Novel potent (dihydro)benzofuranyl piperazines as human histamine receptor ligands - Functional characterization and modeling studies on H3 and H4 receptors.

Histamine acts through four different receptors (H1R-H4R), the H3R and H4R being the most explored in the last years as drug targets. The H3R is a potential target to treat narcolepsy, Parkinson's disease, epilepsy, schizophrenia and several other CNS-related conditions, while H4R blockade leads to anti-inflammatory and immunomodulatory effects. Our group has been exploring the dihydrobenzofuranyl-piperazines (LINS01 series) as human H3R/H4R ligands as potential drug candidates. In the present study, a set of 12 compounds were synthesized from adequate (dihydro)benzofuran synthons through simple reactions with corresponding piperazines, giving moderate to high yields. Four compounds (1b, 1f, 1g and 1h) showed high hH3R affinity (pKi > 7), compound 1h being the most potent (pKi 8.4), and compound 1f showed the best efficiency (pKi 8.2, LE 0.53, LLE 5.85). BRET-based assays monitoring Gαi activity indicated that the compounds are potent antagonists. Only one compound (2c, pKi 7.1) presented high affinity for hH4R. In contrast to what was observed for hH3R, it showed partial agonist activity. Docking experiments indicated that bulky substituents occupy a hydrophobic pocket in hH3R, while the N-allyl group forms favorable interactions with hydrophobic residues in the TM2, 3 and 7, increasing the selectivity towards hH3R. Additionally, the importance of the indole NH in the interaction with Glu5.46 from hH4R was confirmed by the modeling results, explaining the affinity and agonistic activity of compound 2c. The data reported in this work represent important findings for the rational design of future compounds for hH3R and hH4R.

Synthesis and antiseizure effect evaluation of nonimidazole histamine H3 receptor antagonists containing the oxazole moiety.

The use of histamine H3 receptor (H3 R) antagonists is becoming a promising therapeutic approach for epilepsy. In this paper, a series of novel nonimidazole H3 R antagonists was synthesized and screened as antiepileptic drugs. All of these prepared antagonists displayed micromolar or submicromolar H3 R antagonistic activities in the cAMP response element luciferase screening assay. Compounds 5a (IC50 = 0.11 µM), 5b (IC50 = 0.56 µM), and 5f (IC50 = 0.78 µM) displayed the most potent H3 R antagonistic activities, with considerable potency when compared with pitolisant (IC50 = 0.51 µM). In the maximal electroshock (MES)-induced seizure model, compounds 5c, 5e, and 5g showed obvious protection for the electrostimulated mice, and the protection of 5g against the MES-induced seizures was fully abrogated when mice were cotreated with R-(α)-methyl-histamine, a central nervous system-penetrant H3 R agonist, suggesting that the potential therapeutic effect of 5g was observed to work through H3 R. These results indicate that the attempt to find a new antiepileptic drug among H3 R antagonists is practicable, but it is necessary to consider the log P of the molecules to ensure penetration of the blood-brain barrier.

Asymmetric Catalysis upon Helically Chiral Loratadine Analogues Unveils Enantiomer-Dependent Antihistamine Activity.

Analogues of the conformationally dynamic Claritin (loratadine) and Clarinex (desloratadine) scaffolds have been enantio- and chemoselectively N-oxidized using an aspartic acid containing peptide catalyst to afford stable, helically chiral products in up to >99:1 er. The conformational dynamics and enantiomeric stability of the N-oxide products have been investigated experimentally and computationally with the aid of crystallographic data. Furthermore, biological assays show that rigidifying the core structure of loratadine and related analogues through N-oxidation affects antihistamine activity in an enantiomer-dependent fashion. Computational docking studies illustrate the observed activity differences.

Development of New Synthetic Reactions Using Hetero-Heavy Atoms and Their Application to Synthesis of Biofunctional Molecules.

Novel reactions using hetero-heavy atoms (P, S, Si, Se, and Sn) were developed and applied to the synthesis of biofunctional molecules and some medicine-candidates, in which the following items are covered. 1) Development of introduction of C1-unit using cyanophosphates (CPs). 2) Carbene-generation under neutral condition from CPs and its application to organic synthesis. 3) [3,3]Sigmatropic rearrangement-ring expansion reactions of medium-sized cyclic thionocarbonates containing a sulfur atom and their application to natural product synthesis. 4) Stereoselective synthesis of novel ß-imidazole C-nucleosides via diazafulvene intermediates and their application to investigating ribozyme reaction mechanism. 5) Developments of novel histamine H3- and H4-receptor ligands using new synthetic methods involving Se or Sn atoms.

Covalent Inhibition of the Histamine H3 Receptor.

Covalent binding of G protein-coupled receptors by small molecules is a useful approach for better understanding of the structure and function of these proteins. We designed, synthesized and characterized a series of 6 potential covalent ligands for the histamine H3 receptor (H3R). Starting from a 2-amino-pyrimidine scaffold, optimization of anchor moiety and warhead followed by fine-tuning of the required reactivity via scaffold hopping resulted in the isothiocyanate H3R ligand 44. It shows high reactivity toward glutathione combined with appropriate stability in water and reacts selectively with the cysteine sidechain in a model nonapeptide equipped with nucleophilic residues. The covalent interaction of 44 with H3R was validated with washout experiments and leads to inverse agonism on H3R. Irreversible binder 44 (VUF15662) may serve as a useful tool compound to stabilize the inactive H3R conformation and to study the consequences of prolonged inhibition of the H3R.

Design and synthesis of histamine H3/H4 receptor ligands with a cyclopropane scaffold.

We previously designed and synthesized a series of histamine analogues with an imidazolylcyclopropane scaffold and identified potent non-selective antagonists for histamine H3 and H4 receptor subtypes. In this study, to develop H4 selective ligands, we newly designed and synthesized cyclopropane-based derivatives having an indole, benzimidazole, or piperazine structure, which are components of representative H4 selective antagonists such as JNJ7777120 and JNJ10191584. Among the synthesized derivatives, imidazolylcyclopropanes 12 and 13 conjugated with a benzimidazole showed binding affinity to the H3 and H4 receptors comparable to that of a well-known non-selective H3/H4 antagonist, thioperamide. These results suggest that the binding modes of the cyclopropane-based H3/H4 ligands in the H4 receptor can be different from those of the indole/benzimidazole-piperazine derivatives.

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.

Non-Imidazole Histamine H3 Ligands. Part VII. Synthesis, In Vitro and In Vivo Characterization of 5-Substituted-2-thiazol-4-n-propylpiperazines.

H3 receptors present on histaminergic and non-histaminergic neurons, act as autoreceptors or heteroreceptors controlling neurotransmitter release and synthesis. Previous, studies have found that the compound N-methyl-N-3-phenylalkyl-2-[2-(4-n-propylpiperazin-1-yl)-1,3-thiazol-5-yl]ethan-1-amine (ADS-531, 2c) exhibits high in vitro potency toward H3 guinea pig jejunal receptors, with pA2 = 8.27. To optimize the structure of the lead compound ADS-531, a series of 5-substituted-2-thiazol-4-n-propylpiperazines 3 were synthesized and subjected to in vitro pharmacological characterization; the alkyl chain between position 2 of the thiazole ring and the terminal secondary N-methylamino function was elongated from three to four methylene groups and the N-methylamino functionality was substituted by benzyl-, 2-phenylethyl-, and 3-phenyl-propyl- moieties. SAR studies on novel non-imidazole, 5-substituted-2-thiazol-4-n-propyl-piperazines 3 showed that the most active compound 3a (pA2 = 8.38), additionally possessed a weak competitive H1-antagonistic activity. Therefore, compound ADS-531, which did not exhibit any H1-antagonistic activity, was chosen for further evaluation for its affinity to the recombinant rat and human histamine H3 receptors (rH3R and hH3R, respectively). ADS-531 exhibited nanomolar affinity for both rH3R and hH3R receptors. It was also shown that, ADS-531 given subchronically to rats (s.c. 3 mg/kg, 5 days) penetrated the brain, where it affected dopamine, noradrenaline and serotonin concentration; however, it did not affect histamine concentration nor feeding behavior.

Synthesis of Gallic Acid Analogs as Histamine and Pro-Inflammatory Cytokine Inhibitors for Treatment of Mast Cell-Mediated Allergic Inflammation.

Gallic acid (3,4,5-trihydroxybenzoic acid), is a natural product found in various foods and herbs that are well known as powerful antioxidants. Our previous report demonstrated that it inhibits mast cell-derived inflammatory allergic reactions by blocking histamine release and pro-inflammatory cytokine expression. In this report, various amide analogs of gallic acid have been synthesized by introducing different amines through carbodiimide-mediated amide coupling and Pd/C-catalyzed hydrogenation. These compounds showed a modest to high inhibitory effect on histamine release and pro-inflammatory cytokine expression. Among them, the amide bearing (S)-phenylglycine methyl ester 3d was found to be more active than natural gallic acid. Further optimization yielded several (S)- and (R)-phenylglycine analogs that inhibited histamine release in vitro. Our findings suggest that some gallamides could be used as a treatment for allergic inflammatory diseases.

Synthesis and evaluation of a 2-benzothiazolylphenylmethyl ether class of histamine H4 receptor antagonists.

Synthesis and biological evaluation of a new class of histamine H4 receptor ligands, distinct from the previously reported chemotypes, are described. A virtual screening of our corporate compound collection identified a hit with an undesired dual H3R/H4R activity. Chemical exploration led to the discovery of a more potent and selective 2-benzothiazolylphenylmethyl ether lead compound.

Novel 1-Phenyl-3-hydroxy-4-pyridinone Derivatives as Multifunctional Agents for the Therapy of Alzheimer's Disease.

A series of novel 1-phenyl-3-hydroxy-4-pyridinone derivatives were designed and synthesized as multifunctional agents for Alzheimer's disease (AD) therapy through incorporation of 3-hydroxy-4-pyridinone moiety from deferiprone into the scaffold of H3 receptor antagonists. Most of these new compounds displayed designed quadruple functions, H3 receptor antagonism, Aß aggregation inhibition, metal ion chelation, and radical scavenging. Especially, the most promising compound 5c displayed nanomolar IC50 values in H3 receptor antagonism with high selectivity, efficient capability to interrupt the formation of Aß(1-42) fibrils, good copper and iron chelating properties, and more potent 2,2'-azino-bis(3-ethyl-benzothiazoline-6-sulfonic acid) radical cation (ABTS(•+)) scavenging activity than Trolox. Further biological evaluation revealed that it did not show obvious cytotoxicity and hERG potassium channel inhibition at micromolar concentration. In addition, compound 5c demonstrated suitable pharmacokinetic properties and acceptable blood-brain barrier (BBB) permeability in vivo. All these results indicate that compound 5c is a potential multifunctional candidate for AD therapy.

Continuous-Flow Multistep Synthesis of Cinnarizine, Cyclizine, and a Buclizine Derivative from Bulk Alcohols.

Cinnarizine, cyclizine, buclizine, and meclizine belong to a family of antihistamines that resemble each other in terms of a 1-diphenylmethylpiperazine moiety. We present the development of a four-step continuous process to generate the final antihistamines from bulk alcohols as the starting compounds. HCl is used to synthesize the intermediate chlorides in a short reaction time and excellent yields. This methodology offers an excellent way to synthesize intermediates to be used in drug synthesis. Inline separation allows the collection of pure products and their immediate consumption in the following steps. Overall isolated yields for cinnarizine, cyclizine, and a buclizine derivative are 82, 94, and 87 %, respectively. The total residence time for the four steps is 90 min with a productivity of 2 mmol h(-1) .

The cooperative effect of Lewis pairs in the Friedel-Crafts hydroxyalkylation reaction: a simple and effective route for the synthesis of (±)-carbinoxamine.

An efficient C-C bond formation strategy between aromatic/heteroaromatic π-nucleophiles and Lewis acid activated aldehydes is described. This aromatic electrophilic substitution reaction of arenes or heteroarenes is facilitated by Lewis acid AlBr3. Aromatic rings with electron donating substituents are excellent nucleophilic counterparts in this reaction, generating carbinols in excellent yields (61-94%). The formation of triarylmethanes is also witnessed in the case of certain reactive aldehydes and aromatic π-nucleophiles through reactive carbocation formation. The formation of triarylmethane is reduced to a greater extent via retardation of the second π-nucleophile addition through a Lewis base, for example, pyridine, coordination with an aluminium alkoxide intermediate. Various aliphatic aldehydes also underwent Friedel-Crafts type hydroxyalkylation and generated the expected carbinols in moderate yields (41-53%) in the presence of AlBr3. This protocol has been successfully applied to the synthesize of the (±)-carbinoxamine, a therapeutically important histamine H1 antagonist, in a one-pot manner.

Synthesis and characterization of new bivalent agents as melatonin- and histamine H3-ligands.

Melatonin is an endogenous molecule involved in many pathophysiological processes. In addition to the control of circadian rhythms, its antioxidant and neuroprotective properties have been widely described. Thus far, different bivalent compounds composed by a melatonin molecule linked to another neuroprotective agent were synthesized and tested for their ability to block neurodegenerative processes in vitro and in vivo. To identify a novel class of potential neuroprotective compounds, we prepared a series of bivalent ligands, in which a prototypic melatonergic ligand is connected to an imidazole-based H3 receptor antagonist through a flexible linker. Four imidazolyl-alkyloxy-anilinoethylamide derivatives, characterized by linkers of different length, were synthesized and their binding affinity for human MT1, MT2 and H3 receptor subtypes was evaluated. Among the tested compounds, 14c and 14d, bearing a pentyl and a hexyl linker, respectively, were able to bind to all receptor subtypes at micromolar concentrations and represent the first bivalent melatonergic/histaminergic ligands reported so far. These preliminary results, based on binding affinity evaluation, pave the way for the future development of new dual-acting compounds targeting both melatonin and histamine receptors, which could represent promising therapeutic agents for the treatment of neurodegenerative pathologies.

Histamine receptors and antihistamines: from discovery to clinical applications.

The synthesis and the identification of histamine marked a milestone in both pharmacological and immunological research. Since Sir Henry Dale and Patrick Laidlaw described some of its physiological effects in vivo in 1910, histamine has been shown to play a key role in the control of gastric acid secretion and in allergic disorders. Using selective agonists and antagonists, as well as molecular biology tools, four histamine receptors (H1R, H2R, H3R and H4R) have been identified. The Nobel Prize in Physiology and Medicine was awarded to Daniel Bovet in 1957 for the discovery of antihistamines (anti-H1R) and to Sir James Black in 1988 for the identification of anti-H2R antagonists. Anti-H1R and anti-H2R histamine receptor antagonists have revolutionized the treatment of certain allergic disorders and gastric acid-related conditions, respectively. More recently, anti-H3R antagonists have entered early-phase clinical trials for possible application in obesity and a variety of neurologic disorders. The preferential expression of H4R by several immune cells and its involvement in the development of allergic inflammation provide the rationale for the use of anti-H4R antagonists in allergic and in other immune-related disorders.

Benzylpiperidine variations on histamine H3 receptor ligands for improved drug-likeness.

Several hH3R antagonists/inverse agonists entered clinical phases for a broad spectrum of mainly centrally occurring diseases. Nevertheless, many promising candidates failed due to their pharmacokinetic profile, mostly because of their strong lipophilicity and their dibasic character. Analysis of previously, as potential PET ligands synthesized compounds (ST-889, ST-928) revealed promising results concerning physicochemical properties and drug-likeness. Herein, the synthesis, the evaluation of the binding properties at the hH3R and the estimation of different physicochemical and drug-likeness properties of further novel benzylpiperidine variations on H3R antagonists is described. Due to the introduction of various small hydrophilic moieties in the structure, drug-likeness parameters have been improved. For instance, compound 12 (ST-1032) showed in addition to high affinity at the H3R (pKi (hH3R)=9.3) clogS, clogP, LE, LipE, and LELP values of -2.48, 2.18, 0.44, 7.14, and 4.95, respectively. Also, the keto derivative 5 (ST-1703, pKi (hH3R)=8.6) revealed LipE and LELP values of 5.25 and 6.84, respectively.

Discovery and SAR of 6-alkyl-2,4-diaminopyrimidines as histamine H4 receptor antagonists.

This report discloses the discovery and SAR of a series of 6-alkyl-2-aminopyrimidine derived histamine H4 antagonists that led to the development of JNJ 39758979, which has been studied in phase II clinical trials in asthma and atopic dermatitis. Building on our SAR studies of saturated derivatives from the indole carboxamide series, typified by JNJ 7777120, and incorporating knowledge from the tricyclic pyrimidines led us to the 6-alkyl-2,4-diaminopyrimidine series. A focused medicinal chemistry effort delivered several 6-alkyl-2,4-diaminopyrimidines that behaved as antagonists at both the human and rodent H4 receptor. Further optimization led to a panel of antagonists that were profiled in animal models of inflammatory disease. On the basis of the preclinical profile and efficacy in several animal models, JNJ 39758979 was selected as a clinical candidate; however, further development was halted during phase II because of the observation of drug-induced agranulocytosis (DIAG) in two subjects.

Synthesis and antinociception activity of new substituted phenothiazines and ethylenediamines as antihistaminic drugs.

Antihistamines play an important role in medicine when it comes to relieving seasonal or non-seasonal rhinitis, the common cold, and itching. They have also shown many various combinations of pharmacological properties such as anti-inflammatory and analgesic activities. Phenothiazines and ethylenediamines are 2 important classes of antihistamines with analgesic activities in addition to other pharmacological effects. In this study, some new derivatives of these compounds (V-IX) were synthesized and their antinociceptive behaviors were examined by pharmacological tests. The results indicated that new analogue with methyl groups produced a better analgesic activity than chlorine atoms but less than III (without any substitutions) in ethylenediamine class. Also in phenothiazine class, adding pyrimidine and pyridine substituted showed the better analgesic activity compared to other groups. Moreover, the analgesic activities proved that dimethylamine is the best group in amino alkyl side chain of these molecules relative to the substituted piperazines in new analogues.

Structure-activity relationships of new 1-substitutedmethyl-4-[5-(N-methyl-N-propylamino)pentyloxy]piperidines and selected 1-[(N-substituted-N-methyl)-3-propyloxy]-5-(N-methy-l-N-propyl)-pentanediamines as H3 -antagonists.

Novel, potent non-imidazole histamine H3 receptor antagonists have been prepared and in vitro tested as H3 -receptor antagonists (the electrically evoked contraction of the guinea-pig jejunum). The present compounds contain a 4-hydroxypiperidine core, which behaves as a conformationally restricted version of the 3-amino-1-propanol moiety common to the many previously described non-imidazole H3 ligands. Detailed structure-activity studies revealed that 1-(2-benzofuranylmethyl)- 5c (pA2 = 8.47 ± 0.05) and 1-(3-benzofuranylmethyl)-4-[5-(N-methyl-N-propyl)pentyloxy]piperidine 5d (pA2 = 8.15 ± 0.07) exhibit high potency for the H3 histamine receptor. In addition, the potency of selected 1-[(N-substituted-N-methyl)-3-propyloxy]-5-(N-methyl-N-propyl)pentanediamines as antagonist of the H3 histamine receptor was also evaluated. Replacement of the 4-hydroxypiperidine of the leads 7 and 5c by a highly flexible 3-(methylamino)propyloxy chain yields compounds 6a (pA2 = 8.02) and 6b (pA2 = 6.23) with higher and lower potency than their piperidine analogues (7, pA2 = 7.79; 5c, pA2 = 8.47), respectively. The histaminergic H1 antagonism of selected compounds 5c, 5d and 6a has been established on the isolated guinea-pig ileum by conventional methods; the pA2 values have compared with the potency of pyrilamine. None of them showed any H1 -antagonistic activity (pA2 < 4; for pyrilamine pA2 = 8.5).