CpG ODNs are synthetic DNA sequences containing unmethylated cytosine-guanine motifs that have potent immunomodulatory effects. Via Toll-like receptor 9 agonism of dendritic cells and B–cells, CpGs induce cytokines, activate natural killer cells, and elicit vigorous Th1 T-cell responses which lead to significant antitumor effects including improved efficacy of chemotherapeutic agents. Based on known CpG mechanisms and on previously demonstrated in vivo radiopotentiation by bacterial immunotherapeutic agents, CpG ODNs were tested for enhancement of tumor responses to radiotherapy. C3Hf/Kam/Law mice bearing a syngeneic immunogenic sarcoma, designated FSa, received single dose local tumor irradiation to the tumor bearing leg with or without CpG 1826. A control ODN with no CpG motif, ODN 1982, was used as an inactive negative control. Several CpG dose levels and treatment schedules in combination with radiation were tested for efficacy, the most common scheme consisting of 3 x 100 μg CpG sc peritumorally: when tumors grew to 6mm in diameter, at 8 mm in diameter at which time tumors were locally irradiated, and 7 days later. Groups of 7 to 10 mice were compared for tumor growth delay (TGD defined as days for tumor growth from 8 to 12 mm) and for tumor cure. In TGD experiments a single radiation dose of 20 Gy was used, whereas in tumor cure experiments a range of single doses from 10 Gy to 55 Gy were used to determine TCD50 values (radiation dose yielding 50% tumor cure). CpG 1826 alone delayed FSa growth slightly (1-3 days). However, when combined with local tumor irradiation CpG 1826 dramatically increased radiation-induced TGD from 12.9 ± 1.2 days to 29.2 ± 4.2 days [enhancement factor (EF) of 2.6], and 3 of 7 mice were cured. CpG 1826 was similarly effective in its ability to potentiate tumor response measured by TGD when a single dose of 100 μg per mouse was given from 7 days before to 3 days after local tumor irradiation. The control ODN was ineffective in enhancing tumor radioresponse. CpG 1826 also greatly enhanced tumor cure, reducing the control (radiation only) TCD50 value from 39.6 Gy to 20.5 Gy (EF= 1.93). The CpG 1826–induced enhancement of tumor radioresponse was greatly reduced when tumors grew in mice whose immunocompetence was compromised by 6 Gy whole body irradiation delivered one day before tumor cell implantation. This finding implies that the induced enhancement of tumor radioresponse was mediated by an immune response activated by CpG 1826. Overall, our findings show that treatment of mice bearing an immunogenic tumor with CpG 1826 strongly enhanced the radioresponse measured by both TGD and tumor cure. Therefore, CpG ODNs may have potential to improve the efficacy of radiotherapy.

[Proc Amer Assoc Cancer Res, Volume 45, 2004]