This 2-part study intends to define the recommended Phase 2 dose of ifinatamab deruxtecan (I-DXd) based on the efficacy, safety, and pharmacokinetics (PK) results observed in participants with Extensive-stage Small Cell Lung Cancer (ES-SCLC) who received at least 1 prior line of platinum-based chemotherapy and a maximum of 3 prior lines of therapy (Part 1) and a minimum of two previous lines of systemic therapy (Part 2). This study will also investigate I-DXd anti-tumor activity in this population.

This study will consist of 2 parts: dose optimization (Part 1) and extension (Part 2). In the dose optimization part of the study (Part 1), approximately 80 participants with at least 1 prior line of platinum-based chemotherapy and a maximum of 3 prior lines of therapy will be enrolled. Two I-DXd doses will be tested (8 mg/kg once every 3 weeks and 12 mg/kg once every 3 weeks). In the extension part of the study (Part 2), approximately 70 participants with a minimum of two previous lines of systemic therapy will be enrolled. I-DXd will be administered at the selected dose of 12 mg/kg once every 3 weeks (Q3W).

In Part 1, eligible participants will be randomized in a 1:1 ratio to receive one of the two dose levels of I-DXd. Randomization will be stratified by:

1. Prior receipt or of an anti-programmed death-ligand 1 (PD-[L]1) antibody (yes/no)
2. The chemotherapy-free interval (CTFI) from completion of the first-line therapy to the date of documented radiological Progressive Disease of <90 days vs. ≥90 days in second-line participants as well as the number of lines of therapy. Thus, the stratification factor includes three categories: (1) second-line participants with CTFI <90 days, (2) second-line participants with CTFI ≥90 days, and (3) third- and fourth-line participants.

Inclusion Criteria:

Participants must meet all the following criteria to be eligible for enrollment into the study:

• Sign and date the informed consent form (ICF) prior to the start of any study-specific qualification procedures.
• Participant must have at least one lesion, not previously irradiated, amenable to core biopsy.
• Male or female subjects aged ≥18 years (follow local regulatory requirements if the legal age of consent for study participation is >18 years old).
• Histologically or cytologically documented ES-SCLC.
• At least one measurable lesion according to RECIST v1.1 as assessed by the investigator.
• Prior therapy with at least one platinum-based line as systemic therapy for extensive-stage disease with at least two cycles of therapy (except in the case of early objective PD) and beginning with protocol version 3.0, a minimum of two previous lines of systemic therapy.
• Documentation of radiological disease progression on or after most recent systemic therapy.
• Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 or 1.

Exclusion Criteria:

Participants who meet any of the following criteria will be disqualified from entering the study:

• Prior treatment with orlotamab, enoblituzumab, or other B7-H3 targeted agents, including I-DXd.
• Prior discontinuation of an antibody drug conjugate (ADC) that consists of an exatecan derivative (eg, trastuzumab deruxtecan) due to treatment-related toxicities.
• Clinically active brain metastases, spinal cord compression or leptomeningeal carcinomatosis, defined as untreated or symptomatic, or requiring therapy with steroids or anticonvulsants to control associated symptoms.
• Any of the following conditions within the past 6 months: cerebrovascular accident, transient ischemic attack, or another arterial thromboembolic event.
• Clinically significant corneal disease.
• Uncontrolled or significant cardiovascular disease.
• 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.
• Clinically severe pulmonary compromise resulting from intercurrent pulmonary illnesses,
• Chronic steroid treatment (dose of 10 mg daily or more prednisone equivalent), except for low-dose inhaled steroids (for asthma/COPD) or topical steroids (for mild skin conditions) or intra-articular steroid injections.
• History of malignancy other than SCLC within the 3 years prior to enrollment, except adequately resected non-melanoma skin cancer, curatively treated in situ disease, superficial gastrointestinal (GI) tract tumors and non-muscle invasive bladder cancer curatively resected by endoscopic surgery.
• History of allogeneic bone marrow, stem cell, or solid organ transplant.
• Unresolved toxicities from previous anticancer therapy, defined as toxicities (other than alopecia) not yet resolved to National Cancer Institute- Common Terminology Criteria for Adverse Events Version 5.0 (NCI-CTCAE V5.0), Grade ≤1 or baseline.
• History of hypersensitivity to the drug substances, inactive ingredients in the drug product or severe hypersensitivity reactions to other monoclonal antibodies.
• Evidence of ongoing uncontrolled systemic bacterial, fungal, or viral infection.
• Has active or uncontrolled hepatitis B or C infection.
• Active, known, or suspected autoimmune disease.
• Any evidence of severe or uncontrolled systemic diseases (including active bleeding diatheses, psychiatric illness/social situations, substance abuse).
• Has received a live vaccine within 30 days prior to the first dose of study drug.
• Female who is pregnant or breast-feeding or intends to become pregnant during the study.
• Prior or ongoing clinically relevant illness, medical condition, surgical history, physical finding, or laboratory abnormality that, in the investigator's opinion, could affect the safety of the participant.
• Known human immunodeficiency virus (HIV) infection that is not well controlled.

De-identified individual participant data (IPD) and applicable supporting clinical trial documents may be available upon request at https://vivli.org/. In cases where clinical trial data and supporting documents are provided pursuant to our company policies and procedures, Daiichi Sankyo will continue to protect the privacy of our clinical trial participants. Details on data sharing criteria and the procedure for requesting access can be found at this web address: https://vivli.org/ourmember/daiichi-sankyo/

Studies for which the medicine and indication have received European Union (EU) and United States (US), and/or Japan (JP) marketing approval on or after 01 January 2014 or by the US or EU or JP Health Authorities when regulatory submissions in all regions are not planned and after the primary study results have been accepted for publication.

Formal request from qualified scientific and medical researchers on IPD and clinical study documents from clinical trials supporting products submitted and licensed in the United States, the European Union and/or Japan from 01 January 2014 and beyond for the purpose of conducting legitimate research. This must be consistent with the principle of safeguarding study participants' privacy and consistent with provision of informed consent.