Azithromycin--review of key chemical, pharmacokinetic and microbiological features

One of the chemical features that distinguishes the 15-membered ring azalide azithromycin from the 14-membered ring macrolide compound erythromycin is the former's increased stability at acid pH. Azithromycin also differs pharmacokinetically from erythromycin, an important feature being azithromycin's ability to achieve high tissue concentrations, with the agent being delivered to the sites of infection by direct uptake and by targeted delivery via phagocytes. High tissue concentrations are maintained for prolonged periods because of azithromycin's long half-life, leading to once-daily dosing for 3 or 5 days. Notable microbiological features of azithromycin are in-vitro activity against many pyogenic bacteria (e.g. Neisseria gonorrhoeae and Moraxella catarrhalis), as well as organisms against which beta-lactam antibiotics are usually ineffective (e.g. Legionella and Chlamydia spp.), organisms that are resistant to benzylpenicillin and erythromycin (e.g. Haemophilus influenzae) and organisms for which satisfactory therapy is limited (e.g. Toxoplasma gondii and the Mycobacterium avium-intracellulare complex). These properties of azithromycin suggest that it might be a useful agent for the treatment of a wide range of bacterial infections.