A macrolide antibiotic is used in a patient with community-acquired pneumonia. Azithromycin has a much longer half-life (68 hours) than erythromycin (1.5 hours) despite similar molecular structures. The pharmacokinetic reason is:
- A Azithromycin is entirely renally excreted with no hepatic metabolism, reducing clearance
- B Azithromycin undergoes enterohepatic recirculation, recycling the drug repeatedly
- C Azithromycin has superior plasma protein binding, reducing its volume of distribution
- D Azithromycin is concentrated extensively in leukocytes and tissues with a very large volume of distribution (~31 L/kg) ✓
Explanation
Azithromycin's prolonged half-life is due to its extraordinary tissue distribution: it accumulates within lysosomes of macrophages and polymorphonuclear leukocytes to concentrations 10–100 times the plasma level, resulting in a volume of distribution of approximately 31 L/kg—among the highest of any antibiotic. Slow release from these cellular reservoirs back into plasma maintains tissue and intracellular concentrations long after the drug is no longer detectable in plasma. This allows a 3- or 5-day course to produce therapeutic tissue levels for 7–10 days. Erythromycin has a much lower Vd (~0.7 L/kg) and is rapidly cleared.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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