A novel orthotopic murine model of neuroendocrine bladder cancer: Insights into the phenotypic plasticity of small cell bladder cancer (SCBC).

571 Background: SCBC is an aggressive subtype of bladder cancer with high metastatic potential and few effective treatment options. We developed an orthotopic mouse model of SCBC to understand the development of this rare bladder cancer variant and identify epigenetic drivers of neuroendocrine differentiation. Methods: Lentiviral particles carrying Cre recombinase were produced using Lenti-sgNeo#2/Cre. The bladders of Rb1fl/fl Trp53fl/fl MycLSL/LSL (RPM) mice was transduced with Cre recombinase expressing lentivirus via transurethral catheterization. Mice were monitored by micro-ultrasound (mUS) and detected tumors were verified by histology. SCBC morphology was confirmed by H&E staining and synaptophysin IHC. Whole transcriptome (RNAseq) analysis was performed to correlate transcriptomic profile of neuroendocrine mouse tumors to a cohort of human SCBC tumors. Results: Transurethral catheterization successfully transduced the bladder urothelium without evidence of exposure of ectopic (non-urothelial) tissues. RPM mice developed SCBC visible on mUS with a latency of 8-10 weeks. As expected in neuroendocrine tumors, mice developed liver and lung metastases. High grade neuroendocrine morphology and NE markers were confirmed on H&E and IHC, respectively, by a GU pathologist. Western blot analysis confirmed cMyc expression and suppression of TP53 and RB1. Synaptophysin expression was confirmed by IHC. Transcriptomic profiling of both mouse and human SCBC demonstrated concordant gene expression. Gene expression profiling of urothelial and non-urothelial neuroendocrine tumors suggested a phenotypic convergence. Conclusions: We developed a novel genetically engineered murine model of SCBC. Ongoing work seeks to identify epigenetic markers playing role in the development of this aggressive variant of bladder cancer. We are assessing the activity of immune checkpoint inhibitors in this immunocompetent background.