Azithromycin has a longer half-life (~68 hours) and tissue concentration than erythromycin despite similar bacteriostatic mechanism. This pharmacokinetic advantage is clinically exploited by:

• A Once-daily dosing for 3 days treating community-acquired pneumonia equivalent to 10 days of standard beta-lactam therapy
• B Weekly single-dose prophylaxis of MAC infection in HIV that maintains continuous serum MIC-exceeding concentrations
• C The long half-life means it cannot be used in renal failure because accumulation causes QTc prolongation
• D Its high tissue concentration makes a 3–5 day course provide tissue levels therapeutic for ~7–10 days after the last dose; and it concentrates in phagocytes, delivering drug to sites of infection ✓

Explanation

Azithromycin concentrates 10–100-fold in macrophages, neutrophils, and fibroblasts relative to plasma. This 'phagocyte delivery' mechanism directly transports the drug to sites of bacterial infection where phagocytes migrate. After a 3–5 day course, tissue concentrations remain above MIC for typical respiratory pathogens for 7–10 days. Plasma half-life is ~68 hours, but tissue half-life is much longer (particularly in lung). Weekly single-dose azithromycin (1.2g) is standard for MAC prophylaxis in CD4 <50 cells/µL HIV patients. QTc prolongation is a real concern that is dose-independent and involves HERG channel inhibition.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.