Pharmacokinetic profile of topiramate in comparison with other new antiepileptic drugs

A novel functional role for the classic CNS neurotransmitters, GABA, glycine, and glutamate, in the kidney: potent and opposing regulators of the renal vasculature

The presence of a renal GABA/glutamate system has previously been described; however, its functional significance in the kidney remains undefined. We hypothesized, given its extensive presence in the kidney, that activation of this GABA/glutamate system would elicit a vasoactive response from the renal microvessels. The functional data here demonstrate, for the first time, that activation of endogenous GABA and glutamate receptors in the kidney significantly alters microvessel diameter with important implications for influencing renal blood flow. Renal blood flow is regulated in both the renal cortical and medullary microcirculatory beds via diverse signaling pathways. GABA- and glutamate-mediated effects on renal capillaries are strikingly similar to those central to the regulation of central nervous system capillaries, that is, exposing renal tissue to physiological concentrations of GABA, glutamate, and glycine led to alterations in the way that contractile cells, pericytes, and smooth muscle cells, regulate microvessel diameter in the kidney. Since dysregulated renal blood flow is linked to chronic renal disease, alterations in the renal GABA/glutamate system, possibly through prescription drugs, could significantly impact long-term kidney function.NEW & NOTEWORTHY Functional data here offer novel insight into the vasoactive activity of the renal GABA/glutamate system. These data show that activation of endogenous GABA and glutamate receptors in the kidney significantly alters microvessel diameter. Furthermore, the results show that these antiepileptic drugs are as potentially challenging to the kidney as nonsteroidal anti-inflammatory drugs.

Outcome Evaluation of Gabapentin as Add-on Therapy for Partial Seizures

Objective:

The safety, tolerability, efficacy, and impact on quality of life of gabapentin (Neurontin®) as adjunctive therapy to carbamazepine (CBZ) and/or phenytoin (PHT) was assessed in epileptic patients with partial seizures.

Methods:

NEON (Neurontin Evaluation of Outcomes in Neurological Practice) was an open-label, prospective, multicentre study conducted in patients on a stable dose of CBZ and/or PHT and experiencing an average of up to 4 complex partial seizures with or without secondary generalization per month, with no seizure-free months. The treatment lasted 20 weeks. Gabapentin was started at 400 mg/day and was individually titrated to effective tolerable dose up to 2400 mg/day. Quality of life was evaluated using the QOLIE-10 questionnaire.

Results:

A total of 141 patients were enrolled at 36 sites; 114 patients were evaluable for efficacy analyses. The mean maintenance dose of gabapentin was 1600 mg/day (range = 300-3200). A decrease of 50% or more in frequency of complex partial + secondarily generalized seizures was observed in 81 (71 %) patients (p = 0.0001). Fifty two (46%) patients were seizure-free during the last 8 weeks of treatment. A significant improvement (p < 0.05) was observed in 5 of the 10 questions of the QOLIE-10, as well as in the composite QOL score (p = 0.0002). The most frequent adverse events included somnolence (16%), dizziness (9%), and asthenia (6%). Twenty-five (18%) patients prematurely discontinued the study, 16 (11%) of them due to adverse events.

Conclusion:

This study indicates that treatment with gabapentin as adjunctive therapy to standard antiepileptic drugs in this group of patients not only provides significant improvement in seizure control, but also has a positive impact on quality of life. The clinical benefits in efficacy, safety and tolerability demonstrated at 20 weeks are sustained, and no tolerance develops with gabapentin in longer term use.

Novel anticonvulsant drugs

Principles of complex mechanisms of action of anticonvulsants including latest reports concerning new antiepileptic drugs (AED) are considered. Different aspects of new anticonvulsant drugs (2nd generation) from preclinical and clinical testing, pharmacokinetics, and mono or combination therapy in children and adults are summarized. In the following condensed synopsis pharmacological and clinical characteristics of gabapentin (GBP), lamotrigine (LTG), levetiracetam (LEV), oxcarbazepine (OXC), pregabalin (PGB) and tiagabine (TGB) as well as topiramate (TPM) and zonisamide (ZNS) are discussed. In addition to the mechanisms of action, pharmacokinetics, interactions, indications and dosages as well as side effects are considered. Important data concerning the effect and tolerability of anticonvulsant drugs can be obtained from controlled studies. In comparison to drugs of the first generation (phenobarbital [PB], primidon [PRD], phenytoin [PHT], carbamazepine [CBZ] and valproic acid [VPA]) the potential for interactions and side effects due to enzyme induction or inhibition is reduced by most of the anticonvulsant drugs of the second generation. New anticonvulsant drugs increase the spectrum of treatment and represent further steps with regard to the optimization of an individual therapy of the epilepsies.

Topiramate in migraine prophylaxis

Many of the drugs available for the prophylaxis of migraine never underwent rigorous placebo-controlled trials, and the effective drugs have side effects that are problematic in daily practice. Three double-blind studies involving more than 1500 patients (the largest clinical development program ever undertaken for migraine prophylaxis) consistently found topiramate, at doses of 100 and 200 mg/day, to be superior to placebo in migraine prophylaxis. Topiramate has demonstrated efficacy in reducing the frequency of migraines, producing a statistically significant effect at 1 month and maintaining effectiveness over more than 6 months. It is not associated with the side effects that may cause patients to stop taking other prophylactic medications.

Influence of Dosage, Age, and Co-medication on Plasma Topiramate Concentrations in Children and Adults with Severe Epilepsy and Preliminary Observations on Correlations with Clinical Response

The influence of dosage, age, and co-medication on plasma topiramate (TPM) concentrations at steady state was investigated in 51 patients aged 3 to 30 years. All patients had chronic active epilepsy, and most were receiving concomitant medication with enzyme-inducing anticonvulsants (carbamazepine and phenobarbital). Plasma TPM concentrations were determined by a specific immunoassay in samples obtained before the morning dose. Thirty-five patients could be evaluated prospectively at different dose levels, and the relationship between plasma TPM concentration and dosage was linear over the assessed dose range (1.8 to 10.0 mg/kg) both in adults and in children. The influence of age on pharmacokinetic parameters could be assessed only for the 42 patients co-medicated with enzyme inducers. In these patients dose-normalized plasma TPM concentrations correlated positively with age (r = 0.59, P < 0.0001), where apparent oral clearance values (CL/F) were inversely related to age (r = 0.73, P < 0.0001). In particular, CL/F values in children aged less than 10 years (112 +/- 82 mL/kg/h, mean +/- SD, n = 14) were almost three times as high as those observed in patients aged >15 to 30 years (42 +/- 16 mL/kg/h, n = 17), whereas the CL/F value in children aged 10 to 15 years (66 +/- 22 mL/kg/h, n = 11) was intermediate between those found in the two other age groups. Patients not receiving enzyme-inducing AEDs showed lower CL/F values than did age- and gender-matched patients on enzyme-inducing co-medication. A preliminary evaluation of the relationship between plasma TPM concentration and therapeutic response could be made in 41 patients. No significant difference in drug concentration was detected between patients showing a greater than 50% reduction in seizure frequency compared with baseline (5.9 +/- 2.2 micrograms/mL, n = 30) and those having no clinical improvement (5.2 +/- 2.2 micrograms/mL, n = 11). Likewise, there was no consistent relationship between plasma TPM concentration and appearance of adverse effects. These results indicate that plasma TPM concentrations are linearly related to dosage both in adults and in children and that children aged <10 years require much greater body weight-adjusted dosage to achieve drug levels comparable to those observed in young adults. The marked increase in TPM clearance caused by enzyme-inducing co-medication was confirmed.

Antiepileptic drug therapy for adults: when to initiate and how to choose

Although antiepileptic drugs (AEDs) are commonly used to control and prevent seizures, their long-term use carries a considerable risk of morbidity. The decision to start AEDs is made once the risks of further seizures outweigh the risks of treatment. Despite a large body of literature on the subject, this common clinical issue perplexes many practitioners because of its neurologic, psychological, and, at times, legal implications. Adding to the confusion is the recent approval of several new AEDs. This article summarizes the current evidence to support individual clinical decisions regarding initiation of AEDs in adults and considers the use of AEDs as seizure prophylaxis. Recently approved AEDs are discussed to help the practitioner understand when to initiate and how to choose the appropriate AED for the patient with seizures.

Determination of antiepileptic drugs in biological material

Current analytical methodologies applied to the determination of antiepileptic drugs in biological material are reviewed. The role of chromatographic techniques is emphasized. Special attention is focused on new chemical entities as well as current trends such as high-speed liquid chromatographic techniques, hyphenated techniques and electrochromatography techniques. A review with 542 references.

Pharmacologic management of epilepsy in the elderly

OBJECTIVE

To review the epidemiology and pharmacologic management of epilepsy in elderly patients.

DATA SOURCES

Controlled trials, case studies, and review articles identified via MEDLINE using the search terms epilepsy, seizures, elderly, phenobarbital, primidone, phenytoin, carbamazepine, valproic acid, felbamate, gabapentin, lamotrigine, topiramate, tiagabine, levetiracetam, oxcarbazepine, and zonisamide. Recently published standard textbooks on epilepsy were also consulted.

DATA SYNTHESIS

Epilepsy is a common neurologic disorder in the elderly. Cerebrovascular and neurodegenerative diseases are the most common causes of new-onset seizures in these patients. Alterations in protein binding, distribution, elimination, and increased sensitivity to the pharmacodynamic effects of antiepileptic drugs (AEDs) are relatively frequent, and these factors should be assessed at the initiation, and during adjustment, of treatment. Drug-drug interactions are also an important issue in elderly patients, because multiple drug use is common and AEDs are susceptible to many interactions. In addition to understanding age-related changes in the pharmacokinetics and pharmacodynamics of AEDs, clinicians should know the common seizure types in the elderly and the spectrum of AED activity for these seizure types. AEDs with activity against both partial-onset and generalized seizures include felbamate, lamotrigine, levetiracetam, topiramate, valproic acid, and zonisamide. Other AEDs discussed in this review (carbamazepine, gabapentin, phenobarbital, phenytoin, primidone, and tiagabine) are most useful for partial-onset seizures.

CONCLUSION

The provision of safe and effective drug therapy to elderly patients requires an understanding of the unique age-related changes' in the pharmacokinetics and pharmacodynamics of AEDs as well as an appreciation of common seizure types and the drugs that are effective for the specific types seen in the elderly.

Controversies in blood-level monitoring: reexamining its role in the treatment of epilepsy

This article reexamines the role of blood-level monitoring (therapeutic drug monitoring, TDM) of antiepileptic drugs (AEDs) in the current treatment of epilepsy and identifies situations in which TDM can be useful. Basic pharmacokinetic and pharmacodynamic principles are reviewed, with specific emphasis on kinetics of absorption/distribution/metabolism, elimination half-life, time to steady state, and plasma drug concentrations. The relationship between AED intensity of effect (pharmacodynamics) and plasma concentration (pharmacokinetics) is expressed mathematically, examined in the context of the major old and new AEDs, and integrated with a historical look at the role of TDM. Situations in which TDM can be useful in the modern treatment of epilepsy are presented and discussed. For both older and newer AEDs, TDM is useful in six clinical situations: establishing "baseline" effective concentrations, evaluating potential causes for lack of efficacy, evaluating potential causes for toxicity, evaluating potential causes for loss of efficacy, judging "room to move" or when to change AEDs, and minimizing predictable problems. TDM remains a valuable tool in the modern treatment of epilepsy. It can be selectively and appropriately utilized to help maximize seizure control and minimize side effects if levels are obtained in response to a patient-specific pharmacokinetic or pharmacodynamic issue or problem.

A double-blind, placebo-controlled study of topiramate in adult patients with refractory partial epilepsy

PURPOSE

The efficacy and safety of topiramate (TPM) as adjunctive therapy in the treatment of adult Chinese patients with refractory partial epilepsy were investigated in a randomized, double-blind, placebo-controlled study.

METHODS

A total of 46 patients who had four or more complex partial seizures with or without secondary generalization within an 8-week baseline phase were enrolled. Patients were assigned randomly to receive TPM (n = 23) or placebo (n = 23). TPM or placebo was titrated to target doses of 300 mg/d for 6 weeks and maintained at stabilized levels for another 8 weeks. Concomitant antiepileptic drugs remained at constant previous levels during the trial.

RESULTS

In all, 41 patients completed the trial (TPM group, n = 20; placebo group, n = 21). The proportion of patients with a > or =50% reduction from baseline in complex partial seizures was 11 of 23 (47.8%) in the TPM group and 3 of 23 (13.0%) in the placebo group (p = 0.01). In addition, patients treated with TPM had significantly better investigator (p = 0.014) and patient (p = 0.0005) global assessment scores than patients in the placebo group. Adverse events were mostly mild and transient, with no significant differences between treatment groups. Two patients with TPM therapy complained of weight loss. Routine blood cell counts and other laboratory results showed no significant changes from baseline in either treatment group.

CONCLUSIONS

TPM 300 mg/d is effective and well tolerated as treatment for refractory partial epilepsy in adults.

Antiepileptic drug pharmacokinetics and interactions: impact on treatment of epilepsy

An understanding of epilepsy therapy's pharmacokinetic and drug interaction principles-combined with knowledge of antiepileptic drug (AED) clinical pharmacology-allows more effective use of these drugs. The most desirable pharmacokinetic characteristic is a linear relationship between dose and steady-state concentration, as this determines the ease or difficulty in determining the appropriate dose. Drug-drug interactions affecting AED metabolism are common, clinically important, and, until recently, often unpredictable. Advances in molecular biology have identified specific enzymes responsible for AED metabolism and interactions. Clinicians now can identify potential interactions and avoid or manage them by adjusting drug dosage. Most newer AEDs follow or approximate linear pharmacokinetics, are absorbed extensively and consistently, are not significantly bound to plasma proteins, do not form active metabolites, and have few, if any, drug interactions. In cases where interactions occur between newer AEDs and other drugs, knowledge of these interactions reduces the likelihood of serious adverse events. The pharmacokinetics of the newer AEDs simplify drug dosing and monitoring and should lead to improved patient care.

Interaction of topiramate with conventional antiepileptic drugs in mice

Topiramate [2,3:4, 5-bis-O-(1-methyl-ethylidene-)-beta-D-fructopyranose sulfamate], administered intraperitoneally (i.p.) up to 5 mg/kg, did not influence the threshold for electroconvulsions. In doses of 10-30 mg/kg, topiramate significantly raised the threshold. This novel antiepileptic drug, in subprotective doses, enhanced the protective activity of i.p. given valproate, carbamazepine, dihenylhydantoin and phenobarbital against maximal electroshock-induced convulsions in mice. The potentiation induced by topiramate (2.5-5 mg/kg) was most profound for carbamazepine and phenobarbital. The anticonvulsive activity of valproate and diphenylhydantoin was potentiated by topiramate only at 5 mg/kg. Topiramate (5 mg/kg) combined with valproate, phenobarbital and diphenylhydantoin did not alter their free plasma levels but its combination with carbamazepine resulted in an increased free plasma level of this antiepileptic drug. Treatment with topiramate (5 mg/kg) alone or in combination with the studied antiepileptics (providing 50% protection against maximal electroshock) resulted in no adverse effects, as measured in the chimney test (motor coordination) or passive avoidance task (long-term memory). In contrast, valproate administered alone at its ED(50) against maximal electroshock impaired motor coordination. It is noteworthy that valproate and carbamazepine at their respective ED(50) values of 248 and 11.2 mg/kg disturbed long-term memory. The results provide an experimental basis for rational polytherapy.

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.

Clinical pharmacology of topiramate: a review

Clinical success with an antiepileptic drug (AED) depends primarily on its efficacy and tolerability. Clinicians also need to have a basic understanding of an AED's pharmacokinetic characteristics, particularly those affecting the potential for drug interactions such as hepatic enzyme inhibition or induction and protein-binding displacement. Successful treatment may be complicated by pharmacokinetic characteristics such as a short half-life, nonlinear kinetics, and active metabolites. Pharmacokinetic characteristics that make a drug easy to use may affect patient adherence. In general, newer AEDs such as topiramate (TPM) are simpler to use than traditional AEDs because they have more favorable pharmacokinetic characteristics and fewer drug interactions.

Topiramate

Six studies are cited to demonstrate that topiramate is effective as adjunctive therapy for refractory partial-onset seizures in adults. Subsequent studies indicate that topiramate is also effective as monotherapy in adults and as adjunctive therapy for partial-onset seizures in children, tonic-clonic seizures of nonfocal origin in children and adults, and drop attacks in Lennox-Gastaut syndrome. Adverse effects for adults and children included dizziness, fatigue, ataxia, confusion, somnolence, nephrolithiasis, paresthesia, and weight loss. More adverse effects were observed at higher doses. Topiramate exhibits rapid absorption, long duration of action, and minimal interaction with other antiepileptic drugs.

A pilot study of topiramate in children with Lennox-Gastaut syndrome

We conducted an open, add-on study with topiramate (TPM) as adjunctive therapy in Lennox-Gastaut syndrome (LGS), to assess the long-term efficacy and safety and to evaluate quality of life (QL) measurements in the chronic use of TPM. We studied 19 patients (11 male; age ranging from 4 to 14 years) with uncontrolled seizures receiving 2-3 anti-epileptic drugs. Patients were followed up to 36 months of treatment. A questionnaire was used to query parents about QL. Seven patients completed the study at 36 months and seizure frequency was reduced > or = 75% in 4, and < 50% in 3 patients. Two children became seizure free for more than 24 months. Most side effects were CNS related, with the most frequent being somnolence and anorexia. These were generally transient. One patient dropped-out due to powder in the urine. None of the patients required hospitalization. At 36 months, patients' alertness (2/7), interaction with environment (5/7), ability to perform daily activities (5/7), and verbal performance (6/7) improved on TPM. We conclude that TPM may be useful as adjunctive therapy in the treatment of LGS. The efficacy of TPM was maintained in long-term treatment in more than 40% of patients, long term safety was confirmed and QL improved on TPM.

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.

Epilepsy in older adults: causes, consequences and treatment

OBJECTIVES

Seizures and epilepsy are common problems in older adults. Although, the highest incidence of seizures and epilepsy occurs in individuals more than 65 years of age, the magnitude of this public health problem, and its consequences on the quality of life of older adults, are not appreciated. Moreover, there is no consensus on the most appropriate way to diagnose or manage epilepsy in this population. This report reviews the current literature on all aspects of epilepsy in older people.

DESIGN/METHODS

The medical literature was reviewed for all articles pertaining to pathophysiology, diagnosis, and treatment of epilepsy in older people.

RESULTS

Epidemiology, etiology, diagnosis, use of diagnostic tests including EEG and imaging, new medical and surgical treatments, and psychosocial issues as they relate to older epilepsy patient are discussed. Several questions that merit future systematic investigation are presented.

CONCLUSION

Seizures and epilepsy in older people are much more common than is generally known. There are unique issues related to this population with regard to diagnosis and treatment. Several new medical and surgical therapies are now available for all epilepsy patients, some of which may be helpful for the older epilepsy patient.

Predictors of outcome of anterior temporal lobectomy for intractable epilepsy: a multivariate study

OBJECTIVE

To identify presurgical and postsurgical factors that are independently predictive of the outcome of anterior temporal lobectomy (ATL) for intractable epilepsy.

BACKGROUND

There have been reports of prognostic factors in epilepsy surgery, but little is known about factors that independently predict outcome of ATL.

METHODS

We studied 175 consecutive ATL patients who had at least 2 years of postsurgical follow-up. Significant factors on univariate analyses were subjected to stepwise logistic regression analysis.

RESULTS

On univariate analyses, two presurgical conditions were significantly associated with excellent seizure control at last follow-up: (1) unilateral hippocampal formation atrophy as detected on MRI and (2) all scalp interictal epileptiform discharges concordant with the location of ictal onset (p < 0.05). Three postsurgical factors that occurred during the first year were associated with excellent seizure outcome: the absence of interictal epileptiform discharges at 3 months, complete seizure control, and having only nondisabling seizures for those who did not become seizure free. Logistic regression analysis revealed the following to be independently predictive of excellent seizure control: MRI-detected unilateral hippocampal formation atrophy, concordant interictal epileptiform discharges, complete seizure control during the first postsurgical year, and having only nondisabling seizures during the first postsurgical year for those who did not become seizure free.

CONCLUSIONS

Presurgical identification of unilateral hippocampal formation atrophy, or of interictal epileptiform discharges that are all concordant with the location of ictal onset, predict excellent outcome of ATL. However, the probability of excellent outcome is highest (94%) when both factors are present.

New antiepileptic drugs. Overcoming the limitations of traditional therapy

Several new antiepileptic drugs have become available recently. Since seizures and epilepsy are common, primary care physicians are likely to encounter a patient who is taking one of these new medications. Successful medical management of epilepsy requires a proper understanding of medication half-life, indications, and side effects. Felbamate has a broad spectrum of efficacy but is limited by side effects and idiosyncratic reactions. Fosphenytoin has the efficacy of phenytoin and offers the advantage of intramuscular and intravenous dosing without the significant adverse effects associated with intravenous phenytoin; however, it is expensive. Gabapentin has minimal side effects and drug interactions yet has limited efficacy for seizures. Lamotrigine has broad seizure efficacy but requires a slow adjustment to therapeutic levels. Topiramate has minimal drug interactions, but therapy must be initiated slowly to avoid side effects. All of the new antiepileptics hold great promise in the management of patients with recurrent seizures.

Recent advances in the pharmacotherapy of epilepsy

The therapeutic options for the treatment of epilepsy have expanded during the 1990s. Since 1993, four novel agents (felbamate, gabapentin, lamotrigine, and topiramate) have been approved by the US Food and Drug Administration, primarily for adjunctive treatment of partial seizures. In addition, a water-soluble pro-drug of phenytoin, fosphenytoin, and a sustained-release preparation of carbamazepine have been introduced. The novel anticonvulsants represent a potential improvement for patients whose seizures are incompletely controlled or who experience significant adverse effects with older anticonvulsants. Felbamate, lamotrigine, and topiramate appear to have a broad spectrum of action in seizure control, but felbamate use is limited by the potential for serious adverse effects. Gabapentin, lamotrigine, and topiramate are all well tolerated. Gabapentin has no known drug interactions, whereas lamotrigine and topiramate have limited interactions compared with older agents. The sustained-release preparation of carbamazepine may decrease the incidence of adverse effects and increase patient compliance. Fosphenytoin offers a safer method for intravenous administration of phenytoin and the added flexibility of intramuscular administration. Taken together, these recent advances in treatment may bring about improved efficacy and decreased adverse effects for many patients with epilepsy.

A pharmacological and clinical review on topiramate, a new antiepileptic drug

Topiramate is a new antiepileptic drug which has recently become available in the United States and in a number of European countries. Pharmacological studies suggest that its mode of action is multifactorial and involves blockade of voltage-dependent sodium channels, potentiation of GABAergic transmission and inhibition of excitatory pathways through an action at AMPA receptor sites. Carbonic anhydrase inhibiting properties have also been demonstrated but they are considered not to be relevant to anticonvulsant activity. Topiramate is well absorbed from the gastrointestinal tract, peak plasma levels being usually attained in 2-3 hours. The drug is negligibly (9-17%) bound to plasma proteins and is eliminated partly by renal excretion in unchanged form and partly by oxidation and hydrolysis. In healthy volunteers, the half-life is about 20-30 hours, but elimination rate is accelerated in patients taking concomitant enzyme inducing drugs such as phenytoin, carbamazepine and barbiturates. Topiramate has no major effects on plasma levels of concurrent anticonvulsants, except for a rise in plasma phenytoin in occasional patients. In double-blind add-on trials in refractory partial epilepsy, a significant reduction in seizure frequency has been demonstrated in over 40% of topiramate-treated patients (vs about 10% of those treated with placebo), a response rate which compares favourably with that observed with other new antiepileptic drugs. Dosages found to be effective in add-on controlled trials range between 200 and 1000 mg day-1, although most patients are likely to benefit from receiving 400 mg day-1 or less. Preliminary data suggest that topiramate may be effective also in generalized epilepsies, but this needs to be confirmed in prospective studies. The most common adverse effects of topiramate are CNS-related and include dizziness, fatigue, visual disturbances, ataxia, mental slowing and impaired concentration. Paresthesias, anorexia, weight loss and increased risk of nephrolithiasis have been also reported. Many of these effects are related to dose and/or to rate of dose titration. Based on these data, topiramate appears to be a valuable new drug, whose main current indication is in the add-on management of refractory partial and secondarily generalized seizures. Studies on its potential-value as monotherapy are in progress.

New antiepileptic drugs in children: present and future

The pharmacologic management of epilepsy has progressed greatly during the last decade. New medications are available for the management of refractory patients, and more are being developed. For some patients, these productions offer added efficacy, better tolerability, and some pharmacodynamic advantages. Since preapproval studies include few pediatric trials, the final role of these medications in the treatment of childhood epilepsy will be dictated by additional studies and postmarketing experience.

Pharmacokinetics and interaction profile of topiramate: review and comparison with other newer antiepileptic drugs

Standard antiepileptic drugs (AEDs) have a number of pharmacokinetic shortcomings, and AEDs with more favorable profiles would be preferred. The pharmacokinetics and interaction profile of the recently developed AED topiramate (TPM), is reviewed and compared with those of other newer AEDs including lamotrigine (LTG), gabapentin (GBP), vigabatrin (VGB), and oxcarbazepine (OCBZ). Although none of these agents meets all of the criteria of the "ideal" AED from the pharmacokinetic standpoint, a number of these drugs, including TPM, have desirable properties that distinguish them from the older AEDs and should contribute to their clinical utility.

Clinical efficacy of topiramate as add-on therapy in refractory partial epilepsy: the European experience

In three randomized, double-blind, placebo-controlled add-on European trials, target daily topiramate (TPM) dosages of 400, 600, and 800 mg/day (200, 300, and 400 mg bid) were evaluated in adults with refractory partial seizures with or without becoming secondarily generalized. Median reductions from baseline in monthly seizure rate were 41% with TPM 400 mg/day vs. 1% with placebo (n = 0.065), 46% with TPM 600 mg/day compared to -12% (a 12% increase) with placebo (p < or = 0.005), and 36% with TPM 800 mg/day versus -18% (an 18% increase) with placebo (p < 0.001). Differences between TPM and placebo with respect to percent responders (percent of patients demonstrating a 50% or greater reduction in seizures) significantly favored TPM (p < 0.05) at all three target dosages. Significant reductions in secondarily generalized tonic-clonic seizures compared to placebo were also observed with 400 mg/day (p = 0.002) and 800 mg/day (p < 0.05) of TPM. TPM appears to be a promising new antiepileptic drug for use as adjunctive therapy in adults with refractory partial epilepsy.

The clinical pharmacokinetics of the newer antiepileptic drugs. Focus on topiramate, zonisamide and tiagabine

Following the introduction of felbamate, gabapentin, lamotrigine, oxcarbazepine and vigabatrin in the early 1990s, other new antiepileptic drugs have been advancing in clinical development. Those most extensively evaluated to date include topiramate, zonisamide and tiagabine. Topiramate, licensed recently in the UK, acts multifactorially through the blockade of sodium channels and kainate/AMPA receptors, enhancement of gamma-aminobutyric acid (GABA)ergic transmission and inhibition of carbonic anhydrase. It is well absorbed from the gastrointestinal tract and negligibly bound to plasma proteins. When used as a monotherapy, topiramate is eliminated primarily in the urine in an unchanged form with a half-life of 20 to 30 hours; elimination is faster in patients receiving concurrent medication with enzyme-inducing anticonvulsants, in whom the extent of biotransformation becomes more prominent. Zonisamide, which has been commercially available in Japan for some years, also has a multifactorial mode of action, possibly involving the blockade of sodium channels, T-type calcium channels and inhibition of carbonic anhydrase. It is rapidly absorbed, 50% bound to plasma proteins and is eliminated predominantly by biotransformation; zonisamide has a half-life of 50 to 70 hours in monotherapy patients, or 25 to 35 hours in patients comedicated with enzyme-inducing anticonvulsants. Tiagabine, a nipecotic acid derivative which inhibits GABA reuptake, is rapidly and completely absorbed after oral intake. It is highly (96%) bound to plasma proteins and it is eliminated primarily by cytochrome P450 3A-mediated oxidation, with a half-life of about 7 hours in healthy volunteers. Tiagabine metabolism is also enhanced by concurrent medication with enzyme-inducing anticonvulsants, resulting in a need to use dosages larger than those required in monotherapy or valproic acid (sodium valproate)-treated patients. Additional investigational antiepileptic agents included in this article are rufinamide (CGP 33101), fosphenytoin, levetiracetam, losigamone, remacemide and stiripentol. All these drugs have undergone early characterisation with respect to pharmacokinetic features and interaction potential.