Azithromycin's activity against intracellular pathogens (Chlamydia, Legionella) is enhanced by which unique pharmacokinetic property compared to erythromycin?
- A Higher plasma protein binding allowing sustained extravascular drug levels
- B Renal tubular secretion achieving very high urinary concentrations
- C Active transport across the blood-brain barrier accumulating in CNS tissue
- D Extensive tissue accumulation and concentration in phagocytes (neutrophils, macrophages), with tissue:plasma ratios of 10-100:1 ✓
Explanation
Azithromycin is a 15-membered azalide macrolide with exceptional tissue distribution. It accumulates in fibroblasts, macrophages, and neutrophils to tissue concentrations 10- to 100-fold higher than plasma, with an elimination half-life of 68 hours compared to erythromycin's 1.5-2 hours. Phagocytes actively transport azithromycin to sites of infection ('Trojan horse' delivery), making it highly effective against obligate intracellular pathogens like Chlamydia (residing in vacuoles within cells). Despite high tissue levels, it has low plasma concentrations, so it is not appropriate for bacteremia.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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