Highly Potent, Stable, and Selective Dimeric Hetarylpropylguanidine-Type Histamine H2 Receptor Agonists

Cryo-EM structure of cell-free synthesized human histamine 2 receptor/Gs complex in nanodisc environment

Here we describe the cryo-electron microscopy structure of the human histamine 2 receptor (H2R) in an active conformation with bound histamine and in complex with Gs heterotrimeric protein at an overall resolution of 3.4 Å. The complex was generated by cotranslational insertion of the receptor into preformed nanodisc membranes using cell-free synthesis in E. coli lysates. Structural comparison with the inactive conformation of H2R and the inactive and Gq-coupled active state of H1R together with structure-guided functional experiments reveal molecular insights into the specificity of ligand binding and G protein coupling for this receptor family. We demonstrate lipid-modulated folding of cell-free synthesized H2R, its agonist-dependent internalization and its interaction with endogenously synthesized H1R and H2R in HEK293 cells by applying a recently developed nanotransfer technique.

Clonidine stimulates force of contraction via histamine H2 receptors in the human atrium

Clonidine has various clinical effects mediated by agonism of α1- or α2-adrenoceptors and the blocking of hyperpolarization-activated-nucleotide-gated pacemaker channels (HCN). It is unknown whether clonidine can also stimulate human cardiac histamine H2 receptors (hH2Rs). We used isolated electrically stimulated left and spontaneously beating right atrial preparations from mice overexpressing the hH2R specifically in the heart (H2-TG), and spontaneously beating right atrial preparations of guinea pigs for comparison. Moreover, we studied isolated electrically stimulated muscle strips from the human right atrium. Clonidine (1, 3, and 10 µM) increased force of contraction in isolated left atrial preparations from H2-TG mice. In contrast, clonidine reduced the spontaneous beating rate in right atrial preparations from H2-TG. Clonidine raised the beating rate in guinea pig right atrial preparations. Clonidine failed to increase the force of contraction but reduced beating rate in wild-type litter mate mice (WT). In WT, histamine failed to increase the force of contraction in left atrial preparations and beating rate in right atrial preparations. Clonidine (10 µM) increased the force of contraction in isolated human right atrial preparations. The positive inotropic effect in the human atrium was attenuated by cimetidine (10 µM). Clonidine increased the beating rate of the isolated spontaneously beating guinea pig right atrium and acted as a H2R partial agonist. Furthermore, clonidine showed binding to the guinea pig H2R (100 µM) using HEK cells in a recombinant expression system (pKi < 4.5) but hardly to the human H2R. These data suggest that clonidine can functionally activate cardiac human H2R.

Lysergic acid diethylamide stimulates cardiac human H2 histamine and cardiac human 5-HT4-serotonin receptors

Lysergic acid diethylamide (LSD) is an artificial hallucinogenic drug. Thus, we hypothesized that LSD might act 5-HT4 serotonin receptors and/or H2 histamine receptors. We studied isolated electrically stimulated left atrial preparations, spontaneously beating right atrial preparations, and spontaneously beating Langendorff-perfused hearts from transgenic mice with cardiomyocyte-specific overexpression of the human 5-HT4 receptor (5-HT4-TG) or of the H2-histamine receptor (H2-TG). For comparison, we used wild type littermate mice (WT). Finally, we measured isometric force of contraction in isolated electrically stimulated muscle strips from the human right atrium obtained from patients during bypass surgery. LSD (up to 10 µM) concentration dependently increased force of contraction and beating rate in left or right atrial preparations from 5-HT4-TG (n = 6, p < 0.05) in 5-HT4-TG atrial preparations. The inotropic and chronotropic effects of LSD were antagonized by 10 µM tropisetron in 5-HT4-TG. In contrast, LSD (10 µM) increased force of contraction and beating rate in left or right atrial preparations, from H2-TG. After pre-stimulation with cilostamide (1 µM), LSD (10 µM) increased force of contraction in human atrial preparations (n = 6, p < 0.05). The contractile effects of LSD in human atrial preparations could be antagonized by 10 µM cimetidine and 1 µM GR 125487. LSD leads to H2-histamine receptor and 5-HT4-receptor mediated cardiac effects in humans.

Ergometrine stimulates histamine H2 receptors in the isolated human atrium

Ergometrine (6aR,9R)-N-((S)-1-hydroxypropan-2-yl)-7-methyl-4,6,6a,7,8,9-hexa-hydro-indolo-[4,3-fg]chinolin-9-carboxamide or lysergide acid β-ethanolamide or ergonovine) activates several types of serotonin and histamine receptors in the animal heart. We thus examined whether ergometrine can activate human serotonin 5-HT4 receptors (h5-HT4R) and/or human histamine H2 receptors (hH2R) in the heart of transgenic mice and/or in the human isolated atrium. Force of contraction or beating rates were studied in electrically stimulated left atrial or spontaneously beating right atrial preparations or spontaneously beating isolated retrogradely perfused hearts (Langendorff setup) of mice with cardiac specific overexpression of the h5-HT4R (5-HT4-TG) or of mice with cardiac specific overexpression of the hH2R (H2-TG) or in electrically stimulated human right atrial preparations obtained during cardiac surgery. Western blots to assess phospholamban (PLB) phosphorylation on serine 16 were performed. Ergometrine exerted concentration- and time-dependent positive inotropic effects and positive chronotropic effects in atrial preparations starting at 0.3 µM and reaching a plateau at 10 µM in H2-TGs (n = 7). This was accompanied by an increase in PLB phosphorylation at serine 16. Ergometrine up 10 µM failed to increase force of contraction in left atrial preparations from 5-HT4-TGs (n = 5). Ten micrometer ergometrine increased the force of contraction in isolated retrogradely perfused spontaneously beating heart preparations (Langendorff setup) from H2-TG but not 5-HT4-TG. In the presence of the phosphodiesterase inhibitor cilostamide (1 µM), ergometrine at 10 µM exerted positive inotropic effects in isolated electrically stimulated human right atrial preparations, obtained during cardiac surgery, and these effects were eliminated by 10 µM of the H2R antagonist cimetidine but not by 10 µM of the 5-HT4R antagonist tropisetron. Furthermore, ergometrine showed binding to human histamine H2 receptors (at 100 µM and 1 mM) using HEK cells in a recombinant expression system (pKi < 4.5, n = 3). In conclusion, we suggest that ergometrine is an agonist at cardiac human H2Rs.

Discovery of a Tritiated Radioligand with High Affinity and Selectivity for the Histamine H3 Receptor

Radioligands used previously for histamine H3 receptor (H3R) are accompanied by a number of disadvantages. In this study, we report the synthesis of the new H3R radioligand [3H]UR-MN259 ([3H]11) with high (radio)chemical purity and stability. The radioligand exhibits sub-nanomolar affinity for the target receptor (pKi (H3R) = 9.56) and displays an outstanding selectivity profile within the histamine receptor family (>100,000-fold selective). [3H]UR-MN259 is ideally suitable for the characterization of H3R ligands in competition binding and shows one-site binding to the H3R in saturation binding experiments. The radiotracer shows fast association to the receptor (τassoc = 6.11 min), as well as full dissociation from the receptor (τdissoc = 14.48 min) in kinetic binding studies. The distinguished profile of [3H]UR-MN259 makes it a highly promising pharmacological tool to further investigate the role of the H3R in the CNS.

Study of G protein-coupled receptors dimerization: From bivalent ligands to drug-like small molecules

In the past decades an increasing number of studies revealed that G protein-coupled receptors (GPCRs) are capable of forming dimers or even higher-ordered oligomers, which may modulate receptor function and act as potential drug targets. In this review, we briefly summarized the design strategy of bivalent GPCR ligands and mainly focused on how to use them to study and/or detect GPCP dimerization in vitro and in vivo. Bivalent ligands show specific properties relative to their corresponding monomeric ligands because they are able to bind to GPCR homodimers or heterodimers simultaneously. For example, bivalent ligands with optimal length of spacers often exhibited higher binding affinities for dimers compared to that of monomers. Furthermore, bivalent ligands displayed specific signal transduction compared to monovalent ligands. Finally, we give our perspective on targeting GPCR dimers from traditional bivalent ligands to more drug-like small molecules.

Evaluating anti-coronavirus activity of some phosphoramides and their influencing inhibitory factors using molecular docking, DFT, QSAR, and NCI-RDG studies

The recent prevalence of coronavirus disease in 2019 (COVID-19) has triggered widespread global health concerns.Antiviral drugs based on phosphoramides have significant inhibitory activity against the main protease (M^pro) of the virus and prevent transcription and viral replication. Hence, in order to design and introduce a group of inhibitors affecting the coronavirus, 35 phosphoramide compounds based on phospho-guanine and phospho-pyrazine derivatives were selected for molecular docking study. The results showed that most phosphoguanides containing the amino benzimidazole have a high interaction tendency with COVID-19. Among them, compound 19 was identified as the strongest inhibitor with the -9.570 kcal/mol binding energy whereas, the binding energy of Remdesivir is -6.75 kcal/mol. The quantitative structure-activity relationship (QSAR) results demonstrated that the number of aromatic rings, amide's nitrogens and their ability in π-staking, and hydrogen interactions with M^pro active sites are major factors contributing to the inhibitory activity of these compounds.Also, the NCI-RDG and DFT results were in good accordance with those of QSAR and molecular docking. The findings of this investigation can be underlying the synthesis of effective and efficient drugs against COVID-19.

Shining light on the histamine H2 receptor: Synthesis of carbamoylguanidine-type agonists as a pharmacological tool to study internalization

So far, only little is known about the internalization process of the histamine H₂ receptor (H₂R). One promising approach to study such dynamic processes is the use of agonistic fluorescent ligands. Therefore, a series of carbamoylguanidine-type H₂R agonists containing various fluorophores, heterocycles, and linkers (28-40) was synthesized. The ligands were pharmacologically characterized in several binding and functional assays. These studies revealed a significantly biased efficacy (E_max) for some of the compounds, e.g. 32: whereas 32 acted as strong partial (E_max: 0.77, mini-Gs recruitment) or full agonist (E_max: 1.04, [³⁵S]GTPγS binding) with respect to G protein activation, it was only a weak partial agonist regarding β-arrestin1/2 recruitment (E_max: 0.09-0.12) and failed to promote H₂R internalization (confocal microscopy). On the other hand, H₂R internalization was observed for compounds that exhibited moderate agonistic activity in the β-arrestin1/2 pathways (E_max ≥ 0.22). The presented differently-biased fluorescent ligands are versatile molecular tools for future H₂R studies on receptor trafficking and internalization e.g. using fluorescence microscopy.

Specific Engineered G Protein Coupling to Histamine Receptors Revealed from Cellular Assay Experiments and Accelerated Molecular Dynamics Simulations

G protein-coupled receptors (GPCRs) are targets of extracellular stimuli and hence occupy a key position in drug discovery. By specific and not yet fully elucidated coupling profiles with α subunits of distinct G protein families, they regulate cellular responses. The histamine H2 and H4 receptors (H2R and H4R) are prominent members of Gs- and Gi-coupled GPCRs. Nevertheless, promiscuous G protein and selective Gi signaling have been reported for the H2R and H4R, respectively, the molecular mechanism of which remained unclear. Using a combination of cellular experimental assays and Gaussian accelerated molecular dynamics (GaMD) simulations, we investigated the coupling profiles of the H2R and H4R to engineered mini-G proteins (mG). We obtained coupling profiles of the mGs, mGsi, or mGsq proteins to the H2R and H4R from the mini-G protein recruitment assays using HEK293T cells. Compared to H2R-mGs expressing cells, histamine responses were weaker (pEC50, Emax) for H2R-mGsi and -mGsq. By contrast, the H4R selectively bound to mGsi. Similarly, in all-atom GaMD simulations, we observed a preferential binding of H2R to mGs and H4R to mGsi revealed by the structural flexibility and free energy landscapes of the complexes. Although the mG α5 helices were consistently located within the HR binding cavity, alternative binding orientations were detected in the complexes. Due to the specific residue interactions, all mG α5 helices of the H2R complexes adopted the Gs-like orientation toward the receptor transmembrane (TM) 6 domain, whereas in H4R complexes, only mGsi was in the Gi-like orientation toward TM2, which was in agreement with Gs- and Gi-coupled GPCRs structures resolved by X-ray/cryo-EM. These cellular and molecular insights support (patho)physiological profiles of the histamine receptors, especially the hitherto little studied H2R function in the brain, as well as of the pharmacological potential of H4R selective drugs.

A Versatile Sub-Nanomolar Fluorescent Ligand Enables NanoBRET Binding Studies and Single-Molecule Microscopy at the Histamine H3 Receptor

The histamine H₃ receptor (H₃R) is considered an attractive drug target for various neurological diseases. We here report the synthesis of UR-NR266, a novel fluorescent H₃R ligand. Broad pharmacological characterization revealed UR-NR266 as a sub-nanomolar compound at the H₃R with an exceptional selectivity profile within the histamine receptor family. The presented neutral antagonist showed fast association to its target and complete dissociation in kinetic binding studies. Detailed characterization of standard H₃R ligands in NanoBRET competition binding using UR-NR266 highlights its value as a versatile pharmacological tool to analyze future H₃R ligands. The low nonspecific binding observed in all experiments could also be verified in TIRF and confocal microscopy. This fluorescent probe allows the highly specific analysis of native H₃R in various assays ranging from optical high throughput technologies to biophysical analyses and single-molecule studies in its natural environment. An off-target screening at 14 receptors revealed UR-NR266 as a selective compound.

Abolishing Dopamine D2long/D3 Receptor Affinity of Subtype-Selective Carbamoylguanidine-Type Histamine H2 Receptor Agonists

3-(2-Amino-4-methylthiazol-5-yl)propyl-substituted carbamoylguanidines are potent, subtype-selective histamine H₂ receptor (H₂R) agonists, but their applicability as pharmacological tools to elucidate the largely unknown H₂R functions in the central nervous system (CNS) is compromised by their concomitant high affinity toward dopamine D₂-like receptors (especially to the D₃R). To improve the selectivity, a series of novel carbamoylguanidine-type ligands containing various heterocycles, spacers, and side residues were rationally designed, synthesized, and tested in binding and/or functional assays at H_1-4 and D_2long/3 receptors. This study revealed a couple of selective candidates (among others 31 and 47), and the most promising ones were screened at several off-target receptors, showing good selectivities. Docking studies suggest that the amino acid residues (3.28, 3.32, E2.49, E2.51, 5.42, and 7.35) are responsible for the different affinities at the H₂- and D_2long/3-receptors. These results provide a solid base for the exploration of the H₂R functions in the brain in further studies.

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.

Molecular Modeling of Histamine Receptors-Recent Advances in Drug Discovery

The recent developments of fast reliable docking, virtual screening and other algorithms gave rise to discovery of many novel ligands of histamine receptors that could be used for treatment of allergic inflammatory disorders, central nervous system pathologies, pain, cancer and obesity. Furthermore, the pharmacological profiles of ligands clearly indicate that these receptors may be considered as targets not only for selective but also for multi-target drugs that could be used for treatment of complex disorders such as Alzheimer's disease. Therefore, analysis of protein-ligand recognition in the binding site of histamine receptors and also other molecular targets has become a valuable tool in drug design toolkit. This review covers the period 2014-2020 in the field of theoretical investigations of histamine receptors mostly based on molecular modeling as well as the experimental characterization of novel ligands of these receptors.

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.

NanoBRET binding assay for histamine H2 receptor ligands using live recombinant HEK293T cells

Fluorescence/luminescence-based techniques play an increasingly important role in the development of test systems for the characterization of future drug candidates, especially in terms of receptor binding in the field of G protein-coupled receptors (GPCRs). In this article, we present the establishment of a homogeneous live cell-based BRET binding assay for the histamine H₂ receptor with different fluorescently labeled squaramide-type compounds synthesized in the course of this study. Py-1-labeled ligand 8 (UR-KAT478) was found to be most suitable in BRET saturation binding experiments with respect to receptor affinity (pK_d = 7.35) and signal intensity. Real-time kinetic experiments showed a full association of 8 within approximately 30 min and a slow dissociation of the ligand from the receptor. Investigation of reference compounds in BRET-based competition binding with 8 yielded pK_i values in agreement with radioligand binding data. This study shows that the BRET binding assay is a versatile test system for the characterization of putative new ligands at the histamine H₂ receptor and represents a valuable fluorescence-based alternative to canonical binding assays.

Synthesis, crystal structure, insecticidal activities, molecular docking and QSAR studies of some new phospho guanidines and phospho pyrazines as cholinesterase inhibitors

Novel phospho guanidine and phospho pyrazine derivatives were synthesized and characterized by ³¹P, ¹³C, ¹HNMR and IR spectroscopy to obtain novel and human-safe insecticides. Compound 35 [(C₄H₄N₂NH)₂P(O)(C₆H₆)] was investigated by X-ray crystallography. The inhibitory effects of synthesized compounds were evaluated on human and insect acetylcholinesterase (AChE) using in vitro Ellman method. A few of these compounds, which had low human toxicity, were selected for assessing the killing effects (in vivo) on the elm leaf beetle (X.luteola). The in vitro and in vivo results indicated that compounds bearing both phosphoryl groups and aromatic systems were found to possess a good selectivity for the inhibition of insect AChE over human AChE; up to 550-fold selectivity was achieved for compound 19. Docking studies were performed to explain reasons for the selective behavior of AChE inhibitors. Additionally, the quantitative structure-activity relationship (QSAR) and density functional theory (DFT) results of AChEs demonstrated that the size, shape, dipole moment, and ability to form hydrogen bond played the main role in both models. In addition, the aromatic π - π interactions and charge of the amide nitrogen had a major effect on insecticidal activity of the compounds. The present research can be helpful to gain a better understanding of the interactions between the insect AChE and its inhibitors and introduces compounds which are capable of becoming human-safe insecticides.

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.

Light-Switchable Antagonists for the Histamine H1 Receptor at the Isolated Guinea Pig Ileum

The histamine H₁ G protein-coupled receptor (GPCR) plays an important role in allergy and inflammation. Existing drugs that address the H₁ receptor differ in their chemical structure, pharmacology, and side effects. Light-controllable spatial and temporal activity regulation of photochromic H₁ ligands may contribute to a better mechanistic understanding and the development of improved correlations between ligand structure and pharmacologic effects. We report photochromic H₁ receptor ligands, which were investigated in an organ-pharmacological assay. Initially, five photochromic azobenzene derivatives of reported dual H₁ -H₄ receptor antagonists were designed, synthesized, photochemically characterized, and organ-pharmacologically tested on the isolated guinea pig ileum. Among them, one compound [trans-19: (Z)-1-(4-chlorophenyl)-1-(4-methylpiperazin-1-yl)-N-(4-((E)-phenyldiazenyl)phenyl)methanimine] retained the antagonistic activity of its non-photochromic lead, and trans-cis isomerization by irradiation induced a fourfold difference in the pharmacological response. Further structural optimization resulted in two bathochromically shifted derivatives of 19 [NO₂ -substituted 35 {(Z)-1-(4-chlorophenyl)-1-(4-methylpiperazin-1-yl)-N-(4-((E)-(4-nitrophenyl)diazenyl)phenyl)methanimine} and SO₃ ^- -substituted 41 {4-((E)-(4-(((Z)-(4-chlorophenyl)(4-methylpiperazin-1-yl)methylene)amino)phenyl)diazenyl)benzenesulfonate}], which do not require the use of UV light for photoisomerization and which also have improved solubility and show reduced tissue impairment. The trans isomers of both compounds showed a remarkable increase in antagonistic activity relative to their lead trans-19; furthermore, a 46-fold difference in activity on the isolated guinea pig ileum was observed between trans- and cis-35.