A Study to Evaluate the Optimal Dose, Adverse Events and Change in Disease Activity of Intravenous ABBV-706 in Combination With Atezolizumab Versus Standard of Care as First-Line Treatment in Adult Participants With Previously Untreated Extensive Stage Small Cell Lung Cancer

Inclusion Criteria: * Diagnosis of histologically or cytologically confirmed extensive stage small cell lung cancer (ES-SCLC) requiring treatment with first line therapy. * Have an Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 to 1 during the screening period prior to the first dose of study treatment. * Have measurable disease per Response Evaluation Criteria in Solid Tumors (RECIST v1.1). * Suspected brain metastases at screening should have a computed tomography (CT)/ magnetic resonance imaging (MRI) of the brain prior to study entry. Exclusion Criteria: * Have received any kind of treatment for limited stage small cell lung cancer (LS-SCLC). * Known active/symptomatic central nervous system (CNS) metastases should be excluded. * History of interstitial lung disease (ILD) or pneumonitis that required treatment with systemic steroids, or any evidence of active ILD/pneumonitis on screening chest computed tomography (CT) scan should be excluded. * Have any clinically significant conditions that would adversely affect the participant's participation in the study, and the subject should have a life expectancy of at least 3 months.