Debio 0123

The primary purpose of the Phase 1 (Dose Escalation) of this study is to identify the dose-limiting toxicities (DLTs) of Debio 0123 combined with temozolomide (TMZ) (Arm A) and with TMZ and radiotherapy (RT) (Arms B and C) and to characterize the safety and tolerability of these combinations in adult participants with glioblastoma (GBM). Arm B which was previously added to the protocol, has been permanently halted per the safety monitoring committees' decision on the safety findings of this arm. The primary purpose of Phase 1 (Dose expansion) of the study is to assess the doses studied under Phase 1 (Dose Escalation) Arm A and identify the recommended dose (RD) for further development. The Phase 2 will start once the RD Phase 1 has been defined. The primary objective of Phase 2 is to assess the efficacy of Debio 0123 at the RD for further development in combination with TMZ, compared to the standard of care (SOC) in adult participants with GBM.