Carboplatin, Etoposide, and Atezolizumab With or Without Trilaciclib (G1T28), a CDK4/6 Inhibitor, in Extensive-Stage SCLC

Key Inclusion Criteria: * Male or female subjects aged ≥18 years * Unequivocally confirmed diagnosis of SCLC by histology or cytology, preferably including the presence of neuroendocrine features by immunohistochemistry * Extensive-stage SCLC * At least 1 target lesion that is measurable by Response Evaluation Criteria in Solid Tumors (RECIST) Version 1.1 * Adequate organ function * Eastern Cooperative Oncology Group (ECOG) performance status of 0 - 2 * Predicted life expectancy of ≥3 months * Able to understand and sign an informed consent Exclusion Criteria: * Limited-stage SCLC * Prior chemotherapy for limited or extensive-stage SCLC * Prior treatment with immunotherapies including but not limited to cluster of differentiation 137 agonists or immune checkpoint blockade therapies (such as anti-programmed cell death protein 1 (PD-1), anti-programmed death-ligand 1(PD-L1), and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) therapeutic antibodies). * Presence of symptomatic brain metastases requiring immediate treatment with radiation therapy or steroids. * Malignancies other than SCLC within 3 years prior to randomization, with the exception of those with negligible risk of metastasis or death treated with expected curative outcome * History of idiopathic pulmonary fibrosis, organizing pneumonia, drug induced pneumonitis, idiopathic pneumonitis, or evidence of active pneumonitis on screening chest CT scan * Active, known, suspected autoimmune disease requiring systemic treatment in the past 2 years * Uncontrolled ischemic heart disease or uncontrolled symptomatic congestive heart failure * Known history of stroke or cerebrovascular accident within 6 months prior to enrollment * Serious active infection at the time of enrollment * Psychiatric illness/social situations that would limit study compliance * Other uncontrolled serious chronic disease or conditions that in the investigator's opinion could affect compliance or follow-up in the protocol * Known human immunodeficiency virus, known active hepatitis B, or hepatitis C * Radiotherapy to any site or radiotherapy within 2 weeks prior to enrollment * Receipt of any investigational medication within 4 weeks prior to enrollment * Administration of attenuated vaccine within 4 weeks before enrollment or anticipation that such a live attenuated vaccine will be required during the study * Influenza vaccination should be given during influenza season only (approx. Oct to March). Patients must not receive live, attenuated influenza vaccine (eg, FluMist) within 4 weeks prior to enrollment at any time during the study, and at least 5 months after the last dose of atezolizumab. * Patients with a condition requiring systemic treatment with either corticosteroids ( \> 10 mg daily prednisone equivalents) or other immunosuppressive medications (including but not limited to cyclophosphamide, azathioprine, methotrexate, thalidomide, and anti-tumor necrosis factor agents) within 14 days of study drug administration. Inhaled or topical steroids ad adrenal replacement dosed \> 10 mg daily prednisone equivalents are permitted in the absence of active autoimmune disease. * Hypersensitivity to any of the components of the formulation of etoposide or etoposide phosphate * Hypersensitivity to carboplatin or other platinum-containing compounds or to mannitol * History of severe allergic, anaphylactic, or other hypersensitivity reactions to chimeric or humanized antibodies or fusion proteins * Known hypersensitivity to Chinese hamster ovary cell products or other recombinant human antibodies * Legal incapacity or limited legal capacity * Pregnant or lactating women