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

Differences in sequence homology between human (h), mouse (m), and rat (r) histamine H receptors (HR) 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/rHRs, 2,4-diaminopyrimidines were synthesized and pharmacologically investigated. The most notable compounds identified were two (partial) agonists with comparable potencies at the h/m/rHRs: UR-DEBa148 (-neopentyl-4-(1,4,6,7-tetrahydro-5-imidazo[4,5-]pyridin-5-yl)pyrimidin-2-amine bis(2,2,2-trifluoroacetate), ), the most potent [pEC (reporter gene assay) = 9.9/9.6/10.3] compound in the series being slightly G-protein biased and UR-DEBa176 [()-4-[3-(dimethylamino)pyrrolidin-1-yl]--neopentylpyrimidin-2-amine bis(2,2,2-trifluoroacetate), , pEC (reporter gene assay) = 8.7/9.0/9.2], a potential "cold" form of a tritiated HR ligand. After radiolabeling, binding studies with [H]UR-DEBa176 ([H]) at the h/m/rHRs revealed comparable values (41/17/22 nM), low nonspecific binding (11-17%, ∼), and fast associations/dissociations (25-30 min) and disclosed [H]UR-DEBa176 as useful molecular tool to determine h/m/rHR binding affinities for HR ligands.