Overcoming challenges in the delivery of STING agonists for cancer immunotherapy: A comprehensive review of strategies and future perspectives

STING-Activating Small Molecular Therapeutics for Cancer Immunotherapy

Immuno-oncology has become a revolutionary strategy for cancer treatment. Therapeutic interventions based on adaptive immunity through immune checkpoint therapy or chimeric antigen receptor (CAR) T cells have received clinical approval for monotherapy and combination treatment in various cancers. Although these treatments have achieved clinical successes, only a minority of cancer patients show a response, highlighting the urgent need to discover new therapeutic molecules that could be exploited to improve clinical outcomes and pave the way for the next generation of immunotherapy. Given the critical role of the innate immune system against infection and cancer, substantial efforts have been dedicated to developing novel anticancer therapeutics that target these pathways. Targeting the stimulator of interferon genes (STING) pathway is a powerful strategy to generate a durable antitumor response, and activation of the adaptor protein STING induces the initiation of transcriptional cascades, thereby producing type I interferons, pro-inflammatory cytokines and chemokines. Various STING agonists, including natural or synthetic cyclic dinucleotides (CDNs), have been developed as anticancer therapeutics. However, since most CDNs are confined to intratumoral administration, there has been a great interest in developing non-nucleotide agonists for systemic treatment. Here, we review the current development of STING-activating therapeutics in both preclinical and clinical stages.

STING-Activating Polymer-Drug Conjugates for Cancer Immunotherapy

The stimulator of interferon genes (STING) pathway links innate and adaptive antitumor immunity and therefore plays an important role in cancer immune surveillance. This has prompted widespread development of STING agonists for cancer immunotherapy, but pharmacological barriers continue to limit the clinical impact of STING agonists and motivate the development of drug delivery systems to improve their efficacy and/or safety. We developed SAPCon, a STING-activating polymer-drug conjugate platform based on strain-promoted azide-alkyne cycloaddition of a novel dimeric amidobenzimidazole (diABZI) STING prodrug to hydrophilic poly(dimethylacrylamide-co-azido-ethylmethacrylate) polymer chains through a cathepsin B-responsive linker to increase circulation time and enable passive tumor accumulation. We found that intravenously administered SAPCon accumulated at tumor sites, where it was endocytosed by tumor-associated myeloid cells, resulting in increased STING activation in the tumor tissue. Consequently, SAPCon promoted an immunogenic tumor microenvironment characterized by increased frequency of activated macrophages and dendritic cells and improved infiltration of CD8+ T cells, resulting in inhibition of tumor growth, prolonged survival, and enhanced response to anti-PD-1 immune checkpoint blockade in orthotopic breast cancer models. Collectively, these studies position SAPCon as a modular and programmable platform for improving the efficacy of systemically administered STING agonists for cancer immunotherapy.

STING Agonist Induced Innate Immune Responses Drive Anti-Respiratory Virus Activity In Vitro with Limited Antiviral Efficacy In Vivo

The emergence of SARS-CoV-2 and seasonal outbreaks of other respiratory viruses highlight the urgent need for broad-spectrum antivirals to treat respiratory tract infections. Stimulator of interferon genes (STING) is a key component of innate immune signaling and plays a critical role in protection of the host against viral infections. Previously the STING agonist diABZI-4, a diamidobenzimidazole-based compound, demonstrated protection against SARS-CoV-2 both in vitro and in vivo. However, its broad-spectrum antiviral activity against other respiratory viruses in human airway epithelial cells, which are the primary targets of these infections, is not well established. In this study, we demonstrated that diABZI-4 stimulated robust innate immune responses protecting lung cells against a wide range of respiratory viruses, including influenza A virus (IAV), common cold coronaviruses, SARS-CoV-2, human rhinovirus (HRV), and human parainfluenza virus. diABZI-4 was highly active in physiologically relevant human airway epithelial tissues grown at the air-liquid interface, blocking replication of IAV, SARS-CoV-2, and HRV in these tissues. Furthermore, treatment of macrophages with diABZI-4 resulted in the secretion of cytokines that protected the primary airway epithelial cells from IAV infection. Despite the promising in vitro pan-antiviral activity, intranasal administration of diABZI-4 in mice provided early, but not sustained, inhibition of IAV replication in the lungs. These data highlight the complexities of the relationship between timing of STING agonist-driven inflammatory responses and viral replication dynamics, emphasizing the development challenge posed by STING agonists as potential therapeutics against respiratory viruses.

Stimulator of Interferon Genes-Activated Biomimetic Dendritic Cell Nanovaccine as a Chemotherapeutic Booster to Enhance Systemic Fibrosarcoma Treatment

Fibrosarcoma, a malignant mesenchymal tumor, is characterized by aggressive invasiveness and a high recurrence rate, leading to poor prognosis. Anthracycline drugs, such as doxorubicin (DOX), represent the frontline chemotherapy for fibrosarcoma, but often exhibit suboptimal efficacy. Recently, exploiting the stimulator of interferon genes (STING)-mediated innate immunity has emerged as a hopeful strategy for cancer treatment. Integrating chemotherapy with immunomodulators in chemo-immunotherapy has shown potential for enhancing treatment outcomes. Herein, we introduce an advanced dendritic cell (DC) nanovaccine, cGAMP@PLGA@CRTM (GP@CRTM), combined with low-dose DOX to enhance fibrosarcoma chemo-immunotherapy. The nanovaccine consists of poly(lactic-co-glycolic acid) (PLGA) nanoparticles encapsulating the STING agonist 2,3-cGAMP (cGAMP@PLGA, GP) as its core, and a calreticulin (CRT) high-expressing fibrosarcoma cell membrane (CRTM) as the shell. Exposing CRT on the vaccine surface aids in recruiting DCs and stimulating uptake, facilitating efficient simultaneous delivery of STING agonists and tumor antigens to DCs. This dual delivery method effectively activates the STING pathway in DCs, triggering sustained immune stimulation. Simultaneously, low-dose DOX reduces chemotherapy-related side effects, directly kills a subset of tumor cells, and increases tumor immunogenicity, thus further amplifying immune therapeutic performance. Hence, these findings demonstrate the potential of DC nanovaccine GP@CRTM as a booster for chemotherapy. Synergistically combining low-dose DOX with the DC nanovaccine emerges as a powerful chemo-immunotherapy strategy, optimizing systemic fibrosarcoma therapy.

Clinical landscape of macrophage-reprogramming cancer immunotherapies

Tumour-associated macrophages (TAMs) sustain a tumour-supporting and immunosuppressive milieu and therefore aggravate cancer prognosis. To modify TAM behaviour and unlock their anti-tumoural potential, novel TAM-reprogramming immunotherapies are being developed at an accelerating rate. At the same time, scientific discoveries have highlighted more sophisticated TAM phenotypes with complex biological functions and contradictory prognostic associations. To understand the evolving clinical landscape, we reviewed current and past clinically evaluated TAM-reprogramming cancer therapeutics and summarised almost 200 TAM-reprogramming agents investigated in more than 700 clinical trials. Observable overall trends include a high frequency of overlapping strategies against the same therapeutic targets, development of more complex strategies to improve previously ineffective approaches and reliance on combinatory strategies for efficacy. However, strong anti-tumour efficacy is uncommon, which encourages re-directing efforts on identifying biomarkers for eligible patient populations and comparing similar treatments earlier. Future endeavours will benefit from considering the shortcomings of past treatment strategies and accommodating the emerging complexity of TAM biology.

Nature of the Association between Rheumatoid Arthritis and Cervical Cancer and Its Potential Therapeutic Implications

It is now established that patients with rheumatoid arthritis (RA) have an increased risk of developing cervical cancer (CC) or its precursor, cervical intraepithelial neoplasia (CIN). However, the underlying mechanisms of this association have not been elucidated. RA is characterized by unresolved chronic inflammation. It is suggested that human papillomavirus (HPV) infection in RA patients exacerbates inflammation, increasing the risk of CC. The tumor microenvironment in RA patients with CC is also marked by chronic inflammation, which aggravates the manifestations of both conditions. Gut and vaginal dysbiosis are also considered potential mechanisms that contribute to the chronic inflammation and aggravation of RA and CC manifestations. Numerous clinical and pre-clinical studies have demonstrated the beneficial effects of various nutritional approaches to attenuate chronic inflammation, including polyunsaturated fatty acids and their derivatives, specialized pro-resolving mediators (SPMs), probiotics, prebiotics, and certain diets. We believe that successful resolution of chronic inflammation and correction of dysbiosis, in combination with current anti-RA and anti-CC therapies, is a promising therapeutic approach for RA and CC. This approach could also reduce the risk of CC development in HPV-infected RA patients.

Mesoporous Silica Nanoparticles as an Ideal Platform for Cancer Immunotherapy: Recent Advances and Future Directions

Cancer immunotherapy recently transforms the traditional approaches against various cancer malignancies. Immunotherapy includes systemic and local treatments to enhance immune responses against cancer and involves strategies such as immune checkpoints, cancer vaccines, immune modulatory agents, mimetic antigen-presenting cells, and adoptive cell therapy. Despite promising results, these approaches still suffer from several limitations including lack of precise delivery of immune-modulatory agents to the target cells and off-target toxicity, among others, that can be overcome using nanotechnology. Mesoporous silica nanoparticles (MSNs) are investigated to improve various aspects of cancer immunotherapy attributed to the advantageous structural features of this nanomaterial. MSNs can be engineered to alter their properties such as size, shape, porosity, surface functionality, and adjuvanticity. This review explores the immunological properties of MSNs and the use of MSNs as delivery vehicles for immune-adjuvants, vaccines, and mimetic antigen-presenting cells (APCs). The review also details the current strategies to remodel the tumor microenvironment to positively reciprocate toward the anti-tumor immune cells and the use of MSNs for immunotherapy in combination with other anti-tumor therapies including photodynamic/thermal therapies to enhance the therapeutic effect against cancer. Last, the present demands and future scenarios for the use of MSNs for cancer immunotherapy are discussed.

Targeting a STING agonist to perivascular macrophages in prostate tumors delays resistance to androgen deprivation therapy

BACKGROUND

Androgen deprivation therapy (ADT) is a front-line treatment for prostate cancer. In some men, their tumors can become refractory leading to the development of castration-resistant prostate cancer (CRPC). This causes tumors to regrow and metastasize, despite ongoing treatment, and impacts negatively on patient survival. ADT is known to stimulate the accumulation of immunosuppressive cells like protumoral tumor-associated macrophages (TAMs), myeloid-derived suppressor cells and regulatory T cells in prostate tumors, as well as hypofunctional T cells. Protumoral TAMs have been shown to accumulate around tumor blood vessels during chemotherapy and radiotherapy in other forms of cancer, where they drive tumor relapse. Our aim was to see whether such perivascular (PV) TAMs also accumulate in ADT-treated prostate tumors prior to CRPC, and, if so, whether selectively inducing them to express a potent immunostimulant, interferon beta (IFNβ), would stimulate antitumor immunity and delay CRPC.

METHODS

We used multiplex immunofluorescence to assess the effects of ADT on the distribution and activation status of TAMs, CD8+T cells, CD4+T cells and NK cells in mouse and/or human prostate tumors. We then used antibody-coated, lipid nanoparticles (LNPs) to selectively target a STING agonist, 2'3'-cGAMP (cGAMP), to PV TAMs in mouse prostate tumors during ADT.

RESULTS

TAMs accumulated at high density around blood vessels in response to ADT and expressed markers of a protumoral phenotype including folate receptor-beta (FR-β), MRC1 (CD206), CD169 and VISTA. Additionally, higher numbers of inactive (PD-1-) CD8+T cells and reduced numbers of active (CD69+) NK cells were present in these PV tumor areas. LNPs coated with an antibody to FR-β selectively delivered cGAMP to PV TAMs in ADT-treated tumors, where they activated STING and upregulated the expression of IFNβ. This resulted in a marked increase in the density of active CD8+T cells (along with CD4+T cells and NK cells) in PV tumor areas, and significantly delayed the onset of CRPC. Antibody depletion of CD8+T cells during LNP administration demonstrated the essential role of these cells in delay in CRPC induced by LNPs.

CONCLUSION

Together, our data indicate that targeting a STING agonist to PV TAMs could be used to extend the treatment window for ADT in prostate cancer.

The cGAS/STING Pathway-A New Potential Biotherapeutic Target for Gastric Cancer?

Gastric cancer ranks among the top five deadliest tumors worldwide, both in terms of prevalence and mortality rates. Despite mainstream treatments, the efficacy in treating gastric cancer remains suboptimal, underscoring the urgency for novel therapeutic approaches. The elucidation of tumor immunosuppressive microenvironments has shifted focus towards cancer biotherapeutics, which leverage the patient's immune system or biologics to target tumor cells. Biotherapy has emerged as a promising alternative for tumors resistant to traditional chemotherapy, radiation, and immunotherapy. Central to this paradigm is the cGAS-STING pathway, a pivotal component of the innate immune system. This pathway recognizes aberrant DNA, such as that from viral infections or tumor cells, and triggers an immune response, thereby reshaping the immunosuppressive tumor microenvironment into an immune-stimulating milieu. In the context of gastric cancer, harnessing the cGAS-STING pathway holds significant potential for biotherapeutic interventions. This review provides a comprehensive overview of the latest research on cGAS-STING in gastric cancer, including insights from clinical trials involving STING agonists. Furthermore, it assesses the prospects of targeting the cGAS-STING pathway as a novel biotherapeutic strategy for gastric cancer.

Stimulator of Interferon Genes Protein (STING) Expression in Cancer Cells: A Tissue Microarray Study Evaluating More than 18,000 Tumors from 139 Different Tumor Entities

Stimulator of interferon genes protein (STING) activates the immune response in inflammatory cells. STING expression in cancer cells is less well characterized, but STING agonists are currently being evaluated as anticancer drugs. A tissue microarray containing 18,001 samples from 139 different tumor types was analyzed for STING by immunohistochemistry. STING-positive tumor cells were found in 130 (93.5%) of 139 tumor entities. The highest STING positivity rates occurred in squamous cell carcinomas (up to 96%); malignant mesothelioma (88.5%-95.7%); adenocarcinoma of the pancreas (94.9%), lung (90.3%), cervix (90.0%), colorectum (75.2%), and gallbladder (68.8%); and serous high-grade ovarian cancer (86.0%). High STING expression was linked to adverse phenotypes in breast cancer, clear cell renal cell carcinoma, colorectal adenocarcinoma, hepatocellular carcinoma, and papillary carcinoma of the thyroid (p < 0.05). In pTa urothelial carcinomas, STING expression was associated with low-grade carcinoma (p = 0.0002). Across all tumors, STING expression paralleled PD-L1 positivity of tumor and inflammatory cells (p < 0.0001 each) but was unrelated to the density of CD8+ lymphocytes. STING expression is variable across tumor types and may be related to aggressive tumor phenotype and PD-L1 positivity. The lack of relationship with tumor-infiltrating CD8+ lymphocytes argues against a significant IFN production by STING positive tumor cells.

Combination of STING agonist with anti-vascular RGD-(KLAKLAK)2 peptide as a novel anti-tumor therapy

Immunotherapy is one of the most promising anti-cancer treatment. It involves activating the host's own immune system to eliminate cancer cells. Activation of cGAS-STING pathway is promising therapeutic approach for cancer immunotherapy. However, in human clinical trials, targeting cGAS-STING pathway results in insufficient or unsustainable anti-tumor response. To enhance its effectiveness, combination with other anti-cancer therapies seems essential to achieve synergistic systemic anti-tumor response.The aim of this study was to evaluate whether the combination of STING agonist-cGAMP with anti-vascular RGD-(KLAKLAK)2 peptide results in a better anti-tumor response in poorly immunogenic tumors with various STING protein and αvβ3 integrin status.Combination therapy inhibited growth of murine breast carcinoma more effectively than melanoma. In melanoma, the administration of STING agonist alone was sufficient to obtain a satisfactory therapeutic effect. In both tumor models we have noted stimulation of innate immune response following cGAMP administration alone or in combination. The largest population of immune cells infiltrating the TME after therapy were activated NK cells. Increased infiltration of cytotoxic CD8+ T lymphocytes within the TME was only observed in melanoma tumors. However, they also expressed the "exhaustion" PD-1 receptor. In contrast, in breast carcinoma tumors each therapy caused the drop in the number of infiltrating CD8+ T cells.The obtained results indicate an additional therapeutic benefit from combining STING agonist with an anti-vascular agent. However, this effect depends on the type of tumor, the status of its microenvironment and the expression of specific proteins such as STING and αvβ3 family integrin.

STING-Activating Polymer-Drug Conjugates for Cancer Immunotherapy

The stimulator of interferon genes (STING) pathway links innate and adaptive antitumor immunity and therefore plays an important role in cancer immune surveillance. This has prompted widespread development of STING agonists for cancer immunotherapy, but pharmacological barriers continue to limit the clinical impact of STING agonists and motivate the development of drug delivery systems to improve their efficacy and/or safety. To address this challenge, we developed SAPCon, a STING-activating polymer-drug conjugate platform based on strain-promoted azide-alkyne cycloaddition of dimeric-amidobenzimidazole (diABZI) STING agonists to hydrophilic polymer chains through an enzyme-responsive chemical linker. To synthesize a first-generation SAPCon, we designed a diABZI prodrug modified with a DBCO reactive handle a cathepsin B-cleavable spacer for intracellular drug release and conjugated this to pendant azide groups on a 100 kDa poly(dimethyla acrylamide-co-azide methacrylate) copolymer backbone to increase circulation time and enable passive tumor accumulation. We found that intravenously administered SAPCon accumulated at tumor sites where they it was endocytosed by tumor-associated myeloid cells, resulting in increased STING activation in tumor tissue compared to a free diABZI STING agonist. Consequently, SAPCon promoted an immunogenic tumor microenvironment, characterized by increased frequency of activated macrophages and dendritic cells and improved infiltration of CD8+ T cells, resulting in inhibition of tumor growth, prolonged survival, and increased response to anti-PD-1 immune checkpoint blockade in orthotopic models of breast cancer. Collectively, these studies position SAPCon as a modular and programmable platform for improving the efficacy of systemically administered STING agonists for cancer immunotherapy.

Nanomaterial-encapsulated STING agonists for immune modulation in cancer therapy

The cGAS-STING signaling pathway has emerged as a critical mediator of innate immune responses, playing a crucial role in improving antitumor immunity through immune effector responses. Targeting the cGAS-STING pathway holds promise for overcoming immunosuppressive tumor microenvironments (TME) and promoting effective tumor elimination. However, systemic administration of current STING agonists faces challenges related to low bioavailability and potential adverse effects, thus limiting their clinical applicability. Recently, nanotechnology-based strategies have been developed to modulate TMEs for robust immunotherapeutic responses. The encapsulation and delivery of STING agonists within nanoparticles (STING-NPs) present an attractive avenue for antitumor immunotherapy. This review explores a range of nanoparticles designed to encapsulate STING agonists, highlighting their benefits, including favorable biocompatibility, improved tumor penetration, and efficient intracellular delivery of STING agonists. The review also summarizes the immunomodulatory impacts of STING-NPs on the TME, including enhanced secretion of pro-inflammatory cytokines and chemokines, dendritic cell activation, cytotoxic T cell priming, macrophage re-education, and vasculature normalization. Furthermore, the review offers insights into co-delivered nanoplatforms involving STING agonists alongside antitumor agents such as chemotherapeutic compounds, immune checkpoint inhibitors, antigen peptides, and other immune adjuvants. These platforms demonstrate remarkable versatility in inducing immunogenic responses within the TME, ultimately amplifying the potential for antitumor immunotherapy.