Phase I Study 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.