Karl A. Hansford

Glycopeptide antibiotics: back to the future

Glycopeptide antibiotics have been a key weapon in the fight against bacterial infections for over half a century, with the progenitors, vancomycin (1) and teicoplanin (2), still used extensively. The increased occurrence of resistance and the effectiveness of these ‘last resort’ treatments for Gram-positive infections has led to the discovery and clinical development of second generation, semisynthetic lipoglycopeptide derivatives such as telavancin (3), dalbavancin (4) and oritavancin (5), which all possess broader spectra of activity and improved pharmacokinetic properties. Two of these new antibiotics, telavancin (3) and dalbavancin (4), were approved in the past 5 years and the third, oritavancin (5), is awaiting regulatory approval. In this review, the discovery, development and associated resistance of vancomycin (1) and teicoplanin (2), and semi-synthetic glycopeptides, telavancin (3), dalbavancin (4) and oritavancin (5), are detailed. The clinical implications of glycopeptide resistance, especially vancomycin (1), as well as the future prospects for current glycopeptide drugs and the development of new glycopeptides are discussed.

D-Tyrosine as a chiral precusor to potent inhibitors of human nonpancreatic secretory phospholipase A2 (IIa) with antiinflammatory activity.

Few reported inhibitors of secretory phospholipase A2 enzymes truly inhibit the IIa human isoform (hnpsPLA2‐IIa) noncovalently at submicromolar concentrations. Herein, the simple chiral precursor D‐tyrosine was derivatised to give a series of potent new inhibitors of hnpsPLA2‐IIa. A 2.2‐Å crystal structure shows an inhibitor bound in the active site of the enzyme, chelated to a Ca2+ ion through carboxylate and amide oxygen atoms, H‐bonded through an amide NH group to His48, with multiple hydrophobic contacts and a T‐shaped aromatic‐group–His6 interaction. Antiinflammatory activity is also demonstrated for two compounds administered orally to rats.

Comparative protection against rat intestinal reperfusion injury by a new inhibitor of sPLA2, COX‐1 and COX‐2 selective inhibitors, and an LTC4 receptor antagonist

A new group IIa sPLA2 inhibitor was compared with selective inhibitors of COX‐1, COX‐2 and an LTC4 antagonist for effects on local and remote tissue injuries following ischaemia and reperfusion (I/R) of the small intestine in rats. In an acute model of ischaemia (30 min) and reperfusion (150 min) injury in the absence of inhibitors, there was significant intestinal haemorrhage, oedema and mucosal damage, neutropenia, elevated serum levels of aspartate aminotransferase (AST) and hypotension. Preischaemic treatment with the inhibitor of sPLA2 (Group IIa), at 5 mg kg−1 i.v. or 10 mg kg−1 p.o. significantly inhibited I/R‐induced neutropenia, the elevation of serum levels of AST, intestinal oedema and hypotension. Pretreatment with the COX‐2 inhibitor celebrex (10 mg kg−1 i.v.) and the LTC4 antagonist zafirlukast (1 mg kg−1 i.v.) also showed marked improvement with I/R‐induced AST, oedema and neutropenia. Hypotension was only reduced by the LTC4 antagonist. The COX‐1 inhibitor flunixin (1 mg kg−1 i.v.) did not effect improvement in the markers of tissue injury. Histological examination of rat I/R injury showed that all of the drugs offered some protection to the mucosal layer damage compared to no drug treatment. Given i.v., the sPLA2 inhibitor was more effective than either the COX‐1 or COX‐2 inhibitors in preventing rat I/R injury. These results indicate that a potent new inhibitor of sPLA2 (group IIa) protects the rat small intestine from I/R injury after oral or intravenous administration. COX‐2 and LTC4 inhibitors also showed some beneficial effects against intestinal I/R injury. Our study suggests that sPLA2 (Group IIa) may have a pathogenic role in intestinal I/R in rats.

Nitroimidazole carboxamides as antiparasitic agents targeting Giardia lamblia, Entamoeba histolytica and Trichomonas vaginalis

Diarrhoeal diseases caused by the intestinal parasites Giardia lamblia and Entamoeba histolytica constitute a major global health burden. Nitroimidazoles are first-line drugs for the treatment of giardiasis and amebiasis, with metronidazole 1 being the most commonly used drug worldwide. However, treatment failures in giardiasis occur in up to 20% of cases and development of resistance to metronidazole is of concern. We have re-examined ‘old’ nitroimidazoles as a foundation for the systematic development of next-generation derivatives. Using this approach, derivatisation of the nitroimidazole carboxamide scaffold provided improved antiparasitic agents. Thirty-three novel nitroimidazole carboxamides were synthesised and evaluated for activity against G. lamblia and E. histolytica. Several of the new compounds exhibited potent activity against G. lamblia strains, including metronidazole-resistant strains of G. lamblia (EC50 = 0.1–2.5 μM cf. metronidazole EC50 = 6.1–18 μM). Other compounds showed improved activity against E. histolytica (EC50 = 1.7–5.1 μM cf. metronidazole EC50 = 5.0 μM), potent activity against Trichomonas vaginalis (EC50 = 0.6–1.4 μM cf. metronidazole EC50 = 0.8 μM) and moderate activity against the intestinal bacterial pathogen Clostridium difficile (0.5–2 μg/mL, cf. metronidazole = 0.5 μg/mL). The new compounds had low toxicity against mammalian kidney and liver cells (CC50 > 100 μM), and selected antiparasitic hits were assessed for human plasma protein binding and metabolic stability in liver microsomes to demonstrate their therapeutic potential.

Anti-cooperative ligand binding and dimerisation in the glycopeptide antibiotic dalbavancin

Dalbavancin, a semi-synthetic glycopeptide with enhanced antibiotic activity compared to vancomycin and teicoplanin, dimerises strongly in an anti-cooperative manner with ligand binding.

Small-Molecule Ligands of GD2 Ganglioside, Designed from NMR Studies, Exhibit Induced-Fit Binding and Bioactivity

Ganglioside GD2 is a cell surface glycosphingolipid. Targeting of GD2, i.e., by anti-GD2 mAb 3F8, is used clinically for cancer diagnosis, prognosis, and therapy. Here, the conformations of free GD2, and of GD2 bound to mAb 3F8, were resolved by saturation transfer difference NMR and molecular modeling. Then, three small-molecule cyclic peptide ligands that bind to GD2 selectively were designed. Transferred nuclear Overhauser enhancement of the GD2-bound conformation of the peptide ligands showed an induced-fit binding mechanism. The mAb 3F8 and the peptidic GD2 ligands mediate similar biological functions in cell-based assays of calcium fluxes and src activation. Thus, small molecules can selectively and functionally interact with a sugar head group. This work furthers the concept of rationally designing ligands for carbohydrate targets, and may be expanded to other clinically relevant gangliosides.

Synthesis of octapeptin C4 and biological profiling against NDM-1 and polymyxin-resistant bacteria

The first synthesis of octapeptin C4 was achieved using a combination of solid phase synthesis and off-resin cyclisation. Octapeptin C4 displayed antibiotic activity against multi-drug resistant, NDM-1 and polymyxin-resistant Gram-negative bacteria, with moderate activity against Staphylococcus aureus. The linear analogue of octapeptin C4 was also prepared, which showed reduced activity.

Towards the cis-Bromination of Double Bonds

Simple bromination of olefins almost invariably yields the trans-1,2-dibromo-product, and in many cases the corresponding cis-product is an unknown compound. Approaches towards a reagent which will generally, mildly and stereospecifically cis-brominate olefins are now discussed. Manuscript received: 1 March 2001.

Investigating the Interaction of Octapeptin A3 with Model Bacterial Membranes

Octapeptins are cyclic lipopeptides with a broader spectrum of activity against fungi and polymyxin-resistant Gram-negative and Gram-positive bacteria. In the present study, we investigated the interaction of octapeptin A3 with asymmetric outer membrane models of Gram-negative pathogen Pseudomonas aeruginosa using neutron reflectometry, together with fluorimetric and calorimetry methods. For the first time, our neutron reflectometry results reveal that the interaction of octapeptin A3 with the Gram-negative outer membrane involves an initial transient polar interaction with the phospholipid and lipid A headgroups, followed by the penetration of the entire octapeptin molecule into the fatty acyl core of the outer membrane. This mechanism contrasts with that of polymyxin B, which specifically targets lipid A, whereas octapeptins appear to target both lipid A and phospholipids. Furthermore, the mechanism of octapeptins does not appear to be highly dependent on an initial complementary electrostatic interaction with lipid A, which accounts for their ability to bind to lipid A of polymyxin-resistant Gram-negative bacteria that is modified with cationic moieties that act to electrostatically repel the cationic polymyxin molecule. The presented findings shed new light on the mechanism whereby octapeptins penetrate the outer membrane of polymyxin-resistant Gram-negative pathogens and highlight their potential as candidates for development as new antibiotics against problematic multi-drug-resistant pathogens.

HALOGENATED TERPENOIDS. XXIX. THE 1-BROMO 1-BROMOMETHYL CYCLOHEXYL SYSTEM

Bromination of methylene groups exocyclic to cyclohexyl systems normally affords two isomeric products; the axial 1-bromo equatorial 1-bromomethyl compound and the axial 1-bromomethyl equatorial 1-bromo derivative. Free energy differences between these two isomers, and the conformations adopted by the axial 1-bromomethyl group, have been explored by n.m.r. spectroscopy, by X-ray crystallography and by MM3 calculations. Evidence is presented to show that the ax-bromomethyl group exists primarily as those rotamers which site the bromine atom synclinal to the vicinal bromine. The A value for a bromomethyl group in this system is then similar to that of an unsubstituted methyl group.