Triazole ligands reveal distinct molecular features that induce histaine H₄ receptor affinity and subtly govern H₄/H₃ subtype selectivity

The histamine H₃ (H₃R) and H₄ (H ₄R) receptors attract considerable interest from the medicinal chemistry community. Given their relatively high homology yet widely differing therapeutic promises, ligand selectivity for the two receptors is crucial. We interrogated H₄R/H₃R selectivities using ligands with a [1,2,3]triazole core. Cu(I)-assisted "click chemistry" was used to assemble diverse [1,2,3]triazole compounds (6a-w and 7a-f), many containing a peripheral imidazole group. The imidazole ring posed some problems in the click chemistry putatively due to Cu(II) coordination, but Boc protection of the imidazole and removal of oxygen from the reaction mixture provided effective strategies. Pharmacological studies revealed two monosubstituted imidazoles (6h,p) with <10 nM H₄R affinities and >10-fold H ₄R/H₃R selectivity. Both compounds possess a cycloalkylmethyl group and appear to target a lipophilic pocket in H ₄R with high steric precision. The use of the [1,2,3]triazole scaffold is further demonstrated by the notion that simple changes in spacer length or peripheral groups can reverse the selectivity toward H₃R. Computational evidence is provided to account for two key selectivity switches and to pinpoint a lipophilic pocket as an important handle for H₄R over H₃R selectivity.