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Difficulty: HardPsychopharmacology and Psychological Therapies

A 24-year-old woman comes to an urgent care clinic reporting a painful skin rash, sore throat, and a low-grade fever that began 48 hours ago. Her medical history is notable for bipolar I disorder. She has been stable on valproate for three years. Five weeks ago, her psychiatrist initiated a cross-taper to a new mood stabilizer due to the patient's concerns regarding ongoing weight gain and hair thinning. She has been taking both medications exactly as prescribed. Her temperature is 38.8°C (101.8°F), pulse is 108/min, and blood pressure is 110/70 mm Hg. Physical examination reveals a diffuse, tender, erythematous, macular rash that easily coalesces into large blisters. Gentle pressure on the skin causes the top layer of the epidermis to detach. There are multiple painful erosions on her buccal mucosa and lips. Which of the following pharmacokinetic interactions most likely contributed to this patient's current condition?

  1. Inhibition of UDP-glucuronosyltransferase by the established mood stabilizerAnswer
  2. B
    Induction of cytochrome P450 3A4 metabolism by the established mood stabilizer
  3. C
    Competitive displacement of the new mood stabilizer from plasma albumin
  4. D
    Synergistic blockade of presynaptic voltage-gated sodium channels
  5. E
    Inhibition of cytochrome P450 2D6 by the newly initiated mood stabilizer

Answer

The correct answer is the inhibition of UDP-glucuronosyltransferase by the established mood stabilizer, as valproate significantly reduces the clearance of lamotrigine.
This patient presents with fever, mucosal erosions, and a blistering rash with a positive Nikolsky sign (epidermal detachment with gentle pressure), which is the classic presentation of Stevens-Johnson syndrome (SJS) or toxic epidermal necrolysis (TEN). In a patient with bipolar disorder who is being cross-tapered off valproate to a weight-neutral alternative, the newly initiated medication is almost certainly lamotrigine. Lamotrigine is highly efficacious for bipolar maintenance but carries a black box warning for severe, life-threatening rashes. The risk of SJS/TEN is significantly increased when lamotrigine is co-administered with valproate. Valproate is a potent inhibitor of UDP-glucuronosyltransferase (UGT), which is the primary hepatic enzyme responsible for the glucuronidation and elimination of lamotrigine. This pharmacokinetic interaction effectively doubles the half-life and serum concentration of lamotrigine. Consequently, when initiating lamotrigine in a patient already taking valproate, the starting dose must be halved and the titration schedule must be remarkably slower to prevent toxic lamotrigine levels and minimize the risk of SJS/TEN.

Step-by-Step Solution

1
Diagnose the patient's acute dermatologic presentation.
The patient has a blistering rash with mucosal involvement and a positive Nikolsky sign, indicating Stevens-Johnson syndrome (SJS) or toxic epidermal necrolysis (TEN).
Recognizing this life-threatening drug reaction is the first requirement to identify the inciting pharmacologic event.
2
Identify the new psychotropic medication initiated.
The new medication is most likely lamotrigine.
Lamotrigine is a weight-neutral mood stabilizer commonly used for bipolar disorder that carries a well-known black box warning for SJS/TEN.
3
Analyze the pharmacokinetic interaction between the patient's current medications.
Valproate (the established medication) inhibits UDP-glucuronosyltransferase (UGT), the enzyme responsible for clearing lamotrigine.
To determine why the patient developed toxicity despite taking the medications as prescribed, one must understand how valproate alters lamotrigine metabolism.

Key Concept

Valproate is a potent inhibitor of UDP-glucuronosyltransferase (UGT). When co-administered with lamotrigine, it roughly doubles lamotrigine's half-life, significantly increasing the risk of life-threatening rashes like Stevens-Johnson syndrome.
Estimated Time:1m 30s
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Topics

Psychopharmacology and Psychological Therapies

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