Azithromycin and Erythromycin Are Both Macrolide Antibiotics
Yes, azithromycin and erythromycin belong to the same class of antibiotics known as macrolides, although azithromycin is technically an azalide, a subclass of macrolides. 1, 2
Chemical Structure and Classification
- Azithromycin is derived from erythromycin but differs chemically in that it has a methyl-substituted nitrogen atom incorporated into the lactone ring 2
- Azithromycin is classified as an azalide, which is considered a subclass of macrolide antibiotics 2, 3
- Both drugs share similar core chemical structures and mechanisms of action, inhibiting bacterial protein synthesis by binding to the 50S ribosomal subunit 4
Mechanism of Action
- Both medications work by binding to the 50S component of the 70S ribosomal subunit in bacteria, interfering with bacterial protein synthesis 5
- Alteration in this binding site confers simultaneous resistance to all macrolide antibiotics - meaning cross-resistance exists between azithromycin and erythromycin 4
Key Differences Between the Drugs
Antimicrobial Activity
- Azithromycin is marginally less active than erythromycin against gram-positive organisms 3
- Azithromycin demonstrates superior activity against many gram-negative pathogens, including Haemophilus influenzae 3, 5
Pharmacokinetics
- Azithromycin has a longer half-life (approximately 3 days in tissues) compared to erythromycin 4
- Azithromycin achieves higher tissue concentrations but lower serum concentrations than erythromycin 3, 4
- Azithromycin's unique pharmacokinetics allow for once-daily dosing and shorter treatment courses (5 days) 4, 6
- Erythromycin typically requires four divided daily doses for 14 days 7
Drug Interactions
- Erythromycin is an inhibitor of the cytochrome P450 enzyme system (CYP3A subclass) and has numerous drug interactions 7
- Azithromycin has fewer drug interactions compared to erythromycin 8
- Erythromycin should not be administered with astemizole, cisapride, pimazole, or terfenadine due to risk of serious cardiovascular events 7
Side Effects
- Both drugs can cause gastrointestinal side effects, but these are more frequent and severe with erythromycin 7
- Erythromycin has been associated with infantile hypertrophic pyloric stenosis (IHPS) in neonates, while azithromycin has not shown this association 7
Clinical Applications
- Both drugs are used for respiratory tract infections, skin and soft tissue infections 7
- Azithromycin is preferred over erythromycin for infants aged <1 month due to the risk of IHPS with erythromycin 7
- Azithromycin can be given as a single dose for certain infections like Chlamydia trachomatis genital infections 4
- Erythromycin requires a longer treatment course (14 days) for pertussis compared to azithromycin (5 days) 7
In summary, while both drugs belong to the macrolide class of antibiotics and share similar mechanisms of action, they have important differences in their pharmacokinetics, spectrum of activity, dosing regimens, and side effect profiles that influence their clinical applications.