Enantioseparation of warfarin by capillary electrophoresis with UV and LIF detection using single and dual cyclodextrin type chiral selectors

Contemporary theory of enantioseparations in capillary electrophoresis

The first separation of enantiomers in capillary electrophoresis (CE) counts slightly longer than three decades. Fast development of the practice and theory of chiral CE occurred in the past 30 years and today one can consider this technology to have a solid and mature theoretical background. The goal of the present review is not only to summarize the history and contemporary theory of enantioseparations by using CE but also to present the authors personal view where shall we head to with this attractive technology not only from the viewpoint of separation of enantiomers but also for better understanding the mechanisms of non-covalent (enantioselective) interactions in chemistry, biology, medicine and related disciplines.

Twenty years of development of dual and multi-selector models in capillary electrophoresis: a review

It has been 20 years since Lurie et al. first published their model of electromigration of an analyte under simultaneous interaction with two cyclodextrins as chiral selectors. Since then, the theory of (enantio)separation in dual and complex mixtures of (chiral) selectors is well understood. In spite of this, a trial-and-error approach still prevails in analytical practice. Such a situation is likely caused by the fact that the entire theory is spread over numerous papers and the relations between various models are not always clear. The present review condenses the theory for the first time. Available mathematical models and feasible practical approaches are summarized and their advantages and limitations discussed.

Investigation of a dual CD chiral CE system for separation of glitazone compounds

A dual CD-CE method for chiral separation of enantiomers of pioglitazone, rosiglitazone and balaglitazone was investigated for the purpose of optimizing the chiral separation. In a previous work a dual CD chiral CE method was used for investigation of glitazone compounds in drug substance and pharmaceutical formulation and the studies showed that all studied glitazones were racemic mixtures. This CE method could separate the enantiomers with a resolution (R(S)) of about 3. However, another study on single glitazone enantiomers pointed out that a higher R(S) is needed to achieve more accurate results for separation of a small amount of one enantiomer in the presence of a high amount of the other enantiomers. The focus of this investigation was thus directed toward the effect of CDs and the pH of the running buffer to achieve a better enantioseparation. Initially CE systems with each of heptakis(2,6-di-O-methyl)-beta-CD (DM-beta-CD) and heptakis(6-sulfobutylether)-beta-CD (SB-beta-CD) as single CD added were investigated at three different pH values (2.5, 5.0 and 9.3). After having chosen the best of these three pH values a dual CD system was further investigated and optimized. The optimization work was then focused on the concentration of the two CDs and the pH of the running buffer and was performed using factorial design experiments. A mixture of a DM-beta-CD and SB-beta-CD was found to be optimal and necessary to achieve enantioseparation with sufficiently high R(S). In order to further verify the importance of the SB-beta-CD, a CE system with the DM-beta-CD added and substitution or partial substitution of the SB-beta-CD by SDS was studied for comparison. (1)H-NMR studies were performed to get a more detailed understanding of the interactions between the glitazones and the CDs used.The optimized dual CD-CE method for chiral separation of the enantiomers of pioglitazone, rosiglitazone and balaglitazone using a running buffer containing 50 mM borate buffer pH 9.7, 12 mM of SB-beta-CD and 3 mM of DM-beta-CD provided a high R(S) (R(S) between 5.5 and 8.8).

Strategies in method development to quantify enantiomeric impurities using CE

The growing number of chiral new drug substances requires increasing efforts in developing enantioselective methods. According to International conference on Harmonization guidelines, one should quantify the enantiomeric impurity of 0.1% relative to the major constituent. Capillary electrophoresis has evolved into an important tool for the separation of chiral drugs. The common strategies consist of two steps: firstly, initial separation conditions are evaluated. This screening usually focuses on the selection of the appropriate chiral selector. In our study 22 neutral, anionic or cationic cyclodextrins were dissolved in phosphate buffer (pH 2.5, 50 mM, CD conc.: 2.0%). Then they were investigated for the separation of 14 chiral compounds. Secondly, the obtained initial conditions for the enantiomeric separation were optimized in terms of resolution and analysis time. In our approach, important optimized factors including the concentration of the chiral selector (1-10%), the pH of the buffer (2.0-9.0), and the percentage of organic modifier (0-15%) were studied. This common strategy was completed by elaborating final requirements for the quantification of the enantiomeric impurity. A resolution between 3 and 4 was found to be necessary for the racemic mixture during the screening and optimization steps, in order to later allow for peak overloading and thus to sufficiently increase the signal-to-noise ratio. The complete strategy was conducted for atenolol, isoprenaline, verapamil and mandelic acid.

Enantioseparations by using capillary electrophoretic techniques. The story of 20 and a few more years

This paper provides the author's insight on the past, present and future of performing enantioseparations using capillary electrophoretic (CE) techniques. These techniques are discussed from the historical point of view, as well as based on their potential as the separation techniques of today and the future. The overview covers mechanistic as well as practical aspects of CE techniques.

Comparative enantioseparations with native beta-cyclodextrin and heptakis-(2-O-methyl- 3,6-di-O-sulfo)-beta-cyclodextrin in capillary electrophoresis

Twenty-three cationic chiral analytes were resolved in capillary electrophoresis using native beta-cyclodextrin and single isomer heptakis-(2-O-methyl-3,6-di-O-sulfo)-beta-cyclodextrin as chiral selectors. For 12 of 16 chiral analytes resolved with both chiral selectors the enantiomer migration order was opposite. In selected cases the structure of cyclodextrin-analyte complexes in aqueous solution was investigated using one-dimensional transverse rotating frame nuclear Overhauser and exchange spectroscopy. It was found that in contrast to mainly inclusion-type complexes between chiral analytes and beta-cyclodextrin, external complexes are formed between the chiral analytes and structurally crowded, highly charged heptakis-(2-O-methyl-3,6-di-O-sulfo)-beta-cyclodextrin.

Enantioseparation of glutethimide and its 5-OH-metabolite in capillary electrophoresis and study of selector-selectand interactions using one-dimensional rotating frame nuclear Overhauser and exchange spectroscopy

Enantioseparation of glutethimide (GT) and its 5-hydroxy metabolite (5-OH-GT) has been studied with several charged cyclodextrin (CD) derivatives. The emphasis was made on the enantiomer migration order of GT and simultaneous enantioseparation of GT and 5-OH-GT. The possible structural differences of GT complexes with three different single isomer charged CD derivatives were studied using one-dimensional rotating frame nuclear Overhauser and exchange spectroscopy (1-D ROESY).

Enantioseparation of novel COX-2 anti-inflammatory drugs by capillary electrophoresis using single and dual cyclodextrin systems

A capillary electrophoresis method was developed for the enantioseparation of three novel cyclooxygenase-2 (COX-2) inhibitor drugs (E-6259, E-6036 and E-6087) with anti-inflammatory and analgesic activities using sulfobutyl ether-beta-cyclodextrin (SBE-beta-CD) as a chiral selector. The use of 50 mM sodium tetraborate at pH 9.2 with 30% v/v methanol, containing 7.1 mM SBE-beta-CD, as a background electrolyte (BGE) allowed the complete enantioseparation of the three neutral racemic mixtures (resolution = 2.4, 3.0 and 8.7, respectively) and their corresponding metabolites (oxidation products) in a single run. Migration times were shortened with some loss of enantioresolution by adding 1.75 mM dimethyl-beta-cyclodextrin (DM-beta-CD) to the previous BGE (dual CD system). The reversal of the migration order of E-6259 enantiomers in the dual CD system was also studied. Furthermore, the addition of DM-beta-CD to the BGE introduced a new chemoselectivity in the system that allowed E-6259 to be separated from the structurally similar compound E-6036.

Studies on the chiral recognition of peptide enantiomers by neutral and sulfated beta-cyclodextrin and heptakis-(2,3-di-O-acetyl)-beta-cyclodextrin using capillary electrophoresis and nuclear magnetic resonance

The separation of dipeptide and tripeptide enantiomers using a neutral single isomer cyclodextrin (CD) derivative, heptakis-(2,3-di-O-acetyl)-beta-CD (DIAC-beta-CD), was investigated with respect to the amino acid sequence applying standard separation conditions. With only one exception the DD-enantiomers migrated faster than the LL-stereoisomers. Separations obtained for the same set of peptides using beta-CD and the sulfated single isomer derivatives heptakis-(2,3-di-O-acetyl-6-sulfo)-beta-CD (HDAS-beta-CD) and heptakis-6-sulfo-beta-CD (HS-beta-CD) revealed identical enantiomer migration order in the presence of the 2,3-disubstituted derivatives DIAC-beta-CD and HDAS-beta-CD. In contrast, reversed migration sequence was found for beta-CD and HS-beta-CD compared to DIAC-beta CD and HDAS-beta-CD indicating the importance of the substitution pattern on the wider rim of the CD cavity on the chiral recognition of the peptide enantiomers by the CDs. Nuclear magnetic resonance (NMR) experiments indicated different complexation modes between the enantiomers and the CDs depending on the presence of acetyl substituents on the wider rim of the CD torus. Thus, the CD-induced chemical shifts observed in samples containing Ala-Phe or Ala-Tyr and beta-CD or HS-beta-CD were consistent with an inclusion of the aromatic moiety into the CD cavity. Although the CD-induced chemical shifts in the presence of DIAC-beta-CD and HDAS-beta-CD did not allow direct conclusions on the complexation mode they substantially differed from those observed in the presence of 2,3-unsubstituted CDs indicating different structures of the peptide-CD complexes.

Mechanistic study on the opposite migration order of clenbuterol enantiomers in capillary electrophoresis with beta-cyclodextrin and single-isomer heptakis(2,3-diacetyl-6-sulfo)-beta-cyclodextrin

Opposite migration order was observed for the enantiomers of the chiral beta2-adrenergic drug clenbuterol (CL) in capillary electrophoresis (CE) when resolved with native beta-cyclodextrin (beta-CD) and heptakis (2,3-diacetyl-6-sulfo)-beta-CD (HDAS-beta-CD). The possible mechanisms of the affinity reversal of the CL enantiomers depending on the structure of the CD were studied using 1H-nuclear magnetic resonance (1H-NMR) spectrometry and one-dimensional rotating frame nuclear Overhauser and exchange spectrometry (1-D ROESY). Significant differences were observed between the structure of the (+/-)-CL complexes with beta-CD and HDAS-beta-CD.

Enantioseparation of the anticoagulant drug phenprocoumon in capillary electrophoresis with UV and laser-induced fluorescence detection and application of the method to urine samples

The enantioseparation of phenprocoumon (PhC) in capillary electrophoresis (CE) has been studied using various cyclodextrins (CDs) such as native alpha, beta and gamma-CD and several neutral and randomly, as well as selectively substituted charged CD derivatives. Reversal of the enantiomer migration order was observed when using heptakis(2,3,6-tri-O-methyl (TM)-beta-CD as a chiral selector compared to all other CDs used. The detection of PhC was performed using either UV or laser-induced fluorescence (LIF) detection. The limit of detection (LOD) observed with LIF detection was ca. 20 times lower compared to UV. The method has been applied to the analysis of urine samples of the patient under treatment with PhC in combination with other drugs such as ramipril, hydrochlorothiazide, and nifedipine.