Structure‐Based Design of Novel Human Toll‐like Receptor 8 Agonists

Abstract

Toll-like receptor (TLR)-8 agonists activate adaptive immune responses by inducing robust production of T helper 1-polarizing cytokines, suggesting that TLR8-active compounds may be promising candidate adjuvants. We recently reported pure TLR8 agonistic activity in a C2-butyl furo[2,3-c]quinoline. We have obtained the structure of human TLR8 ectodomain co-crystallized with the furoquinoline compound, which indicates ligand-induced reorganization of the binding pocket of TLR8. The loss of a key H-bond between the oxygen atom of the furanyl ring of the agonist and Thr574 in TLR8 suggested that the furan ring was dispensable. We employed a disconnection strategy and examined 3- and 4-substituted aminoquinolines. Focused structure-based ligand design studies led to the identification of 3-pentyl-quinoline-2-amine as a novel, structurally simple, and highly potent human TLR8-specific agonist.

Human Toll-Like Receptor 8-Selective Agonistic Activities in 1-Alkyl-1H-benzimidazol-2-amines

“…28 Aliquots of PBMCs (10 5 cells in 100 μL/well) were stimulated for 12 h with graded concentrations of test compounds. Supernatants were isolated by centrifugation and were assayed in duplicates using analyte-specific multiplexed cytokine/chemokine bead array assays as reported by us previously.…”

“…28 We now report detailed structure–activity relationships in the ring-contracted 1-alkyl-1 H -benzimidazol-2-amines. The best-in-class compound of this novel chemotype, 4-methyl-1-pentyl-1 H -benzo[ d ]imidazol-2-amine, was found to retain a pure TLR8 agonistic activity profile.…”

“…Our discovery of the 2,3-diamino-furo[2,3- c ]pyridines 36 and 4-amino-furo[2,3- c ]quinolines, 32 leading to structure-based design of the 3-alkyl-quinoline-2-amines 28 as pure TLR8 agonists with no detectable TLR7 activity, was also motivated by observations that the strongly Th1-biasing TLR8 agonists could be useful as candidate vaccine adjuvants for the newborn. 53,54 Maternal immunoglobulins acquired by passive transplacental passage confer protection to the neonate for the first few weeks of life; 55 thereafter, the newborn is susceptible to a wide range of pathogens until early infancy.…”

Enrichment Assessment of Multiple Virtual Screening Strategies for Toll‐Like Receptor 8 Agonists Based on a Maximal Unbiased Benchmarking Data Set

“…According to the study of Tanji et al (12) essential interactions of agonists with TLR8 are the benzene rings of ligands stacked on the side chain of Phe405; hydrogen bonds between amidine group of the quinoline moiety and the side chain of Asp543, N atoms of the imidazole or thiazole moieties, and the Thr574 N atoms; and hydrophobic interaction between 2-propyl and the region of Phe346, Tyr348, Gly376, Val378, Ile403, Phe405, Gly572*, and Val573*. What is more, Kokatla et al (14) demonstrated that the key hydrogen bond with Thr574 was dispensable in purely TLR8 agonistic activity. Therefore, we set the hydrophobic centroid at the C2 substituent, the hydrogen donor, and one of the aromatic rings to be the three fixed pharmacophore features, leaving other features to be edited flexibly.…”

“…As yet, current experimental studies on TLR8 agonists are focused on limited chemotypes, and the information of TLR8 agonist binding has not been fully exploited. Recently, six X-ray crystal structures of TLR8 in complex with cognate agonists are available in Protein Data Bank (PDB IDs: 3W3J, 3W3K, 3W3N, 3WN4, 4Q8Z, and 4QC0) (12)(13)(14). Therefore, it is of great interest to identify novel TLR8 smallmolecule agonists through in silico strategies.…”