Psychophysiological and psychometric studies after manipulating the GABA system by vigabatrin, a GABA-transaminase inhibitor

Pharmacokinetic aspects of the anti-epileptic drug substance vigabatrin: focus on transporter interactions

Drug transporters in various tissues, such as intestine, kidney, liver and brain, are recognized as important mediators of absorption, distribution, metabolism and excretion of drug substances. This review gives a current status on the transporter(s) mediating the absorption, distribution, metabolism and excretion properties of the anti-epileptic drug substance vigabatrin. For orally administered drugs, like vigabatrin, the absorption from the intestine is a prerequisite for the bioavailability. Therefore, transporter(s) involved in the intestinal absorption of vigabatrin in vitro and in vivo are discussed in detail. Special focus is on the contribution of the proton-coupled amino acid transporter 1 (PAT1) for intestinal vigabatrin absorption. Furthermore, the review gives an overview of the pharmacokinetic parameters of vigabatrin across different species and drug–food and drug–drug interactions involving vigabatrin.

Mechanism of action of vigabatrin: correcting misperceptions

Discovered more than three decades ago, vigabatrin is approved in more than 50 countries as adjunctive therapy for adult patients with refractory complex partial seizures who have responded inadequately to several alternative treatments and as monotherapy for pediatric patients aged 1 month to 2 years with infantile spasms. Contrary to a fairly common misperception, the compound's mechanism of action is very well-characterized in animal models and cell cultures. γ-Aminobutyric acid (GABA)-ergic synapses comprise approximately 30% of all synapses within the central nervous system, and therein underlies the primary mode of synaptic inhibition. Vigabatrin was rationally designed to have a specific effect on brain chemistry by inhibiting the GABA-degrading enzyme, GABA transaminase, resulting in a widespread increase in GABA concentrations in the brain. The increase in GABA functions as a brake on the excitatory processes that can initiate seizure activity. Despite the short half-life of vigabatrin in the body (5-7 h) and its relatively low concentration in cerebrospinal fluid (10% of the concentration observed in plasma), it has the profound effect of increasing GABA concentration in the brain for more than a week after a single dose in humans. This effect persists steadily over years of vigabatrin administration and results in significant and persistent decreases in seizure activity. Vigabatrin can be effective with once-daily dosing. Because of its specificity, vigabatrin has helped researchers explore the specific mechanisms within the brain that underlie seizure activity.

Cognitive side effects of antiepileptic drugs

Antiepileptic drugs produce global changes in the excitation levels in the central nervous system and often lead to cognitive and behavioral deficits. These deficits vary and must be considered independently in every patient. A number of consistent risk factors have been established. Polypharmacy and high blood levels of an antiepileptic drug (AED) increase the risk of cognitive side effects. Different effects have been demonstrated for some AEDs, but comparative data are incomplete across all of them. Other factors such as patient age and type/frequency of seizures may also be important contributors to the patient's cognitive state. AEDs can have positive or negative effects on mood, providing another consideration in choosing the course of treatment.

Current discoveries on the cognitive effects of antiepileptic drugs

The cognitive effects of antiepileptic drugs (AEDs) are of particular concern to clinicians because these drugs are the primary therapeutic modality for managing epilepsy. In general, the cognitive effects of most AEDs are modest and offset by their benefit in reducing seizures. Nonetheless, the cognitive effects of a particular AED may be clinically significant when treating specific patient populations, such as children and the elderly.

Vigabatrin dosing during haemodialysis

The antiepileptic drug vigabatrin has shown efficacy in the treatment of patients with refractory epilepsy. Unlike many other antiepileptics it is not bound to plasma protein and mainly eliminated by the kidney. Although the therapeutic and toxic serum concentration range is not clearly defined and efficacy and toxicity are not closely correlated with the dose, factors decreasing vigabatrin elimination such as advanced age or renal failure may pose risk of untoward effects. Thus far there are no dose recommendations available for patients on haemodialysis. We report on an epileptic patient who experienced severe, partially reversible renal failure as a consequence of near-drowning. In this patient serum concentrations of vigabatrin were measured repeatedly both during haemodialysis and after partial recovery of renal function. The terminal elimination half-life in this patient was 41 hours during the period of severe renal failure (creatinine clearance < 5 ml/min). As about 60% of vigabatrin was removed from the blood pool by haemodialysis in these patients the antiepileptic should be administered after dialysis. To maintain serum concentrations in the usual range and to control seizure activity only 500 mg vigabatrin every 3 days were necessary.

Cognitive side-effects of chronic antiepileptic drug treatment: a review of 25 years of research

Over 90 investigations have been conducted over the past 25 years to determine what effect AEDs have on cognition. No satisfactory answer to this problem can be given, however, chiefly because there is a paucity of studies that pass fairly basic standards of methodology, design and analysis that apply to the evaluation of any clinical research. This severely limits the precision of statements regarding cognitive AED effects. More particularly, there is little reason to recommend any of the first-line AEDs as the AED of choice from the standpoint of cognitive side-effects. On the basis of the present review we are not in a position to provide a straightforward answer to the most pertinent question, i.e., whether AEDs in therapeutic doses have any cognitive effects at all, good or bad. If we reduce the available database to monotherapy studies in epilepsy that use control group data for comparison, employ an appropriate form of repeated measures analysis, and provide sufficient information, very few studies remain that are directly relevant to this issue. This in itself precludes definitive conclusions. As can be seen from Table 9, absolute effects of CBZ and VPA have been examined in epilepsy patients three times each (in four studies), PB has been examined two times, PHT only once. In addition to the paucity of relevant data, there are miscellaneous validity concerns in all of these studies, one recurring theme being that of inconclusive 'no effect' findings with small samples. Without firm knowledge about absolute effects, relative effects, and particularly their absence, are difficult to interpret. Employing the above criteria (except that concerning controls), ten epilepsy studies that address this issue remain (Table 10). It is instructive to look at the number of times particular AEDs have been compared against each other (Table 11). CBZ has been compared to PHT five times, other comparisons occur only once or twice. Again, this is hardly a basis for definitive statements, particularly because validity concerns occur here as well. Recurring concerns here are scattered significant findings that tend to disappear if adjustment of the significance level for multiple comparisons is done, and inconclusive 'no difference' findings with small samples. Even if there were no conclusion validity concerns in individual studies, comparison between studies would be complicated by considerable variation in the subjects studied. Five of the studies summarized in Tables 8 and 9 use children as subject, nine use adults; results obtained in one group may not be generalizable to the other. Also, subjects may be newly diagnosed cases, or patients already on chronic treatment. The latter choice of subjects may be a factor working against detecting cognitive side-effects, as the damage (if any) may already have been done before the beginning of the trial. In addition, a wide variety of assessment tools have been used to search for cognitive effects of AEDs, ranging from measurements of reaction time and motor speed to intelligence tests. Some of these may be more sensitive to drug induced changes in cognition than others. Still, the tentative overall picture emerging from the creme de la creme of research on cognitive AED effects is that differences in cognitive profiles may not be very large. An important point here, of course, is the magnitude of the difference one considers worth detecting. Very few studies have attempted to answer this question. In the majority of studies we examined, a large treatment effect was anticipated implicitly, judging from the generally limited sample sizes. The choice of a study design based on a large treatment effect size may not always be appropriate, though. Of course, one could argue that it is only large effects that may be of practical or clinical significance anyways [30] and that effects of lesser magnitude are of no consequence. However, there are many examples where even a small benefit of one treatme

Effects of differing dosages of vigabatrin (Sabril) on cognitive abilities and quality of life in epilepsy

Vigabatrin (VGB) prevents seizures by irreversible inhibition of gamma-aminobutyric acid (GABA) transaminase and a resulting increase in GABA levels. We evaluated the cognitive and quality-of-life (QOL) effects of VGB in a double-blinded, add-on, placebo-controlled, parallel group dose-response study of patients with focal epilepsy whose complex partial seizures (CPS) were difficult to control. In a single investigation, patients were randomly assigned to placebo (n = 40), 1 g VGB (n = 36), 3 g VGB (n = 38), or 6 g VGB (n = 32), treated for 12 weeks after a 6-week dose escalation period, and tested at the end of the baseline period and at the end of the treatment period with eight cognitive measures and three tests of mood and adjustment. The patient groups were highly similar at study entry. Results at the end of the study showed substantial relief from seizures. The Digit Cancellation Test showed decreases in performance with increasing doses of VGB. Performance on no other test showed any decrement with increasing dosage. Relief from seizures was not associated with changes on the psychological tests. VGB is a useful antiepileptic drug (AED) that has little impact on tests of either cognitive abilities or QOL, even at a high dose.

Vigabatrin

gamma-Aminobutyric acid (GABA) was first proposed as a putative inhibitory neurotransmitter by Elliot and van Gelder in 1958. Since then, numerous efforts have been made to find ways to increase GABA at its receptor sites, based on the findings that decreased GABA results in convulsions in animals and that agents enhancing GABA-mediated functions can have antiepileptic effects. However, the relationship between GABA levels and seizures is not simple. Seizures can occur even in the presence of elevated GABA levels. Indeed, it is possible that regional biochemical differences in the brain can be important. The antiepileptic effects of GABA depend on the mechanism whereby GABA-mediated inhibition is enhanced. Since the 1970s, several compounds have been developed that are designed to act in some manner on the GABA system. These compounds affect GABA-mediated inhibition at different levels and appear to have varied effects, depending on their mechanism of action. To date, specific antiepileptic drugs (AEDs) with potential GABA-inhibitory effects have been designed either to have GABA agonist properties, to inhibit GABA catabolism, to inhibit GABA uptake, or to facilitate GABA release or facilitate GABAA receptor activity. Vigabatrin (VGB) was designed specifically to inhibit GABA transaminase and thereby increase the availability of GABA in the brain. Study data and clinical experience over the past 14 years have demonstrated VGB to be an effective AED.

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.

Vigabatrin

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

Gamma-vinyl GABA (vigabatrin) and mood disturbances

We explored factors that may predispose patients to adverse mood effects during treatment with vigabatrin (gamma-vinyl GABA; VGB): mood disorders before VGB treatment, type of epilepsy, seizure type and seizure frequency, type and number of comedication, and VGB dose. The clinical relevance of such a study is that it may help identify circumstances in which VGB should be administered with caution. Seventy-three patients (40 males, 33 females), all with refractory epilepsies, who received VGB as add-on therapy, were assessed by the Amsterdamse Stemmingslyst (ASL), a mood-rating scale, before the start of treatment, and demographic and clinical data were recorded. The patients were followed for 6 months after the start of VGB treatment. Treatment with VGB had to be discontinued in 38 patients (52% of the total sample). Mood problems were the main reason for discontinuation in 9 (12.3% of the total sample). In 6 other patients, mood problems were mentioned as the reason for discontinuing treatment, in combination with lack of drug efficacy. Development of adverse mood effects could not be predicted by a specific mood profile on the ASL. Before treatment, the "mood problems discontinuation group" did not show extreme scores for any assessed areas of mood and no significant differences from other patients were noted on the mood scales. Neither did clinical or demographic data show statistically confirmed specific characteristics for the mood problems discontinuation group, though the patients tended to use more antiepileptic drugs (AEDs) as cotherapy, to have a slightly lower daily dose of VGB, to be slightly older, and were mostly female.(ABSTRACT TRUNCATED AT 250 WORDS)

Newer antiepileptic drugs. Towards an improved risk-benefit ratio

Epilepsy is one of the most common neurological disorders. Even though existing antiepileptic drugs can render 80% of newly diagnosed patients seizure free, a significant number of patients have chronic intractable epilepsy causing disability with considerable socioeconomic implications. There is, therefore, a need for more potent and effective antiepileptic drugs and drugs with fewer adverse effects, particularly CNS effects. Drugs for the treatment of partial seizures are particularly needed. With major advances in our understanding of the basic neuropathology, neuropharmacology and neurophysiology of epilepsy, numerous candidate novel antiepileptic drugs have been developed in recent years. This review comparatively evaluates the pharmacokinetics, efficacy and adverse effects of 12 new antiepileptic drugs namely vigabatrin, lamotrigine, gabapentin, oxcarbazepine, felbamate, tiagabine, eterobarb, zonisamide, remacemide, stiripentol, topiramate and levetiracetam (ucb-L059). Of the 12 drugs, vigabatrin, lamotrigine and gabapentin have recently been marketed in the UK. Five of these new drugs have known mechanisms of action (vigabatrin, lamotrigine, tiagabine, oxcarbazepine and eterobarb), which may provide for a more rational approach to the treatment of epilepsy. Oxcarbazepine, remacemide and eterobarb are prodrugs. Vigabatrin, gabapentin and topiramate are more promising on the basis of their pharmacokinetic characteristics in that they are excreted mainly unchanged in urine and not susceptible to significant pharmacokinetic interactions. In contrast, lamotrigine, felbamate and stiripentol exhibit significant drug interactions. Essentially, all the drugs are effective in partial or secondarily generalised seizures and are effective to varying degrees in other seizure types. Particularly welcome is the possible effectiveness of zonisamide in myoclonus and felbamate in Lennox-Gastaut syndrome. In relation to adverse effects, CNS effects are observed with all drugs, however, gabapentin, remacemide and levetiracetam appear to exhibit least. There is also the possibility of rational duotherapy, using drugs with known mechanisms of action, as an additional therapeutic approach. The efficacy of these 12 antiepileptic drug occurs despite the fact that candidate antiepileptic drugs are evaluated under highly unfavourable conditions, namely as add-on therapy in patients refractory to drug management and with high seizure frequency. Thus, whilst candidate drugs which do become licensed are an advance in that they are effective and/or are associated with less adverse effects than currently available antiepileptic drugs in these patients, it is possible that these drugs may exhibit even more improved risk-benefit ratios when used in normal clinical practice.

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

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

Six-year follow-up study on the efficacy and safety of vigabatrin in patients with epilepsy

Twenty-five patients with epilepsy (mostly with partial seizures) who had responded favourably to a short-term trial of add-on vigabatrin entered maintenance treatment. After 52 to 78 months, 15 patients continue to take the drug with good therapeutic response. Median monthly seizure frequency during the last 2 months on vigabatrin in all patients, including drop-outs, was 3.5 (range 0-74) as compared with 10 (range 3-98) during an initial placebo period (p < 0.01). Drop-outs were caused by adverse events in 2 cases (ataxia and psychotic symptoms respectively), seizure breakthrough in 4 cases and reasons unrelated to treatment in 4 patients. In most patients, side effects were absent or mild, the most frequent complaint being weight gain. It is concluded that the antiepileptic efficacy and good clinical tolerability of vigabatrin are generally maintained during long-term treatment for up to 6 years.

Chronic toxicity studies with vigabatrin, a GABA-transaminase inhibitor

The GABA-transaminase inhibitor, vigabatrin, has been shown to have a rather low degree of acute toxicity in several animal species. Oral administration of the drug at 1,000 mg/kg/day for 2-4 weeks caused decreased food consumption and weight loss with resultant prostration and death in both rats and dogs. Dosages of 200 mg/kg/day were tolerated for a year without clinical signs in dogs, although rats suffered reduced weight gains and convulsions after 3-4 months when given the drug in the diet. The convulsions continued to occur frequently throughout the one-yr study, but abated 3-4 months after cessation of treatment. The only consistent histopathologic evidence of toxicity in rats and dogs has been the finding of intramyelinic edema (microvacuolation) in the brain, most notably in certain areas of white matter (cerebellum, reticular formation and optic tract in rats and columns of fornix and optic tract in dogs). No lesions were found in the spinal cord or peripheral nervous system. It took several weeks for the microvacuolation to develop, even at high dosages, but it did not continue to progress thereafter, even though a slight effect was noted at dosages as low as 30-50 mg/kg/day after one yr of treatment. The intramyelinic edemia disappeared within a few weeks after treatment was withdrawn. No residual effects were observed in dogs, whereas rats exhibited swollen axons and microscopic mineralized bodies in the cerebellum. Monkeys exhibited no adverse clinical effects except for occasional loose stools at 300 mg/kg/day. After 16 months of oral treatment at 300 mg/kg/day any suggestion of intramyelinic edema was considered to be equivocal, and there was no evidence of any effect in the 50 or 100 mg/kg/day monkeys after 6 yr of treatment. Higher doses caused chronic diarrhea, thus limiting the dosage in this species. Vigabatrin was shown to be well absorbed in rat, dog and man, whereas dose-limited absorption occurred in the monkey. Metabolism is practically nil in all 4 species and the primary elimination pathway is by glomerular filtration. Because vigabatrin is an irreversible inhibitor of GABA-transaminase and the enzyme has a slow turnover rate, plasma levels of the drug are not indicative of its pharmacologic activity. For this reason cerebrospinal fluid levels of GABA and vigabatrin were evaluated, with considerable species differences being noted. The significance of these differences in relation to the differences in toxic response is discussed.

Effects of single doses of vigabatrin on CSF concentrations of GABA, homocarnosine, homovanillic acid and 5-hydroxyindoleacetic acid in patients with complex partial epilepsy

Vigabatrin, as a single oral dose of 50 mg/kg, was administered to 11 patients with drug-refractory complex partial epilepsy. Serial lumbar punctures were performed prior to and 5 times within the first week following treatment. Cerebrospinal fluid (CSF) concentrations of total GABA, free GABA, homocarnosine, homovanillic acid (HVA), 5-hydroxyindoleacetic acid (5-HIAA) and vigabatrin were determined as well as blood vigabatrin levels. CSF GABA, homocarnosine, HVA and 5-HIAA concentrations increased by 6 h after the single dose and remained elevated for up to 5-7 days. In contrast, CSF and blood vigabatrin levels were maximal within the first 24 h and were no longer detectable thereafter. Hence, these results are consistent with vigabatrin acting as an irreversible inhibitor of GABA-transaminase and suggest that it may also increase biogenic amine turnover.

Acute and subacute CNS effects of milacemide in elderly people: double-blind, placebo-controlled quantitative EEG and psychometric investigations

In a double-blind, placebo-controlled study the encephalotropic and psychotropic properties of acutely and chronically administered milacemide--a new derivative of glycine showing anti-convulsant action by increasing GABA concentrations and endogenous glycine pools in the brain--were investigated in 12 elderly subjects in their late sixties. Each subject had a treatment period of 2 weeks (with 400 mg b.i.d. and 1,200 mg b.i.d. orally administered in week 1 and 2, respectively) and another period of 2 weeks with placebo. A treatment-free interval of 1 week was introduced in between. EEG-recordings, psychometric and psychophysiological tests as well as evaluation of pulse, blood pressure and side effects were carried out at the hours 0, 2, 4, 6 and 8 after the administration of one single dose of 400 mg or placebo (acute effect), after 1 week's and 2 weeks' chronic administration (chronic effect), as well as after one additional superimposed dosage of 400 mg and 1,200 mg on days 8 and 15 of chronic treatment, respectively (super-imposed effect). Computer-assisted spectral analysis of the EEG demonstrated after single doses of 400 mg milacemide significant changes in the resting EEG indicative of improvement in vigilance (beta augmentation, acceleration of the beta centroid), while in the vigilance-controlled recording condition a dissociative vigilance shift occurred seen also after antidepressants of the amitriptyline and imipramine type (increase of both delta/theta and beta activity, decrease of alpha activity). One week chronic treatment resulted in the same V- and R-EEG profiles. However, after the 2nd weeks' treatment with higher doses as well as after one additional superimposed dosage of 1,200 mg milacemide both V- and R-EEG recordings demonstrated dissociative vigilance shifts. Time/treatment-efficacy calculations showed the chronic effect to be more pronounced than the acute effect, which peaked in the 2nd hour after oral drug administration. Psychometric analyses exhibited in noopsychic variables a significant improvement in intellectual and mnestic performance but even more so in the thymopsyche an improvement in subjective well-being and affectivity. Psychophysiological tests showed a decrease in CFF, while static and dynamic pupillometry and skin conductance measures remained unchanged. Pulse rate, systolic and diastolic blood pressure were not altered. The findings are discussed in the light of the involvement of GABA in the pathogenesis and treatment of affective disorders.