A Study in People With Advanced Extrapulmonary Neuroendocrine Cancer to Compare Obrixtamig Plus Carboplatin and Etoposide Treatment With Standard Chemotherapy

Open to people ages 18 years and up

1. Patients with poorly differentiated unresectable locally advanced or metastatic extrapulmonary neuroendocrine carcinoma (epNEC) with Ki-67 >20% or mitotic rate mitotic rate with number of mitoses >20 per 2 mm2, regardless of primary site (including site of unknown origin)
2. Patients with tumours with mixed histologies are eligible only if neuroendocrine carcinoma component is predominant and represents more than 70% of the overall tumour tissue
3. No prior systemic treatment for unresectable locally advanced or metastatic epNEC (except for the completed one cycle of standard platinum + etoposide). Prior peri-operative chemotherapy or -radiation for curative intention is allowed if at least 6 months have elapsed between completion of this therapy and diagnosis of unresectable locally advanced or metastatic disease
4. Patients who have finished one cycle of standard platinum + etoposide regimen as first-line treatment (Cycle 0: etoposide with carboplatin or cisplatin, administered at a minimum dose of cisplatin 75 mg/m2 or carboplatin area under the curve (AUC) 5 and etoposide 80 mg/m2) prior to randomisation
5. Patients must comply with criteria for receiving further chemotherapy treatment as first-line standard of care (SoC) treatment within 28 days after the start of the initial chemotherapy (Cycle 0)
6. Adequate archival Formalin-Fixed Paraffin-Embedded (FFPE) tumour tissue, as specified in the Laboratory Manual, must be available for central laboratory analysis of Delta-like ligand 3 (DLL3) expression status. Tumours must be positive (as defined in the diagnostic study protocol) for DLL3 expression status assessed by investigational VENTANA DLL3 (SP347) RxDx Assay
7. Eastern Cooperative Oncology Group (ECOG) score of 0 or 1
8. Male or female participants ≥18 years old and at least at the legal age of consent in countries where it is greater than 18 years at the time of signature of the informed consent form (ICF) Further inclusion criteria apply.

1. Presence of leptomeningeal disease and/or carcinomatous meningitis
2. Patients with diagnosis of Merkel cell carcinoma or medullary thyroid carcinoma
3. Patients with neuroendocrine prostate cancer
4. Patients with well-differentiated neuroendocrine tumours of any grade according to the world health organization (WHO) classification, 5th edition
5. Patients with a history of well differentiated Neuroendocrine tumour (NET) tumour that transformed into poorly differentiated Neuroendocrine carcinoma (NEC)
6. Previous treatment with obrixtamig or other DLL3-targeting therapies (e.g. T-cell engager (TcEs), cell therapies, antibody-drug conjugates, or radiopharmaceuticals)
7. Previous treatment with anti-PD-1 or programmed death ligand 1 (PD-L1) therapies during the one cycle of standard platinum + etoposide first-line chemotherapy (Cycle 0)
8. Toxicity from previous treatments that has not resolved to ≤ Common Terminology Criteria for Adverse Events (CTCAE) Grade 1 or grade prior to Cycle 0. Participants with alopecia any grade, CTCAE ≤Grade 2 asthenia/fatigue, amenorrhea/menstrual disorders any grade, CTCAE ≤Grade 2 peripheral neuropathy, and/or CTCAE ≤Grade 2 endocrinopathies controlled by replacement therapy, and toxicities, which are considered irreversible but stable for at least 4 weeks, per Investigator judgement may be eligible Further exclusion criteria apply.