Paper II
Question
Compare and contrast: erythromycin and azithromycin.
Answer
Compare and contrast: Erythromycin and Azithromycin
| Feature | Erythromycin | Azithromycin |
|---|---|---|
| Class | Macrolide (older) | Azalide (newer macrolide derivative) |
| Mechanism | Both bind the bacterial 50S ribosomal subunit, blocking translocation and inhibiting protein synthesis | Same |
| Half-life | Short (~1.5 hours), requiring frequent dosing (3-4 times daily) | Long (~68 hours), permitting once-daily dosing and short courses (e.g. 3-5 days) |
| Tissue penetration | Moderate | Excellent, with high and prolonged tissue concentration |
| GI tolerability | Poor — motilin-receptor agonism causes prominent nausea, vomiting, abdominal cramping | Better tolerated, less GI upset |
| CYP450 interaction | Significant CYP3A4 inhibitor — numerous drug interactions | Minimal CYP450 interaction, fewer drug interactions |
| QT prolongation risk | Present | Present, though generally considered somewhat lower |
| Typical uses | Alternative in penicillin allergy, some GI motility indications (as a prokinetic, exploiting the motilin effect) | Respiratory tract infections, STIs (chlamydia — single dose), COPD exacerbations |
Shared feature: both are bacteriostatic (at usual doses) protein synthesis inhibitors with similar spectrum, useful alternatives in penicillin-allergic patients for susceptible organisms.