Detailed analysis of biased histamine H4 receptor signalling by JNJ 7777120 analogues

Background and Purpose The histamine H-4 receptor, originally thought to signal merely through G(i) proteins, has recently been shown to also recruit and signal via -arrestin2. Following the discovery that the reference antagonist indolecarboxamide JNJ 7777120 appears to be a partial agonist in -arrestin2 recruitment, we have identified additional biased hH(4)R ligands that preferentially couple to G(i) or -arrestin2 proteins. In this study, we explored ligand and receptor regions that are important for biased hH(4)R signalling. Experimental Approach We evaluated a series of 48 indolecarboxamides with subtle structural differences for their ability to induce hH(4)R-mediated G(i) protein signalling or -arrestin2 recruitment. Subsequently, a Fingerprints for Ligands and Proteins three-dimensional quantitative structure-activity relationship analysis correlated intrinsic activity values with structural ligand requirements. Moreover, a hH(4)R homology model was used to identify receptor regions important for biased hH(4)R signalling. Key Results One indolecarboxamide (75) with a nitro substituent on position R7 of the aromatic ring displayed an equal preference for the G(i) and -arrestin2 pathway and was classified as unbiased hH(4)R ligand. The other 47 indolecarboxamides were -arrestin2-biased agonists. Intrinsic activities of the unbiased as well as -arrestin2-biased indolecarboxamides to induce -arrestin2 recruitment could be correlated with different ligand features and hH(4)R regions. Conclusion and Implications Small structural modifications resulted in diverse intrinsic activities for unbiased (75) and -arrestin2-biased indolecarboxamides. Analysis of ligand and receptor features revealed efficacy hotspots responsible for biased--arrestin2 recruitment. This knowledge is useful for the design of hH(4)R ligands with biased intrinsic activities and aids our understanding of the mechanism of H4R activation.