International experience with tiagabine add-on therapy

γ-aminobutyric acid as a metabolite: Interpreting magnetic resonance spectroscopy experiments

The current rise in the prevalence of magnetic resonance spectroscopy experiments to measure γ-aminobutyric acid in the living human brain is an exciting and productive area of research. As research spreads into clinical populations and cognitive research, it is important to fully understand the source of the magnetic resonance spectroscopy signal and apply appropriate interpretation to the results of the experiments. γ-aminobutyric acid is present in the brain not only as a neurotransmitter, but also in high intracellular concentrations, both as a transmitter precursor and a metabolite. γ-aminobutyric acid concentrations measured by magnetic resonance spectroscopy are not necessarily implicated in neurotransmission and therefore may reflect a very different brain activity to that commonly suggested. In this perspective, we examine some of the considerations to be taken in the interpretation of any γ-aminobutyric acid signal measured by magnetic resonance spectroscopy.

Measurement of GABA using J-difference edited 1H-MRS following modulation of synaptic GABA concentration with tiagabine

Though GABA is the major inhibitory neurotransmitter in the brain, involved in a wide variety of brain functions and many neuropsychiatric disorders, its intracellular and metabolic presence provides uncertainty in the interpretation of the GABA signal measured by (1)H-MRS. Previous studies demonstrating the sensitivity of this technique to pharmacological manipulations of GABA have used nonspecific challenges that make it difficult to infer the exact source of the changes. In this study, the synaptic GABA reuptake inhibitor tiagabine, which selectively blocks GAT1, was used to test the sensitivity of J-difference edited (1)H-MRS to changes in extracellular GABA concentrations. MEGA-PRESS was used to obtain GABA-edited spectra in 10 male individuals, before and after a 15-mg oral dose of tiagabine. In the three voxels measured, no significant changes were found in GABA+ concentration after the challenge compared to baseline. This dose of tiagabine is known to modulate synaptic GABA and neurotransmission through studies using other imaging modalities, and significant increases in self-reported sleepiness scales were observed. Therefore, it is concluded that recompartmentalization of GABA through transport block does not have a significant impact on total GABA concentration. Furthermore, it is likely that the majority of the magnetic resonance spectroscopy (MRS)-derived GABA signal is intracellular. It should be considered, in individual interpretation of GABA MRS studies, whether it is appropriate to attribute observed effects to changes in neurotransmission.

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.

Antiepileptic drugs and liver disease

Antiepileptic drugs (AEDs) are no longer restricted to the treatment of epilepsy. These are widely used in a broad spectrum of psychiatric and neurological disorders. Liver plays a major role in the metabolism of a majority of these drugs. Hepatotoxicity is rare, but a real concern when initiating therapy. Likewise, liver disease can adversely affect the biotransformation of some of these drugs. This manuscript addresses the significance of elevated liver enzymes associated with AED use, the role of therapeutic drug monitoring, pharmacokinetics during liver disease and potential risk of hepatotoxicity.

Tiagabine as add-on therapy may be more effective with valproic acid--open label, multicentre study of patients with focal epilepsy

The aim of the current study was to review the efficacy of tiagabine (TGB) as add-on therapy in patients with drug-resistant focal epilepsy under normal daily clinical practice, and try to identify those who had improvement. This was an open multicentre study conducted in Poland. A group of 330 patients were analysed. Patients received TGB up to 30-50 mg/day with adjustment within the therapeutic range and titration period. For statistical evaluation chi-square test and logistic analysis were used. At the 16-week follow-up visit, 71.4% patients were reported as responders, i.e. had a 50% or greater decrease in seizure frequency compared with baseline (P<0.001). One-third of patients were seizure-free at 16-week evaluation (P<0.001). The beneficial effect of TGB on seizure reduction was most marked in patients with partial seizures (P<0.001). Patients who used valproic acid (mean dose 1307 mg/day) had 61-85% higher chances for disappearance of seizures or reduction of their number by 50% or more. Patients who used carbamazepine (mean dose 800 mg/day) at a dose 1000 mg or higher mg/day had twice lower chance for reduction of seizures by 50% or more (OR=0.45; 95 CI 0.25-0.82). There was no statistical impact of sex, age and aetiology on probability of therapeutic effect.

Treating refractory epilepsy with tiagabine: clinical experience

Epilepsy remains difficult to treat with more than 30% of patients being refractory to conventional anticonvulsant therapy. Combination therapy may improve seizure control in some of these patients. Tiagabine is a new anticonvulsant that has a unique mechanism of action as a selective gamma-aminobutyric acid (GABA) re-uptake inhibitor (SGRI). Twenty consecutive patients with refractory epilepsy were treated with tiagabine, and prior to tiagabine administration the mean number of anticonvulsants that each patient had taken was five. Tiagabine therapy was initiated at a dose of 5 or 10mg per day and was increased at weekly increments of 5 or 10 mg per day, respectively. Thirty-five percent of patients receiving tiagabine (20-40 mg per day, mean 34.29 mg per day) achieved a > or =50% reduction in seizure frequency. Tiagabine was effective when added to carbamazepine, lamotrigine, or oxcarbazepine. Tiagabine appeared more effective at higher doses. Side effects were predominately central nervous system-related, the most common being dizziness. For optimal results, tiagabine should be initiated at low doses and titrated slowly. This observational study has demonstrated tiagabine to be effective and safe in patients with refractory epilepsy.

Emergent properties of CNS neuronal networks as targets for pharmacology: application to anticonvulsant drug action

CNS drugs may act by modifying the emergent properties of complex CNS neuronal networks. Emergent properties are network characteristics that are not predictably based on properties of individual member neurons. Neuronal membership within networks is controlled by several mechanisms, including burst firing, gap junctions, endogenous and exogenous neuroactive substances, extracellular ions, temperature, interneuron activity, astrocytic integration and external stimuli. The effects of many CNS drugs in vivo may critically involve actions on specific brain loci, but this selectivity may be absent when the same neurons are isolated from the network in vitro where emergent properties are lost. Audiogenic seizures (AGS) qualify as an emergent CNS property, since in AGS the acoustic stimulus evokes a non-linear output (motor convulsion), but the identical stimulus evokes minimal behavioral changes normally. The hierarchical neuronal network, subserving AGS in rodents is initiated in inferior colliculus (IC) and progresses to deep layers of superior colliculus (DLSC), pontine reticular formation (PRF) and periaqueductal gray (PAG) in genetic and ethanol withdrawal-induced AGS. In blocking AGS, certain anticonvulsants reduce IC neuronal firing, while other agents act primarily on neurons in other AGS network sites. However, the NMDA receptor channel blocker, MK-801, does not depress neuronal firing in any network site despite potently blocking AGS. Recent findings indicate that MK-801 actually enhances firing in substantia nigra reticulata (SNR) neurons in vivo but not in vitro. Thus, the MK-801-induced firing increases in SNR neurons observed in vivo may involve an indirect effect via disinhibition, involving an action on the emergent properties of this seizure network.

Tiagabine synergistically interacts with gabapentin in the electroconvulsive threshold test in mice

Polytherapy, based on the rational combining of antiepileptic drugs (AEDs), is required for patients with drug-resistant epilepsy. In such cases, the combinations of AEDs usually offer a significant enhancement of their protective effects against seizures. There has appeared a hypothesis that combining two AEDs, influencing the same neurotransmitter system, results in the potentialization of their anticonvulsant effects. For corroborating this hypothesis, a pharmacological character of interaction between tiagabine (TGB) and gabapentin (GBP)-two novel AEDs affecting the GABA-ergic system, in the maximal electroshock seizure threshold (MEST)-test in mice was evaluated. TGB at the dose of 4 mg/kg and GBP at 75 mg/kg significantly raised the electroconvulsive threshold. Further, using the isobolographic calculations, TGB was coadministered with GBP at three fixed-ratios (1 : 3, 1 : 1, and 3 : 1) of their respective protective drug doses. All examined combinations of TGB with GBP exerted supra-additive (synergistic) interactions against MEST-induced seizures in mice. The interaction index, describing the strength and magnitude of interaction, ranged between 0.25 and 0.50 indicating supra-additivity. Adverse (neurotoxic) effects were evaluated in the chimney (motor performance) and the step-through, light-dark passive avoidance (long-term memory) tests in mice. The examined combinations of TGB with GBP did not affect the motor coordination, except for the fixed-ratio of 1 : 1, at which significant impairment of motor performance was observed. Moreover, all combinations selectively impaired the acquisition of the task in the passive avoidance test, having no impact on consolidation and retrieval in the long-term memory test. The pain threshold test revealed that the observed disturbances in the passive avoidance testing resulted presumably from the antinociceptive activity of these AEDs in combinations. After lengthening the exposing time to the direct current stimulus in the passive avoidance test from 2 to 6 s, the acquisition of the task, in animals receiving the combinations of TGB and GBP was not impaired. Neither the plasma, nor brain concentrations of GBP were affected by TGB application, so pharmacokinetic events that might negatively influence the observed effects are not probable. Results of this study clearly indicate that the activation of the same neurotransmitter system (GABA-ergic) leads to a synergistic interaction. The pain threshold test is a very good paradigm for screening the antinociceptive properties of AEDs, which may disturb the long-term memory testing in animals. Combinations of TGB with GBP (very promising from a preclinical point of view) should be clinically verified for elaborating the most effective treatment regimen in patients with intractable seizures.

GABAergic mechanisms in epilepsy

gamma-Aminobutyric acid (GABA), the principal inhibitory neurotransmitter in the cerebral cortex, maintains the inhibitory tone that counterbalances neuronal excitation. When this balance is perturbed, seizures may ensue. GABA is formed within GABAergic axon terminals and released into the synapse, where it acts at one of two types of receptor: GABAA, which controls chloride entry into the cell, and GABAB, which increases potassium conductance, decreases calcium entry, and inhibits the presynaptic release of other transmitters. GABAA-receptor binding influences the early portion of the GABA-mediated inhibitory postsynaptic potential, whereas GABAB binding influences the late portion. GABA is rapidly removed by uptake into both glia and presynaptic nerve terminals and then catabolized by GABA transaminase. Experimental and clinical study evidence indicates that GABA has an important role in the mechanism and treatment of epilepsy: (a) Abnormalities of GABAergic function have been observed in genetic and acquired animal models of epilepsy; (b) Reductions of GABA-mediated inhibition, activity of glutamate decarboxylase, binding to GABAA and benzodiazepine sites, GABA in cerebrospinal fluid and brain tissue, and GABA detected during microdialysis studies have been reported in studies of human epileptic brain tissue; (c) GABA agonists suppress seizures, and GABA antagonists produce seizures; (d) Drugs that inhibit GABA synthesis cause seizures; and (e) Benzodiazepines and barbiturates work by enhancing GABA-mediated inhibition. Finally, drugs that increase synaptic GABA are potent anticonvulsants. Two recently developed antiepileptic drugs (AEDs), vigabatrin (VGB) and tiagabine (TGB), are examples of such agents. However, their mechanisms of action are quite different (VGB is an irreversible suicide inhibitor of GABA transaminase, whereas TGB blocks GABA reuptake into neurons and glia), which may account for observed differences in drug side-effect profile.

Tiagabine versus phenytoin and carbamazepine as add-on therapies: effects on abilities, adjustment, and mood

The effects of tiagabine (TGB) on abilities and on adjustment and mood are as yet incompletely understood. These effects were compared with those of phenytoin (PHT) and carbamazepine (CBZ) in an add-on study. Patients included in the analysis were adults with uncontrolled partial seizures who at study entry were on CBZ alone (n=153) or on PHT alone (n=124). Of the patients receiving CBZ, 82 were randomized to add-on TGB and 71 were randomized to add-on PHT during the double-blind period. Of the patients receiving PHT, 58 were randomized to add-on TGB and 66 were randomized to add-on CBZ. Eight tests of mental abilities and three of mood and adjustment were given prior to assignment of add-on treatment and after up to 16 weeks of add-on treatment. For the baseline CBZ group, analyses were done to search for differential changes from baseline in the test scores of the add-on TGB and add-on PHT groups, and for the baseline PHT group in the add-on TGB and add-on CBZ groups. In the baseline CBZ group, no differences in test scores were found between PHT and TGB. In the baseline PHT group for the area of abilities, patients treated with TGB had improved verbal fluency, as well as quicker responses on a test of perceptual/motor speed compared with patients treated with CBZ. For the baseline PHT group in the area of adjustment and mood, patients treated with TGB reported less positive mood and more financial concerns compared to patients treated with CBZ. Overall, add-on TGB showed few or no differences in comparison with add-on CBZ and add-on PHT.

Tiagabine adjunctive therapy in children with refractory epilepsy: a single-blind dose escalating study

Tiagabine, a specific gamma-aminobutyric acid-uptake inhibitor, has been shown to be reasonably well tolerated and efficacious as adjunctive treatment for partial seizures in adults and is now being investigated in children. This 4-month, single-blind study evaluated the tolerability, safety and preliminary efficacy of ascending doses (0.25-1.5 mg/kg/day) of tiagabine add-on therapy in 52 children over the age of 2 years with different syndromes of refractory epilepsy. Adverse events, mostly mild to moderate, were reported by 39% of children during the single-blind placebo period and by 83% of children during tiagabine treatment. The events predominantly affected the nervous system with asthenia (19%), nervousness (19%), dizziness (17%) and somnolence (17%) being the most common. Only three children (6%) withdrew because of adverse events. Tiagabine appeared to reduce seizures more in localisation-related epilepsy syndromes than in generalised epilepsy syndromes. Twenty-three patients with localisation-related epilepsy syndromes were included and 17 of these patients entered the fourth dosing period. The 17 patients had a median reduction of seizure rate in the fourth month of treatment of 33% compared with baseline. In comparison, 13 of 22 children with seven different generalised epilepsy syndromes entered the fourth dosing period with a median change of seizure rate of 0%. Two patients experienced single episodes of status epilepticus during treatment; both cases resolved. Tiagabine showed efficacy mainly in localisation-related syndromes and was well tolerated by most children in a group of very refractory patients and warrants further study in children with epilepsy.

Two different anticonvulsant actions of tiagabine in developing rats

PURPOSE

Our goal was to study the anticonvulsant action of tiagabine (TGB) at different levels of brain maturation in rats.

METHODS

Wistar rats in five age groups (7, 12, 18, 25, and 90 days old) were injected intraperitoneally with TGB at doses of 0.5-32 mg/kg. Thirty minutes later, motor seizures were induced by the subcutaneous adminstration of pentylenetetrazol (PTZ) in a dose of 100 mg/kg for all of the groups except the 18-day-old rat pups, which received a 90-mg/kg dose. The incidence and latency of two types of motor seizures, minimal clonic and generalized tonic-clonic seizures (GTCSs), were evaluated, and the seizure severity was scored. The time profile of TGB action at the 8-mg/kg dose was studied in the 12-and 25-day-old rats.

RESULTS

Minimal clonic seizures were reliably induced in rats 18 days old or older, and the seizures were suppressed by TGB in all of these age groups. Although TGB was very effective against this type of seizure in the 18-day-old rats, the efficacy of the drug decreased with the age of the animal. GTCSs were suppressed by TGB in the adult and 25-day-old rats, and a U-shaped dose-response curve was outlined in these two groups. The 18-and 12-day-old rat pups exhibited a selective suppression of the tonic phase of GTCSs. A mixture of these two effects was observed in the youngest group. TGB demonstrated a markedly longer action in the 12-day-old rats than in the 25-day-old rats.

CONCLUSIONS

TGB exhibits anticonvulsant action against both minimal seizures and GTCSs. Ontogenetic development of these two actions is markedly different.

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.

Safety of long-term treatment with tiagabine

The aim of this study was to evaluate the safety of long-term treatment with tiagabine. We reviewed the case report forms of patients with refractory partial epilepsy who took tiagabine for longer than 6 months in two long-term studies. We classified all adverse events based on severity and persistence, and recorded the dose at onset of each adverse event. We then divided patients into those treated for 6-12 months, 12-24 months and > 24 months. We compared the adverse event profile and change in seizure frequency among the three groups. Forty-two patients took tiagabine for longer than 6 months. The mean duration of treatment was 22.6 months. The mean monthly seizure frequency was 12.7 at baseline and 8.1 at study termination (36% decrease). The most common adverse events were: tiredness (56%), headache (46%), dizziness (44%), visual symptoms (blurring, difficulty focusing, diplopia) (39%), altered mentation (32%), and tremor (31%). The adverse event profile was comparable among the three groups. Seizure frequency was significantly more improved in the > 24 months group. Long-term treatment with tiagabine is well tolerated. The most important predictor of long-term therapy with tiagabine was the degree of seizure improvement.

Tiagabine: absence of kinetic or dynamic interactions with ethanol

Tiagabine is a new antiepileptic drug that inhibits the uptake of gamma-aminobutyric acid into neurons and glia. This double-blind, placebo-controlled study investigated the effect of multiple doses of tiagabine on the adverse cognitive effects produced by a single dose of ethanol in 20 healthy volunteers. The effects of each drug on the pharmaco-kinetics of the other were also determined. Compared with placebo, tiagabine produced no statistically significant effects on digit vigilance speed (primary assessment variable) or accuracy, choice reaction time, immediate or delayed word recall, delayed word recognition speed or sensitivity, visual tracking, body sway, or subjective measures of alertness, calmness, and contentment. There was no evidence of a pharmacodynamic interaction between tiagabine and ethanol with respect to these variables. The pharmacokinetic parameters of tiagabine and ethanol (maximum plasma concentration [Cmax], time to Cmax and area under the plasma concentration-time curve) were unchanged during concomitant administration. Adverse events, which mainly affected the central nervous system, occurred with a similar incidence during tiagabine and placebo administration and were more common after the administration of ethanol. There appears to be no need for additional caution regarding driving or operating machinery when ethanol is administered to patients taking tiagabine.

Review of controlled trials of gabitril (tiagabine): a clinician's viewpoint

A recent problem for doctors has been the choice of which new antiepileptic drug (AED) to select for treatment of pharmacoresistant epilepsy. This article summarizes the clinical experience to date regarding the efficacy and safety of tiagabine (TGB; Gabitril) as adjunctive therapy in patients with partial-onset seizures. In its early Phase II development, TGB was evaluated in two multicenter pilot studies. Each had an open-label enrichment phase followed by a treatment phase with randomized, double-blind, two-period, cross-over phases. Between 24 and 50% of patients experienced reductions in seizure rates of > or =50%, depending on the type of partial seizure. In Phase III, three double-blind, parallel group, placebo-controlled adjunctive studies determined the efficacy of TGB in patients with refractory partial seizures. The first was a dose-response study employing doses of TGB-HCl of 16, 32 or 56 mg/day. Significant reductions in seizure rates were found with 32 and 56 mg/day. The second and third studies evaluated the efficacy of dosing TGB twice, three times, and four times daily, all of which showed similar efficacy. TGB efficacy in partial seizures was supported in several open trials, and no tolerance to efficacy was noted in long-term continuation studies. Tolerability was documented in all trials. Most adverse events were mild or moderate and transient, occurring during dose titration. They were clearly dose-related. No relevant changes in hematologic and biochemical tests, vital signs, or body weight were attributable to TGB. TGB appears to be an effective new drug for partial seizures with an acceptable safety profile.

The pharmacologic basis of antiepileptic drug action

The development of medications used in the treatment of epilepsy has accelerated over the past decade, and has benefited from a parallel growth in our knowledge of the basic mechanisms underlying neuronal excitability and synchronization. This understanding of the pharmacologic basis of antiepileptic drug (AED) action has, in large part, arisen from recent advances in cellular and molecular biology, coupled with avenues of drug discovery that have departed somewhat from the largely empiric approaches of the past. Physicians now have available to them an ever-growing armentarium of AEDs, necessitating a firmer appreciation of their mechanisms of action if more rational approaches toward both clinical application and research are to be adopted. An important example in this regard is the concept of rational polypharmacy for patients with epilepsy who are refractory to monotherapy. This review summarizes our current understanding of the molecular targets of clinically significant AEDs, comparing and contrasting their differing mechanisms of action.

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.

Absence of interaction between tiagabine, a new antiepileptic drug, and the benzodiazepine triazolam

In a randomised, double blind, placebo-controlled, four-period cross-over study in 12 healthy volunteers, the potential pharmacodynamic and pharmacokinetic interactions between the new antiepileptic drug, tiagabine, and the benzodiazepine, triazolam, were investigated. A single dose of tiagabine HCl 10 mg did not enhance the sedative or cognitive effects of a single dose of the benzodiazepine triazolam 0.125 mg, although the time-course of the effects was prolonged. Furthermore, tiagabine did not produce any statistically significant effects on the pharmacokinetics of triazolam. Similarly, the pharmacokinetics of tiagabine were not modified by triazolam. Tiagabine was well tolerated when administered alone or with triazolam.

Differential effects on motorcortical inhibition induced by blockade of GABA uptake in humans

1. Blockade of uptake carriers of gamma-aminobutyric acid (GABA) has been shown to modulate inhibition in cortical slices of experimental animals, although little is known about this mechanism in vivo and, in particular, in humans. 2. The effects of blockade of GABA uptake were studied using transcranial magnetic stimulation (TMS) in humans. In eight healthy volunteers several measures of cortical excitation and inhibition were obtained before and approximately 2 h after ingestion of 5-15 mg of tiagabine (TGB). 3. After TGB ingestion, the duration of the TMS-induced silent period observable in the electromyogram of the voluntarily contracted target muscle was prolonged. Similarly, paired-pulse inhibition of the motor-evoked potential (MEP), as tested by delivering two magnetic shocks of equal suprathreshold intensities at 160 ms interstimulus interval (ISI), was more pronounced. In apparent contradistinction, paired-pulse inhibition of the MEPs produced by a subthreshold conditioning stimulus delivered 3 ms prior to a suprathreshold stimulus was reduced. Paired-pulse facilitation elicited by the same double-shock protocol at an ISI of 10 ms was increased. 4. The prolongation of the GABAB receptor-mediated component of the inhibitory postsynaptic potential observed with TGB in in vitro studies probably underlies the increase in cortical silent period duration. The reduction of the paired-pulse inhibition at 3 ms, in turn, probably reflects inhibition of GABAA receptor-mediated inhibition via presynaptic GABAB receptors. 5. These data provide in vivo evidence of differential modulation of cortical inhibition by blockade of GABA uptake. Presynaptic GABA autoreceptors may be involved in modulating cortical inhibition in the human motor cortex.

New antiepileptic drugs: comparison of key clinical trials

PURPOSE

Data accrued from clinical trials of five new antiepileptic drugs (AEDs) are compared for efficacy in reducing seizures and self-reported adverse events as a basis of selection among new AEDs. Drawbacks to use of these data also are demonstrated.

METHODS

A review of double-blind, placebo-controlled clinical trials of a new AED or placebo added to a standard AED provided data on reduction of complex partial seizures (CPSs). Success is > or =50% fewer CPSs with a new AED or placebo; Overall Improvement is the success rate with drug minus the success rate with placebo. Adverse events were tabulated from product-labeling lists of COSTART items (incidence, > or =5%). The Summary Complaint score is the total number of reports of individual events for each AED.

RESULTS

Efficacy data demonstrate differences in Overall Improvement rates among five new AEDs and placebos (p = 0.001). However, rates of response to placebo also differed significantly among trials (p = 0.01). Adverse events predominantly affect central nervous system, psychiatric, and general body systems. However, patients in the placebo control groups did not consistently report adverse effects. Summary Complaint scores differ among the five new AEDs, but variability in use of COSTART terms nullifies comparisons.

CONCLUSIONS

Comparisons of data for five new AEDs provide information for selection among treatments when a second drug is needed to improve control of CPSs. However, significant differences among the control groups and other problems make comparisons between trials problematic. The final choice should be based on the need of the individual patient for superior seizure control versus minimal adverse effects.

Inhibitory nature of tiagabine-augmented GABAA receptor-mediated depolarizing responses in hippocampal pyramidal cells

Tiagabine is a potent GABA uptake inhibitor with demonstrated anticonvulsant activity. GABA uptake inhibitors are believed to produce their anticonvulsant effects by prolonging the postsynaptic actions of GABA, released during episodes of neuronal hyperexcitability. However, tiagabine has recently been reported to facilitate the depolarizing actions of GABA in the CNS of adult rats following the stimulation of inhibitory pathways at a frequency (100 Hz) intended to mimic interneuronal activation during epileptiform activity. In the present study, we performed extracellular and whole cell recordings from CA1 pyramidal neurons in rat hippocampal slices to examine the functional consequences of tiagabine-augmented GABA-mediated depolarizing responses. Orthodromic population spikes (PSs), elicited from the stratum radiatum, were inhibited following the activation of recurrent inhibitory pathways by antidromic conditioning stimulation of the alveus, which consisted of either a single stimulus or a train of stimuli delivered at high-frequency (100 Hz, 200 ms). The inhibition of orthodromic PSs produced by high-frequency conditioning stimulation (HFS), which was always of much greater strength and duration than that produced by a single conditioning stimulus, was greatly enhanced following the bath application of tiagabine (2-100 microM). Thus, in the presence of tiagabine (20 microM), orthodromic PSs, evoked 200 and 800 ms following HFS, were inhibited to 7.8 +/- 2.6% (mean +/- SE) and 34.4 +/- 18.5% of their unconditioned amplitudes compared with only 35.4 +/- 12.7% and 98.8 +/- 12.4% in control. Whole cell recordings revealed that the bath application of tiagabine (20 microM) either caused the appearance or greatly enhanced the amplitude of GABA-mediated depolarizing responses (DR). Excitatory postsynaptic potentials (EPSPs) evoked from stratum radiatum at time points that coincided with the DR were inhibited to below the threshold for action-potential firing. Independently of the stimulus intensity with which they were evoked, the charge transferred to the soma by excitatory postsynaptic currents (EPSCs), elicited in the presence of tiagabine (20 microM) during the large (1,428 +/- 331 pA) inward currents that underlie the DRs, was decreased on the average by 90.8 +/- 1.7%. Such inhibition occurred despite the presence of the GABAB receptor antagonist, CGP 52 432 (10 microM), indicating that GABAB heteroreceptors, located on glutamatergic terminals, do not mediate the observed reduction in the amplitude of excitatory postsynaptic responses. The present results suggest that despite facilitating the induction of GABA-mediated depolarizations, tiagabine application may nevertheless increase the effectiveness of synaptic inhibition during the synchronous high-frequency activation of inhibitory interneurons by enhanced shunting.

Safety of tiagabine: summary of 53 trials

We reviewed the clinical safety of tiagabine HCl (TGB), a selective CNS GABA uptake inhibitor, in nearly 3100 patients from 53 separate clinical trials. TGB was found to have no clinically important effect upon hepatic metabolic processes, serum concentrations of concomitant antiepileptic drugs (AEDs), laboratory values, or important interactions with any common non-AEDs. Adverse effects were usually mild and involved the nervous system. TGB is safe and well-tolerated as add-on therapy for the treatment of partial seizures.

Tiagabine: a novel antiepileptic drug

OBJECTIVE

To provide a comprehensive review of tiagabine, including its pharmacology, toxicology, pharmacokinetics, drug interactions, efficacy, adverse effects, and dosing recommendations.

DATA SOURCES

A computerized search of the MEDLINE database from 1966 to December 1997 was used to identify publications related to tiagabine and nipecotic acid derivatives. Included in this review was information gathered from scientific meetings. Manufacturer's information was used when there was no primary literature.

DATA SYNTHESIS

Tiagabine amplifies gamma-aminobutyric acid (GABA) neurotransmission, the predominant inhibitory neurotransmitter in the brain. Its mechanism of action is selective and has shown promise as an antiepileptic drug (AED) in patients with seizures refractory to other pharmaceutical products. Tiagabine exhibits dose-independent absorption, 90-95% bioavailability, high protein binding (96%), metabolism via hepatic cytochrome P450 enzymes (CYP3A subfamily), and displays first-order elimination pharmacokinetics. The mean plasma half-life is 5-8 hours. Concomitant medications that induce hepatic metabolism enhance tiagabine elimination; metabolism is reduced in patients with hepatic dysfunction. Adverse events of tiagabine typically involve the central nervous system, have been mild to moderate in intensity, and also have been transient in nature.

CONCLUSIONS

Tiagabine has demonstrated a good safety profile and, while it has not been demonstrated to be superior to other second-line AEDs for partial seizures, its safety and select mechanism of action warrant its further evaluation in the clinical setting. Tiagabine should be a good alternative add-on agent for patients with unsatisfactory seizure control or intolerable adverse effects of traditional therapies; thus, this agent should be made available to these patients.

Lack of pharmacokinetic interaction between tiagabine and erythromycin

This study was conducted to investigate the effects on the pharmacokinetics of tiagabine at steady state when coadministered with therapeutic doses of erythromycin. Tiagabine doses of 4 mg twice daily and erythromycin doses of 500 mg twice daily were administered for 4 days in an open-label, crossover, two-period interaction trial in 13 healthy volunteers. No statistically significant differences in maximum plasma concentration (Cmax), area under the concentration-time curve (AUC tau), or half-life (t1/2) of tiagabine were observed when tiagabine was administered alone or in combination with erythromycin. A statistically significant treatment effect was observed for time to maximum concentration (tmax; 0.72 after tiagabine alone versus 0.56 hours after administration with erythromycin). No statistically significant differences were seen between men and women except in tmax and t1/2; these differences were thought to be of no clinical significance. The decrease in tmax seen in women in this study is interpreted as a differential effect of erythromycin on gastric emptying of females and not as an interaction between tiagabine and erythromycin. No changes in laboratory parameters or vital signs were attributable to trial medication. The most common treatment-emergent adverse events that were possibly related to trial medication were central nervous system effects (e.g., headache, dizziness); all adverse events were transient, the majority were rated mild in severity, and did not require additional action. Coadministration of erythromycin in healthy subjects does not significantly affect the pharmacokinetics of tiagabine at the doses tested.

Nonconvulsive status epilepticus in two patients receiving tiagabine treatment

In the course of an open study on the add-on treatment of tiagabine (TGB) in patients with localization-related epilepsy syndromes, 2 of 9 patients developed nonconvulsive status epilepticus (NCSE) with electroclinical features consistent with those of atypical absence seizures. One patient had never had atypical absence seizures before. In both cases, immediate discontinuation of TGB was followed by complete and sustained electroclinical remission; we suggest a possible causative role of TGB. This observation may be consistent with a paradoxical effect of TGB in selected cases. Possible risk factors and a pathophysiological hypothesis are discussed.

Advances in the medical treatment of epilepsy

Treatment options for epilepsy, especially using antiepileptic drugs, have increased substantially in the past five years. Since 1993, four novel antiepileptic drugs have been approved and marketed in the United States: felbamate, gabapentin, lamotrigine, and topiramate. Two others, tiagabine and vigabatrin, are likely to be approved in the near future. For many patients, these agents offer the realistic promise of improved seizure control, often with fewer adverse effects and less significant drug interactions compared with older agents. In addition, fosphenytoin, a water-soluble phenytoin prodrug with a number of advantages over intravenous phenytoin, has been released. There are new administration options for carbamazepine, diazepam, and valproic acid. For drug-resistant or -intolerant patients, there has been renewed interest in alternative therapies, especially the ketogenic diet. Taken together, these represent significant therapeutic advances that are benefiting patients with epilepsy. At the same time, improved understanding of the basic mechanisms of epileptogenesis, and of the cellular and molecular actions of available antiepileptic drugs, creates a framework for designing unique therapeutic strategies that are targeted at key sites of vulnerability involved in the development and maintenance of the epileptic state.

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.

A double-blind, placebo-controlled trial of tiagabine given three-times daily as add-on therapy for refractory partial seizures. Northern European Tiagabine Study Group

In a multicentre, double-blind, parallel-group, placebo-controlled trial, a three-times daily regimen of tiagabine was evaluated as add-on therapy in 154 adult patients with refractory partial seizures. A total of 77 patients were randomised to treatment in each arm. Tiagabine HCl was titrated from an initial dose of 12-30 mg/day over 4 weeks. During the 12-week fixed-dose period, there was a significant reduction in the median 4-weekly seizure rate for all partial seizures and simple partial seizures (P < 0.05 in each case). Furthermore, the proportion of patients with a reduction of 50% or more in all partial seizures was higher in the tiagabine group than in the placebo group (14 versus 6%), though the difference did not achieve statistical significance. The difference with respect to simple partial seizures was significant (21 versus 6%, P < 0.01). The percentage of patients achieving an increase of at least 50% in the proportion of days free of all partial seizures was significantly greater in the tiagabine group compared to placebo (14 versus 4%, P<0.01). Tiagabine did not appear to influence the plasma concentrations of other concomitant antiepileptic drugs and was generally well tolerated, with most drug-related adverse events being mild or moderate in severity. The most common adverse events were dizziness, asthenia, headache and somnolence. Adverse event incidence was similar between tiagabine and placebo groups, except for dizziness which was more common with tiagabine (29 versus 10%, P < 0.01). Tiagabine had no significant effects on laboratory tests or vital signs. The present study shows that tiagabine, at a dose of 10 mg administered three-times daily, which is at the lower end of the usual recommended dose range (30-50 mg/day, tiagabine base), is generally well tolerated and demonstrates efficacy for the treatment of refractory partial seizures.

Effects of tiagabine monotherapy on abilities, adjustment, and mood

PURPOSE

We evaluated the dose-related impacts of tiagabine (TGB) on cognition and mood in a monotherapy study.

METHODS

Patients were 123 adults with uncontrolled partial seizures, each treated with a single currently available antiepileptic drug (AED) for management of clinical epilepsy. They completed a battery of neuropsychological tests during an 8-week prospective baseline period and once again at the end of the 12-week fixed-dose period (or earlier if they dropped out of the study). Sixty-six patients were randomized to 6 mg/day TGB and 57 were randomized to 36 mg/day TGB.

RESULTS

Few changes in either abilities or adjustment and mood were noted when all patients were considered as a single group. However, analysis of both dose and attainment of TGB monotherapy showed that patients receiving TGB monotherapy did best, improving particularly in the areas of adjustment and mood with low-dose TGB and in the area of abilities with high-dose TGB. Patients who did not attain monotherapy showed no change except that the high-dose group did not perform as well on measures of mood and adjustment. Baseline AED and changes in seizure control did not affect the results.

CONCLUSIONS

Patients' attainment of TGB monotherapy was associated with their achievement of positive changes of varying degree on psychological tests. Failure to attain TGB monotherapy was associated with no changes on the tests except in patients receiving high-dose TGB where it appeared that some alterations in mood might have been avoided if a slower titration schedule had been used.

The pharmacokinetics of tiagabine in healthy elderly volunteers and elderly patients with epilepsy

The pharmacokinetics of tiagabine after single-dose (8 mg) and multiple-dose (3 mg, three-times daily for four days) administration of tiagabine HCl were investigated in healthy elderly volunteers (n = 8; Group 1), elderly patients with epilepsy receiving at least one hepatic enzyme-inducing antiepileptic drug (AED) (n = 8; Group 2), and healthy young volunteers (n = 8; Group 3). Participants were matched by gender, age (Groups 1 and 2), alcohol intake, body weight, and whether they smoked tobacco. The pharmacokinetic parameters of tiagabine following single- and multiple-dose administration were similar in both healthy elderly and young volunteers except for a small but significant difference in the area under the concentration-time curve after multiple-doses (103 +/- 29 ng.hr/mL/mg in the elderly versus 72 +/- 20 ng.hr/mL/mg in younger participants). This is not expected to have any clinical relevance because of the large intersubject variability in this parameter. In contrast, and as expected, the pharmacokinetics of tiagabine were altered in the presence of enzyme-inducing antiepileptic drugs: Time to reach maximum plasma concentration, area under the concentration-time curve, and elimination half-lives were significantly lower (e.g. 39 +/- 13 ng.hr/mL/mg for AUC after multiple-dose) compared with corresponding values in the healthy volunteers. These findings suggest that adjusting the dose of tiagabine on the basis of the age of the patient is not necessary, although, irrespective of age, higher doses and/or more frequent administrations will be required in patients taking concomitant enzyme-inducing antiepileptic drugs.

A pharmacological and clinical review on topiramate, a new antiepileptic drug

Topiramate is a new antiepileptic drug which has recently become available in the United States and in a number of European countries. Pharmacological studies suggest that its mode of action is multifactorial and involves blockade of voltage-dependent sodium channels, potentiation of GABAergic transmission and inhibition of excitatory pathways through an action at AMPA receptor sites. Carbonic anhydrase inhibiting properties have also been demonstrated but they are considered not to be relevant to anticonvulsant activity. Topiramate is well absorbed from the gastrointestinal tract, peak plasma levels being usually attained in 2-3 hours. The drug is negligibly (9-17%) bound to plasma proteins and is eliminated partly by renal excretion in unchanged form and partly by oxidation and hydrolysis. In healthy volunteers, the half-life is about 20-30 hours, but elimination rate is accelerated in patients taking concomitant enzyme inducing drugs such as phenytoin, carbamazepine and barbiturates. Topiramate has no major effects on plasma levels of concurrent anticonvulsants, except for a rise in plasma phenytoin in occasional patients. In double-blind add-on trials in refractory partial epilepsy, a significant reduction in seizure frequency has been demonstrated in over 40% of topiramate-treated patients (vs about 10% of those treated with placebo), a response rate which compares favourably with that observed with other new antiepileptic drugs. Dosages found to be effective in add-on controlled trials range between 200 and 1000 mg day-1, although most patients are likely to benefit from receiving 400 mg day-1 or less. Preliminary data suggest that topiramate may be effective also in generalized epilepsies, but this needs to be confirmed in prospective studies. The most common adverse effects of topiramate are CNS-related and include dizziness, fatigue, visual disturbances, ataxia, mental slowing and impaired concentration. Paresthesias, anorexia, weight loss and increased risk of nephrolithiasis have been also reported. Many of these effects are related to dose and/or to rate of dose titration. Based on these data, topiramate appears to be a valuable new drug, whose main current indication is in the add-on management of refractory partial and secondarily generalized seizures. Studies on its potential-value as monotherapy are in progress.

New antiepileptic drugs in children: present and future

The pharmacologic management of epilepsy has progressed greatly during the last decade. New medications are available for the management of refractory patients, and more are being developed. For some patients, these productions offer added efficacy, better tolerability, and some pharmacodynamic advantages. Since preapproval studies include few pediatric trials, the final role of these medications in the treatment of childhood epilepsy will be dictated by additional studies and postmarketing experience.

The new generation of antiepileptic drugs: advantages and disadvantages

1. After a hiatus of over 20 years, several new antiepileptic drugs (vigabatrin, lamotrigine, gabapentin, oxcarbazepine, topiramate, felbamate, zonisamide and tiagabine) have reached or approached the registration phase. 2. Compared with older agents, many new drugs exhibit simpler pharmacokinetics. This is especially true for vigabatrin and gabapentin, which are renally eliminated and have a low interaction potential. 3. Unlike most of the older agents, vigabatrin, lamotrigine, gabapentin and tiagabine are devoid of significant enzyme inducing or inhibiting properties. Topiramate, oxcarbazepine and felbamate may induce the metabolism of steroid oral contraceptives. In addition, felbamate also acts as a metabolic inhibitor. 4. To date, the efficacy of new drugs has been evaluated extensively only under add-on conditions in patients with partial seizures (with or without secondary generalization) refractory to conventional treatment. However, there is evidence that lamotrigine, zonisamide, felbamate and, possibly, topiramate may also be effective in generalized epilepsies. 5. In placebo-controlled studies, typically between 15 and 40% of patients with difficult-to-treat partial epilepsy have shown an improvement (defined as a 50% or greater decrease in seizure frequency) after addition of a new drug. Only a small minority of these patients achieved complete seizure control. 6. Compared with older agents, some of the new drugs may have a better tolerability profile. Felbamate, however, has been associated with a high risk of aplastic anaemia and hepatotoxicity. 7. At present, the main use of the new agents is in patients refractory to first-line drugs such as carbamazepine or valproate, and further studies are required to characterize their activity spectrum as well as their potential value in monotherapy. In most patients, new drugs cannot be recommended for first-line use until evidence is obtained that potential advantages in tolerability or ease of use outweigh the drawback of their high cost.

Tiagabine monotherapy in the treatment of partial epilepsy

Three studies were conducted to assess tiagabine (TGB) hydrochloride monotherapy in patients with partial seizures. The first was a double-blind, placebo-controlled trial of 11 patients (seven TGB, four placebo) undergoing evaluation for epilepsy surgery. Baseline antiepileptic drug (AED) therapy was discontinued abruptly before monotherapy. Although 24-h seizure rates increased during monotherapy in both groups, patients receiving TGB experienced fewer seizures than placebo patients. Subsequent studies (an open-label, dose-ranging study; n=31 and a double-blind, randomized comparison of 6 and 36 mg/day TGB; n=102 and 96, respectively) involved discontinuation of baseline AEDs. In the dose-ranging study, 19 of 31 patients (61%) converted to TGB monotherapy, with a mean final dose of 38.4 mg/day (range 24-54 mg/day) in those who completed the study (n=12). In the low-vs. high-dosage study, median 4-week complex partial seizure rates decreased significantly in patients from both dose groups who completed the monotherapy period (p<0.05 compared with baseline). In the intent-to-treat analysis, significantly more patients in the high-dose group experienced a reduction in seizures of at least 50% compared with the low-dose group (p = 0.038). Overall, the types of adverse events with TGB monotherapy were similar to those observed in add-on trials. These initial trials in difficult-to-treat epilepsy patients indicate that TGB monotherapy may provide a new approach to the treatment of patients with partial seizures refractory to other AEDs.

Tiagabine: the safety landscape

Tiagabine (TGB) hydrochloride is a potential new antiepileptic drug (AED) undergoing clinical development. Experience in humans amounts to 1,810 patient-years of exposure. TGB was found to be tolerated in an integrated safety analysis of five double-blind, add-on therapy trials involving approximately 1,000 patients with epilepsy with difficult-to-control seizures with existing AEDs. Discontinuation resulting from adverse events were infrequent, occurring in 15% of patients receiving TGB compared to 5% receiving placebo. The most frequently reported adverse event was dizziness, which was usually transient and did not require medical intervention. Adverse events that were statistically significantly more common with TGB than placebo were dizziness, asthenia, nervousness, tremor, diarrhea, and depression (not major depression). Adverse events were usually mild to moderate in severity and transient, and most were associated with dose titration. The incidence, type, and severity of adverse events in long-term studies were comparable with those in short-term studies. Serious adverse events were uncommon and no idiosyncratic events were reported.