Study of Tarlatamab in Combination With YL201 With or Without Anti-programmed Death Ligand 1 (PD-L1) in Participants With Extensive Stage (ES) Small Cell Lung Cancer (SCLC)

The primary objective of this study is to evaluate the safety and tolerability of tarlatamab in combination with YL201 with or without anti-PD-L1.

Inclusion Criteria: * Participants ≥ 18 years of age (or legal adult age within country) at time of signing informed consent. * Participants with histologically or cytologically confirmed ES-SCLC. * For Parts 1 and 2, participant must have ES-SCLC that has progressed or recurred following at least 1 line of platinum-based anti-cancer therapy. * For Part 3, participants must have ES-SCLC and no prior systemic treatment for ES SCLC other than 1 cycle of platinum-based chemotherapy, etoposide, and PD-(L)1 inhibitor in the first-line setting. * At least 1 measurable lesion as defined by RECIST 1.1. * Participants must have adequate organ function (cardiac, pulmonary, kidney, bone marrow, and liver). Exclusion Criteria: * Prior delta-like ligand 3 (DLL3) or B7 homolog 3 (B7-H3) targeted therapy. * Prior exposure to topoisomerase I inhibitors or antibody-drug conjugate (ADC) with topoisomerase I inhibitor payload. * Symptomatic central nervous system (CNS) metastases. Note: Participants with asymptomatic brain metastases are eligible as defined in the protocol. * History of (non-infectious) interstitial lung disease (ILD)/pneumonitis that required corticosteroids, current ILD/pneumonitis, or suspected ILD/pneumonitis that cannot be ruled out by imaging at screening. * Baseline requirement of supplemental oxygen.