Identifying and characterizing a novel activating mutation of the FLT3 tyrosine kinase in AML

Mutations of RAS genes identified in acute myeloid leukemia affect glycerophospholipid metabolism pathway

Background

Acute myeloid leukemia (AML) is a malignant disease originating from myeloid hematopoietic stem cells. Recent studies have shown that certain gene mutations promote tumor cell survival and affect the prognosis of patients by affecting metabolic mechanisms in tumor cells. RAS gene mutations are prevalent in AML, and the RAS signaling pathway is closely related to many metabolic pathways. However, the effects of different RAS gene mutations on AML cell metabolism are unclear.

Objectives

The main purpose of this study was to explore the effect of RAS gene mutation on the metabolic pathway of tumor cells.

Methods

In this study, we first used a retrovirus carrying a mutant gene to prepare Ba/F3 cell lines with RAS gene mutations, and then compared full-transcriptome data of Ba/F3 cells before and after RAS gene mutation and found that differentially expressed genes after NRASQ61K and KRASG12V mutation.

Results

We found a total of 1899 differentially expressed genes after NRASQ61K and KRASG12V mutation. 1089 of these genes were involved in metabolic processes, of which 167 genes were enriched in metabolism-related pathways. In metabolism-related pathways, differential genes were associated with the lipid metabolism pathway. Moreover, by comparing groups, we found that the expression of the DGKzeta and PLA2G4A genes in the glycerophospholipid metabolism pathway was significantly upregulated.

Conclusion

In conclusion, our study revealed that RAS gene mutation is closely related to the glycerophospholipid metabolism pathway in Ba/F3 cells, which may contribute to new precision therapy strategies and the development and application of new therapeutic drugs for AML.

Mutation spectrum of FLT3 and significance of non-canonical FLT3 mutations in haematological malignancy

Fms-like tyrosine kinase 3 (FLT3) is frequently mutated in haematological malignancies. Although canonical FLT3 mutations including internal tandem duplications (ITDs) and tyrosine kinase domains (TKDs) have been extensively studied, little is known about the clinical significance of non-canonical FLT3 mutations. Here, we first profiled the spectrum of FLT3 mutations in 869 consecutively newly diagnosed acute myeloid leukaemia (AML), myelodysplastic syndrome and acute lymphoblastic leukaemia patients. Our results showed four types of non-canonical FLT3 mutations depending on the affected protein structure: namely non-canonical point mutations (NCPMs) (19.2%), deletion (0.7%), frameshift (0.8%) and ITD outside the juxtamembrane domain (JMD) and TKD1 regions (0.5%). Furthermore, we found that the survival of patients with high-frequency (>1%) FLT3-NCPM in AML was comparable to those with canonical TKD. In vitro studies using seven representative FLT3-deletion or frameshift mutant constructs showed that the deletion mutants of TKD1 and the FLT3-ITD mutant of TKD2 had significantly higher kinase activity than wild-type FLT3, whereas the deletion mutants of JMD had phosphorylation levels comparable with wild-type FLT3. All tested deletion mutations and ITD were sensitive to AC220 and sorafenib. Collectively, these data enrich our understanding of FLT3 non-canonical mutations in haematological malignancies. Our results may also facilitate prognostic stratification and targeted therapy of AML with FLT3 non-canonical mutations.

Exploring the Resistance Mechanisms of Distal D835V Mutation in FLT3 to Inhibitors

Objective

FMS-like tyrosine kinase 3 (FLT3) is an attractive therapeutic target in acute myeloid leukemia. Unfortunately, secondary FLT3 mutations that developed resistance to inhibitors have become a severe problem. Specifically, ASP-835 (D835F/H/V/Y) mutant within the activation loop of FLT3 is the most commonly encountered drug-resistant and observed secondary FLT3 mutations. In this study, we carried out a set of computational approaches to explore how this mutation influenced the conformation and dynamics of DFG motif in a manner altered inhibitors' susceptibility.

Methods

Molecular dynamics (MD) simulation, dynamic cross-correlation (DCC) analysis, surface area (SASA), binding free energy (MM-GBSA), and structural analysis were used to compare the severe and minor D835V mutation-induced impact to sorafenib and crenolanib, respectively.

Results

The A-loop of the FLT3 protein may experience conformational change in the presence of the resistant mutation, which were mainly positioned at PHE-830. The protein-inhibitor interactions displayed that the motions of PHE-830 influenced that of sorafenib, but not to crenolanib.

Conclusions

These findings indicated that the structural impact brought by D835V mutation should be considered in designing novel drugs to overcome resistance to FLT3-D835V.

Genomic Abnormalities as Biomarkers and Therapeutic Targets in Acute Myeloid Leukemia

Simple Summary

AML is a heterogenous malignancy with a variety of underlying genomic abnormalities. Some of the genetic aberrations in AML have led to the development of specific inhibitors which were approved by the Food and Drug Administration (FDA) and are currently used to treat eligible patients. In this review, we describe five gene mutations for which approved inhibitors have been developed, the response of AML patients to these inhibitors, and the known mechanism(s) of resistance. This review also highlights the significance of developing function-based screens for target discovery in the era of personalized medicine.

Abstract

Acute myeloid leukemia (AML) is a highly heterogeneous malignancy characterized by the clonal expansion of myeloid stem and progenitor cells in the bone marrow, peripheral blood, and other tissues. AML results from the acquisition of gene mutations or chromosomal abnormalities that induce proliferation or block differentiation of hematopoietic progenitors. A combination of cytogenetic profiling and gene mutation analyses are essential for the proper diagnosis, classification, prognosis, and treatment of AML. In the present review, we provide a summary of genomic abnormalities in AML that have emerged as both markers of disease and therapeutic targets. We discuss the abnormalities of RARA, FLT3, BCL2, IDH1, and IDH2, their significance as therapeutic targets in AML, and how various mechanisms cause resistance to the currently FDA-approved inhibitors. We also discuss the limitations of current genomic approaches for producing a comprehensive picture of the activated signaling pathways at diagnosis or at relapse in AML patients, and how innovative technologies combining genomic and functional methods will improve the discovery of novel therapeutic targets in AML. The ultimate goal is to optimize a personalized medicine approach for AML patients and possibly those with other types of cancers.

Gilteritinib is a clinically active FLT3 inhibitor with broad activity against FLT3 kinase domain mutations

Gilteritinib is the first FMS-like tyrosine kinase 3 (FLT3) tyrosine kinase inhibitor (TKI) approved as monotherapy in acute myeloid leukemia with FLT3 internal tandem duplication and D835/I836 tyrosine kinase domain (TKD) mutations. Sequencing studies in patients have uncovered less common, noncanonical (NC) mutations in FLT3 and have implicated secondary TKD mutations in FLT3 TKI resistance. We report that gilteritinib is active against FLT3 NC and TKI resistance-causing mutations in vitro. A mutagenesis screen identified FLT3 F691L, Y693C/N, and G697S as mutations that confer moderate resistance to gilteritinib in vitro. Analysis of patients treated with gilteritinib revealed that 2/9 patients with preexisting NC FLT3 mutations responded and that secondary TKD mutations are acquired in a minority (5/31) of patients treated with gilteritinib. Four of 5 patients developed F691L mutations (all treated at <200 mg). These studies suggest that gilteritinib has broad activity against FLT3 mutations and limited vulnerability to resistance-causing FLT3 TKD mutations, particularly when used at higher doses.

Potential targeting of FLT3 acute myeloid leukemia

Aberrant FLT3 receptor signaling is common in acute myeloid leukemia (AML) and has important implications for the biology and clinical management of the disease. Patients with FLT3-mutated AML frequently present with critical illness, are more likely to relapse after treatment, and have worse clinical outcomes than their FLT3 wild type counterparts. The clinical management of FLT3-mutated AML has been transformed by the development of FLT3 inhibitors, which are now in use in the frontline and relapsed/refractory settings. However, many questions regarding the optimal approach to the treatment of these patients remain. In this paper, we will review the rationale for targeting the FLT3 receptor in AML, the impact of FLT3 mutation on patient prognosis, the current standard of care approaches to FLT3-mutated AML management, and the diverse array of FLT3 inhibitors in use and under investigation. We will also explore new opportunities and strategies for targeting the FLT3 receptor. These include targeting the receptor in patients with non-canonical FLT3 mutations or wild type FLT3, pairing FLT3 inhibitors with other novel therapies, using minimal residual disease (MRD) testing to guide the targeting of FLT3, and novel immunotherapeutic approaches.

Diagnostic and therapeutic pitfalls in NPM1-mutated AML: notes from the field

Mutations of Nucleophosmin (NPM1) are the most common genetic abnormalities in adult acute myeloid leukaemia (AML), accounting for about 30% of cases. NPM1-mutated AML has been recognized as distinct entity in the 2017 World Health Organization (WHO) classification of lympho-haematopoietic neoplasms. WHO criteria allow recognition of this leukaemia entity and its distinction from AML with myelodysplasia-related changes, AML with BCR-ABL1 rearrangement and AML with RUNX1 mutations. Nevertheless, controversial issues include the percentage of blasts required for the diagnosis of NPM1-mutated AML and whether cases of NPM1-mutated myelodysplasia and chronic myelomonocytic leukaemia do exist. Evaluation of NPM1 and FLT3 status represents a major pillar of the European LeukemiaNet (ELN) genetic-based risk stratification model. Moreover, NPM1 mutations are particularly suitable for assessing measurable residual disease (MRD) since they are frequent, stable at relapse and do not drive clonal haematopoiesis. Ideally, combining monitoring of MRD with the ELN prognostication model can help to guide therapeutic decisions. Here, we provide examples of instructive cases of NPM1-mutated AML, in order to provide criteria for the appropriate diagnosis and therapy of this frequent leukaemia entity.

Acute Myeloid Leukemia Case Harboring Unusual FLT3 Variant: Somatic vs Germline?

FLT3 mutations are considered a prognostic and predictive marker. Here we report on a patient with a rare FLT3 germline variant in the context of relapsed acute myeloid leukemia (AML). A female patient aged 57 years presented with AML with mutations in the IDH2, ASXL1, and DNMT3A genes. She underwent allogenic hematopoietic stem cell transplant but relapsed 2 years posttransplant. Targeted next generation sequencing identified a new missense variant in the FLT3 tyrosine kinase domain c.2440G > T (p.A814S). The treating team considered the possibility of patient eligibility for an FLT3 inhibitor. Because both somatic and germline mutations can be identified in tumor tissue with high-throughput sequencing, it becomes important to distinguish the origin of these alterations when possible-especially, in this challenging case, to define the treatment modality. Simultaneous tumor/germline sequencing allows for the identification of rare germline mutations and may help in determining their significance in the pathogenesis of disease.

Pharmacological impact of FLT3 mutations on receptor activity and responsiveness to tyrosine kinase inhibitors

Acute myelogenous leukaemia (AML) is an aggressive blood cancer characterized by the rapid proliferation of immature myeloid blast cells, resulting in a high mortality rate. The 5-year overall survival rate for AML patients is approximately 25%. Circa 35% of all patients carry a mutation in the FLT3 gene which have a poor prognosis. Targeting FLT3 receptor tyrosine kinase has become a treatment strategy in AML patients possessing FLT3 mutations. The most common mutations are internal tandem duplications (ITD) within exon 14 and a single nucleotide polymorphism (SNP) that leads to a point mutation in the D835 of the tyrosine kinase domain (TKD). Variations in the ITD sequence and the occurrence of other point mutations that lead to ligand-independent FLT3 receptor activation create difficulties in developing personalized therapeutic strategies to overcome observed mutation-driven drug resistance. Midostaurin and quizartinib are tyrosine kinase inhibitors (TKIs) with inhibitory efficacy against FLT3-ITD, but exhibit limited clinical impact. In this review, we focus on the structural aspects of the FLT3 receptor and correlate those mutations with receptor activation and the consequences for molecular and clinical responsiveness towards therapies targeting FLT3-ITD positive AML.

Novel Approaches to Target Mutant FLT3 Leukaemia

Simple Summary

Acute myeloid leukemia (AML) is a haematologic disease in which oncogenic mutations in the receptor tyrosine kinase FLT3 frequently lead to leukaemic development. Potent treatment of AML patients is still hampered by inefficient targeting of leukemic stem cells expressing constitutive active FLT3 mutants. This review summarizes the current knowledge about the regulation of FLT3 activity at cellular level and discusses therapeutical options to affect the tumor cells and the microenvironment to impair the haematological aberrations.

Abstract

Fms-like tyrosine kinase 3 (FLT3) is a member of the class III receptor tyrosine kinases (RTK) and is involved in cell survival, proliferation, and differentiation of haematopoietic progenitors of lymphoid and myeloid lineages. Oncogenic mutations in the FLT3 gene resulting in constitutively active FLT3 variants are frequently found in acute myeloid leukaemia (AML) patients and correlate with patient’s poor survival. Targeting FLT3 mutant leukaemic stem cells (LSC) is a key to efficient treatment of patients with relapsed/refractory AML. It is therefore essential to understand how LSC escape current therapies in order to develop novel therapeutic strategies. Here, we summarize the current knowledge on mechanisms of FLT3 activity regulation and its cellular consequences. Furthermore, we discuss how aberrant FLT3 signalling cooperates with other oncogenic lesions and the microenvironment to drive haematopoietic malignancies and how this can be harnessed for therapeutical purposes.

Mutations within the Activation Loop Domain of FLT3 in Two Pediatric Patients with Refractory Infant Acute Myeloid Leukemia

Approximately 24% of all pediatric acute myeloid leukemia (AML) cases have mutations in the FMS-like tyrosine kinase 3 (<i>FLT3</i>) receptor gene. FLT3-TKD point mutations are rare in pediatrics and often occur in younger patients and in combination with 11q23 abnormalities. There is a paucity of data related to their prognostic implications in children. We describe 2 pediatric patients with FLT3-activating mutations as a feature of their AML. Both were diagnosed in infancy. The first experienced induction failure and had refractory disease without expression of FLT3-TKD mutation on subsequent bone marrow evaluations. His disease also harbored a KMT2A-PICALM gene rearrangement. He died of invasive fungal disease nine months after diagnosis. The second had a post-induction remission but developed swelling of the left calcaneus shown on biopsy to be a myeloid sarcoma positive for a new BRAF V600E mutation in addition to his known KMT2A rearrangement but without FLT3-TKD mutation. Despite multiple courses of therapy including BRAF/MEK-inhibition, he died of progressive disease nine months after diagnosis. FLT3 inhibition was not utilized in either patient as studies have largely focused on its role in internal tandem duplication (ITD) mutations and because the mutation was no longer detectable in either patient on subsequent evaluation. However, these cases add to the suggestion that these mutations confer a worse prognosis in pediatric AML patients.

FMS-like Tyrosine Kinase 3/FLT3: From Basic Science to Clinical Implications

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase that is expressed almost exclusively in the hematopoietic compartment. Its ligand, FLT3 ligand (FL), induces dimerization and activation of its intrinsic tyrosine kinase activity. Activation of FLT3 leads to its autophosphorylation and initiation of several signal transduction cascades. Signaling is initiated by the recruitment of signal transduction molecules to activated FLT3 through binding to specific phosphorylated tyrosine residues in the intracellular region of FLT3. Activation of FLT3 mediates cell survival, cell proliferation, and differentiation of hematopoietic progenitor cells. It acts in synergy with several other cytokines to promote its biological effects. Deregulated FLT3 activity has been implicated in several diseases, most prominently in acute myeloid leukemia where around one-third of patients carry an activating mutant of FLT3 which drives the disease and is correlated with poor prognosis. Overactivity of FLT3 has also been implicated in autoimmune diseases, such as rheumatoid arthritis. The observation that gain-of-function mutations of FLT3 can promote leukemogenesis has stimulated the development of inhibitors that target this receptor. Many of these are in clinical trials, and some have been approved for clinical use. However, problems with acquired resistance to these inhibitors are common and, furthermore, only a fraction of patients respond to these selective treatments. This review provides a summary of our current knowledge regarding structural and functional aspects of FLT3 signaling, both under normal and pathological conditions, and discusses challenges for the future regarding the use of targeted inhibition of these pathways for the treatment of patients.

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.

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.

Spotlight on midostaurin in the treatment of FLT3-mutated acute myeloid leukemia and systemic mastocytosis: design, development, and potential place in therapy

The Fms-like tyrosine kinase-3 (FLT3; fetal liver kinase-2; human stem cell tyrosine kinase-1; CD135) is a class III receptor tyrosine kinase that is normally involved in regulating the proliferation, differentiation, and survival of both hematopoietic cells and dendritic cells. Mutations leading it to be constitutively activated make it an oncogenic driver in ~30% of acute myeloid leukemia (AML) patients where it is associated with poor prognosis. The prevalence of oncogenic FLT3 and the dependency on its constitutively activated kinase activity for leukemia growth make this protein an attractive target for therapeutic intervention. Of the numerous small molecule inhibitors under clinical investigation for the treatment of oncogenic FLT3-positive AML, the N-benzoyl-staurosporine, midostaurin (CGP41251; PKC412; Rydapt^®; Novartis Pharma AG, Basel, Switzerland), is the first to be approved by the US Food and Drug Administration for the treatment, in combination with standard chemotherapy, of newly diagnosed adult AML patients who harbor mutations in FLT3. Here, we describe the early design of midostaurin, the preclinical discovery of its activity against oncogenic FLT3, and its subsequent clinical development as a therapeutic agent for FLT3 mutant-positive AML.

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.

Discovery of a Highly Potent and Selective Indenoindolone Type 1 Pan-FLT3 Inhibitor

For a subpopulation of acute myeloid leukemia (AML) patients, the mutationally activated tyrosine kinase FLT3, has emerged as a promising target for therapy. The development of drug resistance due to mutation is a growing concern for mutant FLT3 inhibitors, such as PKC412, Quizartinib, PLX3397, and Crenolanib. Thus, there is a need to develop novel FLT3 inhibitors that overcome these mutations. Here we report the development of a novel type I ATP competitive inhibitor, JH-IX-179, that is extremely potent and selective for FLT3. JH-IX-179 also has the highest affinity for three constitutively active isoforms of FLT3 (FLT3-ITD, FLT3-N841I, and FLT3-D835V) compared to a panel 456 other kinases. The unique and specific kinase inhibition profile suggests that this chemotype may represent an attractive starting point for the development of further improved FLT3 inhibitors with therapeutic potential in tumors harboring deregulated FLT3 activity.

Class III Receptor Tyrosine Kinases in Acute Leukemia - Biological Functions and Modern Laboratory Analysis

Acute myeloid leukemia (AML) is a complex disease caused by deregulation of multiple signaling pathways. Mutations in class III receptor tyrosine kinases (RTKs) have been implicated in alteration of cell signals concerning the growth and differentiation of leukemic cells. Point mutations, insertions, or deletions of RTKs as well as chromosomal translocations induce constitutive activation of the receptor, leading to uncontrolled proliferation of undifferentiated myeloid blasts. Aberrations can occur in all domains of RTKs causing either the ligand-independent activation or mimicking the activated conformation. The World Health Organization recommended including RTK mutations in the AML classification since their detection in routine laboratory diagnostics is a major factor for prognostic stratification of patients. Polymerase chain reaction (PCR)-based methods are well-validated for the detection of fms-related tyrosine kinase 3 (FLT3) mutations and can easily be applied for other RTKs. However, when methodological limitations are reached, accessory techniques can be applied. For a higher resolution and more quantitative approach compared to agarose gel electrophoresis, PCR fragments can be separated by capillary electrophoresis. Furthermore, high-resolution melting and denaturing high-pressure liquid chromatography are reliable presequencing screening methods that reduce the sample amount for Sanger sequencing. Because traditional DNA sequencing is time-consuming, next-generation sequencing (NGS) is an innovative modern possibility to analyze a high amount of samples simultaneously in a short period of time. At present, standardized procedures for NGS are not established, but when this barrier is resolved, it will provide a new platform for rapid and reliable laboratory diagnostic of RTK mutations in patients with AML. In this article, the biological and physiological role of RTK mutations in AML as well as possible laboratory methods for their detection will be reviewed.

Activating FLT3 mutants show distinct gain-of-function phenotypes in vitro and a characteristic signaling pathway profile associated with prognosis in acute myeloid leukemia

About 30% of patients with acute myeloid leukemia (AML) harbour mutations of the receptor tyrosine kinase FLT3, mostly internal tandem duplications (ITD) and point mutations of the second tyrosine kinase domain (TKD). It was the aim of this study to comprehensively analyze clinical and functional properties of various FLT3 mutants.

In 672 normal karyotype AML patients FLT3-ITD, but not FLT3-TKD mutations were associated with a worse relapse free and overall survival in multivariate analysis. In paired diagnosis-relapse samples FLT3-ITD showed higher stability (70%) compared to FLT3-TKD (30%). In vitro, FLT3-ITD induced a strong activating phenotype in Ba/F3 cells. In contrast, FLT3-TKD mutations and other point mutations – including two novel mutations – showed a weaker but clear gain-of-function phenotype with gradual increase in proliferation and protection from apoptosis. The pro-proliferative capacity of the investigated FLT3 mutants was associated with cell surface expression and tyrosine 591 phosphorylation of the FLT3 receptor. Western blot experiments revealed STAT5 activation only in FLT3-ITD positive cell lines, in contrast to FLT3-non-ITD mutants, which displayed an enhanced signal of AKT and MAPK activation. Gene expression analysis revealed distinct difference between FLT3-ITD and FLT3-TKD for STAT5 target gene expression as well as deregulation of SOCS2, ENPP2, PRUNE2 and ART3.

FLT3-ITD and FLT3 point mutations show a gain-of-function phenotype with distinct signalling properties in vitro. Although poor prognosis in AML is only associated with FLT3-ITD, all activating FLT3 mutations can contribute to leukemogenesis and are thus potential targets for therapeutic interventions.

Mutations of FLT3/ITD confer resistance to multiple tyrosine kinase inhibitors

FMS-like tyrosine kinase 3 (FLT3) normally functions in the survival/proliferation of hematopoietic stem/progenitor cells, but its constitutive activation by internal tandem duplication (ITD) mutations correlates with a poor prognosis in AML. The development of FLT3 tyrosine kinase inhibitors (TKI) is a promising strategy, but resistance that arises during the course of treatment caused by secondary mutations within the mutated gene itself poses a significant challenge. In an effort to predict FLT3 resistance mutations that might develop in patients, we used saturation mutagenesis of FLT3/ITD followed by selection of transfected cells in FLT3 TKI. We identified F621L, A627P, F691L and Y842C mutations in FLT3/ITD that confer varying levels of resistance to FLT3 TKI. Western blotting confirmed that some FLT3 TKI were ineffective at inhibiting FLT3 autophosphorylation and signaling through MAP kinase, STAT5 and AKT in some mutants. Balb/c mice transplanted with the FLT3/ITD Y842C mutation confirmed resistance to sorafenib in vivo but not to lestaurtinib. These results indicate a growing number of FLT3 mutations that are likely to be encountered in patients. Such knowledge, combined with known remaining sensitivity to other FLT3 TKI, will be important to establish as secondary drug treatments that can be substituted when these mutants are encountered.

FLT3 and NPM1 mutations in a cohort of AML patients and detection of a novel mutation in tyrosine kinase domain of FLT3 gene from Western India

Acute myeloid leukemia (AML) is an aggressive hematological disorder characterized by the loss of ability of the hematopoietic progenitor cells to differentiate and proliferate normally leading to an accumulation of immature myeloid cells in the bone marrow. Several novel molecular genetic aberrations in FLT3 and NPM1 have been shown to have a prognostic impact in AML, particularly in those having normal karyotype. Though there is substantial amount of data on these mutations from western literature, there is surprisingly little data from Indian subcontinent on the frequency of this mutation in AML patients from India. The present study screens a large cohort of non-acute promyelocytic leukemia (APL) AML patients (207 patients) for the presence of FLT3 and NPM1 mutations and further correlates with cytogenetics, immunophenotypic characteristics and with follow-up data wherever available. During the course of study, 56 APL patients were also studied. Briefly, both FLT3 (internal tandem duplication (ITD) in 19.4% and tyrosine kinase domain (TKD) in 9%) and NPM1 mutations were detected in 28.4% of the total non-APL AML patients screened showing distinct correlations with hematologic, immunophenotypic, cytogenetics characteristics and follow-up. With regards to adult APL patients, 22.2 and 32.6% of the patients showed FLT3 and NPM1 mutation, respectively. In the pediatrics age group (<15 years), 23 and 16% of patients with APL showed FLT3 and NPM1 mutation, respectively, while in non-APL patient is this age group, 23% of patients showed both FLT3 and NPM1 mutation. NPM1 mutation was distinctly uncommon in younger age group of patients. In contrast to report elsewhere, most of our FLT3 mutation was in exon 11 rather than in exon 12. FLT3 mutation due to ITD or TKD mutation was detected in 2:1 ratio in our patients and a new TKD mutation was also detected S840G in an M5 patient who did not go into remission and had a short survival of 3 months from diagnosis. Generally, patients with NPM1 mutation had a very high white cell count but they went into remission more often than those with wild (Wt)-type allele (written as NPM1- and FLT3-, respectively) and FLT3 mutation. These patients also tended to have significantly lower expression of CD34 antigen on flowcytometry. Distinct prognostic subclasses of adult AML patients were identified based on the presence of NPM1 and FLT3 mutations.

An overview on the role of FLT3-tyrosine kinase receptor in acute myeloid leukemia: biology and treatment

Hematopoiesis, the process by which the hematopoietic stem cells and progenitors differentiate into blood cells of various lineages, involves complex interactions of transcription factors that modulate the expression of downstream genes and mediate proliferation and differentiation signals. Despite the many controls that regulate hematopoiesis, mutations in the regulatory genes capable of promoting leukemogenesis may occur. The FLT3 gene encodes a tyrosine kinase receptor that plays a key role in controlling survival, proliferation and differentiation of hematopoietic cells. Mutations in this gene are critical in causing a deregulation of the delicate balance between cell proliferation and differentiation. In this review, we provide an update on the structure, synthesis and activation of the FLT3 receptor and the subsequent activation of multiple downstream signaling pathways. We also review activating FLT3 mutations that are frequently identified in acute myeloid leukemia, cause activation of more complex downstream signaling pathways and promote leukemogenesis. Finally, FLT3 has emerged as an important target for molecular therapy. We, therefore, report on some recent therapies directed against it.

Combination of Sleeping Beauty transposition and chemically induced dimerization selection for robust production of engineered cells

The main methods for producing genetically engineered cells use viral vectors for which safety issues and manufacturing costs remain a concern. In addition, selection of desired cells typically relies on the use of cytotoxic drugs with long culture times. Here, we introduce an efficient non-viral approach combining the Sleeping Beauty (SB) Transposon System with selective proliferation of engineered cells by chemically induced dimerization (CID) of growth factor receptors. Minicircles carrying a SB transposon cassette containing a reporter transgene and a gene for the F36VFGFR1 fusion protein were delivered to the hematopoietic cell line Ba/F3. Stably-transduced Ba/F3 cell populations with >98% purity were obtained within 1 week using this positive selection strategy. Copy number analysis by quantitative PCR (qPCR) revealed that CID-selected cells contain on average higher copy numbers of transgenes than flow cytometry-selected cells, demonstrating selective advantage for cells with multiple transposon insertions. A diverse population of cells is present both before and after culture in CID media, although site-specific qPCR of transposon junctions show that population diversity is significantly reduced after selection due to preferential expansion of clones with multiple integration events. This non-viral, positive selection approach is an attractive alternative for producing engineered cells.

Sorafenib treatment of FLT3-ITD(+) acute myeloid leukemia: favorable initial outcome and mechanisms of subsequent nonresponsiveness associated with the emergence of a D835 mutation

Internal tandem duplication (ITD) of the fms-related tyrosine kinase-3 (FLT3) gene occurs in 30% of acute myeloid leukemias (AMLs) and confers a poor prognosis. Thirteen relapsed or chemo-refractory FLT3-ITD(+) AML patients were treated with sorafenib (200-400 mg twice daily). Twelve patients showed clearance or near clearance of bone marrow myeloblasts after 27 (range 21-84) days with evidence of differentiation of leukemia cells. The sorafenib response was lost in most patients after 72 (range 54-287) days but the FLT3 and downstream effectors remained suppressed. Gene expression profiling showed that leukemia cells that have become sorafenib resistant expressed several genes including ALDH1A1, JAK3, and MMP15, whose functions were unknown in AML. Nonobese diabetic/severe combined immunodeficiency mice transplanted with leukemia cells from patients before and during sorafenib resistance recapitulated the clinical results. Both ITD and tyrosine kinase domain mutations at D835 were identified in leukemia initiating cells (LICs) from samples before sorafenib treatment. LICs bearing the D835 mutant have expanded during sorafenib treatment and dominated during the subsequent clinical resistance. These results suggest that sorafenib have selected more aggressive sorafenib-resistant subclones carrying both FLT3-ITD and D835 mutations, and might provide important leads to further improvement of treatment outcome with FLT3 inhibitors.

Antileukemic Effects of Novel First- and Second-Generation FLT3 Inhibitors: Structure-Affinity Comparison

Constitutively activated mutant FLT3 has emerged as a promising target for therapy for the subpopulation of acute myeloid leukemia (AML) patients who harbor it. The small molecule inhibitor, PKC412, targets mutant FLT3 and is currently in late-stage clinical trials. However, the identification of PKC412-resistant leukemic blast cells in the bone marrow of AML patients has propelled the development of novel and structurally distinct FLT3 inhibitors that have the potential to override drug resistance and more efficiently prevent disease progression or recurrence. Here, we present the novel first-generation "type II" FLT3 inhibitors, AFG206, AFG210, and AHL196, and the second-generation "type II" derivatives and AST487 analogs, AUZ454 and ATH686. All agents potently and selectively target mutant FLT3 protein kinase activity and inhibit the proliferation of cells harboring FLT3 mutants via induction of apoptosis and cell cycle inhibition. Cross-resistance between "type I" inhibitors, PKC412 and AAE871, was demonstrated. While cross-resistance was also observed between "type I" and first-generation "type II" FLT3 inhibitors, the high potency of the second-generation "type II" inhibitors was sufficient to potently kill "type I" inhibitor-resistant mutant FLT3-expressing cells. The increased potency observed for the second-generation "type II" inhibitors was observed to be due to an improved interaction with the ATP pocket of FLT3, specifically associated with introduction of a piperazine moiety and placement of an amino group in position 2 of the pyrimidine ring. Thus, we present 2 structurally novel classes of FLT3 inhibitors characterized by high selectivity and potency toward mutant FLT3 as a molecular target. In addition, presentation of the antileukemic effects of "type II" inhibitors, such as AUZ454 and ATH686, highlights a new class of highly potent FLT3 inhibitors able to override drug resistance that less potent "type I" inhibitors and "type II" first-generation FLT3 inhibitors cannot.

Wnt/β-catenin Pathway Modulates the Sensitivity of the Mutant FLT3 Receptor Kinase Inhibitors in a GSK-3β Dependent Manner

The FLT3 tyrosine kinase receptor is involved in both hematopoiesis and hematological malignancies. The Wnt/β-catenin pathway has been shown to participate in the self-renewal of hematopoietic stem cells and to cooperate with the mutant FLT3 receptors in leukemic transformation. However, the detailed biological impact of such a constitutively activated Wnt pathway remains to be further explored. Here, the authors report that activating mutations of FLT3 constitutively activate β-catenin by inhibition of GSK-3β in a PI3 kinase pathway-dependent manner. Ectopic expression of a dominant negative form of GSK-3β in FLT3-ITD-expressing cells activated β-catenin and blocked the downregulation of the TCF/β-catenin transcriptional activity induced by inhibition of FLT3 kinase. Furthermore, inhibition of cell proliferation and colony formation induced by such suppression of FLT3 kinase activity could be partially reversed by knockdown of GSK-3β and restored by knockdown of either TCF4 or β-catenin. Moreover, exogenous activation of the Wnt pathway also attenuated such inhibitory effect. These findings indicate that the potencies of the inhibitors of FLT3 kinase activity could be modulated by the activity of the Wnt/β-catenin pathway in the cells harboring FLT3-ITD mutations, and FLT3-ITDs signal through GSK-3β to activate β-catenin that this is likely to directly contribute to the leukemic phenotype.

Reversible resistance induced by FLT3 inhibition: a novel resistance mechanism in mutant FLT3-expressing cells

Objectives

Clinical responses achieved with FLT3 kinase inhibitors in acute myeloid leukemia (AML) are typically transient and partial. Thus, there is a need for identification of molecular mechanisms of clinical resistance to these drugs. In response, we characterized MOLM13 AML cell lines made resistant to two structurally-independent FLT3 inhibitors.

Methods

MOLM13 cells were made drug resistant via prolonged exposure to midostaurin and HG-7-85-01, respectively. Cell proliferation was determined by Trypan blue exclusion. Protein expression was assessed by immunoblotting, immunoprecipitation, and flow cytometry. Cycloheximide was used to determine protein half-life. RT-PCR was performed to determine FLT3 mRNA levels, and FISH analysis was performed to determine FLT3 gene expression.

Results and Conclusions

We found that MOLM13 cells readily developed cross-resistance when exposed to either midostaurin or HG-7-85-01. Resistance in both lines was associated with dramatically elevated levels of cell surface FLT3 and elevated levels of phosphor-MAPK, but not phospho-STAT5. The increase in FLT3-ITD expression was at least in part due to reduced turnover of the receptor, with prolonged half-life. Importantly, the drug-resistant phenotype could be rapidly reversed upon withdrawal of either inhibitor. Consistent with this phenotype, no significant evidence of FLT3 gene amplification, kinase domain mutations, or elevated levels of mRNA was observed, suggesting that protein turnover may be part of an auto-regulatory pathway initiated by FLT3 kinase activity. Interestingly, FLT3 inhibitor resistance also correlated with resistance to cytosine arabinoside. Over-expression of FLT3 protein in response to kinase inhibitors may be part of a novel mechanism that could contribute to clinical resistance.

An integrated genomic approach to the assessment and treatment of acute myeloid leukemia

Traditionally, new scientific advances have been applied quickly to the leukemias based on the ease with which relatively pure samples of malignant cells can be obtained. Currently, our arsenal of approaches used to characterize an individual's acute myeloid leukemia (AML) combines hematopathologic evaluation, flow cytometry, cytogenetic analysis, and molecular studies focused on a few key genes. The advent of high-throughput methods capable of full-genome evaluation presents new options for a revolutionary change in the way we diagnose, characterize, and treat AML. Next-generation DNA sequencing techniques allow full sequencing of a cancer genome or transcriptome, with the hope that this will be affordable for routine clinical care within the decade. Microarray-based testing will define gene and miRNA expression, DNA methylation patterns, chromosomal imbalances, and predisposition to disease and chemosensitivity. The vision for the future entails an integrated and automated approach to these analyses, bringing the possibility of formulating an individualized treatment plan within days of a patient's initial presentation. With these expectations comes the hope that such an approach will lead to decreased toxicities and prolonged survival for patients.

Emerging FMS-like tyrosine kinase 3 inhibitors for the treatment of acute myelogenous leukemia

INTRODUCTION

The FMS-like tyrosine kinase 3 (FLT3) is highly expressed in acute leukemias. Mutations involving FLT3 are among the most common molecular abnormalities in acute myelogenous leukemia (AML). Available evidence suggests that these molecular lesions confer a shorter disease-free survival and overall survival in patients with intermediate-risk cytogenetics. Therefore, substantial interest in FLT3 as a therapeutic target has led to the development of several promising inhibitors that target this tyrosine kinase.

AREAS COVERED

This review covers the molecular pathways associated with FLT3 activation in patients with AML, the biological rationale for inhibiting FLT3 and recent clinical progress with FLT3 inhibitors for the treatment of AML. Six FLT3 inhibitors undergoing clinical evaluation are discussed. A review of selected published manuscripts on the subject of FLT3 inhibition in AML and a search of the English language manuscripts in PubMed using the index words FLT3 and AML were conducted and articles of interest selected.

EXPERT OPINION

Mutated forms of FLT3, specifically FLT3-internal tandem duplication, have a significant impact on the prognosis of AML patients, particularly those with a normal karyotype. Inhibiting FLT3 may lead to clinical benefit for patients with AML. Newly developed FLT3 inhibitors have shown encouraging activity as monotherapy and in combination with other therapeutic agents.

Smac mimetics: implications for enhancement of targeted therapies in leukemia

Drug resistance is a growing concern with clinical use of tyrosine kinase inhibitors. Utilizing in vitro models of intrinsic drug resistance and stromal-mediated chemoresistance, as well as functional mouse models of progressive and residual disease, we attempted to develop a potential therapeutic approach designed to suppress leukemia recurrence following treatment with selective kinase inhibitors. The novel IAP inhibitor, LCL161, [corrected] was observed to potentiate the effects of tyrosine kinase inhibition against leukemic disease both in the absence and presence of a stromal-protected [corrected] environment. LCL161 enhanced the proapoptotic effects of nilotinib and PKC412, against leukemic disease in vitro and potentiated the activity of both kinase inhibitors against leukemic disease in vivo. In addition, LCL161 synergized in vivo with nilotinib to reduce leukemia burden significantly below the baseline level suppression exhibited by a moderate-to-high dose of nilotinib. Finally, LCL161 displayed antiproliferative effects against cells characterized by intrinsic resistance to tyrosine kinase inhibitors as a result of expression of point mutations in the protein targets of drug inhibition. These results support the idea of using IAP inhibitors in conjunction with targeted tyrosine kinase inhibition to override drug resistance and suppress or eradicate residual disease.

Discovery and characterization of novel mutant FLT3 kinase inhibitors

For a subpopulation of acute myeloid leukemia (AML) patients, the constitutively activated tyrosine kinase, mutant FLT3, has emerged as a promising target for therapy. The development of drug resistance, however, is a growing concern for mutant FLT3 inhibitors, such as PKC412. Potential therapeutic benefit can arise from the combination of two structurally diverse inhibitors that target-but bind differently to-the same protein or from two inhibitors with completely different mechanisms of action. Thus, there is a need for identification and development of novel FLT3 inhibitors that have the ability to positively combine with PKC412 or standard chemotherapeutic agents used to treat AML as a way to suppress the development of drug resistance and consequently prolong disease remission. Here, we report the effects of the novel type II ATP-competitive inhibitors, HG-7-85-01 and HG-7-86-01, which potently and selectively target mutant FLT3 protein kinase activity and inhibit the proliferation of cells harboring FLT3-ITD or FLT3 kinase domain point mutants via induction of apoptosis and cell cycle inhibition. Antileukemic activity of HG-7-85-01 was shown in vivo to be comparable with that observed with PKC412 in a bioluminescence assay using NCr nude mice harboring Ba/F3-FLT3-ITD-luc+ cells. HG-7-85-01 was also observed to override PKC412 resistance. Finally, HG-7-85-01 and HG-7-86-01 synergized with PKC412 and standard chemotherapeutic agents against mutant PKC412-sensitive and some PKC412-resistant, FLT3-positive cells. Thus, we present a structurally novel class of FLT3 inhibitors that warrants consideration for clinical testing against drug-resistant disease in AML patients.

Proteus in the world of proteins: conformational changes in protein kinases

The 512 protein kinases encoded by the human genome are a prime example of nature's ability to create diversity by introducing variations to a highly conserved theme. The activity of each kinase domain is controlled by layers of regulatory mechanisms involving different combinations of post-translational modifications, intramolecular contacts, and intermolecular interactions. Ultimately, they all achieve their effect by favoring particular conformations that promote or prevent the kinase domain from catalyzing protein phosphorylation. The central role of kinases in various diseases has encouraged extensive investigations of their biological function and three-dimensional structures, yielding a more detailed understanding of the mechanisms that regulate protein kinase activity by conformational changes. In the present review, we discuss these regulatory mechanisms and show how conformational changes can be exploited for the design of specific inhibitors that lock protein kinases in inactive conformations. In addition, we highlight recent developments to monitor ligand-induced structural changes in protein kinases and for screening and identifying inhibitors that stabilize enzymatically incompetent kinase conformations.

Phase I/II study of combination therapy with sorafenib, idarubicin, and cytarabine in younger patients with acute myeloid leukemia

PURPOSE To determine the efficacy and toxicity of the combination of sorafenib, cytarabine, and idarubicin in patients with acute myeloid leukemia (AML) younger than age 65 years. PATIENTS AND METHODS In the phase I part of the study, 10 patients with relapsed AML were treated with escalating doses of sorafenib with chemotherapy to establish the feasibility of the combination. We then treated 51 patients (median age, 53 years; range, 18 to 65 years) who had previously untreated AML with cytarabine at 1.5 g/m(2) by continuous intravenous (IV) infusion daily for 4 days (3 days if > 60 years of age), idarubicin at 12 mg/m(2) IV daily for 3 days, and sorafenib at 400 mg orally twice daily for 7 days. RESULTS Overall, 38 (75%) patients have achieved a complete remission (CR), including 14 (93%) of 15 patients with mutated FMS-like tyrosine kinase-3 (FLT3; the 15th patient had complete remission with incomplete platelet recovery [CRp]) and 24 (66%) of 36 patients with FLT3 wild-type (WT) disease (three additional FLT3-WT patients had CRp). FLT3-mutated patients were more likely to achieve a CR than FLT3-WT patients (P = .033). With a median follow-up of 54 weeks (range, 8 to 87 weeks), the probability of survival at 1 year is 74%. Among the FLT3-mutated patients, 10 have relapsed and five remain in CR with a median follow-up of 62 weeks (range, 10 to 76 weeks). Plasma inhibitory assay demonstrated an on-target effect on FLT3 kinase activity. CONCLUSION Sorafenib can be safely combined with chemotherapy, produces a high CR rate in FLT3-mutated patients, and inhibits FLT3 signaling.

Synthesis of a resorcylic acid lactone (RAL) library using fluorous-mixture synthesis and profile of its selectivity against a panel of kinases

A library of resorcylic acid lactones (RAL) containing a cis-enone moiety targeting kinases bearing a cysteine residue within the ATP-binding pocket was prepared using a fluorous-mixture synthesis and evaluated against a panel of 19 kinases thus providing important structure-activity trends. Two new analogues were then profiled for their selectivity against a panel of 402 kinases providing the broadest evaluation of this pharmacophores' selectivity.

Our changing view of the genomic landscape of cancer

Sporadic tumours, which account for the majority of all human cancers, arise from the acquisition of somatic, genetic and epigenetic alterations leading to changes in gene sequence, structure, copy number and expression. Within the last decade, the availability of a complete sequence-based map of the human genome, coupled with significant technological advances, has revolutionized the search for somatic alterations in tumour genomes. Recent landmark studies, which resequenced all coding exons within breast, colorectal, brain and pancreatic cancers, have shed new light on the genomic landscape of cancer. Within a given tumour type there are many infrequently mutated genes and a few frequently mutated genes, resulting in incredible genetic heterogeneity. However, when the altered genes are placed into biological processes and biochemical pathways, this complexity is significantly reduced and shared pathways that are affected in significant numbers of tumours can be discerned. The advent of next-generation sequencing technologies has opened up the potential to resequence entire tumour genomes to interrogate protein-encoding genes, non-coding RNA genes, non-genic regions and the mitochondrial genome. During the next decade it is anticipated that the most common forms of human cancer will be systematically surveyed to identify the underlying somatic changes in gene copy number, sequence and expression. The resulting catalogues of somatic alterations will point to candidate cancer genes requiring further validation to determine whether they have a causal role in tumourigenesis. The hope is that this knowledge will fuel improvements in cancer diagnosis, prognosis and therapy, based on the specific molecular alterations that drive individual tumours. In this review, I will provide a historical perspective on the identification of somatic alterations in the pre- and post-genomic eras, with a particular emphasis on recent pioneering studies that have provided unprecedented insights into the genomic landscape of human cancer.

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.

FMS-like tyrosine kinase 3-internal tandem duplication tyrosine kinase inhibitors display a nonoverlapping profile of resistance mutations in vitro

FMS-like tyrosine kinase 3 (FLT3) inhibitors have shown activity in the treatment of acute myelogenous leukemia (AML). Secondary mutations in target kinases can cause clinical resistance to therapeutic kinase inhibition. We have previously shown that sensitivity toward tyrosine kinase inhibitors varies between different activating FLT3 mutations. We therefore intended to determine whether different FLT3 inhibitors would produce distinct profiles of secondary, FLT3 resistance mutations. Using a cell-based screening approach, we generated FLT3-internal tandem duplication (ITD)-expressing cell lines resistant to the FLT3 inhibitors SU5614, PKC412, and sorafenib. Interestingly, the profile of resistance mutations emerging with SU5614 was limited to exchanges in the second part of the kinase domain (TK2) with exchanges of D835 predominating. In contrast, PKC412 exclusively produced mutations within tyrosine kinase domain 1 (TK1) at position N676. A mutation at N676 recently has been reported in a case of PKC412-resistant AML. TK1 mutations exhibited a differential response to SU5614, sorafenib, and sunitinib but strongly impaired response to PKC412. TK2 exchanges identified with SU5614 were sensitive to PKC412, sunitinib, or sorafenib, with the exception of Y842D, which caused a strong resistance to sorafenib. Of note, sorafenib also produced a highly distinct profile of resistance mutations with no overlap to SU5614 or PKC412, including F691L in TK1 and exchanges at position Y842 of TK2. Thus, different FLT3 kinase inhibitors generate distinct, nonoverlapping resistance profiles. This is in contrast to Bcr-Abl kinase inhibitors such as imatinib, nilotinib, and dasatinib, which display overlapping resistance profiles. Therefore, combinations of FLT3 inhibitors may be useful to prevent FLT3 resistance mutations in the setting of FLT3-ITD-positive AML.

Antileukemic effects of the novel, mutant FLT3 inhibitor NVP-AST487: effects on PKC412-sensitive and -resistant FLT3-expressing cells

An attractive target for therapeutic intervention is constitutively activated, mutant FLT3, which is expressed in a subpopulation of patients with acute myelocyic leukemia (AML) and is generally a poor prognostic indicator in patients under the age of 65 years. PKC412 is one of several mutant FLT3 inhibitors that is undergoing clinical testing, and which is currently in late-stage clinical trials. However, the discovery of drug-resistant leukemic blast cells in PKC412-treated patients with AML has prompted the search for novel, structurally diverse FLT3 inhibitors that could be alternatively used to override drug resistance. Here, we report the potent and selective antiproliferative effects of the novel mutant FLT3 inhibitor NVP-AST487 on primary patient cells and cell lines expressing FLT3-ITD or FLT3 kinase domain point mutants. NVP-AST487, which selectively targets mutant FLT3 protein kinase activity, is also shown to override PKC412 resistance in vitro, and has significant antileukemic activity in an in vivo model of FLT3-ITD(+) leukemia. Finally, the combination of NVP-AST487 with standard chemotherapeutic agents leads to enhanced inhibition of proliferation of mutant FLT3-expressing cells. Thus, we present a novel class of FLT3 inhibitors that displays high selectivity and potency toward FLT3 as a molecular target, and which could potentially be used to override drug resistance in AML.

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.

An FLT3 gene-expression signature predicts clinical outcome in normal karyotype AML

Acute myeloid leukemia with normal karyotype (NK-AML) represents a cytogenetic grouping with intermediate prognosis but substantial molecular and clinical heterogeneity. Within this subgroup, presence of FLT3 (FMS-like tyrosine kinase 3) internal tandem duplication (ITD) mutation predicts less favorable outcome. The goal of our study was to discover gene-expression patterns correlated with FLT3-ITD mutation and to evaluate the utility of a FLT3 signature for prognostication. DNA microarrays were used to profile gene expression in a training set of 65 NK-AML cases, and supervised analysis, using the Prediction Analysis of Microarrays method, was applied to build a gene expression-based predictor of FLT3-ITD mutation status. The optimal predictor, composed of 20 genes, was then evaluated by classifying expression profiles from an independent test set of 72 NK-AML cases. The predictor exhibited modest performance (73% sensitivity; 85% specificity) in classifying FLT3-ITD status. Remarkably, however, the signature outperformed FLT3-ITD mutation status in predicting clinical outcome. The signature may better define clinically relevant FLT3 signaling and/or alternative changes that phenocopy FLT3-ITD, whereas the signature genes provide a starting point to dissect these pathways. Our findings support the potential clinical utility of a gene expression-based measure of FLT3 pathway activation in AML.

Potentiation of antileukemic therapies by the dual PI3K/PDK-1 inhibitor, BAG956: effects on BCR-ABL- and mutant FLT3-expressing cells

Mediators of PI3K/AKT signaling have been implicated in chronic myeloid leukemia (CML) and acute myeloid leukemia (AML). Studies have shown that inhibitors of PI3K/AKT signaling, such as wortmannin and LY294002, are able to inhibit CML and AML cell proliferation and synergize with targeted tyrosine kinase inhibitors. We investigated the ability of BAG956, a dual PI3K/PDK-1 inhibitor, to be used in combination with inhibitors of BCR-ABL and mutant FLT3, as well as with the mTOR inhibitor, rapamycin, and the rapamycin derivative, RAD001. BAG956 was shown to block AKT phosphorylation induced by BCR-ABL-, and induce apoptosis of BCR-ABL-expressing cell lines and patient bone marrow cells at concentrations that also inhibit PI3K signaling. Enhancement of the inhibitory effects of the tyrosine kinase inhibitors, imatinib and nilotinib, by BAG956 was demonstrated against BCR-ABL expressing cells both in vitro and in vivo. We have also shown that BAG956 is effective against mutant FLT3-expressing cell lines and AML patient bone marrow cells. Enhancement of the inhibitory effects of the tyrosine kinase inhibitor, PKC412, by BAG956 was demonstrated against mutant FLT3-expressing cells. Finally, BAG956 and rapamycin/RAD001 were shown to combine in a nonantagonistic fashion against BCR-ABL- and mutant FLT3-expressing cells both in vitro and in vivo.

Cytogenetic findings and clinical outcomes of adult acute myeloid leukaemia patients

The role of cytogenetic findings in determining the diagnosis, therapy and prognosis of acute myeloid leukaemia (AML) has become more valuable by the day. In this study, the results of conventional and molecular cytogenetic analyses and clinical outcomes of 66 AML patients of different subgroups aged between 16 and 82 were evaluated. Chromosomal abnormalities were detected in 17 (25.7%) patients cytogenetically at the time of diagnosis, whereas molecular cytogenetic abnormalities were detected in 21 (31.8%) patients by fluorescence in situ hybridisation (FISH). Thirty-eight (57.6%) patients had a normal karyotype. In 8 patients, we did not obtain suitable chromosomes for karyotype analysis and in 3 patients conventional cytogenetics were not requested by the physician. During clinical follow-up, 21 patients (31.8%) achieved complete remission (CR), 2 had partial remission (PR) (3.0%) and 4 patients had progressive disease (6.06%). Twenty-eight (42.4%) patients died during treatment and no follow-up data were available in 7 cases. Among patients with chromosome abnormalities detected by either one of the two cytogenetic methods (n=28), 12 had achieved CR, 9 of whom were already categorised in the good prognostic group with t(15;17), inv16 or t(8;21). As for the normal karyotype, each patient displayed a different clinical course, which is probably due to the molecular changes in leukaemia-related genes. Here we report our findings, which correlate with previous reports and conclude that cytogenetics is a crucial marker in leukaemia diagnosis and conventional and molecular cytogenetics should be performed as well as molecular genetic diagnostic methods.

Potentiation of antileukemic therapies by Smac mimetic, LBW242: effects on mutant FLT3-expressing cells

Members of the inhibitor of apoptosis protein (IAP) family play a role in mediating apoptosis. Studies suggest that these proteins may be a viable target in leukemia because they have been found to be variably expressed in acute leukemias and are associated with chemosensitivity, chemoresistance, disease progression, remission, and patient survival. Another promising therapeutic target, FLT3, is mutated in about one third of acute myelogenous leukemia (AML) patients; promising results have recently been achieved in clinical trials investigating the effects of the protein tyrosine kinase inhibitor PKC412 on AML patients harboring mutations in the FLT3 protein. Of growing concern, however, is the development of drug resistance resulting from the emergence of point mutations in targeted tyrosine kinases used for treatment of acute leukemia patients. One approach to overriding resistance is to combine structurally unrelated inhibitors and/or inhibitors of different signaling pathways. The proapoptotic IAP inhibitor, LBW242, was shown in proliferation studies done in vitro to enhance the killing of PKC412-sensitive and PKC412-resistant cell lines expressing mutant FLT3 when combined with either PKC412 or standard cytotoxic agents (doxorubicin and Ara-c). In addition, in an in vivo imaging assay using bioluminescence as a measure of tumor burden, a total of 12 male NCr-nude mice were treated for 10 days with p.o. administration of vehicle, LBW242 (50 mg/kg/day), PKC412 (40 mg/kg/day), or a combination of LBW242 and PKC412; the lowest tumor burden was observed in the drug combination group. Finally, the combination of LBW242 and PKC412 was sufficient to override stromal-mediated viability signaling conferring resistance to PKC412.

Importance of early detection and follow-up of FLT3 mutations in patients with acute myeloid leukemia

Mutations in the fms-like tyrosine kinase 3 (FLT3) gene, such as internal tandem duplication (FLT3/ITD) in the juxtamembrane domain and point mutations in the tyrosine kinase domain, are the most common abnormalities in acute myeloid leukemia (AML). FLT3/ITD and FLT3/D835 mutations were analyzed in 113 Serbian adult AML patients using polymerase chain reaction. Twenty patients were found to be FLT3/ITD positive (17.7%). The mutations occurred most frequently in M5 and M0 subtypes of AML. They were mainly associated with the normal karyotype. All patients harboring FLT3/ITD had a higher number of white blood cells than patients without it (p = 0.027). FLT3/ITD mutations were associated with lower complete remission (CR) rate (chi (2 )= 5.706; p = 0.017) and shorter overall survival (OS; Log rank = 8.76; p = 0.0031). As for disease-free survival, the difference between FLT3/ITD-positive and FLT3/ITD-negative patients was not statistically significant (Log rank = 0.78; p = 0.3764). In multivariate analysis, the presence of FLT3/ITD mutations was the most significant prognostic factor for both OS and CR rate (p = 0.0287; relative risk = 1.73; 95% CI = 1.06-2.82). However, in the group of patients with the intermediate-risk karyotype, the mere presence of FLT3/ITD was not associated with inferior clinical outcome. FLT3/D835 point mutation was found in four patients (3.5%) only. Follow-up of the FLT3/ITD-positive patients revealed stability of this mutation during the course of the disease. However, changes in the pattern of FLT3/D835 mutations in initial and relapsed AML were observed. Our results indicate an association of FLT3/ITD with the adverse outcome in AML patients treated with standard induction chemotherapy. Because FLT3/ITD mutation is a target for specific therapeutic inhibition, its early detection could be helpful in clinical practice.

Anticipating the 1,000 dollar genome

New technologies will soon enable re-sequencing of human genomes at a much lower cost. The $1,000 human genome is an important goal for research and clinical diagnostics.

A new generation of DNA-sequencing platforms will become commercially available over the next few years. These instruments will enable re-sequencing of human genomes at a previously unimagined throughput and low cost. Here, I examine why the $1,000 human genome is an important goal for research and clinical diagnostics, and what will be required to achieve it.

FLT3 tyrosine kinase domain mutations are biologically distinct from and have a significantly more favorable prognosis than FLT3 internal tandem duplications in patients with acute myeloid leukemia

The prognostic impact of tyrosine kinase domain (TKD) mutations of the fms-like tyrosine kinase-3 (FLT3) gene in acute myeloid leukemia (AML) is currently uncertain. To resolve this issue we screened 1107 young adult nonacute promyelocytic leukemia AML patients with known FLT3 internal tandem duplication (ITD) status for FLT3/TKDs; they were detected in 127 (11%) cases. Mutations were associated with a high white cell count (P =.006) and patients with inv(16) (P = .005) but were infrequent in patients with adverse cytogenetics and secondary AML. Overall survival (OS) at 5 years was 53% and 37% for FLT3/TKD mutant and wild-type patients respectively (odds ratio, 0.72; 95% confidence interval, 0.58 to 0.89; P = .002). For both the cumulative incidence of relapse and OS the difference in outcome between FLT3/ITDs and FLT3/TKDs was highly significant (P < .001). In multivariate analysis, impact of FLT3/TKDs on OS when including all mutant-positive patients was not significant, but patients with high-level mutations (more than 25% mutant) had a significantly improved outcome (P = .004). The novel finding that biologically distinct activating mutations of the same gene can be associated with markedly different clinical outcomes has implications for risk stratification and therapy and is significant to the understanding of chemoresistance in AML.

Divergent cytotoxic effects of PKC412 in combination with conventional antileukemic agents in FLT3 mutation-positive versus -negative leukemia cell lines

FMS-like tyrosine kinase-3 (FLT3) is a new therapeutic target for acute myelocytic leukemia (AML), because FLT3 mutations are the most common genetic alterations in AML and are directly related to leukemogenesis. We studied cytotoxic interactions of a FLT3 inhibitor, PKC412, with eight conventional antileukemic agents (cytarabine, doxorubicin, idarubicin, mitoxantrone, etoposide, 4-hydroperoxy-cyclophosphamide, methotrexate and vincristine) using three leukemia cell lines carrying FLT3 mutations (MOLM13, MOLM14 and MV4-11) and five leukemia cell lines without FLT3 mutations (KOPB-26, THP-1, BALL-1, KG-1 and U937). PKC412 showed synergistic effects with all agents studied except methotrexate for FLT3-mutated cell lines in isobologram analysis. In contrast, PKC412 was rather antagonistic to most drugs, except for 4-hydroperoxy-cyclophosphamide and vincristine, in leukemia cell lines without FLT3 mutations. Cell-cycle analysis revealed that PKC412 induced G1 arrest in leukemia cell lines carrying FLT3 mutations, whereas it arrested cells in G2/M phase in the absence of FLT3 mutations, which may underlie the divergent cytotoxic interactions. These results suggest that the simultaneous administration of PKC412 and other agents except methotrexate is clinically effective against FLT3 mutation-positive leukemias, whereas it would be of little benefit for FLT3 mutation-negative leukemias. Our findings may be of help for the design of PKC412-based combination chemotherapy.