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Phenytoin toxicity due to genetic polymorphism.
Lauren K McCluggage1, Stacy A Voils, Malcolm Ross Bullock
1Department of Pharmacy Practice and Pharmacy Administration, University of the Sciences in Philadelphia, Philadelphia, PA, USA.
Genetic variations in cytochrome P450 2C9 can lead to phenytoin toxicity in traumatic brain injury patients. Monitoring drug levels is crucial, especially when pharmacogenomic testing is unavailable.
Area of Science:
- Neuroscience
- Pharmacology
- Genetics
Background:
- Traumatic brain injury (TBI) patients often receive phenytoin for seizure prophylaxis.
- Phenytoin has non-linear pharmacokinetics and a narrow therapeutic window, necessitating concentration monitoring.
- Hepatic metabolism of phenytoin involves cytochrome P450 enzymes, which are subject to genetic polymorphisms.
Observation:
- A 53-year-old female with TBI developed suspected phenytoin toxicity.
- The patient presented with lethargy and elevated free phenytoin levels (4.4 mg/l).
- She was identified as a cytochrome P450 2C9 poor metabolizer.
Findings:
- Genetic polymorphism in CYP2C9 was the cause of elevated phenytoin concentrations.
- Other potential causes like drug interactions, hypoalbuminemia, and lab errors were ruled out.
- This highlights the impact of genetic variations on drug metabolism and dosage requirements.
Implications:
- Clinical significance of genetic polymorphisms in phenytoin dosage adjustment.
- Routine phenytoin concentration monitoring is essential, particularly when pharmacogenomic testing is inaccessible.
- Documenting known polymorphisms can prevent toxicity for drugs metabolized via similar pathways.
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