Targeting Oncogenic Signaling in Mutant FLT3 Acute Myeloid Leukemia: The Path to Least Resistance

Structure-Based Optimization of Pyrazinamide-Containing Macrocyclic Derivatives as Fms-like Tyrosine Kinase 3 (FLT3) Inhibitors to Overcome Clinical Mutations

Secondary mutations in Fms-like tyrosine kinase 3-tyrosine kinase domain (FLT3-TKD) (e.g., D835Y and F691L) have become a major on-target resistance mechanism of FLT3 inhibitors, which present a significant clinical challenge. To date, no effective drugs have been approved to simultaneously overcome clinical resistance caused by these two mutants. Thus, a series of pyrazinamide macrocyclic compounds were first designed and evaluated to overcome the secondary mutations of FLT3. The representative 8v exhibited potent inhibitory activities against FLT3D835Y and FLT3D835Y/F691L with IC50 values of 1.5 and 9.7 nM, respectively. 8v also strongly suppressed the proliferation against Ba/F3 cells transfected with FLT3-ITD, FLT3-ITD-D835Y, FLT3-ITD-F691L, FLT3-ITD-D835Y-F691L, and MV4-11 acute myeloid leukemia (AML) cell lines with IC50 values of 12.2, 10.5, 24.6, 16.9, and 6.8 nM, respectively. Furthermore, 8v demonstrated ideal anticancer efficacy in a Ba/F3-FLT3-ITD-D835Y xenograft model. The results suggested that 8v can serve as a promising macrocycle-based FLT3 inhibitor for the treatment of AML.

Pharmaco-phosphoproteomic analysis of cancer-associated KIT mutations D816V and V560G

The CD117 mast/stem cell growth factor receptor tyrosine kinase (KIT) is critical for haematopoiesis, melanogenesis and stem cell maintenance. KIT is commonly activated by mutation in cancers including acute myeloid leukaemia, melanoma and gastrointestinal stromal tumours (GISTs). The kinase and the juxtamembrane domains of KIT are mutation hotspots; with the kinase domain mutation D816V common in leukaemia and the juxtamembrane domain mutation V560G common in GISTs. Given the importance of mutant KIT signalling in cancer, we have conducted a proteomic and phosphoproteomic analysis of myeloid progenitor cells expressing D816V- and V560G-KIT mutants, using an FDCP1 isogenic cell line model. Proteomic analysis revealed increased abundance of proteases and growth signalling proteins in KIT-mutant cells compared to empty vector (EV) controls. Pathway analysis identified increased oxidative phosphorylation in D816V- and V560G-mutant KIT cells, which was targetable using the inhibitor IACS010759. Dysregulation of RNA metabolism and cytoskeleton/adhesion pathways was identified in both the proteome and phosphoproteome of KIT-mutant cells. Phosphoproteome analysis further revealed active kinases such as EGFR, ERK and PKC, which were targetable using pharmacological inhibitors. This study provides a pharmaco-phosphoproteomic profile of D816V- and V560G-mutant KIT cells, which reveals novel therapeutic strategies that may be applicable to a range of cancers.

Shikonin Exerts an Antileukemia Effect against FLT3-ITD Mutated Acute Myeloid Leukemia Cells via Targeting FLT3 and Its Downstream Pathways

INTRODUCTION

Acute myeloid leukemia (AML) with internal tandem duplication (ITD) mutations in Fms-like tyrosine kinase 3 (FLT3) has an unfavorable prognosis. Recently, using newly emerging inhibitors of FLT3 has led to improved outcomes of patients with FLT3-ITD mutations. However, drug resistance and relapse continue to be significant challenges in the treatment of patients with FLT3-ITD mutations. This study aimed to evaluate the antileukemic effects of shikonin (SHK) and its mechanisms of action against AML cells with FLT3-ITD mutations in vitro and in vivo.

METHODS

The CCK-8 assay was used to analyze cell viability, and flow cytometry was used to detect cell apoptosis and differentiation. Western blotting and real-time polymerase chain reaction were used to examine the expression of certain proteins and genes. Leukemia mouse model was created to evaluate the antileukemia effect of SHK against FLT3-ITD mutated leukemia in vivo.

RESULTS

After screening a series of leukemia cell lines, those with FLT3-ITD mutations were found to be more sensitive to SHK in terms of proliferation inhibition and apoptosis induction than those without FLT3-ITD mutation. SHK suppresses the expression and phosphorylation of FLT3 receptors and their downstream molecules. Inhibition of the NF-κB/miR-155 pathway is an important mechanism through which SHK kills FLT3-AML cells. Moreover, a low concentration of SHK promotes the differentiation of AML cells with FLT3-ITD mutations. Finally, SHK could significantly inhibit the growth of MV4-11 cells in leukemia bearing mice.

CONCLUSION

The findings of this study indicate that SHK may be a promising drug for the treatment of FLT3-ITD mutated AML.

PROTAC'ing oncoproteins: targeted protein degradation for cancer therapy

Molecularly targeted cancer therapies substantially improve patient outcomes, although the durability of their effectiveness can be limited. Resistance to these therapies is often related to adaptive changes in the target oncoprotein which reduce binding affinity. The arsenal of targeted cancer therapies, moreover, lacks coverage of several notorious oncoproteins with challenging features for inhibitor development. Degraders are a relatively new therapeutic modality which deplete the target protein by hijacking the cellular protein destruction machinery. Degraders offer several advantages for cancer therapy including resiliency to acquired mutations in the target protein, enhanced selectivity, lower dosing requirements, and the potential to abrogate oncogenic transcription factors and scaffolding proteins. Herein, we review the development of proteolysis targeting chimeras (PROTACs) for selected cancer therapy targets and their reported biological activities. The medicinal chemistry of PROTAC design has been a challenging area of active research, but the recent advances in the field will usher in an era of rational degrader design.

Blockade of ROS production inhibits oncogenic signaling in acute myeloid leukemia and amplifies response to precision therapies

Mutations in the type III receptor tyrosine kinase FLT3 are frequent in patients with acute myeloid leukemia (AML) and are associated with a poor prognosis. AML is characterized by the overproduction of reactive oxygen species (ROS), which can induce cysteine oxidation in redox-sensitive signaling proteins. Here, we sought to characterize the specific pathways affected by ROS in AML by assessing oncogenic signaling in primary AML samples. The oxidation or phosphorylation of signaling proteins that mediate growth and proliferation was increased in samples from patient subtypes with FLT3 mutations. These samples also showed increases in the oxidation of proteins in the ROS-producing Rac/NADPH oxidase-2 (NOX2) complex. Inhibition of NOX2 increased the apoptosis of FLT3-mutant AML cells in response to FLT3 inhibitors. NOX2 inhibition also reduced the phosphorylation and cysteine oxidation of FLT3 in patient-derived xenograft mouse models, suggesting that decreased oxidative stress reduces the oncogenic signaling of FLT3. In mice grafted with FLT3 mutant AML cells, treatment with a NOX2 inhibitor reduced the number of circulating cancer cells, and combining FLT3 and NOX2 inhibitors increased survival to a greater extent than either treatment alone. Together, these data raise the possibility that combining NOX2 and FLT3 inhibitors could improve the treatment of FLT3 mutant AML.

Synergistic Targeting of DNA-PK and KIT Signaling Pathways in KIT Mutant Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is the most common and aggressive form of acute leukemia, with a 5-year survival rate of just 24%. Over a third of all AML patients harbor activating mutations in kinases, such as the receptor tyrosine kinases FLT3 (receptor-type tyrosine-protein kinase FLT3) and KIT (mast/stem cell growth factor receptor kit). FLT3 and KIT mutations are associated with poor clinical outcomes and lower remission rates in response to standard-of-care chemotherapy. We have recently identified that the core kinase of the non-homologous end joining DNA repair pathway, DNA-PK (DNA-dependent protein kinase), is activated downstream of FLT3; and targeting DNA-PK sensitized FLT3-mutant AML cells to standard-of-care therapies. Herein, we investigated DNA-PK as a possible therapeutic vulnerability in KIT mutant AML, using isogenic FDC-P1 mouse myeloid progenitor cell lines transduced with oncogenic mutant KIT (V560G and D816V) or vector control. Targeted quantitative phosphoproteomic profiling identified phosphorylation of DNA-PK in the T2599/T2605/S2608/S2610 cluster in KIT mutant cells, indicative of DNA-PK activation. Accordingly, proliferation assays revealed that KIT mutant FDC-P1 cells were more sensitive to the DNA-PK inhibitors M3814 or NU7441, compared with empty vector controls. DNA-PK inhibition combined with inhibition of KIT signaling using the kinase inhibitors dasatinib or ibrutinib, or the protein phosphatase 2A activators FTY720 or AAL(S), led to synergistic cell death. Global phosphoproteomic analysis of KIT-D816V cells revealed that dasatinib and M3814 single-agent treatments inhibited extracellular signal-regulated kinase and AKT (RAC-alpha serine/threonine-protein kinase)/MTOR (serine/threonine-protein kinase mTOR) activity, with greater inhibition of both pathways when used in combination. Combined dasatinib and M3814 treatment also synergistically inhibited phosphorylation of the transcriptional regulators MYC and MYB. This study provides insight into the oncogenic pathways regulated by DNA-PK beyond its canonical role in DNA repair and demonstrates that DNA-PK is a promising therapeutic target for KIT mutant cancers.

Phospho-heavy-labeled-spiketide FAIMS stepped-CV DDA (pHASED) provides real-time phosphoproteomics data to aid in cancer drug selection

Global high-throughput phosphoproteomic profiling is increasingly being applied to cancer specimens to identify the oncogenic signaling cascades responsible for promoting disease initiation and disease progression; pathways that are often invisible to genomics analysis. Hence, phosphoproteomic profiling has enormous potential to inform and improve individualized anti-cancer treatment strategies. However, to achieve the adequate phosphoproteomic depth and coverage necessary to identify the activated, and hence, targetable kinases responsible for driving oncogenic signaling pathways, affinity phosphopeptide enrichment techniques are required and often coupled with offline high-pressure liquid chromatographic (HPLC) separation prior to nanoflow liquid chromatography-tandem mass spectrometry (nLC-MS/MS). These complex and time-consuming procedures, limit the utility of phosphoproteomics for the analysis of individual cancer patient specimens in real-time, and restrict phosphoproteomics to specialized laboratories often outside of the clinical setting. To address these limitations, here we have optimized a new protocol, phospho-heavy-labeled-spiketide FAIMS Stepped-CV DDA (pHASED), that employs online phosphoproteome deconvolution using high-field asymmetric waveform ion mobility spectrometry (FAIMS) and internal phosphopeptide standards to provide accurate label-free quantitation (LFQ) data in real-time. Compared with traditional single-shot LFQ phosphoproteomics workflows, pHASED provided increased phosphoproteomic depth and coverage (phosphopeptides = 4617 pHASED, 2789 LFQ), whilst eliminating the variability associated with offline prefractionation. pHASED was optimized using tyrosine kinase inhibitor (sorafenib) resistant isogenic FLT3-mutant acute myeloid leukemia (AML) cell line models. Bioinformatic analysis identified differential activation of the serine/threonine protein kinase ataxia-telangiectasia mutated (ATM) pathway, responsible for sensing and repairing DNA damage in sorafenib-resistant AML cell line models, thereby uncovering a potential therapeutic opportunity. Herein, we have optimized a rapid, reproducible, and flexible protocol for the characterization of complex cancer phosphoproteomes in real-time, a step towards the implementation of phosphoproteomics in the clinic to aid in the selection of anti-cancer therapies for patients.

B-cell Lymphoma 6 (BCL6): From Master Regulator of Humoral Immunity to Oncogenic Driver in Pediatric Cancers

B-cell lymphoma 6 (BCL6) is a protooncogene in adult and pediatric cancers, first identified in diffuse large B-cell lymphoma (DLBCL) where it acts as a repressor of the tumor suppressor TP53, conferring survival, protection, and maintenance of lymphoma cells. BCL6 expression in normal B cells is fundamental in the regulation of humoral immunity, via initiation and maintenance of the germinal centers (GC). Its role in B cells during the production of high affinity immunoglobins (that recognize and bind specific antigens) is believed to underpin its function as an oncogene. BCL6 is known to drive the self-renewal capacity of leukemia-initiating cells (LIC), with high BCL6 expression in acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), and glioblastoma (GBM) associated with disease progression and treatment resistance. The mechanisms underpinning BCL6-driven therapy resistance are yet to be uncovered; however, high activity is considered to confer poor prognosis in the clinical setting. BCL6's key binding partner, BCL6 corepressor (BCOR), is frequently mutated in pediatric cancers and appears to act in concert with BCL6. Using publicly available data, here we show that BCL6 is ubiquitously overexpressed in pediatric brain tumors, inversely to BCOR, highlighting the potential for targeting BCL6 in these often lethal and untreatable cancers. In this review, we summarize what is known of BCL6 (role, effect, mechanisms) in pediatric cancers, highlighting the two sides of BCL6 function, humoral immunity, and tumorigenesis, as well as to review BCL6 inhibitors and highlight areas of opportunity to improve the outcomes of patients with pediatric cancer.

Therapeutic Targeting of FLT3 in Acute Myeloid Leukemia: Current Status and Novel Approaches

FMS-like tyrosine kinase 3 (FLT3) is mutated in approximately 30% of acute myeloid leukemia (AML) patients. The presence of FLT3-ITD (internal tandem duplication, 20-25%) mutation and, to a lesser extent, FLT3-TKD (tyrosine kinase domain, 5-10%) mutation is associated with poorer diagnosis and therapy response since the leukemic cells become hyperproliferative and resistant to apoptosis after continuous activation of FLT3 signaling. Targeting FLT3 has been the focus of many pre-clinical and clinical studies. Hence, many small-molecule FLT3 inhibitors (FLT3is) have been developed, some of which are approved such as midostaurin and gilteritinib to be used in different clinical settings, either in combination with chemotherapy or alone. However, many questions regarding the best treatment strategy remain to be answered. On the other hand, various FLT3-dependent and -independent resistance mechanisms could be evolved during FLT3i therapy which limit their clinical impact. Therefore, identifying molecular mechanisms of resistance and developing novel strategies to overcome this obstacle is a current interest in the field. In this review, recent studies of approved FLT3i and knowledge about major resistance mechanisms of clinically approved FLT3i's will be discussed together with novel treatment approaches such as designing novel FLT3i and dual FLT3i and combination strategies including approved FLT3i plus small-molecule agents targeting altered molecules in the resistant cells to abrogate resistance. Moreover, how to choose an appropriate FLT3i for the patients will be summarized based on what is currently known from available clinical data. In addition, strategies beyond FLT3i's including immunotherapeutics, small-molecule FLT3 degraders, and flavonoids will be summarized to highlight potential alternatives in FLT3-mutated AML therapy.

Analysis of microRNA expression profiles in exosomes derived from acute myeloid leukemia by p62 knockdown and effect on angiogenesis

Objectives

In this study, we aimed to investigate the effect of p62 on angiogenesis and microRNA (miRNA) expression profiles in acute myeloid leukemia (AML) exosomes.

Methods

An Exiqon v19.0 microRNA MicroArray was used to profile miRNAs in exosomes derived from parental and p62-knockdown U937 cells. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases were used to predict the biological functions and potential mechanisms of differentially expressed miRNAs in AML exosomes. Endothelial cell tube formation assays using human umbilical vein endothelial cells (HUVECs) were performed to investigate the effect of AML exosomes on angiogenesis.

Results

We demonstrated that 2,080 miRNAs were expressed in exosomes derived from our cultured cell samples, of which 215 and 208 miRNAs were upregulated and downregulated, respectively, in p62-knockdown U937 cells (fold change ≥ 2, P < 0.05). GO analysis indicated that miRNAs were most enriched in the intercellular pathways. Biological process analysis revealed that 1460 biological processes were associated with downregulated transcripts, including 19 pathways related to vesicles, and 1,515 pathways were upregulated, including 8 pathways related to vesicles. Molecular function analysis indicated that protein binding, transcription regulator activity, and DNA-binding transcription factor activity were enriched (P < 0.05). Pathway analysis indicated that 84 pathways corresponded to upregulated transcripts, and 55 pathways corresponded to downregulated transcripts (P < 0.05). We also found that exosomes derived from U937 cells promoted angiogenesis in HUVECs.

Conclusions

Our data suggest that exosomal miRNAs may play important roles in the pathogenesis of AML, which may be treated by p62 knockdown with exosomal miRNAs to inhibit angiogenesis.

Acute Myeloid Leukaemia Drives Metabolic Changes in the Bone Marrow Niche

Acute myeloid leukaemia (AML) is a highly proliferative cancer characterised by infiltration of immature haematopoietic cells in the bone marrow (BM). AML predominantly affects older people and outcomes, particularly in this difficult to treat population remain poor, in part due to inadequate response to therapy, and treatment toxicity. Normal haematopoiesis is supported by numerous support cells within the BM microenvironment or niche, including adipocytes, stromal cells and endothelial cells. In steady state haematopoiesis, haematopoietic stem cells (HSCs) primarily acquire ATP through glycolysis. However, during stress-responses HSCs rapidly transition to oxidative phosphorylation, enabled by mitochondrial plasticity. Historically it was thought that cancer cells preferentially used glycolysis for ATP production, however recently it has become evident that many cancers, including AML primarily use the TCA cycle and oxidative phosphorylation for rapid proliferation. AML cells hijack the stress-response pathways of their non-malignant counterparts, utilising mitochondrial changes to drive expansion. In addition, amino acids are also utilised by leukaemic stem cells to aid their metabolic output. Together, these processes allow AML cells to maximise their ATP production, using multiple metabolites and fuelling rapid cell turnover which is a hallmark of the disease. This review of AML derived changes in the BM niche, which enable enhanced metabolism, will consider the important pathways and discuss future challenges with a view to understanding how AML cells are able to hijack metabolic pathways and how we may elucidate new targets for potential therapies.

Potential Prognostic Markers for Relapsed/Refractory vs. Responsive Acute Myeloid Leukemia

Simple Summary

Acute myeloid leukemia (AML) is the most common blood cancer in the elderly, which progresses rapidly and is often fatal. The prognosis for AML remains poor in most older patients: only about 15% of patients over 60 years of age can recover. Our aim is to determine new potential AML clinical treatment prognosis markers. We analyzed certain genes, proteins and the epigenome profile in therapy-resistant and responsive AML patients at diagnosis stage and after clinical treatment. We determined that MYC, WT1, IDH1, CDKN1A, HDAC2, TET1, KAT6A and GATAD2A gene expression changes might characterize refractory AML. Therefore, these genes could have an impact for AML prognosis.

Abstract

Acute myeloid leukemia (AML) is a heterogeneous disease. A significant proportion of AML patients is refractory to clinical treatment or relapses. Our aim is to determine new potential AML clinical treatment prognosis markers. We investigated various cell fate and epigenetic regulation important gene level differences between refractory and responsive AML patient groups at diagnosis stage and after clinical treatment using RT-qPCR. We demonstrated that oncogenic MYC and WT1 and metabolic IDH1 gene expression was significantly higher and cell cycle inhibitor CDKN1A (p21) gene expression was significantly lower in refractory patients’ bone marrow cells compared to treatment responsive patients both at diagnosis and after clinical treatment. Moreover, we determined that, compared to clinical treatment responsive patients, refractory patients possess a significantly higher gene expression of histone deacetylase 2 (HDAC2) and epigenetic DNA modulator TET1 and a significantly lower gene expression of lysine acetyltransferase 6A (KAT6A) and nucleosome remodeling and deacetylase (NuRD) complex component GATAD2A. We suggest that MYC, WT1, IDH1, CDKN1A, HDAC2, TET1, KAT6A and GATAD2A gene expression changes might characterize refractory AML. Thus, they might be useful for AML prognosis. Additionally, we suggest that epigenetic modulation might be beneficial in combination with standard treatment.

Molecular Systems Architecture of Interactome in the Acute Myeloid Leukemia Microenvironment

A molecular systems architecture is presented for acute myeloid leukemia (AML) to provide a framework for organizing the complexity of biomolecular interactions. AML is a multifactorial disease resulting from impaired differentiation and increased proliferation of hematopoietic precursor cells involving genetic mutations, signaling pathways related to the cancer cell genetics, and molecular interactions between the cancer cell and the tumor microenvironment, including endothelial cells, fibroblasts, myeloid-derived suppressor cells, bone marrow stromal cells, and immune cells (e.g., T-regs, T-helper 1 cells, T-helper 17 cells, T-effector cells, natural killer cells, and dendritic cells). This molecular systems architecture provides a layered understanding of intra- and inter-cellular interactions in the AML cancer cell and the cells in the stromal microenvironment. The molecular systems architecture may be utilized for target identification and the discovery of single and combination therapeutics and strategies to treat AML.

Inhibition of BCL2A1 by STAT5 inactivation overcomes resistance to targeted therapies of FLT3-ITD/D835 mutant AML

•

BCL2A1 is upregulated and exerts a pro-survival function in FLT3-ITD/D835 AML cells.

•

Upregulation of BCL2A1 attenuates sensitivity to quizartinib in FLT3-ITD/D835 cells.

•

Gilteritinib decreases BCL2A1 through inactivation of STAT5 in FLT3-ITD/D835 cells.

•

Gilteritinib/Venetoclax has a synergistic anti-tumor activity in FLT3-ITD/D835 cells.

Tyrosine kinase inhibitors (TKIs) are established drugs in the therapy of FLT3-ITD mutated acute myeloid leukemia (AML). However, acquired mutations, such as D835 in the tyrosine kinase domain (FLT3-ITD/D835), can induce resistance to TKIs. A cap analysis gene expression (CAGE) technology revealed that the gene expression of BCL2A1 transcription start sites was increased in primary AML cells bearing FLT3-ITD/D835 compared to FLT3-ITD. Overexpression of BCL2A1 attenuated the sensitivity to quizartinib, a type II TKI, and venetoclax, a selective BCL2 inhibitor, in AML cell lines. However, a type I TKI, gilteritinib, inhibited the expression of BCL2A1 through inactivation of STAT5 and alleviated TKI resistance of FLT3-ITD/D835. The combination of gilteritinib and venetoclax showed synergistic effects in the FLT3-ITD/D835 positive AML cells. The promoter region of BCL2A1 contains a BRD4 binding site. Thus, the blockade of BRD4 with a BET inhibitor (CPI-0610) downregulated BCL2A1 in FLT3-mutated AML cells and extended profound suppression of FLT3-ITD/D835 mutant cells. Therefore, we propose that BCL2A1 has the potential to be a novel therapeutic target in treating FLT3-ITD/D835 mutated AML.

Older Patients with Acute Myeloid Leukemia Deserve Individualized Treatment

PURPOSE OF REVIEW

Treatment of elderly patients with acute myeloid leukemia is a known challenge for hematologists due to patient diversity, heterogeneous disease biology, and a rapidly evolving treatment landscape. Here, we highlight the importance of determining fitness, review the latest therapeutic developments, and discuss clinical scenarios to provide guidance on individualized treatment for older AML patients.

RECENT FINDINGS

Several factors, like age, performance status, and comorbidities, play a role in fitness and are associated with outcome. Comorbidity scoring systems and geriatric assessments are tools to help physicians select the most appropriate treatment for each patient. The addition of venetoclax, targeted therapy with IDH1/2 and FLT3 inhibitors, and enhanced formulas of existing drugs like CPX-351 and oral azacitidine have improved responses and outcomes. New drugs and combination therapies have increased the therapeutic options for elderly AML patients but determination of fitness and disease biology is essential to select patient-tailored treatments.

A recent development of new therapeutic agents and novel drug targets for cancer treatment

Despite recent advances in cancer diagnosis, prevention, detection, as well as management, the disease is expected to be the top cause of death globally. The chemotherapy approach for cancer has become more advanced in its design, yet no medication can cure enough against all types of cancer and its stage. Thus, this review aimed to summarize a recent development of new therapeutic agents and novel drug targets for the treatment of cancer. Several obstacles stand in the way of effective cancer treatment and drug development, including inaccessibility of tumor site by appropriate drug concentration, debilitating untoward effects caused by non-selective tissue distribution of chemotherapeutic agents, and occurrence of drug resistance, which leads to cross-resistance to a variety of drugs. Resistance to treatment with anticancer drugs results from multiple factors and the most common reason for acquiring drug resistance is marking and expelling drugs that prevent cancer cells to be targeted by chemotherapeutic agents. Moreover, insensitivity to drug-induced apoptosis, alteration, and mutation of drug target and interference/change of DNA replication are other main causes of treatment failure.

Discover Novel Covalent Inhibitors Targeting FLT3 through Hybrid Virtual Screening Strategy

FMS-like tyrosine kinase 3 (FLT3) plays a very important role in regulating the proliferation, differentiation and survival of normal hematopoietic stem cells. Internal tandem duplications of the FLT3 gene (FLT3-ITD) mutations are present in 25% of all acute myeloid leukemia (AML) patients and are frequently associated with adverse clinical outcomes. Therefore, FLT3-ITD is a promising target for the treatment of AML. The use of covalent virtual screenings has shown that efficient rational approaches for the rapid discovery of new drugs scaffold. Herein, we report a hybrid virtual screening strategy that led to the discovery of FLT3 inhibitors. Using the combination of non-covalent docking and covalent docking, 8 compounds were found to inhibit FLT3, and G856-8335, S346-0154 are also effective against mutant FLT3. These two compounds also show selectivity to receptor tyrosine kinase (C-KIT), which has the potential for optimization. And this work can be extended to the screening of other covalent inhibitors.

Treatment Failure in Acute Myeloid Leukemia: Focus on the Role of Extracellular Vesicles

Acute myeloblastic leukemia (AML) is one of the most common types of blood malignancies that results in an AML-associated high mortality rate each year. Several causes have been reported as prognostic factors for AML in children and adults, the most important of which are cytogenetic abnormalities and environmental risk factors. Following the discovery of numerous drugs for AML treatment, leukemic cells sought a way to escape from the cytotoxic effects of chemotherapy drugs, leading to treatment failure. Nowadays, comprehensive studies have looked at the role of extracellular vesicles (EVs) secreted by AML blasts and how the microenvironment of the tumor changes in favor of cancer progression and survival to discover the mechanisms of treatment failure to choose the well-advised treatment. Reports show that malignant cells secrete EVs that transmit messages to adjacent cells and the tumor's microenvironment. By secreting EVs, containing immune-inhibiting cytokines, AML cells inactivate the immune system against malignant cells, thus ensuring their survival. Also, increased secretion of EVs in various malignancies indicates an unfavorable prognostic factor and the possibility of drug resistance. In this study, we briefly reviewed the challenges of treating AML with a glance at the EVs' role in this process. It is hoped that with a deeper understanding of EVs, new therapies will be developed to eliminate the relapse of leukemic cells.

Reactive Oxygen Species in Acute Lymphoblastic Leukaemia: Reducing Radicals to Refine Responses

Acute lymphoblastic leukaemia (ALL) is the most common cancer diagnosed in children and adolescents. Approximately 70% of patients survive >5-years following diagnosis, however, for those that fail upfront therapies, survival is poor. Reactive oxygen species (ROS) are elevated in a range of cancers and are emerging as significant contributors to the leukaemogenesis of ALL. ROS modulate the function of signalling proteins through oxidation of cysteine residues, as well as promote genomic instability by damaging DNA, to promote chemotherapy resistance. Current therapeutic approaches exploit the pro-oxidant intracellular environment of malignant B and T lymphoblasts to cause irreversible DNA damage and cell death, however these strategies impact normal haematopoiesis and lead to long lasting side-effects. Therapies suppressing ROS production, especially those targeting ROS producing enzymes such as the NADPH oxidases (NOXs), are emerging alternatives to treat cancers and may be exploited to improve the ALL treatment. Here, we discuss the roles that ROS play in normal haematopoiesis and in ALL. We explore the molecular mechanisms underpinning overproduction of ROS in ALL, and their roles in disease progression and drug resistance. Finally, we examine strategies to target ROS production, with a specific focus on the NOX enzymes, to improve the treatment of ALL.

A novel insight into the pathophysiology of autoimmune hepatitis: An immune activator mutation in the FLT3 receptor

Autoimmune hepatitis (AIH) is a chronic progressive autoimmune liver disease characterized by hypergammaglobulinemia, interface hepatitis, a female preponderance, and the presence of autoantibodies in most patients. The presence of HLA-DR3/DR4 and functional impairment in regulatory T cells are associated with AIH. However, AIH is a multifactorial complex disease. This report is a description of a case of seronegative AIH in a girl with chronic hepatitis, a high immunoglobulin E (IgE) level, perforating nodular dermatitis, and sheer eosinophilia. To re-evaluate the diagnosis, whole exon sequencing was performed. It was determined that the patient had ancestral haplotype A1-B8-DR3, which is associated with autoimmunity. Importantly, it was also noted that an undocumented point mutation (Ala627Thr) of the FMS-like tyrosine 3 kinase (FLT3) receptor was present. This FLT3 receptor gain-of-function mutation is associated with the activation of the mechanistic target of rapamycin (mTOR), and dendritic cell activation. In addition, a loss-of-function mutation in the melanocortin-3 receptor gene, which inhibits interleukin 4, was detected. The constellation of these immune deregulatory factors may have propagated auto-aggression of the liver, causing chronic hepatitis with AIH features. The findings of seronegativity with eosinophilia and a high IgE level led us to hypothesize that the pathognomonic mechanism in this case was unlike that of classic AIH pathophysiology. Since mTOR is constitutively activated, mTOR inhibitors may be a useful option to treat AIH and dermatitis.

Mechanism of Human Telomerase Reverse Transcriptase (hTERT) Regulation and Clinical Impacts in Leukemia

The proliferative capacity and continuous survival of cells are highly dependent on telomerase expression and the maintenance of telomere length. For this reason, elevated expression of telomerase has been identified in virtually all cancers, including leukemias; however, it should be noted that expression of telomerase is sometimes observed later in malignant development. This time point of activation is highly dependent on the type of leukemia and its causative factors. Many recent studies in this field have contributed to the elucidation of the mechanisms by which the various forms of leukemias increase telomerase activity. These include the dysregulation of telomerase reverse transcriptase (TERT) at various levels which include transcriptional, post-transcriptional, and post-translational stages. The pathways and biological molecules involved in these processes are also being deciphered with the advent of enabling technologies such as next-generation sequencing (NGS), ribonucleic acid sequencing (RNA-Seq), liquid chromatography-mass spectrometry (LCMS/MS), and many others. It has also been established that TERT possess diagnostic value as most adult cells do not express high levels of telomerase. Indeed, studies have shown that prognosis is not favorable in patients who have leukemias expressing high levels of telomerase. Recent research has indicated that targeting of this gene is able to control the survival of malignant cells and therefore offers a potential treatment for TERT-dependent leukemias. Here we review the mechanisms of hTERT regulation and deliberate their association in malignant states of leukemic cells. Further, we also cover the clinical implications of this gene including its use in diagnostic, prognostic, and therapeutic discoveries.

Identification of survival-related alternative splicing signatures in acute myeloid leukemia

Aberrant RNA alternative splicing (AS) variants play critical roles in tumorigenesis and prognosis in human cancers. Here, we conducted a comprehensive profiling of aberrant AS events in acute myeloid leukemia (AML). RNA AS profile, including seven AS types, and the percent spliced in (PSI) value for each patient were generated by SpliceSeq using RNA-seq data from TCGA. Univariate followed by multivariate Cox regression analysis were used to identify survival-related AS events and develop the AS signatures. A nomogram was developed, and its predictive efficacy was assessed. About 27,892 AS events and 3,178 events were associated with overall survival (OS) after strict filtering. Parent genes of survival-associated AS events were mainly enriched in leukemia-associated processes including chromatin modification, autophagy, and T-cell receptor signaling pathway. The 10 AS signature based on seven types of AS events showed better efficacy in predicting OS of patients than those built on a single AS event type. The area under curve (AUC) value of the 10 AS signature for 3-year OS was 0.91. Gene set enrichment analysis (GSEA) confirmed that these survival-related AS events contribute to AML progression. Moreover, the nomogram showed good predictive performance for patient's prognosis. Finally, the correlation network of AS variants with splicing factor genes found potential important regulatory genes in AML. The present study presented a systematic analysis of survival-related AS events and developed AS signatures for predicting the patient’s survival. Further studies are needed to validate the signatures in independent AML cohorts and might provide a promising perspective for developing therapeutic targets.

Eradication of Measurable Residual Disease in AML: A Challenging Clinical Goal

In non-promyelocytic (non-M3) AML measurable residual disease (MRD) detected by multi-parameter flow cytometry and molecular technologies, which are guided by Consensus-based guidelines and discover very low leukemic cell numbers far below the 5% threshold of morphological assessment, has emerged as the most relevant predictor of clinical outcome. Currently, it is well-established that MRD positivity after standard induction and consolidation chemotherapy, as well as during the period preceding an allogeneic hematopoietic stem cell transplant (allo-HSCT), portends to a significantly inferior relapse-free survival (RFS) and overall survival (OS). In addition, it has become absolutely clear that conversion from an MRD-positive to an MRD-negative state provides a favorable clinical outcome similar to that associated with early MRD negativity. Thus, the complete eradication of MRD, i.e., the clearance of the few leukemic stem cells-which, due to their chemo-radiotherapy resistance, might eventually be responsible of disease recurrence-has become an un-met clinical need in AML. Nowadays, this goal might potentially be achieved thanks to the development of novel innovative treatment strategies, including those targeting driver mutations, apoptosis, methylation patterns and leukemic proteins. The aim of this review is to analyze these strategies and to suggest any potential combination able to induce MRD negativity in the pre- and post-HSCT period.

FLT3 Inhibitors in Acute Myeloid Leukemia: Challenges and Recent Developments in Overcoming Resistance

Mutations in the FMS-like tyrosine kinase 3 (FLT3) gene are often present in newly diagnosed acute myeloid leukemia (AML) patients with an incidence rate of approximately 30%. Recently, many FLT3 inhibitors have been developed and exhibit positive preclinical and clinical effects against AML. However, patients develop resistance soon after undergoing FLT3 inhibitor treatment, resulting in short durable responses and poor clinical effects. This review will discuss the main mechanisms of resistance to clinical FLT3 inhibitors and summarize the emerging strategies that are utilized to overcome drug resistance. Basically, medicinal chemistry efforts to develop new small-molecule FLT3 inhibitors offer a direct solution to this problem. Other potential strategies include the combination of FLT3 inhibitors with other therapies and the development of multitarget inhibitors. It is hoped that this review will provide inspiring insights into the discovery of new AML therapies that can eventually overcome the resistance to current FLT3 inhibitors.

Inhibition of the proteasome and proteaphagy enhances apoptosis in FLT3-ITD-driven acute myeloid leukemia

Acute myeloid leukaemia (AML) is a clonal disorder that affects hematopoietic stem cells or myeloid progenitors. One of the most common mutations that results in AML occurs in the gene encoding fms‐like tyrosine kinase 3 (FLT3). Previous studies have demonstrated that AML cells expressing FLT3‐internal tandem duplication (ITD) are more sensitive to the proteasome inhibitor bortezomib (Bz) than FLT3 wild‐type cells, with this cytotoxicity being mediated by autophagy (Atg). Here, we show that proteasome inhibition with Bz results in modest but consistent proteaphagy in MOLM‐14 leukemic cells expressing the FLT3‐ITD mutation, but not in OCI‐AML3 leukemic cells with wild‐type FLT3. Chemical inhibition of Atg with bafilomycin A simultaneously blocked proteaphagy and resulted in the accumulation of the p62 Atg receptor in Bz‐treated MOLM‐14 cells. The use of ubiquitin traps revealed that ubiquitin plays an important role in proteasome‐Atg cross‐talk. The p62 inhibitor verteporfin blocked proteaphagy and, importantly, resulted in accumulation of high molecular weight forms of p62 and FLT3‐ITD in Bz‐treated MOLM‐14 cells. Both Atg inhibitors enhanced Bz‐induced apoptosis in FLT3‐ITD‐driven leukemic cells, highlighting the therapeutic potential of these treatments.

Mechanisms Underlying Resistance to FLT3 Inhibitors in Acute Myeloid Leukemia

FLT3-ITD and FLT3-TKD mutations were observed in approximately 20 and 10% of acute myeloid leukemia (AML) cases, respectively. FLT3 inhibitors such as midostaurin, gilteritinib and quizartinib show excellent response rates in patients with FLT3-mutated AML, but its duration of response may not be sufficient yet. The majority of cases gain secondary resistance either by on-target and off-target abnormalities. On-target mutations (i.e., FLT3-TKD) such as D835Y keep the TK domain in its active form, abrogating pharmacodynamics of type II FLT3 inhibitors (e.g., midostaurin and quizartinib). Second generation type I inhibitors such as gilteritinib are consistently active against FLT3-TKD as well as FLT3-ITD. However, a “gatekeeper” mutation F691L shows universal resistance to all currently available FLT3 inhibitors. Off-target abnormalities are consisted with a variety of somatic mutations such as NRAS, AXL and PIM1 that bypass or reinforce FLT3 signaling. Off-target mutations can occur just in the primary FLT3-mutated clone or be gained by the evolution of other clones. A small number of cases show primary resistance by an FL-dependent, FGF2-dependent, and stromal CYP3A4-mediated manner. To overcome these mechanisms, the development of novel agents such as covalently-coupling FLT3 inhibitor FF-10101 and the investigation of combination therapy with different class agents are now ongoing. Along with novel agents, gene sequencing may improve clinical approaches by detecting additional targetable mutations and determining individual patterns of clonal evolution.

Allogeneic FLT3 CAR T Cells with an Off-Switch Exhibit Potent Activity against AML and Can Be Depleted to Expedite Bone Marrow Recovery

Patients with relapsed or refractory acute myeloid leukemia (AML) have a dismal prognosis and limited treatment options. Chimeric antigen receptor (CAR) T cells have achieved unprecedented clinical responses in patients with B cell leukemias and lymphomas and could prove highly efficacious in AML. However, a significant number of patients with AML may not receive treatment with an autologous product due to manufacturing failures associated with low lymphocyte counts or rapid disease progression while the therapeutic is being produced. We report the preclinical evaluation of an off-the-shelf CAR T cell therapy targeting Fms-related tyrosine kinase 3 (FLT3) for the treatment of AML. Single-chain variable fragments (scFvs) targeting various epitopes in the extracellular region of FLT3 were inserted into CAR constructs and tested for their ability to redirect T cell specificity and effector function to FLT3⁺ AML cells. A lead CAR, exhibiting minimal tonic signaling and robust activity in vitro and in vivo, was selected and then modified to incorporate a rituximab-responsive off-switch in cis. We found that allogeneic FLT3 CAR T cells, generated from healthy-donor T cells, eliminate primary AML blasts but are also active against mouse and human hematopoietic stem and progenitor cells, indicating risk of myelotoxicity. By employing a surrogate CAR with affinity to murine FLT3, we show that rituximab-mediated depletion of FLT3 CAR T cells after AML eradication enables bone marrow recovery without compromising leukemia remission. These results support clinical investigation of allogeneic FLT3 CAR T cells in AML and other FLT3⁺ hematologic malignancies.

Induction of leukemic stem cell differentiation by aryl hydrocarbon receptor agonist and synergy with gilteritinib in FLT3-ITD + acute myeloid leukemia

Leukemic stem cells (LSCs) are a major cause of treatment failure and recurrence of acute myeloid leukemia (AML). Targeting LSC is essential to developing a potential cure for patients with relapsed/refractory AML. Here we investigated the effect of aryl hydrocarbon receptor (AhR) signaling on AML stem/progenitor proportion and examined the combined effect of AhR agonist and tyrosine kinase inhibitor. The AhR agonist, 6-formylindolo[3,2-b]carbazole (FICZ), significantly decreased the LSC proportion and clonogenicity and increased differentiation markers in AML primary cells. Synergistic/additive effects of FICZ and gilteritinib, FMS-like tyrosine kinase 3 (FLT3) inhibitor, were confirmed in AML cells with FLT3-ITD. We present evidence that combination of both agents inhibits FLT3 downstream molecules and degrades clonogenicity. Collectively, our results suggest that FICZ not only compels LSC differentiation, but also enhances the efficacy of gilteritinib when combined. Clinical application of this combined approach may pave a new therapeutic strategy for patients with FLT3 mutated AML.

Can Hemp Help? Low-THC Cannabis and Non-THC Cannabinoids for the Treatment of Cancer

Cannabis has been used to relieve the symptoms of disease for thousands of years. However, social and political biases have limited effective interrogation of the potential benefits of cannabis and polarised public opinion. Further, the medicinal and clinical utility of cannabis is limited by the psychotropic side effects of ∆⁹-tetrahydrocannabinol (∆⁹-THC). Evidence is emerging for the therapeutic benefits of cannabis in the treatment of neurological and neurodegenerative diseases, with potential efficacy as an analgesic and antiemetic for the management of cancer-related pain and treatment-related nausea and vomiting, respectively. An increasing number of preclinical studies have established that ∆⁹-THC can inhibit the growth and proliferation of cancerous cells through the modulation of cannabinoid receptors (CB1R and CB2R), but clinical confirmation remains lacking. In parallel, the anti-cancer properties of non-THC cannabinoids, such as cannabidiol (CBD), are linked to the modulation of non-CB1R/CB2R G-protein-coupled receptors, neurotransmitter receptors, and ligand-regulated transcription factors, which together modulate oncogenic signalling and redox homeostasis. Additional evidence has also demonstrated the anti-inflammatory properties of cannabinoids, and this may prove relevant in the context of peritumoural oedema and the tumour immune microenvironment. This review aims to document the emerging mechanisms of anti-cancer actions of non-THC cannabinoids.

Quizartinib, a selective FLT3 inhibitor, maintains antileukemic activity in preclinical models of RAS-mediated midostaurin-resistant acute myeloid leukemia cells

FLT3 internal tandem duplication (ITD) mutations are associated with poor prognosis in patients with acute myeloid leukemia (AML). In this preclinical study, we characterized the binding affinity and selectivity of quizartinib, a small-molecule inhibitor of FLT3, and AC886, the active metabolite of quizartinib, compared with those of other FLT3 inhibitors. Selectivity profiling against >400 kinases showed that quizartinib and AC886 were highly selective against FLT3. Quizartinib and AC886 inhibited FLT3 signaling pathways in FLT3-ITD–mutated AML cells, leading to potent growth inhibition with IC₅₀ values of <1 nM. When quizartinib was administered to mice bearing FLT3-ITD mutated tumors, AC886 was rapidly detected and tumor regression was observed at doses of ≥1 mg/kg without severe body weight loss. In addition, quizartinib inhibited the viability of midostaurin-resistant MOLM-14 cells and exerted potent antitumor activity in mouse xenograft models without severe body weight loss, while midostaurin and gilteritinib did not show significant antitumor effects. This is the first detailed characterization of quizartinib and AC886 in comparison with other FLT3 inhibitors under the same experimental conditions. Preclinical antileukemic activity in midostaurin-resistant FLT3-ITD–mutated AML cells suggests the potential value of quizartinib following midostaurin failure in patients with FLT3-ITD mutated AML.

Discovery of orally active indirubin-3'-oxime derivatives as potent type 1 FLT3 inhibitors for acute myeloid leukemia

FMS-like receptor tyrosine kinase-3 (FLT3) is expressed on acute leukemia cells and is implicated in the survival, proliferation and differentiation of hematopoietic cells in most acute myeloid leukemia (AML) patients. Despite recent achievements in the development of FLT3-targeted small-molecule drugs, there are still unmet medical needs related to kinase selectivity and the progression of some mutant forms of FLT3. Herein, we describe the discovery of novel orally available type 1 FLT3 inhibitors from structure-activity relationship (SAR) studies for the optimization of indirubin derivatives with biological and pharmacokinetic profiles as potential therapeutic agents for AML. The SAR exploration provided important structural insights into the key substituents for potent inhibitory activities of FLT3 and in MV4-11 cells. The profile of the most optimized inhibitor (36) showed IC₅₀ values of 0.87 and 0.32 nM against FLT3 and FLT3/D835Y, respectively, along with potent inhibition against MV4-11 and FLT3/D835Y expressed MOLM14 cells with a GI₅₀ value of 1.0 and 1.87 nM, respectively. With the high oral bioavailability of 42.6%, compound 36 displayed significant in vivo antitumor activity by oral administration of 20 mg/kg once daily dosing schedule for 21 days in a mouse xenograft model. The molecular docking study of 36 in the homology model of the DFG-in conformation of FLT3 resulted in a reasonable binding mode in type 1 kinases similar to the reported type 1 FLT3 inhibitors Crenolanib and Gilteritinib.

Hybridization capture-based next generation sequencing reliably detects FLT3 mutations and classifies FLT3-internal tandem duplication allelic ratio in acute myeloid leukemia: a comparative study to standard fragment analysis

FLT3-internal tandem duplication occurs in 20-30% of acute myeloid leukemia and confers an adverse prognosis with its allelic ratio being a key risk stratifier. The US Food and Drug Administration recently approved FLT3 inhibitors midostaurin and gilteritinib in FLT3 mutation-positive acute myeloid leukemia. Historically, FLT3 was tested by fragment analysis, which has become the standard method endorsed by international guidelines. However, next generation sequencing is increasingly used at acute myeloid leukemia diagnosis given its ability to simultaneously evaluate multiple clinically informative markers. As FLT3-internal tandem duplication detection was known to be challenging by next generation sequencing and the results carry profound prognostic and therapeutic implications, it is important to thoroughly examine its performance in FLT3-internal tandem duplication detection and allelic ratio classification. In a comparative study with fragment analysis, we retrospectively reviewed our experience using a custom-designed, hybridization capture-based, targeted next generation sequencing panel. Among 7902 cases, FLT3-internal tandem duplication was detected in 335 with variable sizes (3-231 bp) and insertion sites. Fragment analysis was also performed in 402 cases, demonstrating 100% concordance in FLT3-internal tandem duplication detection. In 136 dual-tested, positive cases, 128/136 (94%) exhibited concordant high/low allelic ratio classifications. The remaining 6% showed borderline low allelic ratio by next generation sequencing. The two methods were concordant in FLT3-tyrosine kinase domain mutation detection at the hotspot D835/I836 targeted by fragment analysis. Furthermore, seven mutations which may benefit from FLT3 inhibitor therapy were detected by next generation sequencing, in regions not covered by fragment analysis. Our study demonstrates that using a hybridization capture-based chemistry and optimized bioinformatics pipeline, next generation sequencing can reliably detect FLT3-internal tandem duplication and classify its allelic ratio for acute myeloid leukemia risk stratification. Next generation sequencing also exhibits superior comprehensiveness in FLT3 mutation detection and may further improve personalized, targeted therapy in acute myeloid leukemia.

Robustaflavone-4'-dimethyl ether from Selaginella uncinata attenuated lipopolysaccharide-induced acute lung injury via inhibiting FLT3-mediated neutrophil activation

Neutrophils act as both messenger and effector which contributed to the pathogenesis of acute lung injury (ALI). Targeting neutrophils could be a novel strategy for prevention and treatment of ALI. Selaginella uncinata is widely used as an antitussive, antipyretic and anti-inflammatory herb to treat various pulmonary diseases, including lung cancer, asthma, pulmonary fibrosis and pneumonia. However, its effective constituents remain unknown. In the present study, the protective effects of flavonoids from S. uncinata (SUF) and its major compound robustaflavone-4'-dimethyl ether (RDE) against lipopolysaccharide (LPS)-induced ALI were investigated in mice and in neutrophils.The results showed that both SUF and RDE had the same inhibition on LPS-induced lung edema and neutrophil infiltration as well as the increased levels of IL-6, TNF-α, P-selectin and ICAM-1 in serum of LPS-challenged mice. Furthermore, RDE significantly inhibited inducible neutrophil activation in a concentration-dependent manner, and also reduced the levels of intracellular calcium as well as the expressions of CCR2. Rescue experiment showed that RDE suppressed FLT3 and its downstream p-p38 and p-AKT, which could be abolished by FLT3 agonist FLT3L but partly by MAPK agonist PDBu or AKT agonist SC79. Therefore, these results indicated that RDE as the main bioactive compound in SUF alleviated LPS-induced acute lung injury and inhibited neutrophil activation via inhibition of FLT3-mediatied AKT and MAPK pathways.

The Unfolded Protein Response: A Novel Therapeutic Target in Acute Leukemias

The unfolded protein response (UPR) is an evolutionarily conserved adaptive response triggered by the stress of the endoplasmic reticulum (ER) due, among other causes, to altered cell protein homeostasis (proteostasis). UPR is mediated by three main sensors, protein kinase RNA-like endoplasmic reticulum kinase (PERK), activating transcription factor 6α (ATF6α), and inositol-requiring enzyme-1α (IRE1α). Given that proteostasis is frequently disregulated in cancer, UPR is emerging as a critical signaling network in controlling the survival, selection, and adaptation of a variety of neoplasias, including breast cancer, prostate cancer, colorectal cancer, and glioblastoma. Indeed, cancer cells can escape from the apoptotic pathways elicited by ER stress by switching UPR into a prosurvival mechanism instead of cell death. Although most of the studies on UPR focused on solid tumors, this intricate network plays a critical role in hematological malignancies, and especially in multiple myeloma (MM), where treatment with proteasome inhibitors induce the accumulation of unfolded proteins that severely perturb proteostasis, thereby leading to ER stress, and, eventually, to apoptosis. However, UPR is emerging as a key player also in acute leukemias, where recent evidence points to the likelihood that targeting UPR-driven prosurvival pathways could represent a novel therapeutic strategy. In this review, we focus on the oncogene-specific regulation of individual UPR signaling arms, and we provide an updated outline of the genetic, biochemical, and preclinical therapeutic findings that support UPR as a relevant, novel target in acute leukemias.

The Role of Reactive Oxygen Species in Acute Myeloid Leukaemia

Acute myeloid leukaemia (AML) is an aggressive haematological malignancy with a poor overall survival. Reactive oxygen species (ROS) have been shown to be elevated in a wide range of cancers including AML. Whilst previously thought to be mere by-products of cellular metabolism, it is now clear that ROS modulate the function of signalling proteins through oxidation of critical cysteine residues. In this way, ROS have been shown to regulate normal haematopoiesis as well as promote leukaemogenesis in AML. In addition, ROS promote genomic instability by damaging DNA, which promotes chemotherapy resistance. The source of ROS in AML appears to be derived from members of the “NOX family” of NADPH oxidases. Most studies link NOX-derived ROS to activating mutations in the Fms-like tyrosine kinase 3 (FLT3) and Ras-related C3 botulinum toxin substrate (Ras). Targeting ROS through either ROS induction or ROS inhibition provides a novel therapeutic target in AML. In this review, we summarise the role of ROS in normal haematopoiesis and in AML. We also explore the current treatments that modulate ROS levels in AML and discuss emerging drug targets based on pre-clinical work.

Effect of Fms-like tyrosine kinase 3 (FLT3) ligand (FL) on antitumor activity of gilteritinib, a FLT3 inhibitor, in mice xenografted with FL-overexpressing cells

Therapeutic effects of FLT3 inhibitors have been reported in acute myeloid leukemia (AML) with constitutively activating FLT3 mutations, including internal tandem duplication (ITD) and point mutation, which are found in approximately one-third of AML patients. One of the critical issues of treatment with FLT3 inhibitors in FLT3-mutated AML is drug resistance. FLT3 ligand (FL) represents a mechanism of resistance to FLT3 inhibitors, including quizartinib, midostaurin, and sorafenib, in AML cells harboring both wild-type and mutant FLT3 (FLT3^wt/FLT3^mut). Here, we investigated the effect of FL on the efficacy of gilteritinib, a FLT3 inhibitor, in AML-derived cells in vitro and in mice. In contrast to other FLT3 inhibitors, FL stimulation had little effect on growth inhibition or apoptosis induction by gilteritinib. The antitumor activity of gilteritinib was also comparable between xenograft mouse models injected with FL-expressing and mock MOLM-13 cells. In the FLT3 signaling analyses, gilteritinib inhibited FLT3^wt and FLT3-ITD to a similar degree in HEK293 and Ba/F3 cells, and similarly suppressed FLT3 downstream signaling molecules (including ERK1/2 and STAT5) in both the presence and absence of FL in MOLM-13 cells. Co-crystal structure analysis showed that gilteritinib bound to the ATP-binding pocket of FLT3. These results suggest that gilteritinib has therapeutic potential in FLT3-mutated AML patients with FL overexpression.

FMS-like Tyrosine Kinase 3/FLT3: From Basic Science to Clinical Implications

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase that is expressed almost exclusively in the hematopoietic compartment. Its ligand, FLT3 ligand (FL), induces dimerization and activation of its intrinsic tyrosine kinase activity. Activation of FLT3 leads to its autophosphorylation and initiation of several signal transduction cascades. Signaling is initiated by the recruitment of signal transduction molecules to activated FLT3 through binding to specific phosphorylated tyrosine residues in the intracellular region of FLT3. Activation of FLT3 mediates cell survival, cell proliferation, and differentiation of hematopoietic progenitor cells. It acts in synergy with several other cytokines to promote its biological effects. Deregulated FLT3 activity has been implicated in several diseases, most prominently in acute myeloid leukemia where around one-third of patients carry an activating mutant of FLT3 which drives the disease and is correlated with poor prognosis. Overactivity of FLT3 has also been implicated in autoimmune diseases, such as rheumatoid arthritis. The observation that gain-of-function mutations of FLT3 can promote leukemogenesis has stimulated the development of inhibitors that target this receptor. Many of these are in clinical trials, and some have been approved for clinical use. However, problems with acquired resistance to these inhibitors are common and, furthermore, only a fraction of patients respond to these selective treatments. This review provides a summary of our current knowledge regarding structural and functional aspects of FLT3 signaling, both under normal and pathological conditions, and discusses challenges for the future regarding the use of targeted inhibition of these pathways for the treatment of patients.

A personalized approach to acute myeloid leukemia therapy: current options

Therapeutic options for acute myeloid leukemia (AML) have remained unchanged for nearly the past 5 decades, with cytarabine and anthracyclines and use of hypomethylating agents for less intensive therapy. Implementation of large-scale genomic studies in the past decade has unraveled the genetic landscape and molecular etiology of AML. The approval of several novel drugs for targeted therapy, including midostaurin, enasidenib, ivosidenib, gemtuzumab–ozogamicin, and CPX351 by the US Food and Drug Administration has widened the treatment options for clinicians treating AML. This review focuses on some of these novel therapies and other promising agents under development, along with key clinical trial findings in AML.

Cytokine production in myelofibrosis exhibits differential responsiveness to JAK-STAT, MAP kinase, and NFκB signaling

The distinct clinical features of myelofibrosis (MF) have been attributed in part to dysregulated inflammatory cytokine production. Circulating cytokine levels are elevated in MF patients; a subset of which have been shown to be poor prognostic indicators. In this study, cytokine overproduction was examined in MF patient plasma and in MF blood cells ex vivo using mass cytometry. Plasma cytokines measured following treatment with ruxolitinib remained markedly abnormal, indicating that aberrant cytokine production persists despite therapeutic JAK2 inhibition. In MF patient samples, 14/15 cytokines measured by mass cytometry were found to be constitutively overproduced, with the principal cellular source for most cytokines being monocytes, implicating a non-cell-autonomous role for monocyte-derived cytokines impacting disease-propagating stem/progenitor cells in MF. The majority of cytokines elevated in MF exhibited ex vivo hypersensitivity to thrombopoietin (TPO), toll-like receptor (TLR) ligands, and/or tumor necrosis factor (TNF). A subset of this group (including TNF, IL-6, IL-8, IL-10) was minimally sensitive to ruxolitinib. All TPO/TLR/TNF-sensitive cytokines, however, were sensitive to pharmacologic inhibition of NFκB and/or MAP kinase signaling. These results indicate that NFκB and MAP kinase signaling maintain cytokine overproduction in MF, and that inhibition of these pathways may provide optimal control of inflammatory pathophysiology in MF.

Novel treatments for relapsed/refractory acute myeloid leukemia with FLT3 mutations

Introduction: Mutations in the gene encoding for the FMS-like tyrosine kinase 3 (FLT3) are present in about 30% of adults with AML and are associated with shorter disease-free and overall survival after initial therapy. Prognosis of relapsed/refractory AML with FLT3 mutations is even more dismal with median overall survival of a few months only. Areas covered: This review will cover current and emerging treatments for relapsed/refractory AML with FLT3 mutations, preclinical rationale and clinical trials with new encouraging data for this particularly challenging population. The authors discuss mechanisms of resistance to FLT3 inhibitors and how these insights serve to identify current and future treatments. As allogeneic stem cell transplant in the first remission is the preferred therapy for newly diagnosed AML patients with FLT3 mutations, the authors discuss the role of maintenance after SCT for the prevention of relapse. Expert opinion: Relapsed/refractory AML with FLT3 mutations remains a therapeutic challenge with currently available treatments. However, the evolution of targeted therapies with next-generation FLT3 inhibitors and their combinations with chemotherapy is showing much promise. Moreover, growing understanding of the pathways of resistance to treatment has led to the identification of various targeted therapies currently being explored, which in time will improve outcomes.

Acute Myeloid Leukemia Mutations: Therapeutic Implications

Acute Myeloid Leukemia (AML) is an extremely heterogeneous group of hematological neoplasms, for which allogeneic stem cell transplantation (HSCT) still represents the only potentially curative option in the majority of cases. However, elderly age and clinically severe comorbidities may often exclude a wide amount of patients from this therapeutic approach, underlying the urgent need for alternative strategies. Thanks to the introduction of advanced high-throughput techniques, light is being shed on the pathogenesis of AML, identifying molecular recurrent mutations as responsible for the onset, as well as progression, of disease. As a consequence, and in parallel, many new compounds, including targeted therapies (FMS-like tyrosine kinase 3 (FLT3) and Isocitrate dehydrogenase 1-2 (IDH1-2) inhibitors), have found a wide room of application in this setting, and are now available in daily practice, or in late phases of clinical development. Moreover, several further innovative molecules are currently under investigation, and promising results for many of them have already been reported. In this review, we will present an update on the most relevant molecular alterations of AML, focusing on the most frequent genomic mutations of the disease, for which compounds have been approved or are still currently under investigation.

Novel Agents for Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a complex hematological disease characterized by genetic and clinical heterogeneity. Recent advances in the understanding of AML pathogenesis have paved the way for the development of new agents targeting specific molecules or mechanisms that contribute to finally move beyond the current standard of care, which is "3 + 7" regimen. In particular, new therapeutic options such as targeted therapies (midostaurin and enasidenib), monoclonal antibodies (gemtuzumab ozogamicin), and a novel liposomal formulation of cytarabine and daunorubicin (CPX-351) have been recently approved, and will be soon available for the treatment of adult patients with AML. In this review, we will present and describe these recently approved drugs as well as selected novel agents against AML that are currently under investigation, and show the most promising results as monotherapy or in combination with chemotherapy. The selection of these emerging treatments is based on the authors' opinion.