Recent advances in antitubercular natural products

meso-Dihydroguaiaretic acid derivatives with antibacterial and antimycobacterial activity

Thirty-three meso-dihydroguaiaretic acid (meso-DGA) derivatives bearing esters, ethers, and amino-ethers were synthesized. All derivatives were tested against twelve drug-resistant clinical isolates of Gram-positive and Gram-negative bacteria, including sensitive (H37Rv) and multidrug-resistant Mycobacterium tuberculosis strains. Among the tested compounds, four esters (7, 11, 13, and 17), one ether (23), and three amino-ethers (30, 31, and 33) exhibited moderate activity against methicillin-resistant Staphylococcus aureus, whereas 30 and 31 showed better results than levofloxacin against vancomycin-resistant Enterococcus faecium. Additionally, nineteen meso-DGA derivatives displayed moderate to potent activity against M. tuberculosis H37Rv with minimum inhibitory concentration (MIC) values ranging from 3.125 to 50µg/mL. Seven meso-DGA derivatives bearing amino-ethers (26-31 and 33) exhibited the lowest MICs against M. tuberculosis H37Rv and G122 strains, with 31 being as potent as ethambutol (MICs of 3.125 and 6.25µg/mL). The presence of positively charged group precursors possessing steric and hydrophobic features (e.g. N-ethylpiperidine moieties in meso-31) resulted essential to significantly increase the antimycobacterial properties of parent meso-DGA as supported by the R-group pharmacophoric and field-based QSAR analyses. To investigate the safety profile of the antimycobacterial compounds, cytotoxicity on Vero cells was determined. The amino-ether 31 exhibited a selectivity index value of 23, which indicate it was more toxic to M. tuberculosis than to mammalian cells. Therefore, 31 can be considered as a promising antitubercular agent for further studies.

Gypmacrophin A, a Rare Pentacyclic Sesterterpenoid, Together with Three Depsides, Functioned as New Chemical Evidence for Gypsoplaca macrophylla (Zahlbr.) Timdal Identification

The phytochemical investigation on 1 g of materials from Gypsoplaca macrophylla (Zahlbr.) Timdal resulted in the discovery of gypmacrophin A, a rare pentacyclic sesterterpenoid; brialmontin III, a new polysubstituted depside and two known ones, brialmontins I and II. The structure and absolute configurations of gypmacrophin A were elucidated by spectroscopic analyses and computational methods. Gypmacrophin A showed weak inhibition of AchE with an IC₅₀ value of 32.03 μM. The four compounds provided new chemical evidence for G. macrophylla identification.

Development of botanicals to combat antibiotic resistance

The discovery of antibiotics in the previous century lead to reduction in mortality and morbidity due to infectious diseases but their inappropriate and irrational use has resulted in emergence of resistant microbial populations. Alteration of target sites, active efflux of drugs and enzymatic degradations are the strategies employed by the pathogenic bacteria to develop intrinsic resistance to antibiotics. This has led to an increased interest in medicinal plants since 25-50% of current pharmaceuticals are plant derived. Crude extracts of medicinal plants could serve as an alternate source of resistance modifying agents owing to the wide variety of secondary metabolites. These metabolites (alkaloids, tannins, polyphenols etc.) could act as potentials for antimicrobials and resistance modifiers. Plant extracts have the ability to bind to protein domains leading to modification or inhibition protein-protein interactions. This enables the herbals to also present themselves as effective modulators of host related cellular processes viz immune response, mitosis, apoptosis and signal transduction. Thus they may exert their activity not only by killing the microorganism but by affecting key events in the pathogenic process, thereby, the bacteria, fungi and viruses may have a reduced ability to develop resistance to botanicals. The article is meant to stimulate research wherein the cidal activity of the extract is not the only parameter considered but other mechanism of action by which plants can combat drug resistant microbes are investigated. The present article emphasizes on mechanisms involved in countering multi drug resistance.

Alcaloides aporfínicos con actividad antituberculosa aislados de Ocotea discolor Kunth (Lauraceae)

La tuberculosis causa miles de muertes a nivel mundial y que, actualmente, los fármacos usados no son suficientes y en ocasiones son obsoletos para su tratamiento, se hace necesaria la búsqueda de nuevos compuestos que ayuden a combatirla. Por tanto, se evaluó la actividad antituberculosis de los alcaloides ocoxilonina (1), ocoteina (2), dicentrina (3) y 1,2-metilendioxi-3, 10,11-trimetoxiaporfina (4), aislados de la madera de Ocotea discolor. Las estructuras fueron identificadas por medio del análisis de los datos espectroscópicos de resonancia magnética nuclear (NMR 1D – 1H, 13C, 2D – COSY, HSQC y HMBC), espectros de masas y comparación con datos de la literatura. Todos los compuestos aislados demostraron actividad antituberculosa, con un rango de variación en la concentración mínima inhibitoria entre 140 y 310 μM, siendo la ocoteina (2) la más activo contra la cepa virulenta de Mycobacterium tuberculosis H37Rv.

QSAR-driven design, synthesis and discovery of potent chalcone derivatives with antitubercular activity

New anti-tuberculosis (anti-TB) drugs are urgently needed to battle drug-resistant Mycobacterium tuberculosis strains and to shorten the current 6-12-month treatment regimen. In this work, we have continued the efforts to develop chalcone-based anti-TB compounds by using an in silico design and QSAR-driven approach. Initially, we developed SAR rules and binary QSAR models using literature data for targeted design of new heteroaryl chalcone compounds with anti-TB activity. Using these models, we prioritized 33 compounds for synthesis and biological evaluation. As a result, 10 heteroaryl chalcone compounds (4, 8, 9, 11, 13, 17-20, and 23) were found to exhibit nanomolar activity against replicating mycobacteria, low micromolar activity against nonreplicating bacteria, and nanomolar and micromolar against rifampin (RMP) and isoniazid (INH) monoresistant strains (rRMP and rINH) (<1 μM and <10 μM, respectively). The series also show low activity against commensal bacteria and generally show good selectivity toward M. tuberculosis, with very low cytotoxicity against Vero cells (SI = 11-545). Our results suggest that our designed heteroaryl chalcone compounds, due to their high potency and selectivity, are promising anti-TB agents.

α-pyrones: Small molecules with versatile structural diversity reflected in multiple pharmacological activities-an update

The investigations in the chemistry and biology of α-pyrone (2-pyrone) are of vital importance as they constitute an essential pharmacophore in many naturally occurring and biologically active synthetic agents. They are a promising class of biorenewable platform chemicals that provide access to an array of chemical products and intermediates. Literature survey reveals that a simple change in the substitution pattern on the 2-pyrone ring system often leads to diverse biological activities. In this review, we present a brief overview of 2-pyrone pharmacophore followed by highlighting their pharmacological properties and potential applicability till date. Particular attention is focused on the distinctive chemotherapeutic activities of 2-pyrones as anti-HIV, anti-TB and anti-cancer agents followed by their potential role against neurodegeneration, hypercholesterolemia, microbial infections, chronic obstructive lung disease, inflammation, antinociception and immunomodulation. Since 2005, when 2-pyrones came in limelight, their detailed pharmacological activities have been well documented. This review has mainly been prepared on the basis of original reports published in recent two decades with an aim to attract the attention of researchers towards this versatile scaffold for future endeavors that may lead to the development of potential drug candidates against above diseases.

Design, synthesis and structure-activity relationship study of wollamide B; a new potential anti TB agent

Wollamide B is a cationic antimycobacterial cyclohexapeptide that exhibits activity against Mycobacterium bovis (M. bovis) (IC₅₀ of 3.1 μM). Aiming to define its structural activity relationship (SAR), optimizing potency and pharmacokinetic properties, libraries of analogues were synthesized following a standard Fmoc-based solid phase peptide synthesis approach. The antimycobacterial activities of wollamide B and all the synthesized analogues were tested against Mycobacterium tuberculosis (Mtb) H37Rv. Parallely, in vitro drug metabolism and pharmacokinetic (ADME) profiling was done for the synthesized compounds to evaluate their drug likeness. Among the 25 synthesized wollamides five of them showed potent activities with MICs ≤ 3.1 μM and found to be nontoxic against human HepG2 cells up to 100 μM. The results of the in vitro ADME profiling revealed the remarkable plasma stability and very good aqueous solubility of the class in general while the metabolic stability was found to be moderate to low. Of particular note, compounds 7c (MIC = 1.1 μM) and 13c (0.6 μM) that exhibited good balance of antimycobacterial activity vs. optimal pharmacokinetic properties could be used as a new lead for further development.

Asymmetric Total Synthesis of Ieodomycin B

Ieodomycin B, which shows in vitro antimicrobial activity, was isolated from a marine Bacillus species. A novel asymmetric total synthetic approach to ieodomycin B using commercially available geraniol was achieved. The approach involves the generation of 1,3-trans-dihydroxyl at C-3 and C-5 positions via a Crimmins-modified Evans aldol reaction and a chelation-controlled Mukaiyama aldol reaction of a p-methoxybenzyl-protected aldehyde, as well as the generation of a lactone ring in a deprotection–lactonization one-pot reaction.

Biosynthetic potential of sesquiterpene synthases: product profiles of Egyptian Henbane premnaspirodiene synthase and related mutants

The plant terpene synthase (TPS) family is responsible for the biosynthesis of a variety of terpenoid natural products possessing diverse biological functions. TPSs catalyze the ionization and, most commonly, rearrangement and cyclization of prenyl diphosphate substrates, forming linear and cyclic hydrocarbons. Moreover, a single TPS often produces several minor products in addition to a dominant product. We characterized the catalytic profiles of Hyoscyamus muticus premnaspirodiene synthase (HPS) and compared it with the profile of a closely related TPS, Nicotiana tabacum 5-epi-aristolochene synthase (TEAS). The profiles of two previously studied HPS and TEAS mutants, each containing nine interconverting mutations, dubbed HPS-M9 and TEAS-M9, were also characterized. All four TPSs were compared under varying temperature and pH conditions. In addition, we solved the X-ray crystal structures of TEAS and a TEAS quadruple mutant complexed with substrate and products to gain insight into the enzymatic features modulating product formation. These informative structures, along with product profiles, provide new insight into plant TPS catalytic promiscuity.

Antitubercular Lanostane Triterpenes from Cultures of the Basidiomycete Ganoderma sp. BCC 16642

Sixteen new lanostane triterpenoids (1-16), together with 26 known compounds (17-42), were isolated from cultures of the basidiomycete Ganoderma sp. BCC 16642. Antitubercular activities of these Ganoderma lanostanoids against Mycobacterium tuberculosis H37Ra were evaluated, and structure-activity relationships are proposed.

Design, Synthesis and Antimycobacterial Activity of Novel Imidazo[1,2-a]pyridine Amide-Cinnamamide Hybrids

We report herein the design and synthesis of a series of novel imidazo[1,2-a]pyridine amide-cinnamamide hybrids linked via an alkyl carbon chain. All 38 new hybrids were evaluated for their antimycobacterial activity against M. tuberculosis (MTB) H37Rv ATCC 27294 using the microplate Alamar Blue assay (MABA). Although the hybrids are less active than the two reference compounds, the promising activity (MICs: 4 μg/mL) of 2,6-dimethylimidazo[1,2-a]pyridine amide-cinnamamide hybrids 11e and 11k could be a good starting point to further find new lead compounds against multi-drug-resistant tuberculosis.

Lipid Metabolites from the MushroomMeripilus giganteus

The phytochemical investigation of the methanolic extract of the white rot fungus Meripilus giganteus resulted in the isolation and identification of compl mixtures of free fatty acids (1), monoacylglycerols (2), cerebrosides (3), ergosterol (4) and ergosterol peroxide (5). The structures of the isolated lip metabolites (1-5) were determined by chemical and spectroscopic methods. The antioxidant activity of the whole MeOH extract of the fungus was evaluat through in vitro model systems, such as 2,2-diphenyl-1-picrylhydrazyl (DPPH) and superoxide anion. In all two systems, the results indicated that the extra of the fungus showed the same free-radical-scavenging activity with SC50data of 47.70 μg/mL, compared with the positive control quercetin (DPPH assa None of the isolated compounds (1-5) showed a significant activity. Compounds 2-4 were isolated from Meripilus giganteus for the first time.

Computational evaluation of phytocompounds for combating drug resistant tuberculosis by multi-targeted therapy

The cell wall of Mycobacterium tuberculosis interacts with the host counterpart during the pathogenesis of tuberculosis. L-rhamnosyl (L-Rha) residue, a linker connects the arabinogalactan and peptidoglycan moieties in the bacterial cell wall. The biosynthesis of L-rhamnose utilizes four successive enzymes RmlA, RmlB, RmlC and RmlD. Neither rhamnose nor the genes responsible for its synthesis are observed in humans. Thus, drugs inhibiting enzymes of this pathway are unlikely to interfere with metabolic pathways in humans. The adverse drug effects of first and second line drugs along with the development of multi-drug resistance tuberculosis have stimulated the research in search of new therapeutic drugs. Thus, it is attractive to hypothesize that inhibition of the biosynthesis of L-Rha would be lethal to the mycobacteria. Nature provides innumerable secondary metabolites with novel structural architectures with reported activity against M. tuberculosis. Combination of structure based virtual screening with physicochemical and pharmacokinetic studies against rhamnose pathway enzymes identified potential leads. The crucial screening studies recognized four phytocompounds butein, diospyrin, indicanine, and rumexneposide A with good binding affinity towards the rhamnose pathway proteins. Furthermore, the high throughput screening methods recognized butein, a secondary metabolite from Butea monosperma with strong anti-tubercular bioactive spectrum. Butein displayed promising anti-mycobacterial activity which is validated by Microplate alamar blue assay (MABA). The focus on novel agents like these phytocompounds which exhibit preference toward the successive enzymes of a single pathway can prevent the development of bacterial resistance.

Design and synthesis of 11α-substituted bile acid derivatives as potential anti-tuberculosis agents

We have synthesized a series of novel 11α-triazoyl bile acid derivatives. In addition, we also have synthesized N-alkyl and N-acyl derivatives of C-11 amino bile acid esters. All the compounds were evaluated for the inhibitory activity against Mycobacterium tuberculosis H37Ra (MTB) at 30 μg/mL level. Four lead compounds (2b, 3, 7 and 8) were further confirmed from their dose dependent effect against MTB. These compounds were found to be active against Dormant and active stage MTB under both in vitro as well as within THP1 host macrophages. The most promising compound 2b showed strong antitubercular activities against MTB under in vitro and ex vivo (IC90 value of ∼3 μg/mL) conditions and almost insignificant cytotoxicity up to 100 μg/mL against THP-1, A549 and PANC-1 human cancer cell lines. Inactivity of all these compounds against Gram positive and Gram negative bacteria indicates their specificity. Molecular docking studies of these compounds into the active site of DprE1 enzyme revealed a similar binding mode to native ligands in the crystal structure thereby helping to establish a structural basis of inhibition of MTB. The synthesized compounds were analyzed for ADME properties and showed potential to develop good oral drug candidates. Our results clearly indicate the identification of some novel, selective and specific inhibitors against MTB that can be explored further for potential antitubercular drug.

Current perspectives in drug discovery against tuberculosis from natural products

Currently, one third of the world's population is latently infected with Mycobacterium tuberculosis (MTB), while 8.9-9.9 million new and relapse cases of tuberculosis (TB) are reported yearly. The renewed research interests in natural products in the hope of discovering new and novel antitubercular leads have been driven partly by the increased incidence of multidrug-resistant strains of MTB and the adverse effects associated with the first- and second-line antitubercular drugs. Natural products have been, and will continue to be a rich source of new drugs against many diseases. The depth and breadth of therapeutic agents that have their origins in the secondary metabolites produced by living organisms cannot be compared with any other source of therapeutic agents. Discovery of new chemical molecules against active and latent TB from natural products requires an interdisciplinary approach, which is a major challenge facing scientists in this field. In order to overcome this challenge, cutting edge techniques in mycobacteriology and innovative natural product chemistry tools need to be developed and used in tandem. The present review provides a cross-linkage to the most recent literature in both fields and their potential to impact the early phase of drug discovery against TB if seamlessly combined.

Aranciamycins I and J, Antimycobacterial Anthracyclines from an Australian Marine-Derived Streptomyces sp

Chemical analysis of an Australian marine-derived Streptomyces sp. (CMB-M0150) yielded two new anthracycline antibiotics, aranciamycins I (1) and J (2), as well as the previously reported aranciamycin A (3) and aranciamycin (4). The aranciamycins 1-4, identified by detailed spectroscopic analysis, were noncytotoxic when tested against selected Gram-negative bacteria and fungi (IC50 >30 μM) and exhibited moderate and selective cytotoxicity against Gram-positive bacteria (IC50 >1.1 μM) and a panel of human cancer cell lines (IC50 > 7.5 μM). Significantly, 1-4 were cytotoxic (IC50 0.7-1.7 μM) against the Mycobacterium tuberculosis surrogate M. bovis bacille Calmette-Guérin.

Natural antimicrobial peptides against Mycobacterium tuberculosis

TB, caused by Mycobacterium tuberculosis, is one the leading infectious diseases worldwide. There is an urgent need to discover new drugs with unique structures and uncommon mechanisms of action to treat M. tuberculosis and combat antimycobacterial resistance. Naturally occurring compounds contain a wide diversity of chemical structures, displaying a wide range of in vitro potency towards M. tuberculosis. A number of recent studies have shown that natural antimycobacterial peptides can disrupt the function of the mycobacterial cell wall through different modes of action and thereafter interact with intracellular targets, including nucleic acids, enzymes and even organelles. More importantly, the probability of antimycobacterial resistance is low. This review presents several natural antimicrobial peptides isolated from different organism sources, including bacteria, fungi, plants and animals. In addition, the molecular features of these molecules are the subject of much attention. Such peptides have common traits among their chemical features, which may be correlated with their biological activities; hence, different parts of the molecular structures can be modified in order to increase penetration into the target cells. This review also summarizes the available information on the properties of antimycobacterial peptides associated with their biological activities.

The tuberculosis drug discovery and development pipeline and emerging drug targets

The recent accelerated approval for use in extensively drug-resistant and multidrug-resistant-tuberculosis (MDR-TB) of two first-in-class TB drugs, bedaquiline and delamanid, has reinvigorated the TB drug discovery and development field. However, although several promising clinical development programs are ongoing to evaluate new TB drugs and regimens, the number of novel series represented is few. The global early-development pipeline is also woefully thin. To have a chance of achieving the goal of better, shorter, safer TB drug regimens with utility against drug-sensitive and drug-resistant disease, a robust and diverse global TB drug discovery pipeline is key, including innovative approaches that make use of recently acquired knowledge on the biology of TB. Fortunately, drug discovery for TB has resurged in recent years, generating compounds with varying potential for progression into developable leads. In parallel, advances have been made in understanding TB pathogenesis. It is now possible to apply the lessons learned from recent TB hit generation efforts and newly validated TB drug targets to generate the next wave of TB drug leads. Use of currently underexploited sources of chemical matter and lead-optimization strategies may also improve the efficiency of future TB drug discovery. Novel TB drug regimens with shorter treatment durations must target all subpopulations of Mycobacterium tuberculosis existing in an infection, including those responsible for the protracted TB treatment duration. This review summarizes the current TB drug development pipeline and proposes strategies for generating improved hits and leads in the discovery phase that could help achieve this goal.

Activation of aqueous hydrogen peroxide for non-catalyzed dihydroperoxidation of ketones by azeotropic removal of water

Cyclic and acyclic ketones were selectively converted to gem-dihydroperoxides in 72–99% yield with 30% aq. hydrogen peroxide by azeotropic distillation of water from the reaction mixture without any catalyst.

Potential mechanism of action of meso-dihydroguaiaretic acid on Mycobacterium tuberculosis H37Rv

The isolation and characterization of the lignan meso-dihydroguaiaretic acid (MDGA) from Larrea tridentata and its activity against Mycobacterial tuberculosis has been demonstrated, but no information regarding its mechanism of action has been documented. Therefore, in this study we carry out the gene expression from total RNA obtained from M. tuberculosis H37Rv treated with MDGA using microarray technology, which was validated by quantitative real time polymerase chain reaction. Results showed that the alpha subunit of coenzyme A transferase of M. tuberculosis H37Rv is present in both geraniol and 1-and 2-methylnaphthalene degradation pathways, which are targeted by MDGA. This assumption was supported by molecular docking which showed stable interaction between MDGA with the active site of the enzyme. We propose that inhibition of coenzyme A transferase of M. tuberculosis H37Rv results in the accumulation of geraniol and 1-and 2-methylnaphtalene inside bacteria, causing membrane destabilization and death of the pathogen. The natural product MDGA is thus an attractive template to develop new anti-tuberculosis drugs, because its target is different from those of known anti-tubercular agents.

BioPhytMol: a drug discovery community resource on anti-mycobacterial phytomolecules and plant extracts

Background

Tuberculosis (TB) is the second leading cause of death from a single infectious organism, demanding attention towards discovery of novel anti-tubercular compounds. Natural products or their derivatives have provided more than 50% of all existing drugs, offering a chemically diverse space for discovery of novel drugs.

Description

BioPhytMol has been designed to systematically curate and analyze the anti-mycobacterial natural product chemical space. BioPhytMol is developed as a drug-discovery community resource with anti-mycobacterial phytomolecules and plant extracts. Currently, it holds 2582 entries including 188 plant families (692 genera and 808 species) from global flora, manually curated from literature. In total, there are 633 phytomolecules (with structures) curated against 25 target mycobacteria. Multiple analysis approaches have been used to prioritize the library for drug-like compounds, for both whole cell screening and target-based approaches. In order to represent the multidimensional data on chemical diversity, physiochemical properties and biological activity data of the compound library, novel approaches such as the use of circular graphs have been employed.

Conclusion

BioPhytMol has been designed to systematically represent and search for anti-mycobacterial phytochemical information. Extensive compound analyses can also be performed through web-application for prioritizing drug-like compounds. The resource is freely available online at http://ab-openlab.csir.res.in/biophytmol/.

Graphical Abstract

Synthesis, characterization, cytotoxicity of mixed ligand complexes of palladium(II) with dipyrido[3,2-d:2',3'-f]quinoxaline/dipyrido[3,2-a:2',3'-c](6,7,8,9-tetrahydro)phenazine and 4-toluensulfonyl-L-amino acid dianion

Ten novel palladium(II) complexes with dipyrido[3,2-d:2',3'-f]quinoxaline (Dpq)/dipyrido[3,2-a:2',3'-c](6,7,8,9-tetrahydro)phenazine (Dpqc) and 4-toluensulfonyl-L-amino acid dianion, [Pd(Dpq)(TsvalNO)]·H2O (1a), [Pd(Dpq)(TsileNO)]·H2O (1b), [Pd(Dpq)(TsserNO)] (1c), [Pd(Dpq)(TsthrNO)]·1.5H2O (1d), [Pd(Dpq)(TsleuNO)]·0.5H2O (1e), [Pd(Dpq)(TspheNO)] (1f), [Pd(Dpqc)(TsvalNO)] (2a), [Pd(Dpqc)(TsileNO)] (2b), [Pd(Dpqc)(TsserNO)]·H2O (2c) and [Pd(Dpqc)(TsthrNO)]·0.5H2O (2d) have been synthesized and characterized by elemental analysis, IR, UV, (1)H NMR and mass spectrometry techniques. Crystal structure of the complex 1f has been determined by X-ray diffraction. The cytotoxicity was tested by MTT assay. The results indicated that the complexes 1a and 2a showed better cytotoxicity than cisplatin against MCF-7. The complex 1e had higher cytotoxicity than cisplatin against K562. Both the N donating ligands and the amino acid have important effects on the cytotoxicity.

Combating pathogenic microorganisms using plant-derived antimicrobials: a minireview of the mechanistic basis

The emergence of antibiotic resistance in pathogenic bacteria has led to renewed interest in exploring the potential of plant-derived antimicrobials (PDAs) as an alternative therapeutic strategy to combat microbial infections. Historically, plant extracts have been used as a safe, effective, and natural remedy for ailments and diseases in traditional medicine. Extensive research in the last two decades has identified a plethora of PDAs with a wide spectrum of activity against a variety of fungal and bacterial pathogens causing infections in humans and animals. Active components of many plant extracts have been characterized and are commercially available; however, research delineating the mechanistic basis of their antimicrobial action is scanty. This review highlights the potential of various plant-derived compounds to control pathogenic bacteria, especially the diverse effects exerted by plant compounds on various virulence factors that are critical for pathogenicity inside the host. In addition, the potential effect of PDAs on gut microbiota is discussed.

Optimisation of the microplate resazurin assay for screening and bioassay-guided fractionation of phytochemical extracts against Mycobacterium tuberculosis

INTRODUCTION

Because of increased resistance to current drugs, there is an urgent need to discover new anti-mycobacterial compounds for the development of novel anti-tuberculosis drugs. The microplate resazurin assay (MRA) is commonly used to evaluate natural products and synthetic compounds for anti-mycobacterial activity. However, the assay can be problematic and unreliable when screening methanolic phytochemical extracts.

OBJECTIVE

To optimise the MRA for the screening and bioassay-guided fractionation of phytochemical extracts using Mycobacterium tuberculosis H37Ra.

METHODS

The effects of varying assay duration, resazurin solution composition, solvent (dimethyl sulphoxide - DMSO) concentration and type of microtitre plate used on the results and reliability of the MRA were investigated. The optimal bioassay protocol was applied to methanolic extracts of medicinal plants that have been reported to possess anti-mycobacterial activity.

RESULTS

The variables investigated were found to have significant effects on the results obtained with the MRA. A standardised procedure that can reliably quantify anti-mycobacterial activity of phytochemical extracts in as little as 48 h was identified. The optimised MRA uses 2% aqueous DMSO, with an indicator solution of 62.5 µg/mL resazurin in 5% aqueous Tween 80 over 96 h incubation.

CONCLUSION

The study has identified an optimal procedure for the MRA when used with M. tuberculosis H37Ra that gives rapid, reliable and consistent results. The assay procedure has been used successfully for the screening and bioassay-guided fractionation of anti-mycobacterial compounds from methanol extracts of Canadian medicinal plants.

Predicting natural product value, an exploration of anti-TB drug space

Covering: January 1990 to December 2012. Mycobacterium tuberculosis (Mtb) still remains a deadly pathogen two decades after the announcement of tuberculosis (TB) as a global health emergency by the World Health Organization. In last few years new drug combinations have shown promising potential to significantly shorten TB treatment times. However there are very few new chemical entities being developed to treat this global threat. From January 1990 to December 2012, 949 anti-mycobacterium natural products were reported in the literature. Here we present a perspective based on an analysis of the drug-like properties of the reported anti-mycobacterium natural products in order to assess drug potential.

Tuberculosis drug discovery and emerging targets

Current tuberculosis (TB) therapies take too long and the regimens are complex and subject to adverse effects and drug-drug interactions with concomitant medications. The emergence of drug-resistant TB strains exacerbates the situation. Drug discovery for TB has resurged in recent years, generating compounds (hits) with varying potential for progression into developable leads. In parallel, advances have been made in understanding TB pathogenesis. It is now possible to apply the lessons learned from recent TB hit generation efforts and newly validated TB drug targets to generate the next wave of TB drug leads. Use of currently underexploited sources of chemical matter and lead-optimization strategies may also improve the efficiency of future TB drug discovery. Novel TB drug regimens with shorter treatment durations must target all subpopulations of Mycobacterium tuberculosis existing in an infection, including those responsible for the protracted TB treatment duration. This review proposes strategies for generating improved hits and leads that could help achieve this goal.

First total syntheses and antimicrobial evaluation of penicimonoterpene, a marine-derived monoterpenoid, and its various derivatives

The first total synthesis of marine-derived penicimonoterpene (±)-1 has been achieved in four steps from 6-methylhept-5-en-2-one using a Reformatsky reaction as the key step to construct the basic carbon skeleton. A total of 24 new derivatives of 1 have also been designed and synthesized. Their structures were characterized by analysis of their 1H NMR, 13C NMR and HRESIMS data. Some of them showed significant antibacterial activity against Aeromonas hydrophila, Escherichia coli, Micrococcus luteus, Staphylococcus aureus, Vibrio anguillarum, V. harveyi and/or V. parahaemolyticus, and some showed activity against plant-pathogenic fungi (Alternaria brassicae, Colletotrichum gloeosporioides and/or Fusarium graminearum). Some of the derivatives exhibited antimicrobial MIC values ranging from 0.25 to 4 μg/mL, which were stronger than those of the positive control. Notably, Compounds 3b and 10 showed extremely high selectively against plant-pathogenic fungus F. graminearum (MIC 0.25 μg/mL) and pathogenic bacteria E. coli (MIC 1 μg/mL), implying their potential as antimicrobial agents. SAR analysis of 1 and its derivatives indicated that modification of the carbon-carbon double bond at C-6/7, of groups on the allylic methylene unit and of the carbonyl group at C-1, effectively enhanced the antimicrobial activity.

Hydnocarpus: an ethnopharmacological, phytochemical and pharmacological review

ETHNOPHARMACOLOGICAL RELEVANCE

The genus Hydnocarpus (Flacourtiaceae) includes forty species that are spread across the globe. In the Indian System of Medicine, Hydnocarpus pentandrus (Buch.-Ham.) Oken. is primarily used for treating leprosy and other skin disorders. It is known as "Chaulmoogra" and is also used to treat other indications including constipation, inflammation, blood disorders, and worm infestations. Various species of Hydnocarpus are also used in traditional medicine in China, Thailand, Malaysia, and Myanmar for several skin disorders. To assess the therapeutic potential of species from the Hydnocarpus genus and to determine future avenues for research.

METHODS

All relevant scientific literature published up to the end of December 2013 was retrieved via a library and electronic search (SciFinder, PubMed, ScienceDirect, and Google Scholar). Manual searches of traditional books like to ancient classics, including Vaidya Yoga Ratnavali, Siddha Materia Medica, and contemporary references including The Ayurvedic Pharmacopoeia of India and The Ayurveda Formulary, were also performed.

RESULTS

Seed oil from species of the Hydnocarpus genus is used for medicinal purposes, predominantly for various skin disorders. This oil is reported to contain a characteristic class of compounds known as cyclopentenyl fatty acids. Furthermore, seeds of this genus are reported to contain triglycerides of fatty acids, sterols, flavonoids, and flavonolignans. Hydnocarpin, a flavonolignan, is reported to potentiate antimicrobial and anticancer activity. The extracts and compounds isolated from this plant show a wide spectrum of pharmacological properties, including antibacterial, antileprotic, antitubercular, antipsoriatic, antirheumatic, hypolipidemic, antidiabetic, anticancer, anti-inflammatory, and antioxidant activities. The antileprotic activity is postulated to be due to the cyclopentenyl fatty acids present in the seed oil.

CONCLUSION

Flavonolignans have an interesting chemical motif, and hydnocarpin and its congeners should be investigated for their activities and the mechanism underlying these activities. Multi-drug-resistant microbes are on the increase, and the possible inhibitory effect of these compounds when used with current antimicrobials should also be evaluated. Furthermore, unique cyclopentenyl fatty acids should also be investigated to understand the exact mechanism of action underlying antileprotic activity. Additional in depth phytochemical investigations of seed oil and extracts are required to tap the true potential of species from the Hydnocarpus genus.

Anti-Mycobacterium tuberculosis activity and cytotoxicity of Calophyllum brasiliense Cambess (Clusiaceae)

We evaluated the in vitro anti-Mycobacterium tuberculosis activity and the cytotoxicity of dichloromethane extract and pure compounds from the leaves of Calophyllum brasiliense. Purification of the dichloromethane extract yielded the pure compounds (-) mammea A/BB (1), (-) mammea B/BB (2) and amentoflavone (3). The compound structures were elucidated on the basis of spectroscopic and spectrometric data. The contents of bioactive compounds in the extracts were quantified using high performance liquid chromatography coupled to an ultraviolet detector. The anti-M. tuberculosis activity of the extracts and the pure compounds was evaluated using a resazurin microtitre assay plate. The cytotoxicity assay was performed in J774G.8 macrophages using the 3-(4,5-dimethyl thiazol-2-yl)-2,5-diphenyl tetrazolium bromide colourimetric method. The quantification of the dichloromethane extract showed (1) and (2) at concentrations of 31.86 ± 2.6 and 8.24 ± 1.1 µg/mg of extract, respectively. The dichloromethane and aqueous extracts showed anti-M. tuberculosis H37Rv activity of 62.5 and 125 µg/mL, respectively. Coumarins (1) and (2) showed minimal inhibitory concentration ranges of 31.2 and 62.5 µg/mL against M. tuberculosis H37Rv and clinical isolates. Compound (3) showed no activity against M. tuberculosis H37Rv. The selectivity index ranged from 0.59-1.06. We report the activity of the extracts and coumarins from the leaves of C. brasiliense against M. tuberculosis.