Cyclic dinucleotides (CDNs) are bacterial messengers that are sensed by the host cytosolic surveillance pathway receptor, STING (Stimulator of Interferon Genes). CDN binding to STING initiates a TLR-independent signaling cascade through the TBK-1/IRF-3 axis, inducing type I interferon (IFN) and other co-regulated genes central to host innate defense. In response to binding cytosolic double-stranded DNA, a recently discovered mammalian enzyme, cyclic GMP-AMP synthase (cGAS), synthesizes CDNs that are structurally distinct from CDNs produced by bacteria. The mammalian CDN has a phosphate bridge with 2′-5′ and 3′-5′ linkages that increases its affinity for STING at least 10-fold. We show that mixed linkage (ML) CDNs were more potent activators of human PBMCs as compared to CDNs with canonical 3′-5′ phosphate linkages. Additionally, particular ML CDN derivatives with sulfur atoms at non-bridging oxygens of the internucleotide phosphate bridge were resistant to digestion by host cell phosphodiesterases, and R,R di-thio CDN diastereomers demonstrated increased potency in vitro and in vivo, as compared to either the R,S di-thio CDN diastereomer, or unmodified CDNs. Strikingly, direct injection of 2-week established flank B16 melanoma tumors with ML R,R dithio-CDN derivatives significantly and profoundly inhibited tumor growth, correlated with cytokine-mediated activation of NK and T lymphocytes, induction of antigen-specific T cell immunity and long-term survival. Anti-tumor efficacy was lost in mice encoding non-functional STING. Similar results were observed in other tumor models. These results demonstrate that CDNs have high translation potential for treatment strategies to induce activation of the tumor microenvironment, leading to effective tumor-initiated CD8+ T cell priming and anti-tumor efficacy.

Citation Format: Laura Hix Glickman, David B. Kanne, Sarah M. McWhirter, Meredith L. Leong, Edward E. Lemmens, Ken Metchette, Russell E. Vance, Drew M. Pardoll, Thomas W. Dubensky. Activation of tumor-initiated T cell priming and tumor destruction with potent STING-activating cyclic dinucleotide derivatives. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2566. doi:10.1158/1538-7445.AM2014-2566