Abstract IA10: Effective immunotherapy regimens incorporating highly active human STING-activating cyclic dinucleotide derivatives
Authors: Glickman Laura Hix , Corrales Leticia , McWhirter Sarah M. , Kanne David B. , Sivick Kelsey E. , Baird Jason R. , Lemmens Edward , Leong Justin J. , Metchette Ken , Crittenden Mark , Gough Michael , Gajewski Thomas F. , Dubensky Thomas W. 
Published: October 2015

Abstract
Innate immune sensing in the tumor microenvironment is a critical step in promoting tumor infiltrating lymphocytes (TILs) and spontaneous anti-tumor T cell priming. Transcriptional profiling analysis of melanoma patients has revealed that tumors with a T cell-inflamed immunophenotype are characterized by a type I IFN (IFN) transcriptional signature. Studies in mice support the notion that IFN produced by tumor-resident dendritic cells (DCs) plays a critical role in spontaneous T cell priming against tumor antigens, which is dependent upon the host Stimulator of Interferon Genes (STING) pathway. STING mediates host innate defense by responding to cytosolic nucleic acids, either through direct binding of cyclic dinucleotides (CDNs) produced by bacteria, or through binding of a structurally distinct CDN produced by host cyclic GMP-AMP synthetase in response to cytosolic double-stranded DNA. While CDN adjuvants have been explored previously in mice, we sought to develop compounds that activate human STING. We therefore synthesized a panel of cyclic dinucleotides (CDNs) that varied by purine nucleotide base, internucleotide phosphate bridge linkage, or by substitution of non-bridging oxygen atoms in the phosphate bridge with sulfur. We screened and selected from among these compounds based on their capacity to activate all known human STING alleles expressed in stably transformed reporter cell lines, stimulate the activation of human PBMCs, and impact significant antitumor efficacy in several mouse tumor models, without significant local or systemic toxicity. Strikingly, direct IT injection of particular CDN derivative molecules into two-week established flank B16 melanoma, CT26 colon, or 4T1 breast tumors profoundly inhibited tumor growth that was durable and correlated with induction of lasting systemic antigen-specific CD8+ T cell immunity that conferred complete protection against tumor re-challenge, or significantly inhibited the growth of distal untreated tumors. Induction of cytokines, tumor antigen-specific immunity, and antitumor efficacy was entirely STING-dependent. We selected dithio-[Rp,Rp]-c[A(2',5')pA(3',5')p], a synthetic CDN molecule that has significantly higher activity than natural STING ligands, as the lead molecule for continued development. We next tested whether direct activation of STING within the tumor microenvironment would enhance the absocopal effect resulting from irradiating the CDN-treated tumor. Treatment of one tumor in mice bearing established bilateral Panc02 flank tumors, with a suboptimal dose of 10 Gy of radiotherapy (RT) in combination with IT CDN injection resulted in rapid local and systemic induction of inflammatory mediators, and vascular damage that spread through the injected tumor without causing detectable damage to normal tissues. Compared to RT alone, CDN injection resulted in significantly enhanced adaptive-immune mediated control of the contralateral tumor. The synthetic CDN molecule described here was significantly more potent than IT TLR ligands, indicating its high translational potential as an approach to elicit effective unbiased T cell priming against an individual's unique tumor antigen repertoire.
Citation Format: Laura Hix Glickman, Leticia Corrales, Sarah M. McWhirter, David B. Kanne, Kelsey E. Sivick, Jason R. Baird, Edward Lemmens, Justin J. Leong, Ken Metchette, Mark Crittenden, Michael Gough, Thomas F. Gajewski, Thomas W. Dubensky, Jr.. Effective immunotherapy regimens incorporating highly active human STING-activating cyclic dinucleotide derivatives. [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy: A New Chapter; December 1-4, 2014; Orlando, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2015;3(10 Suppl):Abstract nr IA10.