Effects of AMN107, a novel aminopyrimidine tyrosine kinase inhibitor, on human mast cells bearing wild-type or mutated codon 816 c-kit

Clinical Use of Molecular Biomarkers in Canine and Feline Oncology: Current and Future

Molecular biomarkers are central to personalised medicine for human cancer patients. It is gaining traction as part of standard veterinary clinical practice for dogs and cats with cancer. Molecular biomarkers can be somatic or germline genomic alterations and can be ascertained from tissues or body fluids using various techniques. This review discusses how these genomic alterations can be determined and the findings used in clinical settings as diagnostic, prognostic, predictive, and screening biomarkers. We showcase the somatic and germline genomic alterations currently available to date for testing dogs and cats in a clinical setting, discussing their utility in each biomarker class. We also look at some emerging molecular biomarkers that are promising for clinical use. Finally, we discuss the hurdles that need to be overcome in going 'bench to bedside', i.e., the translation from discovery of genomic alterations to adoption by veterinary clinicians. As we understand more of the genomics underlying canine and feline tumours, molecular biomarkers will undoubtedly become a mainstay in delivering precision veterinary care to dogs and cats with cancer.

To Investigate Growth Factor Receptor Targets and Generate Cancer Targeting Inhibitors

Receptor tyrosine kinase (RTK) regulates multiple pathways, including Mitogenactivated protein kinases (MAPKs), PI3/AKT, JAK/STAT pathway, etc. which has a significant role in the progression and metastasis of tumor. As RTK activation regulates numerous essential bodily processes, including cell proliferation and division, RTK dysregulation has been identified in many types of cancers. Targeting RTK is a significant challenge in cancer due to the abnormal upregulation and downregulation of RTK receptors subfamily EGFR, FGFR, PDGFR, VEGFR, and HGFR in the progression of cancer, which is governed by multiple RTK receptor signalling pathways and impacts treatment response and disease progression. In this review, an extensive focus has been carried out on the normal and abnormal signalling pathways of EGFR, FGFR, PDGFR, VEGFR, and HGFR and their association with cancer initiation and progression. These are explored as potential therapeutic cancer targets and therefore, the inhibitors were evaluated alone and merged with additional therapies in clinical trials aimed at combating global cancer.

Adverse events associated with nilotinib in chronic myeloid leukemia: mechanisms and management strategies

Introduction: Nilotinib is a second-generation tyrosine kinase inhibitor (TKI) targeting BCR/ABL, which is used for the first-line treatment of newly diagnosed chronic myeloid leukemia (CML) patients and the second-line treatment of most CML patients who are resistant or intolerant to prior therapy that includes imatinib. In addition to common adverse reactions, long-term use of nilotinib shows some toxicities that are different from those of occurring during other BCR/ABL TKI treatments, such as cardiovascular toxicity. It is life-threatening, which would affect not only the choice of initial treatment of CML patients but also the safety of long-term medication.Areas covered: Through searching literature and reports from PubMed and clinical trials, here we review a profile of the adverse effects induced by nilotinib. We also discuss the potential molecular toxicological mechanisms and clinical management, which may provide strategies to prevent or intervene the toxicity associated with nilotinib.Expert opinion: Severe adverse effects associated with nilotinib limit its long-term clinical application. However, the exact mechanisms underlying these toxicities remain unclear. Future research should focus on the developing strategies to reduce the toxicities of nilotinib as well as to avoid similar toxicity in the development of new drugs.

Nanoformulations of small molecule protein tyrosine kinases inhibitors potentiate targeted cancer therapy

Protein tyrosine kinases (PTKs) are closely related to tumor development and usually participate in apoptosis, DNA repair, and cell proliferation by activating signaling pathways. Therefore, PTKs have become the most promising targets for cancer therapy. In recent years, a large number of studies on the mechanism of tyrosine kinase activation have indicated that tyrosine kinase inhibitors (TKIs) have important clinical significance and application prospects as targeted anticancer drugs because they can effectively block certain cellular signaling pathways, inhibit tumor metastases and reduce tumor proliferation. Although the increasing emergence of anticancer drug resistance limits the clinical application of TKIs, emerging nanotechnology has made it possible to solve this problem. In this work, the state-of-art of small molecule protein tyrosine kinase inhibitors and the applications of drug delivery systems for TKIs are reviewed, and the potentials and challenges for future research of small molecule TKIs are addressed.

Comparative aspects of mast cell neoplasia in animals and the role of KIT in prognosis and treatment

Mast cell neoplasia clinical presentation and biological behaviour vary considerably across mammalian species, ranging from a solitary benign mass to an aggressive systemic malignancy. Mutations in the KIT Proto‐Oncogene Receptor Tyrosine Kinase (KIT) gene are common molecular abnormalities involved in mast cell tumorigenesis. KIT mutations often occur in dog, cat and human neoplastic mast cells and result in altered Kit protein structure and function. In dogs, certain KIT mutations are associated with more malignant and lethal disease. In contrast, KIT mutations in feline and human mast cell neoplasms are not correlated with prognosis, but are of value in diagnosis and treatment planning in humans. KIT genetic abnormalities have not been well investigated in other species, although aberrant cytoplasmic Kit protein staining detected in neoplasms of the ferret, horse and cow resembles aberrant Kit staining patterns detected in neoplastic mast cells of dogs, cats and humans. Mutations within KIT are classified as either regulatory‐type or enzymatic pocket‐type mutations according to their location within the KIT Proto‐Oncogene. Mutations within the enzymatic pocket domain confer tumour resistance to tyrosine kinase inhibitors (TKIs). Hence, knowledge of tumour KIT mutation status adds valuable information for optimizing patient treatment strategies. The use of TKIs in combination with conventional chemotherapeutics has opened a new treatment avenue for patients unresponsive to existing drugs. This review highlights the similarities and differences of mast cell neoplasia in mammals with a special focus on the involvement of KIT in the canine and feline forms in comparison to human mast cell neoplasia.

KIT as a therapeutic target for non-oncological diseases

KIT is a receptor tyrosine kinase that after binding to its ligand stem cell factor activates signaling cascades linked to biological processes such as proliferation, differentiation, migration and cell survival. Based on studies performed on SCF and/or KIT mutant animals that presented anemia, sterility, and/or pigmentation disorders, KIT signaling was mainly considered to be involved in the regulation of hematopoiesis, gametogenesis, and melanogenesis. More recently, novel animal models and ameliorated cellular and molecular techniques have led to the discovery of a widen repertoire of tissue compartments and functions that are being modulated by KIT. This is the case for the lung, heart, nervous system, gastrointestinal tract, pancreas, kidney, liver, and bone. For this reason, the tyrosine kinase inhibitors that were originally developed for the treatment of hemato-oncological diseases are being currently investigated for the treatment of non-oncological disorders such as asthma, rheumatoid arthritis, and alzheimer's disease, among others. The beneficial effects of some of these tyrosine kinase inhibitors have been proven to depend on KIT inhibition. This review will focus on KIT expression and regulation in healthy and pathologic conditions other than cancer. Moreover, advances in the development of anti-KIT therapies, including tyrosine kinase inhibitors, and their application will be discussed.

Infliction of proteotoxic stresses by impairment of the unfolded protein response or proteasomal inhibition as a therapeutic strategy for mast cell leukemia

The intensity and duration of endoplasmic reticulum (ER) stress converts the unfolded protein response (UPR) from an adaptive into a terminal response. The first regulates homeostasis, the latter triggers apoptosis. Cells that rapidly proliferate and possess developed secretory capabilities, such as leukemia cells, depend on an efficiently operating UPR to maintain proteostasis. Activation of terminal UPR by either blockade of adaptive UPR or exaggeration of ER stress has been explored as a novel approach in cancer therapy. For mast cell leukemia (MCL) the efficacy of both approaches, by utilizing the KIT^V560G,D816V-positive MCL cell line HMC-1.2, was investigated. We show that HMC-1.2 cells display a tonic activation of the IRE1α arm of the UPR, which constitutively generates spliced XBP1. Inhibition of IRE1α by different types of inhibitors (MKC-8866, STF-083010, and KIRA6) suppressed proliferation at concentrations needed for blockade of IRE1α-mediated XBP1 splicing. At higher concentrations, these inhibitors triggered an apoptotic response. Blocking the proteasome by bortezomib, which confers an exaggerated UPR, resulted in a marked cytotoxic response. Bortezomib treatment also caused activation of the kinase JNK, which played a pro-proliferative and anti-apoptotic role. Hence, the combination of bortezomib with a JNK inhibitor synergized to induce cell death. In summary, the UPR can be addressed as an effective therapeutic target against KIT^D816V-positive MCL.

Nilotinib in patients with systemic mastocytosis: analysis of the phase 2, open-label, single-arm nilotinib registration study

Purpose

Activating KIT mutations are part of the pathogenesis of systemic mastocytosis (SM). Nilotinib is a tyrosine kinase inhibitor that potently inhibits activated forms of KIT. This phase 2, open-label, single-arm study (CAMN107A2101; www.clinicaltrials.gov NCT00109707) evaluated nilotinib in patients with SM.

Methods

Patients with SM [aggressive SM (ASM), indolent SM, or other] received nilotinib 400 mg twice daily. C-findings were collected retrospectively to assess response using criteria proposed after trial initiation. Response was evaluated using improvements in laboratory findings (for all patients) and ASM response criteria (for the ASM subgroup).

Results

In 61 patients enrolled, the median nilotinib exposure was 232 days (range 3–1274 days) with a median follow-up of 34.7 months. In patients with ASM (n = 37), the overall response rate was 21.6 %. In the eight responders, all of whom had a KIT D816V mutation at any time, mast cell infiltration and tryptase level decreased by 70 % and 29.8 %, respectively; absolute neutrophil count increased by 94.7 %. Laboratory parameters also improved in the non-ASM subgroups. Overall survival at 24 months was 81.2 % (95 % CI 70.6–91.8 %) with median survival not yet reached. New or worsening grade 3/4 hematologic adverse events (AEs) included thrombocytopenia (10.3 %), anemia (10.0 %), and neutropenia (6.9 %). The most common grade 3/4 nonhematologic drug-related AEs were diarrhea (6.6 %) and headache (4.9 %). Eleven patients (9 with ASM, 2 with MCL) died, 10 due to progressive disease; 7 deaths occurred ≥28 days after treatment discontinuation.

Conclusions

Nilotinib 400 mg twice daily was effective in some patients with SM, including patients with mutated KIT D816V.

Signal transduction and chemotaxis in mast cells

Mast cells play crucial roles in both innate and adaptive arms of the immune system. Along with basophils, mast cells are essential effector cells for allergic inflammation that causes asthma, allergic rhinitis, food allergy and atopic dermatitis. Mast cells are usually increased in inflammatory sites of allergy and, upon activation, release various chemical, lipid, peptide and protein mediators of allergic reactions. Since antigen/immunoglobulin E (IgE)-mediated activation of these cells is a central event to trigger allergic reactions, innumerable studies have been conducted on how these cells are activated through cross-linking of the high-affinity IgE receptor (FcεRI). Development of mature mast cells from their progenitor cells is under the influence of several growth factors, of which the stem cell factor (SCF) seems to be the most important. Therefore, how SCF induces mast cell development and activation via its receptor, KIT, has been studied extensively, including a cross-talk between KIT and FcεRI signaling pathways. Although our understanding of the signaling mechanisms of the FcεRI and KIT pathways is far from complete, pharmaceutical applications of the knowledge about these pathways are underway. This review will focus on recent progresses in FcεRI and KIT signaling and chemotaxis.

A preclinical study demonstrating the efficacy of nilotinib in inhibiting the growth of pediatric high-grade glioma

Solid tumors arising from malignant transformation of glial cells are one of the leading causes of central nervous system tumor-related death in children. Recurrence in spite of rigorous surgical and chemoradiation therapies remains a major hurdle in management of these tumors. Here, we investigate the efficacy of the second-generation receptor tyrosine kinase inhibitor nilotinib as a therapeutic option for the management of pediatric gliomas. We have utilized two independent pediatric high-grade glioma cell lines with either high platelet-derived growth factor receptor alpha (PDGFRα) or high PDGFRβ expression in in vitro assays to investigate the specific downstream effects of nilotinib treatment. Using in vitro cell-based assays we show that nilotinib inhibits PDGF-BB-dependent activation of PDGFRα. We further show that nilotinib is able to decrease cell proliferation and anchorage-independent growth via suppression of AKT and ERK1/2 signaling pathways. Our results suggest that nilotinib may be effective for management of a PDGFRα-dependent group of pediatric gliomas.

Novel targeted therapies for eosinophil-associated diseases and allergy

Eosinophil-associated diseases often present with life-threatening manifestations and/or chronic organ damage. Currently available therapeutic options are limited to a few drugs that often have to be prescribed on a lifelong basis to keep eosinophil counts under control. In the past 10 years, treatment options and outcomes in patients with clonal eosinophilic and other eosinophilic disorders have improved substantially. Several new targeted therapies have emerged, addressing different aspects of eosinophil expansion and inflammation. In this review, we discuss available and currently tested agents as well as new strategies and drug targets relevant to both primary and secondary eosinophilic diseases, including allergic disorders.

The overexpression of KIT proto-oncogene in acute leukemic cells is not necessarily caused by the gene mutation

KIT is detected in a variety of cells, also in acute leukemia. Inhibition of wild-type KIT is not always satisfactory. The aim of this work was to evaluate the frequency of the most common KIT mutations in acute myeloid leukemia (AML) and determine the correlation between mutation and expression level. Samples were obtained from 75 patients with AL. CD117 presence was shown in 45 of 51 patients with AML and in 1 of 16 patients with acute lymphocytic leukemia (ALL). Asp816Val mutation was found in 3.5% of cases of AML and Val560Gly mutation in 1 sample with acute biclonal leukemia. Other genetic changes were found in 15 of 57 samples with AML: polymorphisms Met541Leu in 14% of cases, Lys546Lys in 7% and 1 case of acute biclonal leukemia, Ile798Ile in 5.3% of cases, Met541Leu in 1 acute biphenotypic leukemia and in 6.3% of ALL. Polymorphism Lys546Lys was also shown in 1 case of acute biclonal leukemia. Nonsilent genetic changes were detected in a total of 23% cases with core binding factor leukemia. There was no statistical significance between KIT expression and genetic changes. There was no correlation between the incidence and types of KIT mutations and its expression on cells in AML.

Mastocytosis: update on pharmacotherapy and future directions

INTRODUCTION

Mastocytosis is a disorder characterized by abnormal mast cell (MC) accumulation in skin and internal organs such as bone marrow. The disease follows a benign course in most patients with cutaneous and indolent systemic mastocytosis (SM); however, advanced variants associated with decreased life expectancy also exist. Pharmacotherapy of mastocytosis is aimed at the control of symptoms caused by MC mediator release, treatment of comorbidities and cytoreductive therapies in advanced variants.

AREAS COVERED

This article will cover the general treatment principles of anti-MC mediator and cytoreductive therapies of mastocytosis. The literature discussed was retrieved with PubMed using the search terms 'treatment of mastocytosis,' 'mastocytosis antimediator therapy' and looking for important cross-references.

EXPERT OPINION

Pharmacotherapy of mastocytosis should be individualized for each patient considering the category of disease, reduction of risk of anaphylaxis, constitutional symptoms and comorbidities including osteoporosis. Cytoreductive therapies are generally reserved for patients with aggressive mastocytosis (ASM), MC leukemia (MCL) and MC sarcoma (MCS); however, some patients with indolent disease and recurrent anaphylactic episodes not responsive to antimediator therapies may also be considered for cytoreduction on a case-by-case basis.

Advanced systemic mastocytosis: the impact of KIT mutations in diagnosis, treatment, and progression

Apart from indolent systemic mastocytosis (SM), which is associated with a favorable prognosis, other subtypes of SM (SM with associated clonal hematologic non–mast cell lineage disease, aggressive SM, and mast cell leukemia – collectively referred to in this review as advanced SM) can be debilitating. The complexity of SM makes both the diagnosis and design of response criteria challenging for clinical studies. The tyrosine kinase KIT has been shown to play a crucial role in the pathogenesis of SM and has been a focal point in the development of targeted therapy. Mutations within various domains of the KIT receptor that lead to constitutive activation have been identified in patients, and those involving the activation loop of the KIT receptor are the mutations most frequently detected in patients with mastocytosis. Aberrant activation of the KIT receptor results in increased production of mast cells in extracutaneous organs that may lead to organ failure or early death. This review discusses the diagnosis and management of patients with advanced SM, including the relevance of KIT in this disease, potential therapies targeting this kinase, and criteria for measuring responses to these therapies.

Proteasome inhibition upregulates Bim and induces caspase-3-dependent apoptosis in human mast cells expressing the Kit D816V mutation

The majority of patients with systemic mastocytosis exhibit a D816V mutation in the activating loop of the Kit receptor expressed on mast cells. The Kit ligand regulates mast cell survival by transcriptional repression of the proapoptotic BH3-only protein Bim and by promoting Bim phosphorylation that makes it vulnerable for proteasomal-dependent degradation. We investigated here whether prevention of Bim degradation by a proteasomal inhibitor, MG132, would induce apoptosis in mast cells with the D816V mutation. Human umbilical cord blood-derived mast cells (CBMCs) with wild-type (wt) Kit and two different subclones of the human mast cell line-1 (HMC-1) were used for the study: HMC-1.1 with the V560G mutation in the juxtamembrane domain and HMC-1.2 carrying the V560G mutation together with the D816V mutation. MG132 at 1 μM induced apoptosis in all cell types, an effect accompanied by increased BH3-only proapoptotic protein Bim. The raise of Bim was accompanied by caspase-3 activation, and a caspase-3 inhibitor reduced MG132-induced apoptosis. Further, MG132 caused a reduction of activated Erk, a negative regulator of Bim expression, and thus Bim upregulation. We conclude that decreased phosphorylation and increased levels of Bim overcome the prosurvival effect of the D816V mutation and that the results warrant further investigations of the clinical effects of proteasomal inhibition in systemic mastocytosis.

Therapeutic targeting of c-KIT in cancer

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.

Mast Cell Leukaemia: c-KIT Mutations Are Not Always Positive

Mast cell leukemia (MCL) is a rare and aggressive disease with poor prognosis and short survival time. D816V c-KIT mutation is the most frequent molecular abnormality and plays a crucial role in the pathogenesis and development of the disease. Thus, comprehensive diagnostic investigations and molecular studies should be carefully carried out to facilitate the therapeutic choice. A MCL patient's case with rare phenotypic and genotypic characteristics is described with review of major clinical biological and therapeutic approaches in MCL.

Targeting c-kit in the therapy of mast cell disorders: current update

Classically, mast cells have been widely associated with allergic reactions and parasite infections, but recent studies have elucidated the important role of these cells in innate and acquired immunity, wound healing, fibrosis, and chronic inflammatory diseases. Mast cells release an impressive array of proinflammatory and immunoregulatory mediators after activation induced by either immunoglobulin-E (IgE)-dependent or IgE-independent mechanisms. Proliferation, differentiation, survival and activation of mast cells are regulated by stem cell factor (SCF), the ligand for the c-kit tyrosine kinase receptor which is expressed on the mast cell surface. Inappropriate c-kit activation causes accumulation of mast cells in tissues resulting in mastocytosis. A number of activating mutations in c-kit have recently been identified and these mutations results in aberrant mast cell growth. Thus, c-kit inhibitors may have potential application in multiple conditions associated with mast cell disorders including systemic mastocytosis, anaphylaxis, and asthma. The present perspective aims to summarize recent findings in mast cell biology and the role of c-kit tyrosine kinase inhibitors in the treatment of different mast cell associated disorders.

Systemic mastocytosis

Systemic mastocytosis (SM) is a clonal disorder of hematopoietic system characterized by abnormal growth and accumulation of mast cells in various tissues. Its clinical spectrum ranges from mild disease to an aggressive course with life-threatening conditions. Some of the clinical signs or symptoms of SM (hyperhidrosis, syncope and hypotensive/tachycardiac attacks) require consideration of pheochromocytoma and carcinoid syndrome in the differential diagnosis. The diagnosis relies on the demonstration of mast cell aggregates in bone marrow or extracutaneous tissues. The World Health Organization categorizes SM into 6 variants: indolent SM, SM with associated clonal hematological nonmast cell lineage disease, aggressive SM, mast cell leukemia, mast cell sarcoma and extracutaneous mastocytosis. Patients with indolent SM have a favorable prognosis with a life expectancy comparable with the healthy population, and symptomatic treatment is usually sufficient. However, more aggressive forms may be life threatening, and cytoreductive treatment is indicated in most cases.

The skeletal effects of the tyrosine kinase inhibitor nilotinib

Nilotinib is a tyrosine kinase inhibitor (TKI) developed to manage imatinib-resistance in patients with chronic myeloid leukemia (CML). It inhibits similar molecular targets to imatinib, but is a significantly more potent inhibitor of Bcr-Abl. Nilotinib exhibits off-target effects in other tissues, and of relevance to bone metabolism, hypophosphataemia has been reported in up to 30% of patients receiving nilotinib. We have assessed the effects of nilotinib on bone cells in vitro and on bone metabolism in patients receiving nilotinib for treatment of CML. We firstly investigated the effects of nilotinib on proliferating and differentiating osteoblastic cells, and on osteoclastogenesis in murine bone marrow cultures and RAW264.7 cells. Nilotinib potently inhibited osteoblast proliferation (0.01-1uM), through inhibition of the platelet-derived growth factor (PDGFR). There was a biphasic effect on osteoblast differentiation such that it was reduced by lower concentrations of nilotinib (0.1-0.5uM), with no effect at higher concentrations (1uM). Nilotinib also potently inhibited osteoclastogenesis, predominantly by stromal-cell dependent mechanisms. Thus, nilotinib decreased osteoclast development in murine bone marrow cultures, but did not affect osteoclastogenesis in RAW264.7 cells. Nilotinib treatment of osteoblastic cells increased expression and secretion of OPG and decreased expression of RANKL. In 10 patients receiving nilotinib, levels of bone turnover markers were in the low-normal range, despite secondary hyperparathyroidism, findings that are similar to those in patients treated with imatinib. Bone density tended to be higher than age and gender-matched normal values. These data suggest that nilotinib may have important effects on bone metabolism. Prospective studies should be conducted to determine the long-term effects of nilotinib on bone density and calcium metabolism.

Tyrosine kinase inhibitors in the treatment of systemic mastocytosis

Systemic mastocytosis (SM) is a heterogeneous disease, vast majority of these patients have a gain of function mutation in the gene encoding the tyrosine kinase KIT (KIT(D816V)). A small subset of SM patients with KIT(D816V) mutation require cytoreductive therapy. In these patients, tyrosine kinase inhibitors (TKIs) have been actively investigated over the last decade because of codon 816 KIT mutations causing constitutive activation of tyrosine kinase activity of the molecule. The main question has been whether the success story with imatinib in chronic myeloid leukemia (CML), another disease associated with a constitutively active tyrosine kinase, could be mimicked in mastocytosis. However, the results from various TKIs in SM with KIT(D816V) mutation have been disappointing to date. Only a few of the TKIs sufficiently block KIT(D816V) activity and have shown promising clinical results. The data from these studies indicate that, apart from KIT(D816V), other kinase targets and target pathways may play a role in disease evolution and progression, especially in patients with SM with an associated clonal hematological non-mast cell lineage disease (SM-AHNMD). Imatinib is effective in patients with increased mast cells and eosinophils associated with FIP1L1/PDGFRA+ (e.g., myeloid neoplasm with eosinophilia and rearrangement of PDGFRA) or rare patients with SM associated with KIT mutations outside of exon 17. This review will focus on the KIT receptor, KIT mutations, and the effects of the mutations in SM. The preclinical and clinical activities of FDA approved TKIs (for CML) as well as novel TKIs in SM will be evaluated.

Pathogenesis, classification and treatment of mastocytosis: state of the art in 2010 and future perspectives

Mastocytosis is a myeloid neoplasm characterized by abnormal accumulation and frequent activation of mast cells (MCs) in various organs. Organ systems typically involved are the bone marrow, skin, liver and gastrointestinal tract. In most adult patients, the systemic form of mastocytosis (SM) is diagnosed, which includes an indolent subvariant, an aggressive subvariant and a leukemic subvariant, also termed MC leukemia. Whereas in pediatric mastocytosis, which is usually confined to the skin, a number of different KIT mutations and other defects may be detected, the KIT mutation D816V is detectable in most (adult) patients with SM. In a subset of these patients, additional oncogenic factors may lead to enhanced survival and growth of MCs and, thus, to advanced SM. Other factors may lead to MC activation, with consecutive anaphylactic reactions that can be severe or even fatal. Treatment of SM usually focuses on symptom relief by histamine receptor antagonists and other supportive therapy. However, in aggressive and leukemic variants, cytoreductive and targeted drugs must be applied. Unfortunately, the prognosis in these patients remains poor, even when treated with novel KIT-targeting agents, polychemotherapy or stem cell transplantation. This article provides a summary of our knowledge on the pathogenesis and on treatment options in SM.

Nilotinib for the treatment of chronic myeloid leukemia

The introduction of targeted therapy has revolutionized the treatment of chronic myeloid leukemia (CML). The pivotal role of the Philadelphia chromosome, resulting from the breakpoint cluster region-Abelson (BCR-ABL) translocation, led to the development of imatinib mesylate, a tyrosine kinase inhibitor with significant activity against the BCR-ABL oncoprotein. Unprecedented clinical activity in CML led to rapid approval and established first-line therapy with imatinib mesylate as the standard of care in most patients. However, the occurrence of imatinib resistance or intolerance has sparked the development of newer drugs with increased activity or specificity. Nilotinib is a second-generation tyrosine kinase inhibitor that has been rationally designed on the basis of imatinib. An overview is given on clinical results in imatinib-resistant or -intolerant patients that led to its current approval as second-line therapy for the chronic and accelerated phases of CML. Future studies will address the role of nilotinib as first-line therapy, in combination strategies and in the context of specific BCR-ABL mutations.

Nilotinib: a novel, selective tyrosine kinase inhibitor

The development of tyrosine kinase inhibitors (TKIs) for the treatment of chronic myelogenous leukemia (CML) was based on the discovery that CML stem and progenitor cells overexpress the abnormal fusion protein kinase BCR-ABL. The prototype TKI, imatinib, selectively inhibits BCR-ABL, as well as several other kinases, including stem cell factor receptor (KIT), discoidin domain receptor (DDR), platelet-derived growth factor receptor (PDGFR), and colony-stimulating factor receptor-1 (CSF-1R). Although the management of CML improved dramatically with the introduction of imatinib, not all patients benefit from treatment because of resistance or intolerance. Consequently, research efforts have focused on developing more potent TKIs with the ability to circumvent imatinib resistance. Nilotinib, a second-generation oral TKI, was rationally designed based on the crystal structure of imatinib to be highly active against a wide range of imatinib-resistant BCR-ABL mutants and is approved for the treatment of newly diagnosed or imatinib-resistant or -intolerant CML, and has shown superiority over imatinib in first-line treatment for newly diagnosed CML. Furthermore, the activity of nilotinib against KIT and PDGFRα has led to its evaluation in advanced gastrointestinal stromal tumors (GIST). The purpose of this review is to describe the development of nilotinib, providing a structural explanation for the differential activity of nilotinib and imatinib in GIST. Activity of nilotinib against KIT and PDGFR and emerging evidence of differences in cellular uptake between nilotinib and imatinib are discussed.

Nilotinib: evaluation and analysis of its role in chronic myeloid leukemia

Nilotinib, formally known as AMN107, is a second-generation tyrosine kinase inhibitor, rationally designed from its revolutionary parent compound imatinib, to produce a 30–40-fold enhancement in the inhibition of the BCR–ABL1-derived oncoprotein associated with chronic myeloid leukemia. In clinical trials, nilotinib has proven to be a useful agent in the treatment of imatinib-refractory disease and was initially approved by both the US FDA and EMA in 2007 for use in adults as a second-line therapy. More recently, data from the first randomized controlled trials of the front-line use of nilotinib in newly diagnosed patients with chronic phase chronic myeloid leukemia have demonstrated superiority in the rates of major molecular responses at 12 months over the gold standard–imatinib 400 mg. As such, in June 2010, the FDA granted accelerated approval for its use in newly diagnosed Philadelphia chromosome-positive chronic myeloid leukemia. Nilotinib is well tolerated, with a favorable side-effect profile. With the emergence of supportive trial data, it is likely to have a leading role both in the front-line management of newly presenting patients and in the second-line treatment of patients resistant to or intolerant of imatinib and other second-line agents.

KIT as a therapeutic target in melanoma

Recently, genetic alterations activating the receptor tyrosine kinase KIT have been shown in some types of melanoma. KIT mutations can be successfully targeted by approved drugs in other cancers. Emerging evidence suggests that melanomas with KIT activation may also respond to KIT inhibitors that are already in clinical use. If confirmed in ongoing clinical trials, this experience would underscore the importance of recognizing the biological diversity among melanomas, representing a first decisive step toward the individualized and mechanism-based treatment of melanoma.

Nilotinib: optimal therapy for patients with chronic myeloid leukemia and resistance or intolerance to imatinib

Chronic myeloid leukemia (CML) is the consequence of a single balanced translocation that produces the BCR-ABL fusion oncogene which is detectable in over 90% of patients at presentation. The BCR-ABL inhibitor imatinib mesylate (IM) has improved survival in all phases of CML and is the standard of care for newly diagnosed patients in chronic phase. Despite the very significant therapeutic benefits of IM, a small minority of patients with early stage disease do not benefit optimally while IM therapy in patients with advanced disease is of modest benefit in many. Diverse mechanisms may be responsible for IM failures, with point mutations within the Bcr-Abl kinase domain being amongst the most common resistance mechanisms described in patients with advanced CML. The development of novel agents designed to overcome IM resistance, while still primarily targeted on BCR-ABL, led to the creation of the high affinity aminopyrimidine inhibitor, nilotinib. Nilotinib is much more potent as a BCR-ABL inhibitor than IM and inhibits both wild type and IM-resistant BCR-ABL with significant clinical activity across the entire spectrum of BCR-ABL mutants with the exception of T315I. The selection of a second generation tyrosine kinase inhibitor to rescue patients with imatinib failure will be based on several factors including age, co-morbid medical problems and ABL kinase mutational profile. It should be noted that while the use of targeted BCR-ABL kinase inhibitors in CML represents a paradigm shift in CML management these agents are not likely to have activity against the quiescent CML stem cell pool. The purpose of this review is to summarize the pre-clinical and clinical data on nilotinib in patients with CML who have failed prior therapy with IM or dasatinib.

Class effects of tyrosine kinase inhibitors in the treatment of chronic myeloid leukemia

Tyrosine kinase inhibitors have revolutionized the treatment of chronic myeloid leukemia (CML), offering patients several targeted therapeutic options that provide the possibility of sustained remissions and prolonged survival. With the availability of imatinib, nilotinib and dasatinib, physicians must weigh the efficacy and safety profile of each agent when choosing the best therapeutic option for individual patients. Each agent targets tyrosine kinases within the cell uniquely to cause the desired antiproliferative effect. In addition to inhibiting the BCR-ABL kinase, imatinib and nilotinib target the same array of other tyrosine kinases, including c-KIT and platelet-derived growth factor receptor (PDGFR), albeit with differing potencies. While targeting BCR-ABL with the highest potency among approved agents in CML, dasatinib also targets a broad array of off-target kinases, including SRC family members, PDGFR and EPHB4. The differences in kinase inhibition profiles among these agents in vitro probably account for the differing clinical safety profiles of these agents. This paper reviews the various kinases inhibited by imatinib, nilotinib and dasatinib, and describes the potential impact of kinase inhibition on the efficacy and safety of each agent.

Advanced mast cell disease: an Italian Hematological Multicenter experience

The aim of the study is to evaluate clinical features, treatments and outcome of patients with systemic mast cell disease (MCD) who arrived to the attention of hematologists. A retrospective study was conducted over 1995-2006 in patients admitted in 18 Italian hematological divisions. Twenty-four cases of advanced MCD were collected: 12 aggressive SM (50%), 8 mast cell leukemia (33%), 4 SM with associated clonal non-mast cell-lineage hematologic disease (17%). Spleen and liver were the principal extramedullary organ involved. The c-kit point mutation D816V was found in 13/18 patients in which molecular biology studies were performed (72%). Treatments were very heterogeneous: on the whole Imatinib was administered in 17 patients, alpha-Interferon in 8, 2-CdA in 3; 2 patients underwent allogeneic hematopoietic stem cell transplantation. The overall response rate to Imatinib, the most frequently employed drugs, was of 29%, registering one complete remission and four partial remission; all responsive patients did not present D816V c-kit mutation. Overall three patients (12%) died for progression of disease. We conclude that MCD is characterized by severe mediator-related symptoms but with a moderate mortality rate. D816V c-kit mutation is frequent and associated with resistance against Imatinib. Because of the rarity of these forms, an effective standard of care is lacking. More data are needed to find new and successful therapeutic strategies.

Molecular drug targets in myeloproliferative neoplasms: mutant ABL1, JAK2, MPL, KIT, PDGFRA, PDGFRB and FGFR1

Therapeutically validated oncoproteins in myeloproliferative neoplasms (MPN) include BCR-ABL1 and rearranged PDGFR proteins. The latter are products of intra- (e.g. FIP1L1-PDGFRA) or inter-chromosomal (e.g.ETV6-PDGFRB) gene fusions. BCR-ABL1 is associated with chronic myelogenous leukaemia (CML) and mutant PDGFR with an MPN phenotype characterized by eosinophilia and in addition, in case of FIP1L1-PDGFRA, bone marrow mastocytosis. These genotype-phenotype associations have been effectively exploited in the development of highly accurate diagnostic assays and molecular targeted therapy. It is hoped that the same will happen in other MPN with specific genetic alterations: polycythemia vera (JAK2V617F and other JAK2 mutations), essential thrombocythemia (JAK2V617F and MPL515 mutations), primary myelofibrosis (JAK2V617F and MPL515 mutations), systemic mastocytosis (KITD816V and other KIT mutations) and stem cell leukaemia/lymphoma (ZNF198-FGFR1 and other FGFR1 fusion genes). The current review discusses the above-listed mutant molecules in the context of their value as drug targets.

Nilotinib: a phenylamino-pyrimidine derivative with activity against BCR-ABL, KIT and PDGFR kinases

The BCR-ABL kinase inhibitor imatinib mesylate is currently the standard therapy for patients with chronic myeloid leukemia (CML). However, mutations within the ABL kinase domain interfering with drug binding have been identified as the main mechanism of resistance to imatinib. Multiple distinct BCR-ABL kinase mutant isoforms conferring varying degrees of resistance to tyrosine kinase inhibitors have been reported. Nilotinib is a tyrosine kinase inhibitor 30-fold more potent than imatinib against BCR-ABL kinase. Nilotinib is active against a wide range of imatinib-resistant BCR-ABL mutant isoforms, except for T315I. Results from Phase II studies of nilotinib for patients with CML after failure or intolerance to imatinib therapy have shown a favorable toxicity profile and confirmed the high efficacy of nilotinib in this setting. Studies addressing the activity of nilotinib in newly-diagnosed patients with CML are underway. Furthermore, nilotinib is a potent inhibitor of KIT and PDGFR kinases. Here, we review the preclinical development of nilotinib and the activity of this agent in patients with CML and in tumors driven by KIT and/or PDGFR mutant kinases, such as gastrointestinal stromal tumors and some forms of clonal hypereosinophilia.

Refining targeted therapies in chronic myeloid leukemia: development and application of nilotinib, a step beyond imatinib

The BCR-ABL kinase inhibitor imatinib mesylate is currently the standard therapy for patients with chronic myeloid leukemia (CML). Despite the remarkable results achieved with imatinib for the treatment of CML, the emergence of resistance to this drug has become a significant problem. Mutations within the ABL kinase domain have been identified as the main mechanism of resistance to imatinib. Other mechanisms include genomic amplification of BCR-ABL and modulation of drug efflux or influx transporters. Several strategies have been developed to overcome the problem of imatinib resistance, including dose escalation of imatinib, combination treatments, or novel targeted agents. Nilotinib is a tyrosine kinase inhibitor 30-fold more potent than imatinib, active against a wide range of mutant clones, except T315I. Phase I-II trials of nilotinib showed high activity in imatinib-resistant CML and Ph+ acute lymphoblastic leukemia. We here review the development of nilotinib and the activity of this agent in CML patients and in other forms of sensitive neoplasms.

KIT and mastocytosis

KIT is a receptor tyrosine kinase that is functionally relevant for hematopoiesis, mast cell development and function, gametogenesis and melanogenesis. Normal KIT signaling requires binding to stem cell factor, and PI3K-Akt is one of the putative effector pathways. In humans, germline loss-of-function KIT mutations have been associated with piebaldism - an autosomal dominant condition characterized by depigmented patches of skin and hair. Gain-of-function KIT mutations are usually acquired and have been associated with myeloid malignancies including core binding factor acute myeloid leukemia and systemic mastocytosis (SM), germ cell tumors, gastrointestinal stromal tumors and sinonasal T cell lymphomas. KITD816V is the most prevalent KIT mutation in mast cell disease and occurs in more than 90% of the cases that fulfill the World Health Organization diagnostic criteria for SM. However, its precise pathogenetic contribution is not well understood. In clinical practice, SM is considered either indolent or aggressive depending on the respective absence or presence of symptomatic target organ dysfunction aside from skin disease. In general, conventional therapy for SM is suboptimal, and efforts are under way to develop and employ small molecule drugs that target mutant KIT.

Pharmacological targeting of the KIT growth factor receptor: a therapeutic consideration for mast cell disorders

KIT is a member of the tyrosine kinase family of growth factor receptors which is expressed on a variety of haematopoietic cells including mast cells. Stem cell factor (SCF)-dependent activation of KIT is critical for mast cell homeostasis and function. However, when KIT is inappropriately activated, accumulation of mast cells in tissues results in mastocytosis. Such dysregulated KIT activation is a manifestation of specific activating point mutations within KIT, with the human D816V mutation considered as a hallmark of human systemic mastocytosis. A number of other activating mutations in KIT have recently been identified and these mutations may also contribute to aberrant mast cell growth. In addition to its role in mast cell growth, differentiation and survival, localized concentration gradients of SCF may control the targeting of mast cells to specific tissues and, once resident within these tissues, mast cell activation by antigen may also be amplified by SCF. Thus, KIT inhibitors may have potential application in multiple conditions linked to mast cells including systemic mastocytosis, anaphylaxis, and asthma. In this review, we discuss the role of KIT in the context of mast cells in these disease states and how recent advances in the development of inhibitors of KIT activity and function may offer novel therapies for the treatment of these disorders.

Concurrent inhibition of kit- and FcepsilonRI-mediated signaling: coordinated suppression of mast cell activation

Although primarily required for the growth, differentiation, and survival of mast cells, Kit ligand (stem cell factor) is also required for optimal antigen-mediated mast cell activation. Therefore, concurrent inhibition of Kit- and FcepsilonRI-mediated signaling would be an attractive approach for targeting mast cell-driven allergic reactions. To explore this concept, we examined the effects of hypothemycin, a molecule that we identified as having such properties, in human and mouse mast cells. Hypothemycin blocked Kit activation and Kit-mediated mast cell adhesion in a similar manner to the well characterized Kit inhibitor imatinib mesylate (imatinib). In contrast to imatinib, however, hypothemycin also effectively inhibited FcepsilonRI-mediated degranulation and cytokine production in addition to the potentiation of these responses via Kit. The effect of hypothemycin on Kit-mediated responses could be explained by its inhibition of Kit kinase activity, whereas the inhibitory effects on FcepsilonRI-dependent signaling were at the level of Btk activation. Because hypothemycin also significantly reduced the mouse passive cutaneous anaphylaxis response in vivo, these data provide proof of principle for a coordinated approach for the suppression of mast cell activation and provide a rationale for the development of compounds with a similar therapeutic profile.

Sensitivity of human cells bearing oncogenic mutant kit isoforms to the novel tyrosine kinase inhibitor INNO-406

The activity of the novel tyrosine kinase inhibitor INNO-406 against human cells with mutated KIT was investigated. Human mast cell (HMC)-1.1 cells with juxtamembrane domain mutation V560G, and HMC-1.2 cells with both V560G and the kinase domain mutation D816V, were treated with INNO-406 (0.02-5.00 microM) or imatinib for 72 h. INNO-406 and imatinib were equipotent against HMC-1 cells regarding cell proliferation (IC50 51 nM and 75 nM, respectively), inhibition of KIT phosphorylation, and induction of apoptosis. In contrast, neither drug was effective against HMC-1.2 cells at the dose range tested. The present results suggest clinical potential for INNO-406 in KIT V560G-expressing malignancies.

Novel Abl kinase inhibitors in chronic myeloid leukemia in blastic phase and Philadelphia chromosome-positive acute lymphoblastic leukemia

Chronic myeloid leukemia (CML) is characterized by the presence of the Philadelphia chromosome, which is associated with a balanced translocation involving chromosomes 9 and 22 to produce a fusion gene (bcr-abl) that gives rise to a constitutively activated Abl tyrosine kinase. This kinase led to the discovery of several small-molecule inhibitors, imatinib being the first and most successful of these. Resistance to imatinib results in some patients from Abl kinase point mutations. Overcoming imatinib resistance represents one of the biggest challenges facing clinicians in the modern management of CML. In this review, we discuss the current understanding of CML pathophysiology and mechanisms of imatinib resistance and how advancing this knowledge has led to the design of novel therapies in the area of blastic phase CML and Philadelphia chromosome-positive acute lymphoblastic leukemia with previous imatinib failure.

Second generation inhibitors of BCR-ABL for the treatment of imatinib-resistant chronic myeloid leukaemia

Imatinib, a small-molecule ABL kinase inhibitor, is a highly effective therapy for early-phase chronic myeloid leukaemia (CML), which has constitutively active ABL kinase activity owing to the expression of the BCR-ABL fusion protein. However, there is a high relapse rate among advanced- and blast-crisis-phase patients owing to the development of mutations in the ABL kinase domain that cause drug resistance. Several second-generation ABL kinase inhibitors have been or are being developed for the treatment of imatinib-resistant CML. Here, we describe the mechanism of action of imatinib in CML, the structural basis of imatinib resistance, and the potential of second-generation BCR-ABL inhibitors to circumvent resistance.