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Radiofrequency ablation and immunostimulant OK-432: combination therapy enhances systemic antitumor immunity for treatment of VX2 lung tumors in rabbits.

PURPOSE
To evaluate whether antitumor immunity is enhanced systemically by combining radiofrequency ablation (RFA) and local injection of an immunostimulant, OK-432.


MATERIALS AND METHODS
Experiments were approved by the institutional animal care committee. Experimental Japanese rabbits inoculated with VX2 tumors in the lung and the auricle were randomized into four groups of eight: control (supportive care), RFA (RFA of lung tumor), OK-432 (direct injection of OK-432 into lung tumor), and combination therapy (lung RFA and direct OK-432 injection into lung tumor). All procedures were performed 1 week after implantation of VX2 tumors (week 1). In addition, a VX2 tumor rechallenge test was performed in the RFA and combination therapy groups. Survival time was evaluated by means of the Kaplan-Meier method and by using the log-rank test for intergroup comparison. Mean auricle tumor volumes were calculated every week. Specific growth rates (SGRs) were calculated and compared by using the Mann-Whitney test.


RESULTS
The median survival times of the control, RFA, OK-432, and combination therapy groups were 23, 36.5, 46.5, and 105 days, respectively. Survival was significantly prolonged in the combination therapy group when compared with the other three groups (P <.05). The mean auricle tumor volume decreased only in the combination therapy group. The mean auricle tumor volumes of the combination therapy group from week 1 to week 7 were 205, 339, 264, 227, 143, 127, and 115 mm(3). SGR in the combination therapy group became significantly smaller than those in the other three groups (P < .05). In the rechallenge test, the volume of all reimplanted tumors decreased.


CONCLUSION
Combining RFA with local injection of immunostimulant OK-432 may lead to indirectly activation of systemic antitumor immunity.