Influence of the nature of the electrolyte on the chiral separation of basic compounds in nonaqueous capillary electrophoresis using heptakis(2,3-di-O-methyl-6-O-sulfo)-beta-cyclodextrin

Application of Experimental Design Methodologies in the Enantioseparation of Pharmaceuticals by Capillary Electrophoresis: A Review

Chirality is one of the major issues in pharmaceutical research and industry. Capillary electrophoresis (CE) is an interesting alternative to the more frequently used chromatographic techniques in the enantioseparation of pharmaceuticals, and is used for the determination of enantiomeric ratio, enantiomeric purity, and in pharmacokinetic studies. Traditionally, optimization of CE methods is performed using a univariate one factor at a time (OFAT) approach; however, this strategy does not allow for the evaluation of interactions between experimental factors, which may result in ineffective method development and optimization. In the last two decades, Design of Experiments (DoE) has been frequently employed to better understand the multidimensional effects and interactions of the input factors on the output responses of analytical CE methods. DoE can be divided into two types: screening and optimization designs. Furthermore, using Quality by Design (QbD) methodology to develop CE-based enantioselective techniques is becoming increasingly popular. The review presents the current use of DoE methodologies in CE-based enantioresolution method development and provides an overview of DoE applications in the optimization and validation of CE enantioselective procedures in the last 25 years. Moreover, a critical perspective on how different DoE strategies can aid in the optimization of enantioseparation procedures is presented.

Enantioseparations in Nonaqueous Capillary Electrophoresis Using Charged Cyclodextrins

The enantioseparation of acidic and basic compounds can be successfully achieved in nonaqueous capillary electrophoresis (NACE) using single-isomer charged β-cyclodextrin (β-CD) derivatives of opposite charge to that of the analytes. This chapter describes how to separate the enantiomers of three basic substances selected as model compounds, i.e., alprenolol, bupranolol, and terbutaline, using the negatively charged heptakis(2,3-di-O-acetyl-6-O-sulfo)-β-CD (HDAS-β-CD). The enantiomers of three acidic drugs (tiaprofenic acid, suprofen, and flurbiprofen) are resolved using a monosubstituted amino β-CD derivative, namely 6-monodeoxy-6-mono(3-hydroxy)propylamino-β-CD (PA-β-CD).

Enantioseparations in nonaqueous capillary electrophoresis using charged cyclodextrins

The enantioseparation of acidic and basic compounds can be successfully achieved in nonaqueous capillary electrophoresis using single-isomer charged β-cyclodextrin (β-CD) derivatives of opposite charge to that of the analytes. This chapter describes how to separate the enantiomers of three basic substances selected as model compounds, i.e., alprenolol, bupranolol, and terbutaline, using the negatively charged heptakis(2,3-di-O-acetyl-6-O-sulfo)-β-CD. The enantiomers of three acidic drugs (tiaprofenic acid, suprofen, and flurbiprofen) are resolved using a monosubstituted amino β-CD derivative, namely, 6-monodeoxy-6-mono(3-hydroxy)propylamino-β-CD.

Combination of capillary electrophoresis, molecular modelling and nuclear magnetic resonance to study the interaction mechanisms between single-isomer anionic cyclodextrin derivatives and basic drug enantiomers in a methanolic background electrolyte

In order to improve our knowledge of the mechanisms of enantiomer recognition pattern in nonaqueous systems, an approach combining nonaqueous CE (NACE), molecular modelling and NMR was undertaken. Bupivacaine and propranolol were selected as model compounds and their interactions with two single-isomer highly charged β-CD derivatives, namely heptakis(2,3-di-O-methyl-6-O-sulfo)-β-CD (HDMS-β-CD) and heptakis(2,3-di-O-acetyl-6-O-sulfo)-β-CD (HDAS-β-CD), were studied. The CD-bupivacaine complexes were evaluated by 2-D Rotating-frame Overhauser Effect SpectroscopY (ROESY) experiments. From these experiments, it can be assumed that inclusion complexes are not formed, whatever the CD derivative used. Molecular modelling was performed at the RHF/MINI-1 or B3LYP/6-31G(d) level. External as well as inclusion type complexes with the alkyl chain of propranolol into both CD cavities were located. Interaction energies calculated for bupivacaine and propranolol correlated with the enantiomer migration order observed in the NACE experiments using both anionic CD derivatives. The interaction of propranolol with HDMS-β-CD or HDAS-β-CD gives rise to a family of external and inclusion complexes in which some are more probably obtained.

Advances in the enantioseparation of β-blocker drugs by capillary electromigration techniques

β-Blocker drugs or β-adrenergic blocking agents are an important class of drugs, prescribed with great frequency. They are used for various diseases, particularly for the treatment of cardiac arrhythmias, cardioprotection after myocardial infarction (heart attack), and hypertension. Almost all β-blocker drugs possess one or more stereogenic centers; however; only some of them are administered as single enantiomers. Since both enantiomers can differ in their pharmacological and toxicological properties, enantioselective analytical methods are required not only for pharmacodynamic and pharmacokinetic studies but also for quality control of pharmaceutical preparations with the determination of enantiomeric purity. In addition to the chromatographic tools, in recent years, capillary electromigration techniques (CE, CEC, and MEKC) have been widely used for enantioselective purposes employing a variety of chiral selectors, e.g. CDs, polysaccharides, macrocyclic antibiotics, proteins, chiral ion-paring agents, etc. The high separation efficiency, rapid analysi,s and low consumption of reagents of electromigration methods make them a very attractive alternative to the conventional chromatographic methods. In this review, the development and applications of electrodriven methods for the enantioseparation of β-blocker drugs are reported. The papers concerning this topic, published from January 2000 until December 2010, are summarised here. Particular attention is given to the coupling of chiral CE and CEC methods to MS, as this detector provides high sensitivity and selectivity.

Generic systems for the enantioseparation of basic drugs in NACE using single-isomer anionic CDs

The enantioseparation of 10 basic drugs was evaluated in NACE systems using heptakis(2-O-methyl-3-O-acetyl-6-O-sulfo)-β-CD (HMAS-β-CD). For this purpose, a D-optimal design with 21 experimental points was applied. Four antifungal agents (econazole, isoconazole, miconazole, sulconazole), three local anesthetics (bupivacaine, mepivacaine and prilocaine), two sympathomimetics (salbutamol and terbutaline) and one β-blocker (carvedilol) were selected as basic model analytes. The influence on the enantiomeric resolution of anionic CD and BGE anion concentrations as well as the BGE anion nature was investigated. For all studied analytes, the enantiomeric resolution was shown to be significantly influenced by the CD concentration. Based on the observed results, a generic NACE system was recommended, namely 20mM HMAS-β-CD and 10mM ammonium camphor SO(3)(-) in methanol acidified with 0.75 M formic acid. Moreover, this NACE system was compared to previous conditions with heptakis(2,3-di-O-methyl-6-O-sulfo)-β-CD (HDMS-β-CD) or heptakis(2,3-di-O-acetyl-6-O-sulfo)-β-CD (HDAS-β-CD). Finally, two generic systems using either HDAS-β-CD or HMAS-β-CD were proposed and evaluated for the enantioseparation of ketamine and norketamine after incubation of ketamine in phenobarbital-induced male rat liver microsomes systems.

Association of two single-isomer anionic CD in NACE for the chiral and achiral separation of fenbendazole, its sulphoxide and sulphone metabolites: application to their determination after in vitro metabolism

A NACE method was developed for the separation of fenbendazole (FBZ), a prochiral drug giving rise to chiral (oxfendazole or OFZ) and nonchiral (FBZ sulphone or FBZSO(2)) metabolites. First, the effect of the nature and the concentration of CD as well as that of the acidic BGE on the enantiomeric separation of OFZ were studied. OFZ enantiomers were completely resolved using a BGE made up of 10 mM ammonium formate and 0.5 M TFA in methanol containing 10 mM heptakis(2,3-di-O-acetyl-6-O-sulfo)-beta-CD and 10 mM heptakis(2,3-di-O-methyl-6-O-sulfo)-beta-CD. Moreover, the NACE method was found to be particularly well suited to the simultaneous determination of FBZ, OFZ enantiomers, and FBZSO(2). Thiabendazole was selected as an internal standard. The CD-NACE potential was then evaluated for in vitro metabolism studies using FBZ as a model case. The OFZ enantiomers and FBZSO(2) could be detected after incubation of FBZ in the phenobarbital-induced male rat liver microsomes systems.

Effect of the nature of the single-isomer anionic CD and the BGE composition on the enantiomeric separation of beta-blockers in NACE

The separation of ten beta-blockers has been investigated in NACE systems using heptakis(2,3-di-O-methyl-6-O-sulfo)-beta-CD (HDMS-beta-CD) and heptakis(2,3-di-O-acetyl-6-O-sulfo)-beta-CD (HDAS-beta-CD). The influence on enantioresolution, mobility difference and selectivity of the nature of both anionic CD and BGE anion as well as their concentrations were studied by means of a multivariate approach. A D-optimal design with 25 experimental points was applied. For all studied analytes, the enantiomeric resolution was shown to be significantly influenced by both CD nature and concentration. Except for two compounds, HDAS-beta-CD was found to give higher enantioresolution values than HDMS-beta-CD. The best enantioseparation for all compounds was achieved in the presence of a high chiral selector concentration and for most of them at a low BGE anion concentration. For each investigated compound, operating conditions leading to the best enantiomeric resolution were deduced. A generic NACE system was then recommended, namely 10 mM ammonium acetate and 40 mM HDAS-beta-CD in methanol acidified with 0.75 M formic acid. This generic system was able to completely resolve the enantiomers of all beta-blockers, with a R(s) value of at least 4. Finally, the optimal conditions obtained modelling resolution, mobility difference and selectivity were compared.

The role of cyclodextrins in chiral capillary electrophoresis

The members of the enantiomeric pairs frequently show rather different biological effects, so their chiral selective synthesis, pharmacological studies and analysis are necessary. CE has unique advantages in chiral analysis. The most frequently used chiral selectors are CDs in this field. This paper gives a short view on the advantages on CE in direct chiral separations, emphasizing the role of CDs. The reason for the broad selectivity spectra of CDs is discussed in detail. The physical background of chiral selective separations is briefly shown in CE. Their interaction mechanisms are shortly defined. The general trend of their use is statistically evaluated. Most frequently used CDs and CD derivatives are characterized. Advantages of ionizable CDs and single-isomer derivatives are shown. The general trend of their use is established.

Role of chemical structure in stereoselective recognition of beta-blockers by cyclodextrins in capillary zone electrophoresis

Most of the beta-blocking drugs for treating diseases of the cardiovascular system are chiral aryloxy-propanolamine derivatives. Tipically, the S(-) enantiomers are more active than the R(+) enantiomers. Only some of them (for example timolol) are used as single enantiomers, the others are employed as racemates. For the determination of the enantiomeric purity of timolol European Pharmacopoeia prescribes an HPLC method using chiral stationary phase. However, the use of chiral capillary zone electrophoresis for the determination of the enantiomeric purity is of pharmaceutical interest. This study describes the application of various cyclodextrin derivatives, hydroxypropyl-beta-cyclodextrin, randomly methylated beta-cyclodextrin, sulphated beta-cyclodextrin and sulphated alpha-cyclodextrin for the stereoselective analyses of beta-blockers. Baseline separation was obtained for bopindolol, carvedilol, mepindolol, pindolol and alprenolol, while only partial separation was observed for sotalol, propranolol, oxprenolol, atenolol, bisoprolol, bupranolol, and metoprolol. The uneven molecular recognition of the enantiomers of the beta-blockers, especially of the optical isomers of labetalol and nadolol, showed the importance of the chemical nature of the separators and the analytes.

Advances in chiral separation using capillary electromigration techniques

This review gives an overview of recent developments in CZE, EKC, and CEC covering the literature since the year 2004. Since there appeared a special issue on applications, this review focuses on the progress in electromigration techniques and new methodological developments. New techniques, new chiral selectors as well as new chiral stationary phases for CEC are discussed.

Nonaqueous capillary electrophoresis in pharmaceutical analysis

This review presents different solvents and electrolytes commonly used as BGEs in NACE for the analysis of pharmaceutical compounds. Most NACE applications carried out since 1998 for the analysis of compounds of pharmaceutical interest are presented in four tables: (i) analysis of drugs and related substances, (ii) analysis of chiral substances, (iii) analysis of phytochemical extracts and (iv) analysis of drugs in biological fluids. These selected examples are used to illustrate the interest in NACE versus conventional aqueous CE.

Nonaqueous capillary electrophoretic behavior of 2-aryl propionic acids in the presence of an achiral ionic liquid

Ionic liquids (ILs) appear really attractive as electrolyte additives in nonaqueous capillary electrophoresis (NACE). These salts may offer new possibilities of interactions to modulate analyte effective mobilities. The presence of 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIM NTf2) in acetonitrile/alcohol background electrolytes (BGEs) was investigated in this work. The aim of this study was to elucidate the influence of the IL concentration on the electrophoretic behavior of four arylpropionic acids and to identify the interactions between the analytes and the IL cation. The influence on mobility of the IL concentration, the nature and the proportion of the organic solvents, and the concentration of the ionic components of the BGE was first studied by a univariate approach. A four-factor D-optimal experimental design was then applied to provide a deeper insight into analyte interaction with IL cation present both free in BGE and adsorbed onto the capillary wall.

Enantiomeric separation of acidic compounds using single-isomer amino cyclodextrin derivatives in nonaqueous capillary electrophoresis

The enantiomeric separation of a series of acidic pharmaceuticals (mostly nonsteroidal anti-inflammatory drugs) has been investigated in NACE systems using single-isomer amino beta-CD derivatives. The first part of this study consisted of the selection of the basic experimental conditions to separate efficiently the enantiomers of acidic drugs. Several parameters, such as the nature of the ionic BGE components, were studied and a methanolic solution of ammonium acetate containing the cationic CD was selected as BGE. A D-optimal design with 20 experimental points was then applied and the nature and concentration of the CD were found to have a significant effect on the enantiomeric resolution for all studied compounds. Resolution (R(s)) values were always higher with 6-monodeoxy-6-mono(3-hydroxy)propylamino-beta-CD (PA-beta-CD) compared to those obtained with 6-monodeoxy-6-mono(2-hydroxy)propylamino-beta-CD (IPA-beta-CD). However, the latter led to shorter migration times. Generic NACE conditions were then selected by means of the multivariate approach in order to obtain the highest R(s) values in a minimum amount of time. Finally, dependence of separation selectivity, resolution, as well as mobility difference on chiral selector concentration was discussed and binding constants with PA-beta-CD were estimated for the two enantiomers of one of the model compounds, suprofen in these NACE systems.

Recent advances in pharmaceutical applications of chiral capillary electrophoresis

This review article summarizes developments and applications of chiral capillary electrophoresis (CE) in the pharmaceutical field published from January 2004 to June 2005. Due to the tremendous number of publications, this article is aimed to focus on major developments in chiral separations and some selected applications rather than to provide a descriptive overview of all published papers. Valuable information is also collected from several excellent reviews published during this period. Developments are classified according to CE modes, namely capillary zone electrophoresis (CZE), micellar electrokinetic capillary chromatography (MEKC), microemulsion electrokinetic chromatography (MEEKC). In the CZE section, different types of chiral selectors including cyclodextrins, oligo- and polysaccharides, crown ethers, macrocyclic antibiotics, ligand exchange systems and proteins are described. Nonaqueous capillary electrophoresis is also included in this section. Coupling CE to MS is discussed in a separate part, followed by a summary of selected pharmaceutical applications of enantioselective CE. Finally, some conclusions are drawn and prospects of CE in chiral analysis are also drafted.

N-Menthoxycarbonylation combined with trimethylsilylation for enantioseparation of β-blockers by achiral dual-column gas chromatography

Solvent extractive two-phase menthoxycarbonyl (MnOC) derivatization was combined with trimethylsilyl (TMS) reaction for enantioseparation of beta-blockers by gas chromatography employing achiral DB-5 and DB-17 dual-columns of different polarity. beta-Blockers in alkaline solution were vortex-mixed with menthyl chloroformate present in dichloromethane to be extracted as diastereomeric N-MnOC derivatives. The subsequent O(N)-TMS reaction allowed complete enantioseparations of two beta-blockers and partial separations of five as N-MnOC/O(N)-TMS derivatives in a single analysis. The temperature-programmed retention index sets were characteristic of each derivative, facilitating chiral discrimination of each enantiomer.