Optimization and preclinical evaluation of novel histamine H3 receptor ligands: Acetyl and propionyl phenoxyalkyl piperazine derivatives

As a continuation of our search for novel histamine H₃ receptor ligands, a series of new acetyl and propionyl phenoxyalkylamine derivatives (2–25) was synthesized. Compounds with three to four carbon atoms alkyl chain spacer, composed of six various 4N-substituted piperazine moieties were evaluated for their binding properties at human histamine H₃ receptors (hH₃R). In vitro test results proved the 4-pyridylpiperazine moiety as crucial element for high hH₃R affinity (hH₃R K_i?=?5.2–115?nM). Moreover introduction of carbonyl group containing residues in the lipophilic part of molecules instead of branched alkyl substituents resulted in increased affinity in correlation to previously described series, whereas propionyl derivatives showed slightly higher affinities than those of acetyl (16 and 22vs.4 and 10; hH₃R K_i?=?5.2 and 15.4?nM vs. 10.2 and 115?nM, respectively). These findings were confirmed by molecular modelling studies, demonstrating multiple ligand-receptor interactions. Furthermore, pharmacological in vivo test results of compound 4 clearly indicate that it may affect the amount of calories consumed, thus act as an anorectic compound. Likewise, its protective action against hyperglycemia and the development of overweight has been shown. In order to estimate drug-likeness of compound 4, in silico and experimental evaluation of metabolic stability in human liver microsomes was performed.     

Synthesis and evaluation of arylpiperazines derivatives of 3,5-dioxo-(2H,4H)-1,2,4-triazine as 5-HT1AR ligands

5-HT_1AR agonist or partial agonists are established drug candidates for psychiatric and neurological disorders. We have reported the synthesis and evaluation of a series of high affinity 5-HT_1AR partial agonist PET imaging agents with greater selectivity over α-1AR. The characteristic of these molecules are 3,5-dioxo-(2H,4H)-1,2,4-triazine skeleton tethered to an arylpiperazine unit through an alkyl side chain. The most potent 5-HT_1AR agonistic properties were found to be associated with the molecules bearing C-4 alkyl group as the linker. Therefore development of 3,5-dioxo-(2H,4H)-1,2,4-triazine bearing arylpiperazine derivatives may provide high affinity selective 5-HT_1AR ligands. Herein we describe the synthesis and evaluation of the binding properties of a series of arylpiperazine analogues of 3,5-dioxo-(2H,4H)-1,2,4-triazine.     

Chlorophenoxy aminoalkyl derivatives as histamine H3R ligands and antiseizure agents

A series of twenty new chlorophenoxyalkylamine derivatives (9–28) was synthesized and evaluated on their binding properties at the human histamine H₃ receptor (hH₃R). The spacer alkyl chain contained five to seven carbon atoms. The highest affinities have shown the 4-chloro substituted derivatives 10 and 25 (K_i = 133 and 128 nM, respectively) classified as antagonists in cAMP accumulation assay (EC₅₀ = 72 and 75 nM, respectively). Synthesized compounds were also evaluated for anticonvulsant activity in Antiepileptic Screening Program (ASP) at National Institute of Neurological Disorders and Stroke (USA). Two compounds (4-chloro substituted derivatives: 20 and 26) were the most promising and showed in the MES seizure model in rats (after ip administration) ED₅₀ values of 14 mg/kg and 13.18 mg/kg, respectively. Protective indexes (PI = TD₅₀/ED₅₀) were 3.2 for 20 and 3.8 for 26. Moreover, molecular modeling and docking studies were undertaken to explain affinity at hH₃R of target compounds, and the experimentally and in silico estimation of properties like lipophilicity and metabolism was performed. Antiproliferative effects have been also investigated in vitro for selected compounds (10 and 25). These compounds neither possessed significant antiproliferative and antitumor activity, nor modulated CYP3A4 activity up to concentration of 10 μM.     

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 (pK_i (hH₃R) = 9.3) c log S, c log P, 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, pK_i (hH₃R) = 8.6) revealed LipE and LELP values of 5.25 and 6.84, respectively.     

Design,synthesis, and anti-HCV activity of thiourea compounds

A series of thiourea derivatives were synthesized and their antiviral activity was evaluated in a cell-based HCV subgenomic replicon assay. SAR studies revealed that the chain length and the position of the alkyl linker largely influenced the in vitro anti-HCV activity of this class of potent antiviral agents. Among this series of compounds synthesized, the thiourea derivative with a six-carbon alkyl linker at the meta-position of the central phenyl ring (10) was identified as the most potent anti-HCV inhibitor (EC₅₀ = 0.047 μM) with a selectivity index of 596.     

Synthesis and biological activity of novel tert-amylphenoxyalkyl (homo)piperidine derivatives as histamine H3R ligands

As a continuation of our search for novel histamine H₃ receptor ligands a series of twenty new tert-amyl phenoxyalkylamine derivatives (2–21) was synthesized. Compounds of four to eight carbon atoms spacer alkyl chain were evaluated on their binding properties at human histamine H₃ receptor (hH₃R). The highest affinities were observed for pentyl derivatives 6–8 (K_i = 8.8–23.4 nM range) and among them piperidine derivative 6 with K_i = 8.8 nM. Structures 6, 7 were also classified as antagonists in cAMP accumulation assay (with EC₅₀ = 157 and 164 nM, respectively). Moreover, new compounds were also evaluated for anticonvulsant activity in Antiepileptic Screening Program (ASP) at National Institute of Neurological Disorders and Stroke (USA). Seven compounds (2–4, 9, 11, 12 and 20) showed anticonvulsant activity at maximal electroshock (MES) test in the dose of 30 mg/kg at 0.5 h. In the subcutaneous pentetrazole (scMET) test compound 4 showed protection at 100 and 300 mg/kg dose at mice, however compounds showed high neurotoxicity in rotarod test at used doses. Also, molecular modeling studies were undertaken, to explain affinity of compounds at hH₃R (taking into the consideration X-ray analysis of compound 18). In order to estimate “drug-likeness” of selected compounds in silico and experimental evaluation of lipophilicity, metabolic stability and cytotoxicity was performed.     

Alkyl derivatives of 1,3,5-triazine as histamine H4 receptor ligands

This study focuses on the design, synthesis, molecular modeling and biological evaluation of a novel group of alkyl-1,3,5-triazinyl-methylpiperazines. New compounds were synthesized and their affinities for human histamine H₄ receptor (hH₄R) were evaluated. Among them, 4-(cyclohexylmethyl)-6-(4-methylpiperazin-1-yl)-1,3,5-triazin-2-amine (14) exhibited hH₄R affinity with a K_i of 160?nM and behaved as antagonist in functional assays: the cellular aequorin-based assay (IC₅₀?=?32?nM) and [³⁵S]GTPγS binding assay (pK_b?=?6.67). In addition, antinociceptive activity of 14 in vivo was observed in Formalin test (in mice) and in Carrageenan-induced acute inflammation test (in rats).     

Rasagiline derivatives combined with histamine H3 receptor properties

The irreversible monoamine oxidase B (MAO B) inhibitor rasagiline has been described with multiple disease modifying effects in vitro on models of Parkinson’s disease. The combination of this established drug to recently developed histamine H₃ receptor (H₃R) antagonist elements gives new impetus to the design of multitargeting ligands. Surprisingly, the 5-substituted 3-piperidinopropyloxy rasagiline derivative 1 was more potent on both targets than its 6-substituted isomer. It showed nanomolar affinities at the desired targets (MAO B IC₅₀ = 256 nM; hH₃R K_i = 2.6 nM) with a high preference over monoamine oxidase A (MAO A) and negligible affinity at histamine H₁, H₄, dopamine D₂, D₃ receptors or acetyl-/butyrylcholinesterases.     

5-HT1A and 5-HT2A receptors affinity,docking studies and pharmacological evaluation of a series of 8-acetyl-7-hydroxy-4-methylcoumarin derivatives

In this work we describe the synthesis, docking studies and biological evaluation of a focused library of novel arylpiperazinyl derivatives of 8-acetyl-7-hydroxy-4-methylcoumarin. The new compounds were screened for their 5-HT_1A and 5-HT_2A receptor affinity. Among the evaluated compounds, six displayed high affinities to 5-HT_1A receptors (4a-0.9?nM, 6a-0.5?nM, 10a-0.6?nM, 3b-0.9?nM, 6b-1.5?nM, 10b-1?nM). Compound 6a and 10a bearing a bromo- or methoxy- substituent in ortho position of the piperazine phenyl ring, were identified as potent antagonists of the 5-HT_1A receptors. In the tail suspension test, mice injected with 6a showed a dose-dependent increase in depressive-like behavior that was related to a decrease in locomotor activity. Compound 10a did not decrease or prolong immobility time nor did it affect home cage activity. Molecular docking studies using 5-HT_1A and 5-HT_2A homology models revealed structural basis of the high affinity of ortho-substituted derivatives and subtle changes in amino acid interactions patterns depending on the length of the alkyl linker.     

Synthesis and evaluation of novel ligands for the histamine H4 receptor based on a pyrrolo[2,3-d]pyrimidine scaffold

Starting from a known H₄R ligand based on a pyrimidine skeleton, a series of novel analogues based on a pyrrolo[2,3-d]pyrimidine scaffold have been prepared. Whereas the original pyrimidine congener shows good affinity at hH₄R (K_i = 0.5 μM), its lacks selectivity with a K_i value for the hH₃R of 1 μM. Within the newly synthesized pyrrolo[2,3-d]pyrimidines, several congeners show K_i values of less than 1 μM at the hH₄R and show a much improved selectivity profile. Therefore, these series represent an interesting starting point for the discovery of novel hH₄R ligands.     

New lead elements for histamine H3 receptor ligands in the pyrrolo[2,3-d]pyrimidine class

This work describes the microwave assisted synthesis of twelve novel histamine H₃ receptor ligands. They display pyrrolo[2,3-d]pyrimidine derivatives with rigidized aliphatic amines as warheads. The compounds were screened for H₃R and H₄R binding affinities in radioligand displacement assays and the most potent compounds were evaluated for H₃R binding properties in vitro and in docking studies. The combination of a rigidized H₃R warhead and the pyrrolo[2,3-d]pyrimidine scaffold resulted in selective activity at the H₃ receptor with a pK_i value of 6.90 for the most potent compound. A bipiperidine warhead displayed higher affinity than a piperazine or morpholine motif, while a naphthyl moiety in the arbitrary region increased affinity compared to a phenyl derivative. The compounds can be starting points for novel, simply synthesized histamine H₃ receptor ligands.     

Structure activity relationship study of benzo[d]thiazol-2(3H)one based σ receptor ligands

Herein we report the SAR study which involved structural modifications to the linker length, aryl substitution and alkylamine ring size of the benzo[d]thiazol-2(3H)one based sigma receptor (σ) ligands. Many compounds in this series displayed low nanomolar affinity for the σ receptor subtypes. In particular, 8a showed high affinity (σ-1 K_i = 4.5 nM) for σ-1 receptors and moderately high selectivity (483-fold) over σ-2 receptors.     

Rigidized 1-aryl sulfonyl tryptamines: synthesis and pharmacological evaluation as 5-HT6 receptor ligands

A series of N₁-arylsulfonyl-3-(pyrrolidin-3-yl)-1H-indole and N₁-arylsulfonyl-3-(4-chloro-2,5-dihydro-1H-pyrrol-3-yl)-1H-indole derivatives (tryptamine derivatives with rigidized side chain) have been prepared and tested for their binding affinity to 5-HT₆ receptor. Several compounds displayed potent binding affinity for the 5-HT₆ receptor when tested in in vitro binding assay. The primary SAR indicates that rigidification of dimethylamino alkyl chain at C₃ of indole carbon maintains the binding affinity to 5-HT₆R. The lead compound N₁-benzenesulfonyl-3-(4-chloro-1-methyl-2,5-dihydro-1H-pyrrol-3-yl)-1H-indole, 10a (K_b = 0.1 nM) has shown excellent in vitro affinity and was active in animal models of cognition like NORT and water maze.     

Anticonvulsant properties of histamine H3 receptor ligands belonging to N-substituted carbamates of imidazopropanol

Ligands targeting central histamine H₃ receptors (H₃Rs) for epilepsy might be a promising therapeutic approach. Therefore, the previously described and structurally strongly related imidazole-based derivatives belonging to carbamate class with high H₃R in vitro affinity, in-vivo antagonist potency, and H₃R selectivity profile were investigated on their anticonvulsant activity in maximal electroshock (MES)-induced and pentylenetetrazole (PTZ)-kindled seizure models in Wistar rats. The effects of systemic injection of H₃R ligands 1–13 on MES-induced and PTZ-kindled seizures were screened and evaluated against the reference antiepileptic drug (AED) Phenytoin (PHT) and the standard histamine H₃R inverse agonist/antagonist Thioperamide (THP) to determine their potential as new antiepileptic drugs. Following administration of the H₃R ligands 1–13 (5, 10 and 15 mg/kg, ip) there was a significant dose dependent reduction in MES-induced seizure duration. The protective action observed for the pentenyl carbamate derivative 4, the most protective H₃R ligand among 1–13, was significantly higher (P <0.05) than that of standard H₃R antagonist THP, and was reversed when rats were pretreated with the selective H₃R agonist R-(α)-methyl-histamine (RAMH) (10 mg/kg), or with the CNS penetrant H₁R antagonist Pyrilamine (PYR) (10 mg/kg). In addition, subeffective dose of H₃R ligand 4 (5 mg/kg, ip) significantly potentiated the protective action in rats pretreated with PHT (5 mg/kg, ip), a dose without appreciable protective effect when given alone. In contrast, pretreatment with H₃R ligand 4 (10 mg/kg ip) failed to modify PTZ-kindled convulsion, whereas the reference drug PHT was found to fully protect PTZ-induced seizure. These results indicate that some of the investigated imidazole-based H₃R ligands 1–13 may be of future therapeutic value in epilepsy.     

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.     

Synthesis and structure–activity relationships of phenothiazine carboxylic acids having pyrimidine-dione as novel histamine H1 antagonists

A series of phenothiazine carboxylic acid derivatives, having 6-amino-pyrimidine-2,4(1H,3H)-dione moiety via a appropriate linker, were synthesized and evaluated for their affinity toward human histamine H₁ receptor and Caco-2 cell permeability. Selected compounds were further evaluated for their oral anti-histaminic activity in mice and bioavailability in rats. Finally, promising compounds were examined for their anti-inflammatory potential in mice OVA-induced biphasic cutaneous reaction model. Among the compounds tested, phenothiazineacetic acid compound 27 showed both histamine H₁-receptor antagonistic activity and anti-inflammatory activity in vivo model.     

New 8-amino-1,2,4-triazolo[4,3-a]pyrazin-3-one derivatives. Evaluation of different moieties on the 6-aryl ring to obtain potent and selective human A2A adenosine receptor antagonists

In this work, further structural investigations on the 8-amino-2-phenyl-6-aryl-1,2,4-triazolo[4,3-a]pyrazin-3-one series were carried out to achieve potent and selective human A_2A adenosine receptor (AR) antagonists. Different ether and amide moieties were attached at the para-position of the 6-phenyl ring, thus leading to compounds 1–9 and 10–18, respectively. Most of these moieties contained terminal basic rings (pyrrolidine, morpholine, piperidine and substituted piperazines) which were thought to confer good physicochemical and drug-like properties.Compounds 11–16, bearing the amide linker, possessed high affinity and selectivity for the hA_2A AR (K_i = 3.6–11.8 nM). Also derivatives 1–9, featuring an ether linker, preferentially targeted the hA_2A AR but with lower affinity, compared to those of the relative amide compounds. Docking studies, carried out at the hA_2A AR binding site, highlighted some crucial ligand-receptor interactions, particularly those provided by the appended substituent whose nature deeply affected hA_2A AR affinity.     

Voltage sensitivities and deactivation kinetics of histamine H3 and H4 receptors

Agonist potency at some neurotransmitter receptors has been shown to be regulated by voltage, a mechanism which has been suggested to play a crucial role in the regulation of neurotransmitter release by inhibitory autoreceptors. Likewise, receptor deactivation rates upon agonist removal have been implicated in autoreceptor function. Using G protein-coupled potassium (GIRK) channel activation in Xenopus oocytes as readout of receptor activity, we have investigated the voltage sensitivities and signaling kinetics of the hH₃⁴⁴⁵ and hH₃³⁶⁵ isoforms of the human histamine H₃ receptor, which functions as an inhibitory auto- and heteroreceptor in the nervous system. We have also investigated both the human and the mouse homologues of the related histamine H₄ receptor, which is expressed mainly on hematopoietic cells. We found that the hH₃⁴⁴⁵ receptor is the most sensitive to voltage, whereas the hH₃³⁶⁵ and H₄ receptors are less affected. We further observed a marked difference in response deactivation kinetics between the hH₃⁴⁴⁵ and hH₃³⁶⁵ isoforms, with the hH₃³⁶⁵ isoform being five to six-fold slower than the hH₃⁴⁴⁵ receptor. Finally, using synthetic agonists, we found evidence for agonist-specific voltage sensitivity at the hH₄ receptor. The differences in voltage sensitivities and deactivation kinetics between the hH₃⁴⁴⁵, hH₃³⁶⁵, and H₄ receptors might be relevant to their respective physiological roles.     

Design,synthesis and pharmacological evaluation of 4-(piperazin-1-yl methyl)-N1-arylsulfonyl indole derivatives as 5-HT6 receptor ligands

4-(Piperazin-1-yl methyl)-N₁-arylsulfonyl indole derivatives were designed and synthesized as 5-HT₆ receptor (5-HT₆R) ligands. The lead compound 6a, from this series shows potent in vitro binding affinity, good PK profile, no CYP liabilities and activity in animal models of cognition.     

Synthesis and evaluation of N-alkyl-S-[3-(piperidin-1-yl)propyl]isothioureas: High affinity and human/rat species-selective histamine H3 receptor antagonists

S-Alkyl-N-alkylisothiourea compounds containing various cyclic amines were synthesized in the search for novel nonimidazole histamine H₃ receptor (H₃R) antagonists. Among them, four N-alkyl S-[3-(piperidin-1-yl)propyl]isothioureas 18, 19, 22, and 23 were found to exhibit potent and selective H₃R antagonistic activities against in vitro human H₃R, but were inactive against in vitro human H₄R. Furthermore, three alkyl homologs 18–20 showed inactivity for histamine release in in vivo rat brain microdialysis, suggesting differences in antagonist affinities between species. In addition, in silico docking studies of N-[4-(4-chlorophenyl)butyl]-S-[3-piperidin-1-yl)propyl]isothiourea 19 and a shorter homolog 17 with human/rat H₃Rs revealed that structural differences between the antagonist-docking cavities of rat and human H₃Rs were likely caused by the Ala122/Val122 mutation.