Study of 225Ac-SS0110 in Subjects With ES-SCLC or MCC (SANTANA-225 )

Key Inclusion Criteria: * Signed Informed Consent Form and willing to comply with all study procedures. * Participants with ES-SCLC (defined as Stage IV per NCCN guidelines Version 1.2024) or recurrent locally advanced or metastatic MCC (per NCCN guidelines Version 1.2024) documented by histology or cytology. * Participants eligible to receive immune checkpoint inhibitor (CPI) monotherapy at the start of treatment with 225Ac-SSO110 or patients already on CPI monotherapy for 7 weeks. * ECOG performance status of 0 or 1. Life expectancy of at least 6 months. * Positive 68Ga-SSO120 scan (Investigator-assessed) during screening. Key Exclusion Criteria: * Participants with unstable spinal cord compression. * Any previous systemic radioligand therapy or extensive radiotherapy. * Participants receiving or planned to receive consolidative chest radiation. * History of primary immunodeficiency, transplantation or CAR-T cell therapy. * Major surgical procedure except diagnostic biopsy within 28 days prior to enrolment or scheduled to have major surgery during the study. * Inadequate organ or marrow function.