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Difficulty: MediumPharmacokinetic and Pharmacodynamic Principles

A 29-year-old woman with a history of generalized tonic-clonic seizures is evaluated in the outpatient clinic for progressive unsteadiness, double vision, and confusion over the past week. Three weeks ago, she had a breakthrough seizure. Her neurologist subsequently increased her daily dose of phenytoin from 300 mg300 \text{ mg} to 400 mg400 \text{ mg}. Her previously stable steady-state serum phenytoin concentration was 14 mg/L14 \text{ mg/L} (therapeutic range: 10−20 mg/L10-20 \text{ mg/L}). Today, physical examination reveals coarse horizontal nystagmus, a broad-based gait, and slurred speech. Laboratory analysis demonstrates a serum phenytoin concentration of 34 mg/L34 \text{ mg/L}. Which of the following pharmacokinetic principles best explains the disproportionate rise in this patient's serum drug concentration?

  1. Saturation of hepatic biotransformation enzymesAnswer
  2. B
    Auto-induction of cytochrome P450 metabolism
  3. C
    Competitive displacement from plasma albumin binding sites
  4. D
    Decreased renal tubular secretion of the active drug
  5. E
    Decreased gastrointestinal motility leading to enhanced absorption

Answer

Saturation of hepatic biotransformation enzymes
The correct answer identifies that hepatic biotransformation enzymes can become saturated. Phenytoin is classically known for displaying Michaelis-Menten (non-linear or capacity-limited) kinetics. At low doses, phenytoin elimination follows first-order kinetics, meaning the amount of drug eliminated is proportional to the plasma concentration. However, as the dose approaches the typical therapeutic range, the cytochrome P450 enzymes (CYP2C9 and CYP2C19) responsible for metabolizing phenytoin become saturated. At this point, the liver can only eliminate a maximum, constant amount of drug per unit of time (zero-order kinetics). Consequently, a relatively small increase in the daily dose can cause a disproportionately massive and potentially toxic increase in steady-state serum drug concentrations.

Step-by-Step Solution

1
Analyze the patient's dose-to-concentration relationship.
The dose was increased by roughly 33% (from 300 mg to 400 mg), but the serum concentration increased by 142% (from 14 mg/L to 34 mg/L), moving from therapeutic to highly toxic.
Establishing the proportionality of the dose change versus the serum level change helps identify the underlying elimination kinetics.
2
Correlate the clinical presentation with the serum concentration.
The patient exhibits classic signs of phenytoin toxicity (nystagmus, ataxia, slurred speech, confusion), which corresponds accurately with the toxic serum level of 34 mg/L.
Confirms that the laboratory value is clinically accurate and represents true systemic accumulation.
3
Identify the pharmacokinetic mechanism specific to the drug (phenytoin) that explains this non-linear accumulation.
Phenytoin exhibits capacity-limited (Michaelis-Menten) kinetics, where hepatic enzymes become fully occupied (saturated) within or just above the therapeutic window.
Saturation prevents the liver from increasing its rate of metabolism to match the increased dose, shifting clearance from first-order to zero-order.

Key Concept

Michaelis-Menten (zero-order) pharmacokinetics of phenytoin
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Topics

Pharmacokinetic and Pharmacodynamic Principles

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