A novel, dual pan-PIM/FLT3 inhibitor SEL24 exhibits broad therapeutic potential in acute myeloid leukemia

Leukemic Stem Cells and Hematological Malignancies

The association between leukemic stem cells (LSCs) and leukemia development has been widely established in the context of genetic alterations, epigenetic pathways, and signaling pathway regulation. Hematopoietic stem cells are at the top of the bone marrow hierarchy and can self-renew and progressively generate blood and immune cells. The microenvironment, niche cells, and complex signaling pathways that regulate them acquire genetic mutations and epigenetic alterations due to aging, a chronic inflammatory environment, stress, and cancer, resulting in hematopoietic stem cell dysregulation and the production of abnormal blood and immune cells, leading to hematological malignancies and blood cancer. Cells that acquire these mutations grow at a faster rate than other cells and induce clone expansion. Excessive growth leads to the development of blood cancers. Standard therapy targets blast cells, which proliferate rapidly; however, LSCs that can induce disease recurrence remain after treatment, leading to recurrence and poor prognosis. To overcome these limitations, researchers have focused on the characteristics and signaling systems of LSCs and therapies that target them to block LSCs. This review aims to provide a comprehensive understanding of the types of hematopoietic malignancies, the characteristics of leukemic stem cells that cause them, the mechanisms by which these cells acquire chemotherapy resistance, and the therapies targeting these mechanisms.

Pim Kinase Inhibitors Increase Gilteritinib Cytotoxicity in FLT3-ITD Acute Myeloid Leukemia Through GSK-3β Activation and c-Myc and Mcl-1 Proteasomal Degradation

Acute myeloid leukemia (AML) with fms-like tyrosine kinase 3 internal tandem duplication (FLT3-ITD) has poor outcomes. FLT3-ITD drives constitutive and aberrant FLT3 signaling, activating STAT5 and upregulating the downstream oncogenic serine/threonine kinase Pim-1. FLT3 inhibitors are in clinical use, but with limited and transient efficacy. We previously showed that concurrent treatment with Pim and FLT3 inhibitors increases apoptosis induction in FLT3-ITD-expressing cells through posttranslational downregulation of Mcl-1. Here we further elucidate the mechanism of action of this dual targeting strategy. Cytotoxicity, apoptosis and protein expression and turnover were measured in FLT3-ITD-expressing cell lines and AML patient blasts treated with the FLT3 inhibitor gilteritinib and/or the Pim inhibitors AZD1208 or TP-3654. Pim inhibitor and gilteritinib cotreatment increased apoptosis induction, produced synergistic cytotoxicity, downregulated c-Myc protein expression, earlier than Mcl-1, increased turnover of both proteins, which was rescued by proteasome inhibition, and increased efficacy and prolonged survival in an in vivo model. Gilteritinib and Pim inhibitor cotreatment of Ba/F3-ITD cells infected with T58A c-Myc or S159A Mcl-1 plasmids, preventing phosphorylation at these sites, did not downregulate these proteins, increase their turnover or increase apoptosis induction. Moreover, concurrent treatment with gilteritinib and Pim inhibitors dephosphorylated (activated) the serine/threonine kinase glycogen synthase kinase-3β (GSK-3β), and GSK-3β inhibition prevented c-Myc and Mcl-1 downregulation and decreased apoptosis induction. The data are consistent with c-Myc T58 and Mcl-1 S159 phosphorylation by activated GSK-3β as the mechanism of action of gilteritinib and Pim inhibitor combination treatment, further supporting GSK-3β activation as a therapeutic strategy in FLT3-ITD AML.

SIGNIFICANCE

FLT3-ITD is present in 25% of in AML, with continued poor outcomes. Combining Pim kinase inhibitors with the FDA-approved FLT3 inhibitor gilteritinib increases cytotoxicity in vitro and in vivo through activation of GSK-3β, which phosphorylates and posttranslationally downregulates c-Myc and Mcl-1. The data support efficacy of GSK-3β activation in FLT3-ITD AML, and also support development of a clinical trial combining the Pim inhibitor TP-3654 with gilteritinib.

Comprehensive Insights that Targeting PIM for Cancer Therapy: Prospects and Obstacles

Proviral integration sitea for Moloney-murine leukemia virus (PIM) kinases are a family of highly conserved serine/tyrosine kinases consisting of three members, PIM-1, PIM-2, and PIM-3. These kinases regulate a wide range of substrates through phosphorylation and affect key cellular processes such as transcription, translation, proliferation, apoptosis, and energy metabolism. Several PIM inhibitors are currently undergoing clinical trials, such as a phase I clinical trial of Uzanserti (5) for the treatment of relapsed diffuse large B-cell lymphoma that has been completed. The current focus encompasses the structural and biological characterization of PIM, ongoing research progress on small-molecule inhibitors undergoing clinical trials, and evaluation analysis of persisting challenges in this field. Additionally, the design and discovery of small-molecule inhibitors targeting PIM in recent years have been explored, with a particular emphasis on medicinal chemistry, aiming to provide valuable insights for the future development of PIM inhibitors.

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.

Dual FLT3/haspin kinase inhibitor based on 3H-pyrazolo[4,3-f]quinoline scaffold with activities against acute myeloid leukemia

The 3H-pyrazolo[4,3-f]quinoline core, a privileged fusion moiety from quinoline and indazole, facilely synthesized in a one flask multi-component Doebner-Povarov reaction, is a newly described kinase hinge binder. Previous works have demonstrated that the 3H-pyrazolo[4,3-f]quinoline moiety can be tuned, via judicious substitution patterns, to selectively inhibit cancer-associated kinases, such as FLT3 and haspin. A first generation 3H-pyrazolo[4,3-f]quinoline-based haspin inhibitor, HSD972, and FLT3 inhibitor, HSD1169, were previously disclosed as inhibitors of various cancer cell lines. Given the recent revelation that haspin is over-expressed and plays critical proliferative roles in many cancers, and compounds with dual activity against FLT3 and other important kinases are now being actively developed by many groups, we became interested in optimizing the 3H-pyrazolo[4,3-f]quinoline-based compounds to improve activity against both FLT3 and haspin. Herein, we report the discovery of new 3H-pyrazolo[4,3-f]quinoline-based dual FLT3/haspin inhibitor, HSK205. HSK205 has remarkable potencies against FLT3-driven AML cell lines, inhibiting proliferation with GI50 values between 2-25 nM. Western blot analyses of treated AML cells confirm that HSK205 inhibit the phosphorylation of both FLT3 and histone H3 (a haspin target) in cells. While multi-component reactions (MCRs) have been used to make many bioactive molecules, there are very few examples of using MCRs to make compounds that target protein kinases, which have emerged as one of the top drug candidates (especially in oncology). This work highlights our recent efforts to make ultrapotent protein kinase inhibitors using multi-component reactions (especially the Doebner-Povarov reaction).

NADPH oxidase mediated oxidative stress signaling in FLT3-ITD acute myeloid leukemia

The internal tandem duplication of the juxtamembrane domain of the FMS-like tyrosine kinase 3 (FLT3-ITD) is the most common genetic change in acute myeloid leukemia (AML), and about 30% of all AMLs harbor a FLT3-ITD mutation. Even though FLT3 inhibitors have displayed encouraging effects in FLT3-ITD-mutated AML, the extent of the clinical response to these compounds is cut short due to the rapid development of drug resistance. Evidence has shown that FLT3-ITD triggered activation of oxidative stress signaling may exert a pivotal role in drug resistance. The downstream pathways of FLT3-ITD, including STAT5, PI3K/AKT, and RAS/MAPK, are considered to be major oxidative stress signaling pathways. These downstream pathways can inhibit apoptosis and promote proliferation and survival by regulating apoptosis-related genes and promoting the generation of reactive oxygen species (ROS) through NADPH oxidase (NOX) or other mechanisms. Appropriate levels of ROS may promote proliferation, but high levels of ROS can lead to oxidative damage to the DNA and increase genomic instability. In addition, post-translational modifications of FLT3-ITD and changes in its subcellular localization can affect downstream signaling which may also be one of the mechanisms leading to drug resistance. In this review, we summarized the research progress on NOX mediated oxidative stress signaling and its relationship with drug resistance in FLT3-ITD AML, and discuss the possible new targets in FLT3-ITD signal blocking to reverse drug resistance in FLT3-ITD-mutated AML.

Discovery of a high potent PIM kinase inhibitor for acute myeloid leukemia based on N-pyridinyl amide scaffold by optimizing the fragments toward to Lys67 and Asp128/Glu171

Despite the recent development of PIM inhibitors based on N-(pyridin-3-yl)acetamide scaffold for acute myeloid leukemia (AML), the structural-activity relationship (SAR) associated with the effects of positional isomerization of N toward to Lys67 and freedom of solvent fragment toward to Asp128/Glu171 still remains an open question. In this work, a structurally novel compound based on N-pyridinyl amide was designed by fragment hybridization and then our SAR exploration revealed that the positional isomerization would lead to a decrease in activity, while increase of the freedom of solvent fragment by breaking the intramolecular hydrogen bond unprecedentedly leads to an increase in activity. These studies finally resulted in the screening out of a potent PIM inhibitor FD1024 (compound 24) which exerts strong antiproliferative activity against the tested AML cell lines and achieves profound antitumor efficacy in mice at well-tolerated dose schedules.

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.

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.

NF-κB: A Druggable Target in Acute Myeloid Leukemia

Acute Myeloid Leukemia (AML) is an aggressive hematological malignancy that relies on highly heterogeneous cytogenetic alterations. Although in the last few years new agents have been developed for AML treatment, the overall survival prospects for AML patients are still gloomy and new therapeutic options are still urgently needed. Constitutive NF-κB activation has been reported in around 40% of AML patients, where it sustains AML cell survival and chemoresistance. Given the central role of NF-κB in AML, targeting the NF-κB pathway represents an attractive strategy to treat AML. This review focuses on current knowledge of NF-κB's roles in AML pathogenesis and summarizes the main therapeutic approaches used to treat NF-κB-driven AML.

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.

Role of Biomarkers in FLT3 AML

Simple Summary

Genetically heterogeneous disorder acute myeloid leukemia (AML) is marked by recurring mutations in FLT3. Current FLT3 inhibitors and other emerging inhibitors have helped in the improvement of the quality of standard of care therapies; however, the overall survival of the patients remains static. This is due to numerous mutations in FLT3, which causes resistance against these FLT3 inhibitors. For effective treatment of AML patients, alternative approaches are required to overcome this resistance. Here, we will summarize the biomarkers for FLT3 inhibitors in AML, as well as the alternative measures to overcome resistance to the current therapies.

Abstract

Acute myeloid leukemia is a disease characterized by uncontrolled proliferation of clonal myeloid blast cells that are incapable of maturation to leukocytes. AML is the most common leukemia in adults and remains a highly fatal disease with a five-year survival rate of 24%. More than 50% of AML patients have mutations in the FLT3 gene, rendering FLT3 an attractive target for small-molecule inhibition. Currently, there are several FLT3 inhibitors in the clinic, and others remain in clinical trials. However, these inhibitors face challenges due to lack of efficacy against several FLT3 mutants. Therefore, the identification of biomarkers is vital to stratify AML patients and target AML patient population with a particular FLT3 mutation. Additionally, there is an unmet need to identify alternative approaches to combat the resistance to FLT3 inhibitors. Here, we summarize the current knowledge on the utilization of diagnostic, prognostic, predictive, and pharmacodynamic biomarkers for FLT3-mutated AML. The resistance mechanisms to various FLT3 inhibitors and alternative approaches to combat this resistance are also discussed and presented.

Inhibition of PIM Kinases in DLBCL Targets MYC Transcriptional Program and Augments the Efficacy of Anti-CD20 Antibodies

The family of PIM serine/threonine kinases includes three highly conserved oncogenes, PIM1, PIM2, and PIM3, which regulate multiple pro-survival pathways and cooperate with other oncogenes such as MYC. Recent genomic CRISPR-Cas9 screens further highlighted oncogenic functions of PIMs in diffuse large B cell lymphoma (DLBCL) cells, justifying development of small molecule PIM inhibitors and therapeutic targeting of PIM kinases in lymphomas. However, detailed consequences of PIM inhibition in DLBCL remain undefined. Using chemical and genetic PIM blockade, we comprehensively characterized PIM kinase-associated pro-survival functions in DLBCL and the mechanisms of PIM inhibition-induced toxicity. Treatment of DLBCL cells with SEL24/MEN1703, a pan PIM inhibitor in clinical development, decreased BAD phosphorylation and cap-dependent protein translation, reduced MCL1 expression, and induced apoptosis. PIM kinases were tightly coexpressed with MYC in diagnostic DLBCL biopsies, and PIM inhibition in cell lines and patient-derived primary lymphoma cells decreased MYC levels as well as expression of multiple MYC-dependent genes, including PLK1. Chemical and genetic PIM inhibition upregulated surface CD20 levels in a MYC-dependent fashion. Consistently, MEN1703 and other clinically available pan-PIM inhibitors synergized with the anti-CD20 monoclonal antibody rituximab in vitro, increasing complement-dependent cytotoxicity and antibody-mediated phagocytosis. Combined treatment with PIM inhibitor and rituximab suppressed tumor growth in lymphoma xenografts more efficiently than either drug alone. Taken together, these results show that targeting PIM in DLBCL exhibits pleiotropic effects that combine direct cytotoxicity with potentiated susceptibility to anti-CD20 antibodies, justifying further clinical development of such combinatorial strategies.

PIM Kinases in Multiple Myeloma

Multiple myeloma (MM) remains an incurable disease and novel therapeutic agents/approaches are urgently needed. The PIM (Proviral insertion in murine malignancies) serine/threonine kinases have 3 isoforms: PIM1, PIM2, and PIM3. PIM kinases are engaged with an expansive scope of biological activities including cell growth, apoptosis, drug resistance, and immune response. An assortment of molecules and pathways that are critical to myeloma tumorigenesis has been recognized as the downstream targets of PIM kinases. The inhibition of PIM kinases has become an emerging scientific interest for the treatment of multiple myeloma and several PIM kinase inhibitors, such as SGI-1776, AZD1208, and PIM447 (formerly LGH447), have been developed and are under different phases of clinical trials. Current research has been focused on the development of a new generation of potent PIM kinase inhibitors with appropriate pharmacological profiles reasonable for human malignancy treatment. Combination therapy of PIM kinase inhibitors with chemotherapeutic appears to create an additive cytotoxic impact in cancer cells. Notwithstanding, the mechanisms by which PIM kinases modulate the immune microenvironment and synergize with the immunomodulatory agents such as lenalidomide have not been deliberately depicted. This review provides a comprehensive overview of the PIM kinase pathways and the current research status of the development of PIM kinase inhibitors for the treatment of MM. Additionally, the combinatorial effects of the PIM kinase inhibitors with other targeted agents and the promising strategies to exploit PIM as a therapeutic target in malignancy are highlighted.

Novel Pan-Pim Kinase Inhibitors With Imidazopyridazine and Thiazolidinedione Structure Exert Potent Antitumor Activities

Pim kinases are overexpressed in various types of hematological malignancies and solid carcinomas, and promote cell proliferation and survival. Here in this study, we investigated the preclinical profile of novel pan-Pim kinase inhibitors with imidazopyridazine and thiazolidinedione structure. Imidazopyridazine-thiazolidinediones inhibited activities of Pim kinases with IC₅₀ values of tens to hundreds nanomolar. With YPC-21440 and/or YPC-21817, which exhibited especially high inhibitory activities against Pim kinases, we investigated in vitro and in vivo activities of imidazopyridazine-thiazolidinediones. In silico analysis of binding mode of YPC-21440 and Pim kinases revealed that it directly bound to ATP-binding pockets of Pim kinases. In the kinase panel tested, YPC-21440 and YPC-21817 were highly specific to Pim kinases. These compounds exerted antiproliferative activities against various cancer cell lines derived from hematological malignancies and solid carcinomas. Furthermore, they suppressed phosphorylation of Pim kinase substrates, arrested cell cycle at the G1 phase, and induced apoptosis in cultured cancer cells. In tumor xenograft models, YPC-21440 methanesulfonate and YPC-21817 methanesulfonate exerted antitumor activities. Furthermore, pharmacodynamic analysis with a xenograft model suggested that YPC-21817 methanesulfonate inhibited Pim kinases in tumors. In conclusion, our data revealed that imidazopyridazine-thiazolidinediones are novel Pim kinases inhibitors, effective on various types of cancer cell lines both in vitro and in vivo.

New Quinoxaline Derivatives as Dual Pim-1/2 Kinase Inhibitors: Design, Synthesis and Biological Evaluation

Proviral integration site for Moloney murine leukemia virus (Pim)-1/2 kinase overexpression has been identified in a variety of hematologic (e.g., multiple myeloma or acute myeloid leukemia (AML)) and solid (e.g., colorectal carcinoma) tumors, playing a key role in cancer progression, metastasis, and drug resistance, and is linked to poor prognosis. These kinases are thus considered interesting targets in oncology. We report herein the design, synthesis, structure–activity relationships (SAR) and in vitro evaluations of new quinoxaline derivatives, acting as dual Pim1/2 inhibitors. Two lead compounds (5c and 5e) were then identified, as potent submicromolar Pim-1 and Pim-2 inhibitors. These molecules were also able to inhibit the growth of the two human cell lines, MV4-11 (AML) and HCT-116 (colorectal carcinoma), expressing high endogenous levels of Pim-1/2 kinases.

Dual Kinase Targeting in Leukemia

Pharmacological cancer therapy is often based on the concurrent inhibition of different survival pathways to improve treatment outcomes and to reduce the risk of relapses. While this strategy is traditionally pursued only through the co-administration of several drugs, the recent development of multi-targeting drugs (i.e., compounds intrinsically able to simultaneously target several macromolecules involved in cancer onset) has had a dramatic impact on cancer treatment. This review focuses on the most recent developments in dual-kinase inhibitors used in acute myeloid leukemia (AML), chronic myelogenous leukemia (CML), and lymphoid tumors, giving details on preclinical studies as well as ongoing clinical trials. A brief overview of dual-targeting inhibitors (kinase/histone deacetylase (HDAC) and kinase/tubulin polymerization inhibitors) applied to leukemia is also given. Finally, the very recently developed Proteolysis Targeting Chimeras (PROTAC)-based kinase inhibitors are presented.

The role of Pim kinase in immunomodulation

Pim kinase, which has three isozymes (Pim-1, Pim-2 and Pim-3), is a serine/threonine kinase abnormally expressed in many cancers. High Pim kinase expression has been recognized to be associated with disease progression and prognosis. It is well accepted that Pim kinase is considered a clinical biomarker and potential therapeutic target for tumor cell. In recent years, researches verified the role of Pim kinase in immunomodulation. The mechanisms by which Pim kinase modulates the immune microenvironment and regulates immune cells, as well as the effects of Pim kinase inhibitors on immunity, have not been systematically described. This review comprehensively focuses on the current research status of Pim kinase pathways and the immune regulation.

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.

Mechanisms Underlying Resistance to FLT3 Inhibitors in Acute Myeloid Leukemia

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

Current perspectives on targeting PIM kinases to overcome mechanisms of drug resistance and immune evasion in cancer

PIM kinases are a class of serine/threonine kinases that play a role in several of the hallmarks of cancer including cell cycle progression, metabolism, inflammation and immune evasion. Their constitutively active nature and unique catalytic structure has led them to be an attractive anticancer target through the use of small molecule inhibitors. This review highlights the enhanced activity of PIM kinases in cancer that can be driven by hypoxia in the tumour microenvironment and the important role that aberrant PIM kinase activity plays in resistance mechanisms to chemotherapy, radiotherapy, anti-angiogenic therapies and targeted therapies. We highlight an interaction of PIM kinases with numerous major oncogenic players, including but not limited to, stabilisation of p53, synergism with c-Myc, and notable parallel signalling with PI3K/Akt. We provide a comprehensive overview of PIM kinase's role as an escape mechanism to targeted therapies including PI3K/mTOR inhibitors, MET inhibitors, anti-HER2/EGFR treatments and the immunosuppressant rapamycin, providing a rationale for co-targeting treatment strategies for a more durable patient response. The current status of PIM kinase inhibitors and their use as a combination therapy with other targeted agents, in addition to the development of novel multi-molecularly targeted single therapeutic agents containing a PIM kinase targeting moiety are discussed.

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

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

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.