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Kinetics of primidone metabolism and excretion in children
Insights
Primidone metabolism in children shows rapid absorption and excretion, with significant conversion to phenylethylmalonamide (PEMA) and phenobarbital (Pb). Elimination rates influence drug levels, impacting treatment efficacy.
Area of Science:
- Pharmacokinetics
- Pediatric Pharmacology
- Drug Metabolism
Background:
- Primidone is an anticonvulsant medication used in treating epilepsy.
- Understanding its metabolism and excretion is crucial for optimizing pediatric dosing and therapeutic outcomes.
- Long-term dosing studies in children are essential to characterize drug behavior in this population.
Purpose of the Study:
- To investigate the metabolism and excretion of orally administered primidone in children aged 7 to 14 years.
- To determine the pharmacokinetic profile of primidone and its major metabolites.
- To identify factors influencing primidone and phenobarbital plasma concentrations.
Main Methods:
- 12 children (aged 7-14 years) with epilepsy received long-term primidone therapy.
- Plasma concentrations of primidone and its metabolites were monitored.
- Urinary excretion of primidone and its metabolites was quantified over 24 hours.
- Rate constants for metabolic conversion and excretion were calculated.
Main Results:
- Primidone plasma concentrations peaked at 4-6 hours with a half-life of 4.5-11 hours.
- Approximately 92% of the primidone dose was recovered in urine as primidone and its metabolites within 24 hours.
- Major metabolites included phenylethylmalonamide (PEMA, 45.2%) and phenobarbital (Pb, 4.9%).
- Excretion rate constant (KE) was a key determinant of phenobarbital steady-state plasma concentration.
Conclusions:
- Primidone is extensively metabolized in children, primarily to PEMA and phenobarbital.
- Individual differences in excretion rates significantly impact phenobarbital levels.
- These findings are vital for personalized primidone therapy in pediatric epilepsy management.
Abstract:
The metabolism and excretion of orally administered primidone was studied in 12 children, aged 7 to 14 yr during long-term dosing. Plasma concentrations of primidone (Pr) peaked at 4 to 6 hr and declined exponentially from 6 to 24 hr, with half-life (t1/2) values ranging from 4.5 to 11 hr. A mean of 92% (72% to 123%) of the administered dose was recovered within 24 hr from the urine as Pr and its metabolites. Of the total Pr daily dose, 42.3% (15.2% to 65.9%) was recovered as unchanged drug, 45.2% (16.3% to 65.3%) as phenylethylmalonamide (PEMA), and 4.9% (1.1% to 8.0%) as phenobarbital (Pb). The mean rate constant for conversion of Pr to PEMA (K1) was 0.0424 hr-1, for conversion of Pr to Pb (K2) was 0.0045 hr-1, and for excretion of unchanged Pr (K3) was 0.0389 hr-1. Of Pb excreted, 43% (13% to 100%) was unchanged, 15% (0% to 27%) was unconjugated p-OH Pb, 20% (0% to 44%) was conjugated p-OH Pb, and 22% (0% to 33%) was conjugated 3,4-OH Pb. KE appears to be important determinant of the steady-state plasma concentration of Pb, but interindividual differences in K2 have little influence on the overall rate constant for elimination of Pr.