Discovery of a G Protein-Biased Radioligand for the Histamine H2 Receptor with Reversible Binding Properties

Currently employed histamine H2 receptor (H2R) radioligands possess several drawbacks, for example, high non-specificity, insurmountable binding, or short half-life. We report the synthesis and the chemical and pharmacological characterization of the highly stable carbamoylguanidine-type radioligand [3H]­UR-KAT479 ([3H]23), a subtype selective histamine H2 receptor G protein-biased agonist. [3H]23 was characterized by saturation, kinetic, and competition binding assays at the human, guinea pig, and mouse H2 receptors (co-)­expressed in HEK293­(T) cells. [3H]23 reversibly bound to the respective H2Rs with moderate to high affinity (human/guinea pig/mouse Kd: 24/28/94 nM). In order to investigate the applicability of carbamoylguanidine-type ligands in animal studies elucidating the role of the H2R in the brain, we performed a preliminary partitioning experiment in the whole human/mouse blood, which indicated a low binding of [3H]23 to red blood cells. These properties turn [3H]23 into a powerful tool for the determination of binding affinities and demonstrate the promising pharmacokinetic profile of carbamoylguanidine-type ligands.