The purpose of this study is to evaluate the safety, tolerability, pharmacokinetics (PK), pharmacodynamics (PD) and preliminary anti-tumor activity of RO7616789. The study will have 3 parts: Dose Escalation (Parts 1 and 2) and Dose Expansion (Part 3). Participants with advanced stage small cell lung cancer (SCLC) and neuroendocrine carcinoma (NEC) will be enrolled in the study.

Inclusion Criteria:

• Life expectancy at least 12 weeks
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
• Adequate hematologic and end organ function
• Negative serum pregnancy test.
• Adequate contraception and no or interruption of breastfeeding
• Histologically confirmed extensive SCLC or poorly differentiated NEC of any other origin, relapsed after at least 1 systemic therapy
• Measurable disease according to Response Evaluation criteria in Solid Tumors (RECIST) Version 1.1
• Confirmed availability of representative archival tumor specimens in formalin-fixed, paraffin-embedded (FFPE) blocks or unstained slides

Exclusion Criteria:

• Pregnant or breastfeeding, or intending to become pregnant during the study or within 40 days after the final dose of study treatment
• Poorly controlled Type 2 diabetes mellitus defined as a screening hemoglobin A1c ≥ 8% or a fasting plasma glucose ≥ 160 mg/dL (or 8.8 mmol/L)
• QT interval corrected using Fridericia's formula (QTcF) > 470 ms. Abnormal electrocardiograms (ECGs) (triplicate) should be performed > 30 minutes apart
• Current treatment with medications that are well known to prolong the QT interval
• Prior treatment with anti-cluster of differentiation (CD)137 agents, anti-CD3 agents and/or delta-like ligand 3 (DLL3) targeted therapies
• Any anti-cancer therapy, whether investigational or approved, including chemotherapy, hormonal therapy, or radiotherapy, within 21 days prior to initiation of study treatment
• Any history of an immune-related Grade 4 adverse event (AE) attributed to prior anti-programmed death ligand-1 (PD-L1) /PD-1 or anti-cytotoxic T-lymphocyte-associated protein (CTLA-4) therapy (other than asymptomatic elevation of serum amylase or lipase)
• Any history of an immune-related Grade 3 adverse event attributed to prior anti-PD-L1 /PD-1 or anti-CTLA-4 therapy (other than asymptomatic elevation of serum amylase or lipase) that resulted in permanent discontinuation of the prior immunotherapeutic agent
• History or clinical evidence of primary central nervous system (CNS) malignancy, symptomatic CNS metastases, CNS metastases requiring any anti-tumor treatment, or leptomeningeal disease and current or history of CNS disease, such as stroke, epilepsy, CNS vasculitis, or neurodegenerative disease
• Spinal cord compression that has not been definitively treated with surgery and/or radiation
• Active or history of clinically significant autoimmune disease
• Positive test for human immunodeficiency virus (HIV) infection
• Positive hepatitis B surface antigen (HbsAg) test, and/or positive total hepatitis B core antibody (HbcAb) test at screening
• Prior allogeneic hematopoietic stem cell transplantation or prior solid organ transplantation
• Administration of a live, attenuated vaccine within 4 weeks before first RO7616789 infusion
• Known allergy or hypersensitivity to any component of the RO7616789 formulation

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