Topiramate Pharmacokinetics in Children and Adults with Epilepsy

Using oral topiramate for primary generalized and focal-to-bilateral tonic-clonic seizures in patients 2 years of age and older: a review of the literature

INTRODUCTION

Topiramate is a drug belonging to the second generation of antiseizure arsenal, used to treat focal onset seizures without generalization, focal-to-bilateral tonic-clonic seizures, and primary generalized tonic-clonic seizures.

AREAS COVERED

The narrative evaluation of topiramate's clinical research that has been published in this article focuses on the medication's effectiveness and safety when used to treat primary generalized and focal-to-bilateral tonic-clonic seizures. From their founding to the present, the databases MEDLINE, SCOPUS, EBSCO, and GOOGLE SCHOLAR were searched.

EXPERT OPINION

Topiramate treatment has the obvious benefit of being effective in treating tonic-clonic seizures; nevertheless, it may have a drawback in that up to 56% of patients discontinue therapy due to its rather poor tolerability, particularly at doses exceeding 600 mg daily. Patients are most bothered by psychiatric and cognitive side effects, and then by appetite and weight decrease. While the onset of anorexia cannot be prevented by changing the dosage regimen, psychiatric and cognitive side effects can be mitigated by slowly titrating the topiramate dose.

Clinical effectiveness and predictors of response to topiramate plus lifestyle modification in youth with obesity seen in a weight management clinical setting

Introduction

Obesity affects approximately 20% of U.S. youth. Anti-obesity medications (AOMs) are promising lifestyle modification adjuncts for obesity treatment, and topiramate is commonly prescribed in pediatric weight management clinics. It is important to determine "real-world" effectiveness of AOMs and, given shifts towards personalized approaches, characteristics potentially predicting better or worse response. We therefore sought to describe clinical effectiveness from topiramate plus lifestyle modification, and to determine if baseline phenotypic characteristics are associated with better or worse response.

Methods

We performed a retrospective cohort study (2012-2020) among youth (<18 years old) followed in a U.S. academic-based weight management clinic. Baseline characteristics (i.e., body mass index (BMI), liver function tests, eating-related behaviors) and outcomes (%BMI of 95th percentile (%BMIp95), BMI, percent %BMI change, weight) were determined through review of electronic health records and clinic intake survey data.

Results

Among 282 youth prescribed topiramate plus lifestyle modifications (mean baseline age 12.7 years, %BMIp95 144%), %BMIp95 and percent BMI change were statistically significantly reduced at each time point (1.5-, 3-, 6-, and 12-month %BMIp95 reductions: -2.2, -3.9, -6.6, and -9.3 percentage points, respectively; percent BMI reduction: -1.2%, -1.9%, -3.2%, and -3.4%, respectively; all p<0.01). Considering multiple comparisons, no baseline characteristics statistically significantly predicted response at any time point.

Conclusions

We found that topiramate plus lifestyle modification reduced %BMIp95 and BMI among youth in a weight management clinical setting, and that no baseline characteristics evaluated were associated with response. These results should be considered preliminary given the observational nature of this study, and prospective studies are needed to further characterize clinical effectiveness and identify and confirm potential predictors of response.

Population Pharmacokinetics of Topiramate in Patients with Epilepsy Using Nonparametric Modeling

BACKGROUND

Topiramate (TPM) is used for the treatment of various epileptic seizures and the prevention of migraine. This study aimed to develop a population pharmacokinetic model and identify covariates that influence TPM behavior in patients with epilepsy in Kuwait.

METHODS

Data were collected retrospectively from 108 patients (2 years old and above) with epilepsy who were treated with oral TPM and 174 TPM blood samples from 3 hospitals in Kuwait from 2009 to 2016. Data were randomly divided into 2 groups for model development and validation. The population pharmacokinetic model was built using the nonparametric modeling algorithm (Pmetrics). The model was evaluated internally through the visual predictive check method and externally using a new data set.

RESULTS

A 1-compartment model with first-order elimination fitted the data well. Covariates showing a significant effect on the elimination rate constant were renal function and coadministration of carbamazepine (CBZ). The mean estimated clearance was 2.11 L/h; this was 50% higher for patients coadministered with CBZ. Age and sex were essential covariates for the volume of distribution (V). The visual predictive check of the final model could predict the measured concentrations. External validation further confirmed the favorable predictive performance of the model with low bias and imprecision for predicting the concentration in a particular population.

CONCLUSIONS

TPM elimination was increased with CBZ coadministration and was affected by renal function. Meanwhile, age and sex were the main predictors for V. The predictive performance of the final model proved to be valid internally and externally.

Monitoring topiramate concentrations at delivery and during lactation

OBJECTIVE

To determine transplacental passage of topiramate and its transport to colostrum, mature maternal milk and breastfed infants, we examined data from 27 women treated with topiramate from 2004 to 2020.

METHODS

In this cohort study, maternal serum, umbilical cord serum, milk and infant serum levels were measured by gas chromatography in the delivery subgroup, the colostrum subgroup (3-4 days postpartum) and the mature milk subgroup (7-30 days postpartum). Paired umbilical cord serum, maternal serum, breastfed infant serum, and milk levels were used to assess the ratios of umbilical cord/maternal serum, milk/maternal serum and infant/maternal serum levels.

RESULTS

Topiramate levels varied from 1.0 to 7.1 mg/L in maternal serum and from 0.8 to 6.2 mg/L in umbilical cord serum, and the mean umbilical cord/maternal serum ratio was 0.93 ± 0.11. At 3-4 days after delivery, topiramate concentrations were 1.4-8.4 mg/L in maternal serum, 1.5-8.6 mg/L in milk and 0.3-4.4 mg/L in infant serum. The mean milk/maternal serum ratio was 0.99 ± 0.45, and the mean infant/maternal serum ratio was 0.25 ± 0.15. At 7-30 days after delivery, maternal serum levels varied from 1.9 to 9.7 mg/L, milk levels ranged from 2.3 to 10.6 mg/L and infant serum levels ranged from 0.3 to 6.5 mg/L. The mean milk/maternal serum ratio was 1.07 ± 0.31, and the mean infant/maternal serum ratio was 0.51 ± 0.27.

CONCLUSIONS

We extended information about free transplacental passage of topiramate and its extensive transport to maternal milk with lower serum concentrations in breastfed infants in the largest group of patients ever reported to our knowledge.

DATA AVAILABILITY STATEMENT

Authors declare that take full responsibility for the data, the analyses and interpretation, and the conduct of the research; that they have full access to all of the data; and that they have the right to publish all data. Authors were not participations in industry-sponsored research and corporate activities for evaluation of a manuscript.

Prediction of Clearance in Children from Adults Following Drug-Drug Interaction Studies: Application of Age-Dependent Exponent Model

Background and Objective

Pharmacokinetic drug–drug interaction (DDI) studies are conducted in adult subjects during drug development but there are limited studies that have characterized pharmacokinetic DDI studies in children. The objective of this study was to evaluate if the DDI clearance values from adults can be allometrically extrapolated from adults to children.

Methods

Fifteen drugs were included in this study and the age of the children ranged from premature neonates to adolescents (30 observations across the age groups). The age-dependent exponent (ADE) model was used to predict the clearance of drugs in children from adults following DDI studies.

Results

The prediction error of drug clearances following DDIs in children ranged from 4 to 67%. Of 30 observations, 17 (57%) and 27 (90%) observations had a prediction error ≤ 30% and ≤ 50%, respectively.

Conclusion

This study indicates that it is possible to predict the clearance of drugs with reasonable accuracy in children from adults following DDI studies using an ADE model. The method is simple, robust, and reliable and can replace other complex empirical models.

Severity of Topiramate-Related Working Memory Impairment Is Modulated by Plasma Concentration and Working Memory Capacity

Drug side effects that impair cognition can lead to diminished quality of life and discontinuation of therapy. Topiramate is an antiepileptic drug that elicits cognitive deficits more frequently than other antiepileptic drugs, impairing multiple cognitive domains including language, attention, and memory. Although up to 40% of individuals taking topiramate may experience cognitive deficits, we are currently unable to predict which individuals will be most severely affected before administration. The objective of this study was to show the contributions of plasma concentration and working memory capacity in determining the severity of an individual's topiramate-related cognitive impairment. Subjects were enrolled in a double-blind, placebo-controlled crossover study during which they received a single dose of either 100, 150, or 200 mg topiramate. Working memory function was assessed using a modified Sternberg working memory task with 3 memory loads administered 4 hours after dosing. After adjustment for differences in working memory capacity, each 1 μg/mL of topiramate plasma concentration was associated with a 3.6% decrease in accuracy for all memory loads. Placebo effects occurred as a function of working memory capacity, with individuals with high working memory capacity experiencing less severe placebo-related impairment compared with those with low working memory capacity. Our results demonstrate that severity of topiramate-related cognitive deficits occurs as a function of both drug exposure and baseline cognitive function. By identifying patient- and exposure-related characteristics that modulate the severity of cognitive side effects, topiramate dosing strategies may be individually tailored in the future to prevent unwanted cognitive impairment.

Dose-Related Concentrations of Neuroactive/Psychoactive Drugs Expected in Blood of Children and Adolescents

PURPOSE

Therapeutic drug monitoring is highly recommended for children and adolescents treated with neurotropic/psychotropic drugs. For interpretation of therapeutic drug monitoring results, drug concentrations (C/D) expected in a "normal" population are helpful to identify pharmacokinetic abnormalities or nonadherence. Using dose-related concentration (DRC) factors obtained from pharmacokinetic data, C/D ranges expected under steady state can be easily calculated by multiplication of DRC by the daily dose. DRC factors, however, are defined only for adults so far. Therefore, it was the aim of this study to estimate DRC factors for children and adolescents and compare them with those of adults.

METHODS

To obtain pharmacokinetic data (apparent total clearance of drugs from plasma after oral administration, elimination half-life, area under the curve, and minimum serum drug concentration) from children and adolescents treated with psychotropic drugs, a systematic review of published literature was performed, and the pharmaceutical companies that market these drugs were contacted. Available information was used for the calculation of DRC factors.

RESULTS

Fourteen of 26 drugs had similar DRC factors to those reported for adults; 8 and 4 had higher and lower factors, respectively. The antidepressants citalopram, clomipramine, fluvoxamine, and imipramine and the antipsychotics haloperidol and olanzapine showed higher DRC factors than those calculated for adults. The DRC factors of amphetamine and methylphenidate were higher in children (6-12 years) but not in adolescents (13-17 years). On the contrary, the antipsychotic quetiapine and the mood-stabilizing antiepileptics lamotrigine, oxcarbazepine, and topiramate showed lower DRC factors than those calculated for adults.

CONCLUSIONS

It was concluded that concentrations of neuroactive/psychoactive drugs to be expected in blood for a given dose may differ between adults and children or adolescents, most probably owing to age-dependent differences in the elimination of these drugs.

Topiramate Blood Levels During Polytherapy for Epilepsy in Children

BACKGROUND

The therapeutic range of topiramate (TPM) blood level is not set because the efficacy and safety are not considered to be related to the level. However, the therapeutic target without side effects is necessary, so the optimal range of TPM blood level was analyzed in this study.

STUDY QUESTION

This study was conducted to evaluate the efficacy of TPM over 2 years and the utility of measuring blood levels of TPM during the follow-up of epileptic patients.

STUDY DESIGN

Thirty patients (18 males, 12 females; age range, 6 months-15 years) were treated with TPM for epilepsy. The initial dosage of TPM was 1-3 mg·kg·d. If the effect proved insufficient after 2 weeks, the dosage was increased to 4-9 mg·kg·d.

MEASURES AND OUTCOMES

Blood levels of TPM were measured by liquid chromatography-tandem mass spectrometry at 1, 6, 12, and 24 months after levels reached steady state. The efficacy of TPM was evaluated by the reduction in epileptic seizure rate (RR) at the time of blood sampling. Statistical analysis was performed using the Mann-Whitney U test.

RESULTS

A positive correlation was seen between blood levels and maintenance dosages, but no correlation was observed between blood levels and RR. Any significant difference was not identified in TPM levels between the effective group (RR ≥50%) and the ineffective group (RR <50%; P = 0.159). In the subgroup of patients who did not use valproic acid, a significant difference in TPM levels was apparent between the effective and ineffective groups (P = 0.029). The optimal range of TPM was advocated 3.5-5.0 μg/mL. The optimal range was set, so that ranges did not overlap between the effective and ineffective groups. No patients experienced any side effects.

CONCLUSIONS

Measuring blood levels of TPM based on the classification of concomitant drugs and adjusting the dosage to reach the optimal range were recommended.

Pharmacotherapy for Seizures in Neonates with Hypoxic Ischemic Encephalopathy

Seizures are common in neonates with moderate and severe hypoxic ischemic encephalopathy (HIE) and are associated with worse outcomes, independent of HIE severity. In contrast to adults and older children, no new drugs have been licensed for treatment of neonatal seizures over the last 50 years, because of a lack of controlled clinical trials. Hence, many antiseizure medications licensed in older children and adults are used off-label for neonatal seizure, which is associated with potential risks of adverse effects during a period when the brain is particularly vulnerable. Phenobarbital is worldwide the first-line drug and is considered standard of care, although there is a limited evidence base for its efficacy. Second-line agents include phenytoin, benzodiazepines, levetiracetam, and lidocaine. These drugs are discussed in more detail along with two emerging drugs (bumetanide and topiramate). More safety, pharmacokinetic, and efficacy data are needed from well-designed clinical trials to develop safe and effective antiseizure regimes for the treatment of neonatal seizures in HIE.

Population Pharmacokinetics of Topiramate in Japanese Pediatric and Adult Patients With Epilepsy Using Routinely Monitored Data

BACKGROUND

Topiramate is a second-generation antiepileptic drug used as monotherapy and adjunctive therapy in adults and children with partial seizures. A population pharmacokinetic (PPK) analysis was performed to improve the topiramate dosage adjustment for individualized treatment.

METHODS

Patients whose steady-state serum concentration of topiramate was routinely monitored at Kyoto University Hospital from April 2012 to March 2013 were included in the model-building data. A nonlinear mixed effects modeling program was used to evaluate the influence of covariates on topiramate pharmacokinetics. The obtained PPK model was evaluated by internal model validations, including goodness-of-fit plots and prediction-corrected visual predictive checks, and was externally confirmed using the validation data from January 2015 to December 2015.

RESULTS

A total of 177 steady-state serum concentrations from 93 patients were used for the model-building analysis. The patients' age ranged from 2 to 68 years, and body weight ranged from 8.6 to 105 kg. The median serum concentration of topiramate was 1.7 mcg/mL, and half of the patients received carbamazepine coadministration. Based on a one-compartment model with first order absorption and elimination, the apparent volume of distribution was 105 L/70 kg, and the apparent clearance was allometrically related to the body weight as 2.25 L·h·70 kg without carbamazepine or phenytoin. Combination treatment with carbamazepine or phenytoin increased the apparent clearance to 3.51 L·h·70 kg. Goodness-of-fit plots, prediction-corrected visual predictive check, and external validation using the validation data from 43 patients confirmed an appropriateness of the final model. Simulations based on the final model showed that dosage adjustments allometrically scaling to body weight can equalize the serum concentrations in children of various ages and adults.

CONCLUSIONS

The PPK model, using the power scaling of body weight, effectively elucidated the topiramate serum concentration profile ranging from pediatric to adult patients. Dosage adjustments based on body weight and concomitant antiepileptic drug help obtain the dosage of topiramate necessary to reach an effective concentration in each individual.

Effect of CYP Inducers/Inhibitors on Topiramate Concentration: Clinical Value of Therapeutic Drug Monitoring

BACKGROUND

This study investigated the pharmacokinetic interactions between topiramate (TPM) and concomitant antiepileptic drugs and evaluated the therapeutic concentration range of TPM and the effect of the achieved plasma concentration on the retention rate of TPM therapy.

METHODS

A total of 1217 plasma samples obtained from 610 patients were retrospectively investigated, and the concentration-to-dose ratio (CD ratio) of TPM was compared among patients on various antiepileptic drug regimens. In addition, the therapeutic concentration of TPM was reviewed in patients on long-term therapy, and factors influencing the retention rate of TPM were analyzed by the Kaplan-Meier method.

RESULTS

Among patients using hepatic enzyme inducers (phenytoin, phenobarbital, and carbamazepine), the CD ratio was reduced by 45.4% in adults and 33.3% in children. Patients taking phenytoin concomitantly had a significantly lower CD ratio than patients taking phenobarbital or carbamazepine. Among noninducers, concomitant use of stiripentol increased the CD ratio. In 276 patients who remained on TPM therapy for more than 2 years, the mean therapeutic concentration was 5.1 mcg/mL (15.0 μmol/L). The estimated retention day was significantly higher for patients with a TPM concentration >5 mcg/mL than that for patients with a concentration <5 mcg/mL (945 versus 802 days; P = 0.007 by the log-rank test). Also, patients without hepatic enzyme inducers had a significantly higher retention rate than patients using such inducers (P = 0.002).

CONCLUSIONS

Concomitant use of hepatic enzyme inducers markedly reduced the plasma TPM concentration and can decrease its antiepileptic effect. A therapeutic concentration of >5 mcg/mL TPM was significantly associated with continuation of therapy, and therapeutic drug monitoring can be helpful.

Topiramate for Treatment in Patients With Migraine and Epilepsy

BACKGROUND

Antiepileptic drugs (AED) are often considered first line for monotherapy in treatment of patients with migraines, and also those with comorbid migraine and epilepsy. Topiramate, a newer generation AED, has broad mechanism of action and evidence of benefit in patients with either episodic or chronic migraine along with epilepsy, both generalized and focal.

METHODS

Our goal is to review the relevant mechanisms of action along with any supportive evidence published to date on the use of topiramate (TPM) in patients with both migraine headache and epilepsy.

CONCLUSIONS

There has been very little published to date on the use of TPM in patients diagnosed with both disorders. Despite this, TPM has been adopted as first line therapy in this patient population. Future studies investigating the effectiveness of this treatment strategy are warranted in order to determine the most effective use of this medication in patients diagnosed with migraine headaches and epilepsy.

Pharmacokinetic-pharmacodynamic modelling of intravenous and oral topiramate and its effect on phonemic fluency in adult healthy volunteers

AIMS

The aim was to develop a quantitative approach that characterizes the magnitude of and variability in phonemic generative fluency scores as measured by the Controlled Oral Word Association (COWA) test in healthy volunteers after administration of an oral and a novel intravenous (IV) formulation of topiramate (TPM).

METHODS

Nonlinear mixed-effects modelling was used to describe the plasma TPM concentrations resulting from oral or IV administration. A pharmacokinetic-pharmacodynamic (PK-PD) model was developed sequentially to characterize the effect of TPM concentrations on COWA with different distributional assumptions.

RESULTS

Topiramate was rapidly absorbed, with a median time to maximal concentration of 1 h and an oral bioavailability of ~100%. Baseline COWA score increased by an average of 12% after the third administration on drug-free sessions. An exponential model described the decline of COWA scores, which decreased by 14.5% for each 1 mg l(-1) increase in TPM concentration. The COWA scores were described equally well by both continuous normal and Poisson distributions.

CONCLUSIONS

This analysis quantified the effect of TPM exposure on generative verbal fluency as measured by COWA. Repetitive administration of COWA resulted in a better performance, possibly due to a learning effect. The model predicts a 27% reduction in the COWA score at the average observed maximal plasma concentration after a 100 mg dose of TPM. The single-dose administration of relatively low TPM doses and narrow range of resultant concentrations in our study were limitations to investigating the PK-PD relationship at higher TPM exposures. Hence, the findings may not be readily generalized to the broader patient population.

Clinical pharmacokinetics of new-generation antiepileptic drugs at the extremes of age: an update

Epilepsies occur across the entire age range, and their incidence peaks in the first years of life and in the elderly. Therefore, antiepileptic drugs (AEDs) are commonly used at the extremes of age. Rational prescribing in these age groups requires not only an understanding of the drugs' pharmacodynamic properties, but also careful consideration of potential age-related changes in their pharmacokinetic profile. The present article, which updates a review published in 2006 in this journal, focuses on recent findings on the pharmacokinetics of new-generation AEDs in neonates, infants, children, and the elderly. Significant new information on the pharmacokinetics of new AEDs in the perinatal period has been acquired, particularly for lamotrigine and levetiracetam. As a result of slow maturation of the enzymes involved in glucuronide conjugation, lamotrigine elimination occurs at a particularly slow rate in neonates, and becomes gradually more efficient during the first months of life. In the case of levetiracetam, elimination occurs primarily by renal excretion and is also slow at birth, but drug clearance increases rapidly thereafter and can even double within 1 week. In general, infants older than 2-3 months and children show higher drug clearance (normalized for body weight) than adults. This pattern was confirmed in recent studies that investigated the pediatric pharmacokinetics of several new AEDs, including levetiracetam, rufinamide, stiripentol, and eslicarbazepine acetate. At the other extreme of age, in the elderly, drug clearance is generally reduced compared with younger adults because of less efficient drug-metabolizing activity, decreased renal function, or both. This general pattern, described previously for several AEDs, was confirmed in recent studies on the effect of old age on the clearance of felbamate, levetiracetam, pregabalin, lacosamide, and retigabine. For those drugs which are predominantly eliminated by renal excretion, aging-related pharmacokinetic changes could be predicted by measuring creatinine clearance (CLCR). Overall, most recent findings confirm that age is a major factor influencing the pharmacokinetic profile of AEDs. However, pharmacokinetic variability at any age can be considerable, and the importance of other factors should not be disregarded. These include genetic factors, co-morbidities, and drug interactions, particularly those caused by concomitantly administered AEDs which induce or inhibit drug-metabolizing enzymes.

Topiramate for the treatment of epilepsy and other nervous system disorders

Initially synthesized as an oral hypoglycemic agent, topiramate was approved for use as an anticonvulsant in 1996. Its broad spectrum efficacy in epilepsy, including as monotherapy and in children, is well established. Topiramate has also been used in the management of nonepileptic neurologic and psychiatric conditions, including migraine prophylaxis (with firmly established efficacy), obesity (with some evidence of long-term maintenance of weight loss), substance dependence, bipolar disorder and neuropathic pain, and it has been investigated as a possible neuroprotective agent. Paresthesias and cognitive side effects are the most common troublesome adverse effects. Recent trends towards lower doses may help achieve the best combination of efficacy and tolerability.

Population pharmacokinetics of topiramate in adult patients with epilepsy using nonlinear mixed effects modelling

The objective of the study was to develop population pharmacokinetic model of topiramate (TPM) using nonlinear mixed effects modelling approach. Data were collected from 78 adult epileptic patients on mono- or co-therapy of TPM and other antiepileptic drugs, such as carbamazepine (CBZ), valproic acid, lamotrigine, levetiracetam, phenobarbital and pregabalin. Steady-state TPM concentrations were determined in blood samples by high performance liquid chromatography with fluorescence detection. A one-compartment model with first order absorption and elimination was used to fit the concentration-time TPM data. Volume of distribution of TPM was estimated at 0.575 l/kg. The influence of demographic, biochemical parameters and therapy characteristics of the patients on oral clearance (CL/F) was evaluated. Daily carbamazepine dose (DCBZ) and renal function estimated by Modification of diet in renal disease (MDRD) formula significantly (p<0.001) influenced CL/F and were included in the final model: CL/F · (l/h)=1.53(l/h) · [1+0.476 · DCBZ(mg/day)/1000(mg/day)] · EXP[0.00476 · [MDRD(ml/ min)-95.72(ml/min)]]. Increase of CL/F with DCBZ and MDRD was best described by linear and exponential models. Mean TPM CL/F during CBZ co-therapy was 2.46 l/h, which is higher for 60.8% than in patients not co-treated with CBZ. Evaluation by bootstrapping showed that the final model was stable. The predictive performance was evaluated by adequate plots and indicated satisfactory precision. This model allows individualisation of TPM dosing in routine patient care, especially useful for patients on different CBZ dosing regimen.

Pharmacokinetic variability of four newer antiepileptic drugs, lamotrigine, levetiracetam, oxcarbazepine, and topiramate: a comparison of the impact of age and comedication

BACKGROUND

Newer antiepileptic drugs (AEDs) are widely used in patients with epilepsy. There is still insufficient documentation regarding pharmacokinetic variability of these AEDs in different patient groups.

PURPOSE

The purpose of this study was to compare age and comedication as factors contributing to pharmacokinetic variability between 4 newer AEDs (lamotrigine, levetiracetam, oxcarbazepine, and topiramate) among patients with refractory epilepsy.

METHODS

Data regarding age, gender, use of AEDs, daily doses, and serum concentration measurements were retrieved from a therapeutic drug monitoring database, from patients admitted to the National Center for Epilepsy, Norway, 2007-2008.

RESULTS

In total, 1050 patients were included, 111 younger children (2-9 years), 137 older children (10-17 years), 720 adults (18-64 years), 82 elderly (65-93 years). Fifty percent of the patients were prescribed polytherapy, in 88 different combinations. The interindividual pharmacokinetic variability was extensive, as illustrated by a 10-fold variability in serum concentration compared with dose. Age affected the apparent clearance of levetiracetam to the largest extent, as shown by a 60% increase in younger children and a 40% reduction in the elderly, respectively, compared with adults. Comedication altered the clearance of lamotrigine to the greatest extent ±70% because it is affected by both enzyme inducers and inhibitors. Hepatic enzyme inducers increased the clearance of levetiracetam and topiramate by 25% and oxcarbazepine by 75%. Valproic acid reduced the clearance of topiramate by 25%.

CONCLUSION

Age and comedication are important contributors to pharmacokinetic variability. Age had the greatest impact on levetiracetam, and comedication affected the clearance of each of the 4 AEDs investigated in this study. Pharmacokinetic drug interactions must be carefully considered when multidrug therapies are prescribed. Therapeutic drug monitoring is a valuable tool for individualizing AED therapy.

Safety and efficacy of topiramate in neonates with hypoxic ischemic encephalopathy treated with hypothermia (NeoNATI)

BACKGROUND

Despite progresses in neonatal care, the mortality and the incidence of neuro-motor disability after perinatal asphyxia have failed to show substantial improvements. In countries with a high level of perinatal care, the incidence of asphyxia responsible for moderate or severe encephalopathy is still 2-3 per 1000 term newborns. Recent trials have demonstrated that moderate hypothermia, started within 6 hours after birth and protracted for 72 hours, can significantly improve survival and reduce neurologic impairment in neonates with hypoxic-ischemic encephalopathy. It is not currently known whether neuroprotective drugs can further improve the beneficial effects of hypothermia. Topiramate has been proven to reduce brain injury in animal models of neonatal hypoxic ischemic encephalopathy. However, the association of mild hypothermia and topiramate treatment has never been studied in human newborns. The objective of this research project is to evaluate, through a multicenter randomized controlled trial, whether the efficacy of moderate hypothermia can be increased by concomitant topiramate treatment.

METHODS/DESIGN

Term newborns (gestational age ≥ 36 weeks and birth weight ≥ 1800 g) with precocious metabolic, clinical and electroencephalographic (EEG) signs of hypoxic-ischemic encephalopathy will be randomized, according to their EEG pattern, to receive topiramate added to standard treatment with moderate hypothermia or standard treatment alone. Topiramate will be administered at 10 mg/kg once a day for the first 3 days of life. Topiramate concentrations will be measured on serial dried blood spots. 64 participants will be recruited in the study. To evaluate the safety of topiramate administration, cardiac and respiratory parameters will be continuously monitored. Blood samplings will be performed to check renal, liver and metabolic balance. To evaluate the efficacy of topiramate, the neurologic outcome of enrolled newborns will be evaluated by serial neurologic and neuroradiologic examinations. Visual function will be evaluated by means of behavioural standardized tests.

DISCUSSION

This pilot study will explore the possible therapeutic role of topiramate in combination with moderate hypothermia. Any favourable results of this research might open new perspectives about the reduction of cerebral damage in asphyxiated newborns.

A new generation of anticonvulsants for the treatment of epilepsy in children

Over the past 12 years, a number of new anticonvulsant drugs have been introduced for the treatment of childhood epilepsy. The present article reviews these new agents and provides the considerations one should use in selecting an antiepileptic drug for use in children. While in some cases the newer antiepileptic drugs have a more favourable pharmacokinetic and toxicity profile than the older drugs, there appears to be no objective evidence that any currently available antiepileptic drug is superior in terms of therapeutic efficacy.

Pharmacokinetic-pharmacodynamic assessment of topiramate dosing regimens for children with epilepsy 2 to <10 years of age

PURPOSE

To identify and validate the efficacious monotherapy dosing regimen for topiramate in children aged 2 to <10 years with newly diagnosed epilepsy using pharmacokinetic-pharmacodynamic (PK-PD) modeling and simulation bridging.

METHODS

Several models were developed in pediatric and adult populations to relate steady-state trough plasma concentrations (C(min)) of topiramate to the magnitude of clinical effect in monotherapy and adjunctive settings. These models were integrated to derive and support the monotherapy dosing regimen for pediatric patients.

KEY FINDINGS

A two-compartmental population PK model with first-order absorption described the time course of topiramate C(min) as a function of dosing regimen. Disposition of topiramate was related to age, body weight, and use of various concomitant antiepileptic drugs. The PK-PD model for monotherapy indicated that the hazard of time to first seizure decreased with increasing C(min) and time since randomization. Higher baseline seizure frequency increased risk for seizures. Age did not significantly influence hazard of time to first seizure after randomization to monotherapy. For adjunctive therapy, the distribution of drug and placebo responses was not significantly different among age groups. Based on the available PK-PD modeling data, the dosing regimen expected to achieve a 65-75% seizure freedom rate after 1 year for pediatric patients age 2-10 years is approximately 6-9 mg/kg per day.

SIGNIFICANCE

This analysis indicated no difference in PK-PD of topiramate between adult and pediatric patients. Effects of indication and body weight on PK were adequately integrated into the model, and monotherapy dosing regimens were identified for children 2-10 years of age.

Topiramate concentrations in neonates treated with prolonged whole body hypothermia for hypoxic ischemic encephalopathy

PURPOSE

Therapeutic hypothermia reduces mortality and neurologic impairment in neonates with hypoxic-ischemic encephalopathy. Topiramate exerts a neuroprotective effect in asphyxiated neonatal animal models. However, no studies have investigated the association of hypothermia and topiramate, because topiramate pharmacokinetics during hypothermia and the optimal administration schedule are unknown. The influence of hypothermia on topiramate pharmacokinetics was evaluated in asphyxiated neonates treated with prolonged whole-body hypothermia and topiramate.

METHODS

Thirteen term newborns were treated with mild or deep whole body hypothermia for 72 h; all received oral topiramate, 5 mg/kg once a day for the first 3 days of life, and seven had concomitant phenobarbital treatment. Topiramate concentrations were measured on serial dried blood spots.

RESULTS

Topiramate concentrations were within the reference range in 11 of 13 newborns, whereas concentrations exceeded the upper limit in 2 of 13, both newborns on deep hypothermia. Topiramate concentrations reached a virtual steady state in nine newborns, for whom pharmacokinetic parameters were calculated. Values of topiramate maximal and minimal concentration, half-life, average concentration, and area under the time-concentration curve resulted in considerably higher values than those reported in normothermic infants. With respect to normothermic infants, time of maximal concentration was mildly delayed and apparent total body clearance was lower, suggesting slower absorption and elimination. Pharmacokinetic parameters did not differ significantly between infants on deep versus mild hypothermia and in those on topiramate monotherapy versus add-on phenobarbital.

CONCLUSION

Most neonates on prolonged hypothermia treated with topiramate 5 mg/kg once a day exhibited drug concentrations within the reference range for the entire treatment duration.

[Drugs for status epilepticus treatment]

The pharmacokinetics and pharmacodynamics of major antiepileptic agents are presented. The onset of action and the factors leading to extraction across the blood brain barrier are described as well as the mechanism and extent of metabolism, and the main interactions with other drugs. For each class, the dosing scheme and practical issues related to administration are described, based on evidence when available in the literature.

Use of second-generation antiepileptic drugs in the pediatric population

Epilepsy is common in the pediatric population. Nine second-generation antiepileptic drugs have been approved in the US for use in epilepsy over the past 15 years: felbamate, gabapentin, lamotrigine, topiramate, tiagabine, levetiracetam, oxcarbazepine, zonisamide, and pregabalin. Their use in pediatric patients is fairly widespread, despite most of these agents not having US FDA indications for use. Felbamate and gabapentin were the first two second-generation antiepileptic drugs to be approved in the US. Felbamate use has been limited because of the occurrence of hepatotoxicity and aplastic anemia. Although gabapentin is a fairly well tolerated antiepileptic drug, its use has also been limited as a result of inconsistent efficacy and concern about seizure exacerbation. Lamotrigine and topiramate are broad-spectrum antiepileptic drugs with efficacy in a wide variety of seizure types. Both agents have some tolerability concerns: rash with lamotrigine and neuropsychiatric events with topiramate. There are very little data on tiagabine use in children, but this agent appears to be effective and to have a good tolerability profile. Levetiracetam is a second-generation antiepileptic agent that is available intravenously. Considering its good efficacy, fast onset of action, and low incidence of serious adverse effects, its use in the acute setting could potentially increase. Oxcarbazepine and zonisamide have been relatively well studied in pediatric seizure patients, including use as monotherapy. Both agents have demonstrated good efficacy and tolerability for patients as young as 1 month old. Vigabatrin and rufinamide are currently not available in the US, but have been shown to have some success in other countries. Pregabalin is the newest antiepileptic agent, but lacks pediatric data currently.

Lack of valproic acid-associated weight gain in prepubertal children

We evaluated whether prepubertal children treated with valproic acid did not gain excessive weight. This retrospective study of children with epilepsy, aged <12 years at enrollment, examined weight gain associated with valproic acid or carbamazepine monotherapy. There was no significant difference between the valproic acid (n = 31) and carbamazepine (n = 49) treated groups in average duration of therapy or mean age. Body mass index scores at the beginning and end of the study were used to evaluate weight gain, while compensating for gains in height. For valproic acid, the linear mixed model detected no gain in body mass index z-scores over time (T = 0.25, DF = 17.3, P = 0.80), though it detected a significant gain in body mass index z-scores for carbamazepine (T = 2.32, DF = 36.7, P = 0.02). Results for McNemar chi-square tests were similar. No significant proportion change occurred among children on valproic acid (chi(2) = 2.0, P = 0.15), whereas a significant increase in the proportion of overweight children occurred on carbamazepine (chi(2) = 4.5, P = 0.03). We detected no excessive weight gain for children on valproic acid, whereas this was demonstrated for a similar socioeconomic group on carbamazepine.

Spotlight on topiramate in epilepsy

Topiramate (Topamax) is a structurally novel broad-spectrum antiepileptic drug (AED) with established efficacy as monotherapy or adjunctive therapy in the treatment of adult and paediatric patients with generalized tonic-clonic seizures, partial seizures with or without generalized seizures, and seizures associated with Lennox-Gastaut syndrome. The incidence and severity of many adverse events, including CNS-related events, may be reduced through the use of slow titration to effective and well tolerated dosages. It is associated with few clinically significant interactions with other drugs, is effective when used with other AEDs, is not associated with drug-induced weight gain and, at lower dosages, does not interfere with the effectiveness of oral contraceptives. Therefore, topiramate is a valuable option as monotherapy or adjunctive therapy in the treatment of epilepsy in adult and paediatric patients.

Topiramate: a review of its use in the treatment of epilepsy

Topiramate (Topamax) is a structurally novel broad-spectrum antiepileptic drug (AED) with established efficacy as monotherapy or adjunctive therapy in the treatment of adult and paediatric patients with generalised tonic-clonic seizures, partial seizures with or without generalised seizures, and seizures associated with Lennox-Gastaut syndrome. The incidence and severity of many adverse events, including CNS-related events, may be reduced through the use of slow titration to effective and well tolerated dosages. It is associated with few clinically significant interactions with other drugs, is effective when used with other AEDs, is not associated with drug-induced weight gain and, at lower dosages, does not interfere with the effectiveness of oral contraceptives. Therefore, topiramate is a valuable option as monotherapy or adjunctive therapy in the treatment of epilepsy in adult and paediatric patients.

The influence of old age and enzyme inducing comedication on the pharmacokinetics of valproic acid at steady-state: A case-matched evaluation based on therapeutic drug monitoring data

PURPOSE

To evaluate the influence of aging on the pharmacokinetics of valproic acid (VPA) at steady-state and on the susceptibility of VPA metabolism to enzyme induction by antiepileptic comedication.

METHODS

The database of the therapeutic drug monitoring service of a large neurological hospital was searched to identify patients aged > or = 65 years stabilized on VPA therapy. Apparent VPA oral clearance (CL/F) calculated for each elderly patient was compared with that determined in an equal number of VPA-treated controls aged 20-50 years and matched for gender, body weight and antiepileptic drug (AED) comedication.

RESULTS

A total of 71 elderly patients aged 70.0+/-4.4 years, including 20 receiving enzyme inducing AEDs, was included in the main evaluation. In the absence of enzyme inducing comedication, VPA CL/F in the elderly was similar to that found in non-elderly controls (9.7+/-4.6 versus 10.2+/-4.6mlh(-1)kg(-1)). Elderly patients on enzyme inducing comedication, on the other hand, had lower CL/F values than enzyme induced younger controls (11.7+/-5.4 versus 16.0+/-6.3mlh(-1)kg(-1), p<0.05). Since VPA CL/F is known to increase with increasing dosage, a lower VPA dosage in elderly patients comedicated with enzyme inducers compared with controls may have contributed to differences in CL/F between the two groups.

CONCLUSIONS

In the absence of enzyme inducing comedication, VPA clearance in the elderly was comparable to that observed in controls. VPA clearance in elderly patients receiving enzyme inducing AEDs was lower than in controls, the difference being probably due to an influence of age as well as to the fact that mean VPA dosage was lower in these patients than in controls. Since our measurements of clearance were based on total serum VPA concentrations and VPA binding to plasma proteins is known to be reduced in old age, it is likely that the clearance of unbound, pharmacologically active, VPA was decreased to an important extent in the elderly, presumably as a result of a decline in drug metabolizing capacity.

Pharmacokinetic variability of new antiepileptic drugs at different ages

Newer generation antiepileptic drugs (AEDs) are increasingly used to treat epilepsies in infants, children, and the elderly. For rational prescribing in these populations, it is essential to understand the pharmacokinetic changes that occur during development and aging. Data obtained in recent years indicate that the apparent oral clearance (CL/F) of lamotrigine, topiramate, levetiracetam, oxcarbazepine, gabapentin, tiagabine, zonisamide, vigabatrin, and felbamate is considerably higher in children than in adults,the magnitude of the difference being on average in the order of 20%to 120%, depending on the drug and the age distribution of the assessed population. Information on the pharmacokinetics of these drugs in newborns is completely lacking or very sparse. Studies in the elderly have demonstrated that significant pharmacokinetic changes also occur at the other extreme of age. On average, CL/F values of newer generation AEDs have been found to be reduced by 10% to 50% compared with those found in young or middle-aged adults. These pharmacokinetic changes are clinically important and con-tribute to age-related differences in dosage requirements.

New generation anticonvulsants for the treatment of epilepsy in children

In the last 12 years, 10 new anticonvulsants have been approved by the U.S. Food and Drug Administration and, as a result, the treatment options for children and adults with epilepsy have been expanded considerably. These new generation antiepileptic drugs offer equal efficacy with improved tolerability, pharmacokinetic properties, and side effect profiles compared with the traditional drugs. With many new medications available, the clinician treating children with epilepsy must be well versed in the application of these drugs to their patient population. This manuscript will review the indications, mechanism of action, pharmacokinetics, adverse effects, and dosing of the new generation of anticonvulsant medications.

Evaluation of toxicity of topiramate exposures reported to poison centers

Published literature on the toxicity of a topiramate overdose is limited to case reports. This retrospective study of poison center data was performed to examine the severity of topiramate overdoses. Data on single substance exposures to topiramate reported to the American Association of Poison Control Centers (AAPCC) Toxic Exposure Surveillance System (TESS) in 2000 and 2001 were retrospectively analysed. A total of 567 cases met the inclusion criteria, of which 39% occurred in adults over 19 years of age and 30.2% in children < or = 4 years old. The majority of patients (62.1%) experienced no toxicity. The most common clinical effects reported were drowsiness/lethargy (15.5%), dizziness/vertigo (4.9%), agitation (4.9%), confusion (3.9%), nausea (2.6%) and vomiting (2.5%). Symptomatic patients were older than asymptomatic patients and adults were more likely to be managed in a healthcare facility (P <0.0001). Patients who received gastrointestinal decontamination experienced less serious outcomes than those without decontamination (P <0.02). It is concluded that clinicians should expect relatively mild mental status changes in adults or children with toxicity from topiramate overdose. Serious toxic effects, such as CNS depression with respiratory depression or persistent non-anion gap metabolic acidosis, are infrequent.

Clinical Pharmacokinetics of New-Generation Antiepileptic Drugs at the Extremes of Age

In recent years, several new-generation antiepileptic drugs (AEDs) have been introduced in clinical practice. These agents, which include felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, tiagabine, topiramate, vigabatrin and zonisamide, are being increasingly used in the treatment of epilepsy at the extremes of age. For a rational prescribing of these drugs in specific age groups, major pharmacokinetic changes that occur during development and aging need to be taken into consideration. A review of available evidence indicates that the apparent oral clearance (CL/F) of new-generation AEDs in children is increased by 20-170% (depending on the type of drug and characteristics of the patients studied) compared with adults, with the highest CL/F values usually being observed in the youngest age groups. These findings do not necessarily apply to the first weeks of life, when drug eliminating capacity is still undergoing maturation, as in the case of lamotrigine for which preliminary data suggest that CL/F in neonates aged <2 months can be much lower than in infants aged 2-12 months. At the other extreme of age, in the elderly, CL/F is almost invariably reduced (on average by 10-50%) compared with values found in non-elderly adults. Age-related CL/F changes, together with the large interindividual pharmacokinetic variability, contribute to the need for individualised dosage requirements in these patients. Measurement of serum drug concentrations can be useful as an aid to dosage individualization in these age groups but interpretation of therapeutic drug monitoring data should also take into account the possibility of age-related changes in pharmacodynamic sensitivity and, for neonates and the elderly, alterations in drug binding to serum proteins.

Pharmacokinetic Variability of Newer Antiepileptic Drugs

A new generation of antiepileptic drugs (AEDs) has reached the market in recent years with ten new compounds: felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, tiagabine, topiramate, vigabatrin and zonisamide. The newer AEDs in general have more predictable pharmacokinetics than older AEDs such as phenytoin, carbamazepine and valproic acid (valproate sodium), which have a pronounced inter-individual variability in their pharmacokinetics and a narrow therapeutic range. For these older drugs it has been common practice to adjust the dosage to achieve a serum drug concentration within a predefined 'therapeutic range', representing an interval where most patients are expected to show an optimal response. However, such ranges must be interpreted with caution, since many patients are optimally treated when they have serum concentrations below or above the suggested range. It is often said that there is less need for therapeutic drug monitoring (TDM) with the newer AEDs, although this is partially based on the lack of documented correlation between serum concentration and drug effects. Nevertheless, TDM may be useful despite the shortcomings of existing therapeutic ranges, by utilisation of the concept of 'individual reference concentrations' based on intra-individual comparisons of drug serum concentrations. With this concept, TDM may be indicated regardless of the existence or lack of a well-defined therapeutic range. The ten newer AEDs all have different pharmacological properties, and therefore, the usefulness of TDM for these drugs has to be assessed individually. For vigabatrin, a clear relationship between drug concentration and clinical effect cannot be expected because of its unique mode of action. Therefore, TDM of vigabatrin is mainly to check compliance. The mode of action of the other new AEDs would not preclude the applicability of TDM. For the prodrug oxcarbazepine, TDM is also useful, since the active metabolite licarbazepine is measured. For drugs that are eliminated renally completely unchanged (gabapentin, pregabalin and vigabatrin) or mainly unchanged (levetiracetam and topiramate), the pharmacokinetic variability is less pronounced and more predictable. However, the dose-dependent absorption of gabapentin increases its pharmacokinetic variability. Drug interactions can affect topiramate concentrations markedly, and individual factors such as age, pregnancy and renal function will contribute to the pharmacokinetic variability of all renally eliminated AEDs. For those of the newer AEDs that are metabolised (felbamate, lamotrigine, oxcarbazepine, tiagabine and zonisamide), pharmacokinetic variability is just as relevant as for many of the older AEDs. Therefore, TDM is likely to be useful in many clinical settings for the newer AEDs. The purpose of the present review is to discuss individually the potential value of TDM of these newer AEDs, with emphasis on pharmacokinetic variability.

Management of Focal-Onset Seizures

Focal-onset seizures are manifestations of abnormal epileptic firing of brain cells in a localised area or areas of the brain. The diagnosis of focal-onset seizures initially entails an EEG, a detailed history from the patient and eyewitnesses, as well as computer tomographic or, preferably, magnetic resonance imaging scans. Video EEG to record ictal events may be necessary to establish the correct diagnosis. Focal seizures are classified according to the International Classification of Epileptic Seizures and International Classification of Epilepsies and Epilepsy Syndromes. It is important to try to decide how the seizure event fits into this system in order to successfully evaluate and optimise treatment, as well as to give detailed information to the patient about their seizures and prognosis. Once the decision to treat the seizures has been made, the physician must choose which medication is the most appropriate to begin with. Carbamazepine, phenytoin or valproic acid (sodium valproate) are often rated as first-line drugs, but factors such as adverse-effect profiles, age, possibility of pregnancy, and concomitant diseases and medication also need to be considered. Most of the newer antiepileptic drugs (AEDs) appear to have good efficacy and better tolerability than the older agents, but evidence to support their superiority is scarce and has led to conflicting advice in several guidelines. Among the newer AEDs, lamotrigine, gabapentin, topiramate and oxcarbazepine have obtained monotherapy indication in many countries. The higher costs of the newer AEDs may inhibit their wider use, especially in poorer countries.

Can Pharmacokinetic Variability Be Controlled for the Patient??s Benefit?

The aim of this brief communication is to update our recent review on therapeutic drug monitoring (TDM) of the newer antiepileptic drugs (AEDs). The potential value of TDM is discussed in relation to their mode of action and their pharmacokinetic proper-ties. Data on the relationships between serum concentrations and clinical efficacy are limited, and few studies have been designed primarily to study these relationships. As yet there are no generally accepted target ranges for any of the new AEDs. For most drugs a wide range in serum concentration is associated with clinical efficacy,and there is a considerable overlap in serum concentrations related to toxicity and lack of clinical efficacy. Although the available documentation is clearly insufficient, the pharmacological properties of some of the drugs suggest that they may be suitable candidates for TDM. The primary role of TDM for both the newer and established AEDs is to identify an individual's optimum concentration and thus establish a reference value in that patient.

A Comparative Study of the Effect of Carbamazepine and Valproic Acid on the Pharmacokinetics and Metabolic Profile of Topiramate at Steady State in Patients with Epilepsy

PURPOSE

To compare the influence of enzyme-inducing comedication and valproic acid (VPA) on topiramate (TPM) pharmacokinetics and metabolism at steady state.

METHODS

Three groups were assessed: (a) patients receiving TPM mostly alone (control group, n =13); (b) patients receiving TPM with carbamazepine (CBZ; n = 13); and (c) patients receiving TPM with VPA (n = 12). TPM and its metabolites were assayed in plasma and urine by liquid chromatography-mass spectrometry (LC-MS).

RESULTS

No significant differences were found in TPM oral (CL/F) and renal (CL(r)) clearance between the VPA group and the control group. Mean TPM CL/F and CL(r) were higher in the CBZ group than in controls (2.1 vs. 1.2 L/h and 1.1 vs. 0.6L/h, respectively; p < 0.05). In all groups, the urinary recovery of unchanged TPM was extensive and accounted for 42-52% of the dose (p > 0.05). Urinary recovery of 2,3-O-des-isopropylidene-TPM (2,3-diol-TPM) accounted for 3.5% of the dose in controls, 2.2% in the VPA group (p > 0.05), and 13% in the CBZ group (p < 0.05). The recovery of 10-hydroxy-TPM (10-OH-TPM) was twofold higher in the CBZ group than in controls, but it accounted for only <2% of the dose. The plasma concentrations of TPM metabolites were severalfold lower than those of the parent drug.

CONCLUSIONS

Renal excretion remains a major route of TPM elimination, even in the presence of enzyme induction. The twofold increase in TPM-CL/F in patients taking CBZ can be ascribed, at least in part, to stimulation of the oxidative pathways leading to formation of 2,3-diol-TPM and 10-OH-TPM. VPA was not found to have any clinically significant influence on TPM pharmacokinetic and metabolic profiles.