A 35-year-old man with cystic fibrosis presents with acute exacerbation of chronic respiratory infection. Sputum culture and sensitivity show *Pseudomonas aeruginosa* susceptible to all aminoglycosides. Which aminoglycoside is the drug of choice for this patient?

Explanation

## Aminoglycoside Selection in *Pseudomonas aeruginosa* Respiratory Infection **Key Point:** Tobramycin is the drug of choice for *Pseudomonas aeruginosa* respiratory infections, particularly in cystic fibrosis, because it achieves superior lung parenchymal and sputum concentrations compared to other aminoglycosides. ### Why Tobramycin for *Pseudomonas* in CF **High-Yield:** Tobramycin is preferred because: 1. **Superior lung penetration:** Achieves 2–3× higher sputum concentrations than gentamicin 2. **Optimal for *Pseudomonas*:** More potent against *P. aeruginosa* than gentamicin in vitro and in vivo 3. **CF-specific advantage:** In CF, chronic *Pseudomonas* colonization requires high local antibiotic concentrations; tobramycin's pharmacokinetics favor lung tissue 4. **Inhaled formulation available:** Tobramycin solution for inhalation (TOBI) is FDA-approved and guideline-recommended for CF airway disease 5. **Proven clinical outcomes:** Reduces *Pseudomonas* density, improves FEV₁, and delays disease progression in CF ### Aminoglycoside Spectrum in *Pseudomonas* Infection | Aminoglycoside | Efficacy vs *P. aeruginosa* | Lung Penetration | Clinical Role | Notes | | --- | --- | --- | --- | --- | | **Gentamicin** | Moderate (variable resistance) | Lower | Not preferred for *Pseudomonas* | Better for Gram-negatives (*E. coli*, *Klebsiella*) | | **Tobramycin** | Excellent | Superior (2–3× higher sputum levels) | **First-line for CF & respiratory *P. aeruginosa*** | Inhaled formulation available; proven CF benefit | | **Amikacin** | Excellent (broader spectrum) | Moderate | Reserved for resistant *Pseudomonas* | Used when tobramycin resistance emerges | | **Netilmicin** | Good | Moderate | Rarely used | No advantage over tobramycin or amikacin | **Clinical Pearl:** In CF patients with *Pseudomonas*, tobramycin is often given as: - **IV/IM:** 8–10 mg/kg once daily (for acute exacerbation) - **Inhaled:** 300 mg twice daily via nebulizer (maintenance/chronic suppression) The inhaled route is unique to tobramycin and delivers high local concentrations while minimizing systemic toxicity. **Mnemonic — *Pseudomonas* Aminoglycosides:** **TOB** for **TOB**ramycin = *Pseudomonas* (CF); **AMIK** for **AMIK**acin = resistant *Pseudomonas*. **Warning:** Do NOT use gentamicin as monotherapy for *Pseudomonas aeruginosa* respiratory infection — it has lower lung penetration and variable efficacy. Tobramycin is the established standard of care in CF guidelines (Cystic Fibrosis Foundation).