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Difficulty: HardInfectious Diseases

During multidisciplinary rounds in the intensive care unit, a clinical pharmacist raises a concern regarding a patient being treated for infective endocarditis. The patient was admitted 8 days ago after presenting with a one-month history of fatigue, low-grade fevers, and painful, erythematous nodules on the distal fingers. Initial blood cultures grew *Enterococcus faecalis* that was widely susceptible, including to ampicillin, vancomycin, and high-level gentamicin. Echocardiography confirmed a 1.2-cm vegetation on the native mitral valve. The patient was promptly started on intravenous ampicillin and scheduled doses of intravenous gentamicin. Today, the patient is afebrile and hemodynamically stable. However, routine laboratory studies reveal a serum creatinine level of 2.6 mg/dL, an increase from 0.8 mg/dL on admission. Urinalysis demonstrates muddy brown casts. Which of the following is the most appropriate modification to this patient's pharmacotherapy?

  1. Discontinue gentamicin and initiate intravenous ceftriaxoneAnswer
  2. B
    Discontinue gentamicin and continue ampicillin monotherapy
  3. C
    Decrease the dose of gentamicin based on renal function and continue ampicillin
  4. D
    Discontinue current antibiotics and initiate intravenous vancomycin
  5. E
    Discontinue current antibiotics and initiate intravenous daptomycin

Answer

The most appropriate modification is to discontinue gentamicin and initiate intravenous ceftriaxone to maintain bactericidal synergy without worsening nephrotoxicity.
The correct answer is to discontinue gentamicin and initiate intravenous ceftriaxone. This patient has subacute infective endocarditis caused by *Enterococcus faecalis* and has developed aminoglycoside-induced acute tubular necrosis, evidenced by the rising creatinine and muddy brown casts on day 8 of therapy. *Enterococcal* endocarditis requires bactericidal synergistic therapy for eradication. Traditionally, this was achieved with a cell-wall active agent (ampicillin) plus an aminoglycoside (gentamicin). However, due to high rates of nephrotoxicity, dual beta-lactam therapy with ampicillin and ceftriaxone has emerged as a first-line standard of care. Despite *E. faecalis* being intrinsically resistant to cephalosporins, ceftriaxone synergizes with ampicillin by saturating penicillin-binding proteins (PBP) 2 and 3. This forces the bacteria to rely entirely on PBP 4, which is then potently inhibited by ampicillin. This combination achieves excellent bactericidal activity while entirely avoiding the nephrotoxicity associated with aminoglycosides, making it the ideal choice for this patient.

Step-by-Step Solution

1
Identify the primary complication of the current therapy.
The patient has developed acute tubular necrosis (indicated by an elevated creatinine and muddy brown casts) secondary to gentamicin toxicity.
Aminoglycosides are highly nephrotoxic, typically manifesting as acute kidney injury 5 to 10 days after initiation.
2
Determine the pharmacological requirements for treating the underlying infection.
Enterococcus faecalis endocarditis requires a synergistic, bactericidal antimicrobial regimen because the organism is intrinsically tolerant to cell-wall active agents alone.
Bacteriostatic monotherapy leads to high rates of clinical failure and relapse in endovascular infections.
3
Select the appropriate alternative regimen.
The combination of ampicillin and ceftriaxone provides the necessary bactericidal synergy without relying on a nephrotoxic aminoglycoside.
Ceftriaxone saturates specific penicillin-binding proteins, allowing ampicillin to effectively eradicate the bacteria.

Key Concept

Treatment of Enterococcus faecalis infective endocarditis requires bactericidal synergy. If aminoglycoside-induced nephrotoxicity occurs, the optimal step is to transition to dual beta-lactam therapy with ampicillin and ceftriaxone.
Estimated Time:1m 30s
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Topics

Infectious Diseases

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