Preclinical/clinical trials of thrice-weekly administration of a combination of tegafur/gimeracil/oteracil (TS-1) and toceranib phosphate in dogs with intranasal tumors.

Intranasal tumors in dogs are malignant solid tumors that are primarily treated with radiotherapy and often recur post-treatment. Combination therapy is pivotal in cancer therapy. Effective drugs include fluoropyrimidine 5-fluorouracil (5-FU) and toceranib phosphate. TS-1, an oral formulation containing the 5-FU prodrug tegafur and enzyme modulators gimeracil and oteracil, is proven to be safe in dogs with solid tumors. While the oral drug toceranib phosphate (Palladia^®) is safely administered, the combined toxicity with TS-1 is unknown. We aimed to determine the dosage of this combination in dogs. In the preclinical/clinical trials conducted here, we used a standard 3+3 cohort design with fixed doses of toceranib phosphate (2.4 mg/kg) administered thrice weekly. TS-1 administration was initiated at a dose of 0.5 mg/kg (upper limit 2.0 mg/kg) thrice weekly. Four cohorts were included to confirm the safety of TS-1 and toceranib phosphate. Each cohort was followed up for 1 month. The intranasal tumor types included in the clinical trial (n=13) were adenocarcinoma (n=7), squamous cell carcinoma (n=1), non-epithelial malignancy (n=2), undifferentiated carcinoma (n=1), and transitional carcinoma (n=2). The TS-1 dosage could be increased up to its dose limit in the preclinical/clinical trials. The TS-1 dose to combine with toceranib phosphate thrice weekly was 2.0 mg/kg. This regimen was well-tolerated in dogs. Thus, combined TS-1 and toceranib phosphate therapy is safe for dogs with intranasal tumors.