In Vitro Antibacterial Activity of Prenylated Guanidine Alkaloids from Pterogyne nitens and Synthetic Analogues

The present investigation deals with the antibiotic activity of eight natural guanidine alkaloids and two synthetic analogues against a variety of clinically relevant methicillin-resistant Staphylococcus aureus strains. Galegine (1) and pterogynidine (2) were the most potent compounds, with a minimum inhibitory concentration of 4 mg/L, to all tested strains. The preliminary chemical features correlating to anti-MRSA activity showed that the size of the side chain and the substitution pattern in the guanidine core played a key role in the antibacterial activity of the imino group. Guanidine alkaloids 1 and 2 are promising molecular models for further synthetic derivatives and, thus, for medicinal chemistry studies.

Amanicadol, a Pimarane-type Diterpene from Phlomis amanica Vierch

Fractionation of the methanol extract of Phlomis amanica resulted in the isolation of a new pimarane type diterpene, amanicadol (1), together with the known glycosides lamiide, verbascoside (= acteoside), syringaresinol-4-O-β -glucoside, liriodendrin, syringin, and a caffeic acid ester, chlorogenic acid. The structure of the new compound was established on the basis of extensive 1D and 2D NMR spectroscopic data interpretation. Molecular modeling studies on 1 were conducted and showed that it exhibited low conformational flexibility. Additionally, NMR chemical shifts were calculated for 1 in vacuo, and calculated values were in very close agreement with those found experimentally.

Flavonoids from Sophora moorcroftiana and their synergistic antibacterial effects on MRSA

Synergy is now a widely recognized approach that has direct applicability for new pharmaceuticals. The ethanolic extract of the aerial parts of the herb Sophora moorcroftiana showed significant antibacterial activity against drug-resistant Staphylococcus aureus, and its minimum inhibitory concentration (MIC) was 8 µg/mL. In a phytochemical study of the extract, five flavonoids were obtained. However, the isolates exhibited antibacterial activity in the range of 32-128 µg/mL, which was weaker than the extract. In combination with antibiotics, the antibacterially inactive compound genistein (1) and diosmetin (4) showed significant synergistic activity against drug-resistant S. aureus. In combination with norfloxacin, genistein (1) reduced the MIC to 16 µg/mL and showed synergy against strain SA1199B with a fractional inhibitory concentration index (FICI) of 0.38. With the antibiotics norfloxacin, streptomycin and ciprofloxacin, diosmetin (4) showed synergy against SA1199B, RN4220 and EMRSA-15, with FICI values of 0.38, 0.38 and 0.09, respectively. In an efflux experiment to elucidate a plausible mechanism for the observed synergy, genistein showed marginal inhibition of the NorA efflux protein.

A new plant-derived antibacterial is an inhibitor of efflux pumps in Staphylococcus aureus

An in-depth evaluation was undertaken of a new antibacterial natural product (1) recently isolated and characterised from the plant Hypericum olympicum L. cf. uniflorum. Minimum inhibitory concentrations (MICs) were determined for a panel of bacteria, including: meticillin-resistant and -susceptible strains of Staphylococcus aureus, Staphylococcus epidermidis and Staphylococcus haemolyticus; vancomycin-resistant and -susceptible Enterococcus faecalis and Enterococcus faecium; penicillin-resistant and -susceptible Streptococcus pneumoniae; group A streptococci (Streptococcus pyogenes); and Clostridium difficile. MICs were 2-8 mg/L for most staphylococci and all enterococci, but were ≥16 mg/L for S. haemolyticus and were >32 mg/L for all species in the presence of blood. Compound 1 was also tested against Gram-negative bacteria, including Escherichia coli, Pseudomonas aeruginosa and Salmonella enterica serovar Typhimurium but was inactive. The MIC for Mycobacterium bovis BCG was 60 mg/L, and compound 1 inhibited the ATP-dependent Mycobacterium tuberculosis MurE ligase [50% inhibitory concentration (IC(50)) = 75 μM]. In a radiometric accumulation assay with a strain of S. aureus overexpressing the NorA multidrug efflux pump, the presence of compound 1 increased accumulation of (14)C-enoxacin in a concentration-dependent manner, implying inhibition of efflux. Only moderate cytotoxicity was observed, with IC50 values of 12.5, 10.5 and 8.9 μM against human breast, lung and fibroblast cell lines, respectively, highlighting the potential value of this chemotype as a new antibacterial agent and efflux pump inhibitor.

Modulators of antibiotic activity from Ipomoea murucoides

Reinvestigation of the CHCl3-soluble extract from the flowers of Ipomoea murucoides, through preparative-scale recycling HPLC, yielded three pentasaccharides of 11-hydroxyhexadecanoic acid, murucoidins XVII-XIX, in addition to the known murucoidin III and V, all of which were characterized by NMR spectroscopy and mass spectrometry. These compounds were found to be macrolactones of the known pentasaccharides simonic acid B and operculinic acid A. The acylating groups corresponded to acetic, (2S)-methyl-butyric, (E)-cinnamic and octanoic acids. The esterification sites were established at the C-2 of the second rhamnose and C-3 and C-4 of the third rhamnose. The aglycone lactonization was placed at C-2 or C-3 of the first rhamnose. Bioassays for modulation of antibiotic activity were performed against multidrug-resistant strains of Staphylococcus aureus, Escherichia coli Rosetta-gami, and two nosocomial pathogens: Salmonella enterica sv. Typhi and Shigella flexneri. The tested glycolipids did not act as cytotoxic (IC50>4 μg/mL) nor as antimicrobial (MIC>128 μg/mL) agents. However, they exerted a potentiation effect on clinically useful antibiotics against the tested bacteria by increasing their antibiotic susceptibility up to four-fold at concentrations of 25 μg/mL.

Biological evaluation of hyperforin and its hydrogenated analogue on bacterial growth and biofilm production

Bacterial biofilms are organized communities of microorganisms, embedded in a self-produced matrix, growing on a biotic surface and resistant to many antimicrobial agents when associated with a medical device. These biofilms require the development of new strategies for the prevention and treatment of infectious disease, including the potential use of natural products. One interesting natural product example is Hypericum, a plant genus that contains species known to have antimicrobial properties. The major constituent of Hypericum perforatum is an unstable compound named hyperforin (1); for this reason it was not believed to play a significant role in the pharmacological effects. In this investigation a hydrogenated hyperforin analogue (2) was tested on several ATCC and clinical isolate strains, in their planktonic and biofilm form (Staphylococcus aureus, MRSA, and Enterococcus faecalis). Compound 2 was effective against planktonic and biofilm cultures, probably due to higher stability, showing the percentage of cells killed in the range from 45% to 52%. These results are noteworthy from the point of view of future development of these polyprenylated phloroglucinols as potential antibiotics.

Flavonoid glycosides from the stem bark of Margaritaria discoidea demonstrate antibacterial and free radical scavenging activities

One new flavonoid glycoside, along with three known flavonoid glycosides were isolated from the stem bark of Margaritaria discoidea, which is traditionally used in the management of wounds and skin infections in Ghana. The new flavonoid glycoside was elucidated as hydroxygenkwanin-8-C-[α-rhamnopyranosyl-(1 → 6)]-β-glucopyranoside (1) on the basis of spectroscopic analysis. The isolated compounds demonstrated free-radical scavenging as well as some level of antibacterial activities. Microorganisms including Staphylococcus aureus are implicated in inhibiting or delaying wound healing. Therefore, any agent capable of reducing or eliminating the microbial load present in a wound as well as decreasing the levels of reactive oxygen species may facilitate the healing process. These findings therefore provide some support to the ethnopharmacological usage of the plant in the management of wounds.

Antimycobacterials from lovage root (Ligusticum officinale Koch)

The n-hexane extract of Lovage root was found to significantly inhibit the growth of both Mycobacterium smegmatis mc²155 and Mycobacterium bovis BCG, and therefore a bioassay-guided isolation strategy was undertaken. (Z)-Ligustilide, (Z)-3-butylidenephthalide, (E)-3-butylidenephthalide, 3-butylphthalide, α-prethapsenol, falcarindiol, levistolide A, psoralen and bergapten were isolated by chromatographic techniques, characterized by NMR spectroscopy and MS, and evaluated for their growth inhibition activity against Mycobacterium tuberculosis H₃₇Rv using the whole-cell phenotypic spot culture growth inhibition assay (SPOTi). Cytotoxicity against RAW 264.7 murine macrophage cells was employed for assessing their degree of selectivity. Falcarindiol was the most potent compound with a minimum inhibitory concentration (MIC) value of 20 mg/L against the virulent H₃₇Rv strain; however, it was found to be cytotoxic with a half-growth inhibitory concentration (GIC₅₀) in the same order of magnitude (SI < 1). Interestingly the sesquiterpene alcohol α-prethapsenol was found to inhibit the growth of the pathogenic mycobacteria with an MIC value of 60 mg/L, being more specific towards mycobacteria than mammalian cells (SI ~ 2). Colony forming unit analysis at different concentrations of this phytochemical showed mycobacteriostatic mode of action.

Antitubercular specific activity of ibuprofen and the other 2-arylpropanoic acids using the HT-SPOTi whole-cell phenotypic assay

OBJECTIVES

Lead antituberculosis (anti-TB) molecules with novel mechanisms of action are urgently required to fuel the anti-TB drug discovery pipeline. The aim of this study was to validate the use of the high-throughput spot culture growth inhibition (HT-SPOTi) assay for screening libraries of compounds against Mycobacterium tuberculosis and to study the inhibitory effect of ibuprofen (IBP) and the other 2-arylpropanoic acids on the growth inhibition of M tuberculosis and other mycobacterial species.

METHODS

The HT-SPOTi method was validated not only with known drugs but also with a library of 47 confirmed anti-TB active compounds published in the ChEMBL database. Three over-the-counter non-steroidal anti-inflammatory drugs were also included in the screening. The 2-arylpropanoic acids, including IBP, were comprehensively evaluated against phenotypically and physiologically different strains of mycobacteria, and their cytotoxicity was determined against murine RAW264.7 macrophages. Furthermore, a comparative bioinformatic analysis was employed to propose a potential mycobacterial target.

RESULTS

IBP showed antitubercular properties while carprofen was the most potent among the 2-arylpropanoic class. A 3,5-dinitro-IBP derivative was found to be more potent than IBP but equally selective. Other synthetic derivatives of IBP were less active, and the free carboxylic acid of IBP seems to be essential for its anti-TB activity. IBP, carprofen and the 3,5-dinitro-IBP derivative exhibited activity against multidrug-resistant isolates and stationary phase bacilli. On the basis of the human targets of the 2-arylpropanoic analgesics, the protein initiation factor infB (Rv2839c) of M tuberculosis was proposed as a potential molecular target.

CONCLUSIONS

The HT-SPOTi method can be employed reliably and reproducibly to screen the antimicrobial potency of different compounds. IBP demonstrated specific antitubercular activity, while carprofen was the most selective agent among the 2-arylpropanoic class. Activity against stationary phase bacilli and multidrug-resistant isolates permits us to speculate a novel mechanism of antimycobacterial action. Further medicinal chemistry and target elucidation studies could potentially lead to new therapies against TB.

Antibacterial sesquiterpenoid derivatives from Ferula ferulaeoides

Three new sesquiterpenoid derivatives 1, 2, and 3 were isolated from Ferula ferulaeoides. To confirm the structure, compound 2 was also synthesized via a condensation reaction between compound 1 and 2,2-dimethoxypropane. The structures of these three compounds were elucidated by means of spectroscopic and chemical methods. Their antibacterial activity against drug-resistant Staphylococcus aureus strains were evaluated with MIC values in the range of 0.5-128 µg/mL. Compounds 1 and 3 were capable of inhibiting efflux of ethidium bromide using an in vitro assay. The cytotoxicity of the compounds was evaluated on cultured HEK293 cells, and none of them showed toxicity to HEK293 cells at a concentration of 125 µg/mL.

An analysis of the synthetic tryptamines AMT and 5-MeO-DALT: emerging 'Novel Psychoactive Drugs'

Novel Psychoactive Drugs (NPD) can be sold without restriction and are often synthetic analogues of controlled drugs. The tryptamines are an important class of NPD as they bind to the various serotonin (5-HT) receptor subtypes and cause psychosis and hallucinations that can lead to injury or death through misadventure. Here we report on the structure elucidation and receptor binding profiles of two widely marketed tryptamine-derived NPDs, namely alpha-methyl-tryptamine and 5-methoxy-N,N-diallyl-tryptamine.

Antimicrobial phenolics and unusual glycerides from Helichrysum italicum subsp. microphyllum

During a large-scale isolation campaign for the heterodimeric phloroglucinyl pyrone arzanol (1a) from Helichrysum italicum subsp. microphyllum, several new phenolics as well as an unusual class of lipids named santinols (5a-c, 6-8) have been characterized. Santinols are angeloylated glycerides characterized by the presence of branched acyl- or keto-acyl chains and represent a hitherto unreported class of plant lipids. The antibacterial activity of arzanol and of a selection of Helichrysum phenolics that includes coumarates, benzofurans, pyrones, and heterodimeric phloroglucinols was evaluated, showing that only the heterodimers showed potent antibacterial action against multidrug-resistant Staphylococcus aureus isolates. These observations validate the topical use of Helichrysum extracts to prevent wound infections, a practice firmly established in the traditional medicine of the Mediterranean area.

Microbial efflux systems and inhibitors: approaches to drug discovery and the challenge of clinical implementation

Conventional antimicrobials are increasingly ineffective due to the emergence of multidrug-resistance among pathogenic microorganisms. The need to overcome these deficiencies has triggered exploration for novel and unconventional approaches to controlling microbial infections. Multidrug efflux systems (MES) have been a profound obstacle in the successful deployment of antimicrobials. The discovery of small molecule efflux system blockers has been an active and rapidly expanding research discipline. A major theme in this platform involves efflux pump inhibitors (EPIs) from natural sources. The discovery methodologies and the available number of natural EPI-chemotypes are increasing. Advances in our understanding of microbial physiology have shed light on a series of pathways and phenotypes where the role of efflux systems is pivotal. Complementing existing antimicrobial discovery platforms such as photodynamic therapy (PDT) with efflux inhibition is a subject under investigation. This core information is a stepping stone in the challenge of highlighting an effective drug development path for EPIs since the puzzle of clinical implementation remains unsolved. This review summarizes advances in the path of EPI discovery, discusses potential avenues of EPI implementation and development, and underlines the need for highly informative and comprehensive translational approaches.

Characterisation of ATP-dependent Mur ligases involved in the biogenesis of cell wall peptidoglycan in Mycobacterium tuberculosis

ATP-dependent Mur ligases (Mur synthetases) play essential roles in the biosynthesis of cell wall peptidoglycan (PG) as they catalyze the ligation of key amino acid residues to the stem peptide at the expense of ATP hydrolysis, thus representing potential targets for antibacterial drug discovery. In this study we characterized the division/cell wall (dcw) operon and identified a promoter driving the co-transcription of mur synthetases along with key cell division genes such as ftsQ and ftsW. Furthermore, we have extended our previous investigations of MurE to MurC, MurD and MurF synthetases from Mycobacterium tuberculosis. Functional analyses of the pure recombinant enzymes revealed that the presence of divalent cations is an absolute requirement for their activities. We also observed that higher concentrations of ATP and UDP-sugar substrates were inhibitory for the activities of all Mur synthetases suggesting stringent control of the cytoplasmic steps of the peptidoglycan biosynthetic pathway. In line with the previous findings on the regulation of mycobacterial MurD and corynebacterial MurC synthetases via phosphorylation, we found that all of the Mur synthetases interacted with the Ser/Thr protein kinases, PknA and PknB. In addition, we critically analyzed the interaction network of all of the Mur synthetases with proteins involved in cell division and cell wall PG biosynthesis to re-evaluate the importance of these key enzymes as novel therapeutic targets in anti-tubercular drug discovery.

The ketamine analogue methoxetamine and 3- and 4-methoxy analogues of phencyclidine are high affinity and selective ligands for the glutamate NMDA receptor

In this paper we determined the pharmacological profiles of novel ketamine and phencyclidine analogues currently used as ‘designer drugs’ and compared them to the parent substances via the resources of the National Institute of Mental Health Psychoactive Drug Screening Program. The ketamine analogues methoxetamine ((RS)-2-(ethylamino)-2-(3-methoxyphenyl)cyclohexanone) and 3-MeO-PCE (N-ethyl-1-(3-methoxyphenyl)cyclohexanamine) and the 3- and 4-methoxy analogues of phencyclidine, (1-[1-(3-methoxyphenyl)cyclohexyl]piperidine and 1-[1-(4-methoxyphenyl)cyclohexyl]piperidine), were all high affinity ligands for the PCP-site on the glutamate NMDA receptor. In addition methoxetamine and PCP and its analogues displayed appreciable affinities for the serotonin transporter, whilst the PCP analogues exhibited high affinities for sigma receptors. Antagonism of the NMDA receptor is thought to be the key pharmacological feature underlying the actions of dissociative anaesthetics. The novel ketamine and PCP analogues had significant affinities for the NMDA receptor in radioligand binding assays, which may explain their psychotomimetic effects in human users. Additional actions on other targets could be important for delineating side-effects.

Neurochemical profiles of some novel psychoactive substances

Fourteen substances from the class of drugs sometimes known as "legal highs" were screened against a battery of human receptors in binding assays, and their potencies as inhibitors of monoamine uptake determined in functional in vitro assays. Thirteen of the test substances acted as inhibitors of monoamine uptake at submicromolar concentrations, including 9 potent inhibitors of the dopamine transporter (DAT), 12 potent inhibitors of the norepinephrine transporter (NET) and 4 potent inhibitors of the serotonin transporter (SERT). Seven compounds acted as submicromolar inhibitors of both DAT and NET, and three substances 1-(benzofuran-5-yl)propan-2-amine (5-APB), 1-naphthalen-2-yl-2-pyrrolidin-1-ylpentan-1-one hydrochloride ("naphyrone") and 1-naphthalen-1-yl-2-pyrrolidin-1-ylpentan-1-one hydrochloride ("1-naphyrone") were submicromolar inhibitors of all three monoamine transporters. There was a lack of correlation between results of functional uptake experiments and in vitro binding assays for the monoamine transporters. There was also no correlation between the human behavioral effects of the substances and the results of bindings assays for a range of receptor targets, although 1-(benzofuran-5-yl)propan-2-amine (5-APB), 1-(benzofuran-6-yl)propan-2-amine hydrochloride (6-APB) and 5-iodo-2,3-dihydro-1H-inden-2-amine hydrochloride (5-iodo-aminoindane) exhibited <100 nM affinities for 5HT(2B) and α(2C) receptors. Functional assays revealed that 5-APB and 6-APB were potent full agonists at 5HT(2B) receptors.

Single chemical entity legal highs: assessing the risk for long term harm

A recent and dramatic increase in the emergence of novel psychoactive substances ('legal highs') has left many governments unable to provide a timely response to an increasing number of potentially harmful drugs now available to the public. In response to this rapid increase in lawful drug use, the UK government intends to implement temporary class drug orders, whereby substances with a potential for misuse and harm can be regulated for a 12 month period. During this period an investigation of the potential for harms induced by these drugs will take place. However, the short time-frame in which information must be gathered, and the paucity of data available on novel psychoactive substances, means that robust pharmacological and toxicological analyses may be replaced by extrapolating data from illegal drugs with similar chemical structures. This review explores the potential pharmacology and toxicology of past and present 'legal highs' and discusses the risks of failing to carry out in-depth scientific research on individual substances.