Comparison of topiramate concentrations in plasma and serum by fluorescence polarization immunoassay

Anti-epileptic drug topiramate upregulates TGFβ1 and SOX9 expression in primary embryonic palatal mesenchyme cells: Implications for teratogenicity

Topiramate is an anti-epileptic drug that is commonly prescribed not just to prevent seizures but also migraine headaches, with over 8 million prescriptions dispensed annually. Topiramate use during pregnancy has been linked to significantly increased risk of babies born with orofacial clefts (OFCs). However, the exact molecular mechanism of topiramate teratogenicity is unknown. In this study, we first used an unbiased antibody array analysis to test the effect of topiramate on human embryonic palatal mesenchyme (HEPM) cells. This analysis identified 40 differentially expressed proteins, showing strong connectivity to known genes associated with orofacial clefts. However, among known OFC genes, only TGFβ1 was significantly upregulated in the antibody array analysis. Next, we validated that topiramate could increase expression of TGFβ1 and of downstream target phospho-SMAD2 in primary mouse embryonic palatal mesenchyme (MEPM) cells. Furthermore, we showed that topiramate treatment of primary MEPM cells increased expression of SOX9. SOX9 overexpression in chondrocytes is known to cause cleft palate in mouse. We propose that topiramate mediates upregulation of TGFβ1 signaling through activation of γ-aminobutyric acid (GABA) receptors in the palate. TGFβ1 and SOX9 play critical roles in orofacial morphogenesis, and their abnormal overexpression provides a plausible etiologic molecular mechanism for the teratogenic effects of topiramate.

A Rapid LC-MS-MS Method for the Quantitation of Antiepileptic Drugs in Urine

Epilepsy is a common neurologic disease that requires treatment with one or more medications. Due to the polypharmaceutical treatments, potential side effects, and drug-drug interactions associated with these medications, therapeutic drug monitoring is important. Therapeutic drug monitoring is typically performed in blood due to established clinical ranges. While blood provides the benefit of determining clinical ranges, urine requires a less invasive collection method, which is attractive for medication monitoring. As urine does not typically have established clinical ranges, it has not become a preferred specimen for monitoring medication adherence. Thus, large urine clinical data sets are rarely published, making method development that addresses reasonable concentration ranges difficult. An initial method developed and validated in-house utilized a universal analytical range of 50-5,000 ng/mL for all antiepileptic drugs and metabolites of interest in this work, namely carbamazepine, carbamazepine-10,11-epoxide, eslicarbazepine, lamotrigine, levetiracetam, oxcarbazepine, phenytoin, 4-hydroxyphenytoin, and topiramate. This upper limit of the analytical range was too low leading to a repeat rate of 11.59% due to concentrations >5,000 ng/mL. Therefore, a new, fast liquid chromatography-tandem mass spectrometry (LC-MS-MS) method with a run time under 4 minutes was developed and validated for the simultaneous quantification of the previously mentioned nine antiepileptic drugs and their metabolites. Urine samples were prepared by solid-phase extraction and analyzed using a Phenomenex Phenyl-Hexyl column with an Agilent 6460 LC-MS-MS instrument system. During method development and validation, the analytical range was optimized for each drug to reduce repeat analysis due to concentrations above the linear range and for carryover. This reduced the average daily repeat rate for antiepileptic testing from 11.59% to 4.82%. After validation, this method was used to test and analyze patient specimens over the course of approximately one year. The resulting concentration data were curated to eliminate specimens that could indicate an individual was noncompliant with their therapy (i.e., positive for illicit drugs) and yielded between 20 and 1,700 concentration points from the patient specimens, depending on the analyte. The resulting raw quantitative urine data set is presented as preliminary reference ranges to assist with interpreting urine drug concentrations for the nine aforementioned antiepileptic medications and metabolites.

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.

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.

A sensitive capillary GC-MS method for analysis of topiramate from plasma obtained from single-dose studies

Topiramate (Topamax®) is an antiepileptic medication used as adjunctive and monotherapy in patients with epilepsy and for migraine prophylaxis. A GC-MS assay was developed that was capable of detecting topiramate plasma concentrations following a single rectal or oral dose administration. Topiramate plasma samples were prepared by solid-phase extraction and were quantified by GC-MS analysis. The topiramate standard curves were split from 0.1 to 4 µg/mL and from 4 to 40 µg/mL in order to give a more accurate determination of the topiramate concentration. The accuracy of the standards ranged from 94.6 to 107.3% and the precision (%CV) ranged from 1.0 to 5.3% for both curves at all concentrations. The %CV for quality controls was <7.6%. The assay is both accurate and precise and will be used to complete future pharmacokinetic studies.

Thermoanalytical investigation of some sulfone-containing drugs

The thermal behavior of some sulfone-containing drugs, namely, dapsone (DDS), dimethylsulfone (MSM), and topiramate (TOP) in drug substances, and products were investigated using different thermal techniques. These include thermogravimetry (TGA), derivative thermogravimetry (DTG), differential thermal analysis (DTA), and differential scanning calorimetry (DSC). The thermogravimetric data allowed the determination of the kinetic parameters: activation energy (E(a)), frequency factor (A), and reaction order (n). The thermal degradation of dapsone and topiramate was followed a first-order kinetic behavior. The calculated data evidenced a zero-order kinetic for dimethylsulfone. The relative thermal stabilities of the studied drugs have been evaluated and follow the order DDS > TOP > MSM. The purity was determined using DSC for the studied compounds, in drug substances and products. The results were in agreement with the recommended pharmacopoeia and manufacturer methods. DSC curves obtained from the tablets suggest compatibility between the drugs, excipients and/or coformulated drugs. The fragmentation pathway of dapsone with mass spectrometry was taken as example, to correlate the thermal decomposition with the resulted MS-EI. The decomposition modes were investigated, and the possible fragmentation pathways were suggested by mass spectrometry.

Modern methods for analysis of antiepileptic drugs in the biological fluids for pharmacokinetics, bioequivalence and therapeutic drug monitoring

Epilepsy is a chronic disease occurring in approximately 1.0% of the world's population. About 30% of the epileptic patients treated with availably antiepileptic drugs (AEDs) continue to have seizures and are considered therapy-resistant or refractory patients. The ultimate goal for the use of AEDs is complete cessation of seizures without side effects. Because of a narrow therapeutic index of AEDs, a complete understanding of its clinical pharmacokinetics is essential for understanding of the pharmacodynamics of these drugs. These drug concentrations in biological fluids serve as surrogate markers and can be used to guide or target drug dosing. Because early studies demonstrated clinical and/or electroencephalographic correlations with serum concentrations of several AEDs, It has been almost 50 years since clinicians started using plasma concentrations of AEDs to optimize pharmacotherapy in patients with epilepsy. Therefore, validated analytical method for concentrations of AEDs in biological fluids is a necessity in order to explore pharmacokinetics, bioequivalence and TDM in various clinical situations. There are hundreds of published articles on the analysis of specific AEDs by a wide variety of analytical methods in biological samples have appears over the past decade. This review intends to provide an updated, concise overview on the modern method development for monitoring AEDs for pharmacokinetic studies, bioequivalence and therapeutic drug monitoring.

Therapeutic Drug Monitoring of the Newer Anti-Epilepsy Medications

In the past twenty years, 14 new antiepileptic drugs have been approved for use in the United States and/or Europe. These drugs are eslicarbazepine acetate, felbamate, gabapentin, lacosamide, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, rufinamide, stiripentol, tiagabine, topiramate, vigabatrin and zonisamide. In general, the clinical utility of therapeutic drug monitoring has not been established in clinical trials for these new anticonvulsants, and clear guidelines for drug monitoring have yet to be defined. The antiepileptic drugs with the strongest justifications for drug monitoring are lamotrigine, oxcarbazepine, stiripentol, and zonisamide. Stiripentol and tiagabine are strongly protein bound and are candidates for free drug monitoring. Therapeutic drug monitoring has lower utility for gabapentin, pregabalin, and vigabatrin. Measurement of salivary drug concentrations has potential utility for therapeutic drug monitoring of lamotrigine, levetiracetam, and topiramate. Therapeutic drug monitoring of the new antiepileptic drugs will be discussed in managing patients with epilepsy.

Therapeutic Drug Monitoring of the Newer Anti-Epilepsy Medications

In the past twenty years, 14 new antiepileptic drugs have been approved for use in the United States and/or Europe. These drugs are eslicarbazepine acetate, felbamate, gabapentin, lacosamide, lamotrigine, levetiracetam, oxcarbazepine, pregabalin, rufinamide, stiripentol, tiagabine, topiramate, vigabatrin and zonisamide. In general, the clinical utility of therapeutic drug monitoring has not been established in clinical trials for these new anticonvulsants, and clear guidelines for drug monitoring have yet to be defined. The antiepileptic drugs with the strongest justifications for drug monitoring are lamotrigine, oxcarbazepine, stiripentol, and zonisamide. Stiripentol and tiagabine are strongly protein bound and are candidates for free drug monitoring. Therapeutic drug monitoring has lower utility for gabapentin, pregabalin, and vigabatrin. Measurement of salivary drug concentrations has potential utility for therapeutic drug monitoring of lamotrigine, levetiracetam, and topiramate. Therapeutic drug monitoring of the new antiepileptic drugs will be discussed in managing patients with epilepsy.

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

Although no randomized studies have demonstrated a positive impact of therapeutic drug monitoring (TDM) on clinical outcome in epilepsy, evidence from nonrandomized studies and everyday clinical experience does indicate that measuring serum concentrations of old and new generation antiepileptic drugs (AEDs) can have a valuable role in guiding patient management provided that concentrations are measured with a clear indication and are interpreted critically, taking into account the whole clinical context. Situations in which AED measurements are most likely to be of benefit include (1) when a person has attained the desired clinical outcome, to establish an individual therapeutic concentration which can be used at subsequent times to assess potential causes for a change in drug response; (2) as an aid in the diagnosis of clinical toxicity; (3) to assess compliance, particularly in patients with uncontrolled seizures or breakthrough seizures; (4) to guide dosage adjustment in situations associated with increased pharmacokinetic variability (e.g., children, the elderly, patients with associated diseases, drug formulation changes); (5) when a potentially important pharmacokinetic change is anticipated (e.g., in pregnancy, or when an interacting drug is added or removed); (6) to guide dose adjustments for AEDs with dose-dependent pharmacokinetics, particularly phenytoin.

A novel high sensitivity HPLC assay for topiramate, using 4-chloro-7-nitrobenzofurazan as pre-column fluorescence derivatizing agent

A new, sensitive and simple high-performance liquid chromatographic method for analysis of topiramate, an antiepileptic agent, using 4-chloro-7-nitrobenzofurazan as pre-column derivatization agent is described. Following liquid-liquid extraction of topiramate and an internal standard (amlodipine) from human serum, derivatization of the drugs was performed by the labeling agent in the presence of dichloromethane, methanol, acetonitrile and borate buffer (0.05 M; pH 10.6). A mixture of sodium phosphate buffer (0.05 M; pH 2.4): methanol (35:65 v/v) was eluted as mobile phase and chromatographic separation was achieved using a Shimpack CLC-C18 (150 x 4.6 mm) column. In this method the limit of quantification of 0.01 microg/mL was obtained and the procedure was validated over the concentration range of 0.01 to 12.8 microg/mL. No interferences were found from commonly co-administrated antiepileptic drugs including phenytoin, phenobarbital carbamazepine, lamotrigine, zonisamide, primidone, gabapentin, vigabatrin, and ethosuximide. The analysis performance was carried-out in terms of specificity, sensitivity, linearity, precision, accuracy and stability and the method was shown to be accurate, with intra-day and inter-day accuracy from -3.4 to 10% and precise, with intra-day and inter-day precision from 1.1 to 18%.

High performance liquid chromatographic determination of topiramate in human serum using UV detection

Topiramate has no ultraviolet, visible or fluorescence absorption. Analysis of the drug in human serum has been reported by high performance liquid chromatography (HPLC) with either mass detector or fluorescence detection after precolumn derivatization using 9-fluorenylmethyl chloroformate as fluorescent labeling agent. This study was aimed to validate derivatization and analysis of topiramate in human serum with HPLC using UV detection. The drug was extracted from human serum by liquid-liquid extraction and subjected to derivatization with 9-fluorenylmethyl chloroformate. Analysis was performed on a phenyl column using of spectrophotometer detection operated at wavelength of 264 nm. A mixture of phosphate buffer (0.05M) containing triethylamine (1 ml/l, v/v; pH 2.3) and methanol (28:72, v/v) at a flow rate of 2.5 ml/min was used as mobile phase. No interference was found with endogenous substances. Validity of the method was studied and the method was precise and accurate with a linearity range from 40 ng/ml to 40 microg/ml. The limit of quantification was 40 ng/ml of serum. The correlation coefficient between HPLC methods using fluorescence and UV detections was studied and found to be 0.992.

Topiramate Pharmacokinetics in Children and Adults with Epilepsy

OBJECTIVE

To compare the steady-state pharmacokinetics of topiramate in a large population of children and adults with epilepsy in a therapeutic drug monitoring setting.

STUDY DESIGN

Retrospective, case-matched pharmacokinetic evaluation.

PATIENTS

Seventy children (aged 1-17 years) with epilepsy and 70 adult controls (aged 18-65 years) with epilepsy, matched for sex and comedication.

METHODS

Topiramate apparent oral clearance (CL/F) values were calculated from steady-state serum concentrations in children and compared with those determined in controls. Comparisons were made by means of the Mann-Whitney's U-test, or the Kruskal-Wallis test in the case of multiple comparisons. A linear regression model was used to assess potential correlation of CL/F values with age. To investigate the influence of different variables on the variability in topiramate CL/F values, a multiple regression model was developed.

RESULTS

In the absence of enzyme-inducing comedication, mean topiramate CL/F was 42% higher in children than in adults (40.3 +/- 21.0 vs 28.4 +/- 15.3 mL/h/kg; p < 0.01). In children and adults comedicated with enzyme-inducing antiepileptic drugs (AEDs), topiramate CL/F values were approximately 1.5- to 2-fold higher than those observed in the absence of enzyme inducers, and the elevation in topiramate CL/F in children compared with adults was also present in the subgroups receiving enzyme inducers (66%; 76.6 +/- 35.1 vs 46.1 +/- 16.7 mL/h/kg; p < 0.0001). In the paediatric population, a negative correlation between CL/F and age was demonstrated, both in the absence (p < 0.01) and in the presence (p < 0.001) of enzyme induction. The independent influence of age and enzyme-inducing AEDs on topiramate CL/F was confirmed by multiple regression analysis.

CONCLUSION

Topiramate CL/F is highest in young children and decreases progressively with age until puberty, presumably due to age-dependent changes in the rate of drug metabolism. As a result of this, younger patients require higher dosages to achieve serum topiramate concentrations comparable with those found in older children and adults. Enzyme-inducing comedication decreases serum topiramate concentration by approximately one-half and one-third in children and adults, respectively.

Age and antiepileptic drugs influence topiramate plasma levels in children

The influence of age and comedication on the dose-to-level ratio of topiramate was examined in 91 children with epilepsy treated with topiramate. The topiramate dosing and plasma concentrations, as well as those of their concomitant antiepileptic drugs were examined retrospectively. The dose-to-level ratio was used as a measure of clearance and was calculated as the weight-normalized topiramate dose (mg/kg/day) divided by the steady-state trough plasma drug level in the child. The children were classified in age groups and treatment groups; topiramate was administered with an enzyme inducer (n = 32), with a nonenzyme inducer (n = 49), or as monotherapy (n = 10). The topiramate clearance in children aged 0-8 years compared with those aged 9-17 years was more than twofold higher if treated with an enzyme-inducing antiepileptic drug and 1.5-fold higher if treated with a nonenzyme inducer. Children receiving enzyme inducers had a more than twofold higher clearance compared with those who did not. Within all age groups, significant differences in topiramate clearance were observed between those receiving enzyme inducers and those receiving nonenzyme inducers or monotherapy. Thus younger age and concomitant enzyme inducers, both acting independently, significantly increased the clearance of topiramate in children. This effect has to be considered to optimize treatment in the individual patient.

Influence of Dosage, Age, and Co-medication on Plasma Topiramate Concentrations in Children and Adults with Severe Epilepsy and Preliminary Observations on Correlations with Clinical Response

The influence of dosage, age, and co-medication on plasma topiramate (TPM) concentrations at steady state was investigated in 51 patients aged 3 to 30 years. All patients had chronic active epilepsy, and most were receiving concomitant medication with enzyme-inducing anticonvulsants (carbamazepine and phenobarbital). Plasma TPM concentrations were determined by a specific immunoassay in samples obtained before the morning dose. Thirty-five patients could be evaluated prospectively at different dose levels, and the relationship between plasma TPM concentration and dosage was linear over the assessed dose range (1.8 to 10.0 mg/kg) both in adults and in children. The influence of age on pharmacokinetic parameters could be assessed only for the 42 patients co-medicated with enzyme inducers. In these patients dose-normalized plasma TPM concentrations correlated positively with age (r = 0.59, P < 0.0001), where apparent oral clearance values (CL/F) were inversely related to age (r = 0.73, P < 0.0001). In particular, CL/F values in children aged less than 10 years (112 +/- 82 mL/kg/h, mean +/- SD, n = 14) were almost three times as high as those observed in patients aged >15 to 30 years (42 +/- 16 mL/kg/h, n = 17), whereas the CL/F value in children aged 10 to 15 years (66 +/- 22 mL/kg/h, n = 11) was intermediate between those found in the two other age groups. Patients not receiving enzyme-inducing AEDs showed lower CL/F values than did age- and gender-matched patients on enzyme-inducing co-medication. A preliminary evaluation of the relationship between plasma TPM concentration and therapeutic response could be made in 41 patients. No significant difference in drug concentration was detected between patients showing a greater than 50% reduction in seizure frequency compared with baseline (5.9 +/- 2.2 micrograms/mL, n = 30) and those having no clinical improvement (5.2 +/- 2.2 micrograms/mL, n = 11). Likewise, there was no consistent relationship between plasma TPM concentration and appearance of adverse effects. These results indicate that plasma TPM concentrations are linearly related to dosage both in adults and in children and that children aged <10 years require much greater body weight-adjusted dosage to achieve drug levels comparable to those observed in young adults. The marked increase in TPM clearance caused by enzyme-inducing co-medication was confirmed.

Vigabatrin, but not gabapentin or topiramate, produces concentration-related effects on enzymes and intermediates of the GABA shunt in rat brain and retina

PURPOSE

The antiepileptic drug (AED) vigabatrin (VGB), which exerts its pharmacologic effects on the gamma-aminobutyric acid (GABA) system, causes concentric visual field constriction in >40% of exposed adults. This may be a class effect of all agents with GABA-related mechanisms of action. We compared the concentration-related effects of VGB in rat brain and eye with those of gabapentin (GBP) and topiramate (TPM), both of which have been reported to elevate brain GABA concentrations in humans.

METHODS

Adult male rats (n = 10) were administered 0.9% saline (control), VGB (250, 500, 1,000 mg/kg), GBP (50, 100, 200 mg/kg), or TPM (12.5, 25, 50, 100 mg/kg). At 2 h after dosing, animals were killed, a blood sample obtained, the brain dissected into eight distinct regions, and the retina and vitreous humor isolated from each eye. Samples were analyzed for several GABA-related neurochemical parameters, and serum and tissue drug concentrations determined.

RESULTS

VGB treatment produced a significant (p < 0.05) dose-related increase in GABA concentrations and decrease in GABA-transaminase activity in all tissues investigated. This effect was most pronounced in the retina, where VGB concentrations were 18.5-fold higher than those in brain. In contrast, GBP and TPM were without effect on any of the neurochemical parameters investigated and did not accumulate appreciably in the retina.

CONCLUSIONS

These findings corroborate a previously reported accumulation of VGB in the retina, which may be responsible for the visual field constriction observed clinically. This phenomenon does not appear to extend to other GABAergic drugs, suggesting that these agents might not cause visual field defects.

Analysis of topiramate and its metabolites in plasma and urine of healthy subjects and patients with epilepsy by use of a novel liquid chromatography-mass spectrometry assay

A novel liquid chromatography-mass spectrometry (LC-MS) method was developed and validated for quantification of topiramate (TPM) and its metabolites 10-hydroxy topiramate (10-OH-TPM), 9-hydroxy topiramate (9-OH-TPM), and 4,5-O-desisopropylidene topiramate (4,5-diol-TPM) in plasma and urine. The method uses 0.5 mL of plasma or 1 mL of urine that is extracted with diethyl ether and analyzed by LC-MS. Positive ion mode detection enables tandem mass spectrometric (MS/MS) identification of the aforementioned four compounds. Calibration curves of TPM, 4,5-diol-TPM, 9-OH-TPM, and 10-OH-TPM in plasma and urine were prepared and validated over the concentration range of 0.625 to 40 microg/mL using TPM-d(12) as an internal standard. Calibration curves were linear over this concentration range for TPM and its metabolites. Accuracy and precision ranged in urine from 83% to 114% and 4% to 13% (%CV), respectively, and in plasma from 82% to 108% and 6% to 13%, respectively. The applicability of the assay was evaluated by analyzing plasma samples from a healthy subject who received a single oral dose of TPM (200 mg) and urine samples from 11 patients with epilepsy treated with TPM (daily dose between 100 to 600 mg) alone or with other antiepileptic drugs. Only TPM was detected and quantified in the plasma samples, and its concentration ranged between 0.7 and 4.3 microg/mL. The concentrations of TPM and 10-OH TPM were quantifiable in all urine samples and ranged from 20 to 300 microg/mL for TPM and from 1 to 50 microg/mL for 10-OH-TPM. The metabolites 4,5-diol-TPM and 9-OH-TPM were also detected in all urine samples, but their concentrations were quantifiable only in 4 patients. An unidentified peak in the chromatograms obtained from patients' urine was attributed to 2,3-O-desisopropylidene topiramate (2,3-diol-TPM). Due to a lack of reference material of 2,3-diol TPM and the similar MS/MS spectrum with 4,5-diol-TPM, the calibration curves of 4,5-diol-TPM were used for the quantification of its isomer 2,3-diol-TPM. Based on these determinations, the apparent 2,3-diol-TPM-to-TPM concentration ratio in patients' urine ranged from 0.05 to 0.51 and the 10-OH-TPM-to-TPM ratio ranged from 0.02 to 0.17. In conclusion, a novel LC-MS method for the assay of TPM and four of its metabolites in plasma and urine was developed. Its utilization for analysis of urine samples from patients with epilepsy showed that the method was suitable for analysis of TPM and its metabolites in clinical samples. Two quantitatively significant TPM metabolites (10-OH-TPM and 2,3-diol-TPM) and two quantitatively minor metabolites (9-OH-TPM and 4,5-diol-TPM) were detected and quantified in urine samples from patients with epilepsy.

Therapeutic drug monitoring of the newer antiepileptic drugs

The aim of the present review is to discuss the potential value of therapeutic drug monitoring (TDM) of the newer antiepileptic drugs (AEDs) felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin, and zonisamide. Studies of the relationship between serum concentrations and clinical efficacy of these drugs are reviewed, and the potential value of TDM of the drugs is discussed based on their pharmacokinetic properties and mode of action. Analytical methods for the determination of the serum concentrations of these drugs are also briefly described. There are only some prospective data on the serum concentration-effect relationships, and few studies have been designed primarily to study these relationships. As TDM is not widely practiced for the newer AEDs, there are no generally accepted target ranges for any of these drugs, and for most a wide range in serum concentration is associated with clinical efficacy. Furthermore, a considerable overlap in drug concentrations related to toxicity and nonresponse is reported. Nevertheless, the current tentative target ranges for felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine (10-hydroxy-carbazepine metabolite), tiagabine, topiramate, vigabatrin, and zonisamide are 125 to 250 micromol/L, 70 to 120 micromol/L, 10 to 60 micromol/L, 35 to 120 micromol/L, 50 to 140 micomol/L, 50 to 250 nmol/L, 15 to 60 micromol/L, 6 to 278 micromol/L, and 45 to 180 micromol/L, respectively. Further systematic studies designed specifically to evaluate concentration-effect relationships of the new AEDs are urgently needed. Although routine monitoring in general cannot be recommended at present, measurements of some of the drugs is undoubtedly of help with individualization of treatment in selected cases in a particular clinical setting.

Interlaboratory variability in the quantification of new generation antiepileptic drugs based on external quality assessment data

PURPOSE

To assess interlaboratory variability in the determination of serum levels of new antiepileptic drugs (AEDs).

METHODS

Lyophilised serum samples containing clinically relevant concentrations of felbamate (FBM), gabapentin (GBP), lamotrigine (LTG), the monohydroxy derivative of oxcarbazepine (OCBZ; MHD), tiagabine (TGB), topiramate (TPM), and vigabatrin (VGB) were distributed monthly among 70 laboratories participating in the international Heathcontrol External Quality Assessment Scheme (EQAS). Assay results returned over a 15-month period were evaluated for precision and accuracy.

RESULTS

The most frequently measured compound was LTG (65), followed by MHD (39), GBP (19), TPM (18), VGB (15), FBM (16), and TGB (8). High-performance liquid chromatography was the most commonly used assay technique for all drugs except for TPM, for which two thirds of laboratories used a commercial immunoassay. For all assay methods combined, precision was <11% for MHD, FBM, TPM, and LTG, close to 15% for GBP and VGB, and as high as 54% for TGB (p < 0.001). Mean accuracy values were <10% for all drugs other than TGB, for which measured values were on average 13.9% higher than spiked values, with a high variability around the mean (45%). No differences in precision and accuracy were found between methods, except for TPM, for which gas chromatography showed poorer accuracy compared with immunoassay and gas chromatography-mass spectrometry.

CONCLUSIONS

With the notable exception of TGB, interlaboratory variability in the determination of new AEDs was comparable to that reported with older-generation agents. Poor assay performance is related more to individual operators than to the intrinsic characteristics of the method applied. Participation in an EQAS scheme is recommended to ensure adequate control of assay variability in therapeutic drug monitoring.

Talampanel, a new antiepileptic drug: single- and multiple-dose pharmacokinetics and initial 1-week experience in patients with chronic intractable epilepsy

PURPOSE

Talampanel (LY300164), a potent and selective alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-receptor antagonist, is a potential new antiepileptic drug (AED). This study examines the single- and multiple-dose pharmacokinetics, safety, and tolerability of talampanel in patients with intractable epilepsy and assesses the potential for pharmacokinetic interaction.

METHODS

Eleven of 14 patients entered into the study completed. Fourteen patients were evaluated for safety, 13 patients were used in the single-dose, and 11 patients in the multiple-dose pharmacokinetic analysis. Each patient initially received a single 35-mg dose of talampanel followed by the measurement of pharmacokinetic profiles. A 21-day t.i.d. dosing regimen was then determined for each patient based on his or her initial pharmacokinetic profile. Adverse events were recorded by patients or their carers.

RESULTS

After oral ingestion, talampanel was rapidly absorbed, with maximal plasma concentrations achieved within 1-3 h. Talampanel concentrations in patients taking enzyme-inducing AEDs were 50% lower than those seen in healthy volunteers. Mean talampanel t1/2 values were 3.0 h compared with 4.2 h in healthy volunteers. After multiple-dose and steady-state, talampanel t1/2 values were increased to 5.6 h Talampanel and valproic acid (VPA) appear to inhibit each other's metabolism mutually. Talampanel had no effect on plasma concentrations of other AEDs. Multiple-dose talampanel administration was associated with nonlinear pharmacokinetics. No serious adverse events were reported; the most frequently reported being dizziness, ataxia, drowsiness, and headaches

CONCLUSIONS

Talampanel dosing strategies may be reliant on concomitant AED medication, as enzyme-inducing AEDs enhance, whereas VPA inhibits its metabolism. Talampanel was well tolerated, although adverse events occurred at lower doses compared with those in healthy subjects, probably because of the additive effect of concomitant AEDs.

Topiramate kinetics during delivery, lactation, and in the neonate: preliminary observations

PURPOSE

To study the pharmacokinetics of topiramate (TPM) during delivery, lactation, and in the neonate.

METHODS

TPM concentrations in plasma and breast milk were measured with fluorescence polarization immunoassay (FPIA) in five women with epilepsy treated with TPM during pregnancy and lactation. Blood samples were obtained at delivery from mothers, from the umbilical cord, and from the newborns on three occasions (24, 48, and 72 h) after delivery. Blood and breast milk also were collected from mothers 2 weeks, and 1 and 3 months postpartum. Blood samples also were drawn from the infants during breast-feeding. Three of the mother-infant pairs were studied both at delivery and during lactation; two contributed with data from delivery only.

RESULTS

The umbilical cord plasma/maternal plasma ratios were close to unity, suggesting extensive transplacental transfer of TPM. The mean milk/maternal plasma concentration ratio was 0.86 (range, 0.67-1.1) at 2-3 weeks after delivery. The milk/maternal plasma concentration ratios at sampling 1 and 3 months after delivery were similar (0.86 and 0.69, respectively). Two to 3 weeks after delivery, two of the breast-fed infants had detectable (>0.9 microM) concentrations of TPM, although below the limit of quantification (2.8 microM), and one had an undetectable concentration.

CONCLUSIONS

Our limited data suggest free passage of TPM over the placenta and an extensive transfer into breast milk. Breast-fed infants had very low TPM concentrations, and no adverse effects were observed in the infants.

Liquid chromatography tandem mass spectrometry assay for topiramate analysis in plasma and cerebrospinal fluid: validation and comparison with fluorescence-polarization immunoassay

SUMMARY

The authors report the development and validation of a liquid chromatography tandem mass spectrometry assay (LC/MS/MS assay) for the analysis of topiramate (2,3:4,5-bis-o-(-1-methyl)-beta-D-fructopyranose sulfamate) in plasma and cerebrospinal fluid (CSF). Comparison is made with the commercially available fluorescence-polarization immunoassay (FPIA).

LC/MS/MS ASSAY

Using the internal standard, 1,2:3,4-bis-o-(1-methylethylidene-alpha-D-galactopyranose sulfamate), a structural isomer, the calibration curve in plasma was linear in the concentration range of 0.02-20.0 mg/L (r(2) = 0.9998). The coefficients of variation in plasma were < or = 3%, and the accuracy ranged from 100% to 101% in the therapeutically relevant concentration range of 0.4-16.0 mg/L. In CSF, the mean recovery was 98%, and there was linearity between the nominal and the estimated concentration in the range of 1.5-20.0 mg/L (r(2)= 0.9996). FPIA: The calibration curve was linear in the concentration interval of 1.6-24.3 mg/L (r(2) = 0.9994), and the mean recovery was 96%. Accuracy in plasma was 99- 104%, and precision was 3.2-6.0%. In CSF, there was linearity between the nominal concentration and the estimated concentration in the range of 1.5-20.0 mg/L (r(2) = 0.9995), and the mean recovery was 100%. COMPARISON BETWEEN FPIA AND LC/MS/MS: There was a high correlation between the FPIA and the LC/MS/MS assay (r(2) = 0.9965 in plasma and r(2) = 0.9996 in CSF, P < 0.001 for both). In plasma and CSF, the two methods showed equal results, evaluated as the ratio between the two methods (plasma: median ratio = 1.00; 95% confidence interval [CI], 0.98-1.02, paired-sample test, P = 0.79; and CSF: median ratio = 1.00, 95% CI, 0.99-1.02, paired-sample test, P = 0.75). The coefficient of variation on the ratios between the two methods had similar levels: 5% in plasma and 3% in CSF.

CONCLUSION

The new LC/MS/MS assay has favorable characteristics, being highly precise and accurate. FPIA also proved precise and accurate, and there was a high agreement with the LC/MS/MS assay in plasma and CSF. Either method displayed sufficient precision and accuracy and may thus be implemented in daily routine.

Lack of an effect of topiramate on lamotrigine serum concentrations

PURPOSE

Pharmacokinetic interactions between the older antiepileptic drugs (AEDs) and topiramate (TPM) were assessed during the clinical development of this drug. Lamotrigine (LTG) has become established as an important new drug in treating a wide spectrum of seizure types, but there are no published data on whether LTG serum concentrations change when TPM is added to treatment.

METHODS

Escalating doses of TPM were added to stable LTG treatment in 24 young patients (8-21 years) with epilepsy. Blood samples taken before the morning dose were collected for drug-concentration measurement in all patients before starting treatment with TPM and after stabilisation at each dose escalation. Several patients had been maintained on unchanged therapy with drug-concentration monitoring for many months before introducing TPM, and a sequence of baseline LTG serum concentrations were available on these patients.

RESULTS

The mean of all baseline LTG concentrations for the group as a whole was 10.4 +/- 4.4 mg/L compared with 9.7 +/- 4.3 mg/L after addition of TPM. A comparison of last baseline LTG concentration with first test LTG concentration (i.e., after 2 weeks' TPM treatment) gave mean values of 10.7 +/- 4.7 and 10.8 +/- 4.6 mg/L, respectively. The mean LTG concentration for patients while taking their highest TPM dose was 9.5 +/- 4.3 mg/L. The analysis-of-variance modeling for the effect of TPM on LTG concentration yielded a mean LTG concentration ratio (with TPM vs. without TPM) of 94.2%, with a 90% confidence interval of 89.5-99.1%.

CONCLUSIONS

TPM did not cause a significant change in LTG serum concentration in this group of patients.

Serum concentrations of topiramate in patients with epilepsy: influence of dose, age, and comedication

Topiramate is a new antiepileptic drug (AED) approved as add-on therapy. Previous studies have shown that topiramate has only a limited effect on other AEDs, but its own metabolism can be induced by enzyme-inducing drugs. The aim of this study was to investigate the influence of topiramate dose, age, and comedication, especially of carbamazepine, phenytoin, phenobarbital, oxcarbazepine, lamotrigine, and valproic acid (VPA) on topiramate serum concentrations in patients with epilepsy. In total, 480 samples of 344 inpatients who fulfilled the inclusion criteria (e.g., trough concentration, body weight available) were investigated. The topiramate serum concentration in relation to topiramate dose per body weight (level-to-dose ratio) was calculated and compared for patients receiving topiramate monotherapy and for patients receiving topiramate plus one other AED. Analysis of covariance (using age as covariate) showed that comedication had a highly significant influence on the topiramate serum concentrations. Regression analysis including all 480 samples confirmed that in combinations with phenytoin, carbamazepine, phenobarbital, and oxcarbazepine, the topiramate concentrations were significantly lower compared with topiramate monotherapy, whereas VPA and lamotrigine had no significant influence. Moreover, regression analysis indicated that primidone and methsuximide lowered topiramate concentrations, whereas gabapentin, bromide, and sulthiame did not. In addition to comedication, the patient's age was significantly correlated with topiramate clearance. In accordance with the results of previous studies, these results indicated that infants and children had lower topiramate concentrations than adults receiving the same topiramate dose per body weight. Comedication and age should be considered in adjusting topiramate dosage. Determination of topiramate serum concentrations may be useful, especially when enzyme-inducing drugs are withdrawn or added.