Synthesis and structure-activity relationships of NG-acylated arylalkylguanidines and related compounds as histamine receptor ligands: searching for selective H4R agonists

In previous studies NG-acylated imidazolylpropylguanidines, originally developed as histamine H2 receptor (H2R) agonists with reduced basicity relative to guanidine-type H2R agonists, turned out to be also highly potent at the histamine H3 receptor (H3R) and histamine H4 receptor (H4R). Particularly at the H4R, the compounds showed high agonistic efficacies. This thesis aimed at the design, synthesis and pharmacological characterization of novel acylguanidine-type compounds and analogs to evaluate structure-activity relationships (SAR) at the histamine receptors (HRs) and to obtain HR subtype selective ligands. As the biological role of the recently discovered H4R is far from being understood, a major interest was the development of potent and selective H4R agonists as pharmacological tools. The prepared compounds were investigated for their pharmacological activities at the human (h) H1R, H2R, H3R and H4R in steady-state GTPase assays, employing membrane preparations of Sf9 insect cells expressing the distinct HR subtype. In addition, selected compounds were evaluated at the guinea pig (gp) ileum for H1R activity and at the guinea pig right atrium for H2R activity (in cooperation with Prof. Seifert (Medical School Hannover) and Prof. Elz (University of Regensburg)). In the first series of compounds the basic central group of the NG-acylated imidazolylpropylguanidines was replaced with a non-basic cyanoguanidine moiety resulting in moderate partial agonists at the hH4R. As efforts to improve potency and efficacy by modifying the second alkyl substituent besides the imidazolylpropyl group were unsuccessful, the focus was turned on the carbon chain connecting the imidazole ring and the cyanoguanidine group. Elongation of the spacer length by one methylene group was the key step toward potent and selective hH4R agonists. Out of these cyanoguanidines, 2-cyano-1-[4-(1H-imidazol-4-yl)butyl]-3-[(2-phenylthio)ethyl]guanidine (UR-PI376) was the most potent hH4R agonist (hH4R: EC50 = 34 nM, Emax = 0.93). Additionally, UR-PI376 showed a more than 25-fold selectivity over the hH3R in the GTPase assay and negligible activities at the hH1R and hH2R. In contrast to other reported selective H4R agonists, UR-PI376 exerted no agonistic activity at the other HR subtypes which makes this compound a promising pharmacological tool for studying the biological function of the hH4R. In addition, the carbamoylguanidine-type compound 2-carbamoyl-1-[2-(1H-imidazol-4-yl)ethyl]-3-(3-phenylpropyl)guanidine (UR-PI97) was identified as highly potent hH3R inverse agonist (KB = 3.8 nM, Emax = -0.97) showing a more than 300-fold selectivity over the hH4R. The next series was focused on the imidazolylpropylguanidine portion of the acylguanidines. Minor structural modifications of this group essentially changed the pharmacological activities at the HR subtypes. For example, all performed modifications abolished hH4R agonistic activity, whereas introduction of a methyl group in position 5 of the imidazole ring remarkably improved efficacy at the hH3R. The study showed the imidazolylalkylguanidine part to be a promising structural motif for the development of HR ligands with new pharmacological profiles. Substitution of the 4-imidazolyl ring with isomers and other heterocycles was critical for the activity at all HRs. The most promising compounds in this series were the 1H-1,2,4-triazol-3-yl analogs that showed (partial) agonistic activity at the hH2R and gpH2R and negligible activities at the other HR subtypes. This suggests the triazole ring to be promising bioisosteric imidazole replacement for the generation of new selective H2R agonists. Attaching small NG-alkanoyl residues to the imidazolylpropylguanidine moiety turned out to be favorable for hH4R activity. Potent (partial) hH4R agonists with up to 1000- and 100-fold selectivity relative to hH1R and hH2R, respectively, and only low efficacy at the hH3R were obtained. Due to the high potency of N1-[3-(1H-imidazol-4-yl)propyl]-N2-propionylguanidine at the hH3R and hH4R, this ligand was prepared as tritiated radioligand ([3H]UR-PI294). This readily available new radioligand specifically bound to the hH3R and hH4R with high affinities (KD (hH3R) = 1.1 nM, KD (hH4R) = 5.1 nM) and proved to be a valuable pharmacological tool for the determination of binding constants of H3R and H4R ligands. In conclusion, the structural modifications of NG-acylated imidazolylpropylguanidines yielded valuable information about SAR and structure-selectivity relationships and revealed new promising pharmacological tools for HR. The imidazolylbutylcyanoguanidines represent a new class of potent and selective hH4R agonists. Moreover, the carbamoylguanidine UR-PI97 was identified as a highly potent and selective hH3R inverse agonist. The synthesized NG-alkanoyl imidazolylpropylguanidines are among the most potent hH4R agonists and paved the way to the new high affinity hH3R and hH4R radioligand [3H]UR PI294