[3H]UR-DEBa176: A 2,4-Diaminopyrimidine-Type Radioligand Enabling Binding Studies at the Human, Mouse, and Rat Histamine H4 Receptors

Differences in sequence homology between human (h), mouse (m), and rat (r) histamine H4 receptors (H4R) cause discrepancies regarding affinities, potencies, and/or efficacies of ligands and therefore compromise translational animal models and the applicability of radioligands. Aiming at a radioligand enabling robust and comparative binding studies at the h/m/rH4Rs, 2,4-diaminopyrimidines were synthesized and pharmacologically investigated. The most notable compounds identified were two (partial) agonists with comparable potencies at the h/m/rH4Rs: UR-DEBa148 (N-neopentyl-4-(1,4,6,7-tetrahydro-5H-imidazo­[4,5-c]­pyridin-5-yl)­pyrimidin-2-amine bis­(2,2,2-trifluoroacetate), 43), the most potent [pEC50 (reporter gene assay) = 9.9/9.6/10.3] compound in the series being slightly G-protein biased and UR-DEBa176 [(R)-4-[3-(dimethylamino)­pyrrolidin-1-yl]-N-neopentylpyrimidin-2-amine bis­(2,2,2-trifluoroacetate), 46, pEC50 (reporter gene assay) = 8.7/9.0/9.2], a potential “cold” form of a tritiated H4R ligand. After radiolabeling, binding studies with [3H]­UR-DEBa176 ([3H]46) at the h/m/rH4Rs revealed comparable Kd values (41/17/22 nM), low nonspecific binding (11–17%, ∼Kd), and fast associations/dissociations (25–30 min) and disclosed [3H]­UR-DEBa176 as useful molecular tool to determine h/m/rH4R binding affinities for H4R ligands.