Background: Proteasome inhibition sensitizes glioma cells to TMZ and RT, providing a novel therapeutic strategy for GBM. MRZ, an irreversible, brain-penetrant, pan-proteasome inhibitor with anti-glioma activity was combined with standard TMZ/RT → TMZ in newly diagnosed GBM (NCT02903069), to determine the recommended dose (RD). The primary endpoint of this expanded phase 1 trial was toxicity, with secondary endpoint of OS. Methods: Patients were enrolled in separate cohorts (TMZ/RT+MRZ→TMZ+MRZ, N=15; TMZ/RT→TMZ+MRZ, N=18) in dose-escalation (3+3 design), followed by dose-expansion (N=20) with TMZ/RT+MRZ at RD → TMZ+MRZ at RD. A separate cohort received TMZ/RT→TMZ+MRZ at RD with Tumor Treating Fields (Optune, N=13). MRZ was infused IV (10 min at 0.55, 0.7, 0.8, and 1.0 mg/m²) on Days 1, 8, 15, 29, 36 (42-day TMZ/RT+MRZ cycle) and Days 1, 8, 15 (28-day TMZ+MRZ cycle). Results: 66 patients treated; median age 58 years, 68% male, 50% receiving corticosteroid at baseline, 52% unmethylated MGMT. Dose-limiting toxicities (DLTs) in dose-escalation cohorts: 1 (fatigue) at 0.7 mg/m² MRZ, 5 (ataxia/diarrhea; ataxia/confusion; myocardial infarction, delirium/ataxia; ataxia/fatigue) in 1.0 mg/m² cohorts. MRZ demonstrated a steep dose-response with treatment-emergent adverse events (TEAEs)/DLTs predominately CNS AEs (Grade ≥3 TEAEs in 12 of 12 patients at 1.0 mg/m² vs 22 of 41 patients at ≤0.8 mg/m²); the RD for MRZ was determined to be 0.8 mg/m². Most common TEAEs (all grades): fatigue, nausea (both 70%), hallucination (54%), vomiting (53%), headache (47%), confusional state (33%), ataxia, constipation, muscular weakness (all 29%). Conclusions: CNS TEAEs were short-lasting, reversible and ameliorated by early dose reductions (29% patients dose-reduced), allowing patients to remain on treatment. For patients receiving MRZ with TMZ/RT→TMZ (N=35), the median OS was 14.8 months (17 deaths, median follow-up 14.3 months), and 7 patients remain active (Cycles 11-23). The MRZ RD + TMZ/Optune combination was tolerated, with 4 of 13 patients treated on this arm remaining active. An international Phase 3 trial (EORTC 1709-BTG/CCTG CE.8, NCT03345095) is ongoing. Clinical trial information: NCT02903069