Inhibition of Mycobacterial Growth by Plumbagin Derivatives

Plant antibacterials: The challenges and opportunities

Nature possesses an inexhaustible reservoir of agents that could serve as alternatives to combat the growing threat of antimicrobial resistance (AMR). While some of the most effective drugs for treating bacterial infections originate from natural sources, they have predominantly been derived from fungal and bacterial species. However, a substantial body of literature is available on the promising antibacterial properties of plant-derived compounds. In this comprehensive review, we address the major challenges associated with the discovery and development of plant-derived antimicrobial compounds, which have acted as obstacles preventing their clinical use. These challenges encompass limited sourcing, the risk of agent rediscovery, suboptimal drug metabolism, and pharmacokinetics (DMPK) properties, as well as a lack of knowledge regarding molecular targets and mechanisms of action, among other pertinent issues. Our review underscores the significance of these challenges and their implications in the quest for the discovery and development of effective plant-derived antimicrobial agents. Through a critical examination of the current state of research, we give valuable insights that will advance our understanding of these classes of compounds, offering potential solutions to the global crisis of AMR. © 2017 Elsevier Inc. All rights reserved.

An overview on the antibacterial properties of juglone, naphthazarin, plumbagin and lawsone derivatives and their metal complexes

Bacterial resistance development represents a serious threat to human health across the globe and has become a very serious clinical problem for many classes of antibiotics. Hence, there is a constant and urgent need for the discovery and development of new effective antibacterial agents to stem the emergence of resistant bacteria. 1,4-naphthoquinones are an important class of natural products and have been known for decades as a privileged scaffold in medicinal chemistry regarding their many biological properties. The significant biological properties of specific 1,4-naphthoquinones hydroxyderivatives have drawn the attention of researchers in order to find new derivatives with an optimized activity, mainly as antibacterial agents. Based on juglone, naphthazarin, plumbagin and lawsone moieties, structural optimization was realized with the purpose of improving the antibacterial activity. Thereupon, relevant antibacterial activities have been observed on different panels of bacterial strains including resistant ones. In this review, we highlight the interest of developing new 1,4-naphthoquinones hydroxyderivatives and some metal complexes as promising antibacterial agents alternatives. Here, we thoroughly report for the first time both the antibacterial activity and the chemical synthesis of four different 1,4-naphthoquinones (juglone, naphthazarin, plumbagin and lawsone) from 2002 to 2022 with an emphasis on the structure-activity relationship, when applicable.

How Structure-Function Relationships of 1,4-Naphthoquinones Combat Antimicrobial Resistance in Multidrug-Resistant (MDR) Pathogens

Antimicrobial resistance (AMR) is one of the top ten health-related threats worldwide. Among several antimicrobial agents, naphthoquinones (NQs) of plant/chemical origin possess enormous structural and functional diversity and are effective against multidrug-resistant (MDR) pathogens. 1,4-NQs possess alkyl, hydroxyl, halide, and metal groups as side chains on their double-ring structure, predominantly at the C-2, C-3, C-5, and C-8 positions. Among 1,4-NQs, hydroxyl groups at either C-2 or C-5 exhibit significant antibacterial activity against Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter spp. (ESKAPE) and MDR categories. 1,4-NQs exhibit antibacterial activities like plasmids curing, reactive oxygen species generation, efflux pumps inhibition, anti-DNA gyrase activity, membrane permeabilization, and biofilm inhibition. This review emphasizes the structure-function relationships of 1,4-NQs against ESKAPE and MDR pathogens based on a literature review of studies published in the last 15 years. Overall, 1,4-NQs have great potential for counteracting the antimicrobial resistance of MDR pathogens.

The effect of C/N ratio and its frequent addition on commensal and pathogenic bacterial abundances in shrimp Litopeaneus vanname gut in a biofloc system: Ratio and frequent addition interaction matters

The environmental biotic and abiotic factors form a complicated relationship with the host intestinal microbiota. In our study, we applied different levels of C/N ratio (10, 15, 20) and frequent addition times (once, twice, triple a day) in a factorial experimental design. GC/LC analysis of filtrated biofloc (BF) samples revealed the highest relative fold change for the untargeted bioactive molecules among different treatments, whereas the 16s rRNA analysis revealed the change in the shrimp gut microbiota composition. Based on the available literature on the relationship between the bioactive molecules and the available bacteria in this study, the next bioactive molecules were discussed. Proline was associated with Bacteroidota, Flavobacteriaceae, Gammaproteobacteria, and Flavobacteriales. Plumbagine was associated with Norcardiaceae. Phytosphingosin was associated with Bacteroidota. Phosphocholine compound was associated with Bacteroidota. The monobutyl ether, benzofuran, and piperidone were associated with Micobacteriaceae and Mycobacterium. Generally, C/N 15 and 20 once a day, and C/N 20 triple a day have showed a merit over other treatments in term of low pathogenic and unfavorable bacteria, and high commensal bacterial abundances. The revealed bioactive molecule composition showed the complicity of BF as a source for novel compounds as biosecurity agents in BF system. These molecules could be developed to feed additives upgrading the biosecurity level in aquaculture systems. Other bioactive molecules require future studies to reveal novel molecules in term of aquaculture biosecurity control.

Filamentous Thermosensitive Mutant Z: An Appealing Target for Emerging Pathogens and a Trek on Its Natural Inhibitors

Simple Summary

Antimicrobial resistance (AMR) is a pressing issue worldwide that must be addressed swiftly. It is driven by spontaneous evolution, bacterial mutation, and the dissemination of resistant genes via horizontal gene transfer. Researchers are working on many novel targets, which can become a pathway to inhibit harmful bacteria. Filamentous Thermosensitive mutant-Z (Fts-Z) is one such bacterial target that has gained popularity amongst scientists due to its conserved nature in bacteria and absence in eukaryotes. The aim of this work was to review the Fts-Z mechanism of action along with current studies on natural inhibitors for Fts-Z.

Abstract

Antibiotic resistance is a major emerging issue in the health care sector, as highlighted by the WHO. Filamentous Thermosensitive mutant Z (Fts-Z) is gaining significant attention in the scientific community as a potential anti-bacterial target for fighting antibiotic resistance among several pathogenic bacteria. The Fts-Z plays a key role in bacterial cell division by allowing Z ring formation. Several in vitro and in silico experiments have demonstrated that inhibition of Fts-Z can lead to filamentous growth of the cells, and finally, cell death occurs. Many natural compounds that have successfully inhibited Fts-Z are also studied. This review article intended to highlight the structural–functional aspect of Fts-Z that leads to Z-ring formation and its contribution to the biochemistry and physiology of cells. The current trend of natural inhibitors of Fts-Z protein is also covered.

Targeting the Achilles Heel of FtsZ: The Interdomain Cleft

Widespread antimicrobial resistance among bacterial pathogens is a serious threat to public health. Thus, identification of new targets and development of new antibacterial agents are urgently needed. Although cell division is a major driver of bacterial colonization and pathogenesis, its targeting with antibacterial compounds is still in its infancy. FtsZ, a bacterial cytoskeletal homolog of eukaryotic tubulin, plays a highly conserved and foundational role in cell division and has been the primary focus of research on small molecule cell division inhibitors. FtsZ contains two drug-binding pockets: the GTP binding site situated at the interface between polymeric subunits, and the inter-domain cleft (IDC), located between the N-terminal and C-terminal segments of the core globular domain of FtsZ. The majority of anti-FtsZ molecules bind to the IDC. Compounds that bind instead to the GTP binding site are much less useful as potential antimicrobial therapeutics because they are often cytotoxic to mammalian cells, due to the high sequence similarity between the GTP binding sites of FtsZ and tubulin. Fortunately, the IDC has much less sequence and structural similarity with tubulin, making it a better potential target for drugs that are less toxic to humans. Over the last decade, a large number of natural and synthetic IDC inhibitors have been identified. Here we outline the molecular structure of IDC in detail and discuss how it has become a crucial target for broad spectrum and species-specific antibacterial agents. We also outline the drugs that bind to the IDC and their modes of action.

Natural-Product-Based Solutions for Tropical Infectious Diseases

About half of the world's population and 80% of the world's biodiversity can be found in the tropics. Many diseases are specific to the tropics, with at least 41 diseases caused by endemic bacteria, viruses, parasites, and fungi. Such diseases are of increasing concern, as the geographic range of tropical diseases is expanding due to climate change, urbanization, change in agricultural practices, deforestation, and loss of biodiversity. While traditional medicines have been used for centuries in the treatment of tropical diseases, the active natural compounds within these medicines remain largely unknown. In this review, we describe infectious diseases specific to the tropics, including their causative pathogens, modes of transmission, recent major outbreaks, and geographic locations. We further review current treatments for these tropical diseases, carefully consider the biodiscovery potential of the tropical biome, and discuss a range of technologies being used for drug development from natural resources. We provide a list of natural products with antimicrobial activity, detailing the source organisms and their effectiveness as treatment. We discuss how technological advancements, such as next-generation sequencing, are driving high-throughput natural product screening pipelines to identify compounds with therapeutic properties. This review demonstrates the impact natural products from the vast tropical biome have in the treatment of tropical infectious diseases and how high-throughput technical capacity will accelerate this discovery process.

Naphthoquinones and Their Derivatives: Emerging Trends in Combating Microbial Pathogens

In the current era, an ever-emerging threat of multidrug-resistant (MDR) pathogens pose serious health challenges to mankind. Researchers are uninterruptedly putting their efforts to design and develop alternative, innovative strategies to tackle the antibiotic resistance displayed by varied pathogens. Among several naturally derived and chemically synthesized compounds, quinones have achieved a distinct position to defeat microbial pathogens. This review unleashes the structural diversity and promising biological activities of naphthoquinones (NQs) and their derivatives documented in the past two decades. Further, realizing their functional potentialities, researchers were encouraged to approach NQs as lead molecules. We have retrieved information that is dedicated on biological applications (antibacterial, antifungal, antiparasitic) of NQs. The multiple roles of NQs offer them a promising armory to combat microbial pathogens including MDR and the ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) group. In bacteria, NQs may exhibit their function in the following ways (1) plasmid curing, (2) inhibiting efflux pumps (EPs), (3) generating reactive oxygen species (ROS), (4) the inhibition of topoisomerase activity. Sparse but meticulous literature suggests the mechanistic roles of NQs. We have highlighted the possible mechanisms of NQs and how the targeted drug synthesis can be achieved via molecular docking analysis. This bioinformatics-oriented approach will explicitly lead to the development of effective and most potent drugs against targeted pathogens. The mechanistic approaches of emerging molecules like NQs might prove a milestone to defeat the battle against microbial pathogens.

Ethnobotany and the Role of Plant Natural Products in Antibiotic Drug Discovery

The crisis of antibiotic resistance necessitates creative and innovative approaches, from chemical identification and analysis to the assessment of bioactivity. Plant natural products (NPs) represent a promising source of antibacterial lead compounds that could help fill the drug discovery pipeline in response to the growing antibiotic resistance crisis. The major strength of plant NPs lies in their rich and unique chemodiversity, their worldwide distribution and ease of access, their various antibacterial modes of action, and the proven clinical effectiveness of plant extracts from which they are isolated. While many studies have tried to summarize NPs with antibacterial activities, a comprehensive review with rigorous selection criteria has never been performed. In this work, the literature from 2012 to 2019 was systematically reviewed to highlight plant-derived compounds with antibacterial activity by focusing on their growth inhibitory activity. A total of 459 compounds are included in this Review, of which 50.8% are phenolic derivatives, 26.6% are terpenoids, 5.7% are alkaloids, and 17% are classified as other metabolites. A selection of 183 compounds is further discussed regarding their antibacterial activity, biosynthesis, structure-activity relationship, mechanism of action, and potential as antibiotics. Emerging trends in the field of antibacterial drug discovery from plants are also discussed. This Review brings to the forefront key findings on the antibacterial potential of plant NPs for consideration in future antibiotic discovery and development efforts.

Isoniazid-phytochemical conjugation: A new approach for potent and less toxic anti-TB drug development

Mycobacterium tuberculosis (Mtb) causes one of the most grievous pandemic infectious diseases, tuberculosis (TB), with long-term morbidity and high mortality. The emergence of drug-resistant Mtb strains, and the co-infection with human immunodeficiency virus, challenges the current WHO-TB stewardship programs. The first-line anti-TB drugs, isoniazid (INH) and rifampicin (RIF), have become extensively obsolete in TB control from chromosomal mutations during the last decades. However, based on clinical trial statistics, the production of well-tolerated anti-TB drug(s) is miserably low. Alternately, semi-synthesis or structural modifications of first-line obsolete antitubercular drugs remain as the versatile approach for getting some potential medicines. The use of any suitable phytochemicals with INH in a hybrid formulation could be an ideal approach for the development of potent anti-TB drug(s). The primary objective of this review was to highlight and analyze available INH-phytochemical hybrid research works. The utilization of phytochemicals through chemical conjugation is a new trend toward the development of safer/non-toxic anti-TB drugs.

Mycobacterium tuberculosis thymidylate synthase (ThyX) is a target for plumbagin, a natural product with antimycobacterial activity

Plumbagin derived from the plant Plumbago indica, known as Chitrak in India, is an example of a medicinal compound used traditionally to cure a variety of ailments. Previous reports have indicated that it can inhibit the growth of Mycobacterium tuberculosis (Mtb), the causative agent of the deadly disease TB. In this investigation, we provide an insight into its mode of action. We show here that a significant mycobacterial target that is inhibited by plumbagin is the enzyme ThyX, a form of thymidylate synthase, that is responsible for the synthesis of dTMP from dUMP in various bacterial pathogens, including Mtb. Using a purified preparation of the recombinant version of Mtb ThyX, we demonstrate that plumbagin, a 2,4 napthoquinone, but not lawsone, a structurally related medicinal compound, inhibits its activity in vitro. We also show that the intracellular [dTTP]/[dATP] ratio in Mycobacterium smegmatis (Msm) cells decrease upon treatment with plumbagin, and this, in turn, leads to cell death. Such a conclusion is supported by the observation that over-expression of thyx in the plumbagin treated Msm cells leads to the restoration of viability. The results of our investigation indicate that plumbagin kills mycobacterial cells primarily by targeting ThyX, a vital enzyme required for their survival.

The assessment of changes to the nontuberculous mycobacterial metabolome in response to anti-TB drugs

Mycobacterium species can cause a range of nontuberculous infections of healthy and immunocompromised people as well as infected people during and after surgical procedures. The similarity of nontuberculous mycobacteria (NTM) to the tuberculosis bacilli (TB) could ultimately enable the use of anti-TB drugs for the genus. Hence, three NTM (Mycobacterium smegmatis, Mycobacterium phlei and Mycobacterium avium) were cultured under different lab conditions, causing two mycobacterial phenotypes (active and dormant), and treated with isoniazid (INH) and ethambutol (EMB) independently or in combination. Metabolite profiling was applied to facilitate the investigation and characterisation of intracellular targets affected by the antibiotics. Aliquots of the cell culture were taken over the treatment period and the metabolite profile of the cells analysed by gas chromatography mass spectrometry. Comparative analysis of the metabolite levels to untreated mycobacteria confirmed the successful action of the antibiotics on the metabolism of all three species. Furthermore, single metabolites and metabolite pathways affected by the antibiotics could be identified and included, besides the known target sites for INH and EMB on mycobacterial cells, changes in e.g. nucleotide and saccharide levels. The combined treatment highlighted the property of EMB to enhance the effects of INH even under hypoxic culture conditions.

Metabolite Profiling: A Tool for the Biochemical Characterisation of Mycobacterium sp

Over the last decades, the prevalence of drug-resistance in Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, has increased. These findings have rekindled interest in elucidating the unique adaptive molecular and biochemistry physiology of Mycobacterium. The use of metabolite profiling independently or in combination with other levels of "-omic" analyses has proven an effective approach to elucidate key physiological/biochemical mechanisms associated with Mtb throughout infection. The following review discusses the use of metabolite profiling in the study of tuberculosis, future approaches, and the technical and logistical limitations of the methodology.

Naphthoquinone Derivatives as Scaffold to Develop New Drugs for Tuberculosis Treatment

Despite being a curable disease, tuberculosis (TB) remains a public health problem worldwide mainly due to lengthy treatment, as well as its toxic effects, TB/HIV co-infection and the emergence of resistant Mycobacterium tuberculosis strains. These barriers reinforcing the need for development of new antimicrobial agents, that ideally should reduce the time of treatment and be active against susceptible and resistant strains. Quinones are compounds found in natural sources and among them, the naphthoquinones show antifungal, antiparasitic, and antimycobacterial activity. Thus, we evaluated the potential antimycobacterial activity of six 1,4-naphthoquinones derivatives. We determined the minimum inhibitory concentration (MIC) of the compounds against three M. tuberculosis strains: a pan-susceptible H37Rv (ATCC 27294); one mono-resistant to isoniazid (ATCC 35822); and one mono-resistant to rifampicin (ATCC 35838); the cytotoxicity in the J774A.1 (ATCC TIB-67) macrophage lineage; performed in silico analysis about absorption, distribution, metabolism, and excretion (ADME) and docking sites. All evaluated naphthoquinones were active against the three strains with MIC between 206.6 and 12.5 μM, and the compounds with lower MIC values have also showed low cytotoxicity. Moreover, two naphthoquinones derivatives 5 and 6 probably do not exhibit cross resistance with isoniazid and rifampicin, respectively, and regarding ADME analysis, no compound violated the Lipinski's rule-of-five. Considering the set of findings in this study, we conclude that these naphthoquinones could be promising scaffolds to develop new therapeutic strategies to TB.

Influence of lipophilicity in O-acyl and O-alkyl derivatives of juglone and lawsone: a structure-activity relationship study in the search for natural herbicide models

BACKGROUND

Naphthoquinones are known for their broad range of biological activities. Given the increasing demands of consumers in relation to food quality and growing concerns about the impact of synthetic herbicides, it is necessary to search for new agrochemicals. Natural products and allelopathy provide new alternatives for the development of pesticides with lower toxicity and greater environmental compatibility.

RESULTS

A structure-activity relationship to evaluate the effect of bioavailability was performed. A total of 44 O-acyl and O-alkyl derivatives of juglone and lawsone with different linear chain lengths were prepared. These compounds were tested on etiolated wheat coleoptiles, standard target species (STS) and four weeds, Echinochloa crus-galli L., Lolium rigidum Gaud., Lolium perenne L. and Avena fatua L. The results showed a strong influence of lipophilicity and, in most cases, the data fitted a logP-dependent quadratic mathematical model.

CONCLUSION

The effects produced were mostly stunting and necrosis caused by growth inhibition. The potential structure and activity behaviour is described. © 2017 Society of Chemical Industry.

Menaquinone biosynthesis inhibition: a review of advancements toward a new antibiotic mechanism

Menaquinone is essential in electron transport and ATP generation in all Gram-positive, and anaerobically respiring Gram-negative bacteria. Inhibition of menaquinone production at different steps of the biosynthesis pathway has shown promising novel antibacterial action.

Stringent Response Factors PPX1 and PPK2 Play an Important Role in Mycobacterium tuberculosis Metabolism, Biofilm Formation, and Sensitivity to Isoniazid In Vivo

Mycobacterium tuberculosis remains a global health threat largely due to the lengthy duration of curative antibiotic treatment, contributing to medical nonadherence and the emergence of drug resistance. This prolonged therapy is likely due to the presence of M. tuberculosis persisters, which exhibit antibiotic tolerance. Inorganic polyphosphate [poly(P)] is a key regulatory molecule in the M. tuberculosis stringent response mediating antibiotic tolerance. The polyphosphate kinase PPK1 is responsible for poly(P) synthesis in M. tuberculosis, while the exopolyphosphatases PPX1 and PPX2 and the GTP synthase PPK2 are responsible for poly(P) hydrolysis. In the present study, we show by liquid chromatography-tandem mass spectrometry that poly(P)-accumulating M. tuberculosis mutant strains deficient in ppx1 or ppk2 had significantly lower intracellular levels of glycerol-3-phosphate (G3P) and 1-deoxy-xylulose-5-phosphate. Real-time PCR revealed decreased expression of genes in the G3P synthesis pathway in each mutant. The ppx1-deficient mutant also showed a significant accumulation of metabolites in the tricarboxylic acid cycle, as well as altered arginine and NADH metabolism. Each poly(P)-accumulating strain showed defective biofilm formation, while deficiency of ppk2 was associated with increased sensitivity to plumbagin and meropenem and deficiency of ppx1 led to enhanced susceptibility to clofazimine. A DNA vaccine expressing ppx1 and ppk2, together with two other members of the M. tuberculosis stringent response, M. tuberculosis rel and sigE, did not show protective activity against aerosol challenge with M. tuberculosis, but vaccine-induced immunity enhanced the killing activity of isoniazid in a murine model of chronic tuberculosis. In summary, poly(P)-regulating factors of the M. tuberculosis stringent response play an important role in M. tuberculosis metabolism, biofilm formation, and antibiotic sensitivity in vivo.

Metabolite analysis of Mycobacterium species under aerobic and hypoxic conditions reveals common metabolic traits

A metabolite profiling approach has been implemented to elucidate metabolic adaptation at set culture conditions in five Mycobacterium species (two fast- and three slow-growing) with the potential to act as model organisms for Mycobacterium tuberculosis (Mtb). Analysis has been performed over designated growth phases and under representative environments (nutrient and oxygen depletion) experienced by Mtb during infection. The procedure was useful in determining a range of metabolites (60-120 compounds) covering nucleotides, amino acids, organic acids, saccharides, fatty acids, glycerols, -esters, -phosphates and isoprenoids. Among these classes of compounds, key biomarker metabolites, which can act as indicators of pathway/process activity, were identified. In numerous cases, common metabolite traits were observed for all five species across the experimental conditions (e.g. uracil indicating DNA repair). Amino acid content, especially glutamic acid, highlighted the different properties between the fast- and slow-growing mycobacteria studied (e.g. nitrogen assimilation). The greatest similarities in metabolite composition between fast- and slow-growing mycobacteria were apparent under hypoxic conditions. A comparison to previously reported transcriptomic data revealed a strong correlation between changes in transcription and metabolite content. Collectively, these data validate the changes in the transcription at the metabolite level, suggesting transcription exists as one of the predominant modes of cellular regulation in Mycobacterium. Sectors with restricted correlation between metabolites and transcription (e.g. hypoxic cultivation) warrant further study to elucidate and exploit post-transcriptional modes of regulation. The strong correlation between the laboratory conditions used and data derived from in vivo conditions, indicate that the approach applied is a valuable addition to our understanding of cell regulation in these Mycobacterium species.

Oxido-molybdenum complexes obtained by Cl/O interchange between MoCl5 and carboxylic acids: a crystallographic, spectroscopic and computational study

The direct interaction of MoCl5 with a series of carboxylic acids has been elucidated for the first time. The reactions proceed with release of hydrogen chloride and Cl/O interchange between the metal centre and one equivalent of organic substrate: this feature is unique in the context of the chemistry generally shown by transition metal halides with carboxylic acids. The dinuclear complexes [MoOCl2(κ(1)-CX3CO2H)(μ-Cl)]2 (X = H, 1a; X = Cl, 1b) and Mo2O2Cl6(μ-CH3CO2H), 2a, were isolated as the prevalent metal products of the 1:2 molar reactions of MoCl5 with CH3COOH and CCl3COOH. Evidence for the formation of Mo2O2Cl6(μ-CCl3CO2H), 2b, was achieved by allowing MoCl5 to react with CCl3COOH in 2:3 ratio. Instead the reactions of MoCl5 with a three-fold molar excess of RCOOH afforded the mononuclear complexes MoOCl3(κ(1)-RCO2H)2 (R = CH3, 3a; CCl3, 3b; CHCl2, 3c; CMe3, 3d) in 50–60% yields. The new compounds were characterized by analytical and spectroscopic techniques, moreover the X-ray molecular structures were ascertained for 1a, 1b and 2a; 1a and 1b display single Mo–Mo bond. DFT calculations were carried out in order to shed light into structural aspects.

Cytotoxicity and apoptosis induced by a plumbagin derivative in estrogen positive MCF-7 breast cancer cells

Plumbagin [5-hydroxy- 2-methyl-1, 4-naphthaquinone] is a well-known plant derived anticancer lead compound. Several efforts have been made to synthesize its analogs and derivatives in order to increase its anticancer potential. In the present study, plumbagin and its five derivatives have been evaluated for their antiproliferative potential in one normal and four human cancer cell lines. Treatment with derivatives resulted in dose- and time-dependent inhibition of growth of various cancer cell lines. Prescreening of compounds led us to focus our further investigations on acetyl plumbagin, which showed remarkably low toxicity towards normal BJ cells and HepG2 cells. The mechanisms of apoptosis induction were determined by APOPercentage staining, caspase-3/7 activation, reactive oxygen species production and cell cycle analysis. The modulation of apoptotic genes (p53, Mdm2, NF-kB, Bad, Bax, Bcl-2 and Casp-7) was also measured using real time PCR. The positive staining using APOPercentage dye, increased caspase-3/7 activity, increased ROS production and enhanced mRNA expression of proapoptotic genes suggested that acetyl plumbagin exhibits anticancer effects on MCF-7 cells through its apoptosis-inducing property. A key highlighting point of the study is low toxicity of acetyl plumbagin towards normal BJ cells and negligible hepatotoxicity (data based on HepG2 cell line). Overall results showed that acetyl plumbagin with reduced toxicity might have the potential to be a new lead molecule for testing against estrogen positive breast cancer.

Synthesis and anticancer activity of some novel 5,6-fused hybrids of juglone based 1,4-naphthoquinones

Six novel 5,6-fused hybrids such as dihydrobenzofuran-quinone (4a and 4b), benzofuran-quinone (5a and 5b) and chromene-quinone (6a and 6b) of juglone based 1,4-naphthoquinones were synthesized by employing a three step protocol with the cyclisation of o-allyl phenol as the key step. The anticancer activity of the newly synthesized compounds was evaluated in vitro against seven human cancer cell lines including cervix (ME-180 and HeLa), breast (MCF-7, MDA-MB-453 and MDA-MB-231), prostate (PC-3) and colon (HT-29) by using MTT assay. The screening results showed that majority of the synthesized compounds exhibited significant anticancer activity. In particular, compounds 6a and 6b showed potent activities than the standard drug etoposide against prostate and breast cancer cell lines respectively. Flow cytometric analysis revealed that compounds 6a and 6b induced apoptosis and arrested the cell cycle at G2/M phase in PC-3 and MDA-MB-453 cells respectively.

Ferrocene and (arene)ruthenium(II) complexes of the natural anticancer naphthoquinone plumbagin with enhanced efficacy against resistant cancer cells and a genuine mode of action

A series of ferrocene and (arene)ruthenium(II) complexes attached to the naturally occurring anticancer naphthoquinones plumbagin and juglone was tested for efficacy against various cancer cell lines and for alterations in the mode of action. The plumbagin ferrocene and (p-cymene)Ru(II) conjugates 1c and 2a overcame the multi-drug drug resistance of KB-V1/Vbl cervix carcinoma cells and showed IC50 (72 h) values around 1 μM in growth inhibition assays using 3-(4,5-dimethyl-2-yl)-2,5-diphenyltetrazolium bromide (MTT). They were further investigated for their influence on the cell cycle of KB-V1/Vbl and HCT-116 colon carcinoma cells, on the generation of reactive oxygen species (ROS) by the latter cell line, for their substrate character for the P-glycoprotein drug eflux pump via the calcein-AM efflux assays, and for DNA affinity by the electrophoretic mobility shift assay (EMSA). The derivatives 1c and 2a increased the number of dead cancer cells (sub-G0/G1 fraction) in a dose- and time-dependent manner. ROS levels were significantly increased upon treatment with 1c and 2a. These compounds also showed a greater affinity to linear DNA than plumbagin. While plumbagin did not affect calcein-AM transport by P-glycoprotein the derivatives 1c and 2a exhibited a 50% or 80% inhibition of the P-glycoprotein-mediated calcein-AM efflux relative to the clinically established sensitizer verapamil.

Plumbagin analogs-synthesis, characterization, and antitubercular activity

Considering the emerging problem of drug resistance in tuberculosis, there is an urgent need of development of new analogs that are useful in curing drug resistant tuberculosis. In India, tuberculosis continues to remain one of the most pressing health problems. India is the highest tuberculosis burden country in the world, accounting one fifth of global incidence - estimated 2.0-2.5 million cases annually. In 2011, approximately 8.7 million new cases of tuberculosis and 1.4 million people die from tuberculosis each year worldwide. Current antitubercular therapies are successful against normal tuberculosis but it is not suitable for drug resistant tuberculosis. In this study Plumbagin analogs, obtained from Plumbago zeylanica (Family-Plumbaginaceae), have been synthesized. Out of the various synthesized analogs, the antitubercular activity of compound a and b was evaluated using standard H₃₇Rv and S, H, R, and E sensitive M tuberculosis strains using LRF assay method. Compound a showed strong activity against both standard H₃₇Rv and S, H, R and E sensitive M. tuberculosis strains as compared to standard Rifampicin. The other compounds are proved to be more active against standard H₃₇Rv and S, H, R and E sensitive M. tuberculosis strain as compared to Rifampicin.

Anti-tumor effects of novel 5-O-acyl plumbagins based on the inhibition of mammalian DNA replicative polymerase activity

We previously found that vitamin K3 (menadione, 2-methyl-1,4-naphthoquinone) inhibits the activity of human mitochondrial DNA polymerase γ (pol γ). In this study, we focused on plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone), and chemically synthesized novel plumbagins conjugated with C2:0 to C22:6 fatty acids (5-O-acyl plumbagins). These chemically modified plumbagins enhanced mammalian pol inhibition and their cytotoxic activity. Plumbagin conjugated with chains consisting of more than C18-unsaturated fatty acids strongly inhibited the activities of calf pol α and human pol γ. Plumbagin conjugated with oleic acid (C18:1-acyl plumbagin) showed the strongest suppression of human colon carcinoma (HCT116) cell proliferation among the ten synthesized 5-O-acyl plumbagins. The inhibitory activity on pol α, a DNA replicative pol, by these compounds showed high correlation with their cancer cell proliferation suppressive activity. C18:1-Acyl plumbagin selectively inhibited the activities of mammalian pol species, but did not influence the activities of other pols and DNA metabolic enzymes tested. This compound inhibited the proliferation of various human cancer cell lines, and was the cytotoxic inhibitor showing strongest inhibition towards HT-29 colon cancer cells (LD50 = 2.9 µM) among the nine cell lines tested. In an in vivo anti-tumor assay conducted on nude mice bearing solid tumors of HT-29 cells, C18:1-acyl plumbagin was shown to be a promising tumor suppressor. These data indicate that novel 5-O-acyl plumbagins act as anti-cancer agents based on mammalian DNA replicative pol α inhibition. Moreover, the results suggest that acylation of plumbagin is an effective chemical modification to improve the anti-cancer activity of vitamin K3 derivatives, such as plumbagin.

Antitubercular and antibacterial activity of quinonoid natural products against multi-drug resistant clinical isolates

Multi-drug resistant Mycobacterium tuberculosis and other bacterial pathogens represent a major threat to human health. In view of the critical need to augment the current drug regime, we have investigated therapeutic potential of five quinonoids, viz. emodin, diospyrin, plumbagin, menadione and thymoquinone, derived from natural products. The antimicrobial activity of quinonoids was evaluated against a broad panel of multi-drug and extensively drug-resistant tuberculosis (M/XDR-TB) strains, rapid growing mycobacteria and other bacterial isolates, some of which were producers of β-lactamase, Extended-spectrum β-lactamase (ESBL), AmpC β-lactamase, metallo-beta-lactamase (MBL) enzymes, as well as their drug-sensitive ATCC counterparts. All the tested quinones exhibited antimycobacterial and broad spectrum antibacterial activity, particularly against M. tuberculosis (lowest MIC 0.25 µg/mL) and Gram-positive bacteria (lowest MIC <4 µg/mL) of clinical origin. The order of antitubercular activity of the tested quinonoids was plumbagin > emodin ~ menadione ~ thymoquinone > diospyrin, whereas their antibacterial efficacy was plumbagin > menadione ~ thymoquinone > diospyrin > emodin. Furthermore, this is the first evaluation performed on these quinonoids against a broad panel of drug-resistant and drug-sensitive clinical isolates, to the best of our knowledge.

Naphthoquinones isolated from Diospyros anisandra exhibit potent activity against pan-resistant first-line drugs Mycobacterium tuberculosis strains

BACKGROUND AND OBJECTIVES

The recent emergence of multidrug-resistant (MDR), extensively drug-resistant (XDR), and totally drug-resistant (TDR) Mycobacterium tuberculosis (MTB) strains have further complicated the control of tuberculosis (TB). There is an urgent need of new molecules candidates to be developed as novel, active, and less toxic anti-tuberculosis (anti-TB) drugs. Medicinal plants have been an excellent source of leads for the development of drugs, particularly as anti-infective agents. In previous studies, the non-polar extract of Diospyros anisandra showed potent anti-TB activity, and three monomeric and five dimeric naphthoquinones have been obtained. In this study, we performed bioguided chemical fractionation and the isolation of eight naphthoquinones from D. anisandra and their evaluation of anti-TB and cytotoxic activities against mammalian cells.

METHODS

The n-hexane crude extract from the stem bark of the plant was obtained by maceration and liquid-liquid fractionation. The isolation of naphthoquinones was carried out by chromatographic methods and identified by gas chromatography and mass spectroscopy data analysis. Anti-TB activity was evaluated against two strains of MTB (H37Rv) susceptible to all five first-line anti-TB drugs and a clinical isolate that is resistant to these medications (pan-resistant, CIBIN 99) by measuring the minimal inhibitory concentration (MIC). Cytotoxicity of naphthoquinones was estimated against two mammalian cells, Vero line and primary cultures of human peripheral blood mononuclear (PBMC) cells, and their selectivity index (SI) was determined.

RESULTS

Plumbagin and its dimers maritinone and 3,3'-biplumbagin showed the strongest activity against both MTB strains (MIC = 1.56-3.33 μg/mL). The bioactivity of maritinone and 3,3'-biplumbagin were 32 times more potent than rifampicin against the pan-resistant strain, and both dimers showed to be non-toxic against PBMC and Vero cells. The SI of maritinone and 3,3'-biplumbagin on Vero cells was 74.34 and 194.11 against sensitive and pan-resistant MTB strains, respectively.

CONCLUSION

Maritinone and 3,3'-biplumbagin possess a very interesting potential for development as new drugs against M. tuberculosis, mainly resistant profile strains.

New targets and inhibitors of mycobacterial sulfur metabolism

The identification of new antibacterial targets is urgently needed to address multidrug resistant and latent tuberculosis infection. Sulfur metabolic pathways are essential for survival and the expression of virulence in many pathogenic bacteria, including Mycobacterium tuberculosis. In addition, microbial sulfur metabolic pathways are largely absent in humans and therefore, represent unique targets for therapeutic intervention. In this review, we summarize our current understanding of the enzymes associated with the production of sulfated and reduced sulfur-containing metabolites in Mycobacteria. Small molecule inhibitors of these catalysts represent valuable chemical tools that can be used to investigate the role of sulfur metabolism throughout the Mycobacterial lifecycle and may also represent new leads for drug development. In this light, we also summarize recent progress made in the development of inhibitors of sulfur metabolism enzymes.

Antimycobacterial assessment of Salicylanilide benzoates including multidrug-resistant tuberculosis strains

The increasing emergence especially of drug-resistant tuberculosis has led to a strong demand for new anti-tuberculosis drugs. Eighteen salicylanilide benzoates were evaluated for their inhibition potential against Mycobacterium tuberculosis, Mycobacterium avium and two strains of Mycobacterium kansasii; minimum inhibitory concentration values ranged from 0.5 to 16 μmol/L. The most active esters underwent additional biological assays. Four benzoates inhibited effectively the growth of five multidrug-resistant strains and one extensively drug-resistant strain of M. tuberculosis at low concentrations (0.25–2 μmol/L) regardless of the resistance patterns. The highest rate of multidrug-resistant mycobacteria inhibition expressed 4-chloro-2-[4-(trifluoromethyl)-phenylcarbamoyl]phenyl benzoate (0.25–1 μmol/L). Unfortunately, the most potent esters were still considerably cytotoxic, although mostly less than their parent salicylanilides.

Open source drug discovery in practice: a case study

Background

Open source drug discovery offers potential for developing new and inexpensive drugs to combat diseases that disproportionally affect the poor. The concept borrows two principle aspects from open source computing (i.e., collaboration and open access) and applies them to pharmaceutical innovation. By opening a project to external contributors, its research capacity may increase significantly. To date there are only a handful of open source R&D projects focusing on neglected diseases. We wanted to learn from these first movers, their successes and failures, in order to generate a better understanding of how a much-discussed theoretical concept works in practice and may be implemented.

Methodology/Principal Findings

A descriptive case study was performed, evaluating two specific R&D projects focused on neglected diseases. CSIR Team India Consortium's Open Source Drug Discovery project (CSIR OSDD) and The Synaptic Leap's Schistosomiasis project (TSLS). Data were gathered from four sources: interviews of participating members (n = 14), a survey of potential members (n = 61), an analysis of the websites and a literature review. Both cases have made significant achievements; however, they have done so in very different ways. CSIR OSDD encourages international collaboration, but its process facilitates contributions from mostly Indian researchers and students. Its processes are formal with each task being reviewed by a mentor (almost always offline) before a result is made public. TSLS, on the other hand, has attracted contributors internationally, albeit significantly fewer than CSIR OSDD. Both have obtained funding used to pay for access to facilities, physical resources and, at times, labor costs. TSLS releases its results into the public domain, whereas CSIR OSDD asserts ownership over its results.

Conclusions/Significance

Technically TSLS is an open source project, whereas CSIR OSDD is a crowdsourced project. However, both have enabled high quality research at low cost. The critical success factors appear to be clearly defined entry points, transparency and funding to cover core material costs.

Open source drug discovery can be an influential model for discovering and developing new medicines and diagnostics for neglected diseases. It offers the opportunity to accelerate the discovery progress while keeping expenditures to a minimum by encouraging incremental contributions from volunteer scientists. Publishing raw data and results in the public domain is positive within the context of neglected diseases since it facilitates open collaboration while obviating the ability to patent any results. In this way it effectively de-links the research and development costs from the sales price of the end product, the new medicine or diagnostic. This case study demonstrates that implementations of the open source model can differ while still achieving the ultimate goal of obtaining high quality research at reduced costs. However, the importance of clearly defined entry points, transparency and funding are shared success factors. These findings present the practical challenges of implementing a theoretical concept and hopefully will assist other scientists in organizing future open source drug discovery projects.

Discovery of selective menaquinone biosynthesis inhibitors against Mycobacterium tuberculosis

Aurachin RE (1) is a strong antibiotic that was recently found to possess 1,4-dihydroxy-2-naphthoate prenyltransferase (MenA) and bacterial electron transport inhibitory activities. Aurachin RE is the only molecule in a series of aurachin natural products that has the chiral center in the alkyl side chain at C9'-position. To identify selective MenA inhibitors against Mycobacterium tuberculosis , a series of chiral molecules were designed based on the structures of previously identified MenA inhibitors and 1. The synthesized molecules were evaluated in in vitro assays, including MenA enzyme and bacterial growth inhibitory assays. We could identify novel MenA inhibitors that showed significant increase in potency of killing nonreplicating M. tuberculosis in the low oxygen recovery assay (LORA) without inhibiting other Gram-positive bacterial growth even at high concentrations. The MenA inhibitors reported here are useful new pharmacophores for the development of selective antimycobacterial agents with strong activity against nonreplicating M. tuberculosis.

Reviews on 1,4-naphthoquinones from Diospyros L

The genus Diospyros is one of the most important sources of bioactive compounds, exclusively 1,4-naphthoquinones. The following information is an attempt to cover the developments in the biology and phytochemistry of 1,4-naphthoquinones isolated from this genus, as well as the studies done and the suggested mechanisms regarding their activities. During the past 60 years, many of these agents have been isolated from Diospyros L. Twelve considerable bioactive structures are reported in this review. The basic 1,4-naphthoquinone skeletons, on which a large number of studies have been done, are plumbagin and diospyrin. Today, the potential for development of leads from 1,4-naphthoquinones obtained from Diospyros L. is growing dramatically, mainly in the area of anticancer and antibacterial investigations. The data prepared and described here are intended to be served as a reference tool to the natural products and chemistry specialists in order to expand the rational drug design.