Felbamate

Pharmacology and Therapeutic Aspects of Antiepileptic Drugs in Pediatrics

In the past year, several new antiepileptic drugs have emerged that have potential benefits for children with epilepsy. The spectrum of adverse effects is the principal feature that differentiates among the older drugs used to treat partial and related seizures, including simple partial, complex partial, and partial secondarily generalized seizures. Based on studies in adults with refractory seizures, the new or investigational compounds felbamate, gabapentin, lamotrigine, and vigabatrin should be active against these types of seizures in children, but none of them have been subjected to pediatric randomized controlled trials, and no studies have been done that compare new and old drugs in this category. Thus, the new drugs hold promise in children with these types of seizures, but their role relative to old drugs has not been elucidated. Several of the new drugs are active against myoclonic and generalized tonic-clonic seizures, but thus far, none have been proven to possess antiabsence activity in children. Open-label investigations suggest that lamotrigine may be helpful in Lennox-Gastaut syndrome, and vigabatrin in infantile spasms. Only felbamate has been evaluated in a randomized controlled study in children, in which it has proven beneficial against astatic and generalized tonic-clonic seizures in children with Lennox-Gastaut syndrome. Whereas investigations of these and other novel drugs are ongoing, this is an active and exciting period in pediatric antiepileptic drug development. (J Child Neurol 1994;9(Suppl):2S1-2S7).

Evaluating the safety and efficacy of felbamate in the context of a black box warning: A single center experience

INTRODUCTION

Felbamate was approved in 1993 to treat partial seizures with and without secondary generalization in adults and in Lennox-Gastaut Syndrome in children. Its use was later restricted when rare but fatal cases of aplastic anemia and hepatic failure were identified.

METHODS

This single center analysis retrospectively evaluated the safety and efficacy of felbamate in a cohort of children, adolescents, and adults with epilepsy.

RESULTS

A chart review identified 103 patients taking felbamate. The range of felbamate dose was 300-4500 mg (mean: 1800 ± 900 mg). The duration of therapy ranged from 1 month to 20 years (mean duration: 35 ± 45 months). Eighteen (17.5%) subjects experienced adverse events including insomnia, nausea, vomiting, decreased appetite, weight loss, gastric discomfort, diarrhea, mood and behavioral problems, high blood pressure, headache, and elevated liver enzymes. Out of these, 6 (5.9%) patients discontinued the therapy. No hepatic failure or agranulocytosis was observed. Fifty-nine (57.72%) patients achieved ≥ 50% reduction in seizure frequency, and 30 (29.12%) patients achieved seizure freedom.

CONCLUSIONS

These findings suggest that felbamate is safe, well tolerated, and effective in treatment of various types of epilepsy syndromes.

The NMDA receptor complex as a therapeutic target in epilepsy: a review

A substantial amount of research has shown that N-methyl-D-aspartate receptors (NMDARs) may play a key role in the pathophysiology of several neurological diseases, including epilepsy. Animal models of epilepsy and clinical studies demonstrate that NMDAR activity and expression can be altered in association with epilepsy and particularly in some specific seizure types. NMDAR antagonists have been shown to have antiepileptic effects in both clinical and preclinical studies. There is some evidence that conventional antiepileptic drugs may also affect NMDAR function. In this review, we describe the evidence for the involvement of NMDARs in the pathophysiology of epilepsy and provide an overview of NMDAR antagonists that have been investigated in clinical trials and animal models of epilepsy.

New antiepileptic drugs in pediatric epilepsy

New antiepileptic drugs (AEDs), introduced since 1993, provide more diverse options in the treatment of epilepsy. Despite the equivalent efficacy and better tolerability of these drugs, more than 25% of patients remain refractory to treatment. Moreover, the issues for pediatric patients are different from those for adults, and have not been addressed in the development and application of the new AEDs. Recently published evidence-based treatment guidelines have helped physicians to choose the most reasonable AED, although they cannot fully endorse new AEDs because of the lack of well-designed, randomized controlled trials. We review the mechanisms of action, pharmacokinetic properties, adverse reactions, efficacy, and tolerability of eight new AEDs (felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, topiramate, vigabatrin, and zonisamide), focusing on currently available treatment guidelines and expert opinions regarding pediatric epilepsy.

NMDA receptors in clinical neurology: excitatory times ahead

Since the N-methyl-D-aspartate receptor (NMDAR) subunits were cloned less than two decades ago, a substantial amount of research has been invested into understanding their physiological function in the healthy CNS. Research has also been directed at their pathological roles in various neurological diseases, including disorders resulting from acute excitotoxic insults (eg, ischaemic stroke, traumatic brain injury), diseases due to chronic neurodegeneration (eg, Alzheimer's, Parkinson's, and Huntington's diseases and amyotrophic lateral sclerosis), disorders arising from sensitisation of neurons (eg, epilepsy, neuropathic pain), and neurodevelopmental disorders associated with NMDAR hypofunction (eg, schizophrenia). Selective NMDAR antagonists have not produced positive results in clinical trials. However, there are other NMDAR-targeted therapies used in current practice that are effective for treating some neurological disorders. In this Review, we describe the evidence for the use of these therapies and provide an overview of drugs being investigated in clinical trials. We also discuss new NMDAR-targeted strategies in clinical neurology.

Use of Antiepileptic Drugs in Patients with Kidney Disease

The number of medications used to treat different types of seizures has increased over the last 10-15 years. Most of the newer antiepileptic drugs (AEDs) are likely to be unfamiliar to many nephrologists. For both the older and newer AEDs, basic pharmacokinetic information, recommendations for drug dosing in patients with reduced kidney function or who are on dialysis, and adverse renal and fluid-electrolyte effects are reviewed. Newer AEDs are less likely to have significant drug-drug interactions than older agents, but are more likely to need dosage adjustment in patients with reduced kidney function. The most common renal toxicities of these drugs include metabolic acidosis, hyponatremia, and nephrolithiasis; interstitial nephritis and other adverse effects are less common. Little is known about the clearance of most of the newer AEDs with high-efficiency hemodialyzers or with peritoneal dialysis. Monitoring of drug levels when available, careful clinical assessment of patients taking AEDs, and close collaboration with neurologists is essential to the management of patients taking AEDs.

Effect of antiepileptic drugs on bodyweight: overview and clinical implications for the treatment of epilepsy

The potential of specific antiepileptic drugs (AEDs) to cause clinically significant changes in bodyweight is a key consideration in the management of epilepsy; changes in weight can pose health hazards, impair body image and self-esteem, and lead to noncompliance with therapy. This article reviews the data regarding the effects of conventional and newer AEDs on weight and discusses the clinical implications of these effects for the management of patients with epilepsy. The data demonstrate that AEDs can differ substantially in their effects on weight. Some, such as valproate and carbamazepine, increase weight; others, such as topiramate and felbamate, decrease it. Still others, such as lamotrigine, levetiracetam and phenytoin, are weight neutral. Because most data regarding the effects of AEDs on weight are circumstantial, the incidence, magnitude and determinants of weight changes with AEDs remain poorly elucidated. Furthermore, little is known about the mechanisms of AED-induced changes in weight. The importance of effects on weight in selecting an AED depends largely upon the individual patient's needs and the risks and benefits of therapy for that patient. The most appropriate therapeutic choice is a weight-neutral medication unless circumstances dictate otherwise.

Therapeutics in pediatric epilepsy, Part 1: The new antiepileptic drugs and the ketogenic diet

Epilepsy is one of the most common and challenging neurologic disorders affecting children. Although various modalities exist to treat pediatric-onset seizures, seizures in 25% of children who are diagnosed as having epilepsy remain refractory to available therapies. Of the 8 new antiepileptic drugs (AEDs) (felbamate, gabapentin, lamotrigine, topiramate, tiagabine, levetiracetam, oxcarbazepine, and zonisamide), all but 2 (zonisamide and levetiracetam) have received Food and Drug Administration approval for adjunctive use in the pediatric population. However, most of the new AEDs used in adults have also been used in children, beyond the AEDs' approved indications. The ultimate goal of patient management is to choose the therapeutic option that provides the best chance of improving the patient's quality of life. Issues that relate to treatment choice include the likelihood of seizure recurrence, type and severity of seizures, available AED efficacies and toxicities, need for hematologic monitoring, ease of dosing, underlying medical conditions, medication interactions, urgency of initiating therapy, and cost. In this review, we discuss these issues for each of the 8 new AEDs; we also discuss the ketogenic diet and briefly review the older AEDs. Knowledge of the available AEDs will enable the practitioner to choose the best drug or drugs for individual patients.

Treatment of partial seizures and seizure-like activity with felbamate in six dogs

Six dogs with partial seizures or partial seizure-like activity were treated with the antiepileptic drug felbamate between 1993 and 1998. All dogs had a history and results of diagnostic testing suggestive of either primary (idiopathic) or occult secondary epilepsy. Dogs ranged between four months and eight years of age at the onset of seizure activity. The median time period between onset of the first seizure and the start of felbamate therapy was 3.8 months (range 0.75 to 36 months). Median duration of therapy was nine months (range two to 22 months). All dogs experienced a reduction in seizure frequency after felbamate administration. Median total number of seizures post-treatment was two (range 0 to 9). Two dogs had an immediate and prolonged cessation of seizure activity. Steady-state trough serum felbamate concentrations measured at two weeks, and one, 12 and 22 months after the commencement of therapy in four dogs ranged between 13 and 55 mg/litre (median 35 mg/litre). Reversible haematological adverse effects were detected in two dogs, with one dog developing concurrent keratoconjunctivitis sicca. These results suggest that felbamate can be an effective antiepileptic drug without life-threatening complications when used as monotherapy for partial seizures in the dog.

The efficacy of felbamate as add-on therapy to valproic acid in the Lennox-Gastaut syndrome

We studied the efficacy of felbamate (FBM) in combination with valproic acid (VPA) in 13 patients with the Lennox-Gastaut syndrome and evaluated the contribution of each drug. Following stabilization on VPA monotherapy, FBM or placebo titration was performed for two observation periods lasting 7 weeks with a washout period between them. 6-h video-electroencephalography was recorded following each observation period. In addition to examining the effects of the drugs with parental reports and video-EEG, we compared video-EEG data with families' seizure reports. Based on parental counts for the 7-week observation periods, patients had 40% fewer drop attacks (p < 0.03, Wilcoxon rank sum test) and 60% fewer total seizures (p < 0.02) on VPA and FBM. VPA level rose by 12.7% when FBM was added (p < 0.01). When the effect of FBM was factored out, VPA had a significant effect on drop attack frequency, although not total number of seizures. FBM's therapeutic effect on drop attacks is due in part to increased VPA levels, although the combination may be synergistic for the effect on total seizure number.

Drug and environmental factors associated with adverse pregnancy outcomes. Part I: Antiepileptic drugs, contraceptives, smoking, and folate

OBJECTIVE

Part I of this review examines the relationship between antiepileptic drugs (AEDs) and pregnancy outcomes. Drug-induced folate deficiency and the role of AED metabolism are emphasized. Part II will discuss periconceptional folate supplementation for prevention of birth defects. Part III will discuss the mechanism of folate's protective effect, therapeutic recommendations, compliance, and cost.

DATA SOURCES

A MEDLINE search was conducted for journal articles published through December 1997. Additional sources were obtained from Current Contents and citations from the references obtained. Search terms included phenytoin, carbamazepine, phenobarbital, primidone, valproic acid, oral contraceptives, clomiphene, drug-induced abnormalities, spina bifida, anencephaly, neural tube defect, folate, folic acid, and folic acid deficiency.

STUDY SELECTION

Relevant animal and human studies examining the effects of AEDs, smoking, and oral contraceptives on folate status and pregnancy outcome are reviewed.

DATA EXTRACTION

Studies and case reports were interpreted. Data extracted included dosing, serum and red blood cell folate concentrations, teratogenicity of anticonvulsant medications, metabolism of AEDs and folate, and genetic susceptibility to AED-induced teratogenicity.

DATA SYNTHESIS

Low serum and red blood cell folate concentrations are associated with adverse pregnancy outcomes. Decreases in serum folate are seen with AEDs, oral contraceptives, and smoking. Since similar birth defects are observed with multiple AEDs, metabolism of aromatic AEDs to epoxide metabolites and genetic factors may play a role in teratogenesis.

CONCLUSIONS

Adequate prepregnancy planning is essential for women who have epilepsy. Women receiving folate-lowering drugs may be at increased risk of adverse pregnancy outcomes. Therefore, epileptic women contemplating pregnancy should be treated with the minimum number of folate-lowering drugs possible and receive folic acid supplementation.

Tolerability and pharmacokinetics of monotherapy felbamate doses of 1,200-6,000 mg/day in subjects with epilepsy

PURPOSE

Felbamate (FBM) pharmacokinetic parameters, safety and tolerability in the dose range of 1,200-6,000 mg/day were assessed in two open-label studies with similar designs.

METHODS

In study A, newly diagnosed subjects with epilepsy receiving FBM monotherapy at a starting dose of 1,200 mg/day (400 mg/three times daily, t.i.d.) and increased 1,200 mg/day, if tolerated, at 14-day intervals to 3,600 mg/day were investigated. In study B, epilepsy subjects with prior FBM monotherapy exposure received ascending FBM doses in five consecutive 14-day periods with a starting dose of 3,600 mg/day (1,200 mg t.i.d.) FBM. In each successive period, if FBM was well tolerated, the dose was increased by 600 mg/day to a maximum of 6,000 mg/day (2,000 mg t.i.d.).

RESULTS

The pharmacokinetic parameter estimates maximum observed concentration (Cmax), area under the concentration-time curve (AUCtau) Ctrough, and Cav showed a linear dependence to dose above the 1,200-6,000 mg/day FBM dose range (F-tests; p < 0.0001) with apparent clearance (Cl/kg) and Tmax (time to Cmax) independent of dose. When AUCtau, Cmax and Ctrough were adjusted for dose, there were no significant differences between the dosing periods.

CONCLUSIONS

The data establish that plasma concentrations of FBM are linear with respect to dose to 6,000 mg/day. In addition, FBM was safely administered at these doses for periods as long as 14 days to epileptic subjects with prior exposure to FBM. FBM-naive subjects appeared to report more adverse experiences (generally of mild to moderate severity) than did subjects with prior FBM exposure.

Totally automated analysis by robotized PrepStation and liquid chromatography: direct-sample analysis of felbamate

A totally automated analysis of felbamate was developed by using a robotized PrepStation for extraction, followed by automated liquid chromatographic (LC) analysis and data reduction. This is one of the newer direct-sample analysis approaches by LC. Felbamate was a previously approved antiepileptic agent used to treat partial seizures with and without generalization and to treat Lennox-Gastaut syndrome in pediatric patients. However, due to the reported incidences of aplastic anemia, its clinical application was recently restricted to the treatment of the latter syndrome. The automated assay using Bench Supervisor, PrepStation, and LC, based on a previously developed manual method, used 200 microliters of serum standards, quality control, or patients' plasma. These were mixed with 600 microliters of internal standard (IS) W509 dissolved in acetonitrile for protein precipitation. After axial centrifugation and standing, aliquots of the clear supernatant were transferred and washed with hexane. Aliquots of the supernatant were transferred and injected into a high-performance liquid chromatograph (HPLC). HPLC parameters included an mu Bondapak C-18 column, phosphate/acetonitrile (8:2) as mobile phase, and detection at 214 nm. Retention times were 2.9 and 4.2 min for felbamate and IS, respectively. Calibration was linear for concentrations from 10 to 200 mg/L with r > 0.994. Precision studies showed coefficients of variation ranging from 2.7% to 8.8%. Correlation with the manual method showed that r = 0.934, slope = 1.048, intercept = -2.642, and n = 21. Phenobarbital coeluted with the IS. This study demonstrated the feasibility of using a robotized, automated method for monitoring felbamate, readily extended to monitoring other antiepileptic drugs with minimal modification.

Pharmacokinetic interactions of the new antiepileptic drugs

Therapy with traditional antiepileptic drugs is associated with a wide range of pharmacokinetic drug-drug interactions. In particular, enzyme induction, enzyme inhibition and displacement from protein binding may result in important changes in serum concentrations of antiepileptics. Relevant interactions have also been described for some new antiepileptics. Felbamate increases serum concentrations of phenytoin, phenobarbital and valproic acid (sodium valproate). On the other hand, it reduces concentrations of carbamazepine and increases concentrations of its metabolite carbamazepine-10,11-epoxide. Concentrations of felbamate itself are reduced by phenytoin and carbamazepine. Concentrations of lamotrigine are considerably increased by valproic acid and decreased by phenytoin, carbamazepine and phenobarbital (phenobarbitone). Vigabatrin reduces serum concentrations of phenytoin by approximately 20%. On the other hand, some new antiepileptics have the important advantage of not interfering with the metabolism of other antiepileptics; this is the case for gabapentin, lamotrigine and oxcarbazepine. Furthermore, the pharmacokinetics of gabapentin, oxcarbazepine and vigabatrin are independent of concomitant drugs. These aspects are especially important as, until now, new antiepileptics have been most often utilised as add-on therapy.

Effect of felbamate on the pharmacokinetics of lamotrigine

To assess the possible interaction between lamotrigine and felbamate, a double-blind, randomized, placebo-controlled, two-way crossover study was conducted in 21 healthy male volunteers. Volunteers were given lamotrigine (100 mg every 12 hours) and felbamate (1,200 mg every 12 hours) or matching placebo for 10 days during each period of the crossover. After morning administration on day 10, blood samples were obtained over 12 hours for measurement of lamotrigine. Felbamate increased the maximum concentration (Cmax) and and area under the concentration-time curve from time 0 to 12 hours (AUC0-12) of lamotrigine by 13% and 14%, respectively, compared with placebo. The 90% confidence intervals of the log-transformed pharmacokinetic parameters were within the 80-125% bioequivalance limits, however. Felbamate had no significant effect on the urinary excretion of lamotrigine (total), unconjugated lamotrigine, or the N-glucuronide. One volunteer discontinued the study after developing a rash while taking lamotrigine and placebo. All other adverse events were primarily related to the central nervous system and gastrointestinal tract, with a higher incidence reported during coadministration of lamotrigine and felbamate than with placebo. Overall, felbamate appears to have no clinically relevant effects on the pharmacokinetics of lamotrigine.

Anticonvulsant Therapy in Children: An Update

Over the years, extensive research has led to the development of a new generation of anticonvulsant medications for the treatment of patients with intractable seizure disorders. Currently three new drugs have been approved in the United States since 1993, and many others have entered into the later stages of development. The purpose of this article is to discuss the pharmacology, pharmacokinetics, drug interactions, clinical use, adverse effects, and dosage and administration of felbamate, gabapentin, lamotrigine, and vigabatrin. Felbamate is indicated in children as adjunctive therapy in the treatment of partial and generalized seizures secondary to Lennox-Gastaut syndrome. Because of life-threatening adverse effects, including aplastic anemia and hepatotoxicity, felbamate is reserved for use only when the benefits of treatment outweigh the risks of toxicity. Presently, gabapentin is indicated as adjunctive treatment of partial seizures with or without generalization in patients older than 12 years of age. To date gabapentin has not been studied in patients younger than age 12 years. Even though lamotrigine is not approved by the Food and Drug Administration (FDA) for pediatric use, preliminary clinical trials show promising results in the treatment of partial and absence seizures as well as Lennox-Gastaut syndrome. Many studies have evaluated the use of vigabatrin for the treatment of intractable seizures. Seizure types most effectively treated include partial seizures, Lennox-Gastaut syndrome, and possibly infantile spasms. Lamotrigine and vigabatrin should be used with caution in patients with myoclonic seizures because an increase in seizure frequency may occur. Copyright © 1996 by W.B. Saunders Company

Neuropharmacology and drug interactions in clinical practice

Absorption, distribution, and clearance are key pharmacokinetic principles. These parameters can be highly variable among patients and among compounds, and are factors that must be considered in the wide variability in response to medications. Current antiepileptic drugs (AEDs) present many challenges in their administration. However, understanding and utilizing pharmacokinetic principles can assist the clinician in the appropriate optimization of AEDs.

Felbamate

After the first year of clinical experience, felbamate (FBM) appears to be a valuable antiepileptic drug (AED) for the treatment of intractable epilepsy. However, many patients experience side effects that may discourage continued usage. These may be decreased by using a slower dose-escalation schedule and/or by being more aggressive in decreasing co-medication. The most common troublesome side effects are nausea and insomnia. With the recent observation of aplastic anemia, FBM should be considered only for persons with intractable epilepsy under the care of a physician familiar with FBM. Nevertheless, many patients have benefited significantly from FBM and have made a decision to continue receiving FBM at the presently known risk profile. A few more years of experience may be needed to more accurately determine the final place of FBM in the treatment of epilepsy.

Potential pharmacokinetic interaction between felbamate and phenobarbital

OBJECTIVE

To report a case of a potential pharmacokinetic interaction between felbamate and phenobarbital in a patient with epilepsy.

CASE SUMMARY

A patient with a history of a mixed seizure disorder and static encephalopathy who was receiving sodium valproate 750 mg/d and phenobarbital 230 mg/d was initiated on felbamate (as part of a compassionate use program). Upon instituting felbamate, valproate dosage was reduced to 500 mg/d and phenobarbital to 200 mg/d. Felbamate dosage was titrated to approximately 50 mg/kg/d over three weeks. In this patient, plasma phenobarbital concentrations increased from 48 micrograms/mL to 68 micrograms/mL, at which point the patient was hospitalized because of clinically significant neurotoxicity. Phenobarbital dosage was subsequently reduced to 150 mg/d; this resulted in phenobarbital trough concentrations of 60 micrograms/mL.

CONCLUSIONS

Felbamate has been shown previously to interact with multiple other anticonvulsant medications, including valproate, phenytoin, and carbamazepine. Felbamate appears to decrease the clearance of valproate, phenytoin, and carbamazepine epoxide to a significant extent, an effect that may be the result of inhibition of the metabolism of these compounds. Carbamazepine plasma concentrations have been demonstrated to decrease following administration of felbamate, suggesting metabolic induction. It is reasonable to suggest that based on these findings and the observations in our patient, felbamate comedication may result in clinically significant increases in plasma phenobarbital concentrations. It would seem prudent, therefore, when initiating or adjusting felbamate therapy in patients receiving this drug combination, to monitor phenobarbital plasma concentrations.