PROJECT SUMMARY Small cell lung cancer (SCLC) is an aggressive malignancy characterized by rapid onset of chemoresistance and poor clinical outcomes. Recent advances in immunotherapy have changed the standard of care for SCLC for the first time in over 30 years. Dr. Allison Stewart (Resarch Specialist) and Dr. Lauren Byers (Unit Director) in the Department of Thoracic Head and Neck Medical Oncology at University of Texas M.D. Anderson Cancer Center aim to address major, unmet needs in SCLC, including development of novel therapeutic targets, investigation of approaches to enhance response to immunotherapy, and the study of heterogeneity and its contribution to drug resistance. Mechanisms underlying treatment-resistance remain obscure due to scarcity of tissue samples from relapsed patients. To address this deficiency, Dr. Stewart has led efforts to establish novel SCLC models and analyze serial blood and tumor biopsies from patients using innovative genomic, transcriptomic, and proteomic assays to study mechanisms driving treatment resistance to chemotherapy, immunotherapy, or targeted therapies in SCLC. Patient liquid biopsies are non-invasive, facilitate both serial and post-relapse tissue sampling and contain sufficient circulating tumor cells (CTCs) for generation of CTC-derived xenograft (CDX) models of SCLC, as well as for direct single-cell transcriptional profiling. CDXs mirror patient disease by expression of SCLC markers, sites of metastatic disease and platinum response. Intratumoral heterogeneity (ITH) of SCLC and its contribution to clinical outcomes has not been fully characterized. To investigate ITH, single-cell analyses (including single-cell RNAseq) were performed on novel platinum-sensitive and -resistant SCLC CDX models, as well as longitudinal analyses of CDX models and patient CTCs over the course of therapy. Dr. Stewart has demonstrated that refractory SCLC is characterized by increased heterogeneity and the development of multiple, disparate resistant cell populations within the same patient. In order to address the scarcity of SCLC tissue samples, especially paired and post-relapse samples, Dr. Byers has spearheaded an effort to integrate serial tissue acquisition into the design of all SCLC- focused clinical trials at MDACC. Dr. Stewart's goal is to coordinate a SCLC platform by working with multi-disciplinary teams, including a clinical team (identify and consent patients), a laboratory processing team (process/bank specimens, as well as generate animal models), an experimental analysis team (perform experiments with CTCs, tumor specimens and animal models) and a bioinformatics team (scientific analysis) to establish a repository of SCLC specimens for understanding treatment resistance and developing methods to recognize patient response.