Breast tumors express the enzyme cyclooxygenase (COX-2), characterizing an inflammatory process or demonstrating the presence of paraneoplastic alterations. This experiment aimed to study the expression of COX-2 in canine mammary tumors and to evaluate the effect of antiinflammatory firocoxib on it. The selected animals were divided into two groups: control (9 animals) and treated (10 animals). The animals were evaluated by clinical examination; serum biochemistry; size, volume, location and staging of tumors by TNM (evaluation of tumor masses, lymph nodes and metastasis); and assessment of quality of life. The treated group received firocoxib at a dosage of 5 mg/kg daily for seven days, and after this period all animals were reevaluated and submitted to mastectomy.

Samples obtained at both moments were evaluated by histopathology and immunohistochemistry for the expression of COX-2. The results showed that, although there was no difference in size of the largest tumor or in the expression of COX-2 between the groups after treatment, increased expression of COX-2 seen in the control group was not observed in the treated group. It was also observed that those animals that received treatment for 7 days showed decreased levels of total serum protein (p = 0.00588) and increased serum levels of urea (p = 0.01388). These results suggest that the use of firocoxib as adjuvant therapy for dogs with breast cancer submitted to mastectomy can be beneficial to halt the progression of the disease, although monitoring of the animals is indicated to ensure there is no gastrointestinal bleeding and health is preserved.