Receptor tyrosine kinases and anticancer therapy

Receptor and non-receptor protein tyrosine kinases (PTKs) are essential enzymes in cellular signaling processes and signal transduction pathways that regulate cell growth, differentiation, migration and metabolism by catalyzing protein phosphorylation and dephosphorylation. In recent years, different tyrosine kinase receptors were identified as regulators of tumor or tumor vessel growth. Their inhibition by specific tyrosine kinase inhibitors and antibodies targeting growth factors and their receptors were recently shown to constitute a new modality for treating cancers. The pathognomonic role of the inhibited tyrosine kinase defines the way of action, whereas the amount of expression in tumor tissue is thought to define the indication for the tumor entity. Various compounds targeting PTKs are under clinical investigation in phase I-III trials or are already approved. This review describes new drugs targeting BCR-Abl, c-kit, EGFR (epidermal growth factor receptor), tumor angiogenesis via VEGF (vascular endothelial growth factor), HER2/neu and "multitarget" tyrosine kinase inhibitors.     

Small molecule tyrosine kinase inhibitors: potential role in pediatric malignant solid tumors

Tyrosine kinase receptors are expressed on the surface of tumor and/or endothelial cells and represent attractive targets for new anti-cancer treatment strategies. The so-called "small molecule" tyrosine kinase inhibitors have been designed to interact with the intracellular ATP binding site of these receptors, subsequently causing arrest of tumor cell proliferation, as well as induction of apoptosis and tumor migration. Furthermore, these molecules can impact on tumor angiogenesis. Tyrosine kinase inhibitors have been evaluated in several clinical trials for various adult malignant tumor entities and are currently being studied in pediatric solid malignancies. In this review, we will address the data available supporting the potential use of tyrosine kinase inhibitors in solid malignancies of childhood.     

Signal transduction pathway targets for anticancer drug discovery

There are currently over 80 agents officially approved for the treatment of cancer world-wide. However, the most common epithelial cancers, which cause greater than 75% of cancer deaths, remain incurable. Most drugs have been developed empirically by testing large numbers of chemicals on rapidly growing transplantable rodent tumors, and more recently, human tumor xenografts. This approach has identified prodeminantly DNA-active drugs that are considerably toxic and have limited efficacy. Novel molecular targets, which are selective for neoplastic cells, are needed for chemotherapeutic agents to improve cure rates of epithelial malignancies, with acceptable toxicity. In recent years, agents inhibiting signal transduction pathway molecules have entered clinical trials. These include antibodies and small molecules, which inhibit growth factor receptors and their receptor tyrosine kinases, inhibitors of cytoplasmic second messengers such as ras, raf and MEK, inhibitors of protein trafficking, and inhibitors of protein degradation.     

Rational combination of targeted therapies as a strategy to overcome the mechanisms of resistance to inhibitors of EGFR signaling

The epidermal growth factor receptor (EGFR) has been widely used as a target for novel anticancer agents, such as blocking antibodies and small molecular weight tyrosine kinase compounds. In spite of recent advances in cancer cell biology, leading to the introduction of clinically active new drugs, such as cetuximab, panitumumab and erlotinib, unfortunately disease control remains unsuccessful due to the presence of constitutive resistance to EGFR inhibitors in most patients and the development of acquired resistance in the responders. A large number of molecular abnormalities in tumor cells seem to partly contribute to their resistance to anti-EGFR therapy: increased angiogenesis, constitutive activation of downstream mediators, overexpression of other tyrosine kinase receptors. Moreover, some mutations in the EGFR receptor kinase domain seem to play a crucial role in determining the sensitivity of cancer cells to specific inhibitors by altering the conformation of the receptor and its activity. The development of rational combinations of anticancer agents and EGFR inhibitors, able to exert synergistic cytotoxic interactions, has been widely accepted and used in both preclinical and clinical studies. Although the failure of large clinical trial based on empirical combination of anti-EGFR and classic chemotherapeutic agents, several preclinical data seems to support the hypothesis that combining EGFR inhibitors and other novel agents could efficiently inhibit tumor growth and overcome intrinsic resistance to a single-agent based therapy. This review focuses on the role of complementary signalling pathways in the development of resistance to EGFR targeting agents and the rationale to combine novel inhibitors as anticancer therapy.     

Molecular diagnosis and response prediction to anti-tyrosine kinase receptors in oncology

Tyrosine kinase receptors are an important family of membrane receptors implicated in oncogenesis. Their activation triggers signaling cascades that activate cell growth, proliferation and controls cell death. Inhibiting these receptors leads to signaling impairments and favors antitumor activity. Different anti-receptor tyrosine kinases are proposed for therapy, mainly consisting in monoclonal antibodies impairing the fixation of ligands onto the receptors, and of tyrosine kinase inhibitors, blocking the phosphorylation of the receptors. These therapeutic agents are being extensively studied and supported by considerable scientific and financial investments in the pharmaceutical industry. In this paper, we will consider Human Epidermal Growth Factor Receptors (HERs). Therapeutic use of targeted therapy agents, directed against these receptors is conditioned by the detection of target receptor expression in the tumor tissues. However, it appears more and more clearly that additional diagnosis molecular markers as well as surrogate response prediction markers are needed to enable more efficacious therapeutics.     

Understanding the causes of multidrug resistance in cancer: a comparison of doxorubicin and sunitinib

Multiple molecular, cellular, micro-environmental and systemic causes of anticancer drug resistance have been identified during the last 25 years. At the same time, genome-wide analysis of human tumor tissues has made it possible in principle to assess the expression of critical genes or mutations that determine the response of an individual patient's tumor to drug treatment. Why then do we, with a few exceptions, such as mutation analysis of the EGFR to guide the use of EGFR inhibitors, have no predictive tests to assess a patient's drug sensitivity profile. The problem urges the more with the expanding choice of drugs, which may be beneficial for a fraction of patients only. In this review we discuss recent studies and insights on mechanisms of anticancer drug resistance and try to answer the question: do we understand why a patient responds or fails to respond to therapy? We focus on doxorubicin as example of a classical cytotoxic, DNA damaging agent and on sunitinib, as example of the new generation of (receptor) tyrosine kinase-targeted agents. For both drugs, classical tumor cell autonomous resistance mechanisms, such as drug efflux transporters and mutations in the tumor cell's survival signaling pathways, as well as micro-environment-related resistance mechanisms, such as changes in tumor stromal cell composition, matrix proteins, vascularity, oxygenation and energy metabolism may play a role. Novel agents that target specific mutations in the tumor cell's damage repair (e.g. PARP inhibitors) or that target tumor survival pathways, such as Akt inhibitors, glycolysis inhibitors or mTOR inhibitors, are of high interest. In order to increase the therapeutic index of treatments, fine-tuned synergistic combinations of new and/or classical cytotoxic agents will be designed. More quantitative assessment of potential resistance mechanisms in real tumors and in real time, such as by kinase profiling methodology, will be developed to allow more precise prediction of the optimal drug combination to treat each patient.     

The importance of c-Kit and PDGF receptors as potential targets for molecular therapy in breast cancer

Molecular therapies target key functional molecules in order to halter viable operation of cancer cells. Receptor tyrosine kinases (RTKs) constitute attractive targets, as quite often their abnormal signaling has been associated with tumor development and growth. Overexpression of growth factor receptors, including IGF, EGF, TGF-alpha, SCF and PDGF receptors, has been associated with poor prognosis in breast cancer. Therefore, a number of RTKs are already targets for novel designed drugs, which involve tyrosine kinase inhibitors and monoclonal antibodies. Despite the fact that c-Kit and PDGF-R have been effective targets in a number of cancers, the experimental results in breast have not yet clarified their importance. The expression and function of c-Kit in breast cancer is a quite controversial subject. Several studies propose that the loss of c-Kit expression has been associated with tumor progress, whereas other reports indicate not only its expression but also the implication of c-Kit in breast cancer. On the other hand, the expression of PDGF-R in breast cancer is not in question. A number of inhibitors against tyrosine kinases are currently in trials as to demonstrate their importance in breast cancer treatment. Imatinib (STI571), which is a selective tyrosine kinase inhibitor and particularly of c-Kit and PDGF-R, exhibited encouraging results in respect to its inhibitory effect in cell growth and invasion potential in a panel of human breast cancer cell lines. In this review, the importance of RTKs in human cancer and of c-Kit and PDGF-R as molecular targets in breast cancer treatment, in the view of their expression profiles and the in vitro effects of STI571 is discussed.     

Drugs interfering with apoptosis in breast cancer

Apoptosis plays an important role in development, growth, differentiation, altered gravity conditions, tissue homeostasis, immune defense, and cancer. It can be initiated by external signals via death receptors, but may also emerge from mitochondria. Today most of the key players in cellular apoptosis regulation are identified and can be targeted by therapeutic strategies. In this review we focus on recent development of drugs, which interfere with apoptosis and are currently used or tested for treatment of breast cancer. These novel agents include those targeting the extrinsic pathway such as Fas, tumor necrosis factor-alpha and tumor necrosis factor related apoptosis-inducing ligand, as well as drugs targeting the intrinsic Bcl-2 family pathway, or drugs inhibiting repair enzymes such as Poly (ADP-ribose) polymerase (PARP) inhibitors. Their function will be explained, their role in tumor biology and actual clinical studies will be discussed.     

Patent focus: inhibitors of tumour cell growth

The following review covers notable patent disclosures (October 1999 - March 2000) involving agents used as inhibitors of tumour cell growth. The primary focus of the review involves specific mechanism-based therapies such as tyrosine kinase inhibitors, cyclin dependent kinase inhibitors and ras-farnesyl transferase inhibitors. Recent disclosures involving new biotechnology, taxanes and other antiproliferative natural products will also be discussed.     

Acquired resistance to drugs targeting receptor tyrosine kinases

Development of resistance to chemotherapeutic drugs represents a significant hindrance to the effective treatment of cancer patients. The molecular mechanisms responsible have been investigated for over half a century and have revealed the lack of a single cause. Rather, a multitude of mechanisms have been delineated ranging from induction and expression of membrane transporters that pump drugs out of cells (multidrug resistance (MDR) phenotype), changes in the glutathione system and altered metabolism to name a few. Treatment of cancer patients/cancer cells with chemotherapeutic agents and/or molecularly targeted drugs is accompanied by acquisition of resistance to the treatment administered. Chemotherapeutic agent resistance was initially assumed to be due to induction of mutations leading to a resistant phenotype. This has also been true for molecularly targeted drugs. Considerable experience has been gained from the study of agents targeting the Bcr-Abl tyrosine kinase including imatinib, dasatinib and sunitinib. It is clear that mutations alone are not responsible for the many resistance mechanisms in play. Rather, additional mechanisms are involved, ranging from epigenetic changes, alternative splicing and the induction of alternative/compensatory signaling pathways. In this review, resistance to receptor tyrosine kinase inhibitors (RTKIs), RTK-directed antibodies and antibodies that inactivate ligands for RTKs are discussed. New approaches and concepts aimed at avoiding the generation of drug resistance will be examined. The recent observation that many RTKs, including the IGF-1R, are dependence receptors that induce apoptosis in a ligand-independent manner will be discussed and the implications this signaling paradigm has on therapeutic strategies will be considered.     

Biochemical and cellular effects of c-Src kinase-selective pyrido[2, 3-d]pyrimidine tyrosine kinase inhibitors

Increased expression or activity of c-Src tyrosine kinase has been associated with the transformed phenotype in tumor cells and with progression of neoplastic disease. A number of pyrido[2, 3-d]pyrimidines have been characterized biochemically and in cells as part of an assessment of their potential as anti-tumor agents. The compounds were ATP-competitive inhibitors of c-Src kinase with IC(50) values < 10 nM and from 6 to >100-fold selectivity for c-Src tyrosine kinase relative to basic fibroblast growth factor receptor (bFGFr) tyrosine kinase, platelet-derived growth factor receptor (PDGFr) tyrosine kinase, and epidermal growth factor receptor (EGFr) tyrosine kinase. The compounds yielded IC(50) values < 5 nM against Lck. Human colon tumor cell growth in culture was inhibited, as was colony formation in soft agar at concentrations < 1 microM. Phosphorylation of the c-Src cellular substrates paxillin, p130(cas), and Stat3 was also inhibited at concentrations < 1 microM. Autophosphorylation of EGFr tyrosine kinase or PDGFr tyrosine kinase was not inhibited by c-Src inhibitors, thus showing the selective nature of the compounds in cells. In a mitogenesis assay measuring thymidine incorporation stimulated by specific mitogens, the c-Src tyrosine kinase inhibitors reduced incorporated thymidine in a manner consistent with previously reported roles of c-Src in mitogenic signaling. Progression through the cell cycle was inhibited at G(2)/M in human colon tumor cells treated with two of the c-Src-selective compounds, which is also consistent with earlier reports describing a requirement for active c-Src tyrosine kinase for G(2) to M phase progression. The compounds described here are selective inhibitors of c-Src tyrosine kinase and have antiproliferative effects in tumor cells consistent with inhibition of c-Src.     

Antitumor therapeutic strategies based on the targeting of epidermal growth factor-induced survival pathways

Cellular receptors for the Epidermal Growth Factor are considered important targets for the experimental treatment of human cancer. Monoclonal antibodies as well as small tyrosine kinase inhibitors have been developed and have undergone extensive evaluation in preclinical and clinical studies. Most of these studies have been conceived on the general idea that epidermal growth factor receptor (EGFR) plays a critical role on the growth and survival of human tumors. This assumption has been derived by the successful development of BCR/ABL tyrosine kinase inhibitors in human chronic myeloid leukemia as well as on the activity of antiCD20 monoclonal antibodies in lymphoproliferative disease and of anti HER2 agents in breast tumors overexpressing the targeted antigens. It is now becoming clear that factors regulating sensitivity to kinase inhibitors may differ from monoclonal antibodies and that the molecules targeted by interferring drugs must be prioritaire for growth and survival of those specific tumors in order to achieve valuable results. Recent evidence of major responses to the EGFR inhibitor Gefitinib in tumors harboring activating mutations in the EGFR appears on line with this concept. In this article we will discuss the significance of targeting the EGFR driven survival pathways. Specifically, we will afford the point of EGFR survival signalling prioritization by means of pharmacological treatment. Finally, we will address the role of profiling technologies and of novel computational system biology-based approaches for identification of innovative strategies for effective targeting of EGFR driven survival pathways.     

Recent advances in the design and discovery of small-molecule therapeutics targeting HER2/neu

The human epidermal growth factor receptor (HER) family is a highly explored and promising anticancer drug target. At present, several investigational agents targeted to the HER family of receptors are in various stages of development. Five drugs are already in the clinic for the treatment of cancers that overexpress HER family receptors. Two FDA-approved small-molecule drugs, gefitinib and erlotinib, inhibit HER1 tyrosine kinase activity. Two mAbs, cetuximab and panitumumab, target the extracellular domain of HER1, and another, trastuzumab, targets the extracellular domain of HER2. HER2 is a prominent member of the HER family of receptor tyrosine kinases and serves as a preferred dimerization partner for other HER family members. This paper reviews recently patented small-molecule inhibitors of HER2 receptor kinase activity, and inhibitors of HER2 expression and shedding. Apart from the well-explored quinazoline class of compounds (e.g., lapatinib), arylazole, benzodithiazole, pyrrolopyridazine, pyrrolotriazine and pyrrolopyrimidine classes of compounds were also claimed as HER2 tyrosine kinase inhibitors. Most of these compounds show considerable activity against all the HER family as well as members from different families of tyrosine kinases. It remains to be established how the combination of selective HER inhibitors compare with the single-agent pan-kinase inhibitors in disrupting HER family mediated signalling pathways. Such information is of paramount importance in the clinical development of HER-targeted inhibitors.     

晚期非小细胞肺癌治疗的新趋势

当今肺癌研究的热点是以与肿瘤发生、发展相关的驱动基因为靶点,研发新的药物,进行有针对性的个体化分子靶向治疗,从而改善患者预后。靶向药物、新型化疗药物、抗血管新生药物以及治疗性疫苗等各种治疗手段在晚期非小细胞肺癌(NSCLC)患者的治疗中均取得了显著的进展。其中表皮生长因子受体酪氨酸激酶抑制剂(EGFR-TKI)对于EGFR突变阳性NSCLC患者无论在一线、维持还是二线治疗中疗效均得到肯定;替吉奥胶囊有望成为晚期NSCLC一线、二线治疗新的选择;克唑替尼是棘皮动物微管结合蛋白4与间变淋巴瘤激酶融合基因(EML4-ALK)阳性患者治疗的新标准;抗血管新生药物在NSCLC的治疗疗效得到了进一步证实;EGF疫苗在晚期NSCLC治疗中的结果值得期待。这些成果将会改变目前晚期NSCLC的治疗策略,而基于患者临床特点及分子分型的个体化治疗将成为晚期非小细胞肺癌治疗的新趋势。     

Development of novel compounds to treat autoimmune and inflammatory diseases and graft versus host reactions

Recently, several new classes of agents were developed to treat patients with malignant diseases. This progress has been based on the advances made in our understanding of critical pathways involved in tumor development and growth. Dysregulated processes leading to uncontrolled regulation of proliferation, cell cycle progression, angiogenesis and apoptosis have provided rational targets for novel therapies. Compounds inhibiting protein phosphorylation and signal transduction like tyrosine kinase inhibitors and inhibitors of proteasomal degradation have demonstrated promising results and were approved for the treatment of patients with malignant diseases. However, based on in vitro and in vivo studies, there is now an emerging evidence that these agents can affect the function and differentiation of normal, non-malignant cells like dendritic cells or T lymphocytes, resulting in immunosuppression. In our review we present recent data on the immune regulatory effects of tyrosine kinase inhibitors like imatinib that is approved to treat chronic myeloid leukemias, or inhibitors of FLT3, currently used to treat acute leukemias, as well as proteasome inhibitors and peroxisome proliferator-activated receptor agonists and discuss their possible role and application in the treatment of autoimmune and graft versus host disease.     

Targeted drugs in oncology: new names, new mechanisms, new paradigm.

The molecular mechanisms of action, clinical development, and efficacy and safety of targeted antineoplastic drugs are discussed. Recently introduced mechanism-based systemic therapies for cancer may be more specific, less toxic, and more effective and represent a paradigm shift in treatment. Currently, receptor tyrosine kinases (RTKs), nonreceptor kinases, the angiogenic molecules, the enzymes involved in extracellular matrix degradation, and the enzymes responsible for protein anchorage to the cytoplasmic membrane are among the targets against which specific interventions have been developed. Monoclonal antibodies against the extracellular portion of RTKs and small-molecule inhibitors of their tyrosine kinase activity are strategies in more advanced phases of clinical development. Over the next few years, one can expect to see the results of many studies of such new pharmacologic agents or combinations. It seems likely, at this point, that targeted drugs will be used in association with existing medical, surgical, and radiotherapeutic modalities and will play an important role in the ultimate goal of reducting the burden of cancer. Targeting of molecular abnormalities that are differentially expressed in tumors may represent a more specific and less toxic way of treating cancer.     

Vascular endothelial growth factor (VEGF), VEGF receptors and their inhibitors for antiangiogenic tumor therapy

Vascular endothelial growth factor (VEGF) and its receptors (VEGFRs) have crucial roles in both physiological and pathological angiogenesis. The VEGF family consists of VEGF-A (generally called VEGF), VEGF-B, VEGF-C, VEGF-D, and placental growth factor (PlGF). These peptides show different affinities for VEGFR subtypes. VEGFR exists as three subtypes, VEGFR-1, VEGFR-2, and VEGFR-3, and is structurally related to platelet-derived growth factor receptors. All subtypes possess seven immunoglobulin-like domains in the extracellular region and a tyrosine kinase domain in the intracellular region. VEGF-A activates VEGFR-1 and VEGFR-2, whereas VEGF-B and PlGF bind to only VEFGR-1. VEGF-C and VEGF-D only bind to VEGFR-3. VEGFR-1 (fms-like tyrosine kinase-1, Flt-1) negatively regulates embryonic vasculogenesis and is involved in tumor angiogenesis via activation of monocytes and macrophages. VEGFR-2 (KDR in humans or Flk-1 in mice) is predominantly responsible for both embryonic vasculogenesis and tumor angiogenesis. In contrast, VEGFR-3 (Flt-4) regulates lymphangiogenesis. Consequently, VEGF-A and VEGFR-2 are currently the main targets for antiangiogenic therapy. Bevacizumab is a humanized monoclonal antibody against VEGF-A, and aflibercept (VEGF-Trap) is a soluble fusion protein of the extracelluar domain of VEGFR-1 and VEGFR-2 and the Fc region of immunoglobulin G (IgG). They neutralize VEGF-A, resulting in prevention of tumor angiogenesis. VEGFR tyrosine kinase inhibitors such as sunitinib and sorafenib are also effective in antiangiogenic tumor therapy by inhibiting VEGFR signaling. Anti-VEGF drugs are a promising therapy for cancer patients.     

Receptor tyrosine kinases and targeted cancer therapeutics

The majority of growth factor receptors are composed of extracellular, transmembrane, and cytoplasmic tyrosine kinase (TK) domains. Receptor tyrosine kinase (RTK) activation regulates many key processes including cell growth and survival. However, dysregulation of RTK has been found in a wide range of cancers, and it has been shown to correlate with the development and progression of numerous cancers. Therefore, RTK has become an attractive therapeutic target. One way to effectively block signaling from RTK is inhibition of its catalytic activity with small-molecule inhibitors. Low-molecular-weight TK inhibitors (TKIs), such as imatinib, targeting tumors with mutant c-Kit, and gefitinib, targeting non-small cell lung cancer with mutant epidermal growth factor receptor (EGFR), have received marketing approval in Japan. MET, fibroblast growth factor receptor (FGFR), and insulin-like growth factor-I receptor (IGF-IR) are frequently genetically altered in advanced cancers. TKIs of these receptors have not yet appeared on the market, but many anticancer drug candidates are currently undergoing clinical trials. Most of these TKIs were designed to compete with ATP at the ATP-binding site within the TK domain. This review will focus on small-molecule TKIs targeting MET, FGFR, and IGF-IR and discuss the merits and demerits of two types of agents, i.e., those with only one or a few targets and those directed at multiple targets. Targeting agents specifically inhibiting the target kinase were previously searched for based on the hypothesis that a narrow target window might reduce unexpected side effects, but agents with multiple targets have been recently developed to overcome tumors resistant against a single-targeting agent.     

Anaplastic lymphoma kinase as a therapeutic target

Introduction: Anaplastic lymphoma kinase (ALK), a tyrosine kinase receptor, has been initially identified through its involvement in chromosomal translocations associated with anaplastic large cell lymphoma. However, recent evidence that aberrant ALK activity is also involved in an expanding number of tumor types, such as other lymphomas, inflammatory myofibroblastic tumor, neuroblastomas and some carcinomas, including non-small cell lung carcinomas, is boosting research progress in ALK-targeted therapies.

Areas covered: The first aim of this review is to describe current understandings about the ALK tyrosine kinase and its implication in the oncogenesis of human cancers as a fusion protein or through mutations. The second goal is to discuss its interest as a therapeutic target and to provide a review of the literature regarding ALK inhibitors. Mechanisms of acquired resistance are also reviewed.

Expert opinion: Several ALK inhibitors have recently been developed, offering new treatment options in tumors driven by abnormal ALK signaling. However, as observed with other tyrosine kinase inhibitors, resistance has emerged in patients treated with these agents. The complexity of mechanisms of acquired resistance recently described suggests that other therapeutic options, including combination of ALK and other kinases targeted drugs, will be required in the future.     

Novel pharmacological approaches in the treatment of breast cancer

Breast cancer is the most common malignancy among women. Novel pharmacological agents, including hormonal, cytotoxic and biological therapies, are currently being developed and tested in clinical trials and may offer patients more treatment options and an improved chance of long-term survival. Signal transduction inhibitors that block endocrine or growth factor pathways have demonstrated exciting antitumour effects in clinical trials. In addition to new chemotherapeutic drugs, numerous biological agents including growth factor receptor-directed monoclonal antibodies and tyrosine kinase inhibitors that target specific molecular lesions are being examined as potential breast cancer treatments.