Major Inclusion Criteria:

          1. Histologically or cytologically confirmed malignancy associated with expression of
             DLL3:

               -  SCLC that has relapsed following at least 1 line of platinum-based chemotherapy

               -  Malignancy other than SCLC with pathologic demonstration of high-grade
                  neuroendocrine features or demonstration of DLL3 expression in a tumor sample,
                  and that the patient has 1 of the following:

               -  Disease that is relapsed/refractory to standard systemic therapy,

               -  Disease for which standard therapy does not exist, or

               -  Disease where standard therapy is not considered appropriate by the Investigator

          2. Available archival tissue sample or fresh biopsy tissue sample must be available for
             shipment prior to enrollment. Patients with no available tumor tissue, who cannot
             safely undergo a biopsy may be eligible if they have documentation of DLL3 expression
             in a tumor sample from a prior biopsy.

          3. Adequate hematologic status, including:

               -  Absolute neutrophil count (ANC) ≥1500 cells/μL

               -  Platelet count ≥100,000/μL

               -  Hemoglobin ≥9 g/dL (no transfusions allowed within 2 weeks prior to screening)

          4. Adequate renal function, including:

             -Calculated creatinine clearance ≥50 mL/min using the formula of Cockcroft and Gault

          5. Adequate liver function, including

               -  Total bilirubin ≤1.5 x upper limit of normal (ULN), regardless of direct
                  bilirubin, unless the patient has documented Gilbert syndrome in which case the
                  maximum total serum bilirubin should be 5 mg/dL

               -  Aspartate and alanine transaminase (AST and ALT) ≤3 x ULN

        Major Exclusion Criteria:

          1. Untreated brain metastases. Participants must have completed treatment for brain
             metastasis, and be neurologically stable off steroids, for at least 7 days prior to
             first dose of study drug

          2. Patients with glioma or other primary CNS malignancy

          3. Patients with spinal cord compression or symptomatic/uncontrolled epidural disease.
             Patients with previously treated spinal cord compression or epidural disease may be
             eligible if stable for at least 1 week prior to first dose of study drug.

          4. Active neurologic paraneoplastic syndrome.

          5. Uncontrolled pleural effusion, pericardial effusion, or ascites requiring recurrent
             drainage procedures (e.g., biweekly or more frequently).