Inhibition of FLT3-mediated transformation by use of a tyrosine kinase inhibitor

C/EBPα Confers Dependence to Fatty Acid Anabolic Pathways and Vulnerability to Lipid Oxidative Stress-Induced Ferroptosis in FLT3-Mutant Leukemia

Although transcription factor CCAAT-enhancer binding protein α (C/EBPα) is critical for normal and leukemic differentiation, its role in cell and metabolic homeostasis is largely unknown in cancer. Here, multiomics analyses uncovered a coordinated activation of C/EBPα and Fms-like tyrosine kinase 3 (FLT3) that increased lipid anabolism in vivo and in patients with FLT3-mutant acute myeloid leukemia (AML). Mechanistically, C/EBPα regulated the fatty acid synthase (FASN)-stearoyl-CoA desaturase (SCD) axis to promote fatty acid (FA) biosynthesis and desaturation. We further demonstrated that FLT3 or C/EBPα inactivation decreased monounsaturated FA incorporation to membrane phospholipids through SCD downregulation. Consequently, SCD inhibition enhanced susceptibility to lipid redox stress that was exploited by combining FLT3 and glutathione peroxidase 4 inhibition to trigger lipid oxidative stress, enhancing ferroptotic death of FLT3-mutant AML cells. Altogether, our study reveals a C/EBPα function in lipid homeostasis and adaptation to redox stress, and a previously unreported vulnerability of FLT3-mutant AML to ferroptosis with promising therapeutic application.

SIGNIFICANCE

FLT3 mutations are found in 30% of AML cases and are actionable by tyrosine kinase inhibitors. Here, we discovered that C/EBPα regulates FA biosynthesis and protection from lipid redox stress downstream mutant-FLT3 signaling, which confers a vulnerability to ferroptosis upon FLT3 inhibition with therapeutic potential in AML. This article is highlighted in the In This Issue feature, p. 1501.

Poor outcome of pediatric patients with acute myeloid leukemia harboring high FLT3/ITD allelic ratios

Activating FLT3 mutations are the most common mutations in acute myeloid leukemia (AML), but the optimal threshold of FLT3/ITD allelic ratio (AR) among pediatric AML patients remains controversial. Here, we present the outcome and prognostic significance of FLT3/ITD AR analysis among pediatric patients with AML from the TARGET dataset. Applying fitting curve models and threshold effect analysis using the restrictive cubic spline function following Cox proportional hazards models identifies the cut-off value of 0.5 on FLT3/ITD AR. Moreover, we observe that high FLT3/ITD AR patients have an inferior outcome when compared to low AR patients. Our study also demonstrates that stem cell transplantation may improve the outcome in pediatric AML patients with high FLT3/ITD AR and may be further improved when combined with additional therapies such as Gemtuzumab Ozogamicin. These findings underline the importance of individualized treatment of pediatric AML.

Activating FLT3 mutations are the most common mutations in AML. Here, the authors explore the relationship between the FLT3/ITD allelic ratio and prognosis in pediatric AML patients and identify an optimal threshold to stratify patients.

Discovery of indirubin-3'-aminooxy-acetamide derivatives as potent and selective FLT3/D835Y mutant kinase inhibitors for acute myeloid leukemia

Mutations in Fms-like tyrosine kinase 3 (FLT3) have been implicated in the pathogenesis of acute myeloid leukemia (AML) by affecting the proliferation and differentiation of hematopoietic stem and progenitor cells. Although several FLT3 inhibitors have been developed, the occurrence of secondary TKD mutations of FLT3 such FLT3/D835Y and FLT3/F691L lead to drug resistance and has become a key area of unmet medical needs. To overcome the obstacle of secondary TKD mutations, a new series of indirubin-3'-aminooxy-acetamide derivatives was discovered as potent and selective FLT3 and FLT3/D835Y inhibitors that were predicted to bind at the DFG-in active conformation of FLT3 in molecular docking studies. Through structure-activity relationship studies, the most optimized compound 13a was developed as a potent inhibitor at FLT3 and FLT3/D835Y with IC₅₀ values of 0.26 nM and 0.18 nM, respectively, which also displayed remarkably strong in vitro anticancer activities, with single-digit nanomolar GI₅₀ values for several AML (MV4-11 and MOLM14) and Ba/F3 cell lines expressed with secondary TKD mutated FLT3 kinases as well as FLT3-ITD. The selectivity profiles of compound 13a in the oncology kinase panel and various human cancer cell lines were prominent, demonstrating that its inhibitory activities were mainly focused on a few members of the receptor tyrosine kinase family and AML versus solid tumor cell lines. Furthermore, significant in vivo anticancer efficacy of compound 13a was confirmed in a xenograft animal model implanted with FLT3-ITD/D835Y-expressing MOLM-14 cells related to secondary TKD mutation.

A method for overcoming plasma protein inhibition of tyrosine kinase inhibitors

Plasma protein binding reduces potency of staurosporine-derived tyrosine kinase inhibitors against Flt3-mutant AML. “Decoy” drugs interfering with the binding, including mifepristone, can be harnessed to restore the antileukemia activity.

FMS-like tyrosine kinase 3 (FLT3) is the most frequently mutated gene in acute myeloid leukemia and a target for tyrosine kinase inhibitors (TKI). FLT3 TKIs have yielded limited improvements to clinical outcomes. One reason for this is TKI inhibition by endogenous factors. We characterized plasma protein binding of FLT3 TKI, specifically staurosporine derivatives (STS-TKI) by alpha-1-acid glycoprotein (AGP), simulating its effects upon drug efficacy. Human AGP inhibits the antiproliferative activity of STS-TKI in FLT3/ITD-dependent cells, with IC₅₀ shifts higher than clinically achievable. This is not seen with nonhuman plasma. Mifepristone cotreatment, with its higher AGP affinity, improves TKI activity despite AGP, yielding IC₅₀s predicted to be clinically effective. In a mouse model of AGP drug inhibition, mifepristone restores midostaurin activity. This suggests combinatorial methods for overcoming plasma protein inhibition of existing TKIs for leukemia as well as providing a platform for investigating the drug–protein interaction space for developing more potent small-molecule agents.

FLT3 inhibitors in the treatment of acute myeloid leukemia: current status and future perspectives

Mutations in the FMS-like tyrosine kinase 3 (FLT3) gene arise in 25-30% of all acute myeloid leukemia (AML) patients. These mutations lead to constitutive activation of the protein product and are divided in two broad types: internal tandem duplication (ITD) of the juxtamembrane domain (25% of cases) and point mutations in the tyrosine kinase domain (TKD). Patients with FLT3 ITD mutations have a high relapse risk and inferior cure rates, whereas the role of FLT3 TKD mutations still remains to be clarified. Additionally, growing research indicates that FLT3 status evolves through a disease continuum (clonal evolution), where AML cases can acquire FLT3 mutations at relapse - not present in the moment of diagnosis. Several FLT3 inhibitors have been tested in patients with FLT3-mutated AML. These drugs exhibit different kinase inhibitory profiles, pharmacokinetics and adverse events. First-generation multi-kinase inhibitors (sorafenib, midostaurin, lestaurtinib) are characterized by a broad-spectrum of drug targets, whereas second-generation inhibitors (quizartinib, crenolanib, gilteritinib) show more potent and specific FLT3 inhibition, and are thereby accompanied by less toxic effects. Notwithstanding, all FLT3 inhibitors face primary and acquired mechanisms of resistance, and therefore the combinations with other drugs (standard chemotherapy, hypomethylating agents, checkpoint inhibitors) and its application in different clinical settings (upfront therapy, maintenance, relapsed or refractory disease) are under study in a myriad of clinical trials. This review focuses on the role of FLT3 mutations in AML, pharmacological features of FLT3 inhibitors, known mechanisms of drug resistance and accumulated evidence for the use of FLT3 inhibitors in different clinical settings.

Evaluation of gilteritinib in combination with chemotherapy in preclinical models of FLT3-ITD+ acute myeloid leukemia

Activating internal tandem duplication (ITD) and tyrosine kinase domain (TKD) point mutations in Fms-like tyrosine kinase 3 (FLT3) occur in approximately 30% of patients with acute myeloid leukemia (AML), and confer a poor prognosis with standard cytarabine/anthracycline or azacitidine-based chemotherapy regimens. Gilteritinib is a highly-specific, potent FLT3/AXL inhibitor with demonstrated activity against FLT3-ITD and FLT3-TKD mutations. Compared with salvage chemotherapy, treatment with once-daily oral gilteritinib demonstrated a clinical benefit in patients with FLT3-mutated relapsed/refractory AML, which led to its recent approval in Japan and the United States. We investigated the effects of gilteritinib combined with cytarabine plus daunorubicin/idarubicin, or combined with azacitidine in human FLT3-ITD–positive (FLT3-ITD⁺) AML cell lines and xenografted mouse models. Gilteritinib induced G₁ arrest and apoptosis in a dose-dependent manner. The addition of cytarabine, daunorubicin, idarubicin, or azacitidine potentiated apoptosis. Gilteritinib alone or combined with cytarabine, daunorubicin, idarubicin, or azacitidine, inhibited anti-apoptotic protein expression in MV4-11 cells. In xenografted mice, administration of cytarabine, idarubicin, or azacitidine in combination with gilteritinib had little impact on plasma or intratumor PK profiles of gilteritinib, cytarabine, idarubicin, or azacitidine. Gilteritinib combined with chemotherapy reduced tumor volume to a greater extent than either gilteritinib or chemotherapy alone. Of note, the addition of cytarabine plus daunorubicin/idarubicin led to tumor regression in mice, with complete regression observed in six out of eight mice in both triple combination groups. These findings support the investigation of gilteritinib combined with chemotherapy in patients with FLT3-ITD⁺ AML, including those who are ineligible for intensive chemotherapy.

Targeted therapies in Acute Myeloid Leukemia: a focus on FLT-3 inhibitors and ABT199

INTRODUCTION

Acute myeloid leukemia (AML) remains a therapeutic challenge. Despite ongoing research, the standard therapy for AML has not changed significantly in the past four decades. With the identification of cytogenetic and molecular abnormalities, several promising therapeutic agents are currently being investigated. FLT3 mutation is a well-recognized target seen in 30% of the cytogenetically normal AML. More recently, the BCL2 family of anti-apoptotic proteins have also generated great interest as a therapeutic target. Areas covered: This review will cover the role of FLT3 inhibitors in AML, discussing trials in relapsed/refractory AML and in the frontline setting, including the young and elderly patient population. Toxicities and potential mechanism of resistance will also be covered. In addition, most current studies demonstrating the role of BCL-2 inhibitors namely ABT-199/venetoclax in AML will also be discussed. Expert commentary: AML is one of the most heterogeneous group of hematological malignancies. It remains a therapeutic challenge with limited therapeutic progress despite ongoing research. With the identification of different mutations in AML, several drugs are being evaluated in clinical trials. Targeted agents such as FLT3 inhibitors and BH3 mimetics so far have shown promising results in terms of response and toxicity profile.

Quinoxaline-Based Scaffolds Targeting Tyrosine Kinases and Their Potential Anticancer Activity

Quinoxaline derivatives, also called benzopyrazines, are an important class of heterocyclic compounds. Quinoxalines have drawn great attention due to their wide spectrum of biological activities. They are considered as an important basis for anticancer drugs due to their potential activity as protein kinase inhibitors. In this review, we focus on the chemistry of the quinoxaline derivatives, the strategies for their synthesis, their potential activities against various tyrosine kinases, and on the structure-activity relationship studies reported to date.

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.

Clinical Response to Sorafenib in a Patient with Metastatic Colorectal Cancer and FLT3 Amplification

Background

A considerable number of patients with metastatic colorectal cancer progress after exhausting all approved standard therapies but maintain an adequate performance status and could be candidates for further treatment. We aim at reviewing our experience with sorafenib treatment of a patient with FLT3 mutation in refractory metastatic colorectal cancer.

Methods

Treatment with sorafenib of a patient with metastatic colorectal cancer and FLT3 translocation who had previously been heavily treated.

Results

The patient with metastatic colorectal cancer, aged 51 years, showed significant symptomatic and laboratory improvement with sorafenib treatment (400 mg twice daily).

Conclusion

The presented case illustrates how an aggressive and refractory colorectal tumor may respond well to targeted therapy.

Targeted Therapy of FLT3 in Treatment of AML-Current Status and Future Directions

Internal tandem duplications (ITDs) of the gene encoding the Fms-Like Tyrosine kinase-3 (FLT3) receptor are present in approximately 25% of patients with acute myeloid leukemia (AML). The mutation is associated with poor prognosis, and the aberrant protein product has been hypothesized as an attractive therapeutic target. Various tyrosine kinase inhibitors (TKIs) have been developed targeting FLT3, but in spite of initial optimism the first generation TKIs tested in clinical studies generally induce only partial and transient hematological responses. The limited treatment efficacy generally observed may be explained by numerous factors; extensively pretreated and high risk cohorts, suboptimal pharmacodynamic and pharmacokinetic properties of the compounds, acquired TKI resistance, or the possible fact that inhibition of mutated FLT3 alone is not sufficient to avoid disease progression. The second-generation agent quizartinb is showing promising outcomes and seems better tolerated and with less toxic effects than traditional chemotherapeutic agents. Therefore, new generations of TKIs might be feasible for use in combination therapy or in a salvage setting in selected patients. Here, we sum up experiences so far, and we discuss the future outlook of targeting dysregulated FLT3 signaling in the treatment of AML.

The Biology and Targeting of FLT3 in Pediatric Leukemia

Despite remarkable improvement in treatment outcomes in pediatric leukemia over the past several decades, the prognosis for high-risk groups of acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL), as well as for relapsed leukemia, remains poor. Intensification of chemotherapy regimens for those at highest risk has improved success rates, but at the cost of significantly increased morbidity and long-term adverse effects. With the success of imatinib in Philadelphia-chromosome-positive leukemia and all-trans retinoic acid in acute promyelocytic leukemia, the quest to find additional molecularly targeted therapies has generated much excitement over recent years. Another such possible target in pediatric acute leukemia is FMS-like tyrosine kinase 3 (FLT3). FLT3 aberrations are among the most frequently identified transforming events in AML, and have significant clinical implications in both high-risk pediatric AML and in certain high-risk groups of pediatric ALL. Therefore, the successful targeting of FLT3 has tremendous potential to improve outcomes in these subsets of patients. This article will give an overview of the molecular function and signaling of the FLT3 receptor, as well as its pathogenic role in leukemia. We review the discovery of targeting FLT3, discuss currently available FLT3 inhibitors in pediatric leukemia and results of clinical trials to date, and finally, consider the future promise and challenges of FLT3 inhibitor therapy.

Systemic sclerosis immunoglobulin induces growth and a pro-fibrotic state in vascular smooth muscle cells through the epidermal growth factor receptor

OBJECTIVE

It has been suggested that autoantibodies in systemic sclerosis (SSc) may induce the differentiation of cultured fibroblasts into myofibroblasts through platelet-derived growth factor receptor (PDGFR) activation. The present study aims to characterize the effects of SSc IgG on vascular smooth muscle cells (VSMCs) and to determine if stimulatory autoantibodies directed to the PDGFR can be detected, and whether they induce a profibrotic response in primary cultured VSMCs.

METHODS

Cultured VSMCs were exposed to IgG fractions purified from SSc-patient or control sera. VSMC responses were then analyzed for ERK1/2 and Akt phosphorylation, PDGFR immunoprecipitation, cellular proliferation, protein synthesis, and pro-fibrotic changes in mRNA expression.

RESULTS

Stimulatory activity in IgG fractions was more prevalent and intense in the SSc samples. SSc IgG immunoprecipitated the PDGFR with greater avidity than control IgG. Interestingly, activation of downstream signaling events (e.g. Akt, ERK1/2) was independent of PDGFR activity, but required functional EGFR. We also detected increased protein synthesis in response to SSc IgG (p<0.001) and pro-fibrotic changes in gene expression (Tgfb1 +200%; Tgfb2 -23%; p<0.001)) in VSMCs treated with SSc IgG.

CONCLUSION

When compared to control IgG, SSc IgG have a higher stimulation index in VSMCs. Although SSc IgG interact with the PDGFR, the observed remodeling signaling events occur through the EGFR in VSMC. Our data thus favour a model of transactivation of the EGFR by SSc-derived PDGFR autoantibodies and suggest the use of EGFR inhibitors in future target identification studies in the field of SSc.

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.

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.

FLT3 inhibitors: a story of the old and the new

PURPOSE OF REVIEW

Ever since the recognition that FMS-like tyrosine kinase 3 (FLT3) mutations exert a profound negative prognostic impact on the clinical outcome of patients with acute myeloid leukemia (AML), researchers have sought to find effective small-molecule inhibitors of this receptor tyrosine kinase. This review will attempt to provide a survey of the FLT3 inhibitors currently under investigation and provide a discussion on their current status in clinical trials.

RECENT FINDINGS

Over the past 10 years, a number of different compounds have been studied in vitro and clinically as FLT3 inhibitors. The first inhibitors studied were hampered by cumbersome pharmacokinetics and a general lack of potency. However, some agents have shown promise in clinical trials with transient responses in AML. Newer compounds, such as AC220, have demonstrated profound selectivity and potency against the FLT3 target, and are currently being investigated in clinical trials.

SUMMARY

Clinical trials have so far demonstrated that inhibitors of FLT3 do have clinical activity in patients with FLT3-mutant AML, although this activity is often transient and correlates with effective in-vivo suppression of the FLT3 target. As newer, more potent agents are now entering advanced clinical trials, opportunities will emerge for real progress against this grim disease.

The multitargeted receptor tyrosine kinase inhibitor linifanib (ABT-869) induces apoptosis through an Akt and glycogen synthase kinase 3β-dependent pathway

The FMS-like receptor tyrosine kinase 3 (FLT3) plays an important role in controlling differentiation and proliferation of hematopoietic cells. Activating mutations in FLT3 occur in patients with acute myeloid leukemia (AML; 15%-35%), resulting in abnormal cell proliferation. Furthermore, both adult and pediatric patients with AML harboring the FLT3 internal tandem duplication (ITD) mutation have a poor prognosis. Several inhibitors have been developed to target mutant FLT3 for the treatment of AML, yet the molecular pathways affected by drug inhibition of the mutated FLT3 receptor alone have not been characterized as yet. Linifanib (ABT-869) is a multitargeted tyrosine kinase receptor inhibitor that suppresses FLT3 signaling. In this article, we show that treatment with linifanib inhibits proliferation and induces apoptosis in ITD mutant cells in vitro and in vivo. We show that treatment with linifanib reduces phosphorylation of Akt and glycogen synthase kinase 3β (GSK3β). In addition, we show that inhibition of GSK3β decreases linifanib-induced apoptosis. This study shows the importance of GSK3 as a potential target for AML therapy, particularly in patients with FLT3 ITD mutations.

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.

Investigational drugs targeting FLT3 for leukemia

FMS-like tyrosine kinase-3 (FLT3) is a member of the class III membrane receptor tyrosine kinase family and is important in survival, proliferation and differentiation of hematopoietic cells. FLT3 is mutated in approximately 30% of acute myelogenous leukemia patients. These mutations involve internal tandem duplications in the juxtamembrane domain of the receptor and tyrosine kinase point mutations in the activation loop. Over the past decade, due to the incidence and poor prognosis associated with FLT3, numerous agents have been developed to directly inhibit the activity of wild type and mutated FLT3. In this review, we focus on the preclinical data demonstrating in vitro activity, inhibition of downstream signaling pathways and potential synergy with traditional chemotherapeutic agents. Also, early clinical trial data specifically focusing on drug toxicity, clinical efficacy and future directions of FLT3-directed anticancer therapy are discussed.

Mutant FLT3 signaling contributes to a block in myeloid differentiation

FLT3 is a member of the class III receptor tyrosine kinase family and is primarily expressed on hematopoietic stem/progenitor cells. Somatic mutations of FLT3 involving internal tandem duplication (ITD) of the juxtamembrane domain or point mutations in the activation loop have been identified in approximately 17 - 34% and 7 - 9% of acute myeloid leukemia (AML) patients, respectively. The ITD mutations appear to activate the tyrosine kinase domain through receptor dimerization in a FLT3 ligand-independent manner. Constitutively activated FLT3 provides cells with proliferative and anti-apoptotic advantages and portends an especially poor prognosis for patients with this mutation. FLT3/ITD mutations also contribute to a block of myeloid differentiation. FLT3 tyrosine kinase inhibitors suppress the growth and induce apoptosis and differentiation of leukemia cells expressing FLT3/ITD mutants. Therefore, FLT3 is a therapeutic target and inhibition of FLT3 tyrosine kinase activity may provide a new approach in the treatment of leukemia carrying these mutations.

FLT3 inhibition and mechanisms of drug resistance in mutant FLT3-positive AML

An appealing therapeutic target in AML is constitutively activated, mutant FLT3, which is expressed in a subpopulation of AML patients and is generally a poor prognostic indicator in patients under the age of 65. There are currently several FLT3 inhibitors that are undergoing clinical investigation. However, the discovery of drug-resistant leukemic blast cells in FLT3 inhibitor-treated AML patients has prompted the search for novel, structurally diverse FLT3 inhibitors that could be alternatively used to circumvent drug resistance. Here, we provide an overview of FLT3 inhibitors under preclinical and clinical investigation, and we discuss mechanisms whereby AML cells develop resistance to FLT3 inhibitors, and the ways in which combination therapy could potentially be utilized to override drug resistance. We discuss how the cross-talk between major downstream signaling pathways, such as PI3K/PTEN/Akt/mTOR, RAS/Raf/MEK/ERK, and Jak/STAT, can be exploited for therapeutic purposes by targeting key signaling molecules with selective inhibitors, such as mTOR inhibitors, HSP90 inhibitors, or farnesyltransferase inhibitors, and identifying those agents with the ability to positively combine with inhibitors of FLT3, such as PKC412 and sunitinib. With the widespread onset of drug resistance associated with tyrosine kinase inhibitors, due to mechanisms involving development of point mutations or gene amplification of target proteins, the use of a multi-targeted therapeutic approach is of potential clinical benefit.

Patient-derived acute myeloid leukemia (AML) bone marrow cells display distinct intracellular kinase phosphorylation patterns

Multiparametric analyses of phospho-protein activation in patients with acute myeloid leukemia (AML) offers a quantitative measure to monitor the activity of novel intracellular kinase (IK) inhibitors. As recent clinical investigation with FMS-like tyrosine-3 inhibitors demonstrated, targeting IK with selective inhibitors can have a modest clinical benefit. Because multiple IKs are active in patients with AML, multikinase inhibitors may provide the necessary inhibition profile to achieve a more sustained clinical benefit. We here describe a method of assessing the activation of several IKs by flow cytometry. In 40 different samples of patients with AML we observed hyper-activated phospho-proteins at baseline, which is modestly increased by adding stem cell factor to AML cells. Finally, AML cells had a significantly different phospho-protein profile compared with cells of the lymphocyte gate. In conclusion, our method offers a way to determine the activation status of multiple kinases in AML and hence is a reliable assay to evaluate the pharmacodynamic activity of novel multikinase inhibitors.

Detection of FLT3 oncogene mutations in acute myeloid leukemia using conformation sensitive gel electrophoresis

FLT3 (fms-related tyrosine kinase 3) is a receptor tyrosine kinase class III that is expressed on by early hematopoietic progenitor cells and plays an important role in hematopoietic stem cell proliferation, differentiation and survival. FLT3 is also expressed on leukemia blasts in most cases of acute myeloid leukemia (AML). In order to determine the frequency of FLT3 oncogene mutations, we analyzed genomic DNA of adult de novo acute myeloid leukemia (AML). Polymerase chain reaction (PCR) and conformation-sensitive gel electrophoresis (CSGE) were used for FLT3 exons 11, 14, and 15, followed by direct DNA sequencing. Two different types of functionally important FLT 3 mutations have been identified. Those mutations were unique to patients with inv(16), t(15:17) or t(8;21) and comprised fifteen cases with internal tandem duplication (ITD) mutation in the juxtamembrane domain and eleven cases with point mutation (exon 20, Asp835Tyr). The high frequency of the flt3 proto-oncogene mutations in acute myeloid leukemia AML suggests a key role for the receptor function. The association of FLT3 mutations with chromosomal abnormalities invites speculation as to the link between these two changes in the pathogenesis of acute myeloid leukemiaAML. Furthermore, CSGE method has shown to be a rapid and sensitive screening method for detection of nucleotide alteration in FLT3 gene. Finally, this study reports, for the first time in Saudi Arabia, mutations in the human FLT3 gene in acute myeloid leukemia AML patients.

FLT3 inhibitors for the treatment of autoimmune disease

BACKGROUND

Autoimmune diseases encompass a broad range of illnesses with a variety of underlying causes, some of which are known and some of which remain elusive.

OBJECTIVE

The focus of this review will be on describing the development of a new type of therapy that could potentially treat T cell-mediated autoimmune diseases. Unlike traditional therapies, which have primarily focused on suppressing T cells directly, targeting the step of antigen presentation may allow a less toxic therapy in which autoimmunity is lessened without compromising the entire immune system. This review will outline the science behind the development of the therapy, the roles of dendritic cells in generating autoimmune disease, and the function of the FLT3 receptor in this process.

Incorporating FLT3 inhibitors into acute myeloid leukemia treatment regimens

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 of this tyrosine kinase by direction inhibition is the focus of both preclinical and clinical research in AML. Several molecules in clinical development inhibit FLT3 with varying degrees of specificity. Preclinical models suggest that these compounds enhance the cytotoxicity of conventional chemotherapeutics against FLT3 mutant leukemia cells. The pharmacodynamic interactions between FLT3 inhibitors and chemotherapy appear to be sequence dependent. When the FLT3 inhibitor is used prior to chemotherapy, antagonism is displayed, while if FLT3 inhibition is instituted after to exposure to chemotherapy, synergistic cytotoxicity is seen. The combination of FLT3 inhibitors with chemotherapy is also complicated by potential pharmacokinetic obstacles, such as plasma protein binding and p-glycoprotein interactions. Ongoing and future studies are aimed at incorporating FLT3 inhibitors into conventional induction and consolidation therapy specifically for patients with FLT3 mutant AML.

Is it important to decipher the heterogeneity of "normal karyotype AML"?

Almost half of adult acute myelogenous leukemia (AML) is normal cytogenetically, and this subgroup shows a remarkable heterogeneity of genetic mutations at the molecular level and an intermediate response to therapy. The finding of recurrent cytogenetic abnormalities has influenced, in a primary way, the understanding and treatment of leukemias. Yet "normal karyotype AML" lacks such obvious abnormalities, but has a variety of prognostically important genetic abnormalities. Thus, the presence of a FLT3-ITD (internal tandem duplication), MLL-PTD (partial tandem duplication), or the increased expression of ERG or EVI1 mRNAs confer a poor prognosis, and an increased risk of relapse. In contrast, the presence of cytoplasmic nucleophosmin or C/EBPA mutations is associated with lower relapse rates and improved survival. Although resistance to treatment is associated with specific mutations, the degree to which the leukemia resembles a stem cell in its functional properties may provide greater protection from the effects of treatment. Although usually all of the circulating leukemia cells are cleared following treatment, a small residual population of leukemic cells in the bone marrow persists, making this disease hard to eradicate. Increased understanding of the biological consequences of at least some of these mutations in "normal karyotype AML" is leading to more targeted approaches to develop more effective treatments for this disease.

FLT3 kinase inhibitors in the management of acute myeloid leukemia

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase (TK) expressed by immature hematopoietic cells and is important for the normal development of stem cells and the immune system. Mutations of the juxtamembranous and TK domain of the gene are described in 30%-35% of patients with acute myeloid leukemia (AML). These mutations alter the biologic properties of AML and are associated with prognosis. In recent years, there has been an enormous development of potential inhibitors of FLT3 mutations. These substances are now being studied in clinical protocols. The initial trials reveal that, unlike in patients with chronic myeloid leukemia, TK inhibitor (TKI) therapy in AML is more complex. To date, most FLT3 TKIs investigated in clinical studies show a favorable toxicity profile with considerable biologic activity. However, refractory disease and/or the rapid development of resistance toward these new drugs remain major challenges. Strategies to circumvent this unsatisfactory clinical potential of FLT3 TKIs are mainly based on the combination with cytotoxic chemotherapy. Herein, we summarize results from studies using FLT3 TKIs as single agents and report on the first clinical trials investigating FLT3 TKIs in combination with chemotherapy.

Roles for deregulated receptor tyrosine kinases and their downstream signaling molecules in hematologic malignancies

Growth, survival and differentiation of hematopoietic cells are regulated by the interactions between hematopoietic growth factors and their receptors. The defect in these interactions results in a failure of hematopoiesis, while aberrantly elevated and/or sustained activation of these signals cause hematologic malignancies. Among them, constitutively activating mutations of the receptor tyrosine kinases (RTKs), such as c-Kit, platelet-derived growth factor receptor (PDGFR) and FLT3, are often involved in the pathogenesis of various types of hematologic malignancies. Constitutive activation of RTKs is provoked by several mechanisms including chromosomal translocations and various mutations involving their regulatory regions. Chromosomal translocations commonly generate chimeric proteins consisting of the cytoplasmic domain of RTKs and the dimerization or multimerization motif of the fusion partner, resulting in the constitutive dimerization of RTKs. On the other hand, missense, insertion or deletion mutations in the regulatory regions, such as juxtamembrane domain, activation loop, and extracellular domain, also cause constitutive activation of RTKs mainly by preventing the auto-inhibitory regulation. Oncogenic RTKs activate downstream signaling molecules such as Ras/MAPK, PI3-K/Akt/mTOR, and STATs as well as ligand-activated wild type RTKs. However, their signals are quantitatively and qualitatively different from wild type RTKs. Based on these findings, several agents that target oncogenic RTKs or their downstream molecules have been developed: imatinib and FLT3 inhibitors for RTKs themselves, farnesyltransferase inhibitors, mTOR inhibitors and MEK inhibitors for the downstream signaling molecules. As promising results have been obtained in several clinical trials using these agents, the establishment of these molecular targeted agents is expected.

Knock-in of an internal tandem duplication mutation into murine FLT3 confers myeloproliferative disease in a mouse model

Constitutive activation of FMS-like tyrosine kinase 3 (FLT3) by internal tandem duplication (ITD) mutations is one of the most common molecular alterations known in acute myeloid leukemia (AML). To investigate the role FLT3/ITD mutations play in the development of leukemia, we generated a FLT3/ITD knock-in mouse model by inserting an ITD mutation into the juxtamembrane domain of murine Flt3. FLT3wt/ITD mice developed myeloproliferative disease, characterized by splenomegaly, leukocytosis, and myeloid hypercellularity, which progressed to mortality by 6 to 20 months. Bone marrow (BM) and spleen from FLT3wt/ITD mice had an increased fraction of granulocytes/monocytes and dendritic cells, and a decreased fraction of B-lymphocytes. No sign of acute leukemia was observed over the lifetime of these mice. BM from FLT3wt/ITD mice showed enhanced potential to generate myeloid colonies in vitro. BM from FLT3wt/ITD mice also produced more spleen colonies in the in vivo colony-forming unit (CFU)-spleen assay. In the long-term competitive repopulation assay, BM cells from FLT3wt/ITD mice outgrew the wild-type competitor cells and showed increased myeloid and reduced lymphoid expansion activity. In summary, our data indicate that expression of FLT3/ITD mutations alone is capable of conferring normal hematopoietic stem/progenitor cells (HSPCs) with enhanced myeloid expansion. It also appears to suppress B lymphoid maturation. Additional cooperative events appear to be required to progress to acute leukemia.

Constitutive Fms-like tyrosine kinase 3 activation results in specific changes in gene expression in myeloid leukaemic cells

Constitutively activating internal tandem duplication (ITD) mutations of the receptor tyrosine kinase FLT3 (Fms-like tyrosine kinase 3) play an important role in leukaemogenesis. We have examined, by cDNA microarray analysis, the changes in gene expression induced by FLT3/ITD or constitutively activated wild type FLT3 signalling. A limited set of genes was consistently affected by FLT3 inhibition. In confirmation of their FLT3 dependence, these genes returned toward pretreatment levels of expression after reversal of FLT3 inhibition. Several of the most significantly affected genes are involved in the RAS/mitogen-activated protein kinase, Janus kinase/signal transducer and activator of transcription and phosphatidylinositol 3 kinase (PI3K)/AKT pathways. These data suggest that constitutively activated FLT3 works through multiple signal transduction pathways. PIM1, MYC and CCND3 were chosen from this gene set to explore their biological roles. Knock-down of these genes by small interfering RNA showed that these genes play important roles in constitutively activated FLT3 expressing cells. The alterations of the gene expression profiles in these cells help to further elucidate the mechanisms of FLT3-mediated leukaemogenesis.

Inhibition of Flt3-activating mutations does not prevent constitutive activation of ERK/Akt/STAT pathways in some AML cells: a possible cause for the limited effectiveness of monotherapy with small-molecule inhibitors

The Flt3 receptor tyrosine kinase is a critical mediator in the pathogenesis of acute myeloid leukaemia (AML). Flt3-activating mutations have been associated with poor prognosis and decreased overall survival of AML patients, thus Flt3 constitutes an ideal target for drug treatment of such disease. Unfortunately, the monotherapy with small-molecule tyrosine kinase inhibitors in clinical trials shows that remission is not permanent, presumably by resistance of Flt3 mutants to inhibitors. An alternative approach for treatment is based on the cooperation between Flt3 and additional intracellular pathways for AML transformation in some patients. Thus, the inhibition of both Flt3 and such pathways may be exploited for successful treatment of the disease. We investigated the importance of Flt3-activating mutations for the constitutive activation of intracellular pathways in primary AML cells, and their effect on cell survival. We found that the main compounds involved in the differentiation, proliferation and survival of AML (MAPK/AKT/STAT) were constitutively activated. However, only four samples showed internal tandem duplications (ITDs) for Flt3. Surprisingly, contrary to previous reports, we found that inhibition of ITD/Flt3 activity did not prevent the phosphorylation of ERK, STAT5 or Akt in some primary AML cells. In parallel, we found that in these cells, Flt3 and ERK or Akt cooperate to regulate cell survival. Our results support the hypothesis that the optimal therapeutic treatment of AML may require not only the oncogenic tyrosine kinase, but also the appropriate combination of different specific inhibitors, thus providing a more effective approach to reverse leukaemogenesis. Thus, we propose that each AML patient should have an individually tailored combination treatment.

Emerging Flt3 kinase inhibitors in the treatment of leukaemia

Acute myeloid leukaemia (AML) is characterised by the infiltration of the bone marrow with highly proliferative leukaemic cells that stop to differentiate at different stages of myeloid development and carry survival advantages. Conventionally, AML is treated with aggressive cytotoxic therapy, in eligible patients followed by allogeneic bone marrow transplantation. However, despite this aggressive treatment, many patients relapse and eventually die from the disease. Activating mutations in the coding sequence of the receptor tyrosine kinase Flt3 are found in leukaemic blasts from approximately 30% of AML patients. The mutations have been described to severely alter the signalling properties of this receptor and to have transforming activity in cell-line models and in primary mouse bone marrow. The prognosis of patients harbouring the most common Flt3 mutations tends to be worse than that of comparable patients without the mutations. Thus, Flt3 seems a promising target for therapeutic intervention. Several small molecules that inhibit Flt3 kinase activity are being evaluated for the treatment of AML in clinical trials. This review article discusses the signal transduction and biological function of Flt3 and its mutations in normal and malignant haematopoiesis and recent progress in drug development aiming at the inhibition of Flt3 kinases.

Constitutively activated FLT3 phosphorylates BAD partially through pim-1

Constitutively activating internal tandem duplication (ITD) mutations of the receptor tyrosine kinase FLT3 (Fms-like tyrosine kinase 3) play an important role in leukaemogenesis and their presence is associated with a poor prognosis in acute myeloid leukaemia (AML). Examining the anti- and proapoptotic proteins in constitutively activated FLT3 signalling in BaF3/ITD and MV4-11 cells, we found that the level of Bcl-2 antagonist of cell death (BAD) phosphorylation was greatly decreased in response to FLT3 inhibition. Both Ser-112 and Ser-136 of BAD are rapidly dephosphorylated after treatment with the FLT3 inhibitor CEP-701 in BaF3/ITD and MV4-11 cells. In confirmation of the cell line data, BAD was highly phosphorylated in both constitutively activated wild-type and mutant FLT3 primary AML samples, and rapidly dephosphorylated after treatment of the primary samples with CEP-701. Upstream proteins known to phosphorylate BAD include Akt, extracellular signal-regulated kinase/mitogen-activated protein kinase (Erk/ MAPK), Pim-1 and Pim-2. We and other groups have shown that constitutively activated FLT3 induces multiple signalling pathways, including phosphatidylinositol 3-kinase (PI3K)/Akt, Erk/MAPK and Janus kinase/signal transducers and activators of transcription (Jak/STAT). Thus, BAD may be a nexus point upon which these multiple signalling pathways converge in FLT3-mediated cell survival. In support of this, siRNA knockdown of BAD expression in MV4-11 cells conferred resistance to CEP-701-mediated apoptosis. Our data suggests that Pim-1 is one of the principal kinases mediating the anti-apoptotic function of FLT3/ITD signalling via the phosphorylation of BAD.

Establishment of a Stroma-Dependent Human Acute Myelomonocytic Leukemia Cell Line, NAMO-2, with FLT3 Tandem Duplication

We have established a stroma-dependent myelomonocytic cell line, NAMO-2, with FLT3 internal tandem duplication (FLT3/ITD). Leukemia cells from a patient with acute myelomonocytic leukemia were administered to form subcutaneous tumors in nude mice, which were maintained successively, although we failed to establish continuously growing cells from the original leukemia cell culture. In the cultures of cells from subcutaneous tumors, there were stroma cells that had originated from the nude mice and showed continuous growth. The leukemia cells showed continuous growth dependent on this stroma, and this cell line was named NAMO-2. Detection of FLT3/ITD by the reverse transcriptase polymerase chain reaction (PCR) and genomic PCR showed that NAMO-2 was homozygous for FLT3/ITD. Constitutive activation of FLT3 was detected by Western blotting, and the phosphorylation of Akt, MEK, and STAT5 was also observed. FLT3 kinase inhibitor AG1296 specifically inhibited cell growth. NAMO-2 provides a useful tool to analyze adherence-dependent survival signaling of leukemia with FLT3/ITD and a model for the screening of FLT3 kinase inhibitors.

Prolonged exposure to FLT3 inhibitors leads to resistance via activation of parallel signaling pathways

Continuous treatment of malignancies with tyrosine kinase inhibitors (TKIs) may select for resistant clones (ie, imatinib mesylate). To study resistance to TKIs targeting FLT3, a receptor tyrosine kinase that is frequently mutated in acute myelogenous leukemia (AML), we developed resistant human cell lines through prolonged coculture with FLT3 TKIs. FLT3 TKI-resistant cell lines and primary samples still exhibit inhibition of FLT3 phosphorylation on FLT3 TKI treatment. However, FLT3 TKI-resistant cell lines and primary samples often show continued activation of downstream PI3K/Akt and/or Ras/MEK/MAPK signaling pathways as well as continued expression of genes involved in FLT3-mediated cellular transformation. Inhibition of these signaling pathways restores partial sensitivity to FLT3 TKIs. Mutational screening of FLT3 TKI-resistant cell lines revealed activating N-Ras mutations in 2 cell lines that were not present in the parental FLT3 TKI-sensitive cell line. Taken together, these data indicate that FLT3 TKI-resistant cells most frequently become FLT3 independent because of activation of parallel signaling pathways that provide compensatory survival/proliferation signals when FLT3 is inhibited. Anti-FLT3 mAb treatment was still cytotoxic to FLT3 TKI-resistant clones. An approach combining FLT3 TKIs with anti-FLT3 antibodies and/or inhibitors of important pathways downstream of FLT3 may reduce the chances of developing resistance.

Clinical implications of FLT3 mutations in pediatric AML

Activating mutations of the FLT3 gene occur because of an internal tandem duplication of the juxta-membrane domain (FLT3/ITD) or point mutation of the activation loop domain (FLT3/ALM). The presence of FLT3 mutations as well as the allelic ratio of FLT3/ITD (ITD-AR, mutant-wild type ratio) may have prognostic significance. FLT3 mutation status of 630 children with de novo acute myeloid leukemia (AML) treated on CCG-2941 and -2961 was determined, and ITD-AR was calculated for patients with FLT3/ITD. Clinical characteristics and outcomes for patients with FLT3/ALM and FLT3/ITD at varying ITD-ARs was determined and compared with those without FLT3 mutations (FLT3/WT). FLT3/ITD and FLT3/ALM were detected in 77 (12%) and 42 (6.7%) of the patients. Progression-free survival (PFS) was similar in patients with FLT3/ALM and FLT3/WT (51% versus 55%, P = .862). In contrast, PFS at 4 years from study entry for patients with FLT3/ITD was inferior to that of patients with FLT3/WT (31% versus 55%, P < .001). PFS decreased with increasing FLT3/ITD-AR (P < .001), and those with ITD-AR greater than 0.4 had a significantly worse PFS than those with lower ITD-AR (16% versus 72%, P = .001) or with FLT3/WT (55%, P < .001). ITD-AR defines the prognostic significance in FLT3/ITD-positive AML, and ITD-AR greater than 0.4 is a significant and independent prognostic factor for relapse in pediatric AML.

IMC-EB10, an anti-FLT3 monoclonal antibody, prolongs survival and reduces nonobese diabetic/severe combined immunodeficient engraftment of some acute lymphoblastic leukemia cell lines and primary leukemic samples

The class III receptor tyrosine kinase FLT3 is expressed on the blasts of >90% of patients with B-lineage acute lymphoblastic leukemias (ALL). In addition, it is expressed at extremely high levels in ALL patients with mixed lineage leukemia rearrangements or hyperdiploidy and is sometimes mutated in these same patients. In this report, we investigate the effects of treating ALL cell lines and primary samples with human anti-FLT3 monoclonal antibodies (mAb) capable of preventing binding of FLT3 ligand. In vitro studies, examining the ability of two anti-FLT3 mAbs (IMC-EB10 and IMC-NC7) to affect FLT3 activation and downstream signaling in ALL cell lines and primary blasts, yielded variable results. FLT3 phosphorylation was consistently inhibited by IMC-NC7 treatment, but in some cell lines, IMC-EB10 actually stimulated FLT3 activation, possibly as a result of antibody-mediated receptor dimerization. Through antibody-dependent, cell-mediated cytotoxicity, such an antibody could still prove efficacious against leukemia cells in vivo. In fact, IMC-EB10 treatment significantly prolonged survival and/or reduced engraftment of several ALL cell lines and primary ALL samples in nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. This occurred even when IMC-EB10 treatment resulted in FLT3 activation in vitro. Moreover, fluorescence-activated cell sorting and PCR analysis of IMC-EB10-treated NOD/SCID mice surviving 150 days post-leukemic cell injection revealed that FLT3 immunotherapy reduced leukemic engraftment below the level of detection in these assays (<0.001%). Furthermore, in vivo IMC-EB10 treatment did not select for resistant cells, because cells surviving IMC-EB10 treatment remain sensitive to IMC-EB10 cytotoxicity upon retransplantation. In vivo studies involving either partial depletion or activation of natural killer (NK) cells show that most of the cytotoxic effect of IMC-EB10 is mediated through NK cells. Therefore, such an antibody, either naked or conjugated to radioactive isotopes or cytotoxic agents, may prove useful in the therapy of infant ALL as well as childhood and adult ALL patients whose blasts typically express FLT3.

Combinations of the FLT3 inhibitor CEP-701 and chemotherapy synergistically kill infant and childhood MLL-rearranged ALL cells in a sequence-dependent manner

Mixed lineage leukemia (MLL) rearrangements occur in 80% of infants and 5% of older children with acute lymphoblastic leukemia (ALL). These cases have a poor prognosis with current therapy. The FLT3 kinase is overexpressed and constitutively activated in MLL-rearranged ALL cells. The FLT3 inhibitor CEP-701 selectively kills these cells, but is unlikely to be curative if used as monotherapy. To identify potentially synergistic combination strategies, we studied CEP-701 and six standard chemotherapeutic agents in three sequences of exposure (S1: chemotherapy followed by CEP-701, S2: simultaneous exposure to both; and S3: CEP-701 followed by chemotherapy) using MLL-rearranged ALL cell lines and patient bone marrow samples. MTT cytotoxicity and annexin V binding apoptosis assays were used to assess antileukemic effects. Combination indices (CI) were calculated for each combination (CI<0.9 - synergistic; CI 0.9-1.1 - additive; CI>1.1 - antagonistic). A striking pattern of sequence-dependent synergy was observed: S1 was markedly synergistic (mean CI=0.59+/-0.10), S2 was additive (mean CI=0.99+/-0.09) and S3 was antagonistic (mean CI=1.23+/-0.10). The sequence dependence is attributable to the effect of CEP-701 on cell cycle kinetics, and is mediated specifically by FLT3 inhibition, as these effects are not seen in control cells without activated FLT3.

Block of C/EBP alpha function by phosphorylation in acute myeloid leukemia with FLT3 activating mutations

Mutations constitutively activating FLT3 kinase are detected in approximately 30% of acute myelogenous leukemia (AML) patients and affect downstream pathways such as extracellular signal-regulated kinase (ERK)1/2. We found that activation of FLT3 in human AML inhibits CCAAT/enhancer binding protein alpha (C/EBPalpha) function by ERK1/2-mediated phosphorylation, which may explain the differentiation block of leukemic blasts. In MV4;11 cells, pharmacological inhibition of either FLT3 or MEK1 leads to granulocytic differentiation. Differentiation of MV4;11 cells was also observed when C/EBPalpha mutated at serine 21 to alanine (S21A) was stably expressed. In contrast, there was no effect when serine 21 was mutated to aspartate (S21D), which mimics phosphorylation of C/EBPalpha. Thus, our results suggest that therapies targeting the MEK/ERK cascade or development of protein therapies based on transduction of constitutively active C/EBPalpha may prove effective in treatment of FLT3 mutant leukemias resistant to the FLT3 inhibitor therapies.

Targeting mutated tyrosine kinases in the therapy of myeloid leukaemias

Myeloid leukaemias are frequently associated with translocations and mutations of tyrosine kinase genes. The products of these oncogenes, including BCR-ABL, TEL-PDGFR, Flt3 and c-Kit, have elevated tyrosine kinase activity and transform haematopoietic cells, mainly by augmentation of proliferation and enhanced viability. Activated ABL kinases are associated with chronic myeloid leukaemia. Mutations in platelet-derived growth factor receptor beta are associated with chronic myelomonocytic leukaemia. Flt3 or c-Kit cooperate with other types of oncogenes to create fully transformed acute leukaemias. Elevated activity of these tyrosine kinases is crucial for transformation, thus making the kinase domain an ideal target for therapeutic intervention. Tyrosine kinase inhibitors for various kinases are currently being evaluated in clinical trials and are potentially useful therapeutic agents in myeloid leukaemias. Here, the authors review the signalling activities, mechanism of transformation and therapeutic targeting of several tyrosine kinase oncogenes important in myeloid leukaemias.

FMS-like tyrosine kinase 3 in normal hematopoiesis and acute myeloid leukemia

Ligand-mediated activation of the FMS-like tyrosine kinase 3 (FLT3) receptor is important for normal proliferation of primitive hematopoietic cells. However, activating mutations in FLT3 induce ligand-independent downstream signaling that promotes oncogenesis through pathways involved in proliferation, differentiation, and survival. FLT3 mutations are identified as the most frequent genetic abnormality in acute myeloid leukemia and are also observed in other leukemias. Multiple small-molecule inhibitors are under development to target aberrant FLT3 activity that confers a poor prognosis in patients.

FLT3 tyrosine kinase inhibitors

The receptor tyrosine kinase FLT3 is an important regulatory molecule in hematopoiesis and is expressed on the blasts in most cases of acute leukemia. Activating mutations of this receptor are present in roughly 30% of acute myeloid leukemia (AML) patients and are associated with a distinctly worse clinical outcome. Efforts to target this mutation and improve out-comes in this subgroup of AML patients have led to the investigation of several novel small-molecule FLT3 tyrosine kinase inhibitors. These compounds derive from a wide variety of chemical classes and differ significantly, both in their potency and in their selectivity. In this review, we discuss the results of preclinical, clinical, and correlative laboratory studies of FLT3 inhibitors in demonstrating how this field represents a truly translational enterprise with multiple ongoing interactions between the laboratory and the clinic.

The FLT3 Internal Tandem Duplication Mutation Prevents Apoptosis in Interleukin-3-Deprived BaF3 Cells Due to Protein Kinase A and Ribosomal S6 Kinase 1–Mediated BAD Phosphorylation at Serine 112

Internal tandem duplication (ITD) mutations in the FLT3 tyrosine kinase have been detected in approximately 20% of acute myeloid leukemia (AML) patients. Patients harboring FLT3/ITD mutations have a relatively poor prognosis. FLT3/ITD results in constitutive autophosphorylation of the receptor and factor-independent survival. Previous studies have shown that FLT3/ITD activates the signal transducers and activators of transcription 5 (STAT5), p42/p44 mitogen-activated protein kinase [MAPK; extracellular signal-regulated kinase (ERK) 1/2], and phosphatidylinositol 3-kinase/Akt pathways. We herein provide biochemical and biological evidence that ribosomal S6 kinase 1 (RSK1) and protein kinase A (PKA) are the two principal kinases that mediate the antiapoptotic function of FLT3/ITD via phosphorylation of BAD at Ser112. Inhibiting both MAPK kinase (MEK)/ERK and PKA pathways by a combination of U0126 (10 micromol/L) and H-89 (5 micromol/L) reduced most of BAD phosphorylation at Ser112 and induced apoptosis to a level comparable with that induced by FLT3 inhibitor AG1296 (5 micromol/L) in BaF3/FLT3/ITD cells. RNA interference of RSK1 or PKA catalytic subunit reduced BAD phosphorylation and induced apoptosis. The MEK inhibitor U0126 and/or the PKA inhibitor H-89 greatly enhanced the efficacy of the FLT3 inhibitor AG1296, suggesting that combining FLT3/ITD downstream pathway inhibition with FLT3 inhibitors may be a viable therapeutic strategy for AML caused by a FLT3/ITD mutation.

FLT3 inhibition in t(4;11)+ adult acute lymphoid leukaemia

The present study was designed to investigate, in t(4;11)+ adult lymphoid leukaemia (ALL) blast cells, the pathogenetic role of the FLT3 protein, its level of mRNA and protein expression, the degree of constitutive phosphorylation, the possible presence of mutations of the sequence, the capacity of signal transduction and the potential therapeutic role of specific inhibitors. We evaluated nine adult ALL patients carrying this translocation. The increased FLT3 mRNA levels, determined by oligonucleotide microarray analysis, was in agreement with the increased protein expression evaluated by Western blot. The protein was constitutively phosphorylated in all cases analysed. Polymerase chain reaction detected no internal tandem duplication or point mutations. The signal transduction apparatus, after stimulation with the specific ligand, was preserved. We then investigated the effect of specific FLT3 inhibition on signal transduction and survival. The PKC412 inhibitor specifically inhibited ligand-induced phosphorylation; the same inhibitor reduced the survival of leukaemic cells when compared with untreated cells. These data indicate that the FLT3 protein might play a role in this subgroup of ALL with a particularly poor prognosis. Specific inhibition of the kinase receptor must be hypothesised as an innovative therapeutic tool for t(4;11)+ ALL patients.