[Research progress on chiral separation of amphetamines, ketamine, cathinones]

Enantiomers are ubiquitous in nature, and they are especially important in the field of pharmaceutical chemistry. Although the enantiomers of chiral drugs have identical chemical structures, they differ notably in their pharmacological, toxicological, pharmacokinetic, metabolic, and other biological activities. The same is true for amphetamines, ketamine, and cathinones, as the chiral separation of these three drugs is representative of drugs. Gas chromatography (GC), high performance liquid chromatography (HPLC), and capillary electrophoresis (CE) are widely used for the chiral separation of these three kinds of drugs. There are some similarities among the three methods for the chiral separation of amphetamines, ketamine, and cathinones: n-trifluoroacetyl-L-prolinyl chloride and (+)R-α-methoxy-α-trifluoromethylphenylacetic acid are the two typical chiral derivatization reagents used in GC. In HPLC, three kinds of chiral stationary phases are used: proteins, polysaccharides, and macrocyclic antibiotics. Cyclodextrin and its derivatives are most commonly used in CE. However, these three methods have inherent shortcomings. In the case of GC, impurities produced during chiral derivatization may interfere with the analysis, and high reaction temperatures affect the efficiency of chiral separation. HPLC has limited application scope and is expensive. In CE, there has no established process to determine the appropriate chiral selector. In recent years, research into application of the chiral separation of the above-mentioned three kinds of drugs has its own characteristics in forensic toxicology. The chiral separation of amphetamine drugs is mostly used to infer the prototype and synthesis route of drugs on the market. The chiral separation of ketamine involves a variety of biological samples. For cathinones, chiral separation methods emphasize their wide applicability. In this review, 66 reports published in professional local and overseas magazines during the past decade are collated. The characteristics of the enantiomers of amphetamines, ketamine, and cathinones as well as the mechanism of chiral recognition are briefly introduced. The commonness of the research and the application of chiral separation in forensic toxicology are reviewed. This paper proposes that the chiral separation of drugs can be further investigated from the following three aspects: 1) the use of computer technology to establish a molecular model for exploring the mechanism of chiral recognition; 2) developing new technologies for chiral separation and carrying out commercial research on the supercritical fluid method; 3) applying chiral separation to judicial practice, pharmaceutical research and development, and other practical fields.

Ezetimibe: A Review of Analytical Methods for the Drug Substance, Pharmaceutical Formulations and Biological Matrices

Ezetimibe (EZM) is a selective inhibitor of the sterol transporter Niemann-Pick C1-Like 1 in the small intestine used as an adjunctive therapy to lower cholesterol levels in cases of hyperlipidemia. The goal of this work was to summarize the main physical-chemical, pharmacological and pharmacokinetic characteristics of EZM, as well as to describe the main analytical methodologies for the quantification of the drug. Methods described in the United States Pharmacopeia for EZM raw material and tablets were also presented. The drug has a large number of process-related impurities and degradation products and needs strict quality control of its impurities. Specific chiral methods for the evaluation of its chiral impurities are also a need for EZM. The main advantages and disadvantages of the compiled analytical methods were presented, as well as the limits of detection and quantitation. The fastest and most efficient methods were highlighted. Most methods for analyzing EZM used C₈ or C₁₈ stationary phases in gradient mode with binary mobile phases containing acetonitrile and an acidic buffer solution with ultraviolet detection. For analysis of EZM in biological matrices, liquid chromatography-tandem mass spectrometry is generally employed using electron spray ionization in negative ionization mode using multiple reaction monitoring. Different methods in the literature evaluate a large number of impurities for EZM, however new stability-indicating high-performance liquid chromatography methods for the drug are still needed.

Comparison in enantioseparation performance of chiral stationary phases prepared from chitosans of different sources and molecular weights

The aim of the present study was to compare the enantioseparation performance of chiral stationary phases (CSPs) which were derived from chitosans of different sources and molecular weights. Therefore, chitosans of shrimp and crab shells were prepared. The viscosity-average molecular weights of the chitosans both prepared from shrimp and crab shells were 2.8 × 10⁵ and 1.4 × 10⁵. The chitosans were isobutyrylated yielding isopropylcarbonyl chitosans which were then derivatized with 4-methylphenyl isocyanate to provide chitosan 3,6-bis(4-methylphenylcarbamate)-2-(isobutyrylamide)s. The chitosan 3,6-bis(4-methylphenylcarbamate)-2-(isobutyrylamide)s were used as chiral selectors (CSs) with which the corresponding CSPs were prepared. With the same chiral analytes and under the same mobile phase conditions, the enantioseparation capability of the CSPs was evaluated by high-performance liquid chromatography. In two CSs prepared from the same source, the one with higher molecular weight showed better enantioseparation capability; in two CSs prepared with the chitosans of the same molecular weight, the one derived from shrimp shell exhibited better performance. With regard to the two shrimp chitosan CSs, most of chiral analytes interacted more strongly with the one with lower molecular weight, and an opposite trend was found for the two crab chitosan CSs. Based on the results observed in the present study and in previous work, we believe that the influence of molecular weight on CSP enantioseparation performance is related to the substituent introduced in the CS molecule.

Chiral separation and thermodynamic investigation of WCK 3023: A novel oxazolidinone antibacterial agent, application to pre-clinical pharmacokinetic study

A chiral liquid chromatographic method was developed and validated for the quantification of R-enantiomer impurity (RE) in WCK 3023 (S-enantiomer), a new drug substance. The separation was achieved on Chiralpak IA (amylose-based immobilized chiral stationary phase), using a mobile phase consisting of n-hexane-ethanol-trifluoroacetic acid (70:30:0.2, v/v/v) at a flow rate of 1.0 mL/min. The method was extensively validated for the quantification of RE in WCK 3023 and proved to be robust. For RE the detector response was linear over the concentration range of 0.11-5 μg/mL. The limit of quantitation and limit of detection for RE were 0.11 and 0.04 μg/mL respectively. Average recovery of the RE was in the range of 98.11-99.55%. The developed method was specific, sensitive, precise and accurate for quantitative determination of RE in WCK 3023. The impact of thermodynamic parameters on the chiral separation was evaluated. The method was employed for controlling the enantiomeric impurity in the lots of WCK 3023 used for pre-clinical studies. The method was successfully applied to evaluate the possible conversion of WCK 3023 to RE in rat serum samples during pre-clinical pharmacokinetic studies.

Structure screening and performance restoration of chiral separation materials based on chitosan derivatives

In order to screen chiral selector structure and find structure-property relationship, ten chitosan 3,6-bis(phenylcarbamate)-2-(cyclopropylformamide)s were synthesized from which corresponding chiral stationary phases were prepared. Enantioseparation capability and mobile phase tolerability of the chiral stationary phases were evaluated. The chiral selectors with 3-chloro-4-methylphenyl, 4-chlorophenyl, 3-chlorophenyl, 3,5-dichlorophenyl and 4-trifluoromethoxyphenyl groups demonstrated powerful enantioseparation capability. Enantioseparation capability was found to be dependent on a match between the substituent at C2 and the one attached to phenyl group at C3/C6 of glucose unit in the chitosan derivatives. Moreover, the tolerability tests revealed that the developed chiral stationary phases were highly tolerable to pure ethyl acetate, pure acetone and n-hexane/tetrahydrofuran of various ratios. In addition, n-hexane/tetrahydrofuran was found to be a modifier to adjust suprastructure of the chitosan derivatives, resulting in an improvement or restoration in enantioseparation. This observation implies n-hexane/tetrahydrofuran may make a damaged chiral selector restore its enantioseparation capability.

Enantiomeric separation of prothioconazole and prothioconazole-desthio on chiral stationary phases

Prothioconazole is a type of broad-spectrum triazole thione fungicide developed by the Bayer Company. Prothioconazole-desthio is the main metabolite of prothioconazole in the environment. In our study, enantiomeric separation of prothioconazole and prothioconazole-desthio was performed on various chiral stationary phases (CSPs) by high-performance liquid chromatography (HPLC). It was found that polysaccharide CSPs showed better ability than brushing CSPs in enantiomeric separation. The successful chiral separation of prothioconazole could be achieved on self-made Chiralcel OD, commercialized Chiralcel OJ-H and Lux Cellulose-1. Chiralpak IA, Chiralpak IB, Chiralpak IC, Chiralcel OD, Chiralpak AY-H, Chiralpak AZ-H, and Lux Cellulose-1 realized the baseline separation of prothioconazole-desthio enantiomers. Simultaneous enantiomeric separation of prothioconazole and prothioconazole-desthio was performed on Lux Cellulose-1 using acetonitrile (ACN) and water as mobile phase. In most cases, low temperature favored the separation of two compounds. The influence of the mobile phase ratio or type was deeply discussed. We obtained larger Rs and longer analysis time with a smaller proportion of isopropanol (IPA) or ethanol and more water content at the same temperature. The ratio of ACN and water had influences on the outflow orders of prothioconazole-desthio enantiomers. This work provides a new approach for chiral separation of prothioconazole and prothioconazole-desthio with a discussion of chiral separation mechanism on different CSPs.

Study on the HPLC-based separation of some ezetimibe stereoisomers and the underlying stereorecognition process

The enantioseparation of ezetimibe stereoisomers by high-performance liquid chromatography on different chiral stationary phases, ie, 3 polysaccharide-based chiral columns, was studied. It was observed that cellulose-based Chiralpak IC column exhibited the best resolving ability. After the optimization of mobile phase compositions in both normal and reversed phase modes, satisfactory separation could be obtained on Chiralpak IC column, especially in normal phase mode. The use of prohibited solvents as nonstandard mobile phase gave rise to better resolution than that of standard mobile phases (n-hexane/alcohol system). In addition, the presence of ethanol in nonstandard mobile phase has played an important role in enhancing chromatographic efficiency and resolution between ezetimibe stereoisomers. Various attempts were made to comprehensively compare the chiral recognition capabilities of immobilized versus coated polysaccharide-based chiral columns, amylose-based versus cellulose-based chiral stationary phases, reversed versus normal phase modes, and standard versus nonstandard mobile phases. Moreover, possible solute-mobile phase-stationary phase interactions were derived to explain how stationary and mobile phases affected the separation. Then the method validation with respect to selectivity, linearity, precision, accuracy, and robustness was carried out, which was demonstrated to be suitable and accurate for the quantitative determination of (RRS)-ezetimibe impurity in ezetimibe bulk drug.

Eluent Tolerance and Enantioseparation Recovery of Chiral Packing Materials Based on Chitosan Bis(Phenylcarbamate)-(n-Octyl Urea)s for High Performance Liquid Chromatography

The goal of the present work was to study the influence of the swelling of chitosan derivatives on the enantioseparation and the separation performance recovery of chiral stationary phases (CSPs) based on these derivatives. Therefore, six chitosan bis(phenylcarbamate)-(n-octyl urea)s were synthesized, which were coated on macroporous 3-aminopropyl silica gel affording new CSPs. Most of the CSPs demonstrated strong enantioseparation capability for the tested chiral compounds. The swelling capacity of the chitosan bis(phenylcarbamate)-(n-octyl urea)s in ethyl acetate, acetone and tetrahydrofuran (THF) was evaluated. Among the chitosan derivatives, the chitosan bis(3,5-dichlorophenylcarbamate)-(n-octyl urea) polymer showed the highest swelling capacity in ethyl acetate and THF. The polymer-based CSPs could be utilized with pure ethyl acetate and a normal phase containing 70% THF, but was damaged by pure THF. On the other hand, the separation performance of the damaged CSP could be recovered after it was allowed to stand for a period of time. The observations are important for the development and application of polysaccharide derivative-based CSPs.