FLT3 inhibitors for acute myeloid leukemia: a review of their efficacy and mechanisms of resistance

Molecular hybridization: a powerful tool for multitarget drug discovery

INTRODUCTION

The current drug discovery paradigm of 'one drug, multiple targets' has gained attention from both the academic medicinal chemistry community and the pharmaceutical industry. This is in response to the urgent need for effective agents to treat multifactorial chronic diseases. The molecular hybridization strategy is a useful tool that has been widely explored, particularly in the last two decades, for the design of multi-target drugs.

AREAS COVERED

This review examines the current state of molecular hybridization in guiding the discovery of multitarget small molecules. The article discusses the design strategies and target selection for a multitarget polypharmacology approach to treat various diseases, including cancer, Alzheimer's disease, cardiac arrhythmia, endometriosis, and inflammatory diseases.

EXPERT OPINION

Although the examples discussed highlight the importance of molecular hybridization for the discovery of multitarget bioactive compounds, it is notorious that the literature has focused on specific classes of targets. This may be due to a deep understanding of the pharmacophore features required for target binding, making targets such as histone deacetylases and cholinesterases frequent starting points. However, it is important to encourage the scientific community to explore diverse combinations of targets using the molecular hybridization strategy.

Oncogenic FLT3 internal tandem duplication activates E2F1 to regulate purine metabolism in acute myeloid leukaemia

Oncogene FLT3 internal tandem duplication (FLT3-ITD) mutation accounts for 30 % of acute myeloid leukaemia (AML) cases and induces transformation. Previously, we found that E2F transcription factor 1 (E2F1) was involved in AML cell differentiation. Here, we reported that E2F1 expression was aberrantly upregulated in AML patients, especially in AML patients carrying FLT3-ITD. E2F1 knockdown inhibited cell proliferation and increased cell sensitivity to chemotherapy in cultured FLT3-ITD-positive AML cells. E2F1-depleted FLT3-ITD⁺ AML cells lost their malignancy as shown by the reduced leukaemia burden and prolonged survival in NOD-Prkdc^scidIl2rg^em1/Smoc mice receiving xenografts. Additionally, FLT3-ITD-driven transformation of human CD34⁺ hematopoietic stem and progenitor cells was counteracted by E2F1 knockdown. Mechanistically, FLT3-ITD enhanced the expression and nuclear accumulation of E2F1 in AML cells. Further study using chromatin immunoprecipitation-sequencing and metabolomics analyses revealed that ectopic FLT3-ITD promoted the recruitment of E2F1 on genes encoding key enzymatic regulators of purine metabolism and thus supported AML cell proliferation. Together, this study demonstrates that E2F1-activated purine metabolism is a critical downstream process of FLT3-ITD in AML and a potential target for FLT3-ITD⁺ AML patients.

PROTACs: Walking through hematological malignancies

Proteolysis targeting chimeras (PROTACs) are heterobifunctional small molecules that uses the proteasome ubiquitin system to target proteins of interest and promote their degradation with remarkable selectivity. Importantly, unlike conventional small molecule inhibitors, PROTACs have proven highly effective in targeting undruggable proteins and those bearing mutations. Because of these considerations, PROTACs have increasingly become an emerging technology for the development of novel targeted anticancer therapeutics. Interestingly, many PROTACs have demonstrated a great potency and specificity in degrading several oncogenic drivers. Many of these, following extensive preclinical evaluation, have reached advanced stages of clinical testing in various cancers including hematologic malignancies. In this review, we provide a comprehensive summary of the recent advances in the development of PROTACs as therapeutic strategies in diverse hematological malignancies. A particular attention has been given to clinically relevant PROTACs and those targeting oncogenic mutants that drive resistance to therapies. We also discus limitations, and various considerations to optimize the design for effective PROTACs.

PCW-A1001, AI-assisted de novo design approach to design a selective inhibitor for FLT-3(D835Y) in acute myeloid leukemia

Treating acute myeloid leukemia (AML) by targeting FMS-like tyrosine kinase 3 (FLT-3) is considered an effective treatment strategy. By using AI-assisted hit optimization, we discovered a novel and highly selective compound with desired drug-like properties with which to target the FLT-3 (D835Y) mutant. In the current study, we applied an AI-assisted de novo design approach to identify a novel inhibitor of FLT-3 (D835Y). A recurrent neural network containing long short-term memory cells (LSTM) was implemented to generate potential candidates related to our in-house hit compound (PCW-1001). Approximately 10,416 hits were generated from 20 epochs, and the generated hits were further filtered using various toxicity and synthetic feasibility filters. Based on the docking and free energy ranking, the top compound was selected for synthesis and screening. Of these three compounds, PCW-A1001 proved to be highly selective for the FLT-3 (D835Y) mutant, with an IC₅₀ of 764 nM, whereas the IC₅₀ of FLT-3 WT was 2.54 μM.

Recent Advances in PROTACs for Drug Targeted Protein Research

Proteolysis-targeting chimera (PROTAC) is a heterobifunctional molecule. Typically, PROTAC consists of two terminals which are the ligand of the protein of interest (POI) and the specific ligand of E3 ubiquitin ligase, respectively, via a suitable linker. PROTAC degradation of the target protein is performed through the ubiquitin–proteasome system (UPS). The general process is that PROTAC binds to the target protein and E3 ligase to form a ternary complex and label the target protein with ubiquitination. The ubiquitinated protein is recognized and degraded by the proteasome in the cell. At present, PROTAC, as a new type of drug, has been developed to degrade a variety of cancer target proteins and other disease target proteins, and has shown good curative effects on a variety of diseases. For example, PROTACs targeting AR, BR, BTK, Tau, IRAK4, and other proteins have shown unprecedented clinical efficacy in cancers, neurodegenerative diseases, inflammations, and other fields. Recently, PROTAC has entered a phase of rapid development, opening a new field for biomedical research and development. This paper reviews the various fields of targeted protein degradation by PROTAC in recent years and summarizes and prospects the hot targets and indications of PROTAC.

FLT3 inhibitors for acute myeloid leukemia: successes, defeats, and emerging paradigms

FLT3 mutations are one of the most common genetic aberrations found in nearly 30% of acute myeloid leukemias (AML). The mutations are associated with poor prognosis despite advances in the understanding of the biological mechanisms of AML. Numerous small molecule FLT3 inhibitors have been developed in an effort to combat AML. Even with the development of these inhibitors, the five-year overall survival for newly diagnosed AML is less than 30%. In 2017, midostaurin received FDA approval to treat AML, which was the first approved FLT3 inhibitor in the U.S. and Europe. Following, gilteritinib received FDA approval in 2018 and in 2019 quizartinib received approval in Japan. This review parallels these clinical success stories along with other pre-clinical and clinical investigations of FLT3 inhibitors.

2-Methoxyestradiol combined with ascorbic acid facilitates the apoptosis of chronic myeloid leukemia cells via the microRNA-223/Fms-like tyrosine kinase 3/phosphatidylinositol-3 kinase/protein kinase B axis

Chronic myeloid leukemia (CML) is a malignant myeloproliferative tumor. 2-Methoxyestradiol (2-ME) is an endogenous estrogen metabolite that shows efficacy in human malignancies. Ascorbic acid (AA) possesses antioxidant activity. This study explored the mechanism of 2-ME combined with AA in the apoptosis of CML cells. Firstly, human CML cell lines were treated with 2-ME and AA. The cell viability, apoptosis, reactive oxygen species (ROS), and mitochondrial membrane potential (MMP) were detected. miR-223 expression in CML cells was detected. In addition, CML cells were transfected with miR-223 inhibitor. The binding relationship between miR-223 and FLT3 was verified. Subsequently, the FLT3 was overexpressed or silenced for the function rescue experiment to confirm the role of FLT3 in CML cell apoptosis. The expression levels of key factors of the PI3K/AKT pathway were detected. Finally, xenograft nude mouse models were established for in vivo verification. 2-ME + AA treatment inhibited CML cell viability and promoted apoptosis, elevated ROS content, and reduced MMP. 2-ME + AA treatment promoted miR-223 expression in CML cells. miR-223 targeted FLT3. Moreover, miR-223 inhibitor or FLT3 overexpression partially annulled the effect of 2-ME + AA on CML cells. 2-ME + AA inhibited the PI3K/AKT pathway via the miR-223/FLT3 axis. Furthermore, 2-ME + AA suppressed CML xenograft growth in mice. Collectively, 2-ME + AA promoted miR-223 expression and suppressed FLT3 and the PI3K/AKT pathway, thereby facilitating the apoptosis of CML cells and inhibiting CML xenograft growth in mice.

Strategies for designing proteolysis targeting chimaeras (PROTACs)

Proteolysis targeting chimaeras (PROTACs) is a cutting edge and rapidly growing technique for new drug discovery and development. Currently, the largest challenge in the molecular design and drug development of PROTACs is efficient identification of potent and drug-like degraders. This review aims to comprehensively summarize and analyse state-of-the-art methods and strategies in the design of PROTACs. We provide a detailed illustration of the general principles and tactics for designing potent PROTACs, highlight representative case studies, and discuss the advantages and limitations of these strategies. Particularly, structure-based rational PROTAC design and emerging new types of PROTACs (e.g., homo-PROTACs, multitargeting PROTACs, photo-control PROTACs and PROTAC-based conjugates) will be focused on.

Gilteritinib-induced upregulation of S100A9 is mediated through BCL6 in acute myeloid leukemia

S100A9 overexpression promotes gilteritinib resistance in FLT3-ITD⁺ AML cells.

Gilteritinib-induced upregulation of S100A9 is mediated through loss of BCL6 enrichment at the S100A9 promoter.

Drug resistance and relapse are common challenges in acute myeloid leukemia (AML), particularly in an aggressive subset bearing internal tandem duplications (ITDs) of the FLT3 receptor (FLT3-ITD⁺). The tyrosine kinase inhibitor gilteritinib is approved for the treatment of relapse/refractory AML with FLT3 mutations, yet resistance to gilteritinib remains a clinical concern, and the underlying mechanisms remain incompletely understood. Using transcriptomic analyses and functional validation studies, we identified the calcium-binding proteins S100A8 and S100A9 (S100A8/A9) as contributors to gilteritinib resistance in FLT3-ITD⁺ AML. Exposure of FLT3-ITD⁺ AML cells to gilteritinib increased S100A8/A9 expression in vivo and in vitro and decreased free calcium levels, and genetic manipulation of S100A9 was associated with altered sensitivity to gilteritinib. Using a transcription factor screen, we identified the transcriptional corepressor BCL6, as a regulator of S100A9 expression and found that gilteritinib decreased BCL6 binding to the S100A9 promoter, thereby increasing S100A9 expression. Furthermore, pharmacological inhibition of BCL6 accelerated the growth rate of gilteritinib-resistant FLT3-ITD⁺ AML cells, suggesting that S100A9 is a functional target of BCL6. These findings shed light on mechanisms of resistance to gilteritinib through regulation of a target that can be therapeutically exploited to enhance the antileukemic effects of gilteritinib.

Proteolysis-Targeting Chimera (PROTAC) Modification of Dovitinib Enhances the Antiproliferative Effect against FLT3-ITD-Positive Acute Myeloid Leukemia Cells

Acute myeloid leukemia (AML) refers to one of the most lethal blood malignancies worldwide. FLT3-ITD mutation is recognized as the most common one that predicted a poorer prognosis. There have been many prominent FLT3-ITD inhibitors approved by the FDA for clinical therapies. However, as impacted by undesirable off-target effects, differentiated metabolic issues, and clinical drug resistance problems, it remains challenging to discover alternative and promising solutions for treating FLT3-ITD⁺ AML. In this study, dovitinib was chemically modified and converted into CRBN-recruiting PROTACs. Two active compounds were identified, which showed enhanced antiproliferative effects against FLT3-ITD⁺ AML cells, both in vitro and in vivo. As demonstrated from further biological evaluation, the compounds could induce the degradation of the FLT3-ITD and KIT proteins in a ubiquitin-proteasome-dependent manner and completely block their downstream signaling pathway. The findings of this study would provide another promising strategy to develop novel therapies for FLT3-ITD⁺ AML.

Identification of Protein Biomarker Signatures for Acute Myeloid Leukemia (AML) Using Both Nontargeted and Targeted Approaches

Acute myeloid leukemia (AML) is characterized by an increasing number of clonal myeloid blast cells which are incapable of differentiating into mature leukocytes. AML risk stratification is based on genetic background, which also serves as a means to identify the optimal treatment of individual patients. However, constant refinements are needed, and the inclusion of significant measurements, based on the various omics approaches that are currently available to researchers/clinicians, have the potential to increase overall accuracy with respect to patient management. Using both nontargeted (label-free mass spectrometry) and targeted (multiplex immunoassays) proteomics, a range of proteins were found to be significantly changed in AML patients with different genetic backgrounds. The inclusion of validated proteomic biomarker panels could be an important factor in the prognostic classification of AML patients. The ability to measure both cellular and secreted analytes, at diagnosis and during the course of treatment, has advantages in identifying transforming biological mechanisms in patients, assisting important clinical management decisions.

Gilteritinib Inhibits Glutamine Uptake and Utilization in FLT3-ITD-Positive AML

Acute myeloid leukemia (AML) with an FLT3 internal tandem duplication (FLT3-ITD) mutation is an aggressive hematologic malignancy associated with frequent relapse and poor overall survival. The tyrosine kinase inhibitor gilteritinib is approved for the treatment of relapse/refractory AML with FLT3 mutations, yet its mechanism of action is not completely understood. Here, we sought to identify additional therapeutic targets that can be exploited to enhance gilteritinib's antileukemic effect. Based on unbiased transcriptomic analyses, we identified the glutamine transporter SNAT1 (SLC38A1) as a novel target of gilteritinib that leads to impaired glutamine uptake and utilization within leukemic cells. Using metabolomics and metabolic flux analyses, we found that gilteritinib decreased glutamine metabolism through the TCA cycle and cellular levels of the oncometabolite 2-hydroxyglutarate. In addition, gilteritinib treatment was associated with decreased ATP production and glutathione synthesis and increased reactive oxygen species, resulting in cellular senescence. Finally, we found that the glutaminase inhibitor CB-839 enhanced antileukemic effect of gilteritinib in ex vivo studies using human primary FLT3-ITD-positive AML cells harboring mutations in the enzyme isocitrate dehydrogenase, which catalyzes the oxidative decarboxylation of isocitrate, producing α-ketoglutarate. Collectively, this work has identified a previously unrecognized, gilteritinib-sensitive metabolic pathway downstream of SLC38A1 that causes decreased glutaminolysis and disruption of redox homeostasis. These findings provide a rationale for the development and therapeutic exploration of targeted combinatorial treatment strategies for this subset of relapse/refractory AML.

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.

FLT3 Amplification as Double Minute Chromosomes in a Patient with Chronic Myelomonocytic Leukemia

Double minute chromosomes (dmins) are a form of gene amplification presenting as small spherical paired chromatin bodies. Dmins are rare in hematologic malignancies and are generally associated with a poor prognosis. Some case reports identified MYC or MLL gene amplification performing as dmin in myeloid neoplasms. FLT3 (FMS-related tyrosine kinase 3) acts as an oncogene in myeloid neoplasms which is associated with several signal transduction pathways. Genomic amplification of FLT3 has not been reported in hematological disease. The current study attempts to demonstrate the existence of double minute chromosomes via FLT3 gene amplification in a patient diagnosed with chronic myelomonocytic leukemia (CMML). Routine G-banded karyotype, array-based comparative genomic hybridization, and fluorescence in situ hybridization analyses were used to characterize the cytogenetic abnormality in the patient's bone marrow. FLT3 amplification as dmins in a patient with CMML was revealed. This case study reports a rare double minute chromosome via FLT3 amplification in CMML by using array-based comparative genomic hybridization and fluorescence in situ hybridization analyses. The study also proposed another possible mechanism of FLT3 genes in leukemogenesis.

Clinical Benefits and Safety of FMS-Like Tyrosine Kinase 3 Inhibitors in Various Treatment Stages of Acute Myeloid Leukemia: A Systematic Review, Meta-Analysis, and Network Meta-Analysis

BACKGROUND

Given the controversial roles of FMS-like tyrosine kinase 3 inhibitors (FLT3i) in various treatment stages of acute myeloid leukemia (AML), this study was designed to assess this problem and further explored which FLT3i worked more effectively.

METHODS

A systematic review, meta-analysis and network meta-analysis (NMA) were conducted by filtering PubMed, Embase, Cochrane library, and Chinese databases. We included studies comparing therapeutic effects between FLT3i and non-FLT3i group in AML, particularly FLT3(+) patients, or demonstrating the efficiency of allogeneic hematopoietic stem cell transplantation (allo-HSCT) in FLT3(+) AML. Relative risk (RR) with 95% confidence intervals (CI) was used for estimating complete remission (CR), early death and toxicity. Hazard ratio (HR) was used to assess overall survival (OS), event-free survival (EFS), relapse-free survival (RFS) and cumulative incidence of relapse (CIR).

RESULTS

After addressing all criteria, 39 studies were eventually analyzed. Better CR was accomplished by FLT3i in untreated AML (RR 0.88, p = 0.04) and refractory and relapsed FLT3(+) AML (rrAML) (RR 0.61, p < 0.01) compared to non-FLT3i arm, followed by improved survival (untreated AML: OS, HR 0.76; EFS, HR 0.67; RFS, HR 0.72; all p < 0.01; FLT3(+) rrAML: OS, HR 0.60, p < 0.01; RFS, HR 0.40, p = 0.01). In addition, allo-HSCT improved survival in FLT3(+) AML (OS, HR 0.53; EFS, HR 0.50; RFS, HR 0.57; CIR, HR 0.26; all p < 0.01), which was further prolonged by FLT3i administrated after allo-HSCT (OS, HR 0.45; RFS, HR 0.34; CIR, HR 0.32; all p < 0.01). Additionally, FLT3i consistently improved OS (p < 0.05) regardless of FLT3-ITD ratio, when compared to non-FLT3i group. Besides, FLT3i showed significantly increased risk of thrombocytopenia, neutropenia, anemia, skin- and cardiac-related adverse effects, increased alanine aminotransferase, and increased risk of cough and dyspnea (p < 0.05). In NMA, gilteritinib showed the highest probability for improved prognosis.

CONCLUSIONS

FLT3i safely improved prognosis in induction/reinduction stage of FLT3(+) AML and further boosted survival benefits from allo-HSCT as maintenance therapy, suggesting better prognosis if FLT3i is combined before and after allo-HSCT. In NMA, gilteritinib potentially achieved the best prognosis, which should be identified in direct trials.

High-throughput proteomic profiling reveals mechanisms of action of AMG925, a dual FLT3-CDK4/6 kinase inhibitor targeting AML and AML stem/progenitor cells

FLT3 mutations, which are found in a third of patients with acute myeloid leukemia (AML), are associated with poor prognosis. Responses to currently available FLT3 inhibitors in AML patients are typically transient and followed by disease recurrence. Thus, FLT3 inhibitors with new inhibitory mechanisms are needed to improve therapeutic outcomes. AMG925 is a novel, potent, small-molecule dual inhibitor of FLT3 and CDK4/6. In this study. we determined the antileukemic effects and mechanisms of action of AMG925 in AML cell lines and primary samples, in particular AML stem/progenitor cells. AMG925 inhibited cell growth and promoted apoptosis in AML cells with or without FLT3 mutations. Reverse-phase protein array profiling confirmed its on-target effects on FLT3-CDK4/6-regulated pathways and identified unrevealed signaling network alterations in AML blasts and stem/progenitor cells in response to AMG925. Mass cytometry identified pathways that may confer resistance to AMG925 in phenotypically defined AML stem/progenitor cells and demonstrated that combined blockade of FLT3-CDK4/6 and AKT/mTOR signaling facilitated stem cell death. Our findings provide a rationale for the mechanism-based inhibition of FLT3-CDK4/6 and for combinatorial approaches to improve the efficacy of FLT3 inhibition in both FLT3 wild-type and FLT3-mutated AML.

Monocytic Maturation Induced by FLT3 Inhibitor Therapy of Acute Myeloid Leukemia: Morphologic and Immunophenotypic Characteristics

BACKGROUND

FMS-like tyrosine kinase-3 (FLT3-ITD) mutations are some of the most common mutations in acute myeloid leukemia (AML), and patient outcomes have improved since the advent of tyrosine kinase inhibitors. First, granulocytic differentiation was described in FLT3-positive AML treated with FLT3 inhibitors, and more recently, monocytic differentiation was reported.

METHODS

Two patients with myelomonocytic cells in their bone marrow were identified during routine follow-up after AML treatment that included FLT3 inhibitors. The bone marrow study was done as standard of care.

RESULTS

Both patients had FLT3-ITD+ AML and showed an atypical maturing monocytic cell population and a decrease in the leukemic blast cell population after FLT3 inhibitor therapy. Concurrent genetic testing revealed persistent genetic abnormalities.

CONCLUSIONS

These cases illustrate monocytic maturation in FLT3+ AML after FLT3 inhibitor treatment. It is critical for pathologists and clinicians to be aware of the differentiation phenomenon, as these patients have persistent molecular abnormalities despite response to treatment and normalization of blast counts.

Gilteritinib: potent targeting of FLT3 mutations in AML

Since the discovery of FMS-like tyrosine kinase-3 (FLT3)-activating mutations as genetic drivers in acute myeloid leukemia (AML), investigators have tried to develop tyrosine kinase inhibitors that could effectively target FLT3 and alter the disease trajectory. Giltertinib (formerly known as ASP2215) is a novel compound that entered the field late, but moved through the developmental process with remarkable speed. In many ways, this drug's rapid development was facilitated by the large body of knowledge gained over the years from efforts to develop other FLT3 inhibitors. Single-agent gilteritinib, a potent and selective oral FLT3 inhibitor, improved the survival of patients with relapsed or refractory FLT3-mutated AML compared with standard chemotherapy. This continues to validate the approach of targeting FLT3 itself and establishes a new backbone for testing combination regimens. This review will frame the preclinical and clinical development of gilteritinib in the context of the lessons learned from its predecessors.

OTS167 blocks FLT3 translation and synergizes with FLT3 inhibitors in FLT3 mutant acute myeloid leukemia

Internal tandem duplication (-ITD) mutations of Fms-like tyrosine kinase 3 (FLT3) provide growth and pro-survival signals in the context of established driver mutations in FLT3 mutant acute myeloid leukemia (AML). Maternal embryonic leucine zipper kinase (MELK) is an aberrantly expressed gene identified as a target in AML. The MELK inhibitor OTS167 induces cell death in AML including cells with FLT3 mutations, yet the role of MELK and mechanisms of OTS167 function are not understood. OTS167 alone or in combination with tyrosine kinase inhibitors (TKIs) were used to investigate the effect of OTS167 on FLT3 signaling and expression in human FLT3 mutant AML cell lines and primary cells. We describe a mechanism whereby OTS167 blocks FLT3 expression by blocking FLT3 translation and inhibiting phosphorylation of eukaryotic initiation factor 4E–binding protein 1 (4E-BP1) and eukaryotic translation initiation factor 4B (eIF4B). OTS167 in combination with TKIs results in synergistic induction of FLT3 mutant cell death in FLT3 mutant cell lines and prolonged survival in a FLT3 mutant AML xenograft mouse model. Our findings suggest signaling through MELK is necessary for the translation and expression of FLT3-ITD, and blocking MELK with OTS167 represents a viable therapeutic strategy for patients with FLT3 mutant AML.

Discovery of novel 4-azaaryl-N-phenylpyrimidin-2-amine derivatives as potent and selective FLT3 inhibitors for acute myeloid leukaemia with FLT3 mutations

Feline McDonough sarcoma (FMS)-like tyrosine kinase 3 (FLT3) is one of the most pursued targets in the treatment of acute myeloid leukaemia (AML) as its gene amplification and mutations, particularly internal tandem duplication (ITD), contribute to the pathogenesis of AML and the resistance to known FLT3 inhibitors. To conquer this challenge, there is a quest for structurally novel FLT3 inhibitors. Herein, we report the discovery of a new series of 4-azaaryl-N-phenylpyrimidin-2-amine derivatives as potent and selective FLT3 inhibitors. Compounds 12b and 12r were capable of suppressing a wide range of mutated FLT3 kinases including ITD and D835Y mutants; the latter isoform is closely associated with acquired drug resistance. In addition, both compounds displayed an anti-proliferative specificity for FLT3-ITD-harbouring cell lines (i.e., MV4-11 and MOLM-13 cells) over those with expression of the wild-type kinase or even without FLT3 expression. In mechanistic studies using MV4-11 cells, 12b was found to diminish the phosphorylation of key downstream effectors of FLT3 and induce apoptosis, supporting an FLT3-ITD-targeted mechanism of its anti-proliferative action.

LT-171-861, a novel FLT3 inhibitor, shows excellent preclinical efficacy for the treatment of FLT3 mutant acute myeloid leukemia

Rationale: Acute myeloid leukemia (AML) is a common type of haematological malignancy. Several studies have shown that neoplasia in AML is enhanced by tyrosine kinase pathways. Recently, given that aberrant activation of Fms-like tyrosine receptor kinase 3 (FLT3) acts as a critical survival signal for cancer cells in 20‒30% patients with AML, inhibition of FLT3 may be a potential therapeutic strategy. Therefore, we identified LT-171-861, a novel kinase inhibitor with remarkable inhibitory activity against FLT3, in preclinical models of AML.

Methods: We determined the inhibitory effects of LT-171-861 in vitro using AML cell lines and transformed BaF3 cells. Target engagement assays were used to verify the interaction between LT-171-861 and FLT3. Finally, a subcutaneous model and a bone marrow engrafted model were used to evaluate the antitumor effects of LT‑171‑861 in vivo.

Results: Our data demonstrated that LT-171-861 had high affinity for FLT3 protein. We also showed that LT-171-861 had an inhibitory effect on FLT3 mutants in cellular assays. Moreover, LT-171-861 had a growth-inhibitory effect on human AML cell lines harboring FLT3 internal tandem duplications (FLT3-ITD) such as FLT3-D835Y, FLT3‑ITD-N676D, FLT3-ITD-D835Y, FLT3-ITD-F691L, FLT3-ITD-Y842C and AML blasts from patients with FLT3-ITD. Furthermore, LT-171-861 showed potent antileukemic efficacy against AML cells. We also show the efficacy of LT‑171-861 in a subcutaneous implantation model and a bone marrow engrafted model in vivo, where administration of LT-171-861 led to almost complete tumor regression and increased survival.

Conclusions: Overall, this study not only identifies LT-171-861 as a potent FLT3 inhibitor, but also provides a rationale for the upcoming clinical trial of LT-171-861 in patients with AML and FLT3-ITD mutations.

Natural and Semisynthetic Chalcones as Dual FLT3 and Microtubule Polymerization Inhibitors

Activating mutations in FLT3 receptor tyrosine kinase are found in a third of acute myeloid leukemia (AML) patients and are associated with disease relapse and a poor prognosis. The majority of these mutations are internal tandem duplications (ITDs) in the juxtamembrane domain of FLT3, which have been validated as a therapeutic target. The clinical success of selective inhibitors targeting oncogenic FLT3, however, has been limited due to the acquisition of drug resistance. Herein the identification of a dual FLT3/microtubule polymerization inhibitor, chalcone 4 (2'-allyloxy-4,4'-dimethoxychalcone), is reported through screening of 15 related chalcones for differential antiproliferative activity in leukemia cell lines dependent on FLT3-ITD (MV-4-11) or BCR-ABL (K562) oncogenes and by subsequent screening for mitotic inducers in the HCT116 cell line. Three natural chalcones (1-3) were found to be differentially more potent toward the MV-4-11 (FLT3-ITD) cell line compared to the K562 (BCR-ABL) cell line. Notably, the new semisynthetic chalcone 4, which is a 2'-O-allyl analogue of the natural chalcone 3, was found to be more potent toward the FLT3-ITD+ cell line and inhibited FLT3 signaling in FLT3-dependent cells. An in vitro kinase assay confirmed that chalcone 4 directly inhibited FLT3. Moreover, chalcone 4 induced mitotic arrest in these cells and inhibited tubulin polymerization in both cellular and biochemical assays. Treatment of MV-4-11 cells with this inhibitor for 24 and 48 h resulted in apoptotic cell death. Finally, chalcone 4 was able to overcome TKD mutation-mediated acquired resistance to FLT3 inhibitors in a MOLM-13 cell line expressing FLT3-ITD with the D835Y mutation. Chalcone 4 is, therefore, a promising lead for the discovery of dual-target FLT3 inhibitors.

Retinoic acid synergizes with the unfolded protein response and oxidative stress to induce cell death in FLT3-ITD+ AML

Acute myeloid leukemia (AML) is often characterized by the expression of fusion or mutant proteins that cause impaired differentiation and enhanced proliferation and survival. The presence of mutant proteins prone to misfolding can render the cells sensitive to endoplasmic reticulum (ER) stress and oxidative stress that could otherwise be overcome. Here, we show that the triple combination of the differentiating agent retinoic acid (RA), the ER stress-inducing drug tunicamycin (Tm), and arsenic trioxide (ATO), able to generate oxidative stress, leads to the death of AML cell lines expressing fusion proteins involving the gene MLL and the internal tandem duplication (ITD) in the FLT3 tyrosine kinase receptor. Importantly, the combination of RA, Tm, and ATO decreased the colony-forming capacity of primary leukemic blasts bearing the FLT-ITD mutation without affecting healthy hematopoietic progenitor cells. We demonstrate in cell lines that combination of these drugs generates ER and oxidative stresses and impairs maturation and causes accumulation of FLT3 protein in the ER. Our data provide a proof of concept that low amounts of drugs that generate ER and oxidative stresses combined with RA could be an effective targeted therapy to hit AML cells characterized by MLL fusion proteins and FLT3-ITD mutation.

Proteolysis targeting chimeras (PROTACs) are emerging therapeutics for hematologic malignancies

Proteolysis targeting chimeras (PROTACs) are heterobifunctional small molecules that utilize the ubiquitin proteasome system (UPS) to degrade proteins of interest (POI). PROTACs are potentially superior to conventional small molecule inhibitors (SMIs) because of their unique mechanism of action (MOA, i.e., degrading POI in a sub-stoichiometric manner), ability to target “undruggable” and mutant proteins, and improved target selectivity. Therefore, PROTACs have become an emerging technology for the development of novel targeted anticancer therapeutics. In fact, some of these reported PROTACs exhibit unprecedented efficacy and specificity in degrading various oncogenic proteins and have advanced to various stages of preclinical and clinical development for the treatment of cancer and hematologic malignancy. In this review, we systematically summarize the known PROTACs that have the potential to be used to treat various hematologic malignancies and discuss strategies to improve the safety of PROTACs for clinical application. Particularly, we propose to use the latest human pan-tissue single-cell RNA sequencing data to identify hematopoietic cell type-specific/selective E3 ligases to generate tumor-specific/selective PROTACs. These PROTACs have the potential to become safer therapeutics for hematologic malignancies because they can overcome some of the on-target toxicities of SMIs and PROTACs.

Hematopoietic cytokines mediate resistance to targeted therapy in FLT3-ITD acute myeloid leukemia

Activating mutations in Fms-like tyrosine kinase 3 (FLT3) occur in ∼30% of adult cases of acute myeloid leukemia (AML). Selective second- and third-generation FLT3 inhibitors have shown significant clinical activity in patients with relapsed FLT3-mutant AML. However, clearance of FLT3-mutant clones does not consistently occur, and disease will progress in most patients after an initial response. This scenario challenges the model of FLT3-mutant AML being oncogene addicted, and it suggests that redundant signaling pathways regulate AML cell survival after FLT3 inhibition. We show that primary FLT3-mutant AML cells escape apoptosis induced by FLT3 inhibition in vitro in the presence of cytokines produced normally in the bone marrow, particularly granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-3 (IL-3). Despite reactivating canonical FLT3-signaling pathways, GM-CSF and IL-3 maintain cell survival without rescuing proliferation. Cytokine-mediated resistance through GM-CSF and IL-3 is dependent on JAK kinase, STAT5, and proviral integration site of Moloney murine leukemia virus (PIM) but not MAPK or mammalian target of rapamycin signaling. Cotreatment with FLT3 inhibitors and inhibitors of JAK or PIM kinases blocks GM-CSF and IL-3 rescue of cell survival in vitro and in vivo. Altogether, these data provide a strong rationale for combination therapy with FLT3 inhibitors to potentially improve clinical responses in AML.

Cutting Edge Molecular Therapy for Acute Myeloid Leukemia

Recently, whole exome sequencing for acute myeloid leukemia (AML) has been performed by a next-generation sequencer in several studies. It has been revealed that a few gene mutations are identified per AML patient. Some of these mutations are actionable mutations that affect the response to an approved targeted treatment that is available for off-label treatment or that is available in clinical trials. The era of precision medicine for AML has arrived, and it is extremely important to detect actionable mutations relevant to treatment decision-making. However, the percentage of actionable mutations found in AML is about 50% at present, and therapeutic development is also needed for AML patients without actionable mutations. In contrast, the newly approved drugs are less toxic than conventional intensive chemotherapy and can be combined with low-intensity treatments. These combination therapies can contribute to the improvement of prognosis, especially in elderly AML patients who account for more than half of all AML patients. Thus, the treatment strategy for leukemia is changing drastically and showing rapid progress. In this review, we present the latest information regarding the recent development of treatment for AML.

Design and Synthesis of New Sulfonamides-Based Flt3 Inhibitors

Background:

Flt3 is an oncogenic kinase involved in different leukemias. It is most prominently associated with acute myeloid leukemia (AML). Flt3-specific inhibitors have shown promising results in interfering with AML.

Methods:

The crystallographic structures of two inhibitors complexed within Flt3, namely, quizartinib and F6M, were used to guide the synthesis of new sulfonamide-based Flt3 inhibitors.

Results:

One of the prepared compounds showed low micromolar anti-Flt3 bioactivity, and interestingly, low micromolar bioactivity against the related oncogenic kinase VEGFR2.

Conclusion:

Sulfonamides were successfully used as privileged scaffolds for the synthesis of novel Flt3 inhibitors of micromolar potencies.

Proteolysis-Targeting Chimeras as Therapeutics and Tools for Biological Discovery

New biological tools provide new techniques to probe fundamental biological processes. Here we describe the burgeoning field of proteolysis-targeting chimeras (PROTACs), which are capable of modulating protein concentrations at a post-translational level by co-opting the ubiquitin-proteasome system. We describe the PROTAC technology and its application to drug discovery and provide examples where PROTACs have enabled novel biological insights. Furthermore, we provide a workflow for PROTAC development and use and discuss the benefits and issues associated with PROTACs. Finally, we compare PROTAC-mediated protein-level modulation with other technologies, such as RNAi and genome editing.

γδ T Cells Number, CD200, and Flt3 Expression Is Associated with Higher Progression Free Survival in Patients with Chronic Myeloid Leukemia

BACKGROUND

Tumor immunoedition involves alterations in cells of immune system, which may play an important role in the immunosurveillance of patients with cancer diseases.

AIM OF THE STUDY

To determine the association between the number of immune cells and the expression of surface markers in leukemic cells of patients with de novo CML who achieved molecular response.

METHODS

A longitudinal study was conducted in 31 patients with de novo CML. Peripheral blood samples were obtained at diagnosis for quantification of immune cells and tumor cells expressing CD200, CD135, GpP, and Bcl-2. Results were compared with a group of 60 healthy donors. Lymphocyte subsets were analyzed during a 48 month follow-up period and molecular response to treatment was assessed simultaneously by QT-PCR. The group of patients with deep molecular response was compared with de novo CML patients; the cut-off value of cell count was determined by ROC analysis. Kaplan-Meier and Cox proportional hazard model were used to determine the significant association between the number of cells and progression-free survival.

RESULTS

Differences in number of CD4, CD4Tregs, NK, γδT, monocytes, and pDC's, tumor-cells expressing CD200⁺, CD135⁺, GpP⁺, and Bcl-2⁺ were observed between patients and healthy donors. The number of γδT lymphocytes, CD200⁺, and CD135⁺ cells were associated with longer progression-free survival (p = 0.0112, p = 0.0012 and p = 0.0201 respectively).

CONCLUSION

A γδT lymphocyte count <63 cel/uL, CD200⁺ <997 cel/uL, and CD135⁺ <23 317 cel/uL at diagnosis is associated with the maintenance of deep molecular response at 48 months in patients with de novo CML.

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.

Gilteritinib induces PUMA-dependent apoptotic cell death via AKT/GSK-3β/NF-κB pathway in colorectal cancer cells

As a highly potent and highly selective oral inhibitor of FLT3/AXL, gilteritinib showed activity against FLT3D835 and FLT3‐ITD mutations in pre‐clinical testing, although its role on colorectal cancer (CRC) cells is not yet fully elucidated. We examined the activity of gilteritinib in suppressing growth of CRC and its enhancing effect on other drugs used in chemotherapy. In this study, we observed that, regardless of p53 status, treatment using gilteritinib induces PUMA in CRC cells via the NF‐κB pathway after inhibition of AKT and activation of glycogen synthase kinase 3β (GSK‐3β). PUMA was observed to be vital for apoptosis in CRC cells through treatment of gilteritinib. Moreover, enhancing induction of PUMA through different pathways could mediate chemosensitization by using gilteritinib. Furthermore, PUMA deficiency revoked the antitumour role of gilteritinib in vivo. Thus, our results indicate that PUMA mediates the antitumour activity of gilteritinib in CRC cells. These observations are critical for the therapeutic role of gilteritinib in CRC.

MEK inhibition enhances the response to tyrosine kinase inhibitors in acute myeloid leukemia

FMS-like tyrosine kinase 3 (FLT3) is a key driver of acute myeloid leukemia (AML). Several tyrosine kinase inhibitors (TKIs) targeting FLT3 have been evaluated clinically, but their effects are limited when used in monotherapy due to the emergence of drug-resistance. Thus, a better understanding of drug-resistance pathways could be a good strategy to explore and evaluate new combinational therapies for AML. Here, we used phosphoproteomics to identify differentially-phosphorylated proteins in patients with AML and TKI resistance. We then studied resistance mechanisms in vitro and evaluated the efficacy and safety of rational combinational therapy in vitro, ex vivo and in vivo in mice. Proteomic and immunohistochemical studies showed the sustained activation of ERK1/2 in bone marrow samples of patients with AML after developing resistance to FLT3 inhibitors, which was identified as a common resistance pathway. We examined the concomitant inhibition of MEK-ERK1/2 and FLT3 as a strategy to overcome drug-resistance, finding that the MEK inhibitor trametinib remained potent in TKI-resistant cells and exerted strong synergy when combined with the TKI midostaurin in cells with mutated and wild-type FLT3. Importantly, this combination was not toxic to CD34+ cells from healthy donors, but produced survival improvements in vivo when compared with single therapy groups. Thus, our data point to trametinib plus midostaurin as a potentially beneficial therapy in patients with AML.

Identification of a Unique Resorcylic Acid Lactone Derivative That Targets Both Lymphangiogenesis and Angiogenesis

We synthesized 11 novel L-783277 derivatives, in which a structure rigidifying phenyl ring is incorporated into the 14-membered chiral resorcylic acid lactone system. The SAR study with these substances demonstrated that 17 possesses excellent kinase selectivity against a panel of 335 kinases in contrast to L-783277 and inhibits VEGFR3, VEGFR2, and FLT3 with single-digit nanomolar IC₅₀ values. Also, we found that 21, a stereoisomer of 17, has excellent potency (IC₅₀ = 9 nM) against VEGFR3 and selectivity over VEGFR2 and FLT3. 17, a potent dual VEGFR3 and VEGFR2 inhibitor, effectively suppresses both lymphangiogenesis and angiogenesis in a 3D-microfluidic tumor lymphangiogenesis assay and in vivo corneal assay while SAR131675 blocks only lymphangiogenesis. In addition, 17 blocks the endothelial tube formation and suppresses proliferation of PHE tumor vascular model. 17 will be a valuable templatefor developing therapeutically active and selective substances that target both lymphangiogenesis and angiogenesis.

Discovery of 4-piperazinyl-2-aminopyrimidine derivatives as dual inhibitors of JAK2 and FLT3

Hybridization strategy is an effective strategy to obtain multi-target inhibitors in drug design. In this study, we assembled the pharmacophores of momelotinib and tandutinib to get a series of 4-piperazinyl-2-aminopyrimidine derivatives. All compounds were tested for the inhibition of JAK2 and FLT3 enzymes, of which, compounds with potent enzyme activities were assayed for antiproliferative activities against three cancer cell lines (HEL, MV4-11, and HL60). The structure-activity relationship studies were conducted through variations in two regions, the "A" phenyl ring and "B" phenyl ring. Compound 14j showed the most balanced in vitro inhibitory activity against JAK2 and FLT3 (JAK2 IC₅₀ = 27 nM, FLT3 IC₅₀ = 30 nM), and it also showed potent inhibition against the above tested cell lines. In the cellular context, 14j strongly induced apoptosis by arresting cell cycle in the G₁/S phase, and was selected as a promising JAK2/FLT3 dual inhibitor.

Targeting Tyrosine Kinases in Acute Myeloid Leukemia: Why, Who and How?

Acute myeloid leukemia (AML) is a myeloid malignancy carrying a heterogeneous molecular panel of mutations participating in the blockade of differentiation and the increased proliferation of myeloid hematopoietic stem and progenitor cells. The historical "3 + 7" treatment (cytarabine and daunorubicin) is currently challenged by new therapeutic strategies, including drugs depending on the molecular landscape of AML. This panel of mutations makes it possible to combine some of these new treatments with conventional chemotherapy. For example, the FLT3 receptor is overexpressed or mutated in 80% or 30% of AML, respectively. Such anomalies have led to the development of targeted therapies using tyrosine kinase inhibitors (TKIs). In this review, we document the history of TKI targeting, FLT3 and several other tyrosine kinases involved in dysregulated signaling pathways.

Dual FLT3 inhibitors: Against the drug resistance of acute myeloid leukemia in recent decade

Acute myeloid leukemia (AML) is a malignant disease characterized by abnormal growth and differentiation of hematopoietic stem cells. Although the pathogenesis has not been fully elucidated, many specific gene mutations have been found in AML. Fms-like tyrosine kinase 3 (FLT3) is recognized as a drug target for the treatment of AML, and the activation mutations of FLT3 were found in about 30% of AML patients. Targeted inhibition of FLT3 receptor tyrosine kinase has shown promising results in the treatment of FLT3 mutation AML. Unfortunately, the therapeutic effects of FLT3 tyrosine kinase inhibitors used as AML monotherapy are usually accompanied by the high risk of resistance development within a few months after treatment. FLT3 dual inhibitors were generated with the co-inhibition of FLT3 and another target, such as CDK4, JAK2, MEK, Mer, Pim, etc., to solve the problems mentioned above. As a result, the therapeutic effect of the drug is significantly improved, while the toxic and side effects are reduced. Besides, the life quality of AML patients with FLT3 mutation has been effectively improved. In this paper, we reviewed the studies of dual FLT3 inhibitors that have been discovered in recent years for the treatment of AML.

How Precision Medicine Is Changing Acute Myeloid Leukemia Therapy

Pretreatment somatic mutations influence acute myeloid leukemia (AML) pathogenesis and responses to chemotherapy. Integration of cytogenetic abnormalities and molecular mutations, co-occurring and in isolation, have resulted in a more refined prognostic assessment. In addition, research performed over the last few years has led to the development of novel therapies and new drug approvals in patients with both newly diagnosed and relapsed/refractory (R/R) AML. Here we discuss the use of these newly approved therapies. Advances in AML have also occurred through development of better tools to assess response to treatment. Both multiparameter flow cytometry and polymerase chain reaction can be used to assess for the presence or absence of measurable residual disease (MRD) and increase the sensitivity of response assessment. The role of MRD assessment is gaining relevance and its integration in clinical trials and treatment decision making will be explored in the second half of this article.

New Targeted Agents in Acute Myeloid Leukemia: New Hope on the Rise

The therapeutic approach for acute myeloid leukemia (AML) remains challenging, since over the last four decades a stagnation in standard cytotoxic treatment has been observed. But within recent years, remarkable advances in the understanding of the molecular heterogeneity and complexity of this disease have led to the identification of novel therapeutic targets. In the last two years, seven new targeted agents (midostaurin, gilteritinib, enasidenib, ivosidenib, glasdegib, venetoclax and gemtuzumab ozogamicin) have received US Food and Drug Administration (FDA) approval for the treatment of AML. These drugs did not just prove to have a clinical benefit as single agents but have especially improved AML patient outcomes if they are combined with conventional therapy. In this review, we will focus on currently approved and promising upcoming agents and we will discuss controversial aspects and limitations of targeted treatment strategies.

Quizartinib (AC220): a promising option for acute myeloid leukemia

Quizartinib is an effective therapy for patients with FLT3-ITD acute myeloid leukemia (AML) by continuing to inhibit the activity of FLT3 gene, leading to apoptosis of tumor cells. Multiple clinical trials have proved that it is effective in relapsed or refractory AML with an FLT3-ITD mutation. In this review, we focus on the characteristics of FLT3/ITD mutations, the mechanism and pharmacokinetics of quizartinib, and the mechanisms of resistance to quizartinib. We also summarize clinical experiences and adverse effects with quizartinib and recommend crucial approaches of quizartinib in the therapy of patients with newly diagnosed AML and patients with relapsed/refractory AML, particularly those with FLT3-ITD mutation. Quizartinib presents its advantages as a very promising agent in the treatment of AML, especially in patients with FLT3-ITD mutations. FLT3/ITD mutation can lead to constitutive autophosphorylation of FLT3 and activation of its downstream effectors including RAS/RAF/MEK, MAPK/ERK, PI3K/AKT/mTOR and JAK/STAT5 signal pathways, while Quizartinib can inhibit these downstream pathways through specific FLT3 inhibition. Quizartinib has received US Food and Drug Administration breakthrough therapy designation in patients with relapsed/refractory FLT3-ITD AML based on clinical trials. A larger sample of clinical trials are needed to verify its safety and efficacy, and the efficacy of quizartinib combined with chemotherapy or allogeneic hematopoietic cell transplantation should also be estimated in clinical trials. Meanwhile, for the side effects of quizartinib, further studies are needed to find a way to reduce its toxicity.

Proteolysis-targeting chimeras for targeting protein for degradation

Proteolysis-targeting chimeras (PROTACs) are an emerging tool for therapeutic intervention by reducing or eliminating disease-causing proteins. PROTACs are bifunctional molecules that consist of a target protein ligand, a linker and an E3 ligase ligand, which mediate the polyubiquitination of the target protein, ultimately leading to the target protein degradation by the ubiquitin–proteasome pathway. We review some of the main PROTACs that have been reported recently and discuss their potential therapeutic benefits over classical enzyme inhibition. Future research is expected to focus on the delivery and bioavailability of PROTACs due to their high molecular weight (700–1000 Da).

High-throughput Identification of FLT3 Wild-type and Mutant Kinase Substrate Preferences and Application to Design of Sensitive In Vitro Kinase Assay Substrates

Acute myeloid leukemia (AML) is an aggressive disease that is characterized by abnormal increase of immature myeloblasts in blood and bone marrow. The FLT3 receptor tyrosine kinase plays an integral role in hematopoiesis, and one third of AML diagnoses exhibit gain-of-function mutations in FLT3, with the juxtamembrane domain internal tandem duplication (ITD) and the kinase domain D835Y variants observed most frequently. Few FLT3 substrates or phosphorylation sites are known, which limits insight into FLT3's substrate preferences and makes assay design particularly challenging. We applied in vitro phosphorylation of a cell lysate digest (adaptation of the Kinase Assay Linked with Phosphoproteomics (KALIP) technique and similar methods) for high-throughput identification of substrates for three FLT3 variants (wild-type, ITD mutant, and D835Y mutant). Incorporation of identified substrate sequences as input into the KINATEST-ID substrate preference analysis and assay development pipeline facilitated the design of several peptide substrates that are phosphorylated efficiently by all three FLT3 kinase variants. These substrates could be used in assays to identify new FLT3 inhibitors that overcome resistant mutations to improve FLT3-positive AML treatment.

Using genomics to define pediatric blood cancers and inform practice

Over the past decade, there has been exponential growth in the number of genome sequencing studies performed across a spectrum of human diseases as sequencing technologies and analytic pipelines improve and costs decline. Pediatric hematologic malignancies have been no exception, with a multitude of next generation sequencing studies conducted on large cohorts of patients in recent years. These efforts have defined the mutational landscape of a number of leukemia subtypes and also identified germ-line genetic variants biologically and clinically relevant to pediatric leukemias. The findings have deepened our understanding of the biology of many childhood leukemias. Additionally, a number of recent discoveries may positively impact the care of pediatric leukemia patients through refinement of risk stratification, identification of targetable genetic lesions, and determination of risk for therapy-related toxicity. Although incredibly promising, many questions remain, including the biologic significance of identified genetic lesions and their clinical implications in the context of contemporary therapy. Importantly, the identification of germ-line mutations and variants with possible implications for members of the patient's family raises challenging ethical questions. Here, we review emerging genomic data germane to pediatric hematologic malignancies.