Pharmacological characterization of seven human histamine H receptor isoforms.

The histamine H receptor (HR) regulates as a presynaptic G protein-coupled receptor the release of histamine and other neurotransmitters in the brain, and is consequently a potential therapeutic target for neuronal disorders. The human HR encodes for seven splice variants that vary in the length of intracellular loop 3 and/or the C-terminal tail but are all able to induce heterotrimeric G protein signaling. The last two decades HR drug discovery and lead optimization has been exclusively focused on the 445 amino acids-long reference isoform HR-445. In this study, we pharmacologically characterized for the first time all seven HR isoforms by determining their binding affinities for reference histamine H receptor agonists and inverse agonists. The HR-453, HR-415, and HR-413 isoforms display similar binding affinities for all ligands as the HR-445. However, increased agonist binding affinities were observed for the three shorter isoforms HR-329, HR-365, and HR-373, whereas inverse agonists such as the approved anti-narcolepsy drug pitolisant (Wakix®) displayed significantly decreased binding affinities for the latter two isoforms. This opposite change in binding affinity of agonist versus inverse agonists on HR-365 and HR-373 is associated with their higher constitutive activity in a cAMP biosensor assay as compared to the other five isoforms. The observed differences in pharmacology between longer and shorter HR isoforms should be considered in future drug discovery programs.