Emergence of polyclonal FLT3 tyrosine kinase domain mutations during sequential therapy with sorafenib and sunitinib in FLT3-ITD-positive acute myeloid leukemia

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Perspectives and challenges of small molecule inhibitor therapy for FLT3-mutated acute myeloid leukemia

Acute myeloid leukemia (AML) is a heterogeneous clonal disease characterized overall by an aggressive clinical course. The underlying genetic abnormalities present in leukemic cells contribute significantly to the AML phenotype. Mutations in FMS-like tyrosine kinase 3 (FLT3) are one of the most common genetic abnormalities identified in AML, and the presence of these mutations strongly influences disease presentation and negatively impacts prognosis. Since mutations in FLT3 were identified in AML, they have been recognized as a valid therapeutic target resulting in decades of research to develop effective small molecule inhibitor treatment that could improve outcome for these patients. Despite the approval of several FLT3 inhibitors over the last couple of years, the treatment of patients with FLT3-mutated AML remains challenging and many questions still need to be addressed. This review will provide an up-to-date overview of our current understanding of FLT3-mutated AML and discuss what the current status is of the available FLT3 inhibitors for the day-to-day management of this aggressive disease.

Venetoclax Overcomes Sorafenib Resistance in Acute Myeloid Leukemia by Targeting BCL2

Sorafenib, a kinase inhibitor, has shown promising therapeutic efficacy in a subset of patients with acute myeloid leukemia (AML). However, despite its clinical effectiveness, sorafenib resistance is frequently observed in clinical settings, and the mechanisms underlying this resistance as well as effective strategies to overcome it remain unclear. We examined both single-cell and bulk transcription data in sorafenib-resistant and control AML patients and integrated a sorafenib resistance gene signature to predict the sensitivity of AML cells and the clinical outcomes of AML patients undergoing sorafenib therapy. In addition, our drug sensitivity analysis of scRNA-seq data using deconvolution methods showed that venetoclax was effective in targeting sorafenib-resistant AML cells. Mechanistically, sorafenib was found to activate the JAK-STAT3 pathway and upregulate BCL2 expression in sorafenib-resistant AML cells. This upregulation of BCL2 expression rendered the cells vulnerable to the BCL2 inhibitor venetoclax. In conclusion, we developed a platform to predict sorafenib resistance and clinical outcomes in AML patients after therapy. Our findings suggest that the combination of sorafenib and venetoclax could be an effective therapeutic strategy for AML treatment.

Resistance to targeted therapies in acute myeloid leukemia

The introduction of new targeted therapies to the treatment algorithm of acute myeloid leukemia (AML) offers new opportunities, but also presents new challenges. Patients diagnosed with AML receiving targeted therapies as part of lower intensity regimens will relapse inevitably due to primary or secondary resistance mechanisms. In this review, we summarize the current knowledge on the main mechanisms of resistance to targeted therapies in AML. Resistance to FLT3 inhibitors is mainly mediated by on target mutations and dysregulation of downstream pathways. Switching the FLT3 inhibitor has a potential therapeutic benefit. During treatment with IDH inhibitors resistance can develop due to aberrant cell metabolism or secondary site IDH mutations. As a unique resistance mechanism the mutated IDH isotype may switch from IDH1 to IDH2 or vice versa. Resistance to gemtuzumab-ozogamicin is determined by the CD33 isotype and the degradation of the cytotoxin. The main mechanisms of resistance to venetoclax are the dysregulation of alternative pathways especially the upregulation of the BCL-2-analogues MCL-1 and BCL-XL or the induction of an aberrant cell metabolism. The introduction of therapies targeting immune processes will lead to new forms of therapy resistance. Knowing those mechanisms will help to develop strategies that can overcome resistance to treatment.

Overcoming Resistance: FLT3 Inhibitors Past, Present, Future and the Challenge of Cure

FLT3 ITD and TKD mutations occur in 20% and 10% of Acute Myeloid Leukemia (AML), respectively, and they represent the target of the first approved anti-leukemic therapies in the 2000s. Type I and type II FLT3 inhibitors (FLT3i) are active against FLT3 TKD/ITD and FLT3 ITD mutations alone respectively, but they still fail remissions in 30-40% of patients due to primary and secondary mechanisms of resistance, with variable relapse rate of 30-50%, influenced by NPM status and FLT3 allelic ratio. Mechanisms of resistance to FLT3i have recently been analyzed through NGS and single cell assays that have identified and elucidated the polyclonal nature of relapse in clinical and preclinical studies, summarized here. Knowledge of tumor escape pathways has helped in the identification of new targeted drugs to overcome resistance. Immunotherapy and combination or sequential use of BCL2 inhibitors and experimental drugs including aurora kinases, menin and JAK2 inhibitors will be the goal of present and future clinical trials, especially in patients with FLT3-mutated (FLT3mut) AML who are not eligible for allogeneic transplantation.

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

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

“FLipping” the Story: FLT3-Mutated Acute Myeloid Leukemia and the Evolving Role of FLT3 Inhibitors

Simple Summary

Patients with acute myeloid leukemia (AML) may have a number of different mutations. Those with mutations in the FLT3 gene have a higher risk of relapse and death than those lacking these mutations. FLT3 is a key receptor on the surface of AML cells, which drives cell survival and growth. Although activation of this receptor is normally tightly controlled, in AML, FLT3 mutations allow it to activate itself, independent of external control. Over the past 5 years, a number of new drugs have been developed to specifically target these mutations. In this article, we discuss these drugs and their uses, as well as the mechanisms by which AML cells may gain resistance to them and how that resistance can be overcome.

Abstract

The treatment of many types of cancers, including acute myeloid leukemia (AML), has been revolutionized by the development of therapeutics targeted at crucial molecular drivers of oncogenesis. In contrast to broad, relatively indiscriminate conventional chemotherapy, these targeted agents precisely disrupt key pathways within cancer cells. FMS-like tyrosine kinase 3 (FLT3)—encoding a critical regulator of hematopoiesis—is the most frequently mutated gene in patients with AML, and these mutations herald reduced survival and increased relapse in these patients. Approximately 30% of newly diagnosed AML carries an FLT3 mutation; of these, approximately three-quarters are internal tandem duplication (ITD) mutations, and the remainder are tyrosine kinase domain (TKD) mutations. In contrast to its usual, tightly controlled expression, FLT3-ITD mutants allow constitutive, “run-away” activation of a large number of key downstream pathways which promote cellular proliferation and survival. Targeted inhibition of FLT3 is, therefore, a promising therapeutic avenue. In April 2017, midostaurin became both the first FLT3 inhibitor and the first targeted therapy of any kind in AML to be approved by the US FDA. The use of FLT3 inhibitors has continued to grow as clinical trials continue to demonstrate the efficacy of this class of agents, with an expanding number available for use as both experimental standard-of-care usage. This review examines the biology of FLT3 and its downstream pathways, the mechanism of FLT3 inhibition, the development of the FLT3 inhibitors as a class and uses of the agents currently available clinically, and the mechanisms by which resistance to FLT3 inhibition may both develop and be overcome.

Preclinical and Pilot Study of Type I FLT3 Tyrosine Kinase Inhibitor, Crenolanib, with Sorafenib in Acute Myeloid Leukemia and FLT3-Internal Tandem Duplication

PURPOSE

To evaluate the safety, activity, and emergence of FLT3-kinase domain (KD) mutations with combination therapy of crenolanib and sorafenib in acute myeloid leukemia (AML) with FLT3-internal tandem duplication (ITD).

PATIENTS AND METHODS

After in vitro and xenograft efficacy studies using AML cell lines that have FLT3-ITD with or without FLT3-KD mutation, a pilot study was performed with crenolanib (67 mg/m2/dose, three times per day on days 1-28) and two dose levels of sorafenib (150 and 200 mg/m2/day on days 8-28) in 9 pediatric patients with refractory/relapsed FLT3-ITD-positive AML. Pharmacokinetic, pharmacodynamic, and FLT3-KD mutation analysis were done in both preclinical and clinical studies.

RESULTS

The combination of crenolanib and sorafenib in preclinical models showed synergy without affecting pharmacokinetics of each agent, inhibited p-STAT5 and p-ERK for up to 8 hours, and led to significantly better leukemia response (P < 0.005) and survival (P < 0.05) compared with single agents. Fewer FLT3-KD mutations emerged with dose-intensive crenolanib (twice daily) and low-intensity sorafenib (three times/week) compared with daily crenolanib or sorafenib (P < 0.05). The crenolanib and sorafenib combination was tolerable without dose-limiting toxicities, and three complete remissions (one with incomplete count recovery) and one partial remission were observed in 8 evaluable patients. Median crenolanib apparent clearance showed a nonsignificant decrease during treatment (45.0, 40.5, and 20.3 L/hour/m2 on days 1, 7, and 14, respectively) without drug-drug interaction. Only 1 patient developed a FLT3-KD mutation (FLT3 F691L).

CONCLUSIONS

The combination of crenolanib and sorafenib was tolerable with antileukemic activities and rare emergence of FLT3-TKD mutations, which warrants further investigation.

Resistance to targeted therapies: delving into FLT3 and IDH

Recent advances in FLT3 and IDH targeted inhibition have improved response rates and overall survival in patients with mutations affecting these respective proteins. Despite this success, resistance mechanisms have arisen including mutations that disrupt inhibitor-target interaction, mutations impacting alternate pathways, and changes in the microenvironment. Here we review the role of these proteins in leukemogenesis, their respective inhibitors, mechanisms of resistance, and briefly ongoing studies aimed at overcoming resistance.

Refining AML Treatment: The Role of Genetics in Response and Resistance Evaluation to New Agents

Simple Summary

Acute myeloid leukemia (AML) is an aggressive cancer of the hematopoietic system. At present, we know that AML is heterogeneous and varies from one patient to another, often characterized by specific changes in the DNA (mutations). Likewise, we know that the mutational landscape of the disease predicts its response to certain therapies and that it can change under the influence of therapy. Since 2017, the number of potential drugs intended to treat AML has substantially increased and so has our knowledge about the role of certain mutations in the prediction of disease response, relapse and resistance. In this article, we review the current state of knowledge of genetic aberrations with respect to clinical decision making.

Abstract

The number of treatment options for acute myeloid leukemia (AML) has greatly increased since 2017. This development is paralleled by the broad implantation of genetic profiling as an integral part of clinical studies, enabling us to characterize mutation–response, mutation–non-response, or mutation–relapse patterns. The aim of this review is to provide a concise overview of the current state of knowledge with respect to newly approved AML treatment options and the association of response, relapse and resistance with genetic alterations. Specifically, we will highlight current genetic data regarding FLT3 inhibitors, IDH inhibitors, hypomethylating agents (HMA), the BCL-2 inhibitor venetoclax (VEN), the anti-CD33 antibody conjugate gemtuzumab ozogamicin (GO) and the liposomal dual drug CPX-351.

A Physiologically Based Pharmacokinetic and Drug-Drug Interaction Model for the CB2 Agonist Lenabasum

BACKGROUND AND OBJECTIVES

Lenabasum is a synthetic agonist of the cannabinoid receptor type 2 (CB2) with anti-inflammatory and antifibrotic properties. Utilizing Simcyp, we developed a physiologically based pharmacokinetic (PBPK) model based on physicochemical properties, cell culture data, and cytochrome P450 (CYP) phenotyping, inhibition, and induction data.

METHODS

Clinical data from healthy volunteers treated with 20 mg of lenabasum in a single ascending dose (SAD) study were used for model development. The model was verified using lenabasum SAD (10 and 40 mg) data as well as multiple dose (20 mg three times per day) data. Lenabasum is a CYP substrate, and the model predicted lenabasum clearance of 51% by CYP2C9, 37% by CYP2C8, and 12% by CYP3A4. Lenabasum is also an inhibitor of these isozymes.

RESULTS

The model accurately described the area under the plasma concentration-time curve (AUC) and maximum plasma concentration (C_max) for lenabasum within 1.19-fold and 1.25-fold accuracy, respectively, of the observed clinical values. The simulations of CYP inducers predicted that the strongest interaction would occur with rifampin, with the AUC decreasing to 0.36 of the control value, whereas the simulations of CYP inhibitors predicted that the greatest effect would occur with fluconazole, with a 1.43-fold increase in AUC.

CONCLUSIONS

Our model is a useful tool for predicting the pharmacokinetics of lenabasum and adjustments to its dosing in possible drug-drug interaction scenarios.

Recurrent Mutations in Cyclin D3 Confer Clinical Resistance to FLT3 Inhibitors in Acute Myeloid Leukemia

PURPOSE

Biomarkers of response and resistance to FLT3 tyrosine kinase inhibitors (TKI) are still emerging, and optimal clinical combinations remain unclear. The purpose of this study is to identify co-occurring mutations that influence clinical response to the novel FLT3 inhibitor pexidartinib (PLX3397).

EXPERIMENTAL DESIGN

We performed targeted sequencing of pretreatment blasts from 29 patients with FLT3 internal tandem duplication (ITD) mutations treated on the phase I/II trial of pexidartinib in relapsed/refractory FLT3-ITD+ acute myeloid leukemia (AML). We sequenced 37 samples from 29 patients with available material, including 8 responders and 21 non-responders treated at or above the recommended phase II dose of 3,000 mg.

RESULTS

Consistent with other studies, we identified mutations in NRAS, TP53, IDH2, and a variety of epigenetic and transcriptional regulators only in non-responders. Among the most frequently mutated genes in non-responders was Cyclin D3 (CCND3). A total of 3 individual mutations in CCND3 (Q276*, S264R, and T283A) were identified in 2 of 21 non-responders (one patient had both Q276* and S264R). No CCND3 mutations were found in pexidartinib responders. Expression of the Q276* and T283A mutations in FLT3-ITD MV4;11 cells conferred resistance to apoptosis, decreased cell-cycle arrest, and increased proliferation in the presence of pexidartinib and other FLT3 inhibitors. Inhibition of CDK4/6 activity in CCND3 mutant MV4;11 cells restored pexidartinib-induced cell-cycle arrest but not apoptosis.

CONCLUSIONS

Mutations in CCND3, a gene not commonly mutated in AML, are a novel cause of clinical primary resistance to FLT3 inhibitors in AML and may have sensitivity to CDK4/6 inhibition.

Single-cell DNA sequencing reveals complex mechanisms of resistance to quizartinib

Single-cell sequencing exposes previously unmeasurable complexity of tumor heterogeneity and clonal evolution on quizartinib. Single-cell sequencing reveals on- and off-target mechanisms of resistance to quizartinib, which can preexist therapy.

A phase 1/2 study of the oral FLT3 inhibitor pexidartinib in relapsed/refractory FLT3-ITD-mutant acute myeloid leukemia

FMS-like tyrosine kinase 3 (FLT3) tyrosine kinase inhibitors (TKIs) have activity in acute myeloid leukemia (AML) patients with FLT3 internal tandem duplication (ITD) mutations, but efficacy is limited by resistance-conferring kinase domain mutations. This phase 1/2 study evaluated the safety, tolerability, and efficacy of the oral FLT3 inhibitor PLX3397 (pexidartinib), which has activity against the FLT3 TKI-resistant F691L gatekeeper mutation in relapsed/refractory FLT3-ITD-mutant AML. Ninety patients were treated: 34 in dose escalation (part 1) and 56 in dose expansion (part 2). Doses of 800 to 5000 mg per day in divided doses were tested. No maximally tolerated dose was reached. Plasma inhibitory assay demonstrated that patients dosed with ≥3000 mg had sufficient levels of active drug in their trough plasma samples to achieve 95% inhibition of FLT3 phosphorylation in an FLT3-ITD AML cell line. Based on a plateau in drug exposure, the 3000-mg dose was chosen as the recommended phase 2 dose. The most frequently reported treatment-emergent adverse events were diarrhea (50%), fatigue (47%), and nausea (46%). Based on modified response criteria, the overall response rate to pexidartinib among all patients was 21%. Twenty-three percent of patients treated at ≥2000 mg responded. The overall composite complete response rate for the study was 11%. Six patients were successfully bridged to transplantation. Median overall survival (OS) of patients treated in dose expansion was 112 days (90% confidence interval [CI], 77-150 days), and median OS of responders with complete remission with or without recovery of blood counts was 265 days (90% CI, 170-422 days). This trial was registered at www.clinicaltrials.gov as #NCT01349049.

Gilteritinib is a clinically active FLT3 inhibitor with broad activity against FLT3 kinase domain mutations

Gilteritinib is the first FMS-like tyrosine kinase 3 (FLT3) tyrosine kinase inhibitor (TKI) approved as monotherapy in acute myeloid leukemia with FLT3 internal tandem duplication and D835/I836 tyrosine kinase domain (TKD) mutations. Sequencing studies in patients have uncovered less common, noncanonical (NC) mutations in FLT3 and have implicated secondary TKD mutations in FLT3 TKI resistance. We report that gilteritinib is active against FLT3 NC and TKI resistance-causing mutations in vitro. A mutagenesis screen identified FLT3 F691L, Y693C/N, and G697S as mutations that confer moderate resistance to gilteritinib in vitro. Analysis of patients treated with gilteritinib revealed that 2/9 patients with preexisting NC FLT3 mutations responded and that secondary TKD mutations are acquired in a minority (5/31) of patients treated with gilteritinib. Four of 5 patients developed F691L mutations (all treated at <200 mg). These studies suggest that gilteritinib has broad activity against FLT3 mutations and limited vulnerability to resistance-causing FLT3 TKD mutations, particularly when used at higher doses.

Maintenance therapy for FLT3-ITD-mutated acute myeloid leukemia

FLT3-ITD is a constitutively activated variant of the FLT3 tyrosine kinase receptor. Its expression in acute myeloid leukemia (AML) is associated with a poor prognosis. Due to this, the development of tyrosine kinase inhibitors (TKI) blocking FLT3-ITD became a rational therapeutic concept. This review describes key milestones in the clinical development of different FLT3-specific TKI with a particular focus on FLT3-TKI maintenance therapy in remission after allogeneic hematopoietic stem cell transplantation (HCT). Recent evidence from randomized trials using sorafenib in FLT3-ITD mutated AML provided a proof of concept that targeted post-HCT maintenance therapy could become a new treatment paradigm in AML.

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

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

TP-0903 is active in models of drug-resistant acute myeloid leukemia

Effective treatment for AML is challenging due to the presence of clonal heterogeneity and the evolution of polyclonal drug resistance. Here, we report that TP-0903 has potent activity against protein kinases related to STAT, AKT, and ERK signaling, as well as cell cycle regulators in biochemical and cellular assays. In vitro and in vivo, TP-0903 was active in multiple models of drug-resistant FLT3 mutant AML, including those involving the F691L gatekeeper mutation and bone marrow microenvironment–mediated factors. Furthermore, TP-0903 demonstrated preclinical activity in AML models with FLT3-ITD and common co-occurring mutations in IDH2 and NRAS genes. We also showed that TP-0903 had ex vivo activity in primary AML cells with recurrent mutations including MLL-PTD, ASXL1, SRSF2, and WT1, which are associated with poor prognosis or promote clinical resistance to AML-directed therapies. Our preclinical studies demonstrate that TP-0903 is a multikinase inhibitor with potent activity against multiple drug-resistant models of AML that will have an immediate clinical impact in a heterogeneous disease like AML.

TP-0903, a multikinase inhibitor, demonstrates preclinical activity in models of drug-resistant AML, including those involving FLT3 mutations, bone marrow microenvironment-mediated factors and recurrent mutations.

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

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

Therapeutic targeting of FLT3 and associated drug resistance in acute myeloid leukemia

Acute myeloid leukemia (AML) is a heterogeneous disease caused by several gene mutations and cytogenetic abnormalities affecting differentiation and proliferation of myeloid lineage cells. FLT3 is a receptor tyrosine kinase commonly overexpressed or mutated, and its mutations are associated with poor prognosis in AML. Although aggressive chemotherapy often followed by hematopoietic stem cell transplant is the current standard of care, the recent approval of FLT3-targeted drugs is revolutionizing AML treatment that had remained unchanged since the 1970s. However, despite the dramatic clinical response to targeted agents, such as FLT3 inhibitors, remission is almost invariably short-lived and ensued by relapse and drug resistance. Hence, there is an urgent need to understand the molecular mechanisms driving drug resistance in order to prevent relapse. In this review, we discuss FLT3 as a target and highlight current understanding of FLT3 inhibitor resistance.

Molecular Mechanisms of Resistance to FLT3 Inhibitors in Acute Myeloid Leukemia: Ongoing Challenges and Future Treatments

Treatment of FMS-like tyrosine kinase 3 (FLT3)-internal tandem duplication (ITD)-positive acute myeloid leukemia (AML) remains a challenge despite the development of novel FLT3-directed tyrosine kinase inhibitors (TKI); the relapse rate is still high even after allogeneic stem cell transplantation. In the era of next-generation FLT3-inhibitors, such as midostaurin and gilteritinib, we still observe primary and secondary resistance to TKI both in monotherapy and in combination with chemotherapy. Moreover, remissions are frequently short-lived even in the presence of continuous treatment with next-generation FLT3 inhibitors. In this comprehensive review, we focus on molecular mechanisms underlying the development of resistance to relevant FLT3 inhibitors and elucidate how this knowledge might help to develop new concepts for improving the response to FLT3-inhibitors and reducing the development of resistance in AML. Tailored treatment approaches that address additional molecular targets beyond FLT3 could overcome resistance and facilitate molecular responses in AML.

Prediction of drug response in multilayer networks based on fusion of multiomics data

Predicting the response of each individual patient to a drug is a key issue assailing personalized medicine. Our study predicted drug response based on the fusion of multiomics data with low-dimensional feature vector representation on a multilayer network model. We named this new method DREMO (Drug Response prEdiction based on MultiOmics data fusion). DREMO fuses similarities between cell lines and similarities between drugs, thereby improving the ability to predict the response of cancer cell lines to therapeutic agents. First, a multilayer similarity network related to cell lines and drugs was constructed based on gene expression profiles, somatic mutation, copy number variation (CNV), drug chemical structures, and drug targets. Next, low-dimensional feature vector representation was used to fuse the biological information in the multilayer network. Then, a machine learning model was applied to predict new drug-cell line associations. Finally, our results were validated using the well-established GDSC/CCLE databases, literature, and the functional pathway database. Furthermore, a comparison was made between DREMO and other methods. Results of the comparison showed that DREMO improves predictive capabilities significantly.

Secondary Resistant Mutations to Small Molecule Inhibitors in Cancer Cells

Secondary resistant mutations in cancer cells arise in response to certain small molecule inhibitors. These mutations inevitably cause recurrence and often progression to a more aggressive form. Resistant mutations may manifest in various forms. For example, some mutations decrease or abrogate the affinity of the drug for the protein. Others restore the function of the enzyme even in the presence of the inhibitor. In some cases, resistance is acquired through activation of a parallel pathway which bypasses the function of the drug targeted pathway. The Catalogue of Somatic Mutations in Cancer (COSMIC) produced a compendium of resistant mutations to small molecule inhibitors reported in the literature. Here, we build on these data and provide a comprehensive review of resistant mutations in cancers. We also discuss mechanistic parallels of resistance.

High incidence of FLT3 mutations in follicular thyroid cancer: potential therapeutic target in patients with advanced disease stage

Background:

Conventional treatments for follicular thyroid cancer (FTC) can be ineffective, leading to poor prognosis. The aim of this study was to identify mutations associated with FTC that would serve as novel molecular markers of the disease and its outcome and could potentially identify new therapeutic targets.

Methods:

FLT3 mutations were first detected in a 29-year-old White female diagnosed with metastasized, treatment-refractory FTC. Analyses of FLT3 mutational status through next-generation sequencing of formalin-fixed, paraffin-embedded FTC specimens were subsequently performed in 35 randomly selected patients diagnosed with FTC.

Results:

FLT3 mutations were found in 69% of patients. FLT3 mutation-positive patients were significantly older than those that were FLT3 mutation-negative [median age at diagnosis 54 (36–82) versus 45 (27–58) (p = 0.023)]. Patients over 60 years were 23 times more likely to be FLT3 mutation-positive (p = 0.006). However, the number of FLT3 mutations did not correlate with age (r-Pearson: –0.244, p-value: 0.25). A total of 26 mutations were identified in the FLT3 gene with 2–16 FLT3 mutations in each FLT3 mutation-positive patient (mean: 5.6 mutations/patient). Tyrosine kinase domain (TKD) mutations in the FLT3 gene were detected in 58% of FLT3 mutation-positive patients. All FLT3 mutation-positive patients with a disease stage of pT2N1 or worse harbored at least one mutation in the TKD of FLT3.

Conclusions:

There is a wide spectrum and high frequency of FLT3 mutations in FTC. The precise role of FLT3 mutations in the genesis of FTC, as well as its potential role as a therapeutic target, requires further investigation.

Nicotinamide-Ponatinib Analogues as Potent Anti-CML and Anti-AML Compounds

Ponatinib is a multikinase inhibitor that is used to treat chronic myeloid leukemia patients harboring mutated ABL1(T315I) kinase. Due to the potent inhibition of FLT3, RET, and fibroblast growth factor receptors (FGFRs), it is also being evaluated against acute myeloid leukemia (AML), biliary, and lung cancers. The multikinase inhibition profile of ponatinib may also account for its toxicity, thus analogs with improved kinase selectivity or different kinase inhibition profiles could be better tolerated. The introduction of nitrogen into drug compounds can enhance efficacy and drug properties (a concept called "necessary nitrogen"). Here, we introduce additional nitrogen into the benzamide moiety of ponatinib to arrive at nicotinamide analogs. A nicotinamide analogue of ponatinib, HSN748, retains activity against FLT3, ABL1, RET, and PDGFRα/β but loses activity against c-Src and P38α. MNK1 and 2 are key kinases that phosphorylate eIF4E to regulate the protein translation complex. MNK also modulates mTORC1 signaling and contributes to rapamycin resistance. Inhibitors of MNK1 and 2 are being evaluated for anticancer therapy. Ponatinib is not a potent inhibitor of MNK1 or 2, but the nicotinamide analogs are potent inhibitors of MNKs. This illustrates a powerful demonstration of the necessary nitrogen concept to alter both the potency and selectivity of drugs.

Preclinical efficacy for a novel tyrosine kinase inhibitor, ArQule 531 against acute myeloid leukemia

BACKGROUND

Acute myeloid leukemia (AML) is the most common type of adult leukemia. Several studies have demonstrated that oncogenesis in AML is enhanced by kinase signaling pathways such as Src family kinases (SFK) including Src and Lyn, spleen tyrosine kinase (SYK), and bruton's tyrosine kinase (BTK). Recently, the multi-kinase inhibitor ArQule 531 (ARQ 531) has demonstrated potent inhibition of SFK and BTK that translated to improved pre-clinical in vivo activity as compared with the irreversible BTK inhibitor ibrutinib in chronic lymphocytic leukemia (CLL) models. Given the superior activity of ARQ 531 in CLL, and recognition that this molecule has a broad kinase inhibition profile, we pursued its application in pre-clinical models of AML.

METHODS

The potency of ARQ 531 was examined in vitro using FLT3 wild type and mutated (ITD) AML cell lines and primary samples. The modulation of pro-survival kinases following ARQ 531 treatment was determined using AML cell lines. The effect of SYK expression on ARQ 531 potency was evaluated using a SYK overexpressing cell line (Ba/F3 murine cells) constitutively expressing FLT3-ITD. Finally, the in vivo activity of ARQ 531 was evaluated using MOLM-13 disseminated xenograft model.

RESULTS

Our data demonstrate that ARQ 531 treatment has anti-proliferative activity in vitro and impairs colony formation in AML cell lines and primary AML cells independent of the presence of a FLT3 ITD mutation. We demonstrate decreased phosphorylation of oncogenic kinases targeted by ARQ 531, including SFK (Tyr416), BTK, and fms-related tyrosine kinase 3 (FLT3), ultimately leading to changes in down-stream targets including SYK, STAT5a, and ERK1/2. Based upon in vitro drug synergy data, we examined ARQ 531 in the MOLM-13 AML xenograft model alone and in combination with venetoclax. Despite ARQ 531 having a less favorable pharmacokinetics profile in rodents, we demonstrate modest single agent in vivo activity and synergy with venetoclax.

CONCLUSIONS

Our data support consideration of the application of ARQ 531 in combination trials for AML targeting higher drug concentrations in vivo.

The growing landscape of FLT3 inhibition in AML

Midostaurin and gilteritinib are FLT3 inhibitors that have been recently approved for use in FLT3-mutant acute myeloid leukemia (AML). These approved drugs represent a new standard of care for patients with FLT3 mutations in both the first-line and salvage settings. The success of midostaurin used in combination with induction chemotherapy has prompted exploration of newer, more potent and targeted inhibitors (including gilteritinib) in the first-line setting in combination with chemotherapy. At the same time, the success of gilteritinib and other newer FLT3 inhibitors as monotherapy in the salvage setting has been tempered by the development of resistance because of diverse mechanisms. Investigational strategies that incorporate FLT3 inhibitors in combination with hypomethylating agents and as maintenance therapy after allogeneic stem cell transplantation have shown promise. Other novel combination strategies are also undergoing clinical investigation. In this article, we review the current landscape of approved and investigational FLT3 inhibitors in AML, including the current standard of care and investigational strategies.

Study on the mechanism and intervention strategy of sunitinib induced nephrotoxicity

Sunitinib is an oral small molecular tyrosine kinase inhibitor that exhibits potent antiangiogenic and antitumor activity. Unfortunately, sunitinib kidney toxicity limits its clinical use. Renal injury caused by sunitinib treatment can not only lead to the failure of cancer treatment, but also jeopardizes the health and life of patients. Currently, there is no better intervention measure for renal injury caused by sunitinib therapy except reducing the dosage or stopping the medication. In this study, we learned from clinical case report that sunitinib can cause severe renal injury. Subsequently, we compiled the clinical trials data of sunitinib found that sunitinib can cause general renal damage. Based on this finding, we conducted a study on the mechanism of sunitinib-induced renal injury. The results showed that sunitinib can inhibit the survival of HK-2 cells (human tubule epithelial cells) in a dose- and time-dependent manner. The survival inhibition is mainly due to the activation apoptotic signaling pathway by sunitinib in HK-2 cells and induces apoptosis of HK-2 cells. Subsequently, we found that natural compound oxypeucedanin can significantly alleviate the apoptosis of HK-2 cells induced by sunitinib. Through clinical investigation and experimental study of sunitinib, we found that sunitinib can cause extensive renal damage by inducing apoptosis of renal tubular epithelial cells and natural compound oxypeucedanin is a potentially effective intervention for nephrotoxicity of sunitinib. Thus, our research will provide a theoretical basis for the future rational use of sunitinib and the search for appropriate interventions for sunitinib-induced kidney damage.

Polyclonal Heterogeneity: The New Norm for Secondary Clinical Resistance to Targeted Monotherapy in Relapsed Leukemia?

In this issue, McMahon and colleagues demonstrate that secondary clinical resistance to the FLT3 inhibitor gilteritinib in relapsed acute myeloid leukemia is often polyclonal and commonly mediated by heterogeneous mutations that activate downstream RAS-MAPK pathways. These findings and recent data from others indicate that emergence of multiple clones, each with distinct mechanisms of resistance, is a common finding at secondary failure of single-agent-targeted therapies for relapsed leukemias.See related article by McMahon et al., p. 1050.

Comparison of effects of midostaurin, crenolanib, quizartinib, gilteritinib, sorafenib and BLU-285 on oncogenic mutants of KIT, CBL and FLT3 in haematological malignancies

Mutations in two type-3 receptor tyrosine kinases (RTKs), KIT and FLT3, are common in both acute myeloid leukaemia (AML) and systemic mastocytosis (SM) and lead to hyperactivation of key signalling pathways. A large number of tyrosine kinase inhibitors (TKIs) have been developed that target either FLT3 or KIT and significant clinical benefit has been demonstrated in multiple clinical trials. Given the structural similarity of FLT3 and KIT, it is not surprising that some of these TKIs inhibit both of these receptors. This is typified by midostaurin, which has been approved by the US Food and Drug Administration for mutant FLT3-positive AML and for KIT D816V-positive SM. Here, we compare the in vitro activities of the clinically available FLT3 and KIT inhibitors with those of midostaurin against a panel of cells expressing a variety of oncogenic FLT3 or KIT receptors, including wild-type (wt) FLT3, FLT3-internal tandem duplication (ITD), FLT3 D835Y, the resistance mutant FLT3-ITD+ F691L, KIT D816V, and KIT N822K. We also examined the effects of these inhibitors in vitro and in vivo on cells expressing mutations in c-CBL found in AML that result in hypersensitization of RTKs, such as FLT3 and KIT. The results show a wide spectrum of activity of these various mutations to these clinically available TKIs.

Preclinical activity and a pilot phase I study of pacritinib, an oral JAK2/FLT3 inhibitor, and chemotherapy in FLT3-ITD-positive AML

Activating FLT3 internal tandem duplication (FLT3-ITD) mutations in acute myeloid leukemia (AML) associate with inferior outcomes. We determined that pacritinib, a JAK2/FLT3 inhibitor, has in vitro activity against FLT3-ITD and tyrosine kinase domain (TKD) mutations. Therefore, we conducted a phase I study of pacritinib in combination with chemotherapy in AML patients with FLT3 mutations to determine the pharmacokinetics and preliminary toxicity and clinical activity. Pacritinib was administered at a dose of 100 mg or 200 mg twice daily following a 3 + 3 dose-escalation in combination with cytarabine and daunorubicin (cohort A) or with decitabine induction (cohort B). A total of thirteen patients were enrolled (five in cohort A; eight in cohort B). Dose limiting toxicities include hemolytic anemia and grade 3 QTc prolongation in two patients who received 100 mg. Complete remission was achieved in two patients in cohort A, one of whom had a minor D835Y clone at baseline. One patient in cohort B achieved morphologic leukemia free state. Seven patients (two in cohort A; five in cohort B) had stable disease. In conclusion, pacritinib, an inhibitor of FLT3-ITD and resistant-conferring TKD mutations, was well tolerated and demonstrated preliminary anti-leukemic activity in combination with chemotherapy in patients with FLT3 mutations.

Sorafenib Therapy Is Associated with Improved Outcomes for FMS-like Tyrosine Kinase 3 Internal Tandem Duplication Acute Myeloid Leukemia Relapsing after Allogeneic Hematopoietic Stem Cell Transplantation

The optimal therapy for patients with acute myeloid leukemia (AML) with FMS-like tyrosine kinase 3 internal tandem duplication (FLT3-ITD) who relapse after allogeneic hematopoietic stem cell transplantation (allo-HSCT) remains unclear. In this study we retrospectively evaluated the efficacy of sorafenib combined with other therapeutic strategies as salvage therapy for these patients. Eighty-three AML patients with FLT3-ITD relapsing after allo-HSCT were enrolled in this study. Fifty-three patients received salvage therapy containing sorafenib and 30 patients did not. Salvage therapy containing sorafenib was superior to that without sorafenib with respect to complete remission rates, overall survival (OS), and progression-free survival (PFS) (66.0% versus 30.0%, 46.8% versus 20.0%, and 44.9% versus 16.7%, respectively; P = .002, P = .003, and P = .001). Further subgroup analysis revealed that the OS and PFS of patients who received sorafenib combined with chemotherapy followed by donor lymphocyte infusion (DLI) were superior to those receiving other therapeutic regimens, including sorafenib combined with chemotherapy, chemotherapy followed by DLI, and monochemotherapy (P = .003, P < .001). Multivariate analysis revealed that salvage therapy including sorafenib was the only protective factor for longer OS (P = .035; hazard ratio [HR], .526); salvage therapy including sorafenib and DLI were the protective factors for longer PFS (P = .011, HR, .423; P = .019, HR, .508). Our data suggest that sorafenib therapy is associated with improved outcomes for FLT3-ITD AML relapsing after allo-HSCT, and whether sorafenib combined with chemotherapy followed by DLI reveals an optimal efficacy merits further study.

Amino alkynylisoquinoline and alkynylnaphthyridine compounds potently inhibit acute myeloid leukemia proliferation in mice

BACKGROUND

Acute myeloid leukemia (AML) remains one of the most lethal, rarely cured cancers, despite decades of active development of AML therapeutics. Currently, the 5-year survival of AML patients is about 30% and for elderly patients, the rate drops to <10%. About 30% of AML patients harbor an activating mutation in the tyrosine kinase domain (TKD) of Fms-Like Tyrosine kinase 3 (FLT3) or a FLT3 internal tandem duplication (FLT3-ITD). Inhibitors of FLT3, such as Rydapt that was recently approved by the FDA, have shown good initial response but patients often relapse due to secondary mutations in the FLT3 TKD, like D835Y and F691 L mutations.

METHODS

Alkynyl aminoisoquinoline and naphthyridine compounds were synthesized via Sonogashira coupling. The compounds were evaluated for their in vitro and in vivo effects on leukemia growth.

FINDINGS

The compounds inhibited FLT3 kinase activity at low nanomolar concentrations. The lead compound, HSN431, also inhibited Src kinase activity. The compounds potently inhibited the viability of MV4-11 and MOLM-14 AML cells with IC50 values <1 nM. Furthermore, the viability of drug-resistant AML cells harboring the D835Y and F691 L mutations were potently inhibited. In vivo efficacy studies in mice demonstrated that the compounds could drastically reduce AML proliferation in mice.

INTERPRETATION

Compounds that inhibit FLT3 and downstream targets like Src (for example HSN431) are good leads for development as anti-AML agents. FUND: Purdue University, Purdue Institute for Drug Discovery (PIDD), Purdue University Center for Cancer Research, Elks Foundation and NIH P30 CA023168.

Clinical resistance to crenolanib in acute myeloid leukemia due to diverse molecular mechanisms

FLT3 mutations are prevalent in AML patients and confer poor prognosis. Crenolanib, a potent type I pan-FLT3 inhibitor, is effective against both internal tandem duplications and resistance-conferring tyrosine kinase domain mutations. While crenolanib monotherapy has demonstrated clinical benefit in heavily pretreated relapsed/refractory AML patients, responses are transient and relapse eventually occurs. Here, to investigate the mechanisms of crenolanib resistance, we perform whole exome sequencing of AML patient samples before and after crenolanib treatment. Unlike other FLT3 inhibitors, crenolanib does not induce FLT3 secondary mutations, and mutations of the FLT3 gatekeeper residue are infrequent. Instead, mutations of NRAS and IDH2 arise, mostly as FLT3-independent subclones, while TET2 and IDH1 predominantly co-occur with FLT3-mutant clones and are enriched in crenolanib poor-responders. The remaining patients exhibit post-crenolanib expansion of mutations associated with epigenetic regulators, transcription factors, and cohesion factors, suggesting diverse genetic/epigenetic mechanisms of crenolanib resistance. Drug combinations in experimental models restore crenolanib sensitivity.

FLT3 is commonly mutated in acute myeloid leukaemia and treatment with FLT3 inhibitors often ends with relapse. Here, the authors perform exome sequencing of samples from patients treated with the FLT3 inhibitor, crenolanib, to show that resistance occurs due to diverse molecular mechanisms, not primarily due to secondary FLT3 mutations.

Targeted Therapies for Pediatric AML: Gaps and Perspective

Acute myeloid leukemia (AML) is a hematopoietic disorder characterized by numerous cytogenetic and molecular aberrations that accounts for ~25% of childhood leukemia diagnoses. The outcome of children with AML has increased remarkably over the past 30 years, with current survival rates up to 70%, mainly due to intensification of standard chemotherapy and improvements in risk classification, supportive care, and minimal residual disease monitoring. However, childhood AML prognosis remains unfavorable and relapse rates are still around 30%. Therefore, novel therapeutic approaches are needed to increase the cure rate. In AML, the presence of gene mutations and rearrangements prompted the identification of effective targeted molecular strategies, including kinase inhibitors, cell pathway inhibitors, and epigenetic modulators. This review will discuss several new drugs that recently received US Food and Drug Administration approval for AML treatment and promising strategies to treat childhood AML, including FLT3 inhibitors, epigenetic modulators, and Hedgehog pathway inhibitors.

P-glycoprotein (MDR1/ABCB1) and Breast Cancer Resistance Protein (BCRP/ABCG2) limit brain accumulation of the FLT3 inhibitor quizartinib in mice

Quizartinib, a second-generation FLT3 inhibitor, is in clinical development for the treatment of acute myeloid leukemia. We studied its pharmacokinetic interactions with the multidrug efflux transporters ABCB1 and ABCG2 and the multidrug metabolizing enzyme CYP3A, using in vitro transport assays and knockout and transgenic mouse models. Quizartinib was transported by human ABCB1 in vitro, and by mouse (m)Abcb1 and mAbcg2 in vivo. Upon oral administration, the brain accumulation of quizartinib was 6-fold decreased by mAbcb1 and 2-fold by mAbcg2 (together: 12-fold). Unexpectedly, the absence of mAbcb1 resulted in a ∼2-fold lower plasma exposure in Abcb1a/1b^-/- and Abcb1a/1b;Abcg2^-/- mice, suggesting that loss of mAbcb1 causes compensatory alterations in alternative quizartinib elimination or uptake systems. mAbcb1 and mAbcg2 themselves did not appear to restrict quizartinib oral availability. Oral and intravenous pharmacokinetics of quizartinib were not substantially altered between wild-type, Cyp3a knockout and CYP3A4-humanized mice. All three strains showed relatively high (33-51%) oral bioavailability. If this also applies in humans, this would suggest a limited risk of CYP3A-related inter-individual variation in exposure for this drug. Our results provide a possible rationale for using pharmacological ABCB1/ABCG2 inhibitors together with quizartinib when treating malignant lesions situated in part or in whole behind the blood-brain barrier.

Discontinuation of sorafenib can lead to the emergence of FLT3-ITD-positive acute myeloid leukemia

A 69-year-old woman who had been diagnosed with unresectable papillary thyroid cancer was referred to our hospital. We initially treated her with sorafenib, but she subsequently developed erythema multiforme, which was suspected to be a drug rush due to sorafenib; therefore, sorafenib was discontinued. At the time of discontinuation, immature blast cells were detected in her peripheral blood. Approximately two weeks later, her skin rash improved substantially, but the proportion of blasts in the peripheral blood increased. We performed a bone marrow examination, and she was diagnosed with FLT3-ITD-positive acute myeloid leukemia. FLT3-ITD expression is found in 20-25% of AML and is a known independent poor prognostic factor. To overcome the poor prognosis associated with FLT3-ITD, molecular drugs targeting FLT3-ITD are attracting much attention. Sorafenib, a multi-kinase inhibitor, also has an effect on FLT3-ITD. Although primary disease flares after tyrosine kinase inhibitor discontinuation have been reported, this is the first report to describe discontinuation of sorafenib treatment as a potential trigger of FLT3-ITD-positive acute myeloid leukemia in papillary thyroid cancer.

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.

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

The identification of recurrent driver mutations in genes encoding tyrosine kinases has resulted in the development of molecularly-targeted treatment strategies designed to improve outcomes for patients diagnosed with acute myeloid leukemia (AML). The receptor tyrosine kinase FLT3 is the most commonly mutated gene in AML, with internal tandem duplications within the juxtamembrane domain (FLT3-ITD) or missense mutations in the tyrosine kinase domain (FLT3-TKD) present in 30–35% of AML patients at diagnosis. An established driver mutation and marker of poor prognosis, the FLT3 tyrosine kinase has emerged as an attractive therapeutic target, and thus, encouraged the development of FLT3 tyrosine kinase inhibitors (TKIs). However, the therapeutic benefit of FLT3 inhibition, particularly as a monotherapy, frequently results in the development of treatment resistance and disease relapse. Commonly, FLT3 inhibitor resistance occurs by the emergence of secondary lesions in the FLT3 gene, particularly in the second tyrosine kinase domain (TKD) at residue Asp835 (D835) to form a ‘dual mutation’ (ITD-D835). Individual FLT3-ITD and FLT3-TKD mutations influence independent signaling cascades; however, little is known about which divergent signaling pathways are controlled by each of the FLT3 specific mutations, particularly in the context of patients harboring dual ITD-D835 mutations. This review provides a comprehensive analysis of the known discrete and cooperative signaling pathways deregulated by each of the FLT3 specific mutations, as well as the therapeutic approaches that hold the most promise of more durable and personalized therapeutic approaches to improve treatments of FLT3 mutant AML.

Arsenic Trioxide and Sorafenib Induce Synthetic Lethality of FLT3-ITD Acute Myeloid Leukemia Cells

Acute myeloid leukemia (AML) with Fms-related tyrosine kinase 3 internal tandem duplication (FLT3-ITD) mutation is notoriously hard to treat. We identified two drugs that together form an effective combination therapy against FLT3-ITD AML. One of the drugs, Sorafenib, an inhibitor of FLT3-ITD and other kinase activity, produces an impressive but short-lived remission in FLT3-ITD AML patients. The second, arsenic trioxide (ATO), at therapeutically achievable concentrations, reduces the level of FLT3-ITD and Mcl-1 proteins, and induces apoptosis in leukemic cell lines and in primary cells expressing FLT3-ITD. We linked this relative sensitivity to ATO to low levels of reduced glutathione. While producing proapoptotic effects, ATO treatment also has an unwanted effect whereby it causes the accumulation of the phosphorylated (inactive) form of glycogen synthase kinase 3β (GSK3β), a kinase necessary for apoptosis. When ATO is combined with Sorafenib, GSK3β is activated, Mcl-1 is further reduced, and proapoptotic proteins Bak and Bax are activated. Mice xenografted with FLT3-ITD MOLM13 cell line treated with the Sorafenib/ATO combination have significantly improved survival. This combination has potential to improve the therapeutic outcome of FLT3-ITD-targeted therapy of AML patients. Mol Cancer Ther; 17(9); 1871-80. ©2018 AACR.

Effect of sorafenib on the outcomes of patients with FLT3-ITD acute myeloid leukemia undergoing allogeneic hematopoietic stem cell transplantation

BACKGROUND

The objective of this study was to evaluate the effect of sorafenib on the outcomes of patients with acute myeloid leukemia (AML) with FMS-like tyrosine kinase 3 (FLT3)-internal tandem duplication (ITD) undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT).

METHODS

A total of 144 patients with FLT3-ITD AML undergoing allo-HSCT between January 2012 and December 2015 were enrolled in this study. Depending on whether they were receiving sorafenib before transplantation or sorafenib maintenance after transplantation, patients were divided into 4 groups: patients receiving sorafenib before transplantation (group A; n = 36), patients receiving sorafenib after transplantation (group B; n = 32), patients receiving sorafenib both before and after transplantation (group C; n = 26), and patients receiving sorafenib neither before nor after transplantation (group D; n = 50). Outcomes were compared among these groups.

RESULTS

The 3-year relapse rates were 22.2%, 18.8%, 15.8%, and 46.1% for groups A, B, C, and D, respectively (P = .006). The 3-year overall survival (OS) rates were 74.9%, 78.1%, 84.6%, and 50.9%, respectively (P = .023), and the 3-year leukemia-free survival (LFS) rates were 69.4%, 78.1%, 80.4%, and 34.8%, respectively (P < .001). The relapse rate was higher and the LFS was shorter in group D versus groups A, B, and C. The OS in group D was shorter than the OS in group C but was similar to the OS in groups A and B. A multivariate analysis revealed that sorafenib before transplantation, sorafenib maintenance after transplantation, and their combined application were protective factors for a lower relapse rate (hazard ratios [HRs], 0.436 [P = .048], 0.431 [P = .046], and 0.173 [P = .002], respectively) and longer LFS (HRs, 0.322 [P = .010], 0.343 [P = .014], and 0.187 [P = .001], respectively).

CONCLUSIONS

Sorafenib before transplantation, sorafenib maintenance after transplantation, and their combined application all could improve the outcomes for patients with FLT3-ITD AML. Further study is needed to determine whether the use of sorafenib both before and after transplantation might be ideal. Cancer 2018;124:1954-63. © 2018 American Cancer Society.

Hypoxia-induced upregulation of BMX kinase mediates therapeutic resistance in acute myeloid leukemia

Oncogenic addiction to the Fms-like tyrosine kinase 3 (FLT3) is a hallmark of acute myeloid leukemia (AML) that harbors the FLT3-internal tandem duplication (FLT3-ITD) mutation. While FLT3 inhibitors like sorafenib show initial therapeutic efficacy, resistance rapidly develops through mechanisms that are incompletely understood. Here, we used RNA-Seq-based analysis of patient leukemic cells and found that upregulation of the Tec family kinase BMX occurs during sorafenib resistance. This upregulation was recapitulated in an in vivo murine FLT3-ITD-positive (FLT3-ITD+) model of sorafenib resistance. Mechanistically, the antiangiogenic effects of sorafenib led to increased bone marrow hypoxia, which contributed to HIF-dependent BMX upregulation. In in vitro experiments, hypoxia-dependent BMX upregulation was observed in both AML and non-AML cell lines. Functional studies in human FLT3-ITD+ cell lines showed that BMX is part of a compensatory signaling mechanism that promotes AML cell survival during FLT3 inhibition. Taken together, our results demonstrate that hypoxia-dependent upregulation of BMX contributes to therapeutic resistance through a compensatory prosurvival signaling mechanism. These results also reveal the role of off-target drug effects on tumor microenvironment and development of acquired drug resistance. We propose that the bone marrow niche can be altered by anticancer therapeutics, resulting in drug resistance through cell-nonautonomous microenvironment-dependent effects.

Molecularly targeted drug combinations demonstrate selective effectiveness for myeloid- and lymphoid-derived hematologic malignancies

Translating the genetic and epigenetic heterogeneity underlying human cancers into therapeutic strategies is an ongoing challenge. Large-scale sequencing efforts have uncovered a spectrum of mutations in many hematologic malignancies, including acute myeloid leukemia (AML), suggesting that combinations of agents will be required to treat these diseases effectively. Combinatorial approaches will also be critical for combating the emergence of genetically heterogeneous subclones, rescue signals in the microenvironment, and tumor-intrinsic feedback pathways that all contribute to disease relapse. To identify novel and effective drug combinations, we performed ex vivo sensitivity profiling of 122 primary patient samples from a variety of hematologic malignancies against a panel of 48 drug combinations. The combinations were designed as drug pairs that target nonoverlapping biological pathways and comprise drugs from different classes, preferably with Food and Drug Administration approval. A combination ratio (CR) was derived for each drug pair, and CRs were evaluated with respect to diagnostic categories as well as against genetic, cytogenetic, and cellular phenotypes of specimens from the two largest disease categories: AML and chronic lymphocytic leukemia (CLL). Nearly all tested combinations involving a BCL2 inhibitor showed additional benefit in patients with myeloid malignancies, whereas select combinations involving PI3K, CSF1R, or bromodomain inhibitors showed preferential benefit in lymphoid malignancies. Expanded analyses of patients with AML and CLL revealed specific patterns of ex vivo drug combination efficacy that were associated with select genetic, cytogenetic, and phenotypic disease subsets, warranting further evaluation. These findings highlight the heuristic value of an integrated functional genomic approach to the identification of novel treatment strategies for hematologic malignancies.