Inhibition of constitutively active forms of mutant kit by multitargeted indolinone tyrosine kinase inhibitors

Novel genomic prognostic biomarkers for dogs with cancer

BACKGROUND

Growing evidence from dogs and humans supports the abundance of mutation-based biomarkers in tumors of dogs. Increasing the use of clinical genomic diagnostic testing now provides another powerful data source for biomarker discovery.

HYPOTHESIS

Analyzed clinical outcomes in dogs with cancer profiled using SearchLight DNA, a cancer gene panel for dogs, to identify mutations with prognostic value.

ANIMALS

A total of 127 cases of cancer in dogs were analyzed using SearchLight DNA and for which clinical outcome information was available.

METHODS

Clinical data points were collected by medical record review. Variables including mutated genes, mutations, signalment, and treatment were fitted using Cox proportional hazard models to identify factors associated with progression-free survival (PFS). The log-rank test was used to compare PFS between patients receiving and not receiving targeted treatment before first progression.

RESULTS

Combined genomic and outcomes analysis identified 336 unique mutations in 89 genes across 26 cancer types. Mutations in 6 genes (CCND1, CCND3, SMARCB1, FANCG, CDKN2A/B, and MSH6) were significantly associated with shorter PFS. Dogs that received targeted treatment before first progression (n = 45) experienced significantly longer PFS compared with those that did not (n = 82, P = .01). This significance held true for 29 dogs that received genomically informed targeted treatment compared with those that did not (P = .05).

CONCLUSION AND CLINICAL IMPORTANCE

We identified novel mutations with prognostic value and demonstrate the benefit of targeted treatment across multiple cancer types. These results provide clinical evidence of the potential for genomics and precision medicine in dogs with cancer.

Toceranib phosphate in the management of canine insulinoma: A retrospective multicentre study of 30 cases (2009-2019)

BACKGROUND

Insulinomas are the most common tumour of the endocrine pancreas in dogs. These malignant tumours have a high metastatic rate and limited chemotherapeutic options. The multi-receptor tyrosine kinase inhibitor sunitinib malate has benefit in the treatment of metastatic insulinoma in people. Toceranib phosphate, an analogous veterinary agent, may provide benefit for dogs.

METHODS

A retrospective study describing the extent and duration of clinical outcomes and adverse events (AEs) in dogs diagnosed with insulinoma and receiving toceranib.

RESULTS

Records for 30 dogs diagnosed with insulinoma and having received toceranib were identified from a medical record search of five university and eight referral hospitals. The median progression-free interval and overall survival time were 561 days (95% confidence interval (CI): [246, 727 days]) and 656 days (95% CI: [310, 1045 days]), respectively. Of the dogs for which the canine Response evaluation criteria for solid tumours tool could be applied, the majority (66.7%) showed either a complete response, partial response or stable disease. Time to clinical progression was associated with prior intervention and type of veterinary practice. Larger dogs were at increased risk for disease progression and death. No novel AEs were reported.

CONCLUSIONS

Most dogs diagnosed with insulinoma and receiving toceranib appeared to have a clinical benefit. Randomised, prospective studies are needed to better elucidate and objectively quantify the potential effect and survival benefit of toceranib therapy for management of insulinoma in dogs.

Synergistic Effect of Toceranib and Nanohydroxyapatite as a Drug Delivery Platform-Physicochemical Properties and In Vitro Studies on Mastocytoma Cells

A new combination of Toceranib (Toc; 5-[(5Z)-(5-Fluoro-2-oxo-1,2-dihydro-3H-indol-3-ylidene)methyl]-2,4-dimethyl-N-[2-(pyrrolidin-1-yl)ethyl]-1H-pyrrole-3-carboxamide) with nanohydroxyapatite (nHAp) was proposed as an antineoplastic drug delivery system. Its physicochemical properties were determined as crystallinity, grain size, morphology, zeta potential and hydrodynamic diameter as well as Toceranib release. The crystalline nanorods of nHAp were synthesised by the co-precipitation method, while the amorphous Toceranib was obtained by its conversion from the crystalline form during nHAp–Toc preparation. The surface interaction between both compounds was confirmed using Fourier-transform infrared spectroscopy (FT-IR), ultraviolet-visible spectroscopy (UV–Vis) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). The nHAp–Toc showed a slower and prolonged release of Toceranib. The release behaviour was affected by hydrodynamic size, surface interaction and the medium used (pH). The effectiveness of the proposed platform was tested by comparing the cytotoxicity of the drug combined with nHAp against the drug itself. The compounds were tested on NI-1 mastocytoma cells using the Alamar blue colorimetric technique. The obtained results suggest that the proposed platform shows high efficiency (the calculated IC50 is 4.29 nM), while maintaining the specificity of the drug alone. Performed analyses confirmed that nanohydroxyapatite is a prospective drug carrier and, when Toceranib-loaded, may be an idea worth developing with further research into therapeutic application in the treatment of canine mast cell tumour.

Nek2 Kinase Signaling in Malaria, Bone, Immune and Kidney Disorders to Metastatic Cancers and Drug Resistance: Progress on Nek2 Inhibitor Development

Cell cycle kinases represent an important component of the cell machinery that controls signal transduction involved in cell proliferation, growth, and differentiation. Nek2 is a mitotic Ser/Thr kinase that localizes predominantly to centrosomes and kinetochores and orchestrates centrosome disjunction and faithful chromosomal segregation. Its activity is tightly regulated during the cell cycle with the help of other kinases and phosphatases and via proteasomal degradation. Increased levels of Nek2 kinase can promote centrosome amplification (CA), mitotic defects, chromosome instability (CIN), tumor growth, and cancer metastasis. While it remains a highly attractive target for the development of anti-cancer therapeutics, several new roles of the Nek2 enzyme have recently emerged: these include drug resistance, bone, ciliopathies, immune and kidney diseases, and parasitic diseases such as malaria. Therefore, Nek2 is at the interface of multiple cellular processes and can influence numerous cellular signaling networks. Herein, we provide a critical overview of Nek2 kinase biology and discuss the signaling roles it plays in both normal and diseased human physiology. While the majority of research efforts over the last two decades have focused on the roles of Nek2 kinase in tumor development and cancer metastasis, the signaling mechanisms involving the key players associated with several other notable human diseases are highlighted here. We summarize the efforts made so far to develop Nek2 inhibitory small molecules, illustrate their action modalities, and provide our opinion on the future of Nek2-targeted therapeutics. It is anticipated that the functional inhibition of Nek2 kinase will be a key strategy going forward in drug development, with applications across multiple human diseases.

Characterization of receptor tyrosine kinase activation and biological activity of toceranib phosphate in canine urothelial carcinoma cell lines

Background

Urothelial carcinoma (UC) accounts for > 90% of canine tumors occurring in the urinary bladder. Toceranib phosphate (TOC) is a multi-target receptor tyrosine kinase (RTK) inhibitor that exhibits activity against members of the split kinase family of RTKs. The purpose of this study was to evaluate primary UC tumors and UC cell lines for the expression and activation of VEGFR2, PDGFRα, PDGFRβ, and KIT to assess whether dysregulation of these RTKs may contribute to the observed biological activity of TOC.

Results

Transcript for VEGFR2, PDGFRα, PDGFRβ, and KIT was detected in all UC tissue samples and UC cell lines. The Proteome Profiler™ Human Phospho-RTK Array Kit (R & D Systems) provided a platform to assess phosphorylation of 42 different RTKs in primary UC tumors and UC cell lines. Evidence of PDGFRα and PDGFRβ phosphorylation was present in only 11% or 33% of UC tumors, respectively, and 25% of UC cell lines. Treatment of UC cell lines with TOC had no significant impact on cell proliferation, including UC cell lines with evidence of PDGFRβ phosphorylation.

Conclusions

Phosphorylation of several key RTKs targeted by TOC is present in a small subset of primary UC tumors and UC cell lines, suggesting that these RTKs do not exist in a state of continuous activation. These data suggest that activation of RTKs targeted by TOC is present in a small subset of UC tumors and UC cell lines and that treatment with TOC at physiologically relevant concentrations has no direct anti-proliferative effect on UC cells.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-021-03027-0.

A canine case of malignant melanoma carrying a KIT c.1725_1733del mutation treated with toceranib: a case report and in vitro analysis

Background

Canine malignant melanoma is highly aggressive and generally chemoresistant. Toceranib is a kinase inhibitor drug that inhibits several tyrosine kinases including the proto-oncogene receptor tyrosine kinase KIT. Although canine malignant melanoma cells often express KIT, a therapeutic effect for toceranib has yet to be reported for this tumor, with only a small number of patients studied to date. This is a case report of a dog with malignant melanoma that experienced a transient response to toceranib. Furthermore, the KIT expressed in the tumor of this case was examined using molecular analysis.

Case presentation

A Shiba Inu dog presented with a gingival malignant melanoma extending into surrounding structures with metastasis to a submandibular lymph node. The dog was treated with toceranib (Palladia®; 2.6–2.9 mg/kg, orally, every other day) alone. Improvement of tumor-associated clinical signs (e.g., halitosis, tumor hemorrhage, trismus, and facial edema) with reduced size of the metastatic lymph node was observed on Day 15. The gingival tumor and associated masses in the masseter and pterygoid muscles decreased in size by Day 29 of treatment. Toceranib treatment was terminated on Day 43 due to disease progression and the dog died on Day 54. The tumor of this dog had a novel deletion mutation c.1725_1733del within KIT and the mutation caused ligand-independent phosphorylation of KIT, which was suppressed by toceranib. This mutation was considered to be an oncogenic driver mutation in the tumor of this dog, thereby explaining the anti-tumor activity of toceranib.

Conclusions

This is the first report that presents a canine case of malignant melanoma that responded to toceranib therapy. KIT encoded by KIT harboring a mutation c.1725_1733del is a potential therapeutic target for toceranib in canine malignant melanoma. Further investigation of the KIT mutation status and toceranib therapy in canine malignant melanoma will need to be undertaken.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-021-02864-3.

Improving human cancer therapy through the evaluation of pet dogs

Comparative oncology clinical trials play an important and growing role in cancer research and drug development efforts. These trials, typically conducted in companion (pet) dogs, allow assessment of novel anticancer agents and combination therapies in a veterinary clinical setting that supports serial biologic sample collections and exploration of dose, schedule and corresponding pharmacokinetic/pharmacodynamic relationships. Further, an intact immune system and natural co-evolution of tumour and microenvironment support exploration of novel immunotherapeutic strategies. Substantial improvements in our collective understanding of the molecular landscape of canine cancers have occurred in the past 10 years, facilitating translational research and supporting the inclusion of comparative studies in drug development. The value of the approach is demonstrated in various clinical trial settings, including single-agent or combination response rates, inhibition of metastatic progression and randomized comparison of multiple agents in a head-to-head fashion. Such comparative oncology studies have been purposefully included in the developmental plan for several US FDA-approved and up-and-coming anticancer drugs. Challenges for this field include keeping pace with technology and data dissemination/harmonization, improving annotation of the canine genome and immune system, and generation of canine-specific validated reagents to support integration of correlative biology within clinical trial efforts.

Transcriptome of Two Canine Prostate Cancer Cells Treated With Toceranib Phosphate Reveals Distinct Antitumor Profiles Associated With the PDGFR Pathway

Canine prostate cancer (PC) presents a poor antitumor response, usually late diagnosis and prognosis. Toceranib phosphate (TP) is a nonspecific inhibitor of receptor tyrosine kinases (RTKs), including vascular endothelial growth factor receptor (VEGFR), platelet-derived growth factor receptor (PDGFR), and c-KIT. This study aimed to evaluate VEGFR2, PDGFR-β, and c-KIT protein expression in two established canine PC cell lines (PC1 and PC2) and the transcriptome profile of the cells after treatment with TP. Immunofluorescence (IF) analysis revealed VEGFR2 and PDGFR-β protein expression and the absence of c-KIT protein expression in both cell lines. After TP treatment, only the viability of PC1 cells decreased in a dose-dependent manner. Transcriptome and enrichment analyses of treated PC1 cells revealed 181 upregulated genes, which were related to decreased angiogenesis and cell proliferation. In addition, we found upregulated PDGFR-A, PDGFR-β, and PDGF-D expression in PC1 cells, and the upregulation of PDGFR-β was also observed in treated PC1 cells by qPCR. PC2 cells had fewer protein-protein interactions (PPIs), with 18 upregulated and 22 downregulated genes; the upregulated genes were involved in the regulation of parallel pathways and mechanisms related to proliferation, which could be associated with the resistance observed after treatment. The canine PC1 cell line but not the PC2 cell line showed decreased viability after treatment with TP, although both cell lines expressed PDGFR and VEGFR receptors. Further studies could explain the mechanism of resistance in PC2 cells and provide a basis for personalized treatment for dogs with PC.

Leveraging Uncertainty in Machine Learning Accelerates Biological Discovery and Design

Machine learning that generates biological hypotheses has transformative potential, but most learning algorithms are susceptible to pathological failure when exploring regimes beyond the training data distribution. A solution to address this issue is to quantify prediction uncertainty so that algorithms can gracefully handle novel phenomena that confound standard methods. Here, we demonstrate the broad utility of robust uncertainty prediction in biological discovery. By leveraging Gaussian process-based uncertainty prediction on modern pre-trained features, we train a model on just 72 compounds to make predictions over a 10,833-compound library, identifying and experimentally validating compounds with nanomolar affinity for diverse kinases and whole-cell growth inhibition of Mycobacterium tuberculosis. Uncertainty facilitates a tight iterative loop between computation and experimentation and generalizes across biological domains as diverse as protein engineering and single-cell transcriptomics. More broadly, our work demonstrates that uncertainty should play a key role in the increasing adoption of machine learning algorithms into the experimental lifecycle.

Evaluation of toceranib for treatment of apocrine gland anal sac adenocarcinoma in dogs

BACKGROUND

There is no widely accepted standard medical treatment for apocrine gland anal sac adenocarcinoma (AGASACA) in dogs. Targeted agents such as toceranib may be effective in treatment of AGASACA, but the number of clinical reports investigating its efficacy is limited.

HYPOTHESIS/AIM

To evaluate the efficacy of toceranib treatment of AGASACA in dogs, and to assess prognostic factors in the study population. Our hypothesis was that toceranib would provide a clinical benefit in the treatment of dogs with AGASACA.

ANIMALS

Thirty-six client-owned dogs with either a cytologic or histologic diagnosis of AGASACA that were treated with toceranib alone or in combination with surgery, nonconcurrent chemotherapy or both.

METHODS

Retrospective study.

RESULT

The median progression-free survival (PFS) and overall survival time (OST) for the study population was 313 days and 827 days, respectively. A clinical benefit from toceranib treatment was observed in 69% of dogs, with 20.7% of dogs experiencing partial response and 48.3% of dogs experiencing stable disease. Dogs that responded to toceranib treatment had significantly prolonged PFS and OST. Hypercalcemia was a negative prognostic factor for clinical outcomes.

CONCLUSIONS

Toceranib is effective in the treatment of AGASACA in dogs. Prospective, controlled clinical trials are needed to determine the efficacy of toceranib in comparison to other treatment protocols for dogs with AGASACA.

Canine Cancer: Strategies in Experimental Therapeutics

Cancer is the most common cause of death in adult dogs. Many features of spontaneously developing tumors in pet dogs contribute to their potential utility as a human disease model. These include similar environmental exposures, similar clonal evolution as it applies to important factors such as immune avoidance, a favorable body size for imaging and serial biopsy, and a relatively contracted time course of disease progression, which makes evaluation of temporal endpoints such as progression free or overall survival feasible in a comparatively short time frame. These criteria have been leveraged to evaluate novel local therapies, demonstrate proof of tumor target inhibition or tumor localization, evaluate potential antimetastatic approaches, and assess the efficacy, safety and immune effects of a variety of immune-based therapeutics. Some of these canine proof of concept studies have been instrumental in informing subsequent human clinical trials. This review will cover key aspects of clinical trials in dogs with spontaneous neoplasia, with examples of how these studies have contributed to human cancer therapeutic development.

Genetic alterations of KIT during clonal expansion and subsequent acquisition of resistance to toceranib in a canine mast cell tumor cell line

One of the potential mechanisms underlying acquired resistance to toceranib in canine mast cell tumor (MCT) is the emergence of a secondary mutation in the KIT gene. Here, genetic alterations of KIT during clonal expansion and subsequent acquisition of resistance to toceranib were investigated in the toceranib-susceptible canine MCT cell line VI-MC, which carries a KIT-activating mutation resulting in a predicted p.(Asn508Ile) amino acid change in the receptor tyrosine kinase protein KIT. Two sublines were cloned from VI-MC and toceranib-resistant sublines then were established by continuous exposure to toceranib. The mutation status of KIT in parental VI-MC and its sublines was investigated using next-generation sequencing (NGS). Additionally, effects of secondary mutations on toceranib sensitivity in p.(Asn508Ile)-mutant KIT were examined. KIT secondary mutations, including those encoding p.(Asn679Lys)-, p.(Asp819Val)-, and p.(Asp819Gly)-mutant KIT, that confer toceranib insensitivity to p.(Asn508Ile)-mutant KIT emerged only in toceranib-resistant VI-MCs. These mutations were not detected by NGS in the parental VI-MC line or in the toceranib-naive cloned VI-MCs, although the parental line and sublines exhibited genetic heterogeneity in KIT that may have been caused by genetic evolution during clonal expansion. VI-MC clones with these secondary mutations in KIT appear to have arisen from subclones during treatment with toceranib rather than being pre-existing. However, further study using a higher resolution technique will be needed to confirm the developmental mechanism of KIT secondary mutation in canine MCT cells with acquired resistance to toceranib.

Comparative oncology: The paradigmatic example of canine and human mast cell neoplasms

In humans, advanced mast cell (MC) neoplasms are rare malignancies with a poor prognosis. Only a few preclinical models are available, and current treatment options are limited. In dogs, MC neoplasms are the most frequent malignant skin tumours. Unlike low‐grade MC neoplasms, high‐grade MC disorders usually have a poor prognosis with short survival. In both species, neoplastic MCs display activating KIT mutations, which are considered to contribute to disease evolution. Therefore, tyrosine kinase inhibitors against KIT have been developed. Unfortunately, clinical responses are unpredictable and often transient, which remains a clinical challenge in both species. Therefore, current efforts focus on the development of new improved treatment strategies. The field of comparative oncology may assist in these efforts and accelerate human and canine research regarding diagnosis, prognostication, and novel therapies. In this article, we review the current status of comparative oncology approaches and perspectives in the field of MC neoplasms.

KTN0158, a Humanized Anti-KIT Monoclonal Antibody, Demonstrates Biologic Activity against both Normal and Malignant Canine Mast Cells

Purpose: KTN0158 is a novel anti-KIT antibody that potently inhibits wild-type and mutant KIT. This study evaluated the safety, biologic activity, and pharmacokinetic/pharmacodynamics profile of KTN0158 in dogs with spontaneous mast cell tumors (MCT) as a prelude to human clinical applications.Experimental Design: Cell proliferation, KIT phosphorylation, and mast cell degranulation were evaluated in vitro KTN0158 was administered to 4 research dogs to assess clinical effects and cutaneous mast cell numbers. Thirteen dogs with spontaneous MCT were enrolled into a prospective phase I dose-escalating open-label clinical study of KTN0158 evaluating 3 dose levels and 2 schedules and with weekly assessments for response and clinical toxicities.Results: KTN0158 was a potent inhibitor of human and dog KIT activation and blocked mast cell degranulation in vitro In dogs, KTN0158 was well tolerated and reduced cutaneous mast cell numbers in a dose-dependent manner. Clinical benefit of KTN0158 administration in dogs with MCT (n = 5 partial response; n = 7 stable disease) was observed regardless of KIT mutation status, and decreased KIT phosphorylation was demonstrated in tumor samples. Histopathology after study completion demonstrated an absence of neoplastic cells in the primary tumors and/or metastatic lymph nodes from 4 dogs. Reversible hematologic and biochemical adverse events were observed at doses of 10 and 30 mg/kg. The MTD was established as 10 mg/kg.Conclusions: KTN0158 inhibits KIT phosphorylation, demonstrates an acceptable safety profile in dogs, and provides objective responses in canine MCT patients with and without activating KIT mutations, supporting future clinical evaluation of KTN0158 in people. Clin Cancer Res; 23(10); 2565-74. ©2016 AACR.

Receptor Tyrosine Kinase Expression Profiles in Canine Cutaneous and Subcutaneous Mast Cell Tumors

The receptor tyrosine kinase (RTK) KIT is a major focus of current research into canine mast cell tumors (MCTs). Little is known about the role of other RTKs, such as vascular endothelial growth factor receptors (VEGFRs) and platelet-derived growth factor receptors (PDGFRs). These RTKs are dysregulated in many human and animal cancers and are key regulators of tumor angiogenesis. The aims of this study were to assess the expression and activation (phosphorylation) status of KIT, VEGFR2, and PDGFR (α and β) in canine MCTs and to examine associations with various clinical outcomes. c- KIT mutational status and KIT cellular localization pattern were also evaluated for these tumors. Twenty-seven MCTs, consisting of 5 subcutaneous and 22 cutaneous tumors, from 25 dogs were evaluated. MCT biopsies, cultured mast cells, and skin from the surgical margin were analyzed through Western blotting. MCT biopsies were also used for KIT immunohistochemical labeling and polymerase chain reaction for c- KIT mutational analysis. MCT had heterogeneous expression profiles for all 3 RTKs, which varied in intensity and activation status. Statistical analyses showed phosphorylated KIT, VEGFR2, and KIT cellular localization to be predictive of decreased survival time, disease-free interval, and increased metastatic rate. Expression of VEGFR2 and KIT diffuse cytoplasmic labeling were also significantly associated with increased rate of local recurrence. The results of the study show that phosphorylated KIT, KIT, VEGFR2, and PDGFRβ, in addition to KIT localization, may be valuable prognostic determinants in MCTs and should be further studied to improve diagnostic and therapeutic modalities.

c-Kit expression, angiogenesis, and grading in canine mast cell tumour: a unique model to study c-Kit driven human malignancies

Canine cutaneous mast cell tumour (CMCT) is a c-Kit driven tumour sharing similar c-Kit aberrations found in human gastrointestinal stromal tumour. CMCT is classified into three forms: well- (G1), intermediately (G2) (more benign diseases), and poorly (G3) differentiated (malignant) forms. We assess a correlation between c-Kit status, grading, and angiogenesis in CMCTs to explore their potential significance in humans. C-Kit receptor (c-KitR) expression, microvascular density (MVD), and mast cell granulated and degranulated status density (MCGD and MCDD, resp.) were analyzed in 97 CMCTs, by means of histochemistry, immunohistochemistry double staining, and image analysis system. Data showed that predominantly diffuse cytoplasmic- and predominantly focal paranuclear- (Golgi-like) c-Kit protein (PDC-c-Kit and PFP-c-Kit, resp.) expression correlate with high MVD, G3 histopathological grade, and MCDD. Moreover, predominant cell membrane-c-KitR (PCM-c-KitR) expression status correlates with low MVD, G1-G2 histopathological grade, and MCGD. These findings underline the key role of c-Kit in the biopathology of canine MCTs, indicating a link between aberrant c-Kit expression, increased angiogenesis, and higher histopathological grade. CMCT seems to be a model to study contributions of c-Kit activated MCs in tumour angiogenesis and to evaluate the inhibition of MCs activation by means of c-Kit tyrosine kinase inhibitors, currently translated in humans.

Evaluation of the adverse event profile and pharmacodynamics of toceranib phosphate administered to dogs with solid tumors at doses below the maximum tolerated dose

Background

The receptor kinase inhibitor toceranib phosphate (Palladia) was approved for use in dogs in 2009 using a dose of 3.25 mg/kg administered every other day. Preliminary data suggests that lower doses of toeceranib may be associated with a reduced adverse event profile while maintaining sufficient drug exposure to provide biologic activity. The purpose of this study was to determine the Cmax of toceranib in dogs with solid tumors receiving 2.5-2.75 mg/kg every other day and to document the adverse events associated with this dose rate. Secondary objectives included determination of plasma VEGF concentrations in treated dogs and response to therapy.

Results

Dogs with solid tumors were administered toceranib at an intended target dose ranging from 2.5-2.75 mg/kg every other day and plasma samples were obtained for analysis of toceranib and VEGF plasma concentrations on days 0, 7, 14 and 30 of the study at 6 and 8 hours post drug administration. Additionally, plasma samples were obtained at 0, 1, 2, 6, 8, and 12 hours from dogs on day 30 for confirmation of Cmax. Response to therapy was assessed using standard RECIST criteria and adverse events were characterized using the VCOG-CTCAE. Toceranib administered at doses between 2.4-2.9 mg/kg every other day resulted in an average 6–8 hr plasma concentration ranging from 100–120 ng/ml, well above the 40 ng/ml concentration associated with target inhibition. Plasma VEGF concentrations increased significantly over the 30 day treatment period indicating that VEGFR2 inhibition was likely achieved in the majority of dogs. The lower doses of toceranib used in this study were associated with a substantially reduced adverse event profile compared to the established label dose of 3.25 mg/kg EOD.

Conclusions

Doses of toceranib ranging from 2.4-2.9 mg/kg every other day provide drug exposure considered sufficient for target inhibition while resulting in an adverse event profile substantially reduced from that associated with the label dose of toceranib. This lower dose range of toceranib should be considered for future use in dogs with cancer.

Tyrosine kinase inhibitors (TKIs) in human and pet tumours with special reference to breast cancer: a comparative review

Tyrosine kinase receptors (TKRs) play a key role in tumour cell proliferation and survival since they are involved in endothelial cell activation leading to tumour neoangiogenesis. In particular, vascular endothelial growth factor receptors (VEGFRs), platelet-derived growth factor receptor (PDGFR), stem cell factor receptor (c-KitR), and colony-stimulating factor 1 (CSF-1) are overexpressed or constitutively activated in human and pet malignancies. A variety of small molecule inhibitors targeting specific tyrosine kinases (known as tyrosine kinase inhibitors or TKIs) have recently been approved, or are under investigation, for the treatment of human cancer. TKI application in animal cancer is however relatively recent. This review aims to illustrate the major aspects of tyrosine kinase dysfunctions, with special regard to human and animal cancer of the mammary gland, providing an update on the background of the anti-angiogenic and anti-neoplastic properties of TKIs in human and veterinary cancer.

A model of study for human cancer: Spontaneous occurring tumors in dogs. Biological features and translation for new anticancer therapies

Murine cancer models have been extremely useful for analyzing the biology of pathways involved in cancer initiation, promotion, and progression. Interestingly, several murine cancer models also exhibit heterogeneity, genomic instability and an intact immune system. However, they do not adequately represent several features that define cancer in humans, including long periods of latency, the complex biology of cancer recurrence and metastasis and outcomes to novel therapies. Therefore, additional models that better investigate the human disease are needed. In the pet population, with special references to the dog, cancer is a spontaneous disease and dogs naturally develop cancers that share many characteristics with human malignancies. More than 40 years ago, optimization of bone marrow transplantation protocols was undertaken in dogs and recently novel targeted therapies such as liposomal muramyl tripeptide phosphatidylethanolamine and several tyrosine kinase inhibitors, namely masitinib (AB1010) and toceranib phosphate (SU11654), have been developed to treat dog tumors which have then been translated to human clinical trials. In this review article, we will analyze biological data from dog tumors and comparative features with human tumors, and new therapeutic approaches translated from dog to human cancer.

SU11652 Inhibits tyrosine kinase activity of FLT3 and growth of MV-4-11 cells

Background

FLT3-ITD and FLT3-TKD mutations are frequently found in acute myeloid leukemia (AML). This makes tyrosine kinase FLT3 a highly attractive target for therapeutic drug development. However, effective drugs have not yet emerged. This study is intended to identify and to characterize new FLT3 inhibitors.

Methods

By using the protein substrate GST-FLT3S to analyze kinase activity of recombinant proteins carrying the catalytic domain of wild type and mutant forms of FLT3, we screened a chemical library containing 80 known protein kinase inhibitors. We identified SU11652 as a potent FLT3 inhibitor and further employed FLT3-ITD-positive MV- 4–11 cells to study its effects on cell growth, apoptosis, cell cycles, and cell signaling.

Results

SU11652 strongly inhibited the activity of wild type, D835Y, and D835H mutant forms of FLT3 with IC₅₀ values of 1.5, 16, and 32 nM, respectively. It effectively blocked the growth of FLT3-ITD -positive MV-4-11 cells at nanomolar concentrations but exhibited much less effects on several other cells which do not carry mutations of FLT3. SU11652 inhibited growth of MV-4-11 cells by inducing apoptosis, causing cell cycle arrest, and blocking activation of the ERK, Akt, and STAT signaling pathways.

Conclusion

SU11652 is a potent FLT3 inhibitor which selectively targets FLT3-ITD-positive cells. It should serve as a good candidate for development of therapeutic drugs to treat AML.

KIT gene exon 11 mutations in canine malignant melanoma

The proto-oncogene KIT encodes a receptor tyrosine kinase which has been shown to be upregulated in canine melanomas. KIT mutations lead to constitutive phosphorylation and activation of KIT in the absence of ligand binding. The presence of KIT mutations and KIT protein expression was examined in a cohort of 49 dogs with canine malignant melanoma. An exon 11 synonymous nucleotide 1743C→T mutation was identified in five cases in which one also harbored a L579P mutation. Tumors that harbored the KIT exon 11 mutation(s) correlated significantly with disease recurrence (P = 0.05). All 36 melanomas available for immunohistochemical analysis showed either weak (16 cases, 44.4%) or strong (20 cases, 55.6%) expression of the KIT protein. The five KIT mutation carriers were all strongly positive for KIT by immunohistochemical staining. These findings suggest that a subset of canine malignant melanomas harbors a KIT exon 11 mutation.

Transcriptome and proteome analysis of tyrosine kinase inhibitor treated canine mast cell tumour cells identifies potentially kit signaling-dependent genes

Background

Canine mast cell tumour proliferation depends to a large extent on the activity of KIT, a tyrosine kinase receptor. Inhibitors of the KIT tyrosine kinase have recently been introduced and successfully applied as a therapeutic agent for this tumour type. However, little is known on the downstream target genes of this signaling pathway and molecular changes after inhibition.

Results

Transcriptome analysis of the canine mast cell tumour cell line C2 treated for up to 72 hours with the tyrosine kinase inhibitor masitinib identified significant changes in the expression levels of approximately 3500 genes or 16% of the canine genome. Approximately 40% of these genes had increased mRNA expression levels including genes associated with the pro-proliferative pathways of B- and T-cell receptors, chemokine receptors, steroid hormone receptors and EPO-, RAS and MAP kinase signaling. Proteome analysis of C2 cells treated for 72 hours identified 24 proteins with changed expression levels, most of which being involved in gene transcription, e.g. EIA3, EIA4, TARDBP, protein folding, e.g. HSP90, UCHL3, PDIA3 and protection from oxidative stress, GSTT3, SELENBP1.

Conclusions

Transcriptome and proteome analysis of neoplastic canine mast cells treated with masitinib confirmed the strong important and complex role of KIT in these cells. Approximately 16% of the total canine genome and thus the majority of the active genes were significantly transcriptionally regulated. Most of these changes were associated with reduced proliferation and metabolism of treated cells. Interestingly, several pro-proliferative pathways were up-regulated, which may represent attempts of masitinib treated cells to activate alternative pro-proliferative pathways. These pathways may contain hypothetical targets for a combination therapy with masitinib to further improve its therapeutic effect.

Advances and controversies in the diagnosis, pathogenesis, and treatment of systemic mastocytosis

The term systemic mastocytosis (SM) encompasses a group of hematopoietic malignancies characterized by excessive proliferation of neoplastic mast cells that accumulate in the bone marrow and visceral organs. Most patients with SM, particularly those who present with aggressive clinical courses, carry somatic mutations of the v-kit Hardy-Zuckerman 4 feline sarcoma viral oncogene homolog (KIT) gene. KIT mutations are considered central events in the pathogenesis of SM and serve as diagnostic markers and putative therapeutic targets. The heterogeneity in the clinical course of patients with SM and recent advances in the genetic and immunophenotypic characterization of neoplastic mast cells may help to improve current diagnostic, taxonomic, and therapeutic approaches in SM.

Masitinib as a chemosensitizer of canine tumor cell lines: a proof of concept study

Masitinib, a selective tyrosine kinase inhibitor, has previously been shown to enhance the antiproliferative effects of gemcitabine in human pancreatic cancer, demonstrating potential as a chemosensitizer. This exploratory study investigated the ability of masitinib to sensitize various canine cancer cell lines to doxorubicin, vinblastine, and gemcitabine. Masitinib strongly sensitized histiocytic sarcoma cells to vinblastine (>70-fold reduction in IC(50) at 5 μM masitinib), as well as osteosarcoma and mammary carcinoma cells to gemcitabine (>70-fold reduction at 5-10 μM). In addition, several cell lines were sensitized to doxorubicin (2-10-fold reduction at 10 μM). These data establish proof-of-concept that masitinib in combination with chemotherapeutic agents can generate synergistic growth inhibition in various canine cancers, possibly through chemosensitization. The findings justify further investigation into those combinations that may potentially yield therapeutic benefit.

Epidermal growth factor enhances the malignant phenotype in canine mammary carcinoma cell lines

Canine mammary gland tumours (CMTs) are the most common malignancies in female dogs. The receptor tyrosine kinase EGFR (erbb1), a receptor for epidermal growth factor (EGF) and related factors, mediates multiple oncogenic functions in human epithelial neoplasms. While previous studies have demonstrated EGFR expression in canine tumours, its function has not been studied in canine cancer. The purpose of this study was to determine the in vitro effects of EGF and vandetanib (ZD6474), a small molecule inhibitor of VEGFR-2, EGFR and RET tyrosine kinases, on proliferation, invasion, survival and chemosensitivity in CMT cells. In low serum, EGF enhanced proliferation and chemotaxis, attenuated apoptosis, and stimulated vascular endothelial growth factor (VEGF) production. Vandetanib dose-dependently inhibited EGFR phosphorylation as well as PI3K/Akt activation, and inhibited all EGF-induced phenotypic effects. In conclusion, EGF stimulates multiple features promoting the malignant phenotype in CMT. Thus, CMT may be an important translational model for the investigation of novel EGFR-directed therapies.

Pharmacokinetic properties of toceranib phosphate (Palladia, SU11654), a novel tyrosine kinase inhibitor, in laboratory dogs and dogs with mast cell tumors

Toceranib phosphate (Palladia, SU11654), an oral tyrosine-kinase inhibitor, is under investigation for the treatment of mast cell tumors in dogs. The pharmacokinetics of toceranib phosphate has been characterized in dogs. Means of the following pharmacokinetic parameters were estimated following a 1.0 mg/kg i.v. dose to laboratory beagles: plasma clearance of 1.45 L/kg/h, volume of distribution of 29.7 L/kg, and terminal half-life of 17.7 h. Following single oral doses of 3.25 mg/kg administered to laboratory beagles, mean C(max) estimates ranged from 68.6 ng/mL to 112 ng/mL with t(max) ranging from 5.3 h and 9.3 h postdose. Terminal half-life was estimated at 31 h. Oral bioavailability was 76.9%. There were no statistically significant (P > 0.05) differences with any pharmacokinetic parameter due to fed/fasted state or with time during 13 weeks of every-other-day dosing at 3.25 mg/kg. Toceranib concentrations were proportional with dose over the range of 2.0 to 6.0 mg/kg. The pharmacokinetics of toceranib in client-owned dogs of a variety of pure and mixed breeds with mast cell tumors was similar to that in healthy laboratory dogs. In summary, toceranib phosphate exhibited moderate clearance, a high volume of distribution, and a moderate elimination half-life. After a single oral dose at 3.25 mg/kg, the concentration vs. time curve showed broad, sustained exposure with measurable concentrations for more than 48 h. These pharmacokinetic parameters support every-other-day administration of toceranib phosphate at an initial dose of 3.25 mg/kg for the treatment of mast cell tumors in dogs.

RT-PCR-based tyrosine kinase display profiling of canine melanoma: IGF-1 receptor as a potential therapeutic target

Canine malignant melanoma (CMM) resembles human malignant melanoma in terms of metastatic behavior, refractoriness to standard therapy, and tumor antigen expression but it is largely unknown how CMM resembles human melanoma with regard to molecular pathogenesis and cellular signaling. No attempt has been made to systematically define the repertoire of tyrosine kinases (TKs) expressed in CMM. This study used a reverse transcription-PCR display technique to evaluate the expression of multiple TKs in the 17CM98 CMM cell line. RT-PCR was performed using degenerate primers coding for highly conserved regions flanking the kinase domains of many TKs and the repertoire of TKs expressed was determined using standard molecular cloning techniques. Sequencing 46 clones yielded canine homologs of insulin-like growth factor-1 receptor (IGF-1R) (50%), JAK1 (17%), PDGFR-a (11%), FGFR1 (9%), Axl (7%), Abl (4%), and PTK2 (2%). Interestingly, IGF-1R, JAK1, and Axl were detected in human melanoma using similar techniques, supporting the cross-species validity of this assay. Given the abundance of IGF-1R clones, we determined the biological effect of rhIGF-1 in 17CM98 cells. IGF-1 stimulated cell proliferation and vascular endothelial growth factor production in 17CM98, and addition of the IGF-1R inhibitor ADW742 abrogated IGF-1-induced phenotypic changes. Expression of IGF-1R mRNA was detected in five of five additional CMM cell cultures, and IGF-1R protein was detected in five of six primary tumors evaluated, suggesting that IGF-1R expression may be common in CMM and may provide a novel target for future therapy. In conclusion, this study suggests that similar TKs are expressed in human and canine melanoma, and shows potential antitumor effects of IGF-1R inhibition in CMM.

AR-42, a novel HDAC inhibitor, exhibits biologic activity against malignant mast cell lines via down-regulation of constitutively activated Kit

Histone hypoacetylation occurs in many cancers and inhibition of histone deacetylation is a promising approach to modulate these epigenetic changes. Our laboratory previously demonstrated that the histone deacetylase inhibitors (HDACis) vorinostat and AR-42 reduced the viability of a canine malignant mast cell line. The purpose of this study was to further investigate the mechanisms of pan-HDAC inhibition in normal and malignant mast cells. Mouse and canine malignant mast cell lines expressing various Kit mutations, normal canine mast cells, and primary canine malignant mast cells were treated with AR-42 (a novel HDACi) and effects on cell viability, cycling, and signaling were evaluated. Treatment with AR-42 induced growth inhibition, cell- cycle arrest, apoptosis, and activation of caspases-3/7. AR-42 promoted hyperacetylation of H3, H4, and alpha-tubulin, and up-regulation of p21. Down-regulation of Kit occurred after AR-42 treatment via inhibition of Kit transcription. Disassociation between Kit and heat shock protein 90 (HSP90) and up-regulation of HSP70 were observed after AR-42 treatment, suggesting potential loss of HSP90 chaperone function. Lastly, AR-42 down-regulated the expression of p-Akt, total Akt, phosphorylated STAT3/5 (pSTAT3/5), and total STAT3/5. In summary, AR-42 exhibits in vitro and ex vivo biologic activity against malignant mast cells, representing a promising therapeutic approach for malignant mast cell disease.

Long-term results of the children's cancer group studies for childhood acute lymphoblastic leukemia 1983-2002: a Children's Oncology Group Report

The Children's Cancer Group enrolled 13 298 young people age <21 years on 1 of 16 protocols between 1983 and 2002. Outcomes were examined in three time periods, 1983-1988, 1989-1995, 1996-2002. Over the three intervals, 10-year event-free survival (EFS) for Rome/National Cancer Institute standard risk (SR) and higher risk (HR) B-precursor patients was 68 and 58%, 77 and 63%, and 78 and 67%, respectively, whereas for SR and HR T-cell patients, EFS was 65 and 56%, 78 and 68%, and 70 and 72%, respectively. Five-year EFS for infants was 36, 38, and 43%, respectively. Seminal randomized studies led to a number of important findings. Stronger post-induction intensification improved outcome for both SR and HR patients. With improved systemic therapy, additional intrathecal (IT) methotrexate effectively replaced cranial radiation. For SR patients receiving three-drug induction, iso-toxic substitution of dexamethasone for prednisone improved EFS. Pegylated asparaginase safely and effectively replaced native asparaginase. Thus, rational therapy modifications yielded better outcomes for both SR and HR patients. These trials provide the platforms for current Children's Oncology Group trials.

Cultivation and characterization of canine skin-derived mast cells

It is essential to develop a technique to culture purified skin-derived mast cells (SMCs) to facilitate immunological research on allergic diseases in dogs. This study was performed to develop an efficient culture system for canine SMCs and to characterize the cells in comparison to canine bone marrow-derived mast cells (BMMCs). Enzymatically digested skin biopsy samples were cultivated in serum-free AIM-V medium supplemented with recombinant canine stem cell factor. Three to five weeks after the initiation of culture, mast cells were collected by a magnetic activated cell separation system using anti-c-Kit antibody. The collected cells were composed of a uniform population showing morphological characteristics of mast cells with a round or oval nucleus and abundant toluidine blue-positive metachromatic granules in the cytoplasm. The results of flow cytometric analysis for the presence of cell membrane c-Kit and Fc epsilon receptor I (FcepsilonRI) indicated that approximately 90% of the cells were mast cells. The cytoplasmic granules were positive for both tryptase and chymase. Apparent dose-dependent degranulation was induced by antibody-mediated cross-linking of immunoglobulin E (IgE) bound to the cells. These cytological and immunological characteristics observed in SMCs were mostly similar to those observed in BMMCs; however, IgE-mediated degranulation was significantly lower in SMCs than BMMCs. The culture system for canine SMCs developed in this study would be useful in understanding the pathophysiology and developing anti-allergic therapeutics in canine allergic dermatitis.

Characterization of STAT3 activation and expression in canine and human osteosarcoma

Background

Dysregulation of signal transducer and activator of transcription 3 (STAT3) has been implicated as a key participant in tumor cell survival, proliferation, and metastasis and is often correlated with a more malignant tumor phenotype. STAT3 phosphorylation has been demonstrated in a subset of human osteosarcoma (OSA) tissues and cell lines. OSA in the canine population is known to exhibit a similar clinical behavior and molecular biology when compared to its human counterpart, and is often used as a model for preclinical testing of novel therapeutics. The purpose of this study was to investigate the potential role of STAT3 in canine and human OSA, and to evaluate the biologic activity of a novel small molecule STAT3 inhibitor.

Methods

To examine STAT3 and Src expression in OSA, we performed Western blotting and RT-PCR. OSA cells were treated with either STAT3 siRNA or small molecule Src (SU6656) or STAT3 (LLL3) inhibitors and cell proliferation (CyQUANT), caspase 3/7 activity (ELISA), apoptosis (Western blotting for PARP cleavage) and/or viability (Wst-1) were determined. Additionally, STAT3 DNA binding after treatment was determined using EMSA. Expression of STAT3 targets after treatment was demonstrated with Western blotting, RT-PCR, or gel zymography.

Results

Our data demonstrate that constitutive activation of STAT3 is present in a subset of canine OSA tumors and human and canine cell lines, but not normal canine osteoblasts. In both canine and human OSA cell lines, downregulation of STAT3 activity through inhibition of upstream Src family kinases using SU6656, inhibition of STAT3 DNA binding and transcriptional activities using LLL3, or modulation of STAT3 expression using siRNA, all resulted in decreased cell proliferation and viability, ultimately inducing caspase-3/7 mediated apoptosis in treated cells. Furthermore, inhibition of either Src or STAT3 activity downregulated the expression of survivin, VEGF, and MMP2, all known transcriptional targets of STAT3.

Conclusion

These data suggest that STAT3 activation contributes to the survival and proliferation of human and canine OSA cells, thereby providing a potentially promising target for therapeutic intervention. Future investigational trials of LLL3 in dogs with spontaneous OSA will help to more accurately define the role of STAT3 in the clinical setting.

RAS, FLT3, and C-KIT mutations in immunophenotyped canine leukemias

OBJECTIVE

To determine the frequency of FLT3, C-KIT, and RAS mutations in canine leukemia patients.

MATERIALS AND METHODS

Ethylenediamine tetra-acetic acid blood samples were recruited from dogs with suspected leukemia, categorized by quantitative and cytological evaluation and immunophenotyping. Flow cytometry was carried out using antibodies against CD3; CD3e; CD4; CD5; CD8; CD11a, b, c, and d; CD14; CD21; CD34; CD45 and 45RA; CD79a; CD90 (THY-1); major histocompatibility complex II; myeloperoxidase; MAC387; and neutrophil-specific antibody. Genomic DNA was extracted from whole blood and analyzed for mutations in N, H, and K-RAS, FLT3, and C-KIT genes by polymerase chain reaction and sequencing.

RESULTS

Fifty-seven (77.0%) of 74 samples submitted from dogs with suspected leukemia had cytologically and immunophenotypically confirmed leukemia. There were 36 (63.2%) acute leukemias, 16 (28.1%) chronic, 3 (5.3%) prolymphocytic, 1 natural killer cell, and 1 chronic leukemia undergoing blast transformation. N-RAS mis-sense mutations were identified in 14 (25%) dogs with acute myeloid (AML) or lymphoid (ALL) leukemia, and also in one dog in the leukemic phase of lymphoma. Mutations in K-RAS were found in two dogs with AML. There were no H-RAS mutations. FLT3 internal tandem duplications were identified in three dogs with ALL, and a mis-sense mutation was found in one dog with ALL. C-KIT mutations were identified in three dogs with AML. Sixty-one percent of dogs with acute leukemia harbored mutations in N/K-RAS, FLT3, or C-KIT.

CONCLUSION

RAS, FLT3, and C-KIT mutations, analogous to those found in human leukemia, occur commonly in acute canine leukemia.

Generation and characterization of novel canine malignant mast cell line CL1

Studies using the currently available malignant canine mast cell lines and bone marrow-derived cultured mast cells (BMCMCs) have provided an in-depth understanding of normal and neoplastic canine mast cell biology. However, many of the currently available malignant canine mast cell lines possess limitations, including loss of cell surface markers and inability to bind canine IgE. We have recently generated a novel mast cell line, CL1, from an 11-year-old spayed female Labrador retriever diagnosed with systemic mastocytosis and neoplastic effusion. The CL1 cells express KIT, FcepsilonRI, CD44, CD45, CD14, CD11a, CD11b and CD18 as well as chymase. Interestingly, these cells express wild-type KIT, with no evidence of autophosphorylation, but are able to proliferate independently without the addition of exogenous stem cell factor (SCF), KIT ligand. However, stimulation of CL1 cells with SCF induces KIT phosphorylation promoting cell proliferation. The CL1 cells retain functional properties of mast cells, degranulating in a dose-dependent manner in response to both IgE cross-linking and chemical stimulation. Lastly, cytogenetic evaluation revealed several recurrent tumor-associated chromosome copy number imbalances in the CL1 line. In summary, the CL1 cell line possesses phenotypic and functional properties similar to those found in canine BMCMCs, and will likely be a useful tool to study mast cell biology, factors regulating transformation of mast cells, cytogenetic abnormalities in mast cell tumors, and novel preclinical therapies.

The novel HSP90 inhibitor STA-9090 exhibits activity against Kit-dependent and -independent malignant mast cell tumors

OBJECTIVE

Mutations of the receptor tyrosine kinase Kit occur in several human and canine cancers. While Kit inhibitors have activity in the clinical setting, they possess variable efficacy against particular forms of mutant Kit and drug resistance often develops over time. Inhibitors of heat shock protein 90 (HSP90), a chaperone for which Kit is a client protein, have demonstrated activity against human cancers and evidence suggests they downregulate several mutated and imatinib-resistant forms of Kit. The purpose of this study was to evaluate a novel HSP90 inhibitor, STA-9090, against wild-type (WT) and mutant Kit in canine bone marrow-derived cultured mast cells (BMCMCs), malignant mast cell lines, and fresh malignant mast cells.

MATERIALS AND METHODS

BMCMCs, cell lines, and fresh malignant mast cells were treated with STA-9090, 17-AAG, and SU11654 and evaluated for loss in cell viability, cell death, alterations in HSP90 and Kit expression/signaling, and Kit mutation. STA-9090 activity was tested in a canine mastocytoma xenograft model.

RESULTS

Treatment of BMCMCs, cell lines, and fresh malignant cells with STA-9090 induced growth inhibition, apoptosis that was caspase-3/7-dependent, and downregulation of phospho/total Kit and Akt, but not extracellular signal-regulated kinase (ERK) or phosphoinositide-3 kinase (PI-3K). Loss of Kit cell-surface expression was also observed. Furthermore, STA-9090 exhibited superior activity to 17-AAG and SU11654, and was effective against malignant mast cells expressing either WT or mutant Kit. Lastly, STA-9090 inhibited tumor growth in a canine mastocytoma mouse xenograft model.

CONCLUSIONS

STA-9090 exhibits broad activity against mast cells expressing WT or mutant Kit, suggesting it may be an effective agent in the clinical setting against mast cell malignancies.

Identification of c-kit mutations-independent neoplastic cell proliferation of canine mast cells

Gain-of-function mutations in the proto-oncogene c-kit have been considered the molecular mechanism of neoplastic proliferation of mast cells. However, the importance of c-kit gene mutations is not well evaluated in canine mast cell tumors (MCTs). In the present study, we established and characterized a mast cell line, HRMC, derived from a dog with MCT. We also examined c-kit mutations in HRMC cells and assessed an inhibitory effect of a tyrosine kinase inhibitor, STI571, on HRMC cells. HRMC cells had cytoplasmic metachromatic granules, chymase and tryptase, and expressed both KIT and FcepsilonRI on the cell surface. HRMC cells contained histamine and released beta-hexosaminidase through FcepsilonRI cross-linking and calcium ionophore stimulation. Nucleotide sequence analysis demonstrated no mutations in an open reading frame of c-kit cDNA and genomic DNA of the juxtamembrane domain of c-kit in HRMC cells. STI571 did not show any inhibitory effects on the proliferation of HRMC cells. These findings clearly demonstrated the existence of c-kit mutations-independent neoplastic canine mast cell proliferation. The growth factor-independent mast cell line established in this study might be valuable to explore novel mechanisms of c-kit mutations-independent neoplastic proliferation of mast cells in dogs.