The STING signaling pathways and bacterial infection.

As antibiotic-resistant bacteria continue to emerge frequently, bacterial infections have become a significant and pressing challenge to global public health. Innate immunity triggers the activation of host responses by sensing "non-self" components through various pattern recognition receptors (PRRs), serving as the first line of antibacterial defense. Stimulator of interferon genes (STING) is a PRR that binds with cyclic dinucleotides (CDN) to exert effects against bacteria, viruses, and cancer by inducing the production of type I interferon and inflammatory cytokines, and facilitating regulated cell death. Currently, drugs targeting the STING signaling pathway are predominantly applied in the fields of modulating host immune defense against cancer and viral infections, with relatively limited application in treating bacterial infections. Given the significant immunomodulatory functions of STING in the interaction between bacteria and hosts, this review summarizes the research progress on STING signaling pathways and their roles in bacterial infection, as well as the novel functions of STING modulators, aiming to offer insights for the development of antibacterial drugs.

Exploration of the Binding Mechanism of Cyclic Dinucleotide Analogs to Stimulating Factor Proteins and the Implications for Subsequent Analog Drug Design.

Cyclic dinucleotides (CDNs) are cyclic molecules consisting of two nucleoside monophosphates linked by two phosphodiester bonds, which act as a second messenger and bind to the interferon gene stimulating factor (STING) to activate the downstream signaling pathway and ultimately induce interferon secretion, initiating an anti-infective immune response. Cyclic dinucleotides and their analogs are lead compounds in the immunotherapy of infectious diseases and tumors, as well as immune adjuvants with promising applications. Many agonists of pathogen recognition receptors have been developed as effective adjuvants to optimize vaccine immunogenicity and efficacy. In this work, the binding mechanism of human-derived interferon gene-stimulating protein and its isoforms with cyclic dinucleotides and their analogs was theoretically investigated using computer simulations and combined with experimental results in the hope of providing guidance for the subsequent synthesis of cyclic dinucleotide analogs.

Fluorinated cGAMP analogs, which act as STING agonists and are not cleavable by poxins: Structural basis of their function.

The cGAS-STING pathway is a crucial part of innate immunity; it serves to detect DNA in the cytoplasm and to defend against certain cancers, viruses, and bacteria. We designed and synthesized fluorinated carbocyclic cGAMP analogs, MD1203 and MD1202D (MDs), to enhance their stability and their affinity for STING. These compounds demonstrated exceptional activity against STING. Despite their distinct chemical modifications relative to the canonical cyclic dinucleotides (CDNs), crystallographic analysis revealed a binding mode with STING that was consistent with the canonical CDNs. Importantly, MDs were resistant to cleavage by viral poxin nucleases and MDs-bound poxin adopted an unliganded-like conformation. Moreover, MDs complexed with poxin showed a conformation distinct from cGAMP bound to poxin, closely resembling their conformation when bound to STING. In conclusion, the development of MD1203 and MD1202D showcases their potential as potent STING activators with remarkable stability against poxin-mediated degradation-a crucial characteristic for future development of antivirals.

The separation of cyclic diadenosine diphosphorothioate and the diastereomers of its difluorinated derivative and the estimation of the binding constants and ionic mobilities of their complexes with 2-hydroxypropyl-ß-cyclodextrin by affinity capillary electrophoresis.

The incorporation of phosphorothioate linkages has recently been extensively employed in therapeutic oligonucleotides. For their separation and quality control, new high-efficient and high-sensitive analytical methods are needed. In this work, a new affinity capillary electrophoresis method has been developed and applied for the separation of a potential anticancer drug, 2',3'-cyclic diadenosine diphosphorothioate (Rp, Rp) (ADU-S100), and three recently newly synthesized diastereomers of its difluorinated derivative, 3',3'-cyclic di(2'-fluoro, 2'-deoxyadenosine phosphorothioate). The separation was performed in the various background electrolytes (BGEs) within a pH range 5-9 using several native and derivatized cyclodextrins (CDs) as chiral additives of the BGE. Relatively good separations were obtained with ß-, γ-, and 2-hydroxypropyl-γ-CDs in some of the BGEs tested. However, the best separation was achieved using the 2-hydroxypropyl-ß-CD chiral selector at 43.5 mM average concentration in the BGE composed of 40 mM Tris, 40 mM tricine, pH 8.1. Under these conditions, all the previous four cyclic dinucleotides (CDNs) were baseline separated within 4 min. Additionally, the average apparent binding constants and the average actual ionic mobilities of the complexes of all four CDNs with 2-hydroxypropyl-ß-CD in the above BGE were determined. The formed complexes were found to be relatively weak, with the average apparent binding constants in the range of 12.2-94.1 L mol-1 and with the actual ionic mobilities spanning the interval (-7.8 to -12.7) × 10-9 m2 V-1 s-1. The developed method can be applied for the separation, analysis, and characterization of the above and similar CDNs.

Phage anti-CBASS protein simultaneously sequesters cyclic trinucleotides and dinucleotides.

Cyclic-oligonucleotide-based anti-phage signaling system (CBASS) is a common immune system that uses cyclic oligonucleotide signals to limit phage replication. In turn, phages encode anti-CBASS (Acb) proteins such as Acb2, which can sequester some cyclic dinucleotides (CDNs) and limit downstream effector activation. Here, we identified that Acb2 sequesters many CDNs produced by CBASS systems and inhibits stimulator of interferon genes (STING) activity in human cells. Surprisingly, the Acb2 hexamer also binds with high affinity to CBASS cyclic trinucleotides (CTNs) 3'3'3'-cyclic AMP-AMP-AMP and 3'3'3'-cAAG at a distinct site from CDNs. One Acb2 hexamer can simultaneously bind two CTNs and three CDNs. Phage-encoded Acb2 provides protection from type III-C CBASS that uses cA3 signaling molecules. Moreover, phylogenetic analysis of >2,000 Acb2 homologs encoded by diverse phages and prophages revealed that most are expected to bind both CTNs and CDNs. Altogether, Acb2 sequesters nearly all known CBASS signaling molecules through two distinct binding pockets and therefore serves as a broad-spectrum inhibitor of cGAS-based immunity.

Acidity constants and protonation sites of cyclic dinucleotides determined by capillary electrophoresis, quantum chemical calculations, and NMR spectroscopy.

Cyclic dinucleotides (CDNs) are important second messengers in bacteria and eukaryotes. Detailed characterization of their physicochemical properties is a prerequisite for understanding their biological functions. Herein, we examine acid-base and electromigration properties of selected CDNs employing capillary electrophoresis (CE), density functional theory (DFT), and nuclear magnetic resonance (NMR) spectroscopy to provide benchmark pKa values, as well as to unambiguously determine the protonation sites. Acidity constants (pKa ) of the NH+ moieties of adenine and guanine bases and actual and limiting ionic mobilities of CDNs were determined by nonlinear regression analysis of the pH dependence of their effective electrophoretic mobilities measured by CE in aqueous background electrolytes in a wide pH range (0.98-11.48), at constant temperature (25°C), and constant ionic strength (25 mM). The thermodynamic pKa values were found to be in the range 3.31-4.56 for adenine and 2.28-3.61 for guanine bases, whereas the pKa of enol group of guanine base was in the range 10.21-10.40. Except for systematic shifts of ∼2 pKa , the pKa values calculated by the DFT-D3//COSMO-RS composite protocol that included large-scale conformational sampling and "cross-morphing" were in a relatively good agreement with the pKa s determined by CE and predict N1 atom of adenine and N7 atom of guanine as the protonation sites. The protonation of the N1 atom of adenine and N7 atom of guanine in acidic background electrolytes (BGEs) and the dissociation of the enol group of guanine in alkaline BGEs was confirmed also by NMR spectroscopy.

Structures of the P. aeruginosa FleQ-FleN master regulators reveal large-scale conformational switching in motility and biofilm control.

Pseudomonas aeruginosa can cause a wide array of chronic and acute infections associated with its ability to rapidly switch between planktonic, biofilm, and dispersed lifestyles, each with a specific arsenal for bacterial survival and virulence. At the cellular level, many of the physiological transitions are orchestrated by the intracellular second messenger c-di-GMP and its receptor-effector FleQ. A bacterial enhancer binding protein, FleQ acts as a master regulator of both flagellar motility and adherence factor secretion and uses remarkably different transcription activation mechanisms depending on its dinucleotide loading state, adenosine triphosphatase (ATPase) activity, interactions with polymerase sigma (σ) factors, and complexation with a second ATPase, FleN. How the FleQ-FleN tandem can exert diverse effects through recognition of a conserved FleQ binding consensus has remained enigmatic. Here, we provide cryogenic electron microscopy (cryo-EM) structures of both c-di-GMP-bound and c-di-GMP-free FleQ-FleN complexes which deepen our understanding of the proteins' (di)nucleotide-dependent conformational switching and fine-tuned roles in gene expression regulation.

Synthesis of 4'-Thiomodified c-di-AMP Analogs.

Cyclic diadenosine monophosphate (c-di-AMP) is a bacterial cyclic dinucleotide (CDN) comprising two adenosine monophosphates covalently linked by two 3',5'-phosphodiester bonds. c-di-AMP works as a second messenger, regulating many biological processes in bacteria such as cell wall homeostasis, DNA integrity, and sporulation via specific protein and/or RNA receptors. Moreover, c-di-AMP can function as an immunomodulatory agent in eukaryote cells via the stimulator of interferon genes (STING) signaling pathway. This protocol describes the chemical synthesis of two c-di-AMP analogs with a sulfur atom at the 4'-position of the furanose ring instead of an oxygen atom: c-di-4'-thioAMP (1) and cAMP-4'-thioAMP (2). Analogs 1 and 2 have resistance to phosphodiesterase-mediated degradation and are therefore useful for understanding the diverse biological phenomena regulated by c-di-AMP. In this protocol, two 4'-thioadenosine monomers are initially prepared via a Pummerer-like reaction assisted by hypervalent iodine. The CDN skeleton is then constructed through two key reactions based on phosphoramidite chemistry: dimerization of two appropriately protected nucleoside monomers to produce a linear dinucleotide, followed by macrocyclization of the resulting linear dinucleotide to form the CDN skeleton. © 2023 Wiley Periodicals LLC. Basic Protocol 1: Preparation of 4'-thioadenosine monomers 13 and 14 Basic Protocol 2: Preparation of c-di-4'-thioAMP (1) and cAMP-4'-thioAMP (2).

Vinylphosphonate-based cyclic dinucleotides enhance STING-mediated cancer immunotherapy.

Cyclic dinucleotides (CDNs) trigger the cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway, which plays a key role in cytosolic DNA sensing and thus in immunomodulation against infections, cell damage and cancer. However, cancer immunotherapy trials with CDNs have shown immune activation, but not complete tumor regression. Nevertheless, we designed a novel class of CDNs containing vinylphosphonate based on a STING-affinity screening assay. In vitro, acyloxymethyl phosphate/phosphonate prodrugs of these vinylphosphonate CDNs were up to 1000-fold more potent than the clinical candidate ADU-S100. In vivo, the lead prodrug induced tumor-specific T cell priming and facilitated tumor regression in the 4T1 syngeneic mouse model of breast cancer. Moreover, we solved the crystal structure of this ligand bound to the STING protein. Therefore, our findings not only validate the therapeutic potential of vinylphosphonate CDNs but also open up opportunities for drug development in cancer immunotherapy bridging innate and adaptive immunity.

Evolution of cyclic di-GMP signalling on a short and long term time scale.

Diversifying radiation of domain families within specific lineages of life indicates the importance of their functionality for the organisms. The foundation for the diversifying radiation of the cyclic di-GMP signalling network that occurred within the bacterial kingdom is most likely based in the outmost adaptability, flexibility and plasticity of the system. Integrative sensing of multiple diverse extra- and intracellular signals is made possible by the N-terminal sensory domains of the modular cyclic di-GMP turnover proteins, mutations in the protein scaffolds and subsequent signal reception by diverse receptors, which eventually rewires opposite host-associated as well as environmental life styles including parallel regulated target outputs. Natural, laboratory and microcosm derived microbial variants often with an altered multicellular biofilm behaviour as reading output demonstrated single amino acid substitutions to substantially alter catalytic activity including substrate specificity. Truncations and domain swapping of cyclic di-GMP signalling genes and horizontal gene transfer suggest rewiring of the network. Presence of cyclic di-GMP signalling genes on horizontally transferable elements in particular observed in extreme acidophilic bacteria indicates that cyclic di-GMP signalling and biofilm components are under selective pressure in these types of environments. On a short and long term evolutionary scale, within a species and in families within bacterial orders, respectively, the cyclic di-GMP signalling network can also rapidly disappear. To investigate variability of the cyclic di-GMP signalling system on various levels will give clues about evolutionary forces and discover novel physiological and metabolic pathways affected by this intriguing second messenger signalling system.

cGLRs are a diverse family of pattern recognition receptors in innate immunity.

Cyclic GMP-AMP synthase (cGAS) is an enzyme in human cells that controls an immune response to cytosolic DNA. Upon binding DNA, cGAS synthesizes a nucleotide signal 2'3'-cGAMP that activates STING-dependent downstream immunity. Here, we discover that cGAS-like receptors (cGLRs) constitute a major family of pattern recognition receptors in innate immunity. Building on recent analysis in Drosophila, we identify >3,000 cGLRs present in nearly all metazoan phyla. A forward biochemical screening of 150 animal cGLRs reveals a conserved mechanism of signaling including response to dsDNA and dsRNA ligands and synthesis of isomers of the nucleotide signals cGAMP, c-UMP-AMP, and c-di-AMP. Combining structural biology and in vivo analysis in coral and oyster animals, we explain how synthesis of distinct nucleotide signals enables cells to control discrete cGLR-STING signaling pathways. Our results reveal cGLRs as a widespread family of pattern recognition receptors and establish molecular rules that govern nucleotide signaling in animal immunity.

Novel nanoadjuvants balance immune activation with modest inflammation: implications for older adult vaccines.

BACKGROUND: Age-associated impairments of immune response and inflammaging likely contribute to poor vaccine efficacy. An appropriate balance between activation of immune memory and inflammatory response may be more effective in vaccines for older adults; attempts to overcome reduced efficacy have included the addition of adjuvants or increased antigenic dose. Next generation vaccine formulations may also use biomaterials to both deliver and adjuvant vaccine antigens. In the context of aging, it is important to determine the degree to which new biomaterials may enhance antigen-presenting cell (APC) functions without inducing potent inflammatory responses of APCs or other immune cell types (e.g., T cells). However, the effect of newer biomaterials on these cell types from young and older adults remains unknown. RESULTS: In this pilot study, cells from young and older adults were used to evaluate the effect of novel biomaterials such as polyanhydride nanoparticles (NP) and pentablock copolymer micelles (Mi) and cyclic dinucleotides (CDN; a STING agonist) on cytokine and chemokine secretion in comparison to standard immune activators such as lipopolysaccharide (LPS) and PMA/ionomycin. The NP treatment showed adjuvant-like activity with induction of inflammatory cytokines, growth factors, and select chemokines in peripheral blood mononuclear cells (PBMCs) of both young (n = 6) and older adults (n = 4), yet the degree of activation was generally less than LPS. Treatment with Mi or CDN resulted in minimal induction of cytokines and chemokine secretion with the exception of increased IFN-α and IL-12p70 by CDN. Age-related decreases were observed across multiple cytokines and chemokines, yet IFN-α, IL-12, and IL-7 production by NP or CDN stimulation was equal to or greater than in cells from younger adults. Consistent with these results in aged humans, a combination nanovaccine composed of NP, Mi, and CDN administered to aged mice resulted in a greater percentage of antigen-specific CD4+ T cells and greater effector memory cells in draining lymph nodes compared to an imiquimod-adjuvanted vaccine. CONCLUSIONS: Overall, our novel biomaterials demonstrated a modest induction of cytokine secretion with a minimal inflammatory profile. These findings suggest a unique role for biomaterial nanoadjuvants in the development of next generation vaccines for older adults.

Supramolecular Cyclic Dinucleotide Nanoparticles for STING-Mediated Cancer Immunotherapy.

Activation of stimulator of interferon genes (STING) can reprogram the immunosuppressive tumor microenvironment (TME) by initiating innate and adaptive immunity. As natural STING agonists, clinical translation of cyclic dinucleotides (CDNs) has been challenged by their short half-life in circulation, poor stability, and low membrane permeability. Herein, we use the natural endogenous small molecules oleic acid and deoxycytidine to construct a ligand for the STING agonist c-di-GMP (CDG), a hydrophobic nucleotide lipid (3',5'-diOA-dC), which can assemble with CDG into stable cyclic dinucleotide nanoparticles (CDG-NPs) through various supramolecular forces driven by molecular recognition. CDG-NPs are homogeneous and stable spherical nanoparticles with an average diameter of 59.0 ± 13.0 nm. Compared with free CDG, CDG-NPs promote the retention and intracellular delivery of CDG in the tumor site, boost STING activation and TME immunogenicity, and potentiate STING-mediated anti-tumor immunity when administered by either intratumoral or systemic routes in melanoma-bearing mice. We propose a flexible supramolecular nanodelivery system for CDG by using endogenous small molecules, which provides a CDN delivery platform for STING-mediated cancer immunotherapy.

Cyclic diguanylate analogues: Facile synthesis, STING binding mode and anti-tumor immunity delivered by cytidinyl/cationic lipid.

Herein 2-cyanoethoxy-N,N,N',N'-tetraisopropyl-phosphorodiamidite(10, PIII, 3.5 eq.) could synergistically react with 3',5'-dihydroxyl groups in a dinucleotide(PV) at the cyclization step for the synthesis of cyclic dinucleotides (CDNs) (c-di-GMP, cGAMP etc.) and their phosphorothioated analogues. A dynamic PIII-PV coordination mechanism has been proposed for the cyclization procedure which is confirmed by the variant 31P NMR data and molecular simulation. Among the mono-phosphorothioated CDNs, two stereoisomers showed different capacity for STING activation and the reason was predicted by molecular modeling. While compound 12b1 showed most potent ability to elicit cytokines (IFNß, IL-6, Cxcl9 and Cxcl10) induction compared to another stereoisomer. Also, 12b1 significantly inhibited the tumor growth in the EO771 model with both 0.1 µg (i.t.) and 2 µg (i.v.) administration through the aid of a Mix delivery system developed by our group, and achieved a 31% long-term survival rate of tumor-bearing mice. 12b1/Mix significantly improved the percentage of CD8+ or CD4+ effector memory T (Tem, CD44highCD62Llow) cells and CD8+ central memory T (Tcm, CD44highCD62Lhigh) cells in the blood of EO771 mice, inducing the immune memory against EO771 tumor cells. Relatively lower dose regimens of 12b1(0.1 µg)/Mix displayed better tumor suppression by more potent STING pathway activation and higher levels of cytokines induction in the tumor.

Comparative Life Cycle Assessment of Chemical and Biocatalytic 2'3'-Cyclic GMP-AMP Synthesis.

Life cycle assessments (LCAs) can provide insights into the environmental impact of production processes. In this study, a comparative LCA was performed for the synthesis of 2'3'-cyclic GMP-AMP (2'3'-cGAMP) in an early development stage. The cyclic dinucleotide (CDN) is of interest for pharmaceutical applications such as cancer immunotherapy. CDNs can be synthesized either by enzymes or chemical catalysis. It is not known which of the routes is more sustainable as both routes have their advantages and disadvantages, such as a poor yield for the chemical synthesis and low titers for the biocatalytic synthesis. The synthesis routes were compared for the production of 200 g 2'3'-cGAMP based on laboratory data to assess the environmental impacts. The biocatalytic synthesis turned out to be superior to the chemical synthesis in all considered categories by at least one magnitude, for example, a global warming potential of 3055.6 kg CO2 equiv. for the enzymatic route and 56454.0 kg CO2 equiv. for the chemical synthesis, which is 18 times higher. This study demonstrates the value of assessment at an early development stage, when the choice between different routes is still possible.

IFN-α/ß Signaling Is Required for CDG-Mediated CTL Generation and B Lymphocyte Activation.

Among cyclic di-nucleotides (CDN), both cyclic di-AMP (CDA) and di-GMP (CDG) are promising adjuvants and immune modulators. These molecules are not only able to induce profuse antibody production but also predominant T helper 1 and cytotoxic CD8 T lymphocytes (CTL) responses, which enable their use for vaccination against intracellular pathogens as well as in cancer immunotherapy. However, for their successful translation into the clinic, a comprehensive understanding of CDN mode of action is still essential. Consistent with evidence in the literature, we show here that IFN-α/ß (Type I IFN) is crucial for CDG-mediated B cell activation. We recently determined the key role of type I IFN signaling for CDA-mediated enhancement of immunogenicity. Based on the biological activities of type I IFN, in this study, we hypothesized that it might also be required for CTL induction by CDG. We disclose here the mode of action of type I IFN signaling in CDG-mediated cross-presentation and subsequent CTL generation.

Recent advances in the activation and regulation of the cGAS-STING pathway.

The cGAS-STING pathway is responsible for cytoplasmic double-stranded DNA (dsDNA) -triggered innate immunity and involved in the pathology of various diseases including infection, autoimmune diseases, neurodegeneration and cancer. Understanding the activation and regulatory mechanisms of this pathway is critical to develop therapeutic strategies toward these diseases. Here, we review the signal transduction, cellular functions and regulations of cGAS and STING, particularly highlighting the latest understandings on the activation of cGAS by dsDNA and/or Manganese (Mn2+), STING trafficking, sulfated glycosaminoglycans (sGAGs)-induced STING polymerization and activation, and also regulation of the cGAS-STING pathway by different biocondensates formed via phase separation of proteins from host cells and viruses.

S-acylthioalkyl ester (SATE)-based prodrugs of deoxyribose cyclic dinucleotides (dCDNs) as the STING agonist for antitumor immunotherapy.

Cancer immunotherapy is a powerful weapon in the fight against cancers. Cyclic dinucleotides (CDNs) have demonstrated the great potential by evoking the immune system to fight cancers. There are still a lot of unmet needs for highly active CDNs in clinical applications due to low cell permeation and serum stability. Here we reported S-acylthioalkyl ester (SATE)-based prodrugs of deoxyribose cyclic dinucleotides (dCDNs) with three different types of internucleotide linkages (3',3':11a; 2',3':11b; 2',2':11c). The parent dCDNs could be efficiently released from SATE-dCDNs by cellular esterases. Compared to 2',3'-cGAMP and ADU-S100, 11a exhibited much higher potency of activating STING pathway and higher serum stability. In a CT26-Luc tumor-bearing animal model, 11a showed the efficient antitumor activity in eliminating the established tumor and induced significant increase of mRNA expression of IFN-ß and other related inflammatory cytokines. Hence, SATE-dCDN prodrugs demonstrated their benefits in promoting cell penetration, improving serum stability, and thus enhancing bioactivity, suggesting their potential application as immunotherapy in a variety of malignancies.

Discovery of isonucleotidic CDNs as potent STING agonists with immunomodulatory potential.

Stimulator of interferon genes (STING) is an adaptor protein of the cGAS-STING signaling pathway involved in the sensing of cytosolic DNA. It functions as a receptor for cyclic dinucleotides (CDNs) and, upon their binding, mediates cytokine expression and host immunity. Besides naturally occurring CDNs, various synthetic CDNs, such as ADU-S100, have been reported to effectively activate STING and are being evaluated in clinical trials for the treatment of cancer. Here, we describe the preparation of a unique new class of STING agonists: isonucleotidic cyclic dinucleotides and the synthesis of their prodrugs. The presented CDNs stimulate STING with comparable efficiency to ADU-S100, whereas their prodrugs demonstrate activity up to four orders of magnitude better due to the improved cellular uptake. The compounds are very potent inducers of inflammatory cytokines by peripheral blood mononuclear cells (PBMCs). We also report the X-ray crystal structure of the lead inhibitor bound to the wild-type (WT) STING.

Discovery of novel Thieno[2,3-d]imidazole derivatives as agonists of human STING for antitumor immunotherapy using systemic administration.

The activation of stimulator of interferon genes (STING) signaling pathways plays an important role in the innate immune response. Although several STING agonists have been developed recently, the majority of clinical CDN STING agonists are administered by intratumoral (IT) injection. Therefore, there remains a need to develop diverse non-CDN small-molecule STING agonists with systemic administration. Herein, by using a scaffold hopping strategy, we designed a series of thieno [2,3-d]imidazole derivatives as novel STING agonists. Further structure-activity relationship study and optimization led to the discovery of compound 45 as a highly potent human STING agonist with an EC50 value of 1.2 nM. Compound 45 was found to bind to multiple human STING isoforms and accordingly activated the downstream TBK1/IRF3 and NF-κB signaling pathways in the reporter cells bearing with different STING isoforms. The activation on STING signaling pathway was abolished in the STING knock-out cells, indicating that it is a specific STING agonist. Compound 45 significantly inhibited the tumor growth in allograft 4T1 and CT26 tumor models by systemic administration, and more significantly, 45 was able to induce tumor regression in CT26 tumor model without inducing weight loss, suggesting that compound 45 is a highly promising candidate worthy for further development.