Discovery of Novel Small Molecule Dual Inhibitor Targeting Toll-Like Receptors 7 and 9

The aberrant secretion of proinflammatory cytokines by immune cells is the principal cause of inflammatory diseases, such as systemic lupus erythematosus and rheumatoid arthritis. Toll-like receptor 7 (TLR7) and TLR9, sequestered to the endosomal compartment of dendritic cells and macrophages, are closely associated with the initiation and progression of these diseases. Therefore, the development of drugs targeting dysregulated endosomal TLRs is imperative to mitigate systemic inflammation. Here, we applied the principles of computer-aided drug discovery to identify a novel low-molecular-weight compound, TLR inhibitory compound 10 (TIC10), and its potent derivative (TIC10g), which demonstrated dual inhibition of TLR7 and TLR9 signaling pathways. Compared to TIC10, TIC10g exhibited a more pronounced inhibition of the TLR7- and TLR9-mediated secretion of the proinflammatory cytokine tumor necrosis factor-α in a mouse macrophage cell line and mouse bone marrow dendritic cells in a concentration-dependent manner. While TIC10g slightly prevented TLR3 and TLR8 activation, it had no impact on cell surface TLRs (TLR1/2, TLR2/6, TLR4, or TLR5), indicating its selectivity for TLR7 and TLR9. Additionally, mechanistic studies suggested that TIC10g interfered with TLR9 activation by CpG DNA and suppressed downstream pathways by directly binding to TLR9. Western blot analysis revealed that TIC10g downregulated the phosphorylation of the p65 subunit of nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) and mitogen-activated protein kinases (MAPKs), including extracellular-signal-regulated kinase, p38-MAPK, and c-Jun N-terminal kinase. These findings indicate that the novel ligand, TIC10g, is a specific dual inhibitor of endosomal TLRs (TLR7 and TLR9), disrupting MAPK- and NF-κB-mediated proinflammatory gene expression.

Design, Synthesis, and Biological Evaluation of New 2,6,7-Substituted Purine Derivatives as Toll-like Receptor 7 Agonists for Intranasal Vaccine Adjuvants

TLR7/8 agonists are versatile immune stimulators capable of treating various diseases such as viral infections, autoimmune, and cancer. Despite the structural similarity of TLR7/8, their immune stimulation mechanisms and time-course responses significantly differ. In this study, a new series of TLR7-selective agonists was synthesized utilizing the economical building block 2,6-dichloropurine. Compound 27b showed the most potent activity on hTLR7 with an EC50 of 17.53 nM and demonstrated high hTLR7 selectivity (224 folds against TLR8). 27b effectively stimulated the secretion of proinflammatory cytokines in mouse macrophages and enhanced intranasal vaccine efficacy against influenza A virus in vivo. Assessment of humoral and mucosal antibody titers confirmed that 27b elevates IgG and IgA levels, protecting against both homologous and heterologous influenza viral infections. These findings suggest that 27b is a promising candidate as a vaccine adjuvant to prevent viral infections or as a robust immunomodulator with prolonged activity for treating immune-suppressed diseases.

Structure-Activity Relationships toward the Identification of a High-Potency Selective Human Toll-like Receptor-7 Agonist

Toll-like receptor (TLR)-7 agonists are immunostimulatory vaccine adjuvants. A systematic structure-activity relationship (SAR) study of TLR7-active 1-benzyl-2-butyl-1H-imidazo[4,5-c]quinolin-4-amine led to the identification of a potent hTLR7-specific p-hydroxymethyl IMDQ 23 with an EC50 value of 0.22 μM. The SAR investigation also resulted in the identification of TLR7 selective carboxamide 12 with EC50 values of 0.32 μM for hTLR7 and 18.25 μM for hTLR8. In the vaccination study, TLR7-specific compound 23 alone or combined with alum (aluminum hydroxide wet gel) showed adjuvant activity for a spike protein immunogen in mice, with enhanced anti-spike antibody production. Interestingly, the adjuvant system comprising carboxamide 12 and alum showed prominent adjuvant activity with high levels of IgG1, IgG2b, and IgG2c in immunized mice, confirming a balanced Th1/Th2 response. In the absence of any apparent toxicity, the TLR7 selective agonists in combination with alum may make a suitable vaccine adjuvant.

Molecular Masking of Synthetic Immunomodulator Evokes Antitumor Immunity With Reduced Immune Tolerance and Systemic Toxicity by Temporal Activity Recovery and Sustained Stimulation

Activation of the innate immune system counteracts tumor-induced immunosuppression. Hence, small molecule-based toll-like receptor 7/8 agonists (TLR7/8a), which can modulate immunosuppression in the tumor microenvironment along with the activation of innate immunity, are emerging as essential components of cancer immunotherapy. However, the clinical application of synthetic TLR7/8a therapies is limited by systemic immune-associated toxicity and immune tolerance induced by uncontrolled stimulatory activities and repeated treatments. To address these limitations, a dynamic immunomodulation strategy incorporating masking and temporal recovery of the activity of TLR7/8a through prodrug-like TLR7/8a (pro-TLR7/8a) at the molecular level and a sustained and controlled release of active TLR7/8a from nanoliposome (pro-TLR7/8a) (NL(pro-TLR7/8)) in a macroscale depot are designed. Immunization with cationic NL(pro-TLR7/8) and anionic antigens triggers robust activation of innate immune cells as well as antigen-specific T cell responses, eliciting reprogramming of immunosuppressive cells into tumor-suppressive cells, with decreased systemic adverse effects and immune tolerance. Combination treatment with NL(pro-TLR7/8a) and immune checkpoint inhibitors (anti-CTLA-4 plus anti-PD-L1) or nanoliposomes (Doxorubicin) has synergistic effects on antitumor immunity in various tumor models. The concept of pro-TLR7/8a suggested herein may facilitate the advancement of small-molecule-based immunomodulators for clinical translation and safe and effective cancer immunotherapy.

Synthesis and Optimization of 1-Substituted Imidazo[4,5-c]quinoline TLR7 Agonists

TLR7 agonists have significant therapeutic potential in a variety of oncology and autoimmune applications. We recently reported a potent TLR7 selective agonist 1 that could be delivered by antibody-drug conjugate (ADC) technology to elicit potent anticancer activity. Herein we report synthetic chemistry and structure-activity relationship studies to develop TLR7 agonists with improved potency for next-generation ADC efforts. We found that the addition of hydrophobic acyl tails to parent compound 1 generally resulted in retained or improved TLR7 agonist activity without sacrificing the permeability or the selectivity over TLR8. In contrast, the addition of a simple alkyl tail at the same position resulted in a dramatic loss in potency. Molecular modeling was performed to provide a rationale for this dramatic loss in potency. We ultimately identified compounds 17b, 16b, and 16d as highly potent TLR7 agonists that potently induced the activation of mouse macrophages and hPBMCs at low-nanomolar concentrations.

Small-Molecule Modulators Targeting Toll-like Receptors for Potential Anticancer Therapeutics

Toll-like receptors (TLRs) are key components of the innate immune system and serve as a crucial link between innate and acquired immunity. In addition to immune function, TLRs are involved in other important pathological processes, including tumorigenesis. TLRs have dual regulatory effects on tumor immunity by activating nuclear factor κ-B signaling pathways, which induce tumor immune evasion or enhance the antitumor immune response. Therefore, TLRs have become a popular target for cancer prevention and treatment, and TLR agonists and antagonists offer considerable potential for drug development. The TLR7 agonist imiquimod (1) has been approved by the U.S. Food and Drug Administration as a treatment for malignant skin cancer. Herein, the structure, signaling pathways, and function of the TLR family are summarized, and the structure-activity relationships associated with TLR selective and multitarget modulators and their potential application in tumor therapy are systematically discussed.

Bioorthogonal Activation of TLR7 Agonists Provokes Innate Immunity to Reinforce Aptamer-Based Checkpoint Blockade

Although cancer immunotherapy based on immune checkpoint blockade has shown promising clinical responses, the limited host response rate and systemic side effects still restrict immunotherapy efficacy. To address these challenges, here, we construct an aptamer-functionalized metal-organic framework (MOF) catalyst for bioorthogonal activation of Toll-like receptors (TLR) 7 agonists and programmed death-ligand 1 (PDL1) blockade for enhanced antitumor immunotherapy. The catalyst contains ultrasmall Pd nanoparticles enabling the local activation of TLR7 agonists in native form, which results in the remodeling of the tumor microenvironment (TME). Meanwhile, the loaded PDL1 aptamers release in response to phosphate and block the PD1/PDL1 signaling pathway between T cells and cancer cells. Thus, synergy between TLR7 agonists and PDL1 blockade induces the infiltration and activation of immune cells to initiate a robust immune response, thereby simultaneously inhibiting primary and distant metastatic tumors. The immunotherapeutic effect of our design has been demonstrated in both single and bilateral subcutaneous colorectal cancer (CT26) models. In situ bioorthogonal activation of agonists may offer an alternative approach to improve the therapeutic efficacy of immunotherapy with minimized systemic toxicity. Our work will provide good inspiration for current checkpoint blockade-based immunotherapy.

Design and Characterization of Immune-Stimulating Imidazo[4,5-c]quinoline Antibody-Drug Conjugates

Traditional antibody-drug conjugate (ADC) technology has employed tumor-targeting antibodies to selectively deliver ultrapotent cytotoxins to tumor tissue. While this technology has been highly successful, resulting in the FDA approval of over 10 ADCs, the field continues to struggle with modest efficacy and significant off-target toxicity. Concurrent with the struggles of the ADC field, a new generation of immune-activating therapeutics has arisen, most clearly exemplified by the PD-1/PD-L1 inhibitors that are now part of standard-of-care treatment regimens for a variety of cancers. The success of these immuno-oncology therapeutic agents has prompted the investigation of a variety of new immuno-stimulant approaches, including toll-like receptor (TLR) activators. Herein, we describe the optimization of ADC technology for the selective delivery of a potent series of TLR7 agonists. A series of imidazole[4,5-c]quinoline agonists (as exemplified by compound 1) were shown to selectively agonize the human and mouse TLR7 receptor at low nanomolar concentrations, resulting in the release of IFNα from human peripheral blood mononuclear cells (hPBMCs) and the upregulation of CD86 on antigen-presenting cells. Compound 1 was attached to a deglycosylated (Fc-γ null) HER2-targeting antibody via a cleavable linker, resulting in an ADC (anti-HER2_vc-1) that potently and selectively activated the TLR7 pathway in tumor-associated macrophages via a "bystander" mechanism. We demonstrated that this ADC rapidly released the TLR7 agonist into the media when incubated with HER2+ cells. This release was not observed upon incubation with an isotype control ADC and furthermore was suppressed by co-administration of the naked antibody. In co-culture experiments with HER2+ HCC1954 cells, this ADC induced the activation of the NFκB pathway in mouse macrophages and the release of IFNα from hPBMCs, while a corresponding isotype control ADC did not. Finally, we demonstrated that IP administration of anti-HER2_vc-1 induced complete tumor regression in an HCC1954 xenograft study in SCID beige mice. Unlike related ADC technology that has been reported recently, our technology relies on the passive diffusion of the TLR7 agonist into tumor-associated macrophages rather than Fc-γ-mediated uptake. Based on these observations, we believe that this ADC technology holds significant potential for both oncology and infectious disease applications.

Evolution of Toll-like receptor 7/8 agonist therapeutics and their delivery approaches: From antiviral formulations to vaccine adjuvants

Imidazoquinoline derivatives (IMDs) and related compounds function as synthetic agonists of Toll-like receptors 7 and 8 (TLR7/8) and one is FDA approved for topical antiviral and skin cancer treatments. Nevertheless, these innate immune system-activating drugs have potentially much broader therapeutic utility; they have been pursued as antitumor immunomodulatory agents and more recently as candidate vaccine adjuvants for cancer and infectious disease. The broad expression profiles of TLR7/8, poor pharmacokinetic properties of IMDs, and toxicities associated with systemic administration, however, are formidable barriers to successful clinical translation. Herein, we review IMD formulations that have advanced to the clinic and discuss issues related to biodistribution and toxicity that have hampered the further development of these compounds. Recent strategies aimed at enhancing safety and efficacy, particularly through the use of bioconjugates and nanoparticle formulations that alter pharmacokinetics, biodistribution, and cellular targeting, are described. Finally, key aspects of the biology of TLR7 signaling, such as TLR7 tolerance, that may need to be considered in the development of new IMD therapeutics are discussed.

Tetrasubstituted imidazoles as incognito Toll-like receptor 8 a(nta)gonists

Small-molecule modulators of TLR8 have drawn much interests as it plays pivotal roles in the innate immune response to single-stranded RNAs (ssRNAs) derived from viruses. However, their clinical uses are limited because they can invoke an uncontrolled, global inflammatory response. The efforts described herein culminate in the fortuitous discovery of a tetrasubstituted imidazole CU-CPD107 which inhibits R848-induced TLR8 signaling. In stark contrast, CU-CPD107 shows unexpected synergistic agonist activities in the presence of ssRNA, while CU-CPD107 alone is unable to influence TLR8 signaling. CU-CPD107’s unique, dichotomous behavior sheds light on a way to approach TLR agonists. CU-CPD107 offers the opportunity to avoid the undesired, global inflammation side effects that have rendered imidazoquinolines clinically irrelevant, providing an insight for the development of antiviral drugs.

Toll-like receptor 8 (TLR8) plays essential roles in the innate immune response to viral single-stranded RNA (ssRNA), so small molecule modulators of TLR8 are of interest, however adverse effects limit their use. Here, the authors report a tetrasubstituted imidazole CU-CPD107 with dichotomous behaviour, which inhibits R848-induced TLR8 signaling, but shows synergistic activity in the presence of ssRNA, making it a potential antiviral agent.

Structural evolution of toll-like receptor 7/8 agonists from imidazoquinolines to imidazoles

Several synthetic heterocyclic small molecules like imiquimod, resiquimod, CL097, CL075, bromopirone, tilorone, loxoribine and isatoribine demonstrated TLR7/8 agonistic activity and relatively modest structural changes in such molecules result in major variation in the TLR7 and/or TLR8 activity. A strict dependency of the electronic configuration of the heterocyclic system was also observed to influence the agonistic activity. In the present review, an evolution of imidazole based TLR7/8 agonist from imidazoquinoline based scaffold is delineated along with the elaboration of detailed structure activity relationship (SAR) in each chemotype. The structural and activity details of not only the active compounds but also the related inactive compounds are included to better understand the SAR. TLR7/8 agonists are emerging as promising vaccine adjuvant candidates and the present SAR and structural information will provide a road map towards the identification of more potent and appropriate candidates for further drug discovery.

Modulation of the Innate Immune Response by Targeting Toll-like Receptors: A Perspective on Their Agonists and Antagonists

Toll-like receptors (TLRs) are a class of proteins that recognize pathogen-associated molecular patterns (PAMPs) and damaged-associated molecular patterns (DAMPs), and they are involved in the regulation of innate immune system. These transmembrane receptors, localized at the cellular or endosomal membrane, trigger inflammatory processes through either myeloid differentiation primary response 88 (MyD88) or TIR-domain-containing adapter-inducing interferon-β (TRIF) signaling pathways. In the last decades, extensive research has been performed on TLR modulators and their therapeutic implication under several pathological conditions, spanning from infections to cancer, from metabolic disorders to neurodegeneration and autoimmune diseases. This Perspective will highlight the recent discoveries in this field, emphasizing the role of TLRs in different diseases and the therapeutic effect of their natural and synthetic modulators, and it will discuss insights for the future exploitation of TLR modulators in human health.

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.

Agonist and antagonist ligands of toll-like receptors 7 and 8: Ingenious tools for therapeutic purposes

The discovery of the TLRs family and more precisely its functions opened a variety of gates to modulate immunological host responses. TLRs 7/8 are located in the endosomal compartment and activate a specific signaling pathway in a MyD88-dependant manner. According to their involvement into various autoimmune, inflammatory and malignant diseases, researchers have designed diverse TLRs 7/8 ligands able to boost or block the inherent signal transduction. These modulators are often small synthetic compounds and most act as agonists and to a much lesser extent as antagonists. Some of them have reached preclinical and clinical trials, and only one has been approved by the FDA and EMA, imiquimod. The key to the success of these modulators probably lies in their combination with other therapies as recently demonstrated. We gather in this review more than 360 scientific publications, reviews and patents, relating the extensive work carried out by researchers on the design of TLRs 7/8 modulators, which are classified firstly by their biological activities (agonist or antagonist) and then by their chemical structures, which total syntheses are not discussed here. This review also reports about 90 clinical cases, thereby showing the biological interest of these modulators in multiple pathologies.

Novel and Selective TLR7 Antagonists among the Imidazo[1,2-a]pyrazines, Imidazo[1,5-a]quinoxalines, and Pyrazolo[1,5-a]quinoxalines Series

The Toll-like receptors (TLRs) 7 and 8 play an important role in the immune system activation, and their agonists may therefore serve as promising candidate vaccine adjuvants. However, the chronic immune activation by excessive TLR stimulation is a hallmark of several clinically important infectious and autoimmune diseases, which warrants the search for TLR antagonists. In this study, we have synthesized and characterized a variety of compounds belonging to three heterocyclic chemical series: imidazo[1,2-a]pyrazine, imidazo[1,5-a]quinoxaline, and pyrazolo[1,5-a]quinoxaline. These compounds have been tested for their TLR7 or TLR8 agonistic and antagonistic activities. Several of them are shown to be selective TLR7 antagonists without any TLR7 or TLR8 agonistic activity. The selectivity was confirmed by a comparative ligand-docking study in TLR7 antagonist pocket. Two compounds of the pyrazolo[1,5-a]quinoxaline series (10a and 10b) are potent selective TLR7 antagonists and may be considered as promising starting points for the development of new therapeutic agents.

Transcriptomal signatures of vaccine adjuvants and accessory immunostimulation of sentinel cells by toll-like receptor 2/6 agonists

An important component of vaccine development is the identification of safe and effective adjuvants. We sought to identify transcriptomal signatures of innate immune stimulating molecules using next-generation RNA sequencing with the goal of being able to utilize such signatures in identifying novel immunostimulatory compounds with adjuvant activity. The CC family of chemokines, particularly CC chemokines 1, 2, 3, 4, 7, 8, 17, 18, 20, and 23, were broadly upregulated by most Toll-like receptor (TLR) and nucleotide-binding domain and leucine-rich repeat-containing receptors (NLR) stimuli. Extracellular receptors such as TLR2, TLR4 and TLR5 induced the transcription of CXC chemokines including CXCL5, CXCL6 and CXCL8, whereas intracellular receptors such as TLR7 and TLR8 upregulated CXC chemokines 11 and 12. Both TLR1/2 and TLR2/6 agonists induced strong chemokine production in human peripheral blood mononuclear cells. Human skeletal muscle cells and fibroblasts respond with chemokine production only to TLR2/6 agonists, but not TLR1/2 agonists, consistent with strong expression of TLR2 and TLR6, but not of TLR1, in fibroblasts. TLR2/6 stimulated fibroblasts demonstrated functional chemotactic responses to human T cell and natural killer cells subsets. The activation of non-hematopoietic, adventitial cells such as fibroblasts and myocytes may contribute.

Toll-like Receptor Agonist Conjugation: A Chemical Perspective

Toll-like receptors (TLRs) are vital elements of the mammalian immune system that function by recognizing pathogen-associated molecular patterns (PAMPs), bridging innate and adaptive immunity. They have become a prominent therapeutic target for the treatment of infectious diseases, cancer, and allergies, with many TLR agonists currently in clinical trials or approved as immunostimulants. Numerous studies have shown that conjugation of TLR agonists to other molecules can beneficially influence their potency, toxicity, pharmacokinetics, or function. The functional properties of TLR agonist conjugates, however, are highly dependent on the ligation strategy employed. Here, we review the chemical structural requirements for effective functional TLR agonist conjugation. In addition, we provide similar analysis for those that have yet to be conjugated. Moreover, we discuss applications of covalent TLR agonist conjugation and their implications for clinical use.

Identification of High-Potency Human TLR8 and Dual TLR7/TLR8 Agonists in Pyrimidine-2,4-diamines

The induction of toll-like receptor 7 (TLR7)-dependent type I interferons (IFN-α/β) from plasmacytoid dendritic cells as well as the production of TLR8-dependent type II interferon (IFN-γ), TNF-α, and IL-12 in myeloid dendritic cells are of importance in generating T helper-1 biased adaptive immune responses. In an effort to identify novel dual TLR7/TLR8-active compounds, we undertook structure-activity relationship studies in pyrimidine 2,4-diamines, focusing on substituents at C5. Several analogues substituted with aminopropyl appendages at C5 displayed dominant TLR8-agonistic activity. N⁴-Butyl-6-methyl-5-(3-morpholinopropyl)pyrimidine-2,4-diamine was found to be a very potent dual TLR7/TLR8 agonist. Employing novel cytokine reporter cell assays, we verified that potency at TLR7 correlates with IFN-α/β production in human blood, whereas IFN-γ and TNF-α induction is largely TLR8-dependent. Dual TLR7/TLR8 agonists markedly upregulate CD80 expression in multiple dendritic cell subsets, providing insight into the immunological basis for the superior adjuvantic properties of such innate immune stimuli.

Syntheses of Human TLR8-Specific Small-Molecule Agonists

Human toll-like receptor (hTLR)-8 is expressed in myeloid dendritic cells, monocytes, and monocyte-derived dendritic cells. Engagement by TLR8 agonists evokes a distinct cytokine profile which favors the development of type 1 helper T cells. Focused exploration of structure-activity relationships in the imidazoquinolines has led to the identification of several novel human TLR8-specific agonists. The synthetic procedures for best-in-class analogues encompassing four chemotypes are described.

An Enzyme-Directed Imidazoquinoline for Cancer Immunotherapy

Herein we report the synthesis and activity of an enzyme-directed immunostimulant with immune cell activation mediated by β-galactosidase, either exogenously added, or on B16 melanoma cells. Covalent attachment of a β-galactopyranoside to an imidazoquinoline immunostimulant at a position critical for activity resulted in a pro-immunostimulant that could be selectively converted by β-galactosidase into an active immunostimulant. The pro-immunostimulant exhibited β-galactosidase-directed immune cell activation as measured by NF-κB transcription in RAW-Blue macrophages or cytokine production (TNF, IL-6, IL-12) in JAWSII monocytes. Conversion of the pro-immunostimulant into an active immunostimulant was also found to occur using β-galactosidase-enriched B16 melanoma cells. In co-culture experiments with either immune cell line, β-galactosidase-enriched B16 cells effected activation of bystander immune cells.

The TLR7 agonist imiquimod induces bronchodilation via a nonneuronal TLR7-independent mechanism: a possible role for quinoline in airway dilation

Toll-like receptor (TLR) 7 agonists are known to reduce allergic airway inflammation. Their recently reported ability to rapidly relax airways has further increased their interest in the treatment of pulmonary disease. However, the mechanisms behind this effect are not fully understood. The present study, therefore, aimed to determine whether airway smooth muscle (ASM)-dependent mechanisms could be identified. TLR7 agonists were added to guinea pig airways following precontraction with carbachol in vitro or histamine in vivo. Pharmacological inhibitors were used to dissect conventional pathways of bronchodilation; tetrodotoxin was used or bilateral vagotomy was performed to assess neuronal involvement. Human ASM cells (HASMCs) were employed to determine the effect of TLR7 agonists on intracellular Ca(2+) ([Ca(2+)]i) mobilization. The well-established TLR7 agonist imiquimod rapidly relaxed precontracted airways in vitro and in vivo. This relaxation was demonstrated to be independent of nitric oxide, carbon monoxide, and cAMP signaling, as well as neuronal activity. A limited role for prostanoids could be detected. Imiquimod induced [Ca(2+)]i release from endoplasmic reticulum stores in HASMCs, inhibiting histamine-induced [Ca(2+)]i The TLR7 antagonist IRS661 failed to inhibit relaxation, and the structurally dissimilar agonist CL264 did not relax airways or inhibit [Ca(2+)]i This study shows that imiquimod acts directly on ASM to induce bronchorelaxation, via a TLR7-independent release of [Ca(2+)]i The effect is paralleled by other bronchorelaxant compounds, like chloroquine, which, like imiquimod, but unlike CL264, contains the chemical structure quinoline. Compounds with quinoline moieties may be of interest in the development of multifunctional drugs to treat pulmonary disease.

Identification of a Human Toll-Like Receptor (TLR) 8-Specific Agonist and a Functional Pan-TLR Inhibitor in 2-Aminoimidazoles

Activation of human toll-like receptor-8 (TLR8), expressed in myeloid dendritic cells, monocytes, and monocyte-derived dendritic cells, evokes a distinct cytokine profile which favors the development of Type 1 helper T cells. Part-structures of the 2-aminobenzimidazole scaffold were examined with a view to identifying structural requisites corresponding to the smallest possible fragment of the benzimidazole core that would allow for retention of TLR8-agonistic activity. TLR8-specific agonistic activity was retained in 1-pentyl-4-phenyl-1H-imidazol-2-amine. The crystal structure of this compound bound to the TLR8 ectodomain displayed binding interactions that are common to other TLR8 agonists. This compound showed markedly attenuated proinflammatory properties in ex vivo human blood models. SAR studies revealed that 4-(2-(benzyloxy)phenyl)-1-pentyl-1H-imidazol-2-amine inhibited TLR signaling in a variety of TLR reporter cell lines, as well as in pharmacologically relevant human blood model systems. A kinase screen of this compound showed relative specificity for calmodulin kinases.

Identification of Adjuvantic Activity of Amphotericin B in a Novel, Multiplexed, Poly-TLR/NLR High-Throughput Screen

Small-molecule agonists have been identified for TLR7, TLR8, TLR4 and TLR2 thus far, and chemotypes other than those of canonical ligands are yet to be explored for a number of innate immune receptors. The discovery of novel immunostimulatory molecules would enhance the repertoire of tools available for interrogating innate immune effector mechanisms, and provide additional venues for vaccine adjuvant development. A multiplexed, reporter gene-based high-throughput assay capable of detecting agonists of TLR2, TLR3, TLR4, TLR5, TLR7, TLR8, TLR9, NOD1 and NOD2 was utilized in screening 123,943 compounds, in which amphotericin B (AmpB) and nystatin were identified as prominent hits. The polyene antifungal agents act as TLR2- and TLR4-agonists. The TLR4-stimulatory activity of AmpB was similar to that of monophosphoryl lipid A, suggestive of TRIF-biased signaling. The adjuvantic activity of AmpB, at a dose of 100 micrograms, was comparable to several other candidate adjuvants in rabbit models of immunization. These results point to its potential applicability as a safe and effective adjuvant for human vaccines.

Structure-Based Design of Human TLR8-Specific Agonists with Augmented Potency and Adjuvanticity

Human Toll-like receptor 8 (hTLR8) is expressed in myeloid dendritic cells, monocytes, and monocyte-derived dendritic cells. Engagement by TLR8 agonists evokes a distinct cytokine profile which favors the development of type 1 helper T cells. Crystal structures of the ectodomain of hTLR8 cocrystallized with two regioisomers of a dual TLR7/8-agonistic N1-substituted imidazoquinolines showed subtle differences in their interactions in the binding site of hTLR8. We hypothesized that the potency of a previously reported best-in-class pure TLR8 agonist, 3-pentylquinoline-2-amine, could be further enhanced by “designing in” functional groups that would mimic key intermolecular interactions that we had observed in the crystal structures. We performed a focused exploration of decorating the quinoline core with alkylamino groups at all possible positions. These studies have led to the identification of a novel TLR8 agonist that was ∼20-fold more potent than the parent compound and displays prominent adjuvantic activity in a rabbit model of immunization.

Relationships Between Pharmacovigilance, Molecular, Structural, and Pathway Data: Revealing Mechanisms for Immune-Mediated Drug-Induced Liver Injury

Immune-mediated drug-induced liver injury (IMDILI) can be devastating, irreversible, and fatal in the absence of successful transplantation surgery. We present a novel approach that combines the methods of pharmacoepidemiology with in silico molecular modeling to identify specific features in toxic ligands that are associated with clinical features of IMDILI. Specifically, from pharmacovigilance data multivariate logistic regression identified 18 drugs associated with IMDILI (P < 0.00015). Eleven of these drugs, along with their known and proposed metabolites, constituted a training set used to develop a four-point pharmacophore model (sensitivity 75%; specificity 85%). Subsequently, this information was combined with information from immune-pathway reviews and genetic-association studies and complemented with ligand-protein docking simulations to support a hypothesis implicating two putative targets within separate, possibly interacting, immune-system pathways: the major histocompatibility complex within the adaptive immune system and Toll-like receptors (TLRs), in particular TLR-7, which represent pattern recognition receptors of the innate immune system.

Toll-like receptor 7 inactive ligands enhanced cytokine induction by conjugation to weak antigens

Toll-like receptors (TLRs) 7/8 are key targets in the design and development of small-molecule drugs serving as anticancer/antiviral agents and vaccine adjuvants. Clinical trials of imiquimod were discontinued owing to its serious adverse side effects. Herein we report the synthesis and biological evaluation of a series of 8-hydroxy-2-(2-methoxyethoxy)adenine derivatives that cannot induce cytokine production and that lack activity toward TLR 7/8. Their ability to triggering remarkable levels of cytokine production were revealed upon their conjugation with antigens that have weak immunogenicity. This discovery demonstrated that TLR 7 can be activated by coupling an antigen to the terminal carboxyl group at N9 of the inactive ligand adenine analogues. These inactive analogues may be well suited as new adjuvants with superior activity after conjugation, effectively decreasing the side effects caused by conventional adjuvants.

Determinants of activity at human Toll-like receptors 7 and 8: quantitative structure-activity relationship (QSAR) of diverse heterocyclic scaffolds

Toll-like receptor (TLR) 7 and 8 agonists are potential vaccine adjuvants, since they directly activate APCs and enhance Th1-driven immune responses. Previous SAR investigations in several scaffolds of small molecule TLR7/8 activators pointed to the strict dependence of the selectivity for TLR7 vis-à-vis TLR8 on the electronic configurations of the heterocyclic systems, which we sought to examine quantitatively with the goal of developing "heuristics" to define structural requisites governing activity at TLR7 and/or TLR8. We undertook a scaffold-hopping approach, entailing the syntheses and biological evaluations of 13 different chemotypes. Crystal structures of TLR8 in complex with the two most active compounds confirmed important binding interactions playing a key role in ligand occupancy and biological activity. Density functional theory based quantum chemical calculations on these compounds followed by linear discriminant analyses permitted the classification of inactive, TLR8-active, and TLR7/8 dual-active compounds, confirming the critical role of partial charges in determining biological activity.

Human Toll-like receptor 8-selective agonistic activities in 1-alkyl-1H-benzimidazol-2-amines

Toll-like receptor (TLR)-8 agonists strongly induce the production of T helper 1-polarizing cytokines and may therefore serve as promising candidate vaccine adjuvants, especially for the very young and the elderly. Earlier structure-based ligand design led to the identification of 3-pentyl-quinoline-2-amine as a novel, human TLR8-specific agonist. Comprehensive structure–activity relationships in ring-contracted 1-alkyl-1H-benzimidazol-2-amines were undertaken, and the best-in-class compound, 4-methyl-1-pentyl-1H-benzo[d]imidazol-2-amine, was found to be a pure TLR8 agonist, evoking strong proinflammatory cytokine and Type II interferon responses in human PBMCs, with no attendant CD69 upregulation in natural lymphocytic subsets. The 1-alkyl-1H-benzimidazol-2-amines represent a novel, alternate chemotype with pure TLR8-agonistic activities and will likely prove useful not only in understanding TLR8 signaling but also perhaps as a candidate vaccine adjuvant.

Stimulation of innate immune cells by light-activated TLR7/8 agonists

The innate immune response is controlled, in part, by the synergistic interaction of multiple Toll-like receptors (TLRs). This multi-receptor cooperation is responsible for the potent activity of many vaccines, but few tools have been developed to understand the spatio-temporal elements of TLR synergies. In this Communication, we present photo-controlled agonists of TLR7/8. By strategically protecting the active agonist moiety based on an agonist-bound crystal structure, TLR activity is suppressed and then regained upon exposure to light. We confirmed NF-κB production upon light exposure in a model macrophage cell line. Primary cell activity was confirmed by examining cytokine and cell surface marker production in bone-marrow-derived dendritic cells. Finally, we used light to activate dendritic cell sub-populations within a larger population.

Structure-based design of novel human Toll-like receptor 8 agonists

Toll-like receptor (TLR)-8 agonists activate adaptive immune responses by inducing robust production of T helper 1-polarizing cytokines, suggesting that TLR8-active compounds might be promising candidate vaccine adjuvants. Recently, a C2-butyl furo[2,3-c]quinoline was reported with purely TLR8 agonistic activity. This compound was successfully co-crystallized with the human TLR8 ectodomain, and the co-crystal structure revealed ligand-induced reorganization of the binding pocket of TLR8. The loss of a key hydrogen bond between the oxygen atom of the furanyl ring of the agonist and Thr 574 in TLR8 suggested that the furan ring is dispensable. Employing a disconnection strategy, 3- and 4-substituted aminoquinolines were investigated. Focused structure-based ligand design studies led to the identification of 3-pentyl-quinoline-2-amine as a novel, structurally simple, and highly potent human TLR8-specific agonist (EC50 =0.2 μM). Preliminary evaluation of this compound in ex vivo human blood assay systems revealed that it retains prominent cytokine-inducing activity. Together, these results indicate the suitability of this compound as a novel vaccine adjuvant, warranting further investigation.