NAMPT Inhibitor KPT-9274 Selectively Targets Self-Renewal Capacity in Acute Myeloid Leukemia

Acute Myeloid Leukemia (AML) is a heterogeneous disease that is characterized by an accumulation of neoplastic myeloid precursor cells in the bone marrow. Recently, multiple agents targeting AML associated mutations in FLT3, IDH1 and IDH2 have been developed. However, a majority of AML patients lack these mutations. Therefore, development of a novel therapeutic approach broadly relevant to AML would be attractive. The clonal capacity of AML cells is maintained by leukemic initiating cells (LICs), which possess self-renewal capabilities and are resistant against cytotoxic combination chemotherapy. Nicotinamide phosphoribosyltransferase (NAMPT) catalyzes the rate-limiting step in the generation of NAD⁺, which is an important enzymatic cofactor and serves as a metabolite required for cellular respiration. LICs show a higher energy turnover rate than normal cells and are heavily reliant on oxidative phosphorylation. This suggests that energy generation processes, such as NAD⁺ biosynthesis, are critically required in myeloid malignancies. Thus, targeting the regeneration of NAD⁺ offers an attractive alternative therapeutic strategy in AML. Unlike many targeted therapies that are limited to one genetic subtype of AML, targeting regeneration of NAD⁺ via NAMPT inhibition could be relevant to a much broader patient population based upon metabolic differences between tumor and normal cells.

We show that inhibition of NAMPT using the agent KPT-9274 induces loss of glycolytic and mitochondrial activity, more specifically a depleted total spare reserve capacity (p<0.001), in MV4-11 and THP-1 cell lines. KPT-9274 treatment decreased colony formation in AML patient samples across multiple genotypes (p<0.001) with a minimal decrease in colony formation by normal CD34+ hematopoietic cells. In addition, KPT-9274 treatment of AML cells decreased re-plating capacity, suggesting decreased self-renewal capacity (p=0.031). In contrast to pre-clinical activity observed in AML, there was minimal depletion of absolute counts observed in cell numbers of B, T and NK-lymphocyte treated ex-vivo as measured by flow cytometry, suggesting potent selectivity of KPT-9274. Furthermore, in a PDX mouse model of AML, KPT-9274 treatment significantly reduced disease burden (p<0.001) and prevented leukemic infiltration in spleen, bone marrow, and liver. In addition, KPT-9274 treatment induced differentiation, which is evident by a decrease in blast percentage and an introduction of segmented and band neutrophils in bone marrow differentials. Mice treated with KPT-9274 showed a reduction of bone marrow CD34+/CD38- cells (p<0.001), which are considered the stem cell population within the bulk leukemic cells. Extending our findings, consecutive re-plating of KPT-9274-treated PDX leukemic cells showed a marked decrease in colony formation, indicative of a loss of self-renewal capacity. KPT-9274 in combination with the anthracycline doxorubicin further decreased the infiltration of AML cells in comparison to either treatment alone. Overall, our data show that the NAMPT inhibitor KPT-9274 targets the leukemia initiating cell population, a potential source of resistance to cytotoxic chemotherapy, which is not adequately eradicated by current therapies in most patients.