SAR exploration of the non-imidazole histamine H₃ receptor ligand ZEL-H16 reveals potent inverse agonism

Histamine H-3 receptor (H3R) agonists without an imidazole moiety remain very scarce. Of these, ZEL-H16 (1) has been reported previously as a high-affinity non-imidazole H3R (partial) agonist. Our structure-activity relationship analysis using derivatives of 1 identified both basic moieties as key interaction motifs and the distance of these from the central core as a determinant for H3R affinity. However, in spite of the reported H3R (partial) agonism, in our hands, 1 acts as an inverse agonist for G alpha(i) signaling in a CRE-luciferase reporter gene assay and using an H3R conformational sensor. Inverse agonism was also observed for all of the synthesized derivatives of 1. Docking studies and molecular dynamics simulations suggest ionic interactions/hydrogen bonds to H3R residues D114(3.32) and E206(5.46) as essential interaction points.