Radiofrequency Ablation of Liver Tumors in Combination with Local OK-432 Injection Prolongs Survival and Suppresses Distant Tumor Growth in the Rabbit Model with Intra- and Extrahepatic VX2 Tumors
Purpose: To evaluate survival and distant tumor growth after radiofrequency ablation (RFA) and local OK-432 injection at a single tumor site in a rabbit model with intra- and extrahepatic VX2 tumors and to examine the effect of this combination therapy, which we termed immuno-radiofrequency ablation (immunoRFA), on systemic antitumor immunity in a rechallenge test. Methods: Our institutional animal care committee approved all experiments. VX2 tumors were implanted to three sites: two in the liver and one in the left ear. Rabbits were randomized into four groups of seven to receive control, RFA alone, OK-432 alone, and immunoRFA treatments at a single liver tumor at 1 week after implantation. Untreated liver and ear tumor volumes were measured after the treatment. As the rechallenge test, tumors were reimplanted into the right ear of rabbits, which survived the 35 weeks and were followed up without additional treatment. Statistical significance was examined by log-rank test for survival and Student's t test for tumor volume. Results: Survival was significantly prolonged in the immunoRFA group compared to the other three groups (P < 0.05). Untreated liver and ear tumor sizes became significantly smaller after immunoRFA compared to controls (P < 0.05). In the rechallenge test, the reimplanted tumors regressed without further therapy compared to the ear tumors of the control group (P < 0.05). Conclusion: ImmunoRFA led to improved survival and suppression of distant untreated tumor growth. Decreases in size of the distant untreated tumors and reimplanted tumors suggested that systemic antitumor immunity was enhanced by immunoRFA.