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Phenobarbital metabolism in adults and in newborn infants
Insights
Newborns and adults excrete similar amounts of phenobarbital and its main metabolite. However, newborns show reduced excretion of the conjugated metabolite, suggesting an immature metabolic pathway.
Area of Science:
- Pharmacology
- Neonatal Medicine
- Drug Metabolism
Background:
- Phenobarbital is commonly used in neonates.
- Understanding its metabolism and excretion in newborns is crucial for safe and effective use.
- Limited data exists on the comparative pharmacokinetics of phenobarbital in newborns versus adults.
Purpose of the Study:
- To compare the excretion of phenobarbital and its metabolites in newborn infants and adult volunteers.
- To investigate the impact of age on phenobarbital pharmacokinetics, specifically focusing on conjugation pathways.
Main Methods:
- Single-dose phenobarbital administration to four newborn infants and two adult volunteers.
- Urine and plasma sampling over 8 days (newborns) and 2-4 weeks (adults).
- Quantification of unchanged phenobarbital, p-hydroxy phenobarbital, and conjugated p-hydroxy phenobarbital.
Main Results:
- Newborns and adults excreted similar proportions of unchanged phenobarbital (16-17%) and p-hydroxy phenobarbital (9-10%) relative to the administered dose.
- Newborns excreted significantly less conjugated p-hydroxy phenobarbital (5%) compared to adults (15%) within the first 8 days.
- Pharmacokinetic constants were calculated, revealing age-related differences in metabolic pathways.
Conclusions:
- Newborns exhibit a reduced capacity for drug conjugation compared to adults.
- The immature conjugation pathway in neonates may not lead to adverse effects with phenobarbital due to alternative excretion routes.
- The clinical significance of immature drug metabolism and excretion in neonates warrants consideration for drug therapy.
Abstract:
Two adult volunteers and four newborn infants were given a single dose of phenobarbital. The output in the urine o f unchanged phenobartital and of the two main metabolites p-hydroxy phenobarbital and conjugated p-hydroxy phenobarbital was followed during 8 days in the newborns and during 2 or 4 weeks in the adults. The plasma levels were also determined and some pharmacokinetic constants calculated. It was found that the newborn patients excreted unchanged phenobartibal and p-hydroxy phenobarbital in the same proportions relative to dose as did the adult volunteers, 2.e. 16--17% unchanged drug and 9--10% of the metabolite during the first 8 days after administration. On the other hand, there was a clear-cut age difference in output of conjugated metabolite where the newborns excreted only 5% of the given dose during the 8-day observation period. The corresponding value for the adults was 15%. It is concluded that a poor conjugating capacity in the newborn may not have any serious consequences with a drug like phenobarbital where major alternative routes of excretion exist (unchanged drug and unconjugated metabolite). The clinical significance of immature drug metabolites and of unchanged drug is taken into consideration.