Felbamate: a new antiepileptic drug

Black-Box Warnings of Antiseizure Medications: What is Inside the Box?

Antiseizure medications can cause serious adverse reactions and have deleterious drug interactions that often complicate the clinical management of patients. When the US Food and Drug Administration (FDA) wants to alert healthcare providers and patients about the risk of potentially serious or fatal drug reactions, the FDA requires the manufacturers of these medications to format these warnings within a "black-box" border, and prominently display this box on the first section of the package insert; such warnings are called "black-box warnings (BBWs)". The BBW is a way for the FDA to urge physicians to evaluate patients more rigorously and carefully weigh the risks and benefits, before prescribing medication that has the potential to cause serious adverse reactions, and to formulate a plan for close monitoring during therapy. The FDA BBW provides the extra layer of safety but many healthcare providers fail to comply with these warnings. Currently, there are 26 FDA-approved antiseizure medications in the US market, 38% of which have received BBWs, and most of the antiseizure medications with BBWs are older-generation drugs. Some antiseizure medications have multiple BBWs; for example, valproic acid has three BBWs including hepatotoxicity, fetal risk, and pancreatitis, carbamazepine has BBWs of serious skin and hematological reactions, and felbamate also has two BBWs including hepatic failure and aplastic anemia. The purpose of this review is to provide insight into each BBW received by antiseizure medications and discuss the FDA recommendations for evaluating the drug benefit/risk, and for monitoring parameters before the initiation of and during treatment.

Pharmacotherapy for children and adolescents with epilepsy

INTRODUCTION

Childhood epilepsies are the most frequent neurological problems that occur in children. Despite the introduction of new antiepileptic drugs (AEDs) 25-30% of children with epilepsy remain refractory to medical therapy.

AREAS COVERED

This review aims to highlight the main published data on the treatment of childhood epilepsy. The electronic database, PubMed, and abstract proceedings were used to identify studies. The aim of antiepileptic therapy should be to provide complete seizure control, if possible without the burden of any side effect. Since 1993, new agents have been approved for use as an antiepileptic. Although there are few published data (especially in pediatric populations) to establish that the second-generation AEDs are more efficacious than the older AEDs, they appear to have better tolerability.

EXPERT OPINION

Old AEDs are efficacious agents that continue to play a major role in the current treatment of epilepsy. These agents actually remain the first-line treatment for many specific seizure types or epileptic syndromes. The new AEDs were initially approved as adjunct agents and--subsequently--as monotherapy for various seizure types in the adult and children. Despite these improvements, few AEDs are now considered to be a first-choice for the treatment of epilepsy in children.

Efficacy of felbamate in the treatment of intractable pediatric epilepsy

The antiepileptic drug felbamate has demonstrated efficacy against a variety of seizure types in the pediatric population, particularly seizures associated with Lennox-Gastaut syndrome. Postmarketing experience, however, revealed serious idiosyncratic adverse effects not observed during clinical trials, including aplastic anemia and liver failure. As a result, many physicians have been hesitant to prescribe felbamate. This retrospective study evaluated the efficacy of felbamate in a pediatric population with intractable epilepsy. Of 38 patients, 22 had Lennox-Gastaut syndrome (58%); 6 had myoclonic-astatic epilepsy of Doose (16%); 5 had symptomatic generalized epilepsy, not otherwise specified (13%); and 5 had symptomatic localization-related epilepsy (13%). Most patients had multiple seizure types and had been tried on a variety of antiepileptic medications. With felbamate treatment, 6 patients (16%) became seizure free, including 4 of the 6 patients with myoclonic-astatic epilepsy of Doose; 24 patients (63%) had a greater than 50% reduction in seizure frequency. In this population felbamate appeared to be safe, with minimal adverse effects. The study is limited by the small number of patients and by its retrospective nature, but nonetheless adds to the evidence that felbamate is an important antiepileptic drug for medically refractory epilepsy in children and is well tolerated with few adverse effects.

Felbamate: Consensus of current clinical experience

An expert panel convened to evaluate data and review current clinical practices regarding the novel antiepileptic drug (AED) felbamate. Felbamate has demonstrated efficacy against a variety of refractory seizures types, including seizures associated with Lennox-Gastaut syndrome, but postmarketing experience revealed serious idiosyncratic adverse effects that were not observed during clinical trials. Although felbamate is not indicated as first-line antiepileptic therapy, its utility in treating seizures that are refractory to other AEDs is undisputed, as shown by the number of patients who continue to use it. New exposures to felbamate number approximately 3200-4200 patients annually, and it is estimated that over the past 10 years, approximately 35,000 new starts have occurred. Recommendations by the American Academy of Neurology and a review of felbamate literature were evaluated in conjunction with the clinical experience of the expert panel to determine current medical opinion and practice regarding felbamate. The past 10 years of clinical experience have demonstrated that when used in accordance with existing recommendations and close clinical monitoring, felbamate is an effective treatment for some patients with seizures refractory to other AEDs. This review of clinical data and discussion of the current understanding of the risk:benefit of felbamate therapy supports its use as an important therapeutic option for some patients with refractory epilepsy.

Characterization of the in vitro antiepileptic activity of new and old anticonvulsant drugs

1. The in vitro antiepileptiform effects of some old and new anticonvulsants in the experimental model of the "epileptiform" hippocampal slice have been reviewed. 2. On the basis of their influence on in vitro epileptogenesis and basal neuronal excitability, anticonvulsants can be classified into three main categories: (1) anticonvulsants (prototypical drug phenytoin) affecting basal neuronal excitability but not epileptogenesis; (2) anticonvulsants (prototypical drugs barbiturates) affecting basal neuronal excitability and epileptogenesis; (3) anticonvulsants (prototypical drug felbamate) affecting epileptogenesis but not basal neuronal excitability. 3. It is concluded that the model of the "epileptiform" hippocampal slices can be considered a previsional test for the study and the screening of new anticonvulsant drugs.

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.

Newer anti-epileptic drugs

During the past few years, a number of drugs have been added to the anti-epileptic arsenal. This review focusses on five of these drugs which have undergone extensive trials: Vigabatrin, Lamotrigine, Gabapentin, Felbamate and Oxcarbazepine. Some of these antiepileptic drugs appear to be helpful for treatment of catastrophic childhood epilepsies. Vigabatrin appears promising in children with infantile spasms who do not respond to ACTH or Prednisolone. Children with Lennox-Gastaut syndrome may respond to treatment with Lamotrigine or Vigabatrin. Gabapentin and vigabatrin have proved to be effective in refractory partial seizures. Oxcarbazepine, a ketoderivative of carbamazepine, is as effective as Carbamazepine but has a better safety profile. Lesser neurotoxicity and fewer drug interactions is another advantage with these drugs. However monitoring is required to determine the long term safety with their usage. These drugs have a definite role in childhood epilepsies refractory to conventional antiepileptic drugs.

Effects of four antiepileptic drugs on sleep and waking in the rat under both light and dark phases

Sedation is a common side effect of anticonvulsant drug therapy. To find out whether the new antiepileptic drugs, felbamate and lamotrigine, are able to produce sedation, we carried out electroencephalographic (EEG) studies in the rat to measure drug effects on sleep-wake patterns, during both light and dark phases. For comparison, the reference drugs, carbamazepine and phenobarbital, were also studied. EEG activity was recorded for 6 h after oral (PO) administration of drugs or vehicle, and the stages of wakefulness, rapid eye movements (REM) sleep and non-REM sleep were classified thereafter. In the light phase, felbamate (30-300 mg/kg) did not produce sedative effects, while lamotrigine (3-30 mg/kg) increased wakefulness at each dose tested. Carbamazepine (10-100 mg/kg) did not produce sleep-wake alterations, and phenobarbital (100 mg/kg) markedly suppressed REM. In the dark phase, felbamate (300 mg/kg), lamotrigine (30 mg/kg), and carbamazepine (100 mg/kg) reduced REM but did not change the total amount of sleep. Phenobarbital, at 100 mg/kg, markedly increased total sleep and greatly reduced REM. This study shows that the anticonvulsant drugs examined have different effects on the states of sleep and wakefulness in the rat. The data are discussed on the basis of the mechanism of action that characterizes each individual drug.

Felbamate selectively blocks in vitro hippocampal kainate-induced irreversible electrical changes

The influence of the anticonvulsant felbamate has been tested on in vitro excitotoxicity induced by treatment of hippocampal slices with elevated concentrations of NMDA, AMPA and kainic acid. For comparison, the effects of the glutamate antagonist 7-chlorokynurenic acid and of the anticonvulsants pentobarbitone and lamotrigine, were also studied. Slice perfusion with 50 microM NMDA or 25 microM AMPA or 12 microM kainic acid produced within 30 min a disappearance or a pronounced irreversible amplitude reduction of the CA1 electrical synaptic responses. Slice perfusion with 1.2-1.6 mM felbamate or 100 microM lamotrigine significantly decreased the incidence of the irreversible disappearance of the CA1 electrical response induced by kainic acid. On the contrary, slice perfusion with the same concentrations of the drugs did not affect the irreversible disappearance of the CA1 electrical response induced by NMDA or AMPA. By contrast, slice perfusion with 100 microM of 7-chlorokynurenic acid significantly prevented the neurotoxic effects induced by both NMDA and kainic acid, while 100 microM of pentobarbitone failed to affect kainic acid-induced neurotoxicity. The different profile of neuroprotection elicited by felbamate with respect to reference drugs indicates that a different mechanism of action than antagonism of NMDA response or potentiation of GABA response underlies the neuroprotectant effects of felbamate.

The use of felbamate to treat infantile spasms

Based on the initial successful use of felbamate for infantile spasms in an infant with tuberous sclerosis, three additional infants with infantile spasms of different etiologies who had failed conventional therapies were treated with felbamate. Three of the four patients have shown complete resolution of infantile spasms. All responding patients did so within 1 week of starting felbamate. The one treatment failure had an initial reduction of seizure frequency and severity but has not maintained that response long term. Controlled studies are needed to firmly establish that felbamate is both safe and effective for the treatment of infantile spasms. As these cases document, felbamate is currently available for use in infantile spasms, and the frequent conversion of infantile spasms to Lennox-Gastaut syndrome, for which felbamate is approved, makes its use in infantile spasms logical.

New antiepileptic drugs--an explosion of activity

The low therapeutic index of established antiepileptic drugs coupled with a better understanding of the pathophysiology of seizure production has led to the development of a range of new therapeutic agents for the treatment of epilepsy. In this review, the three drugs recently licensed in the UK (vigabatrin, lamotrigine and gabapentin) are profiled, together with several of the more promising up-and-coming compounds (oxcarbazepine, felbamate, tiagabine, stiripentol, remacemide and topiramate). Future avenues for clinical research in the pharmacological management of the epilepsies involve their rational use both singly and in combination.

Felbamate displays in vitro antiepileptic effects as a broad spectrum excitatory amino acid receptor antagonist

The in vitro antiepileptic activity of the novel anticonvulsant drug felbamate was tested in rat hippocampal slices on the CA1 epileptiform bursting induced by different chemical epileptogenic agents. The effects of felbamate were compared with those of the anticonvulsant drugs diphenylhydantoin and pentobarbitone and with the effects of excitatory amino acid antagonists acting at both N-methyl-D-aspartate (NMDA) and non-NMDA receptors. Like the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), felbamate at a minimum effective concentration of 1 mM induced a significant (P < 0.01) reduction of the duration of the CA1 epileptiform bursting due to the K+ channel blocker, 4-aminopyridine, and the excitatory amino acids, kainate and quisqualate. Like the NMDA receptor antagonist ketamine, felbamate (1.6 mM) significantly (P < 0.01) decreased the duration of the CA1 epileptiform bursting caused by 'Mg(2+)-free' solutions. Conversely, felbamate (1.6 mM), CNQX (100 microM) and ketamine (100 microM) failed to affect the epileptiform bursting induced by the GABA antagonist penicillin. Pentobarbitone (100 microM) significantly (P < 0.01) decreased the duration of the CA1 epileptiform bursting caused by 'Mg(2+)-free' solutions, 4-aminopyridine or penicillin, while diphenylhydantoin (up to concentrations of 100 microM) failed to have an effect. The results indicate that felbamate displays a unique profile of in vitro antiepileptic effects as a broad spectrum antagonist of excitatory amino acid transmission.

The new anticonvulsant drugs. Implications for avoidance of adverse effects

Several new antiepileptic drugs offer a worthwhile alternative when standard antiepileptic drugs have failed. Suggestions have been made to improve the risk-benefit ratio of the new antiepileptic agents. More specifically, vigabatrin, which is a very useful and well tolerated new antiepileptic drug for refractory partial epilepsy, should be started at a low dosage of 0.5 g/day with increments of 0.5 g/day every week. Daily dosages exceeding 3 g/day should be restricted to patients with improvement. If necessary, the daily dosage of vigabatrin should be withdrawn slowly, i.e. by not more than 1 g/week. Lamotrigine is also a beneficial new drug for refractory partial and generalized seizures. However, the drug is associated with rash. In patients also receiving valproic acid (sodium valproate) [which inhibits the metabolism of lamotrigine], the incidence of rash can be reduced by slow titration of 25mg every other day for the first week and 25mg per day for the second week. Rare hypersensitivity reactions, e.g. Stevens-Johnson syndrome, remain a problem. The risk-benefit ratio of felbamate has recently been compromised by fatal aplastic anaemia and fatal liver disease in a number of patients. In general, patients should be withdrawn from felbamate, if possible, until further clarification of its definitive risk-benefit ratio. Finally, gabapentin is a very safe add-on medication. Its remarkably low potential to cause adverse effects makes it a welcome addition for the treatment of refractory partial epilepsy.