A Study Evaluating the Safety, Pharmacokinetic and Anti-tumor Activity of RO7428731 in Participants With Glioblastoma

Prior treatment

• Any prior anti-tumor treatment for GBM: tumor resection, adjuvant radiotherapy with or without concomitant temozolomide and temozolomide maintenance (Part I only) must be the only tumor-directed treatment that the participant has received for GBM.
• Prior anti-EGFRvIII-targeting agents (including vaccines), anti-angiogenic therapy, and/or gene therapy for the treatment of GBM and gliomas.

Other illness

• Presence of extracranial metastatic or leptomeningeal disease
• Known hypersensitivity to immunoglobulins or to any other component of the investigational medicinal product formulation
• Active bleeding or pathological condition that carries a high risk of bleeding, including inherited and acquired coagulopathies
• Recurrent malignant gliomas

Other

• Participants with infratentorial tumors and tumors primarily located in or close to critical structures (e.g., brain stem)
• Participants unable to undergo an MRI with contrast.
• More than two recurrences of GBM

• Life expectancy of greater than or equal to 12 weeks, in the opinion of the Investigator
• Diagnosis of GBM based on World Health Organization (WHO) classification of central nervous system (CNS) tumors, 5th edition
• Participants must have confirmed EGFRvIII-expression
• Karnofsky Performance Status (KPS) Score of \>=70%
• Adequate organ functions prior to start of study treatment
• Willingness to abide by contraceptive measures for the duration of the study.
• Participants whose tumors have an unmethylated (Part I and Part II) or methylated (Part I only) O6-methylguanine-DNA methyltransferase (MGMT) promotor status based on local assessment
• Participants (in Part I): Adult participants with newly diagnosed EGFRvIII-positive GBM with unmethylated MGMT promotor status who have completed standard of care therapy with surgical resection and adjuvant radiotherapy with or without concomitant temozolomide. Participants are allowed to have received any number of cycles of temozolomide maintenance. Adult participants with newly diagnosed EGFRvIII-positive GBM with methylated MGMT promotor status who have completed standard of care with surgical resection and adjuvant radiotherapy with concomitant and maintenance temozolomide or discontinued temozolomide maintenance due to reasons other than progressive disease.
• Participants (in Part II): Adult participants with newly diagnosed EGFRvIII-positive GBM with unmethylated MGMT promotor status who have completed standard of care therapy with surgical resection and adjuvant radiotherapy with or without concomitant temozolomide.
• Documented first or second recurrence of GBM
• At least one measurable GBM lesion as per Response Assessment in Neuro-Oncology (RANO) criteria prior to initiation of study treatment.