The following discussion of the background of the invention is merely provided to aid the reader in understanding the invention and is not admitted to describe or constitute prior art to the present invention.
New insights into the mechanisms underlying immune-evasion, together with combination treatment regimens that potentiate the potency of therapeutic vaccination-either directly or indirectly-through combination with immune checkpoint inhibitors or other therapies, have served as a basis for the development of vaccines or immune modulators that can prime or boost an effective adaptive immune response, consisting of tumor-specific CD4+ and CD8+ T cells specific for a targeted malignancy, resulting in an antitumor response and clinical benefit. How the innate immune system is engaged by targeted ligands shapes the development of an adaptive response and lends itself to the design of vaccines and immunomodulators (Reed et al., Trends Immunol., 30: 23-32, 2009; Dubensky and Reed, Semin. Immunol., 22: 155-61, 2010; Kastenmuller et al., J. Clin. Invest., 121: 1782-1796, 2011; Coffman et al., Immunity, 33: 492-503, 2010).
The cyclic dinucleotides CDNs cyclic-di-AMP (produced by Listeria monocytogenes and other bacteria) and its analogs cyclic-di-GMP and cyclic-GMP-AMP (cGAMP) are recognized by the host cell as a pathogen associated molecular pattern (PAMP), which bind to the pathogen recognition receptor (PRR) known as Stimulator of INterferon Genes (STING). STING is an adaptor protein in the cytoplasm of host mammalian cells which activates the TANK binding kinase (TBK1)-IRF3 and the NF-κB signaling axis, resulting in the induction of IFN-β and other gene products that strongly activate innate immunity. It is now recognized that STING is a component of the host cytosolic surveillance pathway (Vance et al., 2009), that senses infection with intracellular pathogens and in response induces the production of IFN-β, leading to the development of an adaptive protective pathogen-specific immune response consisting of both antigen-specific CD4 and CD8 T cells as well as pathogen-specific antibodies. Examples of cyclic purine dinucleotides are described in some detail in, for example: U.S. Pat. Nos. 7,709,458 and 7,592,326; patent applications WO2007/054279, WO2014/093936, and WO2014/189805; and Yan et al., Bioorg. Med. Chem Lett. 18: 5631 (2008).
An uncharacterized mouse gene with significant structural homology to the catalytic domain of human oligoadenylate synthase cyclic GMP-AMP synthase was reported to be the enzyme responsible for producing STING-binding CDNs in mammalian cells. Sun et al., Science 339(6121):786-91, 2013. Termed cyclic GMP-AMP Synthase (cGAS), this enzyme catalyzes the synthesis of cGAMP from ATP and GTP in the presence of DNA. This cGAMP then functions as a second messenger that binds to and activates STING. These cGAS-produced CDNs differed structurally from the bacterially produced CDNs in that they possess an unusual phosphodiester linkage. Thus, while the bacterially produced CDNs contain a bis-3′,5′ linkage between the two nucleotides, mammalian CDNs contained one 2′,5′ linkage and one 3′,5′ linkage, or a so-called “mixed linkage (ML) or non-canonical CDNs. These 2′,5′-3′,5′ molecules bind STING with nM affinity, some 300-fold better than bacterial c-di-GMP.
Human STING (hSTING) also has known polymorphisms, including alleles encoding histidine at position 232, which are refractory to bis-3′,5′ (canonical) CDNs, but not 2′,5′-3′,5′ (non-canonical, mixed linkage) CDNs (Diner et al., Cell Reports 3, 1355-61, 2013; Jin et al., Genes and Immunity, 12: 263-9, 2011). Single nucleotide polymorphisms in the hSTING gene have been reported to affect the responsiveness to bacterial-derived canonical CDNs (Diner et al., 2013: Gao et al., Cell 154, 748-762, 2013; Conlon et. al., J. Immunol. 190: 5216-5225, 2013). Five haplotypes of hSTING have been reported (WT, REF, HAQ, AQ and Q alleles), which vary at amino acid positions 71, 230, 232 and 293 (Jin et al., 2011; Yi et al., PLOS One 8: e77846, 2013). Cells expressing hSTING reportedly respond poorly to stimulation with bacterial CDNs cGAMP, c-di-AMP and c-di-GMP having bis-(3′,5′) linkages, but are responsive to the endogenously produced cGAS product, ML cGAMP (Diner et al., 2013). Thus, it has been suggested that the 2′,5′-3′,5′ molecules represent much more potent physiological ligands in terms of hSTING targeting (Zhang et al., Mol. Cell. 51:226-35, 2013; Xiao and Fitzgerald, Mol. Cell 51: 135-39, 2013).