Azithromycin has a much longer half-life (68 hours) than erythromycin (1.5 hours) primarily because of:
- A Concentration in lysosomes and tissue phagocytes due to its basic nature with slow release ✓
- B Extensive protein binding preventing glomerular filtration
- C Inhibition of CYP3A4, preventing its own first-pass metabolism
- D Enterohepatic recirculation with reabsorption from the terminal ileum
Correct answer: A. Concentration in lysosomes and tissue phagocytes due to its basic nature with slow release
Explanation
Azithromycin is a weakly basic macrolide that concentrates extensively in lysosomes and phagocytes (tissue-to-plasma ratio >100:1). This tissue accumulation creates a large volume of distribution (~31 L/kg) with slow redistribution back into plasma, prolonging the effective half-life. This property also allows it to be delivered to sites of infection by phagocytes. Erythromycin does inhibit CYP3A4, but azithromycin does not, and this is not responsible for half-life differences.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.