G-749, a novel FLT3 kinase inhibitor, can overcome drug resistance for the treatment of acute myeloid leukemia

Discovery of 3-amide-pyrimidine-based derivatives as potential fms-like tyrosine receptor kinase 3 (FLT3) inhibitors for treating acute myelogenous leukemia

FLT3-ITD and TKD mutants play a central role in acute myeloid leukemia (AML), making FLT3 an attractive target for AML treatment. To discover next-generation FLT3 inhibitors and gather additional structure-activity relationship (SAR) information, we performed structural modifications of G-749 (denfivontinib) utilizing structure simplification and scaffold hopping strategies. Among these derivatives, MY-10 exhibited the most potent and selective inhibition of MV4-11 cell proliferation, demonstrating potent inhibitory activity against FLT3-ITD (IC50 = 6.5 nM) and FLT3-D835Y (IC50 = 10.3 nM) mutants. Notably, MY-10 exhibited no inhibitory activity against c-KIT kinase (IC50 > 100 μM). Mechanistic studies revealed that MY-10 arrested the cell cycle at the G0/G1 phase and efficiently induced apoptosis. Furthermore, it significantly reduced reactive oxygen species (ROS) production and mitochondrial membrane potential (MMP), and strongly inhibited FLT3-mediated signaling pathways. These findings, along with the obtained SAR information, provide valuable insights for the further development of FLT3 inhibitors.

A systematic review of second-generation FLT3 inhibitors for treatment of patients with relapsed/refractory acute myeloid leukemia

BACKGROUND

Acute myeloid leukemia (AML) is a complex disease with diverse mutations, including prevalent mutations in the FMS-like receptor tyrosine kinase 3 (FLT3) gene that lead to poor prognosis. Recent advancements have introduced FLT3 inhibitors that have improved outcomes for FLT3-mutated AML patients, however, questions remain on their application in complex conditions such as relapsed/refractory (R/R) disease. Therefore, we aimed to evaluate the clinical effectiveness of second-generation FLT3 inhibitors in treating patients with R/R AML.

METHODS

A systematic literature search of PubMed, MEDLINE, SCOPUS and Google Scholar databases was made to identify relevant studies up to January 30, 2024. This study was conducted following the guidelines of the PRISMA.

RESULTS

The ADMIRAL trial revealed significantly improved overall survival and complete remission rates with gilteritinib compared to salvage chemotherapy, with manageable adverse effects. Ongoing research explores its potential in combination therapies, showing synergistic effects with venetoclax and promising outcomes in various clinical trials. The QuANTUM-R trial suggested longer overall survival with quizartinib compared to standard chemotherapy, although concerns were raised regarding trial design and cardiotoxicity. Ongoing research explores combination therapies involving quizartinib, such as doublet or triplet regimens with venetoclax, showing promising outcomes in FLT3-mutated AML patients.

CONCLUSION

These targeted therapies offer promise for managing this subgroup of AML patients, but further research is needed to optimize their use. This study underscores the importance of personalized treatment based on genetic mutations in AML, paving the way for more effective and tailored approaches to combat the disease.

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.

Annual review of PROTAC degraders as anticancer agents in 2022

Following nearly two decades of development, significant advancements have been achieved in PROTAC technology. As of the end of 2022, more than 20 drugs have entered clinical trials, with ARV-471 targeting estrogen receptor (ER) showing remarkable progress by entering phase III clinical studies. In 2022, significant progress has been made on multiple targets. The first reversible covalent degrader designed to target the KRASG12C mutant protein, based on cyclopropionamide, has been reported. Additionally, the activity HDCA1 degrader surpassed submicromolar levels during the same year. A novel FEM1B covalent ligand called EN106 was also discovered, expanding the range of available ligands. Furthermore, the first PROTAC drug targeting SOS1 was reported. Additionally, the first-in-class degraders that specifically target BRD4 isoforms (BRD4 L and BRD4 S) have recently been reported, providing a valuable tool for further investigating the biological functions of these isoforms. Lastly, a breakthrough was also achieved with the first degrader targeting both CDK9 and Cyclin T1. In this review, we aimed to update the PROTAC degraders as potential anticancer agents covering articles published in 2022. The design strategies, degradation effects, and anticancer activities were highlighted, which might provide an updated sight to develop novel PROTAC degraders with great potential as anticancer agents as well as favorable drug-like properties.

Update on Small Molecule Targeted Therapies for Acute Myeloid Leukemia

OPINION STATEMENT

The search for effective therapies for the highly heterogenous disease acute myeloid leukemia (AML) has remained elusive. While cytotoxic therapies can induce complete remission and even, at times, long-term survival, this approach is associated with significant toxic effects to visceral organs and worsening of immune dysfunction and marrow suppression leading to death. Sophisticated molecular studies have revealed defects within the AML cell that can be exploited by utilizing small molecule agents to target these defects, often dubbed "target therapy." Several medications have already established new standards of care for many patients with AML, including FDA-approved agents that inhibitor IDH1, IDH2, FLT3, and BCL-2. Emerging small molecules hold additional to add to the armamentarium of AML treatment options including MCL-1 inhibitors, TP53 inhibitors, menin inhibitors, and E-selectin antagonists. Moreover, the increasing options also mean that future combinations of these agents need to be explored, including with cytotoxic drugs and other newer emerging strategies such as immunotherapies for AML. Recent investigations continue to show that overcoming many of the challenges of treating AML finally is on the horizon.

Small molecule inhibitors targeting the cancers

Compared with traditional therapies, targeted therapy has merits in selectivity, efficacy, and tolerability. Small molecule inhibitors are one of the primary targeted therapies for cancer. Due to their advantages in a wide range of targets, convenient medication, and the ability to penetrate into the central nervous system, many efforts have been devoted to developing more small molecule inhibitors. To date, 88 small molecule inhibitors have been approved by the United States Food and Drug Administration to treat cancers. Despite remarkable progress, small molecule inhibitors in cancer treatment still face many obstacles, such as low response rate, short duration of response, toxicity, biomarkers, and resistance. To better promote the development of small molecule inhibitors targeting cancers, we comprehensively reviewed small molecule inhibitors involved in all the approved agents and pivotal drug candidates in clinical trials arranged by the signaling pathways and the classification of small molecule inhibitors. We discussed lessons learned from the development of these agents, the proper strategies to overcome resistance arising from different mechanisms, and combination therapies concerned with small molecule inhibitors. Through our review, we hoped to provide insights and perspectives for the research and development of small molecule inhibitors in cancer treatment.

PROTACs: great opportunities for academia and industry (an update from 2020 to 2021)

PROteolysis TArgeting Chimeras (PROTACs) technology is a new protein-degradation strategy that has emerged in recent years. It uses bifunctional small molecules to induce the ubiquitination and degradation of target proteins through the ubiquitin-proteasome system. PROTACs can not only be used as potential clinical treatments for diseases such as cancer, immune disorders, viral infections, and neurodegenerative diseases, but also provide unique chemical knockdown tools for biological research in a catalytic, reversible, and rapid manner. In 2019, our group published a review article "PROTACs: great opportunities for academia and industry" in the journal, summarizing the representative compounds of PROTACs reported before the end of 2019. In the past 2 years, the entire field of protein degradation has experienced rapid development, including not only a large increase in the number of research papers on protein-degradation technology but also a rapid increase in the number of small-molecule degraders that have entered the clinical and will enter the clinical stage. In addition to PROTAC and molecular glue technology, other new degradation technologies are also developing rapidly. In this article, we mainly summarize and review the representative PROTACs of related targets published in 2020-2021 to present to researchers the exciting developments in the field of protein degradation. The problems that need to be solved in this field will also be briefly introduced.

A review of FLT3 inhibitors in acute myeloid leukemia

FLT3 mutations are the most common genetic aberrations found in acute myeloid leukemia (AML) and associated with poor prognosis. Since the discovery of FLT3 mutations and their prognostic implications, multiple FLT3-targeted molecules have been evaluated. Midostaurin is approved in the U.S. and Europe for newly diagnosed FLT3 mutated AML in combination with standard induction and consolidation chemotherapy based on data from the RATIFY study. Gilteritinib is approved for relapsed or refractory FLT3 mutated AML as monotherapy based on the ADMIRAL study. Although significant progress has been made in the treatment of AML with FLT3-targeting, many challenges remain. Several drug resistance mechanisms have been identified, including clonal selection, stromal protection, FLT3-associated mutations, and off-target mutations. The benefit of FLT3 inhibitor maintenance therapy, either post-chemotherapy or post-transplant, remains controversial, although several studies are ongoing.

Murine Models of Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a phenotypically and genetically heterogeneous hematologic malignancy. Extensive sequencing efforts have mapped the genomic landscape of adult and pediatric AML revealing a number of biologically and prognostically relevant driver lesions. Beyond identifying recurrent genetic aberrations, it is of critical importance to fully delineate the complex mechanisms by which they contribute to the initiation and evolution of disease to ultimately facilitate the development of targeted therapies. Towards these aims, murine models of AML are indispensable research tools. The rapid evolution of genetic engineering techniques over the past 20 years has greatly advanced the use of murine models to mirror specific genetic subtypes of human AML, define cell-intrinsic and extrinsic disease mechanisms, study the interaction between co-occurring genetic lesions, and test novel therapeutic approaches. This review summarizes the mouse model systems that have been developed to recapitulate the most common genomic subtypes of AML. We will discuss the strengths and weaknesses of varying modeling strategies, highlight major discoveries emanating from these model systems, and outline future opportunities to leverage emerging technologies for mechanistic and preclinical investigations.

Degrading FLT3-ITD protein by proteolysis targeting chimera (PROTAC)

Clinical FLT3 mutations caused poor therapeutic benefits toward the present FLT3 inhibitors, and degradation of the FLT3 mutant protein may be a promising alternative approach to protect against acute myeloid leukemia (AML). Herein, we report the discovery of small molecule FLT3 degraders based on the proteolysis targeting chimera (PROTAC). FLT3 degraders were designed, synthesized, and evaluated for FLT3 degradation. Promising PF15 significantly inhibited the proliferation of FLT3-ITD-positive cells, induced FLT3 degradation and downregulated the phosphorylation of FLT3 and STAT5. An in vivo xenograft model and survival period evaluation verified the efficacy of PROTAC. These findings laid a robust foundation for FLT3-PROTAC molecules as an effective strategy for treating AML.

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 a highly efficient dual type I/II FMS-like tyrosine kinase inhibitor that disrupts the growth of leukemic cells

Internal tandem duplications (ITDs) in the FMS-like tyrosine kinase-3 (FLT3) are causally linked to acute myeloid leukemia (AML) with poor prognosis. Available FLT3 inhibitors (FLT3i) preferentially target inactive or active conformations of FLT3. Moreover, they co-target kinases for normal hematopoiesis, are vulnerable to therapy-associated tyrosine kinase domain (TKD) FLT3 mutants, or lack low nanomolar activity. We show that the tyrosine kinase inhibitor marbotinib suppresses the phosphorylation of FLT3-ITD and the growth of permanent and primary AML cells with FLT3-ITD. This also applies to leukemic cells carrying FLT3-ITD/TKD mutants that confer resistance to clinically used FLT3i. Marbotinib shows high selectivity for FLT3 and alters signaling, reminiscent of genetic elimination of FLT3-ITD. Molecular docking shows that marbotinib fits in opposite orientations into inactive and active conformations of FLT3. The water-soluble marbotinib-carbamate significantly prolongs survival of mice with FLT3-driven leukemia. Marbotinib is a nanomolar next-generation FLT3i that represents a hybrid inhibitory principle.

G-749 Promotes Receptor Tyrosine Kinase TYRO3 Degradation and Induces Apoptosis in Both Colon Cancer Cell Lines and Xenograft Mouse Models

G-749 is an FLT3 kinase inhibitor that was originally developed as a treatment for acute myeloid leukemia. Some FLT3 kinase inhibitors are dual kinase inhibitors that inhibit the TAM (Tyro3, Axl, Mer) receptor tyrosine kinase family and are used to treat solid cancers such as non-small cell lung cancer (NSCLC) and triple-negative breast cancer (TNBC). AXL promotes metastasis, suppression of immune response, and drug resistance in NSCLC and TNBC. G-749, a potential TAM receptor tyrosine kinase inhibitor, and its derivative SKI-G-801, effectively inhibits the phosphorylation of AXL at nanomolar concentration (IC50 = 20 nM). This study aimed to investigate the anticancer effects of G-749 targeting the TAM receptor tyrosine kinase in colon cancer. Here, we demonstrate the potential of G-749 to effectively inhibit tumorigenesis by degrading TYRO3 via regulated intramembrane proteolysis both in vitro and in vivo. In addition, we demonstrated that G-749 inhibits the signaling pathway associated with cell proliferation in colon cancer cell lines HCT15 and SW620, as well as tumor xenograft mouse models. We propose G-749 as a new therapeutic agent for the treatment of colon cancer caused by abnormal TYRO3 expression or activity.

FLT3 Mutations in Acute Myeloid Leukemia: Key Concepts and Emerging Controversies

The FLT3 receptor is overexpressed on the majority of acute myeloid leukemia (AML) blasts. Mutations in FLT3 are the most common genetic alteration in AML, identified in approximately one third of newly diagnosed patients. FLT3 internal tandem duplication mutations (FLT3-ITD) are associated with increased relapse and inferior overall survival. Multiple small molecule inhibitors of FLT3 signaling have been identified, two of which (midostaurin and gilteritinib) are currently approved in the United States, and many more of which are in clinical trials. Despite significant advances, resistance to FLT3 inhibitors through secondary FLT3 mutations, upregulation of parallel pathways, and extracellular signaling remains an ongoing challenge. Novel therapeutic strategies to overcome resistance, including combining FLT3 inhibitors with other antileukemic agents, development of new FLT3 inhibitors, and FLT3-directed immunotherapy are in active clinical development. Multiple questions regarding FLT3-mutated AML remain. In this review, we highlight several of the current most intriguing controversies in the field including the role of FLT3 inhibitors in maintenance therapy, the role of hematopoietic cell transplantation in FLT3-mutated AML, use of FLT3 inhibitors in FLT3 wild-type disease, significance of non-canonical FLT3 mutations, and finally, emerging concerns regarding clonal evolution.

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.

Pharmacological impact of FLT3 mutations on receptor activity and responsiveness to tyrosine kinase inhibitors

Acute myelogenous leukaemia (AML) is an aggressive blood cancer characterized by the rapid proliferation of immature myeloid blast cells, resulting in a high mortality rate. The 5-year overall survival rate for AML patients is approximately 25%. Circa 35% of all patients carry a mutation in the FLT3 gene which have a poor prognosis. Targeting FLT3 receptor tyrosine kinase has become a treatment strategy in AML patients possessing FLT3 mutations. The most common mutations are internal tandem duplications (ITD) within exon 14 and a single nucleotide polymorphism (SNP) that leads to a point mutation in the D835 of the tyrosine kinase domain (TKD). Variations in the ITD sequence and the occurrence of other point mutations that lead to ligand-independent FLT3 receptor activation create difficulties in developing personalized therapeutic strategies to overcome observed mutation-driven drug resistance. Midostaurin and quizartinib are tyrosine kinase inhibitors (TKIs) with inhibitory efficacy against FLT3-ITD, but exhibit limited clinical impact. In this review, we focus on the structural aspects of the FLT3 receptor and correlate those mutations with receptor activation and the consequences for molecular and clinical responsiveness towards therapies targeting FLT3-ITD positive AML.

Discovery of small molecule FLT3 inhibitors that are able to overcome drug-resistant mutations

Herein we report the discovery of 1-(5-(tert-butyl)isoxazol-3-yl)-3- (3-fluorophenyl)urea derivatives as new FLT3 inhibitors that are able to overcome the drug resistance mutations: the secondary D835Y and F691L mutations on the basis of the internal tandem duplications (ITD) mutation of FLT3 (FLT3-ITD/D835Y and FLT3-ITD/F691L, respectively). The most potent compound corresponds to 1-(5-(tert-butyl)isoxazol-3-yl)-3-(4-((6,7-dimethoxyquinolin-4-yl)oxy)-3- fluorophenyl)urea (4d), which showed IC₅₀s (half maximal inhibitory concentrations) of 0.072 nM, 5.86 nM and 3.48 nM against FLT3-ITD, FLT3-ITD/F691L and FLT3-ITD/D835Y, respectively. Compound 4d also showed good selectivity for FLT3 in a kinase profiling assay. Collectively, 4d could be a good lead compound and deserves further in-depth studies.

Small-Molecule Fms-like Tyrosine Kinase 3 Inhibitors: An Attractive and Efficient Method for the Treatment of Acute Myeloid Leukemia

Fms-like tyrosine kinase 3 (FLT3) is an important member of the class III receptor tyrosine kinase (RTK) family, which is involved in the proliferation of hematopoietic cells and lymphocytes. In recent years, increasing evidence have demonstrated that the activation and mutation of FLT3 is closely implicated in the occurrence and development of acute myeloid leukemia (AML). The exploration of small-molecule inhibitors targeting FLT3 has aroused wide interest of pharmaceutical chemists and is expected to bring new hope for AML therapy. In this review, we specifically highlighted FLT3 mediated JAK/STAT, RAS/MAPK, and PI3K/AKT/mTOR signaling. The structural properties and biological activities of representative FLT3 inhibitors reported from 2014 to the present were also summarized. In addition, the major challenges in the current advance of novel FLT3 inhibitors were further analyzed, with the aim to guide future drug discovery.

Recent advances in FLT3 inhibitors for acute myeloid leukemia

Fms-like tyrosine kinase-3 (FLT3) mutations occur in approximately 30% of acute myeloid leukemia (AML) cases, suggesting FLT3 as an attractive target for AML treatment. Early FLT3 inhibitors enhance antileukemia efficacy by inhibiting multiple targets, and thus had stronger off-target activity, increasing their toxicity. Recently, a number of potent and selective FLT3 inhibitors have been developed, many of which are effective against multiple mutations. This review outlines the evolution of AML-targeting FLT3 inhibitors by focusing on their chemotypes, selectivity and activity over FLT3 wild-type and FLT3 mutations as well as new techniques related to FLT3. Compounds that currently enter the late clinical stage or have entered the market are also briefly reported.

A series of novel aryl-methanone derivatives as inhibitors of FMS-like tyrosine kinase 3 (FLT3) in FLT3-ITD-positive acute myeloid leukemia

Mutants of the FLT3 receptor tyrosine kinase (RTK) with duplications in the juxtamembrane domain (FLT3-ITD) act as drivers of acute myeloid leukemia (AML). Potent tyrosine kinase inhibitors (TKi) of FLT3-ITD entered clinical trials and showed a promising, but transient success due to the occurrence of secondary drug-resistant AML clones. A further caveat of drugs targeting FLT3-ITD is the co-targeting of other RTKs which are required for normal hematopoiesis. This is observed quite frequently. Therefore, novel drugs are necessary to treat AML effectively and safely. Recently bis(1H-indol-2-yl)methanones were found to inhibit FLT3 and PDGFR kinases. In order to optimize these agents we synthesized novel derivatives of these methanones with various substituents. Methanone 16 and its carbamate derivative 17b inhibit FLT3-ITD at least as potently as the TKi AC220 (quizartinib). Models indicate corresponding interactions of 16 and quizartinib with FLT3. The activity of 16 is accompanied by a high selectivity for FLT3-ITD.

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.

Identification of Pyrrolo[2,3- d]pyrimidine-Based Derivatives as Potent and Orally Effective Fms-like Tyrosine Receptor Kinase 3 (FLT3) Inhibitors for Treating Acute Myelogenous Leukemia

A series of pyrrolo[2,3- d]pyrimidine derivatives were prepared and optimized for cytotoxic activities against FLT3-ITD mutant cancer cells. Among them, compound 9u possessed nanomolar FLT3 inhibitory activities and subnanomolar inhibitory activities against MV4-11 and Molm-13 cells. It also showed excellent inhibitory activities in FLT3-ITD-D835V and FLT3-ITD-F691L cells which were resistant to quizartinib. Furthermore, 9u exhibited over 40-fold selectivity toward FLT3 relative to c-Kit kinase, which might reduce myelosuppression toxicity. Cellular assays demonstrated that 9u inhibited phosphorylated FLT3 and downstream signaling factors and also induced cell cycle arrest in the G₀/G₁ stage and apoptosis in MV4-11 and Molm-13 cells. Oral administration of 9u at 10 mg/kg could achieve rapid tumor extinction in the MV4-11 xenograft model and significantly inhibit the tumor growth in the MOLM-13 xenograft model with a tumor growth inhibitory rate of 96% without obvious toxicity. Additionally, 9u demonstrated high bioavailability ( F = 59.5%) and suitable eliminated half-life time ( T_1/2 = 2.06 h), suggesting that 9u may be a potent candidate for treating acute myelogenous leukemia.

Midostaurin for the management of FLT3-mutated acute myeloid leukemia and advanced systemic mastocytosis

Purpose

This article reviews the pharmacology, efficacy, safety, cost, and future directions of midostaurin for the treatment of acute myeloid leukemia (AML), aggressive systemic mastocytosis, systemic mastocytosis with associated hematological neoplasm, and mast cell leukemia, collectively known as advanced systemic mastocytosis (SM).

Summary

Midostaurin was approved by the U.S. Food and Drug Administration for the treatment of FMS-like tyrosine kinase-3 (FLT3)- mutated AML. FLT3 is a tyrosine kinase which plays a key role in proliferation of early hematopoietic progenitor cells. In addition to having activity against FLT3, midostaurin inhibits several other tyrosine kinases which led to its approval for the treatment of advanced SM.

Conclusion

Midostaurin offers a novel strategy to treat both FLT3-mutated AML and advanced SM. With a comparable adverse effect profile to other agents and substantial antiproliferative activity, midostaurin offers a therapeutic option for patients who have historically been difficult to treat.

FLT3 inhibitors in acute myeloid leukemia

FLT3 mutations are one of the most common findings in acute myeloid leukemia (AML). FLT3 inhibitors have been in active clinical development. Midostaurin as the first-in-class FLT3 inhibitor has been approved for treatment of patients with FLT3-mutated AML. In this review, we summarized the preclinical and clinical studies on new FLT3 inhibitors, including sorafenib, lestaurtinib, sunitinib, tandutinib, quizartinib, midostaurin, gilteritinib, crenolanib, cabozantinib, Sel24-B489, G-749, AMG 925, TTT-3002, and FF-10101. New generation FLT3 inhibitors and combination therapies may overcome resistance to first-generation agents.

Advances in the drug therapies of acute myeloid leukemia (except acute wpromyelocytic leukemia)

Acute myeloid leukemia (AML) is a heterogeneous hematologic malignancy, characterized by the clonal expansion of myeloid blasts in the peripheral blood, bone marrow, and/or other tissues. The new drugs used for treating AML are facing a big challenge, and the candidates include cytotoxic drugs, targeted small-molecule inhibitors, and monoclonal antibodies. In recent years, active research has focused on several new agents for including them in the large antileukemic drug family. This review aims to introduce some of these new drugs and highlights new advances made in the old drugs, mainly in the last 5 years.

Targeting FLT3 Mutations in Acute Myeloid Leukemia

The FMS-like tyrosine kinase 3 (FLT3) pathway has an important role in cellular proliferation, survival, and differentiation. Acute myeloid leukemia (AML) patients with mutated FLT3 have a large disease burden at presentation and a dismal prognosis. A number of FLT3 inhibitors have been developed over the years. The first-generation inhibitors are largely non-specific, while the second-generation inhibitors are more specific and more potent. These inhibitors are used to treat patients with FLT3-mutated AML in virtually all disease settings including induction, consolidation, maintenance, relapse, and after hematopoietic cell transplantation (HCT). In this article, we will review the use of FLT3 inhibitors in AML.

Dual inhibition of AKT/FLT3-ITD by A674563 overcomes FLT3 ligand-induced drug resistance in FLT3-ITD positive AML

The FLT3-ITD mutation is one of the most prevalent oncogenic mutations in AML. Several FLT3 kinase inhibitors have shown impressive activity in clinical evaluation, however clinical responses are usually transient and clinical effects are rapidly lost due to drug resistance. One of the resistance mechanisms in the AML refractory patients involves FLT3-ligand induced reactivation of AKT and/or ERK signaling via FLT3 wt kinase. Via a screen of numerous AKT kinase inhibitors, we identified the well-established orally available AKT inhibitor, A674563, as a dual suppressor of AKT and FLT3-ITD. A674563 suppressed FLT3-ITD positive AML both in vitro and in vivo. More importantly, compared to other FLT3 inhibitors, A674563 is able to overcome FLT3 ligand-induced drug resistance through simultaneous inhibition of FLT3-ITD- and AKT-mediated signaling. Our findings suggest that A674563 might be a potential drug candidate for overcoming FLT3 ligand-mediated drug resistance in FLT3-ITD positive AML.

Acute myeloid leukaemia genomics

Acute myeloid leukaemia (AML) is a biologically complex, molecularly and clinically heterogeneous disease. Despite major advances in understanding the genetic landscape of AML and its impact on the pathophysiology and biology of the disease, standard treatment options have not significantly changed during the past three decades. AML is characterized by multiple somatically acquired mutations that affect genes of different functional categories. Mutations in genes encoding epigenetic modifiers, such as DNMT3A, ASXL1, TET2, IDH1, and IDH2, are commonly acquired early and are present in the founding clone. By contrast, mutations involving NPM1 or signalling molecules (e.g., FLT3, RAS gene family) are typically secondary events that occur later during leukaemogenesis. This review aims to provide an overview of advances in new prognostic markers, including targetable mutations that will probably guide the development and use of novel molecularly targeted therapies.

Mechanisms of Resistance to FLT3 Inhibitors and the Role of the Bone Marrow Microenvironment

The presence of FLT3 mutations in acute myeloid leukemia (AML) carries a particularly poor prognosis, making the development of FLT3 inhibitors an imperative goal. The last decade has seen an abundance of clinical trials using these drugs alone or in combination with chemotherapy. This culminated with the recent approval by the US Food and Drug Administration of Midostaurin for the treatment of FLT3-mutated AML. Initial success has been followed by the emergence of clinical resistance. Although novel FLT3 inhibitors are being developed, studies into mechanisms of resistance raise hope of new strategies to prevent emergence of resistance and eliminate minimal residual disease.

Targeted Next-Generation Sequencing of Acute Leukemia

Mutation profiling of acute leukemias is a valuable tool for identifying genetic mutations with prognostic, predictive, therapeutic, and diagnostic utility. Technological advances, such as massively parallel sequencing, have allowed laboratories to assess for variation across dozens or hundreds of genes simultaneously with relatively low cost per target.Here, we describe a procedure for designing and using a TruSeq Custom Amplicon assay targeting genes involved in acute leukemias. This method is a fully customizable, amplicon-based assay for targeted resequencing, allowing interrogation of selected genomic regions of interest. The most readily available form of the assay allows sequencing of up to 1536 amplicons in a single reaction using a straightforward workflow. The ability to multiplex up to 1536 amplicons per reaction allows coverage of up to 650 kb of cumulative sequence and supports up to 96 samples per batch, depending on library size and desired sequencing depth.

Novel therapeutic options in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a biologically complex and molecularly and clinically heterogeneous disease, and its incidence is increasing as the population ages. Cytogenetic anomalies and mutation testing remain important prognostic tools for tailoring treatment after induction therapy. Despite major advances in understanding the genetic landscape of AML and its impact on the pathophysiology and biology of the disease, as well as the rapid development of new drugs, standard treatment options have not experienced major changes during the past three decades. Especially for patients with intermediate or high-risk AML, which often show relapse. Allogeneic hematopoietic stem cell transplantation (HSCT) remains the best chance for cure. Here we review the state of the art therapy of AML, with special focus on new developments in immunotherapies and cellular therapies including HSCT and particularly discuss the impact of new conditioning and haplo-identical donor regimens for HSCT, post-transplant strategies for preventing and treating relapse, and emerging novel therapeutic options.

Novel Prognostic and Therapeutic Mutations in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a biologically complex and molecularly and clinically heterogeneous disease, and its incidence increases with age. Cytogenetics and mutation testing remain important prognostic tools for treatment after induction therapy. The post-induction treatment is dependent on risk stratification. Despite rapid advances in determination of gene mutations involved in the pathophysiology and biology of AML, and the rapid development of new drugs, treatment improvements changed slowly over the past 30 years, with the majority of patients eventually experiencing relapse and dying of their disease. Allogenic hematopoietic stem cell transplantation remains the best chance of cure for patients with intermediate- or high-risk disease. This review gives an overview about advances in prognostic markers and novel treatment options for AML, focusing on new prognostic and probably therapeutic mutations, and novel drug therapies such as tyrosine kinase inhibitors.

FLT3 Inhibitors for Treating Acute Myeloid Leukemia

FLT3 (Fms-like tyrosine kinase 3) inhibitors are tyrosine kinase inhibitors. The first-generation FLT3 inhibitors were developed several years ago and include midostaurin, lestaurtinib, sunitinib, and sorafenib. They are relatively nonspecific for FLT3, with other potential targets that include platelet-derived growth factor receptor, vascular endothelial growth factor receptor, KIT, and Janus kinase 2. The second-generation inhibitors, including quizartinib, crenolanib, PLX3397, and ASP2215, are more potent and selective than the first-generation inhibitors. The greater potency and selectivity promises greater efficacy in FLT3-mutated acute myelogenous leukemia (AML) (particularly in patients with a greater allele burden) and less toxicity. A number of receptor tyrosine kinase inhibitors are being studied across virtually all disease settings, including frontline, relapsed and refractory, and maintenance, mainly in patients with FLT3-mutated AML. The future of FLT3 inhibitors in the treatment of AML, in combination with chemotherapy or stem cell transplant, appears bright. The present report reviews the current data on FLT3 inhibitors.

A CD3xCD123 bispecific DART for redirecting host T cells to myelogenous leukemia: preclinical activity and safety in nonhuman primates

Current therapies for acute myeloid leukemia (AML) are largely ineffective, and AML patients may benefit from targeted immunotherapy approaches. MGD006 is a bispecific CD3xCD123 dual-affinity re-targeting (DART) molecule that binds T lymphocytes and cells expressing CD123, an antigen up-regulated in several hematological malignancies including AML. MGD006 mediates blast killing in AML samples, together with concomitant activation and expansion of residual T cells. MGD006 is designed to be rapidly cleared, and therefore requires continuous delivery. In a mouse model of continuous administration, MGD006 eliminated engrafted KG-1a cells (an AML-M0 line) in human PBMC (peripheral blood mononuclear cell)-reconstituted NSG/β2m(-/-) mice at doses as low as 0.5 μg/kg per day for ~7 days. MGD006 binds to human and cynomolgus monkey antigens with similar affinities and redirects T cells from either species to kill CD123-expressing target cells. MGD006 was well tolerated in monkeys continuously infused with 0.1 μg/kg per day escalated weekly to up to 1 μg/kg per day during a 4-week period. Depletion of circulating CD123-positive cells was observed as early as 72 hours after treatment initiation and persisted throughout the infusion period. Cytokine release, observed after the first infusion, was reduced after subsequent administrations, even when the dose was escalated. T cells from animals with prolonged in vivo exposure exhibited unperturbed target cell lysis ex vivo, indicating no exhaustion. A transient decrease in red cell mass was observed, with no neutropenia or thrombocytopenia. These studies support clinical testing of MGD006 in hematological malignancies, including AML.

Acute Myeloid Leukemia: A Concise Review

Acute myeloid leukemia (AML) is a heterogeneous clonal disorder characterized by immature myeloid cell proliferation and bone marrow failure. Cytogenetics and mutation testing remain a critical prognostic tool for post induction treatment. Despite rapid advances in the field including new drug targets and increased understanding of the biology, AML treatment remains unchanged for the past three decades with the majority of patients eventually relapsing and dying of the disease. Allogenic transplant remains the best chance for cure for patients with intermediate or high risk disease. In this review, we discuss the landmark genetic studies that have improved outcome prediction and novel therapies.

Identification of an orally available compound with potent and broad FLT3 inhibition activity

FLT3 internal tandem duplication (FLT3-ITD) is an activating mutation found in 20%-30% of patients with acute myeloid leukemia (AML), which makes FLT3 an attractive target for the treatment of AML. Although FLT3-mutant patients respond to current FLT3 inhibitors, relapse usually happens due to the acquisition of resistant secondary mutations at the FLT3 catalytic domain, which is mainly on D835. In the search for compounds with broad FLT3 inhibition activities, we screened a kinase inhibitor library by using our unique FLT3 substrate and identified JAK3 inhibitor VI (designated JI6 hereafter) as a novel FLT3 inhibitor, which selectively targets FLT3 D835 mutants as well as FLT3-ITD. JI6 effectively inhibited FLT3-ITD-containing MV4-11 cells and HCD-57 cells transformed with FLT3-ITD and D835 mutants. Furthermore, administration of JI6 effectively targeted FLT3 signaling in vivo and suppressed the myeloproliferative phenotypes in FLT3-ITD knock-in mice and significantly prolonged the survival of immunodeficient mice implanted with the transformed HCD-57 cells. Therefore, JI6 is a promising candidate for development of next generation anti-AML drugs.

HMGA2 as a potential molecular target in KMT2A-AFF1-positive infant acute lymphoblastic leukaemia

Acute lymphoblastic leukaemia (ALL) in infants is an intractable cancer in childhood. Although recent intensive chemotherapy progress has considerably improved ALL treatment outcome, disease cure is often accompanied by undesirable long-term side effects, and efficient, less toxic molecular targeting therapies have been anticipated. In infant ALL cells with KMT2A (MLL) fusion, the microRNA let-7b (MIRLET7B) is significantly downregulated by DNA hypermethylation of its promoter region. We show here that the expression of HMGA2, one of the oncogenes repressed by MIRLET7B, is reversely upregulated in infant ALL leukaemic cells, particularly in KMT2A-AFF1 (MLL-AF4) positive ALL. In addition to the suppression of MIRLET7B, KMT2A fusion proteins positively regulate the expression of HMGA2. HMGA2 is one of the negative regulators of CDKN2A gene, which encodes the cyclin-dependent kinase inhibitor p16(INK4A) . The HMGA2 inhibitor netropsin, when combined with demethylating agent 5-azacytidine, upregulated and sustained the expression of CDKN2A, which resulted in growth suppression of KMT2A-AFF1-expressing cell lines. This effect was more apparent compared to treatment with 5-azacytidine alone. These results indicate that the MIRLET7B-HMGA2-CDKN2A axis plays an important role in cell proliferation of leukaemic cells and could be a possible molecular target for the therapy of infant ALL with KMT2A-AFF1.

Current Approaches in the Treatment of Relapsed and Refractory Acute Myeloid Leukemia

The limited sensitivity of the historical treatment response criteria for acute myeloid leukemia (AML) has resulted in a different paradigm for treatment compared with most other cancers presenting with widely disseminated disease. Initial cytotoxic induction chemotherapy is often able to reduce tumor burden to a level sufficient to meet the current criteria for "complete" remission. Nevertheless, most AML patients ultimately die from their disease, most commonly as clinically evident relapsed AML. Despite a variety of available salvage therapy options, prognosis in patients with relapsed or refractory AML is generally poor. In this review, we outline the commonly utilized salvage cytotoxic therapy interventions and then highlight novel investigational efforts currently in clinical trials using both pathway-targeted agents and immunotherapy based approaches. We conclude that there is no current standard of care for adult relapsed or refractory AML other than offering referral to an appropriate clinical trial.

Redox biology in normal cells and cancer: restoring function of the redox/Fyn/c-Cbl pathway in cancer cells offers new approaches to cancer treatment

This review discusses a unique discovery path starting with novel findings on redox regulation of precursor cell and signaling pathway function and identification of a new mechanism by which relatively small changes in redox status can control entire signaling networks that regulate self-renewal, differentiation, and survival. The pathway central to this work, the redox/Fyn/c-Cbl (RFC) pathway, converts small increases in oxidative status to pan-activation of the c-Cbl ubiquitin ligase, which controls multiple receptors and other proteins of central importance in precursor cell and cancer cell function. Integration of work on the RFC pathway with attempts to understand how treatment with systemic chemotherapy causes neurological problems led to the discovery that glioblastomas (GBMs) and basal-like breast cancers (BLBCs) inhibit c-Cbl function through altered utilization of the cytoskeletal regulators Cool-1/βpix and Cdc42, respectively. Inhibition of these proteins to restore normal c-Cbl function suppresses cancer cell division, increases sensitivity to chemotherapy, disrupts tumor-initiating cell (TIC) activity in GBMs and BLBCs, controls multiple critical TIC regulators, and also allows targeting of non-TICs. Moreover, these manipulations do not increase chemosensitivity or suppress division of nontransformed cells. Restoration of normal c-Cbl function also allows more effective harnessing of estrogen receptor-α (ERα)-independent activities of tamoxifen to activate the RFC pathway and target ERα-negative cancer cells. Our work thus provides a discovery strategy that reveals mechanisms and therapeutic targets that cannot be deduced by standard genetics analyses, which fail to reveal the metabolic information, isoform shifts, protein activation, protein complexes, and protein degradation critical to our discoveries.

Overcoming myelosuppression due to synthetic lethal toxicity for FLT3-targeted acute myeloid leukemia therapy

Activating mutations in FLT3 confer poor prognosis for individuals with acute myeloid leukemia (AML). Clinically active investigational FLT3 inhibitors can achieve complete remissions but their utility has been hampered by acquired resistance and myelosuppression attributed to a ‘synthetic lethal toxicity’ arising from simultaneous inhibition of FLT3 and KIT. We report a novel chemical strategy for selective FLT3 inhibition while avoiding KIT inhibition with the staurosporine analog, Star 27. Star 27 maintains potency against FLT3 in proliferation assays of FLT3-transformed cells compared with KIT-transformed cells, shows no toxicity towards normal human hematopoiesis at concentrations that inhibit primary FLT3-mutant AML blast growth, and is active against mutations that confer resistance to clinical inhibitors. As a more complete understanding of kinase networks emerges, it may be possible to define anti-targets such as KIT in the case of AML to allow improved kinase inhibitor design of clinical agents with enhanced efficacy and reduced toxicity.

DOI:http://dx.doi.org/10.7554/eLife.03445.001

Major advances in cancer therapy have improved the treatment options for many patients. However, many cancer treatments are toxic or have severe side effects, making them difficult for patients to tolerate. One cause of these side effects is that many cancer therapies kill both normal cells and cancer cells. Developing cancer therapies that are more targeted is therefore a priority in cancer research.

Acute myeloid leukemia is a type of blood cancer that has proven difficult to treat without causing serious side effects. This cancer is very aggressive and only about 1 in 4 patients are successfully cured of their cancer. At present, physicians treat acute myeloid leukemia with chemotherapy, which kills both the cancer cells and some of the patient's healthy cells.

Many patients with acute myeloid leukemia have mutations in the gene encoding an enzyme called Fms-like tyrosine kinase 3 (FLT3). This mutation makes the enzyme permanently active, and patients with the mutation have a greater risk of their cancer recurring or death. Scientists have recently discovered that treatments that inhibit the FLT3 enzyme can be effective against cancer. However, the drugs investigated so far also interfere with the patient's ability to produce new blood cells, which can lead to infections or an inability to recover from bleeding. Therefore, no new drugs have yet been approved for general use.

Warkentin et al. suspected the reason for the adverse effects of FLT3 inhibitors is that these drugs also inhibit another enzyme necessary for blood cell production. Previous work showed that inhibiting one or the other of the enzymes still allows blood cells to be produced as normal: it is only when both are inhibited that production problems arise. Warkentin et al. therefore looked for a chemical that inhibits only the FLT3 enzyme and found one called Star 27. Tests revealed that this inhibits FLT3 and prevents the growth and spread of cancerous cells but does not impair blood cell production. Additionally, Star 27 continues to work even when mutations arise in the cancer cells that cause resistance to other FLT3 inhibitors.

The findings demonstrate that when it comes to drug development, it is sometimes as important to avoid certain molecular targets as it is to hit others. Understanding the network of enzymes that FLT3 works with could therefore help researchers to develop more effective and safer cancer treatments.

DOI:http://dx.doi.org/10.7554/eLife.03445.002

A novel anticancer diarylurea derivative HL-40 as a multi-kinases inhibitor with good pharmacokinetics in Wistar rats

HL-40, N-(4-(1-(4-chlorine indazole)) phenyl)-N-(4-chloro-3-three fluorine methyl phenyl) urea, is a novel diarylurea derivative. In this study, we investigated the kinases activities and binding constants, pharmacokinetics of HL-40, and then evaluated its anticancer efficacy by both in vitro and in vivo methods. Enzyme activities assays in vitro were employed to identify eight candidate kinase targets. The competition binding assays against eight candidate kinases suggested that HL-40 showed strong affinity to c-Kit, PDGFRβ and FLT3. The pharmacokinetic studies in Wistar rats showed that HL-40 could maintain high compound concentration and long residence time in the blood circulation. HL-40 possessed strong inhibition activities against 12 human cancer cells. Meanwhile, HL-40 effectively delayed the growth of cancer xenografts without significant toxicity to mice. Based on these in vitro and in vivo results, we suggested that HL-40 might be developed as a potential multi-kinases inhibitor for cancer treatment.