MGC018 in Patients With Relapsed or Refractory Extensive-Stage Small-Cell Lung Cancer

The goal of this clinical trial is to test MGC018 in patients with relapsed or refractory Extensive-Stage Small-Cell Lung Cancer (ES-SCLC). The main question it aims to answer is: • Does the administration of MGC018 achieve a clinically meaningful response rate of 25% in patients with relapsed or refractory ES-SCLC? Participants enrolled in the trial will receive MGC018 through an intravenous (IV) infusion, every 28 days until disease progression or unacceptable toxicity. Tumor assessment will be done every 2 cycles (28 day cycles). Blood samples will be taken for biomarker analysis before treatment, on cycle 3 day 1, and at progression. A pretreatment biopsies will be done.

Inclusion Criteria: 1. Age greater than or equal to 18 years at time of signing Informed consent form (ICF) 2. Ability to comply with the study protocol, in the investigator's judgment. 3. Histologically or cytologically confirmed advanced small cell lung cancer that is not amenable to definitive therapy. Patients with epidermal growth factor receptor (EGFR)-mutant Non Small Cell Lung Cancer (NSCLC) that has transformed to Small Cell Lung Cancer (SCLC) will be allowed if their SCLC has progressed following treatment with platinum-based chemotherapy. 4. Disease progression during or following treatment with platinum-based chemotherapy. a) Patients could have received any number of therapies for relapsed or progressive disease. 5. Measurable disease per RECIST v1.1 6. Eastern Cooperative Oncology Group (ECOG) Performance Status of 0-2 7. Adequate hematologic and end-organ function, defined by the following laboratory test results, obtained within 14 days prior to initiation of study treatment: 1. Absolute Neutrophil Count (ANC) greater than or equal to (\>/=) 1.0 x 10\^9/L (1000/uL) without granulocyte colony-stimulating factor support 2. Platelet count \>/=100 x 10\^9/L (100,000/uL) without transfusion 3. Hemoglobin \>/= 80 g/L (8 g/dL) (1) Patients may be transfused to meet this criterion. 4. Aspartate Aminotransferase (AST), Alanine Transaminase (ALT), and alkaline phosphatase (ALP) less than or equal to (\