A rapid procedure for the simultaneous estimation of the anticonvulsant drugs, ethosuximide, phenobarbitone, phenytoin, and carbamazepine in serum using high-pressure liquid chromatography

Development of a useful single-reference HPLC method for therapeutic drug monitoring of phenytoin and carbamazepine in human plasma

A single reference high-performance liquid chromatographic (SR-HPLC) method was developed and validated for the therapeutic drug monitoring (TDM) of phenytoin (PHT) and carbamazepine (CBZ) in plasma from patients. The analytical parameters evaluated were linearity, limit of quantification (LOQ), selectivity, accuracy, and stability according to the US Food and Drug Administration (FDA) guideline. The developed method shows good linearity (r2 > 0.999; LOQ-50 µg/mL), and LOQ values were 1.56 µg/mL for PHT and 0.40 µg/mL for CBZ at 254 nm. For the development of SR-HPLC method, we evaluated to improve the detection wavelength, stirred retention time, and stability for SR, and selected 5-(p-methylphenyl)-5-phenylhydantoin for PHT (relative molar sensitivity, RMS = 0.848) and 10-methoxyiminostilbene for CBZ (RMS = 0.263). The established differential definite quantities of PHT and CBZ in plasma samples were similar using the RMS and absolute calibration methods based on RSD < 5.10%. A preliminary application was performed using chemiluminescent immunoassay and SR-HPLC method, in which the detectable values of the correlation coefficient and the slope of the intercept were PHT: 0.964 and 0.992647, and CBZ: 0.969 and 1.072089, respectively. Based on these results, we propose that the SR-HPLC method with RMS would be offered to the useful and accurate TDM of various medicines in plasma/serum samples.

[Therapeutic drug monitoring of ethosuximide]

Ethosuximide is a minor antiepileptic drug, available in France since 1965, indicated in the epilepsy absence, whose interest was reassessed from recent clinical trials, showing that it was the first choice, in term of risk benefit relationship, in this indication. It is a chiral molecule that presents a high bioavailability, a lack of protein binding, hepatic metabolism and urinary excretion. Its elimination half-life is long, between 40 and 60 h in adults, 30 and 40 h in children. The therapeutic range is established at 40-100 mg/L (283-708 µmol/L), but the upper limit is probably underestimated. The clinical studies of relation exposure effects, although ancient (from the 1970s) and realized with methodologies that do not meet current criteria, show concentration-efficacy and -toxicity relationship and the risk of drug interactions is proven. It is a drug preponderantly prescribed in children, a vulnerable population with physiological change with age. To benefit at best of its effectiveness, it is necessary to have relatively high plasma concentrations. Despite these arguments and due to the lack of studies providing a sufficient level of evidence, the recommendation can only be "potentially useful", assessment probably underestimated.

Automated sequential trace enrichment of dialysates and robotics. A technique for the preparation of biological samples prior to high-performance liquid chromatography

The development of the sample preparation process, the automated sequential trace enrichment of dialysates, in association with a cartesian robotic sampler is described. The system has been applied to the total automation of the preparation of biological samples and high-performance liquid chromatographic analysis. Concepts of the technique are reported together with an examination of its application to free and total analyte estimation. Examples of chromatographic separations obtained from the preparation of a variety of different analytes and sample materials are given.

Rapid measurement of anticonvulsant drug concentrations in the out-patient clinic, using HPLC with direct injection of plasma

A manual column-switching technique is described for the measurement of phenytoin, phenobarbitone, carbamazepine, and carbamazepine 10,11-epoxide. The analytical system is designed to be portable for use at the out-patient clinic and comprises an isocratic pump, UV detector and injection valve, together with a preparation column. Diluted plasma or serum is injected, without pre-extraction, onto a preparation column which replaces the sample loop on the injection valve. After washing unwanted material to waste, the preparation column is switched in-line with the analytical column, where separation of analytes occurs. The precision, accuracy and carryover of this extra-laboratory system are comparable with those obtained with laboratory-based immunoassay systems. Operation of the system allows the reporting of results within 5 min of sample injection and requires no specialist skills. The technique should be of particular interest to district general hospital laboratories where workload does not justify the cost of an automated HPLC system as the total capital cost is comparable to that of a portable glucose analyser. In contrast to immunoassay systems consumable costs are minimal. The equipment is easy to transport and may be used in the out-patient department to provide an analytical service similar to that provided for the determination of prothrombin time at the anticoagulant clinic.

Automated sequential process for preparing samples for analysis by high-performance liquid chromatography

A new method for the preparation of biological samples prior to high-performance liquid chromatographic analysis is described. The automated technique combines dialysis and trace enrichment. In order to investigate its potential, it has been applied to the estimation of total anticonvulsants and theophylline in serum.

A study on carbamazepine levels, including estimation of 10-11 epoxy-carbamazepine and levels in free plasma and saliva

Blood and saliva were collected from 48 children, aged three to 17 years, before a morning dose of carbamazepine and then at three, six and eight hours afterward. There was no significant difference in the ratio of 10-11 epoxy-carbamazepine to total carbamazepine for children on carbamazepine alone (0.18) and those on an additional anticonvulsant (0.19). 11 children were studied on their first day of therapy. Though four of them achieved levels of total carbamazepine which would have given a readily measurable 10-11 epoxide on the above ratio, 10 of the 11 consistently had levels less than 4 mumol/l. This may be of help in detecting non-compliance. For 16 children free carbamazepine also was estimated and there was an over-all correlation coefficient of 0.85 between saliva and free-drug levels. The proportion of free to total carbamazepine varied between 19 and 54 per cent. There was a significantly higher proportion of free drug at the pre-dose sampling time than at other times. Protein binding of carbamazepine therefore is very variable and saliva estimations can contribute usefully to the monitoring of drug levels.

Simultaneous determination of fluzinamide and three of its active metabolites in plasma by high-performance liquid chromatography

A sensitive and selective high-performance liquid chromatographic method has been developed for a new anticonvulsant, fluzinamide, and three of its active metabolites. This method requires only 0.5 ml of plasma, and it involves a single extraction with a mixture of hexane--dichloromethane--butanol (55:40:5). The plasma extract is chromatographed on a 10-micron, C18 reversed-phase column and quantitated by ultraviolet absorbance at 220 nm. The concentration--response curves for all four compounds are linear from 0.05 micrograms/ml to at least 10 micrograms/ml. The extraction efficiency of this method is greater than 90%. The accuracy and precision of the method were tested by analyzing spiked unknown samples that had been randomly distributed across the concentration range. The mean concentrations found were within +/- 9% of the various amounts added with a standard deviation of +/- 3.5%. This method has been successfully applied to the analysis of samples obtained from fluzinamide-dosed dogs, healthy unmedicated volunteers, and patients who were at steady state with phenytoin, carbamazepine, and fluzinamide.

Routine determination of eight common anti-epileptic drugs and metabolites by high-performance liquid chromatography using a column-switching system for direct injection of serum samples

A simple and rapid high-performance liquid chromatographic method for determining eight common anti-epileptic drugs and metabolites in serum is described. A column-switching system including one analytical column and two precolumns for sample enrichment offers the possibility of directly injecting patients' sera without any pretreatment. The two precolumns are alternately switched over to avoid time loss in analysis due to the sample washing step. The samples are flushed with dilute phosphoric acid, as the purge liquid, onto the precolumns which consist of very short cartridges (length 0.5 cm) filled with spherical ODS silica gel (particle size 30 micron). The retained substances are carried over, after purification, onto the analytical column in the same direction of flow as in the flushing step. A mixture of acetonitrile and phosphoric acid--sodium phosphate buffer solution is thereby used as solvent for the gradient elution. The separation was carried out using an analytical column, which was filled with ODS material of particle size 5 micron.