10549 Background: LSD1 (KDM1A) is FAD-dependent histone H3K4Me2 demethylase. Inhibition of LSD1 increases H3K4Me3-a permissive mark for gene expression, and inhibits growth of pluripotent cancer cells. We have previously noted that treatment with the histone deacetylase (HDAC) inhibitor panobinostat (PS) depletes EZH2 (the catalytic subunit of the polycomb repressive complex 2, PRC2) and disrupts its interaction with the other PRC2 proteins, attenuates LSD1, and de-represses growth inhibitory and pro-apoptosis genes. Methods: In the present studies, we determined the chromatin effects and the pre-clinical in vitro and in vivo anti-AML activity of the novel, non-monoamine oxidase, reversible inhibitor of LSD1, HCI2509 (100 to 500 nM), alone and in combination with PS, utilizing cultured (OCI-AML3 and HL-60) and primary human AML blast progenitor cells. Results: Treatment with HCI2509 dose-dependently increased the levels of H3K4Me3, p16, p27, and CEBPα in cultured AML cells. This correlated with inhibition of cell growth and induction of morphologic differentiation in AML cells. Exposure to HCI2509 disrupted the binding of LSD1 with the co-repressor CoREST and HDAC1. Treatment with PS (10 to 50 nM) dose-dependently depleted not only LSD1 but also EZH2, SUZ12 and BMI1 in AML cells. Co-treatment with PS enhanced the chromatin modifying effects of HCI2509. The combination also synergistically induced loss of viability of cultured and primary AML cells (combination indices, CI <1.0). Following tail vein infusion and establishment of AML by OCI-AML3 cells in NOD-SCID mice, treatment with HCI2509 (25 mg/kg, b.i.w, IP, for 3 weeks) improved survival, as compared to the control mice (p <0.001). HCI2509 treatment (15 mg/kg IP) also dramatically improved survival of NSG mice with established human AML following tail-vein injection of primary AML blasts expressing FLT3-ITD. Survival was further improved upon co-treatment with HCI2509 and PS (5 mg/kg IP, MWF). Mice did not experience any toxicity or weight loss. Conclusions: Combined epigenetic therapy with HCI2509 and PS exerts superior pre-clinical activity, warranting further in vivo development and testing of HCI2509 against human AML.