∙ Common Criteria:

• Participant is \>= 18 years on the day of signing informed consent form

• Histologically confirmed glioblastoma

• Adequate bone marrow, organ function and electrolyte values

∙ Newly diagnosed glioblastoma (Group 1):

• Presence of Gadolinium enhancing tumor in pre-surgery magnetic resonance imaging (MRI)

• Karnofsky Performance Score (KPS) \>= 70 %

∙ Recurrent glioblastoma (Group 3 dose Escalation only):

∙ • Participant has experienced first or second recurrence of their glioblastoma, after standard or experimental therapy that includes prior EBRT

∙ Recurrent glioblastoma (Group 3 dose escalation and expansion):

• Evidence of recurrent disease demonstrated by disease progression using modified Response Assessment in Neuro-Oncology (mRANO) criteria

• KPS \>= 60 %

• \[68Ga\]Ga-DOTA-TATE uptake by positron emission tomography/computed tomography (PET/CT) or PET/MRI at the tumor region

• Presence of Gadolinium enhancement in the tumor region in MRI at the time of diagnosis of tumor recurrence

• A second surgery for glioblastoma is allowed provided that the following criteria are met:

‣ Residual and measurable disease post-surgery is not required but surgery must have confirmed the diagnosis of recurrence

⁃ Surgery completed at least 2 weeks prior to study treatment initiation, with post-surgery recovery without any complications related to surgical procedure

‣ Recurrent glioblastoma (Group 3 Dose Expansion only):

• Patients experiencing first recurrence of their glioblastoma, after standard or experimental therapy that includes prior EBRT