The oncolytic effects of reovirus in various cancers have been proven in many clinical trials in human medicine. Oncolytic virotherapy using reovirus for canine cancers is being developed in our laboratory. The objective of this study was to examine the synergistic anti-cancer effects of a combination of reovirus and low doses of various chemotherapeutic agents on mammary gland tumors (MGTs) in dogs. The first part of this study demonstrated the efficacy of reovirus in canine MGTs in vitro and in vivo. Reovirus alone exerted significant cell death by means of caspase-dependent apoptosis in canine MGT cell lines. A single injection of reovirus impeded growth of canine MGT tumors in xenografted mice, but was insufficient to induce complete tumor regression. The second part of this study highlighted the anti-tumor effects of reovirus in combination with low doses of paclitaxel, carboplatin, gemcitabine, or toceranib. Enhanced synergistic activity was observed in the MGT cell line treated concomitantly with reovirus and in all the chemotherapeutic agents except toceranib. In addition, combining reovirus with paclitaxel or gemcitabine at half dosage of half maximal inhibitory concentration (IC50) enhanced cytotoxicity by activating caspase 3. Our data suggest that the combination of reovirus and low dose chemotherapeutic agents provides an attractive option in canine cancer therapy.
