In the new histologic classification proposed by Goldschmidt in 2011 [
10], solid carcinomas are considered highly malignant canine mammary carcinomas. They share the poorest prognosis with anaplastic carcinomas, comedocarcinomas, and “carcinoma and malignant myoepitheliomas”, if inflammatory carcinomas are excluded [
11,
12,
13]. In the reported case, the tumor was larger than 5 cm; grew very fast, with poor cellular differentiation; invaded lymphatic vessels, with severe carcinomatosis; and infiltrated sacral, iliac and suprasternal LNs. All of these elements were very-poor-prognosis parameters [
11,
14]. Histologic stage IIIb has the worst prognosis, with a median survival time of 163 days after surgery [
15,
16]. Moreover, with numerous infiltrated lymph nodes, without other tissue infiltration demonstrated by tomodensitometric examination, the stage of the reported tumor was at least IV, which further worsened the prognosis [
12]. As mammary carcinoma cells tend to invade bone marrow [
17,
18,
19,
20], we cannot be certain that bone marrow metastases were not already present, but they were not visible upon the first tomodensitometric examination as bone lesions. We could have carried out a bone marrow puncture and used the cell block technique or a biopsy to look for the presence of epithelial cells in the marrow by immunohistochemistry examination [
21], but the round cellular appearance of the tumor cells and their CK5/6 negativity would probably have produced a false-negative result in that case. A cell-free DNA test could have detected the mammary tumor cells, but the detection rate in blood is only about 61%, and that in bone marrow is unknown [
22].
Very few published case series reported an effect of maximum-tolerated-dose chemotherapy alone on specific survival in dogs bearing canine mammary tumors [
1,
2,
3,
4,
5,
6,
7,
23,
24]. Carboplatin alone [
3], in addition to naltrexone [
7] or with concurrent metronomic cyclophosphamide chemotherapy [
24] seemed to have an interesting effect on survival in female dogs with mammary carcinomas. Cox2 overexpression promotes angiogenesis and invasion [
25]; the use of anti-cox2 anti-inflammatory drugs has previously shown an effect on certain types of mammary carcinoma, namely, inflammatory mammary carcinomas [
23,
26,
27,
28,
29]. It seems that firocoxib could also promote the apoptosis of mammary cancer cells, particularly in the case of triple-negative tumors, in vitro and in vivo [
30]. In Lavalle’s study, carboplatin was first instated without anti-inflammatory drugs, which were given after the last chemotherapy treatment for 6 months [
3]. Previously published studies indicated that toceranib had effects on inflammatory mammary carcinomas [
28,
31], which also invade lymphatic vessels, as did the solid carcinoma described in our case.
In the described case, the owner still requested treatment in spite of the extremely unfavorable prognosis. Given such an aggressive mammary carcinoma, growing very quickly, the treatment had to be intensive to hopefully have an effect. In multi-drug chemotherapy, “the combined agents should possess a demonstrable individual activity against the tumor type being treated, with the potential for synergistic and/or additive activity, as well as disparate mechanisms of action, and as a result distinct adverse event (AE) profiles. Moreover, the agents should ideally be able to be administered concurrently at an optimized therapeutic dose and interval” [
32]. The proposed treatment consisted in high-dose surgery and combined maximum-tolerated-dose and metronomic chemotherapy treatments with carboplatin, firocoxib, chloraminophene and toceranib. To the best of our knowledge, metronomic chemotherapy and toceranib were previously used together after completing carboplatin sessions, and not at the same time, to treat osteosarcoma [
33], and metronomic chemotherapy and piroxicam or firocoxib were administered after completing carboplatin sessions [
3]. In a study by London, which included toceranib (2.75 mg/kg EOD), piroxicam (0.3 mg/kg) and cyclophosphamide (10 mg/m
2 EOD), greater toxicity was described than with piroxicam (0.3 mg/kg) and cyclophosphamide (10 mg/m
2 EOD) alone [
33]. The combination of cyclophosphamide, cox2 inhibitors and toceranib at lower doses has been proven to be well tolerated in dogs bearing inflammatory carcinomas [
31]. Toceranib phosphate was administered in the latter study at doses from 2.4 to 2.7 mg/kg PO on a Monday–Wednesday–Friday schedule, and cyclophosphamide was given at a dose of 12.5 mg/m
2/d PO. In another study, which included piroxicam at 0,3 mg/kg and toceranib at 3.25 mg/kg EOD, 12% of tumor-bearing dogs developed azotemia (where one case was grade IV), and few developed gastrointestinal adverse effects [
34]. In our study, the protocol was progressively implemented with the introduction of the different molecules in stages. The dose of carboplatin was adapted and progressively reduced according to the neutropenia at nadir and on the day of each chemotherapy. We selected a lower dose of toceranib compared with the doses described above in the other studies for three reasons: its frequent side-effects at the marketing authorization doses [
33] or slightly lowered doses [
34]; the risk of cumulative effects on liver or kidney function with firocoxib and carboplatin; lastly, the fact that using a 15 mg tablet would have led to a large overdose (3.75 mg/kg). As described in a previous study, where female dogs bearing mammary carcinoma were treated with carprofen for 3 months without any effect on renal function [
35], no azotemia nor proteinuria occurred with firocoxib, in spite of its association with toceranib and carboplatin. No hepatitis, anemia, diarrhea nor vomiting occurred in our case either, in spite of the concomitant use of carboplatin, toceranib and firocoxib. No thrombocytopenia was noted during treatment, except at the end. Such thrombocytopenia has been previously described [
24], but as having occurred cumulatively during the carboplatin and metronomic cyclophosphamide regimen. In Machado’s study [
24], the dose of carboplatin used was 300 mg/m
2 and was not changed in spite of progressive thrombocytopenia during the treatment. In published trials with ITK and concomitant maximal-tolerated-dose chemotherapy, it has been shown that chemotherapy doses had to be reduced. For instance, in a study combining toceranib and carboplatin in tumor-bearing dogs, carboplatin was used at the dose of 200 mg/m
2 and toceranib at 2.75 mg/kg EOD. No unique or novel adverse events were observed. A dose of carboplatin of 200 mg/m
2 induced neutropenia (grade I), anemia (grade I) and thrombocytopenia (grade I–III), as well as other events of low grade (gastrointestinal events, hypertension, etc.). Escalating carboplatin doses to 250 or 225 mg/m
2 increased myelosuppressive effects [
36]. The thrombocytopenia we noted occurred around the sixth dose of carboplatin and its subsequent discontinuation; it could have occurred due to the bone metastatic process, the cumulative/amplifying effect of the myelosuppression-associated drugs carboplatin, chloraminophene and toceranib or the dysimmune paraneoplastic mechanism. It did not resolve with chloraminophene and toceranib discontinuation and shortly preceded the discovery of spinal metastases. Paraneoplastic thrombocytopenia is more likely, either due to a dysimmune mechanism or the invasion of bone marrow by metastatic carcinoma cells (as shown by the tomodensitometric examination) or both; as we did not carry out a bone marrow examination at the time, we cannot conclude as to the cause of the thrombocytopenia.
The initial tumor was further tested after the dog’s death with immunohistochemistry and was subsequently found to be negative for C-kit and Cox2 (the histological and immunohistological examinations were carried out by Dr NGuyen at Abadie’s laboratory). We could not test for PDGF-R receptors due to a lack of antibodies against PDGR-R; firocoxib and toceranib may have had indirect effects on the tumor microenvironment or through PDGF-R.