Clarithromycin has an important advantage over erythromycin regarding pharmacokinetics. Which statement best describes this?
- A Clarithromycin has a shorter half-life than erythromycin, requiring more frequent dosing
- B Clarithromycin penetrates the BBB better than erythromycin, making it useful for CNS infections
- C Clarithromycin has improved acid stability, better oral bioavailability (~55% vs ~25%), and a longer half-life allowing twice-daily dosing; its active metabolite 14-hydroxyclarithromycin adds synergistic activity against H. influenzae ✓
- D Clarithromycin is the only macrolide with activity against Pseudomonas aeruginosa
Correct answer: C. Clarithromycin has improved acid stability, better oral bioavailability (~55% vs ~25%), and a longer half-life allowing twice-daily dosing; its active metabolite 14-hydroxyclarithromycin adds synergistic activity against H. influenzae
Explanation
Clarithromycin is acid-stable, unlike erythromycin which degrades in gastric acid. This improves bioavailability to ~55% vs 25% for erythromycin. Its half-life of 3–7 hours allows twice-daily dosing compared to erythromycin's 6-hourly regimen. The active metabolite 14-hydroxyclarithromycin has additional antibacterial activity, particularly against H. influenzae. Azithromycin has even better pharmacokinetics (once daily, tissue concentrations far exceed plasma). Macrolides have no activity against Pseudomonas.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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