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) prior to 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 by the Investigator to be neurologically stable and meet the following criteria: a. Radiotherapy or surgery for brain metastases was completed at least 2 weeks prior to Cycle 1 Day 1 (C1D1); b. Symptoms are stable and steroid/antiepileptic doses remain unchanged for a minimum of 2 weeks prior to C1D1. * At least 4 weeks from prior major surgery (other than for brain metastases), or at least 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 treatment administration. * Willing to provide archival tumor tissue for central analysis; if unavailable, a pre-study treatment biopsy may be collected and provided. * Female and male patients of childbearing potential agree to use at least 2 highly effective forms of contraception (1 a…