Key Inclusion Criteria:  Participant is >= 18 years on the day of signing informed consent form Histologically or cytologically confirmed ES-SCLC Presence of measurable disease (at least one target lesion) according to RECIST v1.1 assessed by conventional computed tomography (CT) scan SSTR positive [68Ga]Ga-DOTA-TATE imaging positron emission tomography (PET) scan demonstrating uptake in at least one target or non-target lesion No prior systemic treatment for ES-SCLC ECOG status =< 1 Provision of tumor tissue to support exploratory biomarker analysis Life expectancy of >= 6 months  Key Exclusion Criteria:  Participant has received prior therapy with an antibody or drug against immune checkpoint pathways Active leptomeningeal disease or uncontrolled, untreated brain metastasis Active autoimmune diseases or history of autoimmune diseases that may relapse Severe chronic or active infections (including active tuberculosis, HBV, or HCV infection) requiring systemic antibacterial, antifungal or antiviral therapy within 2 weeks before Cycle 1 Day 1 Any major surgical procedure requiring general anesthesia =< 28 days before Cycle 1 Day 1 History or current diagnosis of electrocardiogram (ECG) abnormalities indicating significant risk of safety for participants participating in the study Known hypersensitivity to the active substances or any of the excipients of the study drugs Concurrent participation in another therapeutic clinical study