First-dose and steady-state pharmacokinetics of valproic acid in children with seizures

Phase 1 study of valproic acid in pediatric patients with refractory solid or CNS tumors: a children's oncology group report

PURPOSE

The primary purpose of this trial was to define and describe the toxicities of oral valproic acid (VPA) at doses required to maintain trough concentrations of 100 to 150 mcg/mL or 150 to 200 mcg/mL in children with refractory solid or central nervous system (CNS) tumors. Secondary objectives included assessment of free and total VPA pharmacokinetics (PKs) and histone acetylation in peripheral blood mononuclear cells (PBMC) at steady state.

PATIENTS AND METHODS

Oral VPA, initially administered twice daily and subsequently three times daily, was continued without interruption to maintain trough concentrations of 100 to 150 mcg/mL. First-dose and steady-state PKs were studied. Histone H3 and H4 acetylation in PBMCs was evaluated using an ELISA technique.

RESULTS

Twenty-six children, sixteen of whom were evaluable for toxicity, were enrolled. Dose-limiting somnolence and intratumoral hemorrhage were associated with VPA troughs of 100 to 150 mcg/mL. Therefore, the final cohort of six children received VPA to maintain troughs of 75 to 100 mcg/mL and did not experience any dose-limiting toxicity. First-dose and steady-state VPA PK parameters were similar to values previously reported in children with seizures. Increased PBMC histone acetylation was documented in 50% of patients studied. One confirmed partial response (glioblastoma multiforme) and one minor response (brainstem glioma) were observed.

CONCLUSIONS

VPA administered three times daily to maintain trough concentrations of 75 to 100 mcg/mL was well tolerated in children with refractory solid or CNS tumors. Histone hyperacetylation in PBMCs was observed in half of the patients at steady state. Future trials combining VPA with chemotherapy and/or radiation therapy should be considered, especially for CNS tumors.

Valproate as a mainstay of therapy for pediatric epilepsy

This article reviews relevant pharmacologic and clinical information gathered for valproate since it was introduced into clinical practice 37 years ago and the application of this information for the treatment of childhood epilepsy. Valproate is available for oral and parenteral use. Oral forms are almost completely bioavailable but the rate of absorption varies between formulations. The Chrono tablet formulation has not been adapted for children aged <6 years, in whom the oral solution or syrup, requiring two or three daily administrations, has been used until recently. A new formulation specifically adapted for children, Chronosphere, administrated once or twice daily, is a modified-release formulation of valproate that minimizes fluctuations in serum drug concentrations during a dosage interval. Plasma protein binding is 80-94% and tends to decrease with increasing drug concentration. Valproate elimination is markedly decreased in newborns compared with older children and adults. Elimination by glucuronidation only becomes fully effective by the age of 3-4 years. In children aged 2-10 years receiving valproate, plasma clearances are 50% higher than those in adults. Over the age of 10 years, pharmacokinetic parameters approximate those of adults. Valproate can increase plasma concentrations of concomitant drugs, such as phenobarbital and lamotrigine, by inhibiting their metabolism. As a result of its broad spectrum of efficacy in a wide range of seizure types and epilepsy syndromes, valproate is a drug of choice for children with newly diagnosed epilepsy (focal or generalized), idiopathic generalized epilepsy, epilepsies with prominent myoclonic seizures or with multiple seizure types, and photosensitive epilepsies. In the group of cognitive epilepsies, in which severe spike and wave discharges are accompanied by cognitive deterioration, valproate, ethosuximide, or both should be tested before using corticosteroids. In comparative trials with carbamazepine, phenytoin, and phenobarbital in focal epilepsy and with ethosuximide in absence epilepsy, valproate was as effective and showed a favorable tolerability profile, with minimal adverse cognitive and CNS effects. The low potential for paradoxical seizure aggravation and the long-term efficacy of the drug are additional important factors that contribute to its excellent profile. Intravenous valproate may be effective for the treatment of convulsive and non-convulsive status epilepticus that is refractory to conventional drugs. In infants, potential benefits should be carefully weighed against the risk of liver toxicity. Gastrointestinal intolerance is a relatively frequent, dose-related adverse effect of the drug in children. Bodyweight increase and tremor may be observed in older children and adolescents. Despite the challenge of newer drugs, valproate remains a gold standard antiepileptic drug for the treatment of children.

Valproate population pharmacokinetics in children

OBJECTIVE

A population analysis of the kinetics of valproic acid (VPA) in children with epilepsy was performed in order to characterize the covariates which influence VPA clearance (CL).

METHODS

A total of 770 steady-state serum concentration samples was analysed. These were collected during VPA therapy from 255 children, aged 0.1-14 years and weighting 4-74 kg. Age, total body weight (TBW), VPA daily dose, sex and comedication with carbamazepine (CBZ) were considered as covariates. Population analysis was made with NONMEM program, assuming a one-compartment model, fixing the VPA absorption rate, bioavailability and distribution volume at values found in the literature. The results of the population pharmacokinetics analysis were validated in a group of 45 epileptic patients.

RESULTS

The final regression model for VPA clearance, that included TBW (kg), daily dose (mg/kg) and CBZ comedication as covariates with a significant influence on this parameter, was as follows: CL (L/h) = 0.012 TBW0.715 DOSE0.306(1.359 CBZ). The coefficient of variation for interpatient variability in CL was 21.4% and the residual variability estimated was 23.9% for a concentration of 65 mg/l. In order to estimate the predictive performance of the selected final model, predictions of the VPA serum concentrations were calculated and compared with VPA measured concentrations in the validation group. This assessment revealed an important improvement in the predictive performance of VPA concentrations in comparison with the basic model that did not include any covariates (root squared mean error: 19.50 vs. 39.73 mg/l).

CONCLUSION

A population pharmacokinetic model is proposed to estimate the individual CL for paediatric patients receiving VPA in terms of patient's dose, weight and concomitant CBZ, in order to establish a priori dosage regimens.

A model for estimating individualized valproate clearance values in children

To evaluate the population pharmacokinetics of valproic acid in children, 97 steady-state serum valproate concentration measurements were gathered during normal, routine, outpatient care of 52 children with epilepsy (1.2-16 years of age). Levels were obtained from patients receiving valproate monotherapy (49%) or valproate with concomitant carbamazepine (32%), phenytoin (11%), or phenobarbitone (8%). A one-compartment model was used to fit the data with the Nonlinear Mixed Effects Model (NONMEM) computer program. The final model for clearance (L/hr) was CL = [EXP (0.022WT-1.38)] X M, where EXP = the base of the natural logarithm, WT = patient weight (kg) and M = a scaling factor for concomitant medication with a value of 1 for patients on valproate monotherapy and 1.61 for those receiving concomitant carbamazepine. Although phenytoin and phenobarbitone also were expected to increase valproate clearance, this could not be demonstrated, possibly because of the small number of samples taken from patients receiving these agents. Weight-adjusted values of valproate clearance decreased with increasing age. The actual mean value of 0.021 L/hr/kg for children taking monotherapy was slightly higher than values shown in most previously published reports, whereas the mean value of 0.028 L/hr/kg for patients taking concomitant carbamazepine was similar to those found previously in children taking other antiepileptic drugs.

Effect of unbound clearance on binding parameters of valproic acid to serum proteins

Nine healthy subjects received 400 mg sodium valproate orally in the fasting state. Binding parameters of valproic acid to serum proteins were determined by Scatchard analysis for individual series of valproic acid data. Total and unbound (intrinsic) clearances (CLt and CLu) were calculated by dividing the dose by the appropriate area under the serum drug concentration-time curve. Unbound clearance correlated positively with the product of association constant (Ka) and concentration of free protein ((P)) (P < .05). Conversely, no significant correlation was found between CLt and binding parameters. The average unbound concentration correlated negatively with both CLu and ka(P) values. The result indicates an effect of CLu on Ka(P) value of valproic acid.

Circadian changes of valproate kinetics depending on meal condition in humans

The objective of this study was to examine the effect of meal condition on circadian changes in valproic acid (VPA) kinetics. Two experiments were performed in 16 healthy men that were synchronized with diurnal activity and nocturnal rest as their routine life. In both experiments, four 200-mg tablets of VPA were given orally on two occasions in the morning (8:30 AM) or in the evening (8:30 PM). In each study, a randomized, single-dose, two-way crossover design was used, and 2 weeks elapsed between morning and evening trials. In experiment 1, eight subjects took a light meal as breakfast between 8:00 and 8:15 AM and a heavy meal as dinner between 6:00 and 6:30 PM to fit the subject's usual food amount. The mean peak plasma concentration (Cmax) was significantly higher (P less than .01), the time to peak concentration (tmax) was shorter (P less than .05), and the absorption rate (Ka) was larger (P less than .05) after the morning dose than after the evening dose. In experiment 2, the size and contents of meal in breakfast and dinner were prepared in the same manner as the standard breakfast for the subjects. Namely, eight subjects took the same light meal between 8:00 and 8:15 AM in the morning and between 8:00 and 8:15 PM in the evening. There was no significant circadian change in VPA kinetics under this same meal condition. These results suggest that the differences of meal condition between morning and evening in our daily life play a major role in the mechanism underlying the circadian changes of VPA absorption in man.

Effect of sodium valproate on psychomotor performance in children as a function of dose, fluctuations in concentration, and diagnosis

Forty-six children receiving sodium valproate monotherapy, and with well-controlled seizures, were tested three times, at weekly intervals in the morning, on an extensive battery of psychomotor tests. The first session was a practice session to minimize subsequent practice effects. On one of the remaining 2 test days, morning medication was delayed until after testing (low concentration condition), whereas sodium valproate was given before testing on the alternate session (high concentration day). The data were analyzed for the effects of diagnosis (partial vs. generalized epilepsy), dose (above or below 20 mg/kg/day), and time of medication (low and high drug concentration days). Four variables discriminated between the diagnostic groups, all favoring children with generalized epilepsy. Another four measures were significantly associated with dose, with those on lower doses performing at a superior level. Time of medication had virtually no effect on performance, however. The findings were related to previous diagnostic and dosage research. Although a deterioration in performance at higher doses appears to be consistent with other dosage research, the absence of an effect due to time of medication suggests that performance does not fluctuate between drug administrations.

A multivariable analysis of factors governing the steady-state pharmacokinetics of valproic acid in 52 young epileptics

A number of factors have been implicated in the interpatient variability of valproic acid (VPA) pharmacokinetics. These include patient age, concurrent anticonvulsant therapy, and dosage of VPA. In the clinical setting, it is important to determine which of these variables exert a major effect on the observed differences in VPA disposition. Accordingly, we analyzed the data from 52 young epileptic patients using multivariate statistical methods. Concurrent anticonvulsant therapy was the major determinant of VPA clearance in this patient population. The half-life of VPA was significantly related to age, but volume of distribution and clearance were not. The dosage of VPA affected the volume of distribution of VPA but not the half-life or clearance of this drug. These results provide a more rational understanding of VPA pharmacokinetics in the clinical setting and have implications for the monitoring and manipulation of VPA therapy in the epileptic population.

An update on sodium valproate

Sodium valproate has been in clinical use for the treatment of epilepsy in Great Britain since 1973 and in the United States since 1978. It is chemically quite different from the existing antiepileptic drugs. Although most authorities concentrate on its modification of GABAergic inhibitory transmission in the central nervous system, its mechanism of action remains obscure. It has been shown to be an effective antiepileptic drug in a wide variety of seizure types, but clinically, its major use to date has been in generalized seizures. It is particularly effective in photosensitive epilepsy and myoclonus. Most adverse reactions to sodium valproate are mild and reversible, but with increasing experience, the drug's rare, idiosyncratic, adverse effects are becoming apparent, particularly hepatotoxicity and teratogenicity. The role of therapeutic drug monitoring in the management of patients taking sodium valproate is controversial.

Comparison of steady-state pharmacokinetics of valproic acid in children between monotherapy and multiple antiepileptic drug treatment

Steady-state pharmacokinetics of valproic acid (VPA) with or without other antiepileptic drug (AED) treatment were studied in 37 children. Children (N = 16) receiving multiple AED therapy had a higher clearance (23.5 vs 13.0 ml/hr/kg, P less than 0.001), larger volume of distribution (0.30 vs 0.22 L/kg, P less than 0.01), and shorter half-life (9.4 vs 12.3 hours, P less than 0.01) than did those (n = 21) receiving VPA only. Inverse correlations of age with clearance (R = -0.559, P less than 0.01) and apparent volume of distribution of VPA (r = -0.490, P less than 0.05) were observed in children receiving monotherapy. In determining the dose and dosing interval of VPA, consider a possible alteration in the pharmacokinetics relating to age and other concurrent AED therapy.

Free valproic acid: steady-state pharmacokinetics in patients with intractable epilepsy

Free and total valproic acid (VPA) pharmacokinetic evaluation was carried out at steady state in six young epileptics who were also receiving other anticonvulsants. Subjects received their usual morning dose of VPA after an overnight fast. Blood samples for free and total VPA were taken prior to the dose and frequently thereafter for 12 hours. The calculated pharmacokinetic parameters for total VPA and free VPA were: half-lives of 7.5 +/- 1.6 hours and 5.0 +/- 1.5 hours, volumes of distribution of 0.189 +/- 0.038 l/kg and 1.51 +/- 0.98 l/kg, and clearances of 0.30 +/- 0.06 and 3.6 +/- 2.0 ml/min/kg., respectively. There was a strong correlation between percent free VPA and total VPA (r = 0.81) but marked inter- and intra-subject variations were seen. Studies attempting to correlate VPA levels to clinical response must take such data into account.