br Histamine H R The cloning of the H
Histamine H4R The cloning of the H3R (Lovenberg et al., 1999) provided a template for the search of other brincidofovir receptors, and resulted in the most recent discovery of histamine H4R. This concluded with six independent groups that have reported the cloning of the H4R (Leurs et al., 2009, Liu et al., 2001, Morse et al., 2001, Nakamura et al., 2000, O'Reilly et al., 2002, Oda et al., 2000, Zhu et al., 2001). Since that time much research has dedicated on the characterization of the receptor's function in inflammatory process since it initially appeared to be mainly expressed in hematopoietic cells. Interestingly, ligand-supported homology models of the hH4R were developed based on the crystal structure of bovine rhodopsin and different known H4R ligands (histamine, OUP-16, JN-J7777120). It has been found that two agonists (histamine and OUP-16) form complementary interactions with Asp94, Glu182 and Thr323, whereas the reference antagonists JNJ-7777120 interacts with Asp94 and Glu182 only (Fig. 2D) (Pappalardo et al., 2014). These important results suggest a role of Thr323 in ligand binding and likely also in receptor activation. Accordingly, interesting complementarities can be found between the properties of hH4R antagonists and their receptor binding pocket. The antagonists contain at least one positively charged group, preferably two. In the hH4R binding pocket, these features are complementary to two negatively charged residues, namely Asp94 in TM3 and Glu182 in TM5, known to be important for the interaction of high affinity ligands (Fig. 2D) (Pappalardo et al., 2014). A large variety of known ligands of the H3R, particularly imidazole-containing compounds, also possess significant affinity for the H4R. These include (R)-α-methylhistamine (28), imetit (31), immepip (32) and clobenpropit (36), which all exhibit agonist activity at the H4R (Fig. 5). Unlikely, the H3R neutral antagonist thioperamide (35) was revealed to be an inverse agonist of the H4R (Lim et al., 2005, Liu et al., 2001, Oda et al., 2000).
H4R agonists The initially described H4R agonists were methylcyanoguanidine derivatives of 2,5-disubstituted tetrahydrofuranylimidazoles. These compounds show moderate affinity for the H4R and some selectivity over H3R (Lim et al., 2005). The best example is OUP-16 (49) with a high binding affinity for the human H4R and an 18-fold selectivity over H3R (Hashimoto et al., 2003, Leurs et al., 2009). Although firstly developed as a H2R-ligand, 4(5)-methylhistamine (16) is the most selective H4R agonist presently described, with more than 100-fold selectivity over the H1R, H2R and H3R. Interestingly, structural modification of dimaprit (18), a non-imidazol with mixed affinity for H2R/H3R/H4R, led to the discovery of VUF-8430 (50), a full agonist at the H4R with around 30-fold selectivity over the H3R (Lim et al., 2006). Noticeably, the aminopyrimidine ligand ST-1006 (51) has been characterized as the most potent H4R agonist reported to date, with a PEC50 of 8.95 (Fig. 6) (Gschwandtner et al., 2013).
H4R antagonists The first potent and selective non-imidazole H4 antagonist was the indole carboxamide compound JNJ-7777120 (52). It was described to bind to the human H4R with high affinity and has excellent selectivity over a broad variety of GPCRs, including the H1R, H2R and H3R (Thurmond, 2015, Thurmond et al., 2004). Also, JNJ-7777120 possesses high affinity for the mouse and rat H4R (Thurmond, 2015, Thurmond et al., 2004) and it has been used widely as the reference compound to determine the role of H4R antagonism in a range of experimental animal models of disease (Gschwandtner et al., 2013, Thurmond, 2015, Thurmond et al., 2004). Following the structural development of JNJ-7777120, a range of structurally diverse classes have been developed, as exemplified by the benzimidazole VUF-6002, also known as JNJ-10191584 (53) (Terzioglu et al., 2004) and the thienopyrrole derivative (54) (Fig. 6) (Venable et al., 2005). Both compounds 53 and 54 revealed binding affinities close to that of JNJ-7777120. Further exploration of the SAR led to the identification of piperazin-1-yl quinoxalines, including VUF-10214 (55) having retained good H4R potency and demonstrated anti-inflammatory efficacy in a rat model of carrageenan-induced paw oedema (Smits et al., 2008).