Study of 225Ac-ABD147 to Establish Optimal Dose in Patients With SCLC and LCNEC of the Lung That Previously Received Platinum-based Chemotherapy

Key Inclusion Criteria:

• Has confirmed, locally advanced or metastatic SCLC or LCNEC of the lung.

• Has received platinum-based chemotherapy.

• Mentally competent and able to understand and sign an Institutional Review Board (IRB)/Independent Ethics Committee (IEC)-approved Informed Consent Form (ICF) priorto any study specific evaluation.

• Age ≥18 years old at the time the ICF is signed.

• Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 to 1.

• Expected life expectancy of >12 weeks per the Investigator.

• Has disease that is measurable by RECIST v1.1.

• Patients with known brain metastases are eligible provided they are considered bythe Investigator to be neurologically stable and meet the following criteria: a.Radiotherapy or surgery for brain metastases was completed at least 2 weeks prior toCycle 1 Day 1 (C1D1); b. Symptoms are stable and steroid/antiepileptic doses remainunchanged for a minimum of 2 weeks prior to C1D1.

• At least 4 weeks from prior major surgery (other than for brain metastases), or atleast 7 days from prior non-study-related minor surgery prior to C1D1. In all cases,the patient must be sufficiently recovered and stable before the study treatmentadministration.

• Willing to provide archival tumor tissue for central analysis; if unavailable, apre-study treatment biopsy may be collected and provided.

• Female and male patients of childbearing potential agree to use at least 2 highlyeffective forms of contraception (1 at least must be barrier method) or agree tocompletely remain abstinent for duration of study and for 6 months after the lastadministration of study drug for both female patients and male patients.

• Patients agree to not make semen/egg donations during treatment, within 2 weeksfollowing the last dose of 111In-ABD147, and for 6 months following the last dose of 225Ac-ABD147.

Key Exclusion Criteria:

• Was previously treated with 225Ac-ABD147.

• Has a history of steroid dependent hepatitis caused by treatment with a checkpointinhibitor.

• Is actively enrolled in another clinical study unless it is an observational (noninterventional) clinical study or the follow-up component of an interventionalstudy.

• Use of an anticancer therapy, radiotherapy (external beam radiotherapy [EBRT],brachytherapy, inoperative radiation therapy, radiopharmaceuticals), orimmunotherapy within 3 weeks prior to C1D1. Prior treatment with DLL3-targetingagent is acceptable with appropriate washout.

• Has a medical history of myocardial infarction or unstable angina within 6 monthsbefore C1D1.

• Has clinically significant cardiac disease not controlled by medical therapy (eg,congestive cardiac failure, arrhythmia, coronary heart disease).

• Has evidence of active infection requiring intravenous (IV) antibiotics duringScreening requiring therapy within 7 days prior to C1D1.

• Has active uncontrolled bleeding or a bleeding diathesis within 28 days prior toC1D1.

• Has serious or non-healing wound, fistula, skin ulcer, or non-healing bone fracturewithin 7 days prior to C1D1.

• Has received any thoracic radiotherapy within 8 weeks prior to C1D1.

• Has a history of idiopathic pulmonary fibrosis, organizing pneumonia (eg,bronchiolitis obliterans), drug-induced pneumonitis (requiring steroids orimmunosuppressive agents), or idiopathic pneumonitis, or evidence of activepneumonitis on the Screening chest computed tomography (CT) scan. Note: History ofradiation pneumonitis in the radiation field (fibrosis) is permitted.

• Has known hypersensitivity to Ac-225; for patients participating in the 111In-ABD147dosimetry substudy, also has known hypersensitivity to In-111.

• Has known hypersensitivity to Chinese hamster ovary cell products.

• Has a history of severe allergic anaphylactic reactions to chimeric or humanizedantibodies or fusion proteins.

• Has human immunodeficiency virus infection; patients who are taking an effectiveantiviral therapy with undetectable viral load prior to C1D1 are eligible.

• Has chronic hepatitis B virus (HBV) infection. Patients who are taking an effectivesuppressive therapy and have an undetectable HBV viral load are eligible.

• Has a history of hepatitis C virus (HCV) infection, unless treated and cured;patients with HCV infection who are currently on treatment and have an undetectableHCV viral load are eligible.

• Has carcinomatous meningitis.

• Has active symptomatic cord compression.

• Has active symptomatic superior vena cava syndrome.

• Has another primary malignancy that has not been treated with curative intent (discuss with Medical Monitor), except for non-metastatic cutaneous basal cell orsquamous cell carcinoma, or non-muscle invasive bladder cancer.

• Is unwilling or unable to follow protocol requirements.

• Has any condition that, in the opinion of the Investigator, would interfere withevaluation of the investigational product or interpretation of the patient's safetyor study results.