A Study to Assess Adverse Events and Change in Disease Activity of Intravenous (IV) ABBV-706 in Combination With Alveltamig and Atezolizumab for Adults Participants With Extensive Stage-Small Cell Lung Cancer

Small cell lung cancer (SCLC) is a type of lung cancer that accounts for approximately 15-17% of all lung cancers. It is characterized by aggressive and rapid growth and early spread to distant sites. Extensive stage-small cell lung cancer (ES-SCLC) includes cases where disease has spread beyond one side of the chest. This study will evaluate the safety and efficacy of ABBV-706 in combination with atezolizumab and alveltamig in previously untreated ES-SCLC. ABBV-706 and alveltamig are investigational drugs being developed for the treatment of ES-SCLC. This is an open-label study, meaning participants and doctors know which treatment is being given. Participants are placed into 1 of 2 treatment arms. Each arm receives a different dose of ABBV-706 in combination with alveltamig and atezolizumab. Adult participants with a diagnosis of previously untreated extensive stage-small cell lung cancer will be enrolled. Around 80 participants will be enrolled in the study at approximately 25 sites worldwide. Participants will receive intravenous (IV) infusion of ABBV-706 in combination with IV infusion of atezolizumab and IV infusion of alveltamig. The estimated duration of the study is approximately 61 months. There may be higher treatment burden for participants in this trial compared to their standard of care due to study procedures. Participants will attend regular visits during the study at a hospital or clinic. The effects of the treatment will be checked by medical assessments, blood tests, imaging scans, checking for side effects and completing questionnaires.

Inclusion Criteria: * Diagnosis of histologically or cytologically confirmed extensive stage-small cell lung cancer (ES-SCLC) requiring treatment with first-line therapy * Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1 * Must have measurable disease per RECIST v1.1 * Has an archival or recent diagnostic FFPE tumor specimen collected no more than 6 months prior to Cycle 1 Day 1 * No clinically significant conditions that would adversely affect the participation in the study, and the subject should have a life expectancy of at least 3 months. Exclusion Criteria: * Prior systemic therapy for the treatment of limited stage-SCLC and/or first-line ES-SCLC * Prior treatment with topoisomerase 1 inhibitors, including belotecan, camptothecin, exatecan, irinotecan, rubitecan and topotecan, and any ADC that consists of a Top1i payload. * History of interstitial lung disease (ILD) or pneumonitis that required treatment with systemic steroids, or any evidence of active ILD/pneumonitis on screening chest CT scan