Tyrosine kinases as targets in cancer therapy - successes and failures

Protein kinases play a crucial role in signal transduction and also in cellular proliferation, differentiation and various regulatory mechanisms. The inhibition of growth-related kinases, especially tyrosine kinases, might therefore provide new therapies for diseases such as cancer. Due to the enormous progress that has been made in the past few years in the identification of the human genome, in molecular and cell biology technologies, in structural biology and in bioinformatics, the number of receptor and non-receptor tyrosine kinases that have been identified as valuable molecular targets has greatly increased. Currently, more than 20 different tyrosine kinase targets are under evaluation in drug discovery projects in oncology. The progress made in the crystallisation of protein kinases, in most cases complexed with ATP-site-directed inhibitors, has confirmed that the ATPbinding domain of tyrosine kinases is an attractive target for rational drug design; more than 20 ATP-competitive, low molecular weight inhibitors are in various phases of clinical evaluation. Meanwhile, clinical proof-of-concept (POC) has been achieved with several antibodies and small molecules targeted against tyrosine kinases. With Herceptin, Glivec and Iressa (registered in Japan), the first kinase drugs have entered the market. This review describes the preclinical and clinical status of low molecular weight drugs targeted against different tyrosine kinases (e.g., epidermal growth factor receptor [EGFR], vascular endothelial growth factor receptor [VEGFR], platelet-derived growth factor receptor [PDGFR], Kit, Fms-like tyrosine kinase [Flt]-3), briefly describes new targets, and provides a critical analysis of the current situation in the area of tyrosine kinase inhibitors.     

Tyrosine kinases as targets in cancer therapy – successes and failures

Protein kinases play a crucial role in signal transduction and also in cellular proliferation, differentiation and various regulatory mechanisms. The inhibition of growth-related kinases, especially tyrosine kinases, might therefore provide new therapies for diseases such as cancer. Due to the enormous progress that has been made in the past few years in the identification of the human genome, in molecular and cell biology technologies, in structural biology and in bioinformatics, the number of receptor and non-receptor tyrosine kinases that have been identified as valuable molecular targets has greatly increased. Currently, more than 20 different tyrosine kinase targets are under evaluation in drug discovery projects in oncology. The progress made in the crystallisation of protein kinases, in most cases complexed with ATP-site-directed inhibitors, has confirmed that the ATPbinding domain of tyrosine kinases is an attractive target for rational drug design; more than 20 ATP-competitive, low molecular weight inhibitors are in various phases of clinical evaluation. Meanwhile, clinical proof-of-concept (POC) has been achieved with several antibodies and small molecules targeted against tyrosine kinases. With Herceptin, Glivec and Iressa (registered in Japan), the first kinase drugs have entered the market. This review describes the preclinical and clinical status of low molecular weight drugs targeted against different tyrosine kinases (e.g., epidermal growth factor receptor [EGFR], vascular endothelial growth factor receptor [VEGFR], platelet-derived growth factor receptor [PDGFR], Kit, Fms-like tyrosine kinase [Flt]-3), briefly describes new targets, and provides a critical analysis of the current situation in the area of tyrosine kinase inhibitors.     

Inhibition of receptor tyrosine kinase-based signal transduction as specific target for cancer treatment

Every cell in a multicellular organism receives signals from the extracellular matrix and neighboring cells. These signals are transmitted, via transmembrane receptors and cascade proteins of the intracellular message system, inside the cell and often to the nucleus, regulating almost every physiological function of the cell. Protein tyrosine kinases constitute a family of receptors that regulate major cellular events, such as cell proliferation, differentiation, cell adhesion and apoptosis. Mutant tyrosine kinases and/or their aberrant activity are associated with human cancer and other hyper-proliferative diseases. Strategies for inhibition of aberrant tyrosine kinase activity, such as antisense oligonucleotides, antigenic stimulation and small molecular inhibitors have been developed. STI571, a phenylaminopyrimidine derivative, is considered to be the pioneer of the small molecular inhibitors available to date. It is a successful tyrosine kinase inhibitor, which is currently approved and used for the treatment of chronic myelogenous leukemia and gastrointestinal tumors. In this article we review the mechanisms of cell signaling, the signal transduction pathways related to tyrosine kinases, their relationship with cancer, and the strategies developed to inhibit the aberrant tyrosine kinase receptor-based signal transduction. Drug resistance and future perspectives for combination therapies are also discussed.     

Targeting protein kinases in cancer therapy

Over the years, many different tyrosine and serine/threonine protein kinases have been selected as candidates for drug discovery activities in oncology research, based either on their overexpression and/or dysfunction in a particular organ or tissue, or through their association in deregulated signal transduction/cell cycle pathways. This review summarises current preclinical and clinical knowledge of ATP-competitive, small molecule kinase inhibitors, and receptor or ligand-competitive antibodies of selected protein kinases in which drug discovery and development activities have advanced with some success.     

Tyrosine kinase inhibitors in cancer treatment (Part II)

In the last few years, enormous progress in the field of signal transduction inhibition has been made. Many companies have entered the field. Along with the epidermal growth factor receptor (EGFR) tyrosine kinase, many other tyrosine kinases have been identified as interesting targets for drug discovery projects. X-ray data of more than 40 crystal structures of protein kinases, in most cases complexed with an inhibitor, have been published. Pharmacophore models for the binding of inhibitors in the ATP-binding site of protein kinases have been developed that are generally applicable, enabling the rational design of tyrosine as well as serine/threonine kinase inhibitors. It has been proven by numerous examples that the ATP-binding of protein kinases is an exciting target for the design of anticancer drugs. In many cases, it has also been demonstrated that through rational design it is possible to modify a lead structure in such a way that inhibitors with an altered selectivity profile are obtained. Chemical optimisation of several lead structures led to development candidates with potent in vitro and in vivo activity fulfilling the pharmacodynamic, pharmacokinetic, toxicological and technical (synthesis, formulation) requirements for a clinical candidate. Currently, there are seven tyrosine kinase inhibitors in early phases of clinical trials. In addition, several candidates are close to entering Phase I trials this year or at the beginning of next year. It is expected that positive results from clinical trials will greatly contribute to the clinical proof of concept of the value of signal transduction inhibition and will greatly stimulate further research in this area. This review is a continuation of a review with the same title of last year and summarises published patent literature and related publications between 1997 and September 1998.     

Small-molecule inhibitors binding to protein kinase. Part II: the novel pharmacophore approach of type II and type III inhibition

Background: Protein kinases are essential enzymes propagating cellular signal transduction processes and consequently emerged as central targets for drug discovery against a wide range of diseases with a strong historical focus on oncological disorders. Several high-resolution crystal structures of various ATP-competitive inhibitors in complex with their target protein kinases have been determined and represent a wealth of detailed information about binding modes, inhibition mechanisms, and associated structure– activity relationships of target-bound small molecules. Objective: In this second part of a two-part review, we discuss the binding mode of inhibitors that target protein kinases in their inactive state. Methods: The scope of this review covers inhibitors for which crystal structures in complex with their respective kinases in the inactive state are available. Results: Structural parameters of both inhibitors and kinases contribute to the complexity of designing kinase inhibitors. Kinase inhibitors that target the inactive state of a kinase have become a novel rule in the design of highly active and selective compounds. The combination of high-resolution structures of ligand-enzyme complexes with especially detailed kinetic studies will in the long-term help to develop new low-molecular weight type II inhibitors.     

新型蛋白酪氨酸激酶抑制剂类抗肿瘤药物的研究进展

目的探讨蛋白酪氨酸激酶抑制剂抗肿瘤作用机理及其研究进展。方法综述了最新发现的小分子酪氨酸激酶抑制剂的化学结构、抗肿瘤作用及其作用机制及其发展方向等内容。结果结论蛋白酪氨酸激酶与细胞的增殖、分化、迁移和凋亡有着密切的关系,在细胞生命活动的信号转导途径中扮演着十分关键的角色,筛选酪氨酸激酶抑制剂已经成为开发抗肿瘤药物的新途径。     

Cardiovascular & Renal: AT2 antagonist inhibition of vascular restenosis

Competition in the field of tyrosine kinase inhibitors has increased in the last few years. New receptor and non-receptor tyrosine kinases have been identified as attractive targets for drug discovery programs. A pharmacophore model of the ATP-binding site of the epidermal growth factor-receptor (EGF-R) tyrosine kinase has been developed and successfully used for the rational design of tyrosine kinase inhibitors. Several inhibitor classes containing a phenylamino-pyrimidine moiety in their structure have provided highly selective ATP-competitive tyrosine kinase inhibitors, active in the low nanomolar or even picomolar range, thus proving that the ATP-binding site of tyrosine kinases is an attractive target for the design of anticancer drugs. More and more examples of inhibitors against several tyrosine kinases are being described which, in addition to showing potent and selective in vitro<> activity, exhibit potency in relevant cellular and in vivo models.<> Presently, several tyrosine kinase inhibitors with an interesting in vitro and in vivo profile are in preclinical evaluation and are expected to enter Phase I clinical trials later this year or early next year. This review summarises development of the last two years in the design and biological profiling of tyrosine kinase inhibitors with special focus on the phenylamino-pyrimidine-containing classes of compounds.     

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.     

The design of drug candidate molecules as selective inhibitors of therapeutically relevant protein kinases

The human genome encompasses some 2,000 proteins that utilize adenosine 5'-triphosphate (ATP) in one way or another and some 500 of these are protein-tyrosine and protein-serine/threonine kinases (PTKs & PSTKs). Substrate phosphorylation by these enzymes is nature's predominant molecular way of organizing cellular signal transduction and regulating biochemical processes in general. It is not surprising, therefore, that abnormal phosphorylation of cellular proteins is a hallmark of disease and that there has been a growing interest in the use of kinase inhibitors as drugs. In fact the search for such agents has recently culminated in the approval of the first kinase inhibitor drugs for medical use. Although it has been demonstrated exhaustively that potent and structurally diverse ATP-antagonistic small molecule kinase inhibitors can be found through mass screening and structure-guided design, the question of biochemical, cellular, and in vivo selectivity of such inhibitors remains much less clear. Here the medicinal chemistry of kinase inhibitors is reviewed critically with particular emphasis on target selectivity and specificity. Approaches based on chemical genomics, combinatorial target-guided ligand assembly, computational chemistry, and structural biology techniques, which aim at classifying both inhibitors and kinase targets, are given special emphasis. The various strategies in which differences in biochemical mechanism of kinase function can be exploited in order to attain selective inhibition are also discussed. Furthermore, recent developments in the design of inhibitors to selected individual validated therapeutic kinase targets, including cell cycle kinases and receptor PTKs, etc. are summarised.     

蛋白酪氨酸激酶抑制剂的研究进展

蛋白酪氨酸激酶（PTK）抑制剂是一类作用于细胞信号转导通路的分子靶向药物,针对特定靶点发挥抗肿瘤作用。目前已有十余种蛋白酪氨酸激酶抑制剂上市,且它们在多种实体瘤的治疗中显示出较好的疗效。该文对近年来蛋白酪氨酸激酶抑制剂的研究进展做简要综述。     

The development of phosphatidylinositol ether lipid analogues as inhibitors of the serine/threonine kinase, Akt

The serine/threonine kinase Akt is a component of the phosphatidylinositol 3'-kinase/Akt signal transduction pathway that is activated by receptor tyrosine kinases, activated Ras and integrins. As Akt regulates many processes crucial to carcinogenesis, and Akt activation has been observed in human cancers, intense efforts are underway to develop Akt inhibitors as cancer therapeutics. Towards this aim, phosphatidylinositol ether lipid analogues (PIAs), which are structurally similar to the products of phosphatidylinositol 3'-kinase, have been synthesised. PIAs inhibit Akt translocation, phosphorylation and kinase activity. Furthermore, they selectively induce apoptosis in cancer cell lines that depend on Akt for survival. This review will trace the development of PIAs, cover the biological activities of PIAs and discuss future steps and challenges in their development.     

Tyrosine Kinase Inhibitors in Preclinical Development

Due to the limited efficacy of cytotoxic chemotherapy in the treatment of advanced malignancy and its excessive toxicity precluding its use in chemoprevention, new therapeutic and preventive strategies have been sought. One of the most interesting of these new approaches is the manipulation of signal transduction pathways. Among the approaches being considered to eventuate such a strategy is the inhibition of autophosphorylation, a critical first step in the signal transduction pathways of many cell surface receptor tyrosine kinases, as well as of non-receptor tyrosine kinases. This article is intended to review those tyrosine kinase inhibitors that are currently in preclinical development, for which there are data to support consideration for their use in chemoprevention or cancer treatment. We will focus upon those agents that have received attention in the past several years.     

Structural biology in drug design: selective protein kinase inhibitors

Protein kinases have a fundamental role in signal transduction pathways, and aberrant kinase activity has been observed in many diseases. In recent years, kinase inhibition has become a major area for therapeutic intervention and a variety of kinase inhibitor pharmacophores has been described. This review illustrates some of the efforts and results in the field of structure-based design of protein kinase inhibitors. The methods and results discussed here illustrate the power of structure-based design in lead discovery, for example via virtual screening and in guiding the optimization of the pharmacological properties of these molecules.     

Signal transduction pathways triggered by selective formylpeptide analogues in human neutrophils

Human neutrophils are highly specialised for their primary function, i.e. phagocytosis and destruction of microorganisms. Leukocyte recruitment to sites of inflammation and infection is dependent upon the presence of a gradient of locally produced chemotactic factors. The bacterial peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP) was one of the first of these to be identified and is a highly potent leukocyte chemoattractant. It interacts with its receptor on the neutrophil membrane, activating these cells through a G-protein-coupled pathway. Two functional fMLP receptors have thus far been cloned and characterized, namely FPR (formyl peptide receptor) and FPRL1 (FPR like-1), with high and low affinities for fMLP, respectively. FMLP is known to activate phospholipase C (PLC), PLD, PLA2 and phosphatidylinositol-3-kinase (PI3K), and it also activates tyrosine phosphorylation. The second messengers resulting from the fMLP receptor interaction act on various intracellular kinases, including protein kinase C (PKC) and mitogen-activated protein kinases (MAPKs). The activation of these signal transduction pathways is known to be responsible for various biochemical responses which contribute to physiological defence against bacterial infection and cell disruption. This review will consider the ability of selective analogues (ligands able to discriminate between different biological responses) to activate a single spectrum of signal transduction pathways capable of producing a unique set of cellular responses, hypothesising that a distinctive imprint of signal protein activation may exist. Through more complete understanding of intracellular signaling, new drugs could be developed for the selective inflammatory blockade.     

The development of phosphatidylinositol ether lipid analogues as inhibitors of the serine/threonine kinase,Akt

The serine/threonine kinase Akt is a component of the phosphatidylinositol 3′-kinase/Akt signal transduction pathway that is activated by receptor tyrosine kinases, activated Ras and integrins. As Akt regulates many processes crucial to carcinogenesis, and Akt activation has been observed in human cancers, intense efforts are underway to develop Akt inhibitors as cancer therapeutics. Towards this aim, phosphatidylinositol ether lipid analogues (PIAs), which are structurally similar to the products of phosphatidylinositol 3′-kinase, have been synthesised. PIAs inhibit Akt translocation, phosphorylation and kinase activity. Furthermore, they selectively induce apoptosis in cancer cell lines that depend on Akt for survival. This review will trace the development of PIAs, cover the biological activities of PIAs and discuss future steps and challenges in their development.     

Tyrosine kinase inhibitors targeted to the epidermal growth factor receptor subfamily: role as anticancer agents

Abnormal cell signal transduction arising from protein tyrosine kinases has been implicated in the initiation and progression of a variety of human cancers. Over the past 2 decades pharmaceutical and university laboratories have been involved in a tremendous effort to develop compounds that can selectively modulate these abnormal signalling pathways. Targeting receptor tyrosine kinases, especially the epidermal growth factor receptor subfamily, has been at the forefront of this effort as a result of strong clinical data correlating over-expression of these receptors with more aggressive cancers. There are a variety of strategies under development for inhibiting the kinase activity of these receptors, targeting both the extracellular and intracellular domains. Antibody-based approaches, immunotoxins and ligand-binding cytotoxic agents use the extracellular domain for targeted tumour therapy. Small molecule inhibitors target the intracellular catalytic region by interfering with ATP binding, while nonphosphorylatable peptides are aimed at the intracellular substrate binding region. Compounds that inhibit subsequent downstream signals from the receptor by interrupting intracellular protein recognition sequences are also being investigated. In the past 5 years enormous progress has been made in developing tyrosine kinase inhibitor compounds with sufficient potency, bioavailability and selectivity against this subfamily of receptor tyrosine kinases. The anti-HER2 monoclonal antibody, trastuzumab, for patients with metastatic breast cancer is the first of these inhibitor compounds to gain FDA approval. However, preclinical and clinical trials are ongoing with a variety of other monoclonal antibodies, immunotoxins, and small molecule quinazoline and pyrimidine-based inhibitors. Although their cytotoxic and cytostatic potential has been proven, they are not likely to replace standard chemotherapy regimens as single-agent, first-line therapeutics. Instead, their promising additive and synergistic antitumour effects in combination with standard chemotherapeutics suggest that these novel agents will find their greatest utility and efficacy in conjunction with existing anticancer agents.     

The use of selective pharmacological inhibitors to delineate signal transduction pathways activated during complement receptor-mediated degranulation in chicken heterophils

Complement receptors (CRs), along with Fc receptors, play a primary role in the removal of bacterial pathogens in poultry. The binding of serum-opsonized bacteria to CR results in the secretion of both toxic oxygen metabolites and antibacterial granules. We have previously shown that the stimulation of chicken heterophils with serum-opsonized Salmonella enteritidis induced tyrosine kinase-dependent phosphorylation regulated degranulation. In the present studies, we used selective pharmacological inhibitors to investigate the roles of protein tyrosine kinases, phospholipases C and D (PLC and PLD), phosphatidylinositol 3'-kinase (PI3-K), and the super family of mitogen-activated protein kinases (MAPKs) on CR-mediated heterophil degranulation. Inhibitors of receptor-linked tyrosine kinases (the tryphostins AG1478 and AG1296) had no attenuating effects on CR-mediated degranulation. However, PP2, a selective inhibitor of the src family of protein tyrosine kinases, and piceatannol, an inhibitor of Syk tyrosine kinases, both significantly attenuated the CR-mediated degranulation. Additionally, the specific inhibitors of PLC, U73122, and PI3-K, LY294002, significantly decreased CR-mediated heterophil degranulation. Two inhibitors of PLD-mediated signaling, 2,3-diphosphoglycerate (2,3-DPG) and 1-butanol, hindered degranulation. Addition of purified PLD restored control levels of degranulation in heterophils in which PLD was inhibited. Lastly, SP600125, a selective inhibitor of c-Jun N-terminal kinase (JNK), inhibited degranulation; whereas neither PD98059, the inhibitor of p38 MAPK, nor SB203580, the inhibitor of extracellular signal-regulated kinase, had any effect on CR-mediated heterophil degranulation. These studies demonstrate that CRs on chicken heterophils lack intrinsic tyrosine kinase activity, but that binding of serum-opsonized bacteria activates both proximal tyrosine kinases (src and Syk kinases), but differentially activates downstream tyrosine kinases (JNK, but not p38 nor ERK). Activation of src and Syk kinases plays a significant role in signal transduction of heterophil degranulation probably by stimulating downstream phosphorylation of PLC, PLD, and PI3-K. PI3-K has also been recently shown to be an upstream mediator of JNK activation, suggesting that this enzyme can induce signaling as both a lipid kinase and protein kinase. Engaging CRs on chicken heterophils activates a proximal tyrosine kinase (src and Syk kinases)-->PLC (PLD)-->PI3-K-->JNK signal transduction pathway that induces degranulation.     

Recent advances in the discovery of Src kinase inhibitors

Src family kinases are involved in the regulation of a wide variety of normal cellular signal transduction pathways, such as cell growth, differentiation, survival, adhesion and migration. Considerable evidence implicates elevated expression and/or activity of Src kinases in many human cancers, osteoporosis, cardiovascular disorders and immune system dysfunction; thus, this family of protein tyrosine kinases now exists as intriguing targets for both basic research and drug discovery. Herein, a number of examples of currently developed Src family kinase inhibitors in selected patents from 2002 – 2005 will be described. Special attention will be made to the chemical diversity of ATP binding site inhibitors, potency, selectivity and therapeutic application of the compounds.