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.

Toll 样受体 8 (TLR8) 是一种内体 TLR，在人类先天免疫系统中具有重要作用，涉及多种病理状况。TLR8 的过度激活会导致炎症和自身免疫性疾病，这凸显了开发 TLR8 调节剂的必要性。然而，仅开发了少数选择性靶向 TLR8 的小分子调节剂。在这里，我们报告了基于先前报道的 6-(三氟甲基)嘧啶-2-胺衍生物的一系列新型 TLR8 负调节剂的构效关系的合成和系统研究。四种化合物在 HEK293 细胞中显示出对 TLR8 介导的信号传导的低微摩尔浓度依赖性抑制。这些数据证实 6-三氟甲基和 2 位和 4 位上的其他两个取代基是嘧啶基 TLR8 调节剂的重要结构元素。用甲基磺酰基取代位置 2 的主支架或对羟基/羟甲基取代的苄胺对于 TLR8 的有效负调节至关重要。我们同类最佳的 TLR8 选择性调节剂53具有 6.2 μM 的 IC ₅₀值，代表了一种有前途的小分子化学探针，可进一步优化具有有效免疫调节特性的先导化合物。