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Supplementary Table S1. AML patients. Supplementary Table S2. RT-qPCR primers. Supplementary Table S3. IC50 concentrations (nM) of FLT3 inhibitors, PP2A activators and GSK-3beta inhibitors. Supplementary Figure S1. Concurrent PP2A-activating drug treatment increases cytotoxicity of FLT3 inhibitors in Ba/F3-ITD cells with FLT3-ITD and in an MV4-11 orthotopic mouse model. Supplementary Figure S2. Chou-Talalay analysis of FLT3-WT cell lines. Supplementary Figure S3. Images of all mice in the in vivo experiment. Supplementary Figure S4. Concurrent treatment with PP2A-activating drug and FLT3 inhibitor does not induce apoptosis in wild-type FLT3 AML, AML complete remission (CR) or AML complete remission with incomplete platelet recovery (CRp) marrows. Supplementary Figure S5. Concurrent treatment with PP2A-activating drug and FLT3 inhibitor downregulates c-Myc and Pim-1 protein expression in cells with FLT3-ITD. Supplementary Figure S6. p-c-Myc (T58) and p-c-Myc (S62) expression in Ba/F3-ITD and MV4-11 cells treated with gilteritinib and/or FTY720. Supplementary Figure S7. Concurrent treatment with PP2A-activating drug and FLT3 inhibitor produces variable changes in c-Myc and Pim-1 mRNA expression in cells with FLT3-ITD. Supplementary Figure S8. c-Myc and Pim-1 expression in FLT3-WT AML blasts treated with gilteritinib and/or FTY720. Supplementary Figure S9. Proteasome inhibition inhibits downregulation of c-Myc and Pim-1 expression by concurrent PP2A-activating drug and FLT3 inhibitor treatment. Supplementary Figure S10. Concurrent PP2A-activating drug and FLT3 inhibitor treatment increases c-Myc and Pim-1 protein turnover in cells with FLT3-ITD. Supplementary Figure S11. Concurrent PP2A activator and FLT3 inhibitor treatment downregulates p-ERK later than p-AKT in cells with FLT3-ITD. Supplementary Figure S12. The GSK-3beta inhibitor TWS119 prevents c-Myc and Pim-1 downregulation and apoptosis induction by PP2A-activating drug and FLT3 inhibitor combination in Ba/F3-ITD cells.

Fms-like tyrosine-like kinase 3 internal tandem duplication (FLT3-ITD) is present in acute myeloid leukemia (AML) in 30% of patients and is associated with short disease-free survival. FLT3 inhibitor efficacy is limited and transient but may be enhanced by multitargeting of FLT3-ITD signaling pathways. FLT3-ITD drives both STAT5-dependent transcription of oncogenic Pim-1 kinase and inactivation of the tumor-suppressor protein phosphatase 2A (PP2A), and FLT3-ITD, Pim-1, and PP2A all regulate the c-Myc oncogene. We studied mechanisms of action of cotreatment of FLT3-ITD–expressing cells with FLT3 inhibitors and PP2A-activating drugs (PADs), which are in development. PADs, including FTY720 and DT-061, enhanced FLT3 inhibitor growth suppression and apoptosis induction in FLT3-ITD–expressing cell lines and primary AML cells in vitro and MV4-11 growth suppression in vivo. PAD and FLT3 inhibitor cotreatment independently downregulated c-Myc and Pim-1 protein through enhanced proteasomal degradation. c-Myc and Pim-1 downregulation was preceded by AKT inactivation, did not occur in cells expressing myristoylated (constitutively active) AKT1, and could be induced by AKT inhibition. AKT inactivation resulted in activation of GSK-3β, and GSK-3β inhibition blocked downregulation of both c-Myc and Pim-1 by PAD and FLT3 inhibitor cotreatment. GSK-3β activation increased c-Myc proteasomal degradation through c-Myc phosphorylation on T58; infection with c-Myc with T58A substitution, preventing phosphorylation, blocked downregulation of c-Myc by PAD and FLT3 inhibitor cotreatment. GSK-3β also phosphorylated Pim-1L/Pim-1S on S95/S4. Thus, PADs enhance efficacy of FLT3 inhibitors in FLT3-ITD–expressing cells through a novel mechanism involving AKT inhibition–dependent GSK-3β–mediated increased c-Myc and Pim-1 proteasomal degradation.

The transcription factor c-Myc regulates diverse biological processes including growth, proliferation, differentiation, quiescence, and senescence or apoptosis. c-Myc proto-oncogene overexpression has been implicated in the genesis of many human malignancies. Identification of compounds that selectively and potently inhibit the expression of c-Myc hence could be beneficial for the treatment of these diseases. We and others have recently shown that inhibiting the binding of bromodomain-containing proteins such as Brd4 to their acetylated histone substrates leads to the specific suppression of c-Myc transcription. We report here the characterization of CPI-267203, a potent, selective and competitive inhibitor of BET domain proteins (Brd4 IC50 = 26 nM and CBP IC50 = 6.0 uM). In vitro, this molecule inhibits IL-6 production (EC50 = 11 nM) induced by lipopolysaccharide (LPS) in acute monocytic leukemia cells (THP1) and c-Myc mRNA production (EC50 = 27 nM) in Burkitt lymphoma cells (Raji). c-Myc mRNA and protein levels decline as early as 4hrs after treatment with CPI-267203, leading to cell cycle arrest and apoptosis. In vivo, CPI-267203 has 84% oral bioavailability and a plasma half-life of 1.9 h. In a xenograft model of human acute myelogenous leukemia (MV4:11), CPI-267203 inhibits the production of c-Myc mRNA and protein, inhibits tumor growth, and triggers tumor apoptosis. Reduction of c-Myc protein levels can also be detected in the normal skin of treated animals, suggesting that the level of c-Myc in the skin might be a useful surrogate pharmacodynamic marker to aid clinical trials. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-237. doi:1538-7445.AM2012-LB-237

The cellular response to DNA double-strand break (DSB) formation is an essential component of normal cell survival, following exposure to DNA-damaging chemicals and ionising radiation. The serine/threonine kinase DNA-dependent protein kinase (DNA-PK) is a member of the phosphatidylinositol (PI) 3-kinase related kinase (PIKK) family of enzymes, and plays an important role in DNA DSB repair via the non-homologous end-joining (NHEJ) pathway. DNA-PK inhibitors may, therefore, be useful as agents to improve the activity of radio- and chemo-therapy in the treatment of cancer. Identification of the lead benzo[h]chromen-4-one DNA-PK inhibitor NU7026 (IC50 = 0.23 uM) guided the development of the potent and selective ATP-competitive chromenone NU7441 (DNA-PK IC50 = 30 nM). Structure-activity relationship studies for DNA-PK inhibition by chromenone-derivatives were conducted in conjunction with homology modelling. Library synthesis was undertaken employing a solution multiple-parallel approach, by O-alkylation or N-acylation of the appropriately substituted NU7441 derivatives, respectively, followed by reaction with a range of amines to afford the target compounds. These studies resulted in the identification of the highly potent inhibitor KU-0060648 (IC50 = 8.6 nM). The further development of KU-0060648 and analogues will be described. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5780.