Antibacterial diterpenes from Plectranthus ernstii.

Three new diterpenoids including two pimaranes (1 and 2) and a labdane (3) were isolated from the whole herb of Plectranthus ernstii. The structures of these compounds were determined as rel-15(zeta),16-epoxy-7alpha-hydroxypimar-8,14-ene (1) and rel-15(zeta),16-epoxy-7-oxopimar-8,14-ene (2), and compound 3 was elucidated as 1R,11S-dihydroxy-8R,13R-epoxylabd-14-ene on the basis of single-crystal X-ray structural analysis. Compound 1 exhibited moderate antistaphylococcal activity against a range of multidrug-resistant (MDR) and methicillin-resistant (MRSA) strains of Staphylococcus aureus with a minimum inhibitory concentration (MIC) of 32 microg/mL. All three diterpenes exhibited antimycobacterial activity against three strains of rapidly growing mycobacteria with MIC values ranging from 8 to 128 microg/mL.

Antibacterial cannabinoids from Cannabis sativa: a structure-activity study.

Marijuana (Cannabis sativa) has long been known to contain antibacterial cannabinoids, whose potential to address antibiotic resistance has not yet been investigated. All five major cannabinoids (cannabidiol (1b), cannabichromene (2), cannabigerol (3b), Delta (9)-tetrahydrocannabinol (4b), and cannabinol (5)) showed potent activity against a variety of methicillin-resistant Staphylococcus aureus (MRSA) strains of current clinical relevance. Activity was remarkably tolerant to the nature of the prenyl moiety, to its relative position compared to the n-pentyl moiety (abnormal cannabinoids), and to carboxylation of the resorcinyl moiety (pre-cannabinoids). Conversely, methylation and acetylation of the phenolic hydroxyls, esterification of the carboxylic group of pre-cannabinoids, and introduction of a second prenyl moiety were all detrimental for antibacterial activity. Taken together, these observations suggest that the prenyl moiety of cannabinoids serves mainly as a modulator of lipid affinity for the olivetol core, a per se poorly active antibacterial pharmacophore, while their high potency definitely suggests a specific, but yet elusive, mechanism of activity.

Guaianolide sesquiterpenes from Pulicaria crispa (Forssk.) Oliv.

A phytochemical study of the asteraceous herb Pulicaria crispa (Forssk.) Oliv. resulted in the characterisation of three guaianolide sesquiterpenes, 2alpha,4alpha-dihydroxy-7alphaH,8alphaH,10alphaH-guaia-1(5),11(13)-dien-8beta,12-olide (1), 1alpha,2alpha-epoxy-4beta-hydroxy-5alphaH,7alphaH,8alphaH,10alphaH-guaia-11(13)-en-8beta,12-olide (2) and 5,10-epi-2,3-dihydroaromatin (3). The structures were assigned on the basis of extensive 1 and 2D NMR experiments. Compound 3 exhibited weak antimycobacterial activity against Mycobacterium phlei with a minimum inhibitory concentration of 0.52 mM and cytotoxicity (IC50 of 5.8+/-0.2 microM) in a human bladder carcinoma cell line, EJ-138.

Antimycobacterial polyacetylenes from Levisticum officinale.

No conflicts of interest concerning financial matters or personal relationships exist between the authors and those who might bias this work. The present work is in part included the PhD thesis of A. Schinkovitz (University of Graz) but has not been published elsewhere previously. The dichloromethane extract of the roots of Levisticum officinale L. (Apiaceae) exhibited significant antimycobacterial activity against Mycobacterium fortuitum and Mycobacterium aurum in a microtiter plate dilution assay and was further analysed following a bioassay-guided fractionation strategy. 3(R)-Falcarinol (3(R)-(-)-1,9-heptadecadien-4,6-diin-3-ol] and 3(R)-8(S)-falcarindiol [3(R)-8(S)-(+)-1,9-heptadecadien-4,6-diin-3,8-diol] could be identified as the active components in this extract. The minimal inhibitory concentration (MIC) of 3(R)-falcarinol against M. fortuitum and M. aurum was 16.4 microM while that of 3(R)-8(S)-falcarindiol was 30.7 microM against M. fortuitum and 61.4 microm against M. aurum, respectively. Previously, 3(R),8(R)-dehydrofalcarindiol was isolated from Artemisia monosperma and surprisingly this polyacetylene exhibited no antimycobacterial activity at 128 microg/mL. This indicates that the terminal methyl group is vital for retention of antimycobacterial activity. Reference antibiotics ethambutol and isoniazid exhibited an activity of 115.5 microM and 14.6 microM against M. fortuitum, and 3.4 microM and 29.2 microM against M. aurum, respectively.

N-caffeoylphenalkylamide derivatives as bacterial efflux pump inhibitors.

As part of an ongoing project to identify plant natural products as efflux pump inhibitors (EPIs), bioassay-guided fractionation of the methanolic extract of Mirabilis jalapa Linn. (Nyctaginaceae) led to the isolation of an active polyphenolic amide: N-trans-feruloyl 4'-O-methyldopamine. This compound showed moderate activity as an EPI against multidrug-resistant (MDR) Staphylococcus aureus overexpressing the multidrug efflux transporter NorA, causing an 8-fold reduction of norfloxacin MIC at 292 microM (100 microg/mL). This prompted us to synthesize derivatives in order to provide structure-activity relationships and to access more potent inhibitors. Among the synthetic compounds, some were more active than the natural compound and N-trans-3,4-O-dimethylcaffeoyl tryptamine showed potentiation of norfloxacin in MDR S. aureus comparable to that of the standard reserpine.

Bacterial efflux pump inhibitors from natural sources.

The rapid spread of bacteria expressing multidrug resistance (MDR) has necessitated the discovery of new antibacterials and resistance-modifying agents. Since the initial discovery of bacterial efflux pumps in the 1980s, many have been characterized in community- and hospital-acquired Gram-positive and Gram-negative pathogens, such as Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli and, more recently, in mycobacteria. Efflux pumps are able to extrude structurally diverse compounds, including antibiotics used in a clinical setting; the latter are rendered therapeutically ineffective. Antibiotic resistance can develop rapidly through changes in the expression of efflux pumps, including changes to some antibiotics considered to be drugs of last resort. It is therefore imperative that new antibiotics, resistance-modifying agents and, more specifically, efflux pump inhibitors (EPIs) are characterized. The use of bacterial resistance modifiers such as EPIs could facilitate the re-introduction of therapeutically ineffective antibiotics back into clinical use such as ciprofloxacin and might even suppress the emergence of MDR strains. Here we review the literature on bacterial EPIs derived from natural sources, primarily those from plants. The resistance-modifying activities of many new chemical classes of EPIs warrant further studies to assess their potential as leads for clinical development.

An antibacterial hydroxy fusidic acid analogue from Acremonium crotocinigenum.

A fusidane triterpene, 16-deacetoxy-7-beta-hydroxy-fusidic acid (1), was isolated from a fermentation of the mitosporic fungus Acremonium crotocinigenum. Full unambiguous assignment of all (1)H and (13)C data of 1 was carried out by extensive one- and two-dimensional NMR studies employing HMQC and HMBC spectra. Compound 1 was tested against a panel of multidrug-resistant (MDR) and methicillin-resistant Staphylococcus aureus (MRSA) strains and showed minimum inhibitory concentration values of 16 microg/ml.

Antimicrobial constituents of Scrophularia deserti.

A study of the chemistry and antibacterial activity of Scrophularia deserti led to the isolation of eight compounds, including the metabolite 3(zeta)-hydroxy-octadeca-4(E),6(Z)-dienoic acid (1). The known compounds ajugoside (2), scropolioside B (3), 6-O-alpha-L-rhamnopyranosylcatalpol (4), buddlejoside A(8) (5), scrospioside A (6), laterioside (7) and 3R-1-octan-3-yl-3-O-beta-D-glucopyranoside (8) were also isolated. Compounds 1-3 exhibited moderate antibacterial activity against strains of multidrug and methicillin-resistant Staphylococcus aureus (MRSA) and a panel of rapidly growing mycobacteria with minimum inhibitory concentration (MIC) values ranging from 32 to 128 microg/ml.

Antibacterial galloylated alkylphloroglucinol glucosides from myrtle (Myrtus communis).

An investigation of the polar glycosidic fraction from the leaves of myrtle afforded four galloylated nonprenylated phloroglucinol glucosides (3a-d) related to the endoperoxide hormone G3 (4) in terms of structure and biogenesis. Despite their close similarity, significant antibacterial activity was shown only by one of these compounds (3b, gallomyrtucommulone B), while the G3 hormone (4) was inactive.

The antimycobacterial constituents of dill (Anethum graveolens).

As part of a project to characterize selected members of the Kuwaiti flora for their phytochemistry and antimycobacterial activity, a new furanocoumarin, 5-[4''-hydroxy-3''-methyl-2''-butenyloxy]-6,7-furocoumarin (3), was isolated from the whole herb of Anethum graveolens. The known compounds oxypeucedanin (1), oxypeucedanin hydrate (2) and falcarindiol (4) were also isolated from this plant. The structure of each compound was determined by interpretation of NMR and mass spectrometric data. The three known compounds exhibited antibacterial activity against a panel of rapidly growing mycobacteria with minimum inhibitory concentration (MIC) values in the range 2-128 microg/mL.

An anti-staphylococcal acylphloroglucinol from Hypericum foliosum.

An investigation into the antibacterial properties of Hypericum foliosum Aiton. (Guttiferae) has led to the isolation of a new bioactive acylphloroglucinol natural product which by NMR spectroscopy and mass spectrometry was characterised as 1,3,5-trihydroxy-6-[2''',3'''-epoxy-3'''-methyl-butyl]-2-[2''-methyl-butanoyl]-4-[3'-methyl-2''-butenyl]-benzene and is described here for the first time. This metabolite was evaluated against a panel of multidrug-resistant strains of Staphylococcus aureus and minimum inhibitory values ranged from 16 to 32 microg/ml.

Bioactive constituents of Artemisia monosperma.

During a study on the chemistry and biological activity of Kuwaiti plants, new metabolites including 4,6-dihydroxy-3-[3'-methyl-2'-butenyl]-5-[4''-hydroxy-3''-methyl-2''-butenyl]-cinnamic acid (1), the 3R,8R stereoisomer of the C17 polyacetylene dehydrofalcarindiol (2) and a C10 polyacetylene glucoside (3) were characterised by spectroscopic means. Additionally, the previously characterised natural products 1,3R,8R-trihydroxydec-9-en-4,6-yne (4), spathulenol (5) and eriodyctiol-7-methyl ether (6) were also isolated. Compounds 2, 3, and 4 were evaluated for their ability to inhibit the enzyme 12-lipoxygenase and 3 and 4 showed moderate activity at 30 microg/ml. Compound 2 was evaluated against a panel of colorectal and breast cancer cell lines and IC50 values ranged from 5.8 to 37.6 microg/ml. Against a panel of fast-growing mycobacteria and a standard ATCC strain of Staphylococcus aureus, compound 6 exhibited minimum inhibitory concentrations in the range of 64-128 microg/ml.

Antimycobacterial coumarins from the sardinian giant fennel (Ferula communis).

The structure of a new prenylated coumarin (E-omega-benzoyloxyferulenol, 1b) from the Sardinian giant fennel (Ferula communis) has been confirmed by synthesis. The parent compound ferulenol (1a) showed sub-micromolar antimycobacterial activity, which was partly retained in 1b and in the simplified synthetic analogue 3, but diminished in its omega-hydroxy and omega-acetoxy derivatives (1c and 1d, respectively). The outstanding activity of 1a, its low toxicity, and the evidence for definite structure-activity relationships make this prenylated 4-hydroxycoumarin an interesting antibacterial chemotype worth further investigation.

The antimycobacterial components of hops (Humulus lupulus) and their dereplication.

Bioassay-guided fractionation of a hexane extract of strobile hops (Humulus lupulus) was undertaken to isolate and characterize the antimycobacterial constituents using the fast-growing mycobacterial species Mycobacterium fortuitum. Activity was associated with a low polarity fraction and 1H NMR spectra indicated the presence of a fatty acid mixture with unsaturated components. GC-MS of the derivatives indicated the presence of palmitic, stearic and oleic acids with small quantities of lignoceric, arachidic, behenic and linoleic acids. These compounds were assessed against M. fortuitum and all saturated fatty acids were inactive at concentrations greater than 256 microg/ml, whereas the unsaturated fats oleic and linoleic acids displayed minimum inhibitory concentrations of between 4 and 16 microg/ml against the fast-growing species tested. The widespread occurrence of these components could render screening for antimycobacterials from natural sources highly problematic without adequate dereplication. We propose that GC-MS of derivatised components of lipophilic extracts be a first step before any antimycobacterial bioassay-guided study, as this technique is the method of choice for dereplication of fatty acids.

New constituents of Artemisia monosperma.

A new eudesmane sesquiterpene (1) and a C(10) diyne (2) were isolated from the aerial parts of Artemisia monosperma. The structures of these compounds were determined as rel-1beta,3alpha,6beta-trihydroxyeudesm-4-ene (1) and 1,3R,8R-trihydroxydec-9-en-4,6-yne (2) on the basis of spectral data interpretation. The absolute stereochemistry of 2 was determined using Mosher ester methodology in which the terminal primary hydroxyl group was first protected to simplify the stereochemical analysis.

The anti-staphylococcal activity of Angelica dahurica (Bai Zhi).

Bioassay-guided fractionation of a hexane extract prepared from the roots of the Chinese drug Angelica dahurica (Bai Zhi) led to the isolation of the polyacetylenic natural product falcarindiol (1). The absolute stereochemistry of this compound was confirmed by careful 1H NMR analysis of its (R)- and (S)-Mosher ester derivatives as the 3(R), 8(S) isomer. Activity was tracked using a Mycobacterium fortuitum screening assay and the purified product was evaluated against multidrug-resistant and methicillin-resistant strains of Staphylococcus aureus (MRSA). The minimum inhibitory concentrations (MIC) of this metabolite ranged from 8 to 32 microg/ml highlighting the potential of the acetylene natural product class as antibiotic-lead compounds. These MIC values compare favourably with some of the newest agents in development for the treatment of MRSA infection and indicate that further evaluation of the antibiotic activity of acetylenes is warranted.

Pangelin, an antimycobacterial coumarin from Ducrosia anethifolia.

The aerial parts of Ducrosia anethifolia afforded the monoterpene glucoside 8-debenzoylpaeoniflorin ( 1) and the prenylated furanocoumarin pangelin [5-[2"( R)-hydroxy-3"-methyl-3"-butenyloxy]furocoumarin] ( 2). Their structures were determined by extensive 1- and 2-dimensional NMR studies. Compound 2 demonstrated activity against a panel of fast growing mycobacteria, namely Mycobacterium fortuitum, M. aurum, M. phlei and M. smegmatis and minimum inhibitory concentration (MIC) values ranged from 64 - 128 microg/mL. Whilst compounds 1 and 2 have previously been reported as an antihyperglycaemic component from Paeonia lactiflora, and as a constituent of Angelica pancici, respectively, this is the first report of the full (1)H- and (13)C-NMR data for these natural products.

Ostruthin: an antimycobacterial coumarin from the roots of Peucedanum ostruthium.

Following a bioassay-guided fractionation, ostruthin (6-geranyl-7-hydroxycoumarin) was isolated from the roots of Peucedanum ostruthium Koch (Apiaceae) as a compound with pronounced in vitro activity against several species of rapidly growing Mycobacteria, namely Mycobacterium abscesus, M. aurum, M. fortuitum, M. phlei and M. smegmatis. Minimum inhibitory concentrations (MIC) ranged between 3.4 to 107.4 microM and were comparable to those of ethambutol and isoniazid. Imperatorin (8-isopent-2-enyloxy-6,7-furanocoumarin) showed no activity at concentrations up to 1.9 mM. Umbelliferone (7-hydroxycoumarin) was only weakly active (MIC = 0.79 mM).