ALBERT

Description: Constitutively activating internal tandem duplication (ITD) mutations of the receptor tyrosine kinase FLT3 play an important role in leukemogenesis. They are the most common genetic alteration in AML and their presence is associated with poor prognosis. 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 signaling by cDNA microarray analysis. In order to minimize gene expression changes that might be drug specific and not related to FLT3 inhibition or might be cell-type specific, we used three different FLT3 inhibitors, CEP-701, CEP-5214, and AG1296, and three different constitutively activated FLT3-expressing leukemia-derived cell lines, EOL-1, MOLM-14 and MV4-11. We considered to be FLT3 responsive only those genes whose expression consistently changed in response to FLT3 inhibition by each of the three FLT3 inhibitors in all of the cell lines. RNA for hybridization to the microarrays was harvested from cells both before and after increasing times of FLT3 inhibition to determine genes which decreased or increased in response to FLT3 signaling. In addition, because the inhibitors are all reversible, RNA was also harvested from the cells at increasing times after release from FLT3 inhibition. This enabled us to confirm that, for example, genes whose expression appeared FLT3 dependent and were thus down-regulated by FLT3 inhibition, returned towards normal levels after FLT3 signaling was allowed to resume. Statistical analysis of the microarray results indicated a limited set of genes are highly and consistently affected by FLT3 inhibition and return toward pretreatment levels after release from inhibitor. We confirmed the cDNA microarray data using quantitative real-time PCR. Several of the most significantly affected genes are involved in the Ras/MAPK pathway including DUSP6, DUSP7, MAPK6, TNF, and cMyc. Other sets of genes are involved in JAK/STAT or Wnt signaling pathways including Pim-1, cMyc, Cyclin D3, IL4 receptor, and CISH. These genes are all consistently down-regulated after FLT3 inhibition. These data further confirm the role of constitutively activating FLT3 in mediating multiple signal transduction pathways. We also found several transcriptional factors (RUNX1/AML1, MAFG, XBP1, TGFBI4, and BRD8), several genes involved in receptor-mediated signaling (IL1RAP, CDC42EP3, PLAUR, LY64), and several genes involved in cell proliferation, metabolism, or structure (BCL7A, APTX, GHRH, SET7, ATAD2, CLIC1, CRIP2, MRPL12, VIM) to be consistently differentially expressed. In summary, we have found by cDNA microarray analysis and confirmed by QPCR, a consistent pattern of FLT3 dependent gene expression. The alteration of the gene expression profile in these cells is likely the mechanism of FLT3-mediated leukemogenisis.

Description: The FLT3 receptor is a potential target in AML due to its role in leukemogenesis and its high degree of expression on blasts from approximately 90% of acute myeloid leukemia (AML) patients. In addition, mutant forms of FLT3, including internal tandem duplications (ITD) in the juxtamembrane region and point mutations in the kinase domain, constitutively activate FLT3 signaling. ITD mutations in particular are also associated with poor prognosis. A number of small molecule tyrosine kinase inhibitors (TKI) against FLT3 are currently in clinical trials and have shown some clinical activity. However, TKIs have various limitations, including their lack of specificity, which may produce toxicities, and can select for drug resistant cells. In an attempt to overcome some of these limitations and to generate new agents which might cooperate in targeting FLT3, we generated a fully humanized phage display monoclonal antibody (EB10). This antibody is capable of inhibiting both ligand-activated wild-type and, to a lesser degree, ligand-independent mutant FLT3 signaling. When EB10 is used to treat cells expressing activated FLT3, inhibition of downstream pathways including STAT5, AKT and MAPK are also frequently seen. EB10 treatment of cells expressing FLT3 in the presence of NK cells leads to antibody-dependent cell-mediated cytotoxicity (ADCC). EB10 treatment of NOD/SCID mice injected with FLT3 expressing AML cell lines or with primary AML blasts significantly prolongs survival and/or reduces engraftment of leukemic cells. EB10 proved efficacious in vivo against cells even when in vitro EB10 treatment did not significantly reduce FLT3 signaling. This indicates that ADCC may be the primary mechanism mediating cytotoxicity as opposed to direct FLT3 inhibition. In contrast to the effects on AML cell lines and primary samples, EB10 treatment did not significantly reduce NOD/SCID engraftment of normal human CD34+ hematopoietic stem cells. Anti-FLT3 antibodies, like EB10, may be a promising therapeutic agent that can specifically target malignant cells with limited toxicities against normal hematopoietic stem cells and should be considered for clinical trials.

Description: The class III receptor tyrosine kinase, FLT3, is expressed by 〉90% of B-lineage acute lymphoblastic leukemias (ALL) blasts. In addition, it is expressed at extremely high levels in ALL patients with MLL-rearrangements or hyperdiploidy and sometimes mutated in these same patients. In this report, we investigated the effects of EB10, an anti-human FLT3 monoclonal antibody capable of preventing binding of FLT3 ligand (FL), on ALL cell lines and primary cells. In vitro studies, examining the ability of EB10 to inhibit FLT3 activation and downstream signaling in ALL cell lines and primary blasts, yielded variable results. In some cell lines FLT3 phosphorylation was inhibited and with it, downstream activation of pathways involving MAPK, AKT, and STAT5 phosphorylation. However, several cell lines actually exhibited FLT3 activation upon antibody treatment, possibly because of antibody-mediated receptor dimerization, and subsequent activation of downstream pathways. Nevertheless, through antibody-mediated cellular cytotoxicity (ADCC) such an antibody could still prove efficacious against leukemia cells in vivo. In fact, EB10 treatment significantly prolongs survival and/or reduces engraftment of ALL cell lines and primary ALL blasts in NOD/SCID mice. This effect might be even more pronounced in a host that was less immune compromised than are NOD/SCID mice. The leukemic cells surviving EB10 treatment in the mice were characterized by FACS analysis and found to express low levels or no FLT3. In contrast to the reduction in engraftment of human ALL primary blasts, EB10 treatment of NOD/SCID mice did not reduce engraftment of human hematopoietic CD34+ cells. Taken together, these data demonstrate that EB10 is selectively cytotoxic to ALL blasts while having little effect on normal hematopoiesis. Such an antibody, either naked or conjugated to radioactive isotopes or cytotoxic agents, may prove useful in the therapy of infant ALL as well as childhood and adult ALL patients whose blasts typically express FLT3.

Description: 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)

Description: The FLT3 receptor tyrosine kinase is highly expressed in most acute leukemias and frequently mutated in acute myeloid leukemia (AML). The mutated form of the receptor is constitutively activated and known to play an important role in AML, but the activation state of the overexpressed wild-type (wt) receptor is, at present, unknown. In this study, we examined the activation state of the wild-type receptor in AML. We found that the wild-type receptor was constitutively phosphorylated/activated in 8 of 12 primary AML samples and 4 of 13 leukemia cell lines. To explain why wtFLT3 is often activated, we investigated the expression of its ligand, FL, by these same cells. Coexpression of FL with FLT3 was a universal finding in both primary AML samples and leukemic-derived cell lines. To further prove that autocrine signaling was accounting for the activation, we showed that conditioned media but not fresh media was able to activate FLT3. In addition, an antibody that blocks binding of ligand to the receptor blocks FLT3 activation. Finally, depletion of FL from conditioned media is able to block the activation of FLT3. Taken together, these findings represent strong evidence that wtFLT3 is often constitutively activated in AML and thus, like its mutated form, might contribute to the altered signaling that characterizes leukemogenesis.

Description: FMS-like tyrosine kinase-3 (FLT3) is a Class III receptor tyrosine kinase that is important for normal hematopoiesis. Activating mutations of FLT3 by internal tandem duplications (ITDs) in the juxtamembrane region are the most common molecular aberrations found in acute myeloid leukemia (AML). The contributions of FLT3 activating mutations in the leukemic transformation of normal human hematopoietic stem/progenitor cells (HSCs) have not yet been fully elucidated. In this study, using a single lentiviral vector containing two promoters, we achieved consistent and efficient coexpression of FLT3/ITD and green fluorescent protein (GFP) in transduced human CD34+ HSCs. When cultured in medium containing SCF, TPO and FLT3 ligand (FL), FLT3/ITD-transduced cells survived with enhanced self-renewal and survival potential, which was not affected by the withdrawal of FL. These cells retained a surface immunophenotype typical of HSCs (CD34+CD38−). Compared to cells transduced with a vector expressing GFP alone, FLT3/ITD-transduced HSCs had a higher fraction of cells in cell cycle. Clonogenic assays showed that FLT3/ITD-transduced HSCs produced fewer CFU-GM, implying that they were at least partially blocked in their ability to differentiate along the myeloid lineage. FLT3/ITD-transduced HSCs were more sensitive to the induction of cytotoxicity by CEP-701, a selective FLT3 inhibitor. In the FLT3/ITD-transduced HSCs, we detected increased expression of Pim-1, a serine/threonine kinase with an important role in cell survival, proliferation and differentiation, c-myc, a transcription factor involved in cell proliferation and cell cycle regulation, and Cyclin D3, a key factor in cell cycle regulation, each of which may contribute to the altered genetic program instituted by FLT3/ITD signaling. These results together indicate that FLT3/ITD mutations may contribute to leukemic transformation of normal HSCs by prolonging survival, promoting proliferation, and blocking differentiation. CEP-701 may act as a potent agent for AML stem cells harboring FLT3/ITD mutations.

Description: Constitutive activation of FMS-like tyrosine kinase 3 (FLT3) by internal tandem duplication (ITD) mutations is one of the most common molecular alterations known in acute myeloid leukemia (AML). To investigate the role FLT3/ITD mutations play in the development of leukemia, we generated a FLT3/ITD knock-in mouse model by inserting an ITD mutation into the juxtamembrane domain of murine Flt3. FLT3wt/ITD mice developed myeloproliferative disease, characterized by splenomegaly, leukocytosis, and myeloid hypercellularity, which progressed to mortality by 6 to 20 months. Bone marrow (BM) and spleen from FLT3wt/ITD mice had an increased fraction of granulocytes/monocytes and dendritic cells, and a decreased fraction of B-lymphocytes. No sign of acute leukemia was observed over the lifetime of these mice. BM from FLT3wt/ITD mice showed enhanced potential to generate myeloid colonies in vitro. BM from FLT3wt/ITD mice also produced more spleen colonies in the in vivo colony-forming unit (CFU)–spleen assay. In the long-term competitive repopulation assay, BM cells from FLT3wt/ITD mice outgrew the wild-type competitor cells and showed increased myeloid and reduced lymphoid expansion activity. In summary, our data indicate that expression of FLT3/ITD mutations alone is capable of conferring normal hematopoietic stem/progenitor cells (HSPCs) with enhanced myeloid expansion. It also appears to suppress B lymphoid maturation. Additional cooperative events appear to be required to progress to acute leukemia.

Description: Activation of FLT3 by internal tandem duplication (ITD) mutations in the juxtamembrane domain is the most common molecular alteration known in AML and confers poor prognosis. Homozygous or hemizygous ITD mutations, with the loss of wild type FLT3, result in even worse prognosis. It has been previously reported that FLT3/ITD activates different signaling pathways as compared to wt FLT3. Several models designed to study the roles of FLT3/ITD in leukemogenesis have been reported, utilizing either retroviral infection of murine BM or transgenic expression off strong promoters. Those models have shown myeloproliferative disease (MPD) with FLT3/ITD expression alone and full transformation to leukemia when additional genetic alterations are added. However, these models utilize high level expression of FLT3/ITD, and potentially express the gene at inappropriate stages of development. In addition, retroviral integration sites could be playing an active role. To more closely simulate and study the in vivo biological impact of FLT3/ITD mutations in the development of leukemia, we generated a FLT3/ITD “knock-in” mouse model by inserting an ITD mutation into the juxtamembrane region of murine FLT3 genomic DNA. Young FLT3 wt/ITD mice showed signs of MPD which progressed to fatality at the age of 6–16 months. Older FLT3wt/ITD mice had significant splenomegaly with disruption of the normal splenic architecture. Increased WBC and reduced RBC counts were detected in the peripheral blood of these mice, with elevated monocyte and neutrophil counts. BM was hypercellular with an increased fraction of granulocytes/monocytes and their progenitors, dendritic cells (DCs), and a reduction in the fraction of B lymphocytes. No signs of leukemia were observed over the lifetime of these mice. Clonogenic assay demonstrated that BM from FLT3wt/ITD mice contained increased granulocytic/monocytic colony-forming cells. In addition, these cells were immortalized as they could undergo long-term serial culture in cytokine-supplemented liquid medium. BM from FLT3wt/ITD mice also generated more colonies in the in vivo spleen-CFU assay, and showed an enhanced ability to repopulate lethally irradiated recipients in the long-term competitive repopulation assay. FLT3ITD/ITD mice developed fatal MPD with a shorter latency. Leukocytosis and BM hypercellularity were more pronounced for these mice, with an even higher fraction of granulocytic/monocytic progenitors and a tremendous suppression of B lymphocytes in the BM. BM from these mice displayed reduced potential for reopulation in the long-term competitive repopulation assay. Some FLT3ITD/ITD mice spontaneously developed leukemia-like disease with a lag of 2–6 months. Histopathology revealed complete effacement of the normal spleen architecture, extensive infiltration of the liver and meninges, and complete replacement of the BM with immature myeloid cells. In summary, these data indicate that expression of a FLT3/ITD mutation alone is capable of partially transforming normal hematopoietic stem cells and progenitors to a phenotype of MPD. Additional cooperative events are likely required to progress to leukemia. These will serve as excellent models to study the pathways by which FLT3/ITD signaling contributes to leukemogenesis.

Description: There is a critical need for new agents with novel therapeutic targets and improved safety profiles in high-risk acute lymphoblastic leukemia (ALL), which is a significant cause of morbidity and mortality in pediatric and adult populations. Phenotypic high-throughput chemical screens allow for discovery of small molecules that modulate complex phenotypes and provide lead compounds for novel therapies; however, identification of their mechanistically relevant targets remains a major experimental challenge. We applied a chemical genetics approach involving sequential unbiased high-throughput chemical and ultra-complex, genome-scale shRNA screens to address this challenge and identify novel agents in ALL. A cell-based phenotypic high-throughput chemical screen of 115,000 compounds identified 640 compounds that inhibited growth of one or both ALL cell lines with high-risk Mixed Lineage Leukemia (MLL) genetic abnormalities, but did not inhibit the growth of a cell line lacking MLL rearrangement. The most potent and selective 64 were tested on an expanded panel of eight human B-ALL cell lines to identify lead compound STF-118804. STF-118804 inhibited the growth of most B-ALL cell lines with high potency demonstrating IC50 values in the low nanomolar range. Leukemic samples from five pediatric ALL patients were also sensitive to STF-118804 in the low nanomolar range. STF-118804 displayed 5–10 fold more potency against most leukemias in comparison to cycling human (lineage-negative cord blood) and murine (c-kit+ bone marrow) progenitor cells, demonstrating a therapeutic index. STF-118804 displays distinctive cytotoxicity by inducing apoptosis without causing a phase-specific cell cycle arrest. To discover the molecular target of STF-118804, a functional genomic screen was performed to identify shRNAs that conferred sensitivity or resistance to STF-118804, utilizing an ultra-complex (∼25 shRNAs per gene) library targeting in total ∼9300 human genes and 1000s of negative control shRNAs. NAMPT was the most statistically significant gene to confer sensitivity to STF-118804, suggesting that STF-118804 functioned as a NAMPT inhibitor. NAMPT encodes nicotinamide phosphoribosyl transferase, a rate-limiting enzyme in the biosynthesis of nicotinamide adenine dinucleotide (NAD+), a crucial cofactor in many biochemical processes. STF-118804 was confirmed as a novel class of NAMPT inhibitor through metabolic rescue, enzymatic, and genetic studies. STF-118804 displayed strong inhibitory activity in in vitro NAMPT enzymatic assays. Over-expression of wild-type or mutant NAMPT in cells indicated that STF-118804 cytotoxicity is a result of its ability to inhibit NAMPT, and that STF-118804 does not have significant off-target effects on cell viability. The potential efficacy of STF-118804 in vivo was assessed in an orthotopic xenograft model of ALL. Sublethally irradiated immunodeficient mice were transplanted with human ALL cells engineered to constitutively express firefly luciferase. Dosing of STF-118804 was initiated two weeks post-transplant when ALL cells had engrafted and bioluminescent signal was detectable. Mice treated with STF-118804 showed regression of leukemia by bioimaging and significantly extended survival. The leukemia initiating cell (LIC) frequency in STF-118804 treated mice was significantly lower (∼8 fold) than vehicle treated mice, showing that STF-118804 was effective in reducing LICs. In summary, tandem high-throughput screening identified a highly-specific, potent, and structurally novel small molecule inhibitor of NAMPT that is active in ALL. Tandem high throughput screening using chemical and ultra-complex shRNA libraries provides a rapid chemical genetics approach for seamless progression from small molecule lead identification to target discovery and validation. Disclosures: No relevant conflicts of interest to declare.