Receptor-specific functional efficacies of alkyl imidazoles as dual histamine H3/H4 receptor ligands

Comprehensive review in current developments of imidazole-based medicinal chemistry

Imidazole ring is an important five-membered aromatic heterocycle widely present in natural products and synthetic molecules. The unique structural feature of imidazole ring with desirable electron-rich characteristic is beneficial for imidazole derivatives to readily bind with a variety of enzymes and receptors in biological systems through diverse weak interactions, thereby exhibiting broad bioactivities. The related research and developments of imidazole-based medicinal chemistry have become a rapidly developing and increasingly active topic. Particularly, numerous imidazole-based compounds as clinical drugs have been extensively used in the clinic to treat various types of diseases with high therapeutic potency, which have shown the enormous development value. This work systematically gives a comprehensive review in current developments of imidazole-based compounds in the whole range of medicinal chemistry as anticancer, antifungal, antibacterial, antitubercular, anti-inflammatory, antineuropathic, antihypertensive, antihistaminic, antiparasitic, antiobesity, antiviral, and other medicinal agents, together with their potential applications in diagnostics and pathology. It is hoped that this review will be helpful for new thoughts in the quest for rational designs of more active and less toxic imidazole-based medicinal drugs, as well as more effective diagnostic agents and pathologic probes.

Bodilisant-a novel fluorescent, highly affine histamine h3 receptor ligand

A piperidine-based lead structure for the human histamine H3 receptor (hH3R) was coupled with the BODIPY fluorophore and resulted in a strong green fluorescent (quantum yield, 0.92) hH3R ligand with affinity in the nanomolar concentration range (K i hH3R = 6.51 ± 3.31 nM), named Bodilisant. Screening for affinities at histamine and dopamine receptor subtypes showed high hH3R preference. Bodilisant was used for visualization of hH3R in hH3R overexpressing HEK-293 cells with fluorescence confocal laser scanning microscopy. In addition, in native human brain tissues, Bodilisant showed clear and displaceable images of labeled hH3R.

Novel chalcone-based fluorescent human histamine h(3) receptor ligands as pharmacological tools

Novel fluorescent chalcone-based ligands at human histamine H₃ receptors (hH₃R) have been designed, synthesized, and characterized. Compounds described are non-imidazole analogs of ciproxifan with a tetralone motif. Tetralones as chemical precursors and related fluorescent chalcones exhibit affinities at hH₃R in the same concentration range like the reference antagonist ciproxifan (hH₃R pK_i value of 7.2). Fluorescence characterization of our novel ligands shows emission maxima about 570 nm for yellow fluorescent chalcones and ≥600 nm for the red fluorescent derivatives. Interferences to cellular autofluorescence could be excluded. All synthesized chalcone compounds could be used to visualize hH₃R proteins in stably transfected HEK-293 cells using confocal laser scanning fluorescence microscopy. These novel fluorescent ligands possess high potential to be used as pharmacological tools for hH₃R visualization in different tissues.

Histamine H4 receptor ligands and their potential therapeutic applications: an update

INTRODUCTION

Several studies underlined the critical role of histamine H4 receptor (H4R) in inflammation, thus H4 modulators have been suggested as promising drug candidates in inflammatory diseases. First H4 ligands typically have indole or amino-pyrimidine scaffolds. During the last few years, however, serious efforts have been made to identify novel H4 chemotypes with improved pharmacodynamic and pharmacokinetic properties.

AREAS COVERED

Areas covered in this review include an overview on H4 ligands published in scientific papers, as well as in patent applications between 2009 and 2011. Recently discovered scaffolds possessing significant H4 activity were analyzed and their therapeutic potential was reviewed.

EXPERT OPINION

Recent results from the scientific literature and novel patent applications reinforce the major role of H4R in inflammatory diseases such as pruritus, asthma, inflammatory pain and allergic rhinitis. Novel studies suggest further indications of H4 modulators in cancer, neuropathic pain, vestibular disorders and type 2 diabetes. The number of active H4 chemotypes was increased significantly. The first H4 antagonist entered to clinics and the results from a proof-of-concept Phase II clinical study is expected to be disclosed soon.

The histamine H₄ receptor: targeting inflammatory disorders

The discovery of the histamine H(4) receptor has added a new chapter to the century of extensive biogenic amine research. The human histamine H(4) receptor is mainly expressed in cells of the human immune system (e.g. mast cells, eosinophils, monocytes, dendritic cells, T cells) and mediates several effects on chemotaxis with numerous cell types. The distinct expression pattern and the immunomodulatory role highlight its physiological relevance in inflammatory and immunological processes. Inflammatory conditions, e.g. allergy, asthma and autoimmune diseases, were for a long time thought to be mainly mediated by activation of the histamine H(1) receptor subtype. However, in the treatment of diseases as chronic pruritus, asthma and allergic rhinitis the use of histamine H(1) receptor antagonists is unsatisfying. Selective H(4) receptor ligands and/or synergism of histamine H(1) and H(4) receptor modulation may be more effective in such pathophysiological conditions. Promising preclinical studies underline its role as an attractive target in the treatment of inflammatory and autoimmune disorders. Meanwhile, first histamine H(4) receptor antagonist has reached clinical phases for the treatment of respiratory diseases.

Histamine H1, H3 and H4 receptors are involved in pruritus

Histamine has long been recognised as a classical inducer of pruritus. However, the specific mechanism of histamine-induced itch has still not been fully understood. The H1 and H4 receptor appear to be key components in the induction of itch. The specific role of the H3 receptor in histamine-induced itch remains unclear. The aim of our study was to investigate the role of the four known histamine receptors (H1-4) in acute itch in mice. Intradermal injection of the selective H3R inverse agonist pitolisant induced strong itch in mice. Pitolisant (50 nmol/injection)-induced pruritus could be completely blocked by a combined treatment with the H1R antagonist cetirizine (15 mg/kg) and the H4R antagonist JNJ 7777120 (15 mg/kg), whereas the H2R antagonist ranitidine (15 mg/kg) failed to inhibit the scratch response. Next, expression and function of histamine receptors on sensory neurons isolated from dorsal root ganglia of mice were investigated. As the itch sensation results from the excitation of sensory nerves in the skin, we further focused on skin specific sensory neurons. Therefore, neurons were retrograde labelled from the skin by means of a fluorescent tracer. Expression of H1R, H3R and H4R on skin innervating sensory neurons was detected. By single-cell calcium imaging, it was demonstrated that histamine induces a calcium increase in a subset of (skin-specific) sensory neurons via activation of the H1R and H4R as well as inhibition of the H3R. It is assumed that the decreased threshold in response to H3R antagonism activates H1R and H4R on sensory neurons, which in turn results in the excitation of histamine-sensitive afferents and therefore elicits the sensation of itch.