Cyclopropane-based conformational restriction of GABA by a stereochemical diversity-oriented strategy: Identification of an efficient lead for potent inhibitors of GABA transports

A series of cyclopropane-based conformationally restricted γ-aminobutyric acid (GABA) analogs with stereochemical diversity, that is, the trans- and cis-2,3-methano analogs Ia and Ib and their enantiomers ent-Ia and ent-Ib, and also the trans- and cis-3,4-methano analogs IIa and IIb and their enantiomers ent-IIa and ent-Iib, were synthesized from the chiral cyclopropane units Type-a and Type-b that we developed. These analogs were systematically evaluated with four GABA transporter (GAT) subtypes. The trans-3,4-methano analog IIa had inhibitory effects on GAT3 (IC₅₀ = 23.9 μM) and betaine-GABA transporter1 (5.48 μM), indicating its potential as an effective lead compound for the development of potent GAT inhibitors due to its hydrophilic and low molecular weight properties and excellent ligand efficiency.     

Synthesis and structural and pharmacological properties of cyclopropane-based conformationally restricted analogs of 4-methylhistamine as histamine H3/H4 receptor ligands

On the basis of the previous results on a histamine H₄ receptor agonist 4-methylhistamine and a cyclopropane-based conformationally restricted analog CEIC (3) with potent H₃/H₄ receptor antagonistic effect, 4-methylhistamine analogs 4 and 5 of CEIC were designed and synthesized. Compound 4 showed strong affinity (K_i = 38.7 nM) for the H₃ receptor, which was more potent than a well-known H₃ antagonist thioperamide. Stable tautomer and conformation of 3 and 4, which can affect the pharmacological activity, were analyzed by ab initio calculations.     

Unsaturated Analogues of the Neurotransmitter GABA: <Emphasis Type="Italic">trans</Emphasis>-4-Aminocrotonic, <Emphasis Type="Italic">cis</Emphasis>-4-Aminocrotonic and 4-Aminotetrolic Acids

Analogues of the neurotransmitter GABA containing unsaturated bonds are restricted in the conformations they can attain. This review traces three such analogues from their synthesis to their use as neurochemicals. trans-4-Aminocrotonic acid was the first conformationally restricted analogue to be extensively studied. It acts like GABA across a range of macromolecules from receptors to transporters. It acts similarly to GABA on ionotropic receptors. cis-4-Aminocrotonic acid selectively activates bicuculline-insensitive GABA_C receptors. 4-Aminotetrolic acid, containing a triple bond, activates bicuculline-sensitive GABA_A receptors. These findings indicate that GABA activates GABA_A receptors in extended conformations and GABA_C receptors in folded conformations. These and related analogues are important for the molecular modelling of ionotropic GABA receptors and to the development of new agents acting selectively on these receptors.     

Synthesis of 1-arylpiperazyl-2-phenylcyclopropanes designed as antidopaminergic agents: cyclopropane-based conformationally restricted analogs of haloperidol

A series of the cyclopropane-based conformationally restricted analogs of haloperidol were designed as potential antidopaminergic agents and were effectively synthesized using highly stereoselective Grignard reaction with tert-butanesulfinyl imines as the key step. Pharmacological evaluation of the compounds showed that the conformational restriction method can effectively work for improving the pharmacological selectivity of a parent compound and also for investigating the bioactive conformation.     

Cyclobutane analogs of GABA

Bothcis-andtrans-3-aminocyclobutane-1-carboxylic acid have been synthesized as conformationally restricted analogs of GABA. The cis isomer displayed weak to moderate GABA-like activity with respect to (1) inhibition of GABA uptake in rat brain minislices, (2) inhibition of sodium-independent binding of GABA to rat brain membranes, (3) activity as a substrate for GABA aminotransferase, and (4) depression of the firing rate of cat spinal neurons in vivo. The trans isomer was less effective on all four assays. The result has been interpreted in terms of the conformational pinning back of the polar groups by the cyclobutane ring in the trans GABA analog so that unfavorable steric interactions would occur between one of the methylene groups and a region of steric hindrance at the active sites for particular GABA processes.     

Syntheses and evaluation of fluorinated conformationally restricted analogues of GABA as potential inhibitors of GABA aminotransferase

Inhibition of gamma-aminobutyric acid aminotransferase (GABA-AT) could raise the concentration of GABA, an inhibitory neurotransmitter in the human brain, and could have therapeutic applications for a variety of neurological diseases including epilepsy. Four fluorine-containing analogues of GABA with conformations restricted by a cyclohexane ring system were designed and synthesized, but unlike some of their five-membered ring counterparts, minimal inhibition of GABA-AT was observed. It is likely that the rigid chair conformation of these compounds cannot be accommodated well in the enzyme's active site.     

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.     

Novel autophagy modulators: Design and synthesis of (+)-epogymnolactam analogues and structure-activity relationship

(+)-Epogymnolactam (1) was discovered as a novel autophagy inducer from a culture of Gymnopus sp. in our laboratory. To determine structure-activity relationships among (+)-epogymnolactam analogues comparing with cerulenin (2), we synthesized 5 analogues including (?)-epogymnolactam (3) having each different functional group, and 3 analogues with different side-chain lengths. Five analogues, 3, 4, 5, 6, and 7 did not significantly increase the ratio of LC3-II to LC3-I as an autophagy marker in NIH3T3 cells. These results suggest that presence and stereochemistry of (2R,3S)-epoxy group and cyclic syn-form (1b) of 1 are important for the activity as autophagy inducer. Hexyl analogue (8) as well as 1 having butyl side-chain dose-dependently increased the ratio of LC3-II to LC3-I, whereas octyl analogue (9) and 2 rather decreased the ratio. Decyl analogue (10) did not give a change in the ratio. Although 8 seemed to be an excellent autophagy inducer, it dose-dependently increased SQSTM1 (p62) as in the case of 2, whereas 1 showed a slight dose-dependent decrease of p62 as an index of autophagic protein degradation. These observations suggest that 8 is an autophagy modulator with different molecular target from 1 or 2.     

Conformational restriction approach to BACE1 inhibitors II: SAR study of the isocytosine derivatives fixed with a cis-cyclopropane ring

To improve the efficacy of the conformationally restricted BACE1 inhibitors, structural modifications were investigated using two strategies: (a) modification of the terminal aromatic ring and (b) insertion of a spacer between the aromatic rings. In the latter approach, another type of inhibitor 17 bearing an ethylene spacer between two aromatic rings was found to exhibit good BACE1 inhibitory activity, while the corresponding conformationally unrestricted compound 25 showed no activity. This result revealed an interesting effect of a conformational restriction with a cyclopropane ring.     

Preparation and characterization of N-(3-pyridinyl) spirocyclic diamines as ligands for nicotinic acetylcholine receptors

Several N-pyridin-3-yl spirobicyclic diamines, designed as conformationally restricted analogs of tebanicline (ABT-594), were synthesized as novel ligands for nicotinic acetylcholine receptors (nAChR). The spirocyclic compounds exhibited weaker binding affinity, than other constrained analogs in accord with a pharmacophore model. Nevertheless, some (1a, 1b) possessed (partial) agonist potencies comparable to nicotine at the α4β2 subtype, but with greatly improved selectivity relative to the α3β4* nAChR.     

Synthesis and analgesic profile of conformationally constrained N-acylhydrazone analogues: Discovery of novel N-arylideneamino quinazolin-4(3H)-one compounds derived from natural safrole

In this work we reported the synthesis and evaluation of the analgesic, anti-inflammatory, and platelet anti-aggregating properties of new 3-(arylideneamino)-2-methyl-6,7-methylenedioxy-quinazolin-4(3H)-one derivatives (3a–j), designed as conformationally constrained analogues of analgesic 1,3-benzodioxolyl-N-acylhydrazones (1) previously developed at LASSBio. Target compounds were synthesized in very good yields exploiting abundant Brazilian natural product safrole (2) as starting material. The pharmacological assays lead us to identify compounds LASSBio-1240 (3b) and LASSBio-1272 (3d) as new analgesic prototypes, presenting an antinociceptive profile more potent and effective than dipyrone and indomethacin used, respectively, as standards in AcOH-induced abdominal constrictions assay and in the formalin test. These results confirmed the success in the exploitation of conformation restriction strategy for identification of novel cyclic N-acylhydrazone analogues with optimized analgesic profile.     

Structure-based design of some isonicotinic acid hydrazide analogues as potential antitubercular agents

New pyridine derivatives were designed and synthesized as Isonicotinic acid hydrazide (INH) analogues. The synthesized compounds were evaluated for their antitubercular activity against Mycobacterium tuberculosis strain H₃₇R_v. Ten compounds (3c, 3e-g, 5a-c, 6h, 10 and 11b) showed promising antitubercular activity with MIC range 7.30 µM–19.39 µM. Compounds 3e, 3g, 5b and 11b were the most potent analogues, with MIC 7.30–8.74 µM. They were equipotent to the standard drug Ethambutol (MIC 7.64 µM) and more active than the standard drug Pyrazinamide (MIC 50.77 µM). They were further examined for cytotoxicity in human embryonic kidney (HEK) cell line at the concentration of 50 µg/mL using MTT assay. Results declared that most compounds showed acceptable safety margin. Molecular Docking studies into 2-trans-enoyl-acyl carrier protein reductase, called InhA have been conducted for compounds 3e, 3g, 5b and 11b using Molecular Operating Enviroment software (MOE 2016.0802), where reasonable binding interactions have been identified and effective overall docking scores have been recorded. Their drug-likeness has been assessed using Molinspiration and Osiris software.     

Syntheses of lipophilic chalcones and their conformationally restricted analogues as antitubercular agents

Lipophilic chalcones and their conformationally restricted analogues were synthesized and evaluated for their antitubercular efficacy against Mycobacterium tuberculosis H37Rv strain. Compounds 16, 24, 25a and 25c were found to be active MIC at 60, 30, 3.5 and 7.5 μg-mL^?1. In vitro cytotoxicity of compounds 16, 24, 25a, 25c and 26 in non-cancerous human epithelial kidney cell line (HEK-293) showed that most active compound 25a was approximately 2.85 times selective towards tubercular versus healthy cells whereas compound 24 was found to be 16 times selective.     

Discovery of stereospecific cytotoxicity of (8R,8′R)-trans-arctigenin against insect cells and structure-activity relationship on aromatic ring

One of the arctigenin stereoisomers, (8R,8′R)-trans-form 1, showed stereospecific cytotoxicity against insect cells, Sf9 and NIAS-AeAl-2 cells. By the comparison with other stereoisomers, the most importance of the 8′R stereochemistry for the higher activities was clarified. On the other hand, the wider range of activity level among stereoisomers against cancer cells, HL-60, was not observed. The structure-activity relationship research using derivatives bearing (8R,8′R)-trans-form was performed to show the same level of activities of 3-iodo, 4-iodo, and 3,4-methylenedioxy derivatives 28, 29, and 36 as (8R,8′R)-trans-arctigenin 1. In the examination of thiono derivatives, 4-iodo thiono and 3,4-methylenedioxy thiono derivatives 66, 67 showed similar level of activities to that of (8R,8′R)-trans-arctigenin 1. The expression of ribosomal 28S rRNA gene of Sf9 cells was increased by (8R,8′R)-trans-arctigenin 1, whereas a degradation of DNA was not observed.     

Synthesis and structure-function analysis of Fe(II)-form-selective antibacterial inhibitors of Escherichia coli methionine aminopeptidase

Methionine aminopeptidase (MetAP) is a promising target for the development of novel antibacterial, antifungal and anticancer therapy. Based on our previous results, catechol derivatives coupled with a thiazole or thiophene moiety showed high potency and selectivity toward the Fe(II)-form of Escherichia coli MetAP, and some of them clearly showed antibacterial activity, indicating that Fe(II) is likely the physiologically relevant metal for MetAP in E. coli and other bacterial cells. To further understand the structure-function relationship of these Fe(II)-form selective MetAP inhibitors, a series of catechol derivatives was designed and synthesized by replacement of the thiazole or thiophene moiety with different five-membered and six-membered heterocycles. Inhibitory activities of these newly synthesized MetAP inhibitors indicate that many five- and six-membered rings can be accommodated by MetAP and potency on the Fe(II)-form can be improved by introducing substitutions on the heterocyles to explore additional interactions with the enzyme. The furan-containing catechols 11–13 showed the highest potency at 1 μM on the Fe(II)-form MetAP, and they were also among the best inhibitors for growth inhibition against E. coli AS19 strain. These findings provide useful information for the design and discovery of more effective MetAP inhibitors for therapeutic applications.     

Novel benzothiazole hydrazine carboxamide hybrid scaffolds as potential in vitro GABA AT enzyme inhibitors: Synthesis,molecular docking and antiepileptic evaluation

In the present study, a series of newer benzothiazole derivatives containing thiazolidin-4-one (5a-g) and azetidin-2-one (6a-g), were synthesized by the cyclization of benzothiazolyl arylidene hydrazine carboxamide derivatives with thioglycolic acid and chloroacetyl chloride, respectively. Results of in vivo anticonvulsant screening revealed that compounds having 2,4-dicholoro (5c and 6c) and 4-nitro substituent (5g) at the phenyl ring have promising anticonvulsant activities without any neurotoxicity. Selected compounds were also evaluated for their in vitro GABA AT inhibition. The results indicated that compound 5c (IC₅₀ 15.26 μM) exhibited excellent activity as compared to the standard drug vigabatrin (IC₅₀ 39.72 μM) suggesting the potential of these benzothiazole analogues as new anticonvulsant agents.     

Synthesis and biological evaluation of novel N-substituted nipecotic acid derivatives with a trans-alkene spacer as potent GABA uptake inhibitors

Our study presents the synthesis and structure-activity relationship (SAR) of novel N-substituted nipecotic acid derivatives closely related to DDPM-1457 [(S)-2a], a chemically stable analog of (S)-SNAP-5114 (1), in the pursuit of finding new and potent mGAT4 selective inhibitors. Iminium ion chemistry served as key step for the preparation of the desired, new N-substituted nipecotic acid derivatives containing a variety of different heterocycles attached to the nipecotic acid moiety via a trans-alkene spacer. The target compounds were characterized with regard to their potency at and subtype selectivity for the GABA transporters mGAT1-mGAT4.     

Delineation of the Role of Astroglial GABA Transporters in Seizure Control

Studies of GABA transport in neurons and astrocytes have provided evidence that termination of GABA as neurotransmitter is brought about primarily by active transport into the presynaptic, GABAergic nerve endings. There is, however, a considerable transport capacity in the astrocytes surrounding the synaptic terminals, a transport which may limit the availability of transmitter GABA leading to a higher probability of seizure activity governed by the balance of excitatory and inhibitory neurotransmission. Based on this it was hypothesized that selective inhibition of astrocytic GABA transport might prevent such seizure activity. A series of GABA analogs of restricted conformation were synthesized and in a number of collaborative investigations between Prof. Steve White at the University of Utah and medicinal chemists and pharmacologists at the School of Pharmacy and the University of Copenhagen, Denmark, GABA analogs with exactly this pharmacological property were identified. The most important analogs identified were N-methyl-exo-THPO (N-methyl-3-hydroxy-4-amino-4,5,6,7-tetrahydro-1,2-benzisoxazole) and its lipophilic analog EF-1502 ((RS)-4-[N-[1,1-bis(3-methyl-2-thienyl)but-1-en-4-yl]-N-methylamino]-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol) both of which turned out to be potent anticonvulsants in animal models of epilepsy.     

Conformationally rigid nucleoside probes help understand the role of sugar pucker and nucleobase orientation in the thrombin-binding aptamer

Modified thrombin-binding aptamers carrying 2′-deoxyguanine (dG) residues with locked North- or South-bicyclo[3.1.0]hexane pseudosugars were synthesized. Individual 2′-deoxyguanosines at positions dG5, dG10, dG14 and dG15 of the aptamer were replaced by these analogues where the North/anti and South/syn conformational states were confined. It was found that the global structure of the DNA aptamer was, for the most part, very accommodating. The substitution at positions 5, 10 and 14 with a locked South/syn-dG nucleoside produced aptamers with the same stability and global structure as the innate, unmodified one. Replacing position 15 with the same South/syn-dG nucleoside induced a strong destabilization of the aptamer, while the antipodal North/anti-dG nucleoside was less destabilizing. Remarkably, the insertion of a North/anti-dG nucleoside at position 14, where both pseudosugar conformation and glycosyl torsion angle are opposite with respect to the native structure, led to the complete disruption of the G-tetraplex structure as detected by NMR and confirmed by extensive molecular dynamics simulations. We conclude that conformationally locked bicyclo[3.1.0]hexane nucleosides appear to be excellent tools for studying the role of key conformational parameters that are critical for the formation of a stable, antiparallel G-tetrad DNA structures.