1.1 Canine cutaneous mast cell tumor (cMCT)
Canine cMCT is one of the most common cutaneous neoplasms in dogs, comprising 16% to 21% of all cutaneous tumors [1]. The incidence is higher in older dogs, with the mean age of 8 to 9 years; however, they have also been reported in younger dogs. There is no apparent sex predilection found. Although mixed-breed dogs have the highest incidence, several breeds are also predisposed, including Boxer, Boston terrier, English bulldog, Pug, Labrador, Golden retriever, and Schnauzer.
The clinical appearances of cMCT are variable, may present with tiny nodules that are a few millimeters in diameter, or masses that are several centimeters in diameter. The lesions are solitary in most of the cases, but multiple lesions are presented in 11% to 14%
cases, and they are most commonly found on the trunk, perineum, and limbs [1]. Release of mast cell granules, which contain histamine, heparin, and other vasoactive amines, may lead to erythema, edema, and ulceration of the surrounding tissue, and may even cause gastrointestinal (GI) ulceration, leading to clinical signs such as anorexia, vomiting, and diarrhea. The incidence of nodal and distant metastasis, including invasion of the spleen and liver, is reported to be 18% and 4.1%, respectively, for all cMCTs at initial diagnosis [2]. Although rarely seen, bone marrow infiltration can also occur in aggressive cMCTs.
In the majority of cases, cMCTs can be diagnosed based on fine-needle aspiration (FNA) cytology. Clinical staging of cMCT is based on World Health Organization (WHO) clinical staging system for MCTs [1] (Table 1), in which the number of lesions and the status of nodal or distant metastasis are taken into account, so the minimal diagnostic workup should include complete blood count, serum biochemistry, blood smear, three-view thoracic radiographs, abdominal ultrasound and FNA of the regional lymph nodes
(LNs).
The histologic grade of cMCTs was primarily based on the Patnaik 3-tier grading system [3], with grade I defined as low grade, grade II defined as an intermediate grade, and grade III defined as high grade; however, the Patnaik grade II cMCTs might present with some histologic variation among tumors, which resulted in unpredictable prognosis [4]. Kiupel 2-tier grading system was then developed to classified tumors into low-grade and high-grade to minimize the disagreement between pathologist with objective criteria, in which tumors were classified as high-grade if they possessed at least seven mitotic figures per 10 high-power field (HPF), at least three multinucleated cells per 10 HPF, at least three bizarre nuclei per 10 HPF or karyomegaly [5]. The 2-tier grading system was demonstrated to provide a more accurate prognosis than the 3-tier grading system [6].
1.2 Treatment of canine cMCT
As the biological behavior of canine cMCTs can vary significantly among cases, treatment decisions should take various factors into account, for example, clinical stage, tumor size, and tumor location.
Surgical excision should be attempted for local control of tumors in the area amenable for wide-margin excision, which includes up to a 3-cm lateral margin and one uninvolved fascial plane deep to the tumor. Adequate surgical resection may provide long-term control for low- and intermediate-grade cMCT [1], and re-excision of the dirty surgical margin or adjuvant radiation therapy (RT) may help achieve tumor-free margin in the case of incomplete excision.
In the cases of “high-risk” cMCTs, including dogs with Patnaik grade II MCTs with nodal or distant metastasis and dogs with Patnaik grade III or Kiupel high-grade MCTs,
local recurrence and distant metastasis will eventually develop following local control alone in most instances [1]. Hence, systemic treatments, including chemotherapy or TKI, are warranted for dogs with high-risk MCTs. Another indication for systemic treatments is treating measurable cMCTs that are not amenable for local control. Some studies have evaluated the efficacy of several chemotherapeutic agents for measurable cMCT, for example, VBL, CTX, and lomustine [7-10]. TKIs such as toceranib (TOC) and masitinib (MAS), have also been studied in such cases [11, 12].
1.3 VBL-based chemotherapy
VBL is an antimicrotubule agent commonly used in canine cMCTs, it interferes with the polymerization or depolymerization of the microtubules, resulting in the arrest of cell division. In one study, single-agent VBL resulted in a 12% to 27% objective response in 51 dogs with non-resectable grade II or grade III cMCTs [9]. In another study, 18 dogs with measurable grade II or grade III cMCTs were treated with the combination therapy of VBL and prednisone (VP protocol). VBL was given as a rapid intravenous (IV) bolus at 2 mg/m2 every 1-2 weeks; prednisone was administered orally at an initial dose of 2 mg/kg daily, then tapered and discontinued over 12-26 weeks. This protocol resulted in a 47% objective response and a median response duration of 154 days. Adverse events were noted in 20% of patients and considered mild and self-limited in most cases [7] (Table 2).
CTX is an alkylating anticancer agent that functions by interfering with DNA replication as well as RNA transcription and replication. A study evaluated the combination of vinblastine-cyclophosphamide-prednisone (VCP protocol) for the treatment of canine high-grade MCTs. VBL was administered as a rapid IV bolus at 2-2.2 mg/m2 every three weeks (on day 1 of the 21-day protocol); CTX was administered at
200-250 mg/m2 every three weeks either orally (over day 8-11 of the 21-day protocol) or as a rapid IV bolus (on day 8 of the 21-day protocol); prednisone was administered orally at an initial dose of 1 mg/kg daily, then tapered and discontinued over 24-32 weeks.
Eleven dogs were treated in the presence of gross lesions, resulting in a 64% objective response and a median progression-free survival time of 74 days. Only minimal toxicity was seen in all treated dogs [8].
1.4 TKIs
1.4.1 Dysregulation of tyrosine kinases in canine MCT
Tyrosine kinases are cellular proteins that take part in normal cell signal transduction.
Binding of external signals generate from growth factors or other stimuli initiates the phosphorylation of tyrosine kinases, leading to the generation of intracellular signaling that regulates cell growth, differentiation, and motility. Dysregulation of tyrosine kinases results in persistent autophosphorylation in the absence of external signals and eventually leads to uncontrolled cell proliferation and survival [12].
The mutation of tyrosine kinase KIT, which is encoded by the c-kit gene, has been identified in about 8.3% to 17% of canine cMCTs, with a higher incidence of up to 35%
reported in higher grade cMCTs [13-17]. The presence of c-kit mutations is associated with worse prognosis, including increased risk of local and systemic recurrence, shorter median progression-free survival, shorter median overall survival, and increased risk of MCT-related death [1, 13, 14].
1.4.2 TKIs in veterinary medicine
TKIs work by blocking the ATP-binding site of tyrosine kinases, inhibiting the
orally bioavailable TKIs, TOC, and MAS were approved for treating canine MCTs, and limited studies have also been performed with the human TKI, imatinib mesylate (Gleevec, Novartis) [1, 12].
1.4.3 Imatinib mesylate
Imatinib mesylate is a TKI used initially in the human patient against the BCR-ABL fusion protein in chronic myeloid leukemia. It has also been demonstrated to have antitumor activity against other tumors by targeting several mutated kinases, for example, KIT, platelet-derived growth factor receptor (PDGFR), ABL1, and ABL2 [12, 18].
Imatinib has been used off-label for dogs and cats, similarly to most chemotherapeutic drugs in veterinary medicine [18]. Although no study has been performed to evaluate the pharmacokinetics of imatinib yet, some clinical studies have already shown the efficacy of imatinib against canine MCTs since 2008. In a study that enrolled 21 dogs with measurable cMCTs treated with imatinib (10 mg/kg daily), objective response was achieved in 10 dogs within 14 days of treatment initiation with response duration up to 63 days [19]. In another study, two dogs with aggressive MCTs presented with bone marrow involvement all achieved complete remission soon after treated with imatinib (4.4 mg/kg daily) [20]. In a review study, a total of 38 cases of MCTs treated with imatinib (10-12.7 mg/kg daily) have been reported in veterinary literature; of these, 16 cases had a detectable mutation in c-kit exon 8, 9 or 11, and all of them achieved complete or partial remission; of the 22 cases without a detectable mutation, objective response was only achieved in 5 cases [18]. Imatinib appears to be well-tolerated at an approximate dose of 10 mg/kg daily or less. Adverse events were only reported in a small number of dogs in previous studies, including neutropenia, vomiting, and elevation of serum liver enzymes, which were usually mild and improved following dose reduction or
temporary withdrawal of imatinib [18] (Table 2).
1.4.4 Treatment decision between TKI and chemotherapy
The decision-making between TKI or chemotherapy for an individual patient with canine cMCT has been of interest in recent studies, especially for the c-kit mutation status.
In the registration trial and a preliminary study for TOC, the ORRs were significantly higher for tumors with c-kit mutation than those without; however, the long-term outcome was not reported [12, 21]. A similar finding was also reported in the registration trial for MAS and a study with a small number of patients treated with imatinib [19, 22]. However, in a recent study, there was no significant difference in ORR and overall survival time between patients with or without c-kit mutation treated with MAS, and the initial response to MAS was the most reliable prognostic factor for survival time [23]. In a prospective, randomized trial enrolling 88 dogs with grade II or grade III macroscopic cMCTs, the use of either KIT pattern or c-kit mutation status alone was not sufficient to make treatment decisions between TOC and VBL as the clinical outcomes were similar between two groups [24]. These results suggest that possession of c-kit mutation may indicate a better response rate to TKIs, but not the long-term outcome, and its utility for selection of MCT treatments remains equivocal [25, 26].
1.5 Prognostic factors
Various prognostic indicators have been identified for canine MCTs, and no single factor is entirely predictive of the biological behavior and clinical outcome. Histologic grade is the most reliable prognostic factor, dogs with well-differentiated MCTs usually experience long-term survival after adequate local control [27-30]; however, local
recurrence or metastasis eventually develop in 55% to 96% of dogs with undifferentiated MCTs, and most dogs die within one year despite managed with aggressive local treatment and adjuvant systemic treatment [1, 31, 32]. Several markers of proliferation are associated with more aggressive behavior, for example, Ki-67 and argyrophilic nucleolar organizer regions (AgNOR) [1].
Clinical-stage is also an important indicator. Stage 0 and stage 1, which represent local disease without nodal or distant metastasis, are associated with a better prognosis compared to the later stage. However, the prognostic significance of stage 3 remains controversial, some studies found an inferior outcome in dogs with stage 3 MCTs [33, 34], but other studies did not [7, 27, 32, 35, 36], and it is still unclear if multiple tumors represent a form of metastasis or multiple unrelated MCTs.
Other prognostic factors have also been reported. Certain breeds tend to develop MCTs with more benign behavior, for example, Boxer, Pug, Boston terrier, and Bulldog [37]. MCTs at prepuce, scrotum, and mucous membrane sites are associated with more aggressive behavior compared to that at other locations [27, 35, 38]. Local recurrence after surgical excision usually indicates a worse prognosis [7, 23, 39]. The presence of clinical signs, such as anorexia, vomiting, or diarrhea, may also relate to worse prognosis [27].