Clinical application value of detecting the mutation load of the FLT3-ITD gene before allogeneic hematopoietic stem cell transplantation using PCR-NGS technology

A consistent pattern of response has been observed when FMS-like tyrosine kinase 3 (FLT3) tyrosine kinase inhibitors (TKIs) have been used as monotherapy to treat patients with relapsed or refractory FLT3- internal tandem duplication (ITD) acute myeloid leukaemia (AML). Circulating blasts are cleared from the peripheral blood, while bone marrow blasts are either unaffected or are cleared from the marrow at a much slower rate. We used an in vitro model of FLT3-ITD AML blasts co-cultured with normal human bone marrow stromal cells to investigate the basis for this dichotomous response pattern to FLT3 inhibitors. We have found that in blasts on stroma, potent FLT3 inhibition predominantly results in cell cycle arrest rather than apoptosis. The anti-apoptotic effect is mediated through a combination of direct cell-cell contact and soluble factors. The addition of exogenous FLT3 ligand (FL) augments the protection, primarily by shifting the 50% inhibitory concentration for FLT3 inhibition upwards. Cytokine-activated extracellular regulated kinase (ERK), rather than STAT5, appears to be the most important downstream signalling protein mediating the protective effect, and inhibition of MEK significantly abrogates stromal-mediated resistance. These findings explain the phenomenon of peripheral blood versus bone marrow blast responses and suggest that the combination of potent FLT3 inhibition and MEK inhibition is a promising strategy for the treatment of FLT3-ITD AML.

Evaluating Impact of Insertion Length on Clinical Outcomes in FLT3-ITD Mutated AML: A Single Center Retrospective Analysis

A Comprehensive Analysis of FLT3 Mutation Profiles and Clinical Outcomes in Adult Patients with Acute Myeloid Leukemia: A Single Institution Experience

FLT3 Inhibitor Treatment Increases FLT3 Expression That Exposes FLT3-ITD+ AML Blasts to Elimination By FLT3 CAR-T Cells

Concurrent Inhibition of Pim-1 and FLT3 Kinases in FLT3-ITD Acute Myeloid Leukemia Post-Translationally Downregulates the Anti-Apoptotic Protein Mcl-1 through Downregulation of the Mcl-1 Deubiquitinase USP9X

Acute myeloid leukemia (AML) with FMS-like tyrosine kinase 3 (FLT3) mutations is associated with poor prognosis with a high risk of relapse after therapy and reduces overall survival. Approximately 30 % of AML patients carry FLT3 internal tandem duplication (ITD) or tyrosine kinase domain (TKD) mutations. Sustained FLT3 inhibition can result in the emergence of resistance-conferring genetic alteration in TKD domain, usually at residues D835 and F691. Therefore, acquired TKD mutations has become a critical therapeutic target in AML therapy. On the other hand, SYK is one of kinases deeply implicated in many hematologic malignancies and highly activated in FLT3 mutation AML. SYK overexpression is known to promote over transformation of FLT3 driven AML and induce resistance to FLT3 targeted therapy. In this study, we characterized HM43239, a novel FLT3 inhibitor with SYK inhibitory activity, and assessed its potential as a novel therapeutic agent to overcome the resistances against AML therapy using current FLT3 inhibitors. HM43239 is an orally active small molecule inhibitor and it exhibited sub-nanomolar potency on binding affinity for FLT3 wild type, ITD, TKDs and ITD/TKDs mutants. HM43239 potently inhibited phosphorylation of FLT3 and its downstream such as p-STAT3/p-STAT5 dose dependently in both MOLM-14 cells harboring FLT3 ITD/F691L and FLT3 ITD/D835Y. In KG-1a cells, HM43239 potently inhibited phosphorylation of SYK, STAT3 and STAT5. Moreover, it inhibited the proliferation and induced the apoptosis of leukemic stem cell (LSC) marker-expressing KG-1a cells (CD34+/CD38- cells), suggesting the possibility of targeting LSC. Furthermore, HM43239 showed to be exhibited good inhibitory activity against FLT3 mutated AML cell lines and effectively regress the tumors in MOLM-14 cells expressing FLT3 ITD/F691L or FLT3 ITD/D835Y xenograft mice models. On the other hand, it was confirmed that HM43239 strongly inhibited the phosphorylation of SYK in the medium of co-culture of stromal cells and AML cells. As a result, HM43239 alone more effectively induced tumor regression and prolonged the survival duration of animals than an approved FLT3 inhibitor (e.g. gilteritinib) in resistant FLT3 ITD/D835Y or ITD/F691L mutated MOLM-14 xenograft mice models. These results suggest that HM43239 could overcome the resistance induced by bone marrow microenvironment in AML patients. Taken together, HM43239 showed strong anticancer activity through various in vitro and in vivo preclinical models of AML, implicating the mechanism of overcoming resistance and preventing relapse. The effect of HM43239 in human would be demonstrated in ongoing Phase I/II clinical trials (NCT03850574) to develop promising therapeutics for patients with AML. Citation Format: Inhwan Bae, Jaeyul Choi, Jiyoung Song, Joo-Yun Byun, Eunyoung Lee, Taehun Song, Yu-Yon Kim, Yesol Bak, Young Hoon Kim, Young Gil Ahn, Kwee Hyun Suh. HM43239, a novel FLT3 inhibitor, has the potential to inhibit mutations resistant to FLT3 inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1257.

FLT3 Inhibitor Treatment in FLT3-Mutated AML Is Associated with Development of Secondary FLT3-TKD Mutations

Tyrosine Kinase Inhibitor-Induced Defects in DNA Repair Sensitize FLT3(ITD)-Positive Acute Myeloid Leukemia Quiescent and Proliferative Cells to PARP Inhibitors

Expert Curation of Somatic FLT3 Variants By the Clingen Somatic Hematologic Cancer Taskforce (ClinGen HCT)

Introduction: Acute myeloid leukemia (AML) with FMS-like tyrosine kinase 3 (FLT3) mutation is associated with poor prognosis with a high risk of relapse after therapy and reduced overall survival. Currently, FLT3 inhibitors have shown clinical benefits in the corresponding AML patients. Activating mutations within internal tandem duplication (ITD) and tyrosine kinase domain (TKD) point mutations of FLT3 have been reported as oncogenic driver mutations in about 30% of AML. The acquired D835Y and F691L point mutations of FLT3-TKD are associated with resistance to FLT3-targeted AML therapy. In this study, we have characterized HM43239, a novel FLT3 inhibitor, and assessed its potential as a novel therapy in overcoming resistance for AML patients. Materials and Methods: In vitro site-directed competition binding assay was performed to measure interactions between HM43239 and FLT3 mutations. Standard proliferation assay, immunoblotting, and apoptosis analysis were carried out to validate the potency of HM43239 in AML resistance cell lines. In vivo study, HM43239 was evaluated in Ba/F3 cells expressing FLT3 ITD/F691L or FLT3 ITD/D835Y xenograft mice models. Combination studies were evaluated in Acute Myeloid Leukemia xenograft mice models. Results: HM43239 potently inhibited both FLT3 ITD/D835Y and FLT3 ITD/F691L mutations in preclinical evaluation. It showed high in vitro binding affinity to both mutations, and exhibited potent inhibitory activity in in vitro and in vivo models using Ba/F3 cells expressing FLT3 ITD/D835Y or FLT3 ITD/F691L. Moreover, HM43239 could overcome the FL-induced drug resistance with a higher cytotoxic potency in MOLM-14 cells harboring FLT3 ITD. In KG-1a cells, HM43239 potently inhibited phosphorylation of SYK, STAT3 and STAT5. In addition, it inhibited the proliferation and induced the apoptosis of leukemic stem cell (LSC) marker-expressing KG1a cells (CD34+/CD38- cells), suggesting the possibility of targeting LSC. Also, HM43239 significantly inhibited p-FLT3 and p-STAT5 under normal human plasma milieu in a dose-dependent manner in Ba/F3 and MOLM-14 cell line harboring FLT3 ITD. Furthermore, the combination treatment of HM43239 with various reagents (e.g., IAP inhibitor, chemotherapy, etc) demonstrated synergistic efficacy in mouse models, xenografted with both MV-4-11 and MOLM-13 cell lines without any significant toxicity. Conclusion: Taken together, HM43239 demonstrated the potential therapeutic efficacy for the treatment of AML patients, and implicated the mechanism of overcoming resistance and preventing relapse. Citation Format: JiSook Kim, InHwan Bae, JaeYul Choi, MinJeong Kim, JooYun Byun, MiJin Moon, EunYoung Lee, Yu-Yon Kim, Hyun Jeong Kang, Eunyoung Kim, SunYoung Jung, YoungGil Ahn, YoungHoon Kim, Kwee Hyun Suh. HM43239, a novel FLT3 inhibitor in overcoming resistance for acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1293.

Adoptive immunotherapy using chimeric antigen receptor (CAR)-modified T cells targeting CD19 has shown remarkable therapeutic efficacy against B cell leukemia and lymphoma, and provided proof of concept for therapeutic potential in other hematologic malignancies. Acute myeloid leukemia (AML) is an entity with an unmet medical need for effective and curative treatments. Therefore, there is a strong desire for development of potentially curative CAR-T cell immunotherapy for AML treatment. FMS-like tyrosine kinase 3 (FLT3) is a homodimeric transmembrane protein expressed uniformly by AML blasts. FLT3 plays a vital role in the survival of AML blasts and is a key driver of leukemia-genesis in AML cases with internal tandem duplication (FLT3ITD) and tyrosine kinase domain (TKD) mutations. These attributes suggest that FLT3 could be an excellent target for CAR-T cell immunotherapy. Here, we engineered human CD4+ and CD8+ T cells to express FLT3-specific CARs and demonstrate that they confer potent reactivity against AML cell lines and primary AML blasts that express either wild-type FLT3 or FLT3-ITD. Further, we show that FLT3 CAR-T cells exert potent antileukemia activity in xenograft models of AML and induce complete remissions. We also demonstrate that FLT3-expression on FLT3-ITD+ AML cells can be augmented by FLT3 inhibitors, which lead to increased recognition by CARs and improved efficacy of FLT3 CAR-T cells. We confirmed this principle with three different FLT3 inhibitors which are at distinct stages of clinical development i.e. Phase II/III clinical trial (crenolanib, quizartinib) and clinically approved (midostaurin). Further, we observed the strongest anti-leukemia activity of FLT3 CAR-T cells in combination with crenolanib in vivo. FLT3 is known to be expressed by normal hematopoietic stem and progenitor cells. We evaluated FLT3-expression on normal hematopoietic stem cells (HSCs) using flow cytometry and confirmed lower level of FLT3-expression on HSCs and progenitors compared to AML cells. As anticipated, we found that FLT3 CAR-T cells recognize normal HSCs in vitro and in vivo, and compromise normal hematopoiesis, suggesting that adoptive therapy with FLT3 CAR-T cells will require successive CAR-T cell depletion and allogeneic HSC transplantation (HSCT) to reconstitute the hematopoietic system. Moreover, an FLT3 inhibitor treatment does not increase FLT3-expression on HSCs. Accordingly, we demonstrate that the depletion of FLT3 CAR-T cells is possible with inducible Caspase 9 (iCasp9) safety switch. Collectively, our data establish FLT3 as a novel CAR target in AML with particular relevance in high-risk FLT3-ITD+ AML. Our data demonstrate that FLT3 CAR-T cells act synergistically with FLT3 inhibitors in FLT3-ITD+ AML. i.e. FLT3 inhibitors-induced upregulation of FLT3 in FLT3-ITD+ AML cells enhances their recognition and elimination by FLT3 CAR-T cells. Due to recognition of normal HSCs, the clinical use of FLT3 CART cells is likely restricted to a defined therapeutic window and must be followed by CART cell depletion and allogeneic HSCT for hematopoietic reconstitution. The data provide rational to use FLT3 CAR-T cells in combination with FLT3 inhibitors to augment the anti-leukemia efficacy of FLT3 CAR-T cells in high-risk FLT3-ITD+ AML patients, and to mitigate the risk of relapse with FLT3-negative AML variants, which could otherwise develop under therapeutic pressure. The data provide proof of concept for synergistic use of CAR-T cell immunotherapy and small molecule targeted therapy and encourage the clinical evaluation of this combination treatment in high-risk patients with FLT3-ITD+ AML.

e15103 Background: Mutations of the FMS-like tyrosine kinase 3 (FLT3) gene occur in approximately 30% of all acute myeloid leukemia (AML) cases, with the internal tandem duplication (ITD) representing the most common type of FLT3 mutation (FLT3-ITD; approximately 25% of all AML cases). Although several FLT3 inhibitors have been developed, occurrence of secondary TKD mutations of FLT3 such as FLT3/D835Y and FLT3/F691L causes the acquired resistance to the current FLT3 inhibitors and eventually become a key area of unmet medical needs. Here, we have revealed that PLM-102, a novel, orally active FLT3 and RET dual inhibitor, has a potential to overcome the acquired resistance to current FLT3 inhibitors. Methods: 1. Kinase assay- Biochemical assays for FLT3 (WT and D835Y) and RET (WT and Mutants) were performed according to ADP-Glo kinase assay protocol(Promega). 2. Cell proliferation and Apoptosis- Human leukemia cell line MV4-11 and MOLM-14 were purchased from ATCC and DSMZ. Cells were seeded at a density of 2 X 103 cells per well and treated with the indicated concentrations of inhibitors for 72 hours at 37°C. Cell viability was determined by an Alamar Blue assay (Bio-Rad). Caspase-3/7 activity was measured by using the Caspase-Glo 3/7 assay (Promega). 3. Western blot analysis- Immunoblotting using MOLM-14 cells was performed using anti-phospho-FLT3 (Cell Signaling Technology #3461) and anti-FLT3 antibody (Cell Signaling Technology #3462). 4. In vivo mouse models- The MV4-11 and MOLM-14 cells are implanted into the subcutaneous space of the left flank of the mice. Resulting tumors are monitored by calipering twice weekly. Treatment started after randomization when tumor volumes had reached a size of approximately 100-150 mm3. For statistical analysis, analysis of variance (ANOVA) was performed using Prism 9.0 to examine statistical differences. Results: In compared to FDA-approved Gilteritinib, PLM-102 showed stronger sub-nanomolar IC50 values in FLT3 kinases regardless of wild-type, ITD, TKD and ITD/TKD mutants in both kinase- and cell-based assays. PLM-102 inhibited phosphorylation of FLT3 and its downstream signaling pathways, and induced apoptosis as evidenced by PARP-cleavage and caspase-3 activation. Moreover, PLM-102 showed an excellent anti-tumor activity in mouse xenograft models implanted with MV4-11 and MOLM-14 AML cells. Conclusions: Taken together, PLM-102 showing potent anti-cancer activity against various in vitro and in vivo AML models could be developed as a valuable agent overcoming the acquired resistance to the current FLT3 inhibitor.

The Pan-FLT3/BTK Multi-Kinase Inhibitor CG '806 Induces AML Killing in FLT-Mutant and Wild Type Cells, and Exerts Synergistic Pro-Apoptotic Effects with Concomitant Targeting of Anti-Apoptotic Bcl-2 and/or Mcl-1

Background: Acute myeloid leukemia (AML) is the most common type of blood cancer. Fmslike tyrosine kinase 3 (FLT3) is a member of the class III receptor tyrosine kinase family. Overexpression of FLT3 was found in 70-100% of patients with acute myeloid leukaemia. FLT3 internal tandem duplication alteration (ITD) and the tyrosine kinase domain (TKD) are the most common molecular alteration in AML, and FLT3 has become a promising drug target for AML. Objective: A series of 6-phenylisoxazolo[3,4-b]pyridin-3-amine derivatives F1–F15 with amide bonds as FLT3 inhibitors were designed and synthesized in order to find a new lead compound to treat AML. Methods: We designed an original scaffold-hopping protocol by combing the RECAP tool with the Gilde-Based Core-Hopping tool to design novel FLT3 inhibitors based on Linifanib. Inhibitors assembled were ranked by the docking scores generated by Glide. Compounds undisclosed among the top 10 were selected to design a series of 6-phenylisoxazolo[3,4-b]pyridin-3-amine derivatives as FLT3 inhibitors. The kinase inhibitory activities of the fifteen compounds were assayed on FLT3 and FLT3-ITD. The antitumor activities of the structurally modified compounds F1–F15 were evaluated against MOLM-13 and MV4-11, typical FLT3-dependent human AML cells carrying FLT3-ITD mutants and the FLT3- independent human cervical carcinoma cell line HL-60 (harboring wide-type FLT3). Results: Structure–activity relationship (SAR) analysis showed that F14 could inhibit FLT3 and FLT3- ITD by 52% and 45.55%, respectively, at the concentration of 1 mΜ. F14 exhibited potent activity against FLT3-dependent human acute myeloid leukemia (AML) cell lines, MOLM-13, and MV4-11 (harboring FLT3-ITD mutant) with IC50 values of 2.558 μM and 1.785 μM, respectively. Conclusion: F14 could be used as a novel lead compound to further develop FLT3 inhibitors against AML with FLT3-ITD mutant.

Introduction: Acute myeloid leukemia (AML) with FMS-like tyrosine kinase 3 (FLT3) mutations is associated with poor prognosis with a high risk of relapse after therapy and reduced overall survival. Activating mutations of internal tandem duplication (ITD) and tyrosine kinase domain point mutations (TKD) of FLT3 have been reported in approximately 30% of AML as oncogenic driver mutations. Currently, FLT3 inhibitors showed clinical benefits in the corresponding AML patients. In this study, we have characterized a novel FLT3 inhibitor, HM43239, and assessed the potential as a novel therapy for AML patients. Materials and Methods: HM43239 is a novel, highly potent FLT3 kinase inhibitor. This compound showed tight binding to FLT3 kinase in in silico docking model as a reversible Type I inhibitor. In vitro kinase assay was performed to identify kinase selectivity of HM43239. Standard proliferation assay, immunoblotting, and apoptosis analysis were carried out to validate the potency of HM43239 in AML cell lines. HM43239 was evaluated in MV-4-11 and MOLM-13 xenograft mice models. Tumor sizes were measured and tumor samples were analyzed of the mechanisms of action. Results: Among 191 kinases biochemically assayed, HM43239 showed the high selectivity toward FLT3 and AML associated other kinases (e.g. SYK, JAK and TAK1). IC50s' of HM43239 against FLT3 WT, FLT3 ITD and FLT3 D835Y kinases were 1.1 nM, 1.8 nM and 1.0 nM, respectively. HM43239 potently inhibited the growth of AML cell lines harboring FLT3 ITD mutation, such as MV4-11 (IC50: 1.3 nM), MOLM-13 (5.1 nM) and MOLM-14 (2.9 nM). Furthermore, HM43239 effectively inhibited the phosphorylation levels of FLT3 and of downstream kinases related with cell proliferation. In addition, caspase 3/7-dependent apoptosis was induced by HM43239 in AML cell lines expressing FLT3 ITD mutant. And HM43239 inhibited proliferation and induced apoptosis of leukemic stem cell (LSC) marker-expressing KG1a cells (CD34+/CD38- cells) suggesting that the possibility for targeting LSC. HM43239 showed the excellent dose proportional antitumor activity in mouse models xenografted with both MV4-11 and MOLM-13 cell line without any significant toxicity. Moreover, we identified in vivo modulation of related targets (p-FLT3 & p-STAT5) in AML cell with FLT3 mutant. Conclusion: Taken together, HM43239 has demonstrated the potential therapeutic efficacy for the treatment of AML patients and implicated the mechanism of overcoming resistance and preventing relapse. Citation Format: Miyoung Lee, Young Eun Ha, Mi Jin Moon, Joo-Yun Byun, HyunKyung Yu, SeokJong Kang, JaeHo Lee, Kyuhang Lee, Eunkyung Kim, Eunyoung Kim, Ho Jeong Lee, YoungHoon Kim, YoungGil Ahn, KweeHyun Suh, Sun-Jin Kim. Antitumor activity of the potent and novel FLT3 inhibitor HM43239 in acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 804.