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 atthe 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 enrolmentor scheduled to have major surgery during the study.

• Inadequate organ or marrow function.