Inclusion Criteria:  18 to 75 years old (at the time of inform consent obtained). Be able and willing to provide written informed consent and to comply with all requirements of study participation (including all study procedures). Histologically or cytologically confirmed ES-SCLC (per the Veterans Administration Lung Study Group [VALG] staging system). Be able to provide formalin fixed, paraffin-embedded (FFPE) tumor tissue obtained from either a core or excisional tumor biopsy. Have a life expectancy of at least 3 months. Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 Have measurable disease based on Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 assessed by investigator Has adequate organ function All female and male subjects of reproductive potential must agree to use an effective method of contraception, as determined by the Investigator, during and for 120 days after the last dose of study treatment.  Exclusion Criteria:  Undergone major surgery within 30 days prior to the first dose of study treatment History of prior malignancy except that basal cell carcinoma of the skin, squamous cell carcinoma of the skin that has undergone potentially curative therapy or in situ cervical cancer Active central nervous system (CNS) metastases History of active autoimmune disease that has required systemic treatment in the past 2 years (i.e.,corticosteroids or immunosuppressive drugs). Active infection requiring systemic therapy Active Hepatitis B or Hepatitis C History of myocardial infarction, unstable angina, cardiac or other vascular stenting, angioplasty, or surgery within 12 months prior to day 1 of study treatment; Pregnant or lactating women