A Phase I, Open-label, Multi-center Study to Evaluate the Safety, Tolerability, Dosimetry, and Preliminary Activity of [225Ac]Ac-ETN029 in Patients With Advanced DLL3-expressing Solid Tumors

Inclusion Criteria: * Age ≥ 18 years old * Patients with one of the following indications: * Locally advanced, unresectable, or metastatic SCLC with disease progression following, or intolerance to, at least 1 line of systemic therapy, including platinum-containing chemotherapy, unless patient was ineligible to receive such therapy. Prior DLL3-targeted therapy is allowed. For dose expansion, patients should have received no more than 2 prior lines of systemic therapy. * Dose escalation only: LCNEC of the lung with disease progression following, or intolerance to, at least 1 line of systemic therapy, including platinum-containing chemotherapy, unless patient was ineligible to receive such therapy. * Dose expansion only: Locally advanced, unresectable, or metastatic de novo or castration-resistant, treatment-emergent NEPC with neuroendocrine differentiation confirmed by local histology and NEPC marker expression (e.g., chromogranin, synaptophysin) confirmed by local IHC. Prior PSMA-targeted, Lu-177-based RLT is allowed. Patients must have at least one measurable lesion (per RECIST 1.1) that shows 111In-ETN029 uptake higher than surrounding tissues on SPECT/CT as assessed by the Investigator. * Dose expansion only: Locally advanced, unresectable, or metastatic GEP-NEC with disease progression following, or intolerance to, at least 1 line of systemic therapy, including platinum-containing chemotherapy, unless patient was ineligible to receive such therapy. Patients must have at least one measurable lesion (per RECIST 1.1) that shows 111In-ETN029 uptake higher than surrounding tissues on SPECT/CT as assessed by the Investigator. Exclusion Criteria: * Absolute neutrophil count (ANC) \< 1.0 x 109/L, hemoglobin \< 9 g/dL, or platelet count \< 75 x 109/L * QT interval corrected by Fridericia's formula (QTcF) ≥ 470 msec * eGFR \< 60 mL/min (\<0.835 mL/s), calculated using the CKD-EPI 2021 formula or measured * Unmanageable urinary tract obstruction or urinary incontinence * Presence of leptomeningeal disease, of symptomatic CNS metastases or of CNS metastases that require local CNS-directed therapy * History of or current interstitial lung disease or pneumonitis ≥ Grade 2 * Any prior DLL3-targeted therapy (except for SCLC) and any prior RLT (except for NEPC) Other protocol-defined inclusion/exclusion criteria may apply.