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

Microextraction combined with microderivatization for drug monitoring and protein modification analysis from limited blood volume using mass spectrometry

In the clinic, ethosuximide is commonly used to treat generalized absence seizures but has recently been repurposed for other diseases. Because of adverse effects and drug interactions, high-throughput therapeutic drug monitoring of ethosuximide is necessary. Microextraction is a simple, effective, rapid, and low consumption of organic solvents method for sample preparation. In this study, microderivatization-increased detection (MDID)-combined microextraction was used to detect ethosuximide by mass spectrometry. Ethosuximide is a difficult to retain and ionize compound in the C18 nano-flow column and ionization interface, respectively. Hence, we developed a fast method for detecting ethosuximide in human plasma by using the MDID strategy (within 2 min). Chemical microderivatization parameters were studied and optimized to increase the sensitivity of ethosuximide detection at trace levels. The linear range for the analysis of ethosuximide in 10 μL plasma was 5-500 μg/mL with a coefficient of determination (r²) ≥ 0.995. The precision and accuracy of intraday and interday analyses of ethosuximide were below 13.0%. Furthermore, modifications of major proteins in plasma and blood cells, induced by ethosuximide, were identified. The proposed method effectively utilizes microliter samples to detect drug plasma concentrations under suitable microextraction procedures toward the eco-friendly goal of low consumption of organic solvents. Graphical abstract ᅟ.

High Doses of Drugs Extensively Metabolized by CYP3A4 Were Needed to Reach Therapeutic Concentrations in Two Patients Taking Inducers

INTRODUCTION

In the last 20 years of clinical practice, the senior author has identified these 2 rare cases in which the patients needed extremely high doses of drugs metabolized by CYP3A4 to reach and maintain serum therapeutic concentrations.

METHODS

The high metabolic ability of these 2 patients was demonstrated by the low concentration-to-dose ratios (C/D ratios) of several drugs metabolized by CYP3A4.

RESULTS

Case 1 was characterized by a history of high carbamazepine doses (up to 2,000mg/day) and needed 170 mg/day of diazepam in 2 days to cooperate with dental cleaning. The high activity of the CYP3A4 isoenzyme was manifested by fast metabolism for quetiapine and diazepam, which took more than 1 year to normalize after the inducer, phenytoin, was stopped. Case 2 was also very sensitive to CYP3A4 inducers as indicated by very low C/D ratios for carbamazepine, risperidone and paliperidone. The carbamazepine (2,800 mg/day) and risperidone (20 mg/day) dosages for this second patient are the highest doses ever seen for these drugs by the senior author. Risperidone induction appeared to last for many months and metabolism was definitively normal 3 years after stopping carbamazepine. On the other hand, olanzapine C/D ratios were normal for induction.

CONCLUSIONS

The literature has never described similar cases of very high doses of drugs metabolized by CYP3A4. We speculate that these 2 patients may have unusual genetic profiles at the nuclear receptor levels; these receptors regulate induction of drugs.

Interaction between ABCG2 421C>A polymorphism and valproate in their effects on steady-state disposition of lamotrigine in adults with epilepsy

AIMS

To investigate the impact of glucuronidation enzyme (UGT1A4*3 142T>G, UGT1A4*2 70C>A, UGT2B7 -161C>T) and transporter (MDR1/ABCB1 1236C>T, ABCG2 421C>A) polymorphisms on steady-state disposition of lamotrigine and on the lamotrigine-valproate interaction.

METHODS

Adults with epilepsy on lamotrigine monotherapy (n = 131) or lamotrigine + valproate treatment (n = 74) were genotyped and steady-state lamotrigine and valproate morning troughs were determined as a part of routine therapeutic drug monitoring.

RESULTS

No effect of UGT and MDR1/ABCB1 polymorphisms was observed. In the entire cohort, ABCG2 421A allele had no effect however an interaction between the variant allele and valproate was observed: (i) in lamotrigine-only patients, variant allele (vs. wild type homozygosity) was independently (adjustments: age, sex, body mass index, lamotrigine dose, other polymorphisms) associated with mildly lower lamotrigine troughs [geometric means ratio (GMR) = 0.76, 95% confidence interval (CI) 0.59-0.98], whereas in lamotrigine + valproate patients it was associated with higher troughs (GMR = 1.72, 95%CI 1.14-2.62); (ii) valproate cotreatment was overall associated with markedly higher troughs vs. lamotrigine monotherapy (GMR = 3.49, 95%CI 2.73-4.44), but more so in variant allele carriers (GMR = 5.24, 95%CI 3.38-8.15) than in wild type homozygotes (GMR = 2.32, 95%CI 1.89-2.83); (iii) variant allele effects in two treatment subsets and valproate effects in two genotype subsets differed by 2.36-fold (95%CI 1.39-3.67); (iv) increase in lamotrigine troughs associated with increasing valproate troughs was greater in variant allele carriers than in wild type homozygotes, i.e. variant allele effect increased with increasing valproate troughs.

CONCLUSION

This study is first to indicate a potentially relevant interaction between ABCG2 421C>A polymorphism and valproate in their effects on lamotrigine disposition.

In vitro changes in the proportion of protein-unbound-free propofol induced by valproate

PURPOSE

It has been reported that oral valproate (VPA) reduces the dose of propofol required for sedation. As  a potential reason for this, it is considered that VPA displaces serum protein-bound propofol and increases the proportion of protein-unbound-free propofol. To examine this hypothesis, the present in vitro study investigated the influence of VPA on the proportion of protein-unbound-free propofol in human serum samples.

METHODS

Serum samples were collected from 10 healthy volunteers, who were not taking any medication. VPA (final concentration: 0.05, 0.1 or 1 mg/mL) and propofol (final concentration: 1 or 5 µg/mL) were mixed with serum samples with normal (4.0 g/dL) or low (2.5 g/dL) albumin concentrations. Then, protein-unbound-free propofol was extracted from the samples, and its concentration was measured using high-performance liquid chromatography. We compared the proportion of protein-unbound-free propofol in each of the VPA-containing samples with that in serum samples without VPA (control).

RESULTS

In the serum samples with normal albumin concentrations, 1 mg/mL VPA significantly increased the proportion of protein-unbound-free propofol at 1 and 5 µg/mL propofol. Furthermore, in the serum samples with low albumin concentrations, the proportion of protein-unbound-free propofol was significantly increased by both 0.1 and 1 mg/mL VPA at propofol concentrations of 1 and 5 µg/mL.

CONCLUSION

VPA might increase the proportion of protein-unbound-free propofol in human serum via displacement reactions.

Pharmacokinetic and Pharmacodynamic Evaluation of Intravenous Levetiracetam in Children With Epilepsy

This study aimed to evaluate the safety and tolerability of intravenous (IV) levetiracetam (LEV) as a monotherapy in children aged 1 month-16 years and to explore the pharmacokinetics (PK) of IV LEV and the time to seizure after IV then oral administration of LEV in pediatric children with epilepsy. Children diagnosed with acute unprovoked seizures requiring in-hospital IV LEV administration were included. After administration, the clinical seizure outcomes, side effects, and the Korean-Child Behavior Checklist were monitored and the PK and repeated time to seizure were analyzed via modeling using NONMEM software. Overall, 37 children with epilepsy were enrolled and underwent a PK analysis (median age, 4.6 years; median weight, 18.0 kg). Nine children (24.3%) had seizure recurrence during the follow-up period (median, 3.8 months) and 5 children (13.5%) experienced LEV-associated adverse events such as irritability (n = 2; 5.4%) and somnolence (n = 3; 8.1%). The plasma LEV concentrations after IV LEV were best described by a one-compartment linear PK model. Only body weight was associated with both the clearance and volume of distribution of LEV. The Weibull distribution model described the time to seizure recurrence well; no statistically significant predictor for the time to seizure was identified. Therefore, IV LEV was a well-tolerated and effective alternative in children with acute unprovoked seizures, and models for the PK and time to repeated seizure recurrence after LEV were successfully developed. In particular, the current use of a weight-based IV LEV dosing regimen in pediatric children is practical.

Reduced Clearance of Phenobarbital in Advanced Cancer Patients near the End of Life

BACKGROUND AND OBJECTIVES

Little is known about the pharmacokinetics of phenobarbital in terminally ill cancer patients. We investigated whether phenobarbital clearance alters depending on the length of survival.

METHODS

We retrospectively reviewed the clinical, laboratory, and therapeutic drug monitoring (TDM) records of patients who received parenteral or oral phenobarbital for 21 consecutive days or longer between 2000 and 2016. Patients were divided into non-cancer and cancer groups. Cancer patients were further stratified according to the survival interval after TDM: those who survived > 3 months were classified as long-surviving and the remainders short-surviving cancer patients. Phenobarbital clearance (CL_PB) was calculated at steady state. Multiple comparisons of median CL_PB were conducted among the three groups.

RESULTS

Data were collected from 44 non-cancer patients and 34 cancer patients comprising 24 long-surviving and 10 short-surviving cancer patients. Among 10 short-surviving cancer patients, 4 had hepatic metastasis. Median CL_PB (range) in short-surviving cancer patients [0.076 (0.057‒0.114) L/kg/day] was significantly (p < 0.05) lower than that in non-cancer patients [0.105 (0.060‒0.226) L/kg/day] and in long-surviving cancer patients [0.100 (0.082‒0.149) L/kg/day].

CONCLUSION

Terminally ill patients with advanced cancer may have reduced CL_PB, thereby TDM is recommended for these patients particularly near the end of life.

Clinical Use and Monitoring of Antiepileptic Drugs

Background

Antiepileptic drugs (AEDs) have been used for the treatment of epilepsy and other neurological disorders since the late 19th century. There are currently several classes of AEDs available for epilepsy management, many of which are also used to treat migraines, bipolar disorder, schizophrenia, depression, and neuropathic pain. Because of their molecular and mechanistic diversity, as well as the potential for drug–drug interactions, AEDs are prescribed and monitored in a highly personalized manner.

Content

This review provides a general overview of the use of AEDs with a focus on the role of therapeutic drug monitoring. Discussed topics include mechanisms of action, guidelines on the clinical applications of AEDs, clinical tests available for AED monitoring, and genetic factors known to affect AED efficacy.

Summary

Implementation of AED therapies is highly individualized, with many patient-specific factors considered for drug and dosage selection. Both therapeutic efficacy and target blood concentrations must be established for each patient to achieve seizure mitigation or cessation. The use of an AED with any additional drug, including other AEDs, requires an evaluation of potential drug–drug interactions. Furthermore, AEDs are commonly used for nonepilepsy indications, often in off-label administration to treat neurological or psychiatric disorders.

Pharmacokinetics of rectal levetiracetam as add-on treatment in dogs affected by cluster seizures or status epilepticus

Background

Levetiracetam can be used for seizure control alone or in combination with other antiepileptic medications. A previous study achieved the minimum targeted serum drug concentration after rectal administration of levetiracetam in healthy dogs. The purpose of the present study was to determine the pharmacokinetics of rectal LEV in dogs presented for cluster seizures or status epilepticus and potentially in treatment with other anti-epileptic drugs. Furthermore, preliminary information on response to this treatment as add-on to the standard treatment protocol is reported.

Results

Eight client-owned dogs were enrolled. Plasma levetiracetam concentrations (measured at 0, 30, 60, 90, 120, 180, 240, 360, 720, and 1440 min after drug administration) reached the minimum target concentration (5 μg/ml) at 30 min in all but one patient. At T1 (30 min) the mean concentration was 28.2 ± 15.5 μg/ml. Plasma concentrations remained above the targeted minimum concentration in all patients until 240 min and in 7/8 until 360 min. Six out of eight patients experienced no seizures in the 24-h period after hospitalization and were classified as “responders”.

Conclusions

Minimum plasma levetiracetam concentration can be reached after rectal administration of 40 mg/kg in dogs affected by cluster seizures and status epilepticus and concurrently receiving other antiepileptic drugs. These preliminary results may encourage the evaluation of rectal levetiracetam as an additional treatment option for cluster seizures and status epilepticus in a larger number of dogs.

Electronic supplementary material

The online version of this article (10.1186/s12917-018-1522-0) contains supplementary material, which is available to authorized users.

Repurposing Valproate, Enteral Clonidine, and Phenobarbital for Comfort in Adult ICU Patients: A Literature Review with Practical Considerations

Provision of adequate sedation is a fundamental part of caring for critically ill patients. Propofol, dexmedetomidine, and benzodiazepines are the most commonly administered sedative medications for adult patients in the intensive care unit (ICU). These agents are limited by adverse effects, need for a monitored environment for safe administration, and lack of universal effectiveness. Increased interest has recently been expressed about repurposing older pharmacologic agents for patient comfort in the ICU. Valproate, enteral clonidine, and phenobarbital are three agents with increasing evidence supporting their use. Potential benefits associated with their utilization are cost minimization and safe administration after transition out of the ICU. This literature review describes the historical context, pharmacologic characteristics, supportive data, and practical considerations associated with the administration of these agents for comfort in critically ill adult patients.

Role of l-Type Amino Acid Transporter 1 at the Inner Blood-Retinal Barrier in the Blood-to-Retina Transport of Gabapentin

Gabapentin is an antiseizure drug that is known to also have beneficial effects on the retinal cells. To use gabapentin in retinal pharmacotherapy, it is critical to understand gabapentin distribution in the retina. The purpose of this study was to clarify the kinetics of gabapentin influx transport across the inner and outer blood-retinal barrier (BRB), which regulates the exchange of compounds/drugs between the circulating blood and the retina. In vivo blood-to-retina gabapentin transfer was evaluated by the rat carotid artery injection technique. In addition, gabapentin transport was examined using in vitro models of the inner (TR-iBRB2 cells) and outer BRB (RPE-J cells). The in vivo [³H]gabapentin transfer to the rat retina across the BRB was significantly reduced in the presence of unlabeled gabapentin, suggesting transporter-mediated blood-to-retina distribution of gabapentin. Substrates of the Na⁺-independent l-type amino acid transporter 1 (LAT1), such as 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid (BCH), also significantly inhibited the in vivo [³H]gabapentin transfer. [³H]Gabapentin uptake in TR-iBRB2 and RPE-J cells exhibited Na⁺-independent and saturable kinetics with a K_m of 735 and 507 μM, respectively. Regarding the effect of various transporter substrates/inhibitors on gabapentin transport in these cells, LAT1 substrates significantly inhibited [³H]gabapentin uptake in TR-iBRB2 and RPE-J cells. In addition, preloaded [³H]gabapentin release from TR-iBRB2 and RPE-J cells was trans-stimulated by LAT1 substrates through the obligatory exchange mechanism as LAT1. Immunoblot analysis indicates the protein expression of LAT1 in TR-iBRB2 and RPE-J cells. These results imply that LAT1 at the inner and outer BRB takes part in gabapentin transport between the circulating blood and retina. Moreover, treatment of LAT1-targeted small interfering RNA to TR-iBRB2 cells significantly reduced both the level of LAT1 protein expression and [³H]gabapentin uptake activities in TR-iBRB2 cells. In conclusion, data from the present study indicate that LAT1 at the inner BRB is involved in retinal gabapentin transfer, and also suggest that LAT1 mediates gabapentin transport in the RPE cells.

Adverse placental effects of valproic acid: Studies in perfused human placentas

OBJECTIVE

In utero exposure to valproic acid (VPA) has been associated with worse pregnancy outcomes compared to all other antiepileptic drugs. We have previously shown that VPA alters the expression of placental transporters for hormones and nutrients in vitro and in pregnant mice. Here, our aim was to characterize the effects of short exposure to VPA on the expression of carriers for compounds essential for fetal development in human placentas ex vivo, under controlled conditions.

METHODS

Placentas were obtained from cesarean deliveries of women with no known epilepsy. Cotyledons were cannulated and perfused in the absence or the presence of VPA (42, 83, or 166 μg/mL; n = 6/group) in the maternal perfusate over 180 minutes. A customized gene panel array was used to analyze the expression of carrier genes in the perfused cotyledons. We additionally measured in the perfused placentas folic acid concentrations and histone acetylation.

RESULTS

VPA significantly altered the mRNA levels of major carriers for folic acid, glucose, choline, thyroid hormones, and serotonin (P < .05) and reduced placental folate concentrations by 25%-35% (P = .059). The effects were observed at therapeutic concentrations sufficient to enhance placental histone acetylation, and some were concentration-dependent.

SIGNIFICANCE

Our results point to the placenta as a novel target of VPA, implying potential involvement of the placenta in VPA's adverse fetal outcomes.

Development and full validation of an innovative HPLC-diode array detection technique to simultaneously quantify lacosamide, levetiracetam and zonisamide in human plasma

AIM

To implement pharmacokinetic drug monitoring and individualize the posology of new antiepileptic drugs, the first HPLC-diode array detection method was developed and validated to simultaneously quantify lacosamide, levetiracetam and zonisamide in human plasma.

MATERIALS & METHODS

Preceded by a reproducible liquid-liquid extraction, chromatographic separation was achieved by using a C18 column of 5 cm length and a mobile phase of water/acetonitrile. Full validation was performed according to international guidelines.

RESULTS

The method was linear within 0.5-30, 2.5-40.0 and 0.5-50.0 μg ml-1 for lacosamide, levetiracetam and zonisamide, respectively (r2 ≥ 0.998), accurate (-12.411-8.303%), precise (≤8.875%).

CONCLUSION

This innovative HPLC-diode array detection method was successfully employed in clinical practice and is expected to empower epileptic patients with a personalized pharmacotherapy service. [Formula: see text].

CYP2C9 polymorphisms in epilepsy: influence on phenytoin treatment

Phenytoin (PHT) is an antiepileptic drug widely used in the treatment of focal epilepsy and status epilepticus, and effective in controlling focal seizures with and without tonic-clonic generalization and status epilepticus. The metabolization of PHT is carried out by two oxidative cytochrome P450 enzymes CYP2C9 and CYP2C19; 90% of this metabolization is done by CYP2C9 and the remaining 10% by CYP2C19. Genetic polymorphism of CYP2C9 may reduce the metabolism of PHT by 25-50% in patients with variants *2 and *3 compared to those with wild-type variant *1. The frequency distribution of CYP2C9 polymorphism alleles in patients with epilepsy around the world ranges from 4.5 to 13.6%, being less frequent in African-Americans and Asians. PHT has a narrow therapeutic range and a nonlinear pharmacokinetic profile; hence, its poor metabolization has significant clinical implications as it causes more frequent and more serious adverse effects requiring discontinuation of treatment, even if it had been effective. There is evidence that polymorphisms of CYP2C9 and the use of PHT are associated with an increase in the frequency of some side effects, such as cerebellar atrophy, gingival hypertrophy or acute cutaneous reactions. The presence of HLA-B*15:02 and CYP2C9 *2 or *3 in the same patient increases the risk of Stevens-Johnson syndrome and toxic epidermal necrolysis; hence, PHT should not be prescribed in these patients. In patients with CYP2C9 *1/*2 or *1/*3 alleles (intermediate metabolizers), the usual PHT maintenance dose (5-10 mg/kg/day) must be reduced by 25%, and in those with CYP2C9 *2/*2, *2/*3 or *3/*3 alleles (poor metabolizers), the dose must be reduced by 50%. It is controversial whether CYP2C9 genotyping should be done before starting PHT treatment. In this paper, we aim to review the influence of CYP2C9 polymorphism on the metabolization of PHT and the clinical implications of poor metabolization in the treatment of epilepsies.

A population pharmacokinetic model taking into account protein binding for the sustained-release granule formulation of valproic acid in children with epilepsy

PURPOSE

The objective of this work was to develop a population pharmacokinetic model for a prolonged-release granule formulation of valproic acid (VPA) in children with epilepsy and to determine the doses providing a VPA trough concentration (C_trough) within the target range (50-100 mg/L).

METHODS

Ninety-eight children (1-17.6 years, 325 plasma samples) were included in the study. The model was built with NONMEM 7.3. The probability to obtain C_trough between 50 and 100 mg/L was determined by the Monte Carlo simulations for doses of 20, 30, 40, and 60 mg/kg/day and body weights between 10 and 70 kg.

RESULTS

A one compartment model, with first-order absorption and flip-flop parameterization and linear elimination, but taking protein binding into account, was used to describe the data. Typical values for unbound VPA clearance and distribution volume were 6.24 L/h/70 kg and 130 L/h/70 kg respectively. Both parameters were related to body weight via allometric models. The highest probability to obtain a C_trough within the target range for 10-kg children was obtained with a 40 mg/kg daily dose, whereas daily doses of 30 and 20 mg/kg were found appropriate for 20 to 30- and ≥ 40-kg children respectively. However, for these same doses, the exposure to unbound VPA could differ by 40%.

CONCLUSIONS

If the present study supports the current dose recommendations of 20-30 mg/kg/day, except for children under 20 kg, who may need higher doses, it also highlights the need for further research on the pharmacokinetics/pharmacodynamic profile of unbound VPA.

Effect of antiepileptic drug comedication on lamotrigine concentrations

Aim

To estimate the effect size of concomitant antiepileptic therapy on the concentrations of lamotrigine, a drug often prescribed in combination with other antiepileptic drugs (AED), which can act as enzyme inducers or inhibitors.

Methods

A total of 304 patients with epilepsy, aged 18-70 years, were divided into a lamotrigine monotherapy group and groups receiving lamotrigine with AEDs that act as enzyme inducers, enzyme inhibitors, or both. We compared lamotrigine monotherapy serum concentrations with those where lamotrigine was administered with a metabolic inhibitor valproate, metabolic inducers carbamazepine, oxcarbazepine, phenobarbital, phenytoin, or topiramate, and both an inducer and an inhibitor.

Results

Comparison of trough lamotrigine monotherapy concentrations and lamotrigine polytherapy concentrations showed an almost similar median concentration in case of drug-inducers, and higher lamotrigine concentration in case of comedication with valproate as an inhibitor. A significant difference was confirmed after dose correction (P < 0.001). Significant positive correlations of lamotrigine trough serum concentrations with valproate were observed before and after the dose correction (r = 0.480, P < 0.001 and r = 0.561, P < 0.001, respectively). Positive correlations between the dose-corrected lamotrigine trough concentration and carbamazepine (r = 0.439; P < 0.001) or monohydroxy metabolite of oxcarbazepine (MHD) (r = 0.675; P < 0.001) were also significant.

Conclusion

Higher valproate levels resulted in higher inhibition potency and higher lamotrigine levels. Increased dose-corrected concentrations of inducers carbamazepine and MHD, after the process of induction was finished, did not lower lamotrigine concentrations. These findings can be of clinical significance for optimal AED dosing.

Comparison of Plasma, Saliva, and Hair Levetiracetam Concentrations

BACKGROUND

Previous findings revealed high correlations between serum/plasma and saliva levetiracetam concentrations, indicating saliva as an alternative matrix for monitoring levetiracetam therapy. Levetiracetam concentration in the hair, which could reflect long-term drug exposure and patients' compliance, has not been systematically tested, as yet. The aim of this study was to determine the correlation between plasma, saliva, and hair levetiracetam concentrations in 47 patients with epilepsy.

METHODS

Plasma, saliva, and hair levetiracetam concentrations were measured by liquid chromatography-tandem mass spectrometry with positive ionization.

RESULTS

Levetiracetam saliva and plasma concentrations were highly correlated (r = 0.93). Plasma concentrations were not influenced by sex, age, and other concomitant antiepileptic drugs. Levetiracetam hair concentrations correlated with plasma concentrations (r = 0.36) but not daily dose (mg/kg). Drug hair concentrations were not influenced by hair color or treatment (dyed).

CONCLUSIONS

The results tend to indicate that saliva may be a reliable alternative to plasma for monitoring levetiracetam concentrations. Levetiracetam can also be detected in human hair.

[The clinical significance of the lamotrigine pharmacokinetic variability]

AIM

To estimate pharmacokinetic variability of lamotrigine (LTG) and its clinical significance.

MATERIAL AND METHODS

One hundred patients, including 74 women, aged from 18 to 77 years (38.23±14.37 years), with focal epilepsy were examined. Monotherapy with LTG was administered to 54 patients, duotherapy to 46 patients (LTG and valproic acid combination to 27 patients, LTG and liver enzymes inducers to 19 patients). Patients underwent procedures of therapeutic drug monitoring (TDM). Minimal (Css_min) and maximal (Css_max) steady-state LTG plasma concentrations, and concentration-to-weight ratio (CDR) were calculated.

RESULTS AND CONCLUSION

In patients who used LTG in monotherapy, LTG Css_min was 5.6±4.65 mg/l, Css_max 7.59±5.54 mg/l. In the group that received LTG in combination with valproate, LTG Сss_min was 7.8 [5.4; 11.8] mg / l and Css_max 11.4 [7.3; 15.3] mg/l. In the group that received LTG in combination with drug-inducers of glucuronidation, Css_min was 2.5 [1.99; 4.32] mg/l, Css_max 4.73 [2.91; 6.70] mg/l. Statistically significant differences in CDR parameter between groups with LTG monotherapy and duotherapy, both with inducer and with inhibitors, as well as between groups of duotherapy with inductors and with inhibitors were obtained. The results of the study indicate a pronounced pharmacokinetic variability of the LTG. Conducting TDM allows the establishment of individual therapeutic concentrations of LTG in blood plasma and setting a correction vector for antiepileptic therapy.

Assessment of the correlations of lacosamide concentrations in saliva and serum in patients with epilepsy

Therapeutic drug monitoring of antiepileptic drugs is based on patient serum samples. In this study, we evaluated the correlation between lacosamide (LCM) steady state concentrations in serum and saliva samples. Additionally, we investigated the relation with daily dose, and assessed the feasibility of saliva collection. This was an open-label, single center study including data from 25 patients at the Bethel Epilepsy Center treated with LCM (50-650 mg/d). Samples were collected in the morning (fasting values) and in selected cases at 50 minutes to 5 hours after the morning dose. Nonsignificant differences in the mean LCM morning (trough) concentration in serum and saliva were observed. Serum and saliva concentrations across all samples were highly correlated, (r = .874), with a slightly lower correlation when only fasting values were analyzed (r = .860). Higher correlation with daily dosages was observed in serum samples (r = .773) than in saliva samples (r = .604). Serum and saliva concentrations increased significantly after intake of the LCM morning dose (P < .001). The median absolute and percentage increase of LCM in serum were moderately lower than in saliva samples, with a few outliers in saliva samples. Consequently, saliva could offer great clinical potential to monitor drug concentrations and guide LCM treatment in epileptic patients.

An Updated Overview on Therapeutic Drug Monitoring of Recent Antiepileptic Drugs

Given the distinctive characteristics of both epilepsy and antiepileptic drugs (AEDs), therapeutic drug monitoring (TDM) can make a significant contribution to the field of epilepsy. The measurement and interpretation of serum drug concentrations can be of benefit in the treatment of uncontrollable seizures and in cases of clinical toxicity; it can aid in the individualization of therapy and in adjusting for variable or nonlinear pharmacokinetics; and can be useful in special populations such as pregnancy. This review examines the potential for TDM of newer AEDs such as eslicarbazepine acetate, felbamate, gabapentin, lacosamide, lamotrigine, levetiracetam, perampanel, pregabalin, rufinamide, retigabine, stiripentol, tiagabine, topiramate, vigabatrin, and zonisamide. We describe the relationships between serum drug concentration, clinical effect, and adverse drug reactions for each AED as well as the different analytical methods used for serum drug quantification. We discuss retrospective studies and prospective data on the serum drug concentration-efficacy of these drugs and present the pharmacokinetic parameters, oral bioavailability, reference concentration range, and active metabolites of newer AEDs. Limited data are available for recent AEDs, and we discuss the connection between drug concentrations in terms of clinical efficacy and nonresponse. Although we do not propose routine TDM, serum drug measurement can play a beneficial role in patient management and treatment individualization. Standardized studies designed to assess, in particular, concentration-efficacy-toxicity relationships for recent AEDs are urgently required.

Pharmacokinetic variability of valproate during pregnancy - Implications for the use of therapeutic drug monitoring

BACKGROUND AND PURPOSE

Use of valproate (VPA) in women of childbearing age is restricted due to dose-dependent risk of teratogenicity. The purpose of this study was to characterise pharmacokinetic variability of VPA in pregnancy, and discuss use of therapeutic drug monitoring (TDM) as guidance to exposure in women.

METHOD

Measurements of trough total and unbound VPA concentrations before, during and after pregnancy, at assumed steady-state were collected from the TDM-database (2006-2016) at the National Center for Epilepsy in Norway. Additional clinical data were obtained from the Oppland county Perinatal Database (1994-2011).

RESULTS

Data from 51 pregnancies in 33 women aged 19-40 years were included. Each woman underwent 1-4 pregnancies, and 1-7 measurements per pregnancy were performed. The variability in total concentration/dose (C/D)-ratios between women was 13-fold, and intra-patient variability extensive. Total C/D-ratios were reduced by 46% from before pregnancy to third trimester (0.48-0.29 μmol/L/mg). Unbound concentrations of VPA were only requested in 10% of the pregnancies. Repeated measurements from two pregnancies in one women revealed increased unbound concentration of VPA during pregnancy. There were 19 with idiopathic generalized epilepsy and two focal based on clinical data from 21 women and 38 pregnancies; 1 major congenital malformation was noted.

CONCLUSION

There is pronounced pharmacokinetic variability of VPA during pregnancy. Unbound concentrations are rarely requested. TDM should be used by measurements of both total and unbound concentrations since total concentrations may be misleading for efficacy and fetal exposure of VPA.

Reference ranges for antiepileptic drugs revisited: a practical approach to establish national guidelines

BACKGROUND AND OBJECTIVE

Laboratories sometimes use different reference ranges for the same antiepileptic drug (AED), particularly for new and poorly investigated drugs. This may contribute to misunderstandings, concerns or inappropriate dose changes, which in turn may affect therapeutic effect, drug safety or treatment adherence. Therefore, the Norwegian Association of Clinical Pharmacology wished to update and harmonize the reference ranges for AEDs and establish national guidelines for Norway.

METHODS

A working group collected information on the reference ranges used by Norwegian laboratories for all commonly used AEDs. These reference ranges were compared to recent recommendations by the International League Against Epilepsy, current literature, applicable clinical studies, reference ranges used by leading Northern European epilepsy centers outside of Norway, and routine data derived from Norwegian laboratory databases.

RESULTS

Reference ranges varied between laboratories for four of 23 available AEDs (lamotrigine, valproate, eslicarbazepine and oxcarbazepine). For four AEDs (brivaracetam, perampanel, stiripentol and sulthiame), reference ranges had not previously been established. In total, 13 reference ranges were either harmonized, updated or newly established. No changes were applied to the remaining 10 AEDs.

CONCLUSION

Updated and harmonized reference ranges are now available for 22 of the 23 AEDs available in Norway. The exception is vigabatrin (reference range not applicable). Revision of reference ranges is an important part of pharmacovigilance of AEDs and must be a continuous process based on current literature and clinical experience.

Perampanel Serum Concentrations in Adults With Epilepsy: Effect of Dose, Age, Sex, and Concomitant Anti-Epileptic Drugs

BACKGROUND

Perampanel (PMP), a noncompetitive α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor antagonist, is a novel anti-epileptic drug (AED) licensed for the adjunctive treatment of focal and generalized epilepsy. There is limited information on PMP's pharmacokinetics and drug interaction characteristics with concomitant AEDs. We have investigated the effects of PMP dose, age, sex, and coprescribed AEDs on serum PMP concentrations.

METHODS

We used the database of a therapeutic drug monitoring unit at a tertiary epilepsy referral center to identify patients who had PMP as part of their treatment and extracted clinical information from their medical notes. Sera PMP concentrations were determined using liquid chromatography/mass spectroscopy.

RESULTS

In total, 160 sera from 107 patients (66 females) aged 18-70 years and weighing 40-125 kg were identified. They were prescribed a median PMP dose of 6 mg/d (range 2-12 mg/d) and were coprescribed a variety of AEDs, including enzyme-inducing [carbamazepine (CBZ) and oxcarbazepine (OXC)] and enzyme-inhibiting (valproic acid) AEDs. A linear relationship was observed between PMP dose and serum concentrations (r = 0.714, P < 0.0005). Sex and age were found not to influence PMP serum concentration. Enzyme-inducing AEDs dose-dependently decreased PMP concentrations, with CBZ and OXC decreasing mean values by 69% and 37%, respectively. In contrast, although topiramate and phenytoin also decreased mean PMP concentrations by 18% and 13%, respectively, these changes did not achieve statistical significance.

CONCLUSIONS

PMP exhibits a linear dose-concentration relationship, with serum PMP concentrations being age and sex independent. CBZ and OXC can significantly decrease PMP concentrations, probably through an induction of CYP3A4-mediated metabolism.

Therapeutic Drug Monitoring of Clobazam and Its Metabolite-Impact of Age and Comedication on Pharmacokinetic Variability

BACKGROUND

Clobazam (CLB) has been used as an antiepileptic drug for several decades. There is still insufficient data regarding its pharmacokinetic variability in clinical practice. The purpose of this study was to investigate pharmacokinetic variability of CLB with emphasis on the impact of age and comedication in patients with epilepsy.

METHODS

Serum concentration measurements of CLB and its metabolite N-desmethylclobazam (NCLB), as well as demographic and clinical data were retrieved from the routine therapeutic drug monitoring service at the National Center for Epilepsy, Norway, 2009-2013. NCLB/CLB and total (CLB + NCLB), CLB and NCLB concentration/dose (C/D) ratios were calculated.

RESULTS

550 patients (296 women/254 men), average age 27 years (range 1-86), were included. The interindividual pharmacokinetic variability was extensive, as illustrated by a 100-fold variability in serum concentration compared with dose (total C/D ratio 0.03-3.29 µmol·L·mg). The CLB C/D ratio was 36% lower in young children (2-9 years) than in adults (18-64 years), reflecting a higher clearance. In patients receiving phenytoin, felbamate, stiripentol, oxcarbazepine or eslicarbazepine acetate, valproate, phenobarbital, zonisamide or carbamazepine one or more of the calculated ratios were significantly different from that in patients receiving no or neutral comedications. The mean values for the different groups were in the order of 20%-230% of C/D ratios in the neutral group and 200%-950% of the NCLB/CLB ratio.

CONCLUSIONS

The pharmacokinetic variability of CLB and its metabolite NCLB in clinical practice is extensive, and is influenced by drug-drug interactions, age, and pharmacogenetics. Therapeutic drug monitoring of CLB and NCLB is therefore valuable in patient management.

Population Pharmacokinetic Modeling of Levetiracetam in Pediatric and Adult Patients With Epilepsy by Using Routinely Monitored Data

BACKGROUND

Levetiracetam, a second-generation antiepileptic drug, is frequently used for managing partial-onset seizures. About 70% of the administered dose is excreted in urine unchanged, and dosage adjustment is recommended based on the individual's renal function. In this study, a population pharmacokinetic model of levetiracetam was developed using routinely monitored serum concentration data for individualized levetiracetam therapy.

METHODS

Patients whose serum concentrations of levetiracetam at steady-state were routinely monitored at Kyoto University Hospital from April 2012 to March 2013 were enrolled. The influence of patient characteristics on levetiracetam pharmacokinetics was evaluated using the nonlinear mixed-effects modeling (NONMEM) program.

RESULTS

A total of 583 steady-state concentrations from 225 patients were used for the analysis. The median patient age and estimated glomerular filtration rate (eGFR) were 38 (range: 1-89) years and 98 (15-189) mL·min·1.73 m, respectively. Serum concentration-time data of levetiracetam were well described by a 1-compartment model with first-order absorption. Oral clearance was allometrically related to the individual body weight and eGFR. An increase in the dose significantly increased oral clearance. No improvement in model fit was observed by including the covariate of any concomitant antiepileptic drugs. The population mean clearance for an adult weighing 70 kg and with a normal renal function was 4.8 and 5.9 L/h for 500 mg bis in die (bid) and 1500 mg bid, respectively.

CONCLUSIONS

Oral clearance allometrically related with body weight and eGFR can well predict the routine therapeutic drug monitoring data from pediatric to aged patients with varying renal function. Dosage adjustments based on renal function are effective in controlling the trough and peak concentrations in similar ranges.

Which information on women's issues in epilepsy does a community pharmacist need to know? A Delphi consensus study

OBJECTIVE

The aim of this study was to develop and achieve consensus on a core list of important knowledge items that community pharmacists should know on women's issues in epilepsy.

METHODS

This was a consensual study using a modified Delphi technique. Knowledge items were collected from the literature and from nine key contacts who were interviewed on their views on what information community pharmacists should have on women's issues in epilepsy. More knowledge items were suggested by five researchers with interest in women's issues who were contacted to rate and comment on the knowledge items collected. Two iterative Delphi rounds were conducted among a panel of pharmacists (n=30) to achieve consensus on the knowledge items to be included in the core list. Ten panelists ranked the knowledge items by their importance using the Analytical Hierarchy Process (AHP).

RESULTS

Consensus was achieved to include 68 knowledge under 13 categories in the final core list. Items ranked by their importance were related to the following: teratogenicity (10.3%), effect of pregnancy on epilepsy (7.4%), preconception counseling (10.3%), bone health (5.9%), catamenial epilepsy (7.4%), menopause and hormonal replacement therapy (2.9%), contraception (14.7%), menstrual disorders and infertility (8.8%), eclampsia (2.9%), breastfeeding (4.4%), folic acid and vitamin K (5.9%), counseling on general issues (14.7%), and sexuality (4.4%).

CONCLUSION

Using consensual knowledge lists might promote congruence in educating and/or training community pharmacists on women's issues in epilepsy. Future studies are needed to investigate if such lists can improve health services provided to women with epilepsy (WWE).

Pharmacotherapy for Seizures in Neonates with Hypoxic Ischemic Encephalopathy

Seizures are common in neonates with moderate and severe hypoxic ischemic encephalopathy (HIE) and are associated with worse outcomes, independent of HIE severity. In contrast to adults and older children, no new drugs have been licensed for treatment of neonatal seizures over the last 50 years, because of a lack of controlled clinical trials. Hence, many antiseizure medications licensed in older children and adults are used off-label for neonatal seizure, which is associated with potential risks of adverse effects during a period when the brain is particularly vulnerable. Phenobarbital is worldwide the first-line drug and is considered standard of care, although there is a limited evidence base for its efficacy. Second-line agents include phenytoin, benzodiazepines, levetiracetam, and lidocaine. These drugs are discussed in more detail along with two emerging drugs (bumetanide and topiramate). More safety, pharmacokinetic, and efficacy data are needed from well-designed clinical trials to develop safe and effective antiseizure regimes for the treatment of neonatal seizures in HIE.

Population Pharmacokinetics of Topiramate in Japanese Pediatric and Adult Patients With Epilepsy Using Routinely Monitored Data

BACKGROUND

Topiramate is a second-generation antiepileptic drug used as monotherapy and adjunctive therapy in adults and children with partial seizures. A population pharmacokinetic (PPK) analysis was performed to improve the topiramate dosage adjustment for individualized treatment.

METHODS

Patients whose steady-state serum concentration of topiramate was routinely monitored at Kyoto University Hospital from April 2012 to March 2013 were included in the model-building data. A nonlinear mixed effects modeling program was used to evaluate the influence of covariates on topiramate pharmacokinetics. The obtained PPK model was evaluated by internal model validations, including goodness-of-fit plots and prediction-corrected visual predictive checks, and was externally confirmed using the validation data from January 2015 to December 2015.

RESULTS

A total of 177 steady-state serum concentrations from 93 patients were used for the model-building analysis. The patients' age ranged from 2 to 68 years, and body weight ranged from 8.6 to 105 kg. The median serum concentration of topiramate was 1.7 mcg/mL, and half of the patients received carbamazepine coadministration. Based on a one-compartment model with first order absorption and elimination, the apparent volume of distribution was 105 L/70 kg, and the apparent clearance was allometrically related to the body weight as 2.25 L·h·70 kg without carbamazepine or phenytoin. Combination treatment with carbamazepine or phenytoin increased the apparent clearance to 3.51 L·h·70 kg. Goodness-of-fit plots, prediction-corrected visual predictive check, and external validation using the validation data from 43 patients confirmed an appropriateness of the final model. Simulations based on the final model showed that dosage adjustments allometrically scaling to body weight can equalize the serum concentrations in children of various ages and adults.

CONCLUSIONS

The PPK model, using the power scaling of body weight, effectively elucidated the topiramate serum concentration profile ranging from pediatric to adult patients. Dosage adjustments based on body weight and concomitant antiepileptic drug help obtain the dosage of topiramate necessary to reach an effective concentration in each individual.

The Impact of Pharmacokinetic Interactions With Eslicarbazepine Acetate Versus Oxcarbazepine and Carbamazepine in Clinical Practice

BACKGROUND

Eslicarbazepine acetate (ESL) is a new anti-epileptic drug (AED) chemically related to oxcarbazepine (OXC) and carbamazepine (CBZ) and is increasingly used in clinical practice. The purpose of the study was to investigate 2-way pharmacokinetic interactions between ESL and other AEDs as compared to OXC and CBZ.

METHODS

Anonymous data regarding age, gender, use of AEDs, daily doses and serum concentration measurements of ESL, OXC, CBZ and lamotrigine (LTG) and other AEDs were retrieved from 2 therapeutic drug monitoring (TDM) databases in Norway. Drugs were categorized according to their known potential for interactions. Concentration/dose (C/D) ratios were calculated.

RESULTS

Data from 1100 patients were available. The C/D ratios of ESL and OXC were unchanged in combination with enzyme-inducing AEDs or valproate (VPA). The C/D ratio of CBZ decreased by 40% and 22% in combination with other enzyme-inducing AEDs or VPA, respectively, pointing to an increased clearance. ESL demonstrated no significant enzyme-inducing effect on LTG metabolism although there was a 20% and 34% decrease in the C/D ratio of LTG in combination with OXC and CBZ, respectively.

CONCLUSIONS

Possible pharmacokinetic interactions have been studied for ESL as compared to OXC and CBZ. The pharmacokinetics of ESL is not affected by enzyme-inducing AEDs or VPA and does not affect the metabolism of LTG in contrast to OXC and CBZ. The study demonstrates the value of using TDM databases to explore the potential for pharmacokinetic interactions of new AEDs.

Effect of CYP Inducers/Inhibitors on Topiramate Concentration: Clinical Value of Therapeutic Drug Monitoring

BACKGROUND

This study investigated the pharmacokinetic interactions between topiramate (TPM) and concomitant antiepileptic drugs and evaluated the therapeutic concentration range of TPM and the effect of the achieved plasma concentration on the retention rate of TPM therapy.

METHODS

A total of 1217 plasma samples obtained from 610 patients were retrospectively investigated, and the concentration-to-dose ratio (CD ratio) of TPM was compared among patients on various antiepileptic drug regimens. In addition, the therapeutic concentration of TPM was reviewed in patients on long-term therapy, and factors influencing the retention rate of TPM were analyzed by the Kaplan-Meier method.

RESULTS

Among patients using hepatic enzyme inducers (phenytoin, phenobarbital, and carbamazepine), the CD ratio was reduced by 45.4% in adults and 33.3% in children. Patients taking phenytoin concomitantly had a significantly lower CD ratio than patients taking phenobarbital or carbamazepine. Among noninducers, concomitant use of stiripentol increased the CD ratio. In 276 patients who remained on TPM therapy for more than 2 years, the mean therapeutic concentration was 5.1 mcg/mL (15.0 μmol/L). The estimated retention day was significantly higher for patients with a TPM concentration >5 mcg/mL than that for patients with a concentration <5 mcg/mL (945 versus 802 days; P = 0.007 by the log-rank test). Also, patients without hepatic enzyme inducers had a significantly higher retention rate than patients using such inducers (P = 0.002).

CONCLUSIONS

Concomitant use of hepatic enzyme inducers markedly reduced the plasma TPM concentration and can decrease its antiepileptic effect. A therapeutic concentration of >5 mcg/mL TPM was significantly associated with continuation of therapy, and therapeutic drug monitoring can be helpful.

The Effect of Different Carbapenem Antibiotics (Ertapenem, Imipenem/Cilastatin, and Meropenem) on Serum Valproic Acid Concentrations

BACKGROUND

Carbapenem antibiotics (CBPMs) may significantly reduce the serum concentration of valproic acid (VPA), but the extent of this effect among various CBPMs is unknown. This study compared the extent and onset of the interactions among ertapenem, imipenem/cilastatin, and meropenem.

METHODS

A 5-year retrospective study was performed. Hospitalized patients over 18 years old who received VPA and a CBPM concurrently were enrolled via the pharmacy computer system. Patients who lacked VPA serum concentration measurements before or during CBPMs' use, had concurrent medication(s) that might interfere with VPA metabolism, or had a history of liver cirrhosis were excluded. Total VPA serum concentrations before and during CBPMs' use and after its discontinuation were recorded, and differences among various CBPMs were analyzed.

RESULTS

Fifty-two patients were included in this analysis. Irrespective of the route of administration, VPA serum concentrations were subtherapeutic in 90% of the subjects during CBPMs' use. There was a significant decrease (P < 0.001) in VPA serum concentrations during the use of CBPMs: 72% ± 17%, 42% ± 22%, and 67% ± 19% in the ertapenem (N = 9), imipenem/cilastatin (N = 17), and meropenem (N = 26) groups, respectively. The effect of ertapenem and meropenem on VPA was significantly more expressed than that of imipenem/cilastatin (P < 0.005). The onset of this drug interaction occurred within 24 hours of CBPMs' administration, and VPA serum concentrations returned to 90% of baseline within 7 days of CBPMs' discontinuation along with a 20% increase in VPA dose. Increasing VPA dose during the use of ertapenem or meropenem did not result in elevating VPA serum concentrations to therapeutic levels during the combined therapy period.

CONCLUSIONS

CBPMs reduced VPA serum concentration within 24 hours of administration by approximately 60%. Ertapenem and meropenem had a greater effect on VPA serum concentration than imipenem/cilastatin. Because of the dramatic reduction of VPA serum concentration during CBPMs' use, concomitant use of VPA and CBPMs should be avoided.

Biomarkers in epilepsy-A modelling perspective

Biomarkers can be categorised from type 0 (genotype or phenotype), through 6 (clinical scales), each level representing a part of the processes involved in the biological system and drug treatment. This classification facilitates the identification and connection of information required to fully (mathematically) model a disease and its treatment using integrated information from biomarkers. Two recent reviews thoroughly discussed the current status and development of biomarkers for epilepsy, but a path towards the integration of such biomarkers for the personalisation of anti-epileptic drug treatment is lacking. Here we aim to 1) briefly categorise the available epilepsy biomarkers and identify gaps, and 2) provide a modelling perspective on approaches to fill such gaps. There is mainly a lack of biomarker types 2 (target occupancy) and 3 (target activation). Current literature typically focuses on qualitative biomarkers for diagnosis and prediction of treatment response or failure, leaving a need for biomarkers that help to quantitatively understand the overall system to explain and predict differences in disease and treatment outcome. Due to the complexity of epilepsy, filling the biomarker gaps will require collaboration and expertise from the fields of systems biology and systems pharmacology.

Pharmacokinetic interactions and dosing rationale for antiepileptic drugs in adults and children

AIMS

Population pharmacokinetic modelling has been widely used across many therapeutic areas to identify sources of variability, which are incorporated into models as covariate factors. Despite numerous publications on pharmacokinetic drug-drug interactions (DDIs) between antiepileptic drugs (AEDs), such data are not used to support the dose rationale for polytherapy in the treatment of epileptic seizures. Here we assess the impact of DDIs on plasma concentrations and evaluate the need for AED dose adjustment.

METHODS

Models describing the pharmacokinetics of carbamazepine, clobazam, clonazepam, lamotrigine, levetiracetam, oxcarbazepine, phenobarbital, phenytoin, topiramate, valproic acid and zonisamide in adult and paediatric patients were collected from the published literature and implemented in NONMEM v7.2. Taking current clinical practice into account, we explore simulation scenarios to characterize AED exposure in virtual patients receiving mono- and polytherapy. Steady-state, maximum and minimum concentrations were selected as parameters of interest for this analysis.

RESULTS

Our simulations show that DDIs can cause major changes in AED concentrations both in adults and children. When more than one AED is used, even larger changes are observed in the concentrations of the primary drug, leading to significant differences in steady-state concentration between mono- and polytherapy for most AEDs. These results suggest that currently recommended dosing algorithms and titration procedures do not ensure attainment of appropriate therapeutic concentrations.

CONCLUSIONS

The effect of DDIs on AED exposure cannot be overlooked. Clinical guidelines must consider such covariate effects and ensure appropriate dosing recommendations for adult and paediatric patients who require combination therapy.

In vitro screening of dual flavonoid combinations for reversing P-glycoprotein-mediated multidrug resistance: Focus on antiepileptic drugs

The combined use of different P-glycoprotein (P-gp) inhibitors may be a relevant approach to the synergistic and safer inhibition of the P-gp-mediated drug efflux. Herein, we aimed to explore dual combinations of the flavonoids baicalein, (-)-epigallocatechin gallate, kaempferol, quercetin and silymarin to reverse the interference of P-gp on the intracellular accumulation of antiepileptic drugs (AEDs). The intracellular accumulation of rhodamine 123 (a classic P-gp substrate) and of several commonly used AEDs (carbamazepine, phenytoin, oxcarbazepine) or their metabolites (carbamazepine-10,11-epoxide and licarbazepine) was evaluated in MDCK-MDR1 cells in the presence and absence of individual flavonoids and their combinations. A selected flavonoid combination [(-)-epigallocatechin gallate/silymarin] was also evaluated in transepithelial transport experiments using licarbazepine (active metabolite of oxcarbazepine) as a model compound. Most flavonoid combinations increased rhodamine 123 intracellular uptake in a greater extent than their additive individual effects at similar concentrations. Moreover, selected (-)-epigallocatechin gallate/silymarin and kaempferol/baicalein combinations also enhanced the intracellular accumulation of all AEDs and metabolites. Overall, the combination of (-)-epigallocatechin gallate/silymarin was the most promising one. Thus, dual flavonoid combinations may be useful to overcome the P-gp-mediated efflux of AEDs and their metabolites, making their association to AED therapy a potentially valuable approach to circumvent pharmacoresistance in epilepsy.

Factors in Variability of Serial Gabapentin Concentrations in Elderly Patients with Epilepsy

OBJECTIVES

To characterize and quantify the variability of serial gabapentin concentrations in elderly patients with epilepsy.

METHODS

This study included 83 patients (age ≥ 60 yrs) from an 18-center randomized double-blind double-dummy parallel study from the Veterans Affairs Cooperative 428 Study. All patients were taking 1500 mg/day gabapentin. Within-person coefficient of variation (CV) in gabapentin concentrations, measured weekly to bimonthly for up to 52 weeks, then quarterly, was computed. Impact of patient characteristics on gabapentin concentrations (linear mixed model) and CV (linear regression) were estimated.

RESULTS

A total of 482 gabapentin concentration measurements were available for analysis. Gabapentin concentrations and intrapatient CVs ranged from 0.5 to 22.6 μg/ml (mean 7.9 μg/ml, standard deviation [SD] 4.1 μg/ml) and 2% to 79% (mean 27.9%, SD 15.3%), respectively, across all visits. Intrapatient CV was higher by 7.3% for those with a body mass index of ≥ 30 kg/m² (coefficient = 7.3, p=0.04). CVs were on average 0.5% higher for each 1-unit higher CV in creatinine clearance (coefficient = 0.5, p=0.03) and 1.2% higher for each 1-hour longer mean time after dose (coefficient = 1.2, p=0.04).

CONCLUSIONS

Substantial intrapatient variability in serial gabapentin concentration was noted in elderly patients with epilepsy. Creatinine clearance, time of sampling relative to dose, and obesity were found to be positively associated with variability.