Bone marrow stroma‐mediated resistance to FLT3 inhibitors in FLT3‐ITD AML is mediated by persistent activation of extracellular regulated kinase

In approximately 23% of AML patients, Fms-like tyrosine kinase 3 (FLT3) contains a gain-of-function, internal tandem duplication (ITD) mutation that is associated with an unfavourable prognosis. FLT3 is therefore seen as a promising therapeutic avenue for AML and as a result, a more complete understanding of its function and signalling may lead to additional targets and resistance mechanisms being identified. To date, several FLT3 inhibitors including Quizartinib (AC220), a second-generation ITD-selective tyrosine kinase inhibitor, have been developed, but single-agent clinical trials have not been overwhelmingly successful. In most preclinical studies, the inhibitory effects of FLT3 inhibitors are mainly evaluated using mutant-expressing models, however, most AML patients harbour both a wild-type and mutant FLT3 allele as well as presenting with high plasma levels of FLT3 ligand (FLT3L). We hypothesized that FLT3L could act through WT-FLT3 to influence the efficacy of FLT3 inhibitors in cells with heterozygous mutations. In this study, we have examined the role of FLT3 inhibition and FLT3L activation on the cellular localisation and downstream signalling in several AML cell lines, some of which express ITD mutations. We also looked at mechanisms by which FLT3L could impair the efficacy of FLT3 inhibitors. Our results revealed that the majority of FLT3 in the MV4-11 and MOLM-13 cells was intracellular. Inhibition of ITD-FLT3 with quizartinib led to a dramatic relocalisation of FLT3 to the cell surface. This effect was more pronounced in MV4-11 (FLT3ITD/ITD) than the heterozygous MOLM-13 (FLT3ITD/WT) cells that also express WT. This was accompanied by inhibition of ERK, AKT and STAT5 signalling pathways and resulted in cell death. Quizartinib induced cell death only in AML lines expressing FLT3-ITD mutations (MOLM-13 and MV4-11), and this could be antagonised by FLT3 ligand (FLT3L). The largest inhibitory effects were seen in heterozygous cells expressing a mutated and wild-type allele. FLT3 inhibition was associated with downregulation of the anti-apoptotic protein Mcl1 and upregulation of the pro-apoptotic BH3-only protein, Bim. Our experiments indicated that both of these proteins were regulated by both WT and ITD-FLT3 through the MAPK pathway. These results suggest that activation of FLT3 signalling by FLT3L confers resistance to quizartinib through upregulation of Mcl-1 and suppression of Bim expression. Taken together, our data suggest that FLT3 cell surface localisation and expression are controlled by ITD mutations and are key components of drug resistance. The data also suggests a novel therapeutic approach in patients with high plasma FLT3L levels, especially when using type II inhibitors such as quizartinib, is to co-target the MAPK/ERK pathway to abrogate the FLT3Lmediated resistance in FLT3-ITD AML. Citation Format: Taha Alqahtani, David MacEwan. The FLT3 tyrosine kinase receptor ITD mutation controls its expression and drug resistance in acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1966.

The Pim Kinase Inhibitor AZD1208 Enhances Apoptosis Induction By Clinically Active FLT3 Inhibitors in FLT3-ITD Acute Myeloid Leukemia Cells in Vitro and in Vivo through Synergistic Downregulation of Mcl-1 and of Bcl-xL

LAM-003, a Novel Oral Heat Shock Protein 90 Inhibitor for Treatment of Acute Myeloid Leukemia, Including Wild-Type and FMS-like Tyrosine Kinase 3 (FLT3)-Mutant Disease

FLT3-F691L confers kinase-independent resistance via Grb2-dependent MAPK signaling in AML

Responsiveness of FLT3-ITD+ AML to Kinase Inhibition Correlates with High Allele Ratio and Relapsed Disease but Not to Inhibition of Canonical Targets

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

Effective treatment regimens for elderly acute myeloid leukemia (AML) patients harboring internal tandem duplication mutations in the FMS-like tyrosine kinase-3 (FLT3) gene (FLT3/ITD) are lacking and represent a significant unmet need. Recent data on the effects of FLT3 tyrosine kinase inhibitors on FLT3/ITD(+) AML showed promising clinical activity, including in elderly patients. DNA methyltransferase (DNMT) inhibitors such as decitabine (5-aza-2-deoxycytidine, DEC) and 5-azacitidine (AZA) demonstrated clinical benefit in AML, are well tolerated and are associated with minimal increases in FLT3 ligand, which can represent a potential resistance mechanism to FLT3 inhibitors. In addition, both FLT3 and DNMT inhibition are associated with the induction of terminal differentiation of myeloid blasts. Consequently, there is a strong theoretical rationale for combining FLT3 and DNMT inhibition for FLT3/ITD(+) AML. We therefore sought to study the anti-leukemic effects of DEC, AZA and FLT3 inhibitors, either as single agents or in combination, on AML cell lines and primary cells derived from newly diagnosed and relapsed AML patients. Our studies indicate that combined treatment using FLT3 inhibition and hypomethylation confers synergistic anti-leukemic effects, including apoptosis, growth inhibition and differentiation. The simultaneous administration of AZA and FLT3 inhibition appears to be the most efficacious combination in this regard. These drugs may provide a novel therapeutic approach for FLT3/ITD(+) AML, in particular for older patients.

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

FGF2 from Bone Marrow Stroma Protects Acute Myeloid Leukemia Cells from the FLT3 Inhibitor Quizartinib and Facilitates Acquisition of Resistance Mutations

FMS-like tyrosine kinase-3 (FLT3) is a tyrosine kinase receptor involved in the survival and expansion of hematopoietic stem progenitors. A constitutively activated, mutated form of FLT3, is expressed in approximately 30% of de novo acute myeloid leukemia (AML) and about 6% of acute lymphoblastic leukemia (ALL) cases. Since mutant FLT3 has emerged as an attractive therapeutic target, there are several FLT3 inhibitors currently undergoing evaluation in different phases of clinical trials. However, although many aspects of the intracellular signaling mediated by oncogenic FLT3 have been revealed, what is the best strategy to inhibit FLT3 and how FLT3 inhibitors should be developed for AML treatment is poorly defined. Despite promising in vitro studies, where most FLT3 inhibitors show potent efficacy at nanomolar concentrations, clinical responses in AML patients are moderate and temporary. Furthermore, under prolonged therapy, FLT3 mutation-positive leukemic cells rapidly develop resistance to FLT3 inhibitors when used as monotherapy. Considering that there is no uniform mechanism of resistance triggered by FLT3 inhibitors, it will be necessary to develop new agents that target FLT3, and that can be used consecutively or in combination with conventional cytotoxic therapeutics. On the other hand, given that overexpression of FLT3 ligand (FL), occurring after myelosuppressive therapy, reduces the efficacy of FLT3 inhibitors, targeting both FL and FLT3 kinase, might be more effective approach in AML treatment. Here, we summarize up-to-date studies on FLT3 structure, its mutation status and role in malignant signal trafficking. We also review why FLT3 targeted therapies have not revolutionized AML treatment.

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.

Combined FLT3 and EZH1/2 inhibition reduces LSCs and promotes myeloid differentiation in PDX models of AML

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

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.