Background: Standard of care (SOC) treatment achieves poor clinical outcomes in patients with glioblastoma (GBM), particularly in the absence of MGMT promoter hypermethylation. Preclinical models suggest that GBM recurrence is facilitated by CXCL12-mediated recruitment of bone marrow-derived cells capable of vasculogenesis after radiotherapy (RT). We have recently reported favorable safety and feasibility data of the phase I/II GLORIA trial, which combines RT and the CXCL12-neutralizing L-RNA aptamer olaptesed pegol (NOX-A12) in patients with newly diagnosed, incompletely resected or unresected GBM lacking MGMT promoter hypermethylation (NCT04121455). Here we report on clinical outcomes and their correlation with potential biomarkers. Methods: 10 patients with newly diagnosed incompletely resected (n=8) or biopsied (n=2) GBM with ECOG≤2, age≥18 and without MGMT promoter hypermethylation were enrolled. All patients received standard RT (60 Gy in 30 fractions or 40.05 Gy in 15 fractions) and escalating dose levels of continuous (24/7) i.v. infusions of NOX-A12 for 26 weeks. While the previously reported primary endpoint was safety, secondary efficacy endpoints included progression-free survival (PFS) and overall survival (OS). Biomarker assessment was performed with a six-plex immunofluorescence staining (CODEX) of neuropathologically confirmed tumor areas of pretreatment tissues for CXCL12, CD31 (endothelial cells), GFAP (glioma cells), CD68 (macrophages, microglia), aSMA (pericytes), Ki-67 (proliferating cells) and a nuclear marker. An independent patient cohort treated with SOC (n=15) matched by clinical and histopathological features was used for comparisons of efficacy and biomarker assessment. Results: Median PFS of the GLORIA cohort was 5.7 (range 1.9–8.5) months and the median OS was 12.7 (4.7–18.4) months. Biomarker analyses revealed that a higher frequency of CXCL12 expressing endothelial and glioma cells (EG12 score) significantly correlated with PFS (r=0.87; p=0.005) in patients treated with RT and NOX-A12, but not with SOC (r=-0.10; p=0.724). GLORIA patients with a high EG12 score (median classifier) had a significantly longer PFS than those with lower scores (3.0 vs. 6.0 months; p=0.031) and a trend towards prolonged OS (11.1 vs. 15.8 months; p=0.075). These correlations were not seen in the reference cohort treated with SOC (PFS: 6.0 vs. 4.6 months; p=0.502; OS: 10.0 vs. 9.6 months; p=0.243). Conclusions: We show superior clinical efficacy of RT and NOX-A12 in patients with high frequency of CXCL12 expressing endothelial and glioma cells, suggesting the use of the EG12 score as a novel predictive biomarker for CXCL12-directed therapies in GBM. Clinical trial information: NCT04121455.