Valproic acid pharmacokinetics in children: III. Very high dosage requirements

Steady-state pharmacokinetic simulation of intermittent vs. continuous infusion valproic acid therapy in non-critically ill and critically ill patients

OBJECTIVES

Valproic acid (VPA) is a broad-spectrum antiepileptic drug used for a variety of neurologic disorders. The relatively short half-life seen with intermittent intravenous bolus doing may lead to serum concentration variability. Continuous infusion VPA therapy is an approach to mitigate these effects. The objective of this study is to characterize the pharmacokinetics of continuous infusion of VPA in acutely ill patients and to determine dosing regimens that most frequently obtain goal steady-state serum concentrations.

METHODS

This is a retrospective pharmacokinetics study in adult patients receiving continuous infusion VPA per institutional protocol for seizure or status migrainosus. Pharmacokinetic parameters were reviewed for 234 patients (25 critically ill) and compared between the two groups (non-critically ill vs. critically ill). Intermittent and continuous infusion dosing strategies were modeled utilizing Monte Carlo simulations for both cohorts. Frequencies of serum concentration attainment were reported.

RESULTS

The percent target attainment for the non-critically ill group and critically ill group were 69.4 and 58.3% (p = 0.282) post-loading dose and 69.7 and 37.5% (p = 0.004) steady state, respectively. The volume of distribution was significantly different between the two groups (0.35 vs. 0.68 L/kg, p = < 0.0001). Highest frequency of target attainment (50-100 mcg/ml) occurred in the continuous infusion 2 mg/kg/h simulation for both critically ill (45.19%) and acutely ill (48.16%) groups.

DISCUSSION

Critically ill patients have an increased volume of distribution. Increasing the volume of distribution requires higher loading doses of VPA to obtain desired therapeutic concentrations. Continuous infusion VPA provides more consistent serum steady-state concentrations while mitigating pharmacokinetic variability.

Pharmacokinetics and Clinical Utility of Valproic Acid Administered via Continuous Infusion

INTRODUCTION

Valproic acid is a versatile antiepileptic drug that is often used in the acute care setting. Intravenous valproic acid lends itself well to a continuous infusion as it exhibits a relatively short half-life. We evaluated the pharmacokinetics and clinical efficacy of continuous infusion valproic acid in hospitalized patients with migraine and seizures.

METHODS

A retrospective cohort study was performed utilizing information from the medical records of patients receiving an intravenous continuous infusion of valproic acid. Patients were included if they were aged 1 month to 85 years and they received a continuous infusion of valproic acid. Therapeutic response, common adverse effects, and the pharmacokinetic profile of valproic acid were evaluated.

RESULTS

Continuous infusion valproic acid led to a concentration within the desired range (50-100 μg/ml) in 83.4% of patients, a rate that was higher in pediatric patients. The clinical response rate was also higher in pediatric patients with seizures or migraines and appeared to be better when the concentration was >75 μg/ml. Analysis of safety parameters suggests similar safety considerations to valproic acid when administered via intermittent infusion.

CONCLUSIONS

Continuous infusion valproic acid appears to be a safe, effective, and predictable manner by which to administer valproic acid to pediatric and adult patients admitted to the hospital.

Pharmacokinetics and clinical application of intravenous valproate in Thai epileptic children

Roles of intravenous administration of valproate in status epilepticus and serial seizures are documented in adults and children. Pharmacokinetic parameters are necessary to predict the optimum therapeutic level after administration. A cross-sectional study to determine the pharmacokinetic parameters and safety of intravenous valproate for future application was conducted in Thai children from January to December 2008. There were eleven children, age-range 1-15 years (mean age 9.5 years) enrolled. Valproate of 15-20 mg/kg was administrated intravenously at the rate of 3 mg/kg/min, followed by 6 mg/kg every 6 h. Valproate level was determined prior to the initial dose and at ½, 1, 2, 4, 5, and 6 h postdose. Complete blood count, serum ammonia, and liver function tests were collected prior to the initial dose and at 6 h. Median loading dose was 19 mg/kg (range 15-20.5 mg/kg). Median maximum concentration at 30 min after infusion was 98.8 mcg/mL (range 67-161 mcg/mL). Median volume of distribution was 0.20 L/kg (range 0.15-0.53 L/kg). Median half-life was 9.5 h (range 4.4-24.2 h). Median clearance was 0.02 L/h/kg (range 0.01-0.05 L/h/kg). Six hours after initial dose, eight children did not have recurrent seizure. One child had brief seizure at 20 min after initial dose. Seizure recurred in two children at 4th and 5th hour. Asymptomatic transient elevation of serum ammonia was observed in two children. Volume of distribution of 0.20 L/kg could be applied for initial intravenous administration with a favorable efficacy.

What Is the Best Strategy for Converting from Twice-Daily Divalproex to a Once-Daily Divalproex ER Regimen?

OBJECTIVE

To examine if, during conversion from conventional divalproex to once-daily divalproex extended-release (ER) tablets, plasma valproic acid (VPA) concentrations in the first 48 hours after conversion are maintained within the accepted therapeutic range (50-100 mg/L).

METHODS

Four distinct 12-hourly (q12h) divalproex to once-daily divalproex ER conversion strategies were explored: immediate, delayed, stepwise and mixed conversion. These strategies were each used in simulations for hypothetical adult patients being treated under different conditions: monotherapy (uninduced, at 1500 mg/day divalproex ER) and polytherapy on enzyme-inducing co-medications (induced, at 3000 and 4500 mg/day divalproex ER).

RESULTS

The proportion of uninduced patients expected to have minimum VPA concentrations (C(min)) >50 mg/L was 90% for immediate, 83% for stepwise and 82% for mixed-conversion strategies; only 52% undergoing a delayed-conversion strategy had C(min) >50 mg/L. More importantly, 33% of induced patients under-going delayed conversion to 3000 mg/day divalproex ER maintained an adequate VPA C(min). Maximum VPA concentrations (C(max)) attained after conversion to divalproex ER are unlikely to rise beyond the steady-state C(max) observed with divalproex q12h regimens with any conversion strategy tested in uninduced or induced patients. Marked perturbation in VPA concentration is not likely when converting to once-daily divalproex ER 'all-at-once' 12 hours after the last divalproex q12h dose. Stepwise and mixed-conversion strategies do not offer any advantage; delayed conversion may produce a large drop in VPA concentration.

CONCLUSIONS

An ideal conversion strategy for q12h divalproex to once-daily divalproex ER appears to be an immediate conversion 12 hours after the last divalproex q12h dose; it causes the least perturbation in plasma VPA, even for patients required to take high divalproex ER doses.

Lack of an effect of valproate concentration on lamotrigine pharmacokinetics in developmentally disabled patients with epilepsy

PURPOSE

to describe the population pharmacokinetics of lamotrigine (LTG) in developmentally disabled (DD) patients with epilepsy and (2) to determine if there is an effect of valproate (VPA) concentration on the extent of the pharmacokinetic interaction between VPA and LTG.

METHOD

a NONMEM population analysis of steady-state LTG serum concentrations was conducted in patients receiving LTG either as mono or polytherapy with either an enzyme inducer (IND)-carbamazepine (CBZ), phenytoin (PHT), phenobarbital (PB) or an inhibitor (VPA).

RESULTS

sixty-two patients (33.6+/-11.3 years, 47+/-9.9 kg) receiving LTG monotherapy (n=19) or polytherapy with VPA (n=15), inducer(s) (n=32) or both (n=5) were evaluated. LTG dose of 369+/-236 mg per day (8.1+/-5.9 mg/kg per day) achieved LTG plasma concentrations of 6.8+/-3.3 microg/ml. The observed LTG monotherapy, LTG+IND, and LTG+VPA oral clearance (Cl/F) were 0. 69+/-0.2, 1.60+/-0.65 and 0.2+/-0.05 ml/kg per min, respectively. The final LTG Cl/F model was dependent on body weight, concomitant VPA, and either single or multiple inducers. Including the serum concentrations of CBZ, PHT, or VPA in the model, did not significantly improve estimates of Cl/F.

CONCLUSION

LTG Cl/F in DD patients is similar to literature values for ambulatory adult patients; however, low weight adult patients have higher elimination rates, as well as an increased response to enzyme induction. VPA inhibition of LTG Cl/F is maximal within the usually accepted therapeutic range for VPA.

Mood stabilizers in children and adolescents

OBJECTIVE

The efficacy of mood stabilizers in children and adolescents has not been studied adequately. This article will review existing studies and highlight some important issues in designing future studies on these agents.

METHOD

Electronic databases including Medline, Psycholnfo, and CRISP were searched for data in children receiving compounds that have mood-stabilizing properties in adults.

RESULTS

Some open clinical data and an extremely modest amount of controlled research data suggest lithium, carbamazepine, and valproate may be effective mood stabilizers in children and adolescents. There are no controlled data on other potential mood stabilizers in children.

CONCLUSIONS

The disorders that may be responsive to mood stabilizers are among the most morbid in child psychiatry. More studies are needed to clarify the efficacy of these compounds in children and adolescents and to provide a rational basis for choosing among them.