Pharmacology: Azithromycin is an azalide antibiotic, a subclass of the macrolides, and is a derivative of erythromycin. It acts by binding to the 50S ribosomal subunit and prevents the translocation of peptidyl-tRNA of susceptible organisms, and thus interfering with microbial protein synthesis.
Azithromycin is active against the following organisms both in vitro and in clinical infections: Gram-positive aerobes: In vitro and clinical infections: Staphylococcus aureus
In vitro: Streptococcus sp.
Azithromycin demonstrates cross-resistance with erythromycin-resistant Gram-positive strains.
Most strains of Enterococcus faecalis and methicillin-resistant staphylococci are resistant to Azithromycin.
Gram-negative aerobes: In vitro and clinical infections: Haemophilus influenzae
In vitro: Bordetella pertussis
Anaerobic bacteria: In vitro and clinical infections: Bacteroides bivius
Other organisms: In vitro and clinical infections: Chlamydia trachomatis
In vitro: Borrelia burgdorferi
Beta-lactamase production should have no effect on Azithromycin activity.
Azithromycin is rapidly absorbed and widely distributed throughout the body after oral administration. Rapid distribution into tissues and high concentration within cells result in significantly higher Azithromycin concentration in tissues than in plasma or serum.
The pharmacokinetic parameters of Azithromycin in plasma after administering 500 mg on day 1 followed by 250 mg once daily on days 2 through 5 in healthy young adults (ages 18 to 40 years old) are described in the following table: (see table)
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Biliary excretion of Azithromycin, predominantly as unchanged drug, is a major route of elimination. Over the course of a week, approximately 6% of the administered dose appears as unchanged drug in urine.