Specificity of vigabatrin for the GABAergic system in human epilepsy

The pharmacokinetics of vigabatrin in rat blood and cerebrospinal fluid

PURPOSE

Data on the blood pharmacokinetics of vigabatrin, an antiepileptic drug with a unique and novel mechanism of action, in the rat are sparse. Additionally, little is known of the kinetics of vigabatrin in the central cerebrospinal fluid (CSF) compartment. We therefore investigated the rate of penetration into and the inter-relationship between serum and CSF compartments following systemic administration of vigabatrin in the rat.

METHODS

Sprague-Dawley rats were implanted with a jugular vein catheter and a cisterna magna catheter for blood and CSF sampling, respectively. Vigabatrin was administered by intraperitonial injection at three different doses (250, 500 and 1000mg/kg) and blood and CSF collected at timed intervals up to 8h. Vigabatrin concentrations in sera and CSF were determined by high performance liquid chromatography.

RESULTS

Vigabatrin concentrations in blood and CSF rose linearly and dose-dependently and the time to maximum concentration (Tmax) was 0.4 and 1.0h, respectively. Vigabatrin is not protein bound in serum and its elimination from serum (mean t1/2 values, 1.1-1.4 h) is rapid and dose-independent. The efflux of vigabatrin from CSF was significantly slower than that seen for serum (mean t1/2 values, 2.2-3.3h).

CONCLUSIONS

The kinetics of vigabatrin are linear with rapid entry into CSF. However, although vigabatrin CSF kinetics parallel that seen in serum, CSF vigabatrin concentrations represent only 2% of concentrations seen in serum and do not reflect free drug concentrations in serum.

Elevated endogenous GABA level correlates with decreased fMRI signals in the rat brain during acute inhibition of GABA transaminase

Vigabatrin and gabaculine, both highly specific inhibitors of GABA (gamma-aminobutyric acid) transaminase, cause significant elevation of endogenous GABA levels in brain. The time course of GABA concentration after acute GABA transaminase inhibition was measured quantitatively in the alpha-chloralose-anesthetized rat brain using in vivo selective homonuclear polarization transfer spectroscopy. The blood oxygenation level-dependent (BOLD) effect in functional magnetic resonance imaging (fMRI) has been considered to be coupled tightly to neuronal activation via the metabolic demand of associated glutamate transport. Correlated with the rise in endogenous GABA level after vigabatrin or gabaculine treatment, the intensity of BOLD-weighted fMRI signals in rat somatosensory cortex during forepaw stimulation was found to be reduced significantly. These results are consistent with previous findings that inhibition of GABA transaminase leads to augmented GABA release and potentiation of GABAergic inhibition.

Effect of vigabatrin addition on carbamazepine blood serum levels in patients with epilepsy

Because vigabatrin (VGB) is not metabolized by liver enzymes and does not bind with serum proteins, there is little theoretical chance of it interacting with other antiepileptic drugs. However, our observations have shown that if VGB is added to carbamazepine (CBZ) monotherapy, some patients respond with adverse, toxic symptoms suggesting possible carbamazepine-vigabatrin interaction. This article presents the results of a study of 66 epileptic patients (27 women and 39 men), age 10-66 years (mean, 28.2 years), with focal seizure onset with or without secondary generalization. In these patients, in addition to CBZ therapy with an average dose of 16.7 mg/kg per day (8.6-26.8), VGB, average dose 31.1 mg/kg per day (7.1-57.9), was added. CBZ concentration was measured twice: prior to VGB introduction and 5-12 weeks after the final dose of VGB was reached. In our study 69.7% of patients responded to VGB addition with a significant increase (by at least 10%) in CBZ concentration. A correlation between the value of the increase and the initial level of CBZ prior to VGB addition was found also. Correlational analysis (Pearson's r) revealed a negative correlation between CBZ concentration and increased concentration after VGB addition (r = -0.47, df = 64, P < 0.001). This negative correlation means that if the initial CBZ level is lower, its concentration value after VGB addition is higher.

Cerebrospinal fluid somatostatin levels in febrile seizures and epilepsy in children

We analysed the level of cerebrospinal fluid (CSF) somatostatin in children with febrile seizures and epilepsy. In the febrile seizure group (n = 23), the somatostatin level was 83.9 +/- 11.2 pg/ml, which was significantly higher than that of age-matched controls. CSF samples obtained within 3 h of the last seizure had higher somatostatin levels (106.1 +/- 12.4 pg/ml;n = 14) than did the CSF obtained after 3 h (49.4 +/- 15.6 pg/ml;n = 9). The mean somatostatin level in the epilepsy group was 35.3 +/- 4.3 pg/ml (n = 34), and was distributed as follows: 27.6 +/- 3.6 pg/ml in the idiopathic generalized epilepsy group (n = 16), 44.0 +/- 9.4 pg/ml in the symptomatic generalized epilepsy group (n = 13), and 37.2 +/- 10.1 pg/ml in the partial epilepsy group (n = 5). The levels in each group were significantly higher than those in age-matched controls. Somatostatin is a hypothalamic tetradecapeptide with excitatory effects on neurons in children with febrile seizures and epilepsy. The finding that patients with convulsive disease had elevated levels of CSF somatostatin suggests that somatostatin release is somehow related to seizure activity. It remains to be determined whether this is due to increased release from over-active excitatory neurons or leakage from damaged or anoxic neurons, secondary to seizure activity.

Cerebrospinal fluid somatostatin in West syndrome: changes in response to combined treatment with high-dose pyridoxal phosphate and low-dose corticotropin

Eighteen children with West syndrome (5-11 months of age) were selected to receive an oral dose of pyridoxal phosphate, (20-50 mg/kg) for 14 d. Seizures disappeared in one patient. The remaining 17 patients were treated with 0.01 mg/kg synthesized corticotropin intramuscularly for 2 weeks as an additional therapy. Seizures disappeared in all 17 patients within a few days after initiation of the corticotropin. Levels of somatostatin in the cerebrospinal fluid were as follows: 61.0+/-10.7 pg/ml before therapy, 34.2+/-6.4 pg/ml during pyridoxal phosphate therapy, and 26.8+/-4.2 pg/ml after 2 weeks corticotropin therapy. Somatostatin levels in untreated patients were higher (p < 0.05) than those of age-matched controls (35.7+/-11.8 pg/ml) and decreased (p < 0.05) after pyridoxal phosphate treatment. Somatostatin is a hypothalamic tetradecapeptide with excitatory effects on neurons and pyridoxal phosphate might subclinically influence neuronal excitation.

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.

Vigabatrin in refractory childhood epilepsy. The Brazilian Multicenter Study

Children, 47, with various types of severe drug-resistant epilepsy were entered into a prospective, add-on, open trial with vigabatrin. Patients with West syndrome and idiopathic generalized epilepsies were excluded. Seven children had the drug withdrawn, five because of increase in seizure frequency and two because of adverse effects. Drug efficacy, measured according to seizure type, showed a 100% decrease in seizure frequency in 18.6% of partial seizures and 17.3% of the generalized seizures. There was a higher than 50% decrease in 39.5% of partial and 60.8% of generalized seizures, and less than 50% decrease or increase in seizure frequency in 41.8% and 21.8% of partial and generalized seizures, respectively. Vigabatrin mean dosage during phase 3 was 63.6 mg/kg per day (S.D. = 30.5), ranging from 19.3 to 110.5 mg/kg per day. Parametric statistical analysis (Student's t-test) of seizure frequency between phases 1 and 3 showed a significant decrease in seizure frequency for partial (P = 0.022), and generalized seizures (P < 0.0001). Drug-related adverse effects were observed in 18/47 cases (38.3%), consisting mainly of irritability, hyperactivity, dizziness, somnolence and gastrointestinal symptoms.

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.

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.

The role of dopamine in epilepsy

The clinical benefits of dopamine agonists in the management of epilepsy can be traced back over a century, whilst the introduction of neuroleptics into psychiatry practice 40 years ago witnessed the emergence of fits as a side effect of dopamine receptor blockade. Epidemiologists noticed a reciprocal relationship between the supposed dopaminergic overactivity syndrome of schizophrenia and epilepsy, which came to be regarded as a dopamine underactivity condition. Early pharmacological studies of epilepsy employed nonselective drugs, that often did not permit dopamine's antiepileptic action to be clearly dissociated from that of other monoamines. Likewise, the biochemical search for genetic abnormalities in brain dopamine function, as predeterminants of spontaneous epilepsy, proved largely inconclusive. The discovery of multiple dopamine receptor families (D1 and D2), mediating opposing influences on neuronal excitability, heralded a new era of dopamine-epilepsy research. The traditional anticonvulsant action of dopamine was attributed to D2 receptor stimulation in the forebrain, while the advent of selective D1 agonists with proconvulsant properties revealed for the first time that dopamine could also lower the seizure threshold from the midbrain. Whilst there is no immediate prospect of developing D2 agonists or D1 antagonists as clinically useful antiepileptics, there is a growing awareness that seizures might be precipitated as a consequence of treating other neurological disorders with D2 antagonists (schizophrenia) or D1 agonists (parkinsonism).

Antiepileptic drug mechanisms of action

Established antiepileptic drugs (AEDs) decrease membrane excitability by interacting with neurotransmitter receptors or ion channels. AEDs developed before 1980 appear to act on sodium channels, gamma-aminobutyric acid type A (GABAA) receptors, or calcium channels. Benzodiazepines and barbiturates enhance GABAA receptor-mediated inhibition. Phenytoin (PHT), carbamazepine (CBZ), and possibly valproate (VPA) decrease high-frequency repetitive firing of action potentials by enhancing sodium-channel inactivation. Ethosuximide (ESM) and VPA reduce a low threshold (T-type) calcium-channel current. The mechanisms of action of the new AEDs are not fully established. Gabapentin (GBP) binds to a high-affinity site on neuronal membranes in a restricted regional distribution of the central nervous system. This binding site may be related to a possible active transport process of GBP into neurons; however, this has not been proven, and the mechanism of action of GBP remains uncertain. Lamotrigine (LTG) decreases sustained high-frequency repetitive firing of voltage-dependent sodium action potentials that may result in a preferential decreased release of presynaptic glutamate. The mechanism of action of oxcarbazepine (OCBZ) is not known; however, its similarity in structure and clinical efficacy to CBZ suggests that its mechanism of action may involve inhibition of sustained high-frequency repetitive firing of voltage-dependent sodium action potentials. Vigabatrin (VGB) irreversibly inhibits GABA transaminase, the enzyme that degrades GABA, thereby producing greater available pools of presynaptic GABA for release in central synapses. Increased activity of GABA at postsynaptic receptors may underline the clinical efficacy of VGB.

Acute effects of gamma-vinyl GABA (vigabatrin) on hippocampal GABAergic inhibition in vitro

The acute effects of gamma-vinyl-GABA (GVG) on GABAergic inhibition were investigated in the hippocampal slice preparation using the paired-pulse test of inhibition during extracellular recordings. Superfusion of GVG (100-500 microM) for 60 min resulted in a concentration-dependent decrease in GABAergic inhibition. Slices superfused with higher concentrations of GVG (0.5-1 mM) were hyperexcitable as demonstrated by the appearance of multiple spikes. Binding studies showed that GVG (1 mM) had no effect on the binding of [3H]flunitrazepam or [3H]TBOB and displaced no more than 15% of specific [3H]GABA binding, which indicates that GVG-induced disinhibition is not mediated through an action at the GABAA receptor complex. Consistent with this suggestion is the finding that GVG (500 microM) had little effect on the inhibition of the orthodromically evoked CA1 population spike produced by the GABAA receptor agonist muscimol (10 microM), whereas this inhibition was considerably attenuated by the GABAA receptor antagonist, bicuculline methiodide (5 microM). The results of this study suggest that the acute actions of GVG on the GABAergic neurotransmitter system are not involved in its anticonvulsant effect.

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.

Effect of vigabatrin on striatal dopamine receptors: evidence in humans for interactions of GABA and dopamine systems

Vigabatrin is a specific gamma-aminobutyric acid transaminase inhibitor. The clinical use of this drug in the treatment of epilepsy has been sporadically linked to the development of psychosis. Using 123I-IBZM, a specific dopamine D2 receptor ligand and single photon emission tomography (SPET), one month of treatment with vigabatrin was associated with a decrease in specific binding of 123I-IBZM to D2 receptors in the left hemisphere basal ganglia. This change may provide one explanation for the development of psychosis in vulnerable patients.

Gamma-vinyl GABA (vigabatrin) in epilepsy: clinical, neurochemical, and neurophysiologic monitoring in epileptic patients

We report long-term clinical, neurochemical, and electrophysiologic data of gamma-vinyl GABA (GVG, vigabatrin) in three groups of patients. GVG was started as add-on therapy for 75 patients with refractory complex partial seizures (group A) and for 36 mentally handicapped patients with severe epilepsy (group B). The third group (C) consisted of 20 patients with carbamazepine (CBZ) monotherapy, in half of whom GVG monotherapy was substituted. After 3 months, 55% of patients in group A and 42% in group B were responders (reduction in seizure frequency greater than 50%). After 6 (group A) and 3 years (group B) of follow-up, 27 and 33% of the patients, respectively, still had good response to GVG. Neurochemical measurements showed a twofold increase in CSF GABA concentrations and minimal or no changes in other neurotransmitter-related parameters. In group C, substitution of GVG as medication tended to normalize the lengthened latencies in somatosensory evoked potentials (SEPs) observed during CBZ 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.