Development of a Conformational Histamine H₃ Receptor Biosensor for the Synchronous Screening of Agonists and Inverse Agonists

The histamine H3 receptor (H3R) represents a highly attractive drug target for the treatment of various central nervous system disorders, but the discovery of novel H3R targeting compounds relies on the assessment of highly amplified intracellular signaling events that do not only reflect H3R modulation and carry the risk of high false-positive and -negative screening rates. To address these limitations, we designed an intramolecular H3R biosensor based on the principle of bioluminescence resonance energy transfer (BRET) that reports the receptor's real-time conformational dynamics and provides an advanced tool to screen for both H3R agonists and inverse agonists in a live cell screening-compatible assay format. This conformational G-protein-coupled receptor (GPCR) sensor allowed us to characterize the pharmacological properties of known and new H3 receptor ligands with unprecedented accuracy. Interestingly, we found that one newly developed H3 receptor ligand possesses even stronger inverse agonistic activity than reference H3R inverse agonists including the current gold standard pitolisant. Taken together, we describe here the design and validation of the first screening-compatible H3R conformational biosensor that will aid in the discovery of novel H3R ligands and can be employed to gain deeper insights into the (in-)activation mechanism of this highly attractive drug target.