. Between the NA1 and the placebo groups, lesion growth in the white matter (1.96 mL v 2.77mL), cortical gray matter (3.93 mL v 4.07 mL) and basal ganglia (1.62 mL v 0.89 mL) did not differ significantly. Conclusion: There was substantial infarct growth early after EVT, with the largest absolute growth in cortical grey matter. No significant effect of NA1 on early infarct growth was observed.