Classification and chemical structure
Azithromycin belongs to the macrolide group. Unlike natural 14-membered (erythromycin, oleandomycin) and 16-membered macrolides (spiramycin, josamycin, midecamycin), azithromycin is a 15-membered semisynthetic compound.
The main structural feature that allowed its classification into a separate subclass of azalides is the presence of a nitrogen atom in the lactone ring.
Mechanism and Spectrum of Action
The mechanism of action of the drug is presumably associated with the inhibition of protein synthesis in the microorganism's organelles.
The spectrum of antimicrobial activity of azithromycin has important differences from classical erythromycin:
- Gram-positive flora: less active against staphylococci.
- Gram-negative flora: more active against H. influenzae and various enterobacteria.
- Intracellular and atypical pathogens: exhibits high activity against Chlamydia, Mycoplasma pneumoniae, Legionella pneumophila, as well as legionellae and toxoplasmas (Toxoplasma gondii).
- Specific pathogens: active against Mycobacterium avium and H. pylori.
Pharmacokinetics
Azithromycin has a unique pharmacokinetic profile. It creates record tissue concentrations among all macrolides. The half-life ($t_{1/2}$) is extremely long, ranging from 35 to 55 hours.
Clinical advantage: such kinetics ensures maximum ease of use — the drug is prescribed just once daily, and the standard treatment course is significantly shorter than that of most other antibiotics.
Clinical application (Indications)
The drug has a wide range of indications in various fields of medicine:
- Pulmonology and ENT: streptococcal tonsillopharyngitis, pertussis, diphtheria, as well as pneumonia (especially "atypical" caused by mycoplasmas, chlamydiae, and legionellae).
- Dermatology: skin and soft tissue infections (scarlet fever, erysipelas).
- Specific infections: chlamydiosis, mycoplasma infection, osteomyelitis, oral cavity infections.
- Gastroenterology: used in H. pylori eradication regimens for gastric and duodenal ulcer disease.
- Phthisiology: treatment and prophylaxis of mycobacteriosis caused by Mycobacterium avium.
- Infectious diseases (Toxoplasmosis): used as an alternative regimen. The gold standard of treatment (pyrimethamine + sulfadiazine) is contraindicated in pregnant women due to the risk of the teratogenic effect of antifolates; therefore, macrolides (azithromycin or clarithromycin) become drugs of choice under these restrictions.
- Rheumatology: year-round prophylaxis of rheumatic fever in patients with allergies to benzylpenicillin drugs.
Resistance issue
When bacterial strains develop resistance to macrolides (including azithromycin), a reserve group — ketolides (e.g., telithromycin) — is used in clinical practice. They have a higher affinity for the 50S ribosomal subunit and bind to an additional site on the 23S rRNA, successfully overcoming resistance.
Formulations and prescription rules
According to the prescription reference, Azithromycin is available in the following forms:
- Capsules of 0.125 g and 0.25 g.
- Tablets of 0.25 g.
Standard dosing regimen (for upper and lower respiratory tract infections): per os 0.5 g 1 hour before meals once daily. The treatment course is only 3 days.
Briefly: answers to topical questions
What are the side effects and contraindications for prescribing azithromycin?
Side effects of azithromycin include cardiotoxicity, hearing impairment, and specific risks for newborns, while contraindications depend on the drug's dosage form.
The main side effects include:
- Hearing impairment — occurs with prolonged administration.
- Cardiotoxicity — associated with prolonged therapy.
- Neonatal distress syndrome — risk increases when the drug is taken for 5 days in premature births.
Contraindications to prescription are related to the patient's age and dosage form:
- Suspension — contraindicated in children under 6 months.
- 125 mg tablets — contraindicated in children under 3 years.
Azalide Azithromycin — Nitrogen in the ring, Active against Atypical. Three "A"s for quick memorization of structure and spectrum.
Class and exam questions
Why is azithromycin classified into a separate subclass of azalides?
Due to its chemical structure: it is a 15-membered semisynthetic macrolide with a nitrogen atom incorporated into the lactone ring.
What is the main pharmacokinetic advantage of azithromycin?
It creates record tissue concentrations and has a very long half-life (35–55 hours), allowing it to be taken in a short course just once daily.
How does azithromycin act on staphylococci compared to erythromycin?
Azithromycin is less active against staphylococci than the natural 14-membered macrolide erythromycin, but surpasses it in combating gram-negative and intracellular flora.
What is the role of azithromycin in the treatment of toxoplasmosis?
It is used as an alternative drug. This is especially important for pregnant women for whom the "gold standard" (pyrimethamine + sulfadiazine) is contraindicated due to teratogenic effects.
What else to study on this topic
- Comparative characteristics of 14-, 15-, and 16-membered macrolides
- Mechanisms of bacterial resistance development to macrolides
- The role of macrolides in modern H. pylori eradication regimens
- Ketolides (telithromycin) as a way to overcome azalide resistance
- Alternative treatment regimens for toxoplasmosis in pregnant women
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