∙ Participants are eligible to be included in the study only if all of the following criteria apply:

• Participant has provided informed consent prior to initiation of any study specific activities/procedures.

• Male or female ≥ 18 years of age and willing and able to provide informed consent.

• Histologically or cytologically confirmed malignancy other than de novo (i.e., non-transformed) SCLC or NEPC. Must be stage IV (metastatic); participants with stage III disease are eligible provided that they are not candidates for surgery and/or radiotherapy with curative intent. Acceptable tumor types include the following:

‣ Low and intermediate grade neuroendocrine carcinoma (including carcinoid and atypical carcinoid)

⁃ Gastroenteropancreatic NEN

⁃ Large cell neuroendocrine carcinoma

⁃ SCLC transformed from previously-treated NSCLC

⁃ Extrapulmonary small cell carcinoma, with the exception of NEPC

⁃ Any other tumor type that meets staging and DLL3 positivity criteria

• Positive DLL3 expression by immunohistochemistry on tumor biopsy.

• • Positive DLL3 expression, for purposes of this study, defined as at least 25% for participants enrolling into Stage 1 or 1% for participants enrolling into Stage 2.

• Participants must have progressed on or following at least one line of therapy, if a standard of care therapy exists for the tumor type.

• Measurable disease, as per RECIST 1.1

• ECOG performance status of 0-1.

• Adequate organ function as defined in Table 3 below. System Laboratory Value Hematological Absolute neutrophil count (ANC) ≥ 1.0 x 109/L Platelets ≥ 100 x 109/L Hemoglobin ≥ 9 g/dL Renal Estimated glomerular filtration rate (eGFR) based on Modification of Diet in Renal Disease (MDRD) ≥ 30 mL/min/1.73 m2 Hepatic Serum total bilirubin ≤ 1.5 x ULN, with the exception of participants with Gilbert's disease AST (SGOT) and ALT (SGPT) ≤ 3 x ULN≤ 5 x ULN for patients with liver metastasis or primary liver cancer Coagulation International Normalized Ratio (INR) or Prothrombin Time (PT), and Activated Partial Thromboplastin Time (aPTT) ≤ 1.5 x ULN unless participant is receiving anticoagulant therapy, and then only as long as PT or PTT is within therapeutic range of intended use of anticoagulants