Inclusion Criteria:

          -  Histological or cytological diagnosis of SCLC

          -  Disease progression or recurrence during or after platinum-based therapy, unless
             platinum-based therapy was contraindicated

          -  Phase 1: Measurable or evaluable disease according to RECIST v1.1

          -  Phase 2: Measurable disease according to RECIST v1.1

          -  Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2

          -  Age ≥ 18 years

          -  Adequate bone marrow function as defined below:

               -  Absolute neutrophil count (ANC) ≥ 1,500/mm3

               -  Platelet count ≥ 100,000/mm3

               -  Hemoglobin ≥ 8.0 g/dL

          -  Adequate renal function as defined below:

          -  Serum creatinine ≤ upper limit of normal (ULN) for the lab or a calculated creatinine
             clearance ≥ 40 mL/min

          -  Adequate hepatic function as defined below:

               -  Total bilirubin ≤ 1.5 x ULN for the laboratory

               -  Aspartate aminotransferase (AST) ≤ 2.5 x ULN for the laboratory

               -  Alanine aminotransferase (ALT) ≤ 2.5 x ULN for the laboratory

          -  Persons with known HIV seropositivity are eligible if they meet the following
             criteria:

               -  CD4 count ≥ 200/mm3

               -  Undetectable HIV viral load on standard PCR-based test

               -  On a stable regimen of highly active anti-retroviral therapy (HAART) that does
                  not include protocol contraindicated agents

               -  No ongoing requirement for concurrent antibiotics or antifungal agents for the
                  prevention of HIV-associated opportunistic infections

          -  Ability to understand and the willingness to sign a written informed consent document.

        Exclusion Criteria:

          -  Ongoing requirement for any non-study anticancer therapy

          -  Ongoing or planned treatment with any of the following:

               -  Greater than 10 mg prednisone daily or equivalent

               -  Immunosuppressive agents

               -  Strong or moderate CYP3A inhibitor or inducer, or a narrow-therapeutic sensitive
                  substrate

               -  P-gp inhibitor or narrow-therapeutic sensitive P-gp substrate If any of these
                  agents have been used, patients must be off them for ≥ 1 week before initiation
                  of study treatment.

          -  Any investigational agent within 21 days prior to the first dose of the
             investigational drugs

          -  Has consumed grapefruit, grapefruit products, Seville oranges, or starfruit within 3
             days before initiation of study treatment.

          -  Phase 2 portion only: Previous systemic anticancer therapy other than platinum-based
             therapy

          -  Known leptomeningeal metastases

          -  Known untreated brain metastases

          -  Hypersensitivity to irinotecan, venetoclax, or their excipients

          -  Diarrhea ≥ grade 1

          -  Ongoing need for antidiarrheal agents

          -  Active uncontrolled infection, ongoing or within 2 weeks before initiating treatment

          -  Known homozygosity for the UGT1A1*28 allele Note: Study-specific UGT1A1 testing is not
             required

          -  Inability to swallow oral medications and/or malabsorption

          -  Pregnancy or breastfeeding

          -  Medical, psychological, or social condition that, in the opinion of the investigator,
             may increase the patient's risk or limit the patient's adherence with study
             requirements