Vigabatrin-induced decrease in serum phenytoin concentration does not involve a change in phenytoin bioavailability

Antiepileptic drug therapy: does mechanism of action matter?

This article represents a synthesis of presentations made by the authors during a scientific meeting held in London on 7 June 2010 and organized by GlaxoSmithKline. Each speaker produced a short précis of his lecture to answer a specific question, resulting in an overview of what we know about the relevance of the mechanisms of action of antiepileptic drugs in determining appropriate combination therapies for the treatment of drug-resistant epilepsy.

Pharmacokinetic and pharmacodynamic drug interactions during treatment with vigabatrin

Pharmacokinetic drug interactions take place when one drug interacts with another at the level of metabolism, absorption or excretion. Pharmacodynamic interactions take place at the level of receptor sites, where they may have additive or potentiating effects. Vigabatrin is relatively free of pharmacokinetic interactions, and though it is associated with about a 20% decrease in serum levels of concomitantly administered phenytoin, the reduction is of little clinical significance. The mechanism underlying this effect is unknown. Because vigabatrin increases GABA-mediated inhibition in the brain (an action that is believed to account for its anticonvulsant effects), it might be expected to potentiate the CNS effects of benzodiazepines and alcohol. However, very sensitive eye movement studies have failed to detect any evidence of such an interaction. Overall, vigabatrin appears to be remarkably free of drug interactions. As a result, it is easier to use in clinical practice than older anti-epilepsy agents. Perhaps the most important finding of the interaction studies with vigabatrin is that there is no need for patients receiving the drug to be told to avoid alcohol.

Antiepileptic drugs--best practice guidelines for therapeutic drug monitoring: a position paper by the subcommission on therapeutic drug monitoring, ILAE Commission on Therapeutic Strategies

Although no randomized studies have demonstrated a positive impact of therapeutic drug monitoring (TDM) on clinical outcome in epilepsy, evidence from nonrandomized studies and everyday clinical experience does indicate that measuring serum concentrations of old and new generation antiepileptic drugs (AEDs) can have a valuable role in guiding patient management provided that concentrations are measured with a clear indication and are interpreted critically, taking into account the whole clinical context. Situations in which AED measurements are most likely to be of benefit include (1) when a person has attained the desired clinical outcome, to establish an individual therapeutic concentration which can be used at subsequent times to assess potential causes for a change in drug response; (2) as an aid in the diagnosis of clinical toxicity; (3) to assess compliance, particularly in patients with uncontrolled seizures or breakthrough seizures; (4) to guide dosage adjustment in situations associated with increased pharmacokinetic variability (e.g., children, the elderly, patients with associated diseases, drug formulation changes); (5) when a potentially important pharmacokinetic change is anticipated (e.g., in pregnancy, or when an interacting drug is added or removed); (6) to guide dose adjustments for AEDs with dose-dependent pharmacokinetics, particularly phenytoin.

Vigabatrin

Refractory epilepsies such as infantile spasms (IS) and complex partial seizures (CPS) can have a severe negative impact on the neurological integrity and quality of life of affected patients, in addition to drastically increasing their risk of premature mortality. Early identification of potentially effective pharmacotherapy agents is important. Vigabatrin has been shown to be a generally well tolerated and effective antiepileptic drug (AED) in a wide variety of seizure types affecting both children and adults, particularly those with IS and CPS. A bilateral, concentric constriction of the peripheral visual field characterizes the visual field defect (VFD) associated with vigabatrin, well characterized by numerous studies. This peripheral VFD presents in 30-50% of patients with exposure of several years; however, most of these patients are asymptomatic. In well-controlled studies, the earliest onset in patients with CPS is 11 months and at 5 months in infants, with average onsets being more than 5 years and 1 year, respectively. Patients with a peripheral VFD retain an average 65 degrees of lateral vision (normal, 90 degrees). The fact that many patients never develop the vigabatrin-related peripheral VFD, despite long-term exposure at high doses, may support the hypothesis that the injury is an idiosyncratic adverse drug reaction (as opposed to a strict dose- or duration-dependent toxicity). Effective testing methods are available to aid in the early detection and management of the peripheral VFD. This article discusses issues of importance to clinical decision-making in the use of vigabatrin to assist the physician and patient in assessing the benefits of vigabatrin therapy and understanding the potential risks of the VFD and uncontrolled seizures.

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.

Therapeutic drug monitoring of old and newer anti-epileptic drugs

The aim of the present paper is to provide information concerning the setting up and interpretation of therapeutic drug monitoring (TDM) for anti-epileptic drugs. The potential value of TDM for these drugs (including carbamazepine, clobazam, clonazepam, ethosuximide, felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, pheneturide, phenobarbital, phenytoin, primidone, tiagabine, topiramate, valproic acid, vigabatrin and zonisamide) is discussed in relation to their mode of action, drug interactions and their pharmacokinetic properties. The review is based upon available literature data and on observations from our clinical practice. Up until approximately 15 years ago anti-epileptic therapeutics were restricted to a very few drugs that were developed in the first half of the 20th century. Unfortunately, many patients were refractory to these drugs and a new generation of drugs has been developed, mostly as add-on therapy. Although the efficacy of the newer drugs is no better, there is an apparent improvement in drug tolerance, combined with a diminished potential for adverse drug interactions. All new anticonvulsant drugs have undergone extensive clinical studies, but information on the relationship between plasma concentrations and effects is scarce for many of these drugs. Wide ranges in concentrations have been published for seizure control and toxicity. Few studies have been undertaken to establish the concentration-effect relationship. This review shows that TDM may be helpful for a number of these newer drugs.

Clinically important drug interactions in epilepsy: general features and interactions between antiepileptic drugs

There are two types of interactions between drugs, pharmacokinetic and pharmacodynamic. For antiepileptic drugs (AEDs), pharmacokinetic interactions are the most notable type, but pharmacodynamic interactions involving reciprocal potentiation of pharmacological effects at the site of action are also important. By far the most important pharmacokinetic interactions are those involving cytochrome P450 isoenzymes in hepatic metabolism. Among old generation AEDs, carbamazepine, phenytoin, phenobarbital, and primidone induce the activity of several enzymes involved in drug metabolism, leading to decreased plasma concentration and reduced pharmacological effect of drugs, which are substrates of the same enzymes (eg, tiagabine, valproic acid, lamotrigine, and topiramate). In contrast, the new AEDs gabapentin, lamotrigine, levetiracetam, tiagabine, topiramate, vigabatrin, and zonisamide do not induce the metabolism of other AEDs. Interactions involving enzyme inhibition include the increase in plasma concentrations of lamotrigine and phenobarbital caused by valproic acid. Among AEDs, the least potential interaction is associated with gabapentin and levetiracetam.

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.

The ideal characteristics of antiepileptic therapy: an overview of old and new AEDs

New and improved anti-epileptic drugs (AEDs) have made the concept of choice, according to the individual prognosis and probable response to specific regimens, increasingly feasible. Inter-individual variability in syndrome severity and complexity make individualization necessary. We propose three categories of disorder control according to the individual objectives of the patient: (1) seizure control, (2) epilepsy control and ultimately, (3) "epilepsy cure"; the latter remaining a largely idealistic target today. An AED is likely to be successful if it exhibits "optimal" characteristics, such as drug efficacy, tolerability, pharmacokinetics, interactions and cost-effectiveness. This review discusses the "optimal" characteristics of add-on AEDs, which, in addition to seizure control, will contribute to the achievement of epilepsy control and therefore address the currently unmet clinical needs of epilepsy treatment.

Effect of vigabatrin on the pharmacokinetics of carbamazepine

OBJECTIVE

To evaluate a possible interaction between vigabatrin and carbamazepine in epileptic patients.

METHODS

Steady-state serum concentrations of carbamazepine with and without vigabatrin were compared. The study group consisted of 15 patients (eight females, seven males, and mean age 31 +/- 12 years), with refractory partial epilepsy. They received vigabatrin as add-on therapy. Patients received carbamazepine monotherapy for at least 6 months and the carbamazepine-vigabatrin combination for at least 3 months. Blood samples were obtained in the morning, before the first daily dose and the carbamazepine plasma concentrations were analysed by fluorescence polarization immunoassay (TDx System).

RESULTS

No statistically significant differences were found in mean carbamazepine daily dose. Mean trough concentrations were 7.9 +/- 1.4 microg/mL with carbamazepine alone, and 6.5 +/- 2.0 microg/mL with carbamazepine-vigabatrin association (P < 0.03). The mean values of pharmacokinetic parameters were: level/dose ratio (L/D) = 0.59 +/- 0.20 vs. 0.45 +/- 0.15 (P < 0.05) and plasma clearance (Cl) = 78.5 +/- 25.8 vs. 105.8 +/- 38.9 mL/h/kg (P < 0.05), with carbamazepine alone and carbamazepine-vigabatrin combination, respectively.

CONCLUSION

Vigabatrin produced a statistically significant increase in the plasma clearance of carbamazepine when the two drugs were given simultaneously.

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.

Pharmacokinetic drug interactions between oral contraceptives and second-generation anticonvulsants

Drug interactions between oral contraceptives (OCs) and traditional anticonvulsants have been well described. However, in the past decade, a number of new anticonvulsants have been developed, as well as modifications made in the composition of the OC preparations themselves. Additionally, anticonvulsants are increasingly employed in the therapy of nonseizure-related disorders, placing more women at risk of potential drug interactions that may lead to contraceptive failure. Second-generation anticonvulsants include felbamate, gabapentin, lamotrigine, oxcarbazepine, tiagabine, topiramate, vigabatrin and zonisamide. Most have been approved for adjunctive management of seizures refractory to therapy with traditional anticonvulsants. On the basis of available study data in women receiving concomitant OC preparations, gabapentin, lamotrigine, tiagabine and vigabatrin may be administered without significant pharmacokinetic interactions that potentially diminish contraceptive efficacy. However, additional or alternative contraceptive measures, including using OCs with higher estrogen content, are recommended when using felbamate, oxcarbazepine and topiramate, as these agents have demonstrated enzyme-inducing activity leading to reduced plasma steroid concentrations. The effects of zonisamide in women receiving OCs have yet to be reported. It is important to characterise the properties [e.g. substrate and enzyme activity (particularly cytochrome P450 3A4 induction)] of new anticonvulsants and recognise their potential to interfere with OCs. However, a pharmacokinetic interaction does not in itself indicate loss of OC efficacy. Contraceptive failure should be measured by changes in ovarian hormone concentrations, maturation of ovarian follicle(s) or ovulation.

The clinical pharmacokinetics of the new antiepileptic drugs

Because pharmacokinetics is a major determinant of the magnitude and duration of pharmacologic response, understanding the kinetic properties of the new antiepileptic drugs (AEDs) is essential for the correct use of these compounds in clinical practice. After oral administration, absorption is rapid and relatively efficient for the new AEDs, the most notable exception being gabapentin, whose bioavailability decreases with increasing dosage. None of the new AEDs is extensively bound to plasma proteins except for tiagabine, which is over 95% protein-bound. The route of elimination differs to an important extent from one compound to another, and elimination half-lives range from over 30 h for zonisamide to 5-7 h for gabapentin. For all drugs that are metabolized, half-life is shortened and clearance is increased when patients receive concomitant enzyme-inducing agents such as barbiturates, phenytoin, and carbamazepine. Lamotrigine metabolism is markedly inhibited by valproic acid, and felbamate may increase the serum levels of most other AEDs. Felbamate, topiramate, and oxcarbazepine may also reduce the efficacy of the contraceptive pill by stimulating its metabolism.

Vigabatrin: a novel therapy for seizure disorders

OBJECTIVE

To review the pharmacology, pharmacokinetics, efficacy, and adverse effects of vigabatrin and its role in the management of seizure disorders.

METHODS

A MEDLINE search of English-language literature from January 1993 through January 1999 was conducted using vigabatrin as a search term to identify pertinent studies and review articles. Additional studies were identified from the bibliographies of reviewed literature. The manufacturer provided postmarketing surveillance data. Priority was given to randomized, double-blind, placebo-controlled studies.

FINDINGS

Vigabatrin is a selective and irreversible inhibitor of gamma-aminobutyric acid transaminase. In controlled clinical trials of vigabatrin add-on therapy in patients with uncontrolled partial seizures, 24-67% of patients achieved a < or =50% reduction in seizure frequency. Data from two comparative trials with carbamazepine monotherapy indicate that vigabatrin monotherapy reduces the frequency of partial seizures in patients with newly diagnosed epilepsy. Vigabatrin also controls infantile spasms, particularly those associated with tuberous sclerosis. Vigabatrin is more effective in patients with partial seizures than in those with generalized seizures. The drug is generally well tolerated. Headache and drowsiness were the most common adverse effects observed in controlled clinical trials; visual field defects, psychiatric reactions, and hyperactivity also have been reported. There are no known clinically significant drug interactions.

CONCLUSIONS

Vigabatrin improves seizure control as add-on therapy for refractory partial seizures and may produce therapeutic benefits in the treatment of infantile spasms. Vigabatrin is generally well tolerated, with a convenient administration schedule, a lack of known significant drug interactions, and no need for routine monitoring of plasma concentrations.

A mechanistic approach to antiepileptic drug interactions

OBJECTIVE

To describe the primary types of antiepileptic drug (AED) interactions by using a mechanistic approach.

DATA SOURCES

A literature search was performed using MEDLINE and bibliographies of recent review articles and published abstracts.

DISCUSSION

AEDs are associated with a wide range of drug interactions, including hepatic enzyme induction and inhibition and protein-binding displacement. Hepatic induction by AEDs affects the metabolism of a limited number of drugs with low therapeutic indices. Anticipation of induction interactions and careful clinical monitoring may alleviate potential problems. Most commonly used AEDs are eliminated through hepatic metabolism catalyzed by the cytochrome P450 (CYP) and uridine diphosphate glucuronosyltransferase (UGT) enzymes. Phenytoin, phenobarbital, and carbamazepine induce CYP and UGT enzymes. Lamotrigine is a weak inducer of UGT. Valproate is a broad-spectrum inhibitor of UGT enzymes, epoxide hydrolase, and CYP2C enzymes. Felbamate induces CYP3A4, but inhibits CYP2C19 substrates. Topiramate inhibits only CYP2C19 substrates. Ethosuximide, gabapentin, tiagabine, and vigabatrin are neither inducers nor inhibitors of drug metabolism. Hepatic enzyme inhibition usually occurs because of competition at the enzyme site. Knowledge of the specific metabolic enzymes involved in the metabolism of AEDs allows clinicians to predict potential interactions.

CONCLUSIONS

By understanding the mechanisms of drug interactions, the pharmacist can play a key role in patient care by anticipating and preventing AED drug interactions.

A double-blind, placebo-controlled study on the effect of vigabatrin on in vivo parameters of hepatic microsomal enzyme induction and on the kinetics of steroid oral contraceptives in healthy female volunteers

PURPOSE

This study was conducted to determine whether vigabatrin affects in vivo indices of hepatic microsomal enzyme activity and the pharmacokinetics of steroid oral contraceptives in healthy subjects.

METHODS

Under double-blind conditions, 13 female healthy volunteers received, in random order and with a washout interval of > or = 4 weeks, two oral 4-week treatments with vigabatrin (VGB) (maintenance dosage, 3,000 mg daily) and placebo, respectively. The clearance and half-life of antipyrine (a broad marker of drug oxidation capacity), the urinary excretion of 6-beta-hydroxycortisol (a selective marker of cytochrome CYP3A-mediated oxidation), and the activity of serum gamma-glutamyltransferase (a nonspecific index of microsomal enzyme activity) were determined after 3 weeks of each treatment. The single-dose kinetics of a combined oral contraceptive containing 30 micrograms ethinyl estradiol and 150 micrograms levonorgestrel were also determined after 3 weeks of treatment by specific radioimmunologic assays.

RESULTS

VGB treatment had no influence on antipyrine clearance (28 +/- 5.6 vs. 30 +/- 4.5 ml/h/kg on placebo), antipyrine half-life (15.5 +/- 3.5 vs. 14.1 +/- 2.1 h), urinary 6-beta-hydroxycortisol excretion (488 +/- 164 vs. 470 +/- 228 nmol/ day), 6-beta-hydroxycortisol-to-cortisol concentration ratio (6.8 +/- 3.1 vs. 6.1 +/- 3.1) and serum gamma-glutamyltransferase activity (12 +/- 3 vs. 11 +/- 3 IU/L). No difference in pharmacokinetic parameters between VGB and placebo sessions were found for ethinyl estradiol (half-life, 12.5 +/- 3.2 vs. 13.9 +/- 3.2 h; AUC, 874 +/- 301 vs. 939 +/- 272 ng/ L/h) and levonorgestrel (half-life, 17.7 +/- 5.2 vs. 23.1 +/- 9.8 h; AUC, 27.5 +/- 9.6 vs. 30.0 +/- 12.0 micrograms/L/h). Two subjects, however, showed a 50 and a 39% reduction in ethinyl estradiol AUC during VGB treatment.

CONCLUSIONS

At therapeutic dosages, VGB did not modify in vivo indices of hepatic microsomal enzyme activity and did not interfere significantly with the CYP3A-mediated metabolism of ethinyl estradiol and levonorgestrel. Based on these data, VGB is unlikely to affect consistently the efficacy of steroid oral contraceptives or interact pharmacokinetically with drugs that are eliminated mainly by oxidative pathways, particularly those involving cytochrome CYP3A.

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.

Clinical pharmacokinetics of newer antiepileptic drugs. Lamotrigine, vigabatrin, gabapentin and oxcarbazepine

The clinical pharmacokinetics of the 4 antiepileptic drugs lamotrigine, vigabatrin, gabapentin and oxcarbazepine have been reviewed in this paper. All the drugs have linear kinetics and reliable absorption, although the saturation of transport across the gut may occur at high doses with gabapentin. All the drugs can be conveniently given as a twice daily dosage apart from gabapentin, which has a short half-life and a midday dose is needed. Unlike many of the older drugs, lamotrigine, vigabatrin and gabapentin have a predominantly renal excretion and are not metabolised through the cytochrome P450 system. They do not induce their own metabolism or that of other commonly used anticonvulsants. Similarly, clinically important interactions with other major classes of drugs metabolised this way, such as anticoagulants or steroid hormones, do not occur. Oxcarbazepine, however, can cause oral contraceptive pill failure. Oxcarbazepine is immediately metabolised to a hydroxy metabolite and could be considered a prodrug. It appears to have fewer pharmacokinetic interactions than carbamazepine. Valproic acid (sodium valproate) inhibits the glucuronidation of lamotrigine and increases its half-life; when used together, dosage modification of lamotrigine is needed to avoid toxicity.

Pharmacokinetic interaction studies between felbamate and vigabatrin

To assess the possible occurrence of pharmacokinetic interactions between the antiepileptic agents felbamate and vigabatrin, two randomized, double-blind, placebo-controlled, crossover studies were conducted in healthy male volunteers. In Study I, 18 subjects received oral vigabatrin 1000 mg every 12 h for two 8 days periods with felbamate 1200 mg every 12 h or placebo. In Study II, 18 other volunteers were administered oral felbamate 1200 mg every 12 h for two 8 days periods with vigabatrin 1000 mg every 12 h or placebo. On the eighth day of each treatment period, blood and urine samples were collected over 12 h for determination of the active S(+)- and inactive R(-)-vigabatrin enantiomer concentrations (Study I) or felbamate concentrations (Study II). In Study I, the pharmacokinetic parameters of R(-)-vigabatrin were similar during co-administration with felbamate or placebo. Felbamate produced a 13% increase in AUC(0.12 h) and an 8% increase in urinary excretion of S(+)-vigabatrin. Although these changes were statistically significant, their magnitude was small. In Study II, the pharmacokinetic parameters of felbamate were similar during concurrent administration with vigabatrin or placebo. These data indicate that there are no clinically relevant pharmacokinetic interactions between felbamate and vigabatrin in man.

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.

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.