A Study of Valemetostat in Combination With Atezolizumab in People With Lung Cancer

Inclusion Criteria: * Signed informed consent form (ICF) * Ability to comply with the study protocol as per the investigator's judgment * Age ≥ 18 years at the time of signing the ICF * Life expectancy ≥ 12 weeks * ECOG performance status 0 or 1 * Pathologically confirmed diagnosis of newly diagnosed extensive-stage small cell lung cancer. Patients with a diagnosis of combined small cell lung cancer with other histologies may be considered for inclusion if the predominant histology is SCLC and only after discussion with the study PI. * Radiographically documented RECIST version 1.1 stable disease, partial or complete response after initial treatment with a platinum doublet regimen in combination with atezolizumab for 4 cycles. It is acceptable to have no measurable disease at the start of this study. * Must be able to begin therapy within 4 weeks of completing the fourth cycle of chemotherapy and immunotherapy. * Adequate hematologic and end-organ function, as defined by the following laboratory test results obtained within 14 days prior to initiation of study treatment. Transfusion (red blood cell or platelet) or granulocyte-colony stimulating factor (G-CSF) administration is not allowed within 2 weeks prior to screening laboratory tests: o Adequate bone marrow function as defined by: * Absolute neutrophil count (ANC) ≥ 1.5 x 10\^9/L (1500/µL) * Hemoglobin ≥ 9 g/dL * Platelets ≥ 100 x 10\^9/L o Adequate renal function as defined by: * Creatinine clearance ≥ 30 mL/min as …