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Difficulty: Very hardPharmacologic Interventions

A 34-year-old woman is evaluated in the hospital after experiencing a generalized tonic-clonic seizure lasting approximately 2 minutes. She was admitted 2 days ago for a perforated appendix and underwent an emergent laparoscopic appendectomy. Postoperatively, she has been receiving intravenous fluids and empiric intravenous ertapenem. She has a 12-year history of focal epilepsy with secondary generalization, which had been well-controlled without any seizures for the past 4 years on a stable daily dose of valproic acid. She also uses a levonorgestrel-releasing intrauterine device.

The patient is currently post-ictal but is slowly returning to her baseline mental status. She denies any preceding aura, headache, or focal weakness. Her temperature is 37.6°C (99.7°F), blood pressure is 112/70 mm Hg, heart rate is 84/min, and respiratory rate is 14/min. Physical examination reveals a well-healing abdominal incision with mild right lower quadrant tenderness. Neurologic examination shows no focal deficits.

Laboratory studies obtained immediately after the seizure show:

TestResult
Sodium138 mEq/L
Potassium4.1 mEq/L
Blood urea nitrogen14 mg/dL
Creatinine0.9 mg/dL
Valproic acid12 mcg/mL (Therapeutic range: 50-125 mcg/mL)

Review of her electronic medical record shows an outpatient valproic acid level of 88 mcg/mL drawn 3 weeks ago.

Which of the following is the most appropriate next step in the pharmacologic management of this patient?

  1. Discontinue ertapenem and initiate ceftriaxone and metronidazoleAnswer
  2. B
    Increase the maintenance dose of valproic acid by 100%
  3. C
    Discontinue ertapenem and initiate ciprofloxacin and metronidazole
  4. D
    Maintain ertapenem and add oral levetiracetam
  5. E
    Maintain ertapenem and administer an intravenous loading dose of valproic acid

Answer

Discontinue ertapenem and initiate ceftriaxone and metronidazole
The correct action is to discontinue the interacting drug (ertapenem) and initiate an effective, safe alternative (ceftriaxone and metronidazole). The patient's breakthrough seizure is directly caused by a precipitous drop in her serum valproic acid level, which was triggered by the initiation of ertapenem. Carbapenem antibiotics (e.g., ertapenem, meropenem, imipenem) cause a rapid, irreversible decrease in valproate levels by inhibiting acylpeptide hydrolase, the enzyme responsible for hydrolyzing valproate glucuronide back into active valproate. This leads to massive renal clearance of the glucuronidated form. This pharmacokinetic interaction is insurmountable; therefore, adjusting the valproate dose is futile. The antimicrobial regimen must be changed to one that covers typical intra-abdominal flora (enteric Gram-negative bacilli and anaerobes) without further provoking seizures. Ceftriaxone combined with metronidazole achieves this safely.

Step-by-Step Solution

1
Identify the primary cause of the patient's breakthrough seizure.
The patient has a markedly subtherapeutic valproic acid level (12 mcg/mL) compared to her stable recent baseline (88 mcg/mL).
Recognizing the subtherapeutic drug level points to a pharmacokinetic issue rather than disease progression or medication non-adherence.
2
Determine the etiology of the subtherapeutic valproic acid level.
The recent initiation of ertapenem is responsible for the precipitous drop in valproic acid.
Carbapenems (such as ertapenem, meropenem, and imipenem) have a well-documented and profound drug-drug interaction with valproic acid.
3
Analyze the mechanism of the drug-drug interaction to guide clinical management.
Carbapenems inhibit acylpeptide hydrolase, an enzyme necessary for the de-glucuronidation and recycling of valproate in the liver. This leads to rapid renal excretion of valproate glucuronide.
Because the clearance is massively accelerated, the interaction is insurmountable and cannot be safely or effectively overcome by simply increasing the valproic acid dose or giving a loading dose.
4
Select the appropriate alternative pharmacologic intervention.
The interacting antibiotic (ertapenem) must be discontinued. A suitable alternative for complicated appendicitis (covering enteric Gram-negative rods and anaerobes) that does not lower the seizure threshold is ceftriaxone combined with metronidazole.
While ciprofloxacin and metronidazole would cover the infection appropriately, fluoroquinolones lower the seizure threshold and are relatively contraindicated in patients with active epilepsy.

Key Concept

Carbapenem antibiotics cause a rapid and profound decrease in serum valproic acid levels, often precipitating breakthrough seizures. This interaction cannot be overcome with higher valproate doses; the carbapenem must be switched to a non-interacting alternative that does not lower the seizure threshold.
Estimated Time:1m 30s
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Pharmacologic Interventions

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