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

Population pharmacokinetics of phenytoin in critically ill children

The objective was to study the population pharmacokinetics of bound and unbound phenytoin in critically ill children, including influences on the protein binding profile. A population pharmacokinetic approach was used to analyze paired protein-unbound and total phenytoin plasma concentrations (n = 146 each) from 32 critically ill children (0.08-17 years of age) who were admitted to a pediatric hospital, primarily intensive care unit. The pharmacokinetics of unbound and bound phenytoin and the influence of possible influential covariates were modeled and evaluated using visual predictive checks and bootstrapping. The pharmacokinetics of protein-unbound phenytoin was described satisfactorily by a 1-compartment model with first-order absorption in conjunction with a linear partition coefficient parameter to describe the binding of phenytoin to albumin. The partitioning coefficient describing protein binding and distribution to bound phenytoin was estimated to be 8.22. Nonlinear elimination of unbound phenytoin was not supported in this patient group. Weight, allometrically scaled for clearance and volume of distribution for the unbound and bound compartments, and albumin concentration significantly influenced the partition coefficient for protein binding of phenytoin. The population model can be applied to estimate the fraction of unbound phenytoin in critically ill children given an individual's albumin concentration.

Rapid and simultaneous quantification of levetiracetam and its carboxylic metabolite in human plasma by liquid chromatography tandem mass spectrometry

A simple liquid chromatography tandem mass spectrometry method was developed and validated according to the guidelines of the US Food and Drug Administration and the European Medicines Agency for a simultaneous quantification of levetiracetam (LEV) and its metabolite, UCB L057 in the plasma of patients. A 0.050 mL plasma sample was prepared by a simple and direct protein precipitation with 0.450 mL acetonitrile (ACN) containing 1 µg/mL of internal standard (IS, diphenhydramine), then vortex mixed and centrifuged. A 0.100 mL of the clear supernatant was diluted with 0.400 mL water and well mixed. A 0.010 mL of the resultant solution was injected into an Agilent Zorbax SB-C18 (2.1 mm×100 mm, 3.5 µm) column with an isocratic elution at 0.5 mL/min using a mixture of 0.1% formic acid in water and ACN (40:60 v/v). Detection was performed using an AB Sciex API 3000 triple quadrupole mass spectrometer, equipped with a Turbo Ion Spray source, operating in a positive mode: LEV at transition 171.1>154.1, UCB L057 at 172.5>126.1, and IS at 256.3>167.3; with an assay run time of 2 minutes. The lower limit of quantification (LLOQ) for both LEV and UCB L057 was validated at 0.5 µg/mL, while their lower limit of detection (LOD) was 0.25 µg/mL. The calibration curves were linear between 0.5 and 100 µg/mL for both analytes. The inaccuracy and imprecision of both intra-assay and inter-assay were less than 10%. Matrix effects were consistent between sources of plasma and the recoveries of all compounds were between 100% and 110%. Stability was established under various storage and processing conditions. The carryovers from both LEV and UCB L057 were less than 6% of the LLOQ and 0.13% of the IS. This assay method has been successfully applied to a population pharmacokinetic study of LEV in patients with epilepsy.

Impact of the superoxide dismutase 2 Val16Ala polymorphism on the relationship between valproic acid exposure and elevation of γ-glutamyltransferase in patients with epilepsy: a population pharmacokinetic-pharmacodynamic analysis

BACKGROUND

There has been accumulating evidence that there are associations among γ-glutamyltransferase (γ-GT) elevation and all-cause mortality, cardiovascular diseases and metabolic diseases, including nonalcoholic fatty liver disease. The primary objective of this study was to evaluate the impact of the most common and potentially functional polymorphisms of antioxidant enzyme genes, i.e. superoxide dismutase 2 (SOD2), glutathione S-transferase M1 and glutathione S-transferase T1, on the γ-GT elevation during valproic acid (VPA) therapy.

METHODS AND FINDINGS

This retrospective study included 237 and 169 VPA-treated Japanese patients with epilepsy for population pharmacokinetic and pharmacokinetic-pharmacodynamic analyses, respectively. A nonlinear mixed-effect model represented the pharmacokinetics of VPA and the relationships between VPA exposure and γ-GT elevation. A one-compartment model of the pharmacokinetic parameters of VPA adequately described the data; while the model for the probability of the γ-GT elevation was fitted using a logistic regression model, in which the logit function of the probability was a linear function of VPA exposure. The SOD2 Val16Ala polymorphism and complication with intellectual disability were found to be significant covariates influencing the intercept of the logit function for the probability of an elevated γ-GT level. The predicted mean percentages of the subjects with γ-GT elevation were about 2- to 3-fold, 3- to 4-fold and 4- to 8-fold greater in patients with the SOD2 Val/Val genotype but without any intellectual disability, those with the SOD2 Val/Ala or Ala/Ala genotype and intellectual disability and those with the SOD2 Val/Val genotype and intellectual disability, respectively, compared to those with the SOD2 Val/Ala or Ala/Ala genotype without intellectual disability.

CONCLUSION

Our results showed that the SOD2 Val16Ala polymorphism has an impact on the relationship between VPA exposure and γ-GT elevation in patients with epilepsy. These results suggest that determining the SOD2 genotype could be helpful for preventing the VPA-induced γ-GT elevation.

Impact of a pharmaceutical care programme on health-related quality of life among women with epilepsy: a randomised controlled trial (IPHIWWE study)

Background

Epilepsy is a complex chronic disorder which affects health-related quality of life (HRQOL), especially in women.

Pharmaceutical care (PC) allows direct intervention between the pharmacist, the patient and the other healthcare team members to optimise treatments in order to reduce negative outcomes related to medication and contribute to improving HRQOL.

The aim of the study was to establish the impact of the application of a pharmaceutical care programme on the HRQOL of women with epilepsy.

Methods

This study is a pragmatic randomised controlled trial involving women with epilepsy (WWE) over 18 years of age.

The intervention group (IG) received a pharmaceutical care programme consisting of medication review follow-up according to Dáder’s method, health education and therapeutic drug monitoring of anticonvulsants.

The impact was assessed by changes in seizure frequency, in the self-administered questionnaires (the QOLIE-31, Liverpool AEP, CES-D, Haynes-Sackett test and Moriski-Green test) and between the first interview and the one at the end of six months of follow-up.

A Student’s t-test was performed to compare the final QOLIE-31 score between groups and a paired Student’s t-test was used to determine the change in each group between the start and the end of follow-up.

Results

One hundred eighty-two WWE entered the study and 144 (79.1%) completed it. The t-test for comparing the final QOLIE-31 scores between groups yielded a t = −2.166 and confidence interval (CI) (95%): −10.125; −0.4625, p-value =0.0319. The change (Δ) in the QOLIE-31 score for the IG was 12.45 points (p-value <0.001) and for the control group it was 2.61 (p-value =0.072). With 10.7 as the minimally important change we found a relative risk of 2.17 (CI: 1.37; 3.43) and a number needed to treat (NNT) of 3.5.

Conclusions

The study demonstrated that the application of a pharmaceutical care programme significantly improves HRQOL in WWE. The NNT we found allows a recommendation to implement the PC programme for the additional benefit that would be obtained in patients’ HRQOL.

Trial registration

Current Controlled Trials ISRCTN46864306 IPHIWWE study.

The effect of polymorphic metabolism enzymes on serum phenytoin level

Phenytoin has a widespread use in epilepsy treatment and is mainly metabolized by hepatic cytochrome P450 enzymes (CYP). We have investigated CYP2C9*2, CYP2C9*3, CYP2C19*2 and CYP2C19*3 allelic variants in a Turkish population of patients on phenytoin therapy. Patients on phenytoin therapy (n = 102) for the prevention of epileptic seizures were included. Polymorphic alleles were analyzed by restriction fragment length polymorphism method. Serum concentrations of phenytoin were measured by fluorescence polarization immune assay method. The most frequent genotype was detected for CYP2C9 wild-type alleles (78.43 %), whereas CYP2C19*2/*2 (5.88 %) was the least frequent genotype group. According to the classification made with both enzyme polymorphisms, CYP2C9*1/*1-CYP2C19*1/*1 (G1: 41.17 %) genotype group was the most frequent whereas CYP2C9*1/*2-CYP2C19*1/*3 (G7: 0.98 %) was the least frequent one. The highest mean phenytoin level (27.95 ± 1.85 µg/ml) was detected in the G8 genotype group (CYP2C9*1/*3-CYP2C19*2/*3) and the G1 genotype group showed the lowest mean phenytoin level (7.43 ± 0.73 µg/ml). The mean serum concentration of phenytoin of the polymorphic patients with epilepsy was higher than that for the wild-type alleles both in the monotherapy and polytherapy patients. These results show the importance of the genetic polymorphism analysis of the main metabolizing enzyme groups of phenytoin for the dose adjustment.

Population pharmacokinetics modeling of oxcarbazepine to characterize drug interactions in Chinese children with epilepsy

AIM

To develop a population pharmacokinetics model of oxcarbazepine in Chinese pediatric patients with epilepsy, and to study the interactions between oxcarbazepine and other antiepileptic drugs (AEDs).

METHODS

A total of 688 patients with epilepsy aged 2 months to 18 years were divided into model (n=573) and valid (n=115) groups. Serum concentrations of the main active metabolite of oxcarbazepine, 10-hydroxycarbazepine (MHD), were determined 0.5-48 h after the last dosage. A population pharmacokinetics (PPK) model was constructed using NLME software. This model was internally evaluated using Bootstrapping and goodness-of-fit plots inspection. The data of the valid group were used to calculate the mean prediction error (MPE), mean absolute prediction error (MAE), mean squared prediction error (MSE) and the 95% confidence intervals (95% CI) to externally evaluate the model.

RESULTS

The population values of pharmacokinetic parameters estimated in the final model were as follows: Ka=0.83 h-1, Vd=0.67 L/kg, and CL=0.035 L·kg(-1)·h(-1). The enzyme-inducing AEDs (carbamazepine, phenytoin, phenobarbital) and newer generation AEDs (levetiracetam, lamotrigine, topiramate) increased the weight-normalized CL value of MHD by 17.4% and 10.5%, respectively, whereas the enzyme-inhibiting AED valproic acid decreased it by 3%. No significant association was found between the CL value of MHD and the other covariates. For the final model, the evaluation results (95% CI) were MPE=0.01 (-0.07-0.10) mg/L, MAE=0.46 (0.40-0.51) mg/L, MSE=0.39 (0.27-0.51) (mg/L)(2).

CONCLUSION

A PPK model of OXC in Chinese pediatric patients with epilepsy is established. The enzyme-inducing AEDs and some newer generation AEDs (lamotrigine, topiramate) could slightly increase the metabolism of MHD.

Clinical validation and implications of dried blood spot sampling of carbamazepine, valproic acid and phenytoin in patients with epilepsy

To facilitate therapeutic monitoring of antiepileptic drugs (AEDs) by healthcare professionals for patients with epilepsy (PWE), we applied a GC-MS assay to measure three AEDs: carbamazepine (CBZ), phenytoin (PHT) and valproic acid (VPA) levels concurrently in one dried blood spot (DBS), and validated the DBS-measured levels to their plasma levels. 169 PWE on either mono- or polytherapy of CBZ, PHT or/and VPA were included. One DBS, containing ∼15 µL of blood, was acquired for the simultaneous measurement of the drug levels using GC-MS. Simple Deming regressions were performed to correlate the DBS levels with the plasma levels determined by the conventional immunoturbimetric assay in clinical practice. Statistical analyses of the results were done using MedCalc Version 12.6.1.0 and SPSS 21. DBS concentrations (C_dbs) were well-correlated to the plasma concentrations (C_plasma): r = 0.8381, 0.9305 and 0.8531 for CBZ, PHT and VPA respectively, The conversion formulas from C_dbs to plasma concentrations were [0.89×C_dbsCBZ+1.00]µg/mL, [1.11×C_dbsPHT−1.00]µg/mL and [0.92×C_dbsVPA+12.48]µg/mL respectively. Inclusion of the red blood cells (RBC)/plasma partition ratio (K) and the individual hematocrit levels in the estimation of the theoretical C_plasma from C_dbs of PHT and VPA further improved the identity between the observed and the estimated theoretical C_plasma. Bland-Altman plots indicated that the theoretical and observed C_plasma of PHT and VPA agreed well, and >93.0% of concentrations was within 95% CI (±2SD); and similar agreement (1∶1) was also found between the observed C_dbs and C_plasma of CBZ. As the C_plasma of CBZ, PHT and VPA can be accurately estimated from their C_dbs, DBS can therefore be used for drug monitoring in PWE on any of these AEDs.

Nonadherence to treatment causing acute hospitalizations in people with epilepsy: an observational, prospective study

The aim was to assess the clinical relevance of antiepileptic drug (AED) nonadherence by means of therapeutic drug concentration monitoring (TDM). Two hundred eighty-two consecutive patients with epilepsy acutely admitted to hospital for seizures were included. Nonadherence was defined as having a serum concentration/dose ratio at admission of <75% of the patient's own control value (probable nonadherence: 50-75%; definite: <50%). Nonadherence was identified in 39% of patients (definite 24%; probable 15%). It was significantly more common in patients with generalized seizures compared to those with focal onset seizures, and in patients <30 years compared to older patients. When specifically asked, 44% of nonadherent patients claimed regular intake. Nonadherence is a major cause of seizure breakthrough in patients with epilepsy, particularly in young adults. Many patients seem to be unaware of missed drug intake. Prompt measurements of AED serum concentrations should be available as part of the emergency care for patients acutely hospitalized for seizures to permit this issue to be thoroughly addressed prior to discharge.

Advances in anti-epileptic drug testing

In the past twenty-one years, 17 new antiepileptic drugs have been approved for use in the United States and/or Europe. These drugs are clobazam, ezogabine (retigabine), eslicarbazepine acetate, felbamate, gabapentin, lacosamide, lamotrigine, levetiracetam, oxcarbazepine, perampanel, pregabalin, rufinamide, stiripentol, tiagabine, topiramate, vigabatrin and zonisamide. Therapeutic drug monitoring is often used in the clinical dosing of the newer anti-epileptic drugs. The drugs with the best justifications for drug monitoring are lamotrigine, levetiracetam, oxcarbazepine, stiripentol, and zonisamide. Perampanel, stiripentol and tiagabine are strongly bound to serum proteins and are candidates for monitoring of the free drug fractions. Alternative specimens for therapeutic drug monitoring are saliva and dried blood spots. Therapeutic drug monitoring of the new antiepileptic drugs is discussed here for managing patients with epilepsy.

The quantitative effect of serum albumin, serum urea, and valproic acid on unbound phenytoin concentrations in children

Dosing of phenytoin is difficult in children because of its variable pharmacokinetics and protein binding. Possible covariates for this protein binding have mostly been univariately investigated in small, and often adult, adult populations. We conducted a study to identify and quantify these covariates in children. We extracted data on serum phenytoin concentrations, albumin, triglycerides, urea, total bilirubin and creatinine concentrations and data on coadministration of valproic acid or carbamazepine in 186 children. Using nonlinear mixed effects modeling the effects of covariates on the unbound phenytoin fraction were investigated. Serum albumin, serum urea concentrations, and concomitant valproic acid use significantly influenced the unbound phenytoin fraction. For clinical practice, we recommend that unbound phenytoin concentrations are measured routinely. However, if this is impossible, we suggest to use our model to calculate the unbound concentration. In selected children, close treatment monitoring and dose reductions should be considered to prevent toxicity.

Phenytoin and sodium valproate but not levetiracetam induce bone alterations in female mice

Adverse effects on the bone are amongst the potentially adverse clinical consequences with antiepileptic drugs (AEDs). This study compared the effects of 3 AEDs (phenytoin (PHT), sodium valproate (SVP), and levetiracetam (LTM)) on the bones of a Swiss strain of albino female mice. Drugs were administered daily for 4 months at doses that produced plasma concentrations corresponding to the clinically relevant therapeutic ranges. PHT and SVP (but not LTM) significantly lowered the bone mineral density (BMD) of lumbar vertebrae (L2-L4) as evaluated by dual-energy X-ray absorptiometry (DEXA) scan. The findings were supported by histopathology of vertebral (lumbar) bone and analysis of bone turnover markers. While both PHT and SVP reduced alkaline phosphatase (ALP) and hydroxyproline (HxP) in lumbar vertebrae, and elevated tartarate-resistant acid phosphatase (TRAP) and urinary excretion of calcium, LTM did not affect any of these markers of bone turnover, indicating that the drug might be a safer option in female epileptic patients prone to bone changes.

Antiepileptic efficacy of lamotrigine in phenobarbital-resistant and -responsive epileptic rats: a pilot study

About 25% of patients with epilepsy are refractory to treatment, so that new, more effective antiepileptic drugs (AEDs) are urgently needed. Animal models that simulate the clinical situation with individuals responding and not responding to treatment are important to determine mechanisms of AED resistance and develop novel more effective treatments. We have previously developed and characterized such a model in which spontaneous recurrent seizures (SRS) develop after a status epilepticus induced by sustained electrical stimulation of the basolateral amygdala. In this model, prolonged treatment of epileptic rats with phenobarbital (PB) results in two subgroups, PB responders and PB nonresponders. When PB nonresponders were treated in previous experiments with phenytoin (PHT), 83% of the PB-resistant rats were also resistant to PHT. In the present study we examined if rats with PB resistant seizures are also resistant to lamotrigine (LTG), using continuous EEG/video recording of spontaneous seizures over 10 consecutive weeks. For this purpose, a new group of epileptic rats was produced and selected by treatment with PB into responders and nonresponders. As in previous studies, PB nonresponders had a significantly higher seizure frequency before onset of treatment. During subsequent treatment with LTG, all PB nonresponders and 60% of the PB responders exhibited >75% reduction of seizure frequency and were therefore considered as LTG responders. Plasma levels of LTG did not differ significantly between responders and nonresponders. The data of this pilot study indicate that LTG is more effective than PHT to suppress seizures in PB nonresponders in this model, but that not all PB responders also respond to LTG. Overall, our data provide further evidence that AED studies in post-SE TLE models are useful in determining and comparing AED efficacy and investigating predictors and mechanisms of pharmacoresistance.

Intranasal administration of carbamazepine to mice: a direct delivery pathway for brain targeting

The currently available antiepileptic drugs are typically administered via oral or intravenous (IV) routes which commonly exhibit high systemic distribution into non-targeted tissues, leading to peripheral adverse effects and limited brain uptake. In order to improve the efficacy and tolerability of the antiepileptic drug therapy, alternative administration strategies have been investigated. The purpose of the present study was to assess the pharmacokinetics of carbamazepine administered via intranasal (IN) and IV routes to mice, and to investigate whether a direct transport of the drug from nose to brain could be involved. The similar pharmacokinetic profiles obtained in all matrices following both administration routes indicate that, after IN delivery, carbamazepine reaches quickly and extensively the bloodstream, achieving the brain predominantly via systemic circulation. However, the uneven biodistribution of carbamazepine through the brain regions with higher concentrations in the olfactory bulb and frontal cortex following IN instillation, in comparison with the homogenous brain distribution pattern after IV injection, strongly suggests the involvement of a direct transport of carbamazepine from nose to brain. Therefore, it seems that IN delivery represents a suitable and promising alternative route to administer carbamazepine not only for the chronically use of the drug but also in emergency conditions.

Measurement of levetiracetam drug levels to assist with seizure control and monitoring of drug interactions with other anti-epileptic medications (AEMs)

PURPOSE

Levetiracetam (LEV) therapeutic range (20-40mg/L) and potential drug interactions were assessed in people with epilepsy (PWE).

METHOD

Fifty-two PWE had LEV and concomitant medications [carbamazepine (CBZ); valproate (VPA); lamotrigine (LTG)] blood levels measured and compared to seizure activity. Lacosamide (LCM) levels were unavailable. Adopted therapeutic ranges were: 20-40mg/L - LEV; 25-50μmol/L - total CBZ; 6-13μmol/L - free CBZ; 300-750μmol/L - total VPA; 30-75μmol/L - free VPA; and 40-60μmol/L - LTG. Seizure-freedom was assessed and patients followed for almost two years.

RESULTS

23 of 52 PWE (44%) used LEV monotherapy and 16/23 (70%) had 'therapeutic' LEV with 13/16 (81%) seizure-free. 29 of 52 (56%) used polytherapy and 16/29 (55%) had 'therapeutic' LEV with 7/16 (44%) seizure-free. 11 of 29 (38%) used CBZ: 4/11 (36%) had therapeutic mean LEV levels and 7/11 (64%) were seizure-free. Fourteen (48%) used VPA: 9/14 (64%) had therapeutic mean LEV levels and 8/14 (57%) were seizure-free. 13 of 29 (45%) used LTG: 8/13 (62%) had therapeutic mean LEV levels and 5/13 (38%) were seizure-free. LEV did not alter CBZ, but CBZ affected LEV. LEV elevated VPA free levels but not VPA total levels. Dosage/concentration was lowered with polytherapy.

CONCLUSION

LEV range (20-40mg/L) assisted epilepsy management and anti-epileptic medication interactions were suggested with polytherapy thus possibly explaining the impaired efficacy of LEV with polytherapy.

A missed opportunity - consequences of unknown levetiracepam pharmacokinetics in a peritoneal dialysis patient

Background

Levetiracetam is a frequently used drug in the therapy of partial onset, myoclonic and generalized tonic-clonic seizures. The main route of elimination is via the kidneys, which eliminate 66% of the unchanged drug as well as 24% as inactive metabolite that stems from enzymatic hydrolysis. Therefore dose adjustments are needed in patients with chronic kidney disease stage 5 D, i.e. patients undergoing dialysis treatment. In this patient population a dose reduction by 50% is recommended, so that patients receive 250–750 mg every 12 hours. However “dialysis” can be performed in using different modalities and treatment intensities. For most of the drugs pharmacokinetic data and dosing recommendations for patients undergoing peritoneal dialysis are not available. This is the first report on levetiracetam pharmacokinetics in a peritoneal dialysis patient.

Case presentation

A 73-y-old Caucasian male (height: 160 cm, weight 93 kg, BMI 36.3 kg/m²) was admitted with a Glasgow Coma Scale of 10. Due to diabetic and hypertensive nephropathy he was undergoing peritoneal dialysis for two years. Eight weeks prior he was put on levetiracetam 500 mg twice daily for suspected partial seizures with secondary generalization. According to the patient’s wife, levetiracetam lead to fatigue and somnolence leading to trauma with fracture of the metatarsal bone. Indeed, even 24 hours after discontinuation of levetiracetam blood level was still 29.8 mg/l (therapeutic range: 12 – 46 mg/l). Fatigue and stupor had disappeared five days after discontinuation of the levetiracepam. A single dose pharamockinetic after re-exposure showed an increased half life of 18.4 hours (normal half life 7 hours) and levetiracetam content in the peritoneal dialysate. Both half-life and dialysate content might help to guide dosing in this patient population.

Conclusion

If levetiracetam is used in peritoneal dialysis patients it should be regularly monitored to avoid supratherapeutic levels that could lead to severe sequelae.

Withdrawal of antiepileptic drugs during presurgical video-EEG monitoring: an observational study for evaluation of current practice at a referral center for epilepsy

OBJECTIVES

Withdrawal of antiepileptic drugs (AEDs) before and during video-EEG-monitoring is commonly implemented to reduce time needed to register a sufficient number of seizures during presurgical evaluation. There are, however, few guidelines regarding withdrawal rate and observation time.

MATERIAL AND METHODS

We performed an observational study including sixty patients admitted to the national Norwegian epilepsy centre and registered tapering of AEDs and their effect on seizure rate and possible complications.

RESULTS

The mean daily seizure rate before admission to the EMU was 0.4 (range 0.02-4) increasing to 1.1 (range 0-8) at the EMU. 29 patients (48%) followed a slow tapering rate whereas 31 (52%) had an intermediate tapering rate. There was no significant difference between the patients with a daily seizure rate during LTM of more or <0.7 seizures per day, an increase of seizure frequency from habitual to during LTM of more or <3.3 or 6.9 with regard to rate of tapering (slow vs intermediate) etiology or AED monotherapy vs polytherapy. Twenty-six patients (43%) had a sufficient number of seizures registered within 3 days to conclude regarding the presurgical evaluation. Two patients received escape treatment while 25 patients did have 24 h-seizure-clusters. There was no serious event.

CONCLUSIONS

Less than 50% of the patients got a sufficient number of seizures for a conclusive result within 3 days. An increase in the registration period could increase the number of successful registrations.

Clinical pharmacokinetics of new-generation antiepileptic drugs at the extremes of age: an update

Epilepsies occur across the entire age range, and their incidence peaks in the first years of life and in the elderly. Therefore, antiepileptic drugs (AEDs) are commonly used at the extremes of age. Rational prescribing in these age groups requires not only an understanding of the drugs' pharmacodynamic properties, but also careful consideration of potential age-related changes in their pharmacokinetic profile. The present article, which updates a review published in 2006 in this journal, focuses on recent findings on the pharmacokinetics of new-generation AEDs in neonates, infants, children, and the elderly. Significant new information on the pharmacokinetics of new AEDs in the perinatal period has been acquired, particularly for lamotrigine and levetiracetam. As a result of slow maturation of the enzymes involved in glucuronide conjugation, lamotrigine elimination occurs at a particularly slow rate in neonates, and becomes gradually more efficient during the first months of life. In the case of levetiracetam, elimination occurs primarily by renal excretion and is also slow at birth, but drug clearance increases rapidly thereafter and can even double within 1 week. In general, infants older than 2-3 months and children show higher drug clearance (normalized for body weight) than adults. This pattern was confirmed in recent studies that investigated the pediatric pharmacokinetics of several new AEDs, including levetiracetam, rufinamide, stiripentol, and eslicarbazepine acetate. At the other extreme of age, in the elderly, drug clearance is generally reduced compared with younger adults because of less efficient drug-metabolizing activity, decreased renal function, or both. This general pattern, described previously for several AEDs, was confirmed in recent studies on the effect of old age on the clearance of felbamate, levetiracetam, pregabalin, lacosamide, and retigabine. For those drugs which are predominantly eliminated by renal excretion, aging-related pharmacokinetic changes could be predicted by measuring creatinine clearance (CLCR). Overall, most recent findings confirm that age is a major factor influencing the pharmacokinetic profile of AEDs. However, pharmacokinetic variability at any age can be considerable, and the importance of other factors should not be disregarded. These include genetic factors, co-morbidities, and drug interactions, particularly those caused by concomitantly administered AEDs which induce or inhibit drug-metabolizing enzymes.

Marked differences in the effect of antiepileptic and cytostatic drugs on the functionality of P-glycoprotein in human and rat brain capillary endothelial cell lines

PURPOSE

The expression of P-glycoprotein (Pgp) is increased in brain capillary endothelial cells (BCECs) of patients with pharmacoresistant epilepsy. This may restrict the penetration of antiepileptic drugs (AEDs) into the brain. However, the mechanisms underlying increased Pgp expression in epilepsy patients are not known. One possibility is that AEDs induce the expression and functionality of Pgp in BCECs. Several older AEDs that induce human cytochrome P450 enzymes also induce Pgp in hepatocytes and enterocytes, but whether this extends to Pgp at the human BBB and to newer AEDs is not known.

METHODS

This prompted us to study the effects of various old and new AEDs on Pgp functionality in the human BCEC line, hCMEC/D3, using the rhodamine 123 (Rho123) efflux assay. For comparison, experiments were performed in two rat BCEC lines, RBE4 and GPNT, and primary cultures of rat and pig BCECs. Furthermore, known Pgp inducers, such as dexamethasone and several cytostatic drugs, were included in our experiments.

RESULTS

Under control conditions, GPNT cells exhibited the highest and RBE4 the lowest Pgp expression and Rho123 efflux, while intermediate values were determined in hCMEC/D3. Known Pgp inducers increased Rho123 efflux in all cell lines, but marked inter-cell line differences in effect size were observed. Of the various AEDs examined, only carbamazepine (100 μM) moderately increased Pgp functionality in hCMEC/D3, while valproate (300 μM) inhibited Pgp.

CONCLUSIONS

These data do not indicate that treatment with AEDs causes a clinically relevant induction in Pgp functionality in BCECs that form the BBB.

Routine drug and food interactions during antihelminthic treatment of neurocysticercosis: a reason for the variable efficacy of albendazole and praziquantel?

Neurocysticercosis (NC) or infection of the central nervous system with Taenia solium larvae is a leading cause of preventable seizures and epilepsy in endemic regions across the globe. Albendazole and praziquantel are commonly used antihelminthic agents to treat NC; however, viable cysts persist in the majority of patients, putting them at risk for future seizures and other neurological complications. Because of their pharmacokinetic profiles, albendazole and praziquantel have the potential to interact with many different drugs. During antihelminthic treatment, antiepileptic drugs and corticosteroids are commonly co-administered to manage seizures and cerebral edema; however, the most commonly used agents from these drug classes are known to significantly alter plasma concentrations of albendazole and praziquantel. The overarching issue with drug interactions during the treatment of NC is whether or not they have clinical relevance, as the plasma concentrations of albendazole and praziquantel have not been directly linked with eradication of viable cysts. Future studies should attempt to evaluate the validity of a causal relationship between antihelminthic plasma concentrations and outcomes so that drug interactions can be better understood and managed and so that treatment can be optimized.

What non-neuronal mechanisms should be studied to understand epileptic seizures?

While seizures ultimately result from aberrant firing of neuronal networks, several laboratories have embraced a non-neurocentric view of epilepsy to show that other cells in the brain also bear an etiologic impact in epilepsy. Astrocytes and brain endothelial cells are examples of controllers of neuronal homeostasis; failure of proper function of either cell type has been shown to have profound consequences on neurophysiology. Recently, an even more holistic view of the cellular and molecular mechanisms of epilepsy has emerged to include white blood cells, immunological synapses, the extracellular matrix and the neurovascular unit. This review will briefly summarize these findings and propose mechanisms and targets for future research efforts on non-neuronal features of neurological disorders including epilepsy.

Myoclonus in renal failure: Two cases of gabapentin toxicity

Gabapentin, an AED approved for the adjunctive treatment of partial seizures with/without secondary generalization and for the treatment of postherpetic neuralgia, is frequently used off-label for the treatment of both psychiatric and pain disorders. Since gabapentin is cleared solely by renal excretion, dosing requires consideration of the patient's renal function. Myoclonic activity may occur as a complication of gabapentin toxicity, especially with acute kidney injury or end-stage renal disease. We report 2 cases of myoclonic activity associated with gabapentin toxicity in the setting of renal disease which resolved with discontinuation of gabapentin and treatment with hemodialysis and peritoneal dialysis. As gabapentin has multiple indications and off-label uses, an understanding of myoclonus, neurotoxicity, and renal dosing is important to clinicians in multiple specialties.

An audit of therapeutic drug monitoring services of anticonvulsants at a tertiary care hospital in India

BACKGROUND

Therapeutic drug monitoring (TDM) is an important adjunct to the treatment of epilepsy. However, few studies have actually correlated plasma levels of antiepileptic drugs (AEDs) with treatment response. The present audit aimed to study (i) the association between seizure control and number of AEDs, plasma AED concentration, and concomitant use of antitubercular drugs; (ii) the pattern of indications for TDM requisitions; and (iii) the association between referral for toxicity and plasma AED concentration.

METHODS

This observational and retrospective study was carried out to analyze the TDM data of patients referred between January 2008 and December 2011. As per the International League Against Epilepsy Task Force 2009, patients were categorized into responders and nonresponders. Plasma AED levels were interpreted as below, within, and above the reference range.

RESULTS

Of 3206 TDM requisitions, 67% were monotherapy and 33% were 2 or more AEDs. Only 8% were responders as against 92% nonresponders. Of 95 patients on concomitant antituberculosis treatment, 72 were nonresponders, with odds ratio (95% confidence interval) 3.71 [2.19 to 6.23]. Breakthrough seizure (37%) was the most common indication followed by suspected toxicity and routine monitoring in 22% each and suspected nonadherence in 11% of the total requests. In 52% of patients, plasma levels were below the reference range, and they were equally distributed amongst responders and nonresponders. Among patients referred for suspected phenytoin toxicity, only 59% (50.6 to 67.8) had plasma concentrations above the reference range.

CONCLUSIONS

TDM continues to remain an important tool to support dose individualization when the patient is receiving multiple AEDs or other drugs such as antitubercular medicines, to assess compliance, and to monitor and treat toxicity.

Pharmacokinetics of lamotrigine in paediatric and young adult epileptic patients--nonlinear mixed effects modelling approach

PURPOSE

The purpose of the study was to examine and describe adjunctive lamotrigine (LTG) pharmacokinetics in paediatric and young adult patients using a nonlinear mixed effects modelling (NONMEM) approach.

METHODS

The study included 53 patients (age range 3-35 years) who were concomitantly treated with carbamazepine (CBZ) and/or valproic acid (VPA). A total of 70 blood samples corresponding to trough levels were available for analysis. Data were modelled, and the final model was evaluated using NONMEM and auxiliary software tools.

RESULTS

The final LTG population model included the effects of concomitant drugs and patient's weight (WT) which stratified the population into three groups: ≤25 kg, >25 to <60 kg and ≥60 kg. Based on the final model, the estimated LTG oral clearance (CL/F) for a typical patient weighing ≤25 kg, >25 to <60 kg or ≥60 kg who was concomitantly treated with CBZ was estimated to be 3.28, 4.23, or 7.15 l/h, respectively. If a patient was concomitantly treated with CBZ + VPA, the CL/F decreased on average by 69.5 % relative to LTG + CBZ co-therapy. VPA was found to decrease the LTG CL/F by 87.6 % compared to co-therapy with only CBZ.

CONCLUSION

The LTG population pharmacokinetic model developed in this study may be a reliable method for individualising the LTG dosing regimen in paediatric and young adult patients on combination therapy during therapeutic drug monitoring.

[Adherence to medication and epilepsy: a current issue]

As in other chronic diseases, adherence to medication in epilepsy is critical for seizure control. Its assessment remains challenging in research as in clinical practice. Recent evidences showed another face of nonadherence: the overconsumption of antiepileptic drugs. Some educational interventions with easy implementation were found to be effective in improving adherence and should be therefore more used in everyday practice.

Is switching from brand name to generic formulations of phenobarbital associated with loss of antiepileptic efficacy?: a pharmacokinetic study with two oral formulations (Luminal(®) vet, Phenoleptil(®)) in dogs

Background

In human medicine, adverse outcomes associated with switching between bioequivalent brand name and generic antiepileptic drug products is a subject of concern among clinicians. In veterinary medicine, epilepsy in dogs is usually treated with phenobarbital, either with the standard brand name formulation Luminal® or the veterinary products Luminal® vet and the generic formulation Phenoleptil®. Luminal® and Luminal® vet are identical 100 mg tablet formulations, while Phenoleptil® is available in the form of 12.5 and 50 mg tablets. Following approval of Phenoleptil® for treatment of canine epilepsy, it was repeatedly reported by clinicians and dog owners that switching from Luminal® (human tablets) to Phenoleptil® in epileptic dogs, which were controlled by treatment with Luminal®, induced recurrence of seizures. In the present study, we compared bioavailability of phenobarbital after single dose administration of Luminal® vet vs. Phenoleptil® with a crossover design in 8 healthy Beagle dogs. Both drugs were administered at a dose of 100 mg/dog, resulting in 8 mg/kg phenobarbital on average.

Results

Peak plasma concentrations (C_max) following Luminal® vet vs. Phenoleptil® were about the same in most dogs (10.9 ± 0.92 vs. 10.5 ± 0.77 μg/ml), and only one dog showed noticeable lower concentrations after Phenoleptil® vs. Luminal® vet. Elimination half-life was about 50 h (50.3 ± 3.1 vs. 52.9 ± 2.8 h) without differences between the formulations. The relative bioavailability of the two products (Phenoleptil® vs. Luminal® vet.) was 0.98 ± 0.031, indicating that both formulations resulted in about the same bioavailability.

Conclusions

Overall, the two formulations did not differ significantly with respect to pharmacokinetic parameters when mean group parameters were compared. Thus, the reasons for the anecdotal reports, if true, that switching from the brand to the generic formulation of phenobarbital may lead to recurrence of seizures are obviously not related to a generally lower bioavailability of the generic formulation, although single dogs may exhibit lower plasma levels after the generic formulation that could be clinically meaningful.