Further hit optimization of 6-(trifluoromethyl)pyrimidin-2-amine based TLR8 modulators: Synthesis, biological evaluation and structure-activity relationships.

Toll-like receptor 8 (TLR8) is an endosomal TLR that has an important role in the innate human immune system, which is involved in numerous pathological conditions. Excessive activation of TLR8 can lead to inflammatory and autoimmune diseases, which highlights the need for development of TLR8 modulators. However, only a few small-molecule modulators that selectively target TLR8 have been developed. Here, we report the synthesis and systematic investigation of the structure-activity relationships of a series of novel TLR8 negative modulators based on previously reported 6-(trifluoromethyl)pyrimidin-2-amine derivatives. Four compounds showed low-micromolar concentration-dependent inhibition of TLR8-mediated signaling in HEK293 cells. These data confirm that the 6-trifluoromethyl group and two other substituents on positions 2 and 4 are important structural elements of pyrimidine-based TLR8 modulators. Substitution of the main scaffold at position 2 with a methylsulfonyl group or para hydroxy/hydroxymethyl substituted benzylamine is essential for potent negative modulation of TLR8. Our best-in-class TLR8-selective modulator 53 with IC50 value of 6.2 µM represents a promising small-molecule chemical probe for further optimization to a lead compound with potent immunomodulatory properties.

Identification and validation of a novel dual small-molecule TLR2/8 antagonist.

Toll-like receptor 2 (TLR2) and TLR8 are involved in the recognition of bacterial and viral components and are linked not only to protective antimicrobial immunity but also to inflammatory diseases. Recently, increasing attention has been paid to the receptor crosstalk between TLR2 and TLR8 to fine-tune innate immune responses. In this study, we report a novel dual TLR2/TLR8 antagonist, compound 24 that was developed by a modeling-guided synthesis approach. The modulator was optimized from the previously reported 1,3-benzothiazole derivative, compound 8. Compound 24 was pharmacologically characterized for the ability to inhibit TLR2- and TLR8-mediated responses in TLR-overexpressing reporter cells and THP-1 macrophages. The modulator showed high efficacy with IC50 values in the low micromolar range for both TLRs, selectivity towards other TLRs and low cytotoxicity. At TLR2, a slight predominance for the TLR2/1 heterodimer was found in reporter cells selectively expressing TLR2/1 or TLR2/6 heterodimers. Concentration ratio analysis in the presence of Pam3CSK4 or Pam2CSK4 indicated non-competitive antagonist behavior at hTLR2. In computational docking studies, a plausible alternative binding mode of compound 24 was predicted for both TLR2 and TLR8. Our results provide evidence that it is feasible to simultaneously and selectively target endosomal- and surface-located TLRs. We identified a small-molecule dual TLR2/8 antagonist that may serve as a valuable pharmacological tool to decipher the role of TLR2/8 co-signaling in inflammation.

Identification and characterization of a novel chemotype for human TLR8 inhibitors.

The endosomal Toll-like receptor 8 (TLR8) recognizes single-stranded RNA and initiates early inflammatory responses. Despite the importance of endosomal TLRs for human host defense against microbial pathogens, extensive activation may contribute to autoimmune and inflammatory diseases. In contrast to the recent progress made in the development of modulators of plasma membrane-bound TLRs, little is known about endosomal TLR modulation and very few TLR8 inhibitors have been reported. In this study, we discovered and validated novel small-molecule TLR8 inhibitors. Fourteen potential TLR8 modulators were experimentally validated in HEK293T cells stably overexpressing human TLR8 and THP-1 macrophages. Five compounds inhibited TLR8-mediated signaling, representing a hit rate of 36%. The three most potent compounds neither cause cellular toxicity nor inhibition of TLR signaling induced by other receptor subtypes. Conclusively, we experimentally confirm novel and selective, pyrimidine-based TLR8 inhibitors with low cytotoxicity that are relevant candidates for lead optimization and further mechanistic studies.