Classification and Chemical Structure
Azithromycin belongs to the macrolide antibiotic family. Unlike natural 14-membered (e.g., erythromycin, oleandomycin) and 16-membered macrolides (e.g., spiramycin, josamycin, midecamycin), azithromycin is a 15-membered semi-synthetic compound.
The key structural feature that places it into a separate subclass, azalides, is the inclusion of a nitrogen atom in the lactone ring.
Mechanism and Spectrum of Action
The mechanism of action involves inhibiting protein synthesis in microbial organelles (binding to the 50S ribosomal subunit).
The antimicrobial spectrum of azithromycin differs significantly from classical erythromycin:
- Gram-positive flora: Less active against staphylococci.
- Gram-negative flora: More active against Haemophilus influenzae and various Enterobacteriaceae.
- Intracellular and atypical pathogens: Exhibits high activity against Chlamydia, Mycoplasma pneumoniae, Legionella pneumophila, and Toxoplasma gondii.
- Specific pathogens: Active against Mycobacterium avium complex and Helicobacter pylori.
Pharmacokinetics
Azithromycin possesses a unique pharmacokinetic profile. It achieves record-breaking tissue concentrations compared to all other macrolides. The elimination half-life ($t_{1/2}$) is exceptionally long at 35–55 hours.
Clinical advantage: This kinetic profile provides maximum dosing convenience—the drug is administered just once daily, and the standard treatment course is significantly shorter than that of most other antibiotics.
Clinical Indications
The drug has a wide range of indications across various medical fields:
- Pulmonology and ENT: Streptococcal tonsillopharyngitis, pertussis, diphtheria, and pneumonias (especially "atypical" pneumonias caused by mycoplasma, chlamydia, and legionella).
- Dermatology: Skin and soft tissue infections (scarlet fever, erysipelas).
- Specific infections: Chlamydial infections, mycoplasma infections, osteomyelitis, and oral cavity infections.
- Gastroenterology: Used in H. pylori eradication regimens for peptic ulcer disease.
- Phthisiology: Treatment and prevention of mycobacterial infections caused by Mycobacterium avium.
- Infectious Diseases (Toxoplasmosis): Used as an alternative regimen. The gold standard of treatment (pyrimethamine + sulfadiazine) is contraindicated in pregnancy due to the teratogenic risk of antifolates, making macrolides (azithromycin or clarithromycin) the drugs of choice under these restrictions.
- Rheumatology: Year-round prophylaxis of rheumatic fever in patients allergic to penicillin derivatives.
Resistance Patterns
When bacterial strains develop resistance to macrolides (including azithromycin), clinicians may resort to a reserve group—ketolides (e.g., telithromycin). They exhibit higher affinity for the 50S ribosomal subunit and bind to an additional site on 23S rRNA, successfully overcoming macrolide resistance.
Formulations and Administration
According to standard references, Azithromycin is available in the following forms:
- Capsules: 0.125 g and 0.25 g.
- Tablets: 0.25 g.
Standard dosing regimen (for upper and lower respiratory tract infections): orally, 0.5 g once daily, taken 1 hour before meals. The treatment course is only 3 days.