Azithromycin has a much longer half-life (~68 hours) and tissue concentration relative to plasma compared with erythromycin. This is primarily due to:
- A Extensive lysosomal trapping in phagocytes and high volume of distribution (~31 L/kg), with slow release from tissues ✓
- B High plasma protein binding preventing renal clearance
- C Inhibition of CYP3A4 enzymes that would otherwise metabolise the drug
- D Azithromycin undergoes extensive enterohepatic recirculation
Correct answer: A. Extensive lysosomal trapping in phagocytes and high volume of distribution (~31 L/kg), with slow release from tissues
Explanation
Azithromycin has an exceptionally large volume of distribution (~31 L/kg) owing to extensive binding to and accumulation in tissues (especially phagocytes, macrophages, fibroblasts). It concentrates in lysosomes of phagocytic cells (lysosomal trapping due to its basic pH) and is slowly released back to the plasma, producing tissue concentrations far exceeding plasma levels and a prolonged tissue half-life. This allows 3-day or Z-pack regimens. Erythromycin, unlike azithromycin, does inhibit CYP3A4 significantly; azithromycin has minimal CYP inhibition.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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