Tyrosine kinases as therapeutic targets in BCR-ABL negative chronic myeloproliferative disorders

Acquired constitutive activation of protein tyrosine kinases is a central feature in the pathogenesis of chronic myeloproliferative disorders (CMPDs). The most commonly involved genes are the receptor tyrosine kinases PDGFRA, PDGFRB, FGFR1 or c-KIT and the non-receptor tyrosine kinases JAK2 and ABL. Activation occurs as a consequence of specific point mutations or fusion genes generated by chromosomal translocations, insertions or deletions. Mutant kinases are constitutively active in the absence of the natural ligands resulting in deregulation of haemopoiesis in a manner analogous to BCR-ABL in chronic myeloid leukaemia. With the advent of targeted signal transduction therapy with tyrosine kinase inhibitors, an accurate diagnosis of CMPDs by morphology, karyotyping and molecular genetics has become increasingly important. Imatinib induces high response rates in patients associated with constitutive activation of ABL, PDGFRalpha, PDGFRbeta and some KIT mutants. Other inhibitors under development are promising candidates for effective treatment of patients with constitutive activation of JAK2, FGFR1 and imatinib-resistant KIT mutants.     

FLT3 as a therapeutic target in childhood acute leukemia

During the past few years, a major focus of leukemia research has centered on tyrosine kinases as potential therapeutic targets. This is due in large part to the success of imatinib mesylate (STI571, Gleevec), which has proven effective as a therapy for chronic myeloid leukemias bearing the t(9;22) encoding the BCR-ABL kinase. It has become increasingly evident that mutations producing constitutively active tyrosine kinases play a role in leukemogenesis. Another kinase that has drawn significant attention is the FMS-like tyrosine kinase 3 (FLT3). FLT3 is expressed in most childhood acute leukemias. Select genetic subgroups possess particularly high-level expression, with a significant percentage therein harboring activating mutations. In this review we will discuss FLT3 as a potential therapeutic target in childhood acute leukemias. We will highlight the role of FLT3 in hematopoiesis, and how when activated, it may play a role in the development of acute myeloid or acute lymphoblastic leukemia. We will examine the successes in elucidating FLT3 function in acute leukemias, highlight current FLT3 targeted therapeutics, and discuss how FLT3 inhibitors might be used in combination therapies in the future.     

Targeting mutated tyrosine kinases in the therapy of myeloid leukaemias

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

Mechanisms of resistance to FLT3 inhibitors

The success of the small molecule tyrosine kinase receptor inhibitor (TKI) imatinib mesylate (Gleevec) in the treatment of chronic myeloid leukemia (CML) constitutes an eminent paradigm shift advocating the rational design of cancer therapeutics specifically targeting the transformation events that drive tumorigenicity. In acute myeloid leukemias (AMLs), the most frequent identified transforming events are activating mutations in the FLT3 receptor tyrosine kinase that constitutively activate survival and proliferation pathways. FLT3 TKIs that are in various phases of clinical trials are showing some initial promise. However, primary and secondary acquired resistance stands to severely compromise long-term and durable efficacy of these inhibitors as a therapeutic strategy. Here, we discuss the mechanisms of resistance to FLT3 inhibitors and possible strategies to overcome resistance through closer examination of the events of leukemogenesis and design of combination therapy.     

Targeting mutated tyrosine kinases in the therapy of myeloid leukaemias

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

Translocation t(12;13)(p13;q14) in a patient with imatinib-sensitive MDS/MPD associated with resistance to treatment: review of the literature

The category of myelodysplastic syndromes/myeloproliferative diseases (MDS/MPD) is a relatively new group of malignant hematologic diseases developed by the World Health Organization. These hematologic disorders lack the BCR/ABL fusion gene, although they can be associated with chromosomal translocations that involve genes encoding other protein kinases. Imatinib mesylate was recognized as a potent inhibitor of some of those kinases. We present a patient with a previously treated acute myeloid leukemia, who, after a 9-year-long remission, developed an MDS/MPD with normal karyotype, which initially responded to imatinib mesylate. Translocation t(12;13)(p12;q14) was detected after loss of response to imatinib treatment. Translocation t(12;13) is rare. It has been described in several hematologic malignancies including chronic myelomonocytic leukemia but not in MDS/MPD, previously described as Philadelphia-negative chronic myelogenous leukemia. Moreover, the correlation of this molecular abnormality with loss of efficacy of imatinib is unique in the literature.     

Proteomics approaches to elucidate oncogenic tyrosine kinase signaling in myeloid malignancies

Myeloid malignancies frequently harbor specific mutations in protein tyrosine kinases leading to oncogenic cell signaling. The most extensively investigated example is chronic myeloid leukemia, where the pathogenic tyrosine kinase fusion protein Bcr-Abl is a successful target for disease control by the specific inhibitor imatinib mesylate. In acute myeloid leukemia the receptor tyrosine kinase Flt3 is frequently mutated and inhibitors to impair the oncogenic signaling are in development. In this review we exemplify oncogenic signaling and how signal pathways can be unraveled with help from proteomics-based technologies. The distinction between cell extract and single cell approaches aiming at rigorous standardization and reliable quantitative aspects for future proteomics-based diagnostics is discussed.     

Midostaurin treatment in FLT3-mutated acute myeloid leukemia and systemic mastocytosis

Introduction: A number of tyrosine kinase inhibitors (TKIs) have been developed that inhibit the constitutively activated kinase activity caused by activating tyrosine kinase mutations, such as FLT3 or KIT, thus interrupting signaling pathways. Currently, midostaurin is the only approved TKI as monotherapy for aggressive systemic mastocytosis (SM), SM with associated hematological neoplasm, or mast cell leukemia displaying a KIT mutation as well as in combination with standard intensive chemotherapy for adult patients with newly diagnosed FLT3-mutated acute myeloid leukemia (AML).

Areas covered: We provide a concise review of the pharmacology, tolerability and clinical efficacy of midostaurin and emerging new treatment options for ASM and FLT3-mutated AML.

Expert commentary: Currently, midostaurin is the only approved TKI in aggressive SM, SM with associated hematological neoplasm, or mast cell leukemia inducing responses including complete remissions. With regard to AML, midostaurin is the first drug to receive regulatory approval in this indication in the molecularly defined subgroup of AML with FLT3 mutations. By introduction of this new standard in AML with FLT3 mutations, the bare has been raised for future approvals of next generation FLT3 inhibitors which will be based increasingly on head to head comparisons with midostaurin.     

FLT3 Antibody‐based therapy for leukemia

Technological advances in antibody generation and production have facilitated recent clinical and commercial success with antibody‐based cancer therapeutics. The class III receptor tyrosine kinase FLT3 is highly expressed on the blast cells in most cases of acute myelogenous leukemia (AML) and B‐cell acute lymphoblastic leukemia (ALL). Activating mutations of FLT3 are detected in approximately 37% AML patients. FLT3 expression in normal tissue is limited to myeloid and B‐cell precursor cells. Therefore, over‐expressed or mutated FLT3 is an attractive target for therapeutic intervention using monoclonal antibodies. This review will discuss recent progress in the development of anti‐FLT3 antibodies as well as their therapeutic potentials in the treatment of AML and other hematological malignancies. Drug Dev. Res. 67:495–500, 2006. © 2006 Wiley‐Liss, Inc.     

Mechanisms of resistance imatinib (STI571) in preclinical models and in leukemia patients.

The tyrosine kinase inhibitor imatinib (STI571, Glivec) blocks the activity of the BCR/ABL oncogene and induces hematologic remissions in the majority of patients with chronic myeloid leukemia (CML). Glivec is an aminopyrimidine derivative that interacts with the ATP-binding site within the kinase domain of ABL and several other tyrosine kinases, including c-KIT, PDGF beta receptor, and ARG. The compound is currently in phase III clinical trials. Although patients with chronic phase CML have been found to develop drug resistance only rarely so far, patients in more advanced phases of the leukemia develop resistance frequently. The available information on Glivec resistance will be reviewed.     

ENMD-2076 for hematological malignancies

INTRODUCTION: Aurora kinases are key regulators of mitosis and inhibition of Aurora kinase activity is a rational therapeutic strategy in the treatment of solid tumors and hematological malignancies. AREAS COVERED: This paper will provide an updated summary of preclinical and clinical experience with ENMD-2076 in hematological malignancies. The MEDLINE (OVID) (1980 through 31 January 2012) was searched with the term combinations including Aurora, multiple myeloma, leukemia, lymphoma, myelodysplastic syndrome and myeloproliferative neoplasms. In addition, the American Society of Clinical Oncology (ASCO) (1997 - 2011) and the American Society of Hematology (ASH) (1997 - 2011) conference proceedings were searched for reports of new or ongoing trials. EXPERT OPINION: ENMD-2076 is a multi-kinase inhibitor, with activity against Aurora A kinase, FLT3, c-KIT, c-FMS and VEGFR-2 and -3. It appears to be tolerable, exhibits favorable pharmacokinetic profiles and has activity in patients with acute myeloid leukemia and multiple myeloma. Further evaluation with cytotoxic chemotherapy and targeted agents, which affect different pathways and have non-overlapping toxicities, in patients with hematological malignancies are warranted.     

Omacetaxine as an anticancer therapeutic: what is old is new again

Omacetaxine mepesuccinate was originally identified more than 35 years ago and initial studies in chronic myeloid leukemia (CML) showed promising activity. It has also been studied in other hematologic and solid tumors as both a single agent and in combination with other treatments. However, the introduction of imatinib and related tyrosine kinase inhibitors (TKIs) abated the clinical development of omacetaxine as a treatment for CML. The advent of resistance to imatinib and other TKIs in CML patients (often due to the presence of an ABL mutation at position 315) has led to a revived clinical interest in omacetaxine in CML patients who failed TKIs. Here we review omacetaxine's mechanism of action (MOA) as a protein translation inhibitor, how its MOA may translate into activity in treatment of cancers, its potential to eradicate leukemia initiating cells and other cancer stem cells and the potential significance of this activity in clinical practice.     

Targeting FLT3 kinase in acute myelogenous leukemia: progress, perils, and prospects

Activating mutations of the FLT3 receptor tyrosine kinase are the most common recurring genetic abnormality in acute myelogenous leukemia (AM). Inhibition of FLT3 kinase activity by small molecule inhibitors has been proposed as a novel therapeutic approach AML. The pre-clinical and clinical development of candidate FLT3 inhibitors will be reviewed.     

Small molecule FLT3 tyrosine kinase inhibitors

Activating mutations of FLT3 (FMS-Like Tyrosine kinase-3) are the most common molecular abnormality in acute myeloid leukemia (AML). Their presence is associated with a worse prognosis, and the recognition of this has led to the development of several new small molecule FLT3 tyrosine kinase inhibitors. In this review, we summarize these developments and compare and contrast these novel agents both with regards to the assays used to characterize them as well as to their clinical potential.     

Bosutinib for the treatment of chronic myeloid leukemia in chronic phase

The clinical outcome for patients with chronic myeloid leukemia in chronic phase (CML-CP) is currently very favorable due to the availability of tyrosine kinase inhibitors (TKIs) that are well tolerated and effectively suppress the constitutively activated BCR-ABL1 kinase that underlies the pathogenesis of this malignancy. Three TKIs -imatinib, nilotinib and dasatinib- have been approved as frontline therapy in CML-CP. Another TKI, bosutinib, inhibits with high potency numerous tyrosine kinases, including BCR-ABL1, Src family of kinases and MAPK, among others. Like nilotinib and dasatinib, bosutinib is a second-generation TKI that inhibits the majority of mutations associated with imatinib resistance, with the exception of T315I. In patients with CML-CP with prior intolerance or resistance to imatinib therapy, bosutinib rendered response rates similar to those observed in the same patient population treated with nilotinib or dasatinib. Preliminary results from the ongoing phase III BELA study in which bosutinib is compared in a randomized fashion to imatinib for patients with newly diagnosed CML-CP have been recently reported. We herein summarize the preclinical and clinical experience of bosutinib in CML.     

BCR-ABL inhibitors in chronic myeloid leukemia: process chemistry and biochemical profile

Chronic myeloid leukemia (CML) is a myeloproliferative disease originating from a constitutively active tyrosine kinase, called BCR-ABL, expressed by an oncogene resulting from a reciprocal translocation between chromosome 9 and chromosome 22, coded as (t[9,22][q34;q11]). Inhibition of BCR-ABL with tyrosine kinase inhibitors (TKI) proved to be an efficient targeted therapy of Philadelphia-positive (Ph+) CML in the chronic phase. This review mainly addresses the synthetic pathways and process chemistry leading to the large scale preparation for pre-clinical demands and clinical supply of the three TKIs approved for Ph+ CML, i.e., imatinib, dasatinib and nilotinib and three more investigational drugs, i.e., bosutinib, ponatinib and bafetinib. Recent progress on the biochemical profiling of the six examined TKIs has been also reported.     

FLT3在急性白血病靶向治疗中的研究进展

FMS-like Tyrosine Kinase 3(FLT3)是一种受体酪氨酸激酶,在造血干细胞、前体B细胞等的增殖、分化以及存活中具有重要作用。急性白血病的发病往往伴随着FLT3的异常活化,FLT3突变是造成其异常活化的主要原因,且FLT3突变的患者往往具有较差的预后。FLT3抑制剂的研究为白血病靶向治疗提供了广阔的前景。     

gamma-Aminobutyric acid (GABA) and glutamate neurotransmission during epileptiform activity in in vitro neural models

Importance of the field: Activating mutations of the JAK2 gene are of tumorigenic significance in myeloproliferative neoplasms. Translocations involving the JAK2 locus are of oncogenic importance in acute leukemias, myelodysplastic/myeloproliferative diseases and T-cell lymphomas. JAK2 locus gains, which are recurrent in Hodgkin's- and primary mediastinal B-cell lymphoma, are also efficient mechanisms of JAK2 activation. Recently, specific drugs blocking JAK2 have been developed and are currently in clinical trials.

Areas covered in this review: We discuss possible mechanisms of deregulation and the significance of pericentriolar material 1 (PCM)1–JAK2 fusion/t(8;9)(p21-23;p23-24) in hematolymphoid neoplasms. Such cases show morphological (myeloproliferaton, eosinophilia, myelofibrosis) and clinical (striking male predominance, aggressive course) similarities. Since increased JAK2 oligomerization and tyrosine kinase domain activation is the probable oncogenic mechanism in this instance, such patients are promising candidates for JAK2 inhibitor therapy.

What the reader will gain: The reader will gain important insights considering PCM1–JAK2 fusion in hematologic malignancies.

Take home message: JAK2 is a tyrosine kinase with oncogenic potential in hematologic malignancies. It can be activated by point mutations, translocations and amplifications. Beyond malignancies associated with JAK2 point mutations, those associated with translocations might be suitable for tyrosine kinase inhibitors, which merits prospective evaluation.     

Insight into the inhibitor discrimination by FLT3 F691L

Fms‐like tyrosine kinase 3 (FLT3) belongs to the receptor tyrosine kinase family and expressed in hematopoietic progenitor cells. FLT3 gene mutations are reported in ~30% of acute myeloid leukemia cases. FLT3 kinase domain mutation F691L is one of the common causes of acquired resistance to the FLT3 inhibitors including quizartinib. MZH29 and crenolanib were previously reported to inhibit FLT3 F691L. However, crenolanib was reported for the moderate inhibition. We found that Glu661and Asp829 were the most significant residues to target the FLT3 F691L which contribute most significantly to the binding energy with MZH29 and crenolanib. These interactions were found absent with quizartinib. Further free energy landscape analysis revealed that FLT3 F691L bound to MZH29 and crenolanib was more stable as compared to quizartinib.