PGP and osteosarcoma: P-gp expression has been described in canine cancer cell lines, including lymphoid leukemia, mast cell tumor and osteosarcoma cell lines [61,86,87], as well as clinical tumor samples, including hepatic, adrenal, gastrointestinal, mammary pulmonary and transitional cell carcinoma, mast cell tumors and malignant lymphoma [16,75,78,79,88,89,90,91].