Functional Profiling of 2-Aminopyrimidine Histamine H4 Receptor Modulators

"Of mice and men": the relevance of Cometin and Erythropoietin origin for its effects on murine spiral ganglion neuron survival and neurite outgrowth in vitro

Neurotrophic factors (NTF) play key roles in the survival of neurons, making them promising candidates for therapy of neurodegenerative diseases. In the case of the inner ear, sensorineural hearing loss (SNHL) is characterized over time by a degeneration of the primary auditory neurons, the spiral ganglion neurons (SGN). It is well known that selected NTF can protect SGN from degeneration, which positively influences the outcome of cochlear implants, the treatment of choice for patients with profound to severe SNHL. However, the outcome of studies investigating protective effects of NTF on auditory neurons are in some cases of high variability. We hypothesize that the factor origin may be one aspect that affects the neuroprotective potential. The aim of this study was to investigate the neuroprotective potential of human and mouse Erythropoietin (EPO) and Cometin on rat SGN. SGN were isolated from neonatal rats (P 2-5) and cultured in serum-free medium. EPO and Cometin of mouse and human origin were added in concentrations of 0.1, 1, and 10 ng/mL and 0.1, 1, and 10 μg/mL, respectively. The SGN survival rate and morphology, and the neurite outgrowth were determined and compared to negative (no additives) and positive (brain-derived neurotrophic factor, BDNF) controls. A neuroprotective effect of 10 μg/mL human Cometin comparable to that obtained with BDNF was observed in the SGN-culture. In contrast, mouse Cometin was ineffective. A similar influence of 10 μg/mL human and mouse and 1 μg/mL human Cometin on the length of regenerated neurites compared to BDNF was also detected. No other Cometin-conditions, and none of the EPO-conditions tested had neuroprotective or neuritogenic effects or influenced the neuronal morphology of the SGN. The neuroprotective effect of 10 μg/mL human Cometin on SGN indicates it is a potentially interesting protein for the supportive treatment of inner ear disorders. The finding that mouse Cometin had no effect on the SGN in the parallel-performed experiments underlines the importance of species origin of molecules being screened for therapeutic purpose.

Chemical Probes for Histamine Receptor Subtypes

Ligands with different properties and different selectivity are highly needed for in vitro and in vivo studies on the (patho)physiological influence of the chemical mediator histamine and its receptor subtypes. A selection of well-described ligands for the different receptor subtypes and different studies is shown with a particular focus on affinity and selectivity. In addition, compounds with radioactive or fluorescence elements will be presented with their beneficial use for other species or different investigations.

Acid-catalyzed [2 + 2 + 2] cycloaddition of two cyanamides and one ynamide: highly regioselective synthesis of 2,4,6-triaminopyrimidines

Triflic acid (10 mol%) catalyzes the highly regioselective [2 + 2 + 2] cycloaddition between two cyanamides and one ynamide to grant the 2,4,6-triaminopyrimidine core. The developed synthetic method is effective for the preparation of a family of the diversely substituted heterocyclic products (30 examples; yields up to 94%). The synthesis can be easily scaled up and conducted in gram quantities. As demonstrated by the post-functionalizations involving the amino-substituents, the obtained heterocycles represent a useful platform for the construction of miscellaneous pyrimidine-based frameworks. The performed density functional theory calculations verified a particular role of H+, functioning as an electrophilic activator, in the regioselectivity of the cycloaddition.

[3H]UR-DEBa176: A 2,4-Diaminopyrimidine-Type Radioligand Enabling Binding Studies at the Human, Mouse, and Rat Histamine H4 Receptors

Differences in sequence homology between human (h), mouse (m), and rat (r) histamine H₄ receptors (H₄R) cause discrepancies regarding affinities, potencies, and/or efficacies of ligands and therefore compromise translational animal models and the applicability of radioligands. Aiming at a radioligand enabling robust and comparative binding studies at the h/m/rH₄Rs, 2,4-diaminopyrimidines were synthesized and pharmacologically investigated. The most notable compounds identified were two (partial) agonists with comparable potencies at the h/m/rH₄Rs: UR-DEBa148 (N-neopentyl-4-(1,4,6,7-tetrahydro-5H-imidazo[4,5-c]pyridin-5-yl)pyrimidin-2-amine bis(2,2,2-trifluoroacetate), 43), the most potent [pEC₅₀ (reporter gene assay) = 9.9/9.6/10.3] compound in the series being slightly G-protein biased and UR-DEBa176 [(R)-4-[3-(dimethylamino)pyrrolidin-1-yl]-N-neopentylpyrimidin-2-amine bis(2,2,2-trifluoroacetate), 46, pEC₅₀ (reporter gene assay) = 8.7/9.0/9.2], a potential "cold" form of a tritiated H₄R ligand. After radiolabeling, binding studies with [³H]UR-DEBa176 ([³H]46) at the h/m/rH₄Rs revealed comparable K_d values (41/17/22 nM), low nonspecific binding (11-17%, ∼K_d), and fast associations/dissociations (25-30 min) and disclosed [³H]UR-DEBa176 as useful molecular tool to determine h/m/rH₄R binding affinities for H₄R ligands.

Histamine, histamine receptors, and anti-histamines in the context of allergic responses

Histamine is a bioactive amine which is considered a key player in the allergic response. Thus, histamine receptor blockers (antihistamines) play an important role in the treatment of a number atopic diseases such as allergic rhinitis, conjunctivitis, and acute and chronic forms of urticaria. Histamine is produced by immune cells but also by bacteria in the gut. Beyond its role in the acute allergic response, histamine exerts numerous effects by binding to its 4 pleiotropic G-protein coupled histamine receptors. Here, we describe the roles of these histamine receptors and antihistamines in the human system, clinical applications, side effects, and novel concepts for the usage of antihistamines with different specificity based on guidelines and recommendations.

Statement of novelty: This review provides an overview of histamine receptors and links it to clinical relevance of antagonizing their action in clinical routine.

Novel Tetrahydroquinazolinamines as Selective Histamine 3 Receptor Antagonists for the Treatment of Obesity

The histamine 3 receptor (H3R) is a presynaptic receptor, which modulates several neurotransmitters including histamine and various essential physiological processes, such as feeding, arousal, cognition, and pain. The H3R is considered as a drug target for the treatment of several central nervous system disorders. We have synthesized and identified a novel series of 4-aryl-6-methyl-5,6,7,8-tetrahydroquinazolinamines that act as selective H3R antagonists. Among all the synthesized compounds, in vitro and docking studies suggested that the 4-methoxy-phenyl-substituted tetrahydroquinazolinamine compound 4c has potent and selective H3R antagonist activity (IC₅₀ < 0.04 μM). Compound 4c did not exhibit any activity on the hERG ion channel and pan-assay interference compounds liability. Pharmacokinetic studies showed that 4c crosses the blood brain barrier, and in vivo studies demonstrated that 4c induces anorexia and weight loss in obese, but not in lean mice. These data reveal the therapeutic potential of 4c as an anti-obesity candidate drug via antagonizing the H3R.

Discovery of a Novel Highly Selective Histamine H4 Receptor Antagonist for the Treatment of Atopic Dermatitis

The histamine H4 receptor (H4R), a member of the G-protein coupled receptor family, has been considered as a potential therapeutic target for treating atopic dermatitis (AD). A large number of H4R antagonists have been disclosed, but no efficient agents controlling both pruritus and inflammation in AD have been developed yet. Here, we have discovered a novel class of orally available H4R antagonists showing strong anti-itching and anti-inflammation activity as well as excellent selectivity against off-targets. A pharmacophore-based virtual screening system constructed in-house successfully identified initial hit compound 9, and the subsequent homology model-guided optimization efficiently led us to discover pyrido[2,3- e]tetrazolo[1,5- a]pyrazine analogue 48 as a novel chemotype of a potent and highly selective H4R antagonist. Importantly, orally administered compound 48 exhibits remarkable efficacy on antipruritus and anti-inflammation with a favorable pharmacokinetic (PK) profile in several mouse models of AD. Thus, these data strongly suggest that our compound 48 is a promising clinical candidate for treatment of AD.

Structural characterization of two tetra-chlorido-zincate salts of 4-carb-oxy-1H-imidazol-3-ium: a salt hydrate and a co-crystal salt hydrate

Imidazole-containing compounds exhibit a myriad of pharmacological activities. Two tetra-chlorido-zincate salts of 4-carb-oxy-1H-imidazol-3-ium, ImHCO2H+, are reported. Bis(4-carb-oxy-1H-imidazol-3-ium) tetra-chlorido-zincate monohydrate, (C4H5N2O2)2[ZnCl4]·H2O, (I), crystallizes as a monohydrate salt, while bis-(4-carb-oxy-1H-imidazol-3-ium) tetra-chlorido-zincate bis-(1H-imidazol-3-ium-4-carboxyl-ato) monohydrate, (C4H5N2O2)2[ZnCl4]·2C4H4N2O2·H2O, (II), is a co-crystal salt with six residues: two ImHCO2H+ cations, two formula units of the zwitterionic 1H-imidazol-3-ium-4-carboxyl-ate, ImHCO2, one tetra-chlorido-zincate anion and one water mol-ecule disordered over two sites in a 0.60 (4):0.40 (4) ratio. The geometric parameters of the ImHCO2H+ and the ImHCO2 moieties are the same within the standard uncertainties of the measurements. Both compounds exhibit extensive hydrogen bonding, including involvement of the tetra-chlorido-zincate anion, resulting in inter-connected chains of anions joined by water mol-ecules.

A Single Amino Acid Difference between Mouse and Human 5-Lipoxygenase Activating Protein (FLAP) Explains the Speciation and Differential Pharmacology of Novel FLAP Inhibitors

5-Lipoxygenase activating protein (FLAP) plays a critical role in the metabolism of arachidonic acid to leukotriene A4, the precursor to the potent pro-inflammatory mediators leukotriene B4 and leukotriene C4 Studies with small molecule inhibitors of FLAP have led to the discovery of a drug binding pocket on the protein surface, and several pharmaceutical companies have developed compounds and performed clinical trials. Crystallographic studies and mutational analyses have contributed to a general understanding of compound binding modes. During our own efforts, we identified two unique chemical series. One series demonstrated strong inhibition of human FLAP but differential pharmacology across species and was completely inactive in assays with mouse or rat FLAP. The other series was active across rodent FLAP, as well as human and dog FLAP. Comparison of rodent and human FLAP amino acid sequences together with an analysis of a published crystal structure led to the identification of amino acid residue 24 in the floor of the putative binding pocket as a likely candidate for the observed speciation. On that basis, we tested compounds for binding to human G24A and mouse A24G FLAP mutant variants and compared the data to that generated for wild type human and mouse FLAP. These studies confirmed that a single amino acid mutation was sufficient to reverse the speciation observed in wild type FLAP. In addition, a PK/PD method was established in canines to enable preclinical profiling of mouse-inactive compounds.

Conformational Restriction and Enantioseparation Increase Potency and Selectivity of Cyanoguanidine-Type Histamine H4 Receptor Agonists

2-Cyano-1-[4-(1H-imidazol-4-yl)butyl]-3-[2-(phenylsulfanyl)ethyl]guanidine (UR-PI376, 1) is a potent and selective agonist of the human histamine H4 receptor (hH4R). To gain information on the active conformation, we synthesized analogues of 1 with a cyclopentane-1,3-diyl linker. Affinities and functional activities were determined at recombinant hHxR (x: 1-4) subtypes on Sf9 cell membranes (radioligand binding, [(35)S]GTPγS, or GTPase assays) and in part in luciferase assays on human or mouse H4R (HEK-293 cells). The most potent H4R agonists among 14 racemates were separated by chiral HPLC, yielding eight enantiomerically pure compounds. Configurations were assigned based on X-ray structures of intermediates and a stereocontrolled synthetic pathway. (+)-2-Cyano-1-{[trans-(1S,3S)-3-(1H-imidazol-4-yl)cyclopentyl]methyl}-3-[2-(phenylsulfanyl)ethyl]guanidine ((1S,3S)-UR-RG98, 39a) was the most potent H4R agonist in this series (EC50 11 nM; H4R vs H3R, >100-fold selectivity; H1R, H2R, negligible activities), whereas the optical antipode proved to be an H4R antagonist ([(35)S]GTPγS assay). MD simulations confirmed differential stabilization of the active and inactive H4R state by the enantiomers.