HPLC Analysis of Antiepileptic Drugs in Blood Samples: Microbore Separation of Fourteen Compounds

Serum concentrations of Levetiracetam in epileptic patients: the influence of dose and co-medication

Levetiracetam (LEV) is a new antiepileptic drug approved as add-on therapy. Previous studies indicated that LEV has no relevant interactions with other antiepileptic drugs. The aim of this study was to investigate the influence of LEV dose, age, and co-medication on the serum concentration of LEV. In total, 363 samples of 297 inpatients who fulfilled the inclusion criteria (e.g., trough concentration, body weight available) were investigated. A patient was considered twice only if his co-medication had been changed. The LEV serum concentration in relation to LEV dose/body weight [level-to-dose ratio, LDR, (microgram/mL)/(mg/kg)] was calculated and compared for the most frequent drug combinations. Analysis of covariance (using age as covariate) carried out on the log-transformed data showed that co-medication had a highly significant (P < 0.001) effect on LEV serum concentrations. The median LDR of LEV was 0.32 for LEV + phenytoin, 0.32 for LEV + carbamazepine, 0.34 LEV + oxcarbazepine, 0.45 for LEV + lamotrigine, 0.46 for LEV + phenobarital, 0.52 for LEV monotherapy, 0.53 for LEV + valproic acid, and 0.54 LEV + valproic acid + lamotrigine. In co-medication with phenytoin (P < 0.001), carbamazepine (P < 0.001), and oxcarbazepine (P < 0.004), the LDR of LEV was significantly lower than it was with LEV monotherapy, whereas the LDR of LEV of patients on co-medication with valproic acid or lamotrigine did not differ significantly from the LDR of LEV of patients on LEV monotherapy (P > 0.05). Regression analysis including all 363 samples confirmed that other drugs (e.g., phenytoin, carbamazepine) lower LEV concentrations. In addition to co-medication, age had a significant effect on clearance of LEV. Children had lower LEV concentrations than adults on the same LEV dose per body weight. In contrast to other studies, our data point out that other enzyme-inducing antiepileptic drugs (e.g., phenytoin, carbamazepine) can moderately decrease LEV serum concentrations (by 20-30%). However, our observations should be confirmed by prospective pharmacokinetic studies.

Postictal serum levels of antiepileptic drugs for detection of noncompliance

Medication noncompliance (NC) is thought to be a major cause of insufficient seizure control. In an explorative study we investigated whether postictal serum levels (PISLs) of antiepileptic drugs (AEDs) are a reliable indicator of NC. Young adults with epilepsy on a stable AED regimen were asked to contact medical service as soon as possible when a seizure occurred to obtain serum levels of their AEDs. PISLs were compared with the mean value of two routine serum levels of the same medication. PISLs lower than 50% of the individual reference value were regarded as an indicator of NC. PISL samples in 61 seizures of 52 patients treated mainly with carbamazepine, valproic acid, or lamotrigine were evaluated. A drop in serum levels >50% indicating NC was noted in 44.3% of the seizures. Determination of PISLs seems to be a simple and useful method for detecting or ruling out irregular intake patterns as a cause of "breakthrough" seizures.

Does lamotrigine influence valproate concentrations?

The aim of this study is to investigate the effect of lamotrigine (LTG) on valproate (VPA) concentrations dependent on LTG dose, LTG concentration, and additional enzyme-inducing antiepileptic drugs (AED) as well. For this purpose the following patient groups were compared: VPA monotherapy, VPA + one enzyme-inducing AED, VPA + LTG, and VPA + LTG + one enzyme-inducing AED. A total of 400 serum concentrations from 372 patients were evaluated. Two or more serum samples from the same patient were considered only if the comedication had been changed. For statistical evaluation, regression analytical methods and an analysis of variance were performed. For the analysis of variance, the VPA serum concentration in relation to VPA dose:body weight (level:dose ratio, LDR) was calculated and compared for different drug combinations. The analysis of variance revealed a significant effect of enzyme-inducing comedication (as expected) and age on the VPA LDR. Patients on LTG had a slightly lower VPA LDR, but this effect was not statistically significant. In addition, nonlinear regression analysis confirmed that patients on enzyme-inducing AED (carbamazepine, phenytoin, phenobarbital, methsuximide) had significantly lower VPA concentrations. Patients on ethosuximide had slightly but not significantly lower VPA concentrations. Patients on LTG also had significantly lower VPA levels, but this effect was only minor (-7%) and most probably not of any clinical relevance. Furthermore, the regression analysis showed that the relationship between the VPA dose per body weight and the serum concentration deviates significantly from linearity. Children less than 6 years old had lower VPA levels than older children and adults on a comparable VPA dose per body weight. Gender had no significant influence on VPA serum concentration. In addition, a subgroup of 40 patients was analyzed to see whether changing the LTG dose influences VPA serum concentrations. It did not. Thus, the authors conclude that the effect of LTG on VPA concentrations is not of clinical relevance.

Determination of antiepileptic drugs in biological material

Current analytical methodologies applied to the determination of antiepileptic drugs in biological material are reviewed. The role of chromatographic techniques is emphasized. Special attention is focused on new chemical entities as well as current trends such as high-speed liquid chromatographic techniques, hyphenated techniques and electrochromatography techniques. A review with 542 references.

Micellar electrokinetic capillary chromatography for therapeutic drug monitoring of carbamazepine and its main metabolites

In carbamazepine (CBZ) therapy the concomitant monitoring of concentrations of CBZ and its metabolites is strictly recommended, primarily to avoid toxic side effects. Currently, clinical routine monitoring of CBZ is accomplished by high-performance liquid chromatography or immunological methods. In this study a micellar electrokinetic capillary chromatographic (MECC) method was developed for routine drug monitoring of CBZ and its main metabolites, carbamazepine 10,11-diol and carbamazepine 10,11-epoxide, in human serum or plasma samples. The MECC method enabled baseline separation of all analytes within 2.5 min. The assay revealed sufficient precision and sensitivity and the results of either an automated HPLC or the MECC chromatography assay were in good agreement (r=-0.97). The maximum deviation for CBZ was 0.26 microM.

Influence of ethosuximide on valproic acid serum concentrations

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

Simultaneous determination of carbamazepine, phenytoin, phenobarbital, primidone and their principal metabolites by high-performance liquid chromatography with photodiode-array detection

We have established a precise and accurate high-performance liquid chromatographic method for the simultaneous assay of carbamazepine, phenytoin, phenobarbital, primidone and their principal metabolites. This method has been used for the analysis of these drugs and the metabolites in serum, saliva and urine samples. Acetonitrile is used for the deproteinization of serum and saliva samples while solid-phase extraction is utilized for urine sample pretreatment. Samples of 2 microliters are injected onto a 3-microns ODS-Hypersil column (250 mm x 2 mm I.D.) with a column temperature of 40 degrees C. The drugs and metabolites are eluted with a mobile phase containing potassium phosphate buffer-acetonitrile-methanol (110:50:30, v/v/v) at a flow-rate of 0.2 ml/min. Signals are monitored by a photodiode-array detector at a sample wavelength of 200 nm with a bandwidth of 10 nm. These four commonly used antiepileptic drugs and their six metabolites are well separated from one another within 15 min. Within-day coefficients of variation (C.V.) are within 5% in most cases and between-day C.V. are from 2.32 to 4.75%. The recovery rates range from 95.12 to 104.42%. This method has the necessary sensitivity and linearity for routine therapeutic monitoring of both total and free drug levels and may be employed for pharmacokinetics studies of drug interactions and metabolism as well.

Analysis of antiepileptic drugs in human plasma using micellar electrokinetic capillary chromatography

We describe a method for the simultaneous determination of antiepileptic drugs (ethosuccimide, phenytoin, primidone, phenobarbital, carbamazepine and valproic acid) by micellar electrokinetic capillary chromatography using sodium dodecyl sulphate as the micellar phase. Factors affecting the micellar electrokinetic separation were studied for the quantitative determination of these drugs in human plasma. The confirmation of the peaks and the specificity of the method were investigated by combining multiwavelength detection with micellar electrokinetic capillary chromatography.

Analysis of sulthiame in serum by narrow-bore high-performance liquid chromatography. Comparison of direct sample injection with pre-column switching and extrelut extraction

Two high-performance liquid chromatographic methods for the analysis of sulthiame in serum are described. In the first method direct injection of serum samples onto a 4 x 4 mm I.D. (C18, 25 microns) precolumn in a column-switching device was used. After a purge step, the adsorbed analytes were eluted onto a 250 x 3 mm I.D. (C18, 5 microns) narrow-bore column for chromatographic separation. In the second method the sample pretreatment was an Extrelut extraction with dichloromethane-propanol-2 (95:5). After evaporation of the solvents, the residue was dissolved in methanol. The chromatographic separation was carried out on the same analytical column as used in the column-switching method. Both sample pretreatment methods were compared with respect to their suitability of routine analysis of sera from patients also receiving other antiepileptic drugs.

Use of normal-phase microcolumn high-performance liquid chromatography for the study of hydrolytic stability, metabolic profiling and pharmacokinetics of an antiepileptic drug, benzonal

A normal-phase microcolumn HPLC method is proposed for the quantitative determination of the most frequently used antiepileptic drugs and some compounds developed recently. The main advantage of this method, in comparison with other micro-scale HPLC techniques for antiepileptic drugs, is that it is used under isocratic conditions at room temperature (22 degrees C). The hydrolytic stability of benzonal (BZ) has been studied using this method together with mass spectrometry (MS), and IR, UV and NMR spectroscopy. Three pathways of hydrolytic degradation were established in alkaline conditions, whereas in an acid medium only one of these routes was followed. An investigation of the metabolic profiling of BZ in guinea-pigs showed that the drug undergoes fast hydrolysis in the intestinal tract forming phenobarbitone (PB) and benzoic acid (BA). Only PB was detected in brain tissue and it is probably responsible for the whole therapeutic effect. The pharmacokinetic parameters of this metabolite (PB) were determined in guinea-pigs, and their identity with the pharmacokinetic parameters of PB as the parent drug was confirmed.

Analysis of antiepileptic drugs

This review discussed various analytical methods for the determination of antiepileptic drugs and their metabolites in biological tissues. The emphasis was on the reports published since their last review [J. T. Burke and J. P. Thenot, J. Chromatogr., 340 (1985) 199]. Both chromatographic and immunological procedure were cited and compared. Methods for individual and simultaneous quantitation of standard antiepileptic drugs and their metabolites were considered. In addition, a discussion of free drug determination and procedures for new candidate antiepileptic drugs were included.

Advances in liquid chromatography and related methodologies for therapeutic drug monitoring

In this article the merits of current liquid chromatography (LC) columns and techniques are reviewed, to include the following topics: (1) a brief introduction to rational therapeutic drug monitoring (TDM) to justify drug measurements; (2) selected recent survey results from the College of American Pathologists (CAP) to establish the current utilization pattern of LC for TDM in the USA; (3) LC analyses of major classes of drugs--antiarrhythmics, antidepressants, antiepileptics, antimicrobials, cyclosporine, and others--with emphasis on analysis of these drugs in human serum or plasma, by focusing on the less usual, reversed-phase functional groups such as CN and phenyl, and by the use of "mini" columns, silica and polymeric columns, the emphasis being reduced on the well-established C-18 columns; (4) high-speed LC; (5) various approaches of direct sample analysis--solvent extraction, automated sample processing, column switching, micro-injections, micellar chromatography, electrochemical detection with photolytic derivation, and the internal surface reversed-phase column of Pinkerton; (6) microbore LC drug analysis; (7) clinical chiral separation; and (8) overall conclusions.