Azithromycin is an azalide
antimicrobial agent. Structurally related to the
macrolide antibiotic erythromycin, its mechanism of activity (similar to
erythromycin) is interference with
bacterial protein synthesis by binding to the 50S component of the 70S ribosomal subunit. Although slightly less potent than
erythromycin against gram-positive organisms,
azithromycin demonstrates superior activity in vitro against a wide variety of gram-negative bacilli, including Haemophilus influenzae. Absorption is approximately 37% after a 500-mg oral dose. The large volume of distribution (23 L/kg) and low peak serum level (0.4 micrograms/ml) are consistent with data demonstrating extensive tissue distribution and intracellular accumulation. Metabolism is predominantly hepatic (to inactive metabolites), with biliary excretion a major pathway of elimination. Drug elimination is biphasic, with a terminal half-life of up to 5 days. Published trials have examined the efficacy and safety of
azithromycin in the treatment of adults with upper and lower
respiratory tract infections, skin and skin structure
infections, streptococcal pharyngitis, and
sexually transmitted diseases. Many used a 5-day course of 250 mg once daily, supplemented with a 250-mg dose on the first day of
therapy. Selected trials in
sexually transmitted diseases examined single 1-g doses. Promising results also were obtained with oral daily doses of 500 mg in patients with human immunoviral
infection who also had Mycobacterium avium complex
infection and in animals with
toxoplasmosis. Adverse reactions are primarily gastrointestinal (
nausea,
diarrhea,
abdominal pain), with minimal laboratory abnormalities reported. Gastrointestinal tolerance is better than that of
erythromycin. Drug interactions have not been observed to date, although coadministration of
azithromycin with a large meal may reduce absorption by up to 50%.