Despite the introduction of new AR pathway inhibitors such as abiraterone and MDV3100, clinical responses are transitory and many patients do not respond, demonstrating the importance of ligand and AR-independent mechanisms of PCa progression. This proposal evaluates the novel hypothesis that the chromatin modifying enzyme LSD1 mediates survival of ligand and AR-independent CRPC (extending its previously identified function as a driver of ligand-mediated AR activity). Specifically, we find that i. LSD1 overexpression is ubiquitous in metastatic CRPCs, including AR+ and AR- tumors; ii. the predominant category of ligand-independent LSD1 target genes involves control of the cell cycle and proliferation; and iii. LSD1 acts in an AR-axis independent manner by promoting cMyc driven tumor growth. Our data re-shape the accepted paradigm of LSD1 as a driver of ligand-mediated PCa growth, and additionally place it as a central driver in the progression of both ligand-independent AR+ CRPC and AR null CRPC. These data strongly suggest that suppression of LSDI in the clinical setting will not only inhibit AR-pathway dependent PCa, but will inhibit the growth and potentially prevent progression to fully-androgen independent CRPC, thereby establishing the rationale for LSD1 inhibition as an important, mechanism-based therapeutic target. To fully elucidate the activity of LSD1 in driving CRPC, this project will: 1. Determine the role of c-Myc in LSD1-mediated induction of ligand-independent proliferation pathways in castration sensitive and castration resistant PCa models; 2. Determine the anti-tumor efficacy of LSD1 suppression using the new LSD1 inhibitor SP-2509, alone or in combination with MDV3100, in ligand-independent AR+ and AR- preclinical CRPC models; and 3. Determine the biological effects, safety, and anti-tumor activity of the new LSD1 inhibitor SP-2509 in a phase I trial in men with metastatic CRPC. The need to target and translate key mechanisms such as the activity of LSD1, which is capable of simultaneously interdicting development of both AR-dependent and AR-independent mechanisms of progression and resistance, is an innovative approach of paramount importance and will yield novel data of immediate clinical translational relevance.