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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.

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.

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 analog...