Ligand binding kinetics at histamine H3 receptors by fluorescence-polarization with real-time monitoring

Abstract

Growing evidence recommends incorporating the concept of drug-target residence times within drug development and screening programs. For many targets, systematic research for binding kinetics is emerging and reported, as in case of the histamine H3 receptor. Alternatively, fluorescent methods based on Foerster resonance energy transfer have been reported recently but application of fluorescence polarization to kinetics of unlabeled ligands is not known to us. Thus, we established a radiolabel-free, real-time resolving method that is compatible to high-throughput-screening programs with the objective to explore the underlying binding kinetics. This method takes benefit of bodilisant as H3 receptor ligand. Thereby, we detected short residence times around 5 min for the H3 receptor ligands ciproxifan, clobenpropit, thioperamide as well as pitolisant. Monitoring association rates, remarkably slower association rate constants were examined for ciproxifan and thioperamide when compared to those of pitolisant or clobenpropit. The affinities for the ligands derived by the kinetic approach differ from affinity estimates in literature using radiolabeled agonists in displacement assays. Further investigation raised exceptional pharmacological properties, consistent with occurrence of secondary binding sites at the H3 receptor. Validation of resulting affinity constants was successfully performed by displacement assays based on fluorescence polarization with bodilisant.