Synergistic effect of arsenic trioxide and flt3 inhibition on cells with flt3 internal tandem duplication

Combination Therapies with Kinase Inhibitors for Acute Myeloid Leukemia Treatment

Targeting kinase activity is considered to be an attractive therapeutic strategy to overcome acute myeloid leukemia (AML) since aberrant activation of the kinase pathway plays a pivotal role in leukemogenesis through abnormal cell proliferation and differentiation block. Although clinical trials for kinase modulators as single agents remain scarce, combination therapies are an area of therapeutic interest. In this review, the author summarizes attractive kinase pathways for therapeutic targets and the combination strategies for these pathways. Specifically, the review focuses on combination therapies targeting the FLT3 pathways, as well as PI3K/AKT/mTOR, CDK and CHK1 pathways. From a literature review, combination therapies with the kinase inhibitors appear more promising than monotherapies with individual agents. Therefore, the development of efficient combination therapies with kinase inhibitors may result in effective therapeutic strategies for AML.

Arsenic Trioxide and Venetoclax Synergize against AML Progenitors by ROS Induction and Inhibition of Nrf2 Activation

Venetoclax (VEN) in combination with hypomethylating agents induces disease remission in patients with de novo AML, however, most patients eventually relapse. AML relapse is attributed to the persistence of drug-resistant leukemia stem cells (LSCs). LSCs need to maintain low intracellular levels of reactive oxygen species (ROS). Arsenic trioxide (ATO) induces apoptosis via upregulation of ROS-induced stress to DNA-repair mechanisms. Elevated ROS levels can trigger the Nrf2 antioxidant pathway to counteract the effects of high ROS levels. We hypothesized that ATO and VEN synergize in targeting LSCs through ROS induction by ATO and the known inhibitory effect of VEN on the Nrf2 antioxidant pathway. Using cell fractionation, immunoprecipitation, RNA-knockdown, and fluorescence assays we found that ATO activated nuclear translocation of Nrf2 and increased transcription of antioxidant enzymes, thereby attenuating the induction of ROS by ATO. VEN disrupted ATO-induced Nrf2 translocation and augmented ATO-induced ROS, thus enhancing apoptosis in LSCs. Using metabolic assays and electron microscopy, we found that the ATO+VEN combination decreased mitochondrial membrane potential, mitochondria size, fatty acid oxidation and oxidative phosphorylation, all of which enhanced apoptosis of LSCs derived from both VEN-sensitive and VEN-resistant AML primary cells. Our results indicate that ATO and VEN cooperate in inducing apoptosis of LSCs through potentiation of ROS induction, suggesting ATO+VEN is a promising regimen for treatment of VEN-sensitive and -resistant AML.

Current Understandings of Myeloid Differentiation Inducers in Leukemia Therapy

Differentiation therapy using all-trans retinoic acid for acute promyelocytic leukemia (APL) is well established. Several attempts have been made to treat non-APL, AML patients by employing differentiation inducers, such as hypomethylating agents (HMAs), and low-dose cytarabine (Ara-C) (LDAC), with encouraging results. Other than HMAs and LDAC, various inducers of myeloid cell differentiation have been identified. This review describes and categorizes these inducers, which include glycosylation modifiers, epigenetic modifiers, vitamin derivatives, cytokines, and chemotherapeutic agents. Some of these inducers are currently being used in clinical trials. I highlight the potential applications of glycosylation modifiers and epigenetic modifiers, which are attracting increasing attention in their use as differentiation therapy against AML. Among the agents described in this review, epigenomic modifiers seem particularly promising, and particular attention should also be paid to glycosylation modifiers. These drugs may signal a new era for AML differentiation therapy.

Arsenic trioxide induces autophagic degradation of the FLT3-ITD mutated protein in FLT3-ITD acute myeloid leukemia cells

The prognosis of acute myeloid leukemia (AML) with FMS-like tyrosine kinase 3 internal tandem duplication (FLT3-ITD) mutations is poor. Some studies, including our previous study, have indicated that arsenic trioxide (ATO) exhibited significant anti-carcinogenic activity in FLT3-ITD AML cells and explored the possibility of targeting the FLT3-ITD protein for degradation as a therapy. Autophagy is a critical mechanism of the anti-leukemic effects of ATO. In this study, we explored the therapeutic efficacy of ATO treatment in a mouse model bearing FLT3-ITD AML and found that ATO significantly reduced the leukemic burden in bone marrow and spleen. We also found that autophagy was responsible for, at least in part, the degradation of the FLT3-ITD protein by ATO. After ATO treatment, MV4-11 cells showed complete autophagic flux. The autophagy inhibitor bafilomycin A or down-regulation of the key autophagy genes Atg5 and Atg7 reversed the FLT3 degradation induced by ATO. We also found that p62/SQSTM1 delivered FLT3-ITD proteins to the lysosome, where they were subsequently degraded. These results indicate that ATO can induce autophagic degradation of the FLT3-ITD mutated protein in FLT3-ITD AML.

Mutations of FLT3 receptor affect its surface glycosylation, intracellular localization, and downstream signaling

This review describes the effects of FLT3 mutations that alter its intracellular localization and modify its glycosylation, leading to differences in downstream signaling pathways. The most common type of FLT3 mutation, internal tandem duplication (FLT3-ITD), leads to localization in the endoplasmic reticulum and constitutive strong activation of STAT5. In contrast, the ligand-activated FLT3-wild type is mainly expressed on the cell surface and activates MAP kinases. Based on these backgrounds, several reports have demonstrated that glycosylation inhibitors are effective for inhibition of FLT3-ITD expression and intracellular localization. The general subcellular localization regulatory mechanisms for receptor tyrosine kinases are also discussed.

Combination of ATO with FLT3 TKIs eliminates FLT3/ITD+ leukemia cells through reduced expression of FLT3

Acute myeloid leukemia (AML) patients with FLT3/ITD mutations have a poor prognosis. Monotherapy with selective FLT3 tyrosine kinase inhibitors (TKIs) have shown transient and limited efficacy due to the development of resistance. Arsenic trioxide (ATO, As₂O₃) has been proven effective in treating acute promyelocytic leukemia (APL) and has shown activity in some cases of refractory and relapsed AML and other hematologic malignances. We explored the feasibility of combining FLT3 TKIs with ATO in the treatment of FLT3/ITD+ leukemias. The combination of FLT3 TKIs with ATO showed synergistic effects in reducing proliferation, viability and colony forming ability, and increased apoptosis in FLT3/ITD+ cells and primary patient samples. In contrast, no cooperativity was observed against wild-type FLT3 leukemia cells. ATO reduced expression of FLT3 RNA and its upstream transcriptional regulators (HOXA9, MEIS1), and induced poly-ubiquitination and degradation of the FLT3 protein, partly through reducing its binding with USP10. ATO also synergizes with FLT3 TKIs to inactivate FLT3 autophosphorylation and phosphorylation of its downstream signaling targets, including STAT5, AKT and ERK. Furthermore, ATO combined with sorafenib, a FLT3 TKI, in vivo reduced growth of FLT3/ITD+ leukemia cells in NSG recipients. In conclusion, these results suggest that ATO is a potential candidate to study in clinical trials in combination with FLT3 TKIs to improve the treatment of FLT3/ITD+ leukemia.

Arsenic trioxide and all-trans-retinoic acid selectively exert synergistic cytotoxicity against FLT3-ITD AML cells via co-inhibition of FLT3 signaling pathways

FLT3-ITD mutations occur in approximately 30% of acute myeloid leukemia (AML) and are associated with a poor outcome. Currently available FLT3 inhibitors have in vitro but limited clinical activity in FLT3-ITD AML. Reports have shown that an arsenic trioxide (ATO)/all-trans-retinoic acid (ATRA) combination improves prognosis in acute promyelocytic leukemia, especially with FLT3-ITD, and ATO or ATRA alone enhances apoptosis in FLT3-ITD AML cells treated with FLT3 inhibitors, providing a rationale to investigate the role of ATO/ATRA in FLT3-ITD AML. Here, we demonstrate that an ATO/ATRA combination selectively exerts synergistic cytotoxicity against FLT3-ITD AML cell lines (MV4;11/MOLM-13). The signaling pathways affected by ATO/ATRA include FLT3/STAT5/MYC, FLT3/STAT5/E2F1, FLT3/ERK/ATF5 and FLT3/AKT/ATF5.ATF5 may function as an oncogene in FLT3-ITD AML. Our findings provide experimental evidence that supports further exploration of ATO/ATRA in FLT3-ITD AML in vivo and warrants a clinical evaluation of regimens comprising an ATO/ATRA combination.

Arsenic trioxide and triptolide synergistically induce apoptosis in the SKM‑1 human myelodysplastic syndrome cell line

Although certain combination therapies comprising arsenic trioxide (As₂O₃) with other agents exist for the treatment of several types of human cancer, few As₂O₃ combination therapies are clinically effective for myelodysplastic syndromes (MDS). Triptolide (TL) may be an effective therapeutic agent for the treatment of MDS. However, to date, there is no combination therapy for MDS with As₂O₃ and TL. Therefore, the aim of the present study was to investigate this combination therapy on the apoptosis of MDS SKM-1 cells. The MDS SKM-1 cells were treated with As₂O₃, TL or the two in combination at various concentrations, or were mock-treated. Cell viability, cell apoptosis, levels of reactive oxygen species (ROS) and the expression of the cell apoptosis-associated genes, B cell lymphoma-2 (Bcl-2), Bcl-2-associated X protein (Bax) and caspase-3, were determined using an MTT assay, flow cytometric analysis of annexin V-fluorescein isothiocyanate/propidium iodide double-stained cells, flow cytometic analysis of intracellular 2′,7′-dichlorodihydrofluorescein diacetate fluorescence and reverse transcription-quantitative polymerase chain reaction analysis, respectively. Combination index (CI) analysis was performed to determine whether effects were synergistic (CI<1). The combination treatment was found to synergistically inhibit MDS SKM-1 cell growth, induce cell apoptosis, increase ROS levels, upregulate the expression levels of Bax and caspase-3, and downregulate the mRNA expression of Bcl-2. In conclusion, the combination treatment of As₂O₃ and TL synergistically induced apoptosis in the MDS SKM-1 cells.

FLT3 inhibition: a moving and evolving target in acute myeloid leukaemia

Internal tandem duplication (ITD) of the fms-like tyrosine kinase 3 (FLT3) gene is a gain-of-function mutation common in acute myeloid leukaemia (AML). It is associated with inferior prognosis and response to chemotherapy. Single base mutations at the FLT3 tyrosine kinase domain (TKD) also leads to a gain of function, although its prognostic significance is less well defined because of its rarity. The clinical benefits of FLT3 inhibition are generally limited to AML with FLT3-ITD. However, responses are transient and leukaemia progression invariably occurs. There is compelling evidence that leukaemia clones carrying both ITD and TKD mutations appear when resistance to FLT3 inhibitors occurs. Interestingly, the emergence of double ITD and TKD mutants can be recapitulated in vitro when FLT3-ITD+ leukaemia cell lines are treated with mutagens and FLT3 inhibitors. Furthermore, murine xenotransplantation models also suggest that, in some cases, the FTL3-ITD and TKD double mutants actually exist in minute amounts before treatment with FLT3 inhibitors, expand under the selection pressure of FLT3 inhibition and become the predominant resistant clone(s) during the drug-refractory phase. On the basis of this model of clonal evolution, a multipronged strategy using more potent FLT3 inhibitors, and a combinatorial approach targeting both FLT3-dependent and FLT3-independent pathways, will be needed to improve outcome.

The endothelium: influencing vascular smooth muscle in many ways

The endothelium, although only a single layer of cells lining the vascular and lymphatic systems, contributes in multiple ways to vascular homeostasis. Subsequent to the 1980 report by Robert Furchgott and John Zawadzki, there has been a phenomenal increase in our knowledge concerning the signalling molecules and pathways that regulate endothelial - vascular smooth muscle communication. It is now recognised that the endothelium is not only an important source of nitric oxide (NO), but also numerous other signalling molecules, including the putative endothelium-derived hyperpolarizing factor (EDHF), prostacyclin (PGI(2)), and hydrogen peroxide (H(2)O(2)), which have both vasodilator and vasoconstrictor properties. In addition, the endothelium, either via transferred chemical mediators, such as NO and PGI(2), and (or) low-resistance electrical coupling through myoendothelial gap junctions, modulates flow-mediated vasodilatation as well as influencing mitogenic activity, platelet aggregation, and neutrophil adhesion. Disruption of endothelial function is an early indicator of the development of vascular disease, and thus an important area for further research and identification of potentially new therapeutic targets. This review focuses on the signalling pathways that regulate endothelial - vascular smooth muscle communication and the mechanisms that initiate endothelial dysfunction, particularly with respect to diabetic vascular disease.

Targeting the FMS-like tyrosine kinase 3 in acute myeloid leukemia

Acute myeloid leukemia (AML) is a highly heterogenous disease with multiple signaling pathways contributing to its pathogenesis. A key driver of AML is the FMS-like tyrosine kinase receptor-3 (FLT3). Activating mutations in FLT3, primarily the FLT3-internal tandem duplication (FLT3-ITD), are associated with decreased progression-free and overall survival. Identification of the importance of FLT3-ITD and the FLT3 pathway in the prognosis of patients with AML has stimulated efforts to develop therapeutic inhibitors of FLT3. Although these inhibitors have shown promising antileukemic activity, they have had limited efficacy to date as single agents and may require use in combination with cytotoxic chemotherapies. Here, we review clinical and preclinical results for the clinically mature FLT3 inhibitors currently in development. We conclude that multitargeted FLT3 inhibitors may have more utility earlier in the course of disease, when in vitro evidence suggests that AML cells are less dependent on FLT3 signaling, perhaps because of upregulation of multiple other signaling pathways. More potent agents may have greater utility in relapsed and heavily pretreated patients, in whom high levels of circulating FLT3 ligand may necessitate use of an agent with a very favorable pharmacokinetic/pharmacodynamic profile. Novel combination regimens are also discussed.

Downstream molecular pathways of FLT3 in the pathogenesis of acute myeloid leukemia: biology and therapeutic implications

FLT3 is a type III receptor tyrosine kinase. Mutations of FLT3 comprise one of the most frequently identified types of genetic alterations in acute myeloid leukemia. One-third of acute myeloid leukemia patients have mutations of this gene, and the majority of these mutations involve an internal tandem duplication in the juxtamembrane region of FLT3, leading to constitutive activation of downstream signaling pathways and aberrant cell growth. This review summarizes the current understanding of the effects of the downstream molecular signaling pathways after FLT3 activation, with a particular focus on the effects on transcription factors. Moreover, this review describes novel FLT3-targeted therapies, as well as efficient combination therapies for FLT3-mutated leukemia cells.

VEGF targeted therapy in acute myeloid leukemia

The cooperation of two classes of mutations in hematopoietic cells is hypothesized in a multistep pathogenesis model of acute myeloid leukemia (AML). Class I mutations confer a proliferative and/or survival advantage, whereas Class II mutations block hematopoietic differentiation and impair apoptosis in AML cells. In addition to these two classes of mutations, a relevant role for angiogenesis in the pathophysiology of AML has been recently proposed. The recognition that the vascular endothelial growth factor (VEGF) pathway is a key regulator of angiogenesis has led to the development of several VEGF-targeted approaches. These include neutralizing antibodies, VEGF traps or selective tyrosine kinase inhibitors for VEGFRs. Other drugs that indirectly affect VEGF pathway, such as statins or arsenic trioxide, also have been shown to possess antiangiogenic activity in leukemias. The benefits of these VEGF targeted agents and their current stage of development as novel anti-antiangiogenic therapies in AML are discussed in this review.

The p38 pathway inhibitor SB202190 activates MEK/MAPK to stimulate the growth of leukemia cells

In this study, the biological effects of signal transduction inhibitors on leukemia cells were examined. We found that the p38 inhibitor SB202190 enhanced the growth of THP-1 and MV4-11 cells. To determine the pathway affected by SB202190, we examined the 50% effective dose (ED(50)) values for THP-1 cell growth in combination with several inhibitors. In the presence of SB202190, the ED(50) values for the farnesyltransferase inhibitor FPT inhibitor II and MEK inhibitor U0126 were significantly decreased. Western blot analysis revealed that SB202190 increased the phosphorylation of C-Raf and extracellular regulated kinase (ERK), suggesting that Ras-Raf-MEK-mitogen-activated protein kinase (MAPK) pathway activation is involved in the leukemia cell growth induced by SB202190.

Diagnostic pathways in acute leukemias: a proposal for a multimodal approach

Acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) each represent a heterogeneous complex of disorders, which result from diverse mechanisms of leukemogenesis. Modern therapeutic concepts are based on individual risk stratification at diagnosis and during follow-up. For some leukemia subtypes such as AML M3/M3v with t(15;17)/PML-RARA or Philadelphia-positive ALL targeted therapy options are available. Thus, optimal therapeutic conditions are based on exact classification of the acute leukemia subtype at diagnosis and are guided by exact and sensitive quantification of minimal residual disease during complete hematologic remission. Today, a multimodal diagnostic approach combining cytomorphology, multiparameter flow cytometry, chromosome banding analysis, accompanied by diverse fluorescence in situ hybridization techniques, and molecular analyses is needed to meet these requirements. As the diagnostic process becomes more demanding with respect to experience of personnel, time, and costs due to the expansion of methods, algorithms, which guide the diagnostic procedure from basic to more specific methods and which lead finally to a synopsis of the respective results, are essential for modern diagnostics and therapeutic concepts.