Evaluation of the antitumor activity of interleukin-12 in an experimental murine model of colorectal cancer induced by 1,2 dimethyl-hydrazine (DMH).

OBJECTIVE Interleukin 12 (IL-12) is a cytokine that may enhance the proliferation and cytotoxic activity of T lymphocytes and natural killer (NK) cells. A relationship between extensive intratumoral infiltration of NK cells and longer survival rates in colorectal cancer (CRC) patients was previously noted. Preliminary evidence suggests that the combined administration of IL-12 and IL-2 may produce additive immunomodulatory activity. The purpose of this study was to determine whether the systemic administration of IL-12 (+/- IL-2) may induce an immune response against CRC as induced by 1,2-dimethylhydrazine (DMH). METHODS Sixty-five 6-week-old Wistar rats were treated with weekly subcutaneous injections of DMH for 26 weeks at a dose of 20 mg/kg of body weight. Once tumoral induction was over, the animals were randomly allocated to one of three groups: I, control; II, intraperitoneal injections of IL-12; III, intraperitoneal injections of IL-12 combined with IL-2. At 30 weeks, all surviving animals were sacrificed. We studied the following parameters in each rat--number of tumors, size of tumors, and total tumoral volume. Tumor samples were studied using the monoclonal antibody CD 57 for the detection of NK cells. The extent of NK infiltration was classified as small, less than 50 NK cells/50 high-power field (HPF); moderate, 50 to 150 NK cells/50 HPF, and extensive, more than 150 NK cells/50 HPF. RESULTS Thirty-five rats died before completion of the carcinogen exposure, and 30 rats were randomized (10 each group). In group II, 2 animals died during treatment. All rats in groups I and III developed tumors, while in group II two rats (25%) were tumor-free. Moreover, only one rat in group II developed multiple neoplasms, in contrast with group I and group III, where six rats (60%) and seven rats (70%), respectively, had more than one tumor. We found statistically significant differences in the mean number of tumors found in group II when compared to group I (p = 0.028) and group III (p = 0.019). Other parameters measured, such as biggest tumor size and total tumoral volume were found to be lower in group II, although no statistical differences were found between groups. Only 10% of rats in group I showed moderated/extensive NK cell infiltration, vs. 60% of rats in group II (p = 0.077) and 70% in group III (p = 0.02). CONCLUSION The administration of DMH to rodents provides a reliable and consistent means of inducing CRC that may be suitable for the evaluation of anti-cancer therapies. Our findings suggest that IL-12 is effective against the development of experimental CRC. Its antineoplastic effect could be attributed to the stimulus of this cytokine on the intratumoral infiltration of NK cells.