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

Peptide-TLR7/8a-Coordinated DNA Vaccines Elicit Enhanced Immune Responses against Infectious Diseases

DNA vaccines represent an innovative approach for the immunization of diverse diseases. However, their clinical trial outcomes are constrained by suboptimal transfection efficiency and immunogenicity. In this work, we present a universal methodology involving the codelivery of Toll-like receptor 7/8 agonists (TLR7/8a) and antigen gene using TLR7/8a-conjugated peptide-coated poly(β-amino ester) (PBAE) nanoparticles (NPs) to augment delivery efficiency and immune response. Peptide-TLR7/8a-coated PBAE NPs exhibit advantageous biophysical attributes, encompassing diminutive particle dimensions, nearly neutral ζ potential, and stability in the physiological environment. This synergistic approach not only ameliorates the stability of plasmid DNA (pDNA) and gene delivery efficacy but also facilitates subsequent antigen production. Furthermore, under optimal formulation conditions, the TLR7/8a-conjugated peptide coated PBAE NPs exhibit a potent capacity to induce robust immune responses. Collectively, this nanoparticulate gene delivery system demonstrates heightened transfection efficacy, stability, biodegradability, immunostimulatory effect, and low toxicity, making it a promising platform for the clinical advancement of DNA vaccines.

Generation and structure-activity relationships of novel imidazo-thienopyridine based TLR7 agonists: application as payloads for immunostimulatory antibody drug-conjugates

Pairing immunostimulatory small molecules with the targeting capability of an antibody has emerged as a novel therapeutic modality with the potential to treat a variety of solid tumors. A series of compounds based on an imidazo-thienopyridine scaffold were synthesized and tested for their ability to agonize the innate immune sensors toll-like receptor 7 and 8 (TLR7/8). Structure-activity relationship (SAR) studies revealed that certain simple amino-substituents could enable TLR7 agonism at low nanomolar concentrations. Drug-linkers containing either payload 1 or 20h were conjugated to the HER2-targeting antibody trastuzumab at the interchain disulfide cysteine residues using a cleavable valine-citrulline dipeptide linker and stochastic thiol-maleimide chemistry. In vitro, these immune-stimulating antibody drug-conjugates (ADCs) were found to induce cytokine release in a murine splenocyte assay when co-cultured with the HER2-high NCI-N87 cancer cell line. In vivo, tumor regression was observed with a single dose in an NCI-N87 gastric carcinoma xenograft model in BALB/c nude mice.

Gap Junctions and Connexins in Microglia-Related Oxidative Stress and Neuroinflammation: Perspectives for Drug Discovery

Microglia represent the immune system of the brain. Their role is central in two phenomena, neuroinflammation and oxidative stress, which are at the roots of different pathologies related to the central nervous system (CNS). In order to maintain the homeostasis of the brain and re-establish the equilibrium after a threatening imbalance, microglia communicate with each other and other cells within the CNS by receiving specific signals through membrane-bound receptors and then releasing neurotrophic factors into either the extracellular milieu or directly into the cytoplasm of nearby cells, such as astrocytes and neurons. These last two mechanisms rely on the activity of protein structures that enable the formation of channels in the membrane, namely, connexins and pannexins, that group and form gap junctions, hemichannels, and pannexons. These channels allow the release of gliotransmitters, such as adenosine triphosphate (ATP) and glutamate, together with calcium ion (Ca2+), that seem to play a pivotal role in inter-cellular communication. The aim of the present review is focused on the physiology of channel protein complexes and their contribution to neuroinflammatory and oxidative stress-related phenomena, which play a central role in neurodegenerative disorders. We will then discuss how pharmacological modulation of these channels can impact neuroinflammatory phenomena and hypothesize that currently available nutraceuticals, such as carnosine and N-acetylcysteine, can modulate the activity of connexins and pannexins in microglial cells and reduce oxidative stress in neurodegenerative disorders.

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 and Translational Potential for Agonists and Antagonists of Endosomal Toll-like Receptors

Toll-like receptors (TLRs) are members of a large family of evolutionarily conserved pattern recognition receptors (PRRs), which serve as key components of the innate immune system by playing a pivotal role in sensing "nonself" ligands. Endosomal TLRs (TLR3, TLR7, TLR8, and TLR9) can recognize pathogen-derived nucleic acid and initiate an innate immune response because they react against both self- and non-self-origin nucleic acid molecules. Accordingly, both receptor agonists and antagonists are potentially useful in disparate clinical contexts and thus are globally sought after. Recent research has revealed that agonists and antagonists share an overlapping binding region. This Perspective highlights rational medicinal chemistry approaches to elucidate the structural attributes of small molecules capable of agonism or antagonism or of elegantly switching between the two. The structural evolution of different chemotypes can provide the framework for the future development of endosomal TLR agonists and antagonists.

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.

Structural and functional understanding of the toll-like receptors

Recognition of invading pathogens by the innate immune system is essential to initiate antimicrobial responses and trigger adaptive immunity. This is largely mediated by an array of pattern-recognition receptor families that are essential for recognizing conserved molecular motifs characteristic of pathogenic microbes. One such family is the Toll-like receptors (TLRs). Activation of TLRs induces production of pro-inflammatory cytokines and type I interferons: the former triggers the synthesis of inflammatory mediators which cause fever, pain and other inflammation, and the latter mediates antiviral responses. Over the past decade, significant progress has been made in structural elucidation of TLRs in higher eukaryotes. The TLR structures with and without agonist and antagonist have been revealed by X-ray crystallography and cryo-electron microscopy studies, demonstrating the activated dimer formation induced by the agonistic ligand and the inhibition mechanism of the antagonistic ligand. Intracellular assembled structures and the TLR-chaperone complex are also reported. As the structural understanding of TLRs becomes better integrated with biochemical and immunological studies, a more comprehensive picture of their architectural and functional properties will emerge. This review summarizes recent advances in structural biological and mechanistic studies on TLRs.

Therapeutic Vaccines for Cancer Immunotherapy

The rapid development of nanobiotechnology has enabled progress in therapeutic cancer vaccines. These vaccines stimulate the host innate immune response by tumor antigens followed by a cascading adaptive response against cancer. However, an improved antitumor immune response is still in high demand because of the unsatisfactory clinical performance of the vaccine in tumor inhibition and regression. To date, a complicated tumor immunosuppressive environment and suboptimal design are the main obstacles for therapeutic cancer vaccines. The optimization of tumor antigens, vaccine delivery pathways, and proper adjuvants for innate immune response initiation, along with reprogramming of the tumor immunosuppressive environment, is essential for therapeutic cancer vaccines in triggering an adequate antitumor immune response. In this review, we aim to review the challenges in and strategies for enhancing the efficacy of therapeutic vaccines. We start with the summary of the available tumor antigens and their properties and then the optimal strategies for vaccine delivery. Subsequently, the vaccine adjuvants focused on the intrinsic adjuvant properties of nanostructures are further discussed. Finally, we summarize the combination strategies with therapeutic cancer vaccines and discuss their positive impact in cancer immunity.

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.

Targeting the innate immune receptor TLR8 using small-molecule agents

Toll-like receptors (TLRs) are pattern-recognition receptors that initiate innate immune responses. Among the TLRs, TLR8 (and TLR7) recognizes single-stranded RNA to mediate downstream signals. In recent years, intensive X-ray crystal structural analyses have provided atomic insights into structures of TLR8 complexed with various agonists or antagonists. Here, structural knowledge of the activation and inactivation mechanisms of the ligands is reviewed. In addition, the potential clinical applications of TLR ligands are examined.

BBIQ, a pure TLR7 agonist, is an effective influenza vaccine adjuvant

Better adjuvants are needed for vaccines against seasonal influenza. TLR7 agonists are potent activators of innate immune responses and thereby may be promising adjuvants. Among the imidazoquinoline compounds, 1-benzyl-2-butyl-1H-imidazo[4,5-c]quinolin-4-amine (BBIQ) was reported to be a highly active TLR7 agonist but has remained relatively unexplored because of its commercial unavailability. Indeed, in silico molecular modeling studies predicted that BBIQ had a higher TLR7 docking score and binding free energy than imiquimod, the gold standard TLR7 agonist. To circumvent the availability issue, we developed an improved and higher yield method to synthesize BBIQ. Testing BBIQ on human and mouse TLR7 reporter cell lines confirmed it to be TLR7 specific with significantly higher potency than imiquimod. To test its adjuvant potential, BBIQ or imiquimod were admixed with recombinant influenza hemagglutinin protein and administered to mice as two intramuscular immunizations 2 weeks apart. Serum anti-influenza IgG responses assessed by ELISA 2 weeks after the second immunization confirmed that the mice that received vaccine admixed with BBIQ had significantly higher anti-influenza IgG1 and IgG2c responses than mice immunized with antigen alone or admixed with imiquimod. This confirmed BBIQ to be a TLR7-specific adjuvant able to enhance humoral immune responses.

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.

Discovery and Biological Evaluation of New Selective Acetylcholinesterase Inhibitors with Anti-Aβ Aggregation Activity through Molecular Docking-Based Virtual Screening

Discovery of novel multifunctional inhibitors targeting acetylcholinesterase (AChE) has becoming a hot spot in anti-Alzheimer's disease (AD) drug development. In the present study, four potent small molecule inhibitors (A01, A02, A03 and A04) of AChE with new chemical scaffold were identified. Inhibitor A03 displayed the most potent inhibition activity on AChE at enzymatic level with IC₅₀ value of 180 nM, and high selectivity towards AChE over butyrylcholinesterase (BChE) by more than 100-fold. The binding modes of compounds A01-A04 were carefully analyzed by molecular docking and molecular dynamics (MD) simulation to provide informative clues for further structure modification. Finally, the anti-amyloid beta (Aβ) aggregation and neuroprotective activity were also well investigated. Our findings highlighted the therapeutic promise of AChE inhibitors A01-A04 for AD treatment.

Structural Analyses of Toll-like Receptor 7 Reveal Detailed RNA Sequence Specificity and Recognition Mechanism of Agonistic Ligands

Toll-like receptor 7 (TLR7) is an innate immune receptor for single-stranded RNA (ssRNA) and has important roles in infectious diseases. We previously reported that TLR7 shows synergistic activation in response to two ligands, guanosine and ssRNA. However, the specific ssRNA sequence preference, detailed recognition mode of TLR7 and its ligand, and molecular determinants of TLR7 and TLR8 selectivity remain unknown. Here, we report on TLR7 from a large-scale crystallographic study combined with a multifaceted approach. We reveal that successive uridine-containing ssRNAs fully or moderately bind TLR7, whereas single uridine-containing ssRNAs have reduced affinities. We also reveal the detailed relationships between the chemical structures of ligands and their binding to TLR7. We demonstrate that an engineered TLR8 mutant alters its responsiveness to TLR7-specific ligands. Finally, we identify guanosine 2',3'-cyclic phosphate (2',3'-cGMP) as a possible endogenous ligand for TLR7 with greater affinity than guanosine. The abundant structural information will facilitate future development of treatments targeting TLR7.

Discovery of Selective Butyrylcholinesterase (BChE) Inhibitors through a Combination of Computational Studies and Biological Evaluations

As there are increased levels and activity of butyrylcholiesterase (BChE) in the late stage of Alzheimer’s disease (AD), development of selective BChE inhibitors is of vital importance. In this study, a workflow combining computational technologies and biological assays were implemented to identify selective BChE inhibitors with new chemical scaffolds. In particular, a pharmacophore model served as a 3D search query to screen three compound collections containing 3.0 million compounds. Molecular docking and cluster analysis were performed to increase the efficiency and accuracy of virtual screening. Finally, 15 compounds were retained for biological investigation. Results revealed that compounds 8 and 18 could potently and highly selectively inhibit BChE activities (IC₅₀ values < 10 μM on human BChE, selectivity index BChE > 30). These active compounds with novel scaffolds provided us with a good starting point to further design potent and selective BChE inhibitors, which may be beneficial for the treatment of AD.

Subtle differences in chemical pattern between human toll-like receptor 8 agonists and antagonists: Emerging chemical patterns analysis

Due to the potencies in the treatments of cancer, infectious diseases, and autoimmune diseases, the developments of human TLR8 (hTLR8) agonists and antagonists have attracted widespread attentions. The hTLR8 agonists and antagonists have similar structures but with completely opposite biological effects. Up to date, the subtle differences in the structures between the hTLR8 agonists and antagonists are still unknown. In this work, emerging chemical pattern (ECP) was successfully used to extract the key chemical patterns of the hTLR8 agonists and antagonists. By using CAEP classifier, an optimal ECP model with only 3 descriptors was established with the overall prediction accuracy larger than 90%. Further hierarchical cluster analysis and molecular docking showed that the H-bond and hydrophobic properties are the key features distinguishing the hTLR8 agonists from antagonists. Comparing with the antagonists, the agonists show stronger specific H-bond properties, while antagonists have stronger non-specific hydrophobic properties. The significant differences in the structural properties may be closely related to the activation/inhibition mechanism of hTLR8.

Discovery of New Selective Butyrylcholinesterase (BChE) Inhibitors with Anti-Aβ Aggregation Activity: Structure-Based Virtual Screening, Hit Optimization and Biological Evaluation

In this study, a series of selective butyrylcholinesterase (BChE) inhibitors was designed and synthesized from the structural optimization of hit 1, a 4-((3,4-dihydroisoquinolin-2(1H)-yl)methyl)benzoic acid derivative identified by virtual screening our compound library. The in vitro enzyme assay results showed that compounds 9 ((4-((3,4-dihydroisoquinolin-2(1H)-yl)methyl)phenyl)(pyrrolidin-1-yl)methanone) and 23 (N-(2-bromophenyl)-4-((3,4-dihydroisoquinolin-2(1H)-yl)methyl)benzamide) displayed improved BChE inhibitory activity and good selectivity towards BChE versus AChE. Their binding modes were probed by molecular docking and further validated by molecular dynamics simulation. Kinetic analysis together with molecular modeling studies suggested that these derivatives could target both the catalytic active site (CAS) and peripheral anionic site (PAS) of BChE. In addition, the selected compounds 9 and 23 displayed anti-Aβ_1–42 aggregation activity in a dose-dependent manner, and they did not show obvious cytotoxicity towards SH-SY5Y neuroblastoma cells. Also, both compounds showed significantly protective activity against Aβ_1-42-induced toxicity in a SH-SY5Y cell model. The present results provided a new valuable chemical template for the development of selective BChE inhibitors.

Synthesis and Evaluation of Novel Pyrazole Ethandiamide Compounds as Inhibitors of Human THP-1 Monocytic Cell Neurotoxicity

Neuroinflammation and microglia-mediated neurotoxicity contribute to the pathogenesis of a broad range of neurodegenerative diseases; therefore, identifying novel compounds that can suppress adverse activation of glia is an important goal. We have previously identified a class of trisubstituted pyrazoles that possess neuroprotective and anti-inflammatory properties. Here, we describe a second generation of pyrazole analogs that were designed to improve their neuroprotective activity toward neurons under inflammatory conditions. Pyrazolyl oxalamide derivatives were designed to explore the effects of steric and electronic factors. Three in vitro assays were performed to evaluate the compounds' anti-neurotoxic, neuroprotective, and cytotoxic activity using human THP-1, PC-3, and SH-SY5Y cells. Five compounds significantly reduced the neurotoxic secretions from immune-stimulated microglia-like human THP-1 monocytic cells. One of these compounds was also found to protect SH-SY5Y neuronal cells when they were exposed to cytotoxic THP-1 cell supernatants. While one of the analogs was discarded due to its interference with the cell viability assay, most compounds were innocuous to the cultured cells at the concentrations used (1-100 μM). The new compounds reported herein provide a design template for the future development of lead candidates as novel inhibitors of neuroinflammation and neuroprotective drugs.

Piper cubeba L. Methanol Extract Has Anti-Inflammatory Activity Targeting Src/Syk via NF-κB Inhibition

Piper cubeba L. is a plant in the Piperaceae family that is generally found in tropical countries and acts as an antioxidant and anti-inflammatory agent. Unfortunately, the molecular mechanism of the anti-inflammatory activity has not been fully investigated. In this study, we elucidated the anti-inflammatory mechanism by focusing on NF-κB signaling, which is considered a prototypical inflammatory signaling pathway in both innate and adaptive immune functions. Cellular activity and the molecular target of Pc-ME were identified in macrophage RAW264.7 cells and HEK293T cells by assessing NO production, cytokine expression by RT-PCR, luciferase gene reporter assay, and protein regulation in cytoplasm by Western blot upon NF-κB activation. Pc-ME reduced NO production without any cell toxicity; inhibited expression of proinflammatory cytokines such as iNOS and IL-6; downregulated NF-κB activation mediated by both MyD88 and TRIF; and diminished the phosphorylation of IκBα, IKKα/β, Akt, p85, Src, and Syk. Pc-ME inhibited Syk and Src autophosphorylation during overexpression in HEK cells, which confirmed our hypothesis that Syk and Src were signaling targets of Pc-ME. These findings indicate that Piper cubeba L. has anti-inflammatory activity by targeting Src/Syk in the NF-κB pathway.

The emerging chemical patterns applied in predicting human toll-like receptor 8 agonists

Toll-like receptors (TLRs) are important pattern recognition receptors to human innate immunity, which can recognize pathogen-associated molecular patterns and initiate innate immune responses. As the receptor of single stranded RNA (ssRNA), toll-like receptor 8 (TLR8) has potential in the treatment of tumors, microbial infection, and inflammatory diseases. Herein, an emerging chemical pattern (ECP) method was utilized to predict the key chemical patterns of TLR8 agonists. Based on the ECPs discovered, a robust and predictive ECP model was derived with prediction accuracies of 83.3%, 81.0%, and 80.0% for 132 training samples, 79 validation samples, and 75 test samples, respectively. When the ECP model was applied with a molecular docking method, the hit rate of TLR8 agonists was greatly enhanced. The results of ECP-based hierarchical cluster analysis and Connolly surface analysis of the TLR8 receptor showed that the H-bonding, hydrophilic and hydrophobic potentials as well as the unbalanced degree of property distributions are very important for distinguishing the TLR8 agonists from non-agonists.

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.

Antibacterial Evaluation and Virtual Screening of New Thiazolyl-Triazole Schiff Bases as Potential DNA-Gyrase Inhibitors

The global spread of bacterial resistance to drugs used in therapy requires new potent and safe antimicrobial agents. DNA gyrases represent important targets in drug discovery. Schiff bases, thiazole, and triazole derivatives are considered key scaffolds in medicinal chemistry. Fifteen thiazolyl-triazole Schiff bases were evaluated for their antibacterial activity, measuring the growth inhibition zone diameter, the minimum inhibitory concentration (MIC), and the minimum bactericidal concentration (MBC), against Gram-positive (Staphylococcus aureus, Listeria monocytogenes) and Gram-negative (Escherichia coli, Salmonella typhimurium, Pseudomonas aeruginosa) bacteria. The inhibition of S. aureus and S. typhimurium was modest. Compounds B1, B2, and B9 showed a similar effect as ciprofloxacin, the antimicrobial reference, against L. monocytogenes. B10 displayed a better effect. Derivatives B1, B5–7, B9, and B11–15 expressed MIC values lower than the reference, against L. monocytogenes. B5, B6, and B11–15 strongly inhibited the growth of P. aeruginosa. All compounds were subjected to an in silico screening of the ADMET (absorption, distribution, metabolism, elimination, toxicity) properties. Molecular docking was performed on the gyrA and gyrB from L. monocytogenes. The virtual screening concluded that thiazolyl-triazole Schiff base B8 is the best drug-like candidate, satisfying requirements for both safety and efficacy, being more potent against the bacterial gyrA than ciprofloxacin.

Small-molecule inhibition of TLR8 through stabilization of its resting state

Endosomal Toll-like receptors (TLR3/7/8/9) are highly analogous sensors for various viral or bacterial RNA/DNA molecular patterns. Nonetheless, few small-molecules can selectively modulate these TLRs. In this manuscript, we identified the first human TLR8-specific small-molecule antagonists via a novel inhibition mechanism. Crystal structures of two distinct TLR8-ligand complexes validated a unique binding site on the protein-protein interface of the TLR8 homodimer. Upon binding to this new site, the small-molecule ligands stabilize the preformed TLR8 dimer in its resting state, preventing activation. As a proof of concept of their therapeutic potential, we have demonstrated that these drug-like inhibitors are able to suppress TLR8-mediated proinflammatory signaling in various cell lines, human primary cells, and patient specimens. These results not only suggest a novel strategy for TLR inhibitor design, but also shed critical mechanistic insight into these clinically important immune receptors.

Design and Synthesis of N1-Modified Imidazoquinoline Agonists for Selective Activation of Toll-like Receptors 7 and 8

A series of N1-modified imidazoquinolines were synthesized and screened for Toll-like receptors (TLR) 7 and 8 activities to identify recognition elements that confer high affinity binding and selectivity. These receptors are key targets in the development of immunomodulatory agents that signal the NF-κB mediated transcription of pro-inflammatory chemokines and cytokines. Results are presented showing both TLR7/8 activations are highly correlated to N1-substitution, with TLR8 selectivity achieved through inclusion of an ethyl-, propyl-, or butylamino group at this position. While the structure-activity relationship analysis indicates TLR7 activity is less sensitive to N1-modification, extension of the aminoalkyl chain length to pentyl and p-methylbenzyl elicited high affinity TLR7 binding. Cytokine profiles are also reported that show the pure TLR8 agonist [4-amino-2-butyl-1-(2-aminoethyl)-7-methoxycarbonyl-1H-imidazo[4,5-c]quinoline] induces higher levels of IL-1β, IL-12, and IFNγ when compared with TLR7 selective or mixed TLR7/8 agonists. The results are consistent with previous work suggesting TLR8 agonists are Th1 polarizing and may help promote cell-mediated immunity.

Toward a structural understanding of nucleic acid-sensing Toll-like receptors in the innate immune system

The history of mankind has been plagued by the tug of war with viral infections. Toll-like receptors (TLRs) and other receptors of the innate immune system constitute an early defense system against invading viruses by recognizing the viral genetic material, the nucleic acids (NAs). Agonistic ligands of NA-sensing TLRs play an emerging role in the treatment of viral diseases, demonstrating a crucial role of these receptors. Recently, crystal structures have afforded new insights into TLR recognition of NAs. An aberrant activation by self-NAs, which leads to the inflammation and autoimmunity, is avoided by strict regulation of NA-TLR interaction at multiple check-points. This Review summarizes the novel structural understanding of NA-sensing by TLRs and regulatory mechanisms of these receptors.

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.

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.

Structural aspects of nucleic acid-sensing Toll-like receptors

Invading pathogens elicit potent immune responses in cells through interactions between structurally conserved molecules derived from the pathogens and specialized innate immune receptors such as the Toll-like receptors (TLRs). Nucleic acid is one of the principal TLR ligands. Nucleic acid-sensing TLRs recognize an array of nucleic acids, including double-stranded RNA, single-stranded RNA, and DNAs with specific sequence motifs. Although ligand-induced dimerization is commonly observed followed by TLR activation, both the specific recognition mechanisms and the ligand-receptor interactions vary among different TLRs. In this review, we highlight our current understanding of how these receptors recognize their cognate ligands based on the recent advances in structural biology.

Cancer Immunotherapy: Selected Targets and Small-Molecule Modulators

There is a significant amount of excitement in the scientific community around cancer immunotherapy, as this approach has renewed hope for many cancer patients owing to some recent successes in the clinic. Currently available immuno-oncology therapeutics under clinical development and on the market are mostly biologics (antibodies, proteins, engineered cells, and oncolytic viruses). However, modulation of the immune system with small molecules offers several advantages that may be complementary and potentially synergistic to the use of large biologicals. Therefore, the discovery and development of novel small-molecule modulators is a rapidly growing research area for medicinal chemists working in cancer immunotherapy. This review provides a brief introduction into recent trends related to selected targets and pathways for cancer immunotherapy and their small-molecule pharmacological modulators.