Shining light on the histamine H₂ receptor: Synthesis of carbamoylguanidine-type agonists as a pharmacological tool...

So far, only little is known about the internalization process of the histamine H₂ receptor (H₂R). One promising approach to study such dynamic processes is the use of agonistic fluorescent ligands. Therefore, a series of carbamoylguanidine-type H₂R agonists containing various fluorophores, heterocycles, and linkers (28-40) was synthesized. The ligands were pharmacologically characterized in several binding and functional assays. These studies revealed a significantly biased efficacy (E_max) for some of the compounds, e.g. 32: whereas 32 acted as strong partial (E_max: 0.77, mini-Gs recruitment) or full agonist (E_max: 1.04, [³⁵S]GTPγS binding) with respect to G protein activation, it was only a weak partial agonist regarding β-arrestin1/2 recruitment (E_max: 0.09-0.12) and failed to promote H₂R internalization (confocal microscopy). On the other hand, H₂R internalization was observed for compounds that exhibited moderate agonistic activity in the β-arrestin1/2 pathways (E_max ≥ 0.22). The presented differently-biased fluorescent ligands are versatile molecular tools for future H₂R studies on receptor trafficking and internalization e.g. using fluorescence microscopy.