Design, Synthesis and Biological Evaluation of (2',5' and 3'5'-Linked) cGAMP Analogs that Activate Stimulator of Interferon Genes (STING)

Mn-phenolic networks as synergistic carrier for STING agonists in tumor immunotherapy

The cGAS-STING pathway holds tremendous potential as a regulator of immune responses, offering a means to reshape the tumor microenvironment and enhance tumor immunotherapy. Despite the emergence of STING agonists, their clinical viability is hampered by stability and delivery challenges, as well as variations in STING expression within tumors. In this study, we present Mn-phenolic networks as a novel carrier for ADU-S100, a hydrophilic STING agonist, aimed at bolstering immunotherapy. These nanoparticles, termed TMA NMs, are synthesized through the coordination of tannic acid and manganese ions, with surface modification involving bovine serum albumin to enhance their colloidal stability. TMA NMs exhibit pH/GSH-responsive disintegration properties, enabling precise drug release. This effectively addresses drug stability issues and facilitates efficient intracellular drug delivery. Importantly, TMA NMs synergistically enhance the effects of ADU-S100 through the concurrent release of Mn2+, which serves as a sensitizer of the STING pathway, resulting in significant STING pathway activation. Upon systemic administration, these nanoparticles efficiently accumulate within tumors. The activation of STING pathways not only induces immunogenic cell death (ICD) in tumor cells but also orchestrates systemic remodeling of the immunosuppressive microenvironment. This includes the promotion of cytokine release, dendritic cell maturation, and T cell infiltration, leading to pronounced suppression of tumor growth. Combining with the excellent biocompatibility and biodegradability, this Mn-based nanocarrier represents a promising strategy for enhancing tumor immunotherapy through the cGAS-STING pathway.

Loops Mediate Agonist-Induced Activation of the Stimulator of Interferon Genes Protein

The stimulator of interferon genes (STING) is an important therapeutic target for cancer diseases. The activated STING recruits downstream tank-binding kinase 1 (TBK1) to trigger several important immune responses. However, the molecular mechanism of how agonist molecules mediate the STING-TBK1 interactions remains elusive. Here, we performed molecular dynamics simulations to capture the conformational changes of STING and TBK1 upon agonist binding. Our simulations revealed that multiple helices (α5-α7) and especially three loops (loop 6, loop 8, and C-terminal tail) of STING participated in the allosteric mediation of the STING-TBK1 interactions. Consistent results were also observed in the simulations of the constitutive activating mutant of STING (R284S). We further identified α5 as a key region in this agonist-induced activation mechanism of STING. Free-energy perturbation calculations of multiple STING agonists demonstrated that an alkynyl group targeting α5 is a determinant for agonist activities. These results not only offer deeper insights into the agonist-induced allosteric mediation of STING-TKB1 interactions but also provide a guidance for future drug development of this important therapeutic target.

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, P^III, 3.5 eq.) could synergistically react with 3',5'-dihydroxyl groups in a dinucleotide(P^V) at the cyclization step for the synthesis of cyclic dinucleotides (CDNs) (c-di-GMP, cGAMP etc.) and their phosphorothioated analogues. A dynamic P^III-P^V 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 (IFNβ, 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, CD44^highCD62L^low) cells and CD8⁺ central memory T (Tcm, CD44^highCD62L^high) 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.

CDK inhibitor Palbociclib targets STING to alleviate autoinflammation

Aberrant activation of stimulator of interferon genes (STING) is tightly associated with multiple types of disease, including cancer, infection, and autoimmune diseases. However, the development of STING modulators for the therapy of STING-related diseases is still an unmet clinical need. We employed a high-throughput screening approach based on the interaction of small-molecule chemical compounds with recombinant STING protein to identify functional STING modulators. Intriguingly, the cyclin-dependent protein kinase (CDK) inhibitor Palbociclib was found to directly bind STING and inhibit its activation in both mouse and human cells. Mechanistically, Palbociclib targets Y167 of STING to block its dimerization, its binding with cyclic dinucleotides, and its trafficking. Importantly, Palbociclib alleviates autoimmune disease features induced by dextran sulphate sodium or genetic ablation of three prime repair exonuclease 1 (Trex1) in mice in a STING-dependent manner. Our work identifies Palbociclib as a novel pharmacological inhibitor of STING that abrogates its homodimerization and provides a basis for the fast repurposing of this Food and Drug Administration-approved drug for the therapy of autoinflammatory diseases.

2',3'-Cyclic GMP-AMP Dinucleotides for STING-Mediated Immune Modulation: Principles, Immunotherapeutic Potential, and Synthesis

The cGAS-STING pathway discovered ten years ago is an important component of the innate immune system. Activation of cGAS-STING triggers downstream signalling, such as TBK1-IRF3, NF-κB and autophagy, which in turn leads to antipathogen responses, durable antitumour immunity or autoimmune diseases. 2',3'-Cyclic GMP-AMP dinucleotides (2',3'-cGAMP), the key second messengers produced by cGAS, play a pivotal role in cGAS-STING signalling by binding and activating STING. Thus, 2',3'-cGAMP has immunotherapeutic potential, which in turn has stimulated research on the design and synthesis of 2',3'-cGAMP analogues for clinical applications over the past ten years. This review presents the discovery, metabolism, and function of 2',3'-cGAMP in the cGAS-STING innate immune signalling axis. The enzymatic and chemical syntheses of 2',3'-cGAMP analogues as STING-targeting therapeutics are also summarized.

Replacement of oxygen with sulfur on the furanose ring of cyclic dinucleotides enhances the immunostimulatory effect via STING activation

Cyclic dinucleotides (CDNs) are secondary messengers composed of two purine nucleotides linked via two phosphodiester linkages: c-di-GMP, c-di-AMP, 3',3'-cGAMP, and 2',3'-cGAMP. CDNs activate the stimulator of interferon genes (STING) and trigger immune responses in mammalian species. CDNs are thus fascinating molecules as drug candidates, and chemically stable CDN analogues that act as STING agonists are highly desired at present. We herein report the practical synthesis of 4'-thiomodified c-di-AMP analogues, which have sulfur atoms at the 4'-position on the furanose ring instead of oxygen atoms, using simple phosphoramidite chemistry. The resulting 4'-thiomodified c-di-AMP analogues acted as potent STING agonists with long-term activity. Our results show that replacing O4' on CDNs with sulfur can lead to enhanced immunostimulatory effects via STING activation.