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Difficulty: Very hardPsychopharmacology and Psychological Therapies

An emergency department physician evaluates a 29-year-old man who presents with a rapidly spreading, painful dermatologic eruption. The patient has a known psychiatric history of bipolar I disorder, for which he has been maintained on divalproex sodium for the past four years without any manic relapses. Six weeks ago, his outpatient psychiatrist prescribed an additional medication to address a severe, functionally impairing depressive episode. Upon physical examination, the patient's temperature is 39.2°C (102.6°F). There is a widespread, tender, purpuric maculopapular rash exhibiting epidermal detachment when lateral pressure is applied. Erosions are visible on the buccal mucosa and bilateral conjunctivae. Which of the following prescribing oversights is the most likely cause of this patient's current presentation?

  1. Failure to halve the standard initial dosage and titration rate of the new medicationAnswer
  2. B
    Omission of genetic screening for the HLA-B*1502 allele prior to treatment initiation
  3. C
    Co-administration of two agents that strongly induce hepatic cytochrome P450 enzymes
  4. D
    Initiation of a serotonergic agent that displaced divalproex sodium from plasma proteins
  5. E
    Concurrent use of two medications that compete for proximal renal tubular secretion

Answer

Failure to halve the standard initial dosage and titration rate of the new medication
The patient's presentation is classic for Stevens-Johnson syndrome (SJS), a life-threatening mucocutaneous reaction characterized by fever, a purpuric maculopapular rash, mucosal erosions, and epidermal detachment (positive Nikolsky sign). The newly added medication for his bipolar depression is most likely lamotrigine. When lamotrigine is co-administered with divalproex sodium (valproate), a critical pharmacokinetic interaction occurs. Valproate is a potent inhibitor of UDP-glucuronosyltransferase (UGT), the primary enzyme responsible for lamotrigine metabolism. This inhibition effectively doubles the half-life of lamotrigine, significantly elevating its serum concentration and drastically increasing the risk of SJS. To mitigate this risk, standard psychiatric guidelines mandate that the starting dose and titration rate of lamotrigine be reduced by 50% when added to a valproate regimen. The failure to make this dosage adjustment is the oversight that precipitated this patient's condition.

Step-by-Step Solution

1
Identify the patient's acute dermatologic complication based on the physical examination.
The patient has Stevens-Johnson syndrome (SJS), indicated by fever, a purpuric rash, mucosal erosions, and a positive Nikolsky sign (epidermal detachment with lateral pressure).
Recognizing this life-threatening drug reaction is the first step in determining which pharmacological interaction occurred.
2
Determine the most likely medication added for the patient's bipolar depression.
Lamotrigine is a first-line treatment for bipolar depression and is strongly associated with the development of SJS, especially when titrated too rapidly.
Identifying the new drug allows for the analysis of drug-drug interactions with the patient's baseline regimen.
3
Analyze the pharmacokinetic interaction between the baseline mood stabilizer (divalproex sodium) and the new agent (lamotrigine).
Divalproex sodium (valproate) is a potent inhibitor of UDP-glucuronosyltransferase (UGT), the primary enzyme responsible for metabolizing lamotrigine.
Inhibition of UGT doubles the half-life of lamotrigine, leading to rapid, toxic accumulation if standard dosing is used.
4
Identify the prescribing rule required to prevent this toxicity.
When lamotrigine is added to valproate, the initial starting dose and the subsequent titration rate of lamotrigine must be reduced by exactly 50%.
Failing to make this critical dosage adjustment is a common error that directly precipitates life-threatening cutaneous reactions.

Key Concept

Pharmacokinetic interaction between divalproex sodium (valproate) and lamotrigine via UGT inhibition, requiring a 50% dose reduction of lamotrigine to prevent Stevens-Johnson syndrome.
Estimated Time:2m 0s
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Topics

Psychopharmacology and Psychological Therapies

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