Project Summary/Abstract ? Project 3 Small cell lung cancer (SCLC) leads to >30,000 deaths in the USA each year and therapies resulting in durable responses are greatly needed. This proposal is focused on a potent and selective LSD1 inhibitor, ORY1001. In preliminary data, we found efficacy of ORY1001 as monotherapy in a subset of patient derived xenograft (PDX) models of SCLC. PDX models differed greatly in sensitivity to ORY1001, with one model exhibiting complete and durable regression upon ORY1001 treatment. We found that strong tumor regression was linked to robust NOTCH pathway activation, which led to suppression of ASCL1, a transcription factor critical for SCLC. We hypothesize that robust activation of NOTCH and suppression of ASCL1 drives strong response to LSD1 inhibition in a subset of SCLC models. We also hypothesize that mutation in chromatin regulating genes may contribute to robust NOTCH pathway activation and increased sensitivity to LSD1 inhibition in SCLC. Specific Aim 1: To use genetically engineered mouse and PDX models to test the efficacy of ORY1001 in SCLC. We will expand our PDX studies to identify additional strongly responding models and will include models with mutations in chromatin regulating genes such as CREBBP and KMT2D that we hypothesize may contribute to strong responses. This aim will also test a novel Crebbp-deficient genetically engineered mouse model of SCLC that we generated to clearly determine whether inactivation of Crebbp increases response to LSD1 inhibition in SCLC. Specific Aim 2: To understand roles for LSD1-NOTCH-ASCL1 axis in control of SCLC cell viability. We will use tumors from PDX and GEM models treated in vivo and PDX tumors treated ex vivo with ORY1001 to interrogate roles for a NOTCH-ASCL1 axis in conferring strong responses to ORY1001 in SCLC. We will also perform studies in PDX models of SCLC studied ex vivo, to identify and better understand differences between strongly responding and non-responsive models. This Aim will include RNAseq and ChIPseq studies that will identify key differences between these groups. Specific Aim 3. Perform an Investigator Initiated clinical trial to test ORY1001 in SCLC patients. Here, we will study circulating tumor cells (CTCs) for biomarkers of response to LSD1 inhibition in a clinical trial. The design of this trial may be modified based on work performed in SCLC model systems as we identify potential biomarkers that may predict responsiveness. This work aims to direct LSD1 inhibition to SCLC patients most likely to benefit.