Apoptotic effects of the tyrosine kinase inhibitor, masitinib mesylate, on canine osteosarcoma cells

Evaluation of the therapeutic potential of masitinib and expression of its specific targets c-Kit, PDGFR-α, PDGFR-β, and Lyn in canine prostate cancer cell lines

Canine prostate cancer is classified into adenocarcinoma, transitional cell carcinoma with prostatic involvement, and mixed forms. Early metastatic spread leads to poor prognosis and limited treatment options. Masitinib is approved for the treatment of canine mast cell tumours and inhibits tyrosine kinase c-Kit, tyrosine-protein kinase Lyn (Lyn), and platelet-derived growth factor receptors alpha and beta (PDGFR-α, PDGFR-β), which are known to be expressed in canine prostate cancer. The aim of this study was to evaluate masitinib in an in vitro model consisting of cell lines from primary prostate adenocarcinoma, the associated lymph node metastasis of the same patient, and transitional cell carcinoma. To assess the suitability of the model system, the targets of masitinib were investigated by immunocytochemistry in the cell lines and by immunohistochemistry in the respective formalin-fixed, paraffin-embedded (FFPE) original neoplastic tissue. After exposure to masitinib, cell viability, cell count, apoptosis induction, and protein expression of c-Kit, Lyn, PDGFR-α, and PDGFR-β were assessed. To hedge the efficacy, two application protocols of masitinib (single application or 12-h double-dose regimen) were compared. Immunocytochemical and immunohistochemical analysis revealed increased Lyn, PDGFR-α, and PDGFR-β expression in cell lines and FFPE original neoplastic tissue compared to healthy prostate tissue. Masitinib exposure increased apoptosis, while the cell counts and cell viability decreased in a dose- and application interval-dependent manner, with increased impact in the 12-h double-dose regimen. These in vitro effects of masitinib in canine prostate cancer and associated metastasis support further in vivo research and modifications of the clinical treatment protocol in future studies.

Discovery of Potent and Orally Bioavailable Platelet-Derived Growth Factor Receptor (PDGFR) Inhibitors for the Treatment of Osteosarcoma

Platelet-derived growth factor receptors (PDGFRs) are now considered promising targets for the treatment of osteosarcoma. Herein, the design, synthesis, and structure-activity relationships (SAR) of novel pyrimidine-2,4-diamine derivatives that selectively inhibit PDGFRα/β kinases have been studied. The screening cascades revealed that 7m was the preferred compound among these derivatives, with IC₅₀ values of 2.4 and 0.9 nM for PDGFRα and PDGFRβ, respectively. Moreover, the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) experiment revealed that 7m has a substantial cytotoxic effect against all osteosarcoma cancer cell lines; 7m also displayed robust antitumor effects and low toxicity in a xenograft model. Additionally, 7m showed excellent bioavailability (F = 62.9%), suitable half-life (T_1/2 = 2.12 h), satisfactory metabolic stability, and weak CYP isoform inhibitory activity, suggesting that 7m is a potential drug candidate for PDGFR-driven osteosarcoma.

An Update on Molecular Pathways Regulating Vasculogenic Mimicry in Human Osteosarcoma and Their Role in Canine Oncology

Canine tumors are valuable comparative models for human counterparts, especially to explore novel biomarkers and to understand pathways and processes involved in metastasis. Vasculogenic mimicry (VM) is a unique property of malignant cancer cells which promote metastasis. Thus, it represents an opportunity to investigate both the molecular mechanisms and the therapeutic targets of a crucial phenotypic malignant switch. Although this biological process has been largely investigated in different human cancer types, including osteosarcoma, it is still largely unknown in veterinary pathology, where it has been mainly explored in canine mammary tumors. The presence of VM in human osteosarcoma is associated with poor clinical outcome, reduced patient survival, and increased risk of metastasis and it shares the main pathways involved in other type of human tumors. This review illustrates the main findings concerning the VM process in human osteosarcoma, search for the related current knowledge in canine pathology and oncology, and potential involvement of multiple pathways in VM formation, in order to provide a basis for future investigations on VM in canine tumors.

Effects of tyrosine kinase inhibitor-masitinib mesylate on canine mammary tumour cell lines

Introduction

Masitinib mesylate, a selective tyrosine kinase inhibitor of the c-KIT receptor, is used for the treatment of mast cell tumours in dogs. Masitinib has previously been investigated in various cancers; however, its potential anticancer effect in canine mammary tumours (CMTs) is unknown. In the present paper, we investigated the antiproliferative effect of masitinib in CMT cells and its possible mechanisms of action.

Material and Methods

The effect of masitinib on the proliferation of CMT-U27 and CMT-U309 cells was assessed by MTT assay and DNA fragmentation. Flow cytometric analysis was used to measure the effect of masitinib on apoptosis and the cell cycle. Additionally, vascular endothelial growth factor levels (VEGF) were measured, and the proliferation marker Ki-67 was visualised in immunocytochemical stainings in CMT cells.

Results

Treatment with masitinib inhibited the proliferation of CMT cells in a concentration-dependent manner. Maximal apoptotic activity and DNA fragmentation were observed at approximately IC₅₀ of masitinib in both cell lines. In addition, cell cycle distribution was altered and VEGF levels and Ki-67 proliferation indices were decreased in masitinib-treated cells in comparison with control cells.

Conclusion

In this study, masitinib suppressed cell proliferation concomitantly via induction of apoptosis and cell cycle arrest by decreasing VEGF levels and the Ki-67 proliferation index in CMT-U27 and CMT-U309 cells in vitro, suggesting its potential as a therapeutic tool in the clinical setting of mammary cancer treatment in dogs.

Assessment of PDGFRβ promoter methylation in canine osteosarcoma using methylation-sensitive high-resolution melting analysis

Platelet-derived growth factor signalling pathways play a fundamental role in inducing and sustaining the proliferative and prosurvival stimuli in canine osteosarcomas (cOSAs). The increased expression of platelet-derived growth factor receptors (PDGFRs) α and β, and their cognate ligands, were almost invariably observed in cOSAs and OSA-derived cell lines. In particular, overexpression of PDGFRβ-mediated signalling pathways was found in both the tumour microenvironment, where it drives stromal cell recruitment, and in neoangiogenesis, such as in tumour cells where it triggers aberrant proliferation, migration and local invasion. The majority of the pathological consequences of PDGFRβ signalling are because of aberrant expression. In fact, epigenetic dysregulation of oncogenes throughout demethylation of their promoter has emerged as a pivotal mechanism driving oncogenesis. The aim of this study was to assess the methylation status of the PDGFRβ promoter and to clarify its role in modulating the expression of the tyrosine kinase receptor in canine osteosarcoma. The CpG island of the PDGFRβ promoter was identified using a mixed in silico and experimental approach, and a method based upon the methylation-sensitive high-resolution melting assay for quantitatively and precisely assessing the methylation status of the promoter was then set up. The method herein described was then exploited to assess the methylation status of the promoter in a case series of cOSAa. COSAs consistently but variably expressed PDGFRβ. However, the promoter was almost completely demethylated, and its methylation status did not correlate with the expression levels. This finding supported the hypothesis that post-transcriptional regulatory mechanisms may act in cOSAs.

In vitro and in vivo effects of toceranib phosphate on canine osteosarcoma cell lines and xenograft orthotopic models

Canine osteosarcoma (OSA) is the most common primary malignant bone tumour in dogs, and it has a high metastatic rate and poor prognosis. Toceranib phosphate (TOC; Palladia, Zoetis) is a veterinary tyrosine kinase inhibitor that selectively inhibits VEGFR-2, PDGFRs and c-Kit, but its efficacy is not yet fully understood in the treatment of canine OSA. Here, we evaluated the functional effects of TOC on six OSA cell lines by transwell, wound healing and colony formation assays. Subsequently, two cell lines (Wall and Penny) were selected and were inoculated in mice by intrafemoral injection to develop an orthotopic xenograft model of canine OSA. For each cell line, 30 mice were xenografted; half of them were used as controls, and the other half were treated with TOC at 40 mg/kg body weight for 20 days. TOC inhibited cell growth of all cell lines, but reduced invasion and migration was only observed in Penny and Wall cell lines. In mice engrafted with Penny cells and subjected to TOC treatment, decreased tumour growth was observed, and PDGFRs and c-Kit mRNA were downregulated. Immunohistochemical analyses demonstrated a significant reduction of Ki67 staining in treated mice when compared to controls. The results obtained here demonstrate that TOC is able to slightly inhibit cell growth in vitro, while its effect is evident only in a Penny cell xenograft model, in which TOC significantly reduced tumour size and the Ki67 index without modifying apoptosis markers.

Injection-Site Sarcoma in a Dog: Clinical and Pathological Findings

This case report documents the clinical and pathological findings in a dog that rapidly developed a high-grade sarcoma at the site of multiple vaccinations and follows the response to surgery and adjunct treatment with toceranib. An 11-year-old female spayed Labrador Retriever presented with dorsocervical subcutaneous masses at the injection site three weeks after receiving DA₂PP-Lepto, Rabies, and Bordetella vaccinations. A high-grade soft tissue sarcoma was diagnosed microscopically and immunohistochemistry revealed positive expression of VEGFr, PDGFr, SCF, and EGFR. Repeat surgical resections and targeted treatment with toceranib resulted in a stable remission for nearly two years.

Tyrosine Kinase Receptor Expression in Canine Liposarcoma

The expression of tyrosine kinase receptors is attracting major interest in human and veterinary oncological pathology because of their role as targets for adjuvant therapies. Little is known about tyrosine kinase receptor (TKR) expression in canine liposarcoma (LP), a soft tissue sarcoma. The aim of this study was to evaluate the immunohistochemical expression of the TKRs fibroblast growth factor receptor 1 (FGFR1) and platelet-derived growth factor receptor–β (PDGFRβ); their ligands, fibroblast growth factor 2 (FGF2) and platelet-derived growth factor B (PDGFB); and c-kit in canine LP. Immunohistochemical labeling was categorized as high or low expression and compared with the mitotic count and MIB-1–based proliferation index. Fifty canine LPs were examined, classified, and graded. Fourteen cases were classified as well differentiated, 7 as myxoid, 25 as pleomorphic, and 4 as dedifferentiated. Seventeen cases were grade 1, 26 were grade 2, and 7 were grade 3. A high expression of FGF2, FGFR1, PDGFB, and PDGFRβ was identified in 62% (31/50), 68% (34/50), 81.6% (40/49), and 70.8% (34/48) of the cases, respectively. c-kit was expressed in 12.5% (6/48) of the cases. Mitotic count negatively correlated with FGF2 ( R = –0.41; P < .01), being lower in cases with high FGF2 expression, and positively correlated with PDGFRβ ( R = 0.33; P < .01), being higher in cases with high PDGFRβ expression. No other statistically significant correlations were identified. These results suggest that the PDGFRβ-mediated pathway may have a role in the progression of canine LP and may thus represent a promising target for adjuvant cancer therapies.

Stem cell growth factor receptor in canine vs. feline osteosarcomas

Osteosarcoma is considered the most common bone cancer in cats and dogs, with cats having a much better prognosis than dogs, since the great majority of dogs with osteosarcoma develop distant metastases. In search of a factor possibly contributing to this disparity, the stem cell growth factor receptor KIT was targeted, and the messenger (m)RNA and protein expression levels of KIT were compared in canine vs. feline osteosarcomas, as well as in normal bone. The mRNA expression of KIT was quantified by reverse transcription-quantitative polymerase chain reaction, and was observed to be significantly higher in canine (n=14) than in feline (n=5) osteosarcoma samples (P<0.001). KIT protein expression was evaluated by immunohistochemistry, which revealed that 21% of canine osteosarcoma samples did not exhibit KIT staining in their neoplastic cells, while in 14% of samples, a score of 1 (<10% positive tumour cells) was observed, and in 50% and 14% of samples, a score of 2 (10–50% positivity) and 3 (>50% positivity), respectively, was observed. By contrast, the cancer cells of all the feline bone tumour samples analysed were entirely negative for KIT. Notably, canine and feline osteocytes of healthy bone tissue lacked any KIT expression. These results could be the first evidence that KIT may be involved in the higher aggressiveness of canine osteosarcoma compared with feline osteosarcoma.

Effects of epidermal growth factor receptor kinase inhibition on radiation response in canine osteosarcoma cells

BACKGROUND

Radiation therapy is a palliative treatment modality for canine osteosarcoma, with transient improvement in analgesia observed in many cases. However there is room for improvement in outcome for these patients. It is possible that the addition of sensitizing agents may increase tumor response to radiation therapy and prolong quality of life. Epidermal growth factor receptor (EGFR) expression has been documented in canine osteosarcoma and higher EGFR levels have been correlated to a worse prognosis. However, effects of EGFR inhibition on radiation responsiveness in canine osteosarcoma have not been previously characterized. This study examined the effects of the small molecule EGFR inhibitor erlotinib on canine osteosarcoma radiation responses, target and downstream protein expression in vitro. Additionally, to assess the potential impact of treatment on tumor angiogenesis, vascular endothelial growth factor (VEGF) levels in conditioned media were measured.

RESULTS

Erlotinib as a single agent reduced clonogenic survival in two canine osteosarcoma cell lines and enhanced the impact of radiation in one out of three cell lines investigated. In cell viability assays, erlotinib enhanced radiation effects and demonstrated single agent effects. Erlotinib did not alter total levels of EGFR, nor inhibit downstream protein kinase B (PKB/Akt) activation. On the contrary, erlotinib treatment increased phosphorylated Akt in these osteosarcoma cell lines. VEGF levels in conditioned media increased after erlotinib treatment as a single agent and in combination with radiation in two out of three cell lines investigated. However, VEGF levels decreased with erlotinib treatment in the third cell line.

CONCLUSIONS

Erlotinib treatment promoted modest enhancement of radiation effects in canine osteosarcoma cells, and possessed activity as a single agent in some cell lines, indicating a potential role for EGFR inhibition in the treatment of a subset of osteosarcoma patients. The relative radioresistance of osteosarcoma cells does not appear to be related to EGFR signalling exclusively. Angiogenic responses to radiation and kinase inhibitors are similarly likely to be multifactorial and require further investigation.

Identification of anti-proliferative kinase inhibitors as potential therapeutic agents to treat canine osteosarcoma

Osteosarcoma is the most common primary bone tumour in dogs but various forms of therapy have not significantly improved clinical outcomes. As dysregulation of kinase activity is often present in tumours, kinases represent attractive molecular targets for cancer therapy. The purpose of this study was to identify novel compounds targeting kinases with the potential to induce cell death in a panel of canine osteosarcoma cell lines. The ability of 80 well-characterized kinase inhibitor compounds to inhibit the proliferation of four canine osteosarcoma cell lines was investigated in vitro. For those compounds with activity, the mechanism of action and capability to potentiate the activity of doxorubicin was further evaluated. The screening showed 22 different kinase inhibitors that induced significant anti-proliferative effects across the four canine osteosarcoma cell lines investigated. Four of these compounds (RO 31-8220, 5-iodotubercidin, BAY 11-7082 and an erbstatin analog) showed significant cell growth inhibitory effects across all cell lines in association with variable induction of apoptosis. RO 31-8220 and 5-iodotubercidin showed the highest ability to potentiate the effects of doxorubicin on cell viability. In conclusion, the present study identified several potent kinase inhibitors targeting the PKC, CK1, PKA, ErbB2, mTOR and NF-κB pathways, which may warrant further investigations for the treatment of osteosarcoma in dogs.

Masitinib (AB1010), from canine tumor model to human clinical development: where we are?

Masitinib mesylate (AB1010) is a novel potent and selective tyrosine kinase inhibitor, targeting mainly wild-type and mutated c-Kit receptor (c-KitR), Platelet Derived Growth Factor Receptor-alfa/beta (PDGFRa/ß), Lymphocyte-specific kinase (Lck), Lck/Yes-related protein (LYn), Fibroblast Growth Factor Receptor 3 (FGFR3) and Focal Adhesion Kinase (FAK). It is the first anticancer therapy approved in veterinary medicine for the treatment of unresectable canine mast cell tumors (CMCTs), harboring activating c-KitR mutations, at dose of 12.5mg/kg once daily. Considering its anti-proliferative action, principally given by inhibiting the MCs c-KitR anti-angiogenic pathway that leads cancer progression, and its role as chemosensitizer, masitinib is under clinical investigation in several human malignancies (Gastro-Intestinal Stromal Tumors, acute myeloid leukemia, systemic mastocytosis, pancreatic cancer, multiple myeloma, non-small cell lung cancer, melanoma, ovarian and prostate cancer), which are characterized by similar canine c-KIT proto-oncogene mutations. Here, we analyze masitinib structure activity, its pharmacokinetics compared to imatinib, the c-KitR pathway referring to the most frequent c-KIT mutations sensitive or resistant to this novel drug compared to imatinib, and masitinib safety profile. We, also, explore preclinical and clinical (completed and ongoing) trials with the aim to emphasize as this recent anti-angiogenic therapy, at first approved in CMCTs and, currently in development for the treatment of several human neoplasms, could be represent a milestone in translational oncology, in which the murine experimental model of cancer research could be integrated by canine spontaneous tumor model.

EGFR and microvessel density in canine malignant mammary tumours

The epidermal growth factor receptor (EGFR) is a transmembrane tyrosine kinase receptor which has been shown to have an important role in human breast cancer. Its role appears to be associated with increased angiogenesis and metastasis. In order to clarify its role in canine mammary tumours (CMT), 61 malignant neoplasms were studied by using immunohistochemistry, comparing expression of EGFR, microvessel density (MVD) by CD31 immunolabelling and characteristics of tumour aggressiveness. High EGFR immunoexpression was statistically significantly associated with tumour size, tumour necrosis, mitotic grade, histological grade of malignancy and clinical stage. High CD31 immunoreactivity was statistically significantly associated with tubule formation, histological grade of malignancy and clinical stage. A positive correlation between EGFR and CD31 immunoexpression (r = 0.843; P < 0.001) was also observed. Results suggest that an over-expression of EGFR may contribute to increased angiogenesis and aggression in malignant CMT, presenting the possibility of using EGFR inhibitors in the context of metastatic disease treatment.