The activity of azithromycin in animal models of infection.

High and prolonged tissue levels accompanied by low serum concentrations are a feature of azithromycin, an azalide antibiotic. It has a broad spectrum of activity against gram-positive and gram-negative microorganisms and several intracellular pathogens. A number of animal models of localised infection have been developed which demonstrate that the efficacy of azithromycin correlates with its extravascular pharmacokinetics and not with blood levels. In many instances, because of high tissue bioavailability, azithromycin has better in vivo efficacy than comparative agents, despite a similar or higher minimum inhibitory concentration. Additionally, the extravascular kinetics of azithromycin are associated with bactericidal activity against pathogens such as Staphylococcus aureus, Streptococcus pneumoniae and Escherichia coli. Intracellular pathogens are susceptible to azithromycin and it is believed that the agent penetrates and remains within host cells infected by organisms including Mycobacterium avium, Legionella pneumophila and Borrelia burgdorferi. This paper reviews the in vivo efficacy of azithromycin and standard agents in animal models of infection, especially those involving intracellular pathogens.