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Zorluk: OrtaPharmacokinetic and Pharmacodynamic Principles

A 28-year-old woman is brought to the emergency department by her partner due to a painful, blistering skin rash. She has a history of bipolar I disorder with rapid cycling. For the past year, her psychiatric regimen consisted solely of lamotrigine, but due to poor mood control, her psychiatrist added a second medication 4 weeks ago. Two days ago, the patient developed a fever, malaise, and a sore throat. This morning, a diffuse erythematous maculopapular rash appeared on her face and trunk, which has rapidly progressed to form flaccid blisters with mucosal involvement. A dermatologist is consulted and suspects Stevens-Johnson syndrome. Laboratory studies reveal a significantly supratherapeutic serum lamotrigine concentration. The psychiatric medication added 4 weeks ago most likely increased this patient's risk for her current condition through which of the following pharmacokinetic mechanisms?

  1. Inhibition of hepatic uridine 5'-diphospho-glucuronosyltransferaseCevap
  2. B
    Induction of hepatic cytochrome P450 3A4 enzyme activity
  3. C
    Competitive displacement from plasma albumin binding sites
  4. D
    Reduced renal tubular secretion via organic anion transporters
  5. E
    Decreased gastrointestinal motility leading to prolonged drug absorption

Cevap

Inhibition of hepatic uridine 5'-diphospho-glucuronosyltransferase
The patient is presenting with Stevens-Johnson syndrome (SJS) after the recent addition of a new mood stabilizer to her lamotrigine regimen, which resulted in supratherapeutic lamotrigine levels. Valproic acid is commonly used for rapid-cycling bipolar disorder and was likely the medication added. Lamotrigine is primarily metabolized in the liver via glucuronidation by the enzyme uridine 5'-diphospho-glucuronosyltransferase (UGT). Valproic acid is a strong inhibitor of UGT. When valproic acid is added to a patient taking lamotrigine, it significantly decreases lamotrigine clearance and more than doubles its half-life. This pharmacokinetic interaction necessitates a significant reduction in the lamotrigine dose to prevent toxicity, specifically life-threatening skin rashes like SJS or toxic epidermal necrolysis.

Adım Adım Çözüm

1
Identify the patient's clinical presentation and the implicated medication.
The patient has Stevens-Johnson syndrome (SJS) caused by supratherapeutic levels of lamotrigine following the addition of a new mood stabilizer.
Recognizing the adverse effect and its trigger is necessary to determine the underlying pharmacokinetic interaction.
2
Determine the most likely mood stabilizer added to the patient's regimen.
Valproic acid (valproate) is commonly added for rapid-cycling bipolar disorder and is well-known to interact dangerously with lamotrigine.
Identifying the interacting drug allows for the analysis of its specific effect on drug metabolism.
3
Recall the mechanism of interaction between valproic acid and lamotrigine.
Valproic acid strongly inhibits uridine 5'-diphospho-glucuronosyltransferase (UGT), which is the primary enzyme responsible for the glucuronidation and clearance of lamotrigine.
Understanding Phase II metabolism pathways is required to identify the correct pharmacokinetic mechanism.

Anahtar Kavram

Valproic acid inhibits uridine 5'-diphospho-glucuronosyltransferase (UGT), decreasing the clearance of lamotrigine and significantly increasing the risk of life-threatening rashes such as Stevens-Johnson syndrome.
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Pharmacokinetic and Pharmacodynamic Principles

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