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

Derivatization of γ-Amino Butyric Acid Analogues for Their Determination in the Biological Samples and Pharmaceutical Preparations: A Comprehensive Review

γ-Aminobutyric acid (GABA) plays an important role in regulating neuronal excitability. Four structurally related drugs to GABA including pregabalin (PGB), gabapentin (GBP), vigabatrin (VGB), and baclofen are used for the treatment of central nervous system disorders. These drugs are small aliphatic molecules having neither fluorescent nor strong absorbance in the ultraviolet/visible region; therefore, direct determination of these analytes by optical methods is difficult. Additionally, their high boiling point makes gas chromatography impossible. Accordingly, the amine or acid moiety in these drugs is derivatized in order to improve their selectivity and sensitivity during determination in the biological samples. This review focuses on derivatization based methods and their different reactions for determination of PGB, GBP, VGB, and baclofen in the biological samples and pharmaceutical preparations reported between 1980 and 2020. High-performance liquid chromatography methods coupled with different detectors are a commonly used methods for determination of GABA analogs after derivatization. These methods cover 38.89% of all developed methods for determination of GABA analogs.

Neuropharmacological and neurobiological relevance of in vivo ¹H-MRS of GABA and glutamate for preclinical drug discovery in mental disorders

Proton magnetic resonance spectroscopy ((1)H-magnetic resonance spectroscopy (MRS)) is a translational modality with great appeal for neuroscience since the two major excitatory and inhibitory neurotransmitters, glutamate, and GABA, can be noninvasively quantified in vivo and have served to explore disease state and effects of drug treatment. Yet, if (1)H-MRS shall serve for decision making in preclinical pharmaceutical drug discovery, it has to meet stringent requirements. In particular, (1)H-MRS needs to reliably report neurobiologically relevant but rather small changes in neurometabolite levels upon pharmacological interventions and to faithfully appraise target engagement in the associated molecular pathways at pharmacologically relevant doses. Here, we thoroughly addressed these matters with a three-pronged approach. Firstly, we determined the sensitivity and reproducibility of (1)H-MRS in rat at 9.4 Tesla for detecting changes in GABA and glutamate levels in the striatum and the prefrontal cortex, respectively. Secondly, we evaluated the neuropharmacological and neurobiological relevance of the MRS readouts by pharmacological interventions with five well-characterized drugs (vigabatrin, 3-mercaptopropionate, tiagabine, methionine sulfoximine, and riluzole), which target key nodes in GABAergic and glutamatergic neurotransmission. Finally, we corroborated the MRS findings with ex vivo biochemical analyses of drug exposure and neurometabolite concentrations. For all five interventions tested, (1)H-MRS provided distinct drug dose-effect relationships in GABA and glutamate over preclinically relevant dose ranges and changes as low as 6% in glutamate and 12% in GABA were reliably detected from 16 mm(3) volumes-of-interest. Taken together, these findings demonstrate the value and limitation of quantitative (1)H-MRS of glutamate and GABA for preclinical pharmaceutical research in mental disorders.

Antiepileptic drugs in non-epilepsy disorders: relations between mechanisms of action and clinical efficacy

Antiepileptic drugs (AEDs) are used extensively to treat multiple non-epilepsy disorders, both in neurology and psychiatry. This article provides a review of the clinical efficacy of AEDs in non-epilepsy disorders based on recently published preclinical and clinical studies, and attempts to relate this efficacy to the mechanism of action of AEDs and pathophysiological processes associated with the disorders. Some newer indications for AEDs have been established, while others are under investigation. The disorders where AEDs have been demonstrated to be of clinical importance include neurological disorders, such as essential tremor, neuropathic pain and migraine, and psychiatric disorders, including anxiety, schizophrenia and bipolar disorder. Many of the AEDs have various targets of action in the synapse and have several proposed relevant mechanisms of action in epilepsy and in other disorders. Pathophysiological processes disturb neuronal excitability by modulating ion channels, receptors and intracellular signalling pathways, and these are targets for the pharmacological action of various AEDs. Attention is focused on the glutamatergic and GABAergic synapses. In psychiatric conditions such as schizophrenia and bipolar disorder, AEDs such as valproate, carbamazepine and lamotrigine appear to have clear roles based on their effect on intracellular pathways. On the other hand, some AEDs, e.g. topiramate, have efficacy for nonpsychiatric disorders including migraine, possibly by enhancing GABAergic and reducing glutamatergic neurotransmission. AEDs that seem to enhance GABAergic neurotransmission, e.g. tiagabine, valproate, gabapentin and possibly levetiracetam, may have a role in treating neurological disorders such as essential tremor, or anxiety disorders. AEDs with effects on voltage-gated sodium or calcium channels may be advantageous in treating neuropathic pain, e.g. gabapentin, pregabalin, carbamazepine, oxcarbazepine, lamotrigine and valproate. Co-morbid conditions associated with epilepsy, such as mood disorders and migraine, may often respond to treatment with AEDs. Other possible disorders where AEDs may be of clinical importance include cancer, HIV infection, drug and alcohol abuse, and also in neuroprotection. A future challenge is to evaluate the second-generation AEDs in non-epilepsy disorders and to design clinical trials to study their effects in such disorders in paediatric patients. Differentiation between the main mechanisms of action of the AEDs needs more consideration in drug selection for tailored treatment of the various non-epilepsy disorders.

Vigabatrin transport across the human intestinal epithelial (Caco-2) brush-border membrane is via the H+ -coupled amino-acid transporter hPAT1

The aim of this investigation was to determine if the human proton-coupled amino-acid transporter 1 (hPAT1 or SLC36A1) is responsible for the intestinal uptake of the orally-administered antiepileptic agent 4-amino-5-hexanoic acid (vigabatrin). The Caco-2 cell line was used as a model of the human small intestinal epithelium. Competition experiments demonstrate that [3H]GABA uptake across the apical membrane was inhibited by vigabatrin and the GABA analogues trans-4-aminocrotonic acid (TACA) and guvacine, whereas 1-(aminomethyl)cyclohexaneacetic acid (gabapentin) had no affect. Experiments with 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF)-loaded Caco-2 cells demonstrate that apical exposure to vigabatrin and TACA induce comparable levels of intracellular acidification (due to H+/amino-acid symport) to that generated by GABA, suggesting that they are substrates for a H+ -coupled absorptive transporter such as hPAT1. In hPAT1 and mPAT1-expressing Xenopus laevis oocytes [3H]GABA uptake was inhibited by vigabatrin, TACA and guvacine, whereas gabapentin failed to inhibit [3H]GABA uptake. In Na+ -free conditions, vigabatrin and TACA evoked similar current responses (due to H+/amino-acid symport) in hPAT1-expressing oocytes under voltage-clamp conditions to that induced by GABA (whereas no current was observed in water-injected oocytes) consistent with the ability of these GABA analogues to inhibit [3H]GABA uptake. This study demonstrates that hPAT1 is the carrier responsible for the uptake of vigabatrin across the brush-border membrane of the small intestine and emphasises the therapeutic potential of hPAT1 as a delivery route for orally administered, clinically significant GABA-related compounds.

Pharmacokinetic Variability of Newer Antiepileptic Drugs

A new generation of antiepileptic drugs (AEDs) has reached the market in recent years with ten new compounds: felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, tiagabine, topiramate, vigabatrin and zonisamide. The newer AEDs in general have more predictable pharmacokinetics than older AEDs such as phenytoin, carbamazepine and valproic acid (valproate sodium), which have a pronounced inter-individual variability in their pharmacokinetics and a narrow therapeutic range. For these older drugs it has been common practice to adjust the dosage to achieve a serum drug concentration within a predefined 'therapeutic range', representing an interval where most patients are expected to show an optimal response. However, such ranges must be interpreted with caution, since many patients are optimally treated when they have serum concentrations below or above the suggested range. It is often said that there is less need for therapeutic drug monitoring (TDM) with the newer AEDs, although this is partially based on the lack of documented correlation between serum concentration and drug effects. Nevertheless, TDM may be useful despite the shortcomings of existing therapeutic ranges, by utilisation of the concept of 'individual reference concentrations' based on intra-individual comparisons of drug serum concentrations. With this concept, TDM may be indicated regardless of the existence or lack of a well-defined therapeutic range. The ten newer AEDs all have different pharmacological properties, and therefore, the usefulness of TDM for these drugs has to be assessed individually. For vigabatrin, a clear relationship between drug concentration and clinical effect cannot be expected because of its unique mode of action. Therefore, TDM of vigabatrin is mainly to check compliance. The mode of action of the other new AEDs would not preclude the applicability of TDM. For the prodrug oxcarbazepine, TDM is also useful, since the active metabolite licarbazepine is measured. For drugs that are eliminated renally completely unchanged (gabapentin, pregabalin and vigabatrin) or mainly unchanged (levetiracetam and topiramate), the pharmacokinetic variability is less pronounced and more predictable. However, the dose-dependent absorption of gabapentin increases its pharmacokinetic variability. Drug interactions can affect topiramate concentrations markedly, and individual factors such as age, pregnancy and renal function will contribute to the pharmacokinetic variability of all renally eliminated AEDs. For those of the newer AEDs that are metabolised (felbamate, lamotrigine, oxcarbazepine, tiagabine and zonisamide), pharmacokinetic variability is just as relevant as for many of the older AEDs. Therefore, TDM is likely to be useful in many clinical settings for the newer AEDs. The purpose of the present review is to discuss individually the potential value of TDM of these newer AEDs, with emphasis on pharmacokinetic variability.

Pharmacological and behavioral characteristics of interactions between vigabatrin and conventional antiepileptic drugs in pentylenetetrazole-induced seizures in mice: an isobolographic analysis

To characterize the anticonvulsant effects and types of interactions exerted by mixtures of vigabatrin (VGB) and conventional antiepileptic drugs (valproate (VPA), ethosuximide (ESM), phenobarbital (PB), and clonazepam (CZP)) in pentylenetetrazole (PTZ)-induced seizures in mice, the isobolographic analysis for three fixed-ratio combinations of 1 : 3, 1 : 1, and 3 : 1 was used. The adverse-effect profile of the combinations tested, at the doses corresponding to their median effective doses (ED(50)) at the fixed-ratio of 1 : 1 against PTZ-induced seizures, was determined by the chimney (motor performance), step-through passive avoidance (long-term memory), pain threshold (pain sensitivity), and Y-maze (general explorative locomotor activity) tests in mice. Additionally, the observed isobolographic interactions were verified in terms of a pharmacokinetic interaction existence. VGB combined with PB or ESM exerted supra-additive (synergistic) interactions against the clonic phase of PTZ-induced seizures, which was associated with the increment of PB or ESM concentrations in the brains of examined animals. The remaining combinations tested (ie VGB+VPA and VGB+CZP) occurred additive in the PTZ test, which was associated with no significant changes in the brain concentrations of VPA and CZP. None of the examined combinations exerted motor impairment in the chimney test in mice. In the standard variant of passive avoidance task (current of 0.6 mA; 2 s of stimulus duration), the combinations of VGB+CZP and VGB+VPA significantly affected long-term memory in mice. Moreover, VGB in a dose-dependent manner lengthened the latency to the first pain reaction in the pain threshold test in mice. The modified variant of step-through passive avoidance task (current of 0.6 mA; stimulus duration based on the latency from the pain threshold test) revealed no significant changes in the long-term memory of animals for the combinations of VGB+VPA and VGB+CZP; so the observed effects in the standard variant of passive avoidance task were a result of the antinociceptive effects produced by VGB. In the Y-maze test, VGB also, in a dose-dependent manner, increased the general explorative locomotor activity of the animals tested. Similarly, the total number of arm entries in the Y-maze was significantly increased for the combinations of VGB+CZP and VGB+ESM, but not for VGB+PB and VGB+VPA. The application of VGB in combination with PB, ESM, CZP, and VPA suppressed the clonic phase of PTZ-induced seizures, having no harmful or deleterious effects on behavioral functioning of the animals tested, which might be advantageous in further clinical practice.

Quantitative determination of vigabatrin and gabapentin in human serum by gas chromatography-mass spectrometry

BACKGROUND

Published methods for routine clinical monitoring of vigabatrin and gabapentin are often very laborious. A simple GC-MS method was developed for the simultaneous quantitative determination of vigabatrin and gabapentin in human serum.

METHODS

After protein precipitation, the compounds are derivatized by methylation and analysed on a polydimethylsiloxane column using splitless injection. Cyclobarbital is used as the internal standard. To attain maximal sensitivity, detection is performed in selected ion monitoring mode.

RESULTS

The method was fully validated and linear calibration curves were obtained in the concentration ranges from 5 to 80 microg/mL for vigabatrin and from 5 to 30 microg/mL for gabapentin. The within-day and day-to-day relative standard deviations at three different concentration levels were <10% and <15%, respectively. The limit of quantitation was 2 mug/mL for both compounds.

CONCLUSIONS

The presented method provides high chromatographic resolution, good sensitivity and unequivocal identification potential and can be used for simultaneous analysis of both antiepileptics.

Biphasic action of vigabatrin on cortical epileptic after-discharges in rats

The time course of the anticonvulsant effect of vigabatrin against cortically induced epileptic after-discharges (ADs) was studied in freely moving rats with implanted electrodes. Adult rats (n=30) were exposed to five stimulation sessions each consisting of six stimulation series at 20-min intervals. The first session was a control one, then two groups of animals (n=10 each) were given vigabatrin (600 or 1,200 mg/kg i.p.), the control animals received physiological saline. Stimulation sessions were repeated 1, 24, 48, and 96 hours after the injection. Control animals exhibited an increased transition from the spike-and-wave type of AD to the second, "limbic" type and an increased intensity of movements accompanying stimulation. ADs in the second and subsequent sessions were, however, shorter than in the first session. Vigabatrin facilitated the transition to the second type of AD 1 h after administration but suppressed this transition as well as decreased the number of stimulations eliciting ADs 48 h later. AD duration and the severity of clonic seizures accompanying spike-and-wave ADs were influenced similarly. The effects of the lower dose of vigabatrin were more marked than those of the higher dose. The biphasic action of vigabatrin in our model might be due either to uneven changes of GABA concentration in different brain structures or to an additional mechanism of action. Our results in a cortical model of seizure demonstrate that the sequence of pro- and anticonvulsant actions of vigabatrin is not restricted to seizures of limbic origin and might represent a general phenomenon.

Development of vigabatrin-induced lesions in the rat brain studied by magnetic resonance imaging, histology, and immunocytochemistry

Vigabatrin, the gamma-aminobutyric acid transaminase (GABA-T)-inhibiting anticonvulsant drug, was given orally at a dose of 275 mg/kg/day to rats (n = 6) in their feed for a period of 12 weeks, during which T2-weighted magnetic resonance images (MRIs) and diffusion-weighted MRIs (DWIs) were collected at weeks 1, 3, 6, 9, and 12. Half the rats (n = 3; and half their age-matched littermate controls; n = 3) were then killed for histopathological confirmation of the observed VGB-induced cerebellar and cortical white-matter lesions. VGB was removed from the diet and additional MRIs of the remaining rats taken at weeks 14, 17, 20, and 24, at which time they (n = 3), along with remaining controls (n = 3), were also killed for histopathology. The T2-weighted MRIs acquired were used to compute T2 relaxation time maps. Statistically significant VGB-induced T2 increases were observed in the frontal and occipital cortices and in the cerebellar white matter (CWM). The cerebellar lesions were more clearly discerned by eye in the DWIs than by T2-contrast alone. During the recovery period the VGB-treatment group CWM-T2 and CWM-DWI hyperintensity greatly decreased as the reversible lesion disappeared. As expected, histological and immunocytochemical examinations demonstrated the presence of intra-myelinic edema, microvacuolation, and reactive astrocytosis in the CWM and cortex after 12 weeks VGB-treatment. In the remaining animals microvacuolation of the white matter had not completely resolved during the 12-week recovery phase. The data show that quantitative MRI T2-relaxometry can be used to detect VGB-induced CNS pathology, and also suggest that DWI is particularly sensitive to the cerebellar lesion. The reversible neurotoxicity of global GABA-elevation in experimental animals is discussed.

Therapeutic drug monitoring of the newer antiepileptic drugs

The aim of the present review is to discuss the potential value of therapeutic drug monitoring (TDM) of the newer antiepileptic drugs (AEDs) felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin, and zonisamide. Studies of the relationship between serum concentrations and clinical efficacy of these drugs are reviewed, and the potential value of TDM of the drugs is discussed based on their pharmacokinetic properties and mode of action. Analytical methods for the determination of the serum concentrations of these drugs are also briefly described. There are only some prospective data on the serum concentration-effect relationships, and few studies have been designed primarily to study these relationships. As TDM is not widely practiced for the newer AEDs, there are no generally accepted target ranges for any of these drugs, and for most a wide range in serum concentration is associated with clinical efficacy. Furthermore, a considerable overlap in drug concentrations related to toxicity and nonresponse is reported. Nevertheless, the current tentative target ranges for felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine (10-hydroxy-carbazepine metabolite), tiagabine, topiramate, vigabatrin, and zonisamide are 125 to 250 micromol/L, 70 to 120 micromol/L, 10 to 60 micromol/L, 35 to 120 micromol/L, 50 to 140 micomol/L, 50 to 250 nmol/L, 15 to 60 micromol/L, 6 to 278 micromol/L, and 45 to 180 micromol/L, respectively. Further systematic studies designed specifically to evaluate concentration-effect relationships of the new AEDs are urgently needed. Although routine monitoring in general cannot be recommended at present, measurements of some of the drugs is undoubtedly of help with individualization of treatment in selected cases in a particular clinical setting.

In vivo monitoring of rat brain metabolites during vigabatrin treatment using localized 2D-COSY

A two-dimensional COSY-based localization sequence was designed to allow the in vivo monitoring of proton metabolites in rat brain [particularly gamma-aminobutyric acid (GABA), glutamine, taurine and myo-inositol]. The sequence incorporated OSIRIS signal localization, B1-insensitive water suppression and phase-sensitive COSY acquisition. The method was used to study the effects of the GABA-transaminase inhibitor vigabatrin on rat brain metabolite concentrations. Wistar rats were treated daily for 3 days with an oral dose of vigabatrin (200 mg/kg, n = 4). Localized COSY spectra were obtained during a 120 min acquisition from a 270 microl central brain voxel and compared with nine untreated control animals. Significant elevations were observed in GABA (267% of control, p < 0.005, Mann-Witney test), glutamine (130% of control, p < 0.005) and taurine (113% of control, p < 0.05). Changes in GABA and taurine were consistent with previous data on the action of Vigabatrin, and support a previously hypothesized link between these compounds. The increase in glutamine was more surprising and may reflect the balance between the level and/or site of GABA-transaminase inhibition and downregulation of GABA synthesis.

In vivo GABA+ measurement at 1.5T using a PRESS-localized double quantum filter

A point-resolved spectroscopy (PRESS)-localized double quantum filter was implemented on a 1.5T clinical scanner for the estimation of gamma-amino butyric acid (GABA) concentrations in vivo. Several calibrations were found to be necessary for consistent results to be obtained. The apparent filter yield was approximately 38%; filter strength was sufficient to reduce the singlet metabolite peaks in vivo to below the level of the noise. Metabolite-nulled experiments were performed, which confirmed that significant overlap occurred between macromolecule signals and the GABA resonance at 3.1 ppm. Although the multiplet arm at 2.9 ppm was confirmed to be relatively free of contamination with macromolecules, some contribution from these and from peptides is likely to remain; therefore, the term GABA+ is used. GABA+ concentrations were estimated relative to creatine (Cr) at the same echo time (TE) in a group of controls, studied on two occasions. The GABA+ concentration in 35-ml regions of interest (ROIs) in the occipital lobe was found to be 1.4 +/- 0.2 mM, with scan-rescan repeatability of 38%.

GABA transaminase inhibition induces spontaneous and enhances depolarization-evoked GABA efflux via reversal of the GABA transporter

The GABA transporter can reverse with depolarization, causing nonvesicular GABA release. However, this is thought to occur only under pathological conditions. Patch-clamp recordings were made from rat hippocampal neurons in primary cell cultures. Inhibition of GABA transaminase with the anticonvulsant gamma-vinyl GABA (vigabatrin; 0.05-100 microm) resulted in a large leak current that was blocked by bicuculline (50 microm). This leak current occurred in the absence of extracellular calcium and was blocked by the GABA transporter antagonist SKF-89976a (5 microm). These results indicate that vigabatrin induces spontaneous GABA efflux from neighboring cells via reversal of GABA transporters, subsequently leading to the stimulation of GABA(A) receptors on the recorded neuron. The leak current increased slowly over 4 d of treatment with 100 microm vigabatrin, at which time it reached an equivalent conductance of 9.0 +/- 4.9 nS. Blockade of glutamic acid decarboxylase with semicarbazide (2 mm) decreased the leak current that was induced by vigabatrin by 47%. In untreated cells, carrier-mediated GABA efflux did not occur spontaneously but was induced by an increase in [K(+)](o) from 3 to as little as 6 mm. Vigabatrin enhanced this depolarization-evoked nonvesicular GABA release and also enhanced the heteroexchange release of GABA induced by nipecotate. Thus, the GABA transporter normally operates near its equilibrium and can be easily induced to reverse by an increase in cytosolic [GABA] or mild depolarization. We propose that this transporter-mediated nonvesicular GABA release plays an important role in neuronal inhibition under both physiological and pathophysiological conditions and is the target of some anticonvulsants.

Simultaneous in vivo spectral editing and water suppression

Water suppression is typically performed in vivo by exciting the longitudinal magnetization in combination with dephasing, or by using frequency-selective coherence generation. MEGA, a frequency-selective refocusing technique, can be placed into any pulse sequence element designed to generate a Hahn spin-echo or stimulated echo, to dephase transverse water coherences with minimal spectral distortions. Water suppression performance was verified in vivo using stimulated echo acquisition mode (STEAM) localization, which provided water suppression comparable with that achieved with four selective pulses in 3,1-DRYSTEAM. The advantage of the proposed method was exploited for editing J-coupled resonances. Using a double-banded pulse that selectively inverts a J-coupling partner and simultaneously suppresses water, efficient metabolite editing was achieved in the point resolved spectroscopy (PRESS) and STEAM sequences in which MEGA was incorporated. To illustrate the efficiency of the method, the detection of gamma-aminobutyric acid (GABA) was demonstrated, with minimal contributions from macromolecules and overlying singlet peaks at 4 T. The estimated occipital GABA concentration was consistent with previous reports, suggesting that editing for GABA is efficient when based on MEGA at high field strengths.

Vigabatrin enhances promoted release of GABA in neonatal rat optic nerve

Vigabatrin (gamma-vinyl GABA) is an antiepileptic drug and blocks GABA transaminase activity resulting in elevations in cellular GABA levels in the brain. Nipecotic acid (NPA) promotes release of GABA from neonatal optic nerve astrocytes, resulting in a bicuculline-sensitive depolarization of the optic nerve axons. The NPA-induced depolarization of vigabatrin-treated rats (100 mg/kg, i.p.) more than doubled, suggesting an elevation in free GABA levels; the GABA transporter inhibitor, NO-711 reduced the depolarization. These results are consistent with the known ability of vigabatrin to block the GABA degradation enzyme GABA-transaminase, suggesting that vigabatrin elevates astrocytic GABA levels, thereby favoring greater release of GABA through the GABA transporter.

Metabolite-specific NMR spectroscopy in vivo

An outline is presented of metabolite-specific in vivo NMR spectroscopy (particularly in brain). It reviews from a physical spectroscopist's perspective, the need for and the methods of observation of, individual metabolite resonances.

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

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

Magnetic resonance spectroscopy

Magnetic resonance spectroscopy (MRS) is noninvasive and may be readily combined with magnetic resonance imaging (MRI). Attention has focussed on proton (1H) and phosphorus (31P) MRS, and studies have been undertaken by using single voxels or many voxels simultaneously (chemical-shift imaging, magnetic resonance spectroscopic imaging). The latter is more difficult and prone to artefact but potentially yields significantly more information. 1H MRS has principally yielded data on concentrations of N-acetyl aspartate (NAA), choline, creatine, and phosphocreatine. NAA is located primarily within neurons, and reduction of the ratio of NAA to choline, creatine, and phosphocreatine is a marker of neuronal loss and dysfunction. This technique may be useful as a noninvasive tool for localizing epileptogenic foci, but its role requires further evaluation. As with all functional imaging methods, coregistration with high-quality MRI is essential for interpreting data. 1H MRS can be used also to estimate cerebral concentrations of several neurotransmitters: glutamate, glutamine, and gamma-aminobutyric acid (GABA). This may prove useful for characterizing the neurometabolic profiles of patients with different epilepsy syndromes and for evaluating the effects of medical and surgical treatments. 31P MRS can detect adenosine triphosphate, phosphodiesters, phosphomonoesters, phosphocreatine, and inorganic phosphate, and estimate intracerebral pH. Abnormalities that have been associated with epileptogenic brain areas include increased inorganic phosphate, reduced phosphomonoesters, and increased pH. Only small numbers of patients have been studied, however, so that conclusions are not definitive, and the clinical role of this technique is not yet established.

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

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

Clinical pharmacokinetics of new antiepileptic drugs

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

New techniques in magnetic resonance and epilepsy

Developments in magnetic resonance imaging (MRI), magnetic resonance spectroscopy (MRS), functional magnetic resonance imaging (fMRI), positron emission tomography (PET), and single photon emission tomography (SPECT) have opened new opportunities for noninvasive brain investigation. Functional imaging methods involving noninvasive MRI and minimally invasive PET and SPECT are available that allow investigation of brain abnormality in intractable epilepsy patients. Noninvasive techniques enable the investigation of many aspects of the underlying neuropathologic basis of intractable seizures and of the relationship of functional abnormalities both to structural abnormalities and to the seizure focus. New MRI techniques demonstrate the structure of the brain in fine detail (especially the hippocampus), provide information about the underlying metabolism of brain regions, and demonstrate functional activity of the brain with high spatial and temporal resolution. The clinical impact of this noninvasive information cannot be overstated and these techniques provide indispensable information to neurologists specializing in epileptology. The proper use and interpretation of the findings provided by these new technologies will be a major challenge to epilepsy programs in the next few years.

Lactate efflux and intracellular pH during severe hypoxia in rat cerebral cortex in vitro studied by nuclear magnetic resonance spectroscopy

Intracellular pH (pHi) and lactate were monitored in a superfused brain slice preparation using NMR spectroscopy in order to study the role of lactate washout in maintenance of pHi during hypoxia. Data are consistence with a functioning lactate-H+ cotransport in the energetically intact cerebral cortex. This pathway is not, however, linked to regulation of pHi during energy failure with external pH of 6.8 and thus appears not to have physiological impact in H+ homeostasis during cerebral hypoxia.