Inclusion Criteria:

        The patients must satisfy all of the following inclusion/exclusion criteria in order to be
        eligible for the study:

          -  Must have signed and dated written informed consent form in accordance with regulatory
             and institutional guidelines.

          -  Males and females aged >18 years at time of consent.

          -  Histological or cytological confirmed extensive-stage SCLC

          -  Patients who progressed after at least 1 platinum-based chemotherapy regimen. Patients
             with platinum resistance (defined as recurrence or progression of disease within 90
             days of completion of the platinum-based regimen) are eligible. For phase II, patients
             also must have been treated with at least one prior line of PD-1/PD-L1 therapy.

          -  Measurable disease according to RECIST v1.1 (Section 8) obtained by imaging within 28
             days prior to study registration.

          -  Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 within 14 days
             before registration and minimum life expectancy of at least 12 weeks.

          -  Treatment to be initiated at least 2 weeks since last dose of prior systemic
             anticancer therapy (chemotherapy, radiation, and/or surgery.

          -  Recovery to grade 1 of any clinically significant toxicity (excluding alopecia, grade
             2 fatigue, vitiligo, endocrinopathies on stable replacement therapy) prior to
             initiation of study drugs.

          -  Female patients of childbearing potential have a negative pregnancy test at baseline.
             Females of childbearing potential are defined as sexually mature women without prior
             hysterectomy or who have had any evidence of menses in the past 12 months. However,
             women who have been amenorrheic for 12 or more months are still considered to be of
             childbearing potential if the amenorrhea is possibly due to prior chemotherapy, anti
             estrogens, or ovarian suppression.

               -  Women of childbearing potential (i.e., menstruating women) must have a negative
                  urine pregnancy test (positive urine tests are to be confirmed by serum test)
                  documented within 14 days of study registration and within the 24-hour period
                  prior to the first dose of study drug.

               -  Sexually active women of childbearing potential enrolled in the study must agree
                  to use 2 forms of accepted methods of contraception during the course of the
                  study and for 23 weeks after their last dose of study drug. Effective birth
                  control includes (a) intrauterine device plus 1 barrier method; (b) on stable
                  doses of hormonal contraception for at least 3 months (e.g., oral, injectable,
                  implant, transdermal) plus one barrier method; (c) 2 barrier methods. Effective
                  barrier methods are male or female condoms, diaphragms, and spermicides (creams
                  or gels that contain a chemical to kill sperm); or (d) a vasectomized partner.

               -  For male patients who are sexually active and who are partners of premenopausal
                  women: agreement to use 2 forms of contraception as in criterion 9b above during
                  the treatment period and for 31 weeks after the last dose of study drug.

          -  Adequate laboratory values.

               -  Absolute neutrophil count ≥1,000/µL

               -  Platelet count ≥100,000/µL

               -  Hemoglobin ≥9.0 g/dL

               -  Total bilirubin ≤1.5 x upper limit of normal (ULN) or ≤3 x ULN for subjects with
                  Gilbert's disease

               -  Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤3.0 x ULN
                  (≤5 x ULN if evidence of hepatic involvement by malignant disease)

               -  Creatinine ≤ 1.5 x ULN or estimated glomerular filtration rate (eGFR) ≥40
                  mL/min/1.73m2

               -  Lipase and Amylase ≤1.5 x ULN. Subjects with Lipase >1.5 x ULN may enroll if
                  there are neither clinical nor radiographic signs of a pancreatitis.

        Exclusion Criteria

        Patients with any of the following will be excluded from participation in the study.

          -  Active interstitial lung disease (ILD) or pneumonitis or a history of ILD or
             pneumonitis requiring treatment with steroids. Prior history of radiation pneumonitis
             is allowed if pneumonitis was restricted to the field of radiation.

          -  History of ileus or other significant gastrointestinal disorder known to increase the
             risk of ileus or chronic bowel hypomotility

          -  Pregnant or breastfeeding, or expecting to conceive or father children within the
             projected duration of the trial, starting with the pre-screening or screening visit
             through 23 weeks (female) or 31 weeks (male) after the last dose of study drug.

          -  Must not have received CTLA-4 targeted therapy previously

          -  Treatment with any investigational agent within 28 days prior to registration for
             protocol therapy. Vaccination for SARS-CoV-2 is allowed as well as any therapy as
             required for the treatment of active COVID 19 infection.

          -  Known active symptomatic central nervous system (CNS) metastases and/or carcinomatous
             meningitis. Patients with neurological symptoms must undergo a head computed
             tomography (CT) scan or brain magnetic resonance imaging (MRI) to exclude brain
             metastasis. Patients whose brain metastases have been treated may participate provided
             there is no evidence of progression for at least 2 weeks after CNS-directed treatment,
             as ascertained by clinical examination or brain imaging.

          -  Known history of human immunodeficiency virus (HIV) or active hepatitis B (by surface
             antigen expression or polymerase chain reaction [PCR]) or active hepatitis C (by PCR)
             infection. NOTE: HIV testing is not required; Hepatitis B and C testing are required
             at screening.

          -  Diagnosis of immunodeficiency or is receiving systemic steroid therapy or any other
             form of immunosuppressive therapy within 7 days prior to study registration.

          -  Has active autoimmune disease that has required systemic treatment in the past 2 years
             (i.e., with use of disease modifying agents, corticosteroids, or immunosuppressive
             drugs) or a documented history of clinically severe autoimmune disease, or a syndrome
             that requires systemic steroids or immunosuppressive agents. Vitiligo, alopecia,
             hypothyroidism only requiring hormone replacement, psoriasis not requiring systemic
             treatment, celiac disease controlled by diet alone or conditions not expected to recur
             in the absence of an external trigger are permitted.

          -  A condition requiring systemic treatment with corticosteroids (>10 mg daily prednisone
             equivalent) or other immunosuppressive medications within 14 days prior to
             administration of study drugs.

          -  History of psychiatric illness or social situations that would limit compliance with
             study requirements. Has a history or current evidence of any condition, therapy, or
             laboratory abnormality that might confound the results of the trial, interfere with
             the patient's participation for the full duration of the trial, or is not in the best
             interest of the patient to participate, in the opinion of the treating investigator.

          -  Prior malignancies (except non-melanoma skin cancers, and the following in situ
             cancers: bladder, gastric, colon, endometrial, cervical/dysplasia, melanoma, or
             breast) unless a complete remission was achieved at least 2 years prior to study
             entry.

          -  Documented history of a cerebral vascular event (stroke or transient ischemic attack),
             unstable angina, myocardial infarction, or cardiac symptoms consistent with New York
             Heart Association (NYHA) Class III-IV within 6 months prior to their first dose of
             study drugs.

          -  Evidence of ongoing inadequately controlled hypertension (defined as baseline systolic
             blood pressure >160 mmHg or diastolic blood pressure >100 mmHg).

          -  Any active grade 3 or higher viral, bacterial, or fungal infection within 2 weeks of
             the first dose of the study drugs. Routine antimicrobial prophylaxis is permitted.