Fluorescent H Receptor Squaramide-Type Antagonists: Synthesis, Characterization, and Applications

Fluorescence labeled ligands have been gaining importance as molecular tools, enabling receptor-ligand-binding studies by various fluorescence-based techniques. Aiming at red-emitting fluorescent ligands for the hHR, a series of squaramides labeled with pyridinium or cyanine fluorophores (-) was synthesized and characterized. The highest hHR affinities in radioligand competition binding assays were obtained in the case of pyridinium labeled antagonists - (p : 7.71-7.76) and cyanine labeled antagonists and (p : 7.67, 7.11). These fluorescent ligands proved to be useful tools for binding studies (saturation and competition binding as well as kinetic experiments), using confocal microscopy, flow cytometry, and high content imaging. Saturation binding experiments revealed p values comparable to the p values. The fluorescent probes , , and could be used to localize H receptors in HEK cells and to determine the binding affinities of unlabeled compounds.