Discovery and Mechanistic Study of a Novel Human-Stimulator-of-Interferon-Genes Agonist

IUPHAR ECR review: The cGAS-STING pathway: Novel functions beyond innate immune and emerging therapeutic opportunities

Stimulator of interferon genes (STING) is a crucial innate immune sensor responsible for distinguishing pathogens and cytosolic DNA, mediating innate immune signaling pathways to defend the host. Recent studies have revealed additional regulatory functions of STING beyond its innate immune-related activities, including the regulation of cellular metabolism, DNA repair, cellular senescence, autophagy and various cell deaths. These findings highlight the broader implications of STING in cellular physiology beyond its role in innate immunity. Currently, approximately 10 STING agonists have entered the clinical stage. Unlike inhibitors, which have a maximum inhibition limit, agonists have the potential for infinite amplification. STING signaling is a complex process that requires precise regulation of STING to ensure balanced immune responses and prevent detrimental autoinflammation. Recent research on the structural mechanism of STING autoinhibition and its negative regulation by adaptor protein complex 1 (AP-1) provides valuable insights into its different effects under physiological and pathological conditions, offering a new perspective for developing immune regulatory drugs. Herein, we present a comprehensive overview of the regulatory functions and molecular mechanisms of STING beyond innate immune regulation, along with updated details of its structural mechanisms. We discuss the implications of these complex regulations in various diseases, emphasizing the importance and feasibility of targeting the immunity-dependent or immunity-independent functions of STING. Moreover, we highlight the current trend in drug development and key points for clinical research, basic research, and translational research related to STING.

Nanomaterial-mediated modulation of the cGAS-STING signaling pathway for enhanced cancer immunotherapy

Despite the current promise of immunotherapy, many cancer patients still suffer from challenges such as poor immune response rates, resulting in unsatisfactory clinical efficacy of existing therapies. There is an urgent need to combine emerging biomedical discoveries and innovations in traditional therapies. Modulation of the cGAS-STING signalling pathway represents an important innate immunotherapy pathway that serves as a crucial DNA sensing mechanism in innate immunity and viral defense. It has attracted increasing attention as an emerging target for cancer therapy. The recent advancements in nanotechnology have led to the significant utilization of nanomaterials in cancer immunotherapy, owing to their exceptional physicochemical properties such as large specific surface area and efficient permeability. Given the rapid development of cancer immunotherapy driven by the cGAS-STING activation, this study reviews the latest research progress in employing nanomaterials to modulate this signaling pathway. Based on the introduction of the main activation mechanisms of cGAS-STING pathway, this review focuses on nanomaterials that mediate the agonists involved and effectively activate this signaling pathway. In addition, combination nanotherapeutics based on the activation of the cGAS-STING signaling pathway are also discussed, including emerging strategies combining nanoformulated agonists with chemotherapy, radiotherapy as well as other immunomodulation in tumor targeting therapy. STATEMENT OF SIGNIFICANCE: Given the rapid development of cancer immunotherapy driven by the cGAS / STING activation, this study reviews the latest research advances in the use of nanomaterials to modulate this signaling pathway. Based on the introduction of key cGAS-STING components and their activation mechanisms, this review focuses on nanomaterials that can mediate the corresponding agonists and effectively activate this signaling pathway. In addition, combination nanotherapies based on the activation of the cGAS-STING signaling pathway are also discussed, including emerging strategies combining nanoformulated agonists with chemotherapy, radiotherapy as well as immunomodulation in cancer therapy,.

Emerging mechanisms and implications of cGAS-STING signaling in cancer immunotherapy strategies

The intricate interplay between the human immune system and cancer development underscores the central role of immunotherapy in cancer treatment. Within this landscape, the innate immune system, a critical sentinel protecting against tumor incursion, is a key player. The cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING) pathway has been found to be a linchpin of innate immunity: activation of this signaling pathway orchestrates the production of type I interferon (IFN-α/β), thus fostering the maturation, differentiation, and mobilization of immune effectors in the tumor microenvironment. Furthermore, STING activation facilitates the release and presentation of tumor antigens, and therefore is an attractive target for cancer immunotherapy. Current strategies to activate the STING pathway, including use of pharmacological agonists, have made substantial advancements, particularly when combined with immune checkpoint inhibitors. These approaches have shown promise in preclinical and clinical settings, by enhancing patient survival rates. This review describes the evolving understanding of the cGAS-STING pathway's involvement in tumor biology and therapy. Moreover, this review explores classical and non-classical STING agonists, providing insights into their mechanisms of action and potential for optimizing immunotherapy strategies. Despite challenges and complexities, the cGAS-STING pathway, a promising avenue for enhancing cancer treatment efficacy, has the potential to revolutionize patient outcomes.

Mitochondrial DNA-triggered innate immune response: mechanisms and diseases

Various cellular stress conditions trigger mitochondrial DNA (mtDNA) release from mitochondria into the cytosol. The released mtDNA is sensed by the cGAS-MITA/STING pathway, resulting in the induced expression of type I interferon and other effector genes. These processes contribute to the innate immune response to viral infection and other stress factors. The deregulation of these processes causes autoimmune diseases, inflammatory metabolic disorders and cancer. Therefore, the cGAS-MITA/STING pathway is a potential target for intervention in infectious, inflammatory and autoimmune diseases as well as cancer. In this review, we focus on the mechanisms underlying the mtDNA-triggered activation of the cGAS-MITA/STING pathway, the effects of the pathway under various physiological and pathological conditions, and advances in the development of drugs that target cGAS and MITA/STING.

NIR-II fluorescence and PA imaging guided activation of STING pathway in photothermal therapy for boosting cancer immunotherapy by theranostic thermosensitive liposomes

Photothermal immunotherapy has shown great potential for efficient cancer treatment. However, the immunosuppressive tumor microenvironment forms a heavy barrier for photothermal-induced anti-tumor immunity by inhibiting dendritic cell (DC) maturation and cytotoxic T cell response. Moreover, the lack of reliable spatiotemporal imaging modalities makes photothermal immunotherapy difficult to guide tumor ablation and monitor therapeutic outcomes in real time. Herein, we designed a theranostic thermosensitive liposome (PLDD) as a versatile nanoplatform to boost the adaptive anti-tumor immunity of photothermal immunotherapy and to achieve multiple bioimaging modalities in a real-time manner. PLDD contains two major functional components: a multifunctional photothermal agent (DTTB) and an immune potentiator STING pathway agonist (DMXAA). Upon irradiation, the heat generated by DTTB induced the immunogenic cell death (ICD) of the tumor and dissociated the structure of thermosensitive liposome to release DMXAA, which ultimately activated the STING pathway and promoted the ICD-induced immune response by increasing DC cell maturation and T cell recruitment. Moreover, the DTTB in PLDD displayed excellent second near-infrared (NIR-II) fluorescence and photoacoustic (PA) dual-modal imaging, which provided omnibearing information on the tumor and guided the subsequent therapeutic operation. Therefore, this versatile PLDD with light-triggered promotion of anti-tumor immunity and multiple spatiotemporal imaging profiles holds great potential for the future development of cancer immunotherapy.

Recent trends in STING modulators: Structures, mechanisms, and therapeutic potential

The cyclic GMP-AMP synthase stimulator (cGAS)-stimulator of interferon gene (STING) signaling pathway has an integral role in the host immune response through DNA sensing followed by inducing a robust innate immune defense program. STING has become a promising therapeutic target associated with multiple diseases, including various inflammatory diseases, cancer, and infectious diseases, among others. Thus, modulators of STING are regarded as emerging therapeutic agents. Recent progress has been made in STING research, including recently identified STING-mediated regulatory pathways, the development of a new STING modulator, and the new association of STING with disease. In this review, we focus on recent trends in the development of STING modulators, including structures, mechanisms, and clinical application.

Significance of the cGAS-STING Pathway in Health and Disease

The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway plays a significant role in health and disease. In this pathway, cGAS, one of the major cytosolic DNA sensors in mammalian cells, regulates innate immunity and the STING-dependent production of pro-inflammatory cytokines, including type-I interferon. Moreover, the cGAS-STING pathway is integral to other cellular processes, such as cell death, cell senescence, and autophagy. Activation of the cGAS-STING pathway by "self" DNA is also attributed to various infectious diseases and autoimmune or inflammatory conditions. In addition, the cGAS-STING pathway activation functions as a link between innate and adaptive immunity, leading to the inhibition or facilitation of tumorigenesis; therefore, research targeting this pathway can provide novel clues for clinical applications to treat infectious, inflammatory, and autoimmune diseases and even cancer. In this review, we focus on the cGAS-STING pathway and its corresponding cellular and molecular mechanisms in health and disease.

The battle between the innate immune cGAS-STING signaling pathway and human herpesvirus infection

The incidence of human herpesvirus (HHVs) is gradually increasing and has affected a wide range of population. HHVs can result in serious consequences such as tumors, neonatal malformations, sexually transmitted diseases, as well as pose an immense threat to the human health. The cGAS-STING pathway is one of the innate immune pattern-recognition receptors discovered recently. This article discusses the role of the cGAS-STING pathway in human diseases, especially in human herpesvirus infections, as well as highlights how these viruses act on this pathway to evade the host immunity. Moreover, the author provides a comprehensive overview of modulators of the cGAS-STING pathway. By focusing on the small molecule compounds based on the cGAS-STING pathway, novel targets and concepts have been proposed for the development of antiviral drugs and vaccines, while also providing a reference for the investigation of disease models related to the cGAS-STING pathway. HHV is a double-stranded DNA virus that can trigger the activation of intracellular DNA sensor cGAS, after which the host cells initiate a cascade of reactions that culminate in the secretion of type I interferon to restrict the viral replication. Meanwhile, the viral protein can interact with various molecules in the cGAS-STING pathway. Viruses can evade immune surveillance and maintain their replication by inhibiting the enzyme activity of cGAS and reducing the phosphorylation levels of STING, TBK1 and IRF3 and suppressing the interferon gene activation. Activators and inhibitors of the cGAS-STING pathway have yielded numerous promising research findings in vitro and in vivo pertaining to cGAS/STING-related disease models. However, there remains a dearth of small molecule modulators that have been successfully translated into clinical applications, which serves as a hurdle to be overcome in the future.

Biomaterial-enabled therapeutic modulation of cGAS-STING signaling for enhancing antitumor immunity

cGAS-STING signaling is a central component in the therapeutic action of most existing cancer therapies. The accumulated knowledge of tumor immunoregulatory network in recent years has spurred the development of cGAS-STING agonists for tumor treatment as an effective immunotherapeutic strategy. However, the clinical translation of these agonists is thus far unsatisfactory because of the low immunostimulatory efficacy and unrestricted side effects under clinically relevant conditions. Interestingly, the rational integration of biomaterial technology offers a promising approach to overcome these limitations for more effective and safer cGAS-STING-mediated tumor therapy. Herein, we first outline the cGAS-STING signaling axis and generally discuss its association with tumors. We then symmetrically summarize the recent progress in those biomaterial-based cGAS-STING agonism strategies to generate robust antitumor immunity, categorized by the chemical nature of those cGAS-STING stimulants and carrier substrates. Finally, a perspective is provided to discuss the existing challenges and potential opportunities in cGAS-STING modulation for tumor therapy.

Development of 18F-Labeled Acridone Analogue for Tumor Imaging via Stimulator of Interferon Genes Targeting

The stimulator of interferon genes (STING) is a pivotal protein in the production of STING-dependent type I interferon, which has the potential to enhance tumor rejection. The visualization of STING in the tumor microenvironment is valuable for STING-related treatments, but few STING imaging probes have been reported to date. In this study, we developed a novel ¹⁸F-labeled agent ([¹⁸F]F-CRI1) with an acridone core structure for the positron emission tomography (PET) imaging of STING in CT26 tumors. The probe was successfully prepared with a nanomolar STING binding affinity of K_d = 40.62 nM. [¹⁸F]F-CRI1 accumulated quickly in the tumor sites and its uptake reached a maximum of 3.02 ± 0.42% ID/g after 1 h i.v. injection. The specificity of [¹⁸F]F-CRI1 was confirmed both in in vitro cell uptake and in vivo PET imaging by blocking studies. Our findings suggest that [¹⁸F]F-CRI1 may be a potential agent for visualizing STING in the tumor microenvironment.

Mn-based cGAS-STING activation for tumor therapy

Immunotherapy has efficiently revolutionized the treatment of human neoplastic diseases. However, the overall responsive rate of current immunotherapy is still unsatisfactory, benefiting only a small proportion of patients. Therefore, significant attention has been paid to the modulation of tumor microenvironment (TME) for the enhancement of immunotherapy. Interestingly, recent studies have shown that cyclic GMP-AMP synthase-stimulator of interferon gene (cGAS-STING) was initially found as an innate immune sensor to recognize cytoplasmic DNA (such as bacterial, viral, micronuclei, and mitochondrial). It is a promising signaling pathway to activate antitumor immune responses via type I interferon production. Notably, Mn²⁺ was found to be a critical molecule to sensitize the activation of the cGAS-STING pathway for better immunotherapy. This activation led to the development of Mn²⁺-based strategies for tumor immunotherapy via the activation of the cGAS-STING pathway. In this critical review, we aimed to summarize the recent progress of this field, focusing on the following three aspects. First, we briefly introduced the signaling pathway of cGAS-STING activation, and its regulation effect on the antitumor immunity cycle has been discussed. Along with this, several agonists of the cGAS-STING pathway were introduced with their potential as immunotherapeutic drugs. Then, the basic biological functions of Mn²⁺ have been illustrated, focusing on its critical roles in the cGAS-STING pathway activation. Next, we systematically reviewed the Mn²⁺-based strategies for tumor immunotherapy, which can be classified by the methods based on Mn²⁺ alone or Mn²⁺ combined with other therapeutic modalities. We finally speculated the future perspectives of the field and provided rational suggestions to develop better Mn²⁺-based therapeutics.

Synthetic Stimulator of Interferon Genes (STING) Agonists Induce a Cytokine-Mediated Anti-Hepatitis B Virus Response in Nonparenchymal Liver Cells

Chronic hepatitis B (CHB) remains a major public health problem worldwide, with limited treatment options, but inducing an antiviral response by innate immunity activation may provide a therapeutic alternative. We assessed the cytokine-mediated anti-hepatitis B virus (HBV) potential for stimulating the cyclic GMP-AMP synthase-stimulator of interferon genes (STING) pathway using STING agonists in primary human hepatocytes (PHH) and nonparenchymal liver cells (NPCs). The natural STING agonist, 2',3'-cyclic GMP-AMP, the synthetic analogue 3',3'-c-di(2'F,2'dAMP), and its bis(pivaloyloxymethyl) prodrug had strong indirect cytokine-mediated anti-HBV effects in PHH regardless of HBV genotype. Furthermore, STING agonists induced anti-HBV cytokine secretion in vitro, in both human and mouse NPCs, and triggered hepatic T cell activation. Cytokine secretion and lymphocyte activation were equally stimulated in NPCs isolated from control and HBV-persistent mice. Therefore, STING agonists modulate immune activation regardless of HBV persistence, paving the way toward a CHB therapy.

The cGAS/STING signaling pathway: a cross-talk of infection, senescence and tumors

The cGAS/STING signaling pathway is an important part of the cytoplasmic DNA sensor, which can trigger a type I interferon response to microbial infection when pathogenic DNA is detected. However, continuous inhibition of cGAS/STING signaling by viral infection may be an important cause of tumorigenesis. At the same time, recent studies have shown that although the cGAS/STING signaling pathway also plays a core role in anti-tumor immunity and cell senescence, the inflammatory response induced by cGAS/STING signaling will also promote tumorigenesis in different backgrounds. Here, we discuss the role of cGAS/STING in the context of infection, senescence, and tumors, especially with respect to progression, to facilitate a better understanding of the mechanism of the cGAS/STING pathway.

Exploiting the DNA Damaging Activity of Liposomal Low Dose Cytarabine for Cancer Immunotherapy

Perhaps the greatest limitation for the continually advancing developments in cancer immunotherapy remains the immunosuppressive tumor microenvironment (TME). The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) axis is an emerging immunotherapy target, with the resulting type I interferons and transcription factors acting at several levels in both tumor and immune cells for the generation of adaptive T cell responses. The cGAS-STING axis activation by therapeutic agents that induce DNA damage, such as certain chemotherapies, continues to be reported, highlighting the importance of the interplay of this signaling pathway and the DNA damage response in cancer immunity/immunotherapy. We have developed a multi-targeted mannosylated cationic liposomal immunomodulatory system (DS) which contains low doses of the chemotherapeutic cytarabine (Ara-C). In this work, we show that entrapment of non-cytotoxic doses of Ara-C within the DS improves its ability to induce DNA double strand breaks in human ovarian and colorectal cancer cell lines, as well as in various immune cells. Importantly, for the first time we demonstrate that the DNA damage induced by Ara-C/DS translates into cGAS-STING axis activation. We further demonstrate that Ara-C/DS-mediated DNA damage leads to upregulation of surface expression of immune ligands on cancer cells, coinciding with priming of cytotoxic lymphocytes as assessed using an ex vivo model of peripheral blood mononuclear cells from colorectal cancer patients, as well as an in vitro NK cell model. Overall, the results highlight a broad immunotherapeutic potential for Ara-C/DS by enhancing tumor-directed inflammatory responses.

Chemical regulation of the cGAS-STING pathway

Nucleic acids represent a major class of pathogen and damage signatures, recognized by a variety of host sensors to initiate signaling cascades and immune responses, such as mechanisms of RLR-MAVS, cGAS-STING, TLR-TRIF, and AIM2 inflammasome. Yet, an outstanding challenge is understanding how nucleic acid sensing initiates immune responses and its tethering in various infectious, cancerous, autoimmune, and inflammatory diseases. However, the discovery and application of a plethora of small molecule compounds have substantially facilitated this process. This review provides an overview and recent development of the innate DNA-sensing pathway of cGAS-STING and highlights the multiple agonists and inhibitors in fine-tuning the pathway that can be exploited to improve disease treatment, focusing primarily on crucial pathway components and regulators.

Radioresistant cells initiate lymphocyte-dependent lung inflammation and IFNγ-dependent mortality in STING gain-of-function mice

Pediatric patients with constitutively active mutations in the cytosolic double-stranded-DNA-sensing adaptor STING develop an autoinflammatory syndrome known as STING-associated vasculopathy with onset in infancy (SAVI). SAVI patients have elevated interferon-stimulated gene expression and suffer from interstitial lung disease (ILD) with lymphocyte predominate bronchus-associated lymphoid tissue (BALT). Mice harboring SAVI mutations (STING V154M [VM]) that recapitulate human disease also develop lymphocyte-rich BALT. Ablation of either T or B lymphocytes prolongs the survival of SAVI mice, but lung immune aggregates persist, indicating that T cells and B cells can independently be recruited as BALT. VM T cells produced IFNγ, and IFNγR deficiency prolonged the survival of SAVI mice; however, T-cell-dependent recruitment of infiltrating myeloid cells to the lung was IFNγ independent. Lethally irradiated VM recipients fully reconstituted with wild type bone-marrow-derived cells still developed ILD, pointing to a critical role for VM-expressing radioresistant parenchymal and/or stromal cells in the recruitment and activation of pathogenic lymphocytes. We identified lung endothelial cells as radioresistant cells that express STING. Transcriptional analysis of VM endothelial cells revealed up-regulation of chemokines, proinflammatory cytokines, and genes associated with antigen presentation. Together, our data show that VM-expressing radioresistant cells play a key role in the initiation of lung disease in VM mice and provide insights for the treatment of SAVI patients, with implications for ILD associated with other connective tissue disorders.

Post-Translational Modifications of STING: A Potential Therapeutic Target

Stimulator of interferon genes (STING) is an endoplasmic-reticulum resident protein, playing essential roles in immune responses against microbial infections. However, over-activation of STING is accompanied by excessive inflammation and results in various diseases, including autoinflammatory diseases and cancers. Therefore, precise regulation of STING activities is critical for adequate immune protection while limiting abnormal tissue damage. Numerous mechanisms regulate STING to maintain homeostasis, including protein-protein interaction and molecular modification. Among these, post-translational modifications (PTMs) are key to accurately orchestrating the activation and degradation of STING by temporarily changing the structure of STING. In this review, we focus on the emerging roles of PTMs that regulate activation and inhibition of STING, and provide insights into the roles of the PTMs of STING in disease pathogenesis and as potential targeted therapy.

Development of Potent Immune Modulators Targeting Stimulator of Interferon Genes Receptor

Stimulator of interferon genes (STING) is an endoplasmic reticulum-membrane protein that plays important roles in cancer immunotherapy by activating innate immune responses. We designed and synthesized STING modulators and characterized compounds 4a and 4c that share a crucial amidobenzimidazole moiety. In vitro STING binding and cell-based activity assays demonstrated the potency and efficacy of the compounds that function as direct STING agonists by stimulating STING downstream signaling and promoting type I interferon immune responses. In vitro metabolic studies and the pharmacokinetic properties of the compounds led us to investigate their anticancer activity in an in vivo syngeneic model. Intravenous injection of compounds 4a and 4c significantly decreased tumor volume in a CT26 murine colorectal carcinoma model, and the immunological memory-derived cancer inhibition was observed in 4c-treated mouse models. The present results suggest the therapeutic potential of the compounds for cancer immunotherapy via STING-mediated immune activation.

Advances in cGAS-STING Signaling Pathway and Diseases

Pathogens can produce conserved pathogen-associated molecular patterns (PAMPs) after invading the body, which can be specifically recognized by host pattern recognition receptors (PRRs). In recent years, it has been found that cytoplasmic DNA receptors recognize exogenous DNA inducing activation of interferon 1 (IFN1), which is a rapid advance in various research areas. The cyclic GMP–AMP synthase (cGAS) stimulator of interferon gene (STING) signaling pathway is a critical natural immune pathway in cells. Early studies revealed that it plays a crucial regulatory role in pathogen infection and tumor, and it is associated with various human autoimmune diseases. Recently studies have found that activation of cGAS-STING signaling pathway is related to different organ injuries. The present review elaborates on the regulation of the cGAS-STING signaling pathway and its role in various diseases, aiming to provide a theoretical basis for immunotherapy targeting this pathway.

Nanodelivery of STING agonists against cancer and infectious diseases

Vaccination is a modality that has been widely explored for the treatment of various diseases. To increase the potency of vaccine formulations, immunostimulatory adjuvants have been regularly exploited, and the stimulator of interferon genes (STING) signaling pathway has recently emerged as a remarkable therapeutic target. STING is an endogenous protein on the endoplasmic reticulum that is a downstream sensor to cytosolic DNA. Upon activation, STING initiates a series of intracellular signaling cascades that ultimately generate potent type I interferon-mediated immune responses. Both natural and synthetic agonists have been used to stimulate the STING pathway, but they are usually administered locally due to low bioavailability, instability, and difficulty in bypassing the plasma membrane. With excellent pharmacokinetic profiles and versatility, nanocarriers can address many of these challenges and broaden the application of STING vaccines. Along these lines, STING-inducing nanovaccines are being developed to address a wide range of diseases. In this review, we discuss the recent advances in STING nanovaccines for anticancer, antiviral, and antibacterial applications.

A yellow fever virus NS4B inhibitor not only suppresses viral replication, but also enhances the virus activation of RIG-I-like receptor-mediated innate immune response

Flavivirus infection of cells induces massive rearrangements of the endoplasmic reticulum (ER) membrane to form viral replication organelles (ROs) which segregates viral RNA replication intermediates from the cytoplasmic RNA sensors. Among other viral nonstructural (NS) proteins, available evidence suggests for a prominent role of NS4B, an ER membrane protein with multiple transmembrane domains, in the formation of ROs and the evasion of the innate immune response. We previously reported a benzodiazepine compound, BDAA, which specifically inhibited yellow fever virus (YFV) replication in cultured cells and in vivo in hamsters, with resistant mutation mapped to P219 of NS4B protein. In the following mechanistic studies, we found that BDAA specifically enhances YFV induced inflammatory cytokine response in association with the induction of dramatic structural alteration of ROs and exposure of double-stranded RNA (dsRNA) in virus-infected cells. Interestingly, the BDAA-enhanced cytokine response in YFV-infected cells is attenuated in RIG-I or MAD5 knockout cells and completely abolished in MAVS knockout cells. However, BDAA inhibited YFV replication at a similar extent in the parent cells and cells deficient of RIG-I, MDA5 or MAVS. These results thus provided multiple lines of biological evidence to support a model that BDAA interaction with NS4B may impair the integrity of YFV ROs, which not only inhibits viral RNA replication, but also promotes the release of viral RNA from ROs, which consequentially activates RIG-I and MDA5. Although the innate immune enhancement activity of BDAA is not required for its antiviral activity in cultured cells, its dual antiviral mechanism is unique among all the reported antiviral agents thus far and warrants further investigation in animal models in future.

Emergence and re-emergence of yellow fever (YF) caused by the yellow fever virus (YFV) infection have posed a global public health threat in previously non-epidemic as well as endemic regions. The approximately 30% of mortality rate makes the outbreaks particularly devastating. In addition to the vaccination campaign and mosquito controls, antiviral drugs are important components in the toolbox for combating YF outbreaks. However, only two nucleotide analogue drugs developed for the treatment of other RNA virus infections are currently repurposed for the treatment of YF with uncertain clinical efficacy. BDAA is a benzodiazepine compound discovered as a potent YFV-specific antiviral agent in our laboratory. The work reported herein further demonstrates that BDAA interaction with the YFV NS4B protein may impair the integrity of viral RNA replication organelles, which not only inhibits viral RNA replication, but also results in the leakage of viral RNA into the cytoplasm to activate RIG-I-like RNA receptors and enhances the innate antiviral immune response. The unprecedented antiviral mechanism of BDAA highlights the essential role of the NS4B protein in viral RNA replication and the evasion of host cellular innate immunity.

Therapeutic Development by Targeting the cGAS-STING Pathway in Autoimmune Disease and Cancer

DNA immune recognition regulation mediated by the cGAS-STING pathway plays an important role in immune functions. Under normal physiological conditions, cGAS can recognize and bind to invading pathogen DNA and activate the innate immune response. On the other hand, abnormal activation of cGAS or STING is closely related to autoimmune diseases. In addition, activation of STING proteins as a bridge connecting innate immunity and adaptive immunity can effectively restrain tumor growth. Therefore, targeting the cGAS-STING pathway can alleviate autoimmune symptoms and be a potential drug target for treating cancer. This article summarizes the current progress on cGAS-STING pathway modulators and lays the foundation for further investigating therapeutic development in autoimmune diseases and tumors.

Therapeutic Interventions Targeting Innate Immune Receptors: A Balancing Act

The innate immune system is an organism's first line of defense against an onslaught of internal and external threats. The downstream adaptive immune system has been a popular target for therapeutic intervention, while there is a relative paucity of therapeutics targeting the innate immune system. However, the innate immune system plays a critical role in many human diseases, such as microbial infection, cancer, and autoimmunity, highlighting the need for ongoing therapeutic research. In this review, we discuss the major innate immune pathways and detail the molecular strategies underpinning successful therapeutics targeting each pathway as well as previous and ongoing efforts. We will also discuss any recent discoveries that could inform the development of novel therapeutic strategies. As our understanding of the innate immune system continues to develop, we envision that therapies harnessing the power of the innate immune system will become the mainstay of treatment for a wide variety of human diseases.

Delivery of STING agonists for adjuvanting subunit vaccines

Adjuvant is an essential component in subunit vaccines. Many agonists of pathogen recognition receptors have been developed as potent adjuvants to optimize the immunogenicity and efficacy of vaccines. Recently discovered cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway has attracted much attention as it is a key mediator for modulating immune responses. Vaccines adjuvanted with STING agonists are found to mediate a robust immune defense against infections and cancer. In this review, we first discuss the mechanisms of STING agonists in the context of vaccination. Next, we present recent progress in novel STING agonist discovery and the delivery strategies. We next highlight recent work in optimizing the efficacy while minimizing toxicity of STING agonist-assisted subunit vaccines for protection against infectious diseases or treatment of cancer. Finally, we share our perspectives of current issues and future directions in further developing STING agonists for adjuvanting subunit vaccines.

Prospects for the Global Elimination of Hepatitis B

Chronic hepatitis B virus (HBV) infection is the leading cause of liver cirrhosis and hepatocellular carcinoma, estimated to be globally responsible for ∼800,000 deaths annually. Although effective vaccines are available to prevent new HBV infection, treatment of existing chronic hepatitis B (CHB) is limited, as the current standard-of-care antiviral drugs can only suppress viral replication without achieving cure. In 2016, the World Health Organization called for the elimination of viral hepatitis as a global public health threat by 2030. The United States and other nations are working to meet this ambitious goal by developing strategies to cure CHB, as well as prevent HBV transmission. This review considers recent research progress in understanding HBV pathobiology and development of therapeutics for the cure of CHB, which is necessary for elimination of hepatitis B by 2030.

Nanoparticle-Mediated STING Agonist Delivery for Enhanced Cancer Immunotherapy

Stimulator of interferon genes (STING) are located in the endoplasmic reticulum of cells, which have been demonstrated to show considerable potentials to achieve efficient antitumor immunity by inducing various pro-inflammatory cytokines and chemokines, such as type I interferons. A variety of STING agonists have been prepared for STING activation, and many of them have been promoted to preclinical trials or clinical applications for the immunotherapy of cancers. However, the intrinsic disadvantages of the small molecule STING agonists can limit the in vivo application and final therapeutic efficacy due to low bioavailability of targeting tissues. Moreover, a cascade of physiological barriers for in vivo STING activation also limit the accumulation of STING agonists in targeting tissues. Drug delivery systems play an important role to improve the STING activation efficiency. In recent years, a variety of nanoparticle-mediated STING agonist delivery systems have been engineered and exploited to address the challenges related to the in vivo STING activation, including liposomes, polymeric micelles, polymersomes, and so on. In this review article, the progresses concerning STING agonists and related delivery systems in recent years will be summarized and discussed.

If small molecules immunotherapy comes, can the prime be far behind?

Anti-cancer immunotherapy, which includes cellular immunotherapy, immune checkpoint inhibitors and cancer vaccines, has transformed the treatment strategies of several malignancies in the past decades. Immune checkpoints blockade (ICB) is the most commonly tested therapy and has the potential to induce a durable immune response in different types of cancers. However, all approved immune checkpoint inhibitors (ICIs) are monoclonal antibodies (mAbs), which are fraught with disadvantages including lack of oral bioavailability, prolonged tissue retention and poor membrane permeability. Therefore, the research focus has shifted to developing small molecule inhibitors to obviate the limitations of mAbs. Given the complexity of the tumor micro-environment (TME), the combination of ICIs with various small molecule agonists/inhibitors are currently being tested in clinical trials to improve treatment outcomes and prevent tumor recurrence. In this review, we have summarized the mechanisms and therapeutic potential of several molecular targets, along with the current status of small molecule inhibitors.

Agonist of stimulator of interferon genes as antitumor agents: a patent review (2008-2020)

INTRODUCTION

Stimulator of interferon genes (STING) is a transmembrane protein that localizes in the endoplasmic reticulum. As a crucial adaptor protein in the pathway of sensing cytosolic DNA, STING can regulate innate immune response by inducing the secretion of type Ι interferons and other cytokines after recognizing endogenous or exogenous DNA. Due to the key role of STING in the innate immune system, activation of the STING signaling pathway is expected to be an efficacious immunotherapeutic tactic for cancer and infectious diseases caused by pathogens.

AREAS COVERED

This review summarizes the structures and biological activities of STING agonists published from 2008 to present, the progress in its structural modification of STING agonists, and the development of their clinical study.

EXPERT OPINION

STING is an important adaptor protein in the process of triggering the innate immune response to viral infection. So far, substantial STING agonists and inhibitors have been published, and their viable curative effects for diverse diseases prove that STING is a promising therapeutic target.

STING and liver disease

STING (stimulator of interferon genes) also known as transmembrane protein 173 (TMEM173) is a cytoplasmic DNA sensor which can be activated by the upstream cyclic dinucleotides (CDNs). This activation produces cytokines such as interferons and pro-inflammatory factors via the downstream IRF3 and NF-κB pathways, triggering an innate immune response and adaptive immunity to maintain homeostasis. STING is mainly expressed and activated in non-parenchymal cells, thus exerting a corresponding effect to maintain the homeostasis of the liver. In viral hepatitis, interferons and pro-inflammatory factors produced after STING activation initiate the immune response to inhibit virus replication and assembly. In the case of metabolic diseases of the liver, the activation of STING in kupffer cells and hepatic stellate cells leads to inflammation, the proliferation of connective tissue, and metabolic disorders in the hepatocytes, promoting the occurrence and development of the disease. In hepatocellular carcinoma, STING has two contradictory roles. When STING is activated in dendritic cells and macrophages, a large number of cytokines can be produced to initiate innate immune effects directly and to exert adaptive immunity through the recruitment and activation of T cells; however, aberrant activation of the STING pathway leads to a weakening of immune function and promotes oncogenesis and metastasis. Here, we summarize the interactions between STING and liver disease that have currently been identified and how to achieve therapeutic goals by modulating the activity of the STING pathway.

STING, a promising target for small molecular immune modulator: A review

Stimulator of interferon genes (STING) plays a crucial role in human innate immune system, which is gradually concerned following the emerging immunotherapy. Activated STING induces the production of type I interferons (IFNs) and proinflammatory cytokines through STING-TBK1-IRF3/NF-κB pathway, which could be applied into the treatment of infection, inflammation, and tumorigenesis. Here, we provided a detailed summary of STING from its structure, function and regulation. Especially, we illustrated the canonical or noncanonical cyclic dinucleotides (CDNs) and synthetic small molecules for STING activation or inhibition and their efficacy in related diseases. Importantly, we particularly emphasized the discovery, development and modification of STING agonist or antagonist, attempting to enlighten reader's mind for enriching small molecular modulator of STING. In addition, we summarized biological evaluation methods for the assessment of small molecules activity.

Small molecules targeting the innate immune cGAS‒STING‒TBK1 signaling pathway

Multiple cancer immunotherapies including chimeric antigen receptor T cell and immune checkpoint inhibitors (ICIs) have been successfully developed to treat various cancers by motivating the adaptive anti-tumor immunity. Particularly, the checkpoint blockade approach has achieved great clinic success as evidenced by several U.S. Food and Drug Administration (FDA)-approved anti-programmed death receptor 1/ligand 1 or anti-cytotoxic T lymphocyte associated protein 4 antibodies. However, the majority of cancers have low clinical response rates to these ICIs due to poor tumor immunogenicity. Indeed, the cyclic guanosine monophosphate-adenosine monophosphate synthase‒stimulator of interferon genes‒TANK-binding kinase 1 (cGAS‒STING‒TBK1) axis is now appreciated as the major signaling pathway in innate immune response across different species. Aberrant signaling of this pathway has been closely linked to multiple diseases, including auto-inflammation, virus infection and cancers. In this perspective, we provide an updated review on the latest progress on the development of small molecule modulators targeting the cGAS‒STING‒TBK1 signaling pathway and their preclinical and clinical use as a new immune stimulatory therapy. Meanwhile, highlights on the clinical candidates, limitations and challenges, as well as future directions in this field are also discussed. Further, small molecule inhibitors targeting this signaling axis and their potential therapeutic use for various indications are discussed as well.

The continued advance of vaccine adjuvants - 'we can work it out'

In the last decade there have been some significant advances in vaccine adjuvants, particularly in relation to their inclusion in licensed products. This was proceeded by several decades in which such advances were very scarce, or entirely absent, but several novel adjuvants have now been included in licensed products, including in the US. These advances have relied upon several key technological insights that have emerged in this time period, which have finally allowed an in depth understanding of how adjuvants work. These advances include developments in systems biology approaches which allow the hypotheses first advanced in pre-clinical studies to be critically evaluated in human studies. This review highlights these recent advances, both in relation to the adjuvants themselves, but also the technologies that have enabled their successes. Moreover, we critically appraise what will come next, both in terms of new adjuvant molecules, and the technologies needed to allow them to succeed. We confidently predict that additional adjuvants will emerge in the coming years that will reach approval in licensed products, but that the components might differ significantly from those which are currently used. Gradually, the natural products that were originally used to build adjuvants, since they were readily available at the time of initial development, will come to be replaced by synthetic or biosynthetic materials, with more appealing attributes, including more reliable and robust supply, along with reduced heterogeneity. The recent advance in vaccine adjuvants is timely, given the need to create novel vaccines to deal with the COVID-19 pandemic. Although, we must ensure that the rigorous safety evaluations that allowed the current adjuvants to advance are not 'short-changed' in the push for new vaccines to meet the global challenge as quickly as possible, we must not jeopardize what we have achieved, by pushing less established technologies too quickly, if the data does not fully support it.

Broad-Spectrum Flavivirus Inhibitors: a Medicinal Chemistry Point of View

Infections by flaviviruses, such as Dengue, West Nile, Yellow Fever and Zika viruses, represent a growing risk for global health. There are vaccines only for few flaviviruses while no effective treatments are available. Flaviviruses share epidemiological, structural, and ecologic features and often different viruses can co-infect the same host. Therefore, the identification of broad-spectrum inhibitors is highly desirable either for known flaviviruses or for viruses that likely will emerge in the future. Strategies targeting both virus and host factors have been pursued to identify broad-spectrum antiflaviviral agents. In this review, we describe the most promising and best characterized targets and their relative broad-spectrum inhibitors, identified by drug repurposing/libraries screenings and by focused medicinal chemistry campaigns. Finally, we discuss about future strategies to identify new broad-spectrum antiflavivirus agents.

Challenges and Opportunities in the Clinical Development of STING Agonists for Cancer Immunotherapy

Immune checkpoint inhibitors (ICI) have revolutionised cancer therapy. However, they have been effective in only a small subset of patients and a principal mechanism underlying immune-refractoriness is a 'cold' tumour microenvironment, that is, lack of a T-cell-rich, spontaneously inflamed phenotype. As such, there is a demand to develop strategies to transform the tumour milieu of non-responsive patients to one supporting T-cell-based inflammation. The cyclic guanosine monophosphate-adenosine monophosphate synthase-stimulator of interferon genes (cGAS-STING) pathway is a fundamental regulator of innate immune sensing of cancer, with potential to enhance tumour rejection through the induction of a pro-inflammatory response dominated by Type I interferons. Recognition of these positive immune-modulatory properties has rapidly elevated the STING pathway as a putative target for immunotherapy, leading to a myriad of preclinical and clinical studies assessing natural and synthetic cyclic dinucleotides and non-nucleotidyl STING agonists. Despite pre-clinical evidence of efficacy, clinical translation has resulted into disappointingly modest efficacy. Poor pharmacokinetic and physiochemical properties of cyclic dinucleotides are key barriers to the development of STING agonists, most of which require intra-tumoral dosing. Development of systemically administered non-nucleotidyl STING agonists, or conjugation with liposomes, polymers and hydrogels may overcome pharmacokinetic limitations and improve drug delivery. In this review, we summarise the body of evidence supporting a synergistic role of STING agonists with currently approved ICI therapies and discuss whether, despite the numerous obstacles encountered to date, the clinical development of STING agonist as novel anti-cancer therapeutics may still hold the promise of broadening the reach of cancer immunotherapy.

Chemical Strategies to Boost Cancer Vaccines

Personalized cancer vaccines (PCVs) are reinvigorating vaccine strategies in cancer immunotherapy. In contrast to adoptive T-cell therapy and checkpoint blockade, the PCV strategy modulates the innate and adaptive immune systems with broader activation to redeploy antitumor immunity with individualized tumor-specific antigens (neoantigens). Following a sequential scheme of tumor biopsy, mutation analysis, and epitope prediction, the administration of neoantigens with synthetic long peptide (SLP) or mRNA formulations dramatically improves the population and activity of antigen-specific CD4+ and CD8+ T cells. Despite the promising prospect of PCVs, there is still great potential for optimizing prevaccination procedures and vaccine potency. In particular, the arduous development of tumor-associated antigen (TAA)-based vaccines provides valuable experience and rational principles for augmenting vaccine potency which is expected to advance PCV through the design of adjuvants, delivery systems, and immunosuppressive tumor microenvironment (TME) reversion since current personalized vaccination simply admixes antigens with adjuvants. Considering the broader application of TAA-based vaccine design, these two strategies complement each other and can lead to both personalized and universal therapeutic methods. Chemical strategies provide vast opportunities for (1) exploring novel adjuvants, including synthetic molecules and materials with optimizable activity, (2) constructing efficient and precise delivery systems to avoid systemic diffusion, improve biosafety, target secondary lymphoid organs, and enhance antigen presentation, and (3) combining bioengineering methods to innovate improvements in conventional vaccination, "smartly" re-educate the TME, and modulate antitumor immunity. As chemical strategies have proven versatility, reliability, and universality in the design of T cell- and B cell-based antitumor vaccines, the union of such numerous chemical methods in vaccine construction is expected to provide new vigor and vitality in cancer treatment.

Comprehensive elaboration of the cGAS-STING signaling axis in cancer development and immunotherapy

Cellular recognition of microbial DNA is an evolutionarily conserved mechanism by which the innate immune system detects pathogens. Cyclic GMP-AMP synthase (cGAS) and its downstream effector, stimulator of interferon genes (STING), are involved in mediating fundamental innate antimicrobial immunity by promoting the release of type I interferons (IFNs) and other inflammatory cytokines. Accumulating evidence suggests that the activation of the cGAS-STING axis is critical for antitumor immunity. The downstream cytokines regulated by cGAS-STING, especially type I IFNs, serve as bridges connecting innate immunity with adaptive immunity. Accordingly, a growing number of studies have focused on the synthesis and screening of STING pathway agonists. However, chronic STING activation may lead to a protumor phenotype in certain malignancies. Hence, the cGAS-STING signaling pathway must be orchestrated properly when STING agonists are used alone or in combination. In this review, we discuss the dichotomous roles of the cGAS-STING pathway in tumor development and the latest advances in the use of STING agonists.

Characterization of a Novel Compound That Stimulates STING-Mediated Innate Immune Activity in an Allele-Specific Manner

The innate immune response to cytosolic DNA involves transcriptional activation of type I interferons (IFN-I) and proinflammatory cytokines. This represents the culmination of intracellular signaling pathways that are initiated by pattern recognition receptors that engage DNA and require the adaptor protein Stimulator of Interferon Genes (STING). These responses lead to the generation of cellular and tissue states that impair microbial replication and facilitate the establishment of long-lived, antigen-specific adaptive immunity. Ultimately this can lead to immune-mediated protection from infection but also to the cytotoxic T cell-mediated clearance of tumor cells. Intriguingly, pharmacologic activation of STING-dependent phenotypes is known to enhance both vaccine-associated immunogenicity and immune-based anti-tumor therapies. Unfortunately, the STING protein exists as multiple variant forms in the human population that exhibit differences in their reactivity to chemical stimuli and in the intensity of molecular signaling they induce. In light of this, STING-targeting drug discovery efforts require an accounting of protein variant-specific activity. Herein we describe a small molecule termed M04 that behaves as a novel agonist of human STING. Importantly, we find that the molecule exhibits a differential ability to activate STING based on the allelic variant examined. Furthermore, while M04 is inactive in mice, expression of human STING in mouse cells rescues reactivity to the compound. Using primary human cells in ex vivo assays we were also able to show that M04 is capable of simulating innate responses important for adaptive immune activation such as cytokine secretion, dendritic cell maturation, and T cell cross-priming. Collectively, this work demonstrates the conceivable utility of a novel agonist of human STING both as a research tool for exploring STING biology and as an immune potentiating molecule.

Nanomedicine-mediated alteration of the pharmacokinetic profile of small molecule cancer immunotherapeutics

The advent of immunotherapy is a game changer in cancer therapy with monoclonal antibody- and T cell-based therapeutics being the current flagships. Small molecule immunotherapeutics might offer advantages over the biological drugs in terms of complexity, tissue penetration, manufacturing cost, stability, and shelf life. However, small molecule drugs are prone to rapid systemic distribution, which might induce severe off-target side effects. Nanotechnology could aid in the formulation of the drug molecules to improve their delivery to specific immune cell subsets. In this review we summarize the current efforts in changing the pharmacokinetic profile of small molecule immunotherapeutics with a strong focus on Toll-like receptor agonists. In addition, we give our vision on limitations and future pathways in the route of nanomedicine to the clinical practice.

Bioactive modulators targeting STING adaptor in cGAS-STING pathway

The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING)-pathway triggers innate immune responses by recognizing cytosolic DNA. Recent studies revealed that STING adaptor associates with various diseases, and several modulators targeting STING have been identified including three agonists that have entered clinical trials for treating cancer over the past 2 years. In particular, the efficacy of STING agonists and/or antagonists suggests adaptor STING as a potential therapeutic target for diverse diseases. Herein, we summarize the latest advances in understanding STING functioning and provide an overview of recent STING modulator discoveries, including structural details and the potential therapeutic applications of these modulators.