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