RT Journal Article SR Electronic T1 Structural Requirements for Inverse Agonism and Neutral Antagonism of Indole-, Benzimidazole-, and Thienopyrrole-Derived Histamine H4 Receptor Ligands JF Journal of Pharmacology and Experimental Therapeutics JO J Pharmacol Exp Ther FD American Society for Pharmacology and Experimental Therapeutics SP 513 OP 521 DO 10.1124/jpet.110.165977 VO 334 IS 2 A1 Erich H. Schneider A1 Andrea Strasser A1 Robin L. Thurmond A1 Roland Seifert YR 2010 UL http://jpet.aspetjournals.org/content/334/2/513.abstract AB The human histamine H4 receptor (hH4R), coexpressed with Gαi2 and Gβ1γ2 in Sf9 insect cells, is highly constitutively active, and thioperamide [THIO; N-cyclohexyl-4-(imidazol-4-yl)-1-piperidinecarbothioamide] is one of the most efficacious hH4R inverse agonists. High constitutive hH4R activity may have pathophysiological implications in which case inverse agonists may behave differently than neutral antagonists. To learn more about the structural requirements for hH4R inverse agonism, we investigated 25 compounds (indole, benzimidazole, and thienopyrrole derivatives) structurally related to the standard antagonist JNJ-7777120 [1-[(5-chloro-1H-indol-2-yl)carbonyl]-4-methyl-piperazine]. We characterized the compounds in radioligand binding assays by using [3H]histamine ([3H]HA) and in steady-state GTPase assays in the presence (antagonist mode) and absence (inverse agonist mode) of the agonist HA, yielding the following results: 1) Twenty-two compounds were inverse agonists (efficacy: 15–62% of the THIO effect), and only three compounds (12%) showed neutral antagonism. Thus, inverse agonism is far more common than neutral antagonism. 2) The inverse agonistic efficacy of the R5-monosubstituted indole-derived compounds increased with the volume of R5. R5 may interact with Trp6.48 of the rotamer toggle switch and stabilize the inactive receptor conformation. 3) A subset of compounds showed large differences between the Ki value from [3H]HA competition binding and the EC50 value from steady-state GTPase assays, whereas the Kb values were closer to the Ki values. Thus, the two-state model should be extended to a model comprising a constitutively active hH4R state, which can be discriminated by inverse agonists from a structurally distinct HA-stabilized active state.