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Clinical implications of serum protein binding in epileptic children during sodium valproate maintenance therapy
Insights
This study shows significant variability in total and unbound valproic acid levels in epileptic children. Monitoring both total and unbound valproic acid is recommended for effective epilepsy management.
Area of Science:
- Pharmacology
- Clinical Chemistry
- Pediatric Neurology
Background:
- Valproic acid is a common antiepileptic drug.
- Understanding its serum binding is crucial for therapeutic drug monitoring.
- Variability in protein binding can affect drug efficacy and safety.
Purpose of the Study:
- To investigate steady-state serum levels of total and unbound valproic acid in epileptic children.
- To compare in vivo and in vitro valproic acid binding parameters.
- To develop a method for evaluating variations in valproic acid serum binding.
Main Methods:
- Studied serum levels of total and unbound valproic acid in epileptic children.
- Analyzed valproic acid binding parameters in vivo and in vitro.
- Derived an equation to estimate unbound valproic acid concentration and fraction.
- Evaluated binding variations using the ratio of observed to estimated unbound fraction (fp/fp').
Main Results:
- Observed considerable variation in total and unbound valproic acid concentrations and unbound fractions.
- Found that the unbound fraction of valproic acid is concentration-dependent within the therapeutic range.
- In vivo and in vitro association constants were not significantly different.
- A ratio of fp/fp' > 1.76 suggests a need for further evaluation of binding variations.
Conclusions:
- Determination of both total and unbound valproic acid levels is preferable for clinical monitoring.
- Understanding valproic acid's unbound fraction aids in effective management of epileptic patients.
- Significant deviations in binding warrant investigation for pharmacokinetic alterations.
Abstract:
Steady-state serum levels of total and unbound valproic acid as well as unbound fraction in epileptic children were studied in a clinical setting. Valproic acid binding parameters were analyzed and compared with in vitro findings. Daily dose of sodium valproate ranging from 29 to 73 mg/kg/day were administered per os. Considerable variation in total and unbound concentrations and unbound fractions within and between subjects was observed. In subjects evaluated in this study, serum level of total and unbound valproic acid ranged from 279 to 1,196 mumol/L and from 37 to 410 mumol/L, respectively. The unbound fraction ranged from 10.32 to 48.39%. In vivo binding parameters obtained from clinical material were as follows: association constant, Ka = 4.984 L/mmol; total binding sites, NP = 1.451 mmol/L, where P is the molar concentration of albumin; number of binding sites per molecule of albumin, N = 2.48. Using spiked sera, binding parameters of Ka = 8.032 L/mmol, NP = 1.262 mmol/L, and N = 1.86 were found in the in vitro study. The association constant obtained from in vivo and in vitro studies were not significantly different (p greater than 0.05) from each other. The unbound fraction of valproic acid was concentration dependent even within the therapeutic range. An equation for estimating unbound concentration (Cf') or unbound fraction (fp') from total concentration (Ct) of valproic acid is derived. The ratio of observed unbound fraction to the estimated unbound fraction (fp/fp') was used to evaluate the variation in valproate serum binding of that clinical sample. Nine samples from hospitalized patients whose medication and diet were closely supervised showed an fp/fp' ratio very close to 1 (mean +/- SD 1.04 +/- 0.24). It is suggested that a clinical sample showing a value of fp/fp' greater than 1.76 (mean + 3 SD) should be evaluated for the cause of the decrease in serum binding and for the associated pharmacokinetic alterations. Therefore, in clinical monitoring of valproate, determination of both total and unbound drug levels was preferable to determination of either one alone. Furthermore, an understanding of the unbound fraction of valproic acid would significantly contribute to the effective management of epileptic patients.