A Phase 3 Randomized, Open Label, Multicenter Study to Evaluate the Safety and Efficacy of ABBV-706 versus Standard of Care in Subjects with Relapsed/Refractory Small Cell Lung Cancer (SCLC)

Inclusion Criteria:
*  Diagnosis of histologically or cytologically confirmed Relapsed/Refractory (R/R) Small Cell Lung Cancer (SCLC).
*  Participants must have progressed on prior systemic therapy, with CPI (if eligible) and prior tarlatamab, defined as:
*  In the 1L and 2L setting respectively, platinum-based chemotherapy (i.e., carboplatin or cisplatin and etoposide with CPI, if eligible for CPI) and 2L tarlatamab; or
*  In the 1L and 2L setting, platinum-based chemotherapy (i.e., carboplatin and etoposide with atezolizumab and lurbinectedin in combination with atezolizumab maintenance and 2L tarlatamab; or
*  In the 1L setting, platinum-based chemotherapy (i.e., carboplatin or cisplatin and etoposide with CPI if eligible) in combination with tarlatamab in frontline induction and/or maintenance
*  Participants must be considered suitable to receive SOC comparator (topotecan, lurbinectedin, or amrubicin).
*  Participants must 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.
*  Participants must have measurable disease per Response Evaluation Criteria in Solid Tumors (RECIST) v1.1. Target lesions situated in a previously irradiated area are considered measurable if progression has been demonstrated in such lesions.
*  Participants with brain metastasis from an extracranial solid tumor are eligible if the brain metastases are:
*  Previously treated and not requiring anticonvulsants and steroids for at least 7 days prior to first dose of study treatment. If required for management, subjects on a steroid equivalent of prednisone dose of ≤ 10 mg/day are eligible or;
*  Untreated and asymptomatic not requiring anticonvulsants and steroids for at least 7 days prior to the first dose of study treatment. If required for management, subjects on a steroid equivalent of prednisone of ≤ 10 mg/day are eligible.

Exclusion Criteria:
*  Participants with known active/symptomatic central nervous system metastases.
*  Participants with a history of interstitial lung disease (ILD) or pneumonitis that previously required treatment with systemic steroids, or any evidence of active ILD/pneumonitis on screening chest computed tomography (CT) scan.
*  Participants with any 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.
*  Participants who have received prior treatment with a seizure-related 6 homolog (SEZ6) targeted Antibody drug conjugate (ADC), other targeted ADCs, or any other investigational agent not including tarlatamab in 1L.
*  Participants who have received prior treatment with any Top1i such as topotecan, irinotecan, belotecan, camptothecin, rubitecan, exatecan or locally approved topoisomerase I inhibitor (Top1i) payload.