Enantiomeric resolution of 2-arylpropionic acid nonsteroidal anti-inflammatory drugs by capillary electrophoresis: methods and applications

The 2-arylpropionic acids (2-APAs) are an important group of nonsteroidal anti-inflammatory drugs. These agents, the majority of which are available as racemates, exhibit stereoselectivity in both their action and disposition. Developments in stereoselective separation science methodology, mainly chromatographic, have facilitated an evaluation of the pharmacological properties of the individual enantiomers of these drugs and contributed to our understanding of both their mode(s) of action and disposition. While a number of electrophoretic techniques, including capillary electrophoresis, capillary electrochromatography and isotachophoresis, have been applied to the stereoselective resolution and stereospecific analysis of these agents using a variety of chiral selectors, e.g., cyclodextrins, oligosaccharides, macrocyclic antibiotics, and proteins, the number of published applications in pharmaceutical and biomedical analysis remains relatively limited. However, the utility of electrophoretic techniques for stereospecific analysis may be illustrated using the 2-APAs as typical examples of chiral acidic pharmaceuticals. Applications include: determination of enantiomeric composition following biosynthetic stereoselective hydrolysis; examination of both achiral and chiral impurity profiles in bulk drugs and formulated products; determination of enantiomeric impurities in both bulk drugs and formulated products; examination of configurational stability following stress testing of formulated products; determination of enantiomeric composition and metabolite profile in biological fluids following administration of the racemates and individual enantiomers. It may be anticipated that future exploitation of electrophoretic approaches to the stereospecific analysis of these agents will result in further contributions to our understanding of their stereoselective biological properties and therapeutic use.     

Enantioseparation of 2‐(substituted phenyl)propanoic acids by high‐speed countercurrent chromatography and investigation of the influence of substituents in enantiorecognition

This paper concentrates on the enantioseparation of racemic 2‐(substituted phenyl)propanoic acids by high‐speed countercurrent chromatography with substituted β‐cyclodextrin as the chiral selector, and an investigation of the influence of the substituent on the benzene ring in enantiorecognition between the chiral selector and enantiomer of each racemate is presented. This is an extension research of our previous work on the enantioseparation of 2‐phenyl propanoic acid derivatives, to investigate the relationship between the value of enantioseparation factor and the different substituent on the benzene ring. In total, ten racemic 2‐(substituted phenyl)propanoic acids were investigated, of which four including 2‐(4‐nitrophenyl)propanoic acid, 2‐(4‐methylphenyl)propanoic acid, 2‐(4‐hydroxyphenyl)propanoic acid, and 2‐(4‐chlorophenyl)propanoic acid, were studied by countercurrent chromatography for the first time, and two racemates were successfully enantioseparated. The distribution ratio and enantioseparation factor for all the ten racemates were determined by enantioselective liquid–liquid extraction. The results showed that an electron‐donating group on the benzene ring presents a higher enantiorecognition induced by chiral selector than that of racemates with an electron‐withdrawing group on the benzene ring.     

淀粉及纤维素三(3-三氟甲基苯基氨基甲酸酯)手性固定相的制备及其性能评价

为了扩展多糖类手性固定相的种类,制备了基于淀粉及纤维素三(3-三氟甲基苯基氨基甲酸酯)的涂敷型手性固定相,以正己烷-异丙醇混合液为流动相,对8种手性化合物进行了高效液相色谱拆分。研究表明: 虽然与应用最广泛的分别以淀粉及纤维素三(3,5-二甲基苯基氨基甲酸酯)为手性选择因子的商品化手性柱Chiralpak AD和Chiralcel OD相比,所制备的手性固定相的手性分离能力较低,但纤维素三(3-三氟甲基苯基氨基甲酸酯)手性固定相显示出特异的手性识别能力,一些手性化合物在此固定相上得到了比在Chiracel OD上更好的分离;所制备的手性固定相的手性识别能力随流动相中异丙醇含量的降低而变好,当流动相中正己烷与异丙醇的体积比为95:5时所制备的手性固定相显示出相对较高的手性识别能力;总体来说,淀粉三(3-三氟甲基苯基氨基甲酸酯)手性固定相的手性识别能力稍强于纤维素三(3-三氟甲基苯基氨基甲酸酯)手性固定相,同时两种手性固定相的手性识别能力具有一定的互补性。     

Enantioselective separation of biologically active basic compounds in ultra-performance supercritical fluid chromatography

The enantioseparation of basic compounds represent a challenging task in modern SFC. Therefore this work is focused on development and optimization of fast SFC methods suitable for enantioseparation of 27 biologically active basic compounds of various structures. The influences of the co-solvent type as well as different mobile phase additives on retention, enantioselectivity and enantioresolution were investigated. Obtained results confirmed that the mobile phase additives, especially bases (or the mixture of base and acid), improve peak shape and enhance enantioresolution. The best results were achieved with isopropylamine or the mixture of isopropylamine and trifluoroacetic acid as additives. In addition, the effect of temperature and back pressure were evaluated to optimize the enantioseparation process. The immobilized amylose-based chiral stationary phase, i.e. tris(3,5-dimethylphenylcarbamate) derivative of amylose proved to be useful tool for the enantioseparation of a broad spectrum of chiral bases. The chromatographic conditions that yielded baseline enantioseparations of all tested compounds were discovered. The presented work can serve as a guide for simplifying the method development for enantioseparation of basic racemates in SFC.     

Computer simulation for the convenient optimization of isocratic reversed-phase liquid chromatographic separations by varying temperature and mobile phase strength

The review summarizes the most recent developments in the field of enantioseparation of chiral drugs using capillary electromigration techniques. The basic principles of enantioseparations in CE are discussed. Recent developments in sample introduction, separation and detection in capillary electrophoresis and capillary electrochromatography are summarized. The applications are arbitrarily divided into the following three groups: (a) racemates and artificial mixtures of enantiomers, (b) drug forms and (c) chiral drugs and their metabolites in biological fluids. Among the various techniques involved the relatively new developments such as CEC in aqueous and nonaqueous buffers, on-line CE-MS coupling, etc. are emphasized.     

手性离子液体和纤维素三(3,5-二甲基苯基氨基甲酸酯)混合气相色谱手性固定相研究

合成了手性离子液体[C7H17NO] [(CF3SO2)2]-和纤维素三(3,5-二甲基苯基氨基甲酸酯),将其混合作为气相色谱固定相。研究结果表明:该混合固定相麦氏常数的平均极性值为760;在容量因子大于2的前提下,新型柱的塔板数可达1680块/m;其对手性化合物、特别是对氨基酸类具有良好的分离效果。有10对氨基酸的分离因子大于1.10,展示了该类色谱柱良好的手性应用前景。     

Enantioseparation tuned by solvent polarity on a β‐cyclodextrin clicked chiral stationary phase

The efficient enantioseparation of 26 racemates has been achieved with the perphenylcarbamoylated cyclodextrin clicked chiral stationary phase by screening the optimum composition of mobile phase in high‐performance liquid chromatography. The chromatographic results indicate that both the retention and chiral resolution of racemates are closely related to the polarity of the mobile phases and the structures of analytes. The addition of alcohols can significantly tune the enantioseparation in normal‐phase high‐performance liquid chromatography. The addition of methanol and the ratio of ethanol/methanol or isopropanol/methanol played a key role on the resolution of flavonoids in ternary eluent systems. The chiral separation of flavonoids with pure organic solvent as mobile phase indicates the preferential order for chiral resolution is methanol>ethanol>isopropanol>n‐propanol>acetonitrile.     

Mono‐6A‐(4‐methoxybutylamino)‐6A‐β‐cyclodextrin as a chiral selector for enantiomeric separation

A new member of the family of methoxylalkylamino monosubstituted β‐cyclodextrins, mono‐6^A‐(4‐methoxybutylamino)‐6^A‐β‐cyclodextrin, has been developed as a chiral selector for enantioseparation in capillary electrophoresis. This amino cyclodextrin exhibited good enantioselectivities for 16 model acidic racemates including three dansyl amino acids at an optimum pH of 6.0. Excellent chiral resolutions over six were obtained for α‐hydroxy acids and 2‐phenoxypropionic acids with 3.0 mM chiral selector. The good chiral recognition for α‐hydroxyl acids was attributed to inclusion complexation, electrostatic interactions, and hydrogen bonding. The hydrogen‐bonding‐enhanced chiral recognition was revealed by NMR spectroscopy. The chiral separation of acidic racemates was further improved with the addition of methanol (≤10 vol%) as an organic additive.     

Novel β-cyclodextrin chiral stationary phases with different length spacers for normal-phase high performance liquid chromatography enantioseparation

Cyclodextrin and its derivatives are widely used as selectors of chiral stationary phases (CSPs) for high performance liquid chromatography (HPLC) due to their unique molecular structure and resolution capability. Three mono(6(A)-N-(ω-alkenylamino)-6(A)-deoxy)perphenylcarbamoylated β-cyclodextrin (PICD) based CSPs with different length spacers have been prepared, with their enantioseparation abilities evaluated with 10 model racemates including aromatic alcohols, flavanone compounds, amine and non-protolytic compounds under normal-phase conditions. The effect of spacer length and surface loading on the enantioseparation performance of CSPs is investigated herewith. The results indicate that higher surface loading 6C-PICD displays the best enantioselectivities towards selected racemates under normal-phase conditions.     

Recent studies of docking and molecular dynamics simulation for liquid‐phase enantioseparations

Liquid‐phase enantioseparations have been fruitfully applied in several fields of science. Various applications along with technical and theoretical advancements contributed to increase significantly the knowledge in this area. Nowadays, chromatographic techniques, in particular HPLC on chiral stationary phase, are considered as mature technologies. In the last thirty years, CE has been also recognized as one of the most versatile technique for analytical scale separation of enantiomers. Despite the huge number of papers published in these fields, understanding mechanistic details of the stereoselective interaction between selector and selectand is still an open issue, in particular for high‐molecular weight chiral selectors like polysaccharide derivatives. With the ever growing improvement of computer facilities, hardware and software, computational techniques have become a basic tool in enantioseparation science. In this field, molecular docking and dynamics simulations proved to be extremely adaptable to model and visualize at molecular level the spatial proximity of interacting molecules in order to predict retention, selectivity, enantiomer elution order, and profile noncovalent interaction patterns underlying the recognition process. On this basis, topics and trends in using docking and molecular dynamics as theoretical complement of experimental LC and CE chiral separations are described herein. The basic concepts of these computational strategies and seminal studies performed over time are presented, with a specific focus on literature published between 2015 and November 2018. A systematic compilation of all published literature has not been attempted.     

Enantiomeric separations by nonaqueous capillary electrophoresis

This paper reviews the recent advances in enantioseparations by nonaqueous capillary electrophoresis (NACE) and the effect of organic solvents on mobility of enantiomers, separation selectivity and resolution. In general, the enantioseparation systems in NACE are similar to those of aqueous capillary electrophoresis (CE) except pure organic solvents are used. The influence of important parameters such as concentration and type of chiral selectors, apparent pH, ionic strength, temperature, and control of electroosmotic flow is discussed. In addition, the reported applications of NACE separations of racemates are presented.     

Comparison of the enantioseparation of racemic uridine analogs on Whelk-O 1 and ChiralPak-AD columns

The commercially available, brush-type (S,S)-Whelk-O 1 chiral stationary phase (CSP) has been used to separate 10 racemates of structurally related uridine analogs, potentially anti-viral agents, under various mobile phase compositions, using various temperatures. The enantioseparation was evaluated by comparing the Whelk-O 1 column performance with that of ChiralPak-AD column, reported previously. The comparison involved the role of some distinctive structural features of the racemates, type and composition of the solvent modifiers, as well as effect of temperature on the chiral discrimination. Despite the fact that both columns separate almost all the uridine analogs, significant differences were observed in their chiral recognition, as revealed from their retention, selectivity, resolution and elution order. The chiral recognition processes, responsible for enantioseparation on the Whelk-O 1 column, were relatively more systematic and easier to manipulate than on ChiralPak-AD column. Enantioseparation on the latter are of more complex nature and frequently gave results that were contradictory to the expectations. On the other hand, the performance in the ChiralPak-AD column was superior to that of the Whelk-O 1 column. Limitations in column handling and maintenance (pressure and temperatures) as well as limited solvent choice lead to the preference of the Whelk-O 1 column, in spite of its lower (but adequate) performance.     

Enantiomeric separation of chiral theophylline derivatives by liquid chromatography on cellulose-based sorbents

The enantiomeric resolution of seven closely related theophylline racemates by high-performance liquid chromatography on cellulose-based sorbents, in particular Chiralcel-OD, -OC and -OJ, is described. Although all chiral stationary phases (CSPs) are suitable for the enantioseparation of all racemates investigated, it is obvious that method development is different for each stationary phase. The screenings with the above-mentioned CSP included variation of mobile phase and temperature. It turned out that Chiralcel-OD should be used with 2-propanol in n-hexane as the mobile phase at higher temperatures, whereas Chiralcel-OC performed best with methanol at ambient temperature. Improved enantioseparations were observed for Chiralcel-OJ with increased modifier concentrations in n-hexane at increased temperature.     

Insights into chiral recognition mechanisms in supercritical fluid chromatography. II. Factors contributing to enantiomer separation on tris-(3,5-dimethylphenylcarbamate) of amylose and cellulose stationary phases

In this second part of our work on enantioselective supercritical fluid chromatography (SFC), we investigate the factors participating in the chiral recognition process on tris-(3,5-dimethylphenylcarbamate) of amylose and cellulose chiral stationary phases (CSPs). 135 racemates with diverse structures were analysed under identical SFC conditions on both stationary phases. The possibility of identifying the differential interactions of an enantiomer pair within the chromatographic system is assessed using a modified version of the solvation parameter model and factorial discriminant analysis. It is illustrated that one relationship of intermolecular interactions is insufficient to express the enantioseparation of different groups of racemates. An innovative approach is used in unravelling the interactions taking part in the enantiorecognition process. Different intermolecular interactions participating in the enantiomeric separation are demonstrated between the two stationary phases.     

A novel pillar[3]trianglimine macrocycle with a deep cavity used as a chiral selector to prepare a chiral stationary phase by thiol-ene click reaction for enantioseparation in high-performance liquid chromatography

A chiral pillar[3]trianglimine (C₆₀H₇₂N₆O₆) with a deep cavity has been developed as a chiral selector and bonded to thiolated silica by thiol-ene click reaction to fabricate a novel chiral stationary phase for enantioseparation in high-performance liquid chromatography. The enantioseparation performance of the fabricated chiral stationary phase has been evaluated by separating various racemic compounds, including alcohols, esters, amines, ketones, amino acids, and epoxides, in both normal-phase and reversed-phase elution modes. In total, 14 and 17 racemates have been effectively separated in these two separation modes, respectively. In comparison with two widely used chiral columns (Chiralcel OD-H and Chiralpak AD-H), our novel chiral stationary phase offered good chiral separation complementarity, separating some of the tested racemates that could not be separated or were only partially separated on these two commercial columns. The influences of analyte mass, mobile phase composition, and column temperature on chiral separation have been investigated. Good repeatability, stability, and column-to-column reproducibility of the chiral stationary phase for enantioseparation have been observed. After the fabricated column had been eluted up to 400 times, the relative standard deviations (n = 5) of resolution (Rs) and retention time of the separated analytes were < 0.39% and < 0.20%, respectively. The relative standard deviations (n = 3) of Rs and retention time for column-to-column reproducibility were < 4.6% and < 5.2%, respectively. This study demonstrated that the new chiral stationary phase has great prospects for chiral separation in high-performance liquid chromatography.     

毛细管电泳新型手性分离体系研究进展

在现代分离科学中,手性化合物的分离分析一直是研究的重点和难点。相比于高效液相色谱（HPLC）、气相色谱（GC）等传统色谱分析方法,毛细管电泳（CE）技术凭借其高效率、低消耗、分离模式多样化等诸多优势,已经发展成为手性分离研究领域最有应用前景的分析方法之一。近年来,研究人员在CE手性分析方法的构建过程中,基于毛细管电动色谱（EKC）、配体交换毛细管电泳（LECE）、毛细管电色谱（CEC）等各种基础电泳模式,不断地对传统手性分离体系进行优化和改造,构建出了许多高性能的新型手性CE分离体系。如利用各类功能化离子液体以"手性离子液体协同拆分""手性离子液体配体交换""离子液体手性选择剂"等模式设计出多种基于离子液体的CE手性分离体系;利用纳米材料独特的尺寸效应、多样性、可设计性等特点,直接或与传统手性选择剂有机结合构建CE手性分离体系。此外,金属有机骨架材料修饰、低共熔溶剂修饰、非连续分段式部分填充等各式新颖的CE手性分离体系也都被研究人员成功开发,并表现出较大的发展潜力。该综述将对近年来（尤其是2015~2019年）此类新型CE手性分离体系的发展状况进行梳理,并结合相应的手性识别机理研究和手性CE方法实际应用情况,对该领域存在的问题及发展前景进行分析和展望。     

Simultaneous microbatch screening of enantiorecognition on solid chiral selectors using selected mixtures of test-racemates: a case study on cellulose tris(alpha-phenylpropionate) with configurational diversity

An improved methodology for microbatch screening in the liquid-solid enantioselective adsorption of racemates by solid chiral selector is presented and illustrated by the evaluation of a series of six cellulose tris(alpha-phenylpropionate) presenting a configurational diversity and cellulose tris (4-methylbenzoate). Analyses were performed on 5 mg scale of chiral selector and 150 microL of supernatant containing a mixture of three test-racemates. Fifteen test-racemates were chosen according to their molecular diversity using a hierarchical clustering approach for seven selected three-dimensional molecular properties. The 15 racemates were sorted in five test mixtures composed of three racemates each according to the following constraints: each triplet of test-racemates can be analyzed without peak overlap on a commercially available chiral stationary phase (CSP). The designed five groups of three racemates may be used for the evaluation of other chiral selectors in view of the preparation of CSPs. This methodology improves the throughput of the evaluation and could be automated.     

Simultaneous enantioseparation of four beta2-agonists by capillary electrophoresis with cyclodextrin additives. Study of the enantioselective mechanism

The simultaneous capillary electrophoretic enantioseparation of adrenergic beta(2)-agonists enantiomers (trantinterol, mabuterol, clenbuterol, bambuterol) was studied with beta-cyclodextrin, ethyl-beta-CD, methyl-beta-CD, hydroxypropyl-beta-CD, and hydroxyethyl-beta-CD as chiral selector. The type and concentration of the chiral selector and buffer pH played a very important role in the enantioseparation of the analyzed compounds. Hydroxypropyl-beta-CD was found to be the most effective complexing agent and allowed excellent chiral/achiral resolutions compared to the other CDs. The simultaneous enantioseparation of four beta(2)-agonists was achieved using 100 mM citric acid-10 mM Na(2)HPO(4) buffer at pH 2.5 containing 120 mM hydroxypropyl-beta-CD with an applied voltage of 20 kV. Method validation in terms of repeatability, linearity, and limits of detection and quantification was performed. The effect of structural features of analytes on R(s) and t(m) was studied. Complexation binding constants for the interactions between the four compounds and three different CDs were evaluated for elucidating the enantioseparation mechanism. It was found that very small differences in the chemical structure of the analytes resulted in significant changes in stereoselective recognition.     

Evaluation of experimental strategies for the development of chiral chromatographic methods based on diastereomer formation

The past two decades has seen explosive growth in the field of chirality as illustrated by the rapid progress in the various facets of this intriguing field. Firstly, it is the basic understanding of the importance of chirality and, secondly, the awareness of the therapeutic pitfalls due to failure to recognize chirality that have paved the way for a rejuvenated interest in the field. Needless to say that the impetus for chiral separation advancement and enhancement has been found to be the highest in the past decade and still continues to be an area of high focus. In this regard, both direct and indirect separation approaches have been instrumental in placing into literature stereoselective pharmacokinetic and pharmacodynamic data of plethora of drug racemates. Also, today, the cloud of uncertainty associated with the development of a chiral molecule is a thing of the past because the field is so evolved and numerous options are available for stereoselective analysis. However, the decision to advance either a single enantiomer or a racemate for development has to be made by a rational approach with adequate justification. Although indirect method of chiral separation has been well established with numerous examples of well-documented cases of stereoselective pharmacokinetic data, there is a growing need for a review that provides a strategic overview of considerations and key issues for developing chromatographic methods based on diastereomer formation. This review provides a general framework for the exploratory planning and a definitive game plan for the establishment of chiral methods based on diastereomer formation. Also, it provides an exhaustive list of applications of numerous chiral derivatization reagents that have been used in the generation of stereoselective pharmacokinetic data.     

(3,5-二甲基苯基氨基甲酸酯)衍生的纤维素手性固定相

纤维素三（3,5-二甲基苯基氨基甲酸酯）是液相色谱中使用最广泛的手性柱。该文详细地研究了不同程度衍生的纤维素（3,5-二甲基苯基氨基甲酸酯）以及不同硅胶（粗制硅胶、氨丙基粗制硅胶、精制硅胶、氨丙基精制硅胶、大孔硅胶、氨丙基大孔硅胶）作为支撑体对该柱手性分离能力的影响。自制了13根手性色谱柱,分别考察了其对16种外消旋体的拆分,分离结果显示：三取代纤维素柱 > 二取代纤维素柱 > 纤维素柱;精制硅胶和大孔硅胶优于粗制硅胶,大孔硅胶的柱压更低;硅胶的氨丙基化对手性选择性有一定的影响;这些手性柱之间具有一定的互补性,尤其是纤维素柱。该文有助于人们更深刻地理解和更好地把握高效液相色谱手性柱的制备。