Canine mast cell tumors: How do TK inhibitors fit? (Proceedings)


One of the most common questions an oncologist hears from a pet owner is, "Why did my dog get cancer?" While it is often impossible to provide a specific answer for a specific patient, our understanding of why cancer develops has grown exponentially in the past decade. As are most things in the living organism, the causes are extremely complex, multifactorial, and still not completely understood.

Introduction to signal transduction

One of the most common questions an oncologist hears from a pet owner is, "Why did my dog get cancer?" While it is often impossible to provide a specific answer for a specific patient, our understanding of why cancer develops has grown exponentially in the past decade. As are most things in the living organism, the causes are extremely complex, multifactorial, and still not completely understood. One basis for understanding how a malignant cell develops comes from knowledge of the normal cellular processes involved in cell division, growth, and differentiation. A critical process in the life of a cell is signal transduction, or transmission of a signal from the outside of the cell to the inside. Cells need to respond to their environment, and signal transduction is a major way in which the cell modulates what it does based on outside factors. Certain proteins found on the cell surface can bind factors and then send a signal into the cell once bound. One such type of signal is sent biochemically via phosphorylation of proteins in a cascade and can lead to cell proliferation, differentiation, and survival via their ultimate effect on gene transcription. If this signaling becomes uncontrolled, cell growth can become unregulated and lead to tumor development.

Tyrosine kinases

Tyrosine kinases (TKs) are a very important class of signaling proteins. They are found not only on the cell surface (these are known as receptor tyrosine kinases [RTKs]), but also in the cytoplasm and the nucleus. In the normal cell, a growth factor can bind to its TK receptor, which then becomes activated and passes on the signal internally via binding adenosine triphosphate (ATP) and then adding phosphate groups to itself (autophosphorylation) and to other molecules farther down the pathway ("downstream"). At least 20 types of protein can be found on the cell surface that are included in the family of RTKs. Examples include Kit, epidermal growth factor receptor (EGFR), vascular endothelial growth factor receptor (VEGFR), and platelet-derived growth factor receptor (PDGFR). While TK signaling is critical for normal development and life processes, unregulated signaling can lead to uncontrolled cell growth and survival and thus is one of the underlying causes of some types of cancer. The classic example of TK dysregulation in human cancer is chronic myelogenous leukemia (CML). These cancer cells have a mutation that causes chronic activation of a cytoplasmic kinase (Abl), which leads to chronic abnormal cell growth and survival of these abnormal cells. In dogs, mutations in the RTK Kit, which lead to constant activation, have been found in 20% to 30% of mast cell tumors (MCTs). Dogs with MCTs with Kit mutations have an increased chance of tumor recurrence and a decreased survival time. Mutations in Kit have also been found in gastrointestinal stromal tumors (GISTs–previously called leiomyosarcomas on histopathology). Inhibitors of these kinases, or tyrosine kinase inhibitors (TKIs), are a way to more directly target a cancer cell. TKIs have been approved for treatment of certain types of cancer in humans and, recently, dogs.

Tyrosine kinase inhibitors

The most effective approach to blocking the signaling from RTKs has been to use small molecules that block the ATP binding site of the kinase. With this blockage, TKIs prevent the kinase from phosphorylating and beginning the signaling cascade, which can lead to an inhibitory/fatal effect on the cancer cell itself, or on the blood vessels that are feeding the cancer cells. These TKIs are generally orally bioavailable.

Imatinib mesylate (Gleevec–Novartis) is one of the most successful TKIs in use. It was developed to target the Abl kinase in humans with CML and has dramatically changed the survival rate of that disease: 90% of patients have progression-free survival of at least 5 years. Gleevec binds the KIT kinase as well and has also shown excellent efficacy against human GISTs. Sunitinib (Sutent–Pfizer) was developed to block new blood vessel growth (angiogenesis) and binds RTKs in the split kinase family, including VEGFR, PDGFR, and Kit. Sutent has activity against a number of tumors, including those affecting the colon and breast.

Tyrosine kinase inhibitors in veterinary medicine

Gleevec has been used in a small number of dogs with resistant MCTs; however, potential hepatotoxicity has raised concerns about its safety. Limited publications discuss this drug in dogs; one study showed a response in 10 of 21 dogs with MCTs treated with Gleevec. The dosage used was 10 mg/kg PO once daily and was well-tolerated, although the duration of treatment was brief for most cases. In another series of dogs with systemic MCTs, three dogs treated with Gleevec at 4.4 mg/kg once daily had complete remission; two dogs survived 4 to 5 months, and the third was still alive at 75 days. Thus, the response duration may be short. Gleevec, approved for human use only, is also very expensive.

Masitinib (Kinavet–AB Science) is a TKI that mainly works against Kit. A recent publication described 202 dogs with grade II or III MCTs in a randomized study evaluating Kinavet. Dogs treated with Kinavet at 12.5 mg/kg PO once daily had a longer time to disease progression (178 days) versus dogs receiving placebo (75 days). Kinavet did not improve overall survival time compared with placebo, except in the subset of dogs with KIT mutations. In that group, survival with Kinavet was 417 days versus 182 days with placebo. The most common adverse effects were diarrhea, vomiting, and neutropenia, most of which were mild to moderate. Dogs with preexisting renal disease showed further elevations in blood urea nitrogen and creatinine levels when treated with Kinavet, and four dogs developed hemolytic anemia while on the drug.

Toceranib phosphate (Palladia–Pfizer) is similar to Sutent in its activity against the split kinase family members (VEGFR, PDGFR, and Kit) and is believed to have both direct antitumor and antiangiogenic activity. Palladia is the first anticancer drug approved by the US Food and Drug Administration for veterinary use. An early study showed 54% of dogs with a variety of tumors, including sarcoma, carcinoma, myeloma, melanoma, and MCTs, having some response to the drug. A subsequent study in 149 dogs with bulky (nonresectable) grade II or III mast cell tumors showed a 37% response rate in dogs receiving Palladia (3.25 mg/kg PO every other day) (n = 86) versus an 8% response in dogs treated with placebo (n = 63). After 6 weeks, the study was unblinded and all dogs were eligible to receive Palladia. In the 58 placebo dogs that subsequently received Palladia 40% showed a response. The overall response rate of 145 dogs that received Palladia was thus 43% (21 complete responses, 41 partial responses). If dogs that maintained stable disease for = 10 weeks (12%) are considered to have shown a biological response to Palladia, then the overall response rate increased to 59.5% (some dogs were removed from this analysis for a variety of reasons, thus the total number of responders is 78 dogs out of a total of 131).

Although dogs with KIT mutations were more likely to have a biological response (82%), dogs without mutations still showed a response (54.5%).The effect of Palladia against MCTs lacking the KIT mutation may be due to the fact that blocking even normal KIT can induce the cell to undergo apoptosis or due to the drug's antiangiogenic effects on the tumor's vascular supply. Palladia treatment was continued in almost 25% of the dogs for > 6 months, thus reflecting a prolonged positive benefit in these cases. Dogs without lymph node metastasis had a better response rate at 67% than did those with affected lymph nodes (46% response rate). The most common side effects of Palladia were gastrointestinal, including diarrhea, anorexia, vomiting, and blood in stool; however, less than 10% were severe (grade 3 or 4 on the Veterinary Cooperative Oncology Group toxicity scale), and most were managed with concurrent medications (discussed below). Other side effects noted with Palladia in small subsets of patients included mild neutropenia and an incompletely understood lameness/skeletal pain.

Clinical use

Palladia is approved for use in grade II or III recurrent MCTs with or without lymph node involvement. So, what does that mean? How does a clinician decide when to use Palladia, or another TKI, in a dog with an MCT? To answer that question, a brief discussion of the biological behavior of canine MCTs follows.

Canine mast cell tumors

MCTs are the most common malignant skin cancer in dogs, and significant variability exists in their biological behavior. Most of the tumors are cured with appropriate local therapy, but a subset shows very malignant behavior and the potential to spread to lymph nodes, liver, spleen, bone marrow, and other areas and to thus become a systemic cancer. Some factors that are considered to carry a more guarded prognosis in dogs with MCTs include recent, rapid tumor growth; certain locations (mucocutaneous, muzzle, pinnae, inguinal, nail bed); fixed ulcerated tumors; and lymph node involvement. A histologic grading scheme (grades I to III) was developed for classification of MCTs affecting haired skin and is still one of the strongest prognostic indicators of canine MCT behavior. However, histologic assessment is prone to operator subjectivity, giving rise to variable grading results for the same tumor among different pathologists. This subjectivity and variability between pathologists bring into question the heavy reliance placed on the grade of an MCT to predict its behavior and thus the need for additional therapies beyond primary tumor excision. In addition, most MCTs are designated grade II and most of these are cured with surgery. However, a subset of grade II tumors are aggressive like grade III tumors. Thus, many other prognostic factors, including various proliferative indices, DNA aneuploidy, c-kit–staining pattern, microvessel density, and mitotic index (MI), have been evaluated in an attempt to better predict the behavior of canine MCTs and to pick out the "bad" grade II tumors. Two groups have evaluated the MI (total number of mitotic figures found in 10 high-power fields) and found that it is predictive of survival time, even within the grade II tumor category. The groups unfortunately found different values for the index, with the first paper showing a median survival of > 70 months for an index score <5 and a survival < 2 months for a score = 5. The second group wrote a letter to the editor in response to the first paper and confirmed that high scores have very short survival times, with their index scores broken down into three groups: MI, 0 = median survival time not reached (i.e. most of the dogs did not die of their disease) MI, 1 to 7, median survival time = 18 months; and MI > 7, median survival = 3 months.

Additional therapy (chemotherapy, TKI therapy) needs to be considered in dogs with poor prognostic indicators (e.g., grade III, high MI, poor location) after complete excision of the MCT. Dogs with nonexcisable tumors may also be considered for chemotherapy or TKI therapy; both of these therapies show better success against smaller tumors, but certainly can be beneficial in cases with bulky tumors.

Clinical use, continued

As Palladia is currently the only approved TKI for veterinary patients and there is a significant amount of information available about its use, it will be the TKI discussed in this section. While combination protocols using traditional therapies, such as chemotherapy or radiation, with a TKI could increase tumor response rates, these combinations are only now being evaluated in cases of human and canine cancers. As side effects of a multimodality protocol could be significant, use of Palladia in dogs is currently recommended as a single modality therapy and only for patients that have failed standard therapies or for which no beneficial therapies exist. While Palladia is approved only for MCTs, it has shown effect in other tumor types and off-label use for other types of cancer may be considered.

The most significant side effects of Palladia are gastrointestinal-related. Dogs that are sick before therapy are more likely to be unable to tolerate Palladia. Thus, minimizing the tumor burden via radiation therapy, debulking surgery, chemotherapy, or some combination may help the patient's quality of life and allow them to receive Palladia and to achieve better results. Diphenhydramine and famotidine are important for H1- and H2-receptor antagonism. Omeprazole may be more beneficial than famotidine in helping to decrease gastric ulceration in these cases. Sucralfate may also be helpful. Loperamide (Imodium–McNeil) can be used for diarrhea – it may help to give the loperamide at the same time as the Palladia as a preventive measure. If nausea is a concern, metoclopramide, maropitant citrate (Cerenia –Pfizer), and/or ondansetron may be used. Dogs with bulky MCTs often receive prednisone for the anti–mast cell tumor effects as well as to control local edema, itching, and allergic reactions. The prednisone can be continued once Palladia is begun, but it should only be given on the days that Palladia is not given. Dogs should never be on a nonsteroidal antiinflammatory drug (NSAID) and a steroid concurrently; if the patient is only on an NSAID, it may be given on the days Palladia is not given.

While the side effects and medications may seem daunting, most patients can be managed with excellent quality of life on Palladia. Initial discussions with owners need to be open and complete about the side effect profile, the likely need for concurrent medications, and the critical importance of home monitoring. Side effects, even mild ones, need to be addressed immediately. Taking breaks ("drug holidays") from Palladia may be needed. Most of the adjustments are minor and soon create a smooth period of therapy. Monitoring for the first 6 weeks includes weekly complete blood cell counts, intermittent chemistry panels and urinalysis, and tumor measurement. Progressive disease is an indication to discontinue therapy; all other responses, including stable disease, are indications to continue treatment.


Targeted therapies for cancer are at the forefront of new treatments for battling this disease. TKIs have shown cancer benefits in both humans and dogs. Palladia, a newly approved TKI, is a welcome addition to the therapeutic options for dogs with progressive or aggressive MCTs for which other therapies have failed. With careful monitoring and good communication with pet owners, most dogs will be able to be comfortably maintained on Palladia and will have an improved quality of life. Studies evaluating combining Palladia with standard-of-care therapies for MCTs, as well as using Palladia against other tumor types, are ongoing.

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