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Oral phenytoin in infancy: dose requirement, absorption, and elimination
Insights
Infants require high oral phenytoin doses (18 mg/kg) due to impaired bioavailability and variable absorption. This study investigates phenytoin pharmacokinetics in pediatric populations, highlighting age-dependent absorption and food effects.
Area of Science:
- Pharmacology
- Pediatric Therapeutics
- Drug Metabolism
Background:
- Phenytoin is a widely used antiepileptic drug.
- Optimal therapeutic drug monitoring is crucial for efficacy and safety.
- Infant pharmacokinetics often differ significantly from adults.
Purpose of the Study:
- To determine the required oral phenytoin dosage in infants.
- To characterize the plasma half-life and bioavailability of phenytoin in infants.
- To investigate the influence of age and food on phenytoin absorption.
Main Methods:
- Dose-finding study to achieve target serum concentrations (8-25 µg/mL).
- Plasma half-life determination in infants (6 weeks to 12 months).
- Assessment of urinary metabolite excretion and oral phenytoin bioavailability across pediatric age groups, with and without food.
Main Results:
- High oral phenytoin doses (approx. 18 mg/kg) were needed for infants.
- Plasma half-life ranged from 7.9 to 24.9 hours (mean 12.8 ± 3.6 hours).
- Renal excretion accounted for only ~30% of daily phenytoin; age-dependent absorption and food effects on bioavailability were observed.
Conclusions:
- Infants exhibit impaired oral phenytoin bioavailability, necessitating higher doses.
- Age and food significantly impact phenytoin absorption kinetics in pediatric patients.
- Further research into the mechanisms of impaired bioavailability is warranted for optimized pediatric phenytoin therapy.
Abstract:
Oral phenytoin therapy in infants required unexpectedly high doses of about 18 mg/kg body weight to achieve and to maintain serum concentrations between 8 and 25 micrograms/ml. Plasma half-life determined in 12 infants aged 6 weeks to 12 months ranged between 7.9 to 24.9 hours (mean 12.8 +/- 3.6 hours). Measurement of phenytoin metabolite excretion in urine during steady state revealed that only about 30% of the daily given phenytoin is eliminated through the kidneys. Studying the bioavailability of phenytoin in different age groups (infants, children, and adolescents) before and after ingestion of age-appropriate food showed an age-dependent absorption rate and extent, and an influence of the food upon the absorption pattern could be demonstrated. Mechanisms for this phenomenon of an impaired bioavailability of phenytoin in infancy are discussed.