Automated analysis of free and total concentrations of three antiepileptic drugs in plasma with on-line dialysis and high-performance liquid chromatography

Free phenytoin assessment in patients: measured versus calculated blood serum levels

BACKGROUND

Total serum drug levels are routinely determined for the therapeutic drug monitoring of selected, difficult-to-dose drugs. For some of these drugs, however, knowledge of the free fraction is necessary to adapt correct dosing. Phenytoin, with its non-linear pharmacokinetics, >90 % albumin binding and slow elimination rate, is such a drug requiring individualization in patients, especially if rapid intravenous loading and subsequent dose adaptation is needed. In a prior long-term investigation, we showed the excellent performance of pharmacy-assisted Bayesian forecasting support for optimal dosing in hospitalized patients treated with phenytoin. In a subgroup analysis, we evaluated the suitability of the Sheiner-Tozer algorithm to calculate the free phenytoin fraction in hypoalbuminemic patients.

OBJECTIVE

To test the usefulness of the Sheiner-Tozer algorithm for the correct estimation of the free phenytoin concentrations in hospitalized patients.

SETTING

A Swiss tertiary care hospital.

METHOD

Free phenytoin plasma concentration was calculated from total phenytoin concentration in hypoalbuminemic patients and compared with the measured free phenytoin. The patients were separated into a low (35 ≤ albumin ≥ 25 g/L) and a very low group (albumin <25 g/L) for comparing and statistically analyzing the calculated and the measured free phenytoin concentration.

MAIN OUTCOME MEASURES

Calculated and the measured free phenytoin concentration.

RESULTS

The calculated (1.2 mg/L (SD = 0.7) and the measured (1.1 mg/L (SD = 0.5) free phenytoin concentration correlated. The mean difference in the low and the very low albumin group was: 0.10 mg/L (SD = 1.4) (n = 11) and 0.13 mg/L (SD = 0.24) (n = 12), respectively. Although the variability of the data could be a bias, no statistically significant difference between the groups was found: t test (p = 0.78), the Passing-Bablok regression, the Spearman's rank correlation coefficient of r = 0.907 and p = 0.00. The Bland-Altman plot including the regression analysis revealed no systematic differences between the calculated and the measured value [M = 0.11 (SD = 0.28)].

CONCLUSION

In absence of a free phenytoin plasma concentration measurement also in hypoalbuminemic patients, the Sheiner-Tozer algorithm represents a useful tool to assist therapeutic monitoring to calculate or control free phenytoin by using total phenytoin and the albumin concentration.

Measurement of drug-protein dissociation rates by high-performance affinity chromatography and peak profiling

The rate at which a drug or other small solute interacts with a protein is important in understanding the biological and pharmacokinetic behavior of these agents. One approach that has been developed for examining these rates involves the use of high-performance affinity chromatography (HPAC) and estimates of band-broadening through peak profiling. Previous work with this method has been based on a comparison of the statistical moments for a retained analyte versus nonretained species at a single, high flow rate to obtain information on stationary phase mass transfer. In this study an alternative approach was created that allows a broad range of flow rates to be used for examining solute-protein dissociation rates. Chromatographic theory was employed to derive equations that could be used with this approach on a single column, as well as with multiple columns to evaluate and correct for the impact of stagnant mobile phase mass transfer. The interaction of L-tryptophan with human serum albumin was used as a model system to test this method. A dissociation rate constant of 2.7 (+/-0.2) s(-1) was obtained by this approach at pH 7.4 and 37 degrees C, which was in good agreement with previous values determined by other methods. The techniques described in this report can be applied to other biomolecular systems and should be valuable for the determination of drug-protein dissociation rates.

Analysis of free drug fractions using near-infrared fluorescent labels and an ultrafast immunoextraction/displacement assay

A chromatographic method was developed for measuring free drug fractions based on the use of an ultrafast immunoextraction/displacement assay (UFIDA) with near-infrared (NIR) fluorescent labels. This approach was evaluated by using it to determine the free fraction of phenytoin in serum or samples containing the binding protein human serum albumin (HSA). Items considered in the design of this method included the dissociation rate of HSA-bound phenytoin, the rate of capture of free phenytoin by immunoextraction microcolumns, the behavior of NIR fluorescent labels in a displacement format, and the overall response and stability of the resulting assay. In the final UFIDA method, the free fraction of phenytoin was extracted in approximately 100 ms by a microcolumn containing a small layer of anti-phenytoin antibodies. This gave a displacement peak for a NIR-fluorescent-labeled analogue of phenytoin that appeared within 2-3 min of sample injection, creating a signal proportional to the amount of free phenytoin in the sample. The UFIDA method provided results within 1-5% of those determined by ultrafiltration for reference samples. The lower limit of detection was 570 pM, and the linear range extended up to 10 microM. This approach is not limited to phenytoin but can be adapted for other analytes through the use of appropriate antibodies and labeled analogues.

Automated determination of free mycophenolic acid and its glucuronide in plasma from renal allograft recipients

Mycophenolic acid, the active moiety of mycophenolate mofetil, inhibits the enzyme inosine monophosphate dehydrogenase. The main metabolite, mycophenolic acid glucuronide, has no immunosuppressive effect. Reported protein bindings are 97% for mycophenolic acid and 82% for mycophenolic acid glucuronide. Considerable intraindividual and interindividual variability in mycophenolic acid pharmacokinetics has been observed. Data on the variability of mycophenolic acid free fraction in plasma are sparse but may be relevant when discussing whether therapeutic drug monitoring of this drug is warranted. The authors describe a fully automated method for the determination of free concentrations by dialysis across a membrane followed by concentration of the dialysate on a trace enrichment column and liquid chromatography. Total concentrations are measured by protein precipitation and direct injection on the trace enrichment column. Plasma concentrations as low as 6 ng/mL free mycophenolic acid and 1 microg/mL free mycophenolic acid glucuronide can be measured with between-day coefficient of variation less than 15% and 6%, respectively. Stability testing confirmed that plasma samples could be stored for 14 days at 4 degrees C or -20 degrees C and at room temperature for approximately 12 hours without significant changes in free concentrations. Predose total and free concentrations of mycophenolic acid and mycophenolic acid glucuronide were determined in 27 samples from stable renal allograft recipients treated with mycophenolate mofetil, cyclosporin, and steroids. Total concentrations ranged from 0.57 to 16.2 microg/mL mycophenolic acid and 36 to 199 microg/mL mycophenolic acid glucuronide. Free concentrations ranged from 13 to 210 ng/mL mycophenolic acid and 8 to 58 microg/mL mycophenolic acid glucuronide. The method presented here has been successfully applied to measure free mycophenolic acid and free mycophenolic acid glucuronide in clinical samples. Further investigations may provide important data to support the identification of principles and target ranges for the monitoring of mycophenolic acid in the immunosuppressive therapy of organ transplant recipients.

Strategy for the development of automated methods involving dialysis and trace enrichment as on-line sample preparation for the determination of basic drugs in plasma by liquid chromatography

Among the sample preparation techniques, dialysis followed by clean-up and enrichment of the dialysate on a pre-column has proved to be a useful approach for the LC determination of drugs in plasma. By use of sample processors, like the ASTED system, such bioanalytical methods can be fully automated, the dialysis and trace enrichment steps being directly coupled to LC. In order to facilitate the development of such automated methods, a strategy based on a decision tree has been elaborated. After the selection of appropriate conditions for the LC analysis, the decision tree provides information about suggested starting conditions and guidelines for the optimisation of the most important parameters likely to influence analyte recovery and method selectivity. The plasma samples are dialysed on a cellulose acetate membrane in the static-pulsed mode and the dialysate is enriched on a trace enrichment pre-column packed with octadecyl silica or with a strong cation-exchange material. This decision tree is until now restricted to the analysis of basic drugs in plasma. In order to demonstrate the applicability of this method development strategy, an automated procedure based on the coupling of dialysis with trace enrichment has been developed for the LC determination of antifungal agents (clotrimazole, econazole and miconazole) in plasma.

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.

On-line dialysis as a sample preparation technique for ion chromatography

The use of on-line dialysis as a sample preparation technique for ion chromatography is described. A fully automated sample preparation device coupled to an ion chromatographic system for the determination of anions and cations in various matrices is presented. The method was based on stopped-flow dialysis, where the samples were continuously dialyzed for 10 min while the acceptor solution was stationary within the recipient channel. The matrices examined, without additional sample treatment, included milk, untreated wastewater, fruit juice, engine coolant, and a multivitamin tablet. The analyte recoveries for anions and cations in various matrices ranged from 87 to 106%. In addition, multiple sample injections were performed and repeatabilities were found in the range of 0.2 to 4%.

Development and validation of an automated method for the liquid chromatographic determination of sotalol in plasma using dialysis and trace enrichment on a cation-exchange pre-column as on-line sample preparation

A fully automated method for the determination of sotalol in human plasma was developed, involving dialysis through a cellulose acetate membrane, clean-up and enrichment of the dialysate on a strong cation-exchange pre-column and subsequent liquid chromatographic (LC) analysis with UV detection. All sample handling operations were carried out by means of an ASTED system. Before starting dialysis, the trace enrichment column (TEC) was conditioned. The plasma sample, to which the internal standard (atenolol) was automatically added, was then loaded in the donor channel and was kept static while the dialysis liquid, consisting of 0.017 M acetic acid, was passed through the acceptor channel in successive pulses. After each pulse, the dialysate was dispensed onto the TEC. When dialysis was discontinued, the analytes were eluted from the TEC by the LC mobile phase by rotation of a switching valve and transferred to the analytical column packed with octyl silica. The LC mobile phase was a mixture of methanol and pH 7.0 phosphate buffer containing 1-octanesulfonate at a concentration of 7.5 x 10(-4) M (19:81; v/v). The UV detection was performed at 230 nm. The influence of several parameters of the dialysis and trace enrichment processes on analyte recovery and method selectivity was investigated. The method was then validated. The mean absolute recovery for sotalol was about 60%. The limit of quantitation was 25 ng/ml and R.S.D. for repeatability and intermediate precision obtained at a concentration level of 50 ng/ml were 4.3 and 5.8%, respectively.

Immunoassay-based determination of phenobarbital using size-exclusion chromatography

The measurement of the anti-epileptic drug phenobarbital from serum samples combining immunoassay and size-exclusion chromatography is presented. The immunoreaction is based on the competitive binding of the analyte (unlabelled phenobarbital) and the fluorescent-labelled phenobarbital to anti-phenobarbital antibodies. Mixing of the reagents and the immunoreaction takes place in a flow system. The products are separated on-line on a short gel chromatographic column and the fluorescence intensity of the marker is measured. The calibration curve shows good linearity in the range 5-80 microg/ml, corresponding to therapeutically relevant serum levels. Intra-day precision values are between 7.32 and 9.48%; the accuracy is between 0.97 and 9.43%. Inter-day precision and accuracy measured on 6 different days fall between 5.38 and 10.05% and -8.27 and -4.97%, respectively. The results obtained with the proposed method show a good correlation with those of other methods (radioimmunoassay and fluorescence polarisation immunoassay) already established in clinical laboratories.

Determination of free levels of phenytoin in human plasma by liquid chromatography/tandem mass spectrometry

A liquid chromatography combined with tandem mass spectrometry assay for the determination of free levels of the highly protein bound drug phenytoin (5,5-diphenylhydantoin) in human plasma is described. The assay was demonstrated to be reliable, accurate and precise, and specific for phenytoin. The procedure involves isolation of the unbound drug from the drug/protein complex by ultrafiltration. Liquid-liquid extraction was employed to extract the resultant ultrafiltrate. PHT was separated on a 50 x 3 mm reversed-phase column using isocratic mobile phase conditions that yielded a run time of 1.5 min, enabling high throughput sample analysis. Linearity was obtained over the range 5.00 to 500 ng/ml. Both between-run and within-run coefficients of variation were less than 15% and accuracy's across the assay range were all within 100 +/- 10%. The assay was successfully implemented to support a clinical interaction study with phenytoin.

Determination of phenobarbital in plasma by micellar liquid chromatography

A new liquid chromatographic procedure for the determination of phenobarbital in plasma samples is described. The proposed system uses a Spherisorb octadecyl-silane ODS-2 C(18) analytical column, a guard column of similar characteristics, and a 0.03 M CTAB-3% 1-propanol at pH 7 mobile phase. The UV detector was set at 250 nm. Butabarbital was used as internal standard. Sample preparation only required the addition to the plasma samples of a 0.1 M SDS solution at pH 3 and centrifugation before injection into the chromatographic system. The limit of detection was 0.83 microg/mL of phenobarbital in plasma samples. The coefficients of variation were lower than 7. 5%.

Automated liquid chromatographic determination of atenolol in plasma using dialysis and trace enrichment on a cation-exchange precolumn for sample handling

A fully automated method involving dialysis combined with trace enrichment was developed for the liquid chromatographic (LC) determination of atenolol, a hydrophilic beta-blocking agent, in human plasma. The plasma samples were dialysed on a cellulose acetate membrane and the dialysate was reconcentrated on a short trace enrichment column (TEC) packed with a strong cation-exchange material. All sample handling operations can be executed automatically by a sample processor (ASTED system). After TEC conditioning, the plasma sample, to which the internal standard (sotalol, another hydrophilic beta-blocker) was automatically added, was introduced in the donor channel and dialysed in the static/pulsed mode. The dialysis liquid consisted of 4.3 mM phosphoric acid. When the dialysis process was discontinued, the analytes were eluted from the TEC in the back-flush mode by the LC mobile phase and transferred to the analytical column, packed with octyl silica. The LC mobile phase consisted of phosphate buffer, pH 7.0-methanol (81:19; v/v) with 1-octanesulfonate. Atenolol and the internal standard were monitored photometrically at 225 nm. The different parameters influencing the dialysis and trace enrichment processes were optimised with respect to analyte recovery. The influence of two different kinds of cation-exchange material on analyte recovery and peak efficiency was also studied. The method was then validated in the concentration range 25-1000 ng/ml. The mean recovery for atenolol was 65% and the limit of quantitation was 25 ng/ml.

Determination of albendazole and its main metabolites in ovine plasma by liquid chromatography with dialysis as an integrated sample preparation technique

Albendazole is a benzimidazole derivative with a broad-spectrum activity against human and animal helminth parasites. In order to determine the main pharmacokinetic parameters in sheep after oral and intravenous administration of a new formulation of albendazole (an aqueous solution), a fully automated method was developed for the determination of this drug and its main metabolites, albendazole sulfoxide (active metabolite) and sulfone in ovine plasma. This method involves dialysis as purification step, followed by enrichment of the dialysate on a precolumn and liquid chromatography (LC). All sample handling operations were executed automatically by means of an ASTED XL system. After conditioning of the trace enrichment column (TEC) packed with octadecyl silica with pH 6.0 phosphate buffer containing sodium azide, the plasma sample, in which a protein releasing reagent (1 M HCl) containing Triton X-100 was automatically added, was loaded in the donor channel and dialysed on a cellulose acetate membrane in the static-pulsed mode. The dialysis liquid consisted of pH 2.5 phosphate buffer. By rotation of a switching valve, the analytes were eluted from the TEC in the back-flush mode by the LC mobile phase and transferred to the analytical column, packed with octyl silica. The chromatographic separation was performed at 35 degrees C and the analytes were monitored photometrically at 295 nm. Due to the differences in hydrophobic character between albendazole and its metabolites, a gradient elution was applied. The mobile phase consisted of a mixture of acetonitrile and pH 6.0 phosphate buffer. The proportion of organic modifier was increased from 10.0 to 50.1% in 12.30 min, then from 50.1 to 66.9% in 1.70 min. First, the gradient conditions and the temperature were optimised for the LC separation using the DryLab software. Then, the influence of some parameters of the dialysis process on analyte recovery was investigated. Finally, the method developed was validated. The mean recoveries for albendazole and its metabolites were about 70 and 65%, respectively. The limits of quantification for albendazole and its metabolites were 10 and 7.5 ng/ml, respectively.

Membrane-based sample preparation coupled on-line to chromatography or electrophoresis

A review on the use of membranes for on-line sample preparation prior to chromatographic and electrophoretic analysis is provided. The current state-of-the-art of four membrane-based techniques (dialysis, electrodialysis, filtration and membrane extraction) is described by reviewing their principles and applications. Possible future developments are discussed.

Preconcentration of quinolones by dialysis on-line coupled to high-performance liquid chromatography

The parameters influencing dialytic separation of ciprofloxacin (CF) fluoroquinolone were investigated. Dialysis with a porous cellulose acetate membrane was on-line coupled with HPLC and the analysis of dialysate was made by isocratic ion-pairing liquid chromatography using a reversed-phase analytical column and fluorescence detection. Optimisation of the experimental conditions for selective dialytic enrichment are described and explanations of some phenomena affecting dialysis efficiency discussed. By the use of a neutral donor (pH approximately 7) and acidic acceptor solution (pH < 4) a substantial enrichment of quinolones was achieved. Accumulation of CF in the acidic acceptor phase is based on the protonation of the analyte in the acceptor compartment. Continuous-flow of donor solution and a stagnant acceptor solution gave high dialysis efficiency in 5-15 min. Effects of interfering substances present in real samples on the variation of dialysis efficiency can be minimised by successive dialysis runs of the original and spiked samples.

Influence of fever on cefazolin pharmacokinetics

The influence of fever on the pharmacokinetics of cefazolin was investigated in patients with acute febrile diseases. Nine patients were included in the study. Antibiotic serum concentrations were determined using high performance liquid chromatograpy (HPLC). An analog computer and the SIMULINK software package were used to identify the pharmacokinetic model and PCNONLIN software package to obtain the secondary parameters. In 6 patients a two-compartment pharmacokinetic model of cefazolin was observed during fever and after defervescence. In 2 patients a two-compartment model changed to a one-compartment after defervescence, and a one-compartment model was observed in one patient during both periods. Cefazolin-treated patients with a two-compartment model (6/9) had higher Cmax, mean steady state serum concentrations (Css), and area under the plasma concentration-time curve (AUC(0-->infinity)), smaller central compartment volume (V1), and lower clearance (Cl) during fever. The varying distribution of antibiotics during fever probably reflects different hemodynamic responses to fever.

Column switching in capillary liquid chromatography-tandem mass spectrometry for the quantitation of pg/ml concentrations of the free basic drug tolterodine and its active 5-hydroxymethyl metabolite in microliter volumes of plasma

A capillary column switching system was developed for the determination of low, unbound concentrations of the basic drug tolterodine and its active 5-hydroxymethyl (5-HM) metabolite in human plasma. Free concentrations of tolterodine and 5-HM at pM and nM (pg/ml and ng/ml) levels were obtained by ultrafiltration of 40-400 microliters plasma at 37 degrees C. The free fraction (%) was independent of the plasma concentrations of the analytes. Detection of the analytes was performed by sheathless electrospray tandem mass spectrometry in the multiple-reaction monitoring mode. The selectivity of the mass spectrometric detection and the additional clean-up on the pre-column allowed direct injection of the ultrafiltrated plasma samples. Tolterodine and 5-HM were pre-concentrated on a reversed-phase capillary pre-column (1 cm x 200 microns) and subsequently backflushed onto the separation column (25 cm x 200 microns). The stability of the chromatographic system was good; a large number of ultrafiltrated plasma samples could be injected and the relative standard deviation of the retention times was typically < or = 1% (within-day). The accuracy was between 86 and 105% and the precision was between 1 and 7% without the use of an internal standard. Linear calibration curves were obtained between 100 pM and 100 nM.

Automated on-line dialysis and liquid chromatography of methylenedioxylated amphetamines in plasma and serum samples

An automated on-line dialysis coupled to a trace enrichment method has been developed for the separation and quantification of four methylenedioxylated amphetamines in serum and plasma, using liquid chromatography coupled to a fluorimetric detector. The on-line dialysis method was optimized and validated on fresh human serum and plasma samples. This sample preparation method allowed the quantification of methylenedioxylated amphetamines in serum or plasma, at concentrations as low as ca. 10 ng ml-1, with good repeatability, reproducibility and accuracy. The automated on-line dialysis method took less than 30 min. This method was applied to seven toxicological cases and results showed that the concentration of methylenedioxylated amphetamines in blood was in the range of 20-484 ng ml-1.

Monitoring of phenytoin in human breast milk, maternal plasma and cord blood plasma by solid-phase extraction and liquid chromatography

A rapid liquid chromatographic method for the quantitation of phenytoin in human breast milk, maternal plasma and cord blood plasma was developed using a Develosil C85 micron reverse phase column and a potassium dihydrogen phosphate buffer/acetonitrile mobile phase. Phenytoin and mephenytoin as an internal standard were detected by ultraviolet absorbance at 240 nm. The sample preparation method involves a rapid and simple procedure based on solid-phase extraction using a C18-bonded phase. Phenytoin could be determined in the concentration range of 0.05-3 micrograms ml-1. The recovery of phenytoin added to human breast milk and plasma were 91.6-94.7 and 91.6-96.0%, respectively, with coefficient of variation less than 4.2 and 8.7%. The method has been used for drug level monitoring in the human breast milk, maternal plasma and cord blood plasma samples that were taken from patients treated with phenytoin. The average ratio between the breast milk concentrations versus the plasma concentration was 0.28 +/- 0.1, with a rather poor correlation (r = 0.3033).

Automated on-line dialysis for sample preparation and HPLC analysis of antidepressant drugs in human plasma. Inhibition of interaction with the dialysis membrane

Antidepressant drugs interact with the dialysis membrane and were selected as model substances to study inhibition of analyte-membrane interactions. A chemometric approach based on response surface modelling was used for screening and optimisation of dialysis recoveries. Optimal dialysis recoveries (52-65%) were obtained for the model compounds (mianserine, amitriptyline, nortriptyline, imipramine and desimipramine) when a cationic surfactant added to the donor solution of the dialyser was used to inhibit analyte-membrane interactions. Automated analysis of antidepressants in plasma was performed by connecting the ASTED (Automated Sequential Trace Enrichment of Dialysates) system to high-performance liquid chromatography (HPLC). The drugs were detected by ultraviolet detection and fluorescence detection after post-column photochemical reaction. Validation of the method showed linear standard curves for all the drugs in the concentration range 50-2000 nmol 1-1. Within-and between day relative standard deviations ranged from 1.1 to 5.7%.

Enantioselective determination of oxprenolol in human plasma using dialysis coupled on-line to reversed-phase chiral liquid chromatography

A fully automated method for the determination of the enantiomers of oxprenolol in human plasma was developed, involving dialysis through a cellulose acetate membrane, clean-up and enrichment of the dialysate on a short precolumn and subsequent chiral liquid chromatographic (LC) analysis. All sample handling operations were executed automatically by a sample processor equipped with a robotic arm (ASTED system). The trace enrichment column (TEC) was packed with octadecylsilica. After conditioning of the TEC with the LC mobile phase and pH 3.0 acetate buffer. After the enrichment step, the analyte was transferred by the LC mobile phase to the analytical column by means of a switching valve. The influence of different parameters of the dialysis process on the recovery of oxprenolol was first investigated using achiral LC conditions. The volume as well as the aspirating and dispensing flow rates of the acceptor solution were the main parameters studied. Oxprenolol was separated on a C18 stationary phase used for the enantioseparation of oxprenolol was a Chiralcel OD-R column which contained cellulose tris (3,5-dimethylphenylcarbamate) coated on silica as chiral selector. The corresponding mobile phase consisted of a mixture of pH 6.0 phosphate buffer containing NaClO4 at 0.45 M concentration and acetonitrile (70:30 v/v). UV detection was performed at 273 nm. The method developed was validated. Recoveries for each enantiomer of oxprenolol were about 80%. The method was found to be linear in the 50-2500 ng ml-1 concentration range (r2 = 0.999 for both enantiomers) and good results with respect to intra- and inter-day reproducibility as well as accuracy were obtained.

Automated on-line dialysis, trace enrichment and high-performance liquid chromatography. Inhibition of interaction with the dialysis membrane and disruption of protein binding in the determination of clozapine in human plasma

Problems related to interaction of drugs with the dialysis membrane and to protein binding must be overcome in order to develop automated methods for drug analysis based on on-line dialysis, trace enrichment and HPLC. In order to study these problems, clozapine and its active metabolite N-desmethylclozapine were chosen as model compounds because they were found to interact with the dialysis membrane, and clozapine is highly protein bound. Addition of a cationic surfactant, dodecylethyldimethyl ammonium bromide, to the donor solution and to the plasma samples was found to inhibit interaction of the drugs with surfaces. The protein binding in plasma was disrupted prior to dialysis by lowering the pH with hydrochloric acid and the plasma proteins were solubilised with glycerol. The results obtained were used to develop a fully automated method for the determination of clozapine and N-desmethylclozapine in human plasma. More than 100 samples could be analysed within 24 h. The limit of detection in human plasma was 0.050 mumol/l for clozapine and 0.055 mumol/l for N-desmethylclozapine. Linearity was found for drug concentrations between 0.25-3 mumol/l. The relative standard deviations were between 1.2-6.7% and the method was applicable for therapeutic drug monitoring.

Automated analysis of levosimendan in human plasma by on-line dialysis and liquid chromatography

A fully automated method for quantitation of levosimendan, (R)-(-)-[[4-(1,4,5,6-tetrahydro-4-methyl-6-oxo-3-pyridazinyl) phenyl]hydrazono]propanedinitrile, in human plasma is described. The method involves on-line dialysis of the samples, trace enrichment of the dialysates, and reversed-phase high-performance liquid chromatography with UV detection at 380 nm. An internal standard was used to compensate for variations in the dialysis rate caused by temperature fluctuations. The precision and accuracy of the method were good. The between-day variation (RSD) was 2.7% at a plasma concentration of 15 ng/mL and 1.7% at 450 ng/mL. The limit of quantitation was 5 ng/mL with an RSD of 4.0%. The completion time of the assay was 19 min.

High-speed liquid chromatographic method for the monitoring of carbamazepine and its active metabolite, carbamazepine-10,11-epoxide, in human plasma

The assays of antiepileptic drugs, which are performed by central laboratories in Phase II and III clinical trials, require both a very fast turn-around time and a suitable specificity. In order to decrease the run time and to keep the powerful specificity of the liquid chromatography (HPLC), the use of a reversed-phase 1.5 microns monosized non-porous silicon dioxide microspheres column instead of regular columns containing spherical porous C18 material was studied. The determination of carbamazepine (CBZ) and its active metabolite, carbamazepine-10,11-epoxide (CBZ-E), in human plasma or serum was chosen to demonstrate the utility of these columns. As a prerequisite of this work, no modification of a regular HPLC system was allowed. The samples were prepared in autosampler vials by protein precipitation with acetonitrile, followed by a quick centrifugation. Without any change to a conventional HPLC system, CBZ and CBZ-E are well separated in less than 2.5 min using a Kovasil MS C14 column. No interference was observed with endogenous compounds and with nine antiepileptic drugs commonly prescribed as co-medication, and their metabolites. Due to the very low specific surface area of the packing, the required organic modifier volume per chromatographic run was decreased by a factor of 25. The method was validated. The developed method is well suited for the determination of CBZ and CBZ-E in clinical trials. It can be easily adapted to the monitoring of other antiepileptic drugs. No modification of a regular HPLC system was required.

Solid-phase microextraction for the determination of the free concentration of valproic acid in human plasma by capillary gas chromatography

The potential of solid-phase microextraction in the bioanalysis of drugs is demonstrated. The free concentration of valproic acid in human plasma was determined by equilibrium dialysis at room temperature. To the dialysate was added an internal standard and the pH was adjusted to 2.5. The polymethylsiloxane-coated fused-silica fibre of the solid-phase microextraction device was inserted into the dialysate for 3 min. The sorbed analytes were then thermally desorbed at 210 degrees C in the split-splitless injection port of the gas chromatograph, separated on a Nukol capillary column and detected with a flame ionization detector. The method was shown to be highly reproducible with a detection limit of 1 microgram/ml of free valproic acid in human plasma.