Lack of Interaction of Gabapentin with Carbamazepine or Valproate

Summary: Gabapentin (GBP) studies were conducted in patients with epilepsy receiving carbamazepine (CBZ, n= 12) or valproate (VPA, n = 14) monotherapy. The effects of GBP coadministration on steady-state CBZ or VPA concentrations and of these antiepileptic drugs (AEDs) on GBP pharmacokinetics were investigated. GBP (400 mg) was coadministered every 8 h for 3% days with CBZ or for 5 1/3 days with VPA. GBP was well tolerated. Mean steady-state plasma CBZ/CBZ-10, ll-epoxide (CBZ-E) and serum VPA concentrations before, during, and after GBP administration were not significantly different. Mean steady-state GBP pharmacokinetic parameters during CBZ or VPA coadministration were similar to steady-state parameters reported in healthy subjects. Thus, no pharmacokinetic interaction exists between CBZ or VPA and GBP. No dosage adjustment is necessary when GBP and CBZ or VPA are coadministered.     

Population Analysis of the Pharmacokinetics of Tiagabine in Patients with Epilepsy

Summary: Purpose: In two open-label long-term safety studies, we determined tiagabine (TGB) pharmacokinetics in patients with epilepsy.
Methods: In all, 2,147 plasma samples from 511 patients who participated in the studies were available. The total daily dose ranged from 2 mg administered once daily to 80 mg administered in four doses. A one-compartment model with first-order absorption and elimination was used to fit the TGB plasma concentration-time data, with a population pharmacokinetic approach.
Results: The patients'average (±SD) weight and age were 73.8 ± 20.7 kg and 32.1 ± 12.3 years. The most significantly factor affecting TGB pharmacokinetics was concomitant administration of other antiepileptic drugs (AEDs). The central clearance value in patients receiving AEDs known to induce hepatic drug metabolism was 21.4 L/h, a value 67% higher than the central clearance estimate obtained for the patients receiving AEDs not known to affect hepatic drug metabolism (12.8 L/h). There was no evidence of any dose or time effect, indicating that TGB pharmacokinetics are linear. TGB pharmacokinetics were not different in white, black, or Hispanic patients, although our ability to explore racial effects was limited since 90% of the patients were white. No other demographic variables (including age and smoking) or any clinical chemistry measurements (including bilirubin, SGOT, and SGPT) were important in explaining the variability in the clearance estimates.
Conclusions: TGB pharmacokinetics are linear, influenced by enzyme-inducing AEDs, and largely unaffected by other demographic variables.     

Population pharmacokinetics of pregabalin in healthy subjects and patients with chronic pain or partial seizures

Purpose: Pregabalin, a high‐affinity ligand for α2δ subunits of voltage‐gated calcium channels, is a novel pharmacotherapy for chronic pain, partial seizures, and other disorders. The present study investigated the population pharmacokinetics of pregabalin following single and multiple doses in healthy volunteers and patient populations. Methods: Using nonlinear mixed‐effect modeling, 5,583 plasma pregabalin concentration–time samples from 1,723 subjects were analyzed: 2,868 samples from healthy volunteers or subjects with renal impairment (n = 123), 1,513 from patients with partial seizures (n = 626), and 1,202 from patients with chronic pain (n = 974). A one‐compartment model with first‐order elimination and absorption processes and absorption lag time was used. Key Findings: This pharmacostatistical model showed that: (1) pregabalin oral clearance (CL/F) was directly proportional to creatinine clearance (CLcr), but was independent of gender, race, age, female hormonal status, daily dose, and dosing regimen; (2) apparent volume of distribution was dependent on body weight and gender; (3) absorption rate was decreased when given with food; and (4) coadministration with marketed antiepileptic drugs (AEDs) had no significant effect on pregabalin CL/F. Significance: Pregabalin CL/F is related to CLcr, and this relationship is similar among healthy volunteers and patients with either partial seizures or chronic pain disorders. The only factor having a clinically significant influence on steady‐state plasma pregabalin concentrations is renal function.     

Efficacy and tolerability of the new antiepileptic drugs: comparison of two recent guidelines

BACKGROUND: Until the early 1990s six major compounds (carbamazepine, ethosuximide, phenobarbital, phenytoin, primidone, and valproic acid) were available for the treatment of epilepsy. However, these drugs have pharmacokinetic limitations, teratogenic potential, and a negative effect on cognitive functions that impairs the quality of patients' lives and limits the use of these drugs in some patients. In addition, 20-30% of patients are refractory to these drugs. RECENT DEVELOPMENTS: The development of ten new antiepileptic drugs (vigabatrin, felbamate, gabapentin, lamotrigine, topiramate, tiagabine, oxcarbazepine, levetiracetam, zonisamide, and pregabalin) has expanded treatment options. The newer drugs may be better tolerated, have fewer drug interactions, and seem to affect cognitive functions to a lesser extent than old drugs. Guidelines on the use of new antiepileptic drugs have been developed in the USA and in the UK. Both guidelines offer a clear picture of the efficacy, safety, and tolerability of the new antiepileptic drugs and agree on their use as add-on treatment in patients who do not respond to conventional drugs. The guidelines differ in the type and strength of recommendations. Whereas the US guidelines recommend treatment in newly diagnosed epilepsy with a standard drug or a new drug depending on the individual patient's characteristics, the UK guidelines recommend that a new antiepileptic drug should be considered only if there is no benefit from an old antiepileptic drug, an old drug is contraindicated, there is a previous negative experience with the same drug, or the patient is a woman of childbearing potential. WHERE NEXT: The limited amount of information on the new antiepileptic drugs may explain the discrepancies among the two guidelines and between these and other recommendations. Comparative, pragmatic, long-term and open trials should be done to show long-term efficacy and comparative features of the new antiepileptic drugs, and to better assess the effect on quality-of-life, cost-effectiveness, tolerability, and teratogenic potential. In addition, the conflicts should be resolved between the needs of the regulatory bodies and those of the treating physicians. Finally, there is a need for trial designs to be standardised.     

Refractory epilepsy: treatment with new antiepileptic drugs.

Five antiepileptic drugs have been marketed in the last decade. We report here a retrospective study of patients attending our unit who were prescribed one of the new antiepileptic drugs. All these patients had refractory localization related epilepsy and had failed to respond to a first-line drug. The drugs had a different profile of side-effects but topiramate (42%) was the most common drug to be withdrawn due to side-effects as compared with tiagabine (26%), vigabatrin (16%), gabapentin (16%), and lamotrigine (15%). With regard to efficacy, 31% of the patients receiving gabapentin had a greater than 50% reduction in seizures compared with lamotrigine (25%), topiramate (20%), vigabatrin (19%) and tiagabine (11%). The number of patients remaining seizure free with gabapentin was 8% whilst for lamotrigine this was 5%, vigabatrin 5%, topiramate 1% and tiagabine 4%. In conclusion, all these five antiepileptic drugs are useful in treating refractory localization related epilepsy.     

Interactions of pregabalin with gabapentin, levetiracetam, tiagabine and vigabatrin in the mouse maximal electroshock-induced seizure model: a type II isobolographic analysis

The aim of this study was to characterize the anticonvulsant effects of pregabalin in combination with four second-generation antiepileptic drugs (i.e., gabapentin, levetiracetam, tiagabine, and vigabatrin) in the mouse maximal electroshock (MES)-induced seizure model by using the type II isobolographic analysis. Tonic hind limb extension (seizure activity) was evoked in adult male albino Swiss mice by a current (sine-wave, 25 mA, 500 V, 50 Hz, 0.2 s stimulus duration) delivered via auricular electrodes. The combination of pregabalin with gabapentin at the fixed-ratio of 1:1 was supra-additive (synergistic) in terms of seizure suppression while the combinations at the fixed-ratios of 2:1 and 4:1 were additive in the mouse MES model. Similarly, the combination of pregabalin with tiagabine at the fixed-ratio of 25:1 was supra-additive, whereas the combinations at the fixed-ratios of 100:1 and 50:1 were additive in the mouse MES model. Pregabalin with levetiracetam and vigabatrin at the fixed-ratios of 1:1, 2:1 and 4:1 were additive in this seizure model. The combinations of pregabalin with gabapentin (1:1) and pregabalin with tiagabine (25:1) appear to be favorable combinations exerting supra-additive interaction in suppressing MES-induced seizures. Pregabalin in combination with levetiracetam and vigabatrin appears to be neutral producing only additivity in the mouse MES model.     

Retention rates of new antiepileptic drugs in localization-related epilepsy: a single-center study

Objectives – We evaluated long‐term retention rates of newer antiepileptic drugs (AED) in adults with localization‐related epilepsy retrospectively. Methods – We estimated retention rates by Kaplan–Meier method in all 222 patients (age ≥ 16) with localization‐related epilepsy exposed to new AED at the Tampere University Hospital. Results – There were 141 patients exposed to lamotrigine, 78 to levetiracetam, 97 to topiramate, 68 to gabapentin, and 69 to tiagabine. Three‐year retention rate for lamotrigine was 73.5%, levetiracetam 65.4%, topiramate 64.2%, gabapentin 41.7%, and tiagabine 38.2%. The most common cause for withdrawal of these AED was lack of efficacy. Conclusions – Our study suggests that there are clinically significant differences among gabapentin, lamotrigine, levetiracetam, tiagabine, and topiramate as treatment for focal epilepsy in everyday practice. Gabapentin and tiagabine seem to be less useful than the other three AED. Furthermore, our study supports the value of retention rate studies in assessing outcome of the drugs in clinical practice.     

Tiagabine: a new therapeutic option for people with intellectual disability and partial epilepsy

Tiagabine exerts its antiepileptic drug (AED) activity by selectively inhibiting the uptake of gamma-aminobutyric acid (GABA) onto the transporter molecules, and thus, increasing extracellular concentrations of GABA in the brain. The absorption and elimination of tiagabine follow linear pharmacokinetics. Tiagabine is metabolized by hepatic cytochrome P450 enzymes and enzyme-inducing AEDs increase tiagabine clearance by 50-65%. Tiagabine has shown no clinically important interactions with other drugs, including oral contraceptives. In the perforant pathway stimulation model of status epilepticus, tiagabine reduced the seizure number and severity, and also prevented the loss of pyramidal cells in the hippocampus as well as alleviated impairment of the spatial memory impairment associated with hippocampal damage. Tiagabine has both antiepileptogenic and anticonvulsant effects in the kindling model of epilepsy. Based on the data from the short- and long-term add-on studies, tiagabine is effective adjunctive therapy for all partial seizure types in adolescents and adults. Conversion to tiagabine monotherapy has been also possible in substantial amount of patients with partial seizures in three trials. Tiagabine is generally well-tolerated. The most common adverse events in controlled studies involve the central nervous system; for example, dizziness, asthenia, nervousness, tremor, depressed mood and emotional lability. Special safety analyses with formal neuropsychological testing suggest that tiagabine does not adversely affect cognition or mood. Tiagabine represents an important new therapeutic option for patients with treatment-refractory partial seizures. The role of tiagabine in the management of partial epilepsy of patients with intellectual disability is especially emphasized since tiagabine has a low side-effect profile in the cognitive area.     

Effect of Renal Impairment on the Pharmacokinetics and Tolerability of Tiagabine

Summary: Purpose: We wished to determine the effect of renal impairment on the pharmacokinetics and tolerability of the new antiepileptic drug tiagabine (TGB).
Methods: We assessed TGB pharmacokinetics and tolerability in 25 subjects with various degrees of renal function (based on creatinine clearance, n = 4–6 per group) from healthy (group I) to requiring hemodialysis (group V) in a single and multiple dose (every 12h), one-period (groups I-IV) or a single dose, two-period (group V) study (4-mg oral doses of TGB · HCl). Blood samples were collected after the first dose (both periods for group V) and after the last dose on day 5 (groups I-IV). TGB plasma concentrations and plasma protein binding were determined by high-performance liquid chromatography (HPLC) and ultrafiltration, respectively.
Results: TGB was well tolerated by all study subjects. The pharmacokinetics of TGB were similar in all subjects; no pharmacokinetic parameter (based on either total or unbound concentrations) was statistically correlated with creatinine clearance. For total TGB in plasma, single-dose mean values of the maximum plasma concentration, clearance, and half-life (t1/2) ranged from 52 to 108 ng/ml, from 7.14 to 11.02 I/h, and from 6.4 to 8.4 h, respectively.
Conclusions: TGB pharmacokinetics and tolerability were independent of renal function; therefore, dosage adjustment is unnecessary for epilepsy patients with renal impairment.     

Levetiracetam,a new antiepileptic agent: lack of in vitro and in vivo pharmacokinetic interaction with valproic acid

PURPOSE: The novel antiepileptic drug (AED) levetiracetam (LEV; Keppra) has a wide therapeutic index and pharmacokinetic characteristics predicting limited drug-interaction potential. It is indicated as an add-on treatment in patients with epilepsy, and thus coadministration with valproic acid (VPA) is likely. These studies were performed to determine whether coadministration of LEV with VPA might result in pharmacokinetic interactions. METHODS: In vitro assays were performed to characterize the transformation of LEV into its main in vivo metabolite UCB L057. The reaction was examined for its sensitivity to clinically relevant concentrations of VPA. An open-label, one-way, one-sequence crossover clinical trial was conducted in 16 healthy volunteers to assess further the possibility of any relevant pharmacokinetic interaction. RESULTS: Human whole blood and, to a lesser extent, human liver homogenates were demonstrated to hydrolyze LEV to UCB L057, its main metabolite. The reaction possibly involves type-B esterases and is not affected by 1 mM VPA (i.e., 166 microg/ml). Pharmacokinetic parameters of a single dose of LEV (1,500 mg) coadministered with steady-state concentrations of VPA (8 days of 500 mg, b.i.d.) did not differ significantly from the pharmacokinetics of LEV administered alone [area under the curve (AUC) of 397 and 400 microg/h/ml, respectively]. Furthermore, LEV did not affect the steady-state pharmacokinetics of VPA. CONCLUSIONS: These findings suggest the absence of a pharmacokinetic interaction between VPA and LEV during short-term administration, and suggest that dose adjustment is not required when these two drugs are given together.     

Clinically important drug interactions in epilepsy: general features and interactions between antiepileptic drugs

There are two types of interactions between drugs, pharmacokinetic and pharmacodynamic. For antiepileptic drugs (AEDs), pharmacokinetic interactions are the most notable type, but pharmacodynamic interactions involving reciprocal potentiation of pharmacological effects at the site of action are also important. By far the most important pharmacokinetic interactions are those involving cytochrome P450 isoenzymes in hepatic metabolism. Among old generation AEDs, carbamazepine, phenytoin, phenobarbital, and primidone induce the activity of several enzymes involved in drug metabolism, leading to decreased plasma concentration and reduced pharmacological effect of drugs, which are substrates of the same enzymes (eg, tiagabine, valproic acid, lamotrigine, and topiramate). In contrast, the new AEDs gabapentin, lamotrigine, levetiracetam, tiagabine, topiramate, vigabatrin, and zonisamide do not induce the metabolism of other AEDs. Interactions involving enzyme inhibition include the increase in plasma concentrations of lamotrigine and phenobarbital caused by valproic acid. Among AEDs, the least potential interaction is associated with gabapentin and levetiracetam.     

Three new drugs for epilepsy: levetiracetam, oxcarbazepine, and zonisamide

During the last decade, nine new antiepileptic drugs were approved for use by the US Food and Drug Administration. The characteristics of three of these new antiepileptic drugs-levetiracetam, oxcarbazepine, and zonisamide--are reviewed here. Their individual characteristics, including mechanism of action, efficacy, safety, and pharmacokinetics, are compared, and their effectiveness in treating specific seizure types is noted. As with all antiepileptic drugs, the efficacy, side-effect, and pharmacokinetic profiles must be matched to each individual's clinical profile to attain the maximum benefit. All three are unique and will be useful in expanding the aggregate of therapies available to clinicians treating diverse epilepsy syndromes and seizure types. This review focuses on results in adults; pediatric experience is reported in other articles in this supplement.     

Antiepileptic drug discovery: does mechanism of action matter?

This commentary discusses briefly the role of the mechanism of antiepileptic action in discovery of drugs for the treatment of epilepsy. More specifically, two questions are addressed. (1) Has mechanism-driven antiepileptic drug discovery brought us better epilepsy treatment? Although this question is difficult to answer, the short answer is "not yet." Modern antiepileptic drugs with new or modified mechanisms of action do not seem to have substantially improved the efficacy or the safety of epilepsy treatment. In fact, some modern antiepileptic drugs such as progabide, tiagabine, and vigabatrin have been associated with a number of safety issues. (2) Why do drugs with new mechanisms seem to have failed to deliver better treatment? Although it is always difficult to know why something did not occur, one putative explanation may be worthwhile to consider. The past development of new antiepileptic drugs targeted putative mechanisms of seizure generation. As seizures are only symptoms of the underlying epilepsy, blocking seizure generation can provide at best only symptomatic treatment. It may be that the failure in treating drug-resistant seizures is related, at least in part, to the failure of current drugs in targeting the mechanisms underlying epilepsy. In conclusion, continuing to develop new antiepileptic drugs for drug-resistant epilepsy by targeting seizure generation may be futile and one possible explanation of why we do not seem to make substantial progress in the treatment of drug-resistant epilepsy. Developing antiepileptic drugs with antiepileptogenic activity may be a clue to better treatment of presently drug-resistant epilepsy.     

Pharmacokinetics of Old, New, and Yet-to-Be-Discovered Antiepileptic Drugs

Summary: The pharmacokinetics of antiepileptic drugs have been investigated extensively during the last two decades. It has become evident that the therapeutic management of epileptic patients would be significantly improved with the elimination of the drawbacks associated with existing antiepileptic drugs. Based on the information accumulated to date, the following set of principles is proposed: (1) The requirement of central nervous system (CNS) penetration automatically creates vulnerability to CNS toxicity. (2) The narrower the therapeutic range of a given drug, the more limiting its pharmacokinetic properties become. (3) The requirements of chronic administration and compliance place narrow limits on the optimum half-life (24 h). (4) The practice of polytherapy and the potential for pharmacodynamic and pharmacokinetic interactions make it necessary to simplify metabolic schemes (i.e., use of meta-bolically directed design approaches). (5) New antiepileptic drugs have shown that widely different structures are associated with anticonvulsant properties–it becomes important to minimize toxicity and optimize pharmacokinetic characteristics early in the development of future antiepileptic drugs. (6) Yet-to-be-discovered antiepileptic drugs will probably include biotechnology-based pharmaceuticals. Such biologic agents as neuropeptides present a new set of pharmacokinetic requirements because they are generally receptor targeted, they do not cross absorption barriers easily, and they are labile and subject to degradation.     

Epilepsy in the Elderly: The Use of Tiagabine

Summary: The incidence of epilepsy increases sharply in patients older than 60 years. There is a clear need for clinical trials designed specifically for this age group, as elderly patients differ from younger patients with epilepsy with respect to seizure etiology, coexisting diseases, concomitant drug therapy, and drug disposition. The new antiepileptic drugs (AEDs) are often associated with fewer side effects than are the traditional AEDs and may be particularly useful in the elderly. The pharmacokinetics of tiagabine (TGB) are not significantly modified in elderly patients, although elimination is more rapid in the presence of enzyme-inducing AEDs. Efficacy and tolerability data on TGB in elderly patients is currently limited, and a formal trial of TGB monotherapy in this age group is needed.     

Social and economic aspects of administration of new antiepileptic drugs

The necessity of analysis of the cost of treatment of patients with epilepsy becomes of primary importance in Poland as a consequence of recent economic transformations affecting the efficiency of health service. The reasons are: high number of patients with epilepsy (approaching 400,000 in a population of about 40 mln) and long time course of illness, taking into account steady, gradual rise of the cost of treatment, even if we accept greater efficiency of the new antiepileptic drugs. However, the analysis of questionnaires provided by patients with epilepsy indicates that optimation of their treatment with introduction of new antiepileptic drugs may be a procedure leading to diminution of the global expenses associated with care of epileptic patients. Identification of factors influencing cost of antiepileptic treatment before and after introduction of new antiepileptic drugs. A group of 150 people chosen at random from a population of persons taking new antiepileptic drugs (vigabatrin, lamotrygin, topiramate, gabapentin, tiagabine) received anonymous questionnaires concerning the time course of their illness. 80 questionnaires were returned. The questions concerned the situation before and after treatment. Statistical analysis included t test for dependent samples-including items such as: number of epileptic seizures and number and days of hospitalization, etc. per year of observation. Significant decrease of the number of epileptic seizures (p < 0.05), number of hospitalizations (p < 0.001), days of hospitalizations (p < 0.001) and neurological consultations (p < 0.001) occurred after optimalization of treatment. Results of our research illustrate significant reduction of direct costs of treatment associated with introduction of new antiepileptic drugs.     

Tiagabine in the treatment of epilepsy--a clinical review with a guide for the prescribing physician

Tiagabine is currently recommended mainly as add-on therapy in adults and children above 12 years with partial epilepsy not satisfactorily controlled with other antiepileptic drugs. Based on available evidence and our clinical experience, tiagabine should be used preferably in patients sharing one or more of the following additional features, (i) a history of drug-induced cutaneous adverse events; (ii) mild to moderate epilepsy allowing for a slow titration and gradual onset of anticonvulsant action over a few weeks; (iii) patients for whom it is particularly important to avoid a deterioration in cognitive performance; and, (iv) patients who failed to respond to previous treatment with sodium channel blocker agents as they may particularly benefit from the introduction of tiagabine, due to its GABAergic mechanism of action. Tiagabine can also be used successfully in other patients with refractory partial epilepsy. Tiagabine is not indicated for patients with generalized or unclassified epilepsies and for patients with severely impaired liver function.     

The interface of preclinical evaluation with clinical testing of antiepileptic drugs: role of pharmacogenomics and pharmacogenetics

Despite the release of eight antiepileptic drugs (AEDs) during the last decade, the incidence of pharmacoresistant epilepsy has changed relatively little. Predicting efficacy and safety of AEDs in people with epilepsy from acute seizure models in rodents is difficult and risky. It is becoming increasingly clear that genetic polymorphisms play an integral role in variability in both antiepileptic drug pharmacokinetics and pharmacodynamics. The publication of the human genome and increasing sophisticated and powerful genetic tools offers new methods for screening drugs and predicting deadly idiosyncratic side effects. In this review the use of pharmacogenomic and pharmacokinetic techniques in the development and monitoring of antiepileptic drug therapy is reviewed. Genetic techniques have the potential of identifying novel drug targets, predicting drug response, and identifying individuals at risk for serious idosyncratic reactions.     

Newer antiepileptic drugs]

Ten newer antiepileptic drugs have been developed since 1990s. These drugs have wider therapeutic spectra, fewer side-effects, and lesser drug-to-drug interactions compared with the older typical antiepileptic drugs. Among them, zonisamide was developed in Japan and has been used from 1989. Gabapentin was at length approved in 2006. The other newer antiepileptic drugs are not approved yet in Japan. Felbamate can not be used in Europe because it may induce lethal hepatic toxicity and aplastic anemia. Vigabatrin is not approved in USA because it may induce permanent visual field deficit. The USA guideline for epilepsy treatment recommends that patients with newly diagnosed epilepsy can be treated with gabapentin, lamotrigine, topiramate, and oxcarbazepine. In contrast, based on epilepsy treatment guideline in England, newer antiepileptic drugs are considered only when patients with newly diagnosed epilepsy are unable to use the older antiepileptic drugs for some reasons. All newer antiepileptic drugs are used for intractable partial epilepsies, and lamotrigine and topiramate can also be used for idiopathic generalized epilepsies. The response rate (seizure reduction rate with 50% or more) and drop-out rate are overlapping among all newer antiepileptic drugs. Gabapentin, levetiracetam, and pregabalin are eliminated from kidney, and they had no drug-to-drug interactions and can be titrated rapidly. The serum concentration of lamotrigine is decreased with co-administration of hepatic enzyme inducing drugs and is increased with co-administration of valproic acid. Hypersensitivity reactions are rare with gavapentin, levetiracetam, topiramate, and tiagabin. Psychoses are reported to be induced with zonisamide, however, they can be induced with the other newer drugs (topiramate, levetiracetam, etc.). Drug-induced psychiatric symptoms, especially depression, may be often underdiagnosed. Many of these newer drugs (gabapentine, lamotrigine, levetiracetam, oxycarbazepine, etc.) have effects on chronic neuropathic pain. Some newer drugs show mood stabilizing effects (lamotrigine, oxycarbazepine, etc.), or antianxiety effect (gabapentin, topiramate, levetiracetam, pregavalin, etc.). Wide range of action to central nervous system of these newer antiepileptic drugs may serve not only for clinical seizure suppression, but also for neuroprotection.