Purpose: Nicotinamide phosphoribosyltransferase (NAMPT) inhibitors are currently in development, but may be limited as a single agent therapy due to compound-specific toxicity and cancer metabolic plasticity allowing resistance development. To potentially lower the doses of NAMPT inhibitors required for therapeutic benefit against acute myeloid leukemia (AML), we performed a genome-wide CRISPRi screen to identify rational disease-specific partners for a novel NAMPT inhibitor, KPT-2974. Experimental Design: Cell lines and primary cells were analyzed for cell viability, self-renewal and responses at RNA and protein levels with loss-of-function approaches and pharmacologic treatments. In vivo efficacy of combination therapy was evaluated with a xenograft model. Results: We identified two histone deacetylases, HDAC8 and SIRT6, whose knockout conferred synthetic lethality with KPT-9274 in AML. Furthermore, HDAC8-specific inhibitor, PCI-34051, or clinical Class I HDAC inhibitor, AR-42, in combination with KPT-9274, synergistically decreased the survival of AML cells in a dose-dependent manner. AR-42/KPT-9274 co-treatment attenuated colony-forming potentials of patient cells while sparing healthy hematopoietic cells. Importantly, combined therapy demonstrated promising in vivo efficacy compared with KPT-9274 or AR-42 monotherapy. Mechanistically, genetic inhibition of SIRT6 potentiated the effect of KPT-9274 on PARP-1 suppression by abolishing mono-ADP ribosylation. AR-42/KPT-9274 co-treatment resulted in synergistic attenuation of homologous recombination (HR) and nonhomologous end joining (NHEJ) pathways in cell lines and leukemia initiating cells (LICs). Conclusions: Our findings provide evidence that HDAC8 inhibition- or shSIRT6-induced DNA repair deficiencies are potently synergistic with NAMPT targeting, with minimal toxicity towards normal cells, providing a rationale for a novel-novel combination-based trea tment for AML.
