Covalent Inhibition of the Histamine H(3) Receptor

Covalent binding of G protein-coupled receptors by small molecules is a useful approach for better understanding of the structure and function of these proteins. We designed, synthesized and characterized a series of 6 potential covalent ligands for the histamine H(3) receptor (H(3)R). Starting from a 2-amino-pyrimidine scaffold, optimization of anchor moiety and warhead followed by fine-tuning of the required reactivity via scaffold hopping resulted in the isothiocyanate H(3)R ligand 44. It shows high reactivity toward glutathione combined with appropriate stability in water and reacts selectively with the cysteine sidechain in a model nonapeptide equipped with nucleophilic residues. The covalent interaction of 44 with H(3)R was validated with washout experiments and leads to inverse agonism on H(3)R. Irreversible binder 44 (VUF15662) may serve as a useful tool compound to stabilize the inactive H(3)R conformation and to study the consequences of prolonged inhibition of the H(3)R.