FLT3: ITDoes matter in leukemia

Relationship between FLT3 mutation status, biologic characteristics, and response to targeted therapy in acute promyelocytic leukemia

The prognostic significance of FLT3 mutations in acute promyelocytic leukemia (APL) is not firmly established and is of particular interest given the opportunities for targeted therapies using FLT3 inhibitors. We studied 203 patients with PML-RARA-positive APL; 43% of the patients had an FLT3 mutation (65 internal tandem duplications [ITDs], 19 D835/I836, 4 ITD+D835/I836). Both mutations were associated with higher white blood cell (WBC) count at presentation; 75% of the patients with WBC counts of 10 x 10(9)/L or greater had mutant FLT3. FLT3/ITDs were correlated with M3v subtype (P < .001), bcr3 PML breakpoint (P < .001), and expression of reciprocal RARA-PML transcripts (P = .01). Microarray analysis revealed differences in expression profiles among patients with FLT3/ITD, D835/I836, and wild-type FLT3. Patients with mutant FLT3 had a higher rate of induction death (19% vs 9%; P = .04, but no significant difference in relapse risk (28% vs 23%; P = .5) or overall survival (59% vs 67%; P = .2) at 5 years. In in vitro differentiation assays using primary APL blasts (n = 6), the FLT3 inhibitor CEP-701 had a greater effect on cell survival/proliferation in FLT3/ITD+ cells, but this inhibition was reduced in the presence of ATRA. Furthermore, in the presence of CEP-701, ATRA-induced differentiation was reduced in FLT3/ITD+ cells. These data carry implications for the use of FLT3 inhibitors as frontline therapy for APL.

Clinical Significance of FLT3 in Leukemia

FLT3 is a class III receptor tyrosine kinase together with KIT, FMS and PDGFR. FLT3 mutations were first reported as internal tandem duplication (FLT3/ITD) of the juxtamembrane domain-coding sequence, and subsequently as a missense mutation of D835 (FLT3/KDM) within a kinase domain. Furthermore, point mutations, deletions, and insertions in the codons surrounding D835 have also been found. FLT3/ITD and FLT3/KDM occur in 15% to 35% and 5% to 10%, respectively, of patients with AML. FLT3 mutations are, therefore, the most frequent genetic alterations so far reported in AML. Several large-scale studies have confirmed that FLT3/ITD is strongly associated with leukocytosis and a poor prognosis. Although the clinical significance of FLT3/KDM is controversial, the meta-analysis suggests its adverse effect on the outcome. FLT3/ITD is far less common in patients with ALL, whereas FLT3/KDM is recurrently found in patients with ALL, especially in those harboring an MLL gene rearrangement or hyperdiploidy. The overexpression of FLT3 transcripts has been demonstrated in a pro-portion of the AML patients without FLT3 mutations, which are associated with a poor prognosis for overall survival. Routine screening of FLT3 mutations is recommended to stratify the patients into distinct risk groups, while the optimal treatment strategy for patients with FLT3 mutations should be further evaluated.

Specific detection of Flt3 point mutations by highly sensitive real-time polymerase chain reaction in acute myeloid leukemia

Among activating class III receptor tyrosine kinase (Flt3) mutations, internal tandem duplications of Flt3 (Flt3-ITD) are detected in about 25% of patients with acute myeloid leukemia (AML). In contrast, mutations within the tyrosine kinase domain of Flt3 (Flt3-TKD mutations) are less frequent (approximately 7%), and there are only limited data on the frequency of recently demonstrated activating Flt3 point mutation at codon 592 (Flt3-V592A mutation). We evaluated a new approach for rapid screening of Flt3-TKD and Flt3-V592A mutations using the fluorescence resonance energy transfer (FRET) principle in a group of 122 patients. Based on individual Flt3-TKD mutations, we designed patient-specific primers to perform a highly sensitive polymerase chain reaction (PCR) assay for rapid detection of minimal residual disease (MRD). We also used a model system with MonoMac-6 cells carrying the Flt3-V592A mutation to establish a mutation-specific real-time PCR approach also for this molecular aberration. We identified 9 cases (8%) of Flt3-TKD mutations (5 cases of mutation D835Y, 3 cases of mutation D835H, and 1 case of mutation Del836), and no cases of Flt3-V592A mutation. Screening for Flt3-TKD mutations with fluorescent probes is equivalent to conventional screening using standard PCR followed by EcoRV restriction. We present a real-time PCR protocol that can be used for MRD analyses based on individual Flt3-TKD mutations. Examples of MRD analyses are presented for all 3 subtypes of Flt3-TKD mutation identified in this study. In summary, we demonstrate new methodological approaches for rapid screening of Flt3 point mutations and for detection of MRD based on patient-specific Flt3-TKD mutations.

Analysis of FLT3 internal tandem duplication and D835 mutations in Chinese acute leukemia patients

Genomic aberrations of Fms-like tyrosine kinase 3 (FLT3), including internal tandem duplication (ITD) and point mutations, have been demonstrated in 25-30% of adults acute myeloid leukemia (AML) and are markers of poor prognosis. FLT3/ITD and D835 mutations were analyzed in 194 Chinese patients with acute leukemia and myelodysplastic syndromes (MDS) by polymerase chain reaction (PCR). FLT3/ITDs and D835 mutations were found in 25.9 and 6.3% of 143 AML patients, respectively. Two patients showed both ITD and point mutations. Among the FAB subtypes of AML, the rate of FLT3 aberration was significantly higher in M3 and M5. However, neither aberrations was found in 25 patients with acute lymphoblastic leukemia (ALL), 2 acute hybrid leukemia, 17 MDS and 7 chronic myeloid leukemia in blast crisis (CML-BC). FLT3/ITD was associated to leukocytosis and lower complete remission (CR) rate, and was more prevalent in patients with normal karyotype. In contrast, D835 mutation was not associated with leukocytosis or low CR rate. Our results confirm that FLT3 activating mutations also occur in a significant percentage in Chinese AML patients. FLT3/ITD(+) patients treated with standard induction regimen could achieve lower complete remission rates compared with patients not harboring this defect. Early detection of FLT3 mutations and an intensification of induction therapy might thus be useful for this group of patients to overcome the poor prognosis.

Src family tyrosine kinases are activated by Flt3 and are involved in the proliferative effects of leukemia-associated Flt3 mutations

OBJECTIVE

The hematopoietic growth factor receptor, Fms-like tyrosine kinase-3 (Flt3), modulates survival and proliferation of myeloid and B-cell precursors. Activating mutations of Flt3 are the most common molecular abnormalities in acute myeloid leukemia (AML) and have an apparent role in leukemogenesis. However, signaling pathways mediating Flt3 effects are incompletely understood. The role of Src kinases is unknown, although some, such as Lyn, have also been linked to leukemogenesis. This study examines the role of Src kinases in Flt3 signaling and the oncogenic effects of leukemia-associated Flt3 mutations.

MATERIALS AND METHODS

We examined the activation and functional roles of Src kinases in human leukemic myeloid cell lines expressing wild-type Flt3 or a constitutively active mutant, and in cells stably transduced with human wild-type or mutant Flt3.

RESULTS

Flt3 ligand stimulation of wild-type Flt3 increased phosphorylation of Src kinase Lyn. Constitutive Lyn phosphorylation and activation was found in cells expressing constitutively active Flt3 mutants. Src kinases are implicated in downregulation of closely related receptors, but Src inhibitors had no effect on ligand-stimulated Flt3 degradation, or on the rapid degradation of an Flt3 mutant. However, growth-factor-independent proliferation resulting from mutant Flt3 expression did depend on the activity of Src kinases.

CONCLUSION

Our studies reveal for the first time the involvement of Src kinases in Flt3 signaling, with activation of Lyn by constitutively active Flt3 mutants as well as ligand-stimulated wild-type receptor, and show that Src kinase inhibitors block proliferative effects of Flt3 mutants found in AML. Thus, Src kinases may represent targets for inhibitor therapy in Flt3-related AML.

Therapeutic intervention in leukemias that express the activated fms-like tyrosine kinase 3 (FLT3): opportunities and challenges

PURPOSE OF REVIEW

The fms-like tyrosine kinase 3 (FLT3) receptor tyrosine kinase is now recognized to be a critical mediator in the pathogenesis of myeloid and some lymphoid leukemias. This article reviews recent efforts to disrupt FLT3 signaling in acute myelogenous leukemia and to identify potential therapeutic challenges posed by the acquisition of resistance mutations in these malignancies.

RECENT FINDINGS

Several broad classes of FLT3 protein tyrosine kinase inhibitors are undergoing evaluation in clinical trials. Although the agents are well tolerated by patients, clinical responses in relapsed or refractory acute myelogenous leukemia (AML) are limited and transient. Nevertheless, these agents may hold promise when combined with traditional chemotherapy. Use of tyrosine kinase inhibitors for AML therapy is hindered by the acquisition of mutations in the kinase catalytic domain, and in the case of BCR-ABL, these mutations confer resistance to imatinib. In anticipation of this problem, FLT3 mutations that might confer resistance to kinase inhibitors in the clinical setting have already been identified in the laboratory. Strategies to overcome such resistance are currently under development. New efforts focus on blocking the binding of FLT3 ligand to its receptor as a means of inhibiting autocrine stimulation in leukemogenesis.

SUMMARY

FLT3 is widely expressed in AML and some cases of acute lymphocytic leukemia. Activating mutations in FLT3 confer a poor risk in patients with AML. The development of FLT3 small molecule kinase inhibitors follows from research efforts to understand signal transduction and profiles of gene expression in leukemia pathogenesis. Thus, FLT3 is a promising target for therapeutic intervention. Research priorities will include (1) identification of other groups of patients likely to benefit from FLT3 inhibition, (2) the optimal use of FLT3 inhibitors in combination with other agents, and (3) development of molecules that overcome resistance to FLT3 inhibitors that arise as a result of further acquired mutations in the receptor.

In vitro activity of the flt3-inhibitor su5614 and standard cytotoxic agents in tumour cells from patients with wild type and mutated flt3 acute myeloid leukaemia

The correlation between drug sensitivity in vitro and the mutation status of the FLT3 receptor gene was evaluated in tumour cells from 17 previously untreated AML patients. Tumour cells with internal tandem duplication (ITD) in the FLT3 receptor gene were significantly more sensitive to the FLT3 inhibitor SU5614 than tumour cells with wild type FLT3. Combinations of SU5614 with etoposide and amsacrine showed better effect (p<0.05) compared with the respective single drugs. Our results suggest that the FLT3 inhibitor SU5614 may have a therapeutic potential, especially in combination with other cytotoxic agents, in patients with FLT3-ITD positive AML.

Internal tandem duplications of the FLT3 gene are present in leukemia stem cells

Internal tandem duplication mutations of the FLT3 gene (FLT3/ITD mutations) are the most frequent molecular abnormality in acute myeloid leukemia (AML) and are associated with a poor overall survival. While the normal FLT3 receptor is expressed in early hematopoietic progenitor cells, it has not been determined whether FLT3 mutations are present in the leukemic stem cells. In this study, we sorted primary AML samples into stem cell-enriched CD34+/CD38- fractions and then analyzed the sorted and unsorted cells for the FLT3 mutant-wild-type ratio. In each case, the FLT3 mutant-wild-type ratio was not changed by selection of CD34+/CD38- cells, implying that the mutations are present in the leukemic stem cells. We used the stem cell-enriched fraction to engraft nonobese diabetic-severe combined immunodeficient (NOD-SCID) mice and then confirmed that the FLT3/ITD mutation was present in the resultant engrafted marrow. As a final test of the importance of FLT3/ITD signaling in this engraftment model, we used a small molecule FLT3 inhibitor, CEP-701, to inhibit engraftment of FLT3/ITD stem cells. Taken together, these experiments establish that the FLT3/ITD mutations are present in leukemia stem cells, and that FLT3 inhibitors may have activity against these cells.

Heat shock protein 90 inhibition sensitizes acute myelogenous leukemia cells to cytarabine

Previous studies demonstrated that ataxia telangiectasia mutated- and Rad3-related (ATR) kinase and its downstream target checkpoint kinase 1 (Chk1) facilitate survival of cells treated with nucleoside analogs and other replication inhibitors. Recent results also demonstrated that Chk1 is depleted when cells are treated with heat shock protein 90 (Hsp90) inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG). The present study examined the effects of 17-AAG and its major metabolite, 17-aminogeldanamycin (17-AG), on Chk1 levels and cellular responses to cytarabine in human acute myelogenous leukemia (AML) cell lines and clinical isolates. Cytarabine, at concentrations as low as 30 nM, caused activating phosphorylation of Chk1, loss of the phosphatase Cdc25A, and S-phase slowing. Conversely, treatment with 100 to 300 nM 17-AAG for 24 hours caused Chk1 depletion that was accompanied by diminished cytarabine-induced S-phase accumulation, decreased Cdc25A degradation, and enhanced cytotoxicity as measured by inhibition of colony formation and induction of apoptosis. Additional studies demonstrated that small inhibitory RNA (siRNA) depletion of Chk1 also sensitized cells to cytarabine, whereas disruption of the phosphatidylinositol 3-kinase (PI3k) signaling pathway, which is also blocked by Hsp90 inhibition, did not. Collectively, these results suggest that treatment with 17-AAG might represent a means of reversing checkpoint-mediated cytarabine resistance in AML.

Inhibitory Anti-FLT3 Antibodies Are Capable of Mediating Antibody-Dependent Cell-Mediated Cytotoxicity and Reducing Engraftment of Acute Myelogenous Leukemia Blasts in Nonobese Diabetic/Severe Combined Immunodeficient Mice

Aberrant FLT3 expression and/or mutation plays a significant role in leukemogenesis. This has prompted the development of selective small molecule tyrosine kinase inhibitors against FLT3. However, like most tyrosine kinase inhibitors, those against FLT3 are not completely specific and at the doses required to completely inhibit target, significant toxicities may occur. In addition, tyrosine kinase inhibitors for other kinases have been shown to select for cells that become resistant. To overcome some of these limitations we developed two fully human phage display monoclonal antibodies against FLT3 (IMC-EB10 and IMC-NC7). These antibodies inhibited ligand-mediated activation of wild-type FLT3 and constitutively activated mutant FLT3 and in most cell types affected downstream STAT5, AKT, and mitogen-activated protein kinase activation. In addition to interfering with FLT3 signaling, IMC-EB10 and, to a significantly lesser extent, IMC-NC7 initiated antibody-dependent cell-mediated cytotoxicity on FLT3-expressing cells. When IMC-EB10 was used in vivo to treat nonobese diabetic/severe combined immunodeficient mice given injections of primary FLT3/ITD acute myelogenous leukemia samples or myeloid cell lines with FLT3 expression, it significantly decreased engraftment of leukemic cells and increased survival, respectively. In contrast, IMC-EB10 treatment did not reduce engraftment of normal human CD34+ cord blood cells nor did it show any significant inhibition of normal murine hematopoiesis. Thus, these types of antibodies have the potential to be safe and effective new therapeutic agents for acute myelogenous leukemia and possibly other FLT3-expressing malignancies.

FLT3 inhibition selectively kills childhood acute lymphoblastic leukemia cells with high levels of FLT3 expression

FMS-like tyrosine kinase 3 (FLT3) is almost universally expressed in B-precursor childhood acute lymphoblastic leukemia (ALL). Cases of ALL with MLL gene rearrangements and those with high hyperdiploidy (&gt; 50 chromosomes) express the highest levels of FLT3, and activating mutations of FLT3 occur in 18% of MLL-rearranged and 28% of hyperdiploid ALL cases. We determined the antileukemic activity of CEP-701, a potent and selective FLT3 inhibitor, in 8 ALL cell lines and 39 bone marrow samples obtained at diagnosis from infants and children with various subtypes of ALL. CEP-701 induced pronounced apoptotic responses in a higher percentage of samples that expressed high levels of FLT3 (74%, n = 23) compared with samples with low levels of expression (8%, n = 13; P = .0003). Sensitivity to FLT3 inhibition was particularly high in samples with MLL gene rearrangements (82%, n = 11; P = .0005), high hyperdiploidy (100%, n = 5; P = .0007), and/or FLT3 mutations (100%, n = 4; P = .0021). Seven of 7 sensitive samples examined by immunoblotting demonstrated constitutively phosphorylated FLT3 that was potently inhibited by CEP-701, whereas 0 of 6 resistant samples expressed constitutively phosphorylated FLT3. We conclude that the FLT3 inhibitor CEP-701 effectively suppresses FLT3-driven leukemic cell survival. Clinical testing of CEP-701 as a novel molecularly targeted agent for the treatment of childhood ALL is warranted.

Recent advances in the development of small-molecule inhibitors for the treatment of acute myeloid leukemia

PURPOSE OF REVIEW

This review outlines recent advances in the development of small-molecule inhibitors of molecular signaling pathways for the treatment of acute myeloid leukemia (AML). These compounds are typically targeted against components of the tyrosine kinase-Ras-Map kinase pathway that have been activated by mutation.

RECENT FINDINGS

Several agents have been tested in phase 2 trials, with only modest clinical results thus far. Careful correlative studies have allowed a clearer understanding of the reasons for the success or failure of these agents and have refined our approach to clinical trial design. In some cases, the target molecule has been successfully inhibited, but for an inadequate duration, and in other cases, inhibiting the target has little correlation with clinical effect.

SUMMARY

Small-molecule inhibitors of these molecular pathways clearly have significant promise for the treatment of AML, but several obstacles remain, and this field of pharmacotherapy is still quite new. These inhibitors seem unlikely to be curative when administered as monotherapy but rather will have to be used in combination with one another or with conventional chemotherapy. In addition, pharmacokinetic problems must be overcome with many of them.

Oncogenes as Novel Targets for Cancer Therapy (Part I)

In the past 10 years, progress made in cancer biology, genetics, and biotechnology has led to a major transition in cancer drug design and development. There has been a change from an emphasis on non-specific, cytotoxic agents to specific, molecular-based therapeutics. Mechanism-based therapy is designed to act on cellular and molecular targets that are causally involved in the formation, growth, and progression of human cancers. These agents, which may have greater selectivity for cancer versus normal cells, and which may produce better anti-tumor efficacy and lower host toxicity, can be small molecules, natural or engineered peptides, proteins, antibodies, or synthetic nucleic acids (e.g. antisense oligonucleotides, ribozymes, and siRNAs). Novel targets are identified and validated by state-of-the-art approaches, including high-throughput screening, combinatorial chemistry, and gene expression arrays, which increase the speed and efficiency of drug discovery and development. Examples of oncogene-based, molecular therapeutics that show promising clinical activity include trastuzumab (Herceptin), imatinib (Gleevec), and gefitinib (Iressa). However, the full potential of oncogenes as novel targets for cancer therapy has not been realized and many challenges remain, from the validation of novel targets, to the design of specific agents, to the evaluation of these agents in both preclinical and clinical settings. In maximizing the benefits of molecular therapeutics in monotherapy or combination therapy of cancer, it is necessary to have an understanding of the underlying molecular abnormalities and mechanisms involved. This is the first part of a four-part review in which we discuss progress made in the last decade as it relates to the discovery of novel oncogenes and signal transduction pathways, in the context of their potential as targets for cancer therapy. This part delineates the latest discoveries about the potential use of growth factors and protein tyrosine kinases as targets for therapy. Later parts focus on intermediate signaling pathways, transcription factors, and proteins involved in cell cycle, DNA damage, and apoptotic pathways.

Gene expression profiling in acute myeloid leukemia

PURPOSE OF REVIEW

This review deals with the emerging promises of gene expression profiling (GEP) and the currently accumulating knowledge about the classification and the discovery of novel disease entities in clinical acute myeloid leukemia (AML).

RECENT FINDINGS

Gene expression profiling studies in AML have shown that known and novel classes of disease can be recognized by unsupervised analyses. Prognostically informative molecular signatures can be deduced. Supervised analyses show that particular clinically relevant subsets of AML can be predicted with high accuracy with minimal sets of genes.

SUMMARY

The AML GEP studies published to date show a remarkable level of concordance in findings, especially for similar GEP platforms. This confirms the robustness of the methodology and the promise for future applicability of GEP in clinical diagnostics. For the time being, certain technical hurdles remain to be overcome. These relate, for instance, to the conversion of data between different GEP platforms, the effect of differences between various statistical clustering methods, and the still incomplete understanding of the effect of biologic (eg, morphology) and genetic factors on the expression signature. GEP analyses, perhaps in combination with high-throughput mutation analysis and proteomic approaches, may ultimately result in the establishment of a comprehensive diagnostic approach that will yield a key to the precise pathobiologic nature of AML.

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.

Pim-1 is up-regulated by constitutively activated FLT3 and plays a role in FLT3-mediated cell survival

Constitutively activating internal tandem duplication (ITD) mutations of the receptor tyrosine kinase FLT3 (Fms-like tyrosine kinase 3) play an important role in leukemogenesis, and their presence is associated with poor prognosis in acute myeloid leukemia (AML). To better understand FLT3 signaling in leukemogenesis, we have examined the changes in gene expression induced by FLT3/ITD or constitutively activated wild-type FLT3 expression. Microarrays were used with RNA harvested before and after inhibition of FLT3 signaling. Pim-1 was found to be one of the most significantly down-regulated genes upon FLT3 inhibition. Pim-1 is a proto-oncogene and is known to be up-regulated by signal transducer and activator of transcription 5 (STAT5), which itself is a downstream target of FLT3 signaling. Quantitative polymerase chain reaction (QPCR) confirmed the microarray results and demonstrated approximately 10-fold decreases in Pim-1 expression in response to FLT3 inhibition. Pim-1 protein also decreased rapidly in parallel with decreasing autophosphorylation activity of FLT3. Enforced expression of either the 44-kDa or 33-kDa Pim-1 isotypes resulted in increased resistance to FLT3 inhibition-mediated cytotoxicity and apoptosis. In contrast, expression of a dominant-negative Pim-1 construct accelerated cytotoxicity in response to FLT3 inhibition and inhibited colony growth of FLT3/ITD-transformed BaF3 cells. These findings demonstrate that constitutively activated FLT3 signaling up-regulates Pim-1 expression in leukemia cells. This up-regulation contributes to the proliferative and antiapoptotic pathways induced by FLT3 signaling.

Suppression of leukemia expressing wild-type or ITD-mutant FLT3 receptor by a fully human anti-FLT3 neutralizing antibody

FMS-like tyrosine kinase 3 (FLT3), a class III receptor tyrosine kinase, is expressed at high levels in the blasts of approximately 90% of patients with acute myelogenous leukemia (AML). Internal tandem duplications (ITDs) in the juxtamembrane domain and point mutations in the kinase domain of FLT3 are found in approximately 37% of AML patients and are associated with a poor prognosis. We report here the development and characterization of a fully human anti-FLT3 neutralizing antibody (IMC-EB10) isolated from a human Fab phage display library. IMCEB10 (immunoglobulin G1 [IgG1], κ) binds with high affinity (KD = 158 pM) to soluble FLT3 in enzyme-linked immunosorbent assay (ELISA) and to FLT3 receptor expressed on the surfaces of human leukemia cell lines. IMC-EB10 blocks the binding of FLT3 ligand (FL) to soluble FLT3 in ELISA and competes with FL for binding to cell-surface FLT3 receptor. IMC-EB10 treatment inhibits FL-induced phosphorylation of FLT3 in EOL-1 and EM3 leukemia cells and FL-independent constitutive activation of ITD-mutant FLT3 in BaF3-ITD and MV4;11 cells. Activation of the downstream signaling proteins mitogen-activated protein kinase (MAPK) and AKT is also inhibited in these cell lines by antibody treatment. The antibody inhibits FL-stimulated proliferation of EOL-1 cells and ligand-independent proliferation of BaF3-ITD cells. In both EOL-1 xenograft and BaF3-ITD leukemia models, treatment with IMC-EB10 significantly prolongs the survival of leukemia-bearing mice. No overt toxicity is observed with IMC-EB10 treatment. Taken together, these data demonstrate that IMC-EB10 is a specific and potent inhibitor of wild-type and ITD-mutant FLT3 and that it deserves further study for targeted therapy of human AML. (Blood. 2004;104:1137-1144)

FLT3 mutations in myeloid sarcoma

Myeloid sarcoma is an extramedullary tumour that typically occurs in the setting of acute myeloid leukaemia (AML), or myeloproliferative disorders. In AML, two types of mutations in Fms-like tyrosine kinase 3 (FLT3) have been described; internal tandem duplications (ITD) and point mutations at aspartic acid residue 835 (D835). We analysed 24 myeloid sarcoma specimens from 20 patients for FLT3 ITD and D835 mutations. FLT3 ITD mutations were identified in three of 20 cases (15%); no D835 mutations were identified. The ITD inserts ranged in size from 33 to 198 base pairs (bp) and represented approximately 20-40% of the FLT3 alleles. Two cases showed discordance in FLT3 ITD mutational status. In one case, the leukaemia specimen was positive for a FLT3 ITD mutation and the myeloid sarcoma specimen was negative. In the second case, the myeloid sarcoma was positive for a FLT3 ITD mutation at diagnosis, but negative in subsequent relapse samples. Our findings suggest that small molecule inhibitors of FLT3 may be useful therapeutic agents for treatment of myeloid sarcomas-containing FLT3 mutations, however, the potential for discordance between the leukaemia and myeloid sarcoma, necessitates that the myeloid sarcoma tumour itself be analysed for FLT3 mutations.

Classical and molecular cytogenetic abnormalities and outcome of childhood acute myeloid leukaemia: report from a referral centre in Israel

The incidence of cytogenetic abnormalities in childhood de novo acute myeloid leukaemia (AML) and its prognostic significance was assessed in an Israeli paediatric referral centre. Cytogenetic analysis was successful in 86 of 97 children (< 20 years of age) diagnosed between 1988 and 2002 with de novo AML. Fluorescence in situ hybridization analysis detected new information in 11 of them, leading to reassignment in cytogenetic group classification. The incidence of the various cytogenetic subgroups was as follows: normal - 9%; t(11q23) - 22%; t(8;21) - 13%; t(15;17) - 8%; inv(16) - 3.4%; abn(3q) - 4.6%; 7/7q-(sole or main) - 5.8%; del(9q)(sole) and +21(sole) - 4.6% each; t(8;16) - 2.3%; t(6;9), t(1;22), +8(sole) - 1.1% each; and miscellaneous - 18%. The overall survival (OS) and event-free survival (EFS) (4 years) for 94 patients treated with the modified Berlin-Frankfürt-Münster (BFM) AML protocols (non-irradiated) were 59.9% (SE = 5%) and 55.7% (SE = 5%), respectively, and for the favourable t(8;21), t(15;17) and inv(16), OS was 60% (SE = 15%), 83% (SE = 15%) and 100% respectively. For the normal group it was 62% (SE = 17%), miscellaneous 64% (SE = 12%), t(11q23) 44.6% (SE = 11%) and of the -7/7q-, del(9q)(sole) or t(6;9), none had survived at 4 years. The incidence of cytogenetic subgroups in the Israeli childhood AML population and their outcome were similar to other recently reported paediatric series. Cytogenetic abnormalities still carry clinical relevance for treatment stratification in the context of modern chemotherapy.

In vitro studies of a FLT3 inhibitor combined with chemotherapy: sequence of administration is important to achieve synergistic cytotoxic effects

Patients with acute myeloid leukemia (AML) harboring internal tandem duplication mutations of the FLT3 receptor (FLT3/ITD mutations) have a poor prognosis compared to patients lacking such mutations. Incorporation of FLT3 inhibitors into existing chemotherapeutic regimens has the potential to improve clinical outcomes in this high-risk group of patients. CEP-701, an indolocarbazole-derived selective FLT3 inhibitor, potently induces apoptosis in FLT3/ITD-expressing cell lines and primary leukemic blasts. We conducted a series of in vitro cytotoxicity experiments combining CEP-701 with chemotherapy using the FLT3/ITD-expressing cell lines MV4-11 and BaF3/ITD as well as a primary blast sample from a patient with AML harboring a FLT3/ITD mutation. CEP-701 induced cytotoxicity in a synergistic fashion with cytarabine, daunorubicin, mitoxantrone, or etoposide if used simultaneously or immediately following exposure to the chemotherapeutic agent. In contrast, the combination of pretreatment with CEP-701 followed by chemotherapy was generally antagonistic, particularly with the more cell cycle-dependent agents such as cytarabine. This effect appears to be due to CEP-701 causing cell cycle arrest. We conclude that in FLT3/ITD-expressing leukemia cells, CEP-701 is synergistic with standard AML chemotherapeutic agents, but only if used simultaneously with or immediately following the chemotherapy. These results should be considered when designing trials combining chemotherapy with each of the FLT3 inhibitors currently in clinical development.

Activating FLT3 mutations in CD117/KIT(+) T-cell acute lymphoblastic leukemias

Activating FLT3 mutations are the most common genetic aberrations in acute myeloid leukemia (AML), resulting in the constitutive activation of this receptor tyrosine kinase (RTK), but such mutations are rarely found in acute lymphoblastic leukemia (ALL). Here we describe a unique subset of de novo adult T-cell ALL (T-ALL) cases that coexpress CD117/KIT and cytoplasmic CD3 (CD117/KIT(+) ALL). Activating mutations in the FLT3 RTK gene were found in each of 3 CD117/KIT(+) cases that were analyzed, but not in 52 other adult T-ALL samples from the same series that lacked CD117/KIT expression. Our results indicate the need for clinical trials to test the efficacy of drugs that inhibit the FLT3 RTK in this subset of patients with T-ALL.

Dual mutations in the AML1 and FLT3 genes are associated with leukemogenesis in acute myeloblastic leukemia of the M0 subtype

Point mutations of the transcription factor AML1 are associated with leukemogenesis in acute myeloblastic leukemia (AML). Internal tandem duplications (ITDs) in the juxtamembrane domain and mutations in the second tyrosine kinase domain of the Fms-like tyrosine kinase 3 (FLT3) gene represent the most frequent genetic alterations in AML. However, such mutations per se appear to be insufficient for leukemic transformation. To evaluate whether both AML1 and FLT3 mutations contribute to leukemogenesis, we analyzed mutations of these genes in AML M0 subtype in whom AML1 mutations were predominantly observed. Of 51 patients, eight showed a mutation in the Runt domain of the AML1 gene: one heterozygous missense mutation with normal function, five heterozygous frameshift mutations and two biallelic nonsense or frameshift mutations, resulting in haploinsufficiency or complete loss of the AML1 activities. On the other hand, a total of 10 of 49 patients examined had the FLT3 mutation. We detected the FLT3 mutation in five of eight (63%) patients with AML1 mutation, whereas five of 41 (12%) without AML1 mutation showed the FLT3 mutation (P=0.0055). These observations suggest that reduced AML1 activities predispose cells to the acquisition of the activating FLT3 mutation as a secondary event leading to full transformation in AML M0.

Single-agent CEP-701, a novel FLT3 inhibitor, shows biologic and clinical activity in patients with relapsed or refractory acute myeloid leukemia

Activating mutations of FMS-like tyrosine kinase 3 (FLT3) are present in approximately 30% of patients with de novo acute myeloid leukemia (AML) and are associated with lower cure rates from standard chemotherapy-based treatment. Targeting the mutation by inhibiting the tyrosine kinase activity of FLT3 is cytotoxic to cell lines and primary AML cells harboring FLT3 mutations. Successful FLT3 inhibition can also improve survival in mouse models of FLT3-activated leukemia. CEP-701 is an orally available, novel, receptor tyrosine kinase inhibitor that selectively inhibits FLT3 autophosphorylation. We undertook a phase 1/2 trial to determine the in vivo hematologic effects of single-agent CEP-701 as salvage treatment for patients with refractory, relapsed, or poor-risk AML expressing FLT3-activating mutations. Fourteen heavily pretreated AML patients were treated with CEP-701 at an initial dose of 60 mg orally twice daily. CEP-701-related toxicities were minimal. Five patients had clinical evidence of biologic activity and measurable clinical response, including significant reductions in bone marrow and peripheral blood blasts. Laboratory data confirmed that clinical responses correlated with sustained FLT3 inhibition to CEP-701. Our results show that FLT3 inhibition is associated with clinical activity in AML patients harboring FLT3-activating mutations and indicate that CEP-701 holds promise as a novel, molecularly targeted therapy for this disease.

Interleukin-3 receptor in acute leukemia

Recent studies indicate that abnormalities of the interleukin-3 receptor (IL-3R) are frequently observed in acute myeloid leukemias (AMLs) and may contribute to the proliferative advantage of leukemic blasts. This review analyzes the evidences indicating that the IL-3R represents one of the target molecules involved in the stimulation of proliferation of AMLs, and the overexpression of the IL-3Ralpha chain may represent one of the mechanisms contributing to the development of a highly malignant leukemic phenotype. Furthermore, there is evidence that the IL-3Ralpha is a marker of leukemic stem cells, at variance with normal stem cells that are IL-3Ralpha-. Finally, the IL-3R may represent an important target for the development of new antileukemic drugs.