Topiramate Serum Concentration-to-Dose Ratio

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.

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.

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.

Drug-Level Monitoring on Admission for Presurgical Epilepsy Evaluation

OBJECTIVE

Evaluation for surgical treatment is offered to patients who do not respond to antiepileptic drugs. Pseudo-pharmacoresistance (PPR) has been described in the context of impaired compliance, incorrect diagnosis of epilepsy or pharmacological interference resulting in too low blood levels. We were interested to determine the frequency and causes of PPR in patients admitted for presurgical evaluation.

METHODS

We reviewed 553 drug levels in 199 patients and analyzed the relative frequency of drugs below reference range (10 and 20% below the range).

RESULTS

Patients who had at least one serum level below the 10% cut-off amounted to 33% and 9% of patients had at least one serum level below the 20% cut-off. Only in 2 patients (1%), this was due to poor compliance. Low levels were equally frequent in mono- or polytherapy. Drugs that were most frequently found out of range were phenytoin, valproate, and topiramate. In monotherapy, lamotrigine was often prescribed in too low dosages.

CONCLUSION

Low drug levels are frequently observed in surgical candidates due to pharmacological interference or insufficient dosing. Poor compliance or incorrect diagnosis does not appear to be a significant concern in this patient group. Our data strengthen the need for regular drug monitoring even in advanced chronic epilepsy to avoid unnecessary health costs by too low and ineffective dosages.

The effects of antiepileptic inducers in neuropsychopharmacology, a neglected issue. Part I: A summary of the current state for clinicians

The literature on inducers in epilepsy and bipolar disorder is seriously contaminated by false negative findings. This is part i of a comprehensive review on antiepileptic drug (AED) inducers using both mechanistic pharmacological and evidence-based medicine to provide practical recommendations to neurologists and psychiatrists concerning how to control for them. Carbamazepine, phenobarbital and phenytoin, are clinically relevant AED inducers; correction factors were calculated for studied induced drugs. These correction factors are rough simplifications for orienting clinicians, since there is great variability in the population regarding inductive effects. As new information is published, the correction factors may need to be modified. Some of the correction factors are so high that the drugs (e.g., bupropion, quetiapine or lurasidone) should not co-prescribed with potent inducers. Clobazam, eslicarbazepine, felbamate, lamotrigine, oxcarbazepine, rufinamide, topiramate, vigabatrin and valproic acid are grouped as mild inducers which may (i)be inducers only in high doses; (ii)frequently combine with inhibitory properties; and (iii)take months to reach maximum effects or de-induction, definitively longer than the potent inducers. Potent inducers, definitively, and mild inducers, possibly, have relevant effects in the endogenous metabolism of (i)sexual hormones, (ii) vitamin D, (iii)thyroid hormones, (iv)lipid metabolism, and (v)folic acid.

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.

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.

Antiepileptic drug interactions - principles and clinical implications

Antiepileptic drugs (AEDs) are widely used as long-term adjunctive therapy or as monotherapy in epilepsy and other indications and consist of a group of drugs that are highly susceptible to drug interactions. The purpose of the present review is to focus upon clinically relevant interactions where AEDs are involved and especially on pharmacokinetic interactions. The older AEDs are susceptible to cause induction (carbamazepine, phenobarbital, phenytoin, primidone) or inhibition (valproic acid), resulting in a decrease or increase, respectively, in the serum concentration of other AEDs, as well as other drug classes (anticoagulants, oral contraceptives, antidepressants, antipsychotics, antimicrobal drugs, antineoplastic drugs, and immunosupressants). Conversely, the serum concentrations of AEDs may be increased by enzyme inhibitors among antidepressants and antipsychotics, antimicrobal drugs (as macrolides or isoniazid) and decreased by other mechanisms as induction, reduced absorption or excretion (as oral contraceptives, cimetidine, probenicid and antacides). Pharmacokinetic interactions involving newer AEDs include the enzyme inhibitors felbamate, rufinamide, and stiripentol and the inducers oxcarbazepine and topiramate. Lamotrigine is affected by these drugs, older AEDs and other drug classes as oral contraceptives. Individual AED interactions may be divided into three levels depending on the clinical consequences of alterations in serum concentrations. This approach may point to interactions of specific importance, although it should be implemented with caution, as it is not meant to oversimplify fact matters. Level 1 involves serious clinical consequences, and the combination should be avoided. Level 2 usually implies cautiousness and possible dosage adjustments, as the combination may not be possible to avoid. Level 3 refers to interactions where dosage adjustments are usually not necessary. Updated knowledge regarding drug interactions is important to predict the potential for harmful or lacking effects involving AEDs.

Molecular pharmacodynamics, clinical therapeutics, and pharmacokinetics of topiramate

Topiramate (TPM; TOPAMAX) is a broad-spectrum antiepileptic drug (AED) that is approved in many world markets for preventing or reducing the frequency of epileptic seizures (as monotherapy or adjunctive therapy), and for the prophylaxis of migraine. TPM, a sulfamate derivative of the naturally occurring sugar D-fructose, possesses several pharmacodynamic properties that may contribute to its clinically useful attributes, and to its observed adverse effects. The sulfamate moiety is essential, but not sufficient, for its pharmacodynamic properties. In this review, we discuss the known pharmacodynamic and pharmacokinetic properties of TPM, as well as its various clinically beneficial and adverse effects.

Treatments for late-life bipolar disorder

BACKGROUND

Bipolar affective disorder is not uncommon in the elderly; prevalence rates in the United States range from 0.1% to 0.4%. However, it accounts for 10% to 25% of all geriatric patients with mood disorders and 5% of patients admitted to geropsychiatric inpatient units. These patients often present a tremendous treatment challenge to clinicians. They frequently have differing treatment needs compared with their younger counterparts because of substantial medical comorbidity and age-related variations in response to therapy. Unfortunately, the management of geriatric bipolar disorder has been relatively neglected compared with the younger population. There continues to be a scarcity of published, controlled trials in the elderly, and no treatment algorithms specific to bipolar disorder in the elderly have been devised.

OBJECTIVE

The goal of this article was to review the current literature on both the pharmacologic and nonpharmacologic management of late-life bipolar disorder.

METHODS

English-language articles written on the treatment of bipolar disorder in the elderly were identified. The first step in data collection involved a search for evidence-based clinical practice guidelines in the Cochrane Database of Systematic Reviews (up until the third quarter of 2006). Systematic reviews were then located in the following databases: MEDLINE (1966-September 2006), EMBASE (1980-2006 [week 36]), and PsycINFO (1967-September 2006 [week 1]). Additional use was made of these 3 databases in searching for single randomized controlled trials, meta-analyses, cohort studies, case-control studies, case series, and case reports. "Elderly," used synonymously with "geriatric," was defined as individuals aged > or =60 years. However, to take into account ambiguity in the nomenclature, the key words aged, geriatric, elderly, and older were combined with words indicating pharmacologic treatments such as pharmacotherapy; classes of medications (eg, lithium, antidepressants, antipsychotics, anticonvulsants, benzodiazepines); and names of selected individual medications (eg, lithium, valproic acid, lamotrigine, carbamazepine, oxcarbazepine, topiramate, gabapentin, zonisamide, clozapine, risperidone, olanzapine, quetiapine, ziprasidone, aripiprazole). These terms were then combined with the diagnostic terms bipolar disorder, mania, hypomania, depression, or bipolar depression. Finally, the terms ECT and psychotherapy were also queried in combination with indicators for age and diagnosis. A few articles on "older adults," usually defined as individuals aged 50 to 55 years, were also included. They may allow for possible extrapolation of data to the geriatric population. Additionally, several mixed-age studies were included for similar considerations. Case reports and case series were described for their potential heuristic value.

RESULTS

Unfortunately, there is a considerable dearth of literature involving evidence-based clinical practice guidelines and even randomized controlled trials in elderly individuals with bipolar disorder. Available options for the treatment of bipolar disorder (including those for mania, hypomania, depression, or maintenance) in the elderly include lithium, antiepileptics, antipsychotics, benzodiazepines, antidepressants, electroconvulsive therapy (ECT), and psychotherapy.

CONCLUSIONS

The data for the treatment of late-life bipolar disorder are limited, but the available evidence shows efficacy for some commonly used treatments. Lithium, divalproex sodium, carbamazepine, lamotrigine, atypical antipsychotics, and antidepressants have all been found to be beneficial in the treatment of elderly patients with bipolar disorder. Although there are no specific guidelines for the treatment of these patients, monotherapy followed by combination therapy of the various classes of drugs may help with the resolution of symptoms. ECT and psychotherapy may be useful in the treatment of refractory disease. There is a need for more controlled studies in this age group before definitive treatment strategies can be enumerated.

Topiramate and its clinical applications in epilepsy

Topiramate, a derivative of the monosaccharide d-fructose, has shown a wide spectrum of antiepileptic efficacy in both animal models and clinical trials. Multiple putative mechanisms of action include voltage-sensitive sodium channel blockade, calcium channel inhibition, increase of potassium conductance, GABA-mediated chloride current increment, glutamate-mediated neurotransmission inhibition and carbonic anhydrase isoenzyme inhibition. In general, the clinical response is maintained in the long-term. The most common side effects include somnolence, fatigue, headache, psychomotor slowing, confusion, difficulty with memory, impaired concentration and attention, speech and language problems and weight loss. If slowly titrated and used at a low-to-medium dosage, it is well tolerated and offers a valid therapeutic option, the relevance of which is comparable to that of the most widely used 'old' antiepileptic drugs. As it is not yet wholly clear which specific epilepsy syndromes may benefit most from topiramate with respect to other drugs, more accurate indications for initial monotherapy would require syndrome-oriented trials and more clinical experience.

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 experience with topiramate dosing and serum levels in patients with epilepsy

OBJECTIVE

To investigate the relevance of serum topiramate (TPM) levels (SL) monitoring in the clinical management of epileptic patients.

METHODS

Twenty-seven patients with different epileptic syndromes on TPM therapy were studied. TPM was used as add-on in 26 patients, only in one as monotherapy de novo; one case changed from TPM as add-on to TPM monotherapy. The mean follow-up time was 11 months. TPM SL were measured by fluorescence polarization immunoassay.

RESULTS

We analyzed the TPM SL in 43 samples from 27 patients. Mean TPM dose was 3.9mg/kg, mean TPM SL 13.43mumol/l. The mean level to dose ratio (LDR) was 3.63mumol/l/mg/kg. Four patients became seizure-free, all with TPM dosages lower than the mean. Eleven patients had at least 50% seizure reduction. The comedication with enzyme-inducing AED significantly reduced TPM SL and LDR. On the other hand, the influence of valproic acid (VPA) on TPM LDR was not univocal. Indeed, patients younger than 15 years showed SL values lower than the adults did, although not significant.

CONCLUSION

We could not detect a direct relationship between high TPM SL and efficacy neither between high TPM SL and tolerability. However, the data we collected seem to favour the hypothesis that high TPM dosage and SL might be associated to a greater probability to reduce seizure severity.

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.

Pharmacokinetic and metabolic investigation of topiramate disposition in healthy subjects in the absence and in the presence of enzyme induction by carbamazepine

PURPOSE

To characterize the metabolic profile of topiramate (TPM) in humans and to assess the influence of enzyme induction by carbamazepine (CBZ) on the pharmacokinetics and metabolic profile of TPM.

METHODS

Twelve healthy subjects received a single oral dose of TPM (200 mg) on two randomized occasions. On one occasion, TPM was administered alone, and on the other, it was given on day 18 of a 24-day treatment with CBZ (maintenance dosage, 600 mg/day). Blood and urine samples were collected for > or = 72 h after dosing. TPM and its metabolites were assayed in plasma and urine by a specific liquid chromatography-mass spectroscopy (LC-MS) method.

RESULTS

Mean TPM oral clearance (CL/F) increased from 1.2 L/h (control) to 2.2 L/h after CBZ treatment. Mean TPM half-life decreased from 29 h to 19 h. TPM was excreted extensively in urine both under noninduced (56%) and CBZ-induced conditions (40%). 2,3-O-Des-isopropylidene-TPM (2,3-diol-TPM) was identified as the most prominent urinary metabolite, with a recovery accounting for 3.2% and 7.9% of the TPM dose under noninduced and induced conditions, respectively. Corresponding recovery values for 10-hydroxy-TPM (10-OH-TPM) were 1.2% and 1.8%, respectively. The control AUC(metabolite)/AUC(drug) ratio for 2,3-diol-TPM and 10-OH-TPM were 1.5% and 0.6%, and they increased by threefold and twofold, respectively, after CBZ treatment.

CONCLUSIONS

TPM remains appreciably excreted unchanged in urine (41%) under CBZ-induced conditions, even though TPM CL/F increased by twofold. Although 2,3-diol-TPM and 10-OH-TPM were measured in unconjugated form, the significant increases in their AUC and urinary excretion are consistent with the twofold increase in TPM clearance.