Separation of enantiomers of drugs by capillary electrophoresis. III. Beta-cyclodextrin as chiral solvating agent

Method Development, Optimization, and Validation of the Separation of Ketamine Enantiomers by Capillary Electrophoresis Using Design of Experiments

Capillary electrophoresis was chosen as cost-effective and robust method to separate ketamine enantiomers. For the method development, first different native and modified cyclodextrins were tested. The most promising chiral selector was α-cyclodextrin. A design of experiments (DoE) was carried out, which started with the screening of relevant factors. Based on these results, the method was optimized according to the significant factors (buffer, cyclodextrin concentration, pH value, voltage, temperature) of the screening based on the response resolution and migration time of the later migrating enantiomer. The optimized conditions consisted of a background electrolyte with 275 mM TRIS, adjusted with 85% phosphoric acid to a pH of 2.50, and 50 mM α-cyclodextrin, at a temperature of 15 °C, an applied voltage of 30 kV and an injection pressure of 1.0 psi for 10 s. A fused-silica capillary with a total length of 70 cm and an effective length to the detector of 60 cm was used. The method was validated according to ICH guideline Q2 R(1). The limit of quantification was 3.51 µg mL−1 for S-ketamine and 3.98 µg mL−1 for R-ketamine. The method showed good linearity for racemic ketamine with R2 of 0.9995 for S-ketamine and 0.9994 for R-ketamine. The lowest quantifiable content of S-ketamine found in R-ketamine was 0.45%.

Enantioselective open-tubular capillary electrochromatography using a β-cyclodextrin–gold nanoparticles–polydopamine coating as a stationary phase

A novel β-cyclodextrin–gold nanoparticles–polydopamine modified open tubular column was prepared for the chiral separation of enantiomers in capillary electrochromatography.

Establishment and molecular modeling study of maltodextrin-based synergistic enantioseparation systems with two new hydroxy acid chiral ionic liquids as additives in capillary electrophoresis

Discovering more superior performance of ionic liquids for the separation science has triggered increasing interest. In this work, two new Hydroxy acid-based chiral ionic liquids (CILs) (tertramethylammonium-d-pantothenate (TMA-d-PAN), tertramethylammonium-d-quinate (TMA-d-QUI)) were designed and first used as additives to establish the maltodextrin-based synergistic systems for enantioseparation in capillary electrophoresis (CE). Compared to traditional single maltodextrin chiral separation system, significantly improved separations of all tested drugs in the CIL/Maltodextrin synergistic systems were obtained. Some parameters (CIL concentration, maltodextrin concentration, buffer pH, and applied voltage) in the TMA-d-PAN/Maltodextrin synergistic system have been examined and optimized for analytes. The molecular docking software AutoDock was applied to simulate the recognition process and surmise feasible resolution mechanism in the Maltodextrin/CILs synergistic systems, which has certain guiding value.

Enantiomeric quality control of antihistamines in pharmaceuticals by affinity electrokinetic chromatography with human serum albumin as chiral selector

The present paper deals with the enantiomeric separation of six antihistaminic enantiomers by affinity electrokinetic chromatography (AEKC)-partial filling technique using human serum albumin (HSA) as chiral selector. A multivariate optimization approach of the most critical experimental variables in enantioresolution, running pH, HSA concentration and HSA plug length (SPL) was carried out since there are interactions between variables that could not be considered in an univariate optimization. The estimated and experimental resolution values obtained for antihistaminic enantiomers varied from 1.13 (for orphenadrine) to 2.15 (for brompheniramine). The optimum experimental conditions for enantioresolution of each compound were: brompheniramine, pH 8.5, [HSA] 180 microM, SPL 180 s; chlorcyclizine, pH 6.5, [HSA] 180 microM, SPL 150 s; chlorpheniramine, pH 8.25, [HSA] 160 microM, SPL 150 s; hydroxyzine, pH 7.0, [HSA] 180 microM, SPL 150 s; and orphenadrine, pH 7.8, [HSA] 160 microM, SPL 150 s. pH and the quadratic term of pH seem to be the most critical factors that determine enantioresolution of antihistamines. The validity of the developed methodologies to enantiomeric quality control of antihistamines in pharmaceutical formulations is demonstrated analyzing the content of brompheniramine, chlorpheniramine and hyroxyzine enantiomers in commercially available pharmaceutical formulations containing racemic mixtures of compounds. Resolution, accuracy, reproducibility, cost and sample throughput of the proposed methodologies make them suitable for quality control of the enantiomeric composition of antihistamines in pharmaceutical preparations.

Characterization of the stereoselective biotransformation of ketamine to norketaminevia determination of their enantiomers in equine plasma by capillary electrophoresis

A robust CE method for the simultaneous determination of the enantiomers of ketamine and norketamine in equine plasma is described. It is based upon liquid-liquid extraction of ketamine and norketamine at alkaline pH from 1 mL plasma followed by analysis of the reconstituted extract by CE in the presence of a pH 2.5 Tris-phosphate buffer containing 10 mg/mL highly sulfated beta-CD as chiral selector. Enantiomer plasma levels between 0.04 and 2.5 microg/mL are shown to provide linear calibration graphs. Intraday and interday precisions evaluated from peak area ratios (n = 5) at the lowest calibrator concentration are < 8 and < 14%, respectively. The LOD for all enantiomers is 0.01 microg/mL. After i.v. bolus administration of 2.2 mg/kg racemic ketamine, the assay is demonstrated to provide reliable data for plasma samples of ponies under isoflurane anesthesia, of ponies premedicated with xylazine, and of one horse that received romifidine, L-methadone, guaifenisine, and isoflurane. In animals not premedicated with xylazine, the ketamine N-demethylation is demonstrated to be enantioselective. The concentrations of the two ketamine enantiomers in plasma are equal whereas S-norketamine is found in a larger amount than R-norketamine. In the group receiving xylazine, data obtained do not reveal this stereoselectivity.

Quantitation of the enantiomers of ofloxacin by capillary electrophoresis in the parts per billion concentration range for in vitro drug absorption studies

Ofloxacin, a chiral fluoroquinolone, possesses two optical isomers. The antibacterial activity of S-(-)-ofloxacin is reported to be 8-128 times higher than that of R-(+)-ofloxacin. A capillary zone electrophoresis method has been developed to quantify the enantiomers of ofloxacin in high diluted samples (20-700 ng/ml for each enantiomer). After fluid-fluid extraction of ofloxacin from physiological solution electrokinetic injection was employed to improve the sensitivity. The method was optimised using a central composite design. Four experimental factors were investigated: the background electrolyte concentration, the methyl-beta-cyclodextrin concentration, the buffer pH and the temperature. The amount migrated into the capillary, determined by the peak area, the resolution between the ofloxacin enantiomers, the migration time and the generated current were evaluated as responses. The quantification limit is 11.4 ng/ml for S-ofloxacin and 10.8 ng/ml for R-ofloxacin. The method has shown good validation data in terms of precision and recovery rate.

Stereoselective distribution and stereoconversion of zopiclone enantiomers in plasma and brain tissues in rats

Concentrations of (-)-zopiclone and (+)-zopiclone were determined in plasma and brain after oral administration, to investigate the stereoselectivity of distribution in rats. Zopiclone enantiomers were administered separately to rats and concentrations were determined by chiral HPLC in plasma and brain. In initial experiments, rats were treated with urethane before cannulation for blood sampling but as this drug modified zopiclone pharmacokinetics, it was not used in subsequent studies. This study showed that zopiclone pharmacokinetics after oral gavage in rats are stereoselective. After oral administration of (+)-zopiclone, no stereoconversion was observed in plasma. Conversely, after administration of (-)-zopiclone, both enantiomers were found in plasma and brain with (+)-zopiclone/(-)-zopiclone ratios of 1 and 8.4 in plasma and brain, respectively. Our findings suggest that zopiclone undergoes stereoconversion and that it is stereospecifically distributed to the brain.

The enantioselective determination of chlorpheniramine and its major metabolites in human plasma using chiral chromatography on a beta-cyclodextrin chiral stationary phase and mass spectrometric detection

A sensitive enantioselective high-performance liquid chromatographic (HPLC) method has been developed for the simultaneous determination of plasma concentrations of (-)(R)- and (+)(S)-chlorpheniramine (CP) and their metabolites, desmethyl-chlorpheniramine (DCP), didesmethyl-chorpheniramine (DDCP) and chlorpheniramine N-oxide (CPNO). Enantioselective separations were achieved on a beta-cyclodextrin chiral stationary phase (CYCLOBOND I 2000) with a mobile phase consisting of diethylamine acetate (0.25%, pH 4.4):methanol:acetonitrile [85:7.5:7.5, (v/v/v)]and a flow-rate of 0.5 ml/min. For CP, the enantioselectivity (alpha) of the separation was 1.12 with a resolution factor (R(s)) of 1.17. The method was validated for CP by using mass spectroscopy detection (MSD). Concentrations of each enantiomer could be measured down to 125 pg/ml from a 1-ml plasma sample. Extracted calibration curves were linear from 0.13 to 50.00 ng/ml for each enantiomer. The method was applied to samples from two clinical studies.

Chiral separation by chromatographic and electromigration techniques. A review

This review gives a survey of different chiral separation principles and their use in high-performance liquid chromatography (HPLC), gas chromatography (GC), supercritical fluid chromatography (SFC), thin-layer chromatography (TLC), capillary electrophoresis (CE) and capillary electrochromatography (CEC) highlighting new developments and innovative techniques. The mechanisms of the different separation principles are briefly discussed and some selected applications are shown.

Fast enantiomeric separation of basis drugs by electrokinetic chromatography. Application to the quantitation of terbutaline in a pharmaceutical preparation

Electrokinetic chromatography (EKC) using micelles of bile salts alone or mixed with sodium dodecyl sulfate (SDS) and neutral, anionic, or cationic cyclodextrins (CDs) in the separation buffer has been employed in order to achieve fast enantiomeric separation of basic drugs. A study of the enantiomeric separation ability of these chiral selectors concerning four basic drugs (epinephrine, terbutaline, clenbuterol, and salbutamol) has been carried out under different experimental conditions. The best chiral selectors to perform the enantiomeric separation of these drugs were neutral beta-CD derivatives, specifically permethylated beta-CD PM-beta-CD. The effect of the PM-beta-CD concentration, temperature, and applied voltage on the enantiomeric resolution of the basic drugs was investigated. The use of a 25 mM ammonium acetate buffer (pH 5.0), 30 mM in PM-beta-CD together with an applied voltage of 20 kV and a temperature of 15 degrees C enabled the individual and fast enantiomeric separation of epinephrine, norepinephrine, terbutaline, clenbuterol, and salbutamol each one into its two enantiomers in less than 3 min. The EKC method was validated (precision and accuracy) to quantitate terbutaline in a pharmaceutical preparation, obtaining a limit of detection of 4 microg/mL.

Separation of carvedilol enantiomers in very small volumes of human plasma by capillary electrophoresis with laser-induced fluorescence

A sensitive capillary electrophoretic method for the determination of carvedilol enantiomers in 100 microl of human plasma has been developed and validated. Carvedilol and the internal standard carazolol are isolated from plasma samples by liquid-liquid extraction using diethylether. A sensitive and selective detection is provided by helium-cadmium laser-induced fluorescence. The total analysis time is 17.5 min, about 30 min are needed for the sample preparation. The linearity of the assay ranges from 1.56 to 50 ng/ml per carvedilol enantiomer. The limits of quantification (LOQ) for the carvedilol enantiomers in 100 microl of human plasma are 1.56 ng/ml. The inter-day accuracy for R-carvedilol is between 95.8 and 103% (104% at LOQ) and for S-carvedilol between 97.1 and 103% (107% at LOQ); the inter-day precision values are between 3.81 and 8.64% (10.9% at LOQ) and between 5.47 and 7.86% (7.91% at LOQ) for R- and S-carvedilol, respectively. The small sample volume needed is especially advantageous for the application in clinical studies in pediatric patients. As an application of the assay concentration/time profiles of the carvedilol enantiomers in a 5-year-old patient receiving a test dose of 0.09 mg/kg carvedilol are reported.

Development of a capillary electrophoresis assay for the determination of carvedilol enantiomers in serum using cyclodextrins

A capillary electrophoresis method using cyclodextrins as the chiral selectors was developed for the determination of carvedilol enantiomers in serum. Several types of cyclodextrins were evaluated. The effect of cyclodextrin concentration on enantiomer resolution was investigated. Best results were obtained using 10 mM hydroxypropyl-beta-cyclodextrin in the run buffer. The effect of voltage on efficiency was assessed. Other electrophoretic conditions were optimized. The method was validated for carvedilol enantiomers in serum. Linearity of detection was assessed over the concentration range of 50-4000 ng/ml of each enantiomer in serum. Intra- and inter-assay variability obtained were under 8% for both enantiomers.

Separation of drug enantiomers by capillary electrophoresis in the presence of neutral cyclodextrins

This is a selected review, highlighting our results obtained in an extended screening program ("The German-Chinese Drug Screening Program"), with a focus on a set of original data obtained with heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (TM-beta-CD) as the chiral solvating agent (CSA). The enantioseparation of 86 drugs by capillary zone electrophoresis in the presence of this CSA was successful for 47 drugs. The migration separation factors (alpham) and the migration retardation factors (Rm) were compared with those found for native beta-cyclodextrin (beta-CD). The patterns thus obtained were also compared with those observed for hexakis(2,3,6-tri-O-methyl)-alpha-CD (TM-alpha-CD) and octakis(2,3,6-tri-O-methyl)-gamma-CD (TM-gamma-CD), respectively. From the statistical data, it can be concluded that there is a remarkable influence of the analyte structure on the electrophoretic data. A substructure 4H was found in the analyte structure that has a significant influence on the analytes' behaviour. Thus, analytes bearing the substructure 4H do not only have a strong affinity to the CDs but also a high rate of success of chiral separation in all systems reviewed. In light of this, the different ring sizes of native cyclodextrins (alpha-, beta- and gamma-CD) readily explain their behaviour towards a limited test set of chiral drugs. Sterical considerations point to the significance of side-on-binding versus inclusion in the cavity of the host. In addition to the findings from the screening program, numerous references to the literature are given.

Separation of drugs by capillary electrophoresis, Part 10. Permethyl-alpha-cyclodextrin as chiral solvating agent

Following the German-Chinese Drug Screening Program, 86 racemic drugs were investigated in capillary zone electrophoresis in the presence of the chiral solvating agent (CSA) hexakis-(2,3,6-tri-O-methyl)-alpha-cyclodextrin (TM-alpha-CD). Of the 86 drugs, 23 were separated into enantiomeric pairs. A comparison of the migration separation factors (alpha(m)) and the migration retardation factors (Rm) with previously published data for native alpha-CD revealed that the 'upper-rim' hydroxyl groups do not necessarily facilitate the recognition of the drug enantiomers by the chiral host. In contrast, an overall increase in affinity for the permethylated host led to a higher rate of successful enantiomer separations. A key substructure (4H) was identified in the analyte structure domain, with a crucial influence on the behavior of a particular drug.

Recent chromatographic and electrophoretic enantioseparations of cardiovascular drugs

The chromatographic and electrophoretic enantiomeric separation and analysis of several clinically used cardiovascular drugs have been reviewed. Several examples of recently reported applications of enantioselective analysis and various cardiovascular agents are presented.

Capillary electrophoresis, nuclear magnetic resonance and mass spectrometry studies of opposite chiral recognition of chlorpheniramine enantiomers with various cyclodextrins

Markedly different chiral separation abilities were observed for native beta-cyclodextrin (beta-CD), carboxymethyl-beta-CD (CM-beta-CD) and heptakis (2,3,6-tri-O-methyl)-beta-CD (TM-beta-CD) towards the enantiomers of (+/-)-chlorpheniramine ((+/-)-CHL) in capillary electrophoresis (CE). Native beta-CD afforded almost baseline enantioseparation at a concentration of 18 mg/mL, whereas only 1 mg/mL solution of CM-beta-CD was required for adequate enantioseparation. TM-beta-CD allowed the nearly baseline enantioseparation only at a concentration as high as 80 mg/mL. Moreover, the migration order of (+/-)-CHL in the presence of TM-beta-CD was opposite to that with beta-CD and CM-beta-CD. 1H and 13C-NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS) have been used in order to obtain preliminary information about the stoichiometry and the binding constants in the intermolecular diastereomeric complexes of (+/-)-CHL with these CDs.

Cationic beta-cyclodextrin derivative for chiral separations

A novel hepta-substituted beta-cyclodextrin bearing the methoxyethylamine group linked to the upper cyclodextrin rim was successfully used as a chiral selector for enantiomeric separation of non-steroidal anti-inflammatory drugs (NSAIDs) and phenoxypropionic acid herbicides (PPAHs). Separation parameters such as pH and concentration were found to have major influences on enantiomeric resolution of the NSAIDs and PPAHs. Results indicate that heptakis(6-methoxyethylamine-6-deoxy)-beta-cyclodextrin [beta-CD-OMe (VII)] performs exceptionally well for the enantiomeric resolution of NSAIDs: indoprofen and fenoprofen (Rs = 11 and 14, respectively). In addition, baseline enantiomeric separation of a mixture of six pairs of PPAHs was achieved in under 30 min. Compared to other cationic beta-cyclodextrins reported in the literature, the beta-CD-OMe (VII) showed improved selectivity for both classes of the aforementioned anionic racemates.

Determination of the enantiomers of chlorpheniramine and its main monodesmethyl metabolite in urine using achiral-chiral liquid chromatography

The enantiomers of chlorpheniramine and its monodesmethyl metabolite were determined separately in urine by using a coupled achiral-chiral chromatographic system. The two enantiomers of the studied compound and the internal standard were separated from the biological matrix on a cyanopropyl column and reinjected into a chiral amylose AD column where the two enantiomers were separated and quantified by UV detection. The method was validated for chlorpheniramine and for the metabolite within the range 0-1000 ng/ml. It was also applied in a pilot pharmacokinetic study to samples from a volunteer given 8 mg of racemic chlorpheniramine by mouth.

Controlling enantioselectivity in chiral capillary electrophoresis with inclusion-complexation

The separation of chiral compounds is of key importance in different fields of application, e.g., pharmaceutical, industrial, forensic, biological, clinical etc. Capillary electrophoresis (CE) is a powerful analytical method applied in chiral analysis and inclusion-complexation is one of the most frequently used mechanism to improve the selectivity of the enantiomeric separation. Cyclodextrins and their derivatives or modified crown-ethers have been successfully applied in CE for the enantiomeric separation of a wide number of analytes. This review surveys the separation of enantiomers by CE when chiral selectors, forming inclusion-complexation, are used. The control of enantioselectivity can be done carefully by considering several experimental parameters such as chiral selector type and concentration, pH, ionic strength and concentration of the background electrolyte, electroosmotic flow, organic modifier etc. The review presents a list of the latest separation of enantiomers by CE where inclusion-complexation plays a key role in the stereoselective separation mechanism.

A persubstituted cationic beta-cyclodextrin for chiral separations

The applications of a novel polycationic derivative of beta-cyclodextrin (beta-CD), heptakis(6-hydroxyethylamino-6-deoxy-beta-cyclodextrin) (beta-CD-EA), as a chiral host--guest additive for the enantioseparation of various classes of chiral anionic analytes are presented. The cationic beta-CD described in this paper is persubstituted with seven ethanolamine side arms at the primary rim of each cyclodextrin (CD) molecule. It is found that the electrophoretic mobility of beta-CD-EA can be adjusted to influence the chiral selectivity by changing the pH of the background electrolyte. Most of the observed CD capillary zone electrophoresis (CZE) separations of anionic drugs and herbicides were accomplished in the pH range of 4.0-7.0 with a reverse polarity configuration. At pH 5.0, enantioseparation of a mixture of three structurally related antiinflammatory agents (fenoprofen, flurbiprofen, and ibuprofen) was possible in about 30 min. However, other chiral acids, such as a series of phenoxypropionic acid herbicides and dansylated amino acids (glutamic acid and aspartic acids), were best separated at pH 6.0 or 7.0. An impressive separation of a mixture of six structurally related anionic herbicides [(+/-)-2-phenoxypropionic acid, (+/-)-2-(2-chlorophenoxy)propionic acid, (+/-)-2-(3-chlorophenoxy)propionic acid, (+/-)-2-(4-chlorophenoxy)propionic acid, (+/-)-2-(2,4-dichlorophenoxy)propionic acid, and (+/-)-2-(2,4,5-trichlorophenoxy)propionic acid] was achieved for the first time in about 15 min during a single run with 20 mM beta-CD-EA. The analytical applicability of this cationic CD molecule for chiral separations is discussed in detail.

Capillary electrophoretic separations of enantiomers using cyclodextrin-containing background electrolytes

The 1996 primary literature papers which deal with the separation of enantiomers using cyclodextrins are reviewed here. Though the majority of the papers still use the neutral native cyclodextrins or the neutral derivatized cyclodextrins as resolving agents, there was a significant increase in number of separations which relied on charged cyclodextrins, both weak electrolytes and strong electrolytes, as resolving agents. Also, there was an increase in the number of papers which reported binding constants and correlated them with other physical or chemical characteristics of the analytes. Several successful minor enantiomer determinations were presented, pushing the reliable quantitation levels below 0.1%. Work continued on the simultaneous use of neutral and charged cyclodextrins to improve separation selectivity or peak resolution.