Mechanisms of Clinical Resistance to Small Molecule Tyrosine Kinase Inhibitors Targeting Oncogenic Tyrosine Kinases

A number of highly specific small molecule inhibitors of oncogenic tyrosine kinases have been developed and may potentially improve the treatment of different malignant diseases. However, it became rapidly evident that multiple resistance mechanisms compromise the successful clinical application of these inhibitors, particularly in advanced solid tumors. To develop efficient therapeutic strategies with small molecule inhibitors, one must understand the causes for treatment failure. Three different types of resistance to small molecule inhibitors of oncogenic tyrosine kinases have been observed. The malignant phenotype may be independent of the activity of the target kinase (target-independent resistance). Alternatively, overexpression or mutation of the target kinase can counteract the inhibition of oncogenic tyrosine kinases (target-dependent resistance). Finally, alterations of drug transporters or drug-metabolizing pathways may block the bioavailability of the tyrosine kinase inhibitors (drug-dependent resistance). This article reviews the current knowledge of clinical resistance to small molecule inhibitors approved for treatment of cancer patients.     

Resistance to imatinib (Glivec): update on clinical mechanisms.

Imatinib mesylate, an orally administered 2-phenylaminopyrimidine derivative that inhibits BCR/ABL tyrosine kinase activity, has shown great promise in the treatment of chronic myelogenous leukemia (CML). This small molecule, tyrosine kinase inhibitor, has also been shown to be effective against metastatic gastrointestinal stromal tumors (GISTs) expressing the stem cell factor (SCF) receptor kit. However, the threat of resistance in patients has prompted investigators to uncover the mechanisms whereby malignant cells develop resistance to imatinib, and has also led to the establishment of strategies designed to over-ride imatinib resistance. Here, we provide a comprehensive overview of the effectiveness of imatinib in the treatment of chronic, accelerated and blast crisis-phase CML, Philadelphia chromosome-positive (Ph+) acute lymphoid leukemia (ALL) and metastatic GIST. Established mechanisms of resistance to imatinib are discussed, as are novel therapeutic approaches to improving drug responsiveness by reversing development of imatinib resistance in patients.     

Kit: molecule of interest for the diagnosis and treatment of mastocytosis and other neoplastic disorders

Kit a type III receptor tyrosine kinase, along with its ligand the stem cell factor, play a critical role in normal cell growth, differentiation, development and survival. Ligand independent activation of kit (dysregulated kit function) has been found to be an important component of oncogenesis in a large number of neoplastic disorders such as systemic mastocytosis, gastro intestinal stromal tumors, germ cell tumors, acute myelogenous leukemia with the disruption of the core binding factor, amongst others. The identification of small molecule inhibitors with activity against Kit, has offered a wider and more effective range of therapeutic options in the treatment of these neoplastic processes. Novel tyrosine kinase inhibitors such as imatinib, nilotinib and dasatinib, have been found to be effective in the management of various subtypes of systemic mastocytosis and gastrointestinal stromal tumors. Non-tyrosine kinase inhibitors like rapamycin, 17-AAG and IMD- 0354 have been added to the therapeutic armamentarium, with the hope that combination therapy might have a synergistic effect, or prevent/delay the development of drug resistance.     

Mechanisms of resistance to FLT3 inhibitors

The success of the small molecule tyrosine kinase receptor inhibitor (TKI) imatinib mesylate (Gleevec) in the treatment of chronic myeloid leukemia (CML) constitutes an eminent paradigm shift advocating the rational design of cancer therapeutics specifically targeting the transformation events that drive tumorigenicity. In acute myeloid leukemias (AMLs), the most frequent identified transforming events are activating mutations in the FLT3 receptor tyrosine kinase that constitutively activate survival and proliferation pathways. FLT3 TKIs that are in various phases of clinical trials are showing some initial promise. However, primary and secondary acquired resistance stands to severely compromise long-term and durable efficacy of these inhibitors as a therapeutic strategy. Here, we discuss the mechanisms of resistance to FLT3 inhibitors and possible strategies to overcome resistance through closer examination of the events of leukemogenesis and design of combination therapy.     

Mechanisms of resistance to imatinib in CML patients: a paradigm for the advantages and pitfalls of molecularly targeted therapy

One of the challenges of cancer therapeutics is to discover targets unique to the tumor cell population. Constitutively activated tyrosine kinases play a role in the malignant phenotype in a number of different cancers. While the kinases may be present in the normal cell, the cancer cell is often dependent upon the activation of the kinase for the maintenance of malignant growth. Inhibition of kinase activation may therefore selectively inhibit malignant proliferation. In the case of chronic myelogenous leukemia (CML), the activated tyrosine kinase (BCR-ABL) is due to a chromosomal translocation that defines this disease, and is necessary for malignant transformation. Imatinib mesylate (Gleevec, Novartis) is a small molecule tyrosine kinase inhibitor, developed through the chemical modification to be selected for a small number of tyrosine kinases present in human cells. This agent is also orally bioavailable and has been found to be effective in clinical trials. We have learned much through the clinical use of this agent. 1) Specific targeting of activated signal transduction pathways may be effective in inhibiting cancer cells. 2) Cancer cells may not only be inherently resistant to small molecule inhibitors, but may also develop resistance after exposure to the inhibitor. 3) Increased knowledge regarding critical signal transduction pathways, the structure of the molecules that are being targeted and the inhibitors themselves, will allow us to understand resistance as it develops and create new molecules to bypass resistance. We will discuss imatinib as an important example of the success and pitfalls of targeted therapeutics for cancer.     

The lack of target specificity of small molecule anticancer kinase inhibitors is correlated with their ability to damage myocytes in vitro

Many new targeted small molecule anticancer kinase inhibitors are actively being developed. However, the clinical use of some kinase inhibitors has been shown to result in cardiotoxicity. In most cases the mechanisms by which they exert their cardiotoxicity are not well understood. We have used large scale profiling data on 8 FDA-approved tyrosine kinase inhibitors and 10 other kinase inhibitors to a panel of 317 kinases in order to correlate binding constants and kinase inhibitor binding selectivity scores with kinase inhibitor-induced damage to neonatal rat cardiac myocytes. The 18 kinase inhibitors that were the subject of this study were: canertinib, dasatinib, dovitinib, erlotinib, flavopiridol, gefitinib, imatinib, lapatinib, midostaurin, motesanib, pazopanib, sorafenib, staurosporine, sunitinib, tandutinib, tozasertib, vandetanib and vatalanib. The combined tyrosine kinase and serine-threonine kinase selectivity scores were highly correlated with the myocyte-damaging effects of the kinase inhibitors. This result suggests that myocyte damage was due to a lack of target selectivity to binding of both tyrosine kinases and serine-threonine kinases, and was not due to binding to either group specifically. Finally, the strength of kinase inhibitor binding for 290 kinases was examined for correlations with myocyte damage. Kinase inhibitor binding was significantly correlated with myocyte damage for 12 kinases. Thus, myocyte damage may be multifactorial in nature with the inhibition of a number of kinases involved in producing kinase inhibitor-induced myocyte damage.     

晚期非小细胞肺癌靶向治疗的研究进展

生物标志物检测使得许多晚期非小细胞肺癌(NSCLC)患者获益。近年来,针对表皮生长因子受体(EGFR)和间变性淋巴瘤激酶(ALK)突变呈阳性的NSCLC患者,以吉非替尼、厄洛替尼、阿法替尼为代表的表皮生长因子受体酪氨酸激酶抑制剂(EGFR-TKI)和以克唑替尼为代表的ALK-TKI取得了卓越的疗效。但是,大多数第一代EGFR-TKI和ALK-TKI的疗效因为不可避免的继发性耐药而被减弱。目前,第三代EGFR-TKI正是基于第二代EGFR-TKI的耐药机制研发而成。除此之外,还有许多针对其他突变位点的晚期NSCLC维持治疗的靶向抑制剂。遗憾的是,针对突变比例最大的K-RAS突变,尚无疗效确切的靶向药物。因此,基于肿瘤驱动基因突变机制的探索和靶向药物的开发是目前NSCLC的研究热点。     

Anacetrapib,a cholesteryl ester transfer protein inhibitor

Introduction: Mutated forms of the receptor tyrosine kinase c-KIT are “drivers” in several cancers and are attractive targets for therapy. While benefits have been obtained from use of inhibitors of KIT kinase activity such as imatinib, especially in gastrointestinal stromal tumours (GIST), primary resistance occurs with certain oncogenic mutations. Furthermore, resistance frequently develops due to secondary mutations. Approaches to addressing both of these issues as well as combination therapies to optimise use of KIT kinase inhibitors are discussed.

Areas covered: This review covers the occurrence of oncogenic KIT mutations in different cancers and the molecular basis of their action. The action of KIT kinase inhibitors, especially imatinib, sunitinib, dasatinib and PKC412, on different primary and secondary mutants is discussed. Outcomes of clinical trials in GIST, acute myeloid leukaemia (AML), systemic mastocytosis and melanoma and their implications for future directions are considered.

Expert opinion: Analysis of KIT mutations in individual patients is an essential prerequisite to the use of kinase inhibitors for therapy, and monitoring for development of secondary mutations that confer drug resistance is necessary. However, it is unlikely that KIT inhibitors alone can lead to cure. KIT mutations alone do not seem to be sufficient for transformation; thus identification and co-targeting of synergistic oncogenic pathways should lead to improved outcomes.     

Tyrosine kinase inhibitors as novel drugs for the treatment of diabetes

Introduction: Some inhibitors of tyrosine kinase, as imatinib, erlotinib and sunitinib have antihyperglycemic effects but the mechanisms are not totally clear.

Areas covered: It is well established that insulin resistance and beta-cell failure are hallmarks of type 2 diabetes mellitus (DM2). The present review will discuss the molecular mechanisms that account for insulin resistance and beta-cell failure in DM2, and also the effect of tyrosine kinase inhibitors in these processes.

Expert opinion: A better understanding of how these drugs improve the two most important mechanisms of DM2 associated with suggestions of clinical studies will lead to improve the treatment of this disease.     

Molecular targeted therapy of gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GIST) are mesenchymal tumors that occur predominantly in the stomach and the small bowel. Their pathogenesis is generally based on primary activating mutations in the KIT or PDGFRα genes that result in constitutive activation of receptor tyrosine kinase activity. Imatinib, first designed to competitively inhibit the ATP-binding pocket of the BCR-ABL tyrosin kinase exhibits inhibition also in the KIT and PDGFRα tyrosine kinases, which revolutionized the therapy of gastrointestinal stromal tumors, a disease without any systemic treatment options prior to imatinib. Clinical benefit is achieved in approximately 85% of patients with unresectable or metastatic disease with a median progression-free survival of 19 to 26 months and an overall survival approaching 5 years. Disease progression results from different mechanisms of resistance most frequently involving the emergence of secondary mutations in KIT exons 13, 14, or 17. Several newer drugs have been studied in patients failing or being intolerant to imatinib, including the multitargeted agent sunitinib as well as other KIT targeting tyrosine kinase inhibitors like nilotinib or agents targeting alternative pathways like anti-angiogenic agents, mTOR-, RAF kinase- and chaperone inhibitors.     

Tyrosine kinase inhibitors as modulators of ATP binding cassette multidrug transporters: substrates, chemosensitizers or inducers of acquired multidrug resistance?

INTRODUCTION: Anticancer tyrosine kinase inhibitors (TKIs) are small molecule hydrophobic compounds designed to arrest aberrant signaling pathways in malignant cells. Multidrug resistance (MDR) ATP binding cassette (ABC) transporters have recently been recognized as important determinants of the general ADME-Tox (absorption, distribution, metabolism, excretion, toxicity) properties of small molecule TKIs, as well as key factors of resistance against targeted anticancer therapeutics. AREAS COVERED: The article summarizes MDR-related ABC transporter interactions with imatinib, nilotinib, dasatinib, gefitinib, erlotinib, lapatinib, sunitinib and sorafenib, including in vitro and in vivo observations. An array of methods developed to study such interactions is presented. Transporter-TKI interactions relevant to the ADME-Tox properties of TKI drugs, primary or acquired cancer TKI resistance, and drug-drug interactions are also reviewed. EXPERT OPINION: Based on the concept presented in this review, TKI anticancer drugs are considered as compounds recognized by the cellular mechanisms handling xenobiotics. Accordingly, novel anticancer therapies should equally focus on the effectiveness of target inhibition and exploration of potential interactions of the designed molecules by membrane transporters. Thus, targeted hydrophobic small molecule compounds should also be screened to evade xenobiotic-sensing cellular mechanisms.     

Future options for imatinib mesilate-resistant tumors

The outcome of patients with gastrointestinal stromal tumors has been dramatically improved by therapy with imatinib mesilate (imatinib mesylate), a KIT and platelet-derived growth factor (PDGFR) tyrosine kinase inhibitor. Unfortunately, the majority of patients eventually experience disease progression due to drug resistance. Recent elucidation of the mechanisms of resistance to imatinib, particularly the acquisition of secondary mutations of the KIT and PDGF receptors, has provided significant insight and potential for the development of novel therapies. This review discusses the efficacy of sunitinib, which is approved for the treatment of patients with imatinib-resistant tumors, and highlights a number of emerging second-generation receptor tyrosine kinase inhibitors that show therapeutic potential in imatinib-resistant patients. Also considered are several promising agents targeting pathways downstream of the constitutionally activated KIT and PDGF receptors. Strategies to overcome imatinib resistance by optimizing combination therapy and selecting specific kinase inhibitors based on the secondary mutations identified in tumors of individual patients are presented.     

Developments in targeted therapy of advanced gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GISTs) comprise a recently defined entity of the most common mesenchymal neoplasms of the gastrointestinal tract. Advances in the understanding of the molecular mechanisms of GIST pathogenesis have resulted in the development of a treatment approach which has become a model of targeted therapy in oncology. The introduction of imatinib mesylate (inhibiting KIT/PDGFRA (platelet-derived growth factor receptor-alpha) and their downstream signaling cascade) has revolutionized the therapy of advanced (inoperable and/or metastatic) GISTs. Imatinib has now become the standard of care in the treatment of patients with advanced GIST. However, a majority of patients eventually develop clinical resistance to imatinib. Over the last few years major progress has been made in elucidating the mechanism of disease progression (as secondary mutations in KIT and/or PDGFRA kinase domains) and resistance to imatinib. Currently, the sole approved second-line drug is sunitinib--a multitargeted agent, an inhibitor of tyrosine kinase, of KIT and PDGFRA/B and of the vascular endothelial growth factor receptors (VEGFRs)-1, -2 and 3, FMS-like tyrosine kinase-3 (FLT3), colony stimulating factor 1 receptor (CSF-1R), and glial cell-line derived neurotrophic factor receptor (REarranged during Transfection; RET). However, a number of new generation tyrosine kinase inhibitors, alone or in combination, are being evaluated at present alongside treatment options alternative to inhibiting the KIT signaling pathway (as heat shock protein 90 or mammalian target of rapamycin). This article discusses the factors relating to imatinib resistance as well as upcoming potentially effective treatment options for patients with progressive disease available in 2008 and those under investigation with more individualized treatment methods, which has been recently patented. This review focuses on the current achievements in targeted therapy of advanced GISTs, and how the insight into the resistance mechanisms may allow in the near future to treat patients with advanced GISTs.     

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.     

New tyrosine kinase inhibitors in the treatment of chronic myeloid leukemia

Imatinib mesylate, Abl tyrosine kinase inhibitor, has improved the treatment of Bcr-Abl-positive leukemia such as chronic myeloid leukemia (CML) and Philadelphia chromosome positive acute lymphoblastic leukemia (Ph(+)ALL). However, resistance is often reported in patients with advanced-stage disease. Several novel tyrosine kinase inhibitors, which have been developed to override imatinib resistance mechanisms such as overexpression of Bcr-Abl and point mutations within the Abl kinase domain, are currently competing. Inhibitors of Abl tyrosine kinase are divided into two main groups, namely, ATP-competitive and ATP non-competitive inhibitors. Moreover, ATP-competitive inhibitors are fall into two subclasses, i.e. the Src/Abl inhibitors, and 2-phenylaminopyrimidin-based compounds. Dasatinib (formerly BMS-354825), AP23464, SKI-606 and PD166326 are classified as Src/Abl inhibitors while AMN107 and NS-187 (INNO-406) belong to the latter subclass of inhibitors. Among these agents, clinical studies on dasatinib and AMN107 had started earlier than the others and favorable results are accumulating. Clinical studies of other compounds including NS-187 (INNO-406) will be performed in rapid succession. Because of its strong affinity, most ATP competitive inhibitors may be effective against imatinib-resistant patients. However, to date, an ATP-competitive inhibitor that can inhibit the phosphorylation of T315I Bcr-Abl has not yet been developed. To address this problem, ATP non-competitive inhibitors such as ON012380, Aurora kinase inhibitor VX-680 and p38 MAP kinase inhibitor BIRB-796 have been developed. It may be necessary for the improvement of CML and Ph(+)ALL treatment to be taken into consideration of the combination therapy with novel ATP-competitive inhibitors and these agents.     

Nilotinib: a novel Bcr-Abl tyrosine kinase inhibitor for the treatment of leukemias

The successful introduction of the tyrosine kinase inhibitors has initiated a new era in the management of chronic myeloid leukemia (CML). Imatinib mesilate therapy has significantly improved the prognosis of CML. A minority of patients in chronic-phase CML--and more patients in advanced phases--are resistant to imatinib, or develop resistance during treatment. This is attributed, in 40-50% of cases, to the development of mutations in the Bcr-Abl tyrosine kinase domain that impair imatinib binding. Nilotinib (Tasigna) is a novel potent selective oral kinase inhibitor. Preclinical and clinical investigations demonstrate that nilotinib effectively overcomes imatinib resistance, and has induced high rates of hematologic and cytogenetic responses in CML post imatinib failure.     

Molecular basis for sunitinib efficacy and future clinical development

Sunitinib malate (SU11248/Sutent; Pfizer) is a multitargeted tyrosine kinase inhibitor that has potent anti-angiogenic and antitumour activities. Definitive efficacy has been demonstrated in advanced renal cell carcinoma and in gastrointestinal stromal tumours that are refractory or intolerant to imatinib (Gleevec; Novartis), which has provided the basis for the recent regulatory approvals for these indications. This article summarizes the discovery and development of sunitinib, and discusses key issues for the multitargeted approach in cancer treatment, such as markers of response and development of resistance, and their significance for the future development of sunitinib and other multikinase inhibitors.     

The expanding role of nilotinib in chronic myeloid leukemia

Importance of the field: Several therapeutic options, including tyrosine kinase inhibitors, exist for the treatment of patients with Philadelphia chromosome (Ph)-positive chronic myeloid leukemia (CML). Despite impressive results, there is room for improvement for those patients who are either resistant or intolerant to imatinib.

Areas covered in this review: An overview is given on the clinical results with nilotinib, a rationally designed second-generation tyrosine kinase inhibitor, as first- and second-line therapy in patients with Ph-positive CML. Important factors in predicting resistance to nilotinib and guiding therapeutic decisions are addressed.

What the reader will gain: Knowledge on the clinical efficacy and safety of nilotinib after imatinib failure and as first-line treatment. Point mutations in the kinase domain (KD) of BCR-ABL1 are important determinants of clinical sensitivity to currently available tyrosine kinase inhibitors, including nilotinib. Information on specific BCR-ABL1 KD mutations and safety profiles assist in therapeutic decision making.

Take home message: Nilotinib is a highly effective and well-tolerated therapeutic option in patients with Ph-positive CML after imatinib failure. Early evidence demonstrating increased efficacy has allowed expanding nilotinib to previously untreated patients in chronic phase. Insights into mechanisms of resistance to tyrosine kinase inhibitors and predictive factors for response will allow for a more individualized use of these agents.     

Mechanisms of acquired resistance to epidermal growth factor receptor tyrosine kinase inhibitors and new therapeutic perspectives in non small cell lung cancer

EGFR somatic mutations define a subset of NSCLCs that are most likely to benefit from EGFR tyrosine kinase inhibitors (TKIs). These tumors are dependent on EGFR-signaling for survival. Recently, tyrosine kinase domain somatic mutations have been approved as criterion to decide first-line therapy in this group of advanced NSCLCs. Anyway, all patients ultimately develop resistance to these drugs. Acquired resistance is linked to a secondary EGFR mutation in about a half of patients. Uncontrolled activation of MET, another tyrosine kinase receptor, has been implicated in neoplastic invasive growth. MET is overexpressed, activated and sometimes mutated in NSCLC cell lines and tumor tissues. MET increased gene copy number has also been documented in NSCLC and has been studied as negative prognostic factor. It has also been found in about 20% of patients developing acquired resistance to TKIs inhibitors. In this group, it seems to display a new mechanism, which is able to mark tumor independence from EGFR signaling. The study of delayed resistance mechanisms could lead to the development of new therapeutic strategies. Different molecular alterations could be specifically targeted in order to extend disease control in this group of NSCLCs with distinct clinical and molecular features. EGFR irreversible inhibitors, MET inhibitors and dual EGFR/VEGFR inhibitors represent one of the most challenging issues in current clinical research. Ongoing clinical trials and future perspectives are discussed.     

Small molecule tyrosine kinase inhibitors: clinical development of anticancer agents

Numerous small molecule synthetic tyrosine kinase inhibitors are in clinical development for the treatment of human cancers. These fall into three broad categories: inhibitors of the epidermal growth factor receptor tyrosine kinase family (e.g., Iressa trade mark and Tarceva trade mark ), inhibitors of the split kinase domain receptor tyrosine kinase subgroup (e.g., PTK787/ZK 222584 and SU11248) and inhibitors of tyrosine kinases from multiple subgroups (e.g., Gleevec trade mark ). In addition, agents targeting other tyrosine kinases implicated in cancer, such as Met, Tie-2 and Src, are in preclinical development. As experience is gained in the clinic, it has become clear that unleashing the full therapeutic potential of tyrosine kinase inhibitors will require patient preselection, better assays to guide dose selection, knowledge of mechanism-based side effects and ways to predict and overcome drug resistance.