Chronic toxicity studies with vigabatrin, a GABA-transaminase inhibitor

Visual field defects with vigabatrin: epidemiology and therapeutic implications

Vigabatrin is an antiepileptic drug (AED) that acts as a selective irreversible inhibitor of gamma-aminobutyric acid (GABA) transaminase. In 1997, 3 cases of severe symptomatic and persistent visual field constriction associated with vigabatrin treatment were described. During 1997 to 1998, similar concentric visual field constrictions were described in patients with drug-resistant epilepsy who were receiving vigabatrin concurrently with other AEDs. However, a study of patients treated with vigabatrin monotherapy alone showed that there was a causal relationship between vigabatrin treatment and the specific bilateral concentric visual field constriction. The Marketing Authorisation Holders survey (involving 335 vigabatrin recipients aged >14 years) indicated that 31% of patients [95% confidence interval (CI) 26 to 36%] had a visual field defect attributable to vigabatrin, compared with a 0% incidence of visual field defects (upper 95% CI 3%) in an unexposed control group. Other studies in adults have given similar overall prevalences, with a total of 169 of 528 patients diagnosed with vigabatrin-associated field defects (32%, 95% CI 28 to 36%). Male gender seems to be associated with an increase in the relative risk of visual field loss of approximately 2-fold. The pattern of defect is typically a bilateral, absolute concentric constriction of the visual field, the severity of which varies from mild to severe. Data gathered so far suggest that the cumulative incidence increases rapidly during the first 2 years of treatment and within the first 2 kg of vigabatrin intake, stabilising at 3 years and after a total vigabatrin dose of 3 kg. The prevalence of vigabatrin-associated field defects seems to be lower in children, but there are also methodological problems and greater variability in the assessment of visual fields in children. There is particular concern that the increased risk of the visual field defects will outweigh the benefit of the drug in patients who could be controlled with other AEDs. Vigabatrin should currently be used only in combination with other AEDs for patients with resistant partial epilepsy when all other appropriate drug combinations have proved inadequate or have not been tolerated. Regular visual field testing should be performed before the start of treatment and at regular intervals during treatment. Patients with pre-existent visual field defects due to other causes should not be treated with vigabatrin. Currently, the benefits of treating infantile spasms with vigabatrin monotherapy seem to outweigh the risks, but further prospective studies and follow-up of children receiving treatment are needed to evaluate the place of vigabatrin in this indication.

Vigabatrin visual toxicity: evolution and dose dependence

PURPOSE

To investigate the prevalence and prognosis of visual field defects (VFDs) in epilepsy patients with and without vigabatrin (VGB) treatment; to investigate the possible relationship between VFDs and cumulative VGB dose, and to characterise the evolution of VFDs.

METHODS

A cohort of 155 presurgical candidates who had undergone full-field Goldmann perimetry (GP) was studied, 99 (64%) of whom had been treated with VGB. All GPs were reevaluated in 1998 by one experienced examiner, blinded to medication. Duration of treatment and total VGB dose were related to perimetric results.

RESULTS

Twenty-five (16%) of the 155 patients had VFDs: Nineteen (19%) of the 99 VGB-treated patients, and six (11%) of the 56 patients unexposed to VGB. VGB-treated patients with VFDs had been treated significantly longer than those without VFDs. Cumulative VGB dose could be calculated for 84 patients. The prevalence of VFDs increased significantly with increasing total VGB-dose, from 4% in the 51 patients who had been exposed to <or=1 kg VGB, to 75% in the eight patients with a total dose of 3-5 kg of VGB. Sixteen VGB-treated patients were reexamined 2-10 years later. In the 12 where evaluation was possible, all still had VFDs, which had worsened in five cases (42%) and improved in none.

CONCLUSIONS

This study indicates a strong relationship between VFDs and duration and total dose of VGB. VFDs were irreversible and in a substantial percentage progressive. Similar VFDs may, however, also be found in patients unexposed to VGB. A model of the evolution of the VFDs in VGB toxicity is introduced, and a new simple visual field test is proposed.

Chronic elevation of brain GABA levels beginning two days after status epilepticus does not prevent epileptogenesis in rats

Vigabatrin (VGB) treatment is neuroprotective in various models of status epilepticus (SE) and delays the development of kindling via mechanisms that are assumed to relate to the elevation of GABA levels in the brain. Here, we tested the hypothesis that a chronic elevation of brain GABA levels obtained by VGB treatment prevents the development of spontaneous seizures (i.e. epilepsy) following SE in rats. Self-sustained SE (SSSE) was induced by stimulating the lateral nucleus of the amygdala. Two days later, chronic VGB (75 mg/kg/day) or saline treatment was started via subcutaneous osmotic minipumps. The development of spontaneous seizures was monitored once a week (24 h at a time) using video-EEG recording. Rats were perfused for histology either at the end of the 10-week drug treatment, or later at the end of an 8-week drug-free follow-up period. Before perfusion for histology, spatial learning and memory perform was tested in the Morris water-maze. Spontaneous seizures were observed in 55% (6/11) of the saline-treated and 73% (8/11) of the VGB-treated rats during the 10-week treatment period. Seizure frequency, severity, and duration were similar in VGB-treated rats and controls during and after the drug-treatment period. VGB treatment did not decrease neuronal damage in various temporal lobe regions or mossy fiber sprouting. VGB treatment also did not attenuate spatial learning or memory impairments. These findings indicate that the augmentation of GABAergic neurotransmission by VGB does not prevent the development of epilepsy when treatment is started 2 days after SE.

Adverse reactions to new anticonvulsant drugs

A lack of systematic pharmacoepidemiological studies investigating adverse drug reactions (ADRs) to anticonvulsants makes it difficult to assess accurately the incidence of anticonvulsant-related ADRs. Most of the available information in this regard stems from clinical trial experience, case reports and postmarketing surveillance, sources that are not, by any means, structured to provide precise data on adverse event epidemiology. For various ethical, statistical and logistical reasons, the organisation of structured clinical trials that are likely to provide substantial data on ADRs is extremely difficult. This review concentrates on current literature concerning serious and life-threatening ADRs. As with the older anticonvulsants, the majority of ADRs to newer anticonvulsants are CNS-related, although there are several that are apparently unique to some of these new drugs. Gabapentin has been reported to cause aggravation of seizures, movement disorders and psychiatric disturbances. Felbamate should only be prescribed under close medical supervision because of aplastic anaemia and hepatotoxicity. Lamotrigine causes hypersensitivity reactions that range from simple morbilliform rashes to multi-organ failure. Psychiatric ADRs and deterioration of seizure control have also been reported with lamotrigine treatment. Oxcarbazepine has a safety profile similar to that of carbamazepine. Hyponatraemia associated with oxcarbazepine is also a problem; however, it is less likely to cause rash than carbamazepine. Nonconvulsive status epilepticus has been reported frequently with tiagabine, although there are insufficient data at present to identify risk factors for this ADR. Topiramate frequently causes cognitive ADRs and, in addition, also appears to cause word-finding difficulties, renal calculi and bodyweight loss. Vigabatrin has been reported to cause seizure aggravation, especially in myoclonic seizures. There have been rare reports of other neurological ADRs to vigabatrin, such as encephalopathy, aphasia and motor disturbances. Vigabatrin-induced visual field constriction is the latest and most worrying ADR. Many questions regarding the nature of this potentially serious ADR remain unanswered, as no prospective controlled study examining the phenomenon has been published. Rare cases of behavioural ADRs and IgA and IgG2 deficiency associated with the use of zonisamide have been reported. However, relatively few patients so far have been exposed to this drug, and therefore more postmarketing information is required. The relatively late establishment of aplastic anaemia and hepatic failure as potentially fatal ADRs of felbamate, and of visual field constriction with vigabatrin, should serve as ample reminders that ADRs can appear at any time.

Effect of long-term vigabatrin administration on the immature rat brain

PURPOSE

To determine whether the neuropathologic changes produced by vigabatrin (VGB; gamma-vinyl GABA) administration in the developing rat brain are reversible.

METHODS

We injected rats daily with VGB (25-40 mg/kg/day, s.c.) from age 12 days for 2 weeks followed by 2 weeks of a drug-free period. Behavioral testing, magnetic resonance (MR) imaging, biochemical assays, and histologic technique were used to assess the adverse effect of VGB in developing brain and its reversibility.

RESULTS

At the end of 2 weeks' VGB administration: (a) there was a hyperactivity and a shortened latency to escape out of cool water; (b) white matter appeared hyperintense in T2 and diffusion-weighted MR images with 4-15% increases in T2; (c) microvacuolation, TUNEL-positive nuclei, and swollen axons were observed in the corpus callosum; (d) myelin staining indicated a reduction in myelination, as did the reduction in activities of myelin and oligodendrocyte-associated enzymes and the decrease in myelin basic protein on Western blots. Two weeks after stopping VGB administration: (a) MR images were normal, and microvacuolation was no longer in the white matter; (b) reduction in myelination reversed partially; (c) the T2 relaxation time remained elevated in the hypothalamus; and (d) the behavioral response remained abnormal.

CONCLUSIONS

Long-term VGB administration to young rats causes brain injury, which recovers partially on its cessation. The observed cell death, disrupted myelination, and alterations in behavior indicate a need for further safety assessment in infants and children.

GABA(A)-mediated toxicity of hippocampal neurons in vitro

In the present study, we examined whether the elevation of GABA by gamma-vinyl-GABA protects cultured rat fetal hippocampal neurons against toxicity induced by a 20-min incubation with 100 microM L-glutamate. Neither a 24-h pretreatment nor posttreatment with gamma-vinyl-GABA (100 microM) had any neuroprotective effects, as determined by counting microtubule-associated protein-2 positive cells and lactate dehydrogenase assay 24 h after the glutamate treatment. Unexpectedly, gamma-vinyl-GABA alone induced a 20% loss of microtubule-associated protein-2-positive cells in a culture that was grown in medium containing 25 mM KCl. The toxic effect of gamma-vinyl-GABA was mimicked by a 24-h treatment with GABA (100 microM) and the GABA(A) receptor agonist, muscimol (10 microM), but not the GABA(B) receptor agonist, baclofen (10 microM). The GABA(A) receptor antagonist, bicuculline (10 microM), protected against gamma-vinyl-GABA and GABA-evoked toxicity. Neither gamma-vinyl-GABA nor GABA was toxic in culture medium containing 15 mM KCl. These data indicate that, under depolarizing conditions, an increased GABA level is toxic for a subpopulation of developing hippocampal neurons in vitro. The effect is GABA(A) receptor-mediated. These data provide a new view for understanding neurodegenerative processes, and raise a question of the safety of therapies aimed at increasing GABA concentration following brain insults, especially in immature brains.

Vigabatrin-associated visual field defects in children

PURPOSE

Vigabatrin (Sabril), a drug that blocks GABA transaminase, has been used in the treatment of epilepsy since 1989. There have been reports of irreversible constriction of the visual field in adult patients related to vigabatrin (VGB) therapy, resulting in reduced VGB usage in adults. Although used as a second or third line agent in adults, in children it is often considered as a first line treatment for several subgroups of seizures in spite of there being no way, in the majority of cases, to monitor visual fields. Some of these children have a pre-existing visual field defect as part of their primary disorder. We aimed to identify whether visual field loss due to VGB was occurring in our hospital.

METHODS

We have studied the results of ophthalmic examination in 14 children on VGB at Great Ormond Street Hospital who were able to perform Goldmann visual fields.

RESULTS

Ten of the 14 patients had constriction of their visual fields attributed to VGB. In addition there were 2 patients with suspicious visual field defects thought to be due to VGB. There was pre-existing visual pathway damage in 4 cases and in 2 of these optic disc pallor increased in association with constricted visual fields. However, the optic discs were normal in 7 patients in spite of visual field constriction. Visual acuity was generally normal in spite of gross visual field constriction.

CONCLUSIONS

We believe that VGB should be used with great caution where there is pre-existing visual pathway damage. In other cases the benefits should be considered in relation to the risks, which include irreversible visual field damage. At present visual fields can only be monitored by perimetry, which is often not possible in children with epilepsy.

Visual electrophysiological effect of a GABA transaminase blocker

Vigabatrin is an antiepileptic drug for the treatment of partial seizures. The anticonvulsant effect is achieved by irreversible inhibition of the enzyme GABA-transaminase which catalyses the inactivation of GABA. Vigabatrin has been associated with visual field loss and electrophysiological abnormalities. The purpose of the study was to determine any alterations in normal volunteers of the visual field and the visual electrophysiology resulting from a short exposure to vigabatrin. A three-way, double-blind study of placebo, carbamazepine and vigabatrin was undertaken at baseline and on days two, four and nine. Seven subjects completed all three cycles and 14 subjects (six females and eight males; mean age 27.3 years SD 6.7) completed at least one cycle. Static threshold automated perimetry comprised Humphrey Visual Field Analyzer Programs 30-2 and 30/60-2. Electro-oculography and electroretinograms were performed with undilated pupils using the Medelec Ganzfeld stimulator GS2000. The visual field was unaffected by placebo, carbamazepine or vigabatrin. The group mean amplitudes and latencies for the scotopic ERG, 30 Hz flicker ERG and the oscillatory potentials remained unchanged for any cycle. The group mean photopic ERG b-wave latency increased from baseline (p<0.05); no significant change occurred with carbamazepine or placebo. The group mean Arden Index for vigabatrin decreased from baseline to day 9 (p<0.01); no significant differences were present for carbamazepine or placebo. Vigabatrin has a rapid effect on both the photopic ERG and the EOG; however, the changes merely reflect alterations in retinal GABA levels secondary to concomitant blocking of GABA transaminase by existing vigabatrin therapy.

The potential for vigabatrin-induced intramyelinic edema in humans

PURPOSE

Vigabatrin (Sabril, Hoechst Marion Roussel) is an antiepilepsy drug (AED) presently marketed in 64 countries for the treatment of partial and secondarily generalized seizures. Vigabatrin (VGB) is marketed in a subset of these countries for the treatment of infantile spasms. Clinical experience in humans has shown that VGB provides effective seizure control with a wide margin of safety. However, animal toxicity studies raised concern when prolonged administration of VGB was shown to induce intramyelinic edema (IME) in some laboratory animal species.

METHODS

Animal and human data were reviewed with respect to the potential for VGB-induced IME. Surveillance of patients receiving VGB in clinical trials or by prescription has been conducted for >15 years to identify patients developing clinical abnormalities that might be IME related.

RESULTS

The histologic lesions of VGB-induced IME in animals are reliably reproduced and correlate with changes in multimodality evoked potentials (EPs) and magnetic resonance imaging (MRI). Numerous studies of the effects of VGB on EP and MRI in epilepsy patients have demonstrated no clear-cut IME-related changes in these modalities. Additionally, autopsy and surgical brain samples from VGB-treated patients have been scrutinized for potential IME histopathology. In an estimated 350,000 patient-years of VGB exposure (approximately 175,000 patients exposed for 2 years at an average dose of 2 g/day), no definite case of VGB-induced IME has been identified.

CONCLUSIONS

Comprehensive review of a variety of sources of data failed to identify any definite case of IME in humans treated with VGB.

Visual field defects associated with vigabatrin therapy

OBJECTIVE

To estimate the prevalence of visual field defects in patients taking the anticonvulsant drug vigabatrin and to characterise the features of visual dysfunction found.

METHODS

Thirty three unselected patients attending neurology and epilepsy clinics were identified as taking vigabatrin and asked to attend for neuro-ophthalmic evaluation. A control group of 16 patients with epilepsy unexposed to vigabatrin was also evaluated. Visual fields were examined by static perimetry using a Humphrey field analyser. Patients underwent detailed ophthalmic examination, various blood tests, and brain MRI where necessary. Visual evoked responses (VERs), electro-oculograms (EOGs), and electroretinograms (ERGs) were recorded.

RESULTS

Of 31 assessable patients treated with vigabatrin, 16 (52%) had definitely abnormal visual fields, nine (29%) had fields that were inconclusive, four (13%) had normal fields, and two (6%) proved unable to cooperate with testing. In four patients some plausible cause was found for the field abnormality leaving 12 patients (39%) in whom a definite bilateral field defect was found, possibly caused by vigabatrin treatment. Of 16 control patients none had definitely abnormal fields, 12 (75%) had normal fields, and four (25%) had fields that were inconclusive. The field defects associated with vigabatrin treatment showed a characteristic pattern of concentric peripheral field loss with temporal and macular sparing. The VERs and ERGs were normal. The EOG Arden Index was reduced in patients taking vigabatrin, although this returned towards normal when vigabatrin was stopped, even in the presence of persistent field defects. Multifocal ERGs recorded in two patients were abnormal, showing marked reduction in amplitude of the peripheral focal ERG.

CONCLUSIONS

Treatment with vigabatrin was associated with a high prevalence of peripheral visual field defects. This seemed to be the result of a toxic effect of vigabatrin on the retina and seemed to persist if the drug was withdrawn.

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.

Dose-Response Study of Vigabatrin as add-on therapy in patients with uncontrolled complex partial seizures

PURPOSE

This placebo-controlled, randomized, double-blind, multicenter study examined the efficacy and safety of three daily doses of vigabatrin (VGB; 1, 3, or 6 g) as add-on therapy in 174 patients with previously uncontrolled complex partial seizures with or without secondary generalization.

METHODS

A 12-week pretreatment assessment period was followed by drug therapy with a 6-week titration period and a 12-week maintenance phase.

RESULTS

VGB doses of 3 and 6 g/day reduced median monthly frequency of seizures by 4.3 and 4.5 seizures, respectively, compared with 0.2 seizures for placebo (p = 0.0001). The percentages of patients classified as therapeutic successes (> or =50% reduction in seizure frequency) were 7% for placebo and 24, 51, and 54% for patients taking daily VGB doses of 1, 3, and 6 g, respectively; the comparison with placebo was significant for all treatment groups. The linear trend for dose response was highly significant (p< or =0.0001) for both median monthly seizure frequency and therapeutic success. Vigabatrin was well tolerated, causing no clinically significant changes in laboratory parameters, brain magnetic resonance imaging, evoked potentials, cognitive function, or psychosocial tests. Fatigue, drowsiness, and dizziness were the most common treatment-related adverse events in all treatment groups. Dropouts due to adverse events were higher in the 6-g/day group.

CONCLUSIONS

VGB was significantly more effective than placebo as add-on therapy in reducing seizure frequency. VGB at 3 and 6 g/day produced the best efficacy: however, adverse events may limit the use of the 6-g/day dose in some patients.

Effects of vigabatrin treatment on status epilepticus-induced neuronal damage and mossy fiber sprouting in the rat hippocampus

Selective neuronal damage and mossy fiber sprouting may underlie epileptogenesis and spontaneous seizure generation in the epileptic hippocampus. It may be beneficial to prevent their development after cerebral insults that are known to be associated with a high risk of epilepsy later in life in humans. In the present study, we investigated whether chronic treatment with an anticonvulsant, vigabatrin (gamma-vinyl GABA), would prevent the damage to hilar neurons and the development of mossy fiber sprouting. Vigabatrin treatment was started either 1 h, or 2 or 7 days after the beginning of kainic acid-induced (9 mg/kg, i.p.) status epilepticus and continued via subcutaneous osmotic minipumps for 2 months (75 mg/kg per day). Thereafter, rats were perfused for histological analyses. One series of horizontal sections was stained with thionine to estimate the total number of hilar neurons by unbiased stereology. One series was prepared for somatostatin immunohistochemistry and another for Timm histochemistry to detect mossy fiber sprouting. Our data show that vigabatrin treatment did not prevent the decrease in the total number of hilar cells, nor the decrease in hilar somatostatin-immunoreactive (SOM-ir) neurons when SOM-ir neuronal numbers were averaged from all septotemporal levels. However, when vigabatrin was administered 2 days after the onset of status epilepticus, we found a mild neuroprotective effect on SOM-ir neurons in the septal end of the hippocampus (92% SOM-ir neurons remaining; P < 0.05 compared to the vehicle group). Vigabatrin did not prevent mossy fiber sprouting regardless of when treatment was started. Rather, sprouting actually increased in the septal end of the hippocampus when vigabatrin treatment began 1 h after the onset of status epilepticus (P < 0.05 compared to the vehicle group). Our data show that chronic elevation of brain GABA levels after status epilepticus does not have any substantial effects on neuronal loss or mossy fiber sprouting in the rat hippocampus.

A comparison of the neuropathological effects of vigabatrin and carbamazepine in patients with newly diagnosed localization-related epilepsy using MR-based cerebral T2 relaxation time measurements

BACKGROUND

Magnetic resonance (MR)-based T2 relaxation time measurement is a sensitive technique to detect neuropathological changes such as intramyelinic edema in vivo.

OBJECTIVE

To determine whether vigabatrin (VGB) causes an increase in T2 relaxation time in patients with newly diagnosed localization-related epilepsy over 1 year.

METHODS

Patients with newly diagnosed localization-related epilepsy who participated in a VGB-carbamazepine (CBZ) monotherapy trial were included. All were scanned on a 1.5 T Siemens SP63 Magnetom scanner. T2 maps of the brain were obtained at baseline and at follow-up 1 year later. Nine control subjects had repeated hippocampal T2 maps with a median interval of approximately 2 years.

RESULTS

23 patients (12 on VGB and 11 on CBZ) were included. There were no increased T2 relaxation times in the VGB treated group at follow-up and no significant differences between the two antiepileptic drug groups. There was a trend for the temporal and frontal white matter T2 relaxation times to be lower on follow-up in the patients compared to the control subjects.

CONCLUSION

The findings do not suggest that intramyelinic edema occurs in patients taking monotherapy VGB for 1 year.

Low-dose vigabatrin (gamma-vinyl GABA)-induced damage in the immature rat brain

The antiepileptic drug, vigabatrin, inhibits GABA transaminase, thus elevating GABA levels in the brain. In adult animal experiments, high-dose (200 mg/kg/day) chronic vigabatrin administration is associated with potentially reversible myelin vacuolation, a phenomenon not documented in humans. We hypothesized that vigabatrin might adversely affect myelination in the developing brain. Rats were given vigabatrin in doses comparable to those used clinically (15-50 mg/kg/day), from age 12 to 16 days. The rats were killed at age 19-20 days. We observed decreased myelin staining in the external capsule, axonal degeneration in white matter, evidence of glial cell death in the white matter, and reactive astrogliosis in the frontal cortex. We did not detect myelin vacuolation. These findings indicate that vigabatrin can have adverse and potentially irreversible effects on the developing rat brain. The mechanism of damage could be direct toxicity of vigabatrin or an indirect effect mediated through elevated GABA levels. Vigabatrin has been recommended as a treatment for some forms of childhood epilepsy; therefore, further studies are needed to assess the risks in children.

Overview of the safety of newer antiepileptic drugs

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

New drugs for the treatment of epilepsy

OBJECTIVES

This article will review current data on the metabolism, interactions, methods of analysis, and adverse effects observed with the use of new anticonvulsant drugs. The role of the laboratory in the provision of therapeutic drug monitoring for these drugs is discussed.

CONCLUSION

Certain of the newer anticonvulsant drugs require therapeutic drug monitoring for their optimal use in the treatment of epileptic seizures. The requirement for therapeutic drug monitoring has not been established for some of these drugs. Many of the newer anticonvulsant drugs, including lamotrigine, felbamate, vigabatrin, and zonisamide, interact clinically with established drugs, such as phenytoin, phenobarbital, carbamazepine, and valproic acid. Introduction of these new drugs will result in the need for more frequent monitoring of the established drugs during polytherapy. The need for a drug-monitoring service for anticonvulsant drugs overall will continue, due to the frequency of drug interactions, the incidence of adverse effects, and concerns about compliance with the dosing regimen in these patients.

Status epilepticus during vigabatrin treatment: a report of three cases

Vigabatrin (gamma-vinyl-GABA or GVG) is an irreversible inhibitor of gamma-aminobutyric acid transaminase (GABA-T), which is an enzyme responsible for gamma-aminobutyric acid (GABA) catabolism. Inhibition of GABA catabolism increases brain concentration of GABA, a neural inhibitor. GVG has been found to be a potent new anti-epileptic drug, especially in the treatment of refractory epilepsy, in particular of complex partial seizures. Three patients who developed a severe status epilepitus while on GVG treatment are reported. A possible proconvulsive effect of GVG is hypothesized, which might result from disinhibition in the nigro-collicular pathway due to increased GABA-levels.

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.

Use of ex vivo magnetic resonance imaging to detect onset of vigabatrin-induced intramyelinic edema in canine brain

Vigabatrin (VGB) causes intramyelinic edema (microvacuolation) in brain of dogs and rodent, which has encouraged development of noninvasive methods to monitor for this effect during clinical trials. We report the qualitative ex vivo magnetic resonance imaging (MRI) changes observed in a neuropathology study in dogs to detect time of onset and regression of VGB-induced intramyelinic edema. Beagles were randomly assigned to 18 groups of 6 dogs per group and administered vigabatrin orally (p.o.) at a dose of 300 mg/kg/day (2 males, 2 females) or placebo (1 male, 1 female). Animals were killed and examined at weekly intervals during the 12 weeks of treatment and at 1, 2, 4, 8, 12, and 16 weeks after discontinuation of drug treatment. Myelin microvacuolation in thalamus, hypothalamus, and fornix were noted histologically after 4-5 weeks of treatment. Increases in MRI T2 intensity were observed in hypothalamus after 4 weeks and in thalamus and columns of the fornix after 7 weeks. Both MRI T2 intensity and microvacuolation continued to increase during 12-week VGB treatment. When VGB treatment was discontinued after 12 weeks, both MRI T2 intensity and microvacuolation began to decrease. Sixteen weeks after VGB discontinuation, histopathology had returned to normal and MRI examination demonstrated a marked trend toward reversal of the increased T2 signal intensity. MRI thus has potential as a noninvasive surveillance technique in certain experimental and clinical conditions associated with intramyelinic edema.

Vigabatrin

The discovery of gamma-aminobutyric acid (GABA) as the first major inhibitory neurotransmitter and a program exploring the use of enzyme inhibition as a therapeutic tool provided the basis for the conception of vigabatrin (VGB, Sabril). This molecule, an analogue of GABA, has a highly specific activity as an enzyme-activated irreversible inhibitor of GABA-transaminase causing several-fold increases in the concentration of brain GABA. In animal models for epilepsy, it was found to have a rather different spectrum of activity than conventional antiepileptic drugs (AEDs). The clinical development of VGB was delayed by the finding of focal areas of reversible microvacuolation in the white matter of the brains of rodents and dogs. An extensive human safety program has confirmed that this finding is species specific and does not occur in humans. Clinically, VGB is well tolerated and has been shown to be specially effective in the management of partial seizures that have failed to respond to other AEDs. In most controlled studies, about 50% of patients with previously uncontrolled seizures have a 50% reduction in frequency and about 4-5% become seizure-free. In children, it also appears to be especially effective in the management of infantile spasms as well as in partial seizures. VGB offers a significant improvement in the management of epilepsy and is now under development as a first-line agent.

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.

[Quantitative clinical and EEG measures of epileptogenesis in temporal lobe epilepsy]

In this controlled, prospective and partially blind study two groups of patients with temporal lobe epilepsy were evaluated, respectively with good and bad prognosis. Measurements of epileptogenesis were based on frequency of seizures, and on epiletogenic electroencephalographic abnormalities obtained from scalp electrodes over the temporal lobes. The results were analysed by non-parametric analysis of variance, comparison of groups and analysis of correlation. The results indicated the temporal lobe groups were similar to at least one of the control groups in age, sex, educational and social level, therapeutic regime, age at onset and length of history of epilepsy. The quantitative measurements showed a global difference between the group of temporal lobe with bad and good prognosis, reaching statistical significance in clinical epileptogenesis, and a trend towards greater epileptogenesis on the electroencephalogram, in the same group of patients. The results indicate the experimental usefulness of some of the original measurements used in the study, but also their problems. A review of the literature is carried out.

Evaluation of the neurotoxic potential of chemicals in animals

This review provides a scientific view on how to evaluate effectively the neurotoxic potential of chemicals in order to provide adequate safeguards for human health. Detection of compounds that may cause direct, persistent, adverse effects on the nervous system should be given the most critical attention. Evaluation of the neurotoxic potential of a chemical should include descriptions of functional and morphological effects as well as the determination of the dose response, no-observed-effect level, time course and reversibility of effects. Evaluation of the nervous system in the context of standard toxicity studies that use a variety of species and study durations are appropriate screening tests (Tier 1) for the detection of potential neurotoxicity. Studies specifically designed to assess neurotoxicity (Tier 2) should be performed with chemicals for which there is an indication of neurotoxic potential and where the available data are inadequate for risk assessment. Tier 2 studies should evaluate the function and structure of the nervous system by comprehensive clinical examinations and neuropathological assessment. These studies may be conducted in conjunction with standard toxicity studies so that any potential neurotoxicity can be interpreted in the context of other systemic toxicity. More specific neurotoxicity tests (Tier 3) may be necessary for advanced characterization of discovered neurotoxicants.

Quantitative MR relaxometry study of effects of vigabatrin on the brains of patients with epilepsy

Neurotoxic changes have been found in the brains of dogs and rats treated with the antiepileptic drug vigabatrin, and these can be demonstrated in vivo by MRI. Quantification of T2 signal by relaxometry is more sensitive than visual assessment of T2-weighted images in revealing changes in T2 signal. We have therefore undertaken a quantitative MR study of 45 patients with refractory partial seizures during a prospective, randomised, double-blind trial of vigabatrin (1.5 g twice daily), followed by open treatment. T2 relaxometry was performed during a baseline period, after 20 weeks vigabatrin or placebo treatment and again in those who continued the drug for at least 35 weeks. Twenty weeks' vigabatrin treatment was not associated with a significant change in T2 relaxation time in any brain area. There were no significant T2 signal changes in the follow-up study and no correlation between change in T2 and duration of vigabatrin treatment. There was no quantitative MR evidence of vigabatrin-related changes in the white matter of these patients similar to those which have been found in animals treated with the drug.

Vigabatrin-induced lesions in the rat brain demonstrated by quantitative magnetic resonance imaging

Rats treated with 250 mg/kg/day vigabatrin showed lesions detected by magnetic resonance imaging (MRI) in the cerebellar white matter in vivo. No lesions were seen in any control animal. As well as these visually apparent lesions, quantitative T2 relaxation time measurements showed a 12 ms increase in cerebellar white matter from 66 +/- 4 ms (SD, n = 5) to 78 +/- 2 ms (SD, n = 7). This region, as expected from previous studies, showed microvacuolation on post-mortem pathology. Additionally, significant increases in T2 relaxation times of 4-9 ms were found in the cerebral cortex, thalamus and hippocampus. Microvacuolation was not detected by post-mortem histopathology in the cerebral cortex or hippocampus, however, immunohistochemical staining for glial fibrillary acidic protein and for macrophages (ED1) showed reactive astrocytes (gliosis) and in more severe cases, microglial proliferation in these regions; such changes were also seen in association with the microvacuoles. No T2 increase was found in the cerebellar grey matter or olfactory bulbs. MRI techniques, including T2 relaxometry, are therefore sensitive for detecting vigabatrin-induced changes, including reactive astrocytosis, microglial proliferation and vacuolation in the rat brain. These results suggest that quantitative MRI should be a useful method for evaluating whether vigabatrin has neuropathological effects when given to patients.

Nuclear magnetic resonance detection of increased cortical GABA in vigabatrin-treated rats in vivo

1H Nuclear magnetic resonance ([1H]NMR) spectroscopy was used to detect elevation of gamma-aminobutyric acid (GABA) in rat brain after administration of the antiepileptic drug vigabatrin (VGB). Rats were treated for 3 weeks with VGB added to their drinking water to deliver a dose of 250 mg/kg body weight per day. NMR spectroscopy was performed noninvasively in vivo, and a GABA concentration of 6.0 +/- 2.3 mmol/kg wet weight (mean +/- SD, n = 5) was measured. GABA could not be detected in control animals in vivo. Postmortem GABA levels of 1.3 +/- 0.5 and 4.5 +/- 1.0 mmol/kg (mean +/- SD, n = 5) were measured in perchloric acid extracts of frozen brain from control and treated animals, respectively. Noninvasive measurement of increased cerebral GABA should allow detailed studies of the pharmacology of GABA-increasing drugs in vivo. With future developments, these measurements may be feasible in human subjects.

Effect of vigabatrin on sedation and cognitive function in patients with refractory epilepsy

Twenty-four patients with refractory epilepsy on one or more antiepileptic drugs were given additional vigabatrin (1 g twice daily for six weeks, followed by 1.5 g twice daily for a further six weeks) and matched placebo in a double blind, randomised, crossover study. A battery of neuropsychological tests was administered at baseline and at weeks two, six and 12 of both treatment periods. No significant differences were found between vigabatrin and placebo at any time point for any of the objective tests of cognitive function. Patients, however, reported a greater degree of sedation after two and six weeks on vigabatrin than during the equivalent placebo phase (p < 0.01), although no such difference was apparent at 12 weeks. Follow up over a mean of 14.75 months in 12 responders, who continued on vigabatrin, revealed a significant improvement (all p < 0.01) on each of three composite scales (three psychomotor tests, four memory tests, three self rating scales) compared with their scores during the double blind trial. Vigabatrin did not cause cognitive impairment either acutely or in the long term. Phased introduction, however, seems a prudent policy to allow tolerance to early subjective sedation.

Sequential neuropathology of dogs treated with vigabatrin, a GABA-transaminase inhibitor

Vigabatrin (Sabril) is a gamma-aminobutyric acid-transaminase (GABA-T) inhibitor that is effective in the treatment of certain types of drug-resistant or uncontrolled epilepsy but is known to cause microscopic vacuolation (intramyelinic edema) in the brains of treated rats, mice, and dogs. The effects of high oral doses (300 mg/kg/day) of vigabatrin administered orally to Beagle dogs were studied during treatment weeks 1-12 and recovery weeks 13, 14, 16, 20, 24, and 28. Emesis, loose stools, and anorexia and 3 drug-related deaths were observed during the first 4 wk of treatment but were virtually nonexistent thereafter because of adaptation to the drug aided by food supplementation. In more sensitive areas of the brain (columns of the fornix, thalamus, and hypothalamus), microscopic quantitative differences between background vacuolation in controls and drug-related vacuolation in treated dogs could be delineated after 4 wk, generally reached highest levels of severity between 8 and 12 wk, and were reversible upon cessation of dosing. Inhibition of brain GABA-T and elevation of brain GABA were noted after 1 wk of treatment. During the course of treatment vigabatrin ranged between 4-17 nmol/ml (plasma) and 42-1,570 nmol/ml [cerebrospinal fluid (CSF)] while CSF GABA concentrations were 4-32 nmol/ml (treated dogs) and 0.1-0.6 nmol/ml (control dogs). Although the cause of vigabatrin-induced microvacuolation is unknown, the results of the study demonstrated that GABA-T inhibition with subsequent GABA elevation occurred within the first week of treatment and was followed by the onset of detectable microvacuolation several weeks later.

Vigabatrin (gamma-vinyl-GABA): neuropathologic evaluation in five patients

We performed neuropathologic examination of cerebral cortex specimens from 4 patients who underwent epilepsy surgery and the brain of 1 patient who died suddenly. All had severe epilepsy and had received gamma-vinyl-GABA (GVG, vigabatrin) for 3-5.5 years. Neither the surgically resected temporal lobe specimens nor the frontal and temporal lobes autopsy specimens showed abnormal white matter vacuolation.

Absence of cortical white matter changes in three patients undergoing long-term vigabatrin therapy

Chronic administration of the experimental antiepileptic drug vigabatrin (gamma-vinyl GABA) to animals has been shown to cause dose-dependent neuropathological changes characterized by a microvacuolation in specific white matter tracts. This finding has led to some concern as to whether similar pathologic changes might occur in patients taking this medication. Here we report on analysis of tissue specimens taken during neurosurgery from three patients undergoing chronic vigabatrin therapy (4 g/day). The first patient, a 34-year-old woman, had taken vigabatrin for 2 years prior to surgery, the second, a 50-year-old man, had taken the drug for 1 year, and a 34-year-old man had taken the drug for 5.3 years. For comparison, similar specimens were taken from three other patients not taking vigabatrin who were undergoing surgery for intractable epilepsy. Specimens from each subject were prepared in an identical manner and examined with light and electron microscopy. All specimens were examined in a blinded fashion. There was some minor nonspecific myelinic splitting seen in both controls and vigabatrin-treated patients but there was no evidence for any drug-induced lesions similar to that seen in experimental animals.

Effects of single and multiple increasing doses of vigabatrin on brain GABA metabolism and correlation with vigabatrin plasma concentration

The effects of increasing (50-1600 mg/kg/day) doses of vigabatrin (GVG) both as single doses and after 8 or 28 days of treatment have been studied in 19 groups of 10 adult Wistar rats. The parameters studied were brain gamma-aminobutyric acid-transaminase (GABA-T) activity, GABA concentration and L-glutamate decarboxylase (GAD) activity. Single increasing doses of GVG progressively inhibited GABA-T activity, but a residual activity of about 40% was observed with the highest doses. GABA concentration increased in a dose-dependent manner but a ceiling was not reached. GAD activity was slightly inhibited at low doses and stimulated at high ones. When treatment was continued for 8 days, more marked effects of GVG on GABA-T and GABA, a more severe toxicity and higher GVG plasma concentrations were observed. GAD was inhibited instead of stimulated by high GVG doses. After 28 days of treatment the effects of GVG on GABA-T and GABA were similar to those after 8 days. However, toxic effects decreased and lower GVG plasma concentrations were found.

IN CONCLUSION

(a) the more marked brain GABAergic effects observed after 8 days of treatment with GVG may explain the greater anticonvulsant effects observed by others in animals, and (b) GVG plasma concentrations correlate well with changes in brain GABA-T and GABA, and may partly explain changes in the effects of GVG related to the length of treatment.

Pharmacology of vigabatrin

Vigabatrin (gamma-vinyl GABA) is a relatively new antiepileptic drug. Vigabatrin increases the concentration of gamma-aminobutyric acid (GABA) in the brain by inhibiting the major GABA metabolizing enzyme, GABA transaminase. Controlled clinical trials have demonstrated an excellent antiepileptic effect of vigabatrin, especially in the treatment of partial epilepsies. Long-term evaluations have shown no signs of tolerance development. Vigabatrin decreases the plasma concentration of phenytoin during concomitant therapy, the only drug with which an interaction seems to occur. In general, vigabatrin is well tolerated. Psychotic reactions occur in 3-6% of patients. Other frequent side effects are sedation and weight increase. Chronic vigabatrin intoxication in animals caused development of intramyelinic oedema, appearing as microvacuoles in brain white matter. No microvacuolation has been observed in humans, even after long-term treatment. Vigabatrin seems a very valuable new antiepileptic drug.

Development of tolerance to the anticonvulsant effect of vigabatrin in amygdala-kindled rats

The anticonvulsant, adverse and biochemical effects of the novel antiepileptic drug vigabatrin (gamma-vinyl GABA), which increases GABA (gamma-aminobutyric acid) levels by inhibition of the GABA degrading enzyme GABA aminotransferase, were examined in amygdala-kindled rats after acute and chronic administration. Vigabatrin proved to be a potent anticonvulsant drug at acute doses (100-200 mg/kg), but during chronic administration, the anticonvulsant activity of the treatment was lost already in the second week of treatment. Tolerance also developed to the adverse effects, i.e. hypothermia, sedation and motor impairment. Determination of vigabatrin in plasma indicated that tolerance was not due to declining drug levels. Furthermore, determination of endogenous amino acids in plasma showed that GABA levels were highly elevated throughout the period of treatment, although the extent of GABA accumulation decreased in the second week. After cessation of chronic treatment with vigabatrin, there was no clear indication of withdrawal symptoms, except a prolonged seizure or afterdischarge duration in experiments with 100 mg/kg per day. The data suggest that chronic treatment with vigabatrin may be associated with a loss of anticonvulsant efficacy, at least when the drug is given as monotherapy.