Background: Plerixafor inhibits the binding of stromal cell-derived factor 1 (SDF-1a / CXCL12) to its receptor on CD11b+ monocytes, inhibiting vasculogenesis and tumor recurrence in preclinical models. An early phase trial demonstrated that Plerixafor administration decreased cerebral blood volume (CBV) within the irradiated field of glioblastoma patients, and that the majority of recurrences occurred outside of the irradiated field (Thomas R. et al, Clin Cancer Res). This follow up study investigates the efficacy of Plerixafor in combination with WBRT, with the hypothesis that widening the radiation therapy (RT) field with WBRT may reduce out-of-field tumor recurrence and improve survival. Methods: This was a Phase II, open-label, non-randomized, single-arm trial.Adults between 18-75 years old with newly diagnosed high grade glioma as defined by WHO 2016 criteria and KPS ≥ 60 were eligible for enrollment. Patients underwent maximal safe resection followed by 6 weeks of concurrent radiation therapy and temozolomide (TMZ). Of note, modifications to the conventional treatment paradigm included completing 30 Gy of standard intensity-modulated RT followed by 30 Gy of WBRT, as well as completing a 4-week continuous infusion of Plerixafor dosed at 400 µg/kg/day. Patients then started adjuvant TMZ after completing Plerixafor. Primary endpoint was 6-month progression free survival (PFS6) after chemoradiation. Secondary endpoints included median overall survival (mOS), adverse events (AEs), MRI-based patterns of recurrence, and neurocognitive outcomes measured by the Trail Making Test, Controlled Oral Word Association Test, and the Hopkins Verbal Learning Test. Additionally, maximum arterial spin labeling-cerebral blood flow (ASL-CBF) values of primary lesions were compared over various time points during the study. Results: 14 of 17 enrolled patients completed the Plerixafor infusion, and all 17 patients completed WBRT. Median age was 57 years. 15 patients (88.2%) were IDH wildtype and 13 patients (76.5%) were pMGMT unmethylated. 13 patients (76.5%) underwent gross total resection, 2 (11.8%) underwent subtotal resection, and 2 (11.8%) underwent biopsy only. The PFS6 was 91.7%. The mOS was 15.11 months. 3 patients discontinued Plerixafor early due to rash (n=1), rising creatinine (n=1), or worsening confusion and agitation (n=1). Common Gr2 treatment-related AEs included alopecia (n=4), nausea (n=4), and vomiting (n=2). 1 patient had Gr3 alopecia. There were no Grade 4/5 AEs. 80% of patients scored worse on their neurocognitive assessments at 6 months compared to screening. ASL-CBF values of primary lesions appeared relatively stable throughout the study. Conclusions: The combination of Plerixafor and WBRT did not improve mOS compared to the original study using focused RT only. There was an overall decline in neurocognitive performance over the course of the study. Clinical trial information: NCT03746080.