Structure-Activity Relationship of Hetarylpropylguanidines Aiming at the Development of Selective Histamine Receptor Ligands†

Histamine H2 receptor radioligands: triumphs and challenges

Since the discovery of the histamine H₂ receptor (H₂R), radioligands were among the most powerful tools to investigate its role and function. Initially, radiolabeling was used to investigate human and rodent tissues regarding their receptor expression. Later, radioligands gained increasing significance as pharmacological tools in in vitro assays. Although tritium-labeling was mainly used for this purpose, labeling with carbon-14 is preferred for metabolic studies of drug candidates. After the more-or-less successful application of numerous labeled H₂R antagonists, the recent development of the G protein-biased radioligand [³H]UR-KAT479 represents another step forward to elucidate the widely unknown role of the H₂R in the central nervous system through future studies.

Pharmacological characterization of a new series of carbamoylguanidines reveals potent agonism at the H2R and D3R

Even today, the role of the histamine H₂ receptor (H₂R) in the central nervous system (CNS) is widely unknown. In previous research, many dimeric, high-affinity and subtype-selective carbamoylguanidine-type ligands such as UR-NK22 (5, pK_i = 8.07) were reported as H₂R agonists. However, their applicability to the study of the H₂R in the CNS is compromised by their molecular and pharmacokinetic properties, such as high molecular weight and, consequently, a limited bioavailability. To address the need for more drug-like H₂R agonists with high affinity, we synthesized a series of monomeric (thio)carbamoylguanidine-type ligands containing various spacers and side-chain moieties. This structural simplification resulted in potent (partial) agonists (guinea pig right atrium, [³⁵S]GTPγS and β-arrestin2 recruitment assays) with human (h) H₂R affinities in the one-digit nanomolar range (pK_i (139, UR-KAT523): 8.35; pK_i (157, UR-MB-69): 8.69). Most of the compounds presented here exhibited an excellent selectivity profile towards the hH₂R, e.g. 157 being at least 3800-fold selective within the histamine receptor family. The structural similarities of our monomeric ligands to pramipexole (6), a dopamine receptor agonist, suggested an investigation of the binding behavior at those receptors. The target compounds were (partial) agonists with moderate affinity at the hD_2longR and agonists with high affinity at the hD₃R (e.g. pK_i (139, UR-KAT523): 7.80; pK_i (157, UR-MB-69): 8.06). In summary, we developed a series of novel, more drug-like H₂R and D₃R agonists for the application in recombinant systems in which either the H₂R or the D₃R is solely expressed. Furthermore, our ligands are promising lead compounds in the development of selective H₂R agonists for future in vivo studies or experiments utilizing primary tissue to unravel the role and function of the H₂R in the CNS.

Structure-Activity Relationship of Hetarylpropylguanidines Aiming at the Development of Selective Histamine Receptor Ligands†

New classes of alkylated hetarylpropylguanidines with different functionality and variation in spacer length were synthesized to determine their behavior at the four histamine receptor (H1R, H2R, H3R, H4R) subtypes. Alkylated guanidines with different terminal functional groups and varied basicity, like amine, guanidine and urea were developed, based on the lead structure SK&F 91486 (2). Furthermore, heteroatomic exchange at the guanidine structure of 2 led to simple analogues of the lead compound. Radioassays at all histamine receptor subtypes were accomplished, as well as organ bath studies at the guinea pig (gp) ileum (gpH1R) and right atrium (gpH2R). Ligands with terminal functionalization led to, partially, highly affine and potent structures (two digit nanomolar), which showed up a bad selectivity profile within the histamine receptor family. While the benzoylurea derivative 144 demonstrated a preference towards the human (h) H3R, S-methylisothiourea analogue 143 obtained high affinity at the hH4R (pKi=8.14) with moderate selectivity. The molecular basis of the latter finding was supported by computational studies.