Carbamazepine-10,11-epoxide in therapeutic drug monitoring

Clinical utility of free drug monitoring

Most drugs are bound to serum proteins to a various degree. Only unbound or free drug is pharmacologically active. Usually total drug is measured for therapeutic monitoring because there is equilibrium between bound and free drugs, and concentration of free drug can be predicted from total drug concentration. However, under certain conditions this equilibrium is disturbed and the measured free drug concentration can be significantly higher than expected from total drug concentrations, especially for strongly protein-bound drugs. In such case a patient may experience drug toxicity even if the total drug concentration is within the therapeutic range. Conditions like uremia, liver disease and hypoalbuminemia can lead to significant increases in free drug concentration. Therefore, monitoring free phenytoin and free valproic acid is recommended in these patients. Drug-drug interactions can also lead to a disproportionate increase in free drug concentration. One strongly protein-bound drug can significantly displace another strongly protein-bound drug if both drugs share the same binding site. Several over-the-counter pain medications such as salicylate, naproxen, and ibuprofen can cause significant displacement of both phenytoin and valproic acid from albumin binding site. Interestingly, such interactions are absent in uremic patients. Elderly patients may have increased free phenytoin or valproic acid due to hypoalbuminemia. Elevated free phenytoin concentrations have also been reported in patients with AIDS. Although digoxin is 25% bound to protein, monitoring free digoxin is useful in patients with elevated endogenous digoxin-like immunoreactive substances or in patients overdosed with digoxin and being treated with digibind. Monitoring free digoxin can also eliminate interference of Chinese medicines Chan Su and Danshen in serum digoxin measurement by certain immunoassays. However, free drug monitoring is not a routine procedure in clinical laboratories due to technical difficulties and lack of established reference ranges for free drugs.

Octakis-6-sulfato-gamma-cyclodextrin as additive for capillary electrokinetic chromatography of dibenzoazepines: carbamazepine, oxcarbamazepine and their metabolites

Single isomer octakis-(2,3-dihydroxy-)6-sulfato-gamma-cyclodextrin used as pseudostationary phase of the background electrolyte interacts with dibenzo[b,f]azepines (consisting of a condensed 3-ring system) and forms negatively charged complexes. Hydroxygroups in position 2 and 3 at carbamazepine increase the extent of interaction, whereas substitution by oxygen at position 10 and/or 11 reduces it. The complex constants for the analytes are ranging from few tens L/mol (10-hydroxycarbamazepine, 10,11-dihydroxycarbamazepine, 10,11-epoxycarbamazepine, oxcarbazepine) to several hundreds L/mol (carbamazepine, 2-hydroxycarbamazepine, 3-hydroxycarbamazepine), and are much larger than those of the analytes with octakis-(2,3-dimethyl-)-6-sulfato-gamma-cyclodextrin. Full enantiomeric separation of the chiral metabolites of carbamazepine and oxcarbazepine is obtained at octakis-(2,3-dihydroxy-)-6-sulfato-gamma-cyclodextrin concentrations of about 10 mM (3 mM borate buffer, pH 8.5). Compared to heptakis-6-sulfato-beta-cyclodextrin, selectivity differs and stereoselectivity is more pronounced.

Performance characteristics of four immunoassays for antiepileptic drugs on the IMMULITE 2000 automated analyzer

Carbamazepine, phenobarbital, phenytoin, and valproic acid are commonly used antiepileptic drugs that show complicated pharmacokinetic behavior Nonisotopic immunoassays are used routinely to monitor these drugs, and assay specificity is important to obtain accurate results. By using samples from subjects receiving each of these antiepileptic medications, competitive immunoassays for them were evaluated on an IMMULITE 2000 automated chemiluminescent analyzer (Diagnostic Products, Los Angeles, CA). Phenytoin assays were evaluated using an additional set of samples from patients with abnormal renal function. All 4 methods were linear, had imprecision of less than 10%, and compared well with other commercial immunoassays. A positive bias was observed for phenytoin measured in samples from uremic patients compared with a high-performance liquid chromatography reference method. The molar cross-reactivity of carbamazepine-10,11-epoxide was 12% in the carbamazepine assay. Phenytoin metabolites and fosphenytoin had substantial cross-reactivity in the phenytoin assay. All antiepileptic drug assays performed well and are suitable for use in monitoring patients receiving antiepileptic drug therapy. One possible exception is the phenytoin assay with samples from patients with renal insufficiency.

Comparison between the Cobas Integra immunoassay and high-performance liquid chromatography for therapeutic monitoring of carbamazepine

OBJECTIVES

Carbamazepine (CBZ) is metabolized by the cytochrome P450 into carbamazepine-10,11-epoxide (CBZepox), a metabolite with similar pharmacological activity to the parent drug. Recently it has been indicated that most current immunoassays for the determination of CBZ are unable to quantitatively measure its active epoxide. An evaluation of the Cobas Integra immunoassay for the determination of CBZ was carried out, and the results are compared with those obtained for CBZ+CBZepox using high-performance liquid chromatography (HPLC) in patients on monotherapy and polytherapy.

DESIGN AND METHODS

Steady-state serum trough concentrations of CBZ were determined in 119 epileptic patients using the Cobas Integra immunoassay and HPLC. In 91 cases CBZ was administered in monotherapy and in 28 cases in polytherapy with other anticonvulsant drugs.

RESULTS

The study of within- and between-run imprecisiom for the Cobas Integra immunoassay led to clinically acceptable coefficients of variation. A high correlation was found between the concentrations of CBZ obtained using the immunoassay and HPLC (r = 0.981, p < 0.001). In both the group of patients on monotherapy and those on polytherapy, the levels of CBZepox were greater than the clinically acceptable error for CBZ; consequently, there is a clinically significant difference between the total of CBZ+CBZepox concentrations (HPLC) and the concentrations of CBZ (immunoassay). In the group of patients under monotherapy, a high correlation coefficient was obtained between the levels of CBZ and CBZ+CBZepox (r = 0.975, p < 0.001) with an standard error of the estimate similar to the clinically acceptable value.

CONCLUSIONS

For the patients on monotherapy, it is possible to make a clinically valid estimation of CBZ+CBZepox from the concentration of CBZ obtained by means the immunoassay. In patients on polytherapy, the analytical determination of CBZepox could be of interest in cases where CBZ+CBZepox would be higher than the critical level of CBZ.

Separation of carbamazepine and five metabolites, and analysis in human plasma by micellar electrokinetic capillary chromatography

A rapid and feasible method was developed for the analysis of carbamazepine and its five metabolites (10,11-dihydro-10,11-epoxycarbamazepine, 10,11-dihydro-10,11-dihydroxycarbamazepine, 10,11-dihydro-10-hydroxycarbamazepine, 2-hydroxycarbamazepine and 3-hydroxycarbamazepine) in human plasma. Separation of the analytes is based on micellar electrokinetic chromatography, in untreated fused-silica capillary (48.5/40.0 cm length, 50 microm I.D.) with phosphate buffer (30 mM, pH 8.00) as background electrolyte, containing 50 mM sodium dodecylsulfate, and methanol (15%, v/v) as organic modifier. Clean up of human plasma samples was carried out by means of a solid-phase extraction procedure, which gave a high extraction yield for all six carbamazepines (>88%). The overall precision of the method gives a mean RSD of about 1.8%. The limit of quantitation for all analytes is < or = 0.30 microg ml(-1), the limit of detection < or = 0.12 microg ml(-1).

Factors contributing to carbamazepine-macrolide interactions

Certain macrolides (e.g. clarithromycin or erythromycin) are known to interact with the carbamazepine antiepileptic drug. Carbamazepine-macrolide interaction leads to an increase in the level of carbamazepine in the blood, so inducing carbamazepine toxicity. The aim of this paper is to compare the extent of the interaction for each macrolide and to study the effects of age, gender, weight, the carbamazepine and macrolide dosages and the use of other antiepileptic drugs on the extent of the carbamazepine-macrolide interaction. Case reports published in the literature were reviewed and analysed to this end. The results show that three macrolides (erythromycin, troleandomycin and, to a lesser extent, clarithromycin) may induce carbamazepine toxicity in clinical practice. Furthermore, it was observed that high dosages of carbamazepine or macrolides and the use of concurrent anticonvulsivant drugs in the case of patients below 60 years of age are associated with the highest carbamazepine levels in carbamazepine-macrolide interactions. This study should help physicians choose a macrolide that does not interact with carbamazepine and evaluate the risk of an interaction between carbamazepine and macrolides.

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.

Simultaneous high-performance liquid chromatography determination of carbamazepine and five of its metabolites in plasma of epileptic patients

A high-performance liquid chromatographic method with UV detection for the simultaneous analysis of the antiepileptic drug carbamazepine and five of its metabolites in human plasma has been developed. The analysis was carried out on a reversed-phase column (C8, 150x4.6 mm I.D., 5 microm) using acetonitrile, methanol and a pH 1.9 phosphate buffer as the mobile phase. Under these chromatographic conditions, carbamazepine and its metabolites 10,11-dihydro-10,11-epoxycarbamazepine, 10,11-dihydro-10,11-dihydroxycarbamazepine, 2-hydroxycarbamazepine, 3-hydroxycarbamazepine and 10,11-dihydro-10-hydroxycarbamazepine are baseline separated in less than 18 min. The extraction of the analytes from plasma samples was performed by means of an original solid-phase extraction procedure using Oasis HLB cartridges. The method requires only 250 microl of plasma for one complete analysis. The repeatability (RSD%<2.4), intermediate precision (RSD%<3.5) and extraction yield (84.8-103.0%) were very good for all analytes. The method is suitable for reliable therapeutic drug monitoring of patients undergoing chronic treatment with carbamazepine and for kinetic-metabolic studies of this drug.