This is a Phase 1, single-arm MT-125 dose escalation study followed by a Phase 2, randomized parallel design dose expansion phase, to determine the safety and tolerability of MT-125 administered 5 consecutive days a week with 2 days off for the 6 weeks of outpatient treatment of Radiotherapy (RT) plus TMZ. Participants with newly diagnosed histologically or molecularly confirmed IDH wild type and MGMT methylated GBM will be eligible to enroll. The dose-limiting toxicity (DLT) observation window will be 6 weeks, starting with the first dose of MT-125 administration and chemoradiation. A Bayesian Optimal Interval (BOIN) adaptive trial design will be used to efficiently evaluate up to four dose levels. Secondary endpoints for this Phase 1/2 study will include determination of the MTD, the systemic PK of MT-125, and the RP2D of MT-125. Once the MTD is defined, additional participants will be enrolled as part of an expansion cohort, with a randomized parallel design, which will include up to 2 dose levels which are expected to be MTD and one dose below the MTD. Up to 36 participants will be enrolled in the study (up to 24 participants in the dose escalation phase and up to a total of 12 participants maximum per dose level for the two expansion cohorts).