Lack of tolerance to the anticonvulsant effects of tiagabine following chronic (21 day) treatment

Long-term anxiolytic and antidepressant-like behavioural effects of tiagabine, a selective GABA transporter-1 (GAT-1) inhibitor, coincide with a decrease in HPA system activity in C57BL/6 mice

Gamma-aminobutyric acid (GABA) system plays a pivotal role in the pathophysiology of anxiety and mood disorders. This study was aimed to assess the anxiolytic and antidepressant-like properties of tiagabine, an inhibitor of the GABA transporter-1 (GAT-1), after acute and chronic administration in C57BL/6JOlaHsD mice with paroxetine as a positive control. In first experiments, the acute administration of tiagabine (7.5 mg/kg, orally [PO]) and paroxetine (10 mg/kg PO) induced anxiolytic effects in the elevated plus maze test and the modified hole board test and an antidepressant-like effect in the forced swim test. Chronic application of tiagabine (7.5 mg/kg PO) and paroxetine (10 mg/kg PO) for 22 days revealed an anxiolytic and antidepressant-like efficacy of tiagabine only. In a further experiment, we analysed the impact of chronic tiagabine versus paroxetine treatment on the hypothalamic-pituitary-adrenocortical (HPA) system regulation. GAT-1 blockade induced a setpoint-shift of the stress hormone system toward lower levels as indicated by decreased plasma corticosterone concentrations and attenuated gene expression levels of corticotropin-releasing factor in the paraventricular nucleus of the hypothalamus and of hippocampal steroid receptors. This data indicate that both acute and long-term anxiolytic and antidepressant-like properties of brain GAT-1 inhibition coincide with a reduction in HPA system activity in mice.

Gaboxadol, a selective extrasynaptic GABA(A) agonist, does not generalise to other sleep-enhancing drugs: a rat drug discrimination study

Gaboxadol is a selective extrasynaptic GABA(A) receptor agonist (SEGA) which enhances slow-wave sleep, and may act principally at extrasynaptic GABA(A)alpha4betadelta receptors. Drug discrimination is a very useful approach for exploring in vivo pharmacological similarities and differences between compounds and was therefore used to compare gaboxadol and zolpidem, an established hypnotic drug, against zopiclone, S-zopiclone, indiplon and tiagabine, all of which have been reported to enhance sleep. Gaboxadol generalised to itself, but not to zolpidem, zopiclone, S-zopiclone, R-zopiclone, indiplon or tiagabine. By contrast, zolpidem generalised to itself, zopiclone, S-zopiclone and indiplon, but not to R-zopiclone (the inactive enantiomer of zopiclone), gaboxadol or tiagabine. This suggests that zolpidem, zopiclone, S-zopiclone and indiplon share a discriminative stimulus, which may be mediated by their efficacy at GABA(A)alpha1betagamma receptors. Gaboxadol and tiagabine each have a different discriminative stimulus from all the other drugs tested.

Effect of the administration of tiagabine and gabapentin on rabbit electroencephalogram activity

New generation antiepileptic drugs, including gabapentin and tiagabine, are used in monotherapy or in combination with other drugs for specific seizure types. The multidirectional mechanism of activity and varied pharmacological properties of these drugs suggest that they could also be used in the therapy of other diseases. A possible limitation of new generation antiepileptic drugs is the incidence of CNS-related adverse effects. Few studies have assessed the effect of new antiepileptic drugs on electroencephalogram (EEG) recordings in subjects using these drugs for diseases other than epilepsy. The aim of this study was to determine the effects of tiagabine and gabapentin on EEG recordings from the midbrain reticular formation, dorsal hippocampus and frontal cortex in rabbits. Tiagabine was administered orally at a single dose of 5 and 20 mg kg(-1), or repeatedly at a dose of 5 mg kg(-1) (twice a day) for 14 days. Gabapentin was administered orally at a single dose of 25 and 100 mg kg(-1), or repeatedly at a dose of 25 mg kg(-1) (twice a day) for 14 days. Both tiagabine and gabapentin caused changes indicative of CNS inhibitory properties, which may be associated with the adverse effects of the drugs. After repeated doses of the drugs, the changes in EEG recordings were less pronounced than after single doses, which may indicate adaptive changes. The hippocampus was found to be the least sensitive to the effect of gabapentin.

Experimental and clinical evidence for loss of effect (tolerance) during prolonged treatment with antiepileptic drugs

Development of tolerance (i.e., the reduction in response to a drug after repeated administration) is an adaptive response of the body to prolonged exposure to the drug, and tolerance to antiepileptic drugs (AEDs) is no exception. Tolerance develops to some drug effects much more rapidly than to others. The extent of tolerance depends on the drug and individual (genetic?) factors. Tolerance may lead to attenuation of side effects but also to loss of efficacy of AEDs and is reversible after discontinuation of drug treatment. Different experimental approaches are used to study tolerance in laboratory animals. Development of tolerance depends on the experimental model, drug, drug dosage, and duration of treatment, so that a battery of experimental protocols is needed to evaluate fully whether tolerance to effect occurs. Two major types of tolerance are known. Pharmacokinetic (metabolic) tolerance, due to induction of AED-metabolizing enzymes has been shown for most first-generation AEDs, and is easy to overcome by increasing dosage. Pharmacodynamic (functional) tolerance is due to "adaptation" of AED targets (e.g., by loss of receptor sensitivity) and has been shown experimentally for all AEDs that lose activity during prolonged treatment. Functional tolerance may lead to complete loss of AED activity and cross-tolerance to other AEDs. Convincing experimental evidence indicates that almost all first-, second-, and third-generation AEDs lose their antiepileptic activity during prolonged treatment, although to a different extent. Because of diverse confounding factors, detecting tolerance in patients with epilepsy is more difficult but can be done with careful assessment of decline during long-term individual patient response. After excluding confounding factors, tolerance to antiepileptic effect for most modern and old AEDs can be shown in small subgroups of responders by assessing individual or group response. Development of tolerance to the antiepileptic activity of an AED may be an important reason for failure of drug treatment. Knowledge of tolerance to AED effects as a mechanism of drug resistance in previous responders is important for patients, physicians, and scientists.

Antidystonic efficacy of γ-aminobutyric acid uptake inhibitors in the dt mutant

In the dtsz mutant hamster, a model of paroxysmal dyskinesia in which dystonic episodes occur in response to stress, previous studies suggested that retarded development of gamma-aminobutyric acid (GABA)ergic inhibition plays a critical role in the pathogenesis. In the present study, we therefore examined the effects of selective GABA uptake inhibitors on severity of dystonia in dtsz hamsters. R(-)N-(4,4-di(3-methylthien-2-yl)-but-3-enyl) nipecotic acid hydrochloride (tiagabine, 5-20 mg/kg i.p.) and 1-[2-[[(diphenylmethylene) imino]oxy]ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride (NNC-711, 1-10 mg/kg i.p.) significantly reduced the severity of dystonia. These data suggest that GABA uptake inhibitors may provide novel therapeutic approaches for paroxysmal dyskinesias.

GABA transporter deficiency causes tremor, ataxia, nervousness, and increased GABA-induced tonic conductance in cerebellum

GABA transporter subtype 1 (GAT1) knock-out (KO) mice display normal reproduction and life span but have reduced body weight (female, -10%; male, -20%) and higher body temperature fluctuations in the 0.2-1.5/h frequency range. Mouse GAT1 (mGAT1) KO mice exhibit motor disorders, including gait abnormality, constant 25-32 Hz tremor, which is aggravated by flunitrazepam, reduced rotarod performance, and reduced locomotor activity in the home cage. Open-field tests show delayed exploratory activity, reduced rearing, and reduced visits to the central area, with no change in the total distance traveled. The mGAT1 KO mice display no difference in acoustic startle response but exhibit a deficiency in prepulse inhibition. These open-field and prepulse inhibition results suggest that the mGAT1 KO mice display mild anxiety or nervousness. The compromised GABA uptake in mGAT1 KO mice results in an increased GABA(A) receptor-mediated tonic conductance in both cerebellar granule and Purkinje cells. The reduced rate of GABA clearance from the synaptic cleft is probably responsible for the slower decay of spontaneous IPSCs in cerebellar granule cells. There is little or no compensatory change in other proteins or structures related to GABA transmission in the mGAT1 KO mice, including GAT1-independent GABA uptake, number of GABAergic interneurons, and GABA(A)-, vesicular GABA transporter-, GAD65-, and GAT3-immunoreactive structures in cerebellum or hippocampus. Therefore, the excessive extracellular GABA present in mGAT1 KO mice results in behaviors that partially phenocopy the clinical side effects of tiagabine, suggesting that these side effects are inherent to a therapeutic strategy that targets the widely expressed GAT1 transporter system.

Unaltered control of extracellular GABA-concentration through GAT-1 in the hippocampus of rats after pilocarpine-induced status epilepticus

The uptake of the inhibitory transmitter GABA (gamma-aminobutyric acid) limits the efficacy of synaptic and tonic inhibition in brain tissue. It has been reported that GABA-uptake is down-regulated in temporal lobe epilepsy. This down-regulation may increase the inhibitory action of GABA but may also limit the anticonvulsant activity of GABA-uptake blockers. We have directly compared the function of GABA-uptake in hippocampal slices from normal and chronically epileptic rats. We raised the global extracellular concentration of GABA by bath-application of the agonist in the absence and presence of the GABA-uptake blocker tiagabine. GABA-induced currents were measured in dentate granule cells and CA1 pyramidal neurons in hippocampal slices. The potentiation of currents by tiagabine was taken as a measure for the efficacy of GABA-uptake in the hippocampal tissue. There was no difference between cells from control- or pilocarpine-treated animals in the response to GABA or in the conductance increase following application of tiagabine. Our data show that in the chronic phase of the pilocarpine-model GABA-uptake maintains its ability to control the extracellular background concentration of GABA.

Pharmacokinetic-pharmacodynamic modelling of tiagabine CNS effects upon chronic treatment in rats: lack of change in concentration-EEG effect relationship

The pharmacodynamics of the gamma-aminobutyric acid (GABA) uptake inhibitor (R)-N-(4,4-di-(methylthien-2-yl)but-3-enyl) nipecotic acid (tiagabine) was quantified in rats following chronic (14 days) administration by an integrated pharmacokinetic-pharmacodynamic (PK/PD) modelling approach. The increase in beta activity (11.5-30 Hz) of the EEG as derived by fast Fourier transformation analysis was used as pharmacodynamic endpoint. Two groups of male Wistar rats were treated for 14 days with either tiagabine at a steady-state concentration of 198+/-10 ng ml(-1) or placebo. Chronic treatment with tiagabine resulted in an increase of the EEG effect parameter by 38+/-2 microV. In the PK/PD experiment the time course of the EEG effect was determined in conjunction with the decline of drug concentrations after an i.v. bolus administration of 10 mg kg(-1). The pharmacokinetics of tiagabine was most adequately described by a bi-exponential function. No influence of chronic treatment on the pharmacokinetics was observed. Hysteresis between plasma concentration and EEG effect was accounted for by incorporation of an 'effect-compartment' in the model. The observed relationship between tiagabine concentrations and EEG effect was non-linear and described on the basis of the Hill equation. Between the treatment groups no differences in the pharmacodynamic parameters were observed. The population means for the different pharmacodynamic parameters were: maximum EEG effect 82 microV, EC(50) 486 ng ml(-1), Hill factor 2.0 and k(e0) 0.060 min(-1). In the in vitro [(3)H]GABA uptake assay no changes in affinity or functionality for the GABA uptake transporter were observed, consistent with the absence of adaptation. It is concluded that chronic treatment with tiagabine in an effective dose range for 14 days does not produce functional adaptation to tiagabine-induced GABA-ergic inhibition in vivo.

Age-dependence of the anticonvulsant effects of the GABA uptake inhibitor tiagabine in vitro

Epileptic syndromes frequently start at childhood and therefore it is crucial to test new anticonvulsants at immature stages of the nervous system. We compared the effects of the gamma-aminobutyric acid (GABA) uptake inhibitor tiagabine [(R)-N-(4, 4-bis(3-methyl-2-thienyl)but)3-en-1-yl nipecotic acid] on low-Mg(2+)-induced epileptic discharges in brain slices from rat pups (p 5-8) and juvenile animals (p 15-20). In tissue from rat pups, tiagabine slightly reduced epileptiform activity in hippocampal area CA1 but had no effect in the entorhinal cortex. In juvenile rats, epileptiform discharges were unaffected in CA1 but suppressed by 60% in the entorhinal cortex. While tiagabine increases its efficacy with age, in-situ hybridisation and PCR analysis show that mRNA coding for the neuronal GABA-transporter GAT-1 is already present at p 5. We therefore conclude that the increasing efficacy of tiagabine during ontogenesis is due to functional maturation of GABAergic synapses rather than to up-regulation of GAT-1 expression.

Tiagabine

Tiagabine (TGB) is a recently approved antiepileptic drug (AED) that inhibits y-aminobutyric acid (GABA) reuptake into neurons and glia, a mechanism of action that is specific and unique among the AEDs. TGB is potent and has linear and predictable pharmacokinetics. It has no clinically relevant effects on hepatic metabolism or serum concentrations of other AEDs, effects on laboratory values, or interactions with common non-AEDs. TGB is effective as add-on therapy for partial seizures in patients with medically refractory epilepsy in doses ranging from 30 to 56 mg daily. Conversion to TGB monotherapy can be achieved in patients with medically refractory epilepsy, although additional controlled studies are needed to confirm the efficacy of TGB as monotherapy and to establish the effective dosage range. In controlled studies, the most common adverse events of TGB are dizziness, asthenia, somnolence, accidental injury, infection, headache, nausea, and nervousness. These are usually mild to moderate in severity and almost always resolve without medical intervention.

Activity-dependent enhancement of hyperpolarizing and depolarizing gamma-aminobutyric acid (GABA) synaptic responses following inhibition of GABA uptake by tiagabine

The effects of the 7-aminobutyric acid (GABA) uptake blocker tiagabine on isolated inhibitory postsynaptic potentials (IPSPs) were examined in CA1 pyramidal cells of the rat hippocampal slice preparation. The IPSPs were elicited by either single stimuli or by high frequency (100 Hz, 200 ms) stimulation (HFS) of inhibitory interneurons. Bath applied tiagabine (20 microM) produced little or no increase in the amplitude of IPSPs evoked by low (30-50 microA) or high (200-400 microA) intensity single stimuli. Only the duration of IPSPs evoked by high intensity stimuli was substantially prolonged by tiagabine, the time integral of the hyperpolarizing response being increased 3.2-fold. HFS elicited much larger fast and slow IPSPs than a single stimulus. In addition, with increments in the intensity (80-550 microA) of HFS, a GABA(A) receptor-mediated depolarizing response of progressively larger amplitude appeared between, and overlapped with, the fast and slow hyperpolarizing components of the IPSP. Tiagabine application markedly increased the GABA-mediated responses evoked by both low and high intensity HFS. Increasing the intensity of HFS enhanced the drug effect. Thus, measurements of the time integral of evoked responses showed that with weak (60 microA) HFS, tiagabine caused a 3.6-fold increase in the area of hyperpolarization while, in contrast, with strong (530 microA) HFS, tiagabine produced a 13.5-fold increase in the depolarizing actions of GABA. Our results suggest that tiagabine, a therapeutically effective anticonvulsant, may paradoxically increase, through a GABA(A) receptor-mediated mechanism, neuronal depolarization during the high frequency discharge of neurons involved in epileptiform activity.

Tiagabine. A review of its pharmacodynamic and pharmacokinetic properties and therapeutic potential in the management of epilepsy

Tiagabine is a gamma-aminobutyric acid (GABA) uptake inhibitor which is structurally related to nipecotic acid but has an improved ability to cross the blood-brain barrier. Clinical trials have shown that tiagabine is effective as add-on therapy in the management of patients with refractory partial epilepsy. In short term studies of this indication, tiagabine < or = 64 mg/day for 7 to 12 weeks reduced the complex partial and simple partial seizure frequency by > or = 50% in 8 to 31 and 28.2 to 37% of patients, respectively. Tiagabine appeared to produce a sustained reduction in seizure frequency in studies of up to 12 months' duration. Data from preliminary studies are currently insufficient to confirm the usefulness of tiagabine when used as monotherapy or in the treatment of children with epilepsy. Further studies are, therefore, necessary to more fully elucidate the efficacy of the drug in these settings. Adverse events associated with tiagabine are primarily CNS-related and include dizziness, asthenia, nonspecific nervousness and tremor. Skin rash or psychosis occurred with similar frequencies among tiagabine- and placebo-treated patients. With long term administration (> or = 1 year for many patients), the profile and incidence of adverse events was similar to that for short term therapy. Tiagabine does not appear to affect the hepatic metabolism of other drugs such as carbamazepine and phenytoin. Possible disadvantages of tiagabine include its short plasma elimination half-life, necessitating 2 to 4 times daily administration, and its inducible hepatic metabolism. Thus, tiagabine is a new antiepileptic agent with a novel mechanism of action, which has demonstrated efficacy in the adjunctive treatment of patients with refractory partial epilepsy. Further investigation of the efficacy of tiagabine is expected to provide a clearer definition of its place in the treatment of epilepsy and its relative merits in relation to other antiepileptic drugs.

Long-term cognitive and EEG effects of tiagabine in drug-resistant partial epilepsy

A new anti-epileptic drug, tiagabine, is a potent inhibitor of GABA uptake into neurons and glia. Tiagabine has shown promising efficacy and safety profiles as add-on treatment for partial seizures. We evaluated the long-term effects of tiagabine on cognition and EEG in 37 patients with partial epilepsy. The study protocol consisted of a randomized, double-blind, placebo-controlled, parallel-group add-on study and an open-label extension study. During the 3 month double-blind phase at low doses (30 mg/day) tiagabine treatment did not cause any cognitive or EEG changes as compared with placebo. Tiagabine treatment did not cause deterioration in cognitive performance or produce any rhythmic slow-wave activity or other constant, new abnormalities on EEG during longer follow-up with successful treatment on higher doses after 6-12 months (mean 65.7 mg/day, range 30-80 mg/day) and after 18-24 months (mean dose 67.6 mg/day, range 24-80 mg/day). The daily dosages in the long-term follow-up of the present study are higher than in the previous reports.

Effects of gamma-aminobutyric acid (GABA) agonists and GABA uptake inhibitors on pharmacosensitive and pharmacoresistant epileptiform activity in vitro

1. Lowering of the extracellular Mg(2+)-concentration induces various patterns of epileptiform activity in combined rat entorhinal cortex-hippocampal brain slices. After a prolonged period of exposure to Mg(2+)-free medium seizure-like events in the entorhinal cortex change to a state of late recurrent discharges which cannot be blocked by clinically available antiepileptic drugs. This late epileptiform activity thus represents a useful model to test the effects of new anticonvulsant substances. 2. A mechanism possibly underlying the development of sustained seizure-like activity is the loss of synaptically released gamma-aminobutyric acid (GABA). Drugs which increase the amount of GABA available in presynaptic endings might thus be useful in the treatment of these therapeutically complicated forms of epilepsy. 3. Therefore, we studied the effects of various substances increasing GABA-mediated inhibition on early and late forms of epileptiform activity. GABA and the GABAA receptor agonist muscimol blocked both the pharmacosensitive discharges in the hippocampus and entorhinal cortex as well as the late recurrent discharges in the medial entorhinal cortex. The GABAB receptor agonist baclofen blocked the recurrent short discharges very potently, but did not consistently block seizure-like events and late recurrent discharges in the entorhinal cortex. 4. GABA uptake blockers showed a differential potency to block the various discharge patterns. Whereas nipecotic acid and beta-alanine suppressed all forms of epileptiform activity albeit at high concentrations (1-5 mM), tiagabine was much more potent in blocking the hippocampal recurrent short discharges and the seizure-like events in the medial entorhinal cortex, but could not block the late recurrent discharges. 5. Our data support the idea that prolonged neuronal overactivity might result in a loss of synaptically available GABA. Selective block of uptake into glia cells or substitution of the transmitter may therefore be an efficient strategy for the treatment of severe prolonged epileptic discharges whereas block of neuronal GABA uptake fails to counteract synchronized discharges in this situation.

Comparative effects of the GABA uptake inhibitors, tiagabine and NNC-711, on extracellular GABA levels in the rat ventrolateral thalamus

The primary mechanism by which the action of synaptically released GABA is thought to be terminated is by re-uptake into neurones and glial cells, and the pharmacological inhibition of this uptake may be beneficial in conditions where decreased GABAergic transmission has been implicated, such as epilepsy. We have compared the effects of two of these uptake inhibitors, tiagabine and NNC-711, on extracellular GABA levels in the thalamus of the rat, after both systemic and local administration. Both compounds produced dose-dependent increases in GABA concentration irrespective of the route of administration, but the concentrations required to produce increased extracellular GABA levels were considerably higher than those known to be effective for anticonvulsant purposes. These data suggest that, initially at least, alternative GABA transporters, not susceptible to inhibition by the compounds used, may still be able to remove synaptically released GABA from the extracellular space.

Effects of GABA uptake inhibitors on posthypoxic myoclonus in rats

Male Sprague-Dawley rats developed posthypoxic myoclonus following 10-min cardiac arrest and resuscitation. Previous results showed that dysfunction of central GABAergic neurotransmission may contribute to the disease. In current studies, effects of GABA uptake inhibitors, guvacine hydrochloride (1,2,5,6-tetrahydro-3-pyridine carboxylic acid hydrochloride) and (+/-)-cis-4-hydroxynipecotic acid ([+/-]-cis-4-hydroxy-3-piperidine carboxylic acid), in the pathophysiology of posthypoxic myoclonus were investigated. Administration of guvacine (1 or 10 mg/kg, IP) or nipecotic acid (0.5 or 5 mg/kg, IP) significantly attenuated myoclonus scores of the animals. Tolerance to antimyoclonus effects of these two compounds did not develop after chronic administration (twice a day for 14 days) of guvacine (10 mg/kg, IP) or nipecotic acid (5 mg/kg, IP). On the other hand, tolerance was noticed with clonazepam (2.5 mg/kg, IP twice a day for 7 days). The results indicate that guvacine or nipecotic acid may be used in combination with (at reduced doses) or as alternatives to clonazepam to treat patients with the disease so as to reduce tolerance phenomenon usually associated with clonazepam.

Effects of chronic tiagabine treatment on [3H]GABAA, [3H]GABAB and [3H]tiagabine binding to sections from mice brain

(R)-N-(4,4-Bis(3-methyl-2-thienyl)but-3-en-1-yl)nipecotic acid (tiagabine) is a potent inhibitor of gamma-aminobutyric acid (GABA) uptake, which maintains its initial anticonvulsant effects when administered to mice for a prolonged period (21 days). In the present study, mice received chronic (21 days) p.o. administration of tiagabine (15 mg/kg, twice daily) or vehicle alone and the densities of GABAA and GABAB receptors and of [3H]tiagabine recognition sites were measured in several brain regions. The following changes were observed following chronic administration of tiagabine as compared to vehicle: significant reduction (18-37%) in [3H]tiagabine binding in the temporal and entorhinal cortex and in the molecular and granular layer of the cerebellar cortex; increases in the number of GABAA sites (22-44%) in various regions of the frontal cortex, in caudate putamen and in the lateral septum; decreases in the numbers of GABAB sites (18-42%) in the motor cortex, the more dorsal parts of cortex, the anteroventral thalamic nucleus, medial geniculate, superior colliculus and the molecular layer of the cerebellar cortex. Such data suggest that the GABAergic system is differentially modulated in a regional specific manner in response to chronic elevation of the extracellular levels of GABA. The significance of these findings in relation to the reported lack of development of tolerance to the anticonvulsant effects of tiagabine is discussed.

Tiagabine pharmacology in profile

Tiagabine (TGB) hydrochloride, a nipecotic acid derivative linked to a lipophilic anchor, potently and specifically inhibits uptake of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) into astrocytes and neurons. With microdialysis, TGB has been shown to increase extracellular overflow of GABA in the midbrain of awake rats. TGB administration prolongs neuronal depolarization induced by iontophoretically applied GABA in hippocampal slices. TGB is effective in a wide range of seizure models, including pentylenetetrazol-induced, methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM)-induced tonic, amygdala-kindled and picrotoxin-induced convulsions, and maximal electroshock seizures in rodents. In humans, TGB absorption is rapid and complete. It is metabolized in the liver, largely by isoform 3A of the cytochrome P450 family of enzymes. The process of elimination is linear, with a half-life of 5-8 h. TGB does not induce or inhibit metabolic processes, although it provides a target for enzyme inducers that can lower its elimination half-life to 2-3 h. Accordingly, TGB does not alter the concentrations of other antiepileptic drugs (AEDs), with the possible exception of a small decrease in valproate levels. A controlled-release formulation of TGB would offset any potential clinical disadvantage of the short elimination half-life, particularly in patients receiving concurrent treatment with enzyme-induced AEDs.