Mitogenic signalling and substrate specificity of the Flk2/Flt3 receptor tyrosine kinase in fibroblasts and interleukin 3-dependent hematopoietic cells

Therapeutic Targeting of FLT3 in Acute Myeloid Leukemia: Current Status and Novel Approaches

FMS-like tyrosine kinase 3 (FLT3) is mutated in approximately 30% of acute myeloid leukemia (AML) patients. The presence of FLT3-ITD (internal tandem duplication, 20-25%) mutation and, to a lesser extent, FLT3-TKD (tyrosine kinase domain, 5-10%) mutation is associated with poorer diagnosis and therapy response since the leukemic cells become hyperproliferative and resistant to apoptosis after continuous activation of FLT3 signaling. Targeting FLT3 has been the focus of many pre-clinical and clinical studies. Hence, many small-molecule FLT3 inhibitors (FLT3is) have been developed, some of which are approved such as midostaurin and gilteritinib to be used in different clinical settings, either in combination with chemotherapy or alone. However, many questions regarding the best treatment strategy remain to be answered. On the other hand, various FLT3-dependent and -independent resistance mechanisms could be evolved during FLT3i therapy which limit their clinical impact. Therefore, identifying molecular mechanisms of resistance and developing novel strategies to overcome this obstacle is a current interest in the field. In this review, recent studies of approved FLT3i and knowledge about major resistance mechanisms of clinically approved FLT3i's will be discussed together with novel treatment approaches such as designing novel FLT3i and dual FLT3i and combination strategies including approved FLT3i plus small-molecule agents targeting altered molecules in the resistant cells to abrogate resistance. Moreover, how to choose an appropriate FLT3i for the patients will be summarized based on what is currently known from available clinical data. In addition, strategies beyond FLT3i's including immunotherapeutics, small-molecule FLT3 degraders, and flavonoids will be summarized to highlight potential alternatives in FLT3-mutated AML therapy.

Current Methods of Post-Translational Modification Analysis and Their Applications in Blood Cancers

Post-translational modifications (PTMs) add a layer of complexity to the proteome through the addition of biochemical moieties to specific residues of proteins, altering their structure, function and/or localization. Mass spectrometry (MS)-based techniques are at the forefront of PTM analysis due to their ability to detect large numbers of modified proteins with a high level of sensitivity and specificity. The low stoichiometry of modified peptides means fractionation and enrichment techniques are often performed prior to MS to improve detection yields. Immuno-based techniques remain popular, with improvements in the quality of commercially available modification-specific antibodies facilitating the detection of modified proteins with high affinity. PTM-focused studies on blood cancers have provided information on altered cellular processes, including cell signaling, apoptosis and transcriptional regulation, that contribute to the malignant phenotype. Furthermore, the mechanism of action of many blood cancer therapies, such as kinase inhibitors, involves inhibiting or modulating protein modifications. Continued optimization of protocols and techniques for PTM analysis in blood cancer will undoubtedly lead to novel insights into mechanisms of malignant transformation, proliferation, and survival, in addition to the identification of novel biomarkers and therapeutic targets. This review discusses techniques used for PTM analysis and their applications in blood cancer research.

Strategies targeting FLT3 beyond the kinase inhibitors

Acute myeloid leukemia (AML) is a hematological malignancy characterized by clonal expansion and differentiation arrest of the myeloid progenitor cells, which leads to the accumulation of immature cells called blasts in the bone marrow and peripheral blood. Mutations in the receptor tyrosine kinase FLT3 occur in 30% of normal karyotype patients with AML and are associated with a higher incidence of relapse and worse survival. Targeted therapies against FLT3 mutations using small-molecule FLT3 tyrosine kinase inhibitors (TKIs) have long been investigated, with some showing favorable clinical outcomes. However, major setbacks such as limited clinical efficacy and the high risk of acquired resistance remain unresolved. FLT3 signaling, mutations, and FLT3 inhibitors are topics that have been extensively reviewed in recent years. Strategies to target FLT3 beyond the small molecule kinase inhibitors are expanding, nevertheless they are not receiving enough attention. These modalities include antibody-based FLT3 targeted therapies, immune cells mediated targeting strategies, and approaches targeting downstream signaling pathways and FLT3 translation. Here, we review the most recent advances and the challenges associated with the development of therapeutic modalities targeting FLT3 beyond the kinase inhibitors.

The Flt3L/Flt3 Axis in Dendritic Cell Biology and Cancer Immunotherapy

Dendritic cells (DCs) prime anti-tumor T cell responses in tumor-draining lymph nodes and can restimulate T effector responses in the tumor site. Thus, in addition to unleashing T cell effector activity, current immunotherapies should be directed to boost DC function. Herein, we review the potential function of Flt3L as a tool for cancer immunotherapy. Flt3L is a growth factor that acts in Flt3-expressing multipotent progenitors and common lymphoid progenitors. Despite the broad expression of Flt3 in the hematopoietic progenitors, the main effect of the Flt3/Flt3L axis, revealed by the characterization of mice deficient in these genes, is the generation of conventional DCs (cDCs) and plasmacytoid DCs (pDCs). However, Flt3 signaling through PI3K and mTOR may also affect the function of mature DCs. We recapitulate the use of Flt3L in preclinical studies either as a single agent or in combination with other cancer therapies. We also analyze the use of Flt3L in clinical trials. The strong correlation between type 1 cDC (cDC1) infiltration of human cancers with overall survival in many cancer types suggests the potential use of Flt3L to boost expansion of this DC subset. However, this may need the combination of Flt3L with other immunomodulatory agents to boost cancer immunotherapy.

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.

Discovery and rational design of 2-aminopyrimidine-based derivatives targeting Janus kinase 2 (JAK2) and FMS-like tyrosine kinase 3 (FLT3)

Herein, with the help of computer-aided drug design (CADD), we describe the structure-based rational drug design, structure-activity relationships, and synthesis of a series of 2-aminopyrimidine derivatives that inhibit both JAK2 and FLT3 kinases. These screening cascades revealed that compound 14l demonstrated the most inhibitory activity with IC₅₀ values of 1.8 and 0.68 nM against JAK2 and FLT3 respectively. 14l also showed potent anti-proliferative activities against HEL (IC₅₀ = 0.84 μM) and Molm-13 (IC₅₀ = 0.019 μM) cell lines, but relatively weak cytotoxicity against K562 and PC-3 cell lines, which proved that it might have high target specificity. In vitro metabolism assay, 14l exhibited moderate stability in RLM (Rat Liver Microsomes) with a half-life time of 31 min. In the cellular context of Molm-13, 14l induced cell cycle arrest in G₁/S phase and enhanced apoptosis in a dose-dependent manner. These results indicate that 14l is a promising dual JAK2/FLT3 inhibitor and worthy of further development.

The FMS like Tyrosine Kinase 3 (FLT3) Is Overexpressed in a Subgroup of Multiple Myeloma Patients with Inferior Prognosis

Therapy resistance remains a major challenge in the management of multiple myeloma (MM). We evaluated the expression of FLT3 tyrosine kinase receptor (FLT3, CD135) in myeloma cells as a possible clonal driver. FLT3 expression was analyzed in bone marrow biopsies of patients with monoclonal gammopathy of undetermined significance or smoldering myeloma (MGUS, SMM), newly diagnosed MM (NDMM), and relapsed/refractory MM (RRMM) by immunohistochemistry (IHC). FLT3 gene expression was analyzed by RNA sequencing (RNAseq) and real-time PCR (rt-PCR). Anti-myeloma activity of FLT3 inhibitors (midostaurin, gilteritinib) was tested in vitro on MM cell lines and primary MM cells by ³H-tymidine incorporation assays or flow cytometry. Semi-quantitative expression analysis applying a staining score (FLT3 expression IHC-score, FES, range 1-6) revealed that a high FES (>3) was associated with a significantly shorter progression-free survival (PFS) in NDMM and RRMM patients (p = 0.04). RNAseq and real-time PCR confirmed the expression of FLT3 in CD138-purified MM samples. The functional relevance of FLT3 expression was corroborated by demonstrating the in vitro anti-myeloma activity of FLT3 inhibitors on FLT3-positive MM cell lines and primary MM cells. FLT3 inhibitors might offer a new targeted therapy approach in a subgroup of MM patients displaying aberrant FLT3 signaling.

Design of New Therapeutic Agents Targeting FLT3 Receptor Tyrosine Kinase Using Molecular Docking and 3D-QSAR Approach

Background:

FMS-like tyrosine kinase-3 (FLT3) belongs to the class III Receptor Tyrosine Kinase (RTK) family. FLT3 is involved in normal hematopoiesis and is generally expressed in early hematopoietic progenitor cells. Mutations either with an internal tandem duplication of FMS-like tyrosine kinase-3 (FLT3-ITD) or point mutation at the activation loop leads to the Acute Myeloid Leukemia (AML), a highly heterogeneous disease. Thus, FLT3 is an important therapeutic target for AML.

Method:

In the present work, docking and 3D-QSAR techniques were performed on a series of diaminopyrimidine derivatives as FLT3 kinase antagonists.

Results:

Docking study recognized important active site residues such as Leu616, Gly617, Val624, Ala642, Phe830, Tyr693, Cys694, Cys695, Tyr696 and Gly697 that participate in the inhibition of FLT3 kinase. Receptor-based CoMFA, RF-CoMFA and CoMSIA models were developed. RFCoMFA model revealed relatively better statistical results compared to other models. Furthermore, the selected RF-CoMFA model was evaluated using various validation techniques. Contour maps of the RF-CoMFA illustrated that steric and electronegative substitutions were favored at R1 position whereas steric and electropositive substitutions were favored at R2 position to enhance the potency.

Conclusion:

Based on the designed strategy, we derived from the contour map analysis, 14 novel FLT3 inhibitors were designed and their activities were predicted. These designed inhibitors exhibited more potent activity than the most active compounds of the dataset.

Signalling circuits that direct early B-cell development

In mammals, the B-cell lineage arises from pluripotent progenitors in the bone marrow. During their development, B-cells undergo lineage specification and commitment, followed by expansion and selection. These processes are mediated by regulated changes in gene expression programmes, rearrangements of immunoglobulin (Ig) genes, and well-timed rounds of proliferation and apoptosis. Many of these processes are initiated by environmental factors including cytokines, chemokines, and cell-cell contacts. Developing B-cells process these environmental cues into stage-specific functions via signalling pathways including the PI3K, MAPK, or JAK-STAT pathway. The cytokines FLT3-Ligand and c-Kit-Ligand are important for the early expansion of the B-cell precursors at different developmental stages and conditions. Interleukin 7 is essential for commitment to the B-cell lineage and for orchestrating the Ig recombination machinery. After rearrangement of the immunoglobulin heavy chain, proliferation and apoptosis, and thus selection, are mediated by the clonal pre-B-cell receptor, and, following light chain rearrangement, by the B-cell receptor.

The role of FLT3 inhibitors as maintenance therapy following hematopoietic stem cell transplant

Activating mutations in FLT3 in acute myeloid leukemia (AML) portend a poor prognosis, and targeting FLT3 with a tyrosine kinase inhibitor has been an area of intense research recently. Most FLT3 mutated AML patients undergo hematopoietic stem cell transplantation (HSCT) as standard of care but a significant proportion of patients relapse. Although the use of FLT3 inhibitors in the pre-HSCT perspective is more clearly defined, its use in the post-HSCT scenario, when most relapses occur, remains unclear. In this review, we comprehensively present the data on the recent and ongoing studies evaluating the role of various FLT3 inhibitors in AML with a particular focus in the post-HSCT setting.

Computer simulations of the signalling network in FLT3 +-acute myeloid leukaemia - indications for an optimal dosage of inhibitors against FLT3 and CDK6

Background

Mutations in the FMS-like tyrosine kinase 3 (FLT3) are associated with uncontrolled cellular functions that contribute to the development of acute myeloid leukaemia (AML). We performed computer simulations of the FLT3-dependent signalling network in order to study the pathways that are involved in AML development and resistance to targeted therapies.

Results

Analysis of the simulations revealed the presence of alternative pathways through phosphoinositide 3 kinase (PI3K) and SH2-containing sequence proteins (SHC), that could overcome inhibition of FLT3. Inhibition of cyclin dependent kinase 6 (CDK6), a related molecular target, was also tested in the simulation but was not found to yield sufficient benefits alone.

Conclusions

The PI3K pathway provided a basis for resistance to treatments. Alternative signalling pathways could not, however, restore cancer growth signals (proliferation and loss of apoptosis) to the same levels as prior to treatment, which may explain why FLT3 resistance mutations are the most common resistance mechanism. Finally, sensitivity analysis suggested the existence of optimal doses of FLT3 and CDK6 inhibitors in terms of efficacy and toxicity.

Electronic supplementary material

The online version of this article (10.1186/s12859-018-2145-y) contains supplementary material, which is available to authorized users.

Liposomal Grb2 antisense oligodeoxynucleotide (BP1001) in patients with refractory or relapsed haematological malignancies: a single-centre, open-label, dose-escalation, phase 1/1b trial

BACKGROUND

Activating mutations of tyrosine kinases are common in leukaemias. Oncogenic tyrosine kinases use the growth factor receptor-bound protein 2 (Grb2) for signal transduction, leading to activation of mitogen-activated protein kinase (MAPK) 1 and MAPK3 (ERK2 and ERK1). We hypothesised that inhibition of Grb2 would suppress ERK1 and ERK2 activation and inhibit leukaemia progression. To inhibit Grb2, a liposome-incorporated antisense oligodeoxynucleotide that blocks Grb2 protein expression, BP1001, was developed. We report the first phase 1 findings of BP1001.

METHODS

In this single-centre, open-label, dose-escalation phase 1/1b trial, we enrolled participants (aged ≥18 years) with refractory or relapsed acute myeloid leukaemia, Philadelphia-chromosome-positive chronic myeloid leukaemia (in chronic, accelerated, or blast phase), acute lymphoblastic leukaemia, or myelodysplastic syndrome, at MD Anderson Cancer Center (Houston, TX, USA). We used a 3 + 3 dose escalation strategy, with at least three patients enrolled at each dose level. We administered BP1001 intravenously, twice weekly, for 28 days, with a starting dose of 5 mg/m2. If two or more patients developed toxic effects of grade 3 or higher, that dose level was deemed toxic. The dose was escalated if it did not produce dose-limiting toxic effects, and patients would be sequentially enrolled into cohort 2 (10 mg/m2), cohort 3 (20 mg/m2), cohort 4 (40 mg/m2), cohort 5 (60 mg/m2), or cohort 6 (90 mg/m2). After completion of monotherapy, we assessed the safety and toxicity of BP1001 (60 or 90 mg/m2) in combination with 20 mg low-dose cytarabine (twice-daily subcutaneous injections) in a phase 1b study in patients with refractory or relapsed acute myeloid leukaemia (ie, those who were refractory to at least one previous therapy regimen and no more than one previous salvage regimen). The objectives of this study were to establish the toxicity and tolerance of escalating doses of BP1001 monotherapy in patients with refractory or relapsed leukaemia, to assess the maximum tolerated dose of BP1001, and to determine the optimal biologically active dose of BP1001, defined as a 50% reduction in Grb2 expression in circulating leukaemia cells. We also aimed to assess the in-vivo pharmacokinetics of BP1001 and tumour response. The study is completed and is registered with ClinicalTrials.gov, number NCT01159028.

FINDINGS

Between July 23, 2010, and Feb 23, 2016, we enrolled and treated 39 patients, of whom 27 were assessable for dose-limiting toxicity. The first patient treated had mucositis and hand-foot syndrome, which were assessed as possibly related to BP1001 and counted as a dose-limiting toxicity. We noted no other dose-limiting toxicities, and a maximum tolerated dose was not identified. The highest tested dose of BP1001 was 90 mg/m2. The most common grade 3-4 adverse events were cardiopulmonary disorders (25 [64%] of 39 patients), and fever (including neutropenic fever) and infections (17 [44%] patients). Grade 5 adverse events were cardiopulmonary disorders (two [5%] of 39 patients), fever (including neutropenic fever) and infections (two [5%] of 39 patients), and multi-organ failure (one [3%] of 39 patients). Nine (33%) of 27 patients who had peripheral blood blasts at the start of therapy had a reduction of 50% or more in peripheral blood blasts while receiving BP1001 montherapy. Three (10%) of 29 patients who had bone marrow blasts at the start of therapy had a reduction in bone marrow blasts of 50% or more while receiving BP1001 monotherapy. Per investigator's assessment, seven (22%) of 32 patients benefited from BP1001 monotherapy and had extended cycles of treatment. Of seven patients receiving BP1001 plus low-dose cytarabine combination therapy, two had complete remission, one had complete remission with incomplete haematological recovery, and two had stable disease with no dose-limiting toxicity; one patient died and one withdrew, both because of disease progression. There were eight deaths; none were treatment related.

INTERPRETATION

BP1001 is well tolerated, with early evidence of anti-leukaemic activity in combination with low-dose cytarabine. To further explore this anti-leukaemic activity, the efficacy of BP1001 plus low-dose cytarabine combination is being investigated in an ongoing phase 2 study in patients with previously untreated acute myeloid leukaemia who are ineligible for intensive induction therapy.

FUNDING

Bio-Path Holdings Inc.

Altered compensatory cytokine signaling underlies the discrepancy between Flt3-/- and Flt3l-/- mice

The receptor Flt3 and its ligand Flt3L are both critical for dendritic cell (DC) development, but DC deficiency is more severe in Flt3l^-/- mice than in Flt3^-/- mice. This has led to speculation that Flt3L binds to another receptor that also supports DC development. However, we found that Flt3L administration does not generate DCs in Flt3^-/- mice, arguing against a second receptor. Instead, Flt3^-/- DC progenitors matured in response to macrophage colony-stimulating factor (M-CSF) or stem cell factor, and deletion of Csf1r in Flt3^-/- mice further reduced DC development, indicating that these cytokines could compensate for Flt3. Surprisingly, Flt3^-/- DC progenitors displayed enhanced M-CSF signaling, suggesting that loss of Flt3 increased responsiveness to other cytokines. In agreement, deletion of Flt3 in Flt3l^-/- mice paradoxically rescued their severe DC deficiency. Thus, multiple cytokines can support DC development, and the discrepancy between Flt3^-/- and Flt3l^-/- mice results from the increased sensitivity of Flt3^-/- progenitors to these cytokines.

Grb2 regulates the proliferation of hematopoietic stem and progenitors cells

Although Hematopoietic Stem and Progenitor Cell (HSPC) proliferation, survival and expansion have been shown to be supported by the cooperative action of different cytokines, little is known about the intracellular signaling pathways that are activated by cytokines upon binding to their receptors. Our study showed that Growth factor receptor-bound protein 2 (Grb2) mRNAs are preferentially expressed in HSC compared to progenitors and differentiated cells of the myeloid and erythroid lineages. Conditional deletion of Grb2 induced a rapid decline of erythroid and myeloid progenitors and a progressive decline of HSC numbers in steady state conditions. We showed that when transplanted, Grb2 deleted bone marrow cells could not reconstitute irradiated recipients. Strinkingly, Grb2 deletion did not modify HSPC quiescence, but impaired LT-HSC and progenitors ability to respond a proliferative signal induced by 5FU in vivo and by various cytokines in vitro. We showed finally that Grb2 links IL3 signaling to the ERK/MAPK proliferative pathway and that both SH2 and SH3 domains of Grb2 are crucial for IL3 signaling in progenitor cells. Our findings position Grb2 as a key adaptor that integrates various cytokines response in cycling HSPC.

Midostaurin: a novel therapeutic agent for patients with FLT3-mutated acute myeloid leukemia and systemic mastocytosis

The development of FLT3-targeted inhibitors represents an important paradigm shift in the management of patients with highly aggressive fms-like tyrosine kinase 3-mutated (FLT3-mut) acute myeloid leukemia (AML). Midostaurin is an orally administered type III tyrosine kinase inhibitor which in addition to FLT3 inhibits c-kit, platelet-derived growth factor receptors, src, and vascular endothelial growth factor receptor. Midostaurin is the first FLT3 inhibitor that has been shown to significantly improve survival in younger patients with FLT3-mut AML when given in combination with standard cytotoxic chemotherapy based on the recently completed RATIFY study. Its role for maintenance therapy after allogeneic transplantation and use in combination with hypomethylating agents for older patients with FLT3-mut has not yet been defined. Midostaurin also has recently been shown to have significant activity in systemic mastocytosis and related disorders due to its inhibitory effect on c-kit bearing a D816V mutation. Activation of downstream pathways in both of these myeloid malignancies likely plays an important role in the development of resistance, and strategies to inhibit these downstream targets may be synergistic. Incorporating patient factors and tumor characteristics, such as FLT3 mutant to wild-type allele ratios and resistance mutations, likely will be important in the optimization of midostaurin and other FLT3 inhibitors in the treatment of myeloid neoplasms.

Discovery of a Diaminopyrimidine FLT3 Inhibitor Active against Acute Myeloid Leukemia

Profiling of the kinase-binding capabilities of an aminopyrimidine analogue detected in a cellular screen of the St. Jude small-molecule collection led to the identification of a novel series of FMS-like tyrosine kinase 3 (FLT3) inhibitors. Structure-activity relationship studies led to the development of compounds exhibiting good potency against MV4-11 and MOLM13 acute myelogenous leukemia cells driven by FLT3, regardless of their FLT3 mutation status. In vitro pharmacological profiling demonstrated that compound 5e shows characteristics suitable for further preclinical development.

The Cytokine Flt3-Ligand in Normal and Malignant Hematopoiesis

The cytokine Fms-like tyrosine kinase 3 ligand (FL) is an important regulator of hematopoiesis. Its receptor, Flt3, is expressed on myeloid, lymphoid and dendritic cell progenitors and is considered an important growth and differentiation factor for several hematopoietic lineages. Activating mutations of Flt3 are frequently found in acute myeloid leukemia (AML) patients and associated with a poor clinical prognosis. In the present review we provide an overview of our current knowledge on the role of FL in the generation of blood cell lineages. We examine recent studies on Flt3 expression by hematopoietic stem cells and its potential instructive action at early stages of hematopoiesis. In addition, we review current findings on the role of mutated FLT3 in leukemia and the development of FLT3 inhibitors for therapeutic use to treat AML. The importance of mouse models in elucidating the role of Flt3-ligand in normal and malignant hematopoiesis is discussed.

Radiation induced pulmonary fibrosis as a model of progressive fibrosis: Contributions of DNA damage, inflammatory response and cellular senescence genes

Purpose/Aim of Study: Studies of pulmonary fibrosis (PF) have resulted in DNA damage, inflammatory response, and cellular senescence being widely hypothesized to play a role in the progression of the disease. Utilizing these aforementioned terms, genomics databases were interrogated along with the term, "pulmonary fibrosis," to identify genes common among all 4 search terms. Findings were compared to data derived from a model of radiation-induced progressive pulmonary fibrosis (RIPF) to verify that these genes are similarly expressed, supporting the use of radiation as a model for diseases involving PF, such as human idiopathic pulmonary fibrosis (IPF).

MATERIALS AND METHODS

In an established model of RIPF, C57BL/6J mice were exposed to 12.5 Gy thorax irradiation and sacrificed at 24 hours, 1, 4, 12, and 32 weeks following exposure, and lung tissue was compared to age-matched controls by RNA sequencing.

RESULTS

Of 176 PF associated gene transcripts identified by database interrogation, 146 (>82%) were present in our experimental model, throughout the progression of RIPF. Analysis revealed that nearly 85% of PF gene transcripts were associated with at least 1 other search term. Furthermore, of 22 genes common to all four terms, 16 were present experimentally in RIPF.

CONCLUSIONS

This illustrates the validity of RIPF as a model of progressive PF/IPF based on the numbers of transcripts reported in both literature and observed experimentally. Well characterized genes and proteins are implicated in this model, supporting the hypotheses that DNA damage, inflammatory response and cellular senescence are associated with the pathogenesis of PF.

FYN expression potentiates FLT3-ITD induced STAT5 signaling in acute myeloid leukemia

FYN is a non-receptor tyrosine kinase belonging to the SRC family of kinases, which are frequently over-expressed in human cancers, and play key roles in cancer biology. SRC has long been recognized as an important oncogene, but little attention has been given to its other family members. In this report, we have studied the role of FYN in FLT3 signaling in respect to acute myeloid leukemia (AML). We observed that FYN displays a strong association with wild-type FLT3 as well as oncogenic FLT3-ITD and is dependent on the kinase activity of FLT3 and the SH2 domain of FYN. We identified multiple FYN binding sites in FLT3, which partially overlapped with SRC binding sites. To understand the role of FYN in FLT3 signaling, we generated FYN overexpressing cells. We observed that expression of FYN resulted in slightly enhanced phosphorylation of AKT, ERK1/2 and p38 in response to ligand stimulation. Furthermore, FYN expression led to a slight increase in FLT3-ITD-dependent cell proliferation, but potent enhancement of STAT5 phosphorylation as well as colony formation. We also observed that FYN expression is deregulated in AML patient samples and that higher expression of FYN, in combination with FLT3-ITD mutation, resulted in enrichment of the STAT5 signaling pathway and correlated with poor prognosis in AML. Taken together our data suggest that FYN cooperates with oncogenic FLT3-ITD in cellular transformation by selective activation of the STAT5 pathway. Therefore, inhibition of FYN, in combination with FLT3 inhibition, will most likely be beneficial for this group of AML patients.

How I treat FLT3-mutated AML

FLT3-mutated acute myeloid leukemia (AML), despite not being recognized as a distinct entity in the World Health Organization (WHO) classification system, is readily recognized as a particular challenge by clinical specialists who treat acute leukemia. This is especially true with regards to the patients harboring the most common type of FLT3 mutation, the internal tandem duplication (FLT3-ITD) mutation. Here we present 4 patient cases from our institution and discuss how our management reflects what we have learned about this subtype of the disease. We also reflect on how we anticipate the management might change in the near future, with the emergence of clinically useful tyrosine kinase inhibitors.

Excessive Reactive Oxygen Species and Exotic DNA Lesions as an Exploitable Liability

Although the terms "excessive reactive oxygen species (ROS)" and "oxidative stress" are widely used, the implications of oxidative stress are often misunderstood. ROS are not a single species but a variety of compounds, each with unique biochemical properties and abilities to react with biomolecules. ROS cause activation of growth signals through thiol oxidation and may lead to DNA damage at elevated levels. In this review, we first discuss a conceptual framework for the interplay of ROS and antioxidants. This review then describes ROS signaling using FLT3-mediated growth signaling as an example. We then focus on ROS-mediated DNA damage. High concentrations of ROS result in various DNA lesions, including 8-oxo-7,8-dihydro-guanine, oxazolone, DNA-protein cross-links, and hydantoins, that have unique biological impacts. Here we delve into the biochemistry of nine well-characterized DNA lesions. Within each lesion, the types of repair mechanisms, the mutations induced, and their effects on transcription and replication are discussed. Finally, this review will discuss biochemically inspired implications for cancer therapy. Several teams have put forward designs to harness the excessive ROS and the burdened DNA repair systems of tumor cells for treating cancer. We discuss inhibition of the antioxidant system, the targeting of DNA repair, and ROS-activated prodrugs.

Pathology Consultation on Gene Mutations in Acute Myeloid Leukemia

OBJECTIVES

Acute myeloid leukemia (AML) is a rapidly fatal disease without the use of aggressive chemotherapy regimens. Cytogenetic and molecular studies are commonly used to classify types of AML based on prognosis, as well as to determine therapeutic regimens.

METHODS

Although there are several AML classifications determined by particular translocations, cytogenetically normal AML represents a molecularly, as well as clinically, heterogeneous group of diseases. Laboratory evaluation of AML will become increasingly important as new mutations with both prognostic and therapeutic implications are being recognized. Moreover, because many patients with AML are being treated more effectively, these mutations may become increasingly useful as markers of minimal residual disease, which can be interpreted in an individualized approach.

RESULTS

Current laboratory studies of gene mutations in AML include analysis of NPM1, FLT3, CEBPA, and KIT. In addition to these genes, many other genes are emerging as potentially useful in determining patients' prognosis, therapy, and disease course.

CONCLUSIONS

This article briefly reviews the current most clinically relevant gene mutations and their clinical and immunophenotypic features, prognostic information, and methods used for detection.

Flt3 is a target of coumestrol in protecting against UVB-induced skin photoaging

While skin aging is a naturally occurring process by senescence, exposure to ultraviolet (UV) radiation accelerates wrinkle formation and sagging of skin. UV induces skin aging by degrading collagen via activating matrix metalloproteinases (MMPs). In this study, we show that coumestrol, a metabolite of the soybean isoflavone daidzein, has a preventive effect on skin photoaging in three-dimensional human skin equivalent model. Coumestrol inhibited UVB-induced MMP-1 expression and activity. Whole human kinase profiling assay identified FLT3 kinase as a novel target protein of coumestrol in UVB-induced signaling pathway in skin. Coumestrol suppresses FLT3 kinase activity, and subsequently, Ras/MEK/ERK and Akt/p70 ribosomal S6 kinase pathway. This suppresses AP-1 activity and in turn, diminishes MMP-1 gene transcription. Using X-ray crystallography, the binding of coumestrol to FLT3 was defined and implied ATP-competitive inhibition. Residues Lys644 and Phe830 showed local changes to accommodate coumestrol in the ATP-binding pocket. 4-APIA, a pharmacological inhibitor of FLT3, inhibited MMP-1 expression and induced signal transduction changes similar to coumestrol. Taken together, coumestrol inhibits UVB-induced MMP-1 expression by suppressing FLT3 kinase activity. These findings suggest that coumestrol is a novel dietary compound with potential application in preventing and improving UVB-associated skin aging.

MUC1-C oncoprotein promotes FLT3 receptor activation in acute myeloid leukemia cells

Blasts from approximately one-third of patients with acute myeloid leukemia (AML) harbor activating mutations in the FMS-like tyrosine kinase 3 (FLT3) receptor tyrosine kinase that confer a poor prognosis. The Mucin 1-C-terminal subunit (MUC1-C) oncoprotein is aberrantly expressed in AML blasts and stem cells; however, there is no known interaction between MUC1-C and FLT3. The present studies demonstrate that MUC1-C associates with wild-type and mutant FLT3 in AML cells. Targeting MUC1-C with the cell-penetrating peptide inhibitor GO-203 disrupts MUC1-C/FLT3 complexes and downregulates FLT3 activation. GO-203 treatment of AML cells was also associated with inhibition of the FLT3 downstream effectors AKT, extracellular signal-regulated kinase, and STAT5. The results further show that AML cells with FLT3-activating mutations and resistant to the FLT3 inhibitor midostaurin/PKC412 are sensitive to GO-203-induced growth arrest and death. Moreover, GO-203 increases sensitivity of mutant FLT3 AML cells to FLT3 inhibitor treatment. These results indicate that MUC1-C contributes to FLT3 activation in AML cells and that targeting MUC1-C inhibits the FLT3 signaling pathway. Our findings support the development of MUC1-C inhibitors alone and in combination with agents that target FLT3 for the treatment of wild-type and mutant FLT3 AML.

Protein kinase C (PKC) as a drug target in chronic lymphocytic leukemia

Protein kinase C (PKC) belongs to a family of ten serine/threonine protein kinases encoded by nine genes. This family of proteins plays critical roles in signal transduction which results in cell proliferation, survival, differentiation and apoptosis. Due to differential subcellular localization and tissue distribution, each member displays distinct signaling characteristics. In this review, we have summarized the roles of PKC family members in chronic lymphocytic leukemia (CLL). CLL is a heterogeneous hematological disorder with survival ranging from months to decades. PKC isoforms are differentially expressed in CLL and play critical roles in CLL pathogenesis. Thus, isoform-specific PKC inhibitors may be an attractive option for CLL treatment.

New targets in pediatric Acute Myeloid Leukemia

In the last few years the improvements of chemotherapy regimens and supportive care has progressively ameliorated the prognosis of children suffering from Acute Myeloid Leukemia (AML). However, a still high percentage of children do not respond to first line treatments or relapse and need to undergo further treatments. The need to explore new agents other than chemotherapy has been highlighted in the last years in order to overcome drug related resistance and toxicity. Recently, novel therapies have been studied within early phases pediatric trials and seem to show encouraging results. In fact, the knowledge of molecular abnormalities related to AML pathogenesis has permitted to identify selective drugs that may represent an important tool for the development of patient-tailored treatments. Nowadays, FLT3, Aurora Kinases, mTORS's and proteasome inhibitors represents the most promising drugs that are being used in pediatric AML studies.

The Impact of FLT3 Mutations on the Development of Acute Myeloid Leukemias

The development of the genetic studies on acute myeloid leukemias (AMLs) has led to the identification of some recurrent genetic abnormalities. Their discovery was of fundamental importance not only for a better understanding of the molecular pathogenesis of AMLs, but also for the identification of new therapeutic targets. In this context, it is essential to identify AML-associated “driver” mutations, which have a causative role in leukemogenesis. Evidences accumulated during the last years indicate that activating internal tandem duplication mutations in FLT3 (FLT3-ITD), detected in about 20% of AMLs, represents driver mutations and valid therapeutic targets in AMLs. Furthermore, the screening of FLT3-ITD mutations has also considerably helped to improve the identification of more accurate prognostic criteria and of the therapeutic selection of patients.

Functional pathway analysis using SCNP of FLT3 receptor pathway deregulation in AML provides prognostic information independent from mutational status

FMS-like tyrosine kinase 3 receptor (FLT3) internal tandem duplication (ITD) mutations result in constitutive activation of this receptor and have been shown to increase the risk of relapse in patients with acute myeloid leukemia (AML); however, substantial heterogeneity in clinical outcomes still exists within both the ITD mutated and unmutated AML subgroups, suggesting alternative mechanisms of disease relapse not accounted by FLT3 mutational status. Single cell network profiling (SCNP) is a multiparametric flow cytometry based assay that simultaneously measures, in a quantitative fashion and at the single cell level, both extracellular surface marker levels and changes in intracellular signaling proteins in response to extracellular modulators. We previously reported an initial characterization of FLT3 ITD-mediated signaling using SCNP. Herein SCNP was applied sequentially to two separate cohorts of samples collected from elderly AML patients at diagnosis. In the first (training) study, AML samples carrying unmutated, wild-type FLT3 (FLT3 WT) displayed a wide range of induced signaling, with a fraction having signaling profiles comparable to FLT3 ITD AML samples. Conversely, the FLT3 ITD AML samples displayed more homogeneous induced signaling, with the exception of patients with low (<40%) mutational load, which had profiles comparable to FLT3 WT AML samples. This observation was then confirmed in an independent (verification) cohort. Data from the second cohort were also used to assess the association between SCNP data and disease-free survival (DFS) in the context of FLT3 and nucleophosmin (NPM1) mutational status among patients who achieved complete remission (CR) to induction chemotherapy. The combination of SCNP read outs together with FLT3 and NPM1 molecular status improved the DFS prediction accuracy of the latter. Taken together, these results emphasize the value of comprehensive functional assessment of biologically relevant signaling pathways in AML as a basis for the development of highly predictive tests for guidance of post-remission therapy.

Cloning and expression of porcine Colony Stimulating Factor-1 (CSF-1) and Colony Stimulating Factor-1 Receptor (CSF-1R) and analysis of the species specificity of stimulation by CSF-1 and Interleukin 34

Macrophage Colony Stimulating Factor (CSF-1) controls the survival, differentiation and proliferation of cells of the mononuclear phagocyte system. A second ligand for the CSF-1R, Interleukin 34 (IL-34), has been described, but its physiological role is not yet known. The domestic pig provides an alternative to traditional rodent models for evaluating potential therapeutic applications of CSF-1R agonists and antagonists. To enable such studies, we cloned and expressed active pig CSF-1. To provide a bioassay, pig CSF-1R was expressed in the factor-dependent Ba/F3 cell line. On this transfected cell line, recombinant porcine CSF-1 and human CSF-1 had identical activity. Mouse CSF-1 does not interact with the human CSF-1 receptor but was active on pig. By contrast, porcine CSF-1 was active on mouse, human, cat and dog cells. IL-34 was previously shown to be species-specific, with mouse and human proteins demonstrating limited cross-species activity. The pig CSF-1R was equally responsive to both mouse and human IL-34. Based upon the published crystal structures of CSF-1/CSF-1R and IL34/CSF-1R complexes, we discuss the molecular basis for the species specificity.

Localization of the NRAS:BCL-2 complex determines anti-apoptotic features associated with progressive disease in myelodysplastic syndromes

We have previously demonstrated that two prognostic features of myelodysplastic syndromes (MDS) and acute myelogenous leukemia (AML), mutant NRAS and over-expressing BCL-2, cooperate physically and functionally in vivo. Screening of MDS patient bone marrow (BM) identified NRAS:BCL-2 co-localization in 64% cases, correlating with percentage BM blasts, apoptotic features and disease status (p<0.0001). Localization of the complex at the plasma membrane or the mitochondria correlated with disease and apoptosis features in MDS patients, whilst caspase-9 mediated mechanism was elucidated in vivo and in vitro. The intensity and localization of the RAS:BCL-2 complex merits further evaluation as a novel biomarker of MDS.

Treatment of FLT3-ITD acute myeloid leukemia

Acute myeloid leukemia (AML) is an aggressive hematologic malignancy which is cured in a minority of patients. A FLT3-internal tandem duplication (ITD) mutation, found in approximately a quarter of patients with de novo AML, imparts a particularly poor prognosis. Patients with FLT3-ITD AML often present with more aggressive disease and have a significantly higher propensity for relapse after remission. The therapeutic approach for these patients has traditionally included intensive induction chemotherapy, followed by consolidative chemotherapy or hematopoietic cell transplantation (HCT). In recent years, multiple small molecule inhibitors of the FLT3 tyrosine kinase have been studied preclinically and in clinical trials. The earlier generation of these agents, often non-specific and impacting a variety of tyrosine kinases, produced at best transient peripheral blood responses in early clinical trials. Additionally, the combination of FLT3 inhibitors with cytotoxic regimens has not, as of yet, demonstrated an improvement in overall survival. Nevertheless, multiple current trials, including those with sorafenib, lestaurtinib, and midostaurin, continue to study the combination of FLT3 inhibitors with standard chemotherapy. Factors such as sustained FLT3 inhibition, protein binding, pharmacokinetics, and the presence of elevated FLT3-ligand levels appear to significantly impact the potency of these agents in vivo. In recent years, the development of more specific and potent agents has generated hope that FLT3 inhibitors may play a more prominent role in the treatment of FLT3-ITD AML in the near future. Nevertheless, questions remain regarding the optimal timing and schedule for incorporation of FLT3 inhibitors. The suitability, type, and timing of allogeneic HCT in the therapeutic approach for these patients are also issues which require further study and definition. Recent retrospective data appears to support the efficacy of allogeneic HCT in first complete remission, possibly due to a graft versus leukemia effect. However, larger prospective studies are necessary to further elucidate the role of HCT and its potential combination with FLT3 inhibitor therapy. We are hopeful that current clinical investigation will lead to an optimization and improvement of outcomes for these patients.

FLT3/ITD AML and the law of unintended consequences

Acute myeloid leukemia with a FLT3 internal tandem duplication (FLT3/ITD) mutation is an aggressive hematologic malignancy with a generally poor prognosis. It can be successfully treated into remission with intensive chemotherapy, but it routinely relapses. At relapse, the blasts tend to have higher mutant allelic ratios and, in vitro, are more addicted to the aberrant signaling from the FLT3/ITD oncoprotein. They remain highly responsive to FLT3 ligand, the levels of which rise several-fold during the course of chemotherapy. The question now arises as to whether these high levels of FLT3 ligand are actually promoting relapse, and, if so, how we can use this information to adjust our therapeutic approach and improve the cure rate for acute myeloid leukemia with FLT3/ITD.

Survey of activated FLT3 signaling in leukemia

Activating mutations of FMS-like tyrosine kinase-3 (FLT3) are found in approximately 30% of patients with acute myeloid leukemia (AML). FLT3 is therefore an attractive drug target. However, the molecular mechanisms by which FLT3 mutations lead to cell transformation in AML remain unclear. To develop a better understanding of FLT3 signaling as well as its downstream effectors, we performed detailed phosphoproteomic analysis of FLT3 signaling in human leukemia cells. We identified over 1000 tyrosine phosphorylation sites from about 750 proteins in both AML (wild type and mutant FLT3) and B cell acute lymphoblastic leukemia (normal and amplification of FLT3) cell lines. Furthermore, using stable isotope labeling by amino acids in cell culture (SILAC), we were able to quantified over 400 phosphorylation sites (pTyr, pSer, and pThr) that were responsive to FLT3 inhibition in FLT3 driven human leukemia cell lines. We also extended this phosphoproteomic analysis on bone marrow from primary AML patient samples, and identify over 200 tyrosine and 800 serine/threonine phosphorylation sites in vivo. This study showed that oncogenic FLT3 regulates proteins involving diverse cellular processes and affects multiple signaling pathways in human leukemia that we previously appreciated, such as Fc epsilon RI-mediated signaling, BCR, and CD40 signaling pathways. It provides a valuable resource for investigation of oncogenic FLT3 signaling in human leukemia.

Further activation of FLT3 mutants by FLT3 ligand

Somatic mutations of FLT3 involving internal tandem duplication (ITD) of the juxtamembrane domainor point mutations in the kinase domain (TKD) appear to activate FLT3 in a FLT3 ligand (FL) - independent manner. To determine whether or not FLT3 mutants respond to FL for their activation, a FL-deficient (FL^−/−) murine embryo fibroblast cell line (MEF) was established. Expression of FLT3/ITD and FLT3/TKD mutations in FL^−/− MEF cells resulted in low levels of constitutive phosphorylation of FLT3.However, a more than 4-fold increase of FLT3 autophosphorylation was induced by exogenous FL. Rescue of endogenous FL expression in FL^−/− MEF cells expressing FLT3 mutants led to more than a 3-fold increase of FLT3 phosphorylation. FL addition led to further activation of the FLT3 receptors and enhanced survival and/or decreased apoptosis in leukemia-derived cell lines and primary leukemic cells expressing FLT3 mutations. Functional studies revealed that exogenous FL promoted the colony-forming and recloning abilities of FLT3 mutant transduced primary bone marrow cells derived from FL^−/− mice. Endogenous FL contributes in vivo to functional signaling through FLT3 as noted by the decreased survival of FL^+/+ITD^+/+ mice compared with FL^−/−ITD^+/+ mice. These data suggest that FL leads to further activation of FLT3 mutants and is especially important in light of recent findings of elevated FL levels in AML patients in response to chemotherapy.

FLT3 inhibitors in acute myeloid leukemia

The fms-like tyrosine kinase 3 (FLT3) plays an important role in both normal and malignant hematopoiesis. Activating mutations in the FLT3 receptor can be detected in approximately 30% of acute myeloid leukemias (AMLs) and are associated with a distinctly poor clinical outcome for patients. There are now several classes of FLT3 inhibitors in development with varying degrees of potency and selectivity for the target, including several in late-phase clinical trials in combination with chemotherapy. Major clinical responses in AML patients receiving single-agent FLT3 inhibitors have been rare, although transient peripheral blood blast reduction is common. Given such biological suggestion and preclinical activity, FLT3 inhibitors hold promise in improving the outcome of patients with mutant FLT3 AML. This review summarizes the current attempts to target this molecule, with emphasis on the validity of the target, the results of the clinical trials evaluating the FLT3 inhibitors in AML, the optimal use of these compounds and the mechanisms of resistance.

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.

Differential signaling of Flt3 activating mutations in acute myeloid leukemia: a working model

Receptor tyrosine kinases couple a wide variety of extracellular cues to cellular responses. The class III subfamily comprises the platelet-derived growth factor receptor, c-Kit, Flt3 and c-Fms, all of which relay cell proliferation signals upon ligand binding. Accordingly, mutations in these proteins that confer ligand-independent activation are found in a subset of cancers. These mutations cluster in the juxtamembrane (JM) and catalytic tyrosine kinase domain (TKD) regions. In the case of acute myeloid leukemia (AML), the juxtamembrane (named ITD for internal tandem duplication) and TKD Flt3 mutants differ in their spectra of clinical outcomes. Although the mechanism of aberrant activation has been largely elucidated by biochemical and structural analyses of mutant kinases, the differences in disease presentation cannot be attributed to a change in substrate specificity or signaling strength of the catalytic domain. This review discusses the latest literature and presents a working model of differential Flt3 signaling based on mis-localized juxtamembrane autophosphorylation, to account for the disease variation. This will have bearing on therapeutic approaches in a complex disease such as AML, for which no efficacious drug yet exists.

Functional characterization of FLT3 receptor signaling deregulation in acute myeloid leukemia by single cell network profiling (SCNP)

Background

Molecular characterization of the FMS-like tyrosine kinase 3 receptor (FLT3) in cytogenetically normal acute myeloid leukemia (AML) has recently been incorporated into clinical guidelines based on correlations between FLT3 internal tandem duplications (FLT3-ITD) and decreased disease-free and overall survival. These mutations result in constitutive activation of FLT3, and FLT3 inhibitors are currently undergoing trials in AML patients selected on FLT3 molecular status. However, the transient and partial responses observed suggest that FLT3 mutational status alone does not provide complete information on FLT3 biological activity at the individual patient level. Examination of variation in cellular responsiveness to signaling modulation may be more informative.

Methodology/Principal Findings

Using single cell network profiling (SCNP), cells were treated with extracellular modulators and their functional responses were quantified by multiparametric flow cytometry. Intracellular signaling responses were compared between healthy bone marrow myeloblasts (BMMb) and AML leukemic blasts characterized as FLT3 wild type (FLT3-WT) or FLT3-ITD. Compared to healthy BMMb, FLT3-WT leukemic blasts demonstrated a wide range of signaling responses to FLT3 ligand (FLT3L), including elevated and sustained PI3K and Ras/Raf/Erk signaling. Distinct signaling and apoptosis profiles were observed in FLT3-WT and FLT3-ITD AML samples, with more uniform signaling observed in FLT3-ITD AML samples. Specifically, increased basal p-Stat5 levels, decreased FLT3L induced activation of the PI3K and Ras/Raf/Erk pathways, decreased IL-27 induced activation of the Jak/Stat pathway, and heightened apoptotic responses to agents inducing DNA damage were observed in FLT3-ITD AML samples. Preliminary analysis correlating these findings with clinical outcomes suggests that classification of patient samples based on signaling profiles may more accurately reflect FLT3 signaling deregulation and provide additional information for disease characterization and management.

Conclusions/Significance

These studies show the feasibility of SCNP to assess modulated intracellular signaling pathways and characterize the biology of individual AML samples in the context of genetic alterations.

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.

Structural and functional alterations of FLT3 in acute myeloid leukemia

Hematopoiesis is highly regulated through cytokine-induced stimulation of multiple signal transduction pathways in order to mediate appropriate differentiation and proliferation of specific progenitor populations. Ligand-induced stimulation of the FMS-like tyrosine kinase 3 (FLT3) leads to activation of multiple downstream effector pathways resulting in differentiation and proliferation of specific progenitor cell populations. Genomic alterations of the FLT3 gene, including FLT3 internal tandem duplication (FLT3/ITD) and FLT3 activation loop mutation (FLT3/ALM) lead to autonomous receptor activation, dysregulation of FLT3 signal transduction pathways, contribute to myeloid pathogenesis, and have been linked to response to therapy and clinical outcome. Exploring the mechanisms by which these FLT3 alterations lead to dysregulated proliferation should provide a better understanding of the molecular pathogenesis of acute myeloid leukemia (AML) and may provide insights into potential therapeutic interventions. FLT3 inhibitors are under evaluation for their efficacy in AML patients with FLT3 mutations.

Potential role of sorafenib in the treatment of acute myeloid leukemia

The identification of aberrant cellular pathways and dysfunctional molecules important in neoplastic transformation has begun to provide us with a number of targets for drug development. It is likely that many of these agents will be incorporated into our existing treatment strategies that include cytotoxic agents. Sorafenib, a multi-kinase inhibitor has been approved in the United States for the treatment of renal cell carcinoma as well as hepatocellular cancer. Its potential role in hematological malignancies, particularly acute myeloid leukemia (AML) is under evaluation. Here we describe the biological pathways in AML that are the potential targets of sorafenib action and discuss the early clinical data with the agent in solid tumors and AML.

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.

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.

FLT3/ITD expression increases expansion, survival and entry into cell cycle of human haematopoietic stem/progenitor cells

Activating mutation of FLT3 by internal tandem duplications (ITDs) in the juxtamembrane region is the most common molecular aberration found in acute myeloid leukaemia (AML). In this study, a lentiviral vector containing two promoters achieved consistent and efficient co-expression of FLT3/ITD and GFP in transduced human CD34(+) haematopoietic stem/progenitor cells (HSPCs). When cultured in medium containing stem cell factor, thrombopoietin and FLT3 ligand (FL), FLT3/ITD-transduced cells demonstrated enhanced self-renewal and survival potential, unaffected by the withdrawal of FL. These cells retained a CD34(+)CD38(-/dim) immunophenotype, typical of HSPCs. Compared to cells transduced with a vector expressing GFP alone, FLT3/ITD-transduced HSPCs had a higher fraction of cells in active cell cycle. FLT3/ITD-transduced HSPCs were more sensitive to the induction of cytotoxicity by CEP-701, a selective FLT3 inhibitor, indicating a rapid 'addiction' to signalling through this oncogenic pathway. The FLT3/ITD-transduced HSPCs showed increased expression of Pim-1, c-Myc and Cyclin D3 (CCND3), each of which may contribute to the altered genetic programme instituted by FLT3/ITD signalling. Taken together, these results indicate that FLT3/ITD mutations may contribute to leukaemic transformation of normal HSPCs by prolonging survival, promoting proliferation and partially blocking differentiation. CEP-701 may act as a potent therapeutic agent for AML stem cells harbouring FLT3/ITD mutations.

Uniform sensitivity of FLT3 activation loop mutants to the tyrosine kinase inhibitor midostaurin

Small molecule inhibitors that target fms-like tyrosine kinase 3 (FLT3)-activating mutations have potential in the treatment of leukemias. However, certain mutations can simultaneously activate the tyrosine kinase, and confer resistance to small molecule inhibitors. We therefore tested the sensitivity of 8 FLT3 activation loop mutants to midostaurin. Each mutant conferred IL-3 factor-independent proliferation to Ba/F3 cells, and each resulted in the constitutive activation of FLT3 and its targets, signal transducer and activator of transcription 5 (STAT5) and extracellular stimuli-responsive kinase (ERK). For each mutant tested, midostaurin inhibited cell growth and phosphorylation of FLT3, STAT5, and ERK. In contrast, midostaruin did not inhibit Ba/F3 cells stably transduced with FLT3-internal tandem duplications containing a G697R mutation that confers resistance to midostaurin, demonstrating that midostaurin inhibition of FLT3 activation loop mutants was not due to off-target effects. We conclude that midostaurin is a potent inhibitor of a spectrum of FLT3 activation loop mutations, and that acute myeloid leukemia patients with such mutations are potential candidates for clinical trials involving midostaurin.