The role of Gilbert's syndrome and frequent NAT2 slow acetylation polymorphisms in the pharmacokinetics of retigabine

Pharmacogenomics in epilepsy

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Genetic variation can influence response to antiepileptic drug (AED) treatment through various effector processes.

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Metabolism of many AEDs is mediated by the cytochrome P450 (CYP) family; some of the CYPs have allelic variants that may affect serum AED concentrations.

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‘Precision medicine’ focuses on the identification of an underlying genetic aetiology allowing personalised therapeutic choices.

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Certain human leukocyte antigen, HLA, alleles are associated with an increased risk of idiosyncratic adverse drug reactions.

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New results are emerging from large-scale multinational efforts, likely imminently to add knowledge of value from a pharmacogenetic perspective.

There is high variability in the response to antiepileptic treatment across people with epilepsy. Genetic factors significantly contribute to such variability. Recent advances in the genetics and neurobiology of the epilepsies are establishing the basis for a new era in the treatment of epilepsy, focused on each individual and their specific epilepsy. Variation in response to antiepileptic drug treatment may arise from genetic variation in a range of gene categories, including genes affecting drug pharmacokinetics, and drug pharmacodynamics, but also genes held to actually cause the epilepsy itself.

From a purely pharmacogenetic perspective, there are few robust genetic findings with established evidence in epilepsy. Many findings are still controversial with anecdotal or less secure evidence and need further validation, e.g. variation in genes for transporter systems and antiepileptic drug targets. The increasing use of genetic sequencing and the results of large-scale collaborative projects may soon expand the established evidence.

Precision medicine treatments represent a growing area of interest, focussing on reversing or circumventing the pathophysiological effects of specific gene mutations. This could lead to a dramatic improvement of the effectiveness and safety of epilepsy treatments, by targeting the biological mechanisms responsible for epilepsy in each specific individual.

Whilst much has been written about epilepsy pharmacogenetics, there does now seem to be building momentum that promises to deliver results of use in clinic.

The pharmacogenomics of epilepsy

Genetic factors contribute to the high interindividual variability in response to antiepileptic drugs. However, most genetic markers identified to date have limited sensitivity and specificity, and the value of genetic testing in guiding antiepileptic drug (AED) therapy is limited. The best defined indication for testing relates to HLA-B*15:02 genotyping to identify those individuals of South Asian ethnicity who are at high risk for developing serious adverse cutaneous reactions to carbamazepine. The indication for HLA-A*31:01 testing to identify individuals at risk for skin reactions from carbamazepine, or for CYP2C9 genotyping to identify individuals at risk for serious skin reactions from phenytoin is less compelling. The use of genetic testing to guide epilepsy treatment is likely to increase in the future, as better understanding of the function of epilepsy genes will permit the application of precision medicine targeting the biological mechanisms responsible for epilepsy in the specific individual.

Pharmacokinetic of antiepileptic drugs in patients with hepatic or renal impairment

Many factors influence choice of antiepileptic drugs (AEDs), including efficacy of the drug for the indication (epilepsy, neuropathic pain, affective disorder, migraine), tolerability, and toxicity. The first-generation AEDs and some newer AEDs are predominately eliminated by hepatic metabolism. Other recent AEDs are eliminated by renal excretion of unchanged drug or a combination of hepatic metabolism and renal excretion. The effect of renal and hepatic disease on the dosing will depend on the fraction of the AED eliminated by hepatic and/or renal excretion, the metabolic isozymes involved, as well as the extent of protein binding, if therapeutic drug monitoring is used. For drugs that are eliminated by renal excretion, methods of estimating creatinine clearance can be used to determine dose adjustments. For drugs eliminated by hepatic metabolism, there are no specific markers of liver function that can be used to provide guidance in dosage adjustments. Based on studies with probe drugs, the hepatic metabolic enzymes are differentially affected depending on the cause and severity of hepatic disease, which can aid in predicting dose adjustment when clinical data are not available. Several AEDs are also associated with laboratory markers of mild hepatic dysfunction and, rarely, more severe hepatic injury. In contrast, the risk of renal injury from AEDs is generally low. In general, co-morbid hepatic or renal diseases influence the decision for the selection of an AED. For some patients dosing changes to their existing AEDs may be appropriate. For others, a change to another AED may be a better option.

Ezogabine: An Evaluation of Its Efficacy and Safety as Adjunctive Therapy for Partial-Onset Seizures in Adults

Objective:

To evaluate the safety, efficacy, pharmacokinetics, pharmacodynamic properties, and clinical application of ezogabine (retigabine. INN), an antiepileptic drug approved in 2011.

Data Sources:

Published data from in vitro, animal, and clinical studies were obtained from PubMed and CINAHL searches, from January 1980 to March 31, 2012. Other relevant data regarding the safety and efficacy of ezogabine were obtained from the Food and Drug Administration and the European Medication Agency Web sites.

Study Selection and Data Extraction:

Selected articles were prospective in vitro, animal, and controlled clinical studies of ezogabine. Non-English-language articles were excluded.

Data Synthesis:

In vitro and animal studies show that ezogabine activates voltage-gated potassium channels, leading to reduction of seizure frequency by inhibiting hyperexcitability activity in the central nervous system. Additionally, ezogabine enhances γ-aminobutyric acid (GABA) activity and de novo GABA synthesis. Eight clinical studies of ezogabine have been published, 5 being Phase 1 clinical trials in healthy subjects and 3 being Phase 3 clinical trials in patients with pharmacoresistant partial-onset seizures. Phase 3 clinical trials demonstrated the safety and efficacy of ezogabine in patients with partial-onset seizures.

Conclusions:

Clinical trials have shown that ezogabine is efficacious as an adjunctive agent in patients with pharmacoresistant partial seizures. Careful monitoring of drug interactions and adverse reactions is necessary. While ezogabine is efficacious (or partial seizures, its precise role in the management of patients with epilepsy is yet to be determined.

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.

Pharmacotherapy of the third-generation AEDs: lacosamide, retigabine and eslicarbazepine acetate

INTRODUCTION

The search for new, more effective antiepileptic drugs (AEDs) continues. The three most recently approved drugs, the so-called third-generation AEDs, include lacosamide, retigabine and eslicarbazepine acetate and are licensed as adjunctive treatment of partial epilepsy in adults.

AREAS COVERED

For the above three AEDs, their mechanisms of action, pharmacokinetic characteristics, drug-drug interactions, pharmacotherapeutics, dose and administration and therapeutic drug monitoring are reviewed in this paper.

EXPERT OPINION

Lacosamide and retigabine act through novel mechanisms, while eslicarbazepine acetate, a pro-drug for eslicarbazepine, acts in a similar manner to several other AEDs. All three AEDs are associated with linear pharmacokinetic and rapid absorption and undergo metabolism. Their drug-drug interaction profile is low (lacosamide and retigabine) to modest (eslicarbazepine) in propensity. At the highest approved doses for the three AEDs, responder rates were similar. The most commonly observed adverse effects compared with placebo were dizziness, headache, diplopia and nausea for lacosamide; dizziness, somnolence and fatigue for retigabine and dizziness and somnolence for eslicarbazepine acetate. The precise role that these new AEDs will have in the treatment of epilepsy and whether they will make a significant impact on the prognosis of intractable epilepsy is not yet known and will have to await further clinical experience.

Retigabine (ezogabine): in partial-onset seizures in adults with epilepsy

Retigabine (ezogabine in the US) opens neuronal voltage-gated potassium channels, resulting in resting membrane potential stabilization, neuronal subthreshold excitability control and anticonvulsant effects. The clinical efficacy of adjunctive oral retigabine in adults with inadequately controlled, partial-onset seizures was demonstrated in two large, well designed, phase III trials (RESTORE-1 and RESTORE-2), generally confirming the findings of an earlier phase IIb study. In the RESTORE trials, retigabine 600, 900 or 1200 mg/day was associated with significantly higher rates of response (i.e. reduction in 28-day total partial seizure frequency of ≥50%) than placebo during both the 12-week maintenance period and the entire 16- or 18-week double-blind phase (i.e. titration plus maintenance) of the studies. Retigabine recipients also had significantly greater median reductions from baseline in 28-day total partial seizure frequency than placebo recipients during these treatment periods. These benefits of retigabine were generally seen irrespective of age, gender, race and baseline seizure frequency, and were maintained for up to 12 months according to interim data from subsequent open-label extension studies, with some patients also experiencing seizure-free periods of up to 12 months. Retigabine was generally well tolerated in adults with partial-onset seizures in the RESTORE studies, with most adverse events being of mild or moderate severity.

Neuronal potassium channel openers in the management of epilepsy: role and potential of retigabine

Despite the availability of over 20 antiepileptic drugs, about 30% of epileptic patients do not achieve seizure control. Thus, identification of additional molecules targeting novel molecular mechanisms is a primary effort in today's antiepileptic drug research. This paper reviews the pharmacological development of retigabine, an antiepileptic drug with a novel mechanism of action, namely the activation of voltage-gated potassium channels of the Kv7 subfamily. These channels, which act as widespread regulators of intrinsic neuronal excitability and of neurotransmitter-induced network excitability changes, are currently viewed among the most promising targets for anticonvulsant pharmacotherapy. In particular, the present work reviews the pathophysiological role of Kv7 channels in neuronal function, the molecular mechanisms involved in the Kv7 channel-opening action of retigabine, the activity of retigabine in preclinical in vitro and in vivo studies predictive of anticonvulsant activities, and the clinical status of development for this drug as an add-on treatment for pharmacoresistant epilepsy. Particular efforts are devoted to highlighting the potential advantages and disadvantages of retigabine when compared with currently available compounds, in order to provide a comprehensive assessment of its role in therapy for treatment-resistant epilepsies.