Ethosuximide kinetics: possible interaction with valproic acid

Model-Informed Precision Dosing Guidance of Ethosuximide Developed from a Randomized Controlled Clinical Trial of Childhood Absence Epilepsy

Ethosuximide was identified as the optimal option for new-onset childhood absence epilepsy (CAE) in a randomized, two-phase dose escalation comparative effectiveness trial of ethosuximide, lamotrigine, and valproic acid. However, 47% of ethosuximide initial monotherapy participants experienced short-term treatment failure. This study aimed to characterize the initial monotherapy ethosuximide exposure-response relationship and to propose model-informed precision dosing guidance. Dose titration occurred over a 16-20-week period until patients experienced seizure freedom or intolerable side effects. Subjects with initial monotherapy failure were randomized to one of the other two medications and dose escalation was repeated. A population pharmacokinetic model was created using plasma concentration data (n = 1,320), collected at 4-week intervals from 211 unique participants during both the initial and second monotherapy phases. A logistic regression analysis was performed on the initial monotherapy cohort (n = 103) with complete exposure-response data. Eighty-four participants achieved seizure freedom with a wide range of ethosuximide area under the curves (AUC) ranging from 420 to 2,420 μg·h/mL. AUC exposure estimates for achieving a 50% and 75% probability of seizure freedom were 1,027 and 1,489 μg·h/mL, respectively, whereas the corresponding cumulative frequency of intolerable adverse events was 11% and 16%. Monte Carlo Simulation indicated a daily dose of 40 and 55 mg/kg to achieve 50% and 75% probability of seizure freedom in the overall population, respectively. We identified the need for adjusted mg/kg dosing in different body weight cohorts. This ethosuximide proposed model-informed precision dosing guidance to achieve seizure freedom carries promise to optimize initial monotherapy success for patients with CAE.

Potential problems and recommendations regarding substitution of generic antiepileptic drugs: a systematic review of literature

Despite the availability of generic antiepileptic drugs (AEDs), still patients and neurologists hesitate to make a switch due to assorted reasons. The objectives of this review were to evaluate the risks associated with the generic substitution of AEDs. In this context, we also summarized the recommendations of various international societies to treat epileptic patients. We used a number of electronic databases to identify the relevant published studies which demonstrated the potential problems and recommendations regarding generic substitution of AEDs. Of 204 articles found initially, 153 were selected for additional review. Subsequently, 68 articles were finally selected. This review concluded that potential problems linked with the generic substitution of AEDs could be bioequivalence issues, failure of drug therapy, emergence of adverse events and increase in the frequency of seizures. The reasons could be the pharmacokinetics properties of AEDs and unique characteristics of some epilepsy patients. Consequently, the generic substitution of AEDs affects the successful treatment and quality of life of the patients. Various guidelines recommend the well-controlled epileptic patients to avoid switching from brand-to-generic products, generic-to-brand products or generic to some other generic products.

[Therapeutic drug monitoring of ethosuximide]

Ethosuximide is a minor antiepileptic drug, available in France since 1965, indicated in the epilepsy absence, whose interest was reassessed from recent clinical trials, showing that it was the first choice, in term of risk benefit relationship, in this indication. It is a chiral molecule that presents a high bioavailability, a lack of protein binding, hepatic metabolism and urinary excretion. Its elimination half-life is long, between 40 and 60 h in adults, 30 and 40 h in children. The therapeutic range is established at 40-100 mg/L (283-708 µmol/L), but the upper limit is probably underestimated. The clinical studies of relation exposure effects, although ancient (from the 1970s) and realized with methodologies that do not meet current criteria, show concentration-efficacy and -toxicity relationship and the risk of drug interactions is proven. It is a drug preponderantly prescribed in children, a vulnerable population with physiological change with age. To benefit at best of its effectiveness, it is necessary to have relatively high plasma concentrations. Despite these arguments and due to the lack of studies providing a sufficient level of evidence, the recommendation can only be "potentially useful", assessment probably underestimated.

Host factors affecting antiepileptic drug delivery-pharmacokinetic variability

Antiepileptic drugs (AEDs) are the mainstay in the treatment of epilepsy, one of the most common serious chronic neurological disorders. AEDs display extensive pharmacological variability between and within patients, and a major determinant of differences in response to treatment is pharmacokinetic variability. Host factors affecting AED delivery may be defined as the pharmacokinetic characteristics that determine the AED delivery to the site of action, the epileptic focus. Individual differences may occur in absorption, distribution, metabolism and excretion. These differences can be determined by genetic factors including gender and ethnicity, but the pharmacokinetics of AEDs can also be affected by age, specific physiological states in life, such as pregnancy, or pathological conditions including hepatic and renal insufficiency. Pharmacokinetic interactions with other drugs are another important source of variability in response to AEDs. Pharmacokinetic characteristics of the presently available AEDs are discussed in this review as well as their clinical implications.

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.

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.

Influence of ethosuximide on valproic acid serum concentrations

In the therapy of absence epilepsies, a combination of ethosuximide (ESM) and valproic acid (VPA) is sometimes necessary for a successful seizure control. Previous studies of the interaction between ESM and VPA revealed contradictory results. We investigated the influence of ESM on VPA serum concentrations in children with epilepsy. In case of ineffectiveness of the drug, ESM was withdrawn (n = 9). Four children treated with VPA got ESM additionally because their seizure control was insufficient with VPA alone. Two children had bromide, and one clobazam as comedicament. Both of these antiepileptic drugs (AEDs) do not have any known interactions with ESM or VPA. Serum levels of VPA were higher in monotherapy than in combination with ESM (120.0 +/- 20.1 micrograms/ml; range, 88.9-153.4 micrograms/ml; vs. 87.0 +/- 13.1 micrograms/ml; range, 67.4-108.0 micrograms/ml). The difference was statistically significant (P < 0.01). After stopping ESM the serum concentrations of VPA rose about 36.7%; when combined with ESM they fell about 28.3%. Neither the age of the patients nor the serum concentrations of ESM influenced significantly the changes of VPA serum levels in either group. The mechanism of ESM to influence the serum levels of VPA remains unknown. ESM has no known enzyme inducing properties. The interaction of ESM and VPA ought to be considered in a combination therapy of these drugs.

Synergistic anticonvulsant effect of valproic acid and ethosuximide on pentylenetetrazole-induced epileptic phenomena in rats

The possible synergistic effect of valproic acid and ethosuximide in combination on pentylenetetrazole-induced epilepsy was investigated in rats. Valproic acid and ethosuximide administered intraperitoneally both showed dose-dependent anti-epileptic activity towards pentylenetetrazole-induced myoclonias and tonic-clonic seizures. The valproic acid-ethosuximide combination had a synergistic pharmacological effect. Against myoclonias combined valproic acid-ethosuximide produced a non-significant decrease in the effective dose of both drugs compared with treatment with either drug alone. In the case of tonic-clonic seizures the protective effect against the seizures was significantly increased by combined treatment compared with treatment with either drug alone. Neither plasma concentrations nor any other pharmacokinetic parameters were significantly changed when the same doses of valproic acid and ethosuximide were given, singly or in combination.

Clinical pharmacokinetics of valproic acid--1988

Sodium valproate (valproic acid) has been widely used in the last decade and is now considered a relatively safe and effective anticonvulsant agent. Recently, several investigators have proposed its use in the treatment of anxiety, alcoholism and mood disorders, although these indications require further clinical studies. Valproic acid is available in different oral formulations such as solutions, tablets, enteric-coated capsules and slow-release preparations. For most of these formulations bio-availability approaches 100%, while the absorption half-life varies from less than 30 minutes to 3 or 4 hours depending on the type of preparation used. Once absorbed, valproic acid is largely bound to plasma proteins and has a relatively small volume of distribution (0.1 to 0.4 L/kg). Its concentration in CSF is approximately one-tenth that in plasma and is directly correlated with the concentration found in tears. At therapeutic doses, valproic acid half-life varies from 10 to 20 hours in adults, while it is significantly shorter (6 to 9 hours) in children. Valproic acid undergoes extensive liver metabolism. Numerous metabolites have been positively identified and there is reasonable evidence that several of them contribute to its pharmacological and toxic actions. In fact, several valproic acid metabolites have anti-convulsant properties, while many of the side effects it may cause (e.g. those related to hyperammonaemia or liver damage) are most often observed in patients previously treated with phenobarbitone. This could indicate that induction of liver enzymes is responsible for the formation of toxic valproic acid metabolites.

Pharmacologic interactions between valproate and other drugs

Valproate is often administered with other antiepileptic drugs, a practice that can lead to clinically significant pharmacologic interactions. Concomitant administration of such enzyme-inducing antiepileptic drugs as carbamazepine, phenobarbital, primidone, or phenytoin will markedly accelerate the metabolic conversion of valproate, particularly in children. In response to the effects of enzyme induction, valproate dosage may need to be doubled to maintain therapeutic serum levels. Valproate does not appear to induce enzymatic drug metabolism, but rather acts as a metabolic inhibitor. Because of this inhibition, phenobarbital dosage must often be reduced after valproate is added to the therapeutic regimen. Valproate also may markedly increase concentrations of the active epoxide metabolite of carbamazepine. The interaction between phenytoin and valproate results primarily from displacement from plasma proteins. The resulting increase in the free fraction of phenytoin alters the relationship between total phenytoin concentration and the drug's pharmacologic effect. Thus, clinical evidence of toxicity may be present at concentrations usually considered to be in the therapeutic range.

An update on sodium valproate

Sodium valproate has been in clinical use for the treatment of epilepsy in Great Britain since 1973 and in the United States since 1978. It is chemically quite different from the existing antiepileptic drugs. Although most authorities concentrate on its modification of GABAergic inhibitory transmission in the central nervous system, its mechanism of action remains obscure. It has been shown to be an effective antiepileptic drug in a wide variety of seizure types, but clinically, its major use to date has been in generalized seizures. It is particularly effective in photosensitive epilepsy and myoclonus. Most adverse reactions to sodium valproate are mild and reversible, but with increasing experience, the drug's rare, idiosyncratic, adverse effects are becoming apparent, particularly hepatotoxicity and teratogenicity. The role of therapeutic drug monitoring in the management of patients taking sodium valproate is controversial.

Valproic acid-ethosuximide interaction: a pharmacokinetic study

The present pharmacokinetic study was designed to investigate the possible interaction between valproic acid (VPA) and ethosuximide (ESM) in humans. Six drug-free healthy volunteers, four men and two women, 18-42 years of age, received a single oral dose of 500 mg ESM before and during a treatment with VPA at 800- to 1,600-mg daily doses. The second ESM dose was given 9 days after VPA administration was started. In this latter condition, a significant (p less than 0.05) increase in ESM serum half-life, from 44 to 54 h on average, and a significant (p less than 0.05) decrease in total body clearance, from 11.2 to 9.5 ml/min on average, were observed. Other pharmacokinetic parameters were unchanged and showed values similar to those reported in the literature. Serum VPA levels ranged between 66.8 and 95 micrograms/ml. Two subjects showed no evidence of interaction. Although a great interindividual variability in the occurrence of VPA-ESM interaction can be observed, the present study indicates that VPA is able to inhibit the metabolism of ESM. Possible factors affecting this interaction are hypothesized and discussed.

Drug interactions with valproic acid

Valproic acid undergoes drug-drug interactions with most of the commonly used anticonvulsants. Since it possesses a wide range of indications, concomitant use with other anticonvulsants, and hence interactions, are not infrequent. Many of these interactions are reciprocal and may have important therapeutic consequences. Valproate acts as a protein binding displacer and/or metabolic inhibitor with respect to a number of other anticonvulsants (phenobarbitone, primidone, phenytoin). Inhibition of metabolism would, in most instances, result in a decrease of the dose requirements of the affected drugs. Valproate is a low clearance drug primarily eliminated by metabolism. Its metabolism is highly inducible by some of the major anticonvulsants (e.g. carbamazepine, phenytoin). Valproate is also highly protein bound in plasma and thus is displaced by salicylates and free fatty acids. However, displacement alone, unlike induced metabolism, should not affect the drug's dose-response relationship.