Ring-substituted 8-hydroxyquinoline-2-carboxanilides as potential antimycobacterial agents

Identification of antimycobacterial 8-hydroxyquinoline derivatives as in vitro enzymatic inhibitors of Mycobacterium tuberculosis enoyl-acyl carrier protein reductase

The increasing prevalence of drug-resistant Mycobacterium tuberculosis strains stimulates the discovery of new drug candidates. Among them are 8-hydroxyquinoline (8HQ) derivatives that exhibited antimicrobial properties. Unfortunately, there is a lack of data assessing possible targets for this class mainly against Mycobacterium tuberculosis enoyl-acyl carrier protein reductase (MtInhA), a validated target in this field. Thus, the main purpose of this study was to identify 8HQ derivatives that are active against M. tuberculosis and MtInhA. Initially, the screening against the microorganism of a small antimicrobial library and its new derivatives that possess some structural similarity with MtInhA inhibitors identified four 7-substituted-8HQ (series 5 - 5a, 5c, 5d and 5i) and four 5-substituted-8HQ active derivatives (series 7 - 7a, 7c, 7d and 7j). In general, the 7-substituted 8-HQs were more potent and, in the enzymatic assay, were able to inhibit MtInhA at low micromolar range. However, the 5-substituted-8-HQs that presented antimycobacterial activity were not able to inhibit MtInhA. These findings indicate the non-promiscuous nature of 8-HQ derivatives and emphasize the significance of selecting appropriate substituents to achieve in vitro enzyme inhibition. Finally, 7-substituted-8HQ series are promising new derivatives for structure-based drug design and further development.

Critical view on antimicrobial, antibiofilm and cytotoxic activities of quinazolin-4(3H)-one derived schiff bases and their Cu(II) complexes

A series of nine 2,3-disubstituted-quinazolin-4(3H)-one derived Schiff bases and their three Cu(II) complexes was prepared and tested for their antimicrobial activities against reference strains Staphylococcus aureus ATCC 29213 and Enterococcus faecalis ATCC 29212 and resistant clinical isolates of methicillin-resistant S. aureus (MRSA) and vancomycin-resistant E. faecalis (VRE). All the substances were tested in vitro against Mycobacterium tuberculosis H37Ra ATCC 25177, M. kansasii DSM 44162 and M. smegmatis ATCC 700084. While anti-enterococcal and antimycobacterial activities were insignificant, 3-[(E)-(2-hydroxy-5-nitrobenzylidene)amino]-2-(2-hydroxy-5-nitrophenyl)-2,3-dihydroquinazolin-4(1H)-one (SB3) and its Cu(II) complex (SB3-Cu) demonstrated bacteriostatic antistaphylococcal activity. In addition, both compounds, as well as the other two prepared complexes, showed antibiofilm activity, which resulted in a reduction of biofilm formation and eradication of mature S. aureus biofilm by 80% even at concentrations lower than the values of their minimum inhibitory concentrations. In addition, the compounds were tested for their cytotoxic effect on the human monocytic leukemia cell line THP-1. The antileukemic efficiency was improved by the preparation of Cu(II) complexes from the corresponding non-chelated Schiff base ligands.

Trifluoromethylcinnamanilide Michael Acceptors for Treatment of Resistant Bacterial Infections

A series of thirty-two anilides of 3-(trifluoromethyl)cinnamic acid (series 1) and 4-(trifluoromethyl)cinnamic acid (series 2) was prepared by microwave-assisted synthesis. All the compounds were tested against reference strains Staphylococcus aureus ATCC 29213 and Enterococcus faecalis ATCC 29212 and resistant clinical isolates of methicillin-resistant S. aureus (MRSA) and vancomycin-resistant E. faecalis (VRE). All the compounds were evaluated in vitro against Mycobacterium smegmatis ATCC 700084 and M. marinum CAMP 5644. (2E)-3-[3-(Trifluoromethyl)phenyl]-N-[4-(trifluoromethyl)phenyl]prop-2-enamide (1j), (2E)-N-(3,5-dichlorophenyl)-3-[3-(trifluoromethyl)phenyl]prop-2-enamide (1o) and (2E)-N-[3-(trifluoromethyl)phenyl]-3-[4-(trifluoromethyl)-phenyl]prop-2-enamide (2i), (2E)-N-[3,5-bis(trifluoromethyl)phenyl]-3-[4-(trifluoromethyl)phenyl]-prop-2-enamide (2p) showed antistaphylococcal (MICs/MBCs 0.15-5.57 µM) as well as anti-enterococcal (MICs/MBCs 2.34-44.5 µM) activity. The growth of M. marinum was strongly inhibited by compounds 1j and 2p in a MIC range from 0.29 to 2.34 µM, while all the agents of series 1 showed activity against M. smegnatis (MICs ranged from 9.36 to 51.7 µM). The performed docking study demonstrated the ability of the compounds to bind to the active site of the mycobacterial enzyme InhA. The compounds had a significant effect on the inhibition of bacterial respiration, as demonstrated by the MTT assay. The compounds showed not only bacteriostatic activity but also bactericidal activity. Preliminary in vitro cytotoxicity screening was assessed using the human monocytic leukemia cell line THP-1 and, except for compound 2p, all effective agents did show insignificant cytotoxic effect. Compound 2p is an interesting anti-invasive agent with dual (cytotoxic and antibacterial) activity, while compounds 1j and 1o are the most interesting purely antibacterial compounds within the prepared molecules.

Design, Synthesis and Antimicrobial Properties of New Tetracyclic Quinobenzothiazine Derivatives

A new method for modifying the structure of tetracyclic quinobenzothiazinium derivatives has been developed, allowing introduction of various substituents at different positions of the benzene ring. The method consists of reacting appropriate aniline derivatives with 5,12-(dimethyl)thioquinantrenediinium bis-chloride. A series of new quinobenzothiazine derivatives was obtained with propyl, allyl, propargyl and benzyl substituents in 9, 10 and 11 positions, respectively. The structure of the obtained compounds was analyzed by 1H and 13C NMR (HSQC, HMBC) and X-ray analysis. All the compounds were tested against reference strains Staphylococcus aureus ATCC 29213 and Enterococcus faecalis ATCC 29212, and representatives of multidrug-resistant clinical isolates of methicillin-resistant S. aureus (MRSA) and vancomycin-resistant E. faecalis (VRE). In addition, all the compounds were evaluated in vitro against Mycobacterium smegmatis ATCC 700084 and M. marinum CAMP 5644. 9-Benzyloxy-5-methyl-12H-quino [3,4-b][1,4]benzothiazinium chloride (6j), 9-propoxy-5-methyl-12H-quino[3,4-b][1,4]benzothiazinium chloride (6a) and 9-allyloxy-5-methyl-12H-quino[3,4-b][1,4]benzothiazinium chloride (6d) demonstrated high activity against the entire tested microbial spectrum. The activities of the compounds were comparable with oxacillin, tetracycline and ciprofloxacinagainst staphylococcal strains and with rifampicin against both mycobacterial strains. Compound 6j had a significant effect on the inhibition of bacterial respiration as demonstrated by the MTT assay. The compounds showed not only bacteriostatic activity, but also bactericidal activity. Preliminary in vitro cytotoxicity screening of the compounds performed using normal human dermal fibroblasts (NHDF) proved that the tested compounds showed an insignificant cytotoxic effect on human cells (IC50 > 37 µM), making these compounds interesting for further investigation. Moreover, the intermolecular similarity of novel compounds was analyzed in the multidimensional space (mDS) of the structure/property-related in silico descriptors by means of principal component analysis (PCA) and hierarchical clustering analysis (HCA), respectively. The distance-oriented structure/property distribution was related with the experimental lipophilic data.

Novel Antimicrobial 8-Hydroxyquinoline-Based Agents: Current Development, Structure-Activity Relationships, and Perspectives

The search for new antimicrobials is imperative due to the emergent resistance of new microorganism strains. In this context, revisiting known classes like 8-hydroxyquinolines could be an interesting strategy to discover new agents. The 8-hydroxyquinoline derivatives nitroxoline and clioquinol are used to treat microbial infections; however, these drugs are underused, being available in few countries or limited to topical use. After years of few advances, in the last two decades, the potent activity of clioquinol and nitroxoline against several targets and the privileged structure of 8-hydroxyquinoline nucleus have prompted an increased interest in the design of novel antimicrobial, anticancer, and anti-Alzheimer agents based on this class. Herein, we discuss the current development and antimicrobial structure-activity relationships of this class in the perspective of using the 8-hydroxyquinoline nucleus for the search for novel antimicrobial agents. Furthermore, the most investigated molecular targets concerning 8-hydroxyquinoline derivatives are explored in the final section.

Trimethoxycinnamates and Their Cholinesterase Inhibitory Activity

A series of twelve nature-inspired 3,4,5-trimethoxycinnamates were prepared and characterized. All compounds, including the starting 3,4,5-trimethoxycinnamic acid, were tested for their ability to inhibit acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) in vitro; the selectivity index (SI) was also determined. 2-Fluororophenyl (2E)-3-(3,4,5-trimethoxyphenyl)-prop-2-enoate demonstrated the highest SI (1.71) in favor of BChE inhibition. 2-Chlorophenyl (2E)-3-(3,4,5-trimethoxyphenyl)prop-2-enoate showed the highest AChE-inhibiting (IC50 = 46.18 µM) as well as BChE-inhibiting (IC50 = 32.46 µM) activity with an SI of 1.42. The mechanism of action of the most potent compound was determined by the Lineweaver–Burk plot as a mixed type of inhibition. An in vitro cell viability assay confirmed the insignificant cytotoxicity of the discussed compounds on the two cell lines. Trends between structure, physicochemical properties and activity were discussed.

Consensus-Based Pharmacophore Mapping for New Set of N-(disubstituted-phenyl)-3-hydroxyl-naphthalene-2-carboxamides

A series of twenty-two novel N-(disubstituted-phenyl)-3-hydroxynaphthalene- 2-carboxamide derivatives was synthesized and characterized as potential antimicrobial agents. N-[3,5-bis(trifluoromethyl)phenyl]- and N-[2-chloro-5-(trifluoromethyl)phenyl]-3-hydroxy- naphthalene-2-carboxamide showed submicromolar (MICs 0.16–0.68 µM) activity against methicillin-resistant Staphylococcus aureus isolates. N-[3,5-bis(trifluoromethyl)phenyl]- and N-[4-bromo-3-(trifluoromethyl)phenyl]-3-hydroxynaphthalene-2-carboxamide revealed activity against M. tuberculosis (both MICs 10 µM) comparable with that of rifampicin. Synergistic activity was observed for the combinations of ciprofloxacin with N-[4-bromo-3-(trifluoromethyl)phenyl]- and N-(4-bromo-3-fluorophenyl)-3-hydroxynaphthalene-2-carboxamides against MRSA SA 630 isolate. The similarity-related property space assessment for the congeneric series of structurally related carboxamide derivatives was performed using the principal component analysis. Interestingly, different distribution of mono-halogenated carboxamide derivatives with the –CF₃ substituent is accompanied by the increased activity profile. A symmetric matrix of Tanimoto coefficients indicated the structural dissimilarities of dichloro- and dimetoxy-substituted isomers from the remaining ones. Moreover, the quantitative sampling of similarity-related activity landscape provided a subtle picture of favorable and disallowed structural modifications that are valid for determining activity cliffs. Finally, the advanced method of neural network quantitative SAR was engaged to illustrate the key 3D steric/electronic/lipophilic features of the ligand-site composition by the systematic probing of the functional group.

Silver-catalyzed direct C-H oxidative carbamoylation of quinolines with oxamic acids

A silver-catalyzed efficient and direct C-H carbamoylation of quinolines with oxamic acids to access carbamoylated quinolines has been developed through oxidative decarboxylation reaction. The reaction proceeds smoothly over a broad range of substrates with excellent functional group tolerance and excellent yields under mild conditions.

8-Hydroxyquinolines are bactericidal against Mycobacterium tuberculosis

There is an urgent need for new treatments effective against Mycobacterium tuberculosis, the causative agent of tuberculosis. The 8-hydroxyquinoline series is a privileged scaffold with anticancer, antifungal, and antibacterial activities. We conducted a structure-activity relationship study of the series regarding its antitubercular activity using 26 analogs. The 8-hydroxyquinolines showed good activity against M. tuberculosis, with minimum inhibitory concentrations (MIC90) of <5 μM for some analogs. Small substitutions at C5 resulted in the most potent activity. Substitutions at C2 generally decreased potency, although a sub-family of 2-styryl-substituted analogs retained activity. Representative compounds demonstrated bactericidal activity against replicating M. tuberculosis with >4 log kill at 10× MIC over 14 days. The majority of the compounds demonstrated cytotoxicity (IC₅₀ of <100 μM). Further development of this series as antitubercular agents should address the cytotoxicity liability. However, the 8-hydroxyquinoline series represents a useful tool for chemical genomics to identify novel targets in M. tuberculosis.

Synthesis and anti-phytopathogenic activity of 8-hydroxyquinoline derivatives

Phytopathogenic fungi have become a serious threat to the quality of agricultural products, food security and human health globally, necessitating the need to discover new antifungal agents with de novo chemical scaffolds and high efficiency. A series of 8-hydroxyquinoline derivatives were designed and synthesized, and their antifungal activity was evaluated against five phytopathogenic fungi. In vitro assays revealed that most of the tested compounds remarkably impacted the five target fungi and their inhibitory activities were better than that of the positive control azoxystrobin. Compound 2, in particular, exhibited the highest potency among all the tested compounds, with an EC₅₀ of 0.0021, 0.0016, 0.0124, 0.0059 and 0.0120 mM respectively against B. cinerea, S. sclerotiorum, F. graminearum, F. oxysporum and M. oryzae, followed by compound 5c. The morphological observations of optical microscopy and scanning electron microscopy revealed that compounds 2 and 5c caused mycelial abnormalities of S. sclerotiorum. Futhermore, the results of in vivo antifungal activity of compounds 2 and 5c against S. sclerotiorum showed that 5c possessed stronger protective and curative activity than that of 2, and the curative effects of 5c at 40 and 80 μg mL⁻¹ (84.18% and 95.44%) were better than those of azoxystrobin (77.32% and 83.59%). Therefore, compounds 2 and 5c are expected to be novel lead structures for the development of new fungicides.

Phytopathogenic fungi have become a serious threat to the quality of agricultural products, food security and human health globally, necessitating the need to discover new antifungal agents with de novo chemical scaffolds and high efficiency.

Second generation of primaquine ureas and bis-ureas as potential antimycobacterial agents

Here, we describe design and synthesis of twelve novel compounds bearing primaquine motif and hydroxy- or halogenamine linked by an urea or bis-urea spacer. Preparation of ureas 3a-f started with the conversion of primaquine to benzotriazolide 2 and aminolysis of the later compound by 4-(2-aminoethyl)phenol or amino alcohols bearing fluorine atom, cycloalkyl or trifluoromethyl group under microwave irradiation. The four-step sequence leading to bis-ureas 6a-f included preparation of benzotriazolide 2 and two intermediates, semicarbazide 4 and benzotriazole bis-urea 5, which upon aminolysis with the same aminophenol or amino alcohols gave the title compounds. Antimycobacterial screening detected three active compounds against Mycobacterium marinum and M. tuberculosis, namely 3b, 3f and 6f, derived from cyclobutyl amino alcohol or amino phenol.

Synthesis and Spectrum of Biological Activities of Novel N-arylcinnamamides

A series of sixteen ring-substituted N-arylcinnamamides was prepared and characterized. Primary in vitro screening of all the synthesized compounds was performed against Staphylococcus aureus, three methicillin-resistant S. aureus strains, Mycobacterium tuberculosis H37Ra, Fusarium avenaceum, and Bipolaris sorokiniana. Several of the tested compounds showed antistaphylococcal, antitubercular, and antifungal activities comparable with or higher than those of ampicillin, isoniazid, and benomyl. (2E)-N-[3,5-bis(trifluoromethyl)phenyl]-3-phenylprop-2-enamide and (2E)-3-phenyl-N-[3-(trifluoromethyl)phenyl]prop-2-enamide showed the highest activities (MICs = 22.27 and 27.47 µM, respectively) against all four staphylococcal strains and against M. tuberculosis. These compounds showed an activity against biofilm formation of S. aureus ATCC 29213 in concentrations close to MICs and an ability to increase the activity of clinically used antibiotics with different mechanisms of action (vancomycin, ciprofloxacin, and tetracycline). In time-kill studies, a decrease of CFU/mL of >99% after 8 h from the beginning of incubation was observed. (2E)-N-(3,5-Dichlorophenyl)- and (2E)-N-(3,4-dichlorophenyl)-3-phenylprop-2-enamide had a MIC = 27.38 µM against M. tuberculosis, while a significant decrease (22.65%) of mycobacterial cell metabolism determined by the MTT assay was observed for the 3,5-dichlorophenyl derivative. (2E)-N-(3-Fluorophenyl)- and (2E)-N-(3-methylphenyl)-3-phenylprop-2-enamide exhibited MICs = 16.58 and 33.71 µM, respectively, against B. sorokiniana. The screening of the cytotoxicity of the most effective antimicrobial compounds was performed using THP-1 cells, and these chosen compounds did not shown any significant lethal effect. The compounds were also evaluated for their activity related to the inhibition of photosynthetic electron transport (PET) in spinach (Spinacia oleracea L.) chloroplasts. (2E)-N-(3,5-dichlorophenyl)-3-phenylprop-2-enamide (IC₅₀ = 5.1 µM) was the most active PET inhibitor. Compounds with fungicide potency did not show any in vivo toxicity against Nicotiana tabacum var. Samsun. The structure–activity relationships are discussed.

The Antimicrobial Activity of Annona emarginata (Schltdl.) H. Rainer and Most Active Isolated Compounds against Clinically Important Bacteria

Annona emarginata (Schltdl.) H. Rainer, commonly known as “arachichú”, “araticú”, “aratigú”, and “yerba mora”, is a plant that grows in Argentina. Infusions and decoctions are used in folk medicine as a gargle against throat pain and for calming toothache; another way to use the plant for these purposes is chewing its leaves. Extracts from bark, flowers, leaves, and fruits from A. emarginata were subjected to antibacterial assays against a panel of Gram (+) and Gram (−) pathogenic bacteria according to Clinical and Laboratory Standards Institute protocols. Extracts from the stem bark and leaves showed moderate activity against the bacteria tested with values between 250–1000 µg/mL. Regarding flower extracts, less polar extracts (hexane, dichloromethane) showed very strong antibacterial activity against methicillin-sensitive Staphylococcus aureus ATCC 25923 and methicillin-resistant S. aureus ATCC 43300 with values between 16–125 µg/mL. Additionally, hexane extract showed activity against Klebsiella pneumoniae (MIC = 250 µg/mL). The global methanolic extract of the fruits (MeOHGEF) was also active against the three strains mentioned above, with MICs values 250–500 µg/mL. Bioassay-guided fractionation of MeOHGEF led to the isolation of a new main compound—(R)-2-(4-methylcyclohex-3-en-1-yl)propan-2-yl (E)-3-(4-hydroxyphenyl)acrylate (1). The structure and relative configurations have been determined by means of 1D and 2D NMR techniques, including COSY, HMQC, HMBC, and NOESY correlations. Compound 1 showed strong antimicrobial activity against all Gram (+) species tested (MICs = 3.12–6.25 µg/mL). In addition, the synthesis and antibacterial activity of some compounds structurally related to compound 1 (including four new compounds) are reported. A SAR study for these compounds was performed based on the results obtained by using molecular calculations.

Theoretical insight into the substituent effects on the antioxidant properties of 8-hydroxyquinoline derivatives in gas phase and in polar solvents

The objective of this work is to perform a theoretical analysis of the antioxidant properties of a series of 8-hydroxyquinolines (8-HQs) to rationalize the available experimental results and to design new potent 8-HQ derivatives. The study was carried out in gas phase and in methanol at the DFT/B3LYP/ 6-311++G(d,p) computational level. The formation of stable ArO• radicals is discussed on the basis of different mechanisms, namely, hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT), and single proton loss electron transfer (SPLET). The obtained results show that the HAT mechanism is, thermodynamically, more favoured in gas phase, whereas the SPLET pathway is more favoured in polar solvents. The calculated thermochemical descriptors allow classification of the antioxidant power of the studied compounds.

Primaquine hybrids as promising antimycobacterial and antimalarial agents

Four series of primaquine (PQ) derivatives were screened for antitubercular and antiplasmodial activity: amides 1a-k, ureas 2a-s, semicarbazides 3a-c and bis-ureas 4a-u. Antimycobacterial activity of PQ derivatives against Mycobacterium tuberculosis (MTB), M. avium complex (MAC) and M. avium subsp. paratuberculosis (MAP) were evaluated in vitro and compared with PQ and the standard antitubercular drugs. In general, the PQ derivatives showed higher potency than the parent compound. Most of the compounds of series 1 and 2 showed high activity against MAP, comparable or even higher than the relevant drug ciprofloxacin, and weak or no activity against MTB and MAC. bis-Trifluoromethylated cinnamamide 1k showed low cytotoxicity and high activity against all three Mycobacterium species and their activities were comparable or slightly higher than those of the reference drugs. PQ urea derivatives with hydroxyl, halogen and trifluoromethyl substituents on benzene ring 2f-p exerted very strong antimycobacterial activity towards all tested mycobacteria, stronger than PQ and the relevant standard drug(s). Unfortunately, these compounds had relatively high cytotoxicity, except bromo 2l and trifluoromethyl 2m, 2n derivatives. In general, meta-substituted derivatives were more active than analogues para-derivatives. Phenyl ureas were also more active than cycloalkyl or hydroxyalkyl ureas. Semicarbazide 3a showed similar activity as PQ, while the other two semicarbazides were inactive. Bis-urea derivatives 4 were generally less active than the urea derivatives sharing the same scaffold, differing only in the spacer type. Out of 21 evaluated bis-urea derivatives, only p-Cl/m-CF₃ phenyl derivative 4p, benzhydryl derivatives 4t and 4u and bis-PQ derivative 4s showed high activity, higher than all three reference drugs. After comparison of activity and cytotoxicity, urea 2m and bis-urea 4u could be considered as the most promising agents. Antimalarial potential of PQ derivatives in vitro against the liver stage of P. berghei was evaluated as well. 3-(4-Chlorophenyl)-1-[({4-[(6-methoxyquinolin-8-yl)amino]pentyl}carbamoyl)amino]urea (4l) was the most active compound (IC₅₀ = 42 nM; cytotoxicity/activity ratio >2000). Our results bring new insights into development of novel anti-TB and antimalarial compounds.

Proline-Based Carbamates as Cholinesterase Inhibitors

Series of twenty-five benzyl (2S)-2-(arylcarbamoyl)pyrrolidine-1-carboxylates was prepared and completely characterized. All the compounds were tested for their in vitro ability to inhibit acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), and the selectivity of compounds to individual cholinesterases was determined. Screening of the cytotoxicity of all the compounds was performed using a human monocytic leukaemia THP-1 cell line, and the compounds demonstrated insignificant toxicity. All the compounds showed rather moderate inhibitory effect against AChE; benzyl (2S)-2-[(2-chlorophenyl)carbamoyl]pyrrolidine-1-carboxylate (IC₅₀ = 46.35 μM) was the most potent agent. On the other hand, benzyl (2S)-2-[(4-bromophenyl)-] and benzyl (2S)-2-[(2-bromophenyl)carbamoyl]pyrrolidine-1-carboxylates expressed anti-BChE activity (IC₅₀ = 28.21 and 27.38 μM, respectively) comparable with that of rivastigmine. The ortho-brominated compound as well as benzyl (2S)-2-[(2-hydroxyphenyl)carbamoyl]pyrrolidine-1-carboxylate demonstrated greater selectivity to BChE. The in silico characterization of the structure–inhibitory potency for the set of proline-based carbamates considering electronic, steric and lipophilic properties was provided using comparative molecular surface analysis (CoMSA) and principal component analysis (PCA). Moreover, the systematic space inspection with splitting data into the training/test subset was performed to monitor the statistical estimators performance in the effort to map the probability-guided pharmacophore pattern. The comprehensive screening of the AChE/BChE profile revealed potentially relevant structural and physicochemical features that might be essential for mapping of the carbamates inhibition efficiency indicating qualitative variations exerted on the reaction site by the substituent in the 3′-/4′-position of the phenyl ring. In addition, the investigation was completed by a molecular docking study of recombinant human AChE.

Halogenated 1-Hydroxynaphthalene-2-Carboxanilides Affecting Photosynthetic Electron Transport in Photosystem II

Series of seventeen new multihalogenated 1-hydroxynaphthalene-2-carboxanilides was prepared and characterized. All the compounds were tested for their activity related to the inhibition of photosynthetic electron transport (PET) in spinach (Spinacia oleracea L.) chloroplasts. 1-Hydroxy-N-phenylnaphthalene-2-carboxamides substituted in the anilide part by 3,5-dichloro-, 4-bromo-3-chloro-, 2,5-dibromo- and 3,4,5-trichloro atoms were the most potent PET inhibitors (IC₅₀ = 5.2, 6.7, 7.6 and 8.0 µM, respectively). The inhibitory activity of these compounds depends on the position and the type of halogen substituents, i.e., on lipophilicity and electronic properties of individual substituents of the anilide part of the molecule. Interactions of the studied compounds with chlorophyll a and aromatic amino acids present in pigment-protein complexes mainly in PS II were documented by fluorescence spectroscopy. The section between P₆₈₀ and plastoquinone Q_B in the PET chain occurring on the acceptor side of PS II can be suggested as the site of action of the compounds. The structure-activity relationships are discussed.

N-Alkoxyphenylhydroxynaphthalenecarboxamides and Their Antimycobacterial Activity

A series of nineteen N-(alkoxyphenyl)-2-hydroxynaphthalene-1-carboxamides and a series of their nineteen positional isomers N-(alkoxyphenyl)-1-hydroxynaphthalene-2-carboxamides were prepared and characterized. Primary in vitro screening of all the synthesized compounds was performed against Mycobacterium tuberculosis H37Ra, M. kansasii and M. smegmatis. Screening of the cytotoxicity of the compounds was performed using human monocytic leukemia THP-1 cells. Some of the tested compounds showed antimycobacterial activity comparable with or higher than that of rifampicin. For example, 2-hydroxy-N-(4-propoxyphenyl)-naphthalene-1-carboxamide showed the highest activity (MIC = 12 µM) against M. tuberculosis with insignificant cytotoxicity. N-[3-(But-2-yloxy)phenyl]- and N-[4-(but-2-yloxy)phenyl]-2-hydroxy-naphthalene-1-carboxamide demonstrated high activity against all tested mycobacterial strains and insignificant cytotoxicity. N-(Alkoxyphenyl)-1-hydroxynaphthalene-2-carboxamides demonstrated rather high effect against M. smegmatis and M. kansasii and strong antiproliferative effect against the human THP-1 cell line. Lipophilicity was found as the main physicochemical parameter influencing the activity. A significant decrease of mycobacterial cell metabolism (viability of M. tuberculosis H37Ra) was observed using MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide) assay. Structure-activity relationships are discussed.

Ring-substituted 8-hydroxyquinoline-2-carboxanilides as photosystem II inhibitors

Ring-substituted 8-hydroxyquinoline-2-carboxanilides inhibited photosynthetic electron transport (PET) through photosystem (PS) II. Their inhibitory efficiency depended on the compound lipophilicity, the electronic properties of the substituent R and the position of the substituent R on the benzene ring. The most effective inhibitors showing IC50 values in the range 2.3-3.6μM were substituted in C'(3) by F, CH3, Cl and Br. The dependence of the PET-inhibiting activity on the lipophilicity of the compounds was quasi-parabolic for 3-substituted derivatives, while for C'(2) ones a slight increase and for C'(4) derivatives a sharp decrease of the activity were observed with increasing lipophilicity. In addition, the dependence of PET-inhibiting activity on electronic Hammett's σ parameter of the substituent R was observed with optimum σ value 0.06 for C'(4) and 0.34 for C'(3) substituted derivatives, while the value of σ parameter did not significantly influence the PET-inhibiting activity of C'(2) substituted compounds. Interactions of the studied compounds with chlorophyll a and aromatic amino acids present in the pigment-protein complexes mainly in PS II were documented by fluorescence spectroscopy. The section between P680 and plastoquinone QB occurring on the acceptor side of PS II can be suggested as the site of action of the compounds.

Derivatives (halogen, nitro and amino) of 8-hydroxyquinoline with highly potent antimicrobial and antioxidant activities

8-Hydroxyquinoline (8HQ) compounds have been reported to possess diverse bioactivities. In recent years, drug repositioning has gained considerable attention in drug discovery and development. Herein, 8HQ (1) and its derivatives (2–9) bearing various substituents (amino, nitro, cyano and halogen) were investigated for their antimicrobial against 27 microorganisms (agar dilution method) and antioxidant (DPPH method) activities. The parent 8HQ (1) exerted a highly potent antimicrobial activity against Gram-positive bacteria including diploid fungi and yeast with MIC values in the range of 3.44–13.78 μM. Moreover, the halogenated 8HQ, especially 7-bromo-8HQ (4) and clioquinol (6), displayed a high antigrowth activity against Gram-negative bacteria compared with the parent compound (1). Apparently, the derivatives with a relatively high safely index, e.g., nitroxoline (2), exhibited strong antibacterial activity against Aeromonas hydrophila (MIC=5.26 μM) and selectively inhibited the growth of P. aeruginosa with the MIC value of 84.14 μM; cloxyquin (3) showed a strong activity against Listseria monocytogenes and Plesiomonas shigelloides with MIC values of 5.57 and 11.14 μM, respectively. Most compounds displayed an antioxidant activity. Specifically, 5-amino-8HQ (8) was shown to be the most potent antioxidant (IC₅₀=8.70 μM) compared with the positive control (α-tocopherol) with IC₅₀ of 13.47 μM. The findings reveal that 8HQ derivatives are potential candidates to be further developed as antimicrobial and antioxidant agents.

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8-Hydroxyquinoline exerted highly potent antibacterial activity (Gram positive).

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Nitroxoline exhibited strong antibacterial activity against Pseudomonas aeruginosa.

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Cloxyquin displayed a high growth inhibition against Listeria monocytogenes and Plesiomonas shigelloides.

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5-Amino-8-hydroxyquinoline exerted the most potent antioxidant activity (IC₅₀=8.70 μM).

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Nitroxoline and cloxyquin had a relatively high selectivity index.