Abstract 4985: Radiation potentiates systemic anti-tumor immunity unleashed by a novel TLR9 agonist (SD-101)

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Abstract Background: SD-101 is a CpG oligonucleotide-based TLR9 agonist that engages its target within early and late endosomes of plasmacytoid dendritic cells (pDCs), resulting in DC activation, robust IFN-α responses, and priming of tumor-specific T-cells. We have previously shown that local radiotherapy (RT) may also act as an immune adjuvant and thereby contribute to immune-mediated tumor rejection (J Natl Cancer Inst, 2013: 105:256-265). We hypothesized that the effects of SD-101 may be potentiated by RT and trigger immune-mediated abscopal responses, outside the irradiated field (Int. J. Radiat. Oncol. Biol. Phys., 58:862-870). Methods: BALB/c mice were inoculated s.c. with syngeneic TSA mammary tumor cells in 2 sites (primary [ipsilateral flank] and secondary [contralateral flank]) to detect abscopal responses. When tumors reached an average of 5 mm in diameter, mice were randomly assigned to 1 out of 4 treatment groups: control (untreated), SD-101, RT, or SD-101+RT. RT was delivered to primary tumors (8 Gy x3 fractions) in 3 consecutive days; secondary tumors were completely shielded from RT. SD-101 was injected into primary tumors (50 μg) three times per week starting on the last day of RT. Mice were followed for tumor growth in both the primary and secondary tumors. In a parallel experiment, mice were euthanized 10 days after initiation of therapy for immune analysis. Results: Significant growth delays (p <0.05) were observed in the primary tumors of mice treated with monotherapy (RT or SD-101), as compared to the primary tumors of the untreated controls. In contrast, no significant growth delays were observed in the secondary tumors of these mice, when compared to the secondary tumors of their untreated counterparts. The combination of SD-101+RT, however, effectively controlled the primary tumors (100% of mice exhibited complete tumor rejection) of treated mice, while significant growth delays were also observed in the secondary tumors (p <0.05), when compared to the secondary tumors of mice from the other groups. SD-101 alone or in combination with RT, induced intratumoral enrichment of CD8α+ DCs (3.02% of CD11c+ cells in SD-101-treated mice vs 1.08% in untreated mice), whereby these CD8α+ DCs preferentially expressed high levels of activation markers (CD40, CD80, and CD86). Yet, mice treated with SD-101+RT, when compared to the other groups, also demonstrated an expansion of tumor-reactive T-cells (CD8+/PD-1+) both locally and systemically (J Clin Investig 2014 124(5):2246-2259). Conclusions: Our studies support the rational use of RT as a powerful therapeutic partner to potentiate the anti-tumor effects of SD-101. Citation Format: Karsten Pilones, Jeffrey Kraynak, Sandra Demaria, Encouse Golden, Silvia Formenti. Radiation potentiates systemic anti-tumor immunity unleashed by a novel TLR9 agonist (SD-101). [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4985.