Azithromycin achieves tissue concentrations 10–100 times greater than plasma concentrations. This pharmacokinetic property results in:
- A Rapid bactericidal activity against intracellular pathogens with short duration of effect
- B Prolonged tissue half-life (~68 hours) enabling single-dose or 3–5 day courses effective for intracellular pathogens like Chlamydia ✓
- C Inability to treat systemic bacteremia as drug is sequestered in tissues
- D High hepatic first-pass effect requiring IV formulation for adequate plasma levels
Correct answer: B. Prolonged tissue half-life (~68 hours) enabling single-dose or 3–5 day courses effective for intracellular pathogens like Chlamydia
Explanation
Azithromycin is unique among macrolides for its extensive intracellular accumulation, particularly in phagocytes (neutrophils, macrophages) and tissue cells. This results in a very long tissue half-life (~68 hours versus 14 hours for clarithromycin) despite shorter plasma half-life. Clinically, this allows single-dose treatment of Chlamydia trachomatis urethritis (1g), and a 5-day Z-pack course maintains therapeutic intracellular levels for 10+ days, making it effective against intracellular organisms.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.