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Difficulty: HardPsychopharmacology and Psychological Therapies

A 34-year-old man with treatment-resistant schizophrenia is brought to the emergency department by his partner after experiencing a generalized tonic-clonic seizure lasting approximately 2 minutes. He has no prior history of seizures. His psychotic symptoms have been well controlled on a stable regimen of clozapine 400 mg daily for the past 3 years. Four weeks ago, the patient suffered an uncomplicated myocardial infarction and was discharged on aspirin and atorvastatin. At that time, the patient successfully quit smoking cold turkey after previously smoking 2 packs of cigarettes daily for 15 years. He reports no illicit drug or alcohol use. His temperature is 37.1°C (98.8°F), blood pressure is 118/76 mm Hg, pulse is 88/min, and respirations are 16/min. Physical examination reveals a postictal state but is otherwise unremarkable. Laboratory studies show an absolute neutrophil count of 5,200/mm35,200/mm^3 and normal liver function tests. Which of the following is the most likely mechanism responsible for this patient's new-onset seizure?

  1. Decreased induction of a hepatic cytochrome P450 enzymeAnswer
  2. B
    Increased induction of a hepatic cytochrome P450 enzyme
  3. C
    Central nervous system infection secondary to severe neutropenia
  4. D
    Lowered seizure threshold secondary to nicotine withdrawal
  5. E
    Displacement of the antipsychotic medication from plasma proteins

Answer

Decreased induction of a hepatic cytochrome P450 enzyme
The correct answer identifies the loss of cytochrome P450 enzyme induction as the cause of the seizure. Polycyclic aromatic hydrocarbons present in tobacco smoke are potent inducers of CYP1A2, the primary hepatic enzyme responsible for metabolizing clozapine. When a patient who is stabilized on clozapine abruptly stops smoking, the induction of CYP1A2 is lost. This leads to a rapid decrease in clozapine clearance and a subsequent elevation in serum clozapine levels. Clozapine is well known to lower the seizure threshold in a dose-dependent manner. The sudden, drastic increase in clozapine concentration precipitated this patient's new-onset generalized tonic-clonic seizure.

Step-by-Step Solution

1
Identify the patient's baseline medications and recent lifestyle changes.
The patient is on a high but stable dose of clozapine and recently underwent abrupt smoking cessation following a myocardial infarction.
Clozapine relies heavily on specific metabolic pathways that are influenced by environmental factors.
2
Determine the metabolic interaction between tobacco smoke and clozapine.
Polycyclic aromatic hydrocarbons in tobacco smoke are potent inducers of the hepatic enzyme cytochrome P450 1A2 (CYP1A2), which is the primary enzyme responsible for clozapine metabolism.
Understanding the enzyme system involved explains how the patient's medication levels are altered.
3
Analyze the physiological effect of smoking cessation on clozapine levels.
Abrupt smoking cessation removes the CYP1A2 induction, causing a relative decrease in enzyme activity. This leads to reduced clozapine clearance and a significant rise in serum clozapine levels.
This establishes the mechanism for medication toxicity.
4
Correlate the elevated clozapine levels with the clinical presentation.
Clozapine has a known, dose-dependent risk of causing seizures. The abrupt rise in serum clozapine resulting from the loss of CYP1A2 induction precipitated the tonic-clonic seizure.
This confirms that the physiological change directly caused the patient's symptom.

Key Concept

Polycyclic aromatic hydrocarbons in tobacco smoke strongly induce cytochrome P450 1A2 (CYP1A2). Abrupt smoking cessation in a patient on clozapine (a CYP1A2 substrate) removes this induction, leading to elevated clozapine levels and an increased risk of dose-dependent toxicities, such as seizures.
Estimated Time:1m 30s
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Topics

Psychopharmacology and Psychological Therapies

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