Novel FLT3 tyrosine kinase inhibitors

Target Therapy for Extramedullary Relapse of FLT3-ITD Acute Myeloid Leukemia: Emerging Data from the Field

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase family member. Mutations in FLT3, as well known, represent the most common genomic alteration in acute myeloid leukemia (AML), identified in approximately one-third of newly diagnosed adult patients. In recent years, this has represented an important therapeutic target. Drugs such as midostaurin, gilteritinib, and sorafenib, either alone in association with conventional chemotherapy, play a pivotal role in AML therapy with the mutated FLT3 gene. A current challenge lies in treating forms of AML with extramedullary localization. Here, we describe the general features of myeloid sarcoma and the ability of a targeted drug, i.e., gilteritinib, approved for relapsed or refractory disease, to induce remission of these extramedullary leukemic localizations in AML patients with FLT3 mutation, analyzing how in the literature, there is an important development of cases describing this promising potential for care.

The Structural Effect of FLT3 Mutations at 835th Position and Their Interaction with Acute Myeloid Leukemia Inhibitors: In Silico Approach

FMS-like tyrosine kinase 3 (FLT3) gene mutations have been found in more than one-third of Acute Myeloid Leukemia (AML) cases. The most common point mutation in FLT3 occurs at the 835th residue (D835A/E/F/G/H/I/N/V/Y), in the activation loop region. The D835 residue is critical in maintaining FLT3 inactive conformation; these mutations might influence the interaction with clinically approved AML inhibitors used to treat the AML. The molecular mechanism of each of these mutations and their interactions with AML inhibitors at the atomic level is still unknown. In this manuscript, we have investigated the structural consequence of native and mutant FLT-3 proteins and their molecular mechanisms at the atomic level, using molecular dynamics simulations (MDS). In addition, we use the molecular docking method to investigate the binding pattern between the FLT-3 protein and AML inhibitors upon mutations. This study apparently elucidates that, due to mutations in the D835, the FLT-3 structure loses its conformation and becomes more flexible compared to the native FLT3 protein. These structural changes are suggested to contribute to the relapse and resistance responses to AML inhibitors. Identifying the effects of FLT3 at the molecular level will aid in developing a personalized therapeutic strategy for treating patients with FLT-3-associated AML.

Deciphering the molecular mechanism of FLT3 resistance mutations

FMS-like tyrosine kinase 3 (FLT3) has been found to be mutated in ~ 30% of acute myeloid leukaemia patients. Small-molecule inhibitors targeting FLT3 that are currently approved or still undergoing clinical trials are subject to drug resistance due to FLT3 mutations. How these mutations lead to drug resistance is hitherto poorly understood. Herein, we studied the molecular mechanism of the drug resistance mutations D835N, Y842S and M664I, which confer resistance against the most advanced inhibitors, quizartinib and PLX3397 (pexidartinib), using enzyme kinetics and computer simulations. In vitro kinase assays were performed to measure the comparative catalytic activity of the native protein and the mutants, using a bacterial expression system developed to this aim. Our results reveal that the differential drug sensitivity profiles can be rationalised by the dynamics of the protein-drug interactions and perturbation of the intraprotein contacts upon mutations. Drug binding induced a single conformation in the native protein, whereas multiple conformations were observed otherwise (in the mutants or in the absence of drugs). The end-point kinetics measurements indicated that the three resistant mutants conferred catalytic activity that is at least as high as that of the reference without such mutations. Overall, our calculations and measurements suggest that the structural dynamics of the drug-resistant mutants that affect the active state and the increased conformational freedom of the remaining inactive drug-bound population are the two major factors that contribute to drug resistance in FLT3 harbouring cancer cells. Our results explain the mechanism of drug resistance mutations and can aid to the design of more effective tyrosine kinase inhibitors.

Rational Design, Synthesis and Biological Evaluation of Pyrimidine-4,6-diamine derivatives as Type-II inhibitors of FLT3 Selective Against c-KIT

FMS-like Tyrosine Kinase 3 (FLT3) is a clinically validated target for acute myeloid leukemia (AML). Inhibitors targeting FLT3 have been evaluated in clinical studies and have exhibited potential to treat FLT3-driven AML. A frequent, clinical limitation is FLT3 selectivity, as concomitant inhibition of FLT3 and c-KIT is thought to cause dose-limiting myelosuppression. Through a rational design approach, novel FLT3 inhibitors were synthesized employing a pyridine/pyrimidine warhead. The most potent compound identified from the studies is compound 13a, which exhibited an IC₅₀ value of 13.9 ± 6.5 nM against the FLT3 kinase with high selectivity over c-KIT. Mechanism of action studies suggested that 13a is a Type-II kinase inhibitor, which was also supported through computer aided drug discovery (CADD) efforts. Cell-based assays identified that 13a was potent on a variety of FLT3-driven cell lines with clinical relevance. We report herein the discovery and therapeutic evaluation of 4,6-diamino pyrimidine-based Type-II FLT3 inhibitors, which can serve as a FLT3-selective scaffold for further clinical development.

Phase IIB trial of oral Midostaurin (PKC412), the FMS-like tyrosine kinase 3 receptor (FLT3) and multi-targeted kinase inhibitor, in patients with acute myeloid leukemia and high-risk myelodysplastic syndrome with either wild-type or mutated FLT3

PURPOSE

Mutations leading to constitutive activation of the FMS-like tyrosine kinase 3 receptor (FLT3) occur in blasts of 30% of patients with acute myeloid leukemia (AML). Midostaurin (PKC412; N-benzoylstaurosporin) is a multitargeted tyrosine kinase inhibitor, with demonstrated activity in patients with AML/myelodysplastic syndrome (MDS) with FLT3 mutations.

PATIENTS AND METHODS

Ninety-five patients with AML or MDS with either wild-type (n = 60) or mutated (n = 35) FLT3 were randomly assigned to receive oral midostaurin at 50 or 100 mg twice daily. The drug was discontinued in the absence of response at 2 months, disease progression, or unacceptable toxicity. Response was defined as complete response, partial response (PR), hematologic improvement, or reduction in peripheral blood or bone marrow blasts by ≥ 50% (BR).

RESULTS

The rate of BR for the population in whom efficacy could be assessed (n = 92) was 71% in patients with FLT3-mutant and 42% in patients with FLT3 wild-type. One PR occurred in a patient with FLT3-mutant receiving the 100-mg dose regimen. Both doses were well-tolerated; there were no differences in toxicity or response rate according to the dose of midostaurin.

CONCLUSION

These results suggest that midostaurin has hematologic activity in both patients with FLT3-mutant and wild-type. The degree of clinical activity observed supports additional studies that combine midostaurin and other agents such as chemotherapy especially in FLT3-mutant AML.

Monitoring of FLT3 phosphorylation status and its response to drugs by flow cytometry in AML blast cells

FLT3 mutation and overexpression in most acute myeloid leukaemia (AML) patients make this tyrosine kinase receptor an attractive therapeutic target. FLT3 kinase inhibitors are actually in clinical trials, thus it is critical to develop a reproducible and standardized method for screening of FLT3 activation and for monitoring its inhibition in response to drug in AML patients. We developed a flow cytometry method to analyse phosphorylated FLT3 (P-FLT3) in samples with <10(5) cells. The method was first validated in FLT3 wild-type (HL60/WT) and mutant (MV4-11/ITD(+)) as well as FLT3 negative (K562) cell lines. The method also proved to be reproducible in AML patient samples. Analysis was performed after exposure to drugs (CEP-701 and SU11657), in vitro and in vivo. In response to increasing drug concentrations, there was a linear reduction in P-FLT3. Intracellular flow cytometry analysis correlated with Western blot and XTT assays; flow cytometry data also correlated with FLT3 mutational status. The results highlight a rapid method to detect P-FLT3 protein at the single cell level by flow cytometry which enables an accurate assessment of FLT3 kinase activity in blast cells in response to novel tyrosine kinase inhibitors.

FLT3 inhibitors for the treatment of autoimmune disease

BACKGROUND

Autoimmune diseases encompass a broad range of illnesses with a variety of underlying causes, some of which are known and some of which remain elusive.

OBJECTIVE

The focus of this review will be on describing the development of a new type of therapy that could potentially treat T cell-mediated autoimmune diseases. Unlike traditional therapies, which have primarily focused on suppressing T cells directly, targeting the step of antigen presentation may allow a less toxic therapy in which autoimmunity is lessened without compromising the entire immune system. This review will outline the science behind the development of the therapy, the roles of dendritic cells in generating autoimmune disease, and the function of the FLT3 receptor in this process.

Prognostic relevance of FLT3-TKD mutations in AML: the combination matters—an analysis of 3082 patients

We characterized the mutational status of the FLT3 tyrosine kinase domain (FLT3-TLD) in 3082 patients with newly diagnosed AML. FLT3-TKD mutations were detected in 147 of 3082 (4.8%) patients. Similar to the FLT3 juxtamembrane domain mutations (FLT3-LM), there was a high correlation of FLT3-TKD mutations with normal karyotype (88 of 1472; 6.0%). FLT3-TKD mutations were most frequent in the AML FAB subtypes M5b (15 of 114; 13.2%), M3v (6 of 51; 11.8%), and M4 (39 of 484; 8.1%). Similar to FLT3-LM, the FLT3-TKD mutations show elevated peripheral leukocytes compared with FLT3wt AML. FLT3-TKD had a high incidence in cases with NPM1 mutations (23 of 262; 8.8%), CEBPA mutations (6 of 76; 7.9%), and NRAS mutations (6 of 78; 7.7%). FLT3-TKD in combination with FLT3-LM (17 of 594 patients; 2.9%) and KITD816 (1 of 44; 2.3%) was rare. Unlike the FLT3-LM, which are associated with inferior survival, prognosis was not influenced by FLT3-TKD in the total cohort of 1720 cases, where follow-up data were available (97 FLT3-TKD; 1623 FLT3-WT). In t(15;17)/PML-RARA with FLT3-TKD mutations, in FLT3-LM/TKD double-mutated, and in MLL-PTD/TKD double-mutated cases prognosis was unfavorably influenced by FLT3-TKD mutations. In contrast, we found an additional favorable impact of FLT3-TKD on EFS in prognostically favorable AML with NPM1- or CEBPA mutations.

Internal tandem duplication of FLT3 (FLT3/ITD) induces increased ROS production, DNA damage, and misrepair: implications for poor prognosis in AML

Activating mutations of the FMS-like tyrosine kinase-3 (FLT3) receptor occur in approximately 30% of acute myeloid leukemia (AML) patients and, at least for internal tandem duplication (ITD) mutations, are associated with poor prognosis. FLT3 mutations trigger downstream signaling pathways including RAS-MAP/AKT kinases and signal transducer and activator of transcription-5 (STAT5). We find that FLT3/ITD mutations start a cycle of genomic instability whereby increased reactive oxygen species (ROS) production leads to increased DNA double-strand breaks (DSBs) and repair errors that may explain aggressive AML in FLT3/ITD patients. Cell lines transfected with FLT3/ITD and FLT3/ITD-positive AML cell lines and primary cells demonstrate increased ROS. Increased ROS levels appear to be produced via STAT5 signaling and activation of RAC1, an essential component of ROS-producing NADPH oxidases. A direct association of RAC1-GTP binding to phosphorylated STAT5 (pSTAT5) provides a possible mechanism for ROS generation. A FLT3 inhibitor blocked increased ROS in FLT3/ITD cells resulting in decreased DSB and increased repair efficiency and fidelity. Our study suggests that the aggressiveness of the disease and poor prognosis of AML patients with FLT3/ITD mutations could be the result of increased genomic instability that is driven by higher endogenous ROS, increased DNA damage, and decreased end-joining fidelity.

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.

Transgenic mice expressing Tel-FLT3, a constitutively activated form of FLT3, develop myeloproliferative disease

Evidence is continuing to accumulate that the FMS-like tyrosine kinase 3 (FLT3) receptor plays an important role in acute leukemias. Acute myeloid leukemia patients often express constitutive active mutant forms of the receptor in their leukemic cells. A t(12;13)(p13;q12) translocation between Tel and the FLT3 receptor was recently described in a patient with myeloproliferative disease (MPD). Here a Tel-FLT3 construct mimicking this fusion protein was used to generate transgenic mice. The fusion protein was previously found to constitutively activate FLT3 signaling and transform Ba/F3 cells. Expression of the fusion protein in the transgenic mice was found in all tissues assayed including spleen, bone marrow (BM), thymus and liver. These mice developed splenomegaly and had a high incidence of MPD with extramedullary hematopoiesis in the liver and lymph nodes. Spleens also had increased dendritic and natural killer cell populations. In vitro analysis of the hematopoietic progenitor cells derived from Tel-FLT3 transgenic mice showed a significant increase in the number of CFU-GM in the BM, and CFU-GM, BFU-E and CFU-GEMM in the spleen. BM also showed significant increases of in vivo CFU-S colonies. Thus, transgenic mice expressing constitutively activated Tel-FLT3 develop MPD with a long latency and also result in the expansion of the hematopoietic stem/progenitor cells.

ABT-869, a multitargeted receptor tyrosine kinase inhibitor: inhibition of FLT3 phosphorylation and signaling in acute myeloid leukemia

In 15% to 30% of patients with acute myeloid leukemia (AML), aberrant proliferation is a consequence of a juxtamembrane mutation in the FLT3 gene (FMS-like tyrosine kinase 3-internal tandem duplication [FLT3-ITD]), causing constitutive kinase activity. ABT-869 (a multitargeted receptor tyrosine kinase inhibitor) inhibited the phosphorylation of FLT3, STAT5, and ERK, as well as Pim-1 expression in MV-4-11 and MOLM-13 cells (IC(50) approximately 1-10 nM) harboring the FLT3-ITD. ABT-869 inhibited the proliferation of these cells (IC(50) = 4 and 6 nM, respectively) through the induction of apoptosis (increased sub-G(0)/G(1) phase, caspase activation, and PARP cleavage), whereas cells harboring wild-type (wt)-FLT3 were less sensitive. In normal human blood spiked with AML cells, ABT-869 inhibited phosphorylation of FLT3 (IC(50) approximately 100 nM), STAT5, and ERK, and decreased Pim-1 expression. In methylcellulose-based colony-forming assays, ABT-869 had no significant effect up to 1000 nM on normal hematopoietic progenitor cells, whereas in AML patient samples harboring both FLT3-ITD and wt-FLT3, ABT-869 inhibited colony formation (IC(50) = 100 and 1000 nM, respectively). ABT-869 dose-dependently inhibited MV-4-11 and MOLM-13 flank tumor growth, prevented tumor formation, regressed established MV-4-11 xenografts, and increased survival by 20 weeks in an MV-4-11 engraftment model. In tumors, ABT-869 inhibited FLT3 phosphorylation, induced apoptosis (transferase-mediated dUTP nick-end labeling [TUNEL]) and decreased proliferation (Ki67). ABT-869 is under clinical development for AML.

The effects of lestaurtinib (CEP701) and PKC412 on primary AML blasts: the induction of cytotoxicity varies with dependence on FLT3 signaling in both FLT3-mutated and wild-type cases

The receptor tyrosine kinase FLT3 is a promising molecular therapeutic target in acute myeloid leukemia (AML). Activating mutations of FLT3 are present in approximately one-third of patients, while many nonmutants show evidence of FLT3 activation, which appears to play a significant role in leukemogenesis. We studied the effects of lestaurtinib (CEP701) and PKC412, 2 small molecule inhibitors of FLT3, on 65 diagnostic AML blast samples. Both agents induced concentration-dependent cytotoxicity in most cases, although responses to PKC412 required higher drug concentrations. Cytotoxic responses were highly heterogeneous and were only weakly associated with FLT3 mutation status and FLT3 expression. Importantly, lestaurtinib induced cytotoxicity in a synergistic fashion with cytarabine, particularly in FLT3 mutant samples. Both lestaurtinib and PKC412 caused inhibition of FLT3 phosphorylation in all samples. Translation of FLT3 inhibition into cytotoxicity was influenced by the degree of residual FLT3 phosphorylation remaining and correlated with deactivation of STAT5 and MAP kinase. FLT3 mutant and wild-type cases both varied considerably in their dependence on FLT3 signaling for survival. These findings support the continued clinical assessment of FLT3 inhibitors in combination with cytotoxic chemotherapy: Entry to future clinical trials should include FLT3 wild-type patients and should remain unrestricted by FLT3 expression level.

Aromatic interactions with phenylalanine 691 and cysteine 828: a concept for FMS-like tyrosine kinase-3 inhibition. Application to the discovery of a new class of potential antileukemia agents

FLT3 kinase inhibitors are currently under investigation as a new treatment for acute myeloid leukemia. We report here a molecular concept invoking interactions between an aromatic ring and the side chains of Phe691 and Cys828, two residues of the ATP pocket, to obtain potent and specific inhibitors of this kinase. The hypothesis has been validated by the successful design of a new inhibitor prototype showing promising antiproliferative activity in cellular assays.

Inhibition of FLT3 signaling targets DCs to ameliorate autoimmune disease

Autoimmune diseases often result from inappropriate or unregulated activation of autoreactive T cells. Traditional approaches to treatment of autoimmune diseases through immunosuppression have focused on direct inhibition of T cells. In the present study, we examined the targeted inhibition of antigen-presenting cells as a means to downregulate immune responses and treat autoimmune disease. Dendritic cells (DCs) are the central antigen-presenting cells for the initiation of T cell responses, including autoreactive ones. A large portion of DCs are derived from hematopoietic progenitors that express FLT3 receptor (CD135), and stimulation of the receptor via FLT3 ligand either in vivo or in vitro is known to drive expansion and differentiation of these progenitors toward a DC phenotype. We hypothesized that inhibition of FLT3 signaling would thus produce an inhibition of DC-induced stimulation of T cells, thereby inhibiting autoimmune responses. To this end, we used small-molecule tyrosine kinase inhibitors targeted against FLT3 and examined the effects on DCs and their role in the promulgation of autoimmune disease. Results of our studies show that inhibition of FLT3 signaling induces apoptosis in both mouse and human DCs, and thus is a potential target for immune suppression. Furthermore, targeted inhibition of FLT3 significantly improved the course of established disease in a model for multiple sclerosis, experimental autoimmune encephalomyelitis, suggesting a potential avenue for treating autoimmune disease.

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.

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.

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.

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.