[Structure-activity relationships of histamine analogues. XXI: Synthesis and pharmacology of N alpha-substituted histamines (author's transl)]

Cherry-picked ligands at histamine receptor subtypes

Histamine, a biogenic amine, is considered as a principle mediator of multiple physiological effects through binding to its H1, H2, H3, and H4 receptors (H1-H4Rs). Currently, the HRs have gained attention as important targets for the treatment of several diseases and disorders ranging from allergy to Alzheimer's disease and immune deficiency. Accordingly, medicinal chemistry studies exploring histamine-like molecules and their physicochemical properties by binding and interacting with the four HRs has led to the development of a diversity of agonists and antagonists that display selectivity for each HR subtype. An overview on H1-R4Rs and developed ligands representing some key steps in development is provided here combined with a short description of structure-activity relationships for each class. Main chemical diversities, pharmacophores, and pharmacological profiles of most innovative H1-H4R agonists and antagonists are highlighted. Therefore, this overview should support the rational choice for the optimal ligand selection based on affinity, selectivity and efficacy data in biochemical and pharmacological studies. This article is part of the Special Issue entitled 'Histamine Receptors'.

Histaprodifens: synthesis, pharmacological in vitro evaluation, and molecular modeling of a new class of highly active and selective histamine H(1)-receptor agonists

A new class of histamine analogues characterized by a 3, 3-diphenylpropyl substituent at the 2-position of the imidazole nucleus has been prepared outgoing from 4,4-diphenylbutyronitrile (4b) via cyclization of the corresponding methyl imidate 5b with 2-oxo-4-phthalimido-1-butyl acetate or 2-oxo-1,4-butandiol in liquid ammonia, followed by standard reactions. The title compounds displayed partial agonism on contractile H(1) receptors of the guinea-pig ileum and endothelium-denuded aorta, respectively, except 10 (histaprodifen; 2-[2-(3, 3-diphenylpropyl)-1H-imidazol-4-yl]ethanamine) which was a full agonist in the ileum assay. While 10 was equipotent with histamine (1), methylhistaprodifen (13) and dimethylhistaprodifen (14) exceeded the functional potency of 1 by a factor of 3-5 (13) and 2-3 (14). Compounds 10 and 13-17 relaxed precontracted rat aortic rings (intact endothelium) with relative potencies of 3.3- up to 28-fold (compared with 1), displaying partial agonism as well. Agonist effects were sensitive to blockade by the selective H(1)-receptor antagonist mepyramine (pA(2) approximately 9 (guinea-pig) and pA(2) approximately 8 (rat aorta)). The affinity of 10 and 13-17 for guinea-pig H(1) receptors increased 20- to 100-fold compared with 1. Two lower homologues of 10 were weak partial H(1)-receptor agonists while two higher homologues of 10 were silent antagonists endowed with micromolar affinity for rat and guinea-pig H(1) receptors. In functional selectivity experiments, 10, 13, and 14 did not stimulate H(2), H(3), and several other neurotransmitter receptors. They displayed only low to moderate affinity for these sites (pA(2) < 6). For a better understanding of structure-activity relationships, the interaction of 1 and 10, 13 and 14 within the transmembrane (TM) domains of the human histamine H(1) receptor were studied using molecular dynamics simulations. Remarkable differences were found between the binding modes of 10, 13, and 14 and that of 1. The imidazole ring of 10, 13, and 14 was placed 'upside down' compared with 1, making the interaction of the N(pi)-atom with Tyr431 possible. This new orientation was mainly caused by the space filling substitution at the 2-position of the imidazole ring and influenced the location of the protonated N(alpha)-atom which was positioned more between TM III and TM VI. This orientation can explain both the increased relative potency and the maximum effect of 10, 13, and 14 compared with 1. Compound 13 (methylhistaprodifen; N(alpha)-methyl-2-[2-(3, 3-diphenylpropyl)-1H-imidazol-4-yl]ethanamine) is the most potent histamine H(1)-receptor agonist reported so far in the literature and may become a valuable tool for the study of physiological and pathophysiological H(1)-receptor-mediated effects.

N alpha-alkylated derivatives of 2-phenylhistamines: synthesis and in vitro activity of potent histamine H1-receptor agonists

New potent N alpha-alkylated histamine H1-receptor agonists have been prepared and functionally evaluated for partial agonist potency and selectivity. N alpha-Methyl-2-(3-trifluoromethylphenyl)histamine contracts ileal segments and aortic rings of guinea-pig with a relative potency of 174% (95% confid. lim. 161-188%) and 217% (164-287%), respectively (histamine: 100%) and is the most potent H1 receptor agonist described so far.

Molecular pharmacological aspects of histamine receptors

In this article, we review the recent developments in the field of histamine research. Besides the description of pharmacological tools for the H1, H2 and H3 receptor, specific attention is paid to both the molecular aspects of the receptor proteins, including the recent cloning of the receptor genes, and their respective signal transduction mechanisms.

Histamine receptors: subclasses and specific ligands

In this review the three main types of histamine receptors are discussed together with their specific ligands. For the classical H1-receptors much emphasis is put on the mechanism by which the receptor is stimulated. For the H1- and H2-receptor the review includes information on the several models available for establishing agonistic or antagonistic activity. In the section on the H3-receptor the ligands are discussed as well as the possible physiological role of this receptor. In the final paragraphs some less well defined activities are presented.

Effects of three histaminergic compounds on function and oxygen consumption of isolated guinea-pig hearts

First experiences in man indicate, that even in catecholamine-insensitive congestive cardiomyopathy a considerable improvement of myocardial function can be attained by the H2-receptor agonist impromidine. In an isolated, pressure-volume work performing guinea-pig heart preparation cardiac effects of three histaminergic compounds (Na,5-dimethylhistamine (HC1)2, 5-ethyl-Na-methyl-histamine (HC1)2, Na,Na-dimethyl-histamine (HC1)2) were examined. Influences on function and myocardial oxygen consumption were compared to those obtained by impromidine. Dose-response curves for the histamine derivatives were 1.7-2.5 orders of magnitude right of the impromidine curves. Maximal inotropic stimulation was greater for Na,5-dimethyl-histamine (HC1)2 than for impromidine. All compounds exhibited a high chronotropic effect and, at high concentrations, a net coronary dilating effect.