New strategies for chiral analysis of drugs by capillary electrophoresis

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.

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.

Potentialities of ITP-CZE method with diode array detection for enantiomeric purity control of dexbrompheniramine in pharmaceuticals

The present work illustrates potentialities of on-line combined isotachophoresis-capillary zone electrophoresis (ITP-CZE) separation techniques coupled with on-capillary diode array detector (DAD) for enantiomeric purity testing of drugs in pharmaceuticals. The general advantages of the proposed method are its (i) high selectivity, (ii) low concentration limit of detection (LOD) obtainable, (iii) enhanced sample loadability, and (iv) enhanced reliability. For separation of brompheniramine (BP) enantiomers, serving as model analytes, carboxyethyl-beta-cyclodextrin (CE-beta-CD) was appropriate chiral selector providing complete enantioresolution. Given by a high sample load capacity (30 microl sample injection volume) and preconcentration of the analytes in ITP stage, concentration LOD of levobrompheniramine (LBP), serving as model impurity, was 2.5 ng/ml (8 x 10(-9)mol/l). Such separation and detection conditions enabled to easily determine LBP in samples containing a 10(3) excess of dexbrompheniramine (DBP). DAD detection in comparison with single wavelength detection can enhance value of analytical information when analytes and interferents have different spectra (distinguishing impurities in analyte zone, confirmation of migration positions of migrants). In this context purity of BP zones was confirmed with higher reliability in pharmaceutical sample. Moreover, distinguishing the trace analyte signal superposed on the baseline noise was provided with sufficient reliability (for this purpose the background correction and smoothing procedure had to be applied to the raw DAD spectra). Successful validation and application of the proposed ITP-CZE-DAD method suggest its routine use for the enantiomeric purity testing of pharmaceuticals.

Chiral separations by capillary electrophoresis: Recent developments and applications

This paper provides an overview of the different classes of chiral selectors that are used in CE. The main properties of every class are described, together with the mechanism of enantioseparation. Newly introduced selectors are also discussed. Pharmaceutical and biomedical applications published from January 2004 till March 2005 are summarized.

Determination of the hydrolysis rate constants and activation energy of aesculin with capillary electrophoresis end-column amperometric detection

Aesculetin is the product of the hydrolysis reaction of aesculin. A high sensitivity and good repeatability method based on capillary electrophoresis with amperometric detection (CE-AD) was developed for simultaneous determination of aesculin and aesculetin in the hydrolysate of aesculin. Under the optimum condition: 10mmol/L KH(2)PO(4)-5mmol/L Na(2)B(4)O(7) (pH 6.0) buffer, separation at 18kV and +900mV (versus Ag/AgCl) as the detection potential, the hydrolysis rate constants of aesculin hydrolysis at 25, 30, 35, 40 and 45 degrees C in 0.1mol/L KOH were obtained as 1.45x10(-2)min(-1), 2.01x10(-2)min(-1), 2.93x10(-2)min(-1), 3.76x10(-2)min(-1) and 5.05x10(-2)min(-1), respectively. It was calculated that the activation energy for aesculin hydrolysis was 49.4kJ/mol.

Enantioselective analysis of cetirizine in pharmaceuticals by cyclodextrin-mediated capillary electrophoresis

The present paper demonstrates the potential of cyclodextrin (CD)-mediated CE for the chiral analysis of a drug of zwitterionic nature, viz. cetirizine (CET). Various separation mechanisms were applied and several parameters affecting the separation were studied, including the type and concentration of chiral selector, coselector, and carrier ion, and pH of buffer. The optimal separation conditions were based on a medium buffer pH (approximately 5.2) (migration velocity of CET molecule was near to zero) and a highly substituted CD derivative, sulfated-beta-CD, serving as an analyte carrier in the anionic regime of the separation with suppressed electroosmotic flow. In this way, a baseline enantioseparation, reasonable separation efficiency, and short analysis time could be easily achieved. Acceptable validation criteria for sensitivity, linearity, precision, accuracy, and robustness were obtained using a hydrodynamically closed CE separation system. The proposed method was successfully applied to the enantioselective assay of CET in pharmaceutical formulations using fexofenadine (FEX) as an internal standard.

Polymer-brush stationary phases for open-tubular capillary electrochromatography

Synthesis of poly(2-hydroxyethyl methacrylate) (PHEMA) brushes from the inside of silica capillaries by surface-initiated atom transfer radical polymerization (ATRP) yields unique stationary phases for open-tubular capillary electrochromatography (OT-CEC). Although PHEMA brushes have only a small effect on the separation of a set of phenols and anilines, derivatization of PHEMA with ethylenediamine (en) allows baseline resolution of several anilines that co-elute from bare silica capillaries. Derivatization of PHEMA with octanoyl chloride (C8-PHEMA films) affords even better resolution in the separation of a series of phenols and anilines. Increasing the thickness of C8-PHEMA coatings by a factor of 2 enhances resolution for several solute pairs, presumably because of an increase in the effective stationary phase to mobile phase volume ratio. Thus, this work demonstrates that thick polymer brushes provide a tunable stationary phase with a much larger phase ratio than is available from monolayer wall coatings. Through appropriate choice of derivatizing reagents, these polymer brushes should allow separation of a wide range of neutral molecules as well as compounds with similar electrophoretic mobilities.

Chiral separation of dioxopromethazine in eye drops by CZE with charged cyclodextrin

Capillary zone electrophoresis (CZE) with carboxyethyl-beta-cyclodextrin (CE-beta-CD) dissolved in the operating buffer was used for the separation and determination of enantiomers of phenothiazine antihistaminic, dioxopromethazine, in commercial pharmaceutical preparation, eye drops. This chiral selector, negatively charged under given separating conditions (20 mmol/l epsilon -aminocaproic acid, acetic acid, pH 4.5), was effective in enantioresolution of the antihistamine even at its low concentrations (3-6 mg/ml) in the buffer solution. CZE identification and quantitation of the relevant constituents present in the preparation (dioxopromethazine enantiomers, phenylephrine) were based on the response of photometric absorbency detector, operating at a 275 nm detection wavelength. Changes in pH, type and concentration of chiral selector were studied in relation to chiral resolution. Acceptable validation criteria for sensitivity, precision, linearity and repeatability are included.