The primary objective of this study is to compare the efficacy of tarlatamab with placebo as assessed by progression free survival (PFS) based on blinded independent central review (BCIR) per response evaluation criteria in solid tumors v1.1 (RECIST 1.1) and on prolonging overall survival (OS).

Inclusion Criteria: -Participants are eligible to be included in the study only if all of the following criteria apply:

• Participant has provided informed consent prior to initiation of any study specific activities/procedures.
• Age ≥ 18 years (or ≥ legal age within the country if it is older than 18 years).
• Histologically or cytologically confirmed small-cell lung cancer (SCLC).
• Diagnosed and treated for LS-SCLC with concurrent chemotherapy and radiotherapy.
• Has completed chemoradiotherapy without progression per RECIST 1.1 (ie, achieved complete response [CR], partial response [PR], or stable disease [SD]).
• Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) of 0 or 1.
• Minimum life expectancy of 12 weeks.
• Adequate organ function.
• Toxicities attributed to concurrent chemoradiotherapy resolved to grade ≤ 1, unless otherwise specified. Excluding alopecia or fatigue.

Exclusion Criteria: -Participants are excluded from the study if any of the following criteria apply:

Disease Related

• Extensive-stage SCLC (ES-SCLC).
• Any previous diagnosis of transformed non-small-cell lung cancer (NSCLC), epidermal growth factor receptor (EGFR) activating mutation positive NSCLC that has transformed to SCLC, or mixed SCLC NSCLC histology.
• Evidence of interstitial lung disease or active, non-infectious pneumonitis. Other Medical Conditions
• History of other malignancy within the past 2 years, with certain exceptions.
• History of solid organ transplantation.
• Myocardial infarction and/or symptomatic congestive heart failure (New York Heart Association > class II) within 6 months prior to first dose of study treatment.
• History of arterial thrombosis (eg, stroke or transient ischemic attack) within 6 months prior to first dose of study treatment.
• Exclusion of human immunodeficiency virus (HIV) or active hepatitis infection based on criteria per protocol.
• Participant with symptoms and/or clinical signs and/or radiographic signs that indicate an acute and/or uncontrolled active systemic infection within 7 days prior to the first dose of study treatment.

Prior/Concomitant Therapy

• Received sequential chemotherapy and thoracic radiotherapy (no overlap of thoracic radiotherapy with chemotherapy) during chemoradiation.
• Prior therapy with any selective inhibitor of the delta-like ligand 3 (DLL3) pathway.
• Prior history of severe or life-threatening events from any immune-mediated therapy.
• Receiving another anti-cancer therapy. Adjuvant hormonal therapy for resected breast cancer is permitted.
• Receiving systemic corticosteroid therapy or any other form of immunosuppressive therapy within 7 days prior to enrollment.
• Major surgical procedures within 28 days prior to first dose of study treatment.
• Treatment with live virus, including live-attenuated vaccination, within 14 days prior to the first dose of study treatment. Inactive vaccines and live viral non-replicating vaccines within 3 days prior to first dose of study treatment.

Prior/Concurrent Clinical Study Experience

• Treatment in an alternative investigational trial within 28 days prior to enrollment.

Other Exclusions

• Female participants of childbearing potential unwilling to use protocol specified method of contraception during treatment and for an additional 60 days after the last dose of study treatment.
• Female participants who are breastfeeding or who plan to breastfeed while on study through 60 days after the last dose of study treatment.
• Female participants planning to become pregnant or donate eggs while on study through 60 days after the last dose of study treatment.
• Female participants of childbearing potential with a positive pregnancy test assessed at screening by a highly sensitive serum pregnancy test.
• Male participants with a female partner of childbearing potential who are unwilling to practice sexual abstinence (refrain from heterosexual intercourse) or use contraception during treatment and for an additional 60 days after the last dose of study treatment.
• Male participants with a pregnant partner who are unwilling to practice abstinence or use a condom during treatment and for an additional 60 days after the last dose of study treatment.
• Male participants unwilling to abstain from donating sperm during treatment and for an additional 60 days after the last dose of study treatment.
• Participant has known sensitivity to any of the products or components to be administered during dosing.
• Participant likely to not be available to complete all protocol-required study visits or procedures, and/or to comply with all required study procedures to the best of the participant and investigator's knowledge.
• History or evidence of any other clinically significant disorder, condition, or disease (with the exception of those outlined above) that, in the opinion of the investigator or Amgen physician, if consulted, would pose a risk to participant safety or interfere with the study evaluation, procedures or completion.

De-identified individual patient data for variables necessary to address the specific research question in an approved data sharing request.

Data sharing requests relating to this study will be considered beginning 18 months after the study has ended and either 1) the product and indication have been granted marketing authorization in both the US and Europe or 2) clinical development for the product and/or indication discontinues and the data will not be submitted to regulatory authorities. There is no end date for eligibility to submit a data sharing request for this study.

Qualified researchers may submit a request containing the research objectives, the Amgen product(s) and Amgen study/studies in scope, endpoints/outcomes of interest, statistical analysis plan, data requirements, publication plan, and qualifications of the researcher(s). In general, Amgen does not grant external requests for individual patient data for the purpose of re-evaluating safety and efficacy issues already addressed in the product labelling. Requests are reviewed by a committee of internal advisors. If not approved, a Data Sharing Independent Review Panel will arbitrate and make the final decision. Upon approval, information necessary to address the research question will be provided under the terms of a data sharing agreement. This may include anonymized individual patient data and/or available supporting documents, containing fragments of analysis code where provided in analysis specifications. Further details are available at the URL below.