Topoisomerase II inhibitors in AML: past,present, and future

ABSTRACT

Introduction: Topoisomerase II inhibitors have long been used in the frontline and as salvage therapy for AML, with daunorubicin and idarubicin being prototypical agents in this therapeutic class, classically in combination with nucleoside analogs, e.g. cytarabine. Most recently, several other compounds from this drug class have or are being investigated.

Areas covered: The current paper reviews older and newer topoisomerase II inhibitors in clinical development for the treatment of AML. The authors discuss the clinical use of these agents, current trials involving them as well as their safety profile. Important side effects of these medications including therapy-related AML (t-AML) are also covered.

Expert opinion: Topoisomerase II inhibitors have helped improve outcomes in AML. Recently, the FDA approved several agents including CPX-351 for the treatment of secondary and t-AML. CPX-351 may have applicability in other high-risk myeloid diseases. Future directions include a combination of these agents with other targeted therapies. Finally, the authors believe that small molecule inhibitors, such as venetoclax and possibly immunotherapy options could also be incorporated to our treatment paradigm in selected patients.     

Investigational FMS-like tyrosine kinase 3 inhibitors in treatment of acute myeloid leukemia

Introduction: Outcomes for the majority of patients with acute myeloid leukemia (AML) remain poor. Over the past decade, significant progress has been made in the understanding of the cytogenetic and molecular determinants of AML pathogenesis. One such advance is the identification of recurring mutations in the FMS-like tyrosine kinase 3 gene (FLT3). Currently, this marker, which appears in approximately one-third of all AML patients, not only signifies a poorer prognosis but also identifies an important target for therapy. FLT3 inhibitors have now undergone clinical evaluation in Phase I, II and III clinical trials, as both single agents and in combination with chemotherapeutics. Unfortunately, to date, none of the FLT3 inhibitors have gained FDA approval for the treatment of patients with AML. Yet, several promising FLT3 inhibitors are being evaluated in all phases of drug development.

Areas covered: This review aims to highlight the agents furthest along in their development. It also focuses on those FLT3 inhibitors that are being evaluated in combination with other anti-leukemia agents.

Expert opinion: The authors believe that the field of research for FLT3 inhibitors remains promising, despite the historically poor prognosis of this subgroup of patients with AML. The most promising areas of research will likely be the elucidation of the mechanisms of resistance to FLT3 inhibitors, and development of potent FLT3 inhibitors alone or in combination with hypomethylating agents, cytotoxic chemotherapy or with other targeted agents.     

The role of FLT3 kinase as an AML therapy target

FMS-like tyrosine kinase-3 (FLT3) is a tyrosine kinase receptor involved in the survival and expansion of hematopoietic stem progenitors. A constitutively activated, mutated form of FLT3, is expressed in approximately 30% of de novo acute myeloid leukemia (AML) and about 6% of acute lymphoblastic leukemia (ALL) cases. Since mutant FLT3 has emerged as an attractive therapeutic target, there are several FLT3 inhibitors currently undergoing evaluation in different phases of clinical trials. However, although many aspects of the intracellular signaling mediated by oncogenic FLT3 have been revealed, what is the best strategy to inhibit FLT3 and how FLT3 inhibitors should be developed for AML treatment is poorly defined. Despite promising in vitro studies, where most FLT3 inhibitors show potent efficacy at nanomolar concentrations, clinical responses in AML patients are moderate and temporary. Furthermore, under prolonged therapy, FLT3 mutation-positive leukemic cells rapidly develop resistance to FLT3 inhibitors when used as monotherapy. Considering that there is no uniform mechanism of resistance triggered by FLT3 inhibitors, it will be necessary to develop new agents that target FLT3, and that can be used consecutively or in combination with conventional cytotoxic therapeutics. On the other hand, given that overexpression of FLT3 ligand (FL), occurring after myelosuppressive therapy, reduces the efficacy of FLT3 inhibitors, targeting both FL and FLT3 kinase, might be more effective approach in AML treatment. Here, we summarize up-to-date studies on FLT3 structure, its mutation status and role in malignant signal trafficking. We also review why FLT3 targeted therapies have not revolutionized AML treatment.     

The clinical development of FLT3 inhibitors in acute myeloid leukemia

INTRODUCTION: Activating mutations of the FMS-like tyrosine kinase 3 (FLT3) gene occur at high frequency in acute myeloid leukemia (AML), being detected in > 30% of patients at diagnosis and carrying a profound negative prognostic impact. The development of effective small molecule inhibitors of FLT3 has been the focus of an intensive international research effort in recent years. AREAS COVERED: The published results of the first decade of clinical trials of FLT3-targeted tyrosine kinase inhibitors are critically reviewed. Over this period, a first generation of compounds has followed an orderly progression from monotherapy studies through combination with chemotherapy and into advanced stage international trials in both relapsed and newly-diagnosed AML. Correlative laboratory studies performed alongside several of these studies have been highly illuminating, demonstrating close correlations between clinical activity and effective inhibition of FLT3, and highlighting potential drug resistance mechanisms. EXPERT OPINION: Clinical responses to several of the early multi-targeted agents were hindered by unfavorable pharmacokinetics and lack of potency. Newer, more potent FLT3 inhibitors such as sorafenib and AC220 possess the ability to achieve more sustained in vivo inhibition of FLT3 and have shown highly promising activity in early clinical studies. As these agents enter advanced stage trials, they carry the potential to make a major clinical impact in this disease. In future, FLT3 inhibitors may be effectively used in combination with other molecularly targeted agents.     

Gilteritinib for the treatment of relapsed and/or refractory FLT3-mutated acute myeloid leukemia

ABSTRACT

Introduction: The receptor tyrosine kinase FLT3 is the most commonly mutated gene in acute myeloid leukemia (AML). FLT3-internal tandem duplication mutations are associated with an increased risk of relapse, and a number of small molecule inhibitors of FLT3 have been developed. The highly potent and selective FLT3 kinase inhibitor gilteritinib is the first tyrosine kinase inhibitor approved as monotherapy for the treatment of relapsed and/or refractory FLT3-mutated AML.

Areas covered: We review the biology and prognostic significance of FLT3 mutations in AML and discuss the pharmacology, clinical efficacy, and toxicity profile of gilteritinib. We also summarize important differences among the various FLT3 inhibitors that are currently approved or under development and highlight areas of ongoing research.

Expert opinion: Gilteritinib has been shown to improve survival compared to salvage chemotherapy in relapsed and/or refractory FLT3-mutated AML. Gilteritinib is orally available with a favorable toxicity profile and as such is quickly becoming the standard of care for this patient population. Ongoing clinical trials are evaluating gilteritinib in combination with frontline chemotherapy, in combination with other agents such as venetoclax and azacitidine for patients who are ineligible for standard induction therapy, and as a maintenance agent.     

FLT3 inhibition and mechanisms of drug resistance in mutant FLT3-positive AML

An appealing therapeutic target in AML is constitutively activated, mutant FLT3, which is expressed in a subpopulation of AML patients and is generally a poor prognostic indicator in patients under the age of 65. There are currently several FLT3 inhibitors that are undergoing clinical investigation. However, the discovery of drug-resistant leukemic blast cells in FLT3 inhibitor-treated AML patients has prompted the search for novel, structurally diverse FLT3 inhibitors that could be alternatively used to circumvent drug resistance. Here, we provide an overview of FLT3 inhibitors under preclinical and clinical investigation, and we discuss mechanisms whereby AML cells develop resistance to FLT3 inhibitors, and the ways in which combination therapy could potentially be utilized to override drug resistance. We discuss how the cross-talk between major downstream signaling pathways, such as PI3K/PTEN/Akt/mTOR, RAS/Raf/MEK/ERK, and Jak/STAT, can be exploited for therapeutic purposes by targeting key signaling molecules with selective inhibitors, such as mTOR inhibitors, HSP90 inhibitors, or farnesyltransferase inhibitors, and identifying those agents with the ability to positively combine with inhibitors of FLT3, such as PKC412 and sunitinib. With the widespread onset of drug resistance associated with tyrosine kinase inhibitors, due to mechanisms involving development of point mutations or gene amplification of target proteins, the use of a multi-targeted therapeutic approach is of potential clinical benefit.     

成人急性髓系白血病治疗现状及新药研究进展

急性髓系白血病（Acute myeloid leukemia, AML）是一类高度异质性疾病,过去常规诱导化疗和各类支持治疗为主,但随着二代测序等技术应用以来,AML药物迅速发展。本文从细胞毒药物、小分子靶向药物以及免疫靶向药物等方面,系统概述AML的致病机制,以及临床研究中较为前沿的各类新型药物（如FLT3抑制剂Midostaurin、IDH抑制剂AG-221和CPX-351等）的研究进展,旨在为AML的新药研发提供参考。     

Novel FLT3 tyrosine kinase inhibitors

Acute myeloid leukaemia (AML) is an aggressive haematological malignancy that is curable in approximately 40% of cases. Activating mutations of the receptor tyrosine kinase FLT3 (FMS-like tyrosine kinase-3) are the single most common molecular abnormalities in AML and are associated with a distinctly worse prognosis. In an effort to target this mutation and improve outcomes in this subgroup of AML patients, several novel small-molecule FLT3 tyrosine kinase inhibitors are currently in development. Some of these FLT3 inhibitors are useful only as laboratory tools, while others clearly have clinical potential. These compounds are derived from a wide variety of chemical classes and differ significantly both in their potency and selectivity. This review summarises these developments and examines these novel agents with regard to both the assays used to characterise them and their clinical potential.     

FLT3 and acute myelogenous leukemia: biology, clinical significance and therapeutic applications

Acute myelogenous leukemia (AML) is a difficult disease to treat, and better treatments are needed. Molecular targeted therapy represents a novel therapeutic approach. The FLT3 tyrosine kinase receptor is mutated in approximately one-fourth to one-third of patients with AML. Normally, binding of FLT3 ligand to the FLT3 receptor leads to phosphorylation of tyrosine residues and activation of the receptor. This in turn leads to induction of intracellular signaling pathways essential to regulation of cell proliferation and apoptosis. Two classes of FLT3 activating mutations have been identified in AML patients: internal tandem duplications (ITDS) and point mutations in the activating loop of the kinase domain. Both mutations result in constitutive FLT3 tyrosine kinase activity and lead to transformation of hematopoietic cell lines in vivo and in vitro. FLT3 ITDs are also an independent poor prognostic factor for overall survival and disease free survival in patients with AML. Therefore, targeting FLT3 mutations represents a potential therapeutic target for AML. This review will discuss the biology and clinical significance of FLT3 and FLT3 mutations in cell growth and signaling. In addition, I will discuss some of the novel FLT3 inhibitors which are entering clinical trials for AML.     

The clinical development of FLT3 inhibitors in acute myeloid leukemia

Introduction: Activating mutations of the FMS-like tyrosine kinase 3 (FLT3) gene occur at high frequency in acute myeloid leukemia (AML), being detected in > 30% of patients at diagnosis and carrying a profound negative prognostic impact. The development of effective small molecule inhibitors of FLT3 has been the focus of an intensive international research effort in recent years.

Areas covered: The published results of the first decade of clinical trials of FLT3-targeted tyrosine kinase inhibitors are critically reviewed. Over this period, a first generation of compounds has followed an orderly progression from monotherapy studies through combination with chemotherapy and into advanced stage international trials in both relapsed and newly-diagnosed AML. Correlative laboratory studies performed alongside several of these studies have been highly illuminating, demonstrating close correlations between clinical activity and effective inhibition of FLT3, and highlighting potential drug resistance mechanisms.

Expert opinion: Clinical responses to several of the early multi-targeted agents were hindered by unfavorable pharmacokinetics and lack of potency. Newer, more potent FLT3 inhibitors such as sorafenib and AC220 possess the ability to achieve more sustained in vivo inhibition of FLT3 and have shown highly promising activity in early clinical studies. As these agents enter advanced stage trials, they carry the potential to make a major clinical impact in this disease. In future, FLT3 inhibitors may be effectively used in combination with other molecularly targeted agents.     

Fms样酪氨酸激酶抑制剂治疗急性髓性白血病的新进展

Fms样酪氨酸激酶(FLT3)属于Ⅲ型受体酪氨酸蛋白激酶家族,其异常表达见于大部分急性髓性白血病(AML)病人,包括野生型受体与配体的协同表达,内部串连复制及活化环突变,表达持续性磷酸化活化的FLT3,对预后的判断具有重要意义。FLT3抑制剂主要有5类及热休克蛋白90抑制剂,实验室研究与初步的临床试验显示出其对AML的治疗价值。     

Novel FLT3 tyrosine kinase inhibitors

Acute myeloid leukaemia (AML) is an aggressive haematological malignancy that is curable in ~ 40% of cases. Activating mutations of the receptor tyrosine kinase FLT3 (FMS-like tyrosine kinase-3) are the single most common molecular abnormalities in AML and are associated with a distinctly worse prognosis. In an effort to target this mutation and improve outcomes in this subgroup of AML patients, several novel small-molecule FLT3 tyrosine kinase inhibitors are currently in development. Some of these FLT3 inhibitors are useful only as laboratory tools, while others clearly have clinical potential. These compounds are derived from a wide variety of chemical classes and differ significantly both in their potency and selectivity. This review summarises these developments and examines these novel agents with regard to both the assays used to characterise them and their clinical potential.     

Molecularly targeted therapies for pediatric acute myeloid leukemia: progress to date

While acute myeloid leukemia (AML) is significantly less common than acute lymphoblastic leukemia (ALL) in childhood, it is significantly more deadly with only half as many children likely to be cured with standard therapy. In addition, the typical treatment for AML is among the most toxic of treatments for pediatric cancer; it includes intensive multiagent chemotherapy and, often, hematopoietic stem cell transplantation. Given the poor prognosis of pediatric AML and the significant toxicity of standard AML therapy, novel therapies are needed. Improved understanding of the molecular and cellular biology of leukemia has facilitated the development of molecularly targeted therapies.In this article, we review progress to date with agents that are showing promise in the treatment of pediatric AML including targeted immunoconjugates, inhibitors of signaling molecules (e.g. FMS-like tyrosine kinase 3 [FLT3], farnesyltransferase, and mammalian target of rapamycin [mTOR]), agents that target epigenetic regulation of gene expression (DNA methyltransferase inhibitors and histone deacetylase inhibitors), and proteasome inhibitors. For the specific agents in each of these classes, we summarize the published preclinical data and the clinical trials that have been completed, are in progress, or are being planned for children with AML. Finally, we discuss potential challenges to the success of molecularly targeted therapy including demonstrating adequate targeting of leukemia stem cells, developing synergistic and tolerable combinations of agents, and designing adequately powered clinical trials to test efficacy in molecularly defined subsets of patients.     

Bcl-2抑制剂用于急性髓系白血病靶向治疗的研究进展

摘要： 急性髓系白血病 （AML） 是成人最常见的急性白血病, 诱导化疗仍是AML的一线治疗手段。但是, 由于 AML诱导化疗毒性较高, 且有高失败率及高复发率的特点, 在AML患者中应用受限。近年来, 去甲基化药物地西他滨和阿扎胞苷在AML表观遗传学治疗中得到深入研究, 已经动摇了诱导化疗在AML治疗中的地位。随着对AML生物学研究的深入, 发现越来越多的靶点可影响AML的生物学进程。针对这些靶点的靶向药物分为3类： 第一类是突变位点抑制剂, 如FLT3和IDH抑制剂; 第二类是调节代谢或信号通路的抑制剂, 如Bcl-2拮抗剂及表观遗传学药物;第三类是靶向细胞毒性药物。2017年美国血液学年会报道, AML靶向治疗最有前景的药物为调节代谢或信号通路的抑制剂, 其中BCL-2抑制剂Venetoclax在AML的临床试验报告被评为本届会议最佳。本文就BCL-2抑制剂在 AML的治疗进展做一综述。     

抗艾滋病毒化疗药物的最新进展

迄今共有25种抗HIV-1的化学治疗药物在临床使用, 包括8种核苷和核苷酸类及4种非核苷类逆转录酶抑制剂, 10种蛋白酶抑制剂, 2种病毒进入抑制剂和1种整合酶抑制剂。此外, 还有20多种针对HIV-1不同靶点的新药进入了临床试验研究阶段。高效抗逆转录病毒治疗具有明显降低病毒载量, 显著升高CD4⁺数目, 改善患者生存质量和延长患者生命的效果。通过对于给药时机和药物选择, 病毒对药物的耐受, 药物引发的代谢并发症, 药物间的相互作用等进行的深入研究, HIV-1/AIDS的化学治疗逐步进入了理性的个性化阶段。     

急性髓细胞白血病FLT3-ITD基因的表达及临床意义

目的 通过对初诊急性髓细胞白血病(acute myeloid leukemia,AML)患者进行FLT3 -ITD基因突变检测,探讨FLT3-ITD基因突变与AML临床特征的相关性,为AML的临床分层治疗、分子靶向治疗及预后判断提供理论依据.方法 采用实时定量聚合酶链反应(RT-PCB)检测60例初治AML患者和10例对照组患者骨髓FLT3-ITD基因突变.结果 60例初治AML患者突变阳性率为25.0％( 15/60),10例对照组患者骨髓均未检测到该突变;依据法美英协作组(FAB)分型标准备亚型FLT3 -ITD突变率不同,其中以M3、M5型突变率较高;60例初治AML中,28例老年AML组与32例非老年AML组FLT3-ITD基因表达水平比较,差异有统计学意义(P＜0.05);FLT3-ITD阳性组患者具有外周血白细胞数、骨髓白血病细胞比例高的临床特点,并且FLT3-ITD阳性组化疗后完全缓解率(CR)低于FLT3-ITD阴性组(P＜0.05).结论 FLT3-ITD基因突变在AML患者中存在一定的突变率,且多见于M3、M5型患者,FLT3-ITD阳性患者具有外周血白细胞数、骨髓白血病细胞比例高,CR低的临床特点.     

Midostaurin treatment in FLT3-mutated acute myeloid leukemia and systemic mastocytosis

Introduction: A number of tyrosine kinase inhibitors (TKIs) have been developed that inhibit the constitutively activated kinase activity caused by activating tyrosine kinase mutations, such as FLT3 or KIT, thus interrupting signaling pathways. Currently, midostaurin is the only approved TKI as monotherapy for aggressive systemic mastocytosis (SM), SM with associated hematological neoplasm, or mast cell leukemia displaying a KIT mutation as well as in combination with standard intensive chemotherapy for adult patients with newly diagnosed FLT3-mutated acute myeloid leukemia (AML).

Areas covered: We provide a concise review of the pharmacology, tolerability and clinical efficacy of midostaurin and emerging new treatment options for ASM and FLT3-mutated AML.

Expert commentary: Currently, midostaurin is the only approved TKI in aggressive SM, SM with associated hematological neoplasm, or mast cell leukemia inducing responses including complete remissions. With regard to AML, midostaurin is the first drug to receive regulatory approval in this indication in the molecularly defined subgroup of AML with FLT3 mutations. By introduction of this new standard in AML with FLT3 mutations, the bare has been raised for future approvals of next generation FLT3 inhibitors which will be based increasingly on head to head comparisons with midostaurin.