Infective endocarditis and glycopeptides

pH-triggered release of vancomycin from protein-capped porous silicon films

Objective: An in vitro model system for pH-triggered release of the antibiotic vancomycin from porous Si films is studied. Method: Vancomycin is infused into a mesoporous Si film from a mixed aqueous/acetonitrile solution and trapped by a capping layer containing the protein bovine serum albumin (BSA). The protein effectively traps vancomycin in the porous nanostructure at pH 4.0; the protein dissolves and vancomycin is released into solution when the pH increases to 7.4. The surface chemistry of porous Si exerts a substantial effect on the efficacy of drug loading. The amount of drug loading is larger in freshly-etched (hydrophobic, hydrogen-terminated) porous Si and smaller in methyl-modified, undecylenic acid-modified and thermally oxidized samples. The quantity of drug loaded in a freshly etched porous Si chip is proportional to the thickness of the porous layer, which exhibits a constant volume loading efficiency of 31% (v/v). Flow-cell experiments designed to mimic the transition from pH 4 to 7 that occurs when material moves from the stomach to the upper intestinal tract were performed on the freshly etched films and vancomycin- and BSA-release rates were quantified from the effluent of the flow cell by high-pressure liquid chromatography analysis. Results & conclusion: There is a small, constant rate of vancomycin release at pH 4 that is independent of the amount of drug loaded in the pores. This is attributed to diffusion of vancomycin from the BSA-capping layer. The release rate increases five- to tenfold when the pH of the solution in the flow cell increases to 7.4; 100% of the drug is released within 3 h of this increase.

The use and therapeutic drug monitoring of teicoplanin in the UK

Teicoplanin dosage recommendations for specific infections have been modified in recent years. However, there was no significant increase in the proportion of pre-dose concentrations > 20 mg/L between 1994 and 1998 in samples sent for teicoplanin assay at the Regional Antimicrobial Reference Laboratory, Bristol, UK. A questionnaire on the use of teicoplanin and therapeutic drug monitoring (TDM) was sent to all UK National External Quality Assurance Scheme antibiotic assay users. Teicoplanin was widely used in the UK, although vancomycin was more popular as a choice of glycopeptide. Fewer than 25% recommended teicoplanin TDM during routine use, the main reasons being perceived lack of toxicity and lack of evidence for the use of teicoplanin TDM. Pre-dose concentrations < 20 mg/L were considered appropriate for treatment of bacteraemia caused by methicillin-resistant Staphylococcus aureus by 53% of those responding. Data sheet advice was relied upon more than TDM as an indication of therapeutic dosing. Microbiologists who mainly used vancomycin tended to perform more TDM and seek higher serum concentrations when using teicoplanin than those who preferentially used teicoplanin.

Glycopeptide tolerance in bacteria causing endocarditis

Glycopeptides have been recommended as therapy for endocarditis. MICs and MBCs of vancomycin and teicoplanin were compared for 100 isolates from patients with proven bacterial endocarditis. MICs were generally lower for teicoplanin and tolerance to both agents was common. Almost all isolates of enterococci were tolerant to both glycopeptides. Among the streptococci, 78% were tolerant to teicoplanin and 57% to vancomycin. Similar findings were demonstrated for staphylococci. Although isolates appear sensitive to glycopeptides, bactericidal activity cannot always be predicted. If a glycopeptide is indicated for treatment of endocarditis, combination therapy with a suitable aminoglycoside should be considered unless MBC testing can be performed.