Pharmacokinetic interactions between lamotrigine and other antiepileptic drugs in children with intractable epilepsy

Lamotrigine induced priapism in children: case analysis and literature review

Lamotrigine is an antiepileptic drug that can be used to control many types of seizures as a single-agent or an add-on therapy in patients over 2 years of age. In addition to common adverse reactions, this current case report describes a paediatric male patient with a rare side-effect of persistent penile erectile due to lamotrigine. Previous studies have shown that it can improve sexual function in adult male patients. This patient suffered from refractory epilepsy and pneumonia. He had taken a variety of antiepileptic drugs for a long time and developed priapism after the dosage of lamotrigine had been increased. The priapism improved after drug withdrawal and sedation. Further research is needed to elucidate the mechanism of this rare side-effect.

Dosing Recommendations for Lamotrigine in Children: Evaluation Based on Previous and New Population Pharmacokinetic Models

Lamotrigine is a broad-spectrum antiepileptic drug with high interindividual variability in serum concentrations in children. The aims of this study were to evaluate the predictive performance of pediatric population pharmacokinetic (PPK) models published on lamotrigine, to build a new model with our monitoring data and to evaluate the current recommended doses. A validation cohort included patients treated with lamotrigine who had a serum level assayed during therapeutic drug monitoring (TDM). PPK models published in the literature were first applied to the validation cohort. We assessed their predictive performance using mean prediction errors, root mean squared errors, and visual predictive checks. A new model was then built using the data. Dose simulations were performed to evaluate the doses recommended. We included 270 lamotrigine concentrations ranging from 0.5 to 17.9 mg/L from 175 patients. The median (range) age and weight were 11.8 years (0.8-18 years) and 32.7 kg (8-110 kg). We tested 6 PPK models; most had acceptable bias and precision but underestimated the variability of the cohort. We built a 1-compartment model with first-order absorption and elimination, allometric scaling, and effects of inhibitor and inducer comedications. In our cohort, 22.6% of trough concentrations were below 2.5 mg/L. In conclusion, we proposed a PPK model that can be used for TDM of lamotrigine in children. In our population, a high percentage of children had low trough concentrations of lamotrigine. As the intervals of recommended doses are large, we suggest aiming at the higher range of doses to reach the target concentration.

Effects of UGT2B7 Genetic Polymorphisms on Serum Concentrations of Valproic Acid in Chinese Children With Epilepsy Comedicated With Lamotrigine

BACKGROUND

Valproic acid (VPA) is widely used in the treatment of children with epilepsy. Genetic polymorphisms in genes encoding drug-metabolizing enzymes may be an important source of interindividual variability in VPA metabolism. VPA is a substrate of uridine diphosphate glucuronosyltransferase 2B7 (UGT2B7). In this study, we seek to evaluate the effects of genetic polymorphisms of the UGT2B7 gene on serum VPA concentrations in epileptic children comedicated with lamotrigine (LTG).

METHODS

We recruited 166 Chinese children with epilepsy who were treated with VPA in combination with LTG. Serum VPA and LTG concentrations were measured by fluorescence polarization immunoassay and high performance liquid chromatography, respectively. The UGT2B7 -161C > T in the 5'-upstream regions and 211 G > T, 372A > G, 735A > G, and 802C > T in the coding regions were genotyped using polymerase chain reaction amplification followed by direct automated DNA sequencing.

RESULTS

Our data show that patients carrying the variant UGT2B7 -161C > T or 802C > T genotypes had significantly higher adjusted VPA concentrations than those carrying the wild-type genotypes. The significant associations were potentiated after adjusted by age and adjusted LTG concentration. However, no associations were detected between the other studied UGT2B7 genotypes and adjusted VPA concentrations, even after adjusting by age and comedication.

CONCLUSIONS

These results suggest that the UGT2B7 -161C > T or 802C > T mutations affect VPA pharmacokinetics, which are potentially enhanced by age and concomitant LTG administration. These findings provide a potential mechanism underlying interindividual variation in the disposition of VPA in combination with LTG.

Influence of uridine diphosphate glucuronosyltransferase inducers and inhibitors on the plasma lamotrigine concentration in pediatric patients with refractory epilepsy

This study evaluated the influence of concomitant antiepileptic drugs (AEDs) on the plasma concentration of lamotrigine (LTG) in pediatric patients with epilepsy. We retrospectively reviewed 1653 plasma samples from 709 patients (aged 6 months to 16 years) and compared the concentration-to-dose ratio (CD ratio) of LTG among different AED regimens. The median CD ratio of patients receiving LTG monotherapy was 1.25 μg/mL/mg/kg. In patients receiving LTG plus VPA, the CD ratio was increased by about 140%. The CD ratio was elevated from a low VPA concentration (<40 μg/mL) and the increase was VPA concentration-dependent. In contrast, the median CD ratio of patients treated with LTG plus the inducers phenytoin, phenobarbital, and carbamazepine was 0.42, 0.63, and 0.66, respectively, and phenytoin significantly reduced the CD ratio in comparison with phenobarbital or carbamazepine (p < 0.001). Pediatric patients of all ages beyond infancy showed similar susceptibility to VPA or inducers, but infants had higher CD ratios compared with the other age groups. Among other AEDs, topiramate, ethosuximide, and rufinamide reduced the CD ratio. These findings should be useful for estimating interactions between LTG and concomitant AEDs.

Flow- and voltage-dependent blocking effect of ethosuximide on the inward rectifier K⁺ (Kir2.1) channel

Absence seizures are manifestations of abnormal thalamocortical oscillations characterized by spike-and-wave complexes in EEG. Ethosuximide (ETX) is one of the principal medications against absence seizures. We investigate the effect of ETX on the Kir2.1 channel, a prototypical inward rectifier K(+) channel possibly playing an important role in the setting of neuronal membrane potential. We demonstrate that the outward currents of Kir2.1 channels are significantly inhibited by intracellular ETX. We further show that the movement of neutral molecule ETX in the Kir2.1 channel is accompanied by ∼1.2 K(+), giving rise to the vivid voltage dependence of ETX unbinding rate. Moreover, the apparent affinity (K d ) of ETX in the channels are decreased by single-point mutations involving M183, E224, and S165, and especially by double mutations involving T141/S165, which always also disrupt the flux-coupling feature of ETX block. Molecular dynamics simulation demonstrates narrowing of the pore at ∼D172 by binding of ETX to S165 or T141. ETX block of the Kir2.1 channels may cause a modest but critical depolarization of the relevant neurons, decreasing available T-type Ca(2+) channels and consequently lessening pathological thalamocortical burst discharges.

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.

[Therapeutic drug monitoring of ethosuximide]

Ethosuximide is a minor antiepileptic drug, available in France since 1965, indicated in the epilepsy absence, whose interest was reassessed from recent clinical trials, showing that it was the first choice, in term of risk benefit relationship, in this indication. It is a chiral molecule that presents a high bioavailability, a lack of protein binding, hepatic metabolism and urinary excretion. Its elimination half-life is long, between 40 and 60 h in adults, 30 and 40 h in children. The therapeutic range is established at 40-100 mg/L (283-708 µmol/L), but the upper limit is probably underestimated. The clinical studies of relation exposure effects, although ancient (from the 1970s) and realized with methodologies that do not meet current criteria, show concentration-efficacy and -toxicity relationship and the risk of drug interactions is proven. It is a drug preponderantly prescribed in children, a vulnerable population with physiological change with age. To benefit at best of its effectiveness, it is necessary to have relatively high plasma concentrations. Despite these arguments and due to the lack of studies providing a sufficient level of evidence, the recommendation can only be "potentially useful", assessment probably underestimated.

Possible drug-drug interaction between quetiapine and lamotrigine--evidence from a Swedish TDM database

AIM

The aim of the present study was to investigate a previously proposed interaction between quetiapine and lamotrigine resulting in reduced serum quetiapine concentrations.

METHODS

Data on 402 patients subjected to analysis of quetiapine concentration in serum were extracted from a routine therapeutic drug monitoring database. Among these patients, those concomitantly treated with lamotrigine (n= 22) were identified and matched with 22 controls receiving quetiapine while unexposed to lamotrigine. The dose-corrected quetiapine concentrations (C : D ratios) in the two groups were compared in both paired and unpaired analyses.

RESULTS

Patients co-treated with lamotrigine had a lower mean C : D ratio (0.71, 95% CI 0.46, 0.97) compared with controls (1.64, 95% CI 1.00, 2.28). Dose-corrected quetiapine concentrations were 58% lower in patients co-medicated with lamotrigine.

CONCLUSIONS

This study indicates that lamotrigine exposure is associated with substantially reduced serum concentrations of quetiapine, possibly due to induced glucuronidation. These findings need to be confirmed in experimental studies.

Antiepileptic drug interactions

INTRODUCTION

Antiepileptics are drugs used in the long-term treatment of epilepsy and other conditions such as pain or psychiatric diseases. They are often administered as polytherapy or in combination with other treatments. It is therefore important to know their potential interactions (with each other and with other substances) in order to avoid altering their efficacy or potentiating their side effects.

OBJECTIVE

The purpose of this article is to review these aspects and stress the most important interactions in day-to-day clinical practice.

RESULTS

Older antiepileptic drugs (AEDs) such as phenytoin, carbamazepine, phenobarbital and valproic acid can significantly interfere not only with each other and other AEDs, but also with other treatments. Although newer AEDs have a more favourable pharmacokinetic profile, they are not entirely exempt from interactions and they are also commonly administered in combination with older AEDs. Another aspect that should be considered is the existence of any clinically important pharmacokinetic and pharmacodynamic interactions in patients requiring the continuous administration of other treatments.

CONCLUSION

We must be aware of the pharmacokinetic and pharmacodynamic interactions of AEDs. Because of a lack of significant interactions, drugs such as levetiracetam, gabapentin or pregabalin can be recommended in particular groups such as patients with cancer, transplants, anticoagulant treatments or HIV infection. In all cases, it is important to ensure AED efficacy and prevent serious complications.

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.

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.

Properties of antiepileptic drugs in the treatment of idiopathic generalized epilepsies

Although valproate is considered to be the drug of first choice for the treatment of idiopathic generalized epilepsies (IGEs), other antiepileptic drugs (AEDs), both old (ethosuximide, clobazam, and clonazepam) and new (lamotrigine, levetiracetam, topiramate, and zonisamide) are also available. These AEDs do not appear to have a common mechanism of action in that both inhibitory gamma-aminobutyric acid (GABA; e.g., clobazam, clonazepam, and valproate) and excitatory glutamate (e.g., lamotrigine and topiramate) mechanisms are involved. Ethosuximide primarily acts by blocking T-type voltage-gated calcium channels in thalamic neurones while topiramate and zonisamide have multiple mechanisms of action. In contrast, levetiracetam is unique in that it may act via a specific binding site in the brain. In terms of their pharmacokinetic characteristics, all eight AEDs are rapidly absorbed after oral ingestion with peak blood concentration being achieved within 1-4 hours. Bioavailability is 100% with the exception clonazepam (90%) and topiramate (81-95%). Plasma protein binding is variable with valproate (90%), clobazam (85%) and clonazepam (86%) showing substantial binding, lamotrigine (55%) and zonisamide (50%) intermediate binding, and levetiracetam (0%), ethosuximide (0%) and topiramate (10%) being minimally bound. However, the binding by zonisamide is complicated by its binding to erythrocytes as well as albumin. All AEDs, with the exception of lamotrigine and levetiracetam, undergo elimination as a result of extensive metabolism by hepatic cytochrome P450 enzymes, which are highly amenable to induction and inhibition by other drugs and therefore susceptible to pharmacokinetic interactions. Lamotrigine metabolism is via hepatic glucuronidation, a process that is also susceptible to induction and inhibition by concurrent drugs. Levetiracetam is minimally metabolized (by hydrolysis in blood), is excreted predominantly unchanged in urine, and to date has not been associated with any clinically significant pharmacokinetic interactions. Using a semiquantitative pharmacokinetic rating system, based on 16 pharmacokinetic characteristics, a direct comparison between AEDs is possible. Thus valproic acid, regarded as the drug of first choice in the treatment of IGEs, rates lowest with respect to favorable pharmacokinetic characteristics, mostly because of its nonlinear pharmacokinetics, extensive hepatic metabolism, and its high propensity to interact both with other AEDs and non-AEDs. Levetiracetam rates highest with topiramate in second place.

Does lamotrigine influence valproate concentrations?

The aim of this study is to investigate the effect of lamotrigine (LTG) on valproate (VPA) concentrations dependent on LTG dose, LTG concentration, and additional enzyme-inducing antiepileptic drugs (AED) as well. For this purpose the following patient groups were compared: VPA monotherapy, VPA + one enzyme-inducing AED, VPA + LTG, and VPA + LTG + one enzyme-inducing AED. A total of 400 serum concentrations from 372 patients were evaluated. Two or more serum samples from the same patient were considered only if the comedication had been changed. For statistical evaluation, regression analytical methods and an analysis of variance were performed. For the analysis of variance, the VPA serum concentration in relation to VPA dose:body weight (level:dose ratio, LDR) was calculated and compared for different drug combinations. The analysis of variance revealed a significant effect of enzyme-inducing comedication (as expected) and age on the VPA LDR. Patients on LTG had a slightly lower VPA LDR, but this effect was not statistically significant. In addition, nonlinear regression analysis confirmed that patients on enzyme-inducing AED (carbamazepine, phenytoin, phenobarbital, methsuximide) had significantly lower VPA concentrations. Patients on ethosuximide had slightly but not significantly lower VPA concentrations. Patients on LTG also had significantly lower VPA levels, but this effect was only minor (-7%) and most probably not of any clinical relevance. Furthermore, the regression analysis showed that the relationship between the VPA dose per body weight and the serum concentration deviates significantly from linearity. Children less than 6 years old had lower VPA levels than older children and adults on a comparable VPA dose per body weight. Gender had no significant influence on VPA serum concentration. In addition, a subgroup of 40 patients was analyzed to see whether changing the LTG dose influences VPA serum concentrations. It did not. Thus, the authors conclude that the effect of LTG on VPA concentrations is not of clinical relevance.

Efficacy, tolerability, and kinetics of lamotrigine in infants

OBJECTIVES

To investigate the efficacy, tolerability, and kinetics of lamotrigine during the first year of life.

STUDY DESIGN

We studied 13 infants with intractable seizures; 7 had partial seizures and 7 had infantile spasms (1 had both). Patients received open-label lamotrigine as add-on therapy for 3 months. Seizure frequency, response ratio, and side effects score were determined and compared with the baseline period.

RESULTS

The rate of partial seizures per day decreased from 8.57 +/- 2.29 to 4.00 +/- 2.15 (P =.027) and infantile spasms from 8.71 +/- 2.15 to 3.61 +/- 2.762 (P =.028). Apparent clearance increased during the first year of life, with a break point at 2 months of age (mean, 0.119 +/- 0.021, 0.217 +/- 0.094 L/h per kilogram for infants <2 months and those 2 to 12 months old, respectively,P <.001). Twenty-four-hour concentration to time plots of three 3- to 4-week-old neonates showed a half-life of 23.44 +/- 3.57 hours. Compared with a group of 17 older children, LTG had similar efficacy (response ratios, -0.68 +/- 0.12 and -0.74 +/- 0.11, P =.504), and similar adverse effects scores (0.67 +/- 0.67 and 0.23 +/- 0.166, P =.95).

CONCLUSIONS

Lamotrigine is a useful and well tolerated drug for partial seizures and infantile spasms in infants <1 year of age. However, lamotrigine has age-dependent kinetics that must be taken into consideration.

Therapeutic drug monitoring of lamotrigine

OBJECTIVE

To evaluate the usefulness of routine monitoring of serum lamotrigine concentration.

DATA SOURCE

Literature was accessed through MEDLINE (1990-January 2001). Key search terms included lamotrigine, pharmacokinetics, and epilepsy.

DATA SYNTHESIS

A decision-making algorithm was used to evaluate the clinical evidence to support or refute the routine use of serum lamotrigine concentrations to adjust doses. The value of serum lamotrigine concentration monitoring remains controversial, primarily because clear relationships between concentration and pharmacologic response (either efficacy or toxicity) have not been demonstrated.

CONCLUSIONS

Serum concentration monitoring of lamotrigine is not recommended as a tool for routine dose adjustment.

Plasma concentration of topiramate correlates with cerebrospinal fluid concentration

The authors examined the ratio between the plasma and the cerebrospinal fluid (CSF) concentration of topiramate in 14 adults with epilepsy. Simultaneous trough samples of venous blood and CSF were collected and analyzed as total and unbound concentrations. Concomitant levels were also analyzed of lamotrigine (n = 5) and the relevant oxcarbazepine metabolite, 10-hydroxycarbazepine (n = 3). There was a close correlation between the plasma and the CSF concentration for both the total and unbound concentration of topiramate. The median CSF/plasma ratio of total topiramate was 0.85. The free topiramate concentration in plasma was not different from the free topiramate concentration in CSF. The CSF/plasma ratios showed little variation and were independent of the plasma level for both the total and the unbound levels. The unbound fraction of topiramate was 84% in plasma and 97% in CSF. The CSF concentrations of lamotrigine and 10-hydroxycarbazepine were 50% and 61% of the plasma concentrations, respectively. For topiramate, there is a close correlation between the plasma concentration and the CSF concentration. There does not seem to be a saturable carrier mechanism restricting topiramate transport across the blood-brain barrier. The concentration of topiramate in CSF is equal to the unbound proportion of topiramate in plasma, implying that the delivery of topiramate to the brain occurs via transfer from the unbound plasma pool. Plasma is thus a relevant matrix for therapeutic drug monitoring of topiramate.

Single-Dose Pharmacokinetics of Lamotrigine in Children: Influence of Age and Antiepileptic Comedication

To evaluate the influence of pediatric age and antiepileptic comedication on the single-dose pharmacokinetics of lamotrigine, 19 patients with epilepsy (10 comedicated with enzyme inducers and 9 comedicated with valproic acid) aged 8 months to 30 years received a single oral dose of lamotrigine (0.6 to 2.2 mg/kg) after an overnight fast. Blood samples were collected for at least 36 hours and plasma lamotrigine concentrations were determined by high-performance liquid chromatography. Pharmacokinetic parameters were calculated by noncompartmental analysis. Lamotrigine half-life (T1/2) and oral clearance (Cl/F) values were significantly lower and significantly higher, respectively, in patients comedicated with enzyme inducers than in those receiving valproic acid (T1/2 = 8.1 vs. 41.7 hours respectively, P < 0.001; Cl/F = 0.11 vs. 0.04 L/h per kg respectively, P < 0.005, geometric means), whereas Cmax and Tmax values were comparable in the two groups. The differences in pharmacokinetic parameters persisted when comparisons were made within subgroups stratified according to age. Within groups of patients homogeneous for type of comedication, Cmax and AUC values tended to be lower in children aged less than 12 years than in older patients. There was no significant relationship between half-life values and age. The authors conclude that both age and type of comedication influence lamotrigine pharmacokinetics. The reduction in lamotrigine concentrations caused by enzyme inducers and the elevation caused by valproic acid can be explained by stimulation and inhibition, respectively, of lamotrigine glucuronidation. On the other hand, the lower plasma lamotrigine levels in children than in adolescents and older patients may not be explainable solely by differences in metabolic rate.

Efficacy, pharmacology, and adverse effects of antiepileptic drugs

This article discusses the factors involved in the appropriate selection of anticonvulsant medications. The clinical use of commonly used traditional antiepileptic drugs and the newly marketed antiepileptic drugs is discussed. This includes the specific indications for use, adverse effects, and dosing of each drug. Drug interactions, mechanisms of action, and pharmacological properties of each drug is also reviewed.

The effect of lamotrigine on epileptiform discharges in young patients with drug-resistant epilepsy

PURPOSE

In a double-blind crossover study with lamotrigine (LTG), we investigated a possible relationship between the clinical responses and changes of the amount of epileptiform activity in EEG.

METHODS

Twelve patients, aged 4-21 years, with generalized drug-resistant epilepsy who had responded to LTG, completed the study. Twenty-four-hour video-EEGs were taken during control, placebo, and drug phases. The amounts of epileptiform discharges were estimated and compared with the clinical effects.

RESULTS

The duration of periods of repeated epileptiform discharges was significantly reduced during the LTG phase compared with the placebo phase (n = 12, p = 0.04). Ten patients showed a reduction of the amount of epileptiform discharges in the LTG phase by a mean of 81% (range, 17-100%). Periods of repeated epileptiform discharges with duration longer than 30 s were significantly reduced in length (p = 0.03) and number (p = 0.04) during the LTG phase compared with the placebo phase. Shorter periods of epileptiform discharges and the numbers of single epileptiform discharges were not affected. In five patients there was a seizure reduction (>/=50%) concomitant with the reduction of epileptiform discharges in the EEG. The behavior improved during LTG treatment in all patients. The patients became more alert, and their concentration and performance improved, according to their parents and the medical personnel.

CONCLUSIONS

LTG in dosages of 1-8 mg/kg body weight was found to depress relatively long episodes of interictal, repetitive, epileptiform activity in young patients with drug-resistant epilepsy, whereas short episodes were not affected. It depressed seizures in about half of the patients studied but gave improvements of behavior in all patients.

Validation of a population pharmacokinetic model for adjunctive lamotrigine therapy in children

AIMS

This analysis was performed to validate a previously developed population pharmacokinetic model for lamotrigine in order to establish a basis for dosage recommendations for children.

METHODS

(a) The importance of the covariates in the final model was confirmed using the model validation dataset. Population and individual (Bayesian estimate) pharmacokinetic parameters were estimated using both the initial model, which included none of the covariates, and the final model. Accuracy and precision of parameter estimation and of concentration prediction were compared between the two models. (b) The performance in predicting the validation concentrations by the final model parameters obtained previously from the model development dataset was assessed. (c) The parameters of the final model were refined using a dataset combining both the development and validation data.

RESULTS

Prediction performance of the final pharmacostatistical model was superior to that of the initial model. The results of the validation confirmed that concomitant antiepileptic drugs that increased or reduced lamotrigine clearance in adults had similar effects in children. The validation also verified the linear relationship between weight and clearance. The previously seen small sex effect on clearance was found statistically insignificant.

CONCLUSIONS

The current analysis confirmed the previous findings. To achieve the same concentrations, children receiving enzyme-inducing antiepileptic drugs without valproate require higher doses than those receiving valproate; and heavier children require higher doses.

Enzyme induction and inhibition by new antiepileptic drugs: a review of human studies

The aim of this paper is to review a number of new antiepileptic agents (i.e. felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin and zonisamide) for their inducing and/or inhibitory properties in humans, mainly considering the interactions where they are involved as the cause rather than the object of such interactions. Two aspects have been particularly taken into account: the changes or absence of changes in plasma/serum concentrations of concomitant drugs and the direct or indirect evidence of induction, inhibition or lack of effect on the six major human hepatic CYP isozymes (CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4), as well as on other CYP isozymes or enzyme systems. Felbamate clearly affects the pharmacokinetics of a number of drugs, generally increasing but also decreasing their concentrations. It induces enzymes such as CYP3A4 and inhibits enzymes such as CYP2C19 and those of the beta-oxidation pathway. Topiramate is not devoid of potential interaction properties: it decreases the plasma concentrations of ethinylestradiol, induces CYP3A4 and inhibits CYP2C19. For oxcarbazepine, no inhibitory, only inductive effects have been observed thus far. Felbamate. topiramate and oxcarbazepine may induce the metabolism of steroidal oral contraceptives. In this respect, tiagabine has been studied at a rather low dose. Pharmacodynamic or pharmacokinetic interaction seems to exist between lamotrigine and carbamazepine. Lamotrigine appears to be a weak inducer of UGTs, whereas induction of CYP3A4 seems improbable as the compound does not change the concentrations of oral contraceptives or the urinary excretion of 6beta-hydroxycortisol. Zonisamide has very peculiar pharmacokinetics and an extensive metabolism. Additional information on its enzyme inducing or inhibiting properties would be necessary, as data so far collected on its effect on the pharmacokinetics of other drugs are conflicting. Gabapentin, vigabatrin and in particular levetiracetam appear to be devoid of significant enzyme inducing or inhibiting properties.

Lamotrigine. A review of its use in childhood epilepsy

Lamotrigine is an antiepileptic agent that blocks use-dependent voltage-sensitive sodium channels, thereby preventing excitatory neurotransmitter release. However, this mechanism does not explain the broad range of clinical efficacy of this agent. In noncomparative trials, adjunctive lamotrigine (< or = 15 mg/kg/day) improved seizure control in children and adolescents with various refractory seizure types, with about 29 to 90% of patients showing a > or = 50% reduction in seizure frequency after > or = 3 months' treatment. Lamotrigine was particularly effective in generalised seizures, especially absence seizures and those related to the Lennox-Gastaut syndrome. In one placebo-controlled study, 33% of children and young adults (aged 3 to 25 years) with refractory Lennox-Gastaut syndrome had a reduction in seizure frequency of > or = 50% after 16 weeks of adjunctive lamotrigine treatment, compared with 16% of placebo recipients (p = 0.01). Significant reductions in seizure frequency when compared with placebo were also observed in patients with refractory generalised and partial seizures. The use of lamotrigine has also been associated with beneficial effects on cognition and behaviour. Adverse events associated with lamotrigine are primarily neurological, gastrointestinal and dermatological and are typically mild or moderate and transient with the exception of a potentially serious rash. Maculopapular or erythematous skin rash occurred in approximately 12% of paediatric patients (aged < 16 years) treated with lamotrigine and was the most common reason for treatment discontinuation. More severe forms of rash, including Stevens-Johnson syndrome, occasionally occurred, with a 3-fold higher incidence in children (approximately 1%) than adults (approximately 0.3%). However, lamotrigine treatment in paediatric trials was generally given at higher initial doses and faster dose escalations than recently revised recommendations. These factors, as well as concomitant use of valproic acid (valproate sodium), are associated with an increased risk of rash.

CONCLUSION

Although published clinical evidence is still limited in paediatric populations, lamotrigine is an effective and generally well tolerated broad-spectrum agent for adjunctive treatment of refractory seizures in children, most notably in those with Lennox-Gastaut syndrome. Results of direct comparisons with other antiepileptic agents are needed to determine more clearly the place of lamotrigine, particularly relative to newer agents, in the treatment of childhood epilepsy. The potential for serious rash in recipients of lamotrigine should also be kept in mind. Nonetheless, lamotrigine is a welcome addition to the available treatments for refractory childhood epilepsy, particularly Lennox-Gastaut syndrome.

New antiepileptic drugs in children: which ones for which seizures?

Until 1993, carbamazepine (CBZ), phenytoin (PHT), phenobarbital (PB), and valproate (VPA) accounted for the great majority of the prescriptions written for the treatment of epilepsy. Since 1993, five antiepileptic drugs (AEDs) have been released in the United States, and at least three additional drugs are expected to be released by the end of the year 2000. As a group, these newer drugs differ from the established drugs in terms of their pharmacokinetics, interaction potential, and adverse effects. In addition, any one of the newer drugs may achieve seizure control in situations in which an established drug had not. The newer drugs certainly represent a welcome addition to the existing options for the treatment of epilepsy in children. However, the availability of several new AEDs represents a therapeutic dilemma for the clinician because optimal use of these drugs has not yet been established. This is particularly true in children because (i) newer drugs are often studied less frequently in children, (ii) pharmacokinetics in children differ from those in adults, (iii) children may have different adverse effects, and (iv) children have a broader spectrum of various seizure types and epilepsy syndromes. In the first part of this review, the clinical pharmacology of the currently available newer AEDs is discussed individually, with special emphasis on data in children. In particular, pharmacokinetics, interactions, dosage and titration, efficacy spectrum, and adverse effect profile is discussed for each drug. In the second part, an attempt is made to determine the place for the newer drugs in the treatment of the different pediatric seizures and epilepsy syndromes.

[Lamotrigine therapy in children. Retrospective study of 32 children]

BACKGROUND

Lamotrigine is one of the new anti-epileptic drugs, which is a phenyltriazine derivative. It is considered to act via an inhibitory effect on voltage-sensitive sodium channels and to have no GABAergic action.

PATIENTS AND METHOD

We studied its efficiency in 32 children with refractory epilepsy after a treatment of at least one year with other anti-epileptic drugs. We then compared our results with other publications.

RESULTS

Good efficiency (at least 50% reduction of crises) has been demonstrated for lamotrigine in children with generalized epilepsy (62.5% good results), particularly with absence epilepsy and Lennox-Gastaut syndrome. Results are encouraging for our few patients with epilepsy with continuous spike waves during slow-wave sleep. On the other hand, more precise indications are needed in partial epilepsy.

CONCLUSION

Seizure control was generally maintained during one year of lamotrigine treatment. Association to sodium valproate is relevant for most of the authors. Adverse effects are uncommon, and we did not observe any skin rash. Lastly, improvement of behaviour and cognitive functions represents another important benefit of lamotrigine.

Mechanisms of ischaemic damage to central white matter axons: a quantitative histological analysis using rat optic nerve

The mechanism of ischaemic injury to white matter axons was studied by transiently depriving rat optic nerves in vitro of oxygen and glucose. Light and electron microscopic analysis showed that increasing periods of oxygen/glucose deprivation (up to 1 h) caused, after a 90-min recovery period, the appearance of increasing numbers of swollen axons whose ultrastructure indicated that they were irreversibly damaged. This conclusion was supported by experiments showing that the damage persisted after a longer recovery period (3 h). To quantify the axonal pathology, an automated morphometric method, based on measurement of the density of swollen axons, was developed. Omission of Ca2+ from the incubation solution during 1 h of oxygen/glucose deprivation (and for 15 min either side) completely prevented the axonopathy (assessed following 90 min recovery). Omission of Na+ was also effective, though less so (70% protection). The classical Na+ channel blocker, tetrodotoxin (1 microM), provided 92% protection. In view of this evidence implicating Na+ channels in the pathogenesis of the axonal damage, the effects of three different Na+ channel inhibitors, with known neuroprotective properties towards gray matter in in vivo models of cerebral ischaemia, were tested. The compounds used were lamotrigine and the structurally-related molecules, BW619C89 and BW1003C87. All three compounds protected the axons to varying degrees, the maximal efficacies (observed at 30 to 100 microM) being in the order: BW619C89 (>95% protection) > BW1003C87 (70%) > lamotrigine (50%). At a concentration affording near complete protection (100 microM), BW619C89 had no significant effect on the optic nerve compound action potential. Experiments in which BW619C89 was added at different times indicated that its effects were exerted during two distinct phases, one (accounting for about 50% protection) was during the early stage of oxygen/glucose deprivation itself and the other (also about 50%) during the first 15 min of recovery in normal incubation solution. The results are consistent with a pathophysiological mechanism in which Na+ entry through tetrodotoxin-sensitive Na+ channels contributes to Na+ loading of the axoplasm which then results in a lethal Ca2+ overload through reversed Na(+)-Ca2+ exchange. The identification of BW619C89 as a compound able to prevent oxygen/glucose deprivation-induced injury to white matter axons without affecting normal nerve function opens the way to testing the importance of this pathway in white matter injury in vivo.

Lamotrigine

Clinical use of the antiepileptic drug (AED) lamotrigine (LTG) has dramatically increased since its introduction in Europe in 1991 and in the United States in 1994. This article surveys the English-language literature of LTG published before 1998. This literature is concerned with the molecular mechanisms of LTG's antiepileptic action, evaluation of its clinical antiepileptic efficacy, adverse experiences associated with its clinical use, and current guidelines for its initiation. LTG's efficacy has been extensively confirmed in multiple postmarketing studies, and its applications are broad. The most serious adverse experiences have involved skin rash. Valproic acid affects LTG metabolism, and a specific set of guidelines for the concurrent use of valproic acid and LTG has been developed. Unique issues are also associated with its pediatric use. LTG has a significant place in clinical management of a wide range of epilepsy syndromes, and the scope of its use is expanding. Accumulating clinical data enable the clinician to maximize its efficacy and minimize adverse experiences. Guidelines for its pediatric use must be followed diligently.

Pharmacokinetics of lamotrigine in children in the absence of other antiepileptic drugs

We evaluated the pharmacokinetics of lamotrigine in 12 children with epilepsy who were receiving no other antiepileptic drugs. Each patient received a single oral dose of lamotrigine 2 mg/kg. Plasma concentrations of the drug were measured up to 48 hours after dosing. Pharmacokinetic parameters were calculated using noncompartmental methods. After rapid absorption, the lamotrigine concentration declined monoexponentially. Oral clearance was 0.64 +/- 0.26 ml/min/kg. The apparent volume of distribution was 1.50 +/- 0.51 L/kg. Weight-normalized clearance and volume were higher in children than in adults. The mean half-life was 32 hours, similar to that in adults. Should similar plasma lamotrigine concentrations in adults and children be desirable, children will likely require higher weight-normalized doses at the same dosing frequency.

Lamotrigine serum concentration-to-dose ratio: influence of age and concomitant antiepileptic drugs and dosage implications

Using bivariate and multivariate methods, we retrospectively analyzed the influence of patient age and the use of concomitant antiepileptic drugs (AEDs) on the lamotrigine (LTG) concentration-to-dose (C/D) ratio in samples from 164 patients (68 children, 96 adults) with epilepsy receiving LTG alone (n = 28) or in combination with various antiepileptic drugs (n = 136). The LTG C/D ratio increased with age in children receiving LTG alone (r = 0.60, p < 0.01), but decreased with age in adults receiving LTG and inducers (r = -0.42, p < 0.001). In patients receiving LTG and inducers, the ratio was statistically lower in those younger than 9 years of age (0.23 +/- 0.08) and older than 30 years of age (0.32 +/- 0.15) than it was in those between 9 and 30 years of age (0.44 +/- 0.15). The mean LTG C/D ratio was 0.37 +/- 0.15 in patients receiving LTG and inducers (n = 92), 0.84 +/- 0.41 in patients receiving LTG alone (n = 28), 1.09 +/- 0.44 in those receiving LTG with VPA plus inducers (n = 17), and 3.41 +/- 1.18 in those receiving LTG and VPA (n = 27). Differences in the LTG C/D ratio between treatment groups were similar in children and in adults. We reached the following conclusions: The LTG C/D ratio increased with age in children but may decrease with age in adults receiving concomitant enzyme-inducing AEDs; the LTG C/D ratio was 10 times lower in patients receiving LTG and inducers than in those receiving LTG and VPA (in both children and adults), and this difference was higher than the four-fold difference described for LTG half-life and the two-fold differences currently used in LTG dosage.

Influence of oxcarbazepine and methsuximide on lamotrigine concentrations in epileptic patients with and without valproic acid comedication: results of a retrospective study

The aim of this retrospective study was to investigate the influence of oxcarbazepine (OCBZ) and methsuximide (MSM) on lamotrigine (LTG) serum concentrations. The effect of OCBZ compared to carbamazepine (CBZ) and the effect of MSM on LTG serum concentrations were examined in patients with and without valproic acid (VPA) comedication. Altogether, 376 samples from 222 patients were analyzed in routine drug monitoring. Two or more serum samples from the same patient were considered only if the comedication had been changed. For statistical evaluation, regression analytical methods and an analysis of variance were performed. For the analysis of variance, the LTG serum concentration in relation to LTG dose/ body weight--level-to-dose ratio (LDR), in (microg/mL)/(mg/kg)--was calculated and compared for different drug combinations. The nonlinear regression analysis including the LTG dose per body weight, age, gender, and the different kinds of comedication revealed that these variables have a significant influence on LTG serum concentration (r2 = 0.724). The relationship between LTG dose/body weight and serum concentration deviates only slightly from linearity, the LTG concentration was about 18% lower in women than in men, and age had a significant influence. The data indicate that children have significantly lower LTG concentrations than adults on a comparable LTG dose per body weight and that children may be more prone to enzyme induction by comedicated drugs. Methsuximide has a strong inducing effect on the LTG metabolism and decreases the LTG concentrations markedly (about 70% compared to LTG monotherapy). Carbamazepine also reduces the LTG concentrations considerably (by 54%). The inducing effect of OCBZ (29%) was less pronounced but also significant. The inducing effect of MSM, CBZ, and OCBZ was also seen in combination with VPA: VPA alone increases the LTG concentration approximately 211%, whereas in addition to MSM (8%), CBZ (21%), or OCBZ (111%), the increase of LTG was significantly smaller. The analysis of variance confirmed the results of the regression analysis. The effect of MSM on the LTG concentration should be considered if MSM is added or withdrawn in patients treated with LTG. Oxcarbazepine had a less pronounced inducing effect on LTG metabolism compared to CBZ. If CBZ is replaced by OCBZ as comedication, an increase in LTG serum concentrations should be expected.

Lamotrigine drug interactions in a TDM material

Using therapeutic drug monitoring (TDM) data from our laboratory, we have studied the concentration/dose relationship for lamotrigine and the influence of drug interactions in clinical practice. One hundred forty-nine lamotrigine samples from 104 adult patients were included in the study. The samples were collected as steady-state trough values, 9-16 hours after dose intake. Concomitant drug treatment was specified on the analysis request form. Lamotrigine serum concentrations were determined by high-performance liquid chromatography (HPLC). In 20 patients in monotherapy, the concentration/dose (C/D) ratio was 65 (range: 50-84) nmol/L/mg (mean and 95% confidence interval, antilog from lognormal distribution). In 37 patients with concomitant carbamazepine treatment, the C/D ratio was less than half that of the patients in monotherapy; 31 (2146) nmol/L/mg, and in 14 patients with phenytoin, it was even lower; 17 (13-23) nmol/L/mg. Valproic acid significantly increased the C/D to 251 (200-320) nmol/L/mg in 13 patients. Triple therapy with valproic acid and either carbamazepine or phenytoin (23 patients) yielded a C/D slightly above that of monotherapy, whereas a few patients on phenobarbital had a C/D slightly below that of monotherapy. The within-group C/D variation was comparatively small. The C/D ratio in a mixed lamotrigine TDM material shows a widespread intra- and interindividual variation, which can largely be explained by pharmacokinetic interactions with concomitantly used antiepileptic drugs. These results support the use of TDM in lamotrigine therapy.

Lamotrigine concentrations in human serum, brain tissue, and tumor tissue

PURPOSE

Although lamotrigine (LTG) has been used for years as an antiepilepic drug (AED), there are no data on penetration into the brain with the exception of a single case report. It was our aim to determine the LTG content in the brain and the tumor tissue and to bring the same into relation to the serum concentration and protein binding in neurosurgically operated patients.

METHODS

Neurosurgical intervention was performed on 11 patients with brain tumors. Tumor tissue was removed, and LTG kinetics was carried out for 12 h. LTG was determined by means of (HPLC) and the protein binding by means of ultrafiltration.

RESULTS

At the time of the section of the tumor, the LTG concentrations in the serum were an average of 3.7 microg/ml (range, 1.1-9.8); in the brain, an average of 6.8 microg/g (range, 1.0-14.9); and in the tumor, an average of 4.4 microg/g (range, 2.0-8.3). Brain/serum and tumor/serum ratios of 2.8 and 1.9, respectively, result from these data. The protein binding was on average 68% (range, 46-96).

CONCLUSIONS

LTG is a lipophile AED with a moderate protein binding that penetrates brain tissue well and can be proven even in the tumor tissue.

Monotherapy versus polytherapy in epilepsy: a framework for patient management

The long-standing debate between proponents of monotherapy and those of polytherapy for treatment of epilepsy has been rekindled by the recent development of several new antiepileptic drugs. The likelihood of improved seizure control on polytherapy must be weighed against the risk of increased side effects, complex drug interactions and cost. Providing maximal seizure control while avoiding overtreatment is a challenge which requires an ongoing critical evaluation of each patient's management. This review provides a framework for decision-making by considering issues affecting the choice between monotherapy and polytherapy in five clinical situations: 1) newly diagnosed epilepsy; 2) seizures on monotherapy; 3) seizures controlled on polytherapy; 4) not controlled on polytherapy; 5) change in medical condition.

New drugs for epilepsy

Seizure freedom with no side-effects is the aim of treatment, and new antiepileptic drugs have not lived up to expectations; only a few patients with chronic epilepsy have been rendered seizure-free. These treatments have side-effects but their safety profile may be better than older alternatives, although chronic effects have not yet been established. This article reviews newly marketed antiepileptic drugs. It concentrates on shortcomings of current antiepileptic treatment and on the way drugs are developed. A new approach to treatment is long overdue. The development of rational antiepileptic treatments should be strongly encouraged. More clinically relevant paradigms need to be developed and incorporated into clinical trial programmes as these are presently biased in their designs towards regulatory issues.

High-performance liquid chromatography quantitation of plasma lamotrigine concentrations: application measuring trough concentrations in patients with epilepsy

Lamotrigine is a phenyltriazine anticonvulsant recently approved for clinical use. A high-performance liquid chromatographic (HPLC) method was developed using a silica column (5 microm) with an aqueous methanol mobile phase consisting of 94% methanol, 5.92% water, and 0.08% NH4H2PO4 adjusted to a final apparent pH of 4.0 and pumped at a flow rate of 1.0 ml/minute. Ultraviolet detection was carried out at a wavelength of 280 nm, and plasma samples were prepared for HPLC analysis by extraction into ethyl acetate after basification. Retention times for lamotrigine and its internal standard (BWA725C) were 10.3 and 11.2 minutes, respectively, and there was no chromatographic interference from other commonly coadministered anticonvulsants. Calibration curves were linear over a concentration range of 0.5 to 30 mg/l, with intra-assay and interassay coefficients of variation less than 8%. Assessment of assay performance in an international quality assurance program showed an average bias of 0.3% compared with the consensus mean. A review of 52 patient specimens showed that, if patients were grouped according to coadministered anticonvulsants, a significant correlation between lamotrigine dosage and concentration was evident in those coadministered valproate (in the absence of metabolic inducers) and in those coadministered a combination of valproate and inducers, but not in patients coadministered inducers alone. Mean (SD) trough concentrations were 9.2 (5.2), 2.8 (1.3), and 3.8 (2.8) mg/l in the valproate, inducer, and combination groups, respectively.

Lamotrigine: pharmacokinetics

The pharmacokinetics of lamotrigine have been studied in single and multiple dose studies in animals, normal volunteers, and patients with epilepsy. Lamotrigine exhibits first-order linear pharmacokinetics. Lamotrigine is well absorbed with bioavailability approaching 100%. The absorption is unaffected by food and there is no first-pass metabolism. The volume of distribution is between 1.25 and 1.47 L/kg and protein binding is about 55%. The half-life of lamotrigine is between 24.1 and 35 hours in drug naive adults but may be altered by enzyme inducing and inhibiting drugs. Clinical trials demonstrated no evidence of autoinduction or saturable metabolism. Younger children (0.17 to 5 years) eliminate lamotrigine faster than older children (5 to 10 years). Children may be more prone to enzyme induction than adults.

Lamotrigine in absence and primary generalized epilepsies

Although lamotrigine has been approved in the United States as adjunctive therapy for partial seizures in patients older than 12 years, there is increasing evidence that it is just as effective, if not more effective, in the treatment of generalized seizures. A large number of open-label studies and some single-blind data, all using lamotrigine as add-on therapy in patients with previously refractory generalized seizures, are available. Controlled studies, some on newly diagnosed, previously untreated patients with generalized seizures are ongoing. Investigations have demonstrated that patients with the following generalized seizure types improve with lamotrigine add-on therapy: Typical and atypical absence, atonic, generalized tonic-clonic, myoclonic, and clonic seizures. Response rates, defined as the percentage of patients with better than 50% reduction in seizure frequency, have been, depending on seizure type, in the range of 30% to 56%, with 0 to 33% of the patients becoming seizure free. The best responses have been noted in typical and atypical absences, and atonic seizures. Children and adults appear to have comparable responses. In addition, add-on studies in patients with specific, previously refractory, epilepsy syndromes have demonstrated that the best improvement in seizure control occurs in patients with petit mal epilepsy, "other symptomatic" generalized epilepsies, and in Lennox-Gastaut syndrome, followed by patients with other myoclonic epilepsies, myoclonic absence and West syndrome. Many previously refractory patients are able to achieve lamotrigine monotherapy. However, patients with nonprogressive myoclonic epilepsy have little, if any, response. Early data from ambulatory encephalographic (EEG) recordings in patients with previously refractory absence seizures, and from controlled studies on patients with newly diagnosed typical absence seizures, appear to confirm the efficacy of lamotrigine in those patients. Controlled studies are ongoing in patients with absence seizures, in patients with generalized tonic-clonic seizures, and in patients with Lennox-Gastaut syndrome. Dosing in generalized seizures is similar to that for partial seizures. Because of the shorter half-life of lamotrigine in children, as compared to adults, higher (mg/kg) doses are often needed in young patients. We conclude that lamotrigine is a promising drug for absence and primary generalized seizures in both children and adults.

No increase in carbamazepine-10,11-epoxide during addition of lamotrigine treatment in children

It has been suggested that lamotrigine (LTG) may enhance the toxicity of carbamazepine (CBZ) by increasing the concentration of the active metabolite carbamazepine-10,11-epoxide (CBZ-E) in adult patients. The authors investigated this hypothesis in an add-on study in 11 children and 3 adolescents, aged 6-22 years, who had been treated for more than 1 year with CBZ in monotherapy or with CBZ in combination with one or two other antiepileptic drugs. The LTG dosage was increased step by step until clinical response or side effects were observed. The plasma concentrations of LTG, CBZ, and CBZ-E were monitored during steady state conditions before and after the addition of LTG. It was found that LTG had no effect on mean CBZ concentrations and that it decreased rather than increased the mean plasma concentration of CBZ-E from 6.4 +/- 2.6 to 4.9 +/- 2.4 mumol/l (mean +/- SD, n = 14, P = 0.019). Observed side effects were diplopia in two children, agitation in two, and increased number of seizures in one. None of these five patients had unusually high CBZ-E levels when the side effect developed. It is concluded that addition of lamotrigine in children treated with carbamazepine children does not result in a pharmacokinetic interaction with a toxic accumulation of carbamazepine-10,11-epoxide.