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

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.

Discovery of pyridine-based derivatives as FLT3 inhibitors for the treatment of acute myeloid leukemia

FMS-like tyrosine receptor kinase 3 (FLT3) mutations, the most common genetic alterations found in acute myeloid leukemia (AML) patients, have been pursued as an ideal drug discovery target for the AML therapy. Taking compound 2 as lead, a series of pyridine derivatives bearing 1,2,3-triazole moiety were rationally designed and synthesized. The bioassays confirmed that these derivatives exerted potent antileukemia effects, and compound 12y was found to be the most potent one. 12y displayed double-digital nanomolar inhibitory activities against FLT3-ITD and FLT3-ITD driven human AML MOLM-13 cells as well as high selectivity over FLT3-ITD non-addicted cell lines. In addition, kinase profiling versus over 51 kinases demonstrated that 12y was potent against FLT3-ITD and VEGFR2. Moreover, treatment of MOLM-13 cells with 12y resulted in downregulated phosphorylation levels of FLT3 and STAT5, as well as cell cycle arrest and apoptosis. With the acceptable oral bioavailability of 19.2 % in SD rats, 12y prolonged the survival rate of NSG mice dose-dependently in MOLM-13 inoculated xenograft model without obvious toxicity. Overall, this study might provide a new insight for the development of novel FLT3 inhibitors.

Molecular mechanisms and therapeutic strategies in overcoming chemotherapy resistance in cancer

Cancer remains a leading cause of mortality globally and a major health burden, with chemotherapy often serving as the primary therapeutic option for patients with advanced-stage disease, partially compensating for the limitations of non-curative treatments. However, the emergence of chemotherapy resistance significantly limits its efficacy, posing a major clinical challenge. Moreover, heterogeneity of resistance mechanisms across cancer types complicates the development of universally effective diagnostic and therapeutic approaches. Understanding the molecular mechanisms of chemoresistance and identifying strategies to overcome it are current research focal points. This review provides a comprehensive analysis of the key molecular mechanisms underlying chemotherapy resistance, including drug efflux, enhanced DNA damage repair (DDR), apoptosis evasion, epigenetic modifications, altered intracellular drug metabolism, and the role of cancer stem cells (CSCs). We also examine specific causes of resistance in major cancer types and highlight various molecular targets involved in resistance. Finally, we discuss current strategies aiming at overcoming chemotherapy resistance, such as combination therapies, targeted treatments, and novel drug delivery systems, while proposing future directions for research in this evolving field. By addressing these molecular barriers, this review lays a foundation for the development of more effective cancer therapies aimed at mitigating chemotherapy resistance.

Strategies for the treatment of acute myeloid leukemia with FLT3 mutations: a patent review

INTRODUCTION

Approximately one-third of all AML patients have a mutation in the Fms-like tyrosine kinase 3 (FLT3) gene, which is associated with a poor prognosis in these individuals. The 2017 approval of midostaurin, the first FLT3 inhibitor, spurred extensive development of more potent and selective inhibitors with an improved safety profile.

AREAS COVERED

This review analyzes patent inventions for the treatment of AML using FLT3 inhibitors, covering developments from the earliest to the most recent, disclosed in 2024. Our search using the global Espacenet database identified numerous compounds with low nanomolar inhibitory concentrations against FLT3-ITD and FLT3-TKD mutants. These compounds have shown promise in preclinical studies. Co-inhibition strategies and combinatorial therapies to overcome resistance and enhance anti-leukemic efficacy are also discussed.

EXPERT OPINION

Recent patents highlight advances in the field of FLT3 inhibitors with a focus on overcoming resistance, improving selectivity and potency. Future strategies may include third-generation inhibitors such as type III allosteric inhibitors, irreversible inhibitors, or PROTACs. Personalized medicine approaches utilizing genetic profiling to tailor therapies are emphasized. Exploration of novel combination regimens with emerging therapies like CAR T-cell therapy, immune checkpoint inhibitors, and small molecules targeting critical AML pathways is ongoing to further enhance anti-leukemic efficacy.

Targeting FMS-like tyrosine kinase 3 (FLT3) in acute myeloid leukemia: Novel molecular approaches and therapeutic challenges

Acute myeloid leukemia (AML), a heterogeneous hematologic malignancy, has generally a poor prognosis despite the recent advancements in diagnostics and treatment. Genetic instability, particularly mutations in the FMS-like tyrosine kinase 3 (FLT3) gene, is associated with severe outcomes. Approximately 30 % of AML patients harbor FLT3 mutations, which have been linked to higher relapse and reduced survival rates. Traditional AML treatments employ cytarabine and anthracyclines drugs. Furthermore, the development of FLT3 inhibitors has significantly improved therapy for FLT3-mutated AML patients. For example, the introduction of midostaurin, the first FLT3 inhibitor, improved patient outcomes. However, resistant AML cell clones continue to pose a challenge to the success of AML treatment. This review discusses FLT3 kinase, mutations, and role in AML pathogenesis. It explores the molecular mechanisms of FLT3 activation, signaling pathways, and the structure and function of the FLT3 receptor. Current and emerging therapeutic approaches are presented, while highlighting the latest FLT3 inhibitors in clinical use, and strategies to overcome drug resistance. Future directions, including personalized therapies and novel drug designs, are examined to provide updated insights into FLT3-targeted treatments. This comprehensive review aims to guide clinicians and researchers in the development of innovative therapies to improve AML patient outcomes.

Discovery of SILA-123 as a Highly Potent FLT3 Inhibitor for the Treatment of Acute Myeloid Leukemia with Various FLT3 Mutations

The FLT3-ITD (internal tandem duplication) mutant has been a promising target for acute myeloid leukemia (AML) drug discovery but is now facing the challenge of resistance due to point mutations. Herein, we have discovered a type II FLT3 inhibitor, SILA-123. This inhibitor has shown highly potent inhibitory effects against FLT3-WT (IC50 = 2.1 nM) and FLT3-ITD (IC50 = 1.0 nM), tumor cells with the FLT3-ITD mutant such as MOLM-13 (IC50 = 0.98 nM) and MV4-11 (IC50 = 0.19 nM), as well as BaF3 cells associated with the FLT3-ITD mutant and point mutations like BaF3-FLT3-ITD-G697R (IC50 = 3.0 nM). Moreover, SILA-123 exhibited promising kinome selectivity against 310 kinases (S score (10) = 0.06). In in vivo studies, SILA-123 significantly suppressed the tumor growth in MV4-11 (50 mg/kg/d, TGI = 87.3%) and BaF3-FLT3-ITD-G697R (50 mg/kg/d, TGI = 60.0%) cell-inoculated allograft models. Our data suggested that SILA-123 might be a promising drug candidate for FLT3-ITD-positive AML.

Design, Synthesis, and Biological Activities Evaluation of Type I FLT3 Inhibitors for the Treatment of Acute Myeloid Leukemia

The abnormal overexpression of FLT3 kinase is intimately associated with pathogenesis of acute myeloid leukemia (AML), positioning FLT3 inhibitors as pivotal therapeutic agents. Despite the availability of three FDA-approved FLT3 inhibitors, their clinical utility is hampered by resistance stemming from tyrosine kinase domain (TKD) mutations. Through an integrative analysis of case studies, we identified a potential advantage of type I FLT3 inhibitors in overcoming TKD mutation-induced resistance. Structure-activity relationships (SAR) analysis indicated that FW-1 exhibited over 50% inhibition against FLT3 at a concentration of 1 μM and demonstrated potent activity against AML cell lines MV4-11 (IC50 = 2.68 μM) and MOLM-13 (IC50 = 1.03 μM). In our cellular mechanistic studies, FW-1 also effectively induced apoptosis by arresting cell cycle progression in the G0/G1 phase. This study introduces FW-1 as a promising lead for type I FLT3 inhibitor, warranting further optimization.

Understanding mechanisms of resistance to FLT3 inhibitors in adult FLT3-mutated acute myeloid leukemia to guide treatment strategy

The presence of FLT3 mutations, including the most common FLT3-ITD (internal tandem duplications) and FLT3-TKD (tyrosine kinase domain), is associated with an unfavorable prognosis in patients affected by acute myeloid leukemia (AML). In this setting, in recent years, new FLT3 inhibitors have demonstrated efficacy in improving survival and treatment response. Nevertheless, the development of primary and secondary mechanisms of resistance poses a significant obstacle to their efficacy. Understanding these mechanisms is crucial for developing novel therapeutic approaches to overcome resistance and improve the outcomes of patients. In this context, the use of novel FLT3 inhibitors and the combination of different targeted therapies have been studied. This review provides an update on the molecular alterations involved in the resistance to FLT3 inhibitors, and describes how the molecular monitoring may be used to guide treatment strategy in FLT3-mutated AML.

Classification of FLT3 inhibitors and SAR analysis by machine learning methods

FMS-like tyrosine kinase 3 (FLT3) is a type III receptor tyrosine kinase, which is an important target for anti-cancer therapy. In this work, we conducted a structure-activity relationship (SAR) study on 3867 FLT3 inhibitors we collected. MACCS fingerprints, ECFP4 fingerprints, and TT fingerprints were used to represent the inhibitors in the dataset. A total of 36 classification models were built based on support vector machine (SVM), random forest (RF), eXtreme Gradient Boosting (XGBoost), and deep neural networks (DNN) algorithms. Model 3D_3 built by deep neural networks (DNN) and TT fingerprints performed best on the test set with the highest prediction accuracy of 85.83% and Matthews correlation coefficient (MCC) of 0.72 and also performed well on the external test set. In addition, we clustered 3867 inhibitors into 11 subsets by the K-Means algorithm to figure out the structural characteristics of the reported FLT3 inhibitors. Finally, we analyzed the SAR of FLT3 inhibitors by RF algorithm based on ECFP4 fingerprints. The results showed that 2-aminopyrimidine, 1-ethylpiperidine,2,4-bis(methylamino)pyrimidine, amino-aromatic heterocycle, [(2E)-but-2-enyl]dimethylamine, but-2-enyl, and alkynyl were typical fragments among highly active inhibitors. Besides, three scaffolds in Subset_A (Subset 4), Subset_B, and Subset_C showed a significant relationship to inhibition activity targeting FLT3.

Development of Epigenetic Modifiers with Therapeutic Potential in FMS-Related Tyrosine Kinase 3/Internal Tandem Duplication (FLT3/ITD) Acute Myeloid Leukemia and Other Blood Malignancies

Blood cancers encompass a group of diseases affecting the blood, bone marrow, or lymphatic system, representing the fourth most commonly diagnosed cancer worldwide. Leukemias are characterized by the dysregulated proliferation of myeloid and lymphoid cells with different rates of progression (acute or chronic). Among the chronic forms, hairy cell leukemia (HCL) is a rare disease, and no drugs have been approved to date. However, acute myeloid leukemia (AML) is one of the most aggressive malignancies, with a low survival rate, especially in cases with FLT3-ITD mutations. Epigenetic modifications have emerged as promising strategies for the treatment of blood cancers. Epigenetic modulators, such as histone deacetylase (HDAC) inhibitors, are increasingly used for targeted cancer therapy. New hydroxamic acid derivatives, preferentially inhibiting HDAC6 (5a-q), were developed and their efficacy was investigated in different blood cancers, including multiple myeloma (MM), HCL, and AML, pointing out their pro-apoptotic effect as the mechanism of cell death. Among the inhibitors described, 5c, 5g, and 5h were able to rescue the hematopoietic phenotype in vivo using the FLT3-ITD zebrafish model of AML. 5c (leuxinostat) proved its efficacy in cells from FLT3-ITD AML patients, promoting marked acetylation of α-tubulin compared to histone H3, thereby confirming HDAC6 as a preferential target for this new class of hydroxamic acid derivatives at the tested doses.

Emerging DNA Methylome Targets in FLT3-ITD-Positive Acute Myeloid Leukemia: Combination Therapy with Clinically Approved FLT3 Inhibitors

OPINION STATEMENT

The internal tandem duplication (ITD) mutation of the FMS-like receptor tyrosine kinase 3 (FLT3-ITD) is the most common mutation observed in approximately 30% of acute myeloid leukemia (AML) patients. It represents poor prognosis due to continuous activation of downstream growth-promoting signaling pathways such as STAT5 and PI3K/AKT. Hence, FLT3 is considered an attractive druggable target; selective small FLT3 inhibitors (FLT3Is), such as midostaurin and quizartinib, have been clinically approved. However, patients possess generally poor remission rates and acquired resistance when FLT3I used alone. Various factors in patients could cause these adverse effects including altered epigenetic regulation, causing mainly abnormal gene expression patterns. Epigenetic modifications are required for hematopoietic stem cell (HSC) self-renewal and differentiation; however, critical driver mutations have been identified in genes controlling DNA methylation (such as DNMT3A, TET2, IDH1/2). These regulators cause leukemia pathogenesis and affect disease diagnosis and prognosis when they co-occur with FLT3-ITD mutation. Therefore, understanding the role of different epigenetic alterations in FLT3-ITD AML pathogenesis and how they modulate FLT3I's activity is important to rationalize combinational treatment approaches including FLT3Is and modulators of methylation regulators or pathways. Data from ongoing pre-clinical and clinical studies will further precisely define the potential use of epigenetic therapy together with FLT3Is especially after characterized patients' mutational status in terms of FLT3 and DNA methlome regulators.

The potential of triplet combination therapies for patients with FLT3-ITD -mutated acute myeloid leukemia

INTRODUCTION

Acute myeloid leukemia (AML) encompasses a heterogeneous group of aggressive myeloid malignancies, where FMS-like tyrosine kinase 3 (FLT3) mutations are prevalent, accounting for approximately 25-30% of adult patients. The presence of this mutation is related to a dismal prognosis and high relapse rates. In the lasts years many FLT3 inhibitors have been developed.

AREAS COVERED

This review provides a comprehensive overview of FLT3mut AML, summarizing the state of art of current treatment and available data about combination strategies including an FLT3 inhibitor.

EXPERT OPINION

In addition, the review discusses the emergence of drug resistance and the need for a nuanced approaches in treating patients who are ineligible for or resistant to intensive chemotherapy. Specifically, it explores the historical context of FLT3 inhibitors (FLT3Is) and their impact on treatment outcomes, emphasizing the pivotal role of midostaurin, as well as gilteritinib and quizartinib, and providing detailed insights into ongoing trials exploring the safety and efficacy of novel triplet combinations involving FLT3Is in different AML settings.

Discovery and Anticancer Screening of Novel Oxindole-Based Derivative Bearing Pyridyl Group as Potent and Selective Dual FLT3/CDK2 Kinase Inhibitor

Protein kinases regulate cellular activities and make up over 60% of oncoproteins and proto-oncoproteins. Among these kinases, FLT3 is a member of class III receptor tyrosine kinase family which is abundantly expressed in individuals with acute leukemia. Our previous oxindole-based hit has a particular affinity toward FLT3 (IC50 = 2.49 μM) and has demonstrated selectivity towards FLT3 ITD-mutated MV4-11 AML cells, with an IC50 of 4.3 μM. By utilizing the scaffold of the previous hit, sixteen new compounds were synthesized and screened against NCI-60 human cancer cell lines. This leads to the discovery of a potent antiproliferative compound, namely 5l, with an average GI50 value against leukemia and colon cancer subpanels equalling 3.39 and 5.97 µM, respectively. Screening against a specific set of 10 kinases that are associated with carcinogenesis indicates that compound 5l has a potent FLT3 inhibition (IC50 = 36.21 ± 1.07 nM). Remarkably, compound 5l was three times more effective as a CDK2 inhibitor (IC50 = 8.17 ± 0.32 nM) compared to sunitinib (IC50 = 27.90 ± 1.80 nM). Compound 5l was further analyzed by means of docking and molecular dynamics simulation for CDK2 and FLT3 active sites which provided a rational for the observed strong inhibition of kinases. These results suggest a novel structural scaffold candidate that simultaneously inhibits CDK2 and FLT3 and gives encouragement for further development as a potential therapeutic for leukemia and colon cancer.

Discovery of FLT3-targeting PROTACs with potent antiproliferative activity against acute myeloid leukemia cells harboring FLT3 mutations

Acute myeloid leukemia (AML) patients harboring Fms-like tyrosine kinase 3 (FLT3) mutations often suffer from poor prognosis and relapse. Targeted protein degradation utilizing proteolysis targeting chimeras (PROTACs) is considered as a novel therapeutic strategy in drug discovery and may be a promising modality to target FLT3 mutations for the development of potent anti-AML drugs. Herein, a kind of FLT3-targeting PROTACs was rationally developed based on a FLT3 inhibitor previously reported by us. The representative compound 35 showed potent and selective antiproliferative activities against AML cells harboring FLT3 mutations. Western blot assay demonstrated that compound 35 effectively induced the degradation of FLT3-ITD and decreased the phosphorylation levels of FLT3-ITD, AKT, STAT5 and ERK in MV4-11 cells in a dose-dependent manner. Flow cytometry analysis illustrated that compound 35 strongly induced apoptosis and cell cycle arrest in MV4-11 cells in a dose-dependent manner. Moreover, compound 35 displayed favorable metabolic stability in in-vitro liver microsomes studies. Comparative molecular dynamic (MD) simulation studies further elucidated the underlying mechanism of compound 35 to stabilize the dynamic ensemble of the FLT3-compound 35-cereblon (CRBN) ternary complex. Taken together, compound 35 could serve as a lead molecule for developing FLT3 degraders against AML.

Discovery of LWY713 as a potent and selective FLT3 PROTAC degrader with in vivo activity against acute myeloid leukemia

Fms-like tyrosine kinase 3 (FLT3) has been validated as a therapeutic target for acute myeloid leukemia (AML). While a number of FLT3 kinase inhibitors have been approved for AML treatment, the clinical data revealed that they cannot achieve complete and sustained suppression of FLT3 signaling at the tolerated dose. Here we report a series of new, potent and selective FLT3 proteolysis targeting chimera degraders. The optimal compound LWY713 potently induced the degradation of FLT3 with a DC50 value of 0.64 nM and a Dmax value of 94.8% in AML MV4-11 cells with FLT3-internal tandem duplication (ITD) mutation. Mechanistic studies demonstrated that LWY713 selectively induced FLT3 degradation in a cereblon- and proteasome-dependent manner. LWY713 potently inhibited FLT3 signaling, suppressed cell proliferation, and induced cell G0/G1-phase arrest and apoptosis in MV4-11 cells. Importantly, LWY713 displayed potent in vivo antitumor activity in MV4-11 xenograft models.

Developments of Fms-like Tyrosine Kinase 3 Inhibitors as Anticancer Agents for AML Treatment

BACKGROUND

FMS-like tyrosine kinase 3 (FLT3) is a commonly mutated gene in acute myeloid leukemia. As a receptor tyrosine kinase (RTK), FLT3 plays a role in the proliferation and differentiation of hematopoietic stem cells. As the most frequent molecular alteration in AML, FLT3 has drawn the attention of many researchers, and a lot of small molecule inhibitors targeting FLT3 have been intensively investigated as potential drugs for AML therapy.

METHODS

In this paper, PubMed and SciFinder® were used as a tool; the publications about "FLT3 inhibitor" and "Acute myeloid leukemia" were surveyed from 2014 to the present with an exclusion of those published as patents.

RESULTS

In this study, the structural characterization and biological activities of representative FLT3 inhibitors were summarized. The major challenges and future directions for further research are discussed.

CONCLUSION

Recently, numerous FLT3 inhibitors have been discovered and employed in FLT3-mutated AML treatment. In order to overcome the drug resistance caused by FLT3 mutations, screening multitargets FLT3 inhibitors has become the main research direction. In addition, the emergence of irreversible FLT3 inhibitors also provides new ideas for discovering new FLT3 inhibitors.

Recruitment of hexahydroquinoline as anticancer scaffold targeting inhibition of wild and mutants EGFR (EGFRWT, EGFRT790M, and EGFRL858R)

Hexahydroquinoline (HHQ) scaffold was constructed and recruited for development of new series of anticancer agents. Thirty-two new compounds were synthesised where x-ray crystallography was performed to confirm enantiomerism. Thirteen compounds showed moderate to good activity against NCI 60 cancer cell lines, with GI % mean up to 74% for 10c. Expending erlotinib as a reference drug, target compounds were verified for their inhibiting activities against EGFRWT, EGFRT790M, and EGFRL858R where compound 10d was the best inhibitor with IC50 = 0.097, 0.280, and 0.051 µM, respectively, compared to erlotinib (IC50 = 0.082 µM, 0.342 µM, and 0.055 µM, respectively). Safety profile was validated using normal human lung (IMR-90) cells. 10c and 10d disrupted cell cycle at pre-G1 and G2/M phases in lung cancer, HOP-92, and cell line. Molecular docking study was achieved to understand the potential binding interactions and affinities in the active sites of three versions of EGFRs.

Rational discovery of dual FLT3/HDAC inhibitors as a potential AML therapy

Acute myeloid leukemia (AML) patients often experience poor therapeutic outcomes and relapse after treatment with single-target drugs, representing the urgent need of new therapies. Simultaneous inhibition of multiple oncogenic signals is a promising strategy for tumor therapy. Previous studies have reported that concomitant inhibition of Fms-like tyrosine kinase 3 (FLT3) and histone deacetylases (HDACs) can significantly improve the therapeutic efficacy for AML. Herein, a series of novel dual FLT3/HDAC inhibitors were developed through a rational structure-based drug design strategy for the first time. Among them, multiple compounds showed potent and equivalent inhibitory activities against FLT3-ITD and HDAC1, with the representative compound 63 selectively inhibiting HDAC class I (HDAC1/2/3/8) and IIB isoforms (HDAC6) related to tumorigenesis, and intensively blocking proliferation of MV4-11 cells. The antiproliferation activity was proven to depend on the dual inhibition of FLT3 and HDAC1. Mechanism assays demonstrated that 63 prohibited both FLT3 and HDAC pathways, induced apoptosis and arrested cell cycle in MV4-11 cells in a dose-dependent manner. In summary, this study validated the therapeutic potential of a kind of dual FLT3/HDAC inhibitors for AML and provided novel compounds for further biological investigation on concomitant inhibition of FLT3/HDAC pathways. Additionally, the structure-based drug design strategy described herein may provide profound enlightenment for developing superior anti-AML drugs.

Design, synthesis and biological evaluation of 2-aminopyrimidine derivatives as potent FLT3 inhibitors

Acute myeloid leukemia (AML) is an aggressive cancer, which is characterized by clonal expansion of myeloid progenitors in the bone marrow and peripheral blood. FMS-like tyrosine kinase 3 (FLT3) mutations are the most frequently identified mutations, present in approximately 25-30 % AML patients, making FLT3 inhibitors a crucial treatment option for AML. In this study, we described the design, synthesis and biological evaluation of a series of 2-aminopyrimidine derivatives as potent FLT3 inhibitors. Notably, compound 15 displayed potent kinase inhibitory activities against FLT3 (FLT3-WT IC50 = 7.42 ± 1.23 nM; FLT3-D835Y IC50 = 9.21 ± 0.04 nM) and robust antiproliferative activities against MV4-11 cells (IC50 = 0.83 ± 0.15 nM) and MOLM-13 cells (IC50 = 10.55 ± 1.70 nM). Compound 15 also possessed potent antiproliferative activities against BaF3 cells carrying various FLT3-TKD and FLT3-ITD-TKD mutations, indicating its potential to overcome on-target resistance caused by FLT3 mutations. In summary, compound 15 showed promising potential for further exploration as a treatment of AML.

Combination strategies to overcome drug resistance in FLT+ acute myeloid leukaemia

BACKGROUND

Acute myeloid leukaemia (AML) remains difficult to treat despite the development of novel formulations and targeted therapies. Activating mutations in the FLT3 gene are common among patients and make the tumour susceptible to FLT3 inhibitors, but resistance to such inhibitors develops quickly.

METHODS

We examined combination therapies aimed at FLT3+-AML, and studied the development of resistance using a newly developed protocol. Combinations of FLT3, CDK4/6 and PI3K inhibitors were tested for synergism.

RESULTS

We show that AML cells express CDK4 and that the CDK4/6 inhibitors palbociclib and abemaciclib inhibit cellular growth. PI3K inhibitors were also effective in inhibiting the growth of AML cell lines that express FLT3-ITD. Whereas resistance to quizartinib develops quickly, the combinations overcome such resistance.

CONCLUSIONS

This study suggests that a multi-targeted intervention involving a CDK4/6 inhibitor with a FLT3 inhibitor or a pan-PI3K inhibitor might be a valuable therapeutic strategy for AML to overcome drug resistance. Moreover, many patients cannot tolerate high doses of the drugs that were studied (quizartinib, palbociclib and PI3K inhibitors) for longer periods, and it is therefore of high significance that the drugs act synergistically and lower doses can be used.

Indole-based FLT3 inhibitors and related scaffolds as potential therapeutic agents for acute myeloid leukemia

Fms-like tyrosine kinase 3 (FLT3) mutation mechanisms are among the most common genetic abnormalities detected in about 30% of acute myeloid leukemia (AML) patients. These mutations are accompanied by poor clinical response, although all these progressions in identifying and interpreting biological AML bio-targets. Several small structured FLT3 inhibitors have been ameliorated to struggle against AML. Despite all these developments regarding these inhibitors, the Overall survival rate is about five years or more in less than one-third of diagnosed AML patients. Midostaurin was the first FDA-approved FLT3 inhibitor in 2017 in the United States and Europe for AML remedy. Next, Gilteritinib was an FDA-approved FLT3 inhibitor in 2018 and in the next year, Quizartinib was approved an as FLT3 inhibitor in Japan. Interestingly, indole-based motifs had risen as advantaged scaffolds with unusual multiple kinase inhibitory activity. This review summarises indole-based FLT3 inhibitors and related scaffolds, including FDA-approved drugs, clinical candidates, and other bioactive compounds. Furthermore, their chemotypes, mechanism of action, and interaction mode over both wild and mutated FLT3 target proteins had been judgmentally discussed. Therefore, this review could offer inspiring future perspectives into the finding of new FLT3-related AML therapies.

Novel spiroindoline quinazolinedione derivatives as anticancer agents and potential FLT3 kinase inhibitors

Despite considerable recent progress in therapeutic strategies, cancer still remains one of the leading causes of death. Molecularly targeted therapies, in particular those focused on blocking receptor tyrosine kinases have produced promising outcomes in recent years. In this study, a new series of spiro[indoline-3,2'-quinazoline]-2,4'(3'H)-dione derivatives (5a-5l) were synthesized and evaluated as potential kinase inhibitors with anticancereffects. The anti-proliferative activity was measured by MTT assay, while the cell cycle was studied using flow cytometry. Moreover, kinase inhibition profiles of the most promising compounds were assessed against a panel of 25 oncogenic kinases. Compounds 5f,5g,5i, and 5jshowed anti-proliferative effect against EBC-1, A549, and HT-29 solid tumor models in addition to leukemia cell line K562. In particular, compound 5f, bearing 4-methylphenyl pendant on the isatin ring displayed considerable potency with IC₅₀ values of 2.4 to 13.4 μM against cancer cells. The most potent derivatives also altered the distribution of cells in different phases of cell cycle and increased the sub-G1 phase cells in K562 cells. Moreover, kinase inhibition assays identified FLT3 kinase was as the primary targetof these derivatives. Compound 5f at 25 μM concentration showed inhibitory activities of 55% and 62% against wild-type FLT3 and its mutant, D835Y, respectively. Finally, the docking and simulation studies revealed the important interactions of compound 5f with wild type and mutant FLT3. The results of this study showed that some novel spiroindoline quinazolinedione compounds could be potential candidates for further development as novel targeted anticancer agents.

Combination Therapies with Kinase Inhibitors for Acute Myeloid Leukemia Treatment

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

Rational design of 4-((6-phenoxypyrimidin-4-yl)amino)-N-(4-(piperazin-1-yl)phenyl)-1H-pyrazole-3-carboxamide (LT-540-717) as orally bioavailable FLT3 inhibitor

In recent years, fms-like tyrosine kinase 3 (FLT3) was confirmed as an exciting target for treatment of AML. However, resistance to FLT3 inhibitors caused by acquired point mutations in tyrosine kinase domain (TKD) have limited their sustained efficacious. Thus, there remains an unmet need to develop high-efficacy FLT3 inhibitors against both FLT3 internal tandem duplication (ITD) and FLT3 (TKD) mutations. Herein, we describe the discovery of compound LT-540-717 (32), a potent FLT3 inhibitor (IC₅₀: 0.62 nM), starting from FN-1501. Compound 32 exhibited highly inhibitory activity against several acquired FLT3 mutations including FLT3 (ITD, D835V), FLT3 (ITD, F691L), FLT3 (D835Y) and FLT3 (D835V). Additionally, 32 displayed potent antiproliferative activity against FLT3-mutation driven BaF3 and AML cells. Oral administration of 32 (25 mg/kg, QD) significantly prohibited tumor growth (tumor-inhibition rate is 94.18%), and no obvious side effect was observed even when increasing dose to 50 mg/kg (tumor-inhibition rate is 93.98%). Furthermore, 32 showed an acceptable bioavailability (F = 33.3% in rat and 72.7% in beagles), a suitable half-life time (T_1/2 = 3.5 h in rat and T_1/2 = 11.1 h in beagles), and a satisfactory metabolic stability. In summary, these results show the therapeutic potential of 32 to become a new anti-AML drug, especially for AML harboring dual FLT3 (ITD, TKD) mutations.

Design, synthesis and pharmacological characterization of aminopyrimidine derivatives as BTK/FLT3 dual-target inhibitors against acute myeloid leukemia

A novel class of aminopyrimidine-based Bruton's tyrosine kinase (BTK) and FMS-like tyrosine kinase 3 (FLT3) dual-target inhibitors based on the BTK inhibitor spebrutinib was designed for the treatment of acute myeloid leukemia. Representative compounds 14d, 14g, 14j and 14m effectively inhibited BTK, FLT3, and FLT3(D835Y) mutant activities with low nanomolar IC₅₀'s. These compounds displayed potent antiproliferative activities against leukemia cells with IC₅₀'s of 0.29-950 nM. In particular, 14m had IC₅₀ values 101-1045 times lower than those of spebrutinib against all cancer cell lines tested. Compound 14m effectively induced autophagy and apoptosis in MV-4-11 cells through regulating related proteins in a dose-dependent manner. Finally, intraperitoneal administration of 14m at 20 mg/kg significantly repressed the growth of MV-4-11 cells with a TGI value of 95.68% with no apparent toxicity. These BTK/FLT3 dual-target inhibitors represent promising leads for further structural optimization and antitumor mechanism studies.

RSK inhibitors as potential anticancer agents: Discovery, optimization, and challenges

Ribosomal S6 kinase (RSK) family is a group of serine/threonine kinases, including four isoforms (RSK1/2/3/4). As a downstream effector of the Ras-mitogen-activated protein kinase (Ras-MAPK) pathway, RSK participates in many physiological activities such as cell growth, proliferation, and migration, and is intimately involved in tumor occurrence and development. As a result, it is recognized as a potential target for anti-cancer and anti-resistance therapies. There have been several RSK inhibitors discovered or designed in recent decades, but only two have entered clinical trials. Low specificity, low selectivity, and poor pharmacokinetic properties in vivo limit their clinical translation. Published studies performed structure optimization by increasing interaction with RSK, avoiding hydrolysis of pharmacophores, eliminating chirality, adapting to binding site shape, and becoming prodrugs. Besides enhancing efficacy, the focus of further design will move towards selectivity since there are functional differences among RSK isoforms. This review summarized the types of cancers associated with RSK, along with the structural characteristics and optimization process of the reported RSK inhibitors. Furthermore, we addressed the importance of RSK inhibitors' selectivity and discussed future drug development directions. This review is expected to shed light on the emergence of RSK inhibitors with high potency, specificity, and selectivity.

A Receptor Tyrosine Kinase Inhibitor Sensitivity Prediction Model Identifies AXL Dependency in Leukemia

Despite incredible progress in cancer treatment, therapy resistance remains the leading limiting factor for long-term survival. During drug treatment, several genes are transcriptionally upregulated to mediate drug tolerance. Using highly variable genes and pharmacogenomic data for acute myeloid leukemia (AML), we developed a drug sensitivity prediction model for the receptor tyrosine kinase inhibitor sorafenib and achieved more than 80% prediction accuracy. Furthermore, by using Shapley additive explanations for determining leading features, we identified AXL as an important feature for drug resistance. Drug-resistant patient samples displayed enrichment of protein kinase C (PKC) signaling, which was also identified in sorafenib-treated FLT3-ITD-dependent AML cell lines by a peptide-based kinase profiling assay. Finally, we show that pharmacological inhibition of tyrosine kinase activity enhances AXL expression, phosphorylation of the PKC-substrate cyclic AMP response element binding (CREB) protein, and displays synergy with AXL and PKC inhibitors. Collectively, our data suggest an involvement of AXL in tyrosine kinase inhibitor resistance and link PKC activation as a possible signaling mediator.

Sitravatinib as a potent FLT3 inhibitor can overcome gilteritinib resistance in acute myeloid leukemia

BACKGROUND

Gilteritinib is the only drug approved as monotherapy for acute myeloid leukemia (AML) patients harboring FMS-like tyrosine kinase 3 internal tandem duplication (FLT3-ITD) mutation throughout the world. However, drug resistance inevitably develops in clinical. Sitravatinib is a multi-kinase inhibitor under evaluation in clinical trials of various solid tumors. In this study, we explored the antitumor activity of sitravatinib against FLT3-ITD and clinically-relevant drug resistance in FLT3 mutant AML.

METHODS

Growth inhibitory assays were performed in AML cell lines and BaF3 cells expressing various FLT3 mutants to evaluate the antitumor activity of sitravatinib in vitro. Immunoblotting was used to examine the activity of FLT3 and its downstream pathways. Molecular docking was performed to predict the binding sites of FLT3 to sitravatinib. The survival benefit of sitravatinib in vivo was assessed in MOLM13 xenograft mouse models and mouse models of transformed BaF3 cells harboring different FLT3 mutants. Primary patient samples and a patient-derived xenograft (PDX) model were also used to determine the efficacy of sitravatinib.

RESULTS

Sitravatinib inhibited cell proliferation, induced cell cycle arrest and apoptosis in FLT3-ITD AML cell lines. In vivo studies showed that sitravatinib exhibited a better therapeutic effect than gilteritinib in MOLM13 xenograft model and BaF3-FLT3-ITD model. Unlike gilteritinib, the predicted binding sites of sitravatinib to FLT3 did not include F691 residue. Sitravatinib displayed a potent inhibitory effect on FLT3-ITD-F691L mutation which conferred resistance to gilteritinib and all other FLT3 inhibitors available, both in vitro and in vivo. Compared with gilteritinib, sitravatinib retained effective activity against FLT3 mutation in the presence of cytokines through the more potent and steady inhibition of p-ERK and p-AKT. Furthermore, patient blasts harboring FLT3-ITD were more sensitive to sitravatinib than to gilteritinib in vitro and in the PDX model.

CONCLUSIONS

Our study reveals the potential therapeutic role of sitravatinib in FLT3 mutant AML and provides an alternative inhibitor for the treatment of AML patients who are resistant to current FLT3 inhibitors.

Discovery of 4-(4-aminophenyl)-6-phenylisoxazolo[3,4-b]pyridine-3-amine derivatives as novel FLT3 covalent inhibitors for the intervention of acute myeloid leukemia

Small molecule covalent drugs have proved to be desirable therapies especially on drug resistance related to point mutations. Secondary mutations of FLT3 have become the main mechanism of FLT3 inhibitors resistance which further causes the failure of treatment. Herein, a series of 4-(4-aminophenyl)-6-phenylisoxazolo[3,4-b]pyridine-3-amine covalent derivatives were synthesized and optimized to overcome the common secondary resistance mutations of FLT3. Among these derivatives, compound F15 displayed potent inhibition activities against FLT3 (IC₅₀  = 123 nM) and FLT3-internal tandem duplication (ITD) by 80% and 26.06%, respectively, at the concentration of 1 μM. Besides, F15 exhibited potent activity against FLT3-dependent human acute myeloid leukemia (AML) cell lines MOLM-13 (IC₅₀  = 253 nM) and MV4-11 (IC₅₀  = 91 nM), as well as BaF3 cells with variety of secondary mutations. Furthermore, cellular mechanism assays indicated that F15 inhibited phosphorylation of FLT3 and its downstream signaling factors. Notably, F15 could be considered for further development as potential drug candidate to treat AML.

Design and synthesis of selective FLT3 inhibitors via exploration of back pocket II

Aim: The clinical benefits of FLT3 inhibitors against acute myeloid leukemia (AML) have been limited by selectivity and resistance mutations. Thus, to identify FLT3 inhibitors possessing high selectivity and potency is of necessity. Methods & results: The authors used computational methods to systematically compare pocket similarity with 269 kinases. Subsequently, based on these investigations and beginning with in-house compound 10, they synthesized a series of 6-methyl-isoxazol[3,4-b]pyridine-3-amino derivatives and identified that compound 45 (IC₅₀: 103 nM) displayed gratifying potency in human AML cell lines with FLT3-internal tandem duplications mutation as well as FLT3-internal tandem duplications-tyrosine kinase domain-transformed BaF3 cells. Conclusion: The integrated biological activity results indicated that compound 45 deserves further development for therapeutic remedies for AML.

Anti-tumor effect of antibody drug conjugate ASP1235 targeting Fms-like tyrosine kinase 3 with venetoclax plus azacitidine in an acute myeloid leukemia xenograft mouse model

Antibody drug conjugates (ADC) are one of the attractive modalities for the treatment of acute myeloid leukemia (AML). Previously, we have developed ASP1235, a novel ADC targeting Fms-like tyrosine kinase 3 (FLT3) which is widely expressed on the leukemic blasts of AML patients. In this study, we sought to evaluate the therapeutic effect of ASP1235 in combination with venetoclax plus azacitidine, a novel standard-of-care treatment for elderly AML patients, in ASP1235 poor sensitive AML cells. To identify the suitable preclinical model, we first evaluated the growth inhibitory effect of ASP1235 on several leukemia cell lines expressing FLT3 and found that THP-1 cells were partially sensitive to ASP1235 in vitro. Furthermore, ASP1235 showed marginal anti-tumor activity in a THP-1 xenograft model. Compared to the leukemic blasts in most of the relapsed or refractory (R/R) AML patients tested, THP-1 cells expressed equivalent protein levels of Bcl-2, suggesting that ASP1235 in combination with venetoclax plus azacitidine is a rational treatment in the THP-1 model. In vitro, ASP1235 showed a cytotoxic effect on THP-1 cells in combination with venetoclax, and the combination effect was greater than the additive effect. Furthermore, ASP1235 also showed a combination effect with venetoclax plus azacitidine treatment. Similarly, the combination of ASP1235, venetoclax and azacitidine showed a superior anti-tumor effect in a THP-1 xenograft model without obvious body weight loss. These findings provide supportive evidence that the triple combination of ASP1235, venetoclax and azacitidine would improve the clinical outcome of ASP1235 monotherapy and venetoclax plus azacitidine regimen in AML patients.

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

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

Cytotoxicity and Lipase Inhibition of Essential Oils from Amazon Annonaceae Species

Essential oils from Amazonian species are gaining increasing interest worldwide due to their medicinal and cosmetic applications; however, the relation among the chemical constituents and their biological properties are not well explored. Therefore, the present research aims to obtain an understanding of the bioactivity of chemical compounds in the essential oils of plants from the Annonaceae family (Bocageopsis pleiosperma, Onychopetalum amazonicum, Unonopsis duckei, U. floribunda, U. rufescens, U.stipitata, U. guatterioides, Duguetia flagellaris and Xylopia benthamii). By means of gas chromatography coupled to mass spectrometry, in vitro cytotoxic and anti-lipase assays, principal component analysis and molecular docking, it was possible to establish the main compounds that may be responsible for the cytotoxic effect of O. amazonicum and B. pleiosperma. Moreover, the anti-lipase potential of D. flagellaris was also established, as well as its composition related to the activity. Thus, by the employed strategy, allo-aromadendrene, cryptomerione, δ-cadinene and β-bisabolene were suggested as plausible cytotoxic agents against cancer cell lines, and dehydroaromadendrene, spathulenol and elemol, against lipase. The present study provides significant information on the chemical profile and bioactivity studies of Amazon Annonaceae aromatic plants.

Developments and challenges of FLT3 inhibitors in acute myeloid leukemia

FLT3 mutations are one of the most common genetic alterations in acute myeloid leukemia (AML) and are identified in approximately one-third of newly diagnosed patients. Aberrant FLT3 receptor signaling has important implications for the biology and clinical management of AML. In recent years, targeting FLT3 has been a part of every course of treatment in FLT3-ITD/TKD-mutated AML and contributes to substantially prolonged survival. At the same time, wide application of next-generation sequencing (NGS) technology has revealed a series of non-canonical FLT3 mutations, including point mutations and small insertions/deletions. Some of these mutations may be able to influence downstream phosphorylation and sensitivity to FLT3 inhibitors, while the correlation with clinical outcomes remains unclear. Exploration of FLT3-targeted therapy has made substantial progress, but resistance to FLT3 inhibitors has become a pressing issue. The mechanisms underlying FLT3 inhibitor tolerance can be roughly divided into primary resistance and secondary resistance. Primary resistance is related to abnormalities in signaling factors, such as FL, CXCL12, and FGF2, and secondary resistance mainly involves on-target mutations and off-target aberrations. To overcome this problem, novel agents such as FF-10101 have shown promising potential. Multitarget strategies directed at FLT3 and anomalous signaling factors simultaneously are in active clinical development and show promising results.

Understanding gilteritinib resistance to FLT3-F691L mutation through an integrated computational strategy

FMS-like tyrosine kinase 3 (FLT3) serves as an important drug target for acute myeloid leukemia (AML), and gene mutations of FLT3 have been closely associated with AML patients with an incidence rate of ~ 30%. However, the mechanism of the clinically relevant F691L gatekeeper mutation conferred resistance to the drug gilteritinib remained poorly understood. In this study, multiple microsecond molecular dynamics (MD) simulations, end-point free energy calculations, and dynamic correlated and network analyses were performed to investigate the molecular basis of gilteritinib resistance to the FLT3-F691L mutation. The simulations revealed that the resistant mutation largely induced the conformational changes of the activation loop (A-loop), the phosphate-binding loop, and the helix αC of the FLT3 protein. The binding abilities of the gilteritinib to the wild-type and the F691L mutant were different through the binding free energy prediction. The simulation results further indicated that the driving force to determine the binding affinity of gilteritinib was derived from the differences in the energy terms of electrostatic and van der Waals interactions. Moreover, the per-residue free energy decomposition suggested that the four residues (Phe803, Gly831, Leu832, and Ala833) located at the A-loop of FLT3 had a significant impact on the binding affinity of gilteritinib to the F691L mutant. This study may provide useful information for the design of novel FLT3 inhibitors specially targeting the F691L gatekeeper mutant.

Advances in clinical studies of FLT3 inhibitors in acute myeloid leukemia

Acute myeloid leukemia (AML) is a highly heterogeneous hematological malignancy. AML patients with FLT3 mutations tend to have a high relapse rate and poor outcome, so FLT3 gene has become an important target for AML treatment, and a series of FLT3 inhibitors have been developed. According to the characteristics of FLT3 inhibitors, they can be divided into first-generation FLT3 inhibitors and second-generation FLT3 inhibitors. So far, totally eight FLT3 inhibitors have been undergone clinical trials and only three were approved for the treatment of AML patients, including Midostourin, Quizartinib and Gilteritinib. FLT3 inhibitors can improve the response rate of patients by combining with standard chemotherapy; in the follow-up maintenance treatment, FLT3 inhibitors can also reduce the disease recurrence rate and improve the overall prognosis of patients. However, the primary drug resistance caused by the bone marrow microenvironment, as well as secondary resistance caused by other mutations may result in poor efficacy of FLT3 inhibitors. For such patients, the combination of FLT3 inhibitor with other drugs may reduce the occurrence of drug resistance and improve the subsequent efficacy of patients. This article reviews the current status of FLT3 inhibitors in clinical research of AML patients and the treatment of FLT3-resistant patients to provide reference for clinicians.

Identification and structural analysis of a selective tropomyosin receptor kinase C (TRKC) inhibitor

Tropomyosin receptor kinases (TRKs) are a family of TRKA, TRKB and TRKC isoforms. It has been widely reported that TRKs are implicated in a variety of tumors with several Pan-TRK inhibitors currently being used or evaluated in clinical treatment. However, off-target adverse events frequently occur in the clinical use of Pan-TRK inhibitors, which result in poor patient compliance, even drug discontinuation. Although a subtype-selectivity TRK inhibitor may avert the potential off-target adverse events and can act as a more powerful tool compound in the biochemical studies on TRKs, the high sequence similarities of TRKs hinder the development of subtype-selectivity TRK inhibitors. For example, no selective TRKC inhibitor has been reported. Herein, a selective TRKC inhibitor (L13) was disclosed, with potent TRKC inhibitory activity and 107.5-/34.9-fold selectivity over TRKA/B (IC₅₀ TRKA/B/C = 1400 nM, 454 nM, 13 nM, respectively). Extensive molecular dynamics simulations illustrated that key interactions of L13 with the residues and diversely conserved water molecules in the ribose regions of different TRKs may be the structural basis of selectivity. This will provide inspiring insights into the development of subtype-selectivity TRK inhibitors. Moreover, L13 could serve as a tool compound to investigate the distinct biological functions of TRKC and a starting point for further research on drugs specifically targeting TRKC.

Novel Mechanisms for Post-Transplant Maintenance Therapy in Acute Myeloid Leukemia

Allogeneic stem cell transplantation has improved survival for patients with acute myeloid leukemia (AML), especially for patients with disease at high risk of relapse. However, relapse remains the most common cause of treatment failure and death in the post-transplant period. Maintenance therapy, an extended course of treatment after achieving remission to reduce the rate of relapse, is an important component of the treatment of various hematologic malignancies; however, its role in the treatment of AML is far less well-defined. Recently, there has been significant interest in the use of novel therapeutic agents as maintenance therapy after allogeneic stem cell transplant, utilizing new mechanisms of treatment and more favorable toxicity profiles. In this review, we will discuss the mechanistic and clinical data for post-transplant maintenance therapies in AML. Then, we will review several emergent and current clinical trials which aim to incorporate novel agents into maintenance therapy regimens.

Medicinal Chemistry Strategies for the Development of Bruton's Tyrosine Kinase Inhibitors against Resistance

Despite significant efficacy, one of the major limitations of small-molecule Bruton's tyrosine kinase (BTK) agents is the presence of clinically acquired resistance, which remains a major clinical challenge. This Perspective focuses on medicinal chemistry strategies for the development of BTK small-molecule inhibitors against resistance, including the structure-based design of BTK inhibitors targeting point mutations, e.g., (i) developing noncovalent inhibitors from covalent inhibitors, (ii) avoiding steric hindrance from mutated residues, (iii) making interactions with the mutated residue, (iv) modifying the solvent-accessible region, and (v) developing new scaffolds. Additionally, a comparative analysis of multi-inhibitions of BTK is presented based on cross-comparisons between 2916 unique BTK ligands and 283 other kinases that cover 7108 dual/multiple inhibitions. Finally, targeting the BTK allosteric site and uding proteolysis-targeting chimera (PROTAC) as two potential strategies are addressed briefly, while also illustrating the possibilities and challenges to find novel ligands of BTK.

Discovery of indirubin-3'-aminooxy-acetamide derivatives as potent and selective FLT3/D835Y mutant kinase inhibitors for acute myeloid leukemia

Mutations in Fms-like tyrosine kinase 3 (FLT3) have been implicated in the pathogenesis of acute myeloid leukemia (AML) by affecting the proliferation and differentiation of hematopoietic stem and progenitor cells. Although several FLT3 inhibitors have been developed, the occurrence of secondary TKD mutations of FLT3 such FLT3/D835Y and FLT3/F691L lead to drug resistance and has become a key area of unmet medical needs. To overcome the obstacle of secondary TKD mutations, a new series of indirubin-3'-aminooxy-acetamide derivatives was discovered as potent and selective FLT3 and FLT3/D835Y inhibitors that were predicted to bind at the DFG-in active conformation of FLT3 in molecular docking studies. Through structure-activity relationship studies, the most optimized compound 13a was developed as a potent inhibitor at FLT3 and FLT3/D835Y with IC₅₀ values of 0.26 nM and 0.18 nM, respectively, which also displayed remarkably strong in vitro anticancer activities, with single-digit nanomolar GI₅₀ values for several AML (MV4-11 and MOLM14) and Ba/F3 cell lines expressed with secondary TKD mutated FLT3 kinases as well as FLT3-ITD. The selectivity profiles of compound 13a in the oncology kinase panel and various human cancer cell lines were prominent, demonstrating that its inhibitory activities were mainly focused on a few members of the receptor tyrosine kinase family and AML versus solid tumor cell lines. Furthermore, significant in vivo anticancer efficacy of compound 13a was confirmed in a xenograft animal model implanted with FLT3-ITD/D835Y-expressing MOLM-14 cells related to secondary TKD mutation.

MBP-11901 Inhibits Tumor Growth of Hepatocellular Carcinoma through Multitargeted Inhibition of Receptor Tyrosine Kinases

Simple Summary

Although various treatments such as surgery and chemotherapy exist for advanced or unresectable HCC, most patients suffer from intractable diseases, having a poor prognosis. While immunotherapy using immune checkpoint inhibitors was recently proposed for HCC, only a small percentage of patients respond. Thus, there remains an unmet need for the development of therapeutic agents for the treatment of liver cancer. Here, we presented multi-RTKi MBP-11901, an innovative targeted anticancer agent for HCC, suggesting it as a new therapeutic strategy for the treatment of liver cancer.

Abstract

Hepatocellular carcinomas (HCCs) are aggressive tumors with a poor prognosis. Approved first-line treatments include sorafenib, lenvatinib, and a combination of atezolizumab and bevacizumab; however, they do not cure HCC. We investigated MBP-11901 as a drug candidate for HCC. Cell proliferation and cytotoxicity were evaluated using normal and cancer human liver cell lines, while Western blotting and flow cytometry evaluated apoptosis. The anticancer effect of MBP-11901 was verified in vitro through migration, invasion, colony formation, and JC-1 MMP assays. In mouse models, the tumor volume, tumor weight, and bodyweight were measured, and cancer cell proliferation and apoptosis were analyzed. The toxicity of MBP-11901 was investigated through GOT/GPT and histological analyses in the liver and kidney. The signaling mechanism of MBP-11901 was investigated through kinase assays, phosphorylation analysis, and in silico docking simulations. Results. MBP-11901 was effective against various human HCC cell lines, leading to the disappearance of most tumors when administered orally in animal models. This effect was dose-dependent, with no differences in efficacy according to administration intervals. MBP-11901 induced anticancer effects by targeting the signaling mechanisms of FLT3, VEGFR2, c-KIT, and PDGFRβ. MBP-11901 is suggested as a novel therapeutic agent for the treatment of advanced or unresectable liver cancer.

Design, synthesis, and biological evaluation of novel pyrido-dipyrimidines as dual topoisomerase II/FLT3 inhibitors in leukemia cells

Dual inhibition of topoisomerase (topo) II and FLT3 kinase, as in the case of C-1311, was shown to overcome the shortcomings of using topo II inhibitors solely. In the present study, we designed and synthesized two series of pyrido-dipyrimidine- and pseudo-pyrido-acridone-containing compounds. The two series were evaluated against topo II and FLT3 as well as the HL-60 promyelocytic leukemia cell line in vitro. Compounds 6, 7, and 20 showed higher potency against topo II than the standard amsacrine (AMSA), whereas compounds 19 and 20 were stronger FLT3 inhibitors than the standard DACA. Compounds 19 and 20 showed to be dual inhibitors of both enzymes. Compounds 6, 7, 19, and 20 were more potent inhibitors of the HL-60 cell line than the standard AMSA. The results of the in vitro DNA flow cytometry analysis assay and Annexin V-FITC apoptosis analysis showed that 19 and 20 induced cell cycle arrest at the G2/M phase, significantly higher total percentage of apoptosis, and late-stage apoptosis in HL-60 cell lines than AMSA. Furthermore, 19 and 20 upregulated several apoptosis biomarkers such as p53, TNFα, caspase 3/7 and increased the Bax/Bcl-2 ratio. These results showed that 19 and 20 deserve further evaluation of their antiproliferative activities, particularly in leukemia. Molecular docking studies were performed for selected compounds against topo II and FLT3 enzymes to investigate their binding patterns. Compound 19 exerted dual fitting inside the active site of both enzymes.

Synthesis of 2H-Imidazo[2',1':2,3] [1,3]thiazolo[4,5-e]isoindol-8-yl-phenylureas with promising therapeutic features for the treatment of acute myeloid leukemia (AML) with FLT3/ITD mutations

Despite progressive advances in understanding the molecular biology of acute myeloid leukemia (AML), the conventional therapeutic approach has not changed substantially, and the outcome for most patients is poor. Thus, continuous efforts on the discovery of new compounds with improved features are required. Following a multistep sequence, we have identified a new tetracyclic ring system with strong antiproliferative activity towards several haematological cell lines. The new compounds possess structural properties typical of inactive-state-binding kinase inhibitors and are structurally related to quizartinib which is known as type-II tyrosine kinase inhibitor. In particular, the high activity found in two cell lines MOLM-13 and MV4-11, expressing the constitutively activated mutant FLT3/ITD, indicates inhibition of FLT3 kinase and on the basis of structure-activity relationship (SAR) the presence of an ureido moiety demonstrates to play a key role in driving the antiproliferative activity towards these cell lines. Molecular modelling studies supported the mechanism of recognition of the most active compounds within the FLT3 pocket where quizartinib binds. Moreover, Molecular Dynamics simulation (MDs) revealed the formation of a recurrent H-bond with Asp829, which more stabilizes the complex of 9c and the FLT3 inactive state. In MV4-11 cell line compound 9c reduces the phosphorylation of FLT3 (Y591) and some of its downstream targets leading to cell cycle arrest at G1 phase and induction of apoptosis. In an MV4-11 xenograft mouse model, 9c significantly reduces the tumor growth at the dose of 1-3 mg/kg without apparent toxicity.

Pyrimidine-fused Dinitrogenous Penta-heterocycles as a Privileged Scaffold for Anti-Cancer Drug Discovery

Pyrimidine-fused derivatives that are the inextricable part of DNA and RNA play a key role in the normal life cycle of cells. Pyrimidine-fused dinitrogenous penta-heterocycles including pyrazolopyrimidines and imidazopyrimidines is a special class of pyrimidine-fused compounds contributing to an important portion in anti-cancer drug discovery, which have been discovered as core structure for promising anti-cancer agents used in clinic or clinical evaluations. Pyrimidine-fused dinitrogenous penta-heterocycles have become one privileged scaffold for anti-cancer drug discovery. This review consists of the recent progress of pyrimidine-fused dinitrogenous penta-heterocycles as anti-cancer agents and their synthetic strategies. In addition, this review also summarizes some key structure-activity relationships (SARs) of pyrimidine-fused dinitrogenous penta-heterocycle derivatives as anti-cancer agents.

The molecular mechanisms behind activation of FLT3 in acute myeloid leukemia and resistance to therapy by selective inhibitors

Acute myeloid leukemia is an aggressive cancer, which, in spite of increasingly better understanding of its genetic background remains difficult to treat. Mutations in the FLT3 gene are observed in ≈30% of the patients. Most of these mutations are internal tandem duplications (ITDs) of a sequence within the protein coding region, an activation mechanism that is almost non-existent with other genes and cancers. As patients each carry their own unique set of mutations, it is challenging to understand how ITDs activate the protein, and ascertain the risk for each individual patient. Available treatment options are limited due to development of drug resistance. Here, recent studies are reviewed that help to better understand the molecular mechanism behind activation of the FLT3 protein due to mutations. It is argued that difference in mutation sequences and especially location might be coupled to prognosis. When it comes to FLT3 inhibitors, key differences between them can be attributed to the mode of inhibition (type-1 and type-2 inhibitors), effective inhibitory coefficient in the blood plasma and off-target binding. Accounting for the position and length of insertions may in the future be used to predict prognosis and rationalise treatment. Development of new inhibitors must take into account the potential for resistance mutations. Inhibitors aimed at multiple specific targets are currently being developed. These, and as well as combination therapies will hopefully lead to longer periods during which targeted FLT3 therapy will remain effective.

Discovery of imidazo[1,2-a]pyridine-thiophene derivatives as FLT3 and FLT3 mutants inhibitors for acute myeloid leukemia through structure-based optimization of an NEK2 inhibitor

FMS-like tyrosine kinase 3 (FLT3) with an internal tandem duplication (ITD) mutation has been validated as a driver lesion and a therapeutic target for acute myeloid leukemia (AML). Currently, several potent small-molecule FLT3 kinase inhibitors are being evaluated or have completed evaluation in clinical trials. However, many of these inhibitors are challenged by the secondary mutations on kinase domain, especially the point mutations at the activation loop (D835) and gatekeeper residue (F691). To overcome the resistance challenge, we identified a novel series of imidazo[1,2-a]pyridine-thiophene derivatives from a NIMA-related kinase 2 (NEK2) kinase inhibitor CMP3a, which retained inhibitory activities on FTL3-ITD^D835V and FLT3-ITD^F691L. Through this study, we identified the imidazo[1,2-a]pyridine-thiophene derivatives as type-I inhibitors of FLT3. Moreover, we observed compound 5o as an inhibitor displaying equal anti-proliferative activities against FLT3-ITD, FTL3-ITD^D835Y and FLT3-ITD^F691L driven acute myeloid leukemia (AML) cell lines. Meanwhile, the apoptotic effects of compound supported its mechanism of anti-proliferative action. These results indicate that the imidazo[1,2-a]pyridine-thiophene scaffold is promising for targeting acquired resistance caused by FLT3 secondary mutations and compound 5o is an interesting lead in this direction.

Molecular Modeling Studies of N-phenylpyrimidine-4-amine Derivatives for Inhibiting FMS-like Tyrosine Kinase-3

Overexpression and frequent mutations in FMS-like tyrosine kinase-3 (FLT3) are considered risk factors for severe acute myeloid leukemia (AML). Hyperactive FLT3 induces premature activation of multiple intracellular signaling pathways, resulting in cell proliferation and anti-apoptosis. We conducted the computational modeling studies of 40 pyrimidine-4,6-diamine-based compounds by integrating docking, molecular dynamics, and three-dimensional structure-activity relationship (3D-QSAR). Molecular docking showed that K644, C694, F691, E692, N701, D829, and F830 are critical residues for the binding of ligands at the hydrophobic active site. Molecular dynamics (MD), together with Molecular Mechanics Poison-Boltzmann/Generalized Born Surface Area, i.e., MM-PB(GB)SA, and linear interaction energy (LIE) estimation, provided critical information on the stability and binding affinity of the selected docked compounds. The MD study suggested that the mutation in the gatekeeper residue F691 exhibited a lower binding affinity to the ligand. Although, the mutation in D835 in the activation loop did not exhibit any significant change in the binding energy to the most active compound. We developed the ligand-based comparative molecular field analysis (CoMFA) and comparative molecular similarity index analysis (CoMSIA) models. CoMFA (q² = 0.802, r² = 0.983, and QF32 = 0.698) and CoMSIA (q² = 0.725, r² = 0.965 and QF32 = 0.668) established the structure-activity relationship (SAR) and showed a reasonable external predictive power. The contour maps from the CoMFA and CoMSIA models could explain valuable information about the favorable and unfavorable positions for chemical group substitution, which can increase or decrease the inhibitory activity of the compounds. In addition, we designed 30 novel compounds, and their predicted pIC₅₀ values were assessed with the CoMSIA model, followed by the assessment of their physicochemical properties, bioavailability, and free energy calculation. The overall outcome could provide valuable information for designing and synthesizing more potent FLT3 inhibitors.

Discovery and structure - activity relationship exploration of pyrazolo[1,5-a]pyrimidine derivatives as potent FLT3-ITD inhibitors

Internal tandem duplications of FLT3 (FLT3-ITD) occur in approximately 25% of all acute myeloid leukemia (AML) cases and confer a poor prognosis. Optimization of the screening hit 1 from our in-house compound library led to the discovery of a series of pyrazolo[1,5-a]pyrimidine derivatives as potent and selective FLT3-ITD inhibitors. Compounds 17 and 19 displayed potent FLT3-ITD activities both with IC₅₀ values of 0.4 nM and excellent antiproliferative activities against AML cell lines. Especially, compounds 17 and 19 inhibited the quizartinib resistance- conferring mutations, FLT3^D835Y, both with IC₅₀ values of 0.3 nM. Moreover, western blot analysis indicated that compounds 17 and 19 potently inhibited the phosphorylation of FLT3 and attenuated downstream signaling in AML cells. These results indicated that pyrazolo[1,5-a]pyrimidine derivatives could be promising FLT3-ITD inhibitors for the treatment AML.