Synthetic oligodeoxynucleotides (ODNs) containing unmethylated cytosine-guanine (CpG) motifs detected by Toll-like receptor 9 (TLR9) of dendritic cells and B cells have potent immunomodulatory effects. They induce cytokines, activate NK cells, and elicit T-cell responses leading to antitumor effects, including improved efficacy of chemotherapeutic agents, and synergy between CpG ODN 1826 and single dose radiotherapy of an immunogenic mouse FSa (Milas et al., Cancer Res. 64: 5074-5077, 2004). The present study extends this finding to fractionated radiotherapy of FSa, a more clinically relevant treatment schedule, and to the radioresponse of the non-immunogenic mouse NFSa whose biological response to CpG ODNs may be more akin to human tumors. Response of tumors growing in the leg of mice was assessed by tumor growth delay and (or) tumor cure rate (TCD50, radiation dose yielding 50% tumor cure). Multiple sc peritumoral or intratumoral (it) administrations of CpG ODN 1826, 100 μg per mouse, were started when tumors were 6 mm in diameter. Local tumor irradiation was initiated when tumors grew to 8 mm in diameter: 1-9-Gy fractions given twice daily separated by 6-7 h for 5 consecutive days to achieve a total dose of 10 to 90 Gy. CpG ODN 1826 alone had minimal antitumor effect, but it dramatically enhanced FSa response to radiotherapy. TCD50 was reduced from the control value of 83.1 (79.2 - 90.0) Gy total dose for mice treated with radiation only to 23.0 (11.5 - 32.7) Gy total dose in mice treated with both CpG ODN 1826 and radiotherapy. The magnitude of radiation enhancement at the TCD50 level was a factor of 3.61, much higher than the factor of 1.93 that we previously reported for single-dose radiotherapy. Mice cured of their tumors by combined CpG ODN 1826 plus radiotherapy were highly resistant to sc tumor take or development of tumor nodules in the lung from iv injected tumor cells when re-challenged with FSa cells 100 to 120 days after treatment, suggesting the development of an immunological memory response. CpG ODN 1826 also increased radioresponse of the non-immunogenic NFSa tumor by factors of 1.41 and 1.73 when CpG ODN 1826 was given sc and it, respectively. These findings show that CpG ODNs are highly potent enhancers of radiotherapy for both immunogenic and nonimmunogenic tumors. Therefore, CpG ODNs combined with radiotherapy have potential to improve clinical treatment outcome.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]