An Overview of the Efficacy and Tolerability of New Antiepileptic Drugs

Summary: To evaluate the efficacy and tolerability of recently developed antiepileptic drugs (AEDs), a systematic review of placebo-controlled, randomized controlled trials (RCTs) of the AEDs as add-on therapy in refractory partial epilepsy was conducted. Two or more RCTs meeting our inclusion criteria were found for gabapentin (GBP), lamotrigine (LTG), tiagabine (TGB), topiramate (TPM), vigabatrin (VGB), and zonisamide (ZNS). The outcome selected for estimation of efficacy was the proportion of patients experiencing a ≥50% reduction in seizure frequency from baseline. Tolerability was estimated on the basis of rates of patient withdrawal from study for any reason. Efficacy and tolerability odds ratios (ORs) and 95% confidence intervals (95% CIs) for each measure were generated for each trial included in the analysis, and overall efficacy and tolerability ORs were calculated for each AED across all trials and drug dosages evaluated. Because 95% CIs for both efficacy and tolerability overlapped for the six drugs, conclusive evidence of between-drug differences in effectiveness or safety were not obtained from the analysis. However, the data suggest that the drug with the highest OR for efficacy (TPM) may be approximately twice as effective as the AED with the lowest OR for efficacy (GBP), and that the treatment that appears to most frequently cause withdrawal (ZNS) may be about four times more likely to do so that the AED with the lowest withdrawal rate (LTG). RCTs comparing newer AEDs with the older standard drugs and with each other are needed to further evaluate their relative utility.     

Clinical Administration of New Antiepileptic Drugs: An Overview of Safety and Efficacy

Summary: Gabapentin, lamotrigine, tiagabine, topira-mate, vigabatrin, and zonisamide are all administered as add-on therapy for treatment of patients with refractory epilepsy. To date, no comparative randomized trials have been performed that could potentially allow an evidence-based choice to be made between these antiepileptic drugs (AEDs). We report a series of meta-analyses of placebo-controlled, randomized add-on trials in patients with partial epilepsy. Results of these meta-analyses are compared, thus giving broad estimates of the comparative efficacy and tolerability of these AEDs. The efficacy out come is the odds ratio for the number of patients with a ≥50% reduction in seizure frequency. Reported side ef fects are also used as tolerability outcomes, and study withdrawal is used as a global outcome measure. Results are summarized as odds ratios with 95% confidence in tervals (CIs). When each outcome is compared among drugs, the 95% CIS overlap. Therefore, no conclusive ev idence of a difference in efficacy or tolerability between these AEDs was derived, even though the apparently most effective agent (topiramate) may be twice as effec tive as the apparently least effective agent (lamotrigine). Comparative randomized studies are needed to further evaluate these drugs.     

Overview of the Safety of Newer Antiepileptic Drugs

Summary: Standard antiepileptic drugs (AEDs) are associated with a wide variety of acute and chronic adverse events and with many interactions with each other and with non-AEDs that complicate patient management. The safety and interaction profiles of the newer AEDs have also been intensively studied. Safety data are available for six of the newer AEDs, lamotrigine (LTG), vigabatrin (VGB), gabapentin (GBP), tiagabine (TGB), felbamate (FBM), and topiramate (TPM). The potential for the most recently developed AEDs for producing rare idiosyncratic reactions cannot be ascertained until additional patient exposures have been reported from careful postmarketing surveillance.     

Clinical Significance of Animal Seizure Models and Mechanism of Action Studies of Potential Antiepileptic Drugs

Summary: More than 50 million persons worldwide suffer from epilepsy, many of whom are refractory to treatment with standard antiepileptic drugs (AEDs). Fortunately, new AEDs commercialized since 1990 are improving the clinical outlook for many patients. Our growing understanding of anticonvulsant mechanisms and the relevance of preclinical animal studies to clinical antiepileptic activity have already contributed to the design of several new AEDs and should be increasingly beneficial to further efforts at drug development. Mechanisms have been identified for older AEDs [phenytoin (PHT), carbamazepine (CBZ), valproate (VPA), barbiturates, benzodiazepines (BZDs), ethosuximide (ESM)] and newer AEDs [vigabatrin (VGB), lamotrigine (LTG), gabapentin (GBP) tiagabine (TGB), felbamate (FBM), topiramate (TPM)]. Several novel anticonvulsant mechanisms have recently been discovered. FBM appears to be active at the strychnine-insensitive glycine binding site of the NMDA receptor. TPM is active on the kainate/AMPA subtype of glu-tamate receptor and at a potentially novel site on the GABA_A receptor. For several reasons, availability of a single AED with multiple mechanisms of action may be preferred over availability of multiple AEDs with single mechanisms of action. These reasons include ease of titration, lack of drug-drug interactions, and reduced potential for pharmacodynamic tolerance.     

Pharmacokinetics of New Antiepileptic Drugs

Summary: This article surveys the pharmacokinetic parameters for the new antiepileptic drugs (AEDs): felba-mate, gabapentin, lamotrigine, oxcarbazepine, tiagabine, topiramate, and vigabatrin. Compared to the pharmaco kinetics of standard AEDs, these new AEDs have progressed in terms of (a) longer half-lives, permitting once-or twice-daily dosing, (b) greatly reduced potential for drug interactions, thus increasing ease of treatment, and (c) general lack of hepatic enzyme induction, which facilitates polytherapy as well as other aspects of treatment.     

Update on the Mechanism of Action of Antiepileptic Drugs

Summary: Novel antiepileptic drugs (AEDs) are thought to act on voltage-sensitive ion channels, on inhibitory neurotransmission or on excitatory neurotransmission. Two successful examples of rational AED design that po tentiate GABA-mediated inhibition are vigabatrin (VGB) by irreversible inhibition of GABA-transaminase, and ti-agabine (TGB) by blocking GAB A uptake. Lamotrigine (LTG) prolongs inactivation of voltage-dependent sodium channels. The anticonvulsant action of remacemide (RCM) is probably largely due to blockade of NMDA receptors and prolonged inactivation of sodium channels induced by its desglycinated metabolite. Felbamate (FBM) apparently blocks NMDA receptors, potentiates GABA-mediated responses, blocks L-type calcium channels, and possibly also prolongs sodium channel inactivation. Similarly, to piramate (TPM) has multiple probable sites of action, including sodium channels, GABA receptors, and glutamate (AMPA) receptors. Gabapentin (GBP) apparently has a completely novel type of action, probably involving potentiation of GABA-mediated inhibition and possibly also inactivation of sodium channels. The therapeutic advantages of the novel AEDs are as yet only partially explained by our present understanding of their. Mechanisms of action.     

Pharmacokinetics and Interaction Profile of Topiramate: Review and Comparison with Other Newer Antiepileptic Drugs

Summary: 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.     

Central nervous system adverse effects of new antiepileptic drugs: A meta-analysis of placebo-controlled studies

OBJECTIVE: Systematic review and meta-analysis of the most frequent treatment-emergent central nervous system adverse events (CNS AEs) of new antiepileptic drugs (AEDs) from double-blind, add-on, placebo-controlled studies conducted in adult epileptic patients and identification of dose-adverse effect relationships. METHODS: Trial reports found by searching Medline and journals. Outcome was the number of patients complaining of treatment-emergent CNS AEs. Sixteen predefined CNS AEs were considered. Risk differences (RDs) were calculated for individual studies and summary statistics estimated using the random effect model. Predefined CNS AEs in patients treated with active drug (broken down into dose levels) or placebo were extracted and the RDs (95% CI) for CNS AEs were calculated. RESULTS: Thirty-six suitable studies identified. No meta-analysis was possible for oxcarbazepine and tiagabine (only one study each included). For these drugs RDs were calculated from single studies. Gabapentin was significantly associated with somnolence 0.13 (0.06-0.2) and dizziness 0.11 (0.07-0.15); lamotrigine with dizziness 0.11 (0.05-0.17), ataxia 0.12 (0.01-0.24) and diplopia 0.12 (0.00-0.24); levetiracetam with somnolence 0.06 (0.01-0.11); pregabalin with somnolence 0.11 (0.07-0.15), dizziness 0.22 (0.16-0.28), ataxia 0.10 (0.06-0.14) and fatigue 0.04 (0.01-0.08); topiramate with somnolence 0.09 (0.04-0.14), dizziness 0.06 (0.00-0.11), cognitive impairment 0.14 (0.06-0.22) and fatigue 0.06 (0.01-0.12); zonisamide with somnolence 0.06 (0.02-0.11) and dizziness 0.06 (0.00-0.12). The dose-response relationship was analysed only for those CNS AEs significantly associated with the AED. CONCLUSIONS: No comparison between drugs was possible. One CNS AE was significantly more frequent for levetiracetam, two for zonisamide and gabapentin, three for lamotrigine and four for pregabalin and topiramate.     

Antiepileptic Drugs and the Electroencephalogram

The usefulness of electroencephalography (EEG) as an aid to diagnosis of seizure disorders is established, but its role as a guide to monitoring treatment is much less certain. For those patients with classical absences and 3-s spike wave activity there is a very close correlation between control of clinically detected seizures and EEG events. In some, but not all, patients with other seizure disorders there is a positive correlation between numbers of seizures and amount of interictal epileptiform activity (IEA). Intravenous benzodiazepines and phenytoin result in both acute seizure control and suppression of IEA. For seizures other than absences, and antiepileptic drugs (AEDs) given in the medium and long term, there is generally not a clear relationship between control of seizures and IEA. In studies of children whose epilepsy is in remission, persistent IEA has been associated with a higher risk of seizure relapse should AEDs be discontinued, but in adults the relevance of persistent IEA appears to be much less certain. Benzodiazepines and barbiturates result in increased fast activity. All AEDs may result in slowing of the dominant rhythm and increased slow activity. Carbamazepine, in particular, is often associated with apparent deterioration of background activity, even in the face of clinical improvement. Further studies are necessary to determine the mechanisms and significance of AED-induced changes in EEG background activity.     

Clobazam, Oxcarbazepine, Tiagabine, Topiramate, and Other New Antiepileptic Drugs

Summary: Clinical investigators recently have studied at least 21 new antiepileptic drugs (AEDs) in people with epilepsy. This review briefly examines 15 of these new AEDs: clobazam (CLB), dezinamide, flunarizine (FNR), loreclezole, milacemide (MLM), MK-801, nafimidone, ORG-6370, oxcarbazepine (OCBZ), progabide (PGB), ralitoline, stiripentol, tiagabine (TGB), topiramate (TPM), and zonisamide (ZNS). CLB, PGB, and TGB represent agents that act on the GABA system, and MLM acts on the glycine system. MK-801 and ZNS (in part) are excitatory amino acid antagonists, and FNR is a calcium-channel antagonist. OCBZ is a keto analogue of carbam-azepine, which is not metabolized to the epoxide and may have fewer side effects. The remaining agents are novel compounds with a variety of suspected mechanisms. TPM appears especially effective for intractable partial seizures but has a high incidence of cognitive side effects. None of these new AEDs is useful for all patients with inadequate seizure control or ongoing toxicity. The role of each will require further clinical study and experience.     

Interlaboratory variability in the quantification of new generation antiepileptic drugs based on external quality assessment data

PURPOSE: To assess interlaboratory variability in the determination of serum levels of new antiepileptic drugs (AEDs). METHODS: Lyophilised serum samples containing clinically relevant concentrations of felbamate (FBM), gabapentin (GBP), lamotrigine (LTG), the monohydroxy derivative of oxcarbazepine (OCBZ; MHD), tiagabine (TGB), topiramate (TPM), and vigabatrin (VGB) were distributed monthly among 70 laboratories participating in the international Heathcontrol External Quality Assessment Scheme (EQAS). Assay results returned over a 15-month period were evaluated for precision and accuracy. RESULTS: The most frequently measured compound was LTG (65), followed by MHD (39), GBP (19), TPM (18), VGB (15), FBM (16), and TGB (8). High-performance liquid chromatography was the most commonly used assay technique for all drugs except for TPM, for which two thirds of laboratories used a commercial immunoassay. For all assay methods combined, precision was <11% for MHD, FBM, TPM, and LTG, close to 15% for GBP and VGB, and as high as 54% for TGB (p < 0.001). Mean accuracy values were <10% for all drugs other than TGB, for which measured values were on average 13.9% higher than spiked values, with a high variability around the mean (45%). No differences in precision and accuracy were found between methods, except for TPM, for which gas chromatography showed poorer accuracy compared with immunoassay and gas chromatography-mass spectrometry. CONCLUSIONS: With the notable exception of TGB, interlaboratory variability in the determination of new AEDs was comparable to that reported with older-generation agents. Poor assay performance is related more to individual operators than to the intrinsic characteristics of the method applied. Participation in an EQAS scheme is recommended to ensure adequate control of assay variability in therapeutic drug monitoring.     

Children of School Age: The Influence of Antiepileptic Drugs on Behavior and Intellect

Summary: The role of antiepileptic drugs in behavior and cognitive function in children is well documented in the literature. In general, behavioral problems occur most frequently with phenobarbital and clonazepam, and appear least often with valproate and carbamazepine. Cognitive impairments occur with phenytoin, are less evident with valproate, and minimal with carbamazepine. Monotherapy, as with adults, leads to improvements in both cognitive abilities and behavior.