Pharmacokinetics of sodium valproate in epileptic patients: prediction of maintenance dosage by single-dose study

A population pharmacokinetic model taking into account protein binding for the sustained-release granule formulation of valproic acid in children with epilepsy

PURPOSE

The objective of this work was to develop a population pharmacokinetic model for a prolonged-release granule formulation of valproic acid (VPA) in children with epilepsy and to determine the doses providing a VPA trough concentration (C_trough) within the target range (50-100 mg/L).

METHODS

Ninety-eight children (1-17.6 years, 325 plasma samples) were included in the study. The model was built with NONMEM 7.3. The probability to obtain C_trough between 50 and 100 mg/L was determined by the Monte Carlo simulations for doses of 20, 30, 40, and 60 mg/kg/day and body weights between 10 and 70 kg.

RESULTS

A one compartment model, with first-order absorption and flip-flop parameterization and linear elimination, but taking protein binding into account, was used to describe the data. Typical values for unbound VPA clearance and distribution volume were 6.24 L/h/70 kg and 130 L/h/70 kg respectively. Both parameters were related to body weight via allometric models. The highest probability to obtain a C_trough within the target range for 10-kg children was obtained with a 40 mg/kg daily dose, whereas daily doses of 30 and 20 mg/kg were found appropriate for 20 to 30- and ≥ 40-kg children respectively. However, for these same doses, the exposure to unbound VPA could differ by 40%.

CONCLUSIONS

If the present study supports the current dose recommendations of 20-30 mg/kg/day, except for children under 20 kg, who may need higher doses, it also highlights the need for further research on the pharmacokinetics/pharmacodynamic profile of unbound VPA.

Investigation of phenobarbital-carbamazepine-valproic acid interactions using population pharmacokinetic analysis for optimisation of antiepileptic drug therapy: an overview

Antiepileptic drugs are associated with a wide range of drug interactions. Although monotherapy with antiepileptic drugs is preferred, patients with multiple seizure types or refractory disease generally require various combinations of antiepileptic drugs. Pharmacokinetic interactions between antiepileptic drugs represent a major complication of epilepsy treatment with polytherapy. It is important to be aware of possible interactions, so as to anticipate clinical effects and to reduce the risk of both toxicity and seizures worsening when a drug is added to, or withdrawn from, the patient's antiepileptic drug regimen. This report is an overview of the drug-drug interactions between antiepileptic drugs (phenobarbital<-->carbamazepine, carbamazepine<-->valproic acid and phenobarbital<-->valproic acid) by population pharmacokinetic analysis.

Population estimation of valproic acid clearance in adult patients using routine clinical pharmacokinetic data

The aim of the present study was to estimate valproic acid (VPA) clearance values for adult patients with epilepsy, using serum concentrations gathered during their routine clinical care. Retrospective steady state serum concentrations data (n=534) collected from 208 adult patients receiving VPA were studied. Data were analysed according to a one-compartment model using the NONMEM program. The influence of VPA daily dose (Dose), gender, age, total body weight (TBW), and comedication with carbamazepine (CBZ), phenytoin (PHT) and phenobarbital (PB) were investigated. The results of the population pharmacokinetics analysis were validated in a group of 30 epileptic patients. The final regression model for VPA clearance (Cl) was: $¿rm Cl¿left (¿rm L/h ¿right )=0¿rm. 004¿times TBW¿times Dose ¿0.304¿¿rm ¿times 1.363¿,¿rm CBZ¿times 1. 541¿,¿rm PHT¿times 1.397¿,¿rm PB.$ The inter-individual variability in VPA clearance, described by a proportional error model, had a variation coefficient (CV) of 23.4% and the residual variability, described using an additive model, was 11.4 mg/L. These results show that VPA clearance increased linearly with TBW, but increases nonlinearly with increasing VPA daily dose. Concomitant administration of CBZ, PHT and PB led to a significant increase in VPA clearance. The model predictions in the validation group were found to have satisfactory precision and bias. In conclusion, inter-individual variability in VPA clearance can be partly explained by TBW, daily dose and bitherapy with CBZ, DPH or PB. Inclusion of these factors allows this variability to be reduced by 37.23% which may be very useful for clinicians when establishing the initial VPA dosage regimen. However, the magnitude of inter-individual plus residual variabilities, remaining in the final model, render these dosage predictions imprecise and justify the need for VPA serum level monitoring in order to individualize dosage regimens more accurately.

Population-based investigations of drug relative clearance using nonlinear mixed-effect modelling from information generated during the routine clinical care of patients

Interpatient variability in drug disposition and response is a therapeutic premise, and thus evaluation and management of such variability are the basis for individualized pharmacotherapy. If the mathematical approach to determining drug doses were accurate and practical, the use of calculated doses could reduce the potential for toxicity and decrease the need for repetitious drug assays. The major strength of the population pharmacokinetics approach is that useful information can be extracted from sparse data collected during routine clinical care. Population pharmacokinetics can be defined as the study of the variability in serum drug concentrations between individuals when standard dosage regimens are administered. An approach to population pharmacokinetic data analysis has been implemented in the Nonlinear Mixed Effects Model (NONMEM) computer program. This report shows the feasibility of using a simple pharmacokinetic screen approach to estimate the population mean relative drug clearance and detecting drug-drug interaction by use of NONMEM. In clinical application of multiple trough screen or multiple peak screen, the variability of drug relative clearance within the population is assessed and a mathematical relationship between drug relative clearance and individual patient characteristics, such as age, body weight, gender, disease state or drug interaction with concomitant drug is derived. In this report I describe this approach and its application using several examples previously reported by us and others.

Population-based investigation of valproic acid relative clearance using nonlinear mixed effects modeling: influence of drug-drug interaction and patient characteristics

Nonlinear mixed effects modeling (NONMEM) was used to estimate the effects of drug-drug interaction on valproic acid relative clearance values using 792 serum levels gathered from 400 pediatric and adult patients with epilepsy (age range, 0.3-54.8 years) during their clinical routine care. Patients received valproic acid as monopharmacy or in combination with either the antiepileptic drugs, phenobarbital, or carbamazepine. The final model describing valproic acid relative clearance was CL (mL/hr/kg) = 15.6.TBW (kg)-0.252.DOSE (mg/kg/day)0.183.0.898GEN.COPB.COCBZ, where COPB equals 1.10 if the patient is treated with phenobarbital, a value of unity otherwise, and COCBZ equals 0.769.DOSE (mg/kg/day)0.179 if the patient is treated with carbamazepine, a value of unity otherwise. Valproic acid relative clearance was highest in the very young and decreased in a weight-related fashion in children, with minimal changes observed in adults. This pattern was consistent whether valproic acid was administered alone or coadministered with phenobarbital or carbamazepine. When valproic acid was coadministered with phenobarbital or carbamazepine, valproic acid relative clearance increased as compared with that in monopharmacy. Its magnitude in the presence of carbamazepine increased in a valproic acid daily dose-related fashion. Concomitant administration of phenobarbital and valproic acid resulted in a 10% increase on valproic acid relative clearance. The clearance in female patients was approximately 10% less than that in male patients.

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.

Intensive follow-up monitoring in patients with once daily evening administration of sodium valproate

For 18 previously untreated patients with absence, myoclonic, or grand mal seizures--or combinations--results of clinical and electroencephalographical monitoring are reported. Sodium valproate was given once daily in the evening. Monitoring included repeated 24-48 h EEG recordings and drug blood level measurements. Results indicated the evening monodose to be an adequate therapeutic schedule for a considerable number of patients. Apart from the aspect of simplification, a further important aim is to individually minimize the drug dose. An average of 15.6 mg/kg sodium valproate (range, 10.0-25.5 mg/kg) per day was administered. In some cases the EEG discharge activity continued to be lowered even after the drug blood level had reached steady state. With medication, frequency and total duration of paroxysms were significantly lowered (by more than 90%) in over 80% of the patients, whereas the mean duration of paroxysmal activity did not change uniformly. Before treatment, short paroxysms (1-5 s) were seen together with longer ones in 11 patients. During treatment either all paroxysms disappeared or, in cases of remaining activity, most discharges were short and not accompanied by seizure manifestations. Blood level profiles over 24 h showed maximal values between midnight and 2 a.m. The minimal values (about half of the maximum) were found between 10 p.m. and midnight. The 8 a.m. value was 70-80% of the maximum. Only three patients complained of slight side effects (temporary drowsiness, loss of hair). Because of the simplified handling, the relatively low dose per day, and the few side effects, it seems possible that for primary generalized epilepsies once daily evening administration of sodium valproate is appropriate without diminishing the antiepileptic effect.

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

The serum concentration-time curve of valproic acid was followed in 25 children after single oral doses of the drug and at steady-state. Total body clearance (CL), half-life (t 1/2), and apparent volume of distribution (Vd) were calculated from the terminal portion of the curve and from the area under the serum concentration-time curve (AUC). The CL and Vd were significantly greater at steady-state (0.42 +/- 0.20 ml/min/kg and 0.231 +/- 0.067 L/kg, respectively) than after a single dose (0.32 +/- 0.13 ml/min/kg and 0.191 +/- 0.055 L/kg, respectively). This difference was most pronounced in patients with valproic acid dosage increases in excess of 20% and no change in their concurrent anticonvulsant therapy between the single-dose and steady-state study periods. The t 1/2 was not significantly different between the 2 study periods. There was a significant correlation between age and both CL and Vd after single doses and at steady-state. The t 1/2 did not appear to be age related. These results suggest that the adequacy of the dosage regimen must be determined during maintenance therapy rather than extrapolated from data obtained after a single dose. Re-evaluation of therapy as the child grows older may also be necessary in view of the age-related differences in valproic acid pharmacokinetics which this study has demonstrated.

The effects of peritoneal dialysis on the single dose and steady state pharmacokinetics of valproic acid in a uremic epileptic child

The pharmacokinetics of valproic acid (VPA) have been studied during peritoneal dialysis in a uremic male epileptic child following a single 500 mg dose and after multiple doses over 5 months (700 mg daily) of valproic acid as the syrup. Serum level decline was biphasic in both instances with a terminal half-life of 27.2 after the single dose and 10.2 h at steady-state. Total serum clearance was 0.0236 l/h/kg after the single dose and increased to 0.0408 l/h/kg after 5 months. Free (intrinsic) serum clearances were 0.1489 and 0.1518 l/h/kg and serum free fractions were 0.224 and 0.272 respectively for the single dose and steady-state studies. Peritoneal dialysis for periods of 12 or 24 h removed an average of 4.5% of the VPA dose.

In vivo uptake of valproic acid into brain

The pharmacokinetics of valproic acid (VPA) penetration into the central nervous system of cats were studied. VPA levels in cortical gray matter and plasma were measured at timed intervals after rapid intravenous drug infusion. Brain uptake of the drug was maximal at 1 min postinfusion and decayed rapidly with a mean elimination half-life of 41 min. After a rapid distribution phase, plasma VPA levels remained stable for 90 min. The brain:plasma ratio was maximal at 1 min and also declined rapidly. The volume of distribution was 0.125 1/kg. The small volume of distribution, low brain:plasma ratios and rapid clearance from brain indicate that VPA is not significantly bound in cerebral cortex after a single dose.

Individualized patient assessment in the treatment of epilepsy with sodium valproate

This study records a comprehensive clinical protocol developed to assess seizure control and unwanted effects in 19 patients treated with sodium valproate (VPA). The assessment is based on six parameters individually ranked: seizure frequency, seizure duration, EEG comparisons, intellectual and cognitive function tests, sociological profile, and unwanted effects. Each parameter was ranked from one to five, and the mean was used to assess the overall patient state. Using this technique, this study assesses the therapeutic efficacy of VPA and the benefits of individualization of therapy as predicted by the use of pharmacokinetic parameters in a controlled trial.