Introduction: Oncolytic virotherapy with tumor selective viruses, such as Vaccinia viruses (VV), offers a promising treatment modality for cancer. TG6002 is a recombinant oncolytic VV deleted in two viral genes (thymidine kinase and ribonucleotide reductase) and armed with the suicide gene FCU1 that encodes a bifunctional chimeric protein which efficiently catalyses the direct conversion of the nontoxic 5-fluorocytosine (5-FC) into the toxic metabolite 5-fluorouracil (5-FU). Canine tumors are relevant predictive preclinical surrogates for human oncology. The first objective was to evaluate the susceptibility, the replication rate and the oncolytic potency of VV in canine tumor cell lines. The second objective was to evaluate oncolytic potency of TG6002 in xenograft model and fresh canine tumor biopsies. The third objective was to assess safety and viral shedding of TG6002 in healthy dogs.

Materials and Methods: Transduction efficiency, replication and oncolytic potency of TG6002 were evaluated in vitro in a variety of different canine cancer cell lines. In vivo anti-tumor effect of TG6002 was examined in a canine tumor xenograft model. TG6002 was injected intratumorally or intravenously with or without 5-FC. Three canine mammary adenocarcinoma explants were infected with TG6002 in presence of 5-FC during 6 days. Oncolytic potency was assessed by histological exams. Concentrations of 5-FC and 5-FU were monitored. TG6002 was administered intramuscularly for 7 healthy dogs and intravenously for 4 healthy dogs. Clinical exams, complete blood count and biochemistry analysis were performed. Blood, saliva, urine, feces were collected for virus detection by qPCR and plaque assay.

Results: Canine cell lines were highly susceptible to VV infection. A replication factor of 106 to 107 was determined 4 days after infection and a significant reduction of cell viability was noticed 5 days after infection. In xenograft model, intratumoral or intravenous injections of TG6002 with oral 5-FC induced a significant inhibition of tumor growth compared to control groups. In canine mammary adenocarcinoma biopsies, a lysis of 90% of tubular cells was observed on histological exams. Conversion of more than 50% of 5-FC to 5-FU was noticed. In healthy dogs, a good tolerance of intramuscular and intravenous injections of TG6002 without viral shedding was assessed.

Conclusion: This study demonstrates that TG6002 is able to infect and replicate in canine tumor cell lines and is oncolytic in both cell lines, xenograft model and canine mammary adenocarcinoma samples. This study also confirms that TG6002 can be safely administered in dogs. These promising results support the use of TG6002 in a clinical trial for both human and canine species. This study emphasizes the importance of a One Health approach in oncology.

Citation Format: Jérémy Béguin, Johann Foloppe, Eve Laloy, Virginie Nourtier, Isabelle Farine, Murielle Gantzer, Christelle Pichon, Sandrine Cochin, Pascale Cordier, Dominique Tierny, Jean Marc Balloul, Eric Quémeneur, Christelle Maurey, Bernard Klonjkowski, Philippe Erbs. Characterization, evaluation and safety studies of the oncolytic Vaccinia virus TG6002 for canine cancer therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1446.