New antiepileptic drugs--an explosion of activity

Outcome Evaluation of Gabapentin as Add-on Therapy for Partial Seizures

Objective:

The safety, tolerability, efficacy, and impact on quality of life of gabapentin (Neurontin®) as adjunctive therapy to carbamazepine (CBZ) and/or phenytoin (PHT) was assessed in epileptic patients with partial seizures.

Methods:

NEON (Neurontin Evaluation of Outcomes in Neurological Practice) was an open-label, prospective, multicentre study conducted in patients on a stable dose of CBZ and/or PHT and experiencing an average of up to 4 complex partial seizures with or without secondary generalization per month, with no seizure-free months. The treatment lasted 20 weeks. Gabapentin was started at 400 mg/day and was individually titrated to effective tolerable dose up to 2400 mg/day. Quality of life was evaluated using the QOLIE-10 questionnaire.

Results:

A total of 141 patients were enrolled at 36 sites; 114 patients were evaluable for efficacy analyses. The mean maintenance dose of gabapentin was 1600 mg/day (range = 300-3200). A decrease of 50% or more in frequency of complex partial + secondarily generalized seizures was observed in 81 (71 %) patients (p = 0.0001). Fifty two (46%) patients were seizure-free during the last 8 weeks of treatment. A significant improvement (p < 0.05) was observed in 5 of the 10 questions of the QOLIE-10, as well as in the composite QOL score (p = 0.0002). The most frequent adverse events included somnolence (16%), dizziness (9%), and asthenia (6%). Twenty-five (18%) patients prematurely discontinued the study, 16 (11%) of them due to adverse events.

Conclusion:

This study indicates that treatment with gabapentin as adjunctive therapy to standard antiepileptic drugs in this group of patients not only provides significant improvement in seizure control, but also has a positive impact on quality of life. The clinical benefits in efficacy, safety and tolerability demonstrated at 20 weeks are sustained, and no tolerance develops with gabapentin in longer term use.

Modulation of the GABAergic pathway for the treatment of fragile X syndrome

Fragile X syndrome (FXS) is the most common genetic cause of intellectual disability and the most common single-gene cause of autism. It is caused by mutations on the fragile X mental retardation gene (FMR1) and lack of fragile X mental retardation protein, which in turn, leads to decreased inhibition of translation of many synaptic proteins. The metabotropic glutamate receptor (mGluR) hypothesis states that the neurological deficits in individuals with FXS are due mainly to downstream consequences of overstimulation of the mGluR pathway. The main efforts have focused on mGluR5 targeted treatments; however, investigation on the gamma-aminobutyric acid (GABA) system and its potential as a targeted treatment is less emphasized. The fragile X mouse models (Fmr1-knock out) show decreased GABA subunit receptors, decreased synthesis of GABA, increased catabolism of GABA, and overall decreased GABAergic input in many regions of the brain. Consequences of the reduced GABAergic input in FXS include oversensitivity to sensory stimuli, seizures, and anxiety. Deficits in the GABA receptors in different regions of the brain are associated with behavioral and attentional processing deficits linked to anxiety and autistic behaviors. The understanding of the neurobiology of FXS has led to the development of targeted treatments for the core behavioral features of FXS, which include social deficits, inattention, and anxiety. These symptoms are also observed in individuals with autism and other neurodevelopmental disorders, therefore the targeted treatments for FXS are leading the way in the treatment of other neurodevelopmental syndromes and autism. The GABAergic system in FXS represents a target for new treatments. Herein, we discuss the animal and human trials of GABAergic treatment in FXS. Arbaclofen and ganaxolone have been used in individuals with FXS. Other potential GABAergic treatments, such as riluzole, gaboxadol, tiagabine, and vigabatrin, will be also discussed. Further studies are needed to determine the safety and efficacy of GABAergic treatments for FXS.

Indirect photometric assay determination of gabapentin in bulk drug and capsules by capillary electrophoresis

A capillary electrophoresis (CE) method was developed for the assay determination of gabapentin (GBP) in bulk drug and capsules. Separation was carried out on 74cm (62.5cm effective length) x 75microm i.d. fused silica capillary by applying a potential of -20kV at ambient temperature. Background electrolyte (BGE) consisting of 5mM 5-sulphosalicylic acid and 0.5mM cetyltrimethylammonium bromide (CTAB), pH 11.0 was employed for the separation. Indirect method of UV detection was performed at a wavelength of 215nm using sulphosalicylate ion as a chromophore. A linear calibration curve was obtained over a concentration range 20-200microg/ml of GBP in deionized water with a correlation coefficient (r) of 0.9998. Recoveries were shown to be >/=98% both in bulk drug and capsules with standard deviation (S.D.) </=2.3%. No placebo (matrix) peaks were observed at the migration times of GBP and L-glutamic acid (internal standard, I.S.).

High-performance liquid chromatographic method for the determination of gabapentin in human plasma

A sensitive high-performance liquid chromatography (HPLC) method using UV detection for the determination of gabapentin in human plasma has been developed. In this method, gabapentin was extracted from human plasma with a reversed-phase solid-phase extraction (SPE) cartridge followed by derivatization with phenylisothiocyanate. Analysis was achieved by using a HPLC system that was equipped with a UV detector. The quantitation limit of gabapentin in human plasma was 0.03 microg/ml. The method is sensitive with excellent selectivity and reproducibility and it has been applied to a bioequivalence clinical study with great success.

Liquid chromatography tandem mass spectrometry assay for topiramate analysis in plasma and cerebrospinal fluid: validation and comparison with fluorescence-polarization immunoassay

SUMMARY

The authors report the development and validation of a liquid chromatography tandem mass spectrometry assay (LC/MS/MS assay) for the analysis of topiramate (2,3:4,5-bis-o-(-1-methyl)-beta-D-fructopyranose sulfamate) in plasma and cerebrospinal fluid (CSF). Comparison is made with the commercially available fluorescence-polarization immunoassay (FPIA).

LC/MS/MS ASSAY

Using the internal standard, 1,2:3,4-bis-o-(1-methylethylidene-alpha-D-galactopyranose sulfamate), a structural isomer, the calibration curve in plasma was linear in the concentration range of 0.02-20.0 mg/L (r(2) = 0.9998). The coefficients of variation in plasma were < or = 3%, and the accuracy ranged from 100% to 101% in the therapeutically relevant concentration range of 0.4-16.0 mg/L. In CSF, the mean recovery was 98%, and there was linearity between the nominal and the estimated concentration in the range of 1.5-20.0 mg/L (r(2)= 0.9996). FPIA: The calibration curve was linear in the concentration interval of 1.6-24.3 mg/L (r(2) = 0.9994), and the mean recovery was 96%. Accuracy in plasma was 99- 104%, and precision was 3.2-6.0%. In CSF, there was linearity between the nominal concentration and the estimated concentration in the range of 1.5-20.0 mg/L (r(2) = 0.9995), and the mean recovery was 100%. COMPARISON BETWEEN FPIA AND LC/MS/MS: There was a high correlation between the FPIA and the LC/MS/MS assay (r(2) = 0.9965 in plasma and r(2) = 0.9996 in CSF, P < 0.001 for both). In plasma and CSF, the two methods showed equal results, evaluated as the ratio between the two methods (plasma: median ratio = 1.00; 95% confidence interval [CI], 0.98-1.02, paired-sample test, P = 0.79; and CSF: median ratio = 1.00, 95% CI, 0.99-1.02, paired-sample test, P = 0.75). The coefficient of variation on the ratios between the two methods had similar levels: 5% in plasma and 3% in CSF.

CONCLUSION

The new LC/MS/MS assay has favorable characteristics, being highly precise and accurate. FPIA also proved precise and accurate, and there was a high agreement with the LC/MS/MS assay in plasma and CSF. Either method displayed sufficient precision and accuracy and may thus be implemented in daily routine.

Mechanisms influencing acquisition and recall of motor memories

An internal model of the dynamics of a tool or an object is part of the motor memory acquired when learning to use the tool or to manipulate the object. Changes in synaptic efficacy may underlie acquisition and storage of memories. Here we studied the effect of pharmacological agents that interfere with synaptic plasticity on acquisition of new motor memories and on recall of a previously learned internal model. Forty-nine subjects, divided into six groups, made reaching movements while holding a robotic arm that applied forces to the hand. On day 1, all subjects learned to move in force field A. On day 2, each group of subjects was tested on their ability to recall field A and their ability to learn a new internal model in field B. Four groups participated in the experiments of day 2 under the effects of lorazepam (LZ; a GABA type A receptor-positive allosteric modulator), dextromethorphan [DM; an N-methyl-D-aspartate (NMDA) receptor blocker], lamotrigine (LG, a drug that blocks voltage-gated Na(+) and Ca(2+) channel), or scopolamine (SP; muscarinic receptor antagonist). Two control groups were tested in a drug-free condition: one group that was not exposed to additional experimental protocols (NP) and another group was tested under ~24 h of sleep deprivation between completion of learning on day 1 and start of testing on day 2 (SD). Recall of field A was normal in all groups. Learning of field B was reduced by LZ and DM but not by SP, LG, SD or in the NP condition. These results suggest that a 24-h sleep-deprivation period may have little or no effect on consolidation of this motor memory and that NMDA receptor activation and GABAergic inhibition are mechanisms operating in the acquisition but not recall of new motor memories in humans.

Future prospects for the drug treatment of epilepsy

Great progress has been made in the last 150 years in the pharmacological management of epilepsy, and, despite the increasing number of technological advances available, antiepileptic drugs (AEDs) remain the mainstay of treatment for the vast majority of patients with epilepsy. This review looks at possible avenues of development in the drug treatment of epilepsy. The strengths and weaknesses of those AEDs which are currently licensed are examined, and ways in which their use may be improved are discussed (e.g. rational combinations, use of new formulations). Potentially new targets that may allow the development of effective treatments are highlighted (neuroimmunological manipulation, decreasing inherent drug resistance mechanisms, and modification of adenosine neurotransmission), and a summary of the most promising AEDs currently in development is provided [e.g. carabersat, ganaxolone, harkoseride, MDL 27192, safinamide (NW 1015), pregabalin, retigabine, talampanel, valrocemide, losigamone and BIA 2093].

Mechanisms underlying rapid experience-dependent plasticity in the human visual cortex

Visual deprivation induces a rapid increase in visual cortex excitability that may result in better consolidation of spatial memory in animals and in lower visual recognition thresholds in humans. gamma-Aminobutyric acid (GABA)ergic, N-methyl-d-aspartate (NMDA), and cholinergic receptors are thought to be involved in visual cortex plasticity in animal studies. Here, we used a pharmacological approach and found that lorazepam (which enhances GABA(A) receptor function by acting as a positive allosteric modulator), dextrometorphan (NMDA receptor antagonist), and scopolamine (muscarinic receptor antagonist) blocked rapid plastic changes associated with light deprivation. These findings suggest the involvement of GABA, NMDA, and cholinergic receptors in rapid experience-dependent plasticity in the human visual cortex.

Mechanisms influencing stimulus-response properties of the human corticospinal system

BACKGROUND

Stimulus-response (S-R) properties of the corticospinal system in humans depend on the interactions that take place at different sites along the corticospinal pathway. The mechanisms influencing stimulus-response curves elicited by transcranial magnetic stimulation (TMS) and their operation site along the human neuraxis are poorly understood. In this study, we investigated the effects of CNS-active drugs with distinct mechanisms of action on S-R curves. Effects of each of these drugs on S-R curves would point to the involvement of specific mechanisms. Additionally, relative sensitivity of S-R curves compared with other measures of corticospinal excitability was studied.

METHODS

We studied the effects of lorazepam, which is a positive allosteric modulator of GABA(A) receptors; lamotrigine, an inhibitor of voltage-gated Na(+) and Ca(2+) channels; and D-amphetamine, an indirect agonist of the dopaminergic-adrenergic system on S-R curves, motor thresholds (MT), and intracortical inhibition (ICI) and facilitation (ICF) with a double-pulse technique. Maximum peripheral M responses and F waves were investigated as measures of the total alpha-motoneuron pool and its excitability.

RESULTS

F and M waves were unaffected by either the drugs or placebo. S-R curves were significantly depressed by lorazepam and lamotrigine without changes in ICI and ICF. Both S-R curves and ICF were enhanced by D-amphetamine. MT increased only with lamotrigine.

CONCLUSIONS

S-R curves were influenced by changes in the GABAergic and monoaminergic system and Na(+) and Ca(2+) channel properties. Our results indicate that, out of different parameters of motor system excitability, S-R curves were the most sensitive.

Evaluation of a series of anticonvulsant 1,2,3,4-tetrahydroisoquinolinyl-benzamides

SAR studies around a series of N-(tetrahydroisoquinolinyl)-2-methoxybenzamides, identified by high-throughput screening at the novel SB-204269 binding site, have provided compounds such as 13, 29-33 with high affinity and excellent anticonvulsant activity in animal models.

Enzyme induction and inhibition by new antiepileptic drugs: a review of human studies

The aim of this paper is to review a number of new antiepileptic agents (i.e. felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin and zonisamide) for their inducing and/or inhibitory properties in humans, mainly considering the interactions where they are involved as the cause rather than the object of such interactions. Two aspects have been particularly taken into account: the changes or absence of changes in plasma/serum concentrations of concomitant drugs and the direct or indirect evidence of induction, inhibition or lack of effect on the six major human hepatic CYP isozymes (CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4), as well as on other CYP isozymes or enzyme systems. Felbamate clearly affects the pharmacokinetics of a number of drugs, generally increasing but also decreasing their concentrations. It induces enzymes such as CYP3A4 and inhibits enzymes such as CYP2C19 and those of the beta-oxidation pathway. Topiramate is not devoid of potential interaction properties: it decreases the plasma concentrations of ethinylestradiol, induces CYP3A4 and inhibits CYP2C19. For oxcarbazepine, no inhibitory, only inductive effects have been observed thus far. Felbamate. topiramate and oxcarbazepine may induce the metabolism of steroidal oral contraceptives. In this respect, tiagabine has been studied at a rather low dose. Pharmacodynamic or pharmacokinetic interaction seems to exist between lamotrigine and carbamazepine. Lamotrigine appears to be a weak inducer of UGTs, whereas induction of CYP3A4 seems improbable as the compound does not change the concentrations of oral contraceptives or the urinary excretion of 6beta-hydroxycortisol. Zonisamide has very peculiar pharmacokinetics and an extensive metabolism. Additional information on its enzyme inducing or inhibiting properties would be necessary, as data so far collected on its effect on the pharmacokinetics of other drugs are conflicting. Gabapentin, vigabatrin and in particular levetiracetam appear to be devoid of significant enzyme inducing or inhibiting properties.

Double-blind substitution of vigabatrin and valproate in carbamazepine-resistant partial epilepsy. 012 Study group

Patients from 12 countries reporting two or more partial seizures per month despite treatment with optimal doses of CBZ were randomised to additional vigabatrin (VGB, 2-4 g daily) or sodium valproate (VPA, 1-2 g daily) using a double-blind, double-dummy design. The study included a 6 month retrospective baseline on unchanged CBZ dosage, a month's prospective baseline, a short titration phase, and an assessment period lasting 3 months on duotherapy. CBZ was withdrawn over a further 2 months in responders ( > or = 50% monthly seizure reduction compared with baseline), who continued on alternative monotherapy for 3 or more months. If seizure control deteriorated, CBZ was reinstated and these patients were also followed up for 3 months. A total of 215 patients (108 VGB, 107 VPA) reporting a mean of seven partial seizures per month fulfilled the criteria for the intention-to-treat analysis. 53 and 51% of patients in the VGB and VPA group respectively achieved a monthly reduction in seizure numbers > or = 50%, respectively. 27 and 31% maintained alternative monotherapy. Overall, 17% (7% monotherapy, 10% duotherapy) of the VGB treated patients and 19% (8% monotherapy, 11% duotherapy) of the VPA group remained seizure-free during the final 3 month treatment period. VGB and VPA, which increase neuronal inhibition mediated by gamma aminobutyric acid, can be added to or substituted for CBZ when this Na+ channel blocker fails to control partial seizures. This lends credence to the hypothesis in support of a mechanistic approach to the management of epilepsy.

Treatment of seizures in patients with learning disabilities

People with learning disabilities commonly have seizures. Combined electroencephalogram and video investigations improve diagnostic accuracy, while neuroimaging may indicate a role for surgery. When epilepsy is diagnosed, individually tailored care is necessary. Rational antiepileptic drug use is advocated, with emphasis upon the newer agents due to their better tolerability and ease of use. Regular clinical review will prevent over-medicating. Although an optimistic approach can now be adopted, future developments require a more solid evidence base together with a rationality to all aspects of care, including drug therapy, carer education, closer collaboration among specialists, and mutual skill awareness of all involved.

Epilepsy and learning disabilities--a challenge for the next millennium?

People with learning disabilities often have seizures in addition to other disorders. Precise diagnosis may be difficult, but accuracy can be improved using electroencephalographic and video investigations. Following the establishment of a diagnosis of epilepsy, individually tailored care is necessary taking into account other health, behavioural and therapeutic issues. Neuroimaging may indicate a need for surgery which should not be automatically excluded as a treatment option. Rational antiepileptic drug use is advised, with emphasis upon the newer agents due to their better tolerance and ease of use. A programme of regular review will prevent over-medicating. Drug therapy may be withdrawn in a seizure-free patient. Realistic goals should be established for each individual coupled with an optimistic approach to care. However, future developments require a solid evidence base combined with rationality in all aspects of management. The community learning disability epilepsy nurse specialist is the key health-care professional who can ensure that a learning disabled individual with epilepsy is able to take full advantage of all available services. Education, closer collaboration and the mutual recognition of skills will ensure more cohesive and comprehensive care for this disadvantaged patient population.

Neurochemical actions of the desglycinyl metabolite of remacemide hydrochloride (ARL 12495AA) in mouse brain

1. Remacemide hydrochloride, a recently developed antiepileptic drug, is believed to exert its effects, at least in part, via its desglycinyl metabolite, ARL 12495AA. 2. We have investigated the effects of ARL 12495AA on several neurochemical parameters in mouse brain. Adult male ICR mice were randomized into two groups and administered ARL 12495AA (0-75 mg kg-1) intraperitoneally, either as a single dose or once daily for 5 days. 3. Six hours after the final dose, animals were killed and their brains removed. Brain tissues were analysed for concentrations of gamma-aminobutyric acid (GABA), glutamine and glutamate and for the activities of GABA-transaminase (GABA-T) and glutamic acid decarboxylase (GAD). 4. Single dose ARL 12495AA was without effect on any of the parameters investigated. 5. Repeated ARL 12495AA treatment did not alter brain concentrations of GABA and glutamine, but at a high dose there was a trend toward reduced brain glutamate concentrations (P = 0.10). 6. Repeated administration of ARL 12495AA at a high dose significantly increased GABA-T activity (P < 0.05) and decreased that of GAD (P < 0.05). 7. These findings may have relevance to the clinical use of remacemide hydrochloride in human epilepsy.

Effects of new anticonvulsant medications on porphyrin synthesis in cultured liver cells: potential implications for patients with acute porphyria

Some patients with acute hereditary porphyrias have seizures and require anticonvulsant therapy, but many anticonvulsants induce exacerbations of the hepatic porphyrias. Recently, several new anticonvulsants have become available. Among these are gabapentin, vigabatrin, felbamate, lamotrigine, and tiagabine. Little is known about their potential for induction of porphyric attacks. We used a cell culture model of primary chicken embryo liver cells, which maintain intact heme synthesis and regulation, to study the effects of these new anticonvulsants on porphyrin accumulation. Treatment of the cells with deferoxamine (250 microM) led to a partial block in heme synthesis, simulating the conditions encountered in human beings with porphyria. Concomitant exposure of these cells to phenobarbital (2 mM) strongly induced accumulation of porphyrins, serving as a positive control in this model. Cells were treated for 20 hours with increasing doses (3.2 to 1,000 microM) of the newer anticonvulsants, with or without deferoxamine. For most of these anticonvulsants 5 to 100 microM is representative of the concentrations achieved in humans with therapeutic doses. Porphyrins were measured spectrofluorometrically as uro-, copro-, and protoporphyrins. Results were confirmed by high-pressure liquid chromatography. Neither vigabatrin nor gabapentin treatment, with or without deferoxamine, led to any increase in porphyrin accumulation. Similar doses of felbamate (with deferoxamine) led to a marked increase in (mainly proto-) porphyrin levels, qualitatively and quantitatively almost identical to the accumulation produced by phenobarbital. Lamotrigine or tiagabine (with deferoxamine) caused similar porphyrin accumulation. Tiagabine treatment up to 100 microM (with deferoxamine) also resulted in very high levels of predominantly proto-porphyrin. In contrast to the other anticonvulsants tested, tiagabine without deferoxamine led to mild porphyrin accumulation. In the presence of deferoxamine, phenobarbital, felbamate, lamotrigine, or tiagabine, but not gabapentin or vigabatrin, increased levels of the mRNA of ALA synthase, the first and rate-controlling enzyme of porphyrin synthesis. Such enzyme induction is a sine qua non for acute porphyric attacks. We conclude that neither vigabatrin nor gabapentin is porphyrogenic, whereas felbamate, lamotrigine, and, especially, tiagabine lead to much accumulation of porphyrins. The latter three anticonvulsants, therefore, may precipitate or exacerbate acute porphyric attacks in humans. We recommend use of vigabatrin or gabapentin, but not felbamate, lamotrigine, or tiagabine, in patients with acute porphyria and seizures.

Neurochemical actions of vigabatrin and tiagabine alone and in combination in mouse cortex

1. The effects of repeated administration of the anticonvulsant compounds, vigabatrin (VGB) and tiagabine (TGB), on gamma-aminobutyric acid (GABA) concentration and the activities of GABA-transaminase (GABA-T) and glutamic acid decarboxylase (GAD) were investigated in mouse cortex. 2. VGB alone increased GABA levels and decreased GABA-T and GAD activities. 3. TGB alone was essentially without effect. 4. Low doses of VGB and TGB in combination increased GABA levels when neither drug had such an effect alone. 5. Despite this observation, this study failed to establish any conclusive evidence for an interaction between VGB and TGB that might help to explain their reported clinical synergism.

Gabapentin for parkinsonism: a double-blind, placebo-controlled, crossover trial

PURPOSE

Gabapentin is a recently available anticonvulsant whose mechanism of action remains unknown. We suspected efficacy from serendipitous observations of gabapentin in patients with parkinsonism. This led us to a double-blind, placebo-controlled, crossover trial.

PATIENTS AND METHODS

We administered gabapentin in a placebo-controlled, double-blind, crossover trial to 19 subjects with advanced parkinsonism. We measured the effect of placebo and gabapentin on subjects' symptoms with the Unified Parkinson's Disease Rating Scale, the Webster Scale, and the Hoehn and Yahr Scale. We assessed tremor with surface-recorded electromyography.

RESULTS

Total Unified Parkinson's Disease Rating Scale improved with gabapentin compared with placebo (P = 0.0005). Likewise, activities of daily living and examination subscore of the Unified Parkinson's Disease Rating Scale improved with gabapentin compared with placebo but did not achieve statistical significance. Webster Scale showed improvement but neither Hoehn and Yahr Scale nor Webster Scale changes reached statistical significance. Tremor as measured by the Unified Parkinson's Disease Rating Scale improved with gabapentin but the use of the root mean square of the rectified electromyography as a measure of tremor activity was not statistically significant.

CONCLUSIONS

This study demonstrates that gabapentin improves rigidity, bradykinesia, and tremor of parkinsonism including both Parkinson's disease and Parkinson's syndrome. The rigidity and bradykinesia of parkinsonism improve on the drug even when the effects of gabapentin on tremor are discounted.

Pharmacokinetic interactions of the new antiepileptic drugs

Therapy with traditional antiepileptic drugs is associated with a wide range of pharmacokinetic drug-drug interactions. In particular, enzyme induction, enzyme inhibition and displacement from protein binding may result in important changes in serum concentrations of antiepileptics. Relevant interactions have also been described for some new antiepileptics. Felbamate increases serum concentrations of phenytoin, phenobarbital and valproic acid (sodium valproate). On the other hand, it reduces concentrations of carbamazepine and increases concentrations of its metabolite carbamazepine-10,11-epoxide. Concentrations of felbamate itself are reduced by phenytoin and carbamazepine. Concentrations of lamotrigine are considerably increased by valproic acid and decreased by phenytoin, carbamazepine and phenobarbital (phenobarbitone). Vigabatrin reduces serum concentrations of phenytoin by approximately 20%. On the other hand, some new antiepileptics have the important advantage of not interfering with the metabolism of other antiepileptics; this is the case for gabapentin, lamotrigine and oxcarbazepine. Furthermore, the pharmacokinetics of gabapentin, oxcarbazepine and vigabatrin are independent of concomitant drugs. These aspects are especially important as, until now, new antiepileptics have been most often utilised as add-on therapy.

Effects of tiagabine and vigabatrin on GABA uptake into primary cultures of rat cortical astrocytes

Tiagabine (TGB) and vigabatrin (VGB) are two novel anticonvulsant compounds reported to exert their pharmacological effects via an action on the gamma-aminobutyric acid (GABA) system. We have investigated the effects of acute exposure of these drugs on the uptake of GABA into rat cortical astrocytes in primary culture. Astrocytes were prepared from the cerebral cortices of one day-old rat pups by a mechanical dissociation technique and were assayed for GABA uptake activity after 21 days in culture. Tiagabine (100-300 nM) and VGB (100 microM) reduced GABA uptake when compared to control at four hours post-exposure. GABA uptake was also reduced following eight and 24 hour exposures to 200 nM TGB. A combination of TGB (200 nM) and VGB (100 microM) treatments reduced GABA uptake when compared to both control and VGB treated cultures. These results support the efficacy of TGB as a GABA uptake inhibitor and suggest that VGB may also exert an effect by this mechanism.

Determination of gabapentin in plasma by high-performance liquid chromatography

A rapid and simple method for determination of the novel antiepileptic compound gabapentin [1-(aminomethyl)cyclohexaneacetic acid] in plasma is described. Blank human plasma was spiked with gabapentin (1.0-10.0 micrograms/ml) and internal standard [1-(aminomethyl)-cycloheptaneacetic acid; 5.0 micrograms/ml]. Individual samples were treated with 2 M perchloric acid, centrifuged and then derivatised with o-phthalaldehyde-3-mercaptopropionic acid. Separation was achieved on a Beckman Ultrasphere 5 microns reversed-phase column with mobile phase consisting of 0.33 M acetate buffer (pH 3.7; containing 100 mg/l EDTA)-methanol-acetonitrile (40:30:30, v/v). Eluents were monitored by fluorescence spectroscopy with excitation and emission wavelengths of 330 and 440 nm, respectively. The calibration curve for gabapentin in plasma was linear (r = 0.9997) over the concentration range 1.0-10.0 micrograms/ml. Recovery was seen to be > or = 90%. The inter- and intra-assay variations for three different gabapentin concentrations were < or = 10% throughout. The lower limit of quantitation was found to be 0.5 microgram/ml. Chromatography was unaffected by a range of commonly employed antiepileptic drugs or selected amino acids.

Deliberate overdose with the novel anticonvulsant tiagabine

Tiagabine is a novel antiepileptic drug which acts by decreasing gamma aminobutyric acid uptake in astrocytes and neurones. Here the first case of deliberate overdose with this compound in a patient on concomitant phenytoin is reported. On admission to hospital his conscious level deteriorated to grade III coma. No changes in the electrocardiogram were noted. Recovery from the initial effects was rapid, and there were no sequelae. Plasma levels of tiagabine (3.1 micrograms/ml) 4 hours after ingestion were 30 times higher than at typical steady state during therapeutic dosing. The effects of poisoning with current first-line antiepileptic drugs are reviewed. The newer agents, particularly those with greater biochemical specificity, may be safer in overdose than the more established anticonvulsants.