Sensitivity toward tyrosine kinase inhibitors varies between different activating mutations of the FLT3 receptor

Characterisation of FLT3 alterations in childhood acute lymphoblastic leukaemia

BACKGROUND

Alterations of FLT3 are among the most common driver events in acute leukaemia with important clinical implications, since it allows patient classification into prognostic groups and the possibility of personalising therapy thanks to the availability of FLT3 inhibitors. Most of the knowledge on FLT3 implications comes from the study of acute myeloid leukaemia and so far, few studies have been performed in other leukaemias.

METHODS

A comprehensive genomic (DNA-seq in 267 patients) and transcriptomic (RNA-seq in 160 patients) analysis of FLT3 in 342 childhood acute lymphoblastic leukaemia (ALL) patients was performed. Mutations were functionally characterised by in vitro experiments.

RESULTS

Point mutations (PM) and internal tandem duplications (ITD) were detected in 4.3% and 2.7% of the patients, respectively. A new activating mutation of the TKD, G846D, conferred oncogenic properties and sorafenib resistance. Moreover, a novel alteration involving the circularisation of read-through transcripts (rt-circRNAs) was observed in 10% of the cases. Patients presenting FLT3 alterations exhibited higher levels of the receptor. In addition, patients with ZNF384- and MLL/KMT2A-rearranged ALL, as well as hyperdiploid subtype, overexpressed FLT3.

DISCUSSION

Our results suggest that specific ALL subgroups may also benefit from a deeper understanding of the biology of FLT3 alterations and their clinical implications.

Indole-based FLT3 inhibitors and related scaffolds as potential therapeutic agents for acute myeloid leukemia

Fms-like tyrosine kinase 3 (FLT3) mutation mechanisms are among the most common genetic abnormalities detected in about 30% of acute myeloid leukemia (AML) patients. These mutations are accompanied by poor clinical response, although all these progressions in identifying and interpreting biological AML bio-targets. Several small structured FLT3 inhibitors have been ameliorated to struggle against AML. Despite all these developments regarding these inhibitors, the Overall survival rate is about five years or more in less than one-third of diagnosed AML patients. Midostaurin was the first FDA-approved FLT3 inhibitor in 2017 in the United States and Europe for AML remedy. Next, Gilteritinib was an FDA-approved FLT3 inhibitor in 2018 and in the next year, Quizartinib was approved an as FLT3 inhibitor in Japan. Interestingly, indole-based motifs had risen as advantaged scaffolds with unusual multiple kinase inhibitory activity. This review summarises indole-based FLT3 inhibitors and related scaffolds, including FDA-approved drugs, clinical candidates, and other bioactive compounds. Furthermore, their chemotypes, mechanism of action, and interaction mode over both wild and mutated FLT3 target proteins had been judgmentally discussed. Therefore, this review could offer inspiring future perspectives into the finding of new FLT3-related AML therapies.

19S Proteasome Subunits as Oncogenes and Prognostic Biomarkers in FLT3-Mutated Acute Myeloid Leukemia (AML)

26S proteasome non-ATPase subunits 1 (PSMD1) and 3 (PSMD3) were recently identified as prognostic biomarkers and potential therapeutic targets in chronic myeloid leukemia (CML) and multiple solid tumors. In the present study, we analyzed the expression of 19S proteasome subunits in acute myeloid leukemia (AML) patients with mutations in the FMS-like tyrosine kinase 3 (FLT3) gene and assessed their impact on overall survival (OS). High levels of PSMD3 but not PSMD1 expression correlated with a worse OS in FLT3-mutated AML. Consistent with an oncogenic role for PSMD3 in AML, shRNA-mediated PSMD3 knockdown impaired colony formation of FLT3+ AML cell lines, which correlated with increased OS in xenograft models. While PSMD3 regulated nuclear factor-kappa B (NF-κB) transcriptional activity in CML, we did not observe similar effects in FLT3+ AML cells. Rather, proteomics analyses suggested a role for PSMD3 in neutrophil degranulation and energy metabolism. Finally, we identified additional PSMD subunits that are upregulated in AML patients with mutated versus wild-type FLT3, which correlated with worse outcomes. These findings suggest that different components of the 19S regulatory complex of the 26S proteasome can have indications for OS and may serve as prognostic biomarkers in AML and other types of cancers.

FLT3 inhibitors for acute myeloid leukemia: successes, defeats, and emerging paradigms

FLT3 mutations are one of the most common genetic aberrations found in nearly 30% of acute myeloid leukemias (AML). The mutations are associated with poor prognosis despite advances in the understanding of the biological mechanisms of AML. Numerous small molecule FLT3 inhibitors have been developed in an effort to combat AML. Even with the development of these inhibitors, the five-year overall survival for newly diagnosed AML is less than 30%. In 2017, midostaurin received FDA approval to treat AML, which was the first approved FLT3 inhibitor in the U.S. and Europe. Following, gilteritinib received FDA approval in 2018 and in 2019 quizartinib received approval in Japan. This review parallels these clinical success stories along with other pre-clinical and clinical investigations of FLT3 inhibitors.

Diagnosis and Management of Atypical Chronic Myeloid Leukemia with a t(2;13)(q33;q12) Translocation

Atypical chronic myeloid leukemia (aCML) is a rare myeloproliferative disorder that shares clinical features with chronic myeloid leukemia but lacks the classic t(9;22) BCR-ABL1 translocation and features prominent dysgranulopoiesis and granulocytic dysplasia. Challenges of this diagnosis include clinical and biologic heterogeneity, the high risk of transformation to acute myeloid leukemia, and the lack of standard treatment options. Allogeneic hematopoietic stem cell transplant is likely the preferred treatment, but this can be limited by patient psychosocial support, age, concomitant medical conditions, and availability of an appropriate donor. We report the case of a 61-year-old male with no significant past medical history diagnosed with aCML with a rare t(2;13)(q33;q12). He presented with weight loss, night sweats, splenomegaly, hyperleukocytosis, a leukoerythroblastic differential with a predominant neutrophilia, anemia, and thrombocytopenia. Subsequent peripheral blood and bone marrow studies lead to the diagnosis of aCML. He was recommended to undergo an allogeneic stem cell transplant evaluation and declined. He was initially treated with hydroxyurea and imatinib to which he responded for approximately three years. After clinical progression, he was treated with sorafenib, a multiprotein kinase inhibitor more commonly used in the treatment of hepatocellular and renal cell carcinoma due to its off target FLT3 inhibition. The patient achieved complete hematologic response which has been sustained for 7 years with tolerable side effects.

A novel approach for relapsed/refractory FLT3mut+ acute myeloid leukaemia: synergistic effect of the combination of bispecific FLT3scFv/NKG2D-CAR T cells and gilteritinib

BACKGROUND

Patients with relapsed/refractory acute myeloid leukaemia (AML) with FMS-like tyrosine kinase 3-internal tandem duplication (FLT3-ITD) have limited treatment options and poor prognosis. Therefore, novel treatment modalities are needed. Since high expression of natural killer group 2 member D ligands (NKG2DLs) can be induced by FLT3 inhibitors, we constructed dual-target FLT3 single-chain fragment variable (scFv)/NKG2D-chimeric antigen receptor (CAR) T cells, and explored whether FLT3 inhibitors combined with FLT3scFv/NKG2D-CAR T cells could have synergistic anti-leukaemia effects.

METHODS

FLT3scFv and NKG2D expression in CAR T cells, FLT3 and NKG2DL expression in AML cells, and the in vitro cytotoxicity of combining CAR T cells with gilteritinib were assessed by flow cytometry. The therapeutic effect was evaluated in a xenograft mouse model established by injection of MOLM-13 cells. Mechanisms underlying the gilteritinib-induced NKG2DL upregulation were investigated using siRNA, ChIP-QPCR and luciferase assays.

RESULTS

The FLT3scFv/NKG2D-CAR T cells specifically lysed AML cells both in vitro and in the xenograft mouse model. The efficacy of FLT3scFv/NKG2D-CAR T cells was improved by gilteritinib-pretreatment. The noncanonical NF-κB2/Rel B signalling pathway was found to mediate gilteritinib-induced NKG2DL upregulation in AML cells.

CONCLUSIONS

Bispecific FLT3scFv/NKG2D-CAR T cells can effectively eradicate AML cells. The FLT3 inhibitor gilteritinib can synergistically improve this effect by upregulating NF-κB2-dependent NKG2DL expression in AML cells.

A review of FLT3 inhibitors in acute myeloid leukemia

FLT3 mutations are the most common genetic aberrations found in acute myeloid leukemia (AML) and associated with poor prognosis. Since the discovery of FLT3 mutations and their prognostic implications, multiple FLT3-targeted molecules have been evaluated. Midostaurin is approved in the U.S. and Europe for newly diagnosed FLT3 mutated AML in combination with standard induction and consolidation chemotherapy based on data from the RATIFY study. Gilteritinib is approved for relapsed or refractory FLT3 mutated AML as monotherapy based on the ADMIRAL study. Although significant progress has been made in the treatment of AML with FLT3-targeting, many challenges remain. Several drug resistance mechanisms have been identified, including clonal selection, stromal protection, FLT3-associated mutations, and off-target mutations. The benefit of FLT3 inhibitor maintenance therapy, either post-chemotherapy or post-transplant, remains controversial, although several studies are ongoing.

Murine Oncostatin M Has Opposing Effects on the Proliferation of OP9 Bone Marrow Stromal Cells and NIH/3T3 Fibroblasts Signaling through the OSMR

The IL-6 family cytokine Oncostatin M (OSM) is involved in cell development, growth, hematopoiesis, inflammation, and cancer. Intriguingly, OSM has proliferative and antiproliferative effects depending on the target cell. The molecular mechanisms underlying these opposing effects are not fully understood. Previously, we found OSM upregulation in different myeloproliferative syndromes. However, OSM receptor (OSMR) expression was detected on stromal cells but not the malignant cells themselves. In the present study, we, therefore, investigated the effect of murine OSM (mOSM) on proliferation in stromal and fibroblast cell lines. We found that mOSM impairs the proliferation of bone marrow (BM) stromal cells, whereas fibroblasts responded to mOSM with increased proliferation. When we set out to reveal the mechanisms underlying these opposing effects, we detected increased expression of the OSM receptors OSMR and LIFR in stromal cells. Interestingly, Osmr knockdown and Lifr overexpression attenuated the OSM-mediated effect on proliferation in both cell lines indicating that mOSM affected the proliferation signaling mainly through the OSMR. Furthermore, mOSM induced activation of the JAK-STAT, PI3K-AKT, and MAPK-ERK pathways in OP9 and NIH/3T3 cells with differences in total protein levels between the two cell lines. Our findings offer new insights into the regulation of proliferation by mOSM.

Activators and Inhibitors of Protein Kinase C (PKC): Their Applications in Clinical Trials

Protein kinase C (PKC), a family of phospholipid-dependent serine/threonine kinase, is classed into three subfamilies based on their structural and activation characteristics: conventional or classic PKC isozymes (cPKCs; α, βI, βII, and γ), novel or non-classic PKC isozymes (nPKCs; δ, ε, η, and θ), and atypical PKC isozymes (aPKCs; ζ, ι, and λ). PKC inhibitors and activators are used to understand PKC-mediated intracellular signaling pathways and for the diagnosis and treatment of various PKC-associated diseases, such as cancers, neurological diseases, cardiovascular diseases, and infections. Many clinical trials of PKC inhibitors in cancers showed no significant clinical benefits, meaning that there is a limitation to design a cancer therapeutic strategy targeting PKC alone. This review will focus on the activators and inhibitors of PKC and their applications in clinical trials.

Loss of the Fanconi anemia-associated protein NIPA causes bone marrow failure

Inherited bone marrow failure syndromes (IBMFSs) are a heterogeneous group of disorders characterized by defective hematopoiesis, impaired stem cell function, and cancer susceptibility. Diagnosis of IBMFS presents a major challenge due to the large variety of associated phenotypes, and novel, clinically relevant biomarkers are urgently needed. Our study identified nuclear interaction partner of ALK (NIPA) as an IBMFS gene, as it is significantly downregulated in a distinct subset of myelodysplastic syndrome-type (MDS-type) refractory cytopenia in children. Mechanistically, we showed that NIPA is major player in the Fanconi anemia (FA) pathway, which binds FANCD2 and regulates its nuclear abundance, making it essential for a functional DNA repair/FA/BRCA pathway. In a knockout mouse model, Nipa deficiency led to major cell-intrinsic defects, including a premature aging phenotype, with accumulation of DNA damage in hematopoietic stem cells (HSCs). Induction of replication stress triggered a reduction in and functional decline of murine HSCs, resulting in complete bone marrow failure and death of the knockout mice with 100% penetrance. Taken together, the results of our study add NIPA to the short list of FA-associated proteins, thereby highlighting its potential as a diagnostic marker and/or possible target in diseases characterized by hematopoietic failure.

Treatment of Acute Myeloid Leukemia in the Era of Genomics-Achievements and Persisting Challenges

Acute myeloid leukemia (AML) represents a malignant disorder of the hematopoietic system that is mainly characterized by rapid proliferation, dysregulated apoptosis, and impaired differentiation of leukemic blasts. For several decades, the diagnostic approach in AML was largely based on histologic characteristics with little impact on the treatment decision-making process. This perspective has drastically changed within the past years due to the advent of novel molecular technologies, such as whole genome next-generation sequencing (NGS), and the resulting knowledge gain in AML biology and pathogenesis. After more than four decades of intensive chemotherapy as a "one-size-fits-all" concept, several targeted agents have recently been approved for the treatment of AML, either as single agents or as part of combined treatment regimens. Several other compounds, directed against regulators of apoptotic, epigenetic, or microenvironmental pathways, as well as modulators of the immune system, are currently in development and being investigated in clinical trials. The constant progress in AML research has started to produce improved survival rates and fueled hopes that a once rapidly fatal disease can be transformed into a chronic condition. In this review, the authors provide a summary of recent advances in the development of targeted AML therapies and discuss persistent challenges.

Targeting on glycosylation of mutant FLT3 in acute myeloid leukemia

Objective: To summarize the abnormal location of FLT3 caused by different glycosylation status which further leads to the distinguishing signaling pathways and discuss targeting on FLT3 glycosylation by drugs reported in recent literatures. Methods: We review FLT3 glycosylation in endoplasmic reticulum. The abnormal signal of mutant FLT3 with different glycosylation status is discussed. We also address potential FLT3 glycosylation-targeting strategies for the treatment. Results: Inhibition of FLT3 mutant cells by drugs reported in recent literatures involves the influence of glycosylation of FLT3: 2-deoxy-D-glucose, Tunicamycin and Fluvastatin are reported to inhibit N-glycosylation of FLT3; Pim-1 inhibitors are proved to block the inhibition of Pim-1 on FLT3 Oglycosylation; HSP90 inhibitors and Tyrosine Kinase Inhibitors are shown to increase fully glycosylated form of FLT3. Discussion: The FMS-like tyrosine kinase 3 (FLT3) gene expressed only in CD34+ progenitor cells in bone marrow is located on chromosome 13q12 encoding FLT3 protein. FLT3 is initially synthesized as a 110 KD protein, which glycosylated in the endoplasmic reticulum to a 130 KD immature protein rich in mannose, and further processed into a mature 160 KD protein in the Golgi apparatus, which could be transferred to the cell surface. Therapy targeting on FLT3 glycosylation is a promising direction for AML treatment. Conclusions: The abnormal location of FLT3 caused by different glycosylation status leads to the distinguishing signaling pathways. Targeting on FLT3 glycosylation may provide a new perspective for therapeutic strategies. Abbreviations: ABCG2: ATP-binding cassette transporter breast cancer resistance protein; ATF: activating transcription factor; AML: acute myeloid leukemia; CHOP: CCAAT-enhancer-binding protein homologous protein; 2-DG: 2-deoxy-D-glucose; EFS: event free survival; EPO: erythropoietin; EPOR: erythropoietin receptor; ERS: endoplasmic reticulum stress; FLT3: FMS-like tyrosine kinase 3; GPI: glycosylphosphatidylinositol; HSP: heat shock protein; ITD: internal tandem duplication; IRE1a: inositol-requiring enzyme 1 alpha; JNK: c-Jun N-terminal kinase; JMD: juxtamembrane domain; JAK: janus kinase; MAPK/ERK: mitogen activated protein kinase/extracellular signal-regulated protein kinase; OS: overall survival; PI3K/AKT: phosphatidylinositide 3-kinases/protein kinase B; PERK: RNA-activated protein kinase-like endoplasmic reticulum kinase; Pgp: P-glycoprotein; PTX3: human pentraxin-3; STAT: signal transducer and activator of transcriptions; TKD: tyrosine-kinase domain; TKI: tyrosine kinase inhibitor; TM: Tunicamycin; UPR: unfolded protein reaction.

Deciphering the molecular mechanism of FLT3 resistance mutations

FMS-like tyrosine kinase 3 (FLT3) has been found to be mutated in ~ 30% of acute myeloid leukaemia patients. Small-molecule inhibitors targeting FLT3 that are currently approved or still undergoing clinical trials are subject to drug resistance due to FLT3 mutations. How these mutations lead to drug resistance is hitherto poorly understood. Herein, we studied the molecular mechanism of the drug resistance mutations D835N, Y842S and M664I, which confer resistance against the most advanced inhibitors, quizartinib and PLX3397 (pexidartinib), using enzyme kinetics and computer simulations. In vitro kinase assays were performed to measure the comparative catalytic activity of the native protein and the mutants, using a bacterial expression system developed to this aim. Our results reveal that the differential drug sensitivity profiles can be rationalised by the dynamics of the protein-drug interactions and perturbation of the intraprotein contacts upon mutations. Drug binding induced a single conformation in the native protein, whereas multiple conformations were observed otherwise (in the mutants or in the absence of drugs). The end-point kinetics measurements indicated that the three resistant mutants conferred catalytic activity that is at least as high as that of the reference without such mutations. Overall, our calculations and measurements suggest that the structural dynamics of the drug-resistant mutants that affect the active state and the increased conformational freedom of the remaining inactive drug-bound population are the two major factors that contribute to drug resistance in FLT3 harbouring cancer cells. Our results explain the mechanism of drug resistance mutations and can aid to the design of more effective tyrosine kinase inhibitors.

Effect of Fms-like tyrosine kinase 3 (FLT3) ligand (FL) on antitumor activity of gilteritinib, a FLT3 inhibitor, in mice xenografted with FL-overexpressing cells

Therapeutic effects of FLT3 inhibitors have been reported in acute myeloid leukemia (AML) with constitutively activating FLT3 mutations, including internal tandem duplication (ITD) and point mutation, which are found in approximately one-third of AML patients. One of the critical issues of treatment with FLT3 inhibitors in FLT3-mutated AML is drug resistance. FLT3 ligand (FL) represents a mechanism of resistance to FLT3 inhibitors, including quizartinib, midostaurin, and sorafenib, in AML cells harboring both wild-type and mutant FLT3 (FLT3^wt/FLT3^mut). Here, we investigated the effect of FL on the efficacy of gilteritinib, a FLT3 inhibitor, in AML-derived cells in vitro and in mice. In contrast to other FLT3 inhibitors, FL stimulation had little effect on growth inhibition or apoptosis induction by gilteritinib. The antitumor activity of gilteritinib was also comparable between xenograft mouse models injected with FL-expressing and mock MOLM-13 cells. In the FLT3 signaling analyses, gilteritinib inhibited FLT3^wt and FLT3-ITD to a similar degree in HEK293 and Ba/F3 cells, and similarly suppressed FLT3 downstream signaling molecules (including ERK1/2 and STAT5) in both the presence and absence of FL in MOLM-13 cells. Co-crystal structure analysis showed that gilteritinib bound to the ATP-binding pocket of FLT3. These results suggest that gilteritinib has therapeutic potential in FLT3-mutated AML patients with FL overexpression.

Midostaurin In Acute Myeloid Leukemia: An Evidence-Based Review And Patient Selection

Fms-related-tyrosine kinase 3 (FLT3) mutations occur in approximately a third of acute myeloid leukemia (AML) patients and confer an adverse prognosis. Numerous studies have evaluated FLT3 targeting as single agent and in combination approaches in frontline and relapsed AML. At this time, midostaurin, a multikinase inhibitor, is the only FLT3-inhibitor that is US FDA approved to be used in combination with induction therapy in the frontline FLT3-mutated AML setting based on improved overall survival noted in the RATIFY Phase III trial. The utility of midostaurin in maintenance post stem cell transplantation has shown promising results and further studies are still ongoing. In this review, we discuss the studies that led to the inception of midostaurin as a targeted kinase inhibitor, its evaluation in AML, the early clinical trials and the large Phase III clinical trial that led to its eventual US FDA-approval in FLT3-mutated AML. Our review also discusses data on midostaurin adverse effects, mechanisms of resistance and limitations of its utility. We further discuss emerging second-generation FLT3 inhibitors, with a focus on quizartinib and gilteritinib and future directions to enhance FLT3-inhibitor efficacy and overcome mechanisms of resistance.

Proteomic Phosphosite Analysis Identified Crucial NPM-ALK-Mediated NIPA Serine and Threonine Residues

Anaplastic large-cell lymphoma (ALCL) is an aggressive non-Hodgkin lymphoma that shows in 60% of cases a translocation t(2;5)(p23;q35), which leads to the expression of the oncogenic kinase NPM-ALK. The nuclear interaction partner of ALK (NIPA) defines an E3-SCF ligase that contributes to the timing of mitotic entry. It has been shown that co-expression of NIPA and NPM-ALK results in constitutive NIPA phosphorylation. By mass spectrometry-based proteomics we identified nine serine/threonine residues to be significantly upregulated in NIPA upon NPM-ALK expression. Generation of phospho-deficient mutants of the respective phospho-residues specified five serine/threonine residues (Ser-338, Ser-344, Ser-370, Ser-381 and Thr-387) as key phosphorylation sites involved in NPM-ALK-directed phosphorylation of NIPA. Analysis of the biological impact of NIPA phosphorylation by NPM-ALK demonstrated that the ALK-induced phosphorylation does not change the SCF^NIPA-complex formation but may influence the localization of NIPA and NPM-ALK. Biochemical analyses with phospho-deficient mutants elucidated the importance of NIPA phosphorylation by NPM-ALK for the interaction of the two proteins and proliferation potential of respective cells: Silencing of the five crucial NIPA serine/threonine residues led to a highly enhanced NIPA-NPM-ALK binding capacity as well as a slightly reduced proliferation in Ba/F3 cells.

Phosphorylation of BECLIN-1 by BCR-ABL suppresses autophagy in chronic myeloid leukemia

Autophagy is a genetically regulated process of adaptation to metabolic stress and was recently shown to be involved in the treatment response of chronic myeloid leukemia (CML). However, in vivo data are limited and the molecular mechanism of autophagy regulators in the process of leukemogenesis is not completely understood. Here we show that Beclin-1 knockdown, but not Atg5 deletion in a murine CML model leads to a reduced leukemic burden and results in a significantly prolonged median survival of targeted mice. Further analyses of murine cell lines and primary patient material indicate that active BCR-ABL directly interacts with BECLIN-1 and phosphorylates its tyrosine residues 233 and 352, resulting in autophagy suppression. By using phosphorylation-deficient and phosphorylation-mimic mutants, we identify BCR-ABL induced BECLIN-1 phosphorylation as a crucial mechanism for BECLIN-1 complex formation: interaction analyses exhibit diminished binding of the positive autophagy regulators UVRAG, VPS15, ATG14 and VPS34 and enhanced binding of the negative regulator Rubicon to BCR-ABL-phosphorylated BECLIN-1. Taken together, our findings show interaction of BCR-ABL and BECLIN-1 thereby highlighting the importance of BECLIN-1-mediated autophagy in BCR-ABL⁺ cells.

The mutational spectrum of FLT3 gene in acute lymphoblastic leukemia is different from acute myeloid leukemia

Mutations in FMS-like tyrosine kinase 3 (FLT3) gene occur frequently in acute myeloid leukemia (AML) and are rare in acute lymphoblastic leukemia (ALL). We aimed to analyze the incidence and characteristics of FLT3 mutations in ALL. Amplicon-targeted next-generation sequencing of 58 genes was performed on 1571 patients (AML, n = 829; ALL, n = 742). FLT3 mutations were identified in 5.12% (38/742) of ALL patients. Four types of FLT3 mutations were disclosed, including internal tandem duplication (ITD), tyrosine kinase domain (TKD), juxtamembrane insertion and deletion (JM-INDEL), and juxtamembrane point mutation (JM-PM), which were respectively identified in 1.21, 1.89, 0.67, and 1.89% of patients. The incidence of FLT3-JM-PM (1.89 vs 0.48%, P = 0.009) and the proportion of TKD non-D835 mutations that accounted for the total TKD mutations (57.14 vs 18.18%, P = 0.013) were significantly higher in ALL when compared with AML. FLT3-JM-INDEL were mainly found in B-ALL. In addition, FLT3-JM-INDEL and FLT3-JM-PM were first reported in patients with B-ALL. Patients with FLT3 mutations besides of ITD and/or TKD had a potential response to tyrosine kinase inhibitors. We showed that the mutation spectrum of FLT3 gene in ALL is distinct from AML that will facilitated an in-depth understand of the pathogenesis and provide a guidance for treatment.

Emerging treatment paradigms with FLT3 inhibitors in acute myeloid leukemia

Mutations in the fms-like tyrosine kinase 3 (FLT3) gene are detected in approximately one-third of patients with newly diagnosed acute myeloid leukemia (AML). These consist of the more common FLT3-internal tandem duplication (ITD) in approximately 20-25% of AML cases, and point mutations in the tyrosine kinase domain (TKD) in approximately 5-10%. FLT3 mutations, especially FLT3-ITD, are associated with proliferative disease, increased risk of relapse, and inferior overall survival when treated with conventional regimens. However, the recent development of well tolerated and active FLT3 inhibitors has significantly improved the outcomes of this aggressive subtype of AML. The multikinase inhibitor midostaurin was approved by the United States Food and Drug Administration (US FDA) in April 2017 for the frontline treatment of patients with FLT3-mutated (either ITD or TKD) AML in combination with induction chemotherapy, representing the first new drug approval in AML in nearly two decades. In November 2018, the US FDA also approved the second-generation FLT3 inhibitor gilteritinib as a single agent for patients with relapsed or refractory FLT3-mutated AML. Promising phase I and II efficacy data for quizartinib is likely to lead to a third regulatory approval in relapsed/refractory AML in the near future. However, despite the significant progress made in managing FLT3-mutated AML, many questions remain regarding the best approach to integrate these inhibitors into combination regimens, and also the optimal sequencing of different FLT3 inhibitors in various clinical settings. This review comprehensively examines the FLT3 inhibitors currently in clinical development, with an emphasis on their spectra of activity against different FLT3 mutations and other kinases, clinical safety and efficacy data, and their current and future roles in the management of AML. The mechanisms of resistance to FLT3 inhibitors and potential combination strategies to overcome such resistance pathways are also discussed.

Therapeutic Vulnerabilities in FLT3-Mutant AML Unmasked by Palbociclib

While significant progress has been made in the treatment of acute myeloid leukemia (AML), not all patients can be cured. Mutated in about 1/3 of de novo AML, the FLT3 receptor tyrosine kinase is an attractive target for drug development, activating mutations of the FLT3 map to the juxtamembrane domain (internal tandem duplications, ITD) or the tyrosine kinase domain (TKD), most frequently at codon D835. While small molecule tyrosine kinase inhibitors (TKI) effectively target ITD mutant forms, those on the TKD are not responsive. Moreover, FLT3 inhibition fails to induce a persistent response in patients due to mutational resistance. More potent compounds with broader inhibitory effects on multiple FLT3 mutations are highly desirable. We describe a critical role of CDK6 in the survival of FLT3⁺ AML cells as palbociclib induced apoptosis not only in FLT3–ITD⁺ cells but also in FLT3–D835Y⁺ cells. Antineoplastic effects were also seen in primary patient-derived cells and in a xenograft model, where therapy effectively suppressed tumor formation in vivo at clinically relevant concentrations. In cells with FLT3–ITD or -TKD mutations, the CDK6 protein not only affects cell cycle progression but also transcriptionally regulates oncogenic kinases mediating intrinsic drug resistance, including AURORA and AKT—a feature not shared by its homolog CDK4. While AKT and AURORA kinase inhibitors have significant therapeutic potential in AML, single agent activity has not been proven overly effective. We describe synergistic combination effects when applying these drugs together with palbociclib which could be readily translated to patients with AML bearing FLT3–ITD or –TKD mutations. Targeting synergistically acting vulnerabilities, with CDK6 being the common denominator, may represent a promising strategy to improve AML patient responses and to reduce the incidence of selection of resistance-inducing mutations.

Targeting Oncogenic Signaling in Mutant FLT3 Acute Myeloid Leukemia: The Path to Least Resistance

The identification of recurrent driver mutations in genes encoding tyrosine kinases has resulted in the development of molecularly-targeted treatment strategies designed to improve outcomes for patients diagnosed with acute myeloid leukemia (AML). The receptor tyrosine kinase FLT3 is the most commonly mutated gene in AML, with internal tandem duplications within the juxtamembrane domain (FLT3-ITD) or missense mutations in the tyrosine kinase domain (FLT3-TKD) present in 30–35% of AML patients at diagnosis. An established driver mutation and marker of poor prognosis, the FLT3 tyrosine kinase has emerged as an attractive therapeutic target, and thus, encouraged the development of FLT3 tyrosine kinase inhibitors (TKIs). However, the therapeutic benefit of FLT3 inhibition, particularly as a monotherapy, frequently results in the development of treatment resistance and disease relapse. Commonly, FLT3 inhibitor resistance occurs by the emergence of secondary lesions in the FLT3 gene, particularly in the second tyrosine kinase domain (TKD) at residue Asp835 (D835) to form a ‘dual mutation’ (ITD-D835). Individual FLT3-ITD and FLT3-TKD mutations influence independent signaling cascades; however, little is known about which divergent signaling pathways are controlled by each of the FLT3 specific mutations, particularly in the context of patients harboring dual ITD-D835 mutations. This review provides a comprehensive analysis of the known discrete and cooperative signaling pathways deregulated by each of the FLT3 specific mutations, as well as the therapeutic approaches that hold the most promise of more durable and personalized therapeutic approaches to improve treatments of FLT3 mutant AML.

Resistance to FLT3 inhibitors in acute myeloid leukemia: Molecular mechanisms and resensitizing strategies

FMS-like tyrosine kinase 3 (FLT3) is classified as a type III receptor tyrosine kinase, which exerts a key role in regulation of normal hematopoiesis. FLT3 mutation is the most common genetic mutation in acute myeloid leukemia (AML) and represents an attractive therapeutic target. Targeted therapy with FLT3 inhibitors in AML shows modest promising results in current ongoing clinical trials suggesting the complexity of FLT3 targeting in therapeutics. Importantly, resistance to FLT3 inhibitors may explain the lack of overwhelming response and could obstruct the successful treatment for AML. Here, we summarize the molecular mechanisms of primary resistance and acquired resistance to FLT3 inhibitors and discuss the strategies to circumvent the emergency of drug resistance and to develop novel treatment intervention.

FLT3 inhibitors in acute myeloid leukemia: Choosing the best when the optimal does not exist

Despite significant advances in deciphering the molecular and cytogenetic pathways governing acute myeloid leukemia, improvements in treatment strategies and clinical outcomes have been limited. The discovery of FLT3 pathway and its potential role in leukemogenesis has generated excitement in the field and has provided a potential target for drug development. Despite setbacks encountered with first-generation inhibitors, we are witnessing an outbreak of novel agents with potent activity and improved pharmacodynamics which continue to generate promising results. The disease, however, remains a challenge to both patients and physicians with rapid emergence of resistance and subsequent treatment failure. Multiple unanswered questions remain as to which are the optimal FLT3-inhibitors and which strategies and combinations are likely to overcome resistance. This review revisits the development of FLT3-inhibitors, the pathways incriminated in their failure and summarizes available molecularly-designed strategies to design better clinical trials.

FLT3 activating mutations display differential sensitivity to multiple tyrosine kinase inhibitors

Fms-like tyrosine kinase-3 (FLT3) is a receptor tyrosine kinase that normally functions in hematopoietic cell survival, proliferation and differentiation. Constitutively activating mutations of FLT3 map predominately to the juxtamembrane domain (internal tandem duplications; ITD) or the activation loop (AL) of the kinase domain and are detected in about 1/3 of de novo acute myeloid leukemia (AML) patients. Small molecule tyrosine kinase inhibitors (TKI) effectively target FLT3/ITD mutations, but some activating mutations, particularly those on the AL, are relatively resistant to many FLT3 TKI. We reproduced many of the AL or other non-ITD activating mutations and tested 13 FLT3 TKI for their activity against these and wild-type FLT3. All 13 TKI tested inhibited BaF3/ITD cell proliferation in a concentration-dependent manner as reported, but most TKI exhibited a wide range of differential activity against AL and other point mutants. Western blotting results examining inhibition of FLT3 autophosphorylation and signaling pathways indicate that many AL mutations reduce TKI binding. Most FLT3 TKI effectively target wild-type FLT3 signaling. As a demonstration of this differential activity, treatment of BaF3 D835Y cells transplanted in BALB/c mice with sorafenib showed no effect in vivo against this mutant whereas lestaurtinib proved effective at reducing disease burden. Thus, while FLT3 TKI have been selected based on their ability to inhibit FLT3/ITD, the selection of appropriate TKI for AML patients with FLT3 AL and other activating point mutations requires personalized consideration.

Mechanisms of Resistance to FLT3 Inhibitors and the Role of the Bone Marrow Microenvironment

The presence of FLT3 mutations in acute myeloid leukemia (AML) carries a particularly poor prognosis, making the development of FLT3 inhibitors an imperative goal. The last decade has seen an abundance of clinical trials using these drugs alone or in combination with chemotherapy. This culminated with the recent approval by the US Food and Drug Administration of Midostaurin for the treatment of FLT3-mutated AML. Initial success has been followed by the emergence of clinical resistance. Although novel FLT3 inhibitors are being developed, studies into mechanisms of resistance raise hope of new strategies to prevent emergence of resistance and eliminate minimal residual disease.

Regain control of p53: Targeting leukemia stem cells by isoform-specific HDAC inhibition

Leukemia stem cells (LSCs) are self-renewable, leukemia-initiating populations that are often resistant to traditional chemotherapy and tyrosine kinase inhibitors currently used for treatment of acute or chronic myeloid leukemia. The persistence and continued acquisition of mutations in resistant LSCs represent a major cause of refractory disease and/or relapse after remission. Understanding the mechanisms regulating LSC growth and survival is critical in devising effective therapies that will improve treatment response and outcome. Several recent studies indicate that the p53 tumor suppressor pathway is often inactivated in de novo myeloid leukemia through oncogenic-specific mechanisms, which converge on aberrant p53 protein deacetylation. Here, we summarize our current understanding of the various mechanisms underlying deregulation of histone deacetylases (HDACs), which could be exploited to restore p53 activity and enhance targeting of LSCs in molecularly defined patient subsets.

Chronic FLT3-ITD Signaling in Acute Myeloid Leukemia Is Connected to a Specific Chromatin Signature

Acute myeloid leukemia (AML) is characterized by recurrent mutations that affect the epigenetic regulatory machinery and signaling molecules, leading to a block in hematopoietic differentiation. Constitutive signaling from mutated growth factor receptors is a major driver of leukemic growth, but how aberrant signaling affects the epigenome in AML is less understood. Furthermore, AML cells undergo extensive clonal evolution, and the mutations in signaling genes are often secondary events. To elucidate how chronic growth factor signaling alters the transcriptional network in AML, we performed a system-wide multi-omics study of primary cells from patients suffering from AML with internal tandem duplications in the FLT3 transmembrane domain (FLT3-ITD). This strategy revealed cooperation between the MAP kinase (MAPK) inducible transcription factor AP-1 and RUNX1 as a major driver of a common, FLT3-ITD-specific gene expression and chromatin signature, demonstrating a major impact of MAPK signaling pathways in shaping the epigenome of FLT3-ITD AML.

Secondary mutations as mediators of resistance to targeted therapy in leukemia

The advent of small molecule-based targeted therapy has improved the treatment of both acute and chronic leukemias. Resistance to small molecule inhibitors has emerged as a common theme. The most frequent mode of acquired resistance is the acquisition of point mutations in the kinase domain. FLT3 inhibitors have improved response rates in FLT3-mutated acute myeloid leukemia (AML). The occurrence of the ATP-binding site and activation loop mutations confers varying degrees of resistance to the individual FLT3 inhibitors. Second-generation FLT3 inhibitors such as crenolanib may overcome the resistance of these mutations. Furthermore, nonmutational mechanisms of resistance such as prosurvival pathways and bone marrow signaling may be upregulated in FLT3 inhibitor-resistant AML with secondary kinase domain mutations. More recently, point mutations conferring resistance to the Bruton tyrosine kinase inhibitor ibrutinib in chronic lymphocytic leukemia, arsenic trioxide in acute promyelocytic leukemia, and the BH3-mimetic ABT199 in lymphoma have been identified. In chronic myeloid leukemia, the emergence of tyrosine kinase domain mutations has historically been the dominant mechanism of resistance. The early identification of secondary point mutations and their downstream effects along with the development of second- or third-generation inhibitors and rationally designed small molecule combinations are potential strategies to overcome mutation-mediated resistance.

Small molecule inhibitors in acute myeloid leukemia: from the bench to the clinic

Many patients with acute myeloid leukemia will eventually develop refractory or relapsed disease. In the absence of standard therapy for this population, there is currently an urgent unmet need for novel therapeutic agents. Targeted therapy with small molecule inhibitors represents a new therapeutic intervention that has been successful for the treatment of multiple tumors (e.g., gastrointestinal stromal tumors, chronic myelogenous leukemia). Hence, there has been great interest in generating selective small molecule inhibitors targeting critical pathways of proliferation and survival in acute myeloid leukemia. This review highlights a selective group of intriguing therapeutic agents and their presumed targets in both preclinical models and in early human clinical trials.

HD chromoendoscopy coupled with DNA mass spectrometry profiling identifies somatic mutations in microdissected human proximal aberrant crypt foci

Despite increased implementation of screening colonoscopy, interval cancers in the proximal colon remain a major public health concern. This fact underscores the limitations of current screening paradigms and the need for developing advanced endoscopic techniques. The density of aberrant crypt foci (ACF), the earliest identifiable mucosal abnormality, may serve as a surrogate marker for colon cancer risk, but has rarely been studied in the proximal colon. To this end, high-definition (HD) chromoendoscopy was conducted to define the relevance of ACF in the proximal colon. In addition, due to limited ACF size, the development of a combinatorial approach was required to maximize data acquisition obtained from individual biopsy samples. Proximal and distal ACF samples were characterized for a total of 105 mutations across 22 known tumor suppressor and proto-oncogenes using high-throughput Sequenom MassARRAY analysis. From this profiling, a discrete number of somatic mutations were identified, including APC(R876*) and FLT3(I836M), as well as a deletion within the EGFR gene. Combined, these data highlight the significance of ACF within the context of colon cancer pathogenesis, particularly in the proximal colon.

IMPLICATIONS

The identification of cancer-related mutations in commonly overlooked mucosal lesions underscores the preventive benefit of implementing advanced endoscopic screening to larger patient populations, particularly in the proximal colon. Visual Overview: http://mcr.aacrjournals.org/content/early/2014/05/22/1541-7786.MCR-13-0624/F1.large.jpg. Mol Cancer Res; 12(6); 823-9. ©2014 AACR.

Evaluation of the antitumor effects of BPR1J-340, a potent and selective FLT3 inhibitor, alone or in combination with an HDAC inhibitor, vorinostat, in AML cancer

Overexpression or/and activating mutation of FLT3 kinase play a major driving role in the pathogenesis of acute myeloid leukemia (AML). Hence, pharmacologic inhibitors of FLT3 are of therapeutic potential for AML treatment. In this study, BPR1J-340 was identified as a novel potent FLT3 inhibitor by biochemical kinase activity (IC₅₀ approximately 25 nM) and cellular proliferation (GC₅₀ approximately 5 nM) assays. BPR1J-340 inhibited the phosphorylation of FLT3 and STAT5 and triggered apoptosis in FLT3-ITD⁺ AML cells. The pharmacokinetic parameters of BPR1J-340 in rats were determined. BPR1J-340 also demonstrated pronounced tumor growth inhibition and regression in FLT3-ITD⁺ AML murine xenograft models. The combination treatment of the HDAC inhibitor vorinostat (SAHA) with BPR1J-340 synergistically induced apoptosis via Mcl-1 down-regulation in MOLM-13 AML cells, indicating that the combination of selective FLT3 kinase inhibitors and HDAC inhibitors could exhibit clinical benefit in AML therapy. Our results suggest that BPR1J-340 may be further developed in the preclinical and clinical studies as therapeutics in AML treatments.

Emergence of polyclonal FLT3 tyrosine kinase domain mutations during sequential therapy with sorafenib and sunitinib in FLT3-ITD-positive acute myeloid leukemia

PURPOSE

To evaluate the clinical activity of sequential therapy with sorafenib and sunitinib in FMS-like tyrosine kinase 3 (FLT3)-internal tandem duplication (ITD)-positive acute myelogenous leukemia (AML) and monitor the emergence of secondary FLT3 tyrosine kinase domain (TKD) mutations during treatment.

EXPERIMENTAL DESIGN

Six children with relapsed/refractory AML were treated with sorafenib in combination with clofarabine and cytarabine, followed by single-agent sorafenib if not a candidate for transplantation. Sunitinib was initiated after sorafenib relapse. Bone marrow samples were obtained for assessment of FLT3 TKD mutations by deep amplicon sequencing. The phase of secondary mutations with ITD alleles was assessed by cloning and sequencing of FLT3 exons 14 through 20. Identified mutations were modeled in Ba/F3 cells, and the effect of kinase inhibitors on FLT3 signaling and cell viability was assessed.

RESULTS

Four patients achieved complete remission, but 3 receiving maintenance therapy with sorafenib relapsed after 14 to 37 weeks. Sunitinib reduced circulating blasts in two patients and marrow blasts in one. Two patients did not respond to sorafenib combination therapy or sunitinib. FLT3 mutations at residues D835 and F691 were observed in sorafenib resistance samples on both ITD-positive and -negative alleles. Deep sequencing revealed low-level mutations and their evolution during sorafenib treatment. Sunitinib suppressed leukemic clones with D835H and F691L mutations, but not D835Y. Cells expressing sorafenib-resistant FLT3 mutations were sensitive to sunitinib in vitro.

CONCLUSIONS

Sunitinib has activity in patients that are resistant to sorafenib and harbor secondary FLT3 TKD mutations. The use of sensitive methods to monitor FLT3 mutations during therapy may allow individualized treatment with the currently available kinase inhibitors.

FLT3 inhibitors for acute myeloid leukemia: a review of their efficacy and mechanisms of resistance

Since the Food and Drug Administration approval of imatinib for treatment of chronic myeloid leukemia in 2001, tyrosine kinase inhibitors (TKIs) have become a mainstay in the care of many malignancies. In acute myeloid leukemia (AML), activating mutations in the FMS-like tyrosine kinase 3 (FLT3) gene result in survival and proliferation of leukemic blasts and are associated with adverse prognosis. Therefore, the FLT3 receptor is an appealing target for inhibition. Multiple small molecule TKIs are currently in development for FLT3-mutated AML, and agents are beginning to show promising efficacy. In other malignancies, the development of resistance to TKIs during the course of therapy has proven to be a challenge, and thus far, in clinical trials of FLT3 TKIs, resistance to inhibition represents a significant barrier to successful FLT3 inhibition. Understanding the mechanisms of resistance and overcoming these obstacles to target inhibition will be central to the success of these agents.

A genome-wide RNAi screen identifies proteins modulating aberrant FLT3-ITD signaling

Fms-like tyrosine kinase-3 is a commonly mutated gene in acute myeloid leukemia, with about one-third of patients carrying an internal-tandem duplication of the juxtamembrane domain in the receptor (FLT3-ITD). FLT3-ITD exhibits altered signaling quality, including aberrant activation of STAT5. To identify genes affecting FLT3-ITD-mediated STAT5 signaling, we performed an esiRNA-based RNAi screen utilizing a STAT5-driven reporter assay. Knockdowns that caused reduced FLT3-ITD-mediated STAT5 signaling were enriched for genes encoding proteins involved in protein secretion and intracellular protein transport, indicating that modulation of protein transport processes could potentially be used to reduce constitutive STAT5 signaling in FLT3-ITD-positive cells. The relevance of KDELR1, a component involved in the Golgi-ER retrograde transport, was further analyzed. In FLT3-ITD-expressing leukemic MV4-11 cells, downregulation of KDELR1 resulted in reduced STAT5 activation, proliferation and colony-forming capacity. Stable shRNA-mediated depletion of KDELR1 in FLT3-ITD-expressing 32D cells likewise resulted in reduced STAT5 signaling and cell proliferation. Importantly, these cells also showed a reduced capacity to generate a leukemia-like disease in syngeneic C3H/HeJ mice. Together our data suggest intracellular protein transport as a potential target for FLT3-ITD driven leukemias, with KDELR1 emerging as a positive modulator of oncogenic FLT3-ITD activity.

FLT3-ITD knockin impairs hematopoietic stem cell quiescence/homeostasis, leading to myeloproliferative neoplasm

Internal tandem duplication (ITD) mutations within the FMS-like tyrosine kinase-3 (FLT3) render the receptor constitutively active driving proliferation and survival in leukemic blasts. Expression of FLT3-ITD from the endogenous promoter in a murine knockin model results in progenitor expansion and a myeloproliferative neoplasm. In this study, we show that this expansion begins with overproliferation within a compartment of normally quiescent long-term hematopoietic stem cells (LT-HSCs), which become rapidly depleted. This depletion is reversible upon treatment with the small molecule inhibitor Sorafenib, which also ablates the disease. Although the normal LT-HSC has been defined as FLT3(-) by flow cytometric detection, we demonstrate that FLT3 is capable of playing a role within this compartment by examining the effects of constitutively activated FLT3-ITD. This indicates an important link between stem cell quiescence/homeostasis and myeloproliferative disease while also giving novel insight into the emergence of FLT3-ITD mutations in the evolution of leukemic transformation.

Targeting CDK1 promotes FLT3-activated acute myeloid leukemia differentiation through C/EBPα

Mutations that activate the fms-like tyrosine kinase 3 (FLT3) receptor are among the most prevalent mutations in acute myeloid leukemias. The oncogenic role of FLT3 mutants has been attributed to the abnormal activation of several downstream signaling pathways, such as STAT3, STAT5, ERK1/2, and AKT. Here, we discovered that the cyclin-dependent kinase 1 (CDK1) pathway is also affected by internal tandem duplication mutations in FLT3. Moreover, we also identified C/EBPα, a granulopoiesis-promoting transcription factor, as a substrate for CDK1. We further demonstrated that CDK1 phosphorylates C/EBPα on serine 21, which inhibits its differentiation-inducing function. Importantly, we found that inhibition of CDK1 activity relieves the differentiation block in cell lines with mutated FLT3 as well as in primary patient-derived peripheral blood samples. Clinical trials with CDK1 inhibitors are currently under way for various malignancies. Our data strongly suggest that targeting the CDK1 pathway might be applied in the treatment of FLT3ITD mutant leukemias, especially those resistant to FLT3 inhibitor therapies.

FLT3 inhibition: a moving and evolving target in acute myeloid leukaemia

Internal tandem duplication (ITD) of the fms-like tyrosine kinase 3 (FLT3) gene is a gain-of-function mutation common in acute myeloid leukaemia (AML). It is associated with inferior prognosis and response to chemotherapy. Single base mutations at the FLT3 tyrosine kinase domain (TKD) also leads to a gain of function, although its prognostic significance is less well defined because of its rarity. The clinical benefits of FLT3 inhibition are generally limited to AML with FLT3-ITD. However, responses are transient and leukaemia progression invariably occurs. There is compelling evidence that leukaemia clones carrying both ITD and TKD mutations appear when resistance to FLT3 inhibitors occurs. Interestingly, the emergence of double ITD and TKD mutants can be recapitulated in vitro when FLT3-ITD+ leukaemia cell lines are treated with mutagens and FLT3 inhibitors. Furthermore, murine xenotransplantation models also suggest that, in some cases, the FTL3-ITD and TKD double mutants actually exist in minute amounts before treatment with FLT3 inhibitors, expand under the selection pressure of FLT3 inhibition and become the predominant resistant clone(s) during the drug-refractory phase. On the basis of this model of clonal evolution, a multipronged strategy using more potent FLT3 inhibitors, and a combinatorial approach targeting both FLT3-dependent and FLT3-independent pathways, will be needed to improve outcome.

Receptor kinase profiles identify a rationale for multitarget kinase inhibition in immature T-ALL

Constitutively activated FLT3 signaling is common in acute myeloid leukemia, and is currently under evaluation for targeted therapy, whereas little data is available in T-cell acute lymphoblastic leukemia (T-ALL). We analyzed 357 T-ALL cases for FLT3 mutations and transcript expression. FLT3 mutations (3% overall) and overexpression (FLT3 high expresser (FLT3(High))) were restricted to immature/TCRγδ T-ALLs. In vitro FLT3 inhibition induced apoptosis in only 30% of FLT3(High) T-ALLs and did not correlate with mutational status. In order to investigate the mechanisms of primary resistance to FLT3 inhibition, a broad quantitative screen for receptor kinome transcript deregulation was performed by Taqman Low Density Array. FLT3 deregulation was associated with overexpression of a network of receptor kinases (RKs), potentially responsible for redundancies and sporadic response to specific FLT3 inhibition. In keeping with this resistance to FLT3 inhibition could be reversed by dual inhibition of FLT3 and KIT with a synergistic effect. We conclude that immature T-ALL may benefit from multitargeted RK inhibition and that exploration of the receptor kinome defines a rational strategy for testing multitarget kinase inhibition in malignant diseases.

SRC is a signaling mediator in FLT3-ITD– but not in FLT3-TKD–positive AML

Mutations of Fms-like tyrosine kinase 3 (FLT3) are among the most frequently detected molecular abnormalities in AML patients. Internal tandem duplications (ITDs) are found in approximately 25% and point mutations within the second tyrosine kinase domain (TKD) in approximately 7% of AML patients. Patients carrying the FLT3-ITD but not the FLT3-TKD mutation have a significantly worse prognosis. Therefore, both FLT3 mutations seem to exert different biologic functions. FLT3-ITD but not FLT3-TKD has been shown to induce robust activation of the STAT5 signaling pathway. In the present study, we investigated the mechanisms leading to differential STAT5 activation and show that FLT3-ITD but not FLT3-TKD uses SRC to activate STAT5. Coimmunoprecipitation and pull-down experiments revealed an exclusive interaction between SRC but not other Src family kinases and FLT3-ITD, which is mediated by the SRC SH2 domain. We identified tyrosines 589 and 591 of FLT3-ITD to be essential for SRC binding and subsequent STAT5 activation. Using site-specific Abs, we found that both residues were significantly more strongly phosphorylated in FLT3-ITD compared with FLT3-TKD. SRC inhibition and knock-down blocked STAT5 activation and proliferation induced by FLT3-ITD but not by FLT3-TKD. We conclude that SRC might be a therapeutic target in FLT3-ITD+ AML.

Pacritinib (SB1518), a JAK2/FLT3 inhibitor for the treatment of acute myeloid leukemia

FMS-like tyrosine kinase 3 (FLT3) is the most commonly mutated gene found in acute myeloid leukemia (AML) patients and its activating mutations have been proven to be a negative prognostic marker for clinical outcome. Pacritinib (SB1518) is a tyrosine kinase inhibitor (TKI) with equipotent activity against FLT3 (IC₅₀=22 n) and Janus kinase 2 (JAK2, IC₅₀=23 n). Pacritinib inhibits FLT3 phosphorylation and downstream STAT, MAPK and PI3 K signaling in FLT3-internal-tandem duplication (ITD), FLT3-wt cells and primary AML blast cells. Oral administration of pacritinib in murine models of FLT3-ITD-driven AML led to significant inhibition of primary tumor growth and lung metastasis. Upregulation of JAK2 in FLT3-TKI-resistant AML cells was identified as a potential mechanism of resistance to selective FLT3 inhibition. This resistance could be overcome by the combined FLT3 and JAK2 activities of pacritinib in this cellular model. Our findings provide a rationale for the clinical evaluation of pacritinib in AML including patients resistant to FLT3-TKI therapy.

Protein-tyrosine phosphatase DEP-1 controls receptor tyrosine kinase FLT3 signaling

Fms-like tyrosine kinase 3 (FLT3) plays an important role in hematopoietic differentiation, and constitutively active FLT3 mutant proteins contribute to the development of acute myeloid leukemia. Little is known about the protein-tyrosine phosphatases (PTP) affecting the signaling activity of FLT3. To identify such PTP, myeloid cells expressing wild type FLT3 were infected with a panel of lentiviral pseudotypes carrying shRNA expression cassettes targeting different PTP. Out of 20 PTP tested, expressed in hematopoietic cells, or presumed to be involved in oncogenesis or tumor suppression, DEP-1 (PTPRJ) was identified as a PTP negatively regulating FLT3 phosphorylation and signaling. Stable 32D myeloid cell lines with strongly reduced DEP-1 levels showed site-selective hyperphosphorylation of FLT3. In particular, the sites pTyr-589, pTyr-591, and pTyr-842 involved in the FLT3 ligand (FL)-mediated activation of FLT3 were hyperphosphorylated the most. Similarly, acute depletion of DEP-1 in the human AML cell line THP-1 caused elevated FLT3 phosphorylation. Direct interaction of DEP-1 and FLT3 was demonstrated by "substrate trapping" experiments showing association of DEP-1 D1205A or C1239S mutant proteins with FLT3 by co-immunoprecipitation. Moreover, activated FLT3 could be dephosphorylated by recombinant DEP-1 in vitro. Enhanced FLT3 phosphorylation in DEP-1-depleted cells was accompanied by enhanced FLT3-dependent activation of ERK and cell proliferation. Stable overexpression of DEP-1 in 32D cells and transient overexpression with FLT3 in HEK293 cells resulted in reduction of FL-mediated FLT3 signaling activity. Furthermore, FL-stimulated colony formation of 32D cells expressing FLT3 in methylcellulose was induced in response to shRNA-mediated DEP-1 knockdown. This transforming effect of DEP-1 knockdown was consistent with a moderately increased activation of STAT5 upon FL stimulation but did not translate into myeloproliferative disease formation in the 32D-C3H/HeJ mouse model. The data indicate that DEP-1 is negatively regulating FLT3 signaling activity and that its loss may contribute to but is not sufficient for leukemogenic cell transformation.

Downstream molecular pathways of FLT3 in the pathogenesis of acute myeloid leukemia: biology and therapeutic implications

FLT3 is a type III receptor tyrosine kinase. Mutations of FLT3 comprise one of the most frequently identified types of genetic alterations in acute myeloid leukemia. One-third of acute myeloid leukemia patients have mutations of this gene, and the majority of these mutations involve an internal tandem duplication in the juxtamembrane region of FLT3, leading to constitutive activation of downstream signaling pathways and aberrant cell growth. This review summarizes the current understanding of the effects of the downstream molecular signaling pathways after FLT3 activation, with a particular focus on the effects on transcription factors. Moreover, this review describes novel FLT3-targeted therapies, as well as efficient combination therapies for FLT3-mutated leukemia cells.

Bench to bedside targeting of FLT3 in acute leukemia

FMS-Like-Tyrosine kinase-3 (FLT3) mutations are found in about 30% of cases of acute myeloid leukemia and confer an increased relapse rate and reduced overall survival. Targeting this tyrosine kinase by direction inhibition is the focus of both preclinical and clinical research in AML. Several molecules are in clinical development inhibit FLT3, but thus far clinical responses have been limited. Correlative studies from monotherapy trials have established that responses require sustained, effective FLT3 inhibition in vivo. Studies combining FLT3 inhibitors with chemotherapy have demonstrated increased remission rates to date but have yet to produce a survival advantage. Currently the only approved FLT3 inhibitor available for off-label use is sorafenib, which clearly has clinical activity but does not commonly lead to a complete response. Several FLT3 inhibitors are currently being tested as single agents and in combination with chemotherapy, and it seems likely that a clinically useful drug will eventually emerge.

FLT3 as a therapeutic target in AML: still challenging after all these years

Mutations within the FMS-like tyrosine kinase 3 (FLT3) gene on chromosome 13q12 have been detected in up to 35% of acute myeloid leukemia (AML) patients and represent one of the most frequently identified genetic alterations in AML. Over the last years, FLT3 has emerged as a promising molecular target in therapy of AML. Here, we review results of clinical trials and of correlative laboratory studies using small molecule FLT3 tyrosine kinase inhibitors (TKIs) in AML patients. We also review mechanisms of primary and secondary drug resistance to FLT3-TKI, and from the data currently available we summarize lessons learned from FLT3-TKI monotherapy. Finally, for using FLT3 as a molecular target, we discuss novel strategies to overcome treatment failure and to improve FLT3 inhibitor therapy.

PIM serine/threonine kinases in the pathogenesis and therapy of hematologic malignancies and solid cancers

The identification as cooperating targets of Proviral Integrations of Moloney virus in murine lymphomas suggested early on that PIM serine/threonine kinases play an important role in cancer biology. Whereas elevated levels of PIM1 and PIM2 were mostly found in hematologic malignancies and prostate cancer, increased PIM3 expression was observed in different solid tumors. PIM kinases are constitutively active and their activity supports in vitro and in vivo tumor cell growth and survival through modification of an increasing number of common as well as isoform-specific substrates including several cell cycle regulators and apoptosis mediators. PIM1 but not PIM2 seems also to mediate homing and migration of normal and malignant hematopoietic cells by regulating chemokine receptor surface expression. Knockdown experiments by RNA interference or dominant-negative acting mutants suggested that PIM kinases are important for maintenance of a transformed phenotype and therefore potential therapeutic targets. Determination of the protein structure facilitated identification of an increasing number of potent small molecule PIM kinase inhibitors with in vitro and in vivo anticancer activity. Ongoing efforts aim to identify isoform-specific PIM inhibitors that would not only help to dissect the kinase function but hopefully also provide targeted therapeutics. Here, we summarize the current knowledge about the role of PIM serine/threonine kinases for the pathogenesis and therapy of hematologic malignancies and solid cancers, and we highlight structural principles and recent progress on small molecule PIM kinase inhibitors that are on their way into first clinical trials.