In vivo treatment of mutant FLT3-transformed murine leukemia with a tyrosine kinase inhibitor

Comparison of the Kinase Profile of Midostaurin (Rydapt) with That of Its Predominant Metabolites and the Potential Relevance of Some Newly Identified Targets to Leukemia Therapy

The multitargeted protein kinase inhibitor midostaurin is approved for the treatment of both newly diagnosed FLT3-mutated acute myeloid leukemia (AML) and KIT-driven advanced systemic mastocytosis. AML is a heterogeneous malignancy, and investigational drugs targeting FLT3 have shown disparate effects in patients with FLT3-mutated AML, probably as a result of their inhibiting different targets and pathways at the administered doses. However, the efficacy and side effects of drugs do not just reflect the biochemical and pharmacodynamic properties of the parent compound but are often comprised of complex cooperative effects between the properties of the parent and active metabolites. Following chronic dosing, two midostaurin metabolites attain steady-state plasma trough levels greater than that of the parent drug. In this study, we characterized these metabolites and determined their profiles as kinase inhibitors using radiometric transphosphorylation assays. Like midostaurin, the metabolites potently inhibit mutant forms of FLT3 and KIT and several additional kinases that either are directly involved in the deregulated signaling pathways or have been implicated as playing a role in AML via stromal support, such as IGF1R, LYN, PDPK1, RET, SYK, TRKA, and VEGFR2. Consequently, a complex interplay between the kinase activities of midostaurin and its metabolites is likely to contribute to the efficacy of midostaurin in AML and helps to engender the distinctive effects of the drug compared to those of other FLT3 inhibitors in this malignancy.

Regulation of protein turnover by heat shock proteins

Protein turnover reflects the balance between synthesis and degradation of proteins, and it is a crucial process for the maintenance of the cellular protein pool. The folding of proteins, refolding of misfolded proteins, and also degradation of misfolded and damaged proteins are involved in the protein quality control (PQC) system. Correct protein folding and degradation are controlled by many different factors, one of the most important of which is the heat shock protein family. Heat shock proteins (HSPs) are in the class of molecular chaperones, which may prevent the inappropriate interaction of proteins and induce correct folding. On the other hand, these proteins play significant roles in the degradation pathways, including endoplasmic reticulum-associated degradation (ERAD), the ubiquitin-proteasome system, and autophagy. This review focuses on the emerging role of HSPs in the regulation of protein turnover; the effects of HSPs on the degradation machineries ERAD, autophagy, and proteasome; as well as the role of posttranslational modifications in the PQC system.

The role of quizartinib in the treatment of acute myeloid leukemia

INTRODUCTION

Approximately one-third of the patients with acute myeloid leukemia (AML) harbor internal tandem duplication (ITD) in the gene encoding FMS-like tyrosine kinase 3 (FLT3-ITD), which is associated with poor prognosis. Over the course of the last decade, several FLT3 inhibitors have been developed. Nevertheless, the pharmacokinetic limitations of some of these compounds as well as their potency have limited their therapeutic efficacy. Quizartinib (AC220) is a second-generation FLT3 inhibitor that has shown promising activity in AML in Phase II clinical trials.

AREAS COVERED

The pharmacokinetic, mechanism of action and resistance as well as clinical studies of quizartinib in AML are reported here in detail.

EXPERT OPINION

Quizartinib is potent and selective FLT3 tyrosine kinase inhibitor with significant activity in both FLT3-mutant and wild-type AML. The quality and duration of achievable response thus far seen with this agent is suboptimal. Quizartinib in combination with chemotherapy might result in improved outcome and results of these trials are eagerly awaited. In addition, quizartinib in combination with other agents tackling the bone marrow microenvironment and FLT3 cooperative pathways may enhance response to quizartinib.

Receptor tyrosine kinase alterations in AML - biology and therapy

Acute myeloid leukemia (AML) is the most common form of leukemia in adults, and despite some recent progress in understanding the biology of the disease, AML remains the leading cause of leukemia-related deaths in adults and children. AML is a complex and heterogeneous disease, often involving multiple genetic defects that promote leukemic transformation and drug resistance. The cooperativity model suggests that an initial genetic event leads to maturational arrest in a myeloid progenitor cell, and subsequent genetic events induce proliferation and block apoptosis. Together, these genetic abnormalities lead to clonal expansion and frank leukemia. The purpose of this chapter is to review the biology of receptor tyrosine kinases (RTKs) in AML, exploring how RTKs are being used as novel prognostic factors and potential therapeutic targets.

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.

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.

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

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

FLT3 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.

Hsp90 inhibition decreases mitochondrial protein turnover

Background

Cells treated with hsp90 inhibitors exhibit pleiotropic changes, including an expansion of the mitochondrial compartment, accompanied by mitochondrial fragmentation and condensed mitochondrial morphology, with ultimate compromise of mitochondrial integrity and apoptosis.

Findings

We identified several mitochondrial oxidative phosphorylation complex subunits, including several encoded by mtDNA, that are upregulated by hsp90 inhibitors, without corresponding changes in mRNA abundance. Post-transcriptional accumulation of mitochondrial proteins observed with hsp90 inhibitors is also seen in cells treated with proteasome inhibitors. Detailed studies of the OSCP subunit of mitochondrial F1F0-ATPase revealed the presence of mono- and polyubiquitinated OSCP in mitochondrial fractions. We demonstrate that processed OSCP undergoes retrotranslocation to a trypsin-sensitive form associated with the outer mitochondrial membrane. Inhibition of proteasome or hsp90 function results in accumulation of both correctly targeted and retrotranslocated mitochondrial OSCP.

Conclusions

Cytosolic turnover of mitochondrial proteins demonstrates a novel connection between mitochondrial and cytosolic compartments through the ubiquitin-proteasome system. Analogous to defective protein folding in the endoplasmic reticulum, a mitochondrial unfolded protein response may play a role in the apoptotic effects of hsp90 and proteasome inhibitors.

FLT3 regulates beta-catenin tyrosine phosphorylation, nuclear localization, and transcriptional activity in acute myeloid leukemia cells

Deregulated accumulation of nuclear beta-catenin enhances transcription of beta-catenin target genes and promotes malignant transformation. Recently, acute myeloid leukemia (AML) cells with activating mutations of FMS-like tyrosine kinase-3 (FLT3) were reported to display elevated beta-catenin-dependent nuclear signaling. Tyrosine phosphorylation of beta-catenin has been shown to promote its nuclear localization. Here, we examined the causal relationship between FLT3 activity and beta-catenin nuclear localization. Compared to cells with wild-type FLT3 (FLT3-WT), cells with the FLT3 internal tandem duplication (FLT3-ITD) and tyrosine kinase domain mutation (FLT3-TKD) had elevated levels of tyrosine-phosphorylated beta-catenin. Although beta-catenin was localized mainly in the cytoplasm in FLT3-WT cells, it was primarily nuclear in FLT3-ITD cells. Treatment with FLT3 kinase inhibitors or FLT3 silencing with RNAi decreased beta-catenin tyrosine phosphorylation and nuclear localization. Conversely, treatment of FLT3-WT cells with FLT3 ligand increased tyrosine phosphorylation and nuclear accumulation of beta-catenin. Endogenous beta-catenin co-immunoprecipitated with endogenous activated FLT3, and recombinant activated FLT3 directly phosphorylated recombinant beta-catenin. Finally, FLT3 inhibitor decreased tyrosine phosphorylation of beta-catenin in leukemia cells obtained from FLT3-ITD-positive AML patients. These data demonstrate that FLT3 activation induces beta-catenin tyrosine phosphorylation and nuclear localization, and thus suggest a mechanism for the association of FLT3 activation and beta-catenin oncogeneic signaling in AML.

A Novel FLT3 Inhibitor FI-700 Selectively Suppresses the Growth of Leukemia Cells with FLT3 Mutations

PURPOSE

The aim of this study was to evaluate the antileukemia activity of a novel FLT3 kinase inhibitor, FI-700.

EXPERIMENTAL DESIGN

The antileukemia activity of FI-700 was evaluated in human leukemia cell lines, mutant or wild-type (Wt)-FLT3-expressing mouse myeloid precursor cell line, 32D and primary acute myeloid leukemia cells, and in xenograft or syngeneic mouse leukemia models.

RESULTS

FI-700 showed a potent IC(50) value against FLT3 kinase at 20 nmol/L in an in vitro kinase assay. FI-700 showed selective growth inhibition against mutant FLT3-expressing leukemia cell lines and primary acute myeloid leukemia cells, whereas it did not affect the FLT3 ligand (FL)-driven growth of Wt-FLT3-expressing cells. These antileukemia activities were induced by the significant dephosphorylations of mutant FLT3 and STAT5, which resulted in G(1) arrest of the cell cycle. Oral administration of FI-700 induced the regression of tumors in a s.c. tumor xenograft model and increased the survival of mice in an i.v. transplanted model. Furthermore, FI-700 treatment eradicated FLT3/ITD-expressing leukemia cells, both in the peripheral blood and in the bone marrow. In this experiment, the depletion of FLT3/ITD-expressing cells by FI-700 was more significant than that of Ara-C, whereas bone marrow suppression by FI-700 was lower than that by Ara-C.

CONCLUSIONS

FI-700 is a novel and potent FLT3 inhibitor with promising antileukemia activity.

FLT3 inhibition in acute myeloid leukaemia

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase that appears to play a significant role in leukaemogenesis. Activating mutations of FLT3 are present in approximately one-third of acute myeloid leukaemia patients and are associated with adverse clinical outcome, while many non-mutated cases also show evidence of FLT3 activation. FLT3 thus represents a potentially exciting molecular therapeutic target. A number of small-molecule tyrosine kinase inhibitors with anti-FLT3 activity have been developed and several of these compounds have entered early phase clinical trials where clinical anti-leukaemic activity has been demonstrated. The depth and duration of clinical responses to FLT3 inhibitor monotherapy have been modest, however, and a number of mechanisms by which blasts may acquire resistance have been proposed. Based on preclinical evidence of synergy with conventional chemotherapy, several combination trials are now underway. FLT3 inhibition may also be effective used in combination with other molecularly targeted agents, in postchemotherapy stem-cell-directed maintenance therapy and in MLL-rearranged infant acute lymphoblastic leukaemia.

Pediatric oncology

Intensive use of cytotoxic agents in multimodality therapeutic regimens has resulted in almost 80% five-year disease-free survival and cure in the majority of childhood cancer patients. However, such success has come at the expense of severe acute or delayed toxicities and an increased occurrence of secondary cancers. With an increasing understanding of the genetic changes that underlie transformation in childhood cancer, rational approaches using agents that target these transforming events are being developed. Current and future strategies in developing tumor-selective therapy using inhibitors of signaling pathways dysregulated in leukemias (FLT3, NOTCH1) and solid/brain tumors (ErbB1-4, IGF-IR, PTCH1), and the challenges in developing less toxic, but equally effective treatments in pediatric oncology are presented.

The emerging role of Wnt signaling in the pathogenesis of acute myeloid leukemia

Wnt signaling plays an important role in stem cell self-renewal and proliferation. Aberrant activation of Wnt signaling and its downstream targets are intimately linked with several types of cancer with colon cancer being the best-studied example. However, recent results also suggest an important role of Wnt signaling in normal as well as leukemic hematopoietic stem cells. Aberrant activation of Wnt signaling and downstream effectors has been demonstrated in acute myeloid leukemia. Here, mutant receptor tyrosine kinases, such as Flt3 and chimeric transcription factors such as promyelocytic leukemia-retinoic acid receptor-alpha and acute myeloid leukemia1-ETO, induce downstream Wnt signaling events. These findings suggest that the Wnt signaling pathway is an important target in several leukemogenic pathways and may provide a novel opportunity for targeting leukemic stem cells.

Lyn is an important component of the signal transduction pathway specific to FLT3/ITD and can be a therapeutic target in the treatment of AML with FLT3/ITD

Fms-like tyrosine kinase 3 (FLT3) is expressed in hematopoietic progenitor cells. An internal tandem duplication (ITD) of FLT3 (FLT3/ITD) is the most frequent mutation in human adult acute myeloid leukemia (AML). FLT3/ITD contributes to the constitutive activation of FLT3 itself and its downstream signal components, mitogen-activated protein kinase and signal transducers and activators of transcription 5 (STAT5), and enables interleukin (IL)-3-dependent cell lines to grow autonomously. In the present study, we showed the specific association of FLT3/ITD with Lyn, which led to the phosphorylation of Lyn in vivo. We also demonstrated that FLT3/ITD receptors displayed a higher affinity to bind to Lyn than wild-type FLT3 receptors in vitro and that this affinity was relative to the intensity of tyrosil phosphorylation of the receptor. Both treatment with small interfering RNA (siRNA) targeting Lyn and the Src family kinase inhibitor PP2 suppressed the IL-3-independent growth of FLT3/ITD-expressing 32D cells (FLT3/ITD-32D), reducing the constitutive phosphorylation of Lyn and STAT5. PP2 treatment of mice transplanted with FLT3/ITD-32D cells blocked the onset of tumors and decreased the size of established tumors. These results demonstrate that Lyn is an important component of the signal transduction pathway specific to FLT3/ITD and can be a therapeutic target in the treatment of AML with FLT3/ITD.

Emerging Flt3 kinase inhibitors in the treatment of leukaemia

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

Biology, clinical relevance, and molecularly targeted therapy in acute leukemia with FLT3 mutation

Overexpression and activating mutations of receptor tyrosine kinases (RTKs) are known to be involved in the pathophysiology of several kinds of cancer cells. FMS-like receptor tyrosine kinase 3 (FLT3), together with KIT, FMS, and platelet-derived growth factor receptor, is a class III RTK. FLT3 mutations were first reported as internal tandem duplication (FLT3/ITD) of the juxtamembrane domain-coding sequence; subsequently, a missense point mutation at the D835 residue and point mutations, deletions, and insertions in the codons surrounding D835 within a FLT3 tyrosine kinase domain (FLT3/KDMs) have been found. FLT3 mutations are the most frequent genetic alterations so far reported in acute myeloid leukemia and are involved in the signaling pathway of autonomous proliferation and differentiation block in leukemia cells. Several large-scale studies have confirmed that FLT3/ITD is strongly associated with leukocytosis and a poor prognosis. Therefore, routine screening for FLT3 mutations is recommended to stratify patients into distinct risk groups. However, because high-dose chemotherapy and stem cell transplantation cannot overcome the adverse effects of FLT3 mutations, the development of FLT3 kinase inhibitors is expected to produce a more efficacious therapeutic strategy for leukemia therapy.

Gene expression profiling data in lymphoma and leukemia: review of the literature and extrapolation of pertinent clinical applications

CONTEXT

Gene expression (GE) analyses using microarrays have become an important part of biomedical and clinical research in hematolymphoid malignancies. However, the methods are time-consuming and costly for routine clinical practice.

OBJECTIVES

To review the literature regarding GE data that may provide important information regarding pathogenesis and that may be extrapolated for use in diagnosing and prognosticating lymphomas and leukemias; to present GE findings in Hodgkin and non-Hodgkin lymphomas, acute leukemias, and chronic myeloid leukemia in detail; and to summarize the practical clinical applications in tables that are referenced throughout the text.

DATA SOURCE

PubMed was searched for pertinent literature from 1993 to 2005.

CONCLUSIONS

Gene expression profiling of lymphomas and leukemias aids in the diagnosis and prognostication of these diseases. The extrapolation of these findings to more timely, efficient, and cost-effective methods, such as flow cytometry and immunohistochemistry, results in better diagnostic tools to manage the diseases. Flow cytometric and immunohistochemical applications of the information gained from GE profiling assist in the management of chronic lymphocytic leukemia, other low-grade B-cell non-Hodgkin lymphomas and leukemias, diffuse large B-cell lymphoma, nodular lymphocyte-predominant Hodgkin lymphoma, and classic Hodgkin lymphoma. For practical clinical use, GE profiling of precursor B acute lymphoblastic leukemia, precursor T acute lymphoblastic leukemia, and acute myeloid leukemia has supported most of the information that has been obtained by cytogenetic and molecular studies (except for the identification of FLT3 mutations for molecular analysis), but extrapolation of the analyses leaves much to be gained based on the GE profiling data.

Antineoplastic effect of a single oral dose of the novel Flt3 inhibitor KRN383 on xenografted human leukemic cells harboring Flt3-activating mutations

Activating mutations of Fms-like tyrosine kinase 3 (Flt3) are the most common genetic abnormalities found in acute myeloid leukemia (AML) and represent potential therapeutic targets. The novel Flt3 inhibitor KRN383 inhibited the autophosphorylation of Flt3 bearing internal tandem duplications (ITDs) and the Asp835Tyr (D835Y) point mutation with half-maximal inhibitory concentration (IC(50)) values of < or =5.9 and 43 nM, respectively. KRN383 also inhibited the proliferation of the ITD-positive cell lines with IC(50) values of < or =2.9 nM. A single oral administration of 80 mg/kg of KRN383 eradicated ITD-positive xenograft tumors in nude mice and prolonged the survival of SCID mice carrying ITD-positive AML cells. The effectiveness of a single oral dose of KRN383 suggests that it has the potential to be used in a wide variety of clinical regimens, including multicycle and combination therapies.

Constitutive activation of Akt by Flt3 internal tandem duplications is necessary for increased survival, proliferation, and myeloid transformation

Up to 30% of patients with acute myeloid leukemia (AML) harbor internal tandem duplications (ITD) within the FLT3 gene, encoding a receptor tyrosine kinase. These mutations induce constitutive tyrosine kinase activity in the absence of the natural Flt3 ligand and confer growth factor independence, increased proliferation, and survival to myeloid precursor cells. The signaling pathways and downstream nuclear targets mediating leukemic transformation are only partly identified. Here, we show that the presence of Flt3-ITD constitutively activates Akt (PKB), a key serine-threonine kinase within the phosphatidylinositol 3-kinase pathway. Constitutive activation of Akt phosphorylated and inhibited the transcription factor Foxo3a. Restored Foxo3a activity reversed Flt3-ITD-mediated growth properties and dominant-negative Akt prevented Flt3-ITD-mediated cytokine independence. Conditional Akt activation targeted to the cell membrane induced cytokine-independent survival, cell cycle progression, and proliferation. Importantly, Akt activation was sufficient to cause in vitro transformation of 32D myeloid progenitor cells and in vivo promoted the development of a leukemia-like myeloid disease. Akt phosphorylation was found in myeloid blasts of 86% of AML patients, suggesting an important role in leukemogenesis. In summary, Akt is necessary for increased survival, proliferation, and leukemic transformation by Flt3-ITD, possibly by inactivation of Foxo transcription factors. These findings indicate that Akt and Foxo transcription factors are attractive targets for therapeutic intervention in AML.

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

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

Flt3 tandem duplication mutations cooperate with Wnt signaling in leukemic signal transduction

Activating Flt3 mutations occur in about 30% of patients with acute myeloid leukemia (AML), often as in-frame internal tandem duplication (ITD) at the juxtamembrane domain of the receptor. These mutations transform hematopoietic cell lines and primary mouse bone marrow. Here, we analyzed the interaction between oncogenic Flt3-ITD mutations and the Wingless-type (Wnt) signaling pathway in the myeloid progenitor cell line 32D. Microarray analyses revealed higher mRNA expression of Frizzled-4, a receptor for Wnt ligands in 32D/Flt3-ITD cells. Findings were verified by quantitative realtime reverse transcription–polymerase chain reaction (RT-PCR) and on the protein level. Compared with 32D/Flt3-WT (wild-type) cells, 32D/Flt3-ITD cells also showed greatly enhanced β-catenin protein levels, irrespective of their exposure to Wnt3a, a ligand inducing the canonical Wnt signal transduction pathway. In addition, 5 of 7 AML samples with Flt3-ITD mutations expressed high β-catenin protein levels, whereas patients with wild-type Flt3 did not. Also, Flt3-ITD induced enhanced T-cell factor (TCF)–dependent transcriptional activity and the induction of the Wnt target gene c-myc. In the presence of Flt3-WT or Flt3-ITD signaling, Wnt3a slightly increased 32D cell proliferation. However, transfection experiments with dominant-negative (dn) TCF4 revealed a strong dependence of Flt3-ITD–mediated clonogenic growth on TCF activity. Taken together, our results indicate that Flt3-ITD and Wnt-dependent signaling pathways synergize in myeloid transformation.

FLT3/ITD mutation signaling includes suppression of SHP-1

Mutations in the FLT3 gene are the most common genetic alteration found in AML patients. FLT3 internal tandem duplication (ITD) mutations result in constitutive activation of FLT3 tyrosine kinase activity. The consequences of this activation are an increase in total phosphotyrosine content, persistent downstream signaling, and ultimately transformation of hematopoietic cells to factor-independent growth. The Src homology (SH)2 domain-containing protein-tyrosine phosphatase (SHP)-1 is involved in the down-regulation of a broad range of growth factor and cytokine-driven signaling cascades. Loss-of-function or deficiency of SHP-1 activity results in a hyperproliferative response of myelomonocytic cell populations to growth factor stimulation. In this study, we examined the possible role of SHP-1 in regulating FLT3 signaling. We found that transformation of TF-1 cells with FLT3/ITD mutations suppressed the activity of SHP-1 by approximately 3-fold. Suppression was caused by decreased SHP-1 protein expression, as analyzed at both the protein and RNA levels. In contrast, protein levels of SHP-2, a phosphatase that plays a stimulatory role in signaling through a variety of receptors, did not change significantly in FLT3 mutant cells. Suppressed SHP-1 protein levels in TF-1/ITD cells were partially overcome after cells were exposed to CEP-701, a selective FLT3 inhibitor. SHP-1 protein levels also increased in naturally occurring FLT3/ITD expressing AML cell lines and in primary FLT3/ITD AML samples after CEP-701 treatment. Furthermore, a small but reproducible growth/survival advantage was observed in both TF-1 and TF-1/ITD cells when SHP-1 expression was knocked down by RNAi. Taken together, these data provide the first evidence that suppression of SHP-1 by FLT3/ITD signaling may be another mechanism contributing to the transformation by FLT3/ITD mutations.

The expression of P-glycoprotein in AML cells with FLT3 internal tandem duplications is associated with reduced apoptosis in response to FLT3 inhibitors

P-glycoprotein (pgp), a membrane efflux pump, is recognized to have an anti-apoptotic function. Internal tandem duplications (ITDs) of the Fms-like tyrosine kinase 3 (FLT3) receptor are the most common mutations in acute myeloid leukaemia (AML). Both ITDs and pgp positivity confer an adverse clinical prognosis. FLT3 inhibitors induce variable apoptosis in cell lines transfected with FLT3 ITDs. We studied the effect of herbimycin A, AG1296 and PKC412 on primary AML blasts. All compounds showed significantly higher cell kill after 48-h incubation in samples with an ITD compared with wild type (Herbimicin P < 0.001; AG1296 P = 0.001, PKC412, P = 0.002). Pgp-positive samples were significantly less sensitive to herbimycin and AG1296 than pgp-negative samples, although neither molecule inhibited the efflux function of pgp. The concurrent incubation with the pgp inhibitor PSC833 resulted in an enhanced cell kill in 4/5 ITD pgp-positive samples versus two of nine ITD pgp-negative samples. PKC412 inhibited pgp function and induced cell death in FLT3 ITD/pgp-positive samples. We conclude that AML samples with a FLT3 ITD are more susceptible to these inhibitors than wild-type samples. However, the expression of pgp in cells with FLT3 ITDs can reduce their sensitivity to FLT3 inhibitors and therefore pgp expression should be assessed in clinical trials of FLT3 inhibitors.

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

The FLT3 receptor is activated by juxtamembrane insertion mutations and by activation loop point mutations in patients with acute myeloid leukemia (AML). In a systematic tyrosine kinase gene exon resequencing study, 21 of 24 FLT3 exons were sequenced in samples from 53 patients with AML, 9 patients with acute lymphoblastic leukemia (ALL), and 3 patients with myelodysplasia samples. Three patients had novel point mutations at residue N841 that resulted in a change to isoleucine in 2 samples and to tyrosine in 1 sample. Introduction of FLT3-N841I cDNA into Ba/F3 cells led to interleukin-3 (IL-3)–independent proliferation, receptor phosphorylation, and constitutive activation of signal transducer and activator of transcription 5 (STAT5) and extracellular regulatory kinase (ERK), suggesting that the N841I mutation confers constitutive activity to the receptor. An FLT3 inhibitor (PKC412) inhibited the growth of Ba/F3-FLT3N841I cells (IC50 10 nM), but not of wild-type Ba/F3 cells cultured with IL-3. PKC412 also reduced tyrosine phosphorylation of the mutant receptor and inhibited STAT5 phosphorylation. Examination of the FLT3 autoinhibited structure showed that N841 is the key residue in a hydrogen-bonding network that likely stabilizes the activation loop. These results suggest that mutations at N841 represent a significant new activating mutation in patients with AML and that patients with such mutations may respond to small-molecule FLT3 inhibitors such as PKC412.

After chronic myelogenous leukemia: tyrosine kinase inhibitors in other hematologic malignancies

Tyrosine kinases phosphorylate proteins on tyrosine residues, producing a biologic signal that influences many aspects of cellular function including cell growth, proliferation, differentiation, and death. Constitutive or unregulated activity through mutation or overexpression of these enzymes is a common pathologic feature in many acute and chronic leukemias. Inhibition of tyrosine kinases represents a strategy to disrupt signaling pathways that promote neoplastic growth and survival in hematologic malignancies and likely in other neoplasias as well. This review focuses on tyrosine kinases that have been implicated in the pathogenesis of hematologic diseases other than chronic myelogenous leukemia and discusses the evidence for the use of small molecules to target these kinases.

Variable sensitivity of FLT3 activation loop mutations to the small molecule tyrosine kinase inhibitor MLN518

FLT3 is constitutively activated by internal tandem duplications (ITDs) in the juxtamembrane domain or by activation loop mutations in acute myeloid leukemia (AML). We tested the sensitivity of 8 activation loop mutations to the small molecule FLT3 inhibitor, MLN518. Each FLT3 activation loop mutant, including D835Y, D835A, D835E, D835H, D835N, D835V, D835del, and I836del, transformed Ba/F3 cells to factor-independent proliferation and had constitutive tyrosine kinase activation, as assessed by FLT3 autophosphorylation and activation of downstream effectors, including STAT5 and ERK. MLN518 inhibited FLT3 autophosphorylation and phosphorylation of STAT5 and ERK in FLT3-ITD-transformed Ba/F3 cells with an IC(50) (50% inhibition of cell viability) of approximately 500 nM. However, there was a broad spectrum of sensitivity among the 8 activation loop mutants, with IC(50) ranging from approximately 500 nM to more than 10 microM for the inhibition of phosphorylation of FLT3, STAT5, and ERK. The relative sensitivity of the mutants to MLN518 in biochemical assays correlated with the cellular IC(50) for cytokine-independent proliferation of FLT3-transformed Ba/F3 cells in the presence of MLN518. Thus, certain activation loop mutations in FLT3 simultaneously confer resistance to small molecule inhibitors. These findings have implications for the evaluation of responses in clinical trials with FLT3 inhibitors and provide a strategy to screen for compounds that can overcome resistance.

FLT3 Inhibitors

The area of molecularly-targeted cancer therapeutics is generating tremendous interest and excitement. While clinical investigation of these agents has been largely limited to adults, clinical trials for paediatric cancer patients with many of these agents are now underway. This paper reviews the current status of molecularly-targeted therapies for paediatric malignancies, with special attention given to one class of agents, inhibitors of the FLT3 receptor tyrosine kinase. FLT3 is expressed and activated in many human leukemias, including a significant percentage of pediatric AML and infant and childhood ALL, especially in the setting of MLL gene rearrangement. Activating mutations of FLT3 portend a poor prognosis in pediatric AML. Activated FLT3 can be effectively and selectively targeted by small molecule inhibitors, and these agents have shown promise in early phase clinical trials in adults with AML. Limited preclinical data with FLT3 inhibitors in MLL-rearranged ALL have also been reported. Challenges and future directions for the use of FLT3 inhibitors and other targeted agents in paediatric cancer are discussed.

FLT3 receptors with internal tandem duplications promote cell viability and proliferation by signaling through Foxo proteins

In about 30% of the patients with acute myeloid leukemia, activating FLT3 receptor mutations have been identified, often as in-frame internal tandem duplications (ITD) at the juxtamembrane domain of the receptor. FLT3-ITD receptors exhibit constitutive tyrosine kinase activity in the absence of FLT3 ligand (FL) binding, and when expressed in cytokine-dependent cell lines and primary hematopoietic cells suppress programmed cell death and increase cell division. However, the signaling pathways important for transformation, in particular the nuclear targets, are unknown. Here we demonstrate that FLT3-ITD expression in Ba/F3 cells results in activation of Akt and concomitant phosphorylation of the Forkhead family member Foxo3a. Phosphorylation of Foxo proteins through FLT3-ITD signaling promotes their translocation from the nucleus into the cytoplasm, which requires the presence of conserved Akt phosphorylation sites in Forkhead transcription factors and PI3K activity. Induction of Foxo3a phosphorylation by FLT3-ITD receptors in Ba/F3 cells correlates with the suppression of Foxo-target genes p27Kip1 and the proapoptotic Bcl-2 family member Bim. Specifically, FLT3-ITD expression prevents Foxo3a-mediated apoptosis and upregulation of p27Kip1 and Bim gene expression. These data indicate that the oncogenic tyrosine kinase FLT3 can negatively regulate Foxo transcription factors, thereby promoting cell survival and proliferation.

Targeted therapies in myeloid leukemia

Targeted therapies for hematological malignancies have come of age since the advent of all trans retinoic acid (ATRA) for treating APL and STI571/Imatinib Mesylate/Gleevec for CML. There are good molecular targets for other malignancies and several new drugs are in clinical trials. In this review, we will concentrate on individual abnormalities that exist in the myelodysplastic syndromes (MDS) and myeloid leukemias that are targets for small molecule therapies (summarised in Fig. 1). We will cover fusion proteins that are produced as a result of translocations, including BCR-ABL, the FLT3 tyrosine kinase receptor and RAS. Progression of diseases such as MDS to secondary AML occur as a result of changes in the balance between cell proliferation and apoptosis and we will review targets in both these areas, including reversal of epigenetic silencing of genes such as p15(INK4B).

The role of FLT3 in haematopoietic malignancies

Normal haematopoietic cells use complex systems to control proliferation, differentiation and cell death. The control of proliferation is, in part, accomplished through the ligand-induced stimulation of receptor tyrosine kinases, which signal to downstream effectors through the RAS pathway. Recently, mutations in the FMS-like tyrosine kinase 3 (FLT3) gene, which encodes a receptor tyrosine kinase, have been found to be the most common genetic lesion in acute myeloid leukaemia (AML), occurring in approximately 25% of cases. Exploring the mechanism by which these FLT3 mutations cause uncontrolled proliferation might lead to a better understanding of how cells become cancerous and provide insights for the development of new drugs.

Therapeutic efficacy of prenylation inhibitors in the treatment of myeloid leukemia

Farnesyltransferase inhibitors (FTIs) represent a new class of anticancer agents that specifically target post-translational farnesylation of various proteins that mediate several cellular processes such as signal transduction, growth, differentiation, angiogenesis and apoptosis. These compounds were originally designed to block oncogenic RAS-induced tumor growth by impeding RAS localization to the membrane, but it is now evident that FTIs also affect processing of several other proteins. The need for novel therapies in myeloid leukemia is underscored by the high rate of treatment failure due to high incidences of relapse- and treatment-related toxicities. As RAS deregulation is important in the pathogenesis of myeloid leukemias, targeting of RAS signaling may provide a new therapeutic strategy. Several FTIs (eg BMS-214662, L-778,123, R-115777 and SCH66336) have entered phase I and phase II clinical trials in myeloid leukemias. This review discusses recent clinical results, potential combination therapies, mechanisms of resistance and the clinical challenges of toxicities associated with prenylation inhibitors.

The use of genetic microarray analysis to classify and predict prognosis in haematological malignancies

The introduction of microarrays offers the opportunity to examine the expression of many thousands of genes in a single experiment. Investigations in leukaemia and lymphoma have led to the identification of a number of subgroups, with a defined gene expression pattern, not previously identified by morphology, cytogenetics or molecular techniques. In many cases these expression patterns can be linked to the tumour cells normal developmental counterpart, and represent distinct disease subgroups with different clinical presentations and outcomes. The technology has also identified genes that may be important in tumour cell biology including key genes in cell proliferation, adhesion, apoptosis, and the development of drug resistance. These early studies demonstrate that genetic microarrays will be useful in classifying haematological malignancies, predicting response to treatment, predicting prognosis, and identifying novel targets for therapy.

Requirement for the PI3K/Akt pathway in MEK1-mediated growth and prevention of apoptosis: identification of an Achilles heel in leukemia

The Raf/MEK/ERK kinase cascade plays a critical role in transducing growth signals from activated cell surface receptors. Using DeltaMEK1:ER, a conditionally active form of MEK1 which responds to either beta-estradiol or the estrogen receptor antagonist 4 hydroxy-tamoxifen (4HT), we previously documented the ability of this dual specificity protein kinase to abrogate the cytokine-dependency of human (TF-1) and murine (FDC-P1 and FL5.12) hematopoietic cells lines. Here we demonstrate the ability of DeltaMEK1:ER to activate the phosphatidylinositol 3-kinase (PI3K)/Akt/p70 ribosomal S6 kinase (p70(S6K)) pathway and the importance of this pathway in MEK1-mediated prevention of apoptosis. MEK1-responsive cells can be maintained long term in the presence of beta-estradiol, 4HT or IL-3. Removal of hormone led to the rapid cessation of cell proliferation and the induction of apoptosis in a manner similar to cytokine deprivation of the parental cells. Stimulation of DeltaMEK1:ER by 4HT resulted in ERK, PI3K, Akt and p70(S6K) activation. Treatment with PI3K, Akt and p70(S6K) inhibitors prevented MEK-responsive growth. Furthermore, the apoptotic effects of PI3K/Akt/p70(S6K) inhibitors could be enhanced by cotreatment with MEK inhibitors. Use of a PI3K inhibitor and a constitutively active form of Akt, [DeltaAkt(Myr(+))], indicated that activation of PI3K was necessary for MEK1-responsive growth and survival as activation of Akt alone was unable to compensate for the loss of PI3K activity. Cells transduced by MEK or MEK+Akt displayed different sensitivities to signal transduction inhibitors, which targeted these pathways. These results indicate a requirement for the activation of the PI3K pathway during MEK-mediated transformation of certain hematopoietic cells. These experiments provide important clues as to why the identification of mutant signaling pathways may be the Achilles heel of leukemic cell growth. Leukemia treatment targeting multiple signal transduction pathways may be more efficacious than therapy aimed at inhibiting a single pathway.

Prognostic implications of the presence of FLT3 mutations in patients with acute myeloid leukemia

Several studies have shown that mutations in the FLT3 gene are common events in AML, with approximately one third of adult patients harbouring either an internal tandem duplication in the juxtramembrane domain or a D835 mutation in the kinase domain. The majority of studies in pediatric and adult AML have shown that FLT3 mutations are powerful prognostic factors predicting for increased relapse risk and adverse overall survival. Some reports have suggested that loss of the wild type allele might be associated with an even worse prognosis. Changes in the pattern of FLT3 mutations between disease presentation and relapse restrict their value as a marker of minimal residual disease, and have significant implications for therapy. The optimum treatment for patients with FLT3 mutations remains unknown and large prospective studies are warranted to evaluate the efficacy of various treatment modalities such as bone marrow transplantation and targeted therapy with tyrosine kinase inhibitors.

Molecular targets in acute myelogenous leukemia

Acute myeloid leukemia (AML) remains the most common form of leukemia and the most common cause of leukemia death. Although conventional chemotherapy can cure between 25 and 45% of AML patients, most patients will either die of relapse or die from the complications associated with treatment. Thus, more specific and less toxic treatments for AML patients are needed. Recently, a small molecular inhibitor (STI571 or Gleevec) that targets the BCR-ABL gene was found to have a dramatic clinical effect in patients with chronic myelogenous leukemia (CML). These results have encouraged investigators to search for additional small molecular inhibitors and other targeted therapies that may be applicable to other forms of leukemia. In this review, we examine some of the signaling pathways that are aberrantly regulated in AML, focusing on the tyrosine kinase/RAS/MAP kinase and JAK/STAT pathways. After reviewing these two pathways, we explore some of the targeted therapies directed at these pathways that are under development for AML, many of which are already in clinical trials.

SU5416 and SU5614 inhibit kinase activity of wild-type and mutant FLT3 receptor tyrosine kinase

Internal tandem duplication (ITD) in the juxtamembrane portion of Fms-like tyrosine kinase 3 (FLT3), a type III receptor tyrosine kinase (RTK), is the most common molecular defect associated with acute myeloid leukemia (AML). The high prevalence of this activating mutation makes it a potential target for molecularly based therapy. Indolinone tyrosine kinase inhibitors have known activity against KIT, another member of the type III RTK family. Given the conserved homology between members of this family, we postulated that the activity of some KIT inhibitors would extend to FLT3. We used various leukemic cell lines (BaF3, MV 4-11, RS 4;11) to test the activity of indolinone compounds against the FLT3 kinase activity of both wild-type (WT) and ITD isoforms. Both SU5416 and SU5614 were capable of inhibiting autophosphorylation of ITD and WT FLT3 (SU5416 concentration that inhibits 50% [IC(50)], 100 nM; and SU5614 IC(50) 10 nM). FLT3-dependent activation of the downstream signaling proteins mitogen-activated protein kinase (MAPK) and signal transducer and activator of transcription 5 (STAT5) was also inhibited by treatment in the same concentration ranges. FLT3 inhibition by SU5416 and SU5614 resulted in reduced proliferation (IC(50), 250 nM and 100 nM, respectively) and induction of apoptosis of FLT3 ITD-positive leukemic cell lines. Treatment of these cells with an alternative growth factor (granulocyte-macrophage colony-stimulating factor [GM-CSF]) restored MAPK signaling and cellular proliferation, demonstrating specificity of the observed inhibitory effects. We conclude that SU5416 and SU5614 are potent inhibitors of FLT3. Our finding that inhibition of FLT3 induces apoptosis of leukemic cells supports the feasibility of targeting FLT3 as a novel treatment strategy for AML.

Studies of FLT3 mutations in paired presentation and relapse samples from patients with acute myeloid leukemia: implications for the role of FLT3 mutations in leukemogenesis, minimal residual disease detection, and possible therapy with FLT3 inhibitors

FLT3 mutations, either internal tandem duplications (ITDs) or aspartate residue 835 (D835) point mutations, are present in approximately one third of patients with acute myeloid leukemia (AML) and have been associated with an increased relapse rate. We have studied FLT3 mutations in paired presentation and relapse samples to ascertain the biology of these mutations and to evaluate whether they can be used as markers of minimal residual disease. At diagnosis, 24 patients were wild-type FLT3, and 4 acquired a FLT3 mutation at relapse (2 D835(+), 2 ITD(+)), with a further patient acquiring an ITD at second relapse. Of 20 patients positive at diagnosis (18 ITD(+), 2 D835(+)), 5 who were all originally ITD(+) had no detectable mutation at relapse, as determined by a sensitive radioactive polymerase chain reaction. One of these patients had acquired an N-Ras mutation not detectable at presentation. Furthermore, another patient had a completely different ITD at relapse, which could not be detected in the presentation sample. These results indicate that FLT3 mutations are secondary events in leukemogenesis, are unstable, and thus should be used cautiously for the detection of minimal residual disease.

Bis(1H-2-indolyl)-1-methanones as inhibitors of the hematopoietic tyrosine kinase Flt3

Aberrant expression and activating mutations of the class III receptor tyrosine kinase Flt3 (Flk-2, STK-1) have been linked to poor prognosis in acute myeloid leukemia (AML). Inhibitors of Flt3 tyrosine kinase activity are, therefore, of interest as potential therapeutic compounds. We previously described bis(1H-2-indolyl)-1-methanones as a novel class of selective inhibitors for platelet-derived growth factor receptors (PDGFR). Several bis(1H-2-indolyl)-1-methanone derivatives, represented by the compounds D-64406 and D-65476, are also potent inhibitors of Flt3. They inhibit proliferation of TEL-Flt3-transfected BA/F3 cells with IC(50) values of 0.2-0.3 microM in the absence of IL-3 but >10 microM in the presence of IL-3. Ligand-stimulated autophosphorylation of Flt3 in EOL-1 cells and corresponding downstream activation of Akt/PKB are effectively inhibited by bis(1H-2-indolyl)-1-methanones whereas autophosphorylation of c-Kit/SCF receptor or c-Fms/CSF-1 receptor is less sensitive or insensitive, respectively. Flt3 kinase purified by different methods is potently inhibited in vitro, demonstrating a direct mechanism of inhibition. 32D cells, expressing a constitutively active Flt3 variant with internal tandem duplication are greatly sensitized to radiation-induced apoptosis in the presence of D-64406 or D-65476 in the absence but not in the presence of IL-3. Thus, bis(1H-2-indolyl)-1-methanones are potential candidates for the treatment of Flt3-driven leukemias.

FLT3 in human hematologic malignancies

FLT3, a member of the receptor tyrosine kinase (RTK) class III, is preferentially expressed on the surface of a high proportion of acute myeloid leukemia (AML) and B-lineage acute lymphocytic leukemia (ALL) cells in addition to hematopoietic stem cells, brain, placenta and liver. An interaction of FLT3 and its ligand has been shown to play an important role in the survival, proliferation and differentiation of not only normal hematopoetic cells but also leukemia cells. Mutations of the FLT3 gene was first reported as an internal tandem duplication (ITD) of the juxtamembrane (JM) domain-coding sequence, subsequently as a missense mutation of D835 within a kinase domain. ITD- and D835-mutations are essentially found in AML and their frequencies are approximately 20 and 6% of adults with AML, respectively. Thus, mutation of the FLT3 gene is so far the most frequent genetic alteration reported to be involved in AML. Several large-scale studies in well-documented patients published to date have demonstrated that ITD-mutation is strongly associated with leukocytosis and a poor prognosis. In this review, we summarize the clinical and biological significance of FLT3-mutations and discuss the possibility of targeting FLT3 kinase for the treatment of leukemia.

Selective apoptosis of tandemly duplicated FLT3-transformed leukemia cells by Hsp90 inhibitors

An internal tandem duplication of the juxtamembrane (JM) domain of FLT3, a family of ligand-activated receptor tyrosine kinases, has been found in 20% of cases of acute myeloid leukemia (AML), and this mutation is correlated with leukocytosis and a poor prognosis. As a therapeutic approach, we previously reported that herbimycin A (HA) inhibited the growth of tandemly duplicated FLT3 (TDFLT3)-transformed cells (Leukemia 2000; 14: 374). Here, we have investigated the mechanism behind the cytotoxicity of HA, an ansamycin derivative which is now known to target Hsp90. The treatment with HA or another Hsp90 inhibitor, radicicol, induced selective apoptosis in TDFLT3-transformed 32D cells (TDFLT3/32D). The tyrosine-phosphorylation of TDFLT3 was inhibited by HA, whereas FLT3 ligand-induced phosphorylation of wild-type FLT3 (WtFLT3) was not. The downstream signal molecules MAPK, Akt and STAT5a were also dephosphorylated by HA in TDFLT3/32D. Immunoprecipitation analysis showed that TDFLT3 but not WtFLT3 formed a complex with Hsp90, and that the HA treatment dissociated TDFLT3 from the Hsp90 chaperone complex. These findings imply that targeting of Hsp90 will facilitate the development of anti-TDFLT3 therapy, and that Hsp90 is closely involved in the oncogenic activation of FLT3.

Acute myeloid leukemia

Through the hard work of a large number of investigators, the biology of acute myeloid leukemia (AML) is becoming increasingly well understood, and as a consequence, new therapeutic targets have been identified and new model systems have been developed for testing novel therapies. How these new therapies can be most effectively studied in the clinic and whether they will ultimately improve cure rates are questions of enormous importance. In this article, Dr. Jacob Rowe presents a summary of the current state-of-the-art therapy for adult AML. His contribution emphasizes the fact that AML is not a single disease, but a number of related diseases each distinguished by unique cytogenetic markers which in turn help determine the most appropriate treatment. Dr. Jerald Radich continues on this theme, emphasizing how these cytogenetic abnormalities, as well as other mutations, give rise to abnormal signal transduction and how these abnormal pathways may represent ideal targets for the development of new therapeutics. A third contribution by Dr. Frederick Appelbaum describes how AML might be made the target of immunologic attack. Specifically, strategies using antibody-based or cell-based immunotherapies are described including the use of unmodified antibodies, drug conjugates, radioimmunoconjugates, non-ablative allogeneic transplantation, T cell adoptive immunotherapy and AML vaccines. Finally, Dr. John Dick provides a review of the development of the NOD/SCID mouse model of human AML emphasizing both what it has taught us about the biology of the disease as well as how it can be used to test new therapies. Taken together, these reviews are meant to help us understand more about where we are in the treatment of AML, where we can go and how we might get there.

Analysis of FLT3 length mutations in 1003 patients with acute myeloid leukemia: correlation to cytogenetics, FAB subtype, and prognosis in the AMLCG study and usefulness as a marker for the detection of minimal residual disease

FLT3 length mutation (FLT3-LM) is a molecular marker potentially useful for the characterization of acute myeloid leukemia (AML). To evaluate the distribution of FLT3-LM within biologic subgroups, we screened 1003 patients with AML at diagnosis for this mutation. FLT3-LM was found in 234 (23.5%) of all patients and thus is the most frequent mutation in AML described so far. Of all positive patients, 165 (70.5%) revealed a normal karyotype. Of the 69 patients with chromosome aberrations, 24 (34.8%) had a t(15;17). The mutation was rare in AML with t(8;21), inv(16) 11q23 rearrangements, and complex karyotypes. FLT3-LM was not distributed equally within different French-American-British (FAB) subtypes and was correlated with a high peripheral blood count in FAB M1, M2, and M4 (P <.0001). In addition, the median age of patients with the mutation was lower (54.9 vs 57.6 years; P =.043), and, at a ratio of 1.36:1 (P =.023), the mutation was more frequent in females than in males. Within the AMLCG study, FLT3-LM was of intermediate prognostic significance. The complete remission rate of 70.3% in patients with FLT3-LM was similar to that (70.4%) in patients without FLT3-LM. Overall survival was not different between patients with or without FLT3-LM. In contrast, patients with FLT3-LM had a significantly shorter event-free survival (7.4 vs 12.6 months; P =.0072) because of a higher relapse rate. Besides the importance of FLT3-LM for biologic and clinical characterization of AML, we show its value as a marker for disease monitoring based on 120 follow-up samples of 34 patients.

PML/RARalpha and FLT3-ITD induce an APL-like disease in a mouse model

Acute promyelocytic leukemia (APL) cells invariably express aberrant fusion proteins involving the retinoic acid receptor alpha (RARalpha). The most common fusion partner is promyelocytic leukemia protein (PML), which is fused to RARalpha in the balanced reciprocal chromosomal translocation, t(15;17)(q22:q11). Expression of PML/RARalpha from the cathepsin G promoter in transgenic mice causes a nonfatal myeloproliferative syndrome in all mice; about 15% go on to develop APL after a long latent period, suggesting that additional mutations are required for the development of APL. A candidate target gene for a second mutation is FLT3, because it is mutated in approximately 40% of human APL cases. Activating mutations in FLT3, including internal tandem duplication (ITD) in the juxtamembrane domain, transform hematopoietic cell lines to factor independent growth. FLT3-ITDs also induce a myeloproliferative disease in a murine bone marrow transplant model, but are not sufficient to cause AML. Here, we test the hypothesis that PML/RARalpha can cooperate with FLT3-ITD to induce an APL-like disease in the mouse. Retroviral transduction of FLT3-ITD into bone marrow cells obtained from PML/RARalpha transgenic mice results in a short latency APL-like disease with complete penetrance. This disease resembles the APL-like disease that occurs with long latency in the PML/RARalpha transgenics, suggesting that activating mutations in FLT3 can functionally substitute for the additional mutations that occur during mouse APL progression. The leukemia is transplantable to secondary recipients and is ATRA responsive. These observations document cooperation between PML/RARalpha and FLT3-ITD in development of the murine APL phenotype.

Inhibition of mutant FLT3 receptors in leukemia cells by the small molecule tyrosine kinase inhibitor PKC412

Constitutively activating FLT3 receptor mutations have been found in 35% of patients with acute myeloblastic leukemia (AML). Here we report the identification of a small molecule FLT3 tyrosine kinase inhibitor PKC412, which selectively induced G1 arrest and apoptosis of Ba/F3 cell lines expressing mutant FLT3 (IC(50) < 10 nM) by directly inhibiting the tyrosine kinase. Ba/F3-FLT3 cell lines made resistant to PKC412 demonstrated overexpression of mutant FLT3, confirming that FLT3 is the target of this drug. Finally, progressive leukemia was prevented in PKC412-treated Balb/c mice transplanted with marrow transduced with a FLT3-ITD-expressing retrovirus. PKC412 is a potent inhibitor of mutant FLT3 and is a candidate for testing as an antileukemia agent in AML patients with mutant FLT3 receptors.