Phenytoin is a relatively old anticonvulsant, but it has been commonly prescribed for more than half a century. The variability of phenytoin pharmacokinetic characteristics presents a challenge in therapeutic drug monitoring; hence, in the past twenty years its pharmacokinetic characteristics have been studied extensively. Up to now the studies were done with either healthy individuals or patients with normal liver functions. In this study a multifactorial scale of liver function, Pugh-Modified CTC (Child-Turcotte Criteria), has been incorporated to develop and evaluate a population pharmacokinetic model for phenytoin to be used in patients with liver dysfunction. Nonlinear Mixed Effects Model (NONMEM), a regression computer program, was utilized to develop the population pharmacokinetic model on the data of this study. The predictive performance of this model was evaluated by means of bootstrapping of the prediction error (PE) with the improved prediction-error (PE imp ) serving as an estimate of internal validity. The developed and validated final population pharmacokinetic model for phenytoin in patients with liver dysfunction is presented as follows: [special characters omitted] where Vmax is the maximum metabolic rate (mg/h); &thetas; 1 , the intercept for Vmax, is 7.41 mg/h; WT is the body weight (Kg); LS indicates one of three liver statuses: normal (CTC ≤ 6), mild dysfunction (CTC scores of 7–9), and moderate dysfunction (CTC scores of 10–12); Vd, the apparent volume of distribution (L), is 184 L; &thetas; WT is 0.126 and &thetas; LS is 2.14 for moderate liver-dysfunction. The maximum metabolic rate increased in patients with liver dysfunction, and there was weak statistical evidence that Vmax might increase in patients with chronic alcohol abuse. Based on the aforementioned longitudinal (population) pharmacokinetic model, a dosing method was also developed. By utilizing the dosing method, it may be possible to improve phenytoin dosage regimens, initial doses, and Bayesian estimates of pharmacokinetic parameters. Improved initial doses and more accurate estimates of pharmacokinetic parameters may lead to fewer required measured phenytoin concentrations and fewer dose changes. A decrease in the number of dose changes should result in less time expended in the writing and processing prescriptions and orders, and there may also be fewer wasted doses. Additionally, the improved initial doses should result in concentrations more frequently in the therapeutic window; thereby, resulting in less toxicity, greater efficacy, and improved patient compliance. All of these effects should decrease the cost of therapy in patients receiving phenytoin, a factor which is an important consideration in this age of cost containment and managed care.
| Identifer | oai:union.ndltd.org:pacific.edu/oai:scholarlycommons.pacific.edu:uop_etds-3705 |
| Date | 01 January 1999 |
| Creators | Hui, Tina Hsiao-Tin |
| Publisher | Scholarly Commons |
| Source Sets | University of the Pacific |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | University of the Pacific Theses and Dissertations |
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