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

Endosomal Toll-like receptors (TLR3, TLR7, TLR8, and TLR9) are highly analogous sensors for various viral or bacterial RNA and 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.

A novel Toll-like receptor 7/8-specific antagonist E6742 ameliorates clinically relevant disease parameters in murine models of lupus.

The sensing of self RNA by the endosomal Toll-like receptors (TLRs) 7 and 8 initiates pathogenic mechanisms underlying the autoimmune disease lupus. A blockade of the TLR7/8 signals may, therefore, be a novel therapeutic intervention for lupus. To test the hypothesis, a novel compound E6742 that blocks TLR7/8 activation was identified. The mode of action of E6742 was investigated by analysis of the tertiary structure of TLR7 and 8 in complex with E6742. The in vitro activities of the compound were examined in cellular systems and its therapeutic potential was evaluated in murine lupus models. Tertiary structures of the extracellular domain of TLR7 and 8 in complex with E6742 showed that E6742 binds specifically and non-covalently to the hydrophobic pocket located at the interface of TLR7 or TLR8 homodimers. E6742 potently and selectively inhibited several TLR7/8-mediated cytokine responses in human PBMC. In two mouse models of lupus, oral dosing of E6742 after the onset of disease suppressed increase in autoantibodies and blocked the advance of organ damage. Collectively, the data show that TLR7/8 activation contributes to disease progression and its blocking by E6742 has potential as a therapeutic intervention for lupus.

TLR3 forms a laterally aligned multimeric complex along double-stranded RNA for efficient signal transduction.

Toll-like receptor 3 (TLR3) is a member of the TLR family, which plays an important role in the innate immune system and is responsible for recognizing viral double-stranded RNA (dsRNA). Previous biochemical and structural studies have revealed that a minimum length of approximately 40-50 base pairs of dsRNA is necessary for TLR3 binding and dimerization. However, efficient TLR3 activation requires longer dsRNA and the molecular mechanism underlying its dsRNA length-dependent activation remains unknown. Here, we report cryo-electron microscopy analyses of TLR3 complexed with longer dsRNA. TLR3 dimers laterally form a higher multimeric complex along dsRNA, providing the basis for cooperative binding and efficient signal transduction.

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.

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.

Targeting the innate immune receptor TLR8 using small-molecule agents. Corrigendum.

Three of the figures in the article by Sakaniwa & Shimizu [(2020), Acta Cryst. D76, 621-629] were incorrectly annotated. Corrected figures are published here.

Rationally Designed Small-Molecule Inhibitors Targeting an Unconventional Pocket on the TLR8 Protein-Protein Interface.

Rational designs of small-molecule inhibitors targeting protein-protein interfaces have met little success. Herein, we have designed a series of triazole derivatives with a novel scaffold to specifically intervene with the interaction of TLR8 homomerization. In multiple assays, TH1027 was identified as a highly potent and specific inhibitor of TLR8. A successful solution of the X-ray crystal structure of TLR8 in complex with TH1027 provided an in-depth mechanistic insight into its binding mode, validating that TH1027 was located between two TLR8 monomers and recognized as an unconventional pocket, thereby preventing TLR8 from activation. Further biological evaluations showed that TH1027 dose-dependently suppressed the TLR8-mediated inflammatory responses in both human monocyte cell lines, peripheral blood mononuclear cells, and rheumatoid arthritis patient specimens, suggesting a strong therapeutic potential against autoimmune diseases.

Discovery of Novel Small Molecule Dual Inhibitors Targeting Toll-Like Receptors 7 and 8.

Endosomal toll-like receptors (TLRs) 7 and 8 recognize viral single-stranded RNAs, a class of imidazoquinoline compounds, 8-oxo-adenosines, 8-aminobenzodiazepines, pyrimidines, and guanosine analogues. Substantial evidence is present linking chronic inflammation mediated specifically by TLR7 to the progression of autoimmunity. We identified a new TLR7/8 dual inhibitor (1) and a TLR8-specific inhibitor (2) based on our previous screen targeting TLR8. Compound 1, bearing a benzanilide scaffold, was found to inhibit TLR7 and TLR8 at low micromolar concentrations. We envisioned making modifications on the benzanilide scaffold of 1 resulting in a class of highly specific TLR7 inhibitors. Our efforts led to the discovery of a new TLR8 inhibitor (CU-115) and identification of a TLR7/8 dual inhibitor (CU-72), bearing a distinct diphenyl ether skeleton, with potential for TLR7 selectivity optimization. Given the role of TLR8 in autoimmunity, we also optimized the potency of 2 and developed a new TLR8 inhibitor bearing a 1,3,4-oxadiazole motif.

Small-Molecule TLR8 Antagonists via Structure-Based Rational Design.

Rational design of drug-like small-molecule ligands based on structural information of proteins remains a significant challenge in chemical biology. In particular, designs targeting protein-protein interfaces have met little success given the dynamic nature of the protein surfaces. Herein, we utilized the structure of a small-molecule ligand in complex with Toll-like receptor 8 (TLR8) as a model system due to TLR8's clinical relevance. Overactivation of TLR8 has been suggested to play a prominent role in the pathogenesis of various autoimmune diseases; however, there are still few small-molecule antagonists available, and our rational designs led to the discovery of six exceptionally potent compounds with ∼picomolar IC50 values. Two X-ray crystallographic structures validated the contacts within the binding pocket. A variety of biological evaluations in cultured cell lines, human peripheral blood mononuclear cells, and splenocytes from human TLR8-transgenic mice further demonstrated these TLR8 inhibitors' high efficacy, suggesting strong therapeutic potential against autoimmune disorders.