Effects of single doses of vigabatrin on CSF concentrations of GABA, homocarnosine, homovanillic acid and 5-hydroxyindoleacetic acid in patients with complex partial epilepsy

Early vigabatrin augmenting GABA-ergic pathways in post-anoxic status epilepticus (VIGAB-STAT) phase IIa clinical trial study protocol

Background

Nearly one in three unconscious cardiac arrest survivors experience post-anoxic status epilepticus (PASE). Historically, PASE has been deemed untreatable resulting in its exclusion from status epilepticus clinical trials. However, emerging reports of survivors achieving functional independence following early and aggressive treatment of PASE challenged this widespread therapeutic nihilism. In the absence of proven therapies specific to PASE, standard of care treatment leans on general management strategies for status epilepticus. Vigabatrin—an approved therapy for refractory focal-onset seizures in adults—inhibits the enzyme responsible for GABA catabolism, increases brain GABA levels and may act synergistically with anesthetic agents to abort seizures. Our central hypothesis is that early inhibition of GABA breakdown is possible in the post-cardiac arrest period and may be an effective adjunctive treatment in PASE.

Methods

This is a phase IIa, single-center, open-label, pilot clinical trial with blinded outcome assessment, of a single dose of vigabatrin in 12 consecutive PASE subjects. Subjects will receive a single loading dose of 4500 mg of vigabatrin (or dose adjusted in moderate and severe renal impairment) via enteric tube within 48 h of PASE onset. Vigabatrin levels will be monitored at 0- (baseline), 0.5-, 1-, 2-, 3-, 6-, 12-, 24-, 48-, 72- and 168-h (7 days) post-vigabatrin. Serum biomarkers of neuronal injury will be measured at 0-, 24-, 48-, 72- and 96-h post-vigabatrin. The primary feasibility endpoint is the proportion of enrolled subjects among identified eligible subjects receiving vigabatrin within 48 h of PASE onset. The primary pharmacokinetic endpoint is the measured vigabatrin level at 3 h post-administration. Descriptive statistics with rates and proportions will be obtained regarding feasibility outcomes, along with the noncompartmental method for pharmacokinetic analyses. The area under the vigabatrin concentration-time curve in plasma from zero to the time of the last quantifiable concentration (AUC0-tlqc) will be calculated to estimate dose-linear pharmacokinetics.

Perspective

Vigabatrin demonstrates high potential for synergism with current standard of care therapies. Demonstration of the feasibility of vigabatrin administration and preliminary safety in PASE will pave the way for future efficacy and safety trials of this pharmacotherapeutic.

Trial Registration NCT04772547.

OV329, a novel highly potent γ-aminobutyric acid aminotransferase inactivator, induces pronounced anticonvulsant effects in the pentylenetetrazole seizure threshold test and in amygdala-kindled rats

OBJECTIVE

An attractive target to interfere with epileptic brain hyperexcitability is the enhancement of γ-aminobutyric acidergic (GABAergic) inhibition by inactivation of the GABA-metabolizing enzyme GABA aminotransferase (GABA-AT). GABA-AT inactivators were designed to control seizures by raising brain GABA levels. OV329, a novel drug candidate for the treatment of epilepsy and addiction, has been shown in vitro to be substantially more potent as a GABA-AT inactivator than vigabatrin, an antiseizure drug approved as an add-on therapy for adult patients with refractory complex partial seizures and monotherapy for pediatric patients with infantile spasms. Thus, we hypothesized that OV329 should produce pronounced anticonvulsant effects in two different rat seizure models.

METHODS

We therefore examined the effects of OV329 (5, 20, and 40 mg/kg ip) on the seizure threshold of female Wistar Unilever rats, using the timed intravenous pentylenetetrazole (ivPTZ) seizure threshold model as a seizure test particularly sensitive to GABA-potentiating manipulations, and amygdala-kindled rats as a model of difficult-to-treat temporal lobe epilepsy.

RESULTS

GABA-AT inactivation by OV329 clearly increased the threshold of both ivPTZ-induced and amygdala-kindled seizures. OV329 further showed a 30-fold greater anticonvulsant potency on ivPTZ-induced myoclonic jerks and clonic seizures compared to vigabatrin investigated previously. Notably, all rats were responsive to OV329 in both seizure models.

SIGNIFICANCE

These results reveal an anticonvulsant profile of OV329 that appears to be superior in both potency and efficacy to vigabatrin and highlight OV329 as a highly promising candidate for the treatment of seizures and pharmacoresistant epilepsies.

In Vivo Detection of CPP-115 Target Engagement in Human Brain

CPP-115, a next-generation γ-amino butyric acid (GABA)-aminotransferase (AT) inhibitor, shows comparable pharmacokinetics, improved safety and tolerability, and a more favorable toxicity profile when compared with vigabatrin. The pharmacodynamic characteristics of CPP-115 remain to be evaluated. The present study employed state-of-the-art proton magnetic resonance spectroscopy techniques to measure changes in brain GABA+ (the composite resonance of GABA, homocarnosine, and macromolecules) concentrations in healthy subjects receiving oral daily doses of CPP-115 or placebo. Six healthy adult males were randomized to receive either single daily 80 mg doses of CPP-115 (n=4) or placebo (n=2) for 6, 10, or 14 days. Metabolite-edited spectra and two-dimensional J-resolved spectroscopy data were acquired from the parietal-occipital cortex and supplementary motor area in all subjects. Four scans were performed in each subject that included a predrug baseline measure, two scans during the dosing timeframe, and a final scan that occurred 1 week after drug cessation. CPP-115 induced robust and significant increases in brain GABA+ concentrations that ranged between 52 and 141% higher than baseline values. Elevated GABA+ concentrations returned to baseline values following drug clearance. Subjects receiving placebo showed no significant changes in GABA+ concentration. CPP-115-induced changes were exclusive to GABA and homocarnosine, and CPP-115 afforded brain GABA+ concentration changes comparable to or greater than previous vigabatrin spectroscopy studies in healthy epilepsy-naive subjects. The return to baseline GABA+ concentration indicates the reversible GABA-AT resynthesis following drug washout. These preliminary data warrant further spectroscopy studies that characterize the acute pharmacodynamic effects of CPP-115 with additional dose-descending measures.

Increasing GABA reverses age-related alterations in excitatory receptive fields and intensity coding of auditory midbrain neurons in aged mice

A key feature of age-related hearing loss is a reduction in the expression of inhibitory neurotransmitters in the central auditory system. This loss is partially responsible for changes in central auditory processing, as inhibitory receptive fields play a critical role in shaping neural responses to sound stimuli. Vigabatrin (VGB), an antiepileptic agent that irreversibly inhibits γ-amino butyric acid (GABA) transaminase, leads to increased availability of GABA throughout the brain. This study used multi-channel electrophysiology measurements to assess the excitatory frequency response areas in old CBA mice to which VGB had been administered. We found a significant post-VGB reduction in the proportion of V-type shapes, and an increase in primary-like excitatory frequency response areas. There was also a significant increase in the mean maximum driven spike rates across the tonotopic frequency range of all treated animals, consistent with observations that GABA buildup within the central auditory system increases spike counts of neural receptive fields. This increased spiking is also seen in the rate-level functions and seems to explain the improved low-frequency thresholds.

Monitoring vigabatrin in head injury patients by cerebral microdialysis: obtaining pharmacokinetic measurements in a neurocritical care setting

Aims

The aims were to determine blood–brain barrier penetration and brain extracellular pharmacokinetics for the anticonvulsant vigabatrin (VGB; γ-vinyl-γ-aminobutyric acid) in brain extracellular fluid and plasma from severe traumatic brain injury (TBI) patients, and to measure the response of γ-aminobutyric acid (GABA) concentration in brain extracellular fluid.

Methods

Severe TBI patients (n = 10) received VGB (0.5 g enterally, every 12 h). Each patient had a cerebral microdialysis catheter; two patients had a second catheter in a different region of the brain. Plasma samples were collected 0.5 h before and 2, 4 and 11.5 h after the first VGB dose. Cerebral microdialysis commenced before the first VGB dose and continued through at least three doses of VGB. Controls were seven severe TBI patients with microdialysis, without VGB.

Results

After the first VGB dose, the maximum concentration of VGB (C_max) was 31.7 (26.9–42.6) μmol l⁻¹ (median and interquartile range for eight patients) in plasma and 2.41 (2.03–5.94) μmol l⁻¹ in brain microdialysates (nine patients, 11 catheters), without significant plasma–brain correlation. After three doses, median C_max in microdialysates increased to 5.22 (4.24–7.14) μmol l⁻¹ (eight patients, 10 catheters). Microdialysate VGB concentrations were higher close to focal lesions than in distant sites. Microdialysate GABA concentrations increased modestly in some of the patients after VGB administration.

Conclusions

Vigabatrin, given enterally to severe TBI patients, crosses the blood–brain barrier into the brain extracellular fluid, where it accumulates with multiple dosing. Pharmacokinetics suggest delayed uptake from the blood.

Mechanism of action of vigabatrin: correcting misperceptions

Discovered more than three decades ago, vigabatrin is approved in more than 50 countries as adjunctive therapy for adult patients with refractory complex partial seizures who have responded inadequately to several alternative treatments and as monotherapy for pediatric patients aged 1 month to 2 years with infantile spasms. Contrary to a fairly common misperception, the compound's mechanism of action is very well-characterized in animal models and cell cultures. γ-Aminobutyric acid (GABA)-ergic synapses comprise approximately 30% of all synapses within the central nervous system, and therein underlies the primary mode of synaptic inhibition. Vigabatrin was rationally designed to have a specific effect on brain chemistry by inhibiting the GABA-degrading enzyme, GABA transaminase, resulting in a widespread increase in GABA concentrations in the brain. The increase in GABA functions as a brake on the excitatory processes that can initiate seizure activity. Despite the short half-life of vigabatrin in the body (5-7 h) and its relatively low concentration in cerebrospinal fluid (10% of the concentration observed in plasma), it has the profound effect of increasing GABA concentration in the brain for more than a week after a single dose in humans. This effect persists steadily over years of vigabatrin administration and results in significant and persistent decreases in seizure activity. Vigabatrin can be effective with once-daily dosing. Because of its specificity, vigabatrin has helped researchers explore the specific mechanisms within the brain that underlie seizure activity.

Extended-release formulations of antiepileptic drugs: rationale and comparative value

Extended-release products are designed to prolong the absorption of drugs with short half-lives, thereby allowing longer dosing intervals while minimizing fluctuations in serum drug levels. The relationship between serum drug concentration and clinical effects of antiepileptic drugs (AEDs) can be complex and reducing fluctuations in serum drug levels is not equally advantageous for all AEDs. Extended-release formulations have been shown to be particularly valuable for carbamazepine, whereas for other AEDs advantages, other than prolongation of the dosing interval, have not been clearly demonstrated. Differences in bioavailability may exist between extended-release and immediate-release formulations and among different brands of extended-release products. Therefore, when switching from one formulation to another, careful monitoring of clinical response and attention to the need for dose adjustment are warranted.

Vigabatrin's complicated journey--to be or not to be?

Visual Fields at School-Age in Children Treated with Vigabatrin in Infancy. Gaily E, Jonsson H, Lappi M. Epilepsia 2009;50(2):206–216. PURPOSE: The use of vigabatrin (VGB) as an antiepileptic drug (AED) has been limited by evidence showing that it causes vigabatrin-attributed visual field loss (VAVFL) in at least 20–40% of patients exposed at school age or later. VGB is an effective drug for infantile spasms, but there are no reports on later visual field testing after such treatment. Our aim was to investigate the risk of VAVFL in school-age children who had received VGB in infancy. METHODS: Visual fields of 16 children treated with VGB for infantile spasms were examined by Goldmann kinetic perimetry at age 6–12 years. Normal fields were defined as the temporal meridian extending to more than 70°, and mild VAVFL between 50 and 70°. Abnormal findings were always confirmed by repeating the test. Exposure data were collected from hospital charts. RESULTS: Vigabatrin was started at a mean age of 7.6 (range, 3.2–20.3) months. The mean duration of therapy was 21.0 (9.3–29.8) months and cumulative dose 655 g (209–1,109 g). Eight children were never treated with other AEDs, five received only adrenocorticotropic hormone (ACTH) in addition to VGB, and three children had been treated with other AEDs. Fifteen children had normal visual fields. Mild VAVFL was observed in one child (6%) who had been treated with VGB for 19 months and who received a cumulative dose of 572 g. CONCLUSIONS: The risk of VAVFL may be lower in children who are treated with VGB in infancy compared to patients who receive VGB at a later age.

Magnetic Resonance Imaging Abnormalities Associated with Vigabatrin in Patients with Epilepsy. Wheless JW, Carmant L, Bebin M, Conry JA, Chiron C, Elterman RD, Frost M, Paolicchi JM, Donald Shields W, Thiele EA, Zupanc ML, Collins SD. Epilepsia 2009;50(2):195–205. PURPOSE: Vigabatrin used to treat infantile spasms (IS) has been associated with transient magnetic resonance imaging (MRI) abnormalities. We carried out a retrospective review to better characterize the frequency of those abnormalities in IS and in children and adults treated with vigabatrin for refractory complex partial seizures (CPS). METHODS: Medical records and 332 cranial MRIs from 205 infants (aged ≤24 months) with IS treated at 10 sites in the United States and Canada were collected. Similarly, 2,074 images from 668 children (aged 2–16 years) and adults (aged >16 years) with CPS were re-reviewed. Prespecified MRI abnormalities were defined as any hyperintensity on T2-weighted or fluid-attenuated inversion-recovery (FLAIR) sequences with or without diffusion restriction not readily explained by a radiographically well-characterized pathology. MRIs were read by two neuroradiologists blinded to treatment group. The incidence and prevalence of MRI abnormalities associated with vigabatrin were estimated. RESULTS: Among infants with IS, the prevalence of prespecified MRI abnormalities was significantly higher among vigabatrin-treated versus vigabatrin-naive subjects (22% vs. 4%; p < 0.001). Of nine subjects in the prevalence population with at least one subsequent determinate MRI, resolution of MRI abnormalities occurred in six (66.7%)—vigabatrin was discontinued in four. Among adults and children treated with vigabatrin for CPS, there was no statistically significant difference in the incidence or prevalence of prespecified MRI abnormalities between vigabatrin-exposed and vigabatrin-naive subjects. DISCUSSION: Vigabatrin is associated with transient, asymptomatic MRI abnormalities in infants treated for IS. The majority of these MRI abnormalities resolved, even in subjects who remained on vigabatrin therapy.

Vigabatrin: effect on brain GABA levels measured by nuclear magnetic resonance spectroscopy

Vigabatrin is undoubtedly one of the most exciting anti-epilepsy drugs in use today. Many open and controlled clinical trials have confirmed that it is particularly effective in controlling partial epileptic seizures with or without secondary generalization. Vigabatrin acts to increase GABA levels in the presynaptic nerve terminal by inhibiting the activity of GABA-transaminase. There is no direct correlation between the blood or brain concentration of vigabatrin and its clinical effect, so monitoring vigabatrin levels is not predictive of patient response. However, it is possible to relate the activity of vigabatrin to levels of GABA in the brain, measured by nuclear magnetic resonance spectroscopy (NMRS). NMRS studies show that following administration of vigabatrin, brain concentrations of GABA rise to about 2-3 times their baseline values. This 'extra' GABA is held within the nerve terminal, and is only released during synaptic transmission. Although there appears to be a clear dose-response relationship up to 3 g/day, it is not well documented if higher doses result in proportionately higher brain GABA levels. This finding seems to support the results of clinical studies suggesting that the optimal dose of vigabatrin may be 3 g/day. There is also some evidence for a correlation between the concentration of GABA in the brain and the clinical outcome. Continuing investigations using NMRS aim to confirm these preliminary findings, and to determine the time course and extent of changes in brain GABA levels after vigabatrin administration.

Treatment of refractory complex partial seizures: role of vigabatrin

Vigabatrin (VGB) is an antiepileptic drug that was designed to inhibit GABA-transaminase, and increase levels of gamma-amino-butyric acid (GABA), a major inhibitory neurotransmitter in the brain. VGB has demonstrated efficacy as an adjunctive antiepileptic drug for refractory complex partial seizures (CPS) and for infantile spasms (IS). This review focuses on its use for complex partial seizures. Although VGB is well tolerated, there have been significant safety concerns about intramyelinic edema and visual field defects. VGB is associated with a risk of developing bilateral concentric visual field defects. Therefore, the use of VGB for complex partial seizures should be limited to those patients with seizures refractory to other treatments. Patients must have baseline and follow-up monitoring of visual fields, early assessment of its efficacy, and ongoing evaluation of the benefits and risks of VGB therapy.

The ketogenic diet influences the levels of excitatory and inhibitory amino acids in the CSF in children with refractory epilepsy

The ketogenic diet (KD) is an established treatment for medically refractory pediatric epilepsy. Its anticonvulsant mechanism is still unclear. We examined the influence of the KD on the CSF levels of excitatory and inhibitory amino acids in 26 children (mean age 6.1 years) with refractory epilepsy. Seventeen amino acids were determined before and at a mean of 4 months after the start of the KD. Seizures were quantified. Highly significant changes were found in eight amino acids: increases in GABA, taurine, serine, and glycine and decreases in asparagine, alanine, tyrosine and phenylalanine. However, aspartate, glutamate, arginine, threonine, citrulline, leucine, isoleucine and valine/methionine remained unchanged. A significant correlation with seizure response was found for threonine (P=0.016). The GABA levels were higher in responders (>50% seizure reduction) than in nonresponders during the diet (P=0.041). In the very good responders (>90% seizure reduction), the GABA levels were significantly higher at baseline as well as during the diet. Age differences were found with significantly larger decreases in glutamate and increases in GABA in connection with the diet in younger children. Our results indicate that the KD significantly alters the levels of several CSF amino acids that may be involved in its mechanism of action and the increase in GABA is of particular interest.

Novel mechanisms of action of three antiepileptic drugs, vigabatrin, tiagabine, and topiramate

Epilepsy, a functional disturbance of the CNS and induced by abnormal electrical discharges, manifests by recurrent seizures. Although new antiepileptic drugs have been developed during recent years, still more than one third of patients with epilepsy are refractory to treatment. Therefore, the search for new mechanisms that can regulate cellular excitability are of utmost importance. Three currently available drugs are of special interest because they have novel mechanisms of action and are especially effective for partial onset seizures. Vigabatrin is a selective and irreversible GABA-transaminase inhibitor that greatly increases whole-brain levels of GABA. Tiagabine is a potent inhibitor of GABA uptake into neurons and glial cells. Topiramate is considered to produce its antiepileptic effect through several mechanisms, including modification of Na(+)-and/or Ca(2+)-dependent action potentials, enhancement of GABA-mediated Cl- fluxes into neurons, and inhibition of kainate-mediated conductance at glutamate receptors of the AMPA/kainate type. This review will discuss these mechanisms of action at the cellular and molecular levels.

Long-term open multicentre, add-on trial of vigabatrin in adult resistant partial epilepsy. The Canadian Vigabatrin Study Group

Vigabatrin (VGB) has been shown in a number of clinical trials with varying designs to be effective and well-tolerated as both add-on therapy and monotherapy in epilepsy with partial seizures with or without secondary generalization as well as in infantile spasms. The present study is an open, long-term (1 year) extension of a randomized double-blind placebo-controlled multicentre Canadian trial of VGB in resistant partial adult epilepsy. The present study was designed to examine the safety and long-term efficacy of VGB. Completers of the preceding double-blind study had their dose of VGB titrated to 4 g/day over 3 weeks. Patients were evaluated every 2-4 weeks and at week 14 were allowed to continue only if they achieved a 50% seizure reduction compared with pre-VGB baseline. In addition to neurological and physical examinations, safety was assessed by a cognitive psychosocial test battery, visual and somatosensory evoked potentials and MRI scans. Ninety-seven of 100 eligible patients entered the study, 53 of whom completed the 52 weeks. Fifty-eight percent of the patients had a greater than 50% seizure reduction in seizures vs. pre-VGB baseline. Seizure reductions of 56% and 45%, respectively, were seen in the VGB and placebo groups from the preceding study. Fifty-four percent of patients were judged by the investigators to have experienced at least a moderate therapeutic effect. Discontinuations were 29% for lack of efficacy and 12% for adverse effects. There was a mean weight gain of 3.7 +/- 0.2 kg by end of study. Neurologica/psychiatric side effects were the most common reason for withdrawal including three behavioral reactions attributed to the drug which required temporary hospitalization. There were no abnormalities on laboratory or special tests and there was a tendency for improvement on most tests of cognitive function and mood. Vigabatrin, as an add-on agent, is well-tolerated and can be of long-term benefit in a substantial proportion of patients with intractable partial epilepsy.

Vigabatrin: a novel therapy for seizure disorders

OBJECTIVE

To review the pharmacology, pharmacokinetics, efficacy, and adverse effects of vigabatrin and its role in the management of seizure disorders.

METHODS

A MEDLINE search of English-language literature from January 1993 through January 1999 was conducted using vigabatrin as a search term to identify pertinent studies and review articles. Additional studies were identified from the bibliographies of reviewed literature. The manufacturer provided postmarketing surveillance data. Priority was given to randomized, double-blind, placebo-controlled studies.

FINDINGS

Vigabatrin is a selective and irreversible inhibitor of gamma-aminobutyric acid transaminase. In controlled clinical trials of vigabatrin add-on therapy in patients with uncontrolled partial seizures, 24-67% of patients achieved a < or =50% reduction in seizure frequency. Data from two comparative trials with carbamazepine monotherapy indicate that vigabatrin monotherapy reduces the frequency of partial seizures in patients with newly diagnosed epilepsy. Vigabatrin also controls infantile spasms, particularly those associated with tuberous sclerosis. Vigabatrin is more effective in patients with partial seizures than in those with generalized seizures. The drug is generally well tolerated. Headache and drowsiness were the most common adverse effects observed in controlled clinical trials; visual field defects, psychiatric reactions, and hyperactivity also have been reported. There are no known clinically significant drug interactions.

CONCLUSIONS

Vigabatrin improves seizure control as add-on therapy for refractory partial seizures and may produce therapeutic benefits in the treatment of infantile spasms. Vigabatrin is generally well tolerated, with a convenient administration schedule, a lack of known significant drug interactions, and no need for routine monitoring of plasma concentrations.

Vigabatrin

Vigabatrin (VGB) is a structural analogue of the inhibitory neurotransmitter gamma-amino butyric acid (GABA), which produces its antiepileptic effect by irreversibly inhibiting the degradative enzyme GABA-transaminase. This produces an increase in central nervous system (CNS) GABA levels. VGB is among the few antiepileptic drugs (AEDs) that was synthesized with a specific targeted mechanism in mind and was subsequently demonstrated to function by that mechanism. Tiagabine, a GABA reuptake blocker, is the only other "designer drug" among the currently available AEDs. Therefore, VGB is among the few AEDs for which the mechanism of action is well understood. Recently, safety issues have been raised with regard to the use of vigabatrin. This article reviews the mechanism of action, pharmacokinetics, safety, and efficacy of VGB.

Aminoaciduria resulting from vigabatrin administration in children with epilepsy

Vigabatrin (gamma-vinyl gamma aminobutyric acid), a recently developed antiepileptic drug, has been extensively evaluated in the treatment of drug-resistant epilepsy. Several case reports demonstrated that vigabatrin affects urinary excretion of several amino and organic acids. Fourteen children were investigated for the presence of abnormal urinary amino acids before and after treatment with vigabatrin. All demonstrated increased urinary excretion of amino acids, particularly beta-alanine, gamma-aminobutyric acid, and beta-aminoisobutyric acid while on vigabatrin, which were not detected when off medication. These results emphasize the importance of obtaining urine for metabolic evaluation before the administration of vigabatrin.

Acute effects of vigabatrin on brain GABA and homocarnosine in patients with complex partial seizures

PURPOSE

The acute, subacute, and chronic effects of vigabatrin (VGB) were studied in patients with refractory complex partial seizures. VGB increases human brain gamma-aminobutyric acid (GABA) and the related metabolites, homocarnosine and 2-pyrrolidinone.

METHODS

In vivo measurements of GABA and homocarnosine were made of a 14-cc volume in the occipital cortex by using 1H spectroscopy with a 2.1-Tesla magnetic resonance spectrometer and an 8-cm surface coil. Six patients (three women) were studied serially during the initiation and maintenance of VGB as adjunct therapy.

RESULTS

The first, 3 g dose of VGB increased brain GABA by 2.0 micromol/g within 81 min of oral administration. After 2 h, median edited GABA remained essentially the same for 2 days. The response to the second, 3-g dose of VGB given at 48 h was considerably less than that to the first dose, with a median increase of 0.5 micromol/g within 72 min. After 2-3 months, rechallenging patients taking 1.5-g VGB twice daily with 6 g increased GABA by 0.4 micromol/g within 87 min. Homocarnosine increased more gradually than GABA to above-normal levels after a week of VGB therapy.

CONCLUSIONS

VGB promptly elevates brain GABA and presumably offers partial protection against further seizures within hours of the first oral dose. Once-a-day dosing is sufficient to increase GABA. Patients may be expected to experience the effects of increased homocarnosine within 1 week.

Once-daily versus twice-daily vigabatrin: is there a difference? The results of a double-blind pilot study

PURPOSE

Vigabatrin (VGB) has been approved in Europe and is prescribed for either once or twice-daily administration. This choice has been based on the pharmacodynamic activity of VGB. The purpose of this study was to compare the efficacy and tolerability of these two different medication regimens.

METHODS

The study design was a double-blind randomized two-period cross-over study in adults who had responded to add-on VGB for previously uncontrolled seizures. Each study period consisted of three months. Patients were maintained on the same daily dose of VGB to which they had demonstrated a clinical response. In addition to the primary efficacy criteria of seizure frequency on the two treatment regimens, this study included blinded ratings of overall efficacy and "well being" by both physician and patient. The primary tolerability criterion was the reported incidence of adverse events by phase.

RESULTS

Fifty patients were initially entered into the study, and 13 patients withdrew before completion, only one reported as due to an adverse event. There was no statistical difference in seizure frequency or the tolerability of the medication. Blinded physician and patient rating scales for seizure control, and patient well being showed a nonstatistical trend toward once-daily administration as compared with twice-daily administration.

CONCLUSIONS

This clinical study provides support for the pharmacological evidence that this preparation may be administered on a once or twice daily basis, depending on the individual patient's preference, total dosage and co-medication.

Vigabatrin increases human brain homocarnosine and improves seizure control

Homocarnosine, a dipeptide of gamma-aminobutyric acid (GABA) and histidine, is thought to be an inhibitory neuromodulator synthesized in subclasses of GABAergic neurons. Homocarnosine is present in human brain in greater amounts (0.4-1.0 micromol/g) than in other animals. The antiepileptic drug vigabatrin increases human cerebrospinal fluid homocarnosine linearly with daily dose. By using 1H nuclear magnetic resonance spectroscopy, serial occipital lobe GABA and homocarnosine concentrations were measured in 11 patients started on vigabatrin. Daily low-dose (2 g) vigabatrin increased both homocarnosine and GABA. Larger doses of vigabatrin (4 g) further increased homocarnosine but changed GABA levels minimally. Seizure control improved with increasing homocarnosine and GABA concentrations. Patients whose seizure control improved with the addition of vigabatrin had higher mean homocarnosine, but the same mean GABA concentrations, than those whose seizure control did not improve. Increased homocarnosine may contribute to improved seizure control.

Measuring human brain GABA in vivo: effects of GABA-transaminase inhibition with vigabatrin

Gamma-aminobutyric acid (GABA) plays a pivotal role in suppressing the origin and spread of seizure activity. Low occipital lobe GABA was associated with poor seizure control in patients with complex partial seizures. Vigabatrin irreversibly inhibits GABA-transaminase, raising brain and cerebrospinal fluid (CSF) GABA concentrations. The effect of vigabatrin on occipital lobe GABA concentrations was measured by in vivo nuclear magnetic-resonance spectroscopy. Using a single oral dose of vigabatrin, the rate of GABA synthesis in human brain was estimated at 17% of the Krebs cycle rate. As the daily dose of vigabatrin was increased to up to 3 g, the fractional elevation of brain GABA was similar to CSF increase. Doubling the daily dose from 3 to 6 g failed to increase brain GABA further. Increased GABA concentrations appear to reduce GABA synthesis in humans as it does in animals. With traditional antiepileptic drugs, remission of the seizure disorder was associated with normal GABA levels. With vigabatrin, elevated CSF and brain GABA was associated with improved seizure control. Vigabatrin enhances the vesicular and nonvesicular release of GABA. The release of GABA during seizures may be mediated in part by transporter reversal that may serve as an important protective mechanism. During a seizure, this mechanism may be critical in stopping the seizure or preventing its spread.

The influence of established and new antiepileptic drugs on visual perception. II. A controlled study in patients with epilepsy under long-term antiepileptic medication

In this study, we investigated visual performance under chronic antiepileptic drug treatment. Patients were under carbamazepine (CBZ) (n = 18), valproic acid (VPA) (n = 9), CBZ and vigabatrin (VGB) (n = 4), CBZ and gabapentin (GBP) (n = 8), and under CBZ and topiramate (TPR) (n = 6), respectively. Seven untreated patients with epilepsy and 42 healthy volunteers served as controls. The test battery comprised the Lanthony-D15-désaturé colour perception test, increment, postadaptation and transient tritanopia (TT) threshold measurements, visual perception threshold assessments for monochromatic and chromatic gratings and gaussian dots, and critical flicker fusion (CFF) tests. No differences were seen between naive patients and healthy controls. Patients under drug treatment always showed alterations of visual perception. Postadaptation and TT thresholds were altered under each drug regimen after short delays between switching off the adaptation light and switching on the blue test light. Threshold elevations were maximum under the combination of CBZ and TPR and lowest under CBZ and GBP. Consistent impairment of the CFF was seen under combined CBZ and TPR whereas VPA as well as combined CBZ and VGB led to ameliorations the mechanisms of which are discussed. The other tests were less sensitive. In conclusion, alterations of visual function were apparent under chronic antiepileptic drug treatment both with established and new agents. However, it may be difficult to distinguish between effects based on specific modes of action and nonspecific retino- and neurotoxicity.

Multicentre clinical evaluation of vigabatrin (Sabril) in mild to moderate partial epilepsies. French Neurologists Sabril Study Group

Vigabatrin (VGB) has been shown through several studies to be safe and effective as add-on therapy, particularly for the treatment of partial seizures in patients with severe epilepsies followed for years in hospital-based clinics. We now report additional clinical experience with VGB arising from an open trial of add-on VGB therapy in patients with relatively few seizures followed by qualified neurologists in private practice (the French Neurologists Sabril Study Group). VGB was administered to 397 patients aged 12-74 years (mean age = 37.5 +/- 13.8 years) who presented with no more than seven partial seizures of any type per month during a 3-month baseline period (mean number of seizures = 3.7 +/- 1.9/month). Simple partial seizures were reported in 121 (30.5%) patients, complex partial seizures in 282 (71.0%) and seizures with secondary generalization were reported in 111 (28.0%). The mean number of associated antiepileptic drugs (AEDs) was 1.9 +/- 0.9 and the mean dose of VGB was 2.21 +/- 0.64 g/day. Following introduction of VGB, 53 (13.4%) became seizure-free and remained so during the whole trial. During the fourth month of treatment, 158 patients (39.8%) had no seizures at all and a further 69 (17.4%) had their seizure frequency reduced by more than 50%. Secondary generalization was controlled during the whole period of treatment in 55 out of 97 patients (56.7%), 17 of which remained free of all types of partial seizures. VGB showed a good tolerability profile; adverse experiences more frequently reported were drowsiness and sleep disturbances. No action was necessary in the great majority of cases; the dose was reduced in 26 (6.5%) and VGB was discontinued in 32 (8%) patients. These data provide additional evidence that VGB can be used safely early on to treat patients with mild to moderate partial epilepsies. Secondary generalization was controlled in the majority of patients. Factors associated with the everyday clinical use of VGB, that resulted from a series of organized meetings with the investigators, are discussed.

Effects of vigabatrin on motor potentials evoked with magnetic stimulation

We studied the effect of an acute loading dose of vigabatrin on threshold of motor responses and duration of silent period elicited with cortical magnetic stimulation in normal subjects. In contrast to phenytoin, vigabatrin does not increase the motor threshold of first dorsal interosseus muscle. We also show that, although vigabatrin increases GABA concentrations in the central nervous system, duration of silent period studied at various stimulus intensities is not modified after vigabatrin administration.

GABA Levels in the Brain: A Target for New Antiepileptic Drugs

A basic strategy for the pharmacological treatment of epilepsy is to develop drugs that reduce the excitability of CNS neurons at times preceding or during the onset of seizure discharge with minimal effects on normal electrical activity. Several antiepileptic drugs currently in use exert their action by modulating sodium channels or receptors of the abundant inhibitory neurotransmitter, GABA. These approaches, which are often successful in reducing the number or severity of seizures, have some effects that limit their clinical use. More recently, a new class of antiepileptic drugs such as vigabatrin, which blocks GABA degradation enzymes, have been developed as effective antiepileptics and are associated with minimal side effects. Although these drugs do not display agonist or antagonist properties at GABA receptor sites, they do appear to interact with brain GABA systems because NMR spectroscopy studies indicate that subjects given these drugs have elevated brain GABA levels, and in vitro electrophysiological studies on CNS tissue reveal elevated GABA release. The precise cellular mechanisms of antiepileptic action of these GABA metabolic modulators are not clear, but current work on the cellular effects of these drugs suggests a model that may explain their action.

Seizure frequency and CSF parameters in a double-blind placebo controlled trial of gabapentin in patients with intractable complex partial seizures

Gabapentin (GBP) is a non-protein-bound gamma amino acid which is not subjected to metabolic degradation in man. As part of a placebo-controlled double-blind study, patients suffering from intractable complex partial seizures with or without secondary generalization were followed with lumbar punctures at baseline and after three months of GBP treatment (900 mg/day or 1200 mg/day). Cerebrospinal fluid (CSF) was analyzed for concentrations of GBP, amino acids including GABA, homovanillic acid (HVA), and 5 hydroxyindoleacetic acid (5-HIAA). The results indicate that there were no changes in the selected amino acids, HVA, or 5-HIAA after GBP treatment. At steady state the CSF/plasma ratios of GBP ranged from 0.056 to 0.34, indicating that there may be some type of active out-transport of GBP across the blood-brain barrier. No linear relationship was observed between plasma and CSF levels in these patients.

Initial observations on effect of vigabatrin on in vivo 1H spectroscopic measurements of gamma-aminobutyric acid, glutamate, and glutamine in human brain

Recent developments involving 1H nuclear magnetic resonance (NMR) spectroscopic editing techniques have allowed noninvasive measurements of gamma-aminobutyric acid (GABA) in human cerebrum. The additional information gained from GABA and macromolecule measurements permitted more precise glutamate (Glu) and glutamine (Gln) measurements. Occipital lobe GABA in 10 nonepileptic, healthy subjects was 1.0 mumol/g brain [95% confidence interval (CI) 0.9-1.1]. Vigabatrin (VGB) is a safe and effective antiepileptic drug (AED) that irreversibly inhibits neuronal and glial GABA-transaminase. GABA levels were increased in all patients treated with VGB. With a standard dose of 3-6 g/day, GABA levels were 2.6 mumol/g (95% CI 2.3-2.8). Mean occipital GABA level measured in epileptic patients not receiving VGB was 0.9 mumol/g (95% CI 0.7-1.1). Gln was increased by 1.9 mumol/g and Glu was decreased by 0.8 mumol/g in patients receiving VGB as compared with patients receiving standard medications alone.

Effects of vagus nerve stimulation on amino acids and other metabolites in the CSF of patients with partial seizures

Electrical stimulation of the vagus nerve (VNS) is a new method for the treatment of patients with medically intractable epilepsy. Sixteen patients, ten of whom participated in a larger multicenter double-blind trial on the efficacy of VNS in epilepsy, and six who participated in pilot studies, consented to participate in the present study. Ten patients received HIGH stimulation and six patients LOW stimulation for the 3-month trial. Cerebrospinal fluid (CSF) samples (16 ml) were collected both before and after 3 months of VNS. Amino acid and neurotransmitter metabolites were analyzed. Four patients responded to VS with more than a 25% seizure reduction after 3 months. Mean and median concentrations of phosphoethanolamine (PEA) increased in responders and decreased in nonresponders. Free GABA increased in both groups but more so in the nonresponders. After 9 months of VS (6-9 months on HIGH stimulation) 4 of 15 patients had more than 40% seizure reduction. There were significant correlations between seizure reduction and increases in asparagine, phenylalanine, PEA, alanine and tryptophan concentrations. Comparison between patients with HIGH or LOW stimulation showed a significant increase in ethanolamine (EA) in the HIGH group and a decrease in glutamine in the LOW group. All patients regardless of response or stimulation intensity showed significantly increased total and free GABA levels. A decrease in CSF aspartate was marginally significant. Other trends were decreases in glutamate and increases in 5-hydroxyindoleacetic acid. Chronic VNS appears to have an effect on various amino acids pools in the brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Vigabatrin

gamma-Aminobutyric acid (GABA) was first proposed as a putative inhibitory neurotransmitter by Elliot and van Gelder in 1958. Since then, numerous efforts have been made to find ways to increase GABA at its receptor sites, based on the findings that decreased GABA results in convulsions in animals and that agents enhancing GABA-mediated functions can have antiepileptic effects. However, the relationship between GABA levels and seizures is not simple. Seizures can occur even in the presence of elevated GABA levels. Indeed, it is possible that regional biochemical differences in the brain can be important. The antiepileptic effects of GABA depend on the mechanism whereby GABA-mediated inhibition is enhanced. Since the 1970s, several compounds have been developed that are designed to act in some manner on the GABA system. These compounds affect GABA-mediated inhibition at different levels and appear to have varied effects, depending on their mechanism of action. To date, specific antiepileptic drugs (AEDs) with potential GABA-inhibitory effects have been designed either to have GABA agonist properties, to inhibit GABA catabolism, to inhibit GABA uptake, or to facilitate GABA release or facilitate GABAA receptor activity. Vigabatrin (VGB) was designed specifically to inhibit GABA transaminase and thereby increase the availability of GABA in the brain. Study data and clinical experience over the past 14 years have demonstrated VGB to be an effective AED.

Adverse effects of established and new antiepileptic drugs: an attempted comparison

Seizures are but one aspect of the negative impact epilepsy has on patients' lives. Adverse effects of antiepileptic treatment may affect the patient's quality of life to an even greater extent than the occurrence of seizures. Adverse effects of antiepileptic drugs (AEDs) are common, and because the differences in efficacy are often marginal, adverse effects may be the most important factor in choosing the best AED for the patient. The search for more efficient and less toxic agents is constantly ongoing. Current evidence suggests that the new generation of AEDs is as efficient as the established AEDs and exhibits fewer adverse effects, but the scientific evidence from randomised clinical trials comparing established and new AEDs with each other is still pending.

Clinical pharmacokinetics of new antiepileptic drugs

We have reviewed the pharmacokinetics of six antiepileptic drugs that are marketed (felbamate, gabapentin, lamotrigine, oxcarbazepine, vigabatrin, and zonisamide) and six drugs that are undergoing evaluation (levetiracetam, ralitoline, remacemide, stiripentol, tiagabine, and topiramate). In addition, we have compared the prodrugs eterobarb and fosphenytoin and the controlled-release formulations of valproic acid and carbamazepine with their parent compounds. Finally, we have devised a scoring system to compare the pharmacokinetics of new antiepileptic drugs. Using this system, vigabatrin, levetiracetam, gabapentin, and topiramate appea to have the most favourable pharmacokinetic profiles, whilst ralitoline and stiripentol have the least favourable.

Vigabatrin: effects on human brain GABA levels by nuclear magnetic resonance spectroscopy

Vigabatrin (VGB, Sabril) is a new antiepileptic drug used for treatment of partial and secondarily generalized tonic-clonic seizures. Many controlled short- and long-term trials have established efficacy as add-on therapy. Side effects have been infrequent. VGB acts as an irreversible substrate for gamma-aminobutyric acid (GABA) transaminase that leads to elevated brain GABA levels. Although this mechanism has been confirmed in animals and in cerebrospinal fluid of humans, we report the first study of brain GABA levels using noninvasive nuclear magnetic resonance spectroscopy. GABA elevation in brain closely parallels VGB dosage and reaches concentrations 2-3 times control values at daily dosage of 3 g. This technique offers promising potential to monitor changes induced by VGB as a function of time, dose, and clinical effect.

Newer antiepileptic drugs. Towards an improved risk-benefit ratio

Epilepsy is one of the most common neurological disorders. Even though existing antiepileptic drugs can render 80% of newly diagnosed patients seizure free, a significant number of patients have chronic intractable epilepsy causing disability with considerable socioeconomic implications. There is, therefore, a need for more potent and effective antiepileptic drugs and drugs with fewer adverse effects, particularly CNS effects. Drugs for the treatment of partial seizures are particularly needed. With major advances in our understanding of the basic neuropathology, neuropharmacology and neurophysiology of epilepsy, numerous candidate novel antiepileptic drugs have been developed in recent years. This review comparatively evaluates the pharmacokinetics, efficacy and adverse effects of 12 new antiepileptic drugs namely vigabatrin, lamotrigine, gabapentin, oxcarbazepine, felbamate, tiagabine, eterobarb, zonisamide, remacemide, stiripentol, topiramate and levetiracetam (ucb-L059). Of the 12 drugs, vigabatrin, lamotrigine and gabapentin have recently been marketed in the UK. Five of these new drugs have known mechanisms of action (vigabatrin, lamotrigine, tiagabine, oxcarbazepine and eterobarb), which may provide for a more rational approach to the treatment of epilepsy. Oxcarbazepine, remacemide and eterobarb are prodrugs. Vigabatrin, gabapentin and topiramate are more promising on the basis of their pharmacokinetic characteristics in that they are excreted mainly unchanged in urine and not susceptible to significant pharmacokinetic interactions. In contrast, lamotrigine, felbamate and stiripentol exhibit significant drug interactions. Essentially, all the drugs are effective in partial or secondarily generalised seizures and are effective to varying degrees in other seizure types. Particularly welcome is the possible effectiveness of zonisamide in myoclonus and felbamate in Lennox-Gastaut syndrome. In relation to adverse effects, CNS effects are observed with all drugs, however, gabapentin, remacemide and levetiracetam appear to exhibit least. There is also the possibility of rational duotherapy, using drugs with known mechanisms of action, as an additional therapeutic approach. The efficacy of these 12 antiepileptic drug occurs despite the fact that candidate antiepileptic drugs are evaluated under highly unfavourable conditions, namely as add-on therapy in patients refractory to drug management and with high seizure frequency. Thus, whilst candidate drugs which do become licensed are an advance in that they are effective and/or are associated with less adverse effects than currently available antiepileptic drugs in these patients, it is possible that these drugs may exhibit even more improved risk-benefit ratios when used in normal clinical practice.

Effect of vigabatrin (GVG) on serotonin (5-HT) uptake and release of human platelets in vitro

In platelets from healthy persons the effect of gamma-vinyl-GABA (GVG) on 5-HT uptake, storage, release and the kinetic parameters Km and Vmax of platelet 5-HT high affinity uptake was investigated in vitro. 5-HT uptake, storage and release in response to increasing GVG concentrations (0-7.74 mM) at a constant incubation time (60 min) and in dependence on time (0-90 min) at a constant GVG concentration (7.74 mM) remained unchanged. Concerning the high affinity 5-HT uptake, GVG (7.74 mM) caused a slight decrease of Vmax from 83.3 +/- 35.0 (SD) pmol 5-HT/10(8) pl./min to 77.0 +/- 33.4 (SD) pmol 5-HT/10(8) pl./min and a significant elevation of Km from 4.2 +/- 1.1 (SD) x 10(-7) M to 6.7 +/- 1.8 (SD) x 10(-7) M (P < 0.002) to about 160% of the control. This means a competitive inhibition of 5-HT uptake induced by GVG. Altogether, the effects of GVG on platelet 5-HT transport appear to be weak. An alteration of the platelet 5-HT system was demonstrated only at therapeutically nonrelevant high GVG concentrations. If platelets represent a model for 5-HT transport processes in presynaptic serotonergic neurons it is concluded that GVG has no effect on serotonergic activity.

Amino acid levels in the cerebrospinal fluid of newly diagnosed epileptic patients: effect of vigabatrin and carbamazepine monotherapies

We studied the CSF amino acid levels of 42 patients with newly diagnosed epilepsy before treatment with antiepileptic medication and during monotherapy with either vigabatrin or carbamazepine. The present study shows that patients with newly diagnosed epilepsy have elevated levels of the excitatory amino acid glutamate in CSF. Vigabatrin monotherapy effectively prevents the appearance of seizures in patients with high baseline CSF glutamate levels. In these patients, vigabatrin not only elevates the levels of gamma-aminobutyric acid, but also decreases the elevated levels of glutamate in CSF, which may also be important to the antiepileptic efficacy of vigabatrin. Patients with low CSF glutamate levels did not benefit from vigabatrin-induced changes in amino acid levels and successful monotherapy with carbamazepine did not affect CSF amino acid levels. The elevation of gamma-aminobutyric acid is thus not the only way to achieve seizure control and there are several factors underlying the generation and control of seizures. Follow-up of the patients with high baseline glutamate CSF levels will show if the observed abnormalities are related to the severity of epilepsy in individual patients and if early treatment with vigabatrin of these patients could prevent the development of intractable epilepsy.

Acute effects of gamma-vinyl GABA on the GABAergic system in rats as studied by microdialysis

The acute effects of the irreversible gamma-aminobutyric acid (GABA) transaminase inhibitor, gamma-vinyl GABA (Vigabatrin), were studied in the central nervous system of the rat. GABA concentrations were monitored in the hippocampus by implantation of microdialysis probes. Two doses of gamma-vinyl GABA (1.6 and 8.0 mM) were administered via the probes and were found to cause a transient increase in the basal GABA outflow (10-fold) during the period of drug administration. In addition, gamma-vinyl GABA pretreatment (1.6 mM) seemed to decrease K(+)-evoked GABA release (P < 0.05). The immediate increase of GABA outflow after gamma-vinyl GABA administration may be the result of direct blockade of GABA uptake sites, a finding which further indicates that the action of GABA transaminase inhibitors may be mediated partly through GABA uptake inhibition.

Vigabatrin and psychosis

We report a series of 14 cases of psychosis occurring in patients with severe intractable epilepsy, following the prescription of vigabatrin, a new antiepileptic drug. Nine patients had no previous history of psychosis. In eight patients the psychosis occurred following a change in the habitual pattern of seizure activity; in four it developed after a period of seizure freedom followed by a cluster of seizures, and in the other four patients an alternating psychosis was observed. In five patients there was no clear relationship to seizure pattern. Another patient developed psychosis after taking an overdose of between eight and 12 g of vigabatrin. A further three patients, who developed psychosis following vigabatrin withdrawal, were not included in this series. The mean dose at onset of the psychosis (excluding the patient who took an overdose) was 2580 mg, and the period from initiation of therapy to the onset of psychosis varied from five days to 32 weeks (and occurred 24 hours after the overdose of vigabatrin). In all cases the psychosis resolved, but necessitated the withdrawal of vigabatrin, except in the single patient who took the overdose. The mechanism of this behaviour change is unclear, but in some instances may reflect vigabatrin's powerful anti-epileptic action. This is clearly not the case for all patients. Vigabatrin should be started with caution in patients with severe epilepsy, particularly in the presence of a previous history of psychosis, and such patients should be carefully monitored.

Administration of vigabatrin (gamma-vinyl-gamma-aminobutyric acid) affects the levels of both inhibitory and excitatory amino acids in rat cerebrospinal fluid

The effect of vigabatrin (gamma-vinyl-gamma-aminobutyric acid), a new anticonvulsant drug, on the transmitter amino acids in rat cisternal CSF was studied. CSF was collected through a permanently implanted polyethylene cannula from freely moving rats at 5, 24, 48, and 96 h after administration of 1,000 mg/kg of vigabatrin. The free gamma-aminobutyric acid (GABA) level was elevated maximally (13.5-fold; p less than 0.01) at 24 h after injection. The homocarnosine (GABA-histidine) level also was increased (123%; p less than 0.01) at 24 h after injection, and its concentration remained at the same level for the next 3 days. Glycine and taurine concentrations had increased [31% (p less than 0.05) and 63% (p less than 0.01), respectively] at 5 h after injection. It is interesting that the levels of glutamate and aspartate increased [330% (p less than 0.05) and 421% (p less than 0.01), respectively] at 96 h after injection, the time when the free GABA level had returned to the baseline concentration and the vigabatrin level was 3% of the maximal concentration. The present study indicates that a single dose of vigabatrin in rats elevates levels of both the inhibitory and excitatory amino acids in CSF. However, the temporal profile of observed changes in relation to vigabatrin injection shows that neither the long-lasting elevation of GABA content nor the increase in glutamate and aspartate levels correlates with the level of vigabatrin in CSF. These findings suggest that the excitatory mechanisms are also augmented following acute administration of vigabatrin, especially when the content of GABA had decreased to the baseline level and the level of vigabatrin was low.

Long-term evaluation of once daily vigabatrin in drug-resistant partial epilepsy

The purpose of this study was to evaluate on an open basis the long-term efficacy and safety of vigabatrin in drug-resistant partial epilepsy as add-on therapy, administered on a once daily basis. Thirty-five patients entered the study. Twenty patients (57%) responded to therapy and are still on drug. This efficacy is in agreement with that seen in double-blind controlled studies on twice daily dose schedules. There did not appear to be any loss of efficacy with continued treatment at this dose regimen in patients responding favourably to the drug. The once daily dosing schedule was well tolerated and side effects were usually mild and always reversible. Vigabatrin seems to be a valuable therapeutic addition for patients with partial seizures resistant to standard anti-epileptic drugs.

Clinical relevance of measuring GABA concentrations in cerebrospinal fluid

Determination of GABA concentrations in human cerebrospinal fluid can be used to assess GABAergic activity in the central nervous system. As CSF free GABA concentrations may vary with age, sex, CSF fraction, and collection and storage conditions, careful attention to these factors are necessary to allow interpretation of results. Longitudinal studies to investigate the influence of pharmacological agents on CSF GABA have proven especially useful to define clinical biochemical activity and have been utilized to attribute the anti-epileptic action of vigabatrin, a selective inhibitor of GABA-transaminase, to its effects on brain GABA metabolism.

Pharmacokinetic effects of vigabatrin on cerebrospinal fluid amino acids in humans

A study was conducted to assess the impact of single dosing and different dosing intervals of vigabatrin [gamma vinyl GABA (GVG)] in 11 patients with drug-resistant complex partial seizures. Cerebrospinal fluid (CSF) concentrations of total GABA, free GABA, homocarnosine, homovanillic acid (HVA), GVG, and 5-hydroxyindolacetic acid were measured up to seven days after a single dose. GVG levels were maximal within 24 h, suggesting that GVG acts to inhibit GABA-transaminase, and may also increase biogenic amines.

Long-term evaluation of vigabatrin (gamma vinyl GABA) in epilepsy

In studies spanning more than 5 years, more than 1,100 patients with epilepsy have been treated with vigabatrin (gamma vinyl GABA, GVG). Sixty-two patients with partial seizures with secondary generalization took part in this trial: 41 patients continued in the trial after 19 months of treatment. After 36 months, the median percentage of baseline seizures was less than 20%. GVG is a very potent antiepileptic drug. It is well tolerated in humans. The side effects are few. Skin rash and other allergic reactions have rarely been seen. Tolerance to the sedative effect is in contrast to the lack of tolerance to the anti-epileptic effect.

Anticonvulsant and proconvulsant effects of inhibitors of GABA degradation in the amygdala-kindling model

The effects of three drugs, namely gamma-vinyl GABA (vigabatrin), gamma-acetylenic GABA, and aminooxyacetic acid, which increase brain GABA concentrations by irreversible inhibition of GABA degradation, were studied in amygdala-kindled rats. Vigabatrin 800 or 1,200 mg/kg i.p. 4 h after its administration, caused prolongation of behavioural seizures and electrographic afterdischarges recorded from the stimulated amygdala. One to three days after administration it dose dependently reduced seizure severity, seizure duration and afterdischarge duration in most animals. Determination of GABA levels in synaptosomes isolated from 12 brain regions of kindled rats 4 or 48 h after injection of 1,200 mg/kg vigabatrin indicated that the variable effects of this drug at different times after its administration could be related to differences in the time course of nerve terminal GABA increases in selective brain regions such as amygdala and corpus striatum. In contrast to vigabatrin, gamma-acetylenic GABA, 100 mg/kg i.p., reduced seizure severity in kindled rats as early as 4 h after its administration but afterdischarge duration increased significantly on subsequent days. Similar late increases in afterdischarge duration (and limbic seizure activity) after the time of maximum anticonvulsant effect had elapsed were also observed with vigabatrin, which could suggest that the anticonvulsant effect of such drugs is followed by withdrawal hyperexcitability. Aminooxyacetic acid, 20 mg/kg i.p., exerted no significant anticonvulsant effect in kindled rats but prolonged afterdischarge duration in several of the animals studied. The data suggest that GABA-T inhibitors, such as vigabatrin, differ from most antiepileptic drugs previously tested in the kindling model in that they may produce both anticonvulsant and proconvulsant effects at the same dose in the same animal as a function of time after administration.