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

Biochemical mechanisms in pathogenesis of infantile epileptic spasm syndrome

The molecular mechanisms leading to infantile epileptic spasm syndrome (IESS) remain obscure. The only common factor seems to be that the spasms are restricted to a limited period of infancy, during a certain maturational state. Here the current literature regarding the biochemical mechanisms of brain maturation in IESS is reviewed, and various hypotheses of the pathophysiology are put together. They include: (1) imbalance of inhibitory (NGF, IGF-1, ACTH, GABA) and excitatory factors (glutamate, nitrites) which distinguishes the different etiological subgroups, (2) abnormality of the hypothalamic pituitary adrenal (HPA) axis linking insults and early life stress, (3) inflammation (4) yet poorly known genetic and epigenetic factors, and (5) glucocorticoid and vigabatrin action on brain development, pinpointing at molecular targets of the pathophysiology from another angle. An altered maturational process may explain why so many, seemingly independent etiological factors lead to the same clinical syndrome and frequently to developmental delay. Understanding these factors can provide ideas for novel therapies.

Neuropathology Evaluation in Juvenile Toxicity Studies in Rodents: Comparison of Developmental Neurotoxicity Studies for Chemicals With Juvenile Animal Studies for Pediatric Pharmaceuticals

The developmental neuropathology examination in juvenile toxicity studies depends on the nature of the product candidate, its intended use, and the exposure scenario (eg, dose, duration, and route). Expectations for sampling, processing, and evaluating neural tissues differ for developmental neurotoxicity studies (DNTS) for chemicals and juvenile animal studies (JAS) for pediatric pharmaceuticals. Juvenile toxicity studies typically include macroscopic observations, brain weights, and light microscopic evaluation of routine hematoxylin and eosin (H&E)-stained sections from major neural tissues (brain, spinal cord, and sciatic nerve) as neuropathology endpoints. The DNTS is a focused evaluation of the nervous system, so the study design incorporates perfusion fixation, plastic embedding of at least one nerve, quantitative analysis of selected brain regions, and sometimes special neurohistological stains. In contrast, the JAS examines multiple systems, so neural tissues undergo conventional tissue processing (eg, immersion fixation, paraffin embedding, H&E staining only). An "expanded neurohistopathology" (or "expanded neuropathology") approach may be performed for JAS if warranted, typically by light microscopic evaluation of more neural tissues (usually additional sections of brain, ganglia, and/or more nerves) or/and special neurohistological stains, to investigate specific questions (eg, a more detailed exploration of a potential neuroactive effect) or to fulfill regulatory requests.

Neuroprotective Effects of Vigabatrin on Spinal Cord Ischemia-Reperfusion Injury

OBJECTIVE

Spinal cord ischemia is a serious and catastrophic clinicopathologic condition. Despite studies reported over the last 20 years, alternative and efficient treatment options remain unclear. We examined the neuroprotective effects of vigabatrin on a spinal ischemia-reperfusion model.

METHODS

We divided 24 New Zealand rabbits into 4 groups (control, ischemia reperfusion, and low-dose and high-dose vigabatrin). The control group underwent only abdominal surgery, whereas an abdominal aortic cross-clamp model of spinal ischemia was performed in the other groups. Clips were removed after 30 minutes and 50 and 150 mg/kg vigabatrin was administered intraperitoneally to the low-dose and high-dose groups, respectively. Neurologic examination was performed for 48 hours, after which the rabbits were sacrificed and a blood sample obtained. Biochemical examination of malondialdehyde, advanced oxidation protein products, total nitric oxide, and glutathione levels and superoxide dismutase activities in plasma and tissue sample, and histopathologic examination of the spinal cord were performed and statistical results compared between the groups.

RESULTS

Low-dose vigabatrin had statistically significant effects of neuroprotection on spinal ischemia. Although high-dose vigabatrin had similar effects, the results were not statistically significant for all parameters of biochemical analysis. In addition, histopathologic examination showed some toxic effects of high-dose vigabatrin.

CONCLUSIONS

Neuroprotective effects of vigabatrin are shown. For clinical use, further studies are needed.

Neurobehavioral effects of vigabatrin and its ability to induce DNA damage in brain cells after acute treatment in rats

RATIONALE

Vigabatrin (VGB) is a drug indicated mostly for the treatment of spasms in childhood and West's syndrome patients. This drug inhibits irreversibly the enzyme GABA-transaminase (GABA-T), increasing GABA concentrations and enhancing GABAergic neurotransmission in the brain, which is known to induce behavioral changes.

OBJECTIVES

The aims of this study were to evaluate the effects of VGB in the short-term memory (STM), long-term memory (LTM), motivation, locomotion, and exploratory behavior tests and to detect deleterious or protective effects on DNA in target tissues of the drug.

METHODS

Male Wistar rats were treated with a single dose of VGB (100, 250, or 500 mg/kg) or saline solution before the inhibitory avoidance and open-field tasks. DNA damage was evaluated using the alkaline comet assay in peripheral blood, cerebral cortex, and hippocampus after behavioral testing.

RESULTS

There was no significant difference in the inhibitory avoidance task between the treated groups and the saline group. In all tested doses, VGB reduced the number of rearings in the open-field task. Besides, VGB 500 mg/kg affected locomotion, though it was not able to induce any DNA damage.

CONCLUSIONS

VGB did not affect STM and LTM, but the drug impaired the exploration and locomotion likely associated with its sedative effect. In addition, no DNA damage in cortex and hippocampus was detected after behavioral testing, when brain GABA levels are already increased.

Genotoxic and mutagenic effects of vigabatrin, a γ-aminobutyric acid transaminase inhibitor, in Wistar rats submitted to rotarod task

Vigabatrin (VGB) is an antiepileptic drug thatincreases brain γ-aminobutyric acid (GABA) levels through irreversible inhibition of GABA transaminase. The aim of this study was to evaluate neurotoxicological effects of VGB measuring motor activity and genotoxic and mutagenic effects after a single and repeated administration. Male Wistar rats received saline, VGB 50, 100, or 250 mg/kg by gavage for acute and subchronic (14 days) treatments and evaluated in the rotarod task. Genotoxicity was evaluated using the alkaline version of the comet assay in samples of blood, liver, hippocampus, and brain cortex after both treatments. Mutagenicity was evaluated using the micronucleus test in bone marrow of the same animals that received subchronic treatment. The groups treated with VGB showed similar performance in rotarod compared with the saline group. Regarding the acute treatment, it was observed that only higher VGB doses induced DNA damage in blood and hippocampus. After the subchronic treatment, VGB did not show genotoxic or mutagenic effects. In brief, VGB did not impair motor activities in rats after acute and subchronic treatments. It showed a repairable genotoxic potential in the central nervous system since genotoxicity was observed in the acute treatment group.

Are vigabatrin induced T2 hyperintensities in cranial MRI associated with acute encephalopathy and extrapyramidal symptoms?

Reversible T2-hyperintensities in cranial MRI have been recently observed in infants with infantile spasms, who were treated with vigabatrin. In most cases, this phenomenon is solely been reported in neuroimaging practice without clinical relevance. We report two patients with infantile spasms, who not only developed transient T2-hyperintensities, but also presented acute encephalopathy, and extrapyramidal symptoms under vigabatrin therapy.

GABA Not Only a Neurotransmitter: Osmotic Regulation by GABA(A)R Signaling

Mature macroglia and almost all neural progenitor types express γ-aminobutyric (GABA) A receptors (GABA_ARs), whose activation by ambient or synaptic GABA, leads to influx or efflux of chloride (Cl⁻) depending on its electro-chemical gradient (E_Cl). Since the flux of Cl⁻ is indissolubly associated to that of osmotically obliged water, GABA_ARs regulate water movements by modulating ion gradients. In addition, since water movements also occur through specialized water channels and transporters, GABA_AR signaling could affect the movement of water by regulating the function of the channels and transporters involved, thereby affecting not only the direction of the water fluxes but also their dynamics. We will here review recent observations indicating that in neural cells GABA_AR-mediated osmotic regulation affects the cellular volume thereby activating multiple intracellular signaling mechanisms important for cell proliferation, maturation, and survival. In addition, we will discuss evidence that the osmotic regulation exerted by GABA may contribute to brain water homeostasis in physiological and in pathological conditions causing brain edema, in which the GABAergic transmission is often altered.

Abnormal axial diffusivity in the deep gray nuclei and dorsal brain stem in infantile spasm treated with vigabatrin

We evaluated the DTI changes in the deep gray nuclei and dorsal brain stem, which demonstrated abnormal T2 and/or diffusion signal intensity, in 6 patients with infantile spasm treated with vigabatrin compared with 6 age-matched controls. Regions of interest were placed in the globi pallidi, thalami, and dorsal brain stem; FA, trace, D(‖), and D(⊥) were measured. Patients on vigabatrin had significantly lower FA in both globi pallidi (P = .01) and the dorsal brain stem (P < .01), significantly lower trace in both globi pallidi (P = .01) and the thalami (P = .02 and .01 for right and left, respectively), and significantly lower D(‖) in both globi pallidi (P ≤ .01), the thalami (P < .01), and the dorsal brain stem (P = .03). There were no significant differences in D(⊥) of the globi pallidi, thalami, or dorsal brain stem in patients compared with controls. The findings suggest that axonal changes play a greater role in the observed abnormal signal intensity, with lesser contribution from myelin changes.

Pathological evidence of vacuolar myelinopathy in a child following vigabatrin administration

Vigabatrin, a gamma-aminobutyric acid (GABA) aminotransferase- inhibiting drug used for seizure control, has been associated with white matter vacuolation and intramyelinic edema in animal studies. Similar pathological lesions have never been described in the central nervous system of human participants treated with the drug. Described here is a child with quadriparetic cerebral palsy secondary to hypoxic-ischemic brain injury following premature birth, who received vigabatrin for the treatment of infantile spasms at 9 months of age. A severe deterioration of neurologic function immediately followed the initiation of vigabatrin, and the child died 3 weeks later. Neuropathological examination revealed white matter vacuolation and intramyelinic edema. This represents the first reported case of vigabatrin-induced intramyelinic edema in humans. It validates the concerns regarding vigabatrin safety in infants and individuals with preexisting abnormalities of myelin.

Neurotransmitter receptors in the life and death of oligodendrocytes

Oligodendrocytes are crucial to the function of the mammalian brain: they increase the action potential conduction speed for a given axon diameter and thus facilitate the rapid flow of information between different brain areas. The proliferation and differentiation of developing oligodendrocytes, and their myelination of axons, are partly controlled by neurotransmitters. In addition, in models of conditions like stroke, periventricular leukomalacia leading to cerebral palsy, spinal cord injury and multiple sclerosis, oligodendrocytes are damaged by glutamate and, contrary to dogma, it has recently been discovered that this damage is mediated in part by N-methyl-D-aspartate receptors. Mutations in oligodendrocyte neurotransmitter receptors or their interacting proteins may cause defects in CNS function. Here we review the roles of neurotransmitter receptors in the normal function, and malfunction in pathological conditions, of oligodendrocytes.

The latest on infantile spasms

PURPOSE OF REVIEW

This article reviews the most significant advances in the field of infantile spasm during the past year, with emphasis on best practise for treatment, and on some new etiological genetic and metabolic causes for the spasms, and new advances in the knowledge of tuberous sclerosis.

RECENT FINDINGS

Up-to-date information comparing corticotrophin, oral steroids and vigabatrin shows that hormonal treatment is the most effective therapy in the short term. In a recent randomized trial, large doses of prednisolone were as effective as corticotrophin. There are insufficient data to recommend any treatment schedule for infantile spasms. Vigabatrin is the choice for infants with tuberous sclerosis. Visual field defects in (older) children seem to be as common as in adults. In animals, vigabatrin can induce apoptosis of the neurons in the developing brain. New rare factors associated with infantile spasms are mitochondrial diseases, mutations of the Aristales-related homeobax gene and posterior quadrantic dysplasia syndrome. The outcome in children with tuberous sclerosis and infantile spasms is better understood.

SUMMARY

The accurate determination of etiology is now becoming increasingly possible. There is still a lack of consensus about the treatment of first choice for infantile spasms. However, recent data show that hormonal treatment is the most effective therapy in the short term. Frequency of visual field defects in children treated with vigabatrin should be studied in addition to the long-term outcome in general. Advances in our understanding of brain maturation, etiologies, mechanisms and genetics underlying catastrophic epilepsy may facilitate more effective pharmacologic interventions.

The effects of vigabatrin on electrophysiology and visual fields in epileptics: a controlled study with a discussion of possible mechanisms

PURPOSE

To compare the visual electrophysiology and visual fields of patients taking vigabatrin to those of a control group of epileptics on other anti-epileptic drugs (AEDs).

METHODS

Fourteen epileptics treated with vigabatrin and 10 control patients treated with other AEDs underwent ERG and EOG. Goldmann visual fields were performed and analysed using standard software to measure areas contained within I4e isopters.

RESULTS

The cone and rod b-waves of the ERG, the oscillatory potential amplitudes and Arden indices were reduced in vigabatrin-treated subjects and the oscillatory potentials delayed. The Arden indices were reduced due to an increased dark trough. The areas contained within the I4e isopter of vigabatrin treated subjects were reduced compared to the control group and these areas correlated well with oscillatory potential amplitudes and b-wave amplitudes in the vigabatrin group only.

CONCLUSIONS

The use of vigabatrin is associated with a reduction of the ERG cone b-wave amplitude and oscillatory potentials which correlates with visual field loss. The Arden ratio is reduced in subjects taking vigabatrin but may recover after cessation. However, visual loss may persist in the presence of a recovered EOG. These findings suggest further effects of the drug than those mediated by GABA receptors, and support the contention that the cause of the field loss may be at least in part due to retinal effects. Possible mechanisms are discussed.

Visual field defects and other ophthalmological disturbances associated with vigabatrin

Vigabatrin has been an important anticonvulsant drug for over 10 years with a reputation for high efficacy and excellent tolerability. However, since 1997, there have been over 25 reports in the literature of visual field defects attributable to the use of this agent. Most are case reports and many have only been reported as abstracts or posters or as letters or short communications. Only a small number of papers give details of patient characteristics. Typically, case reports detail ophthalmological tests such as visual acuity, funduscopic examination, integrity of colour vision, and the nature of the field cut. Many also include various electrophysiological tests which were performed in an attempt to further describe the nature of the visual changes. In order to shed light on the mechanism underlying these visual field changes, many investigators also tested various electrophysiological parameters. However, because electrophysiological testing requires considerable expertise on the part of the technician, this could be a source of variability in results and may also pose a challenge with data interpretation. The magnitude of the problem is difficult to assess. The manufacturer's estimate of incidence of visual field defects with vigabatrin was approximately 0.1%, but incidences estimated in the literature range from 6 to 30%. Since the majority of the published data are in case report form, proof of causation is also very difficult. Two papers that used proper scientific methodology to investigate this condition suggest that vigabatrin causes these changes; however, there needs to be further studies with larger populations to answer this question definitively. There is a lack of data on the dose-response characteristics of vigabatrin and the development of visual field defects. The only available data are reports of trends that implicate duration of therapy or cumulative dose. Perhaps the most important area to elucidate is whether or not the visual field defects are reversible. Data are scare on this subject, but we can hope that data will emerge as follow-up periods become more substantial. There is a need for more complete information regarding several aspects of the mechanistic basis of visual field defects associated with vigabatrin that will allow rational clinical decision making. The treatment choices, both pharmacological and nonpharmacological, for patients with refractory epilepsy have grown substantially in the last few years. Thus, it is doubtful that the clinical positioning of vigabatrin is likely to change in the future from that of a very valuable 'niche drug', with emphasis on paediatric usage.

The role of vigabatrin in childhood seizure disorders: results from a clinical audit

PURPOSE

The emergence of visual field defects attributed to vigabatrin (VGB) treatment and intramyelinic edema in animal experiments has raised concerns about its future role in the treatment of childhood seizures.

METHODS

We evaluated our experience with this antiepileptic agent with retrospective analysis of database and chart audit.

RESULTS

Of 73 patients, 43 girls and 33 boys were treated with VGB over a 7-year period. The mean age of patients at the introduction of VGB was 87 months (range, 5-257 months). In 12 of 73 cases, VGB was used as monotherapy; in 61 of 73 cases, it was used as an add-on drug. Seizure types included secondarily generalized seizures (21), mixed seizures (21), partial seizures (18), and generalized seizures (13). Seizure etiology was idiopathic/cryptogenic in 22 patients, symptomatic in 50, and undetermined in a single patient. The mean duration of therapy was 16 months (median, 10 months; range, 1-144 months). VGB was effective in 30 (seven seizure free, 23 with >90% reduction in seizures), partially effective in four (50-90% reduction in seizures), and ineffective in 38 (<50% reduction in seizures). Nearly 50% of patients with infantile spasms responded to VGB. All patients underwent ophthalmic evaluation; two (16%) of 12 patients who could undergo static threshold perimetry were demonstrated to have the characteristic visual field constriction.

CONCLUSIONS

VGB is effective in producing a significant reduction in seizure frequency in nearly half the patients with childhood seizures, including refractory epilepsy. Despite emerging concerns regarding visual side effects, this drug retains an important role in the medical management of childhood epilepsy.

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.