Fusidic acid

The in vitro activity of fusidic acid against Staphylococcus aureus is confirmed in clinical studies which demonstrate that this antibiotic in combination with other agents, particularly beta-lactams, is efficacious in non-MRSA septicaemia. Some reports suggest that fusidic acid when used in combination, may significantly diminish the risk of relapse after cessation of therapy. However, in spite of over 30 years of use and its recommendation in a number of guidelines, published evidence is insufficient to allow reliable comment on the efficacy of the antibiotic in the treatment of endocarditis.

Fusidic acid comes in a variety of formulations for oral, intravenous and topical use. After oral administration of 500 mg Cmax values range from 14.5-3.3 mg/l and an elimination half-life of 8.9-11.0 h. Similar values are obtained with intravenous administration of the sodium salt, although peaks tend to be higher. Bioavailability for the new film-coated tablet is approximately 91% while that of the suspension formulation appears to be much lower. Repeated dosing results in substantial drug accumulation when given 8-hourly, and to a variable extent depending on dose when administered 12-hourly. One study has demonstrated a modest dose-dependency for pharmacokinetics, with decreased clearance at higher doses. Fusidic acid is primarily eliminated by non-renal mechanisms, and a proportion of the drug is metabolised to seven or more breakdown products that can be detected in bile. Hypoalbuminaemia increases fusidic acid clearance, while clearance is decreased in the presence of severe cholestasis, and essentially unchanged in renal failure. Fusidic acid is highly protein-bound (91-98S), but has good penetration to a number of tissues including skin blisters, burns, infected bone and joints. Topical application of fusidic acid results in poor penetration through skin but good penetration into aqueous and vitreous humour. Little is known about the pharmacodynamics of fusidic acid, apart from the fact that it is slowly bactericidal against Staphylococcus aureus, and produces moderate post-antibiotic effects in vitro.

This review summarises current knowledge of the microbiological and clinical aspects of fusidic acid resistance in Staphylococcus aureus, and makes recommendations about fusidic acid prescribing and further research.