Background: Glioblastoma (GBM) and tumor endothelial cells express phosphatidylserine (PS), a highly immunosuppressive membrane phospholipid. PS receptors engage with immune cells, leading to expansion of myeloid-derived suppressor cells (MDSCs) which promote an immunosuppressive and pro-angiogenic tumor microenvironment. Bavituximab (BAV) – a chimeric monoclonal antibody – binds to β2-glycoprotein 1 (β2-GP1) to form a complex of β2-GP1 with PS, resulting in immune activation against tumor cells and anti-angiogenic effects. Pre-clinical data in GBM models suggest synergistic effects of PS blockade, radiation (RT), and temozolomide (TMZ). Here, we present results from a phase II trial (NCT03139916) of BAV, RT and TMZ in GBM patients. Methods: 33 adults with newly diagnosed IDH-wild-type GBM were enrolled and received 6 weeks of RT+TMZ, followed by 6 cycles of TMZ. BAV (3 mg/kg) was given weekly, starting at week 1 of RT+TMZ, for 18 weeks with the option to continue if tolerated. The primary endpoint was the proportion of patients alive at 12 months (OS-12). The null hypothesis would be rejected if OS-12 was ≥ 72%. As an exploratory endpoint, the immune profile in tumor tissue and peripheral blood mononuclear cells (PBMCs) was assessed using nanoString and multispectral immunofluorescence, with the goal to assess on-target effects of BAV in longer vs. shorter surviving patients (split based on median survival). Relative cerebral blood flow (rCBF) from dynamic susceptibility contrast MRI was also obtained. Results: 24 patients were alive at 12 months and OS-12 was 73% (95% CI 59-90%) so the study met its primary endpoint. Median OS was 15.4 months. As best response, 79% of patients had stable disease, 12% had a partial response and 9% had progressive disease. Eight grade 3 or 4 adverse events were seen (no grade 5 AEs). Ten pre-treatment and 7 post-treatment tissue samples were available. Analysis of RNA from pre-treatment tumor specimens showed a significantly positive shift in myeloid-related gene expression in patients with longer survival, with enrichment of 116 and 120 transcripts as well as downregulation of 2 and 1 gene for PFS and OS, respectively. There was no differential expression in PBMCs. Including all tissue samples, there was a marked reduction of MDSCs after BAV compared to time of diagnosis (p = 0.011). Decreased rCBF post-RT/pre-cycle 1 TMZ was associated with improved OS (HR 4.63, p = 0.029). Conclusions: OS-12 was 73%, meeting the primary endpoint and suggesting potential activity of BAV in newly diagnosed GBM. BAV leads to on-target depletion of intratumoral immunosuppressive MDSCs and anti-angiogenic effects. As expected, based on the mechanism of action of BAV, there was no difference in PBMC gene expression profile in patients with long and short survival. Combining BAV with immune checkpoint inhibitors in the future may augment tumor immune response. Clinical trial information: NCT03139916.