The azalide
azithromycin, which is derived from
erythromycin, contains a methyl-substituted
nitrogen in the
lactone ring. This 15-membered expanded
lactone ring results in improved
acid stability and oral bioavailability compared with
erythromycin.
Azithromycin possesses a broad spectrum of activity against Gram-positive and Gram-negative bacteria, including enhanced activity compared with the
macrolides against Haemophilus influenzae and Moraxella catarrhalis. In vitro activity of
azithromycin against intracellular and clinically atypical pathogens is also good.
Azithromycin has a distinct pharmacokinetic profile compared with other
antimicrobial agents, the most prominent feature is its high tissue selectivity. Concentrations of
azithromycin in respiratory tract, gynaecological tissue and prostate remain above minimum inhibitory concentrations of pathogens for several days, thus making it possible to use a short-course, once-daily dosing regimen. Another feature of
azithromycin is that it rapidly penetrates phagocytic cells, with the release of the
antibiotics at local sites of
infection. Comparative clinical trials have shown that
azithromycin given once daily for 3 or 5 days is comparable to comparator drugs given for 7 or 10 days in the treatment of
otitis media,
sinusitis,
pharyngitis, acute
bronchitis, acute infectious exacerbations of
chronic bronchitis, community-acquired
pneumonia and skin and
soft tissue infections in adults.
Azithromycin given once daily for 3 days has also been shown to be effective in the treatment of respiratory tract and skin and
soft tissue infections in children. In addition, some
sexually-transmitted diseases are effectively treated by a single 1-g dose of
azithromycin; clinical and microbiological responses were comparable to those recorded using
doxycycline given twice daily for 7 days. The short-duration, once-daily dosing regimen is well tolerated in adults and children, and there is no evidence of interaction between
azithromycin and
theophylline,
terfenadine, or
cimetidine.