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

Synthesis of Diverse Ureas from Amines and CO2 at Atmospheric Pressure and Room Temperature

A metal-free method is developed to perform the synthesis of urea derivatives utilizing CO2 as the C1 building block at atmospheric pressure and room temperature. In addition to diverse symmetric and dissymmetric ureas, benzimidazolones and quinazolinone can also be easily prepared using this protocol. Most importantly, the gram-scale preparation of fungicide pencycuron and antipsychotic drug pimavanserin proceeded smoothly under the mild conditions.

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.

N-Terminus to Arginine Side-Chain Cyclization of Linear Peptidic Neuropeptide Y Y4 Receptor Ligands Results in Picomolar Binding Constants

The family of neuropeptide Y (NPY) receptors comprises four subtypes (Y₁R, Y₂R, Y₄R, Y₅R), which are addressed by at least three endogenous peptides, i.e., NPY, peptide YY, and pancreatic polypeptide (PP), the latter showing a preference for Y₄R. A series of cyclic oligopeptidic Y₄R ligands were prepared by applying a novel approach, i.e., N-terminus to arginine side-chain cyclization. Most peptides acted as Y₄R partial agonists, showing up to 60-fold higher Y₄R affinity compared to the linear precursor peptides. Two cyclic hexapeptides (18, 24) showed higher Y₄R potency (Ca²⁺ aequorin assay) and, with pK_i values >10, also higher Y₄R affinity compared to human pancreatic polypeptide (hPP). Compounds such as 18 and 24, exhibiting considerably lower molecular weight and considerably more pronounced Y₄R selectivity than PP and previously described dimeric peptidic ligands with high Y₄R affinity, represent promising leads for the preparation of labeled tool compounds and might support the development of drug-like Y₄R ligands.

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.

Cardiac Effects of Novel Histamine H2 Receptor Agonists

In an integrative approach, we studied cardiac effects of recently published novel H₂ receptor agonists in the heart of mice that overexpress the human H₂ receptor (H₂-TG mice) and littermate wild type (WT) control mice and in isolated electrically driven muscle preparations from patients undergoing cardiac surgery. Under our experimental conditions, the H₂ receptor agonists UR-Po563, UR-MB-158, and UR-MB-159 increased force of contraction in left atrium from H₂-TG mice with pEC₅₀ values of 8.27, 9.38, and 8.28, respectively, but not in WT mice. Likewise, UR-Po563, UR-MB-158, and UR-MB-159 increased the beating rate in right atrium from H₂-TG mice with pEC₅₀ values of 9.01, 9.24, and 7.91, respectively, but not from WT mice. These effects could be antagonized by famotidine, a H₂ receptor antagonist. UR-Po563 (1 µM) increased force of contraction in Langendorff-perfused hearts from H₂-TG but not WT mice. Similarly, UR-Po563, UR-MB-158, or UR-MB-159 increased the left ventricular ejection fraction in echocardiography of H₂-TG mice. Finally, UR-Po563 increased force of contraction in isolated human right atrial muscle strips. The contractile effects of UR-Po563 in H₂-TG mice were accompanied by an increase in the phosphorylation state of phospholamban. In summary, we report here three recently developed agonists functionally stimulating human cardiac H₂ receptors in vitro and in vivo. We speculate that these compounds might be of some merit to treat neurologic disorders if their cardiac effects are blocked by concomitantly applied receptor antagonists that cannot pass through the blood-brain barrier or might be useful to treat congestive heart failure in patients. SIGNIFICANCE STATEMENT: Recently, a new generation of histamine H₂ receptor (H₂R) agonists has been developed as possible treatment option for Alzheimer's disease. Here, possible cardiac (side) effects of these novel H₂R agonists have been evaluated.

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.