A 30-year-old patient with Staphylococcus aureus right-sided endocarditis is started on daptomycin. Which clinical scenario would render daptomycin INEFFECTIVE for this patient?

• A Concurrent vancomycin use reducing daptomycin plasma concentrations
• B MRSA strain with thickened cell wall reducing daptomycin binding
• C Pulmonary involvement, as surfactant inactivates daptomycin in the alveoli ✓
• D Severe renal failure increasing daptomycin's volume of distribution

Explanation

Daptomycin is a cyclic lipopeptide that inserts into the bacterial cell membrane in a calcium-dependent manner, causing rapid membrane depolarization and cell death. However, pulmonary surfactant (phosphatidylglycerol and other phospholipids) inactivates daptomycin by competitively binding the drug, making it ineffective for pneumonia and other pulmonary infections. This is why daptomycin should NOT be used for pneumonia despite its activity against S. aureus. It remains highly effective for bacteremia, endocarditis, and skin/soft tissue infections.

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

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Written and medically reviewed by the StethoPrep medical team.