Testing of Tazemetostat in Combination With Topotecan and Pembrolizumab in Patients With Recurrent Small Cell Lung Cancer

Inclusion Criteria: * Patients enrolled to the primary cohort must have limited- or extensive-disease SCLC at diagnosis, with relapse at study entry with measurable disease per Response Evaluation Criteria in Solid Tumors (RECIST) 1.1, and with prior therapy with platinum doublet. Patients with extensive stage disease should have received chemo-immunotherapy. Both platinum-sensitive and platinum-resistant patients will be included * Age \>= 18 years. Because no dosing or adverse event data are currently available on the use of pembrolizumab in combination with tazemetostat and topotecan in patients \< 18 years of age, children are excluded from this study * Eastern Cooperative Oncology Group (ECOG) performance status =\< 2 (Karnofsky \>= 70%) * Leukocytes \>= 3000/mcL * Absolute neutrophil count \>= 1,500/mcL * Platelets \>= 100,000/mcL * Hemoglobin \>= 9 g/dL or \>= 5.6 mmol/L * Total bilirubin =\< 1.5 institutional upper limit of normal (ULN) OR direct bilirubin =\< ULN for patients with total bilirubin levels \> 1.5 × ULN * Aspartate aminotransferase (AST) (serum glutamic-oxaloacetic transaminase \[SGOT\])/alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase \[SGPT\]) =\< 2.5 x institutional ULN * Creatinine =\< 1.5 institutional ULN OR glomerular filtration rate (GFR) \>= 60 mL/min/1.73 m\^2 unless data exists supporting safe use at lower kidney function values, no lower than 30 mL/min/1.73 m\^2 * Note: Creatinine clearance (CrCl) should be calculated…