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BACKGROUND Internal tandem duplication in the fms-like tyrosine kinase 3 receptor tyrosine kinase (FLT3-ITD) is present in acute myeloid leukemia (AML) in 30% of patients, associated with poor treatment outcomes due to rapid relapse. FLT3 inhibitors are used in the clinic, but with incomplete efficacy and development of resistance. Further treatment options are needed. The serine/threonine kinase proviral integration site for Moloney murine leukemia virus (Pim-1) is upregulated downstream of FLT3-ITD; it directly stimulates cell growth and inhibits apoptosis, and also phosphorylates and stabilizes FLT3 in a positive feedback loop in cells with FLT3-ITD. Dual targeting of Pim-1 and FLT3 is a promising treatment strategy. The c-Myc transcription factor contributes to dysregulation of cell growth and apoptosis in cancers, including AML. In addition to transcriptional regulation, c-Myc is regulated post-translationally by T58 phosphorylation by the serine/threonine kinase glycogen synthase kinase-3- β (GSK-3β). Here we show that concurrent treatment of cells with FLT3-ITD with Pim and FLT3 inhibitors activates GSK-3β, which phosphorylates and post-translationally downregulates c-Myc. METHODS Ba/F3-ITD and MV4-11 cells, with FLT3-ITD, and FLT3-ITD AML patient blasts were cultured with the pan-Pim inhibitor AZD1208 (1 μM) and/or the FLT3 inhibitors gilteritinib or quizartinib (15 nM, 1 nM), with and without the GSK-3β inhibitor TCG-24 (20 μM). c-Myc, p-GSK3-α/β (S21/9) and GSK3-α/β protein expression was measured by immunoblotting. c-Myc mRNA was measured by qPCR. Cells were also cultured with cycloheximide (100 μg/mL) with and without the proteasome inhibitor MG-132 (20 μM) to measure protein half-life and proteasomal degradation. To study the role of c-Myc overexpression and activation, Ba/F3-ITD cells were infected with retroviral estrogen receptor (ER)-Myc plasmid, causing c-Myc nuclear translocation when activated by 4-hydroxytamoxifen (4-OHT; 300 nM). To study the role of c-Myc phosphorylation at T58, Ba/F3-ITD cells were infected with MycT58A plasmid, preventing c-Myc phosphorylation at T58. Apoptosis was detected by Annexin V and propidium iodide staining, measured by flow cytometry. RESULTS Treatment with Pim inhibitor AZD1208 and FLT3 inhibitor gilteritinib or quizartinib combination rapidly downregulated c-Myc protein expression in Ba/F3-ITD and MV4-11 cells, with FLT3-ITD, and in primary FLT3-ITD AML patient blasts, compared to quizartinib or gilteritinib alone. Pim inhibitor and FLT3 inhibitor combination treatment did not decrease c-Myc mRNA levels, but markedly decreased c-Myc protein half-life, from 36 mins without drugs and 24 mins with gilteritinib to 18 mins with combination. Half-life did not decrease when cells were pre-treated with the proteasome inhibitor MG-132, consistent with post-translational downregulation through proteasomal degradation. Apoptosis induction by Pim inhibitor and FLT3 inhibitor combination decreased by more than 50% in Ba/F3-ITD cells infected with ER-Myc plasmid and treated with 4-OHT, demonstrating the major role of c-Myc downregulation in apoptosis induction by combination treatment. GSK-3b is inactivated by phosphorylation, and combination treatment rapidly decreased p-GSK-3b levels, while total GSK-3b levels were unchanged, indicating activation of GSK-3b. Treatment of cells with FLT3-ITD with the GSK-3b inhibitor TCG-24 in addition to Pim and FLT3 inhibitors abrogated c-Myc protein downregulation, demonstrating that Pim and FLT3 inhibitor combination downregulates c-Myc through activation of GSK-3b. Finally, Pim and FLT3 inhibitor combination treatment did not downregulate c-Myc in Ba/F3-ITD cells transfected with c-Myc T58A, preventing c-Myc phosphorylation at T58, showing that c-Myc phosphorylation at T58 is necessary for its downregulation by combination treatment. CONCLUSIONS Concurrent treatment of cells with FLT3-ITD with Pim kinase inhibitor enhances the efficacy of FLT3 inhibitors through activation of GSK-3β and GSK-3β-mediated phosphorylation of c-Myc at T58, with resulting c-Myc downregulation through increased proteasomal degradation. This work and previous work in our laboratory on PP2A activating drugs and FLT3 inhibitor combination (Mol Cancer Ther 20:676, 2021) support GSK-3β activation as a mechanism for enhancing efficacy of FLT3 inhibitors in AML with FLT3-ITD. Disclosures No relevant conflicts of interest to declare.

Introduction fms-like tyrosine like kinase 3 internal tandem duplication (FLT3-ITD), present in acute myeloid leukemia (AML) cells of 30% of patients, results in constitutive and aberrant FLT3 signaling and, clinically, short disease-free survival. Efficacy of FLT3 inhibitors is limited and transient, but may be enhanced by dual targeting of FLT3-ITD signaling pathways. The tumor suppressor protein phosphatase 2A (PP2A) is inhibited in cells with FLT3-ITD. The oncogenic serine/threonine kinase Pim-1 is transcriptionally upregulated and also stabilized by PP2A inhibition in cells with FLT3-ITD. Pim-1 contributes directly to FLT3-ITD proliferative and anti-apoptotic effects, and also phosphorylates and stabilizes FLT3-ITD in a positive feedback loop. Moreover FLT3-ITD, PP2A and Pim-1 all regulate the transcription factor c-Myc. PP2A-activating drugs enhance efficacy of FLT3 inhibitors. We sought to identify mechanisms underlying the efficacy of this combination. Methods Ba/F3-ITD and MV4-11 cells, with FLT3-ITD, and blasts from patients with AML with FLT3-ITD were cultured with the FLT3 inhibitors gilteritinib (15 nM) or quizartinib (1 nM) and/or the PP2A-activating drugs FTY720 (2-4 µM) or DT-061 (10 µM), or with DMSO control. Pim-1, c-Myc, p-AKT (S473 and T308) and AKT protein expression was measured by immunoblotting, along with p-STAT5 (Y694), STAT5, p-PP2A (Y307) and PP2A expression. To study post-translational regulation, cells were cultured with cycloheximide (100 µg/mL) with and without the proteasome inhibitor MG-132 (20 µM). Ubiquitinated c-Myc was measured by co-immunoprecipitation and immunoblotting with c-Myc and ubiquitin antibodies. Ba/F3-ITD cells were stably transfected with estrogen receptor (ER)-c-Myc, kinase-dead Pim-1 or myristoylated AKT plasmids or corresponding empty vectors. Apoptosis was detected by Annexin V and propidium iodine staining, measured by flow cytometry. Cells were also cultured with the pan-Pim kinase inhibitor AZD1208 (1 µM), the Myc inhibitor 10058-F4 (100 µM) or the pan-AKT inhibitor MK-2206 (5 µM). Results Concurrent treatment of Ba/F3-ITD and MV4-11 cells and primary AML cells with FLT3-ITD with a FLT3 inhibitor (gilteritinib or quizartinib) and a PP2A-activating drug (FTY720 or DT-061) decreased growth and increased apoptosis induction, relative to treatment with single drugs. Concurrent FLT3 inhibitor and PP2A-activating drug treatment decreased expression of both Pim-1 and c-Myc protein. Concurrent treatment decreased Pim-1 half-life from 15 to 5 minutes, and c-Myc half-life from 30 to 5 minutes, while half-lives were restored by concurrent treatment with the proteasome inhibitor MG-132. Concurrent treatment was also shown to increase c-Myc ubiquitination. Effects of concurrent treatment on Pim-1 and c-Myc were independent, as transfection with kinase-dead Pim-1 or treatment with Pim inhibitor AZD1208 did not alter c-Myc downregulation, and c-Myc overexpression or treatment with Myc inhibitor 10058-F4 did not alter Pim-1 downregulation. Concurrent treatment with FLT3 inhibitor and PP2A-activating drug did not alter expression of c-Myc deubiquitinases, but rapidly decreased AKT S473 and T308 phosphorylation. FLT3 inhibitor and PP2A activator co-treatment did not induce downregulation or increased turnover of Pim-1 and c-Myc protein or apoptosis in cells with constitutive AKT activation caused by transfection of myristoylated AKT. Moreover, AKT inhibition downregulated Pim-1 and c-Myc protein expression, decreased Pim-1 and c-Myc protein half-lives from 15 to 5 minutes and 30 to 10 minutes, respectively, and induced apoptosis of cells with FLT3-ITD, replicating the effects of FLT3 inhibitor and PP2A activator co-treatment. Conclusion PP2A activators enhance the efficacy of FLT3 inhibitors in AML cells with FLT3-ITD through AKT inactivation-dependent increased Pim-1 and c-Myc proteasomal degradation, which is a novel mechanism. The data support further preclinical and clinical testing of this dual targeting approach to treatment of AML with FLT3-ITD. Disclosures Baer: Takeda: Research Funding; Incyte: Research Funding; Kite: Research Funding; Forma: Research Funding; AI Therapeutics: Research Funding; Abbvie: Research Funding; Astellas: Research Funding.

Introduction: In 30% of acute myeloid leukemia (AML) patients, internal tandem duplication of fms-like tyrosine kinase 3 (FLT3-ITD) causes constitutive and aberrant FLT3 signaling, and these patients have short relapse-free and overall survival. FLT3 inhibitors have limited and transient efficacy, but their efficacy may be enhanced by combination with other drugs targeting FLT3 signaling. FLT3 activation also inhibits the tumor suppressor protein phosphatase 2A (PP2A). FLT3 inhibitors and PP2A-activating drugs have been shown to induce synergistic cytotoxicity in cells with FLT3-ITD. To address mechanisms underlying this effect, we studied effects of combination therapy on the oncogenic serine/threonine kinase Pim-1 and the transcription factor c-Myc, both of which are upregulated in cells with FLT3-ITD and are also PP2A substrates. Methods: Ba/F3-ITD and MV4-11 cells and AML patient blasts with FLT3-ITD were cultured with a FLT3 inhibitor, gilteritinib (ASP2215) or quizartinib (AC220), and/or the PP2A-activating drug fingolimod (FTY720) at pharmacologically relevant concentrations, or DMSO control. Drug combination effects were measured by combination index determined by the Chou-Talalay method using CompuSyn software. Apoptosis was measured by Annexin V/propidium iodide staining detected by flow cytometry. c-Myc and GAPDH control mRNA was measured by real-time polymerase chain reaction. Pim-1 kinase, c-Myc, phospho-c-MycSer62, phospho-c-MycThr58, phospho-STAT5Tyr694, STAT5, phospho-PP2ATyr307, PP2A, phospho-BADSer112 and BAD levels were measured by immunoblotting. Cycloheximide treatment was used to assess protein stability. Protein expression and stability were measured with and without the proteasome inhibitor MG-132. Pim-1 kinase was inhibited with the pan-Pim inhibitor AZD1208. Ba/F3-ITD cells were infected with pMX-Flag-K67M kinase-dead (KD) Pim-1 and empty pMX retroviral vectors and with pBABE-ER-cMYC and with empty pBABE-ER retroviral vectors. Results: Concurrent treatment with 15 nM gilteritinib or 1 nM quizartinib and FTY720 2 µM in cell lines and 4 µM in patient samples decreased growth and increased apoptosis of cells with FLT3-ITD, relative to single drug treatments, and produced synergistic cytotoxicity. FLT3 inhibition was confirmed by decrease in phospho-STAT5 and PP2A activation by decreased phospho-PP2A. Concurrent treatment decreased expression of both Pim-1 and c-Myc protein, but not c-Myc mRNA, in Ba/F3-ITD and MV4-11 cells and AML patient blasts with FLT3-ITD, relative to single drug treatments. Additionally, selective decrease in phospho-MycSer62, a stable c-Myc phosphoprotein that is dephosphorylated by PP2A, was seen, with persistence of phospho-c-MycThr58. FLT3 inhibitor and PP2A activator combination treatment was found to decrease stability of c-Myc and Pim-1 protein, in relation to single drugs. Moreover, pretreatment with the proteasome inhibitor MG-132 abrogated downregulation of Pim-1 and c-Myc protein expression and decrease in Pim-1 and c-Myc protein stability in Ba/F3-ITD cells treated with FLT3 inhibitor and PP2A activator. Pretreatment with the pan-Pim kinase inhibitor AZD1208, with Pim-1 inhibition confirmed by decreased phospho-BADS112 had no effect on c-Myc downregulation, and c-Myc was similarly downregulated in Pim-1 kinase-dead cells as in parental and empty-vector cells, demonstrating that combination treatment effects on c-Myc are not Pim-1 kinase-dependent. Additionally, FLT3 inhibitor and PP2A-activating drug combination induced apoptosis in 30% of cells with c-Myc overexpression, compared to 60% of parental and empty vector-infected cells. Finally, c-Myc overexpression did not abrogate Pim-1 downregulation by combination treatment. Conclusions: Concurrent FLT3 inhibitor and PP2A activating drug treatment induces synergistic cytotoxicity in AML cells with FLT3 internal tandem duplication through proteasomal degradation of Pim-1 and c-Myc, and effects on Pim-1 and c-Myc are independent. The data support in vivo testing of FLT3 inhibitor and PP2A-activating drug combinations and development of a clinical trial. Disclosures No relevant conflicts of interest to declare.