Antiepileptic drug interactions

Dietary effects on antiseizure drug metabolism and management of epilepsy

In recent years, there has been growing interest in the influences of food-drug interactions on the metabolism of antiseizure medications (ASM) and the management of epilepsy. Studies have proven the effectiveness of the ketogenic diet (KD) in controlling refractory epilepsy. However, dietary interventions such as the KD or its variants may induce significant changes in serum drug concentrations which counteracts the anticonvulsive effects of ASMs, leading to an increased risk of developing seizures. Interactions with enzymes within the cytochrome P450 system may also explain the dietary influences on serum concentrations of antiseizure drugs. The bioavailability of ASMs is also affected by several foods and nutritional supplements. Nevertheless, more studies are warranted to explore the mechanisms underlying food-drug interactions and the risks and benefits of combined drug-diet therapy.

Selecting Rational Drug Combinations in Epilepsy

Monotherapy remains the standard initial therapy of epilepsy, but when the first antiepileptic drug (AED) fails, combination therapy may be considered. The choice of combination therapy should take into consideration pharmacokinetic interactions, as well as pharmacodynamic interactions related to mechanism of action. There is evidence that an AED combination with different mechanisms of action is more likely to be successful than a combination with the same mechanisms. The combination of lamotrigine and valproate has been demonstrated to be synergistic in its efficacy. However, there are limited data to support other synergistic AED combinations.

Pharmacotherapy Overview of Seizure Management in the Adult Emergency Department

Seizures are a common cause of emergency department visits, and approximately 28% of epilepsy patients present to an emergency department annually for treatment. This article will provide an overview of the pharmacotherapeutic management of seizures and anticonvulsant therapy for patients who present to the adult emergency department, including practical information for pharmacists covering or cross-covering this practice area. The benzodiazepines are reviewed as a class, including dosing strategies, pharmacodynamic considerations, and advantages and disadvantages of lorazepam, diazepam, and midazolam. Indications for the use of phenytoin and fosphenytoin will be reviewed, as well as dosing, adverse effects, and cost-effectiveness data. In addition, dosing, administration, pharmacokinetics, and adverse effects of phenobarbital, carbamazepine, and valproate will be discussed. Clinical indications for serum anticonvulsant concentration monitoring and subsequent calculation of loading doses from serum concentrations are reviewed. Since status epilepticus is a life-threatening emergency, its therapeutic management is reviewed, including the use of continuous infusion midazolam, pentobarbital, and propofol. There are many opportunities for clinical pharmacists to collaborate with other members of the health care team to optimize efficacy and minimize adverse effects of anticonvulsant agents in the emergency department setting.

What are the arguments for and against rational therapy for epilepsy?

Although more than a dozen new anti-seizure drugs (ASDs) have entered the market since 1993, a substantial proportion of patients (~30 %) remain refractory to current treatments. Thus, a concerted effort to identify and develop new therapies that will help these patients continues. Until this effort succeeds, it is reasonable to re-assess the use of currently available therapies and to consider how these therapies might be utilized in a more efficacious manner. This applies to the selection of monotherapies in newly-diagnosed epilepsy, but perhaps, more importantly, to the choice of combination treatments in otherwise drug-refractory epilepsy. Rational polytherapy is a concept that is predicated on the combination of drugs with complementary mechanisms of action (MoAs) that work synergistically to maximize efficacy and minimize the potential for adverse events. Furthermore, rational polytherapy requires a detailed understanding of the MoA subclasses amongst available ASDs and an appreciation of the empirical evidence that supports the use of specific combinations. The majority of ASDs can be loosely categorized into those that target neurotransmission and network hyperexcitability, modulate intrinsic neuronal properties through ion channels, or possess broad-spectrum efficacy as a result of multiple mechanisms. Within each of these categories, there are discrete pharmacological profiles that differentiate individual ASDs. This chapter will consider how knowledge of MoA can help guide therapy in a rational manner, both in the selection of monotherapies for specific seizure types and syndromes, but also in the choice of drug combinations for patients whose epilepsy is not optimally controlled with a single ASD.

Levetiracetam monotherapy in patients with brain tumor-related epilepsy: seizure control, safety, and quality of life

We performed a case series analysis to evaluate the effects of levetiracetam (LEV) monotherapy on seizures, adverse events, cognitive functioning and quality of life (QoL) in patients with brain tumor-related epilepsy (BTRE). We also explored the possible effects of systemic therapies on the efficacy of LEV. Twenty-nine patients were followed (13 female, 16 male; age 24-75 years) with 12 months of follow-up. Patients were evaluated by QoL and neuropsychological tests. At final follow-up, mean LEV dosage was 1991.4 mg/day. Among patients who reached the final follow-up of 12 months (n = 15), 1 patient had ≥50% reduction of seizure frequency (SF), and 14/15 were seizure free. The difference in presence/absence of seizures between baseline and final follow-up was significant (p < 0.001). Responder rate was 100%. We observed five side-effects: four mild (reversible) and one severe. Logistic regression revealed that chemotherapy and radiotherapy did not affect the efficacy of LEV in seizure outcome (p = 0.999). The following statistically significant observations emerged by tests' evaluation: less worry about seizures, effects of antiepileptic, and ability to maintain social functions. Our data suggest that seizure occurrence can be an important warning sign that the clinician should heed throughout the duration of the illness. Patients with BTRE represent a unique patient population that presents difficulties regarding management of two very different pathologies: epilepsy on the one hand, and brain tumor on the other. Our data indicate that LEV, in patients with BTRE, is safe and efficacious, with positive impact on QoL.

Management of epilepsy in drug-resistant patients

Epilepsy affects > 2 million people in the United States, making it one of the most common neurobiological conditions. Typically, epilepsy is treated with one of several available antiepileptic drugs and patients are able to experience freedom from seizures with minimal side effects. However, there are some patients who do not respond to treatment and require the use of multiple drug combinations or surgical intervention. Although there are few studies supporting its use, multi-drug regimens have been known to be helpful for patients, although clinicians should monitor patients for adverse side effects. Vagus nerve stimulation is the only US Food and Drug Administration-approved surgical neurostimulation therapy for epilepsy, and patients' conditions often progress for many years before epilepsy surgery options are considered. Lastly, due to the chronic nature of epilepsy, clinicians should be aware of the presence of comorbid psychiatric conditions as well. This supplement is Part One in the "Case in Point: Evidence-Based Insights for Epilepsy Management" series. In this Expert Review Supplement, Andrew J. Cole, MD, FRCPC, outlines a case of a patient with drug resistant epilepsy, and Brien J. Smith, MD, outlines the best practices for the case patient including discussion on defining drug resistance in patients as well as the benefits and risks of available and emerging drug and surgical treatments.

Rational polytherapy

Monotherapy has been considered the gold standard for drug treatment of epilepsy. However, there is renewed interest in polytherapy because of the advent of new drugs with fewer drug interactions and novel mechanisms of action, and the realization that most patients with refractory epilepsy are eventually treated with drug combinations. Careful consideration must be given to drug additions and conversions; it may be less risky to add a drug than to convert from one monotherapy to another in patients with frequent or severe seizures. Rational choice of drug combinations is, at present, based more on avoidance of pharmacodynamic or pharmacokinetic side effects than on evidence for supra-additive efficacy. There are indications that combinations of two sodium-channel blocking agents are less effective than combinations of drugs with different primary mechanisms of action, and some human studies suggest that lamotrigine and valproate may be synergistic for efficacy. However, more animal and human research is needed, with attention to the effects of various combinations on both toxicity and seizure control.

Antiepileptic drugs and other medications: what interactions may arise?

Many patients with epilepsy are on lifelong therapy with antiepileptic drugs (AEDs), and AEDs are used for other conditions such as mood stabilization and headache prophylaxis. These drugs have high potential for clinically significant interaction with nonepilepsy drugs. Interactions occur largely through altered pharmacokinetics. One drug may increase the hepatic clearance of another, leading to attenuated efficacy of the affected drug. Alternatively, inhibition of liver metabolism by one drug can cause acute toxicity by reducing clearance of another drug. To identify potential drug interactions before they lead to toxicity or therapy failure, the treating clinician should combine knowledge of the patient's overall history with a general knowledge of comorbid conditions in which significant interactions involving AEDs are most likely to occur. Treatments susceptible to interactions include anticoagulants, antiarrhythmics, antibiotics, antiretroviral drugs, immunosuppressives, antineoplastics, and contraceptives. Therefore, it is important to obtain periodically a thorough history of medical problems, use of medications or herbal remedies, and adverse effects of medications. Physicians managing epilepsy patients should also strive to avoid potential drug interactions by favoring low-interaction AEDs in patients taking many other types of drugs. There is quite a large degree of patient heterogeneity in the extent of any given interaction between an AED and another drug. Indeed, some groups of patients may have different susceptibilities to such interactions because of genetic and environmental influences on drug metabolism. Effective treatment with AEDs should include attention to drug interactions.

Antiepileptic drug interactions

INTRODUCTION

Antiepileptics are drugs used in the long-term treatment of epilepsy and other conditions such as pain or psychiatric diseases. They are often administered as polytherapy or in combination with other treatments. It is therefore important to know their potential interactions (with each other and with other substances) in order to avoid altering their efficacy or potentiating their side effects.

OBJECTIVE

The purpose of this article is to review these aspects and stress the most important interactions in day-to-day clinical practice.

RESULTS

Older antiepileptic drugs (AEDs) such as phenytoin, carbamazepine, phenobarbital and valproic acid can significantly interfere not only with each other and other AEDs, but also with other treatments. Although newer AEDs have a more favourable pharmacokinetic profile, they are not entirely exempt from interactions and they are also commonly administered in combination with older AEDs. Another aspect that should be considered is the existence of any clinically important pharmacokinetic and pharmacodynamic interactions in patients requiring the continuous administration of other treatments.

CONCLUSION

We must be aware of the pharmacokinetic and pharmacodynamic interactions of AEDs. Because of a lack of significant interactions, drugs such as levetiracetam, gabapentin or pregabalin can be recommended in particular groups such as patients with cancer, transplants, anticoagulant treatments or HIV infection. In all cases, it is important to ensure AED efficacy and prevent serious complications.

Acetaminophen Overdose with Altered Acetaminophen Pharmacokinetics and Hepatotoxicity Associated with Premature Cessation of Intravenous N-Acetylcysteine Therapy

Objective

To report a case of erratic absorption, double peak serum concentrations, and hepatotoxicity following premature cessation of intravenous N-acetylcysteine (NAC) treatment in the setting of a massive acetaminophen overdose.

Case Summary:

A 78-year-old man reportedly ingested approximately 96 immediate-release acetaminophen 500-mg tablets (48 g) over a one-hour period in an apparent suicide attempt. The acetaminophen concentration at 2.25 hours was 264 μg/mL. Intravenous NAC was initiated 5 hours postingestion. At 6.25 hours postingestion, the acetaminophen concentration was 281 μg/mL. Following administration of intravenous NAC for 21 hours, therapy was discontinued despite a residual acetaminophen concentration of 116 μg/mL. The patient experienced hepatotoxicity, coagulopathy, and renal injury. Pharmacokinetic analysis revealed significantly prolonged acetaminophen absorption and a second peak acetaminophen concentration of 228 μg/mL approximately 48 hours post ingestion. Direct in-hospital monitoring of the patient made a second ingestion unlikely.

Discussion:

Acetaminophen overdose is usually effectively managed with NAC. Patients with massive ingestions may have altered absorption kinetics due to acetaminophen's solubility being exceeded, physiologically or chemically altered gastrointestinal emptying or motility, or other factors. These patients may benefit from gastrointestinal decontamination and prolonged NAC therapy.

Conclusions:

In patients with massive acetaminophen ingestion, erratic absorption may occur, and toxic serum concentrations may persist beyond a standard 21–hour course of intravenous NAC therapy. Acetaminophen concentrations and aminotransferase levels should be evaluated at the completion of the intravenous NAC infusion to ensure complete elimination of acetaminophen and absence of hepatotoxicity and to exclude the need lor prolonged treatment.

Management of common neurologic symptoms in pediatric palliative care: seizures, agitation, and spasticity

Palliative care for children is complex and focuses on patients' comfort. Some of the most troublesome symptoms as patients approach the end of life are seizures, agitation, and spasticity. Many doctors caring for children at the end of life are uncomfortable or untrained in managing these symptoms in children. Our goal is to help physicians recognize and treat these neurologic symptoms optimally.

Antiepileptic drug monotherapy: pediatric concerns

Selecting the optimal antiepileptic drug (AED) begins with accurate epilepsy classification, including seizure type and epilepsy syndrome if possible. Based on the available data, children with focal epilepsy, with or without secondary generalization, can be treated with a traditional or newer narrow-spectrum or broad-spectrum AED. Children with generalized convulsive epilepsy, mixed epilepsy, or seizures of an unknown type are best treated with a broad-spectrum AED. Children with childhood absence epilepsy can be treated with ethosuximide, valproate, or lamotrigine. In all cases, the best choice among the various AED options requires consideration of factors such as seizure frequency, seizure severity, AED adverse event profile, AED titration schedule, patient comorbidities, prescription plan coverage, and cost. Most children with epilepsy achieve the goal of "no seizures and no side effects" and most children eventually become seizure free without AEDs. If accurate epilepsy classification is made, clear differences in efficacy are not evident among the multiple available AEDs. Better comparative data emphasizing adverse event profiles, comorbidities and longer-term outcome are needed between the traditional and newer AEDs.

Impairment of inhibitory control of the hypothalamic pituitary adrenocortical system in epilepsy

Excess comorbidity between depression and epilepsy proposes common pathophysiological patterns in both disorders. Neuroendocrine abnormalities were often observed in depression as well as in epilepsy. Lack of inhibitory control of the hypothalamic pituitary adrenocortical (HPA) system is a core feature of depression; main relay stations of this system are located in the amygdala and hippocampus, which are key regions for both disorders. Therefore we explored the feedback mechanism of the HPA system in epilepsy. In order to control for the impact of depression we focused on epilepsies without depression. We compared patients with epilepsy (subdivided by medication with or without hepatic enzyme inducing antiepileptic medication) with 16 healthy controls and 16 patients with unipolar major depression but without epilepsy. We observed a lack of inhibitory control of the HPA system in patients with epilepsy, also in the absence of enzyme inducing medication. An impact of the temporal lobe location of the epileptic focus could not be observed. Thus, epilepsies share with depression the deficiencies in the feedback mechanism of the HPA system, proposing common pathophysiological features of up to now unknown nature.

Nonparametric population modeling of valproate pharmacokinetics in epileptic patients using routine serum monitoring data: implications for dosage

Therapeutic drug monitoring (TDM) of valproate (VAL) is important in the optimization of its therapy. The aim of the present work was to evaluate the ability of TDM using model-based, goal-oriented Bayesian adaptive control for help in planning, monitoring, and adjusting individualized VAL dosing regimens. USC*PACK software and routine TDM data were used to estimate population and individual pharmacokinetics of two commercially available VAL formulations in epileptic adult and pediatric patients on chronic VAL monotherapy. The population parameter values found were in agreement with values reported earlier. A statistically significant (P < 0.001) difference in median values of the absorption rate constant was found between enteric-coated and sustained-release VAL formulations. In our patients (aged 0.25-53 years), VAL clearance declined with age until adult values were reached at about age 10. Because of the large interindividual variability in PK behavior, the median population parameter values gave poor predictions of the observed VAL serum concentrations. In contrast, the Bayesian individualized models gave good predictions for all subjects in all populations. The Bayesian posterior individualized PK models were based on the population models described here and where most patients had two (a peak and a trough) measured serum concentrations. Repeated consultations and adjusted dosage regimens with some patients allowed us to evaluate any possible influence of dose-dependent VAL clearance on the precision of total VAL concentration predictions based on TDM data and the proposed population models. These nonparametric expectation maximization (NPEM) population models thus provide a useful tool for planning an initial dosage regimen of VAL to achieve desired target peak and trough serum concentration goals, coupled with TDM soon thereafter, as a peak-trough pair of serum concentrations, and Bayesian fitting to individualize the PK model for each patient. The nonparametric PK parameter distributions in these NPEM population models also permit their use by the new method of 'multiple model' dosage design, which allows the target goals to be achieved specifically with maximum precision. Software for both types of Bayesian adaptive control is now available to employ these population models in clinical practice.

Lack of pharmacokinetic interaction between retigabine and phenobarbitone at steady-state in healthy subjects

AIMS

To evaluate potential pharmacokinetic interactions between phenobarbitone and retigabine, a new antiepileptic drug.

METHODS

Fifteen healthy men received 200 mg of retigabine on day 1. On days 4-32, phenobarbitone 90 mg was administered at 22.00 h. On days 26-32, increasing doses of retigabine were given to achieve a final dose of 200 mg every 8 h on day 32. The pharmacokinetics of retigabine were determined on days 1 and 32, and those for phenobarbitone on days 25 and 31.

RESULTS

After administration of a single 200 mg dose, retigabine was rapidly absorbed and eliminated with a mean terminal half-life of 6.7 h, a mean AUC of 3936 ng x ml(-1) x h and a mean apparent clearance of 0.76 l x h(-1) x kg(-1). Similar exposure to the partially active acetylated metabolite (AWD21-360) of retigabine was observed. After administration of phenobarbitone dosed to steady-state, the pharmacokinetics of retigabine at steady-state were similar (AUC of 4433 ng x ml(-1) x h and t1/2 of 8.5 h) to those of retigabine alone. The AUC of phenobarbitone was 298 mg x l(-1) x h when administered alone and 311 mg x ml(-1) x h after retigabine administration. The geometric mean ratios and 90% confidence intervals of the AUC were 1.11 (0.97, 1.28) for retigabine, 1.01 (0.88, 1.06) for AWD21-360 and 1.04 (0.96, 1.11) for phenobarbitone. Individual and combined treatments were generally well tolerated. One subject was withdrawn from the study on day 10 due to severe abdominal pain. Headache was the most commonly reported adverse event. No clinically relevant changes were observed in the electrocardiograms, vital signs or laboratory measurements.

CONCLUSIONS

There was no pharmacokinetic interaction between retigabine and phenobarbitone in healthy subjects. No dosage adjustment is likely to be necessary when retigabine and phenobarbitone are coadministered to patients.

Interactions between antiepileptic and chemotherapeutic drugs

Cancer and epilepsy commonly co-occur, and concomitant administration of antiepileptic (AEDS) and chemotherapeutic drugs (CTDs) is necessary in many cases. Many drugs are metabolised by the hepatic cytochrome P450 (CYP) isoenzyme system, and coadministration of AEDs and CTDs can lead to clinically relevant interactions by induction or inhibition of enzymes in shared metabolic pathways. These interactions can cause insufficient tumour and seizure control or lead to unforeseen toxicity. Enzyme-inducing AEDs reduce the effects of taxanes, vinca alkaloids, methotrexate, teniposide, and camptothecin analogues. Inhibition of the metabolism of nitrosoureas or etoposide by valproic acid can lead to CTD toxicity. Poor seizure control may result from the combinations of phenytoin with cisplatin or corticosteroids, and valproic acid with methotrexate. Increased toxicity of AEDs can occur when phenytoin is combined with 5-fluorouracil. Use of enzyme-inducing AEDs should be avoided in patients with cancer, particularly in association with chemotherapy. Generally, valproic acid-although not free from interactions-would be the agent of first choice. Some of the newer AEDs not metabolised by the P450 system may prove to be good alternatives.

[Antiepileptic drugs used in childhood. New products and new concepts]

INTRODUCTION

Eight new antiepileptic drugs (AED) have been marketed in Spain since 1990 and others will soon follow.

OBJECTIVE

To review the concepts underlying the development of the new drugs, as well as their indications, efficacy and safety.

DEVELOPMENT

Pharmacologic antiepileptic intervention is no longer solely directed towards an anticonvulsant effect, but also to epileptogenic prevention, disease modification and reversal of pharmacoresistance. The development of new AED, initially based on the screening of putative products in animal models, changed during the last half of the century to a rational design based on known facts about excitatory /inhibitory neuronal mechanisms. More recently, attention has focussed on pharmacogenetics. The new AED were initially indicated for partial epilepsies, but some have been shown to have a broader clinical spectrum. Some show the ideal pharmacokinetic mechanisms, avoiding hepatic metabolism and protein binding. Drug interactions and adverse effects, especially severe idiosyncratic adverse effects, are rare, although there are some exceptions. In most cases, however, seizure control does not seem to be better than with the classic AED. Because of the specific characteristics of childhood epilepsy and pharmacokinetics, as well as the regulations governing the development of clinical trials, the use of new products in children is circumspect, which in turn delays the access of such patients to a possible therapeutic benefit.

Effectiveness of muscimol-containing microparticles against pilocarpine-induced focal seizures

PURPOSE

To investigate the efficacy of in situ lipid-protein-sugar particles (LPSPs) in mitigating the epileptogenic and histologic effects of intrahippocampal pilocarpine in rats.

METHODS

LPSPs with and without muscimol were produced by spray-drying, sized by Coulter counter, and muscimol content determined by high-pressure liquid chromatography (HLPC). Particles, free muscimol or saline, were injected into the hippocampi of Sprague-Dawley rats before 40 mM pilocarpine, and seizure activity was scored. The trajectories of behavioral scores between groups were compared with two-way repeated measures analysis of variance. Animals were killed after 2 weeks. Brain sections were stained (Timm and thionin) and scored.

RESULTS

LPSPs were 4 to 5 microm in diameter, and contained 0 or 2% (wt/wt) muscimol. In vitro, muscimol was released over a 5-day period. Intrahippocampal injections of normal saline and blank LPSPs did not deter seizure activity from pilocarpine. The rise of the trajectory in behavior scores in animals given LPSPs containing 5 microg muscimol was significantly slower than in those receiving saline, blank particles, or 5 microg of unencapsulated muscimol. There was less apparent neuronal injury and CA3 and supragranular mossy fiber sprouting in hippocampi of animals receiving muscimol-containing particles than in animals that did not receive muscimol. Hippocampi of animals that received 5 microg of encapsulated muscimol showed less supragranular sprouting than did those receiving 5 microg of unencapsulated muscimol, but showed no difference in cell loss or CA3 sprouting.

CONCLUSIONS

Focally delivered biodegradable microparticles loaded with muscimol are effective in reducing seizure activity from pilocarpine in animals and mitigate the histologic effects.

Epilepsy, psychosis and clozapine

Six patients with epilepsy and severe psychosis were treated with the atypical antipsychotic clozapine. The use of clozapine might be complicated in epileptic patients because of an increased risk of seizures. However, none of the reported patients had an increase of their seizure frequency, in contrast, three patients had a substantial reduction of seizures. One patient had a reduction of non-epileptic seizures as well. In the second part of this paper, combinations of clozapine with newer and older anticonvulsants as well as their interactions and associated risks are discussed.

Epilepsy

Epilepsy may be associated with major social and medical problems, and counselling of patient and family is essential for good management. The workup of a person with a seizure includes history, physical examination, and laboratory testing. An electroencephalogram is essential to help classify the seizure and epilepsy type. Neuroimaging (preferably by magnetic resonance imaging) helps to exclude a structural abnormality. Seizures can be controlled with a single drug (monotherapy) in 70% of patients. The incidence of drug side effects is increased if more than one drug is used. Pregnancy is associated with an increased risk of seizures in 30% of women with epilepsy. Frequent assessment throughout pregnancy is important. There is a slightly increased risk of congenital malformation associated with the antiepileptic drugs. Folic acid supplementation is advisable.