Phenytoin Dosage Predictions in Paediatric Patients

SummaryPhenytoin dosing in paediatric patients is complicated both by alterations in patient requirements due to growth and maturation changes and by the capacity-limited characteristics of Phenytoin metabolism. This study examines 2 pharmacokinetic methods to adjust Phenytoin dosage based on a single dosing-rate/steady-state serum Phenytoin concentration pair. A Bayesian forecaster and a fixed parameter [rate of metabolism (Vmax)] method were examined with previously published sets of a priori parameter estimates. The fixed Vmaxmethod was utilised with the parameter derived from native Japanese (method 1), US Caucasian (method 2) and European (method 3) patients. The Bayesian forecaster used a priori parameter estimates obtained from native Japanese (method 4) and European (method 5) patients. Each method was examined retrospectively in 34 paediatric patients with a total of 48 predictions possible. Measures of absolute predictability, bias (mean error, % dose) and precision (root mean squared error, % dose), were −3.58/12.2, −1.51/12.2, 4.06/9.96, −4.38/13.2, and −3.10/11.5, for methods 1, 2, 3, 4 and 5, respectively. There was no significant difference among the 5 methods. However, the Bayesian algorithm tended to be more robust over a broad range of situations, providing predictions in all cases. The fixed Vmaxmethods could not provide predictions in every case. Finally, all methods had a significant number of overpredictions of dosage. Poorer results were observed when prediction of steady-state serum concentrations were performed, partly due to the retrospective nature of the study. We conclude that close monitoring of patients, regardless of the method chosen to adjust dosage, is recommended.

[1]  S L Beal,et al.  Evaluation of a Bayesian regression-analysis computer program for predicting phenytoin concentration. , 1986, Clinical pharmacy.

[2]  E. Perucca,et al.  Is phenytoin metabolism dose‐dependent by enzyme saturation or by feedback inhibition? , 1978, Clinical pharmacology and therapeutics.

[3]  James H. Torrie,et al.  Principles and procedures of statistics: a biometrical approach (2nd ed) , 1980 .

[4]  J. Wagner,et al.  Explanation of dose-dependent decline of diphenylhydantoin plasma levels by fitting to the integrated form of the Michaelis-Menten equation. , 1972, Research communications in chemical pathology and pharmacology.

[5]  L. Skovsted,et al.  Disposition of Antipyrine and Phenytoin Correlated with Age and Liver Volume in Man , 1981, Clinical pharmacokinetics.

[6]  L. Lund Anticonvulsant effect of diphenylhydantoin relative to plasma levels. A prospective three-year study in ambulant patients with generalized epileptic seizures. , 1974, Archives of neurology.

[7]  T Ishizaki,et al.  Michaelis-Menten pharmacokinetics of diphenylhydantoin and application in the pediatric age patient. , 1980, The Journal of pediatrics.

[8]  K. Arnold,et al.  The rate of decline of diphenylhydantoin in human plasma , 1970, Clinical pharmacology and therapeutics.

[9]  Peter W. Mullen,et al.  Optimal phenytoin therapy: A new technique for individualizing dosage , 1978, Clinical pharmacology and therapy.

[10]  A. Richens,et al.  Predicting Phenytoin Dose—A Revised Nomogram , 1979, Therapeutic drug monitoring.

[11]  A. J. Robins,et al.  Phenytoin dose adjustment in epileptic patients. , 1974, British journal of clinical pharmacology.

[12]  P. Blain,et al.  Steady-state Pharmacokinetics of Phenytoin from Routinely Collected Patient Data , 1983, Clinical pharmacokinetics.

[13]  T. Ludden,et al.  Individualization of phenytoin dosage regimens , 1977, Clinical pharmacology and therapeutics.

[14]  L B Sheiner,et al.  Bayesian individualization of pharmacokinetics: simple implementation and comparison with non-Bayesian methods. , 1982, Journal of pharmaceutical sciences.

[15]  J W Taylor,et al.  Predicting Phenytoin Dosages Using Bayesian Feedback: A Comparison with Other Methods , 1983, Therapeutic drug monitoring.

[16]  L A Bauer,et al.  Phenytoin Michaelis-Menten Pharmacokinetics in Caucasian Paediatric Patients , 1983, Clinical pharmacokinetics.

[17]  William J. Jusko,et al.  Berichten , 2005, Pharmaceutisch weekblad.

[18]  W. Dodson Nonlinear kinetics of phenytoin in children , 1982, Neurology.

[19]  A. Richens,et al.  SERUM-PHENYTOIN LEVELS IN MANAGEMENT OF EPILEPSY , 1975, The Lancet.