Imidazoquinoxaline derivative EAPB0503: A promising drug targeting mutant nucleophosmin 1 in acute myeloid leukemia

BACKGROUND Nucleophosmin 1 (NPM1) is a nucleocytoplasmic shuttling protein mainly localized in the nucleolus. NPM1 is frequently mutated in acute myeloid leukemia (AML). NPM1c oligomerizes with wild‐type nucleophosmin 1 (wt‐NPM1), and this leads to its continuous cytoplasmic delocalization and contributes to leukemogenesis. Recent studies have shown that Cytoplasmic NPM1 (NPM1c) degradation leads to growth arrest and apoptosis of NPM1c AML cells and corrects wt‐NPM1 normal nucleolar localization. METHODS AML cells expressing wt‐NPM1 or NPM1c or transfected with wt‐NPM1 or NPM1c as well as wt‐NPM1 and NPM1c AML xenograft mice were used. Cell growth was assessed with trypan blue or a CellTiter 96 proliferation kit. The cell cycle was studied with a propidium iodide (PI) assay. Caspase‐mediated intrinsic apoptosis was assessed with annexin V/PI, the mitochondrial membrane potential, and poly(adenosine diphosphate ribose) polymerase cleavage. The expression of NPM1, p53, phosphorylated p53, and p21 was analyzed via immunoblotting. Localization was performed with confocal microscopy. The leukemia burden was evaluated by flow cytometry with an anti‐human CD45 antibody. RESULTS The imidazoquinoxaline 1‐(3‐methoxyphenyl)‐N‐methylimidazo[1,2‐a]quinoxalin‐4‐amine (EAPB0503) induced selective proteasome‐mediated degradation of NPM1c, restored wt‐NPM1 nucleolar localization in NPM1c AML cells, and thus yielded selective growth arrest and apoptosis. Introducing NPM1c to cells normally harboring wt‐NPM1 sensitized them to EAPB0503 and led to their growth arrest. Moreover, EAPB0503 selectively reduced the leukemia burden in NPM1c AML xenograft mice. CONCLUSIONS These findings further reinforce the idea of targeting the NPM1c oncoprotein to eradicate leukemic cells and warrant a broader preclinical evaluation and then a clinical evaluation of this promising drug. Cancer 2017;123:1662–1673. © 2017 American Cancer Society. Nucleophosmin 1 (NPM1) is one of the most frequently mutated genes and accounts for approximately one‐third of all acute myeloid leukemia cases. The imidazoquinoxaline 1‐(3‐methoxyphenyl)‐N‐methylimidazo[1,2‐a]quinoxalin‐4‐amine (EAPB0503) induces proteasomal degradation of mutant NPM1 (NPM1c) and restoration of wild‐type NPM1 nucleolar localization, and this results in NPM1c acute myeloid leukemia cell growth arrest and apoptosis. This drug selectively reduces the leukemia burden of NPM1c (cytoplasmic NPM1) acute myeloid leukemia in mice.