Lack of KIT or FMS internal tandem duplications but co-expression with ligands in AML

Gene mutation profile and risk stratification in AML1‑ETO‑positive acute myeloid leukemia based on next‑generation sequencing

Gene mutations play an important role in the development and progression of AML1-ETO-positive acute myeloid leukemia (AE-AML). Nevertheless, the gene mutation profile in this subtype of leukemia remains unclear. In addition, the clinical and prognostic effects of different mutant genes may be underestimated. In the present study, gene sequencing was conducted at diagnosis and relapse with next-generation sequencing (NGS) in 64 patients with newly diagnosed AE-AML, and 44/64 (68.8%) patients were found to present with a median of 2 (1–10) recurrent mutations at diagnosis and 6/11 (54.5%) cases were found to present with genetic alterations at relapse. c-KIT mutation was the most common in this cohort, with an incidence of 27/64 (42.2%) at diagnosis, followed by ASXL1 (n=10, 15.6%), MET (n=8, 12.5%), MLH1 (n=6, 9.4%), TET2 (n=5, 7.8%), and FBXW7, TP53 and DNMT3A (n=5, 7.8%). Survival analysis showed that c-KIT (exon 8, 17) but not exon 10 adversely affected survival. In addition, ASXL1 and TP53 were poor impact factors for recurrence-free survival (RFS) (P<0.05), and ASXL1, MET, FBXW7 and TP53 had a negative impact on overall survival (OS) (P<0.05). Multivariate analysis showed that c-KIT (exon 8, 17) [RFS: hazard ratio (HR) 3.36, 95% confidence interval (CI) 1.54–7.34, P=0.002; OS: HR 2.84, 95% CI 1.20–6.71, P=0.018] and ASXL1 mutations (RFS: HR 3.13, 95% CI 1.34–7.32, P=0.009; OS: HR 3.94, 95% CI 1.62–9.61, P=0.003) were independent adverse factors for survival. Further, co-mutation of these two genes showed even worse effect on disease outcome. Collectively, additional gene mutations play critical role in AE-AML. C-KIT and ASXL1 mutations are the two most common mutations in this subtype of leukemia. C-KIT (exon 8, 17) but not exon 10, and also the ASXL1 mutation poorly affect the disease outcome of this disease.

MEF2C Phosphorylation Is Required for Chemotherapy Resistance in Acute Myeloid Leukemia

In acute myeloid leukemia (AML), chemotherapy resistance remains prevalent and poorly understood. Using functional proteomics of patient AML specimens, we identified MEF2C S222 phosphorylation as a specific marker of primary chemoresistance. We found that Mef2cS222A/S222A knock-in mutant mice engineered to block MEF2C phosphorylation exhibited normal hematopoiesis, but were resistant to leukemogenesis induced by MLL-AF9 MEF2C phosphorylation was required for leukemia stem cell maintenance and induced by MARK kinases in cells. Treatment with the selective MARK/SIK inhibitor MRT199665 caused apoptosis and conferred chemosensitivity in MEF2C-activated human AML cell lines and primary patient specimens, but not those lacking MEF2C phosphorylation. These findings identify kinase-dependent dysregulation of transcription factor control as a determinant of therapy response in AML, with immediate potential for improved diagnosis and therapy for this disease.Significance: Functional proteomics identifies phosphorylation of MEF2C in the majority of primary chemotherapy-resistant AML. Kinase-dependent dysregulation of this transcription factor confers susceptibility to MARK/SIK kinase inhibition in preclinical models, substantiating its clinical investigation for improved diagnosis and therapy of AML. Cancer Discov; 8(4); 478-97. ©2018 AACR.This article is highlighted in the In This Issue feature, p. 371.

Grsf1-induced translation of the SNARE protein Use1 is required for expansion of the erythroid compartment

Induction of cell proliferation requires a concomitant increase in the synthesis of glycosylated lipids and membrane proteins, which is dependent on ER-Golgi protein transport by CopII-coated vesicles. In this process, retrograde transport of ER resident proteins from the Golgi is crucial to maintain ER integrity, and allows for anterograde transport to continue. We previously showed that expression of the CopI specific SNARE protein Use1 (Unusual SNARE in the ER 1) is tightly regulated by eIF4E-dependent translation initiation of Use1 mRNA. Here we investigate the mechanism that controls Use1 mRNA translation. The 5'UTR of mouse Use1 contains a 156 nt alternatively spliced intron. The non-spliced form is the predominantly translated mRNA. The alternatively spliced sequence contains G-repeats that bind the RNA-binding protein G-rich sequence binding factor 1 (Grsf1) in RNA band shift assays. The presence of these G-repeats rendered translation of reporter constructs dependent on the Grsf1 concentration. Down regulation of either Grsf1 or Use1 abrogated expansion of erythroblasts. The 5'UTR of human Use1 lacks the splice donor site, but contains an additional upstream open reading frame in close proximity of the translation start site. Similar to mouse Use1, also the human 5'UTR contains G-repeats in front of the start codon. In conclusion, Grsf1 controls translation of the SNARE protein Use1, possibly by positioning the 40S ribosomal subunit and associated translation factors in front of the translation start site.

Blastic leukaemias (AML): a biologist's view

Acute myeloblastic leukaemia is characterised by the extreme clonal proliferation of haematopoietic precursor cells with abnormal or arrested differentiation. Chemotherapy of acute leukaemia is channelled towards the reduction and eradication of leukaemic cells. However, relapse is generally assumed to occur in residual host cells, which are refractory to or elude therapy. The cancer stem cell hypothesis has gained considerable importance in recent years and could interpret this behaviour. This persuasive theory states that cells within a tumour are organised in a hierarchy similar to that of normal tissues and are maintained by a small subset of cells responsible for tumour dormancy. These cells, defined as 'tumour initiating cells' (TICs), possess several properties of normal tissue stem cells. Recently, the TICs associated with AML have been shown to comprise distinct, hierarchically arranged classes similar to those observed for haematopoietic stem cells. We know now that the growth and survival of blasts in AML are driven by the same growth factors that stimulate normal cells. Furthermore, direct evidence of the role of membrane stem cell factor and its receptor c-Kit in cell-cell interactions and cell survival in primary AML blasts have been provided, defining the importance of juxtacrine stimulation. Inhibition of c-Kit signalling induces combinations of cell death: autophagy (compensatory mechanism towards survival) and apoptosis. While recent work confirmed that c-Kit inhibitors reduce cancer cell proliferation, it also demonstrated that future inappropriate prescriptions could cause normal tissue deterioration. The purpose of this paper was to review some of the salient features of leukaemic blasts in support of the proposal that research into neoplasia be increased. Rather than presenting the details of various studies, I have attempted to indicate general areas in which work has been done or is in progress. It is hoped that this survey of the subject will demonstrate a variety of opportunities for additional research in human neoplasia.

GAS6 expression identifies high-risk adult AML patients: potential implications for therapy

Emerging data demonstrate important roles for the TYRO3/AXL/MERTK receptor tyrosine kinase (TAM RTK) family in diverse cancers. We investigated the prognostic relevance of GAS6 expression, encoding the common TAM RTK ligand, in 270 adults (n=71 aged <60 years; n=199 aged ≥60 years) with de novo cytogenetically normal acute myeloid leukemia (CN-AML). Patients expressing GAS6 (GAS6+), especially those aged ≥60 years, more often failed to achieve a complete remission (CR). In all patients, GAS6+ patients had shorter disease-free (DFS) and overall (OS) survival than patients without GAS6 expression (GAS6−). After adjusting for other prognostic markers, GAS6+ predicted CR failure (P=0.02), shorter DFS (P=0.004) and OS (P=0.04). To gain further biologic insights, we derived a GAS6-associated gene-expression signature (P<0.001) that in GAS6+ patients included overexpressed BAALC and MN1, known to confer adverse prognosis in CN-AML, and overexpressed CXCL12, encoding stromal cell-derived factor, and its receptor genes, CXCR4 and CXCR7. This study reports for the first time that GAS6 expression is an adverse prognostic marker in CN-AML. Although GAS6 decoy receptors are not yet available in the clinic for GAS6+ CN-AML therapy, potential alternative therapies targeting GAS6+-associated pathways, e.g., CXCR4 antagonists may be considered for GAS6+ patients to sensitize them to chemotherapy.

The Src and c-Kit kinase inhibitor dasatinib enhances p53-mediated targeting of human acute myeloid leukemia stem cells by chemotherapeutic agents

The SRC family kinases (SFKs) and the receptor tyrosine kinase c-Kit are activated in human acute myeloid leukemia (AML) cells. We show here that the SFKs LYN, HCK, or FGR are overexpressed and activated in AML progenitor cells. Treatment with the SFK and c-KIT inhibitor dasatinib selectively inhibits human AML stem/progenitor cell growth in vitro. Importantly, dasatinib markedly increases the elimination of AML stem cells capable of engrafting immunodeficient mice by chemotherapeutic agents. In vivo dasatinib treatment enhances chemotherapy-induced targeting of primary murine AML stem cells capable of regenerating leukemia in secondary recipients. Our studies suggest that enhanced targeting of AML cells by the combination of dasatinib with daunorubicin may be related to inhibition of AKT-mediated human mouse double minute 2 homolog phosphorylation, resulting in enhanced p53 activity in AML cells. Combined treatment using dasatinib and chemotherapy provides a novel approach to increasing p53 activity and enhancing targeting of AML stem cells.

Quizartinib (AC220) is a potent second generation class III tyrosine kinase inhibitor that displays a distinct inhibition profile against mutant-FLT3, -PDGFRA and -KIT isoforms

Background

Activating mutations of class III receptor tyrosine kinases (RTK) FLT3, PDGFR and KIT are associated with multiple human neoplasms including hematologic malignancies, for example: systemic mast cell disorders (KIT), non-CML myeloproliferative neoplasms (PDGFR) and subsets of acute leukemias (FLT3 and KIT). First generation tyrosine kinase inhibitors (TKI) are rapidly being integrated into routine cancer care. However, the expanding spectrum of TK-mutations, bioavailability issues and the emerging problem of primary or secondary TKI-therapy resistance have lead to the search for novel second generation TKIs to improve target potency and to overcome resistant clones.

Quizartinib was recently demonstrated to be a selective FLT3 inhibitor with excellent pharmacokinetics and promising in vivo activity in a phase II study for FLT3 ITD + AML patients. In vitro kinase assays have suggested that in addition to FLT3, quizartinib also targets related class III RTK isoforms.

Methods

Various FLT3 or KIT leukemia cell lines and native blasts were used to determine the antiproliferative and proapoptotic efficacy of quizartinib. To better compare differences between the mutant kinase isoforms, we generated an isogenic BaF3 cell line expressing different FLT3, KIT or BCR/ABL isoforms. Using immunoblotting, we examined the effects of quizartinib on activation of mutant KIT or FLT3 isoforms.

Results

Kinase inhibition of (mutant) KIT, PDGFR and FLT3 isoforms by quizartinib leads to potent inhibition of cellular proliferation and induction of apoptosis in in vitro leukemia models as well as in native leukemia blasts treated ex vivo. However, the sensitivity patterns vary widely depending on the underlying (mutant)-kinase isoform, with some isoforms being relatively insensitive to this agent (e.g. FLT3 D835V and KIT codon D816 mutations). Evaluation of sensitivities in an isogenic cellular background confirms a direct association with the underlying mutant-TK isoform – which is further validated by immunoblotting experiments demonstrating kinase inhibition consistent with the cellular sensitivity/resistance to quizartinib.

Conclusion

Quizartinib is a potent second-generation class III receptor TK-inhibitor – but specific, mutation restricted spectrum of activity may require mutation screening prior to therapy.

Autocrine activation of the MET receptor tyrosine kinase in acute myeloid leukemia

Although the treatment of acute myeloid leukemia (AML) has improved substantially in the past three decades, more than half of all patients develop disease that is refractory to intensive chemotherapy. Functional genomics approaches offer a means to discover specific molecules mediating the aberrant growth and survival of cancer cells. Thus, using a loss-of-function RNA interference genomic screen, we identified the aberrant expression of hepatocyte growth factor (HGF) as a crucial element in AML pathogenesis. We found HGF expression leading to autocrine activation of its receptor tyrosine kinase, MET, in nearly half of the AML cell lines and clinical samples we studied. Genetic depletion of HGF or MET potently inhibited the growth and survival of HGF-expressing AML cells. However, leukemic cells treated with the specific MET kinase inhibitor crizotinib developed resistance resulting from compensatory upregulation of HGF expression, leading to the restoration of MET signaling. In cases of AML where MET is coactivated with other tyrosine kinases, such as fibroblast growth factor receptor 1 (FGFR1), concomitant inhibition of FGFR1 and MET blocked this compensatory HGF upregulation, resulting in sustained logarithmic cell killing both in vitro and in xenograft models in vivo. Our results show a widespread dependence of AML cells on autocrine activation of MET, as well as the key role of compensatory upregulation of HGF expression in maintaining leukemogenic signaling by this receptor. We anticipate that these findings will lead to the design of additional strategies to block adaptive cellular responses that drive compensatory ligand expression as an essential component of the targeted inhibition of oncogenic receptors in human cancers.

ENMD-2076 is an orally active kinase inhibitor with antiangiogenic and antiproliferative mechanisms of action

ENMD-2076 is a novel orally active, small molecule kinase inhibitor with a mechanism of action involving several pathways key to tumor growth and survival: angiogenesis, proliferation, and the cell cycle. ENMD-2076 has selective activity against the mitotic kinase Aurora A, as well as kinases involved in angiogenesis (VEGFRs, FGFRs). ENMD-2076 inhibited the growth in vitro of a wide range of human solid tumor and hematopoietic cancer cell lines with IC(50) values ranging from 0.025 to 0.7 μmol/L. ENMD-2076 was also shown to induce regression or complete inhibition of tumor growth in vivo at well-tolerated doses in tumor xenograft models derived from breast, colon, melanoma, leukemia, and multiple myeloma cell lines. Pharmacodynamic experiments in vivo showed that in addition to inhibiting Aurora A, single doses of ENMD-2076 had sustained inhibitory effects on the activation of Flt3 as well as the angiogenic tyrosine kinases, VEGFR2/KDR and FGFR1 and 2. ENMD-2076 was shown to prevent the formation of new blood vessels and regress formed vessels in vivo at doses equivalent to those that gave substantial activity in tumor xenograft models. These results indicate that ENMD-2076 is a well-tolerated, orally active multitarget kinase inhibitor with a unique antiangiogenic/antiproliferative profile and provides strong preclinical support for use as a therapeutic for human cancers. Several phase 1 studies involving ENMD-2076 have been recently completed, and the compound is currently being evaluated in a phase 2 clinical trial in patients with platinum-resistant ovarian cancer.

Targeting the c-kit receptor in the treatment of acute myelogenous leukemia

Acute myelogenous leukemia (AML) is a difficult disease to treat. Novel treatment strategies, including molecular targeted therapy, are being explored. The c-kit receptor represents a potential therapeutic target for AML. The receptor is expressed on more than 10% of blasts in 64% of patients with de novo AML and 95% of those with relapsed AML. It mediates proliferation and anti-apoptotic effects in AML. This review discusses the biology of c-kit in normal and malignant hematopoiesis and the recent clinical trials targeting c-kit in AML.

JNJ-28312141, a novel orally active colony-stimulating factor-1 receptor/FMS-related receptor tyrosine kinase-3 receptor tyrosine kinase inhibitor with potential utility in solid tumors, bone metastases, and acute myeloid leukemia

There is increasing evidence that tumor-associated macrophages promote the malignancy of some cancers. Colony-stimulating factor-1 (CSF-1) is expressed by many tumors and is a growth factor for macrophages and mediates osteoclast differentiation. Herein, we report the efficacy of a novel orally active CSF-1 receptor (CSF-1R) kinase inhibitor, JNJ-28312141, in proof of concept studies of solid tumor growth and tumor-induced bone erosion. H460 lung adenocarcinoma cells did not express CSF-1R and were not growth inhibited by JNJ-28312141 in vitro. Nevertheless, daily p.o. administration of JNJ-28312141 caused dose-dependent suppression of H460 tumor growth in nude mice that correlated with marked reductions in F4/80(+) tumor-associated macrophages and with increased plasma CSF-1, a possible biomarker of CSF-1R inhibition. Furthermore, the tumor microvasculature was reduced in JNJ-28312141-treated mice, consistent with a role for macrophages in tumor angiogenesis. In separate studies, JNJ-28312141 was compared with zoledronate in a model in which MRMT-1 mammary carcinoma cells inoculated into the tibias of rats led to severe cortical and trabecular bone lesions. Both agents reduced tumor growth and preserved bone. However, JNJ-28312141 reduced the number of tumor-associated osteoclasts superior to zoledronate. JNJ-28312141 exhibited additional activity against FMS-related receptor tyrosine kinase-3 (FLT3). To more fully define the therapeutic potential of this new agent, JNJ-28312141 was evaluated in a FLT3-dependent acute myeloid leukemia tumor xenograft model and caused tumor regression. In summary, this novel CSF-1R/FLT3 inhibitor represents a new agent with potential therapeutic activity in acute myeloid leukemia and in settings where CSF-1-dependent macrophages and osteoclasts contribute to tumor growth and skeletal events.

Leukemic cells create bone marrow niches that disrupt the behavior of normal hematopoietic progenitor cells

The host tissue microenvironment influences malignant cell proliferation and metastasis, but little is known about how tumor-induced changes in the microenvironment affect benign cellular ecosystems. Applying dynamic in vivo imaging to a mouse model, we show that leukemic cell growth disrupts normal hematopoietic progenitor cell (HPC) bone marrow niches and creates abnormal microenvironments that sequester transplanted human CD34+ (HPC-enriched) cells. CD34+ cells in leukemic mice declined in number over time and failed to mobilize into the peripheral circulation in response to cytokine stimulation. Neutralization of stem cell factor (SCF) secreted by leukemic cells inhibited CD34+ cell migration into malignant niches, normalized CD34+ cell numbers, and restored CD34+ cell mobilization in leukemic mice. These data suggest that the tumor microenvironment causes HPC dysfunction by usurping normal HPC niches and that therapeutic inhibition of HPC interaction with tumor niches may help maintain normal progenitor cell function in the setting of malignancy.

Expression of stem cell factor and its receptor, c-kit, in human oral mesenchymal cells

Stem cell factor (SCF) is the pleiotropic ligand for the tyrosine kinase receptor, c-kit. Ligand and receptor are usually expressed in different cell types, and binding of SCF to c-kit promotes cell proliferation, differentiation, and recruitment of progenitor cells in various biologic systems. However, the localization of these two molecules in cells of the oral cavity has not been systematically examined. We investigated the expression of SCF and c-kit in human dental pulp (HDP) cells as well as in human gingival fibroblasts (HGF). Both alternatively spliced isoforms of SCF were detected (through reverse transcription-polymerase chain reaction) in RNA obtained from the two cell types. Western analysis established that both cell types express SCF and/or c-kit, whereas flow cytometry demonstrated distinct cell populations expressing only the ligand (SCF), only the receptor (c-kit), or co-expressing the two. HDP cultures showed higher soluble SCF (sSCF) production associated with faster cell growth, as compared with HGF cultures. In both cell types, however, sSCF levels appeared to increase as a result of in vitro aging and/or differentiation.

Co expression of SCF and KIT in gastrointestinal stromal tumours (GISTs) suggests an autocrine/paracrine mechanism

KIT is a tyrosine kinase receptor expressed by several tumours, which has for specific ligand the stem cell factor (SCF). KIT is the main oncogene in gastrointestinal stromal tumours (GISTs), and gain-of-function KIT mutations are present in 70% of these tumours. The aim of the study was to measure and investigate the mechanisms of KIT activation in 80 KIT-positive GIST patients. KIT activation was quantified by detecting phosphotyrosine residues in Western blotting. SCF production was determined by reverse transcriptase-PCR, ELISA and/or immunohistochemistry. Primary cultures established from three GISTs were also analysed. The results show that KIT activation was detected in all cases, even in absence of KIT mutations. The fraction of activated KIT was not correlated with the mutational status of GISTs. Membrane and soluble isoforms of SCF mRNA were present in all GISTs analysed. Additionally, SCF was also detected in up to 93% of GISTs, and seen to be present within GIST cells. Likewise, the two SCF mRNA isoforms were found to be expressed in GIST-derived primary cultures. Thus, KIT activation in GISTs may in part result from the presence of SCF within the tumours.

E2a/Pbx1 oncogene inhibits terminal differentiation but not myeloid potential of pro-T cells

E2a/Pbx1 is a fusion oncoprotein resulting from the t(1;19) translocation found in human pre-B acute lymphocytic leukemia and in a small number of acute T-lymphoid and myeloid leukemias. It was previously suggested that E2a/Pbx1 could cooperate with normal or oncogenic signaling pathways to immortalize myeloid and lymphoid progenitor cells. To address this question, we introduced the receptor of the macrophage-colony-stimulating factor (M-CSF-R) in pro-T cells immortalized by a conditional, estradiol-dependent, E2a/Pbx1-protein, and continuously proliferating in response to stem cell factor and interleukin-7. We asked whether M-CSF-R would be functional in an early T progenitor cell and influence the fate of E2a/Pbx1-immortalized cells. E2a-Pbx1 immortalized pro-T cells could proliferate and shifted from lymphoid to myeloid lineage after signaling through exogenously expressed M-CSF-R, irrespective of the presence of estradiol. However, terminal macrophage differentiation of the cells was obtained only when estradiol was withdrawn from cultures. This demonstrated that M-CSF-R is functional for proliferation and differentiation signaling in a T-lymphoid progenitor cell, which, in addition, unveiled myeloid potential of pro-T progenitors. Moreover, the block of differentiation induced by the E2a/Pbx1 oncogene could be modulated by hematopoietic cytokines such as M-CSF, suggesting plasticity of leukemic progenitor cells. Finally, additional experiments suggested that PU.1 and eight twenty-one transcriptional regulators might be implicated in the mechanisms of oncogenesis by E2a/Pbx1.

Translation initiation factor 4E inhibits differentiation of erythroid progenitors

Stem cell factor (SCF) delays differentiation and enhances the expansion of erythroid progenitors. Previously, we performed expression-profiling experiments to link signaling pathways to target genes using polysome-bound mRNA. SCF-induced phosphoinositide-3-kinase (PI3K) appeared to control polysome recruitment of specific mRNAs associated with neoplastic transformation. To evaluate the role of mRNA translation in the regulation of expansion versus differentiation of erythroid progenitors, we examined the function of the eukaryote initiation factor 4E (eIF4E) in these cells. SCF induced a rapid and complete phosphorylation of eIF4E-binding protein (4E-BP). Overexpression of eIF4E did not induce factor-independent growth but specifically impaired differentiation into mature erythrocytes. Overexpression of eIF4E rendered polysome recruitment of mRNAs with structured 5' untranslated regions largely independent of growth factor and resistant to the PI3K inhibitor LY294002. In addition, overexpression of eIF4E rendered progenitors insensitive to the differentiation-inducing effect of LY294002, indicating that control of mRNA translation is a major pathway downstream of PI3K in the regulation of progenitor expansion.

CHIR-258: A Potent Inhibitor of FLT3 Kinase in Experimental Tumor Xenograft Models of Human Acute Myelogenous Leukemia

PURPOSE

Fms-like tyrosine kinase 3 (FLT3) encodes a receptor tyrosine kinase (RTK) for which activating mutations have been identified in a proportion of acute myelogenous leukemia (AML) patients and associated with poor clinical prognosis. Given the relevance of FLT3 mutations in AML, we investigated the activity of CHIR-258, an orally active, multitargeted small molecule, with potent activity against FLT3 kinase and class III, IV, and V RTKs involved in endothelial and tumor cell proliferation in AML models.

EXPERIMENTAL DESIGN

CHIR-258 was tested on two human leukemic cell lines in vitro and in vivo with differing FLT3 mutational status [MV4;11 cells express FLT3 internal tandem duplications (ITD) versus RS4;11 cells with wild-type (WT) FLT3].

RESULTS

Antiproliferative activity of CHIR-258 against MV4;11 was approximately 24-fold greater compared with RS4;11, indicating more potent inhibition against cells with constitutively activated FLT3 ITD. Dose-dependent down modulation of receptor phosphorylation and downstream signaling [signal transducer and activator of transcription 5 (STAT5) and extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase] in MV4;11 cells with CHIR-258 confirmed the molecular mechanism of action. Target modulation of phospho-FLT3, phospho-STAT5, and phospho-ERK in MV4;11 tumors was achieved at biologically active doses of CHIR-258. Tumor regressions and eradication of AML cells from the bone marrow were shown in s.c. and bone marrow engraftment leukemic xenograft models. Tumor responses were characterized by decreased cellular proliferation and positive immunohistochemical staining for active caspase-3 and cleaved poly(ADP-ribose) polymerase, suggesting cell death was mediated in part via apoptosis.

CONCLUSIONS

Our data indicate that CHIR-258 may be an effective therapy in FLT3-associated AML and warrants clinical trials.