The role of HB-donor groups in the heterocyclic polar fragment of H3-antagonists

Synthesis and Structural Characterization of 1-Arylimidazole-2-thiones and N,N'-Aryldiethoxyethylthioureas with Electronically Diverse Substituents: A Manifold of Hydrogen Bonding Networks

The 1-arylimidazole-2-thiones, (HmimAr) [Ar = 3,4,5-C6H2(OMe)3, 2,4-C6H3(NO2)(OMe), 2,4,6-C6H2Cl3 and 3,5-C6H3(CF3)2], which feature electronically diverse substituents, may be obtained via acid-catalyzed ring closure of the corresponding N,N'-aryldiethoxyethylthiourea derivatives, ArN(H)C(S)N(H)CH2CH(OEt)2, (H2detuAr), which in turn are obtained via treatment of aminoacetaldehyde diethyl acetal, H2NCH2CH(OEt)2, with the respective arylisothiocyanates (ArNCS). The molecular structures of all of the above N,N'-aryldiethoxyethylthioureas and 1-arylimidazole-2-thiones have been determined by X-ray diffraction, thereby demonstrating that the substituents have a profound effect on the crystal structures. For example, each of the N,N'-aryldiethoxyethylthiourea derivatives adopts a different hydrogen bonding pattern. Specifically, the hydrogen-bonding network in (i) H2detuArCl3 consists of chains of 9-membered rings, with an [ [Formula: see text](9)] motif, that feature one N-H ⋯ O and one N-H ⋯ S interaction, (ii) H2detuArOMe,NO2 consists of chains of 6-membered rings, with an [ [Formula: see text](6)] motif, that feature two head-to-tail N-H ⋯ S interactions, (iii) H2detuAr(CF3)2 consists of a dimer that features two pairs of N-H ⋯ O interactions, of which each pair is a component of an 8-membered ring with an [ [Formula: see text](8)] motif, and (iv) H2detuAr(OMe)3 consists of a chain of head-to-head dimeric rings with a basic [ [Formula: see text](16)] motif, a notable feature of which is that sulfur does not play a role as a hydrogen bond acceptor. Each of the 1-arylimidazole-2-thiones exists as a "head-to-head" hydrogen-bonded dimer in the solid state, with an [ [Formula: see text](8)] motif. However, while the hydrogen-bonded motifs for HmimArCl3 and HmimAr(OMe)3 are planar, those for HmimAr(CF3)2 and HmimArOMe,NO2 are extremely puckered, with fold angles of 24.2° (mean value) and 45.7°, respectively.

Molecular modeling of histamine H3 receptor and QSAR studies on arylbenzofuran derived H3 antagonists

Histamine H3 receptors are presynaptic autoreceptors found in both central and peripheral nervous systems of many species. The central effects of these receptors suggest a potential therapeutic role for their antagonists in treatment of several neurological disorders such as epilepsy, schizophrenia, Alzheimer's and Parkinson's diseases. The purpose of this study was to identify the structural requirements for H3 antagonistic activity via quantitative structure-activity relationship (QSAR) studies and receptor modeling/docking techniques. A combination of partial least squares (PLS) and genetic algorithm (GA) was used in the QSAR approach to select the structural descriptors relevant to the receptor binding affinity of a series of 58 H3 antagonists. The descriptors were selected out of a pool of >1000 descriptors calculated by DRAGON, Hyperchem and ACD labs suite of programs. The resulting QSAR models for rat and human H3 binding affinities were validated using different strategies. QSAR models generated in the current work suggested the role of charge transfer interactions in the ligand-receptor interaction verified using the molecular modeling of the receptor and docking two antagonists to the binding site. The 3D model of human H3 receptor was built based on bovine rhodopsin structure and evaluated by molecular dynamics (MD) simulation in a mixed water-vacuum-water environment. The results were indicative of the stability of the model relating the observed structural changes during the MD simulation to the suggested ligand-receptor interactions. The results of this investigation are expected to be useful in the process of design and development of new potent H3 receptor antagonists.

Synthesis and biological evaluation of new non-imidazole H3-receptor antagonists of the 2-aminobenzimidazole series

A novel series of non-imidazole H(3)-receptor antagonists was developed, by chemical modification of a potent lead H(3)-antagonist composed by an imidazole ring connected through an alkyl spacer to a 2-aminobenzimidazole moiety (e.g., 2-[[3-[4(5)-imidazolyl]propyl]amino]benzimidazole), previously reported by our research group. We investigated whether the removal of the imidazole ring could allow retaining high affinity for the H(3)-receptor, thanks to the interactions undertaken by the 2-aminobenzimidazole moiety at the binding site. The imidazole ring of the lead was replaced by a basic piperidine or by a lipophilic p-chlorophenoxy substituent, modulating the spacer length from three to eight methylene groups; moreover, the substituents were moved to the 5(6) position of the benzimidazole nucleus. Within both the 2-alkylaminobenzimidazole series and the 5(6)-alkoxy-2-aminobenzimidazole one, the greatest H(3)-receptor affinity was obtained for the piperidine-substituted compounds, while the presence of the p-chlorophenoxy group resulted in a drop in affinity. The optimal chain length was different in the two series. Even if the new compounds did not reach the high receptor affinity shown by the imidazole-containing lead compound, it was possible to get good H(3)-antagonist potencies with 2-aminobenzimidazoles having a tertiary amino group at appropriate distance.

Validation of a histamine H3 receptor model through structure-activity relationships for classical H3 antagonists

Histamine H(3) receptor is a G protein-coupled receptor whose activation inhibits the synthesis and release of histamine and other neurotransmitters from nerve endings and is involved in the modulation of different central nervous system functions. H(3) antagonists have been proposed for their potential usefulness in diseases characterized by impaired neurotransmission and they have demonstrated beneficial effects on learning and food intake in animal models. In the present work, a 3D model of the rat histamine H(3) receptor, built by comparative modeling from the crystallographic coordinates of bovine rhodopsin, is presented with the discussion of its ability to predict the potency of known and new H(3) antagonists. A putative binding site for classical, imidazole-derived H(3) antagonists was identified by molecular docking. Comparison with a known pharmacophore model and the binding affinity of a new rigid H(3) antagonist (compound 1, pK(i)=8.02) allowed the characterization of a binding scheme which could also account for the different affinities observed in a recently reported series of potent H(3) antagonists, characterized by a 2-aminobenzimidazole moiety. Molecular dynamics simulations were employed to assess the stability and reliability of the proposed binding mode. Two new conformationally constrained benzimidazole derivatives were prepared and their binding affinity was tested on rat brain membranes; compound 9, designed to reproduce the conformation of a known potent H(3) antagonist, showed higher potency than compound 8, as expected from the binding scheme hypothesized.

The First Potent Inhibitors for Human Glutaminyl Cyclase: Synthesis and Structure−Activity Relationship

The first effective inhibitors for human glutaminyl cyclase (QC) are described. The structures are developed by applying a ligand-based optimization approach starting from imidazole. Screening of derivatives of that heterocycle led to compounds of the imidazol-1-yl-alkyl thiourea type as a lead scaffold. A library of thiourea derivatives was synthesized, resulting in an inhibitory improvement by 2 orders of magnitude, leading to 1-(3-(1H-imidazol-1-yl)propyl)-3-(3,4-dimethoxyphenyl)thiourea as a potent inhibitor. Systematic exploitation of the scaffold revealed a strong impact on the inhibitory efficacy and resulted in the development of imidazole-propyl-thioamides as another new class of potent inhibitors. A flexible alignment of the most potent compounds of the thioamide and thiourea class and a QC substrate revealed a good match of characteristic features of the molecules, which suggests a similar binding mode of both inhibitors and the substrate to the active site of QC.

Imidazole H3-antagonists: relationship between structure and ex vivo binding to rat brain H3-receptors

H3-antagonists possess promising pharmacological effects on awakening, learning and memory, but few data on their access to the central nervous system (CNS) have been reported so far. The purpose of this work was to investigate the relationships between structure and brain penetration of a series of H3-antagonists, using ex vivo binding experiments in rats. H3-antagonists belonging to different chemical classes but all having an imidazole ring, an alkyl spacer, a polar fragment and a lipophilic ending group, were selected among the numerous H3-antagonists recently described by us. Ex vivo binding studies were performed by inhibiting specific [3H]-(R)-alpha-methylhistamine ([3H]-RAMHA) binding to rat cerebral cortical membranes following H3-antagonist peripheral administration. Ionization constants and partition coefficients in n-octanol/water and 1,2-dichloroethane/water were determined by the potentiometric pH-metric method and were compared to the ex vivo binding potencies to analyse structure-property relationships (SPR). In the ex vivo assay, the H3-antagonists showed different potencies (pED50) not correlated to their in vitro H3-receptor binding affinities (pKi). Compound 4a, having a benzothiazol-2-yl-thioethyl chain, showed high ex vivo potency (ED50=1.35 mg kg(-1) i.p.) and a fast brain penetration, eliciting maximal displacement of [3H]-RAMHA already 5 min after i.v. or i.p. administration. Ex vivo binding assays of three compounds, following i.v. and i.p. administration, showed that the observed i.p. ex vivo potencies were not significantly affected by biotransformation. Within the set of compounds, those having a better ability to reach the CNS had a logDoct(7.4) in the range 2-3.5, and a DeltalogPoct-dce < 2. The combined use of two easily measurable physicochemical descriptors, namely logDoct(7.4) (apparent lipophilicity at pH 7.4) and DeltalogPoct-dce (a descriptor of H-bond donor capacity) allowed to model brain permeation of the majority of the compounds examined.