Total synthesis of acylphloroglucinols and their antibacterial activities against clinical isolates of multi-drug resistant (MDR) and methicillin-resistant strains of Staphylococcus aureus.

Bioassay-directed drug discovery efforts focusing on various species of the genus Hypericum led to the discovery of a number of new acylphloroglucinols including (S,E)-1-(2-((3,7-dimethylocta-2,6-dien-1-yl)oxy)-4,6-dihydroxyphenyl)-2-methylbutan-1-one (6, olympicin A) from H. olympicum, with MICs ranging from 0.5 to 1 mg/L against a series of clinical isolates of multi-drug-resistant (MDR) and methicillin-resistant Staphylococcus aureus (MRSA) strains. The promising activity and interesting chemistry of olympicin A prompted us to carry out the total synthesis of 6 and a series of analogues in order to assess their structure-activity profile as a new group of antibacterial agents. Following the synthesis of 6 and structurally-related acylphloroglucinols 7-15 and 18-24, their antibacterial activities against a panel of S. aureus strains were evaluated. The presence of an alkyloxy group consisting of 8-10 carbon atoms ortho to a five-carbon acyl substituent on the phloroglucinol core are important structural features for promising anti-staphylococcal activity.

T-shaped Peptide Amphiphiles Self Assemble into Nanofiber Networks.

BACKGROUND: Conventional nanofiber forming peptide amphiphiles comprise a beta sheet forming, short peptide sequence with an alkyl chain attached at one terminus. We report the selfassembly of a peptide amphiphile possessing a mid-chain located alkyl substituent (a T-shaped peptide amphiphile) into nanofiber networks. METHOD: Peptide synthesis was carried out using standard 9-fluorenylmethoxycarbonyl solid phase peptide synthesis protocols, followed by covalent attachment of the alkyl chains to yield target peptide amphiphiles. Self-assembly was then studied using electron microscopy and coarse-grained molecular dynamics simulations. RESULTS: T-shaped peptide amphiphiles self-assembled into nanofibers just like linear peptide amphiphiles, but then unlike linear peptide amphiphiles, T-shaped peptide amphiphiles formed inter-fiber associations and ultimately nanofiber networks. CONCLUSION: Changing the position of the alkyl chain in a peptide amphiphile from the terminal end of the peptide to the middle part of the peptide, to form a T-shaped peptide amphiphile, does not disrupt the molecular interactions required for the self-assembly of the peptide amphiphiles into nanofibers.

Synthesis and antibacterial evaluation of 3-Farnesyl-2-hydroxybenzoic acid from Piper multiplinervium.

3-Farnesyl-2-hydroxybenzoic acid is an antibacterial agent isolated from the leaves of Piper multiplinervium. This compound has activity against both Gram positive and Gram negative bacteria including Escherichia coli, Staphylococcus aureus and Helicobacter pylori. This research aimed to synthesize a natural antibacterial compound and its analogs. The synthesis of 3-Farnesyl-2-hydroxybenzoic acid consists of three steps: straightforward synthesis involving protection of phenolic hydroxyl group, coupling of suitable isoprenyl chain to the protected aromatic ring at ortho position followed by carboxylation with concomitant deprotection to give the derivatives of the salicylic acid. All the three prenylated compounds synthesized were found to exhibit spectrum of activity against S. aureus (ATCC) having MIC: 5.84×10(-3), 41.46×10(-2) and 6.19×10(-1) µmol/ml respectively. The compounds also displayed activity against resistance strain of S. aureus (SA1119B) having MIC: 5.84×10(-3), 7.29×10(-3) and 3.09×10(-1) µmol/ml respectively. This synthesis has been achieved and accomplished with the confirmation of it structure to that of the original natural product, thus producing the first synthesis of the natural product and providing the first synthesis of its analogs with 3-Farnesyl-2-hydroxybenzoic acid having biological activity higher than that of the original natural product.

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.

A prodrug nanoparticle approach for the oral delivery of a hydrophilic peptide, leucine(5)-enkephalin, to the brain.

The oral use of neuropeptides to treat brain disease is currently not possible because of a combination of poor oral absorption, short plasma half-lives and the blood-brain barrier. Here we demonstrate a strategy for neuropeptide brain delivery via the (a) oral and (b) intravenous routes. The strategy is exemplified by a palmitic ester prodrug of the model drug leucine(5)-enkephalin, encapsulated within chitosan amphiphile nanoparticles. Via the oral route the nanoparticle-prodrug formulation increased the brain drug levels by 67% and significantly increased leucine(5)-enkephalin's antinociceptive activity. The nanoparticles facilitate oral absorption and the prodrug prevents plasma degradation, enabling brain delivery. Via the intravenous route, the nanoparticle-prodrug increases the peptide brain levels by 50% and confers antinociceptive activity on leucine(5)-enkephalin. The nanoparticle-prodrug enables brain delivery by stabilizing the peptide in the plasma although the chitosan amphiphile particles are not transported across the blood-brain barrier per se, and are excreted in the urine.

Antibacterial acylphloroglucinols from Hypericum olympicum.

New antibacterial acylphloroglucinols (1-5) were isolated and characterized from the aerial parts of the plant Hypericum olympicum L. cf. uniflorum. The structures of these compounds were confirmed by extensive 1D- and 2D-NMR experiments to be 4,6-dihydroxy-2-O-(3″,7″-dimethyl-2″,6″-octadienyl)-1-(2'-methylbutanoyl)benzene (1), 4,6-dihydroxy-2-O-(7″-hydroxy-3″,7″-dimethyl-2″,5″-octadienyl)-1-(2'-methylbutanoyl)benzene (2), 4,6-dihydroxy-2-O-(6″-hydroxy-3″,7″-dimethyl-2″,7″-octadienyl)-1-(2'-methylbutanoyl)benzene (3), 4,6-dihydroxy-2-O-(6″-hydroperoxy-3″,7″-dimethyl-2″,7″-octadienyl)-1-(2'-methylbutanoyl)benzene (4), and 4,6-dihydroxy-2-O-(6″,7″-epoxy-3″,7″-dimethyloct-2″-enyl)-1-(2'-methylbutanoyl)benzene (5). These new natural products have been given the trivial names olympicins A-E (1-5). All compounds were evaluated against a panel of methicillin-resistant Staph. aureus and multidrug-resistant strains of Staph. aureus. Compound 1 exhibited minimum inhibitory concentrations (MICs) of 0.5-1 mg/L against the tested Staph. aureus strains. Compounds 2 to 5 were also shown to be active, with MICs ranging from 64 to 128 mg/L. Compound 1 was synthesized using a simple four-step method that can be readily utilized to give a number of structural analogues of 1.

Bioactive pyridine-N-oxide disulfides from Allium stipitatum.

From Allium stipitatum, three pyridine-N-oxide alkaloids (1-3) possessing disulfide functional groups were isolated. The structures of these natural products were elucidated by spectroscopic means as 2-(methyldithio)pyridine-N-oxide (1), 2-[(methylthiomethyl)dithio]pyridine-N-oxide (2), and 2,2'-dithio-bis-pyridine-N-oxide (3). The proposed structure of 1 was confirmed by synthetic S-methylthiolation of commercial 2-thiopyridine-N-oxide. Compounds 1 and 2 are new natural products, and 3 is reported for the first time from an Allium species. All compounds were evaluated for activity against fast-growing species of Mycobacterium, methicillin-resistant Staphylococcus aureus, and a multidrug-resistant (MDR) variants of S. aureus. Compounds 1 and 2 exhibited minimum inhibitory concentrations (MICs) of 0.5-8 microg/mL against these strains. A small series of analogues of 1 were synthesized in an attempt to optimize antibacterial activity, although the natural product had the most potent in vitro activity. In a whole-cell assay at 30 microg/mL, 1 was shown to give complete inhibition of the incorporation of (14)C-labeled acetate into soluble fatty acids, indicating that it is potentially an inhibitor of fatty acid biosynthesis. In a human cancer cell line antiproliferative assay, 1 and 2 displayed IC(50) values ranging from 0.3 to 1.8 microM with a selectivity index of 2.3 when compared to a human somatic cell line. Compound 1 was evaluated in a microarray analysis that indicated a similar mode of action to menadione and 8-quinolinol by interfering with the thioredoxin system and up-regulating the production of various heat shock proteins. This compound was also assessed in a mouse model for in vivo toxicity.

DNA binding by analogues of the bifunctional intercalator TANDEM.

We have used DNase I footprinting to study the binding strength and DNA sequence selectivity of novel derivatives of the quinoxaline bis-intercalator TANDEM. Replacing the valine residues in the cyclic octadepsipeptide with lysines does not affect the selectivity for TpA but leads to a 50-fold increase in affinity. In contrast, replacing both of the quinoxaline chromophores with naphthalene rings abolishes binding, while changing a single ring decreases the affinity, and footprints are observed at only the best binding sites (especially TATATA). By using fragments with different lengths of [(AT) n ], we demonstrate that these ligands bind best to the center of the longer (AT) n tracts.

Solid-phase synthesis of chlorofusin analogues.

We report an efficient and versatile solid-phase synthesis through which two series of chlorofusin analogues, one bearing varying chromophores and the other with various amino acid substitutions in the cyclic peptide, were synthesized. These peptides were prepared using a strategy involving side-chain immobilization, on-resin cyclization, and postcyclization modification. The success of these syntheses demonstrates the broad utility of the method. Both series of analogues were evaluated for their inhibitory activity against the p53/MDM2 interaction but were shown to be inactive in the concentration range tested. This suggests that the full chromophore structure may be required for activity.

Bisintercalator natural products with potential therapeutic applications: isolation, structure determination, synthetic and biological studies.

Echinomycin is the prototypical bisintercalator, a molecule that binds to DNA by inserting two planar chromophores between the base-pairs of duplex DNA, placing its cyclic depsipeptide backbone in the minor groove. As such, it has been the focus of an extensive number of investigations into its biological activity, nucleic acid binding and, to some extent, its structure-activity relationships. However, echinomycin is also the parent member of an extended family of natural products that interact with DNA by a similar mechanism of bisintercalation. The structural variety in these compounds leads to changes in sequence selectivity and and biological activity, particularly as anti-tumour and anti-viral agents. One of the more recently identified marine natural products that is moving close to clinical development is thiocoraline, and it therefore seems timely to review the various bisintercalator natural products.

Efficient solid-phase-based total synthesis of the bisintercalator TANDEM.

[reaction: see text] In this article, the first solid-phase-based total synthesis of TANDEM, a synthetic analogue of triostin A, is described. In initial studies, the synthesis incorporated depsipeptide formation, introduction of chromophores, and disulfide bond formation on the solid phase, prior to a final solution-phase macrolactamization, to give the target molecule. Although pure TANDEM was obtained in an overall yield comparable to those for all syntheses to date, the yield of the final cyclization was low (11%). A more efficient approach involved removal from the solid phase prior to disulfide bond formation. The resulting linear peptide underwent macrolactamization under mild conditions and high dilution. Final disulfide bond formation was essentially quantitative and gave the target molecule, TANDEM, in an overall yield of 18%. The final compound was assessed for its ability to bind to 5'-TpA sequences on DNA by DNase I footprinting. This efficient synthesis sets the stage for a study of the structure-activity relationship of TANDEM and the natural product triostin A, with analogues containing "point mutations" at every site within the cyclic compounds.

Solid-phase synthesis of the cyclic peptide portion of chlorofusin, an inhibitor of p53-MDM2 interactions.

The first solid-phase synthesis of the chlorofusin peptide is described. The synthesis involved side-chain immobilization of N(alpha)-Fmoc-Asp-ODmab. Synthesis of the linear peptide, initially incorporating racemic Ade8 and unsubstituted ornithine in place of the chromophore-bearing residue, was followed by cyclization on resin and peptide release to give a mixture of diastereomers. Resynthesis identified (by HPLC) the second isomer as analogous to the natural product. Initial biological assays, using an immunofluorescence method, suggest that the compounds are not cytotoxic but do not inhibit the p53/mdm2 interaction. [structure: see text]