Receptor tyrosine kinase inhibitors

Receptor tyrosine kinases (RTKs) are a diverse group of transmembrane proteins involved in signal transduction. Their function in many cell types is to drive a wide variety of cellular functions, including growth, differentiation and angiogenesis, by transducing growth factor signals from the external milieu to intracellular processes. In malignancies, these pathways are often exploited by tumor cells to optimize tumor growth and metastasis. Indeed, alterations in RTK pathways have been implicated in oncogenic activation, tumor angiogenesis and mitogenic stimulation. Thus, RTKs are logical targets for novel anticancer agent development. There are currently a large number of small-molecule RTK antagonists in phase I to III clinical development. These agents inhibit the intracellular tyrosine kinase activity of receptors for epidermal growth factor (EGF), vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), and fibroblast growth factor (FGF). The biology and results of clinical trials with these agents will be discussed.     

Tetrapeptide tyrosine kinase inhibitors

A convenient enantioselective synthesis of p-hydroxymethyl-L-phenylalaninc was developed which produces a 4/1 ratio of L/D enantiomers resulting from a chiral phase-transfer-catalyzed alkylation. This amino acid was coupled into the p561ck tyrosine kinase substrate Ac-Leu-Pro-Tyr-Ala-NHCH₃ as a replacement for Tyr and can subsequently be elaborated into a variety of potential tyrosine kinase inhibitor designs of general structure Ac-Leu-Pro-AA-Ala-NHCH₃, wherein AA is an unnatural amino acid. The contaminating D enan-tiomer was readily removed after coupling to L-Ala-NHCH₃ of this sequence. The utility of the p-hydroxymethyl functionality in an efficient divergent synthetic strategy leading to various inhibitor designs is illustrated with the synthesis of Ac-Leu-Pro-AA-Ala-NHCH3, wherein AA is p-(R,S-hydroxyphosphonomethyl)-L-phenylalanine.     

Selective tyrosine kinase inhibitors

The tyrosine specific protein kinases (TK) are a subgroup of the largest known gene family, the kinases. Latest estimates suggest that there are over 2000 kinases encoded in the human genome [1]. TKs catalyse the transfer of phosphate to the phenolic hydroxyl of tyrosine residues in substrate proteins, consequently modifying the target protein properties. By working in concert with tyrosine phosphatases, which drive the reverse process, the TKs provide a switching system resulting in the transduction of signals from cell surface receptor to the nucleus. Inappropriate activation of TKs can lead to abnormal, dysregulated cellular proliferation and many of the known oncogenes are kinases. Naturally, there has been great interest in TKs as potential molecular targets for developing drugs for the treatment of cancer and results from the first clinical trials are now being published. Preclinical research is also focused on other therapeutic applications of TK inhibitors. This review concentrates on TK inhibitors which are either already in the clinic or likely to enter Phase I studies in the near future.     

Mechanisms of acquired resistance to tyrosine kinase inhibitors

In recent years, structural and functional studies reveal that tyrosine kinases (TKs) act as the essential components of signal transduction pathways that regulate cancer cell proliferation, apoptosis and angiogenesis, and therefore become potential targets for anticancer therapy. Most of TK inhibitors (TKIs) are small molecular and hydrophobic compounds, thus they can rapidly reach their specific intracellular targets and inhibit the activation of the related TKs. Unfortunately, accompanied with patients who gain great benefit of TKIs therapy, increasing evidences of acquired resistance to these agents have been documented. The unveiling point mutations within the kinase domain, gene amplification or overexpression, or modification of signaling pathway have been implicated in drug resistance. Additionally, overexpression of ABC transporters is likely to set stage for resistant development. In this review, we focus on the discussion of the molecular mechanisms of acquired resistance to TKIs therapy. The mechanistic understanding may help to put forward new hypotheses on drug development and design better therapies to overcome TKIs resistance.     

Forthcoming receptor tyrosine kinase inhibitors

Receptor tyrosine kinases play a significant role in carcinogenesis and have been successfully targeted with monoclonal antibodies and small-molecule inhibitors. There have been recent developments in the understanding of receptor tyrosine kinase signal transduction which have enabled better drug development. The use of receptor tyrosine kinase inhibitors in clinical practice has expanded the knowledge on cancer biology, in particular the understanding of resistant mutations and strategies to overcome such resistance. This has driven drug development from single kinase inhibitors to multi-kinase inhibitors and high-affinity kinase inhibitors. Finally, as the use of receptor tyrosine kinase inhibitors grows in clinical practice, more is learned about appropriate patient selection for such therapies. This is an exciting time in cancer therapeutics, highlighted by the advent of effective targeted therapy with receptor tyrosine kinase inhibitors.     

Forthcoming receptor tyrosine kinase inhibitors

Receptor tyrosine kinases play a significant role in carcinogenesis and have been successfully targeted with monoclonal antibodies and small-molecule inhibitors. There have been recent developments in the understanding of receptor tyrosine kinase signal transduction which have enabled better drug development. The use of receptor tyrosine kinase inhibitors in clinical practice has expanded the knowledge on cancer biology, in particular the understanding of resistant mutations and strategies to overcome such resistance. This has driven drug development from single kinase inhibitors to multi-kinase inhibitors and high-affinity kinase inhibitors. Finally, as the use of receptor tyrosine kinase inhibitors grows in clinical practice, more is learned about appropriate patient selection for such therapies. This is an exciting time in cancer therapeutics, highlighted by the advent of effective targeted therapy with receptor tyrosine kinase inhibitors.     

不可逆性酪氨酸激酶抑制剂的研究进展

酪氨酸激酶的过度表达和过度激活在许多肿瘤的发生和发展中具有重要意义,因此,多种酪氨酸激酶成为抗肿瘤药物的靶点。目前已经上市的小分子酪氨酸激酶抑制剂多属于可逆性抑制剂,这些药物具有选择性差、药效不够强烈和持久以及易引发耐药性等缺点。近些年,不可逆性酪氨酸激酶抑制剂的研究正方兴未艾。这一类药物分子以不可逆的共价键与酪氨酸激酶上ATP结合域进行结合,从而使该靶点永久性失活。由于其独特的作用机制,不可逆性酪氨酸激酶抑制剂可以有效地解决可逆性酪氨酸激酶抑制剂的几个缺点。目前,已经有一批不可逆性酪氨酸激酶抑制剂进入市场或临床研究阶段。该篇综述是对不可逆性酪氨酸激酶抑制剂的结构、药理和药化特征及其研究进展等进行总结和阐述。     

Bcr-Abl tyrosine kinase inhibitors: a patent review

Introduction: Breakpoint cluster region Abelson (Bcr-Abl) tyrosine kinase (TK) is a constitutively activated cytoplasmic TK and is the underlying cause of chronic myeloid leukemia (CML). To date, imatinib represents the frontline treatment for CML therapy. The development of resistance has prompted the search for novel Bcr-Abl inhibitors.

Areas covered: This review presents a short overview of drugs already approved for CML therapy and of the compounds that are in clinical trials. The body of the article deals with Bcr-Abl inhibitors patented since 2008, focusing on their chemical features.

Expert opinion: The search for Bcr-Abl inhibitors is very active. We believe that a number of patented compounds could enter clinical trials and some could be approved for CML therapy in the next few years. Overall, Bcr-Abl inhibitors constitute a very appealing research field that can be expected to expand further.     

表皮生长因子受体酪氨酸激酶抑制剂的耐药机制及其治疗策略

表皮生长因子受体酪氨酸激酶抑制剂在治疗非小细胞肺癌中取得了较好的疗效,但最终仍会出现耐药导致的肿瘤进展。目前的研究发现其中涉及的耐药机制多样,本文就近年来在非小细胞肺癌小分子酪氨酸激酶抑制剂治疗中存在的耐药机制及其治疗策略进行综述。     

Inhibition of platelet activation by tyrosine kinase inhibitors.

Protein tyrosine kinase (PTK) blockers (tyrphostins) inhibit in a dose-dependent fashion thrombin-induced aggregation and serotonin release with IC50 values in the 10-35 microM concentration range. The inhibition of thrombin-induced aggregation correlates with their potency in inhibiting phosphorylation of proteins on tyrosine residues. Using metabolically 32P-labelled human platelets, it was found that the tyrphostins have no effect on the decrease in [32P]phosphatidylinositol bisphosphate but prevent the replenishment of [32P]polyphosphoinositide. Tyrphostins decreased [32P]phosphatidic acid production induced by thrombin, although never by more than 50%, and only delayed the peak of diacylglycerol, suggesting that phospholipase C was still activated. Tyrphostins inhibited the thrombin-elicited early phosphorylation of p43 and p20, substrates for protein kinase C (PKC) and myosin light chain kinase, respectively, at short times of activation. This inhibition, however, was overcome after 1 min of stimulation with thrombin. Tyrphostin AG213 also inhibited platelet aggregation and tyrosine protein phosphorylation induced by phorbol myristate acetate (PMA), but did not inhibit pleckstrin phosphorylation. These results suggest that thrombin induces the phosphorylation of proteins on tyrosine residues which most probably results in the activation of phosphoinositide kinases. The ability of tyrphostins to inhibit phosphorylation of p43 and p20 when induced by thrombin but not when induced by PMA confirms that PTKs may be involved subsequent to PKC activation.     

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.     

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.     

FMS-like tyrosine kinase 3 inhibitors: a patent review

Introduction: Flt3 (FMS-like tyrosine kinase 3) has been presented as a target for novel anti-leukemic drugs because Flt3 mutations have been observed in acute myeloid leukemia (AML) cells. Due to both the poor efficacy and high toxicity of current standard AML therapies, there is an unmet need for new, improved therapies. Flt3 inhibitors have great potential to address this with mutated Flt3.

Areas covered: This paper provides a comprehensive review of the Flt3 inhibitor patents currently available. Information from original research articles in peer-reviewed journals and current clinical developments from several resources is also described.

Expert opinion: Our understanding of Flt3 inhibitors has been increased by findings from recent preclinical and clinical trials. Some Flt3 inhibitors show good efficacy in AML patients, but relapse and resistance to these inhibitors are still unavoidable. To address these problems, structurally diverse inhibitors, which exhibit inhibitory activities against both wild type and mutated Flt3, should be explored.     

Clinical pharmacokinetics of tyrosine kinase inhibitors: focus on 4-anilinoquinazolines

The 4-anilinoquinazolines (gefitinib, erlotinib and lapatinib) are members of a class of potent and selective inhibitors of the human epidermal growth factor receptor (HER) family of tyrosine kinases that have been developed to treat patients with tumours with defined genetic alterations of the HER tyrosine kinase domain. They are characterized by a moderate rate of absorption after oral administration with peak plasma concentrations at several hours post-dose. Absolute bioavailability of gefitinib and erlotinib is about 60%. Low bioavailability is assumed for lapatinib. The drugs are extensively distributed in human tissues, including tumour tissues, have a large volume of distribution at least 3-fold exceeding the volume of body water and are extensively (about 95%) protein bound to α(1)-acid glycoprotein and albumin. Existing human data for gefitinib and erlotinib indicate that these substances penetrate into the central nervous system and accumulate in brain tumours, possibly due to leaks in the blood-brain barrier. Gefitinib, erlotinib and the absorbed fraction of lapatinib undergo extensive metabolism - mainly via hepatic and intestinal cytochrome P450 (CYP) 3A4 and also via CYP2D6 (gefitinib) and CYP1A2 (erlotinib) - and are primarily eliminated by biotransformation. The excretion of unchanged gefitinib, erlotinib, lapatinib and their metabolites occurs predominantly in the faeces and only a minor fraction is excreted in the urine. No relevant effects of age, sex, bodyweight or race on their pharmacokinetics have been reported to date. Limited available data indicate that genetic polymorphisms in enzymes and transporters involved in the pharmacokinetics of gefitinib (CYP2D6) and erlotinib (CYP3A4, CYP3A5 and ABCG2 [breast cancer resistance protein]) alter the exposure to these drugs. Modification of drug dose should be considered in patients with severe hepatic impairment receiving these tyrosine kinase inhibitors and in current smokers receiving erlotinib. Existing recommendations for dose adjustment (i.e. a dose decrement or increment for gefitinib, erlotinib and lapatinib in the presence of CYP3A4 inhibitors or inducers, respectively; a dose increase for erlotinib in smoking patients) need to be validated in clinical studies. Further investigations are required to explain the large interindividual variability in the pharmacokinetics of these drugs and to assess the clinical relevance of interaction potential and inhibitory effects on the metabolizing enzymes and transporters.     

FMS-like tyrosine kinase 3 inhibitors: a patent review

INTRODUCTION: Flt3 (FMS-like tyrosine kinase 3) has been presented as a target for novel anti-leukemic drugs because Flt3 mutations have been observed in acute myeloid leukemia (AML) cells. Due to both the poor efficacy and high toxicity of current standard AML therapies, there is an unmet need for new, improved therapies. Flt3 inhibitors have great potential to address this with mutated Flt3. AREAS COVERED: This paper provides a comprehensive review of the Flt3 inhibitor patents currently available. Information from original research articles in peer-reviewed journals and current clinical developments from several resources is also described. EXPERT OPINION: Our understanding of Flt3 inhibitors has been increased by findings from recent preclinical and clinical trials. Some Flt3 inhibitors show good efficacy in AML patients, but relapse and resistance to these inhibitors are still unavoidable. To address these problems, structurally diverse inhibitors, which exhibit inhibitory activities against both wild type and mutated Flt3, should be explored.     

Epidermal growth factor receptor tyrosine kinase inhibitors

Clinical trials of selective small-molecule inhibitors of epidermal growth factor receptor tyrosine kinase activity have shown that these targeted inhibitors of proliferative signal transduction provide well-tolerated antitumour activity in patients. Preclinical pharmacology studies illustrate the potential use of these new cancer therapeutics in combination with chemotherapy, radiotherapy and hormone therapy, and in chemoprevention, in a spectrum of solid human tumours.     

Thyroid dysfunctions induced by tyrosine kinase inhibitors

Introduction: Recently, tyrosine kinase inhibitors (TKIs) have emerged as a new class of anticancer therapy. Although generally considered less toxic than cytotoxic chemotherapy, TKIs do cause significant side effects including fatigue and hypertension. In addition, thyroid dysfunction is a well-known adverse effect of TKI.

Areas covered: This review provides a comprehensive assessment of TKI-induced thyroid dysfunctions by sunitinib, sorafenib, pazopanib, imatinib, dasatinib, nilotinib, vandetanib, axitinib, motesanib and tivozanib. Furthermore, the potential mechanisms that result in this toxicity, the clinical impact of thyroid dysfunction in these patients and the controversies regarding treatment with thyroid hormone (TH) therapy are evaluated.

Expert opinion: Detection of TKI-induced thyroid dysfunction requires routine monitoring of thyroid function and may necessitate treatment. Potential benefits in developing thyroid dysfunction and potential harm in treating it necessitate controlled studies. Finally, if treatment is pursued, appropriate dosing and timing of TH replacement will require prospective clinical evaluation.