Synthesis and biological activity of a novel class nicotinic acetylcholine receptors (nAChRs) ligands structurally related to anatoxin-a

The introduction of the isoxazole ring as bioisosteric replacement of the acetyl group of anatoxin-a led to a new series of derivatives binding to nicotinic acetylcholine receptors. Bulkier substitutions than methyl at the 3 position of isoxazole were shown to be detrimental for the activity. The binding potency of the most interesting compounds with α1, α7 and α3β4 receptor subtypes, was, anyway, only at micromolar level. Moreover, differently from known derivatives with pyridine, isoxazole condensed to azabicyclo ring led to no activity.     

Comparison of anti-aggregatory activities of 5-phenyl-3-(3-pyridyl)isoxazole and 5-phenyl-3-(3-pyridyl)-1,2,4-oxadiazole

Two bioisosteric analogs, 5-phenyl-3-(3-pyridyl)isoxazole and 5-phenyl-3-(3-pyridyl)-1,2,4-oxadiazole, were synthesized so as to compare their antiaggregatory activities, to determine a pharmacologically active fragment in molecules of this type, and to explore the mechanisms of action of potential antiag-gregatory compounds belonging to the class of 3,5-substituted isoxazoles. Antiaggregatory activities of these compounds were studied in vitro using three aggregation inducers, such as arachidonic acid, adenosine diphosphate, and adrenaline. It was shown that 5-phenyl-3-(3-pyridyl)-1,2,4-oxadiazole and 5-phenyl-3-(3-pyridyl)isoxazole completely suppressed platelet aggregation induced by arachidonic acid and the second wave of platelet aggregation induced by adrenaline or adenosine diphosphate. The antiaggregatory activity of substituted isoxasole was 1.1–1.5 times higher than that of substituted oxadiazole. In contrast to the isoxazole analog, 5-phenyl-3-(3-pyridyl)-1,2,4-oxadiazole in concentrations of 300–400 μM partially suppressed the first wave of aggregation induced by adenosine diphosphate. It was demonstrated that both compounds were not thrombin inhibitors in vitro at concentrations up to 250 μM. Thus, introduction of a nitrogen atom into the C4-position of the isoxazole ring changes the molecule properties. It suggests that the pharmacophoric fragment of the molecule should be the whole isoxazole or 1,2,4-oxadiazole ring but not a part of the ring as was supposed previously.     

Novel N-benzylpyridinium moiety linked to arylisoxazole derivatives as selective butyrylcholinesterase inhibitors: Synthesis,biological evaluation,and docking study

A novel series of N-benzylpyridinium moiety linked to arylisoxazole ring were designed, synthesized, and evaluated for their in vitro acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) inhibitory activities. Synthesized compounds were classified into two series of 5a-i and 5j-q considering the position of positively charged nitrogen of pyridinium moiety (3- or 4- position, respectively) connected to isoxazole carboxamide group. Among the synthesized compounds, compound 5n from the second series of compounds possessing 2,4-dichloroaryl group connected to isoxazole ring was found to be the most potent AChE inhibitor (IC₅₀ = 5.96 µM) and compound 5j also from the same series of compounds containing phenyl group connected to isoxazole ring demonstrated the most promising inhibitory activity against BChE (IC₅₀ = 0.32 µM). Also, kinetic study demonstrated competitive inhibition mode for both AChE and BChE inhibitory activity. Docking study was also performed for those compounds and desired interactions with those active site amino acid residues were confirmed through hydrogen bonding as well as π-π and π-anion interactions. In addition, the most potent compounds were tested against BACE1 and their neuroprotectivity on Aβ-treated neurotoxicity in PC12 cells which depicted negligible activity. It should be noted that most of the synthesized compounds from both categories 5a-i and 5j-q showed a significant selectivity toward BChE. However, series 5j-q were more active toward AChE than series 5a-i.     

Novel isoxazole carboxamides as growth hormone secretagogue receptor (GHS-R) antagonists

Novel isoxazole carboxamides have been identified as growth hormone secretagogue receptor (GHS-R) antagonists. Substituent modification off the 5-position of the isoxazole ring led to analogues with potent binding affinity and functional antagonism of GHS-R. A potent analogue (32) with high aqueous solubility and good GPCR selectivity was also identified as a potential pharmacological tool for in vivo studies.     

Synthesis and in vitro activity of novel isoxazolyl tetrahydropyridinyl oxazolidinone antibacterial agents

A series of isoxazolyl tetrahydropyridinyl oxazolidinones with various substituents at the 3-position of the isoxazole ring have been synthesized and their in vitro antibacterial activities (MIC) were evaluated against several Gram-positive strains including the resistant strains of Staphyloccus and Enterococcus, such as MRSA and VRE. One of the most potent compounds synthesized, 4f, showed comparable or better activity against selected bacterial strains than those of linezolid and vancomycin.     

Isoxazole substituted chromans against oxidative stress-induced neuronal damage

We have previously reported that catechol-bearing regioisomers of 5-isoxazolyl-6-hydroxy-chroman display higher in vitro neuroprotective activity, compared to hybrids with other nitrogen heterocycles, but their activity is hampered by cytotoxicity at higher concentrations. In an effort to discover non-cytotoxic isoxazole substituted chromans of high neuroprotective activity, 20 new 3- and 5-substituted (chroman-5-yl)-isoxazoles and (chroman-2-yl)-isoxazoles were synthesized using the copper(I)-catalysed cycloaddition reaction between in situ generated nitrile oxides and terminal acetylenes. An additional aim was to further explore the effect of the isoxazole ring substituents on the neuroprotective activity. The activity of these compounds against oxidative stress-induced death (oxytosis) of neuronal HT22 cells was evaluated and interesting SARs for this group of analogues were derived. The vast majority of new chroman analogues displayed high in vitro neuroprotective activity displaying EC(50) values below 1 μM and lacked cytotoxicity. The position of substituents on the isoxazole ring influences the activity of the regioisomers, with the 3-aryl-5-(chroman-5-yl)-isoxazoles, 17 and 18 and bis-chroman 20 displaying higher neuroprotective activity (EC(50)～0.3 μM) compared to other (chroman-5-yl) and (chroman-2-yl)-isoxazoles.     

Identification of diphenylalkylisoxazol-5-amine scaffold as novel activator of cardiac myosin

To identify novel potent cardiac myosin activator, a series of diphenylalkylisoxazol-5-amine compounds 4–7 have been synthesized and evaluated for cardiac myosin ATPase activation. Among the 37 compounds, 4a (CMA at 10 µM = 81.6%), 4w (CMA at 10 µM = 71.2%) and 6b (CMA at 10 µM = 67.4%) showed potent cardiac myosin activation at a single concentration of 10 µM. These results suggested that the introduction of the amino-isoxazole ring as a bioisostere for urea group is acceptable for the cardiac myosin activation. Additional structure–activity relationship (SAR) studies were conducted. Para substitution (-Cl, –OCH₃, -SO₂N(CH₃)₂) to the phenyl rings or replacement of a phenyl ring with a heterocycle (pyridine, piperidine and tetrahydropyran) appeared to attenuate cardiac myosin activation at 10 µM. Additional hydrogen bonding acceptor next to the amino group of the isoxazoles did not enhance the activity. The potent isoxazole compounds showed selectivity for cardiac myosin activation over skeletal and smooth muscle myosin, and therefore these potent and selective isoxazole compounds could be considered as a new series of cardiac myosin ATPase activators for the treatment of systolic heart failure.     

Design and diversity-oriented synthesis of novel 1,4-thiazepan-3-ones fused with bioactive heterocyclic skeletons and evaluation of their antioxidant and cytotoxic activities

This study has achieved the design and diversity-oriented synthesis of novel 1,4-thiazepine derivatives embedded with carbazole, pyrazole or isoxazole motif via microwave-assisted multicomponent reactions under solvent-free condition, thus providing a green and facile access to 1,4-thiazepine derivatives with prominent features of high structural diversity, short reaction time, high yields and environmental friendliness. More importantly, these novel compounds have been subjected to the test of in vitro antioxidant and cytotoxic activities, resulting in the finding that these 1,4-thiazepine derivatives not only have significant antioxidant activity, but also exhibit remarkably selective cytotoxicity to carcinoma cell line HCT 116.     

Bioisosteric replacement strategy for the synthesis of 1-azacyclic compounds with high affinity for the central nicotinic cholinergic receptors

Bioisosteric replacement of the isoxazole heterocycle in (3-methyl-5-isoxazolyl)methylene-azacyclic compounds with pyridine, oxadiazole, or an acyl group resulted in ligands with high to moderate affinity for the central nicotinic cholinergic receptors (IC50 = 2.0 to IC50 > 1000 nM) labeled by [3H]methylcarbamylcholine. Additionally, further support of an important distance parameter for high-affinity nicotinic compounds has been provided.     

Synthesis and evaluation of electron-rich curcumin analogues

The natural product curcumin has long been recognized for its medicinal properties and is utilized for the treatment of many diseases. However, it remains unknown whether this activity is based on its presumably promiscuous scaffold, or if it results from the Michael acceptor properties of the α,β-unsaturated 1,3-diketone moiety central to its structure. To probe this issue, electron-rich pyrazole and isoxazole analogues were prepared and evaluated against two breast cancer cell lines, which resulted in the identification of several compounds that exhibit low micromolar to mid nanomolar anti-proliferative activity. A conjugate addition study was also performed to compare the relative electrophilicity of the diketone, pyrazole and isoxazole analogues.     

3- and 5-isoxazolol zwitterions: an ab initio molecular orbital study relating to GABA agonism and antagonism

The Hartree-Fock ab initio molecular orbital method has been applied to eight compounds: GABA (gamma-amino butyric acid) (1), its partially rigidified analog, TACA (trans-4-aminocrotonic acid) (2), six isoxazolol analogs; muscimol (5-aminomethylisoxazol-3-ol (3), THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) (4), THAZ (5,6,7,8-tetrahydro-4H-isoxazolo[4,5-d]azepin-3-ol) (5), isomuscimol (3-aminomethylisoxazol-5-ol) (6), iso-THIP (4,5,6,7-tetrahydroisoxazolo[3,4-c] pyridin-5-ol) (7), and iso-THAZ (5,6,7,8-tetrahydro-4H-isoxazolo[3,4-d]azepin-5-ol) (8). GABA is an endogenous inhibitory transmitter. The four following molecules (2), (3), (4) and (5) are agonist: they bind themselves to the GABA receptors and induce approximately the same effect as GABA. (6) is lightly agonist, presenting a lower affinity. Compounds (7) and (8) are antagonists, giving rise to convulsion. Optimized molecular conformations of GABA (1), muscimol (3) and isomuscimol (6) are discussed. Geometric and electronic parameters showing the presence of intramolecular hydrogen bonds are presented. The permutation of the heteroatoms in the isoxazole ring has no effect on the side-chain orientation explaining maybe the agonist character of isomuscimol, being able to adopt easily and exactly the active conformation. Atomic charge distributions and electronic overlap populations for all compounds have been computed in order to try to understand why their GABAergic activities can be so different. The computed values show that the 3-isoxazolol ring mimics in a good way the carboxylic function of GABA. They also illustrate the larger electronic delocalization within the 5-isoxazolol ring and therefore the resulting antagonist character, except for isomuscimol.     

Electron spin resonance studies of the effects of naturally-occurring excitotoxic amino acid analogues on the physical state of membrane proteins in human erythrocytes

The interaction of the neurotoxic natural products, kainic and ibotenic acids, both of which are also excitatory neurotransmitters and amino acid analogues of glutamic acid, along with the latter compound, with human erythrocyte membranes has been investigated by electron spin resonance methods. Only ibotenic acid caused a statistically significant alteration in the physical state of membrane proteins (P = 0.01) while none of these excitotoxins measurably affected motion of membrane lipids. In order to further investigate some of the molecular characteristics of ibotenic acid that may have contributed to its effect on the conformation of membrane proteins, similar spin labeling studies were performed employing the decarboxylation product and parent ring compound of this excitotoxin, muscimol and isoxazole, respectively. No effect of either of these latter compounds was observed suggesting that the carboxylic acid group of ibotenic acid is essential for its interaction with membrane proteins. These results are discussed in relation to the known different neurotoxic and physiological effects of kainic and ibotenic acids and muscimol.     

Design and synthesis of potent, isoxazole-containing renin inhibitors

The design and optimization of a novel isoxazole S(1) linker for renin inhibitor is described herein. This effort culminated in the identification of compound 18, an orally bioavailable, sub-nanomolar renin inhibitor even in the presence of human plasma. When compound 18 was found to inhibit CYP3A4 in a time dependent manner, two strategies were pursued that successfully delivered equipotent compounds with minimal TDI potential.     

Computational analyses of curcuminoid analogs against kinase domain of HER2

Background

Human epidermal growth factor receptor 2 (HER2) has an important role in cancer aggressiveness and poor prognosis. HER2 has been used as a drug target for cancers. In particular, to effectively treat HER2-positive cancer, small molecule inhibitors were developed to target HER2 kinase. Knowing that curcumin has been used as food to inhibit cancer activity, this study evaluated the efficacy of natural curcumins and curcumin analogs as HER2 inhibitors using in vitro and in silico studies. The curcumin analogs considered in this study composed of 4 groups classified by their core structure, β-diketone, monoketone, pyrazole, and isoxazole.

Results

In the present study, both computational and experimental studies were performed. The specificity of curcumin analogs selected from the docked results was examined against human breast cancer cell lines. The screened curcumin compounds were then subjected to molecular dynamics simulation study. By modifying curcumin analogs, we found that protein-ligand affinity increases. The benzene ring with a hydroxyl group could enhance affinity by forming hydrophobic interactions and the hydrogen bond with the hydrophobic pocket. Hydroxyl, carbonyl or methoxy group also formed hydrogen bonds with residues in the adenine pocket and sugar pocket of HER2-TK. These modifications could suggest the new drug design for potentially effective HER2-TK inhibitors. Two outstanding compounds, bisdemethylcurcumin (AS-KTC006) and 3,5-bis((E)-3,4-dimethoxystyryl)isoxazole (AS-KTC021 ),were well oriented in the binding pocket almost in the simulation time, 30 ns. This evidence confirmed the results of cell-based assays and the docking studies. They possessed more distinguished interactions than known HER2-TK inhibitors, considering them as a promising drug in the near future.

Conclusions

The series of curcumin compounds were screened using a computational molecular docking and followed by human breast cancer cell lines assay. Both AS-KTC006 and AS-KTC021 could inhibit breast cancer cell lines though inhibiting of HER2-TK. The intermolecular interactions were confirmed by molecular dynamics simulation studies. This information would explore more understanding of curcuminoid structures and HER2-TK.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2105-15-261) contains supplementary material, which is available to authorized users.     

Synthesis and SAR of novel isoxazoles as potent c-jun N-terminal kinase (JNK) inhibitors

The design and synthesis of isoxazole 3 is described, a potent JNK inhibitor with two fold selectivity over p38. Optimization of this scaffold led to compounds 27 and 28 which showed greatly improved selectivity over p38 by maintaining the JNK3 potency of compound 3. Extensive SAR studies will be described as well as preliminary in vivo data of the two lead compounds.     

Isoxazole-type derivatives related to combretastatin A-4, synthesis and biological evaluation

Novel combretastatin analogues bearing various five-membered heterocycles with consecutive oxygen and nitrogen atoms, in place of the olefinic bridge of CA4, have been synthesized (isoxazole, isoxazoline, oxadiazole, etc). These compounds have been evaluated for cytotoxicity and their ability to inhibit the tubulin assembly. On the basis of the relative position of the aromatic A- and B-rings on the heterocyclic moiety, they could be split in two classes, the alpha,gamma- or alpha,beta-diaryl heterocyclic derivatives. In the first series, the 3,5-diaryloxadiazole 9a displayed comparable antitubulin activity to that of CA4, but was devoid of cytotoxic effects. Among the alpha,beta-diaryl heterocyclic derivatives, the 4,5-diarylisoxazole 35 exhibited greater antitubulin activity than that of CA4 (0.75 vs 1.2 microM), but modest antiproliferative activity. These data showed that minor alteration in the chemical structure of the heterocyclic ring and its relative orientation with regard to the two phenyl rings of CA4 could dramatically influence the tubulin binding properties.     

Synthesis and evaluation of 4-(substituted styryl/alkenyl)-3,5-bis(4-hydroxyphenyl)-isoxazoles as ligands for the estrogen receptor

A series of 3,5-bis (4-hydroxyphenyl) isoxazoles bearing a styryl/alkyl vinyl group at the 4-position were prepared and evaluated as ligands for the estrogen receptor-alpha (ERα). The target compounds were prepared using the Suzuki reaction to couple an iodo-isoxazole intermediate with a series of styryl/alkenyl boronic acids, followed by O-demethylation. The products were evaluated for their estrogen receptor-α ligand binding domain (ERα-LBD) binding affinity using a competitive binding assay. The 4-(4-hydroxystyryl) derivative 4h displays binding properties similar to those of the previously described pyrazole class of ER ligands, indicating that the ERα-LBD tolerates the presence of the added vinyl group at the 4-position of the isoxazole ring.     

Synthesis of some novel androstanes as potential aromatase inhibitors

Novel pyrazole and isoxazole derivatives (6-9) were synthesized as a aromatase inhibitors. Pyrazole was synthesized from hydrazine hydrate and isoxazoles from hydroxylamine hydrochloride under different conditions. Molecular docking studies were carried out for the synthesized compounds. The best score was obtained for the compound (9) followed by compound (6) while compound (8) afforded poorest of the score. Aromatase inhibitory activity for compound (6) having pyrazole ring at 2,3 position showed highest activity followed by nitrile derivative (9). Isomeric forms of isoxazole (7 and 8) showed very poor activity compared to fadrozole and aminoglutethimide. Preliminary kinetic studies have shown that both of the active compounds (6 and 9) are reversible inhibitors of the enzyme.     

Structure-activity relationships of acyloxyamidine cytomegalovirus DNA polymerase inhibitors

This paper describes the structure activity relationships of a new class of cytomegalovirus DNA polymerase inhibitors having two aryl groups joined by an acyloxyamidine linker. Examination of a series of analogues in which the terminal groups are varied revealed a very narrow SAR around the 2,4-dichlorophenyl group of the lead compound, but a variety of replacements for the benzothiazole ring are compatible with activity. The most notable of these is the isoxazole ring of compound 78, which provides a 30-fold enhancement in potency compared to the lead compound. We also describe the design, synthesis and evaluation of 10 analogues in which the acyloxyamidine linker is modified or replaced by an isosteric group. Structure-activity relationship studies identified the linker -NH2 group as a critical pharmacophoric element. Ab initio molecular orbital calculations combined with qualitative estimates of steric interaction energies suggest that the lowest energy conformations of the acyloxyamidine linker are characterized by an extended planar CAr-C=N-O-C arrangement and either a syn-periplanar or anti-periplanar N-O-C-C(Ar') arrangement. Only the anti-periplanar conformation was observed in the crystal structures of three acyloxyamidines. The most active of the linker-modified compounds designed on the basis of these studies is the amidine carbamate 20, which is approximately one-third as potent in the cytomegalovirus DNA polymerase inhibition assay as the comparator acyloxyamidine 53. The activity of 20 suggests that acyloxyamidines may bind to the cytomegalovirus DNA polymerase via an anti-periplanar conformation similar to that observed in the crystal structure of acyloxyamidine 36.