Clinically relevant anti-epileptic drug interactions

The significance of folic acid for epilepsy patients

The following is a comprehensive review of the current understanding of the many important roles of folic acid in the health of patients with epilepsy. A review of past and current literature reveals that folic acid plays important roles in the areas of hematology, neurology, development, and reproduction. Also highlighted are new areas for exploration.

Carbamazepine toxicity following Oxybutynin and Dantrolene administration: a case report

OBJECTIVE

To report a case of Carbamazepine toxicity following the administration of Oxybutynin and Dantrolene.

STUDY DESIGN

A case report.

SETTING

The Spinal Rehabilitation Department, Loewenstein Hospital, Raanana, Israel.

METHODS

A patient with C6D tetraplegia who sustained intoxication because of drug interaction is presented. She had been treated by Carbamazepine 1000 mg/day for neuropathic pain for 2 years without clinical or laboratory signs of toxicity. After administration of Oxybutynin concomitantly with an increase in the dose of Dantrolene, she presented the clinical symptoms and laboratory finding of Carbamazepine intoxication. Trying to adjust the treatment to the patient's requirements, Carbamazepine together with Oxybutynin and Dantrolene was readministrated in lower doses.

RESULTS

The combination of these drugs, even small doses, caused toxicity. Adding Dantrolene and Oxybutynin elevated the blood level of Carbamazepine, possibly by inhibition of cytochrome P450.

CONCLUSION

A possible pharmacokinetic interaction between Dantrolene and Oxybutynin should be borne in mind when considering Carbamazepine medication for a patient with a spinal cord lesion.

Drug treatment of epilepsy in elderly people: focus on valproic Acid

Despite old age being the commonest time of life to develop epilepsy, relatively little is known about the condition in later years. Antiepileptic drugs (AEDs) are the mainstay of treatment and valproic acid (VPA) has been prescribed for older patients with seizures for over 35 years. VPA is available in a variety of formulations. The drug is generally rapidly absorbed, although there are no data on the extent of oral absorption in the elderly. The volume of distribution (Vd) and elimination half-life have been compared in older and younger patients. One study reported no change in either parameter between elderly and younger patients (Vd: 0.16 vs 0.14 L/kg; elimination half-life: 15.3 vs 13.0h), the other found an increase in both for older patients (Vd: 0.19 vs 0.13 L/kg; elimination half-life 14.9 vs 7.2h). Total VPA clearance is similar in young and elderly subjects. The drug does not induce the metabolism of hepatic enzymes, but can act as a metabolic inhibitor, raising plasma concentrations of lamotrigine, phenobarbital (phenobarbitone), carbamazepine-10-11-epoxide, lorazepam, nimodipine and zidovudine. Concomitant use of VPA may also lead to an elevation in phenytoin, diazepam, warfarin, amitriptyline and chlorpromazine concentrations. A number of enzyme-inducing AEDs such as phenytoin, phenobarbital, primidone and carbamazepine can increase the clearance of VPA. Plasma concentrations of VPA may also rise when the drug is administered with felbamate, stiripentol, aspirin (acetylsalicylic acid), naproxen, phenylbutazone, isoniazid, fluoxetine and chlorpromazine. The majority of elderly patients present with partial and/or secondary generalised seizures, although a few have long-standing primary generalised seizures. Results from meta-analyses and randomised studies of patients comparing VPA with other AED monotherapies suggest that the drug is as effective as carbamazepine, phenytoin and phenobarbital in treating these seizure types. Although some of these studies recruited older patients, there have been no randomised double-blind trials examining the efficacy of VPA with other AEDs in an exclusively elderly cohort. There is no direct correlation between efficacy and plasma VPA concentrations. The majority of older patients require lower doses of AEDs than younger adults. Higher VPA doses may be needed in patients taking drugs which induce hepatic microsomal enzymes. Once-daily dosing of the controlled-release preparation can help to improve compliance and may render some frail elderly people seizure free. There is a perception that the elderly are generally more susceptible to the adverse effects of AEDs than younger adults, although there are few data to validate this claim. Dose-dependent and idiosyncratic reactions may be more frequent. Common adverse effects of VPA include gastrointestinal symptoms and tremor. Slow-dose escalation and controlled-release preparations may minimise these. In summary, VPA is a long established AED. Its broad spectrum of action and dosing schedule are favourable properties for its use in older people. To accurately establish the place of this and other AEDs in treating elderly patients with epilepsy, well designed clinical trials are urgently required in this vulnerable population.

Factors contributing to carbamazepine-macrolide interactions

Certain macrolides (e.g. clarithromycin or erythromycin) are known to interact with the carbamazepine antiepileptic drug. Carbamazepine-macrolide interaction leads to an increase in the level of carbamazepine in the blood, so inducing carbamazepine toxicity. The aim of this paper is to compare the extent of the interaction for each macrolide and to study the effects of age, gender, weight, the carbamazepine and macrolide dosages and the use of other antiepileptic drugs on the extent of the carbamazepine-macrolide interaction. Case reports published in the literature were reviewed and analysed to this end. The results show that three macrolides (erythromycin, troleandomycin and, to a lesser extent, clarithromycin) may induce carbamazepine toxicity in clinical practice. Furthermore, it was observed that high dosages of carbamazepine or macrolides and the use of concurrent anticonvulsivant drugs in the case of patients below 60 years of age are associated with the highest carbamazepine levels in carbamazepine-macrolide interactions. This study should help physicians choose a macrolide that does not interact with carbamazepine and evaluate the risk of an interaction between carbamazepine and macrolides.

Clinical pharmacology and therapeutic use of the new antiepileptic drugs

Although older generation antiepileptic drugs (AEDs) such as carbamazepine, phenytoin and valproic acid continue to be widely used in the treatment of epilepsy, these drugs have important shortcomings such as a highly variable and nonlinear pharmacokinetics, a narrow therapeutic index, suboptimal response rates, and a propensity to cause significant adverse effects and drug interactions. In an attempt to overcome these problems, a new generation of AEDs has been introduced in the last decade. Compared with older agents, some of these drugs offer appreciable advantages in terms of less variable kinetics and, particularly in the case of gabapentin, levetiracetam and vigabatrin, a lower interaction potential. Lamotrigine, topiramate, zonisamide and felbamate protect against partial seizures and a variety of generalized seizure types, vigabatrin is effective against partial seizures (with or without secondary generalization) and infantile spasms, while the use of oxcarbazepine, tiagabine and gabapentin is mainly restricted to patients with partial epilepsy (and, in the case of oxcarbazepine, also primarily generalized tonic-clonic seizures). Levetiracetam, the latest AED to be introduced, has been found to be effective in partial seizures, but its potentially broader efficacy spectrum remains to be determined in clinical studies. Currently, the main use of new generation AEDs is in the adjunctive therapy of patients refractory to older agents. However, due to advantages in terms of tolerability and ease of use, some of these drugs are increasingly used for first-line management in certain subgroups of patients. Due to serious toxicity risks, felbamate and vigabatrin should be prescribed only in patients refractory to other drugs. In the case of vigabatrin, however, first line use may be justified in infants with spasms.

Antidepressant drugs and seizure susceptibility: from in vitro data to clinical practice

The use of antidepressant drugs (ADs) in patients with epilepsy still raises uncertainties because of the widespread conviction that this class of drugs facilitates seizures. A detailed knowledge of this issue in its various aspects may help in optimal management of patients suffering concurrently from epilepsy and depression. This article reviews the available data in vitro in animals and humans concerning the known potential of various ADs to induce epileptic seizures. Emphasis has been placed on those variables that may generate confusion in interpreting the results of the various studies. Most ADs at therapeutic dosages exhibit in nonepileptic patients a seizure risk close to that reported for the first spontaneous seizure in the general population (i.e., <0.1%). In patients taking high AD doses, seizure incidence rises markedly and may reach values up to 40%. With a patient history of epilepsy and/or concomitant drugs that act on neuronal excitability, low or therapeutic AD doses may be sufficient to trigger seizures. Experimental data are in partial conflict with human data on the relative potential seizure risk of the various ADs. Therefore, a reliable scale for assigning a relative value to an individual AD or to single AD classes cannot be made. It appears fair to say that maprotiline and amoxapine exhibit the greatest seizure risk, whereas trazodone, fluoxetine, and fluvoxamine exhibit the least. Some ADs may also display antiepileptic effects, especially in low doses, in experimental models of epilepsy and in humans, but the mechanism of this action is largely unknown. The available data suggest that ADs may display both convulsant and anticonvulsant effects and that the most important factor in determining the direction of a given compound in terms of excitation/inhibition is drug dosage. It is probable that drugs that increase serotonergic transmission are less convulsant or, even, more anticonvulsant than others. Because of mutual pharmacokinetic interactions between antiepileptic drugs and ADs, with consequent marked changes in plasma concentrations, it remains to be established whether or not plasma AD levels that are effective against depression also facilitate seizures. Finally, exploring the mechanisms through which ADs modulate neuronal excitability might open new possibilities in antiepileptic drug development.

Modelling of the pharmacodynamic interaction between phenytoin and sodium valproate

Treatment of epilepsy with a combination of antiepileptic drugs remains the therapeutic choice when monotherapy fails. In this study, we apply pharmacokinetic-pharmacodynamic modelling to characterize the interaction between phenytoin (PHT) and sodium valproate (VPA). Male Wistar rats received a 40 mg kg(-1) intravenous dose of PHT over 5 min either alone or in combination with an infusion of VPA resulting in a steady-state concentration of 115.5+/-4.9 microg ml(-1). A control group received only the infusion of VPA. The increase in the threshold for generalized seizure activity (ATGS) was used as measure of the anticonvulsant effect. PHT pharmacokinetics was described by a pharmacokinetic model with Michaelis-Menten elimination. The concentration-time course and plasma protein binding of PHT were not altered by VPA. The pharmacokinetic parameters Vmax and Km were, respectively, 294+/-63 microg min(-1) and 7.8+/-2.4 microg ml(-1) in the absence of VPA and 562+/-40 microg min(-1) and 15.6+/-0.9 microg ml(-1) upon administration in combination with VPA. A delay of the onset of the effect relative to plasma concentrations of PHT was observed. The assessment of PHT concentrations at the effect site was based on the effect-compartment model, yielding mean ke0 values of 0.128 and 0.107 min(-1) in the presence and absence of VPA, respectively. A nonlinear relationship between effect-site concentration and the increase in the TGS was observed. The concentration that causes an increase of 50% in the baseline TGS (EC50%TGS) was used to compare drug potency. A shift of EC50%TGS from 13.27+3.55 to 4.32+/-0.52 microg ml(-1) was observed upon combination with VPA (P<0.01). It is concluded that there is a synergistic pharmacodynamic interaction between PHT and VPA in vivo.

Established antiepileptic drugs

Despite the recent entry into the market-place of a range of new pharmacological treatments for epilepsy, most patients still receive the standard antiepileptic drugs. This review considers the clinical place and practical use of these agents. Detailed consideration is given to carbamazepine, phenytoin, sodium valproate, phenobarbital and ethosuximide, with lesser emphasis on primidone, clobazam and clonazepam. Individualization of therapy, polypharmacy, refractory epilepsy, therapeutic drug monitoring, pregnancy, withdrawing treatment, epilepsy prophylaxis and referral to an epilepsy centre are also discussed. The paper concludes with a statement of 12 basic rules in prescribing established antiepileptic drugs.

The new generation of antiepileptic drugs: advantages and disadvantages

1. After a hiatus of over 20 years, several new antiepileptic drugs (vigabatrin, lamotrigine, gabapentin, oxcarbazepine, topiramate, felbamate, zonisamide and tiagabine) have reached or approached the registration phase. 2. Compared with older agents, many new drugs exhibit simpler pharmacokinetics. This is especially true for vigabatrin and gabapentin, which are renally eliminated and have a low interaction potential. 3. Unlike most of the older agents, vigabatrin, lamotrigine, gabapentin and tiagabine are devoid of significant enzyme inducing or inhibiting properties. Topiramate, oxcarbazepine and felbamate may induce the metabolism of steroid oral contraceptives. In addition, felbamate also acts as a metabolic inhibitor. 4. To date, the efficacy of new drugs has been evaluated extensively only under add-on conditions in patients with partial seizures (with or without secondary generalization) refractory to conventional treatment. However, there is evidence that lamotrigine, zonisamide, felbamate and, possibly, topiramate may also be effective in generalized epilepsies. 5. In placebo-controlled studies, typically between 15 and 40% of patients with difficult-to-treat partial epilepsy have shown an improvement (defined as a 50% or greater decrease in seizure frequency) after addition of a new drug. Only a small minority of these patients achieved complete seizure control. 6. Compared with older agents, some of the new drugs may have a better tolerability profile. Felbamate, however, has been associated with a high risk of aplastic anaemia and hepatotoxicity. 7. At present, the main use of the new agents is in patients refractory to first-line drugs such as carbamazepine or valproate, and further studies are required to characterize their activity spectrum as well as their potential value in monotherapy. In most patients, new drugs cannot be recommended for first-line use until evidence is obtained that potential advantages in tolerability or ease of use outweigh the drawback of their high cost.

Pharmacological problems in the management of epilepsy in children

Rational prescribing of anti-epilepsy drugs in children may be complicated by a number of problems, which include: (i) difficulties in arriving rapidly at a syndromic diagnosis, at least in some cases, with consequent uncertainties about therapeutic management; (ii) difficulties in evaluating drug response in young age groups, particularly with respect to subjective side-effects affecting cognitive function; (iii) the vulnerability of infants and children to specific aspects of drug toxicity, such as liver damage induced by valproic acid or behavioural disorders caused by barbiturates; and (iv) the need to adjust dosage to account for age-dependent pharmacokinetic changes. In particular, it is known that the rate of drug metabolism changes markedly during development. Metabolic drug elimination is often reduced at birth, but drug metabolizing enzymes mature rapidly and biotransformation in infants and children usually occurs at a faster rate than in adults. The elimination of drug which are excreted unchanged in urine appears to be less influenced by age in paediatric patients, though impairment in renal drug clearance may be seen in newborns. Monitoring of serum drug concentrations may be helpful for dosage adjustments, but it is not a substitute for careful clinical observation.

Single dose pharmacokinetics of carbamazepine-10,11-epoxide in patients on lamotrigine monotherapy

The pharmacokinetics of a single oral dose of carbamazepine-10,11-epoxide (CBZ-E, 100 mg) were compared in 10 patients on chronic monotherapy with lamotrigine (LTG, 200-300 mg/day) and in 10 drug-free healthy control subjects. CBZ-E pharmacokinetic parameters in LTG-treated patients were found to be similar to those observed in controls (half-life: 7.2 +/- 1.6 vs 6.1 +/- 0.9 h; apparent oral clearance: 110.8 +/- 53.1 vs 120.5 +/- 29.9 ml/h/kg; apparent volume of distribution: 1.08 +/- 0.37 vs 1.04 +/- 0.25 l/kg respectively; means +/- s.d.). These data indicate that, contrary to previous suggestions, LTG has no effect on the metabolic disposition of CBZ-E.

The pharmacokinetics of oxcarbazepine and its active metabolite 10-hydroxy-carbazepine in healthy subjects and in epileptic patients treated with phenobarbitone or valproic acid

The kinetics of oxcarbazepine (OXC) and its active metabolite 10-hydroxy-carbazepine (10-OH-CZ) after a single oral OXC dose (600 mg) were compared in healthy control subjects and in epileptic patients treated with phenobarbitone or sodium valproate (n = 8 in each group). In all groups, serum 10-OH-CZ concentrations were much higher than those of the parent drug. In patients on valproate, the kinetics of OXC and 10-OH-CZ did not differ significantly from those observed in controls. In patients on phenobarbitone, AUC values of both OXC and 10-OH-CZ were lower than in controls (2.9 +/- 0.4 vs 5.1 +/- 0.7 microg ml(-1) h and 89 +/- 7 vs 119 +/- 10 microg ml(-1) h respectively, means +/- s.e. mean, P < 0.05), whereas 10-OH-CZ half-lives were only marginally shorter (17 +/- 1 h vs 20 +/- 2 h, NS). These data indicate that the biotransformation of OXC and 10-OH-CZ may be accelerated by concomitant treatment with phenobarbitone but that the magnitude of this effect is unlikely to be of great clinical significance.

Absence of interaction between oxcarbazepine and erythromycin

When erythromycin (ERY) is co-administrated with the antiepileptic carbamazepine (CBZ), a drug interaction may cause an increase in CBZ plasma concentrations, which can result in CBZ related toxic symptoms. This cross-over study was designated to investigate whether ERY influences the pharmacokinetics of the new antiepileptic oxcarbazepine (OXC) and its metabolites. In 8 healthy volunteers there were no significant differences in AUC, peak plasma concentrations or time to peak concentration when OXC was administered either with or without ERY. The results of this study suggest that OXC may offer an important advantage over CBZ especially when concomitant therapy with ERY is required.

Oxcarbazepine does not interact with cimetidine in healthy volunteers

When cimetidine (CIM) is administered together with the anti-epileptic drug carbamazepine (CBZ), a drug interaction may cause a rise in plasma concentrations of CBZ, which can result in CBZ-related toxic symptoms. The aim of this cross-over study was to investigate whether CIM influences the disposition and kinetics of the new anti-epileptic oxcarbazepine (OXC) and its metabolites. In 8 healthy volunteers there was no difference in AUC, Cmax or tmax when OXC was administered either with or without CIM. The results of this study suggest that in the treatment of epilepsy OXC offers an important advantage over the established anti-epileptics, especially when concomitant therapy with CIM is required.

Drug interactions in epilepsy

The abolition of seizures using a single antiepileptic agent can be expected in more than 80% of patients, although not necessarily with the first drug tried. The remainder often receive polypharmacy, and current evidence suggests that perhaps only around 10% of these benefit significantly in terms of improved seizure control. Many more experience complicated drug interactions. Carbamazepine, phenytoin, phenobarbital, and primidone (metabolized in part to phenobarbital) all induce the synthesis of hepatic monooxygenase and conjugating enzymes. This will result in an acceleration in the metabolism of other lipid-soluble drugs with likely attenuation of their pharmacological effects. Valproate, on the other hand, is a minor enzyme inhibitor. Pharmacokinetic interactions are almost invariable when more than one antiepileptic drug is coprescribed. The extent and direction of interactions with combinations of these drugs are varied and unpredictable. Discontinuation of an enzyme inducer or inhibitor will influence the concentrations of the remaining drug(s). Pharmacodynamic interactions also cause problems in epileptic patients. A number of commonly prescribed psychoactive drugs, such as tricyclic antidepressants and neuroleptics, can worsen seizure control by reducing the convulsion threshold. In addition, there seems little doubt that ethanol abuse and withdrawal can precipitate seizures in susceptible patients. Antiepileptic polypharmacy is more likely to impair cognitive function than the same drugs used singly. In addition, the more antiepileptic drugs received by a patient in the first trimester of pregnancy, the higher the risk of teratogenesis in the exposed infant. Drug interactions prolong and complicate the process of new drug assessment, particularly when introduced in treated patients with refractory epilepsy. The candidate antiepileptic drug may alter the concentration of concomitant therapy, or its own breakdown may be influenced by coprescribed enzyme inducers or inhibitors. Even if the new drug is excreted unchanged by the kidney, unexpected interactions can be uncovered. Pharmacodynamic interactions need not always be detrimental. Currently, there is no rational approach to the treatment of intractable epilepsy. As more new drugs with single mechanisms of action become available, the potential exists for combining these synergistically. This approach may revolutionize the pharmacological management of the epileptic patient in the 21st century.

Differential effects of valproic acid and enzyme-inducing anticonvulsants on nimodipine pharmacokinetics in epileptic patients

1. The single dose pharmacokinetics of orally administered nimodipine (60 mg) were investigated in normal subjects and in two groups of epileptic patients receiving chronic treatment with hepatic microsomal enzyme-inducing anticonvulsants (carbamazepine, phenobarbitone or phenytoin) and sodium valproate, respectively. 2. Compared with the values found in the control group, mean areas under the plasma nimodipine concentration curve were lowered by about seven-fold (P less than 0.01) in patients taking enzyme-inducing anticonvulsants and increased by about 50% (P less than 0.05) in patients taking sodium valproate. 3. Nimodipine half-lives were shorter in enzyme-induced patients than in controls (3.9 +/- 2.0 h vs 9.1 +/- 3.4 h, means +/- s.d., P less than 0.01), but this difference could be artifactual since in the patients drug concentrations declined rapidly below the limit of assay, thus preventing identification of a possible slower terminal phase. In valproate-treated patients, half-lives (8.2 +/- 1.8 h) were similar to those found in controls.