New pharmacokinetic methods: I. Estimation of mean serum concentration from trough serum concentration

Phenytoin and phenobarbital stable isotope studies in neonates

A pharmacokinetic study of phenytoin and phenobarbital with nonradioactive isotopes was performed in nine neonates in an intensive care unit setting. A single-pulse dose of either labeled phenobarbital (1,3-(15)N, 2-(13)C) or labeled phenytoin (2-(13)C, 1, 3-(15)N) was administered to neonates who manifested gestation between 25 and 40 weeks and were receiving maintenance medication. Blood samples were collected at fixed intervals, and with a computerized gas chromatography mass spectrometry system, plasma concentrations of the labeled and unlabeled drug in relation to time administered were obtained. According to the calculations obtained from labeled analogue, several kinetic characteristics related to drug absorption, clearance, and elimination were determined. The use of a nonradioactive labeled isotope overcomes the limitations of conventional pharmacokinetic methodology and can be specifically useful in neonates and infants in whom volumes of distribution are rapidly changing and steady state is not achieved.

The influence of polytherapy on the relationships between serum carbamazepine and its metabolites in epileptic children

The influence of polytherapy on the relationships between the age, weight, carbamazepine (CBZ) dose, total clearance, and intrinsic clearance, with concentrations, concentration ratios, and level/dose ratios of CBZ, carbamazepine-10,11-epoxide (CBZ-E) and trans-10,11-dihydroxy-10,11-dihydro-carbamazepine (CBZ-H) are investigated. Three groups of patients with CBZ monotherapy, or receiving CBZ polytherapy by taking CBZ and valproic acid (VPA) or CBZ plus other antiepileptic drugs (AEDs) were studied. The significant correlations between serum CBZ concentrations and CBZ dose in patients taking CBZ alone were no longer significant in patients with polytherapy, and the positive associations between serum CBZ-E concentrations and CBZ dose were lost in patients with CBZ + VPA. Only the concentrations of CBZ-H had significant correlations with CBZ dose in all three groups of patients. Results from this relationship study indicate a heteroinduction effect of other AEDs on CBZ metabolism, and a relatively weak influence on CBZ-E elimination. Data also suggest that there is a block in the biotransformation from CBZ-E to CBZ-H in patients taking CBZ + VPA, presumably caused by the inhibition effect of VPA on epoxide hydrolase. Therapeutic drug monitoring of CBZ will benefit from the knowledge obtained from the relationship study.