This study will evaluate the safety of tobemstomig alone or in combination with tiragolumab compared with atezolizumab in participants with previously untreated, locally advanced or metastatic urothelial cancer (mUC) who are ineligible to receive a platinum containing chemotherapy.

Inclusion Criteria:

• Eastern Cooperative Oncology Group (ECOG) Performance Status of ≤ 2
• Histologically or cytologically documented locally advanced or metastatic transitional cell carcinoma (TCC) of the urothelium. Participants with squamous, sarcomatoid, micropapillary, and glandular variant histologies are eligible for inclusion in the study, provided that a urothelial component is present in the tumor specimen. Participants with other variant histologies or pure variant histologies are not eligible for inclusion in this study
• Ineligible ("unfit") to receive platinum-based chemotherapy
• No prior chemotherapy for inoperable locally advanced or metastatic or recurrent urothelial carcinoma (UC)
• Measurable disease; at least one measurable lesion as defined by response evaluation criteria in solid tumors, version 1.1 (RECIST v1.1)
• Availability of a representative leftover tumor specimen that is suitable for determination of PD-L1 status as assessed by a central laboratory
• Adequate hematologic and end organ function
• Negative for hepatitis B and hepatitis C virus (HCV)
• Adequate cardiovascular function

Exclusion Criteria:

• Pregnancy or breastfeeding
• GFR <15 mL/min/1.73 m2
• Symptomatic, untreated, or actively progressing central nervous system (CNS) metastases
• History of leptomeningeal disease
• Uncontrolled tumor-related pain
• Uncontrolled pleural effusion, pericardial effusion, or ascites requiring recurrent drainage procedures
• Uncontrolled or symptomatic hypercalcemia
• Active or history of autoimmune disease or immune deficiency
• History of idiopathic pulmonary fibrosis, organizing pneumonia, drug-induced pneumonitis, or idiopathic pneumonitis, or evidence of active pneumonitis on screening chest computed tomography (CT) scan
• Active tuberculosis (TB) or acute Epstein-Barr virus (EBV)
• Significant cardiovascular/cerebrovascular disease within 3 months prior to initiation of study treatment
• Major surgical procedure, other than for diagnosis, within 4 weeks prior to initiation of study treatment, or anticipation of need for a major surgical procedure during the study
• History of another primary malignancy other than urothelial carcinoma within 2 years prior to initiation of study treatment, with the exception of malignancies with a negligible risk of metastasis or death
• Severe infection within 4 weeks prior to initiation of study treatment
• Treatment with therapeutic oral or intravenous antibiotics within 2 weeks prior to initiation of study treatment. Participants receiving prophylactic antibiotics (e.g., to prevent a urinary tract infection or chronic obstructive pulmonary disease [COPD] exacerbation), or who are receiving oral antibiotics to treat a urinary tract infection are eligible for the study
• Prior allogeneic stem cell or solid organ transplantation
• Treatment with a live, attenuated vaccine within 4 weeks prior to initiation of study treatment, or anticipation of need for such a vaccine during treatment or within 5 months after the final dose of atezolizumab, 4 months after the final dose of tobemstomig, or 90 days after the final dose of tiragolumab
• Current treatment with anti-viral therapy for HBV
• Treatment with any approved anti-cancer therapy, including chemotherapy or hormonal therapy, within 3 weeks prior to initiation of study treatment
• Treatment with investigational therapy within 4 weeks or 5 drug-elimination half-lives (whichever is longer) prior to initiation of study treatment
• Prior treatment with CD137 agonists or immune checkpoint blockade therapies, including anti-TIGIT and anti-LAG3 therapeutic antibodies or pathways targeting agents
• Treatment with systemic immunostimulatory agents within 4 weeks or 5 drug-elimination half-lives prior to initiation of study treatment
• Treatment with systemic immunosuppressive medication within 2 weeks prior to initiation of study treatment, or anticipation of need for systemic immunosuppressive medication during study treatment
• History of severe allergic anaphylactic reactions to chimeric or humanized antibodies or fusion proteins

For eligible studies, qualified researchers may request access to individual patient level clinical data. See Roche's commitment to transparency of clinical study information here: https://go.roche.com/data\_sharing