Sensitive stereospecific assay of warfarin in plasma: reversed-phase high-performance liquid chromatographic separation using diastereoisomeric esters of (-)-(1S,2R,4R)-endo-1,4,5,6,7,7-hexachlorobicyclo[2.2.1]-hept-5- ene-2-carboxylic acid

High-performance liquid-chromatographic determination of warfarin enantiomers in plasma with automated on-line sample enrichment

A sensitive and selective chiral high performance liquid chromatographic method was developed for the direct determination of R- and S-warfarin enantiomers in human plasma. The method involved direct injection of human plasma onto a semipermeable surface (SPS) guard column, washing the proteins from the column with aqueous acetonitrile and back flushing the analytes onto a reversed phase ovomucoid silica HPLC column using switching valves. After separation, the analytes were simultaneously detected and quantitated with a fluorometer. The recoveries of R-warfarin from human plasma at 25 and 2500 ng/ml were 98.9% and 88.1%, respectively. The recoveries of S-warfarin at 25 and 2500 ng/ml were 105.4% and 93.9%, respectively. Using 100 microl of human plasma, the lower limit of quantification for both R- and S-warfarins was 25 ng/ml. Linear responses in analyte/internal standard peak height ratios were observed for analyte concentrations ranging from 25 to 2500 ng/ml for both enantiomers. Fluorescence chromatograms of drug-free human plasma showed no interfering peaks with retention times similar to those for R- and S-warfarins and the internal standard. Results from a 3-day validation study for both enantiomers demonstrated excellent precision (1.7-9.0%) and accuracy (97-109%) across the calibration range.

Chiral CE separation of warfarin in albumin containing samples

A capillary zone electrophoresis method with ultraviolet (UV) absorbance detection for the chiral separation of warfarin enantiomers using highly sulfated beta-cyclodextrin (beta-CD) was developed and optimized. Enantiomeric separation of warfarin was characterized by high resolution and efficiency. The optimized electrophoretic conditions were subsequently applied to the analysis of warfarin extracted from spiked human serum albumin samples. This assay showed acceptable precision, with linearity in the warfarin enantiomer concentration range of 0.1-25 mg/l. The limits of detection (LOD) and quantitation (LOQ) evaluated as warfarin enantiomer concentrations in the serum samples were 0.05 and 0.15 mg/l, respectively, for each warfarin enantiomer.

General strategies and selection of derivatization reactions for liquid chromatography and capillary electrophoresis

The general strategies, reasons and the different possibilities for the derivatization of biomedically important compounds are reviewed. Different approaches apply for small versus large analyte molecules, different advantages and disadvantages are visualized with pre- and post-column arrangements. Particular interest is focused upon solid-phase derivatization reagents.

Enantiomeric derivatization for biomedical chromatography

Derivatization reactions aimed at creating the basis for the chromatographic resolution of biologically and pharmaceutically important enantiomers are reviewed, with emphasis on the literature published in the last 10 years. Three main aspects of chiral derivatization are discussed. (a) Enantiomers containing suitable functional groups (amino, carboxyl, hydroxyl, epoxy, etc.) are transformed into covalently bonded diastereomeric derivatives using homochiral derivatizing agents. The diastereomers formed (esters, amides, urethanes, urea and thiourea, etc., derivatives) can be separated on achiral stationary phases. The derivatization reactions often afford further advantages, such as the improvement of chromatographic properties and the detectability of the solutes using UV and fluorimetric detectors. (b) Covalent but achiral derivatization is often necessary even with the use of chiral stationary phases enabling in principle direct enantioseparations (Pirkle-type columns, cyclodextrin-bonded phases, glycoprotein column and functionalized cellulose columns). The main goals of these derivatization reactions (which are analogous to those discussed above), are to introduce functional groups into the molecule of the enantiomers that improve the possibilities for chiral interactions or block functional groups to avoid non-specific interactions. (c) In the broader sense, the dynamic formation of diastereomers using chiral mobile phase additives (cyclodextrins, various reagents to form diastereomeric ion pairs, adducts, mixed metal complexes) can also be considered to be chiral derivatization reactions and is therefore briefly discussed also.

Development and validation of a high-performance liquid chromatographic method for the quantitation of warfarin enantiomers in human plasma

An HPLC method for the quantitation of warfarin enantiomers in human plasma has been developed and validated. Baseline separation of S- and R-warfarin was achieved on a silica-bonded beta-cyclodextrin column with a mobile phase of acetonitrile-acetic acid-triethylamine (1000:3:2.5, v/v/v). The detection was performed at 320 nm. The established linearity range was 12.5-2500 ng ml-1 (r > 0.99). The limit of quantitation was 12.5 ng ml-1 for each enantiomer. Inter-day precision and accuracy of 12.5 ng ml-1 standards were 12.1% relative standard deviation (RSD) and +0.67% bias for S-warfarin and 9.7% RSD and +10.8% bias for R-warfarin. The low quality control samples at 37.5 ng ml-1 showed 6.9% RSD and 0.0% bias for S-warfarin, 7.2% RSD and +0.5% bias for R-warfarin. S-Naproxen was used as internal standard. Potential metabolites of warfarin were well resolved from S- and R-warfarin, the internal standard (S-naproxen) and each other. The run time was 25 min. The silica-bonded beta-cyclodextrin column showed excellent stability; over 1000 samples were injected without significant loss of performance. The column variability test showed that the method can be applied on several batches of beta-cyclodextrin columns but not all the beta-cyclodextrin columns were suitable for this method.

Separation and determination of warfarin enantiomers in human plasma samples by capillary zone electrophoresis using a methylated beta-cyclodextrin-containing electrolyte

A methyl beta-cyclodextrin with a degree of substitution of 1.8 proved to be an effective chiral selector, among other cyclodextrins tested, for the separation of warfarin enantiomers by capillary electrophoresis. The operating conditions were optimized with respect to electrolyte composition (buffer pH, ionic strength, cyclodextrin concentration, methanol content) and applied voltage. The influence of a high ionic strength on the resolution was clearly shown. A baseline separation can be obtained in less than 15 min with an efficiency of ca. 250,000 theoretical plates. These conditions were applied to the determination of warfarin enantiomers in the plasma of patients under warfarin therapy. The limit of detection for the whole procedure (dichloromethane extraction followed by evaporation to dryness and capillary electrophoresis) was of the order of 0.2 mg/l (6.5.10(-7) M) of each enantiomer.

Enantioselective assay of warfarin in blood plasma by liquid chromatography on Chiralcel OC

A stereoselective method for the two enantiomers of warfarin has been developed. Warfarin is extracted into dichloromethane-hexane from blood plasma. After concentration by evaporation, the enantiomers are resolved by liquid chromatography on Chiralcel OC employing a non-polar mobile phase. By fluorometric detection, concentrations in plasma down to 80 nmol/l (25 ng/ml) can be determined with a coefficient of variation of less than 15%. At the 200 nmol/l level, a coefficient of variation of ca. 4% was found.