Chiral separation of new designer drugs (Cathinones) on chiral ion-exchange type stationary phases

Zeolite-based chiral ion-exchangers for chromatographic enantioseparations and potential applications in membrane separation processes

Chiral resolution of racemic compounds represents an important task in research and development and, most importantly, in the large-scale production of pharmaceuticals. Zeolites, which are already frequently utilized for their unique properties, represent materials that can be used for the development of new chiral stationary phases for liquid chromatography, simulated moving bed or enantioselective membranes. The aim of this study was to modify a series of MWW zeolites by a chiral anion-exchange type selector thereby creating a chiral stationary phase for enantiomeric resolution of acidic compounds. To evaluate the applicability of the prepared chiral stationary phase in liquid chromatography, we used N-protected amino acids as model analytes. First, we tested the new sorbents preferential sorption using N-(3,5-dinitrobenzoyl)leucine. We observed outstanding sorption properties of a zeolite-based sorbent (MCM-36), which were comparable to spherical chromatographic silica. This particular material was subsequently packed into a chromatographic column, which was tested under polar organic mode HPLC conditions facilitating baseline resolution of 5 out of 8 N-protected amino acids. Although the chromatographic performance shows several drawbacks (high backpressure, low column efficiency), it clearly documents the potential of the novel materials in chiral separation. To the best of our knowledge, this is the first example of the preparation of the chiral stationary phase based on MWW zeolites ever.

Chiral zwitterionic stationary phases based on Cinchona alkaloids and dipeptides - design, synthesis and application in chiral separation

Chiral resolution of polar organic compounds such as amino acids and peptides represents an important chromatographic task due to increasing significance of natural species, which play important signaling and regulatory roles in the living organisms. Despite the number of available chiral stationary phases, this task remains challenging, since not many of the commercially available systems are capable to resolve non-derivatized zwitterionic species. In this study, we present a target-oriented design of a new class of chiral selectors. Pursuing the goal to separate amino acids, and especially short peptides, we have combined Cinchona alkaloids - quinine and quinidine - with three different biogenic dipeptides. We have synthesized six different chiral stationary phases, with selector loading of ∼200 μmol g-1, and tested their chiral recognition capabilities for acidic, basic and zwitterionic analytes using various mobile phases. We have observed that all chiral stationary phases retain the chiral anion exchange capability known for commercially available Cinchona-based columns leading to baseline or partial resolution of six out of ten analytes. The performance in chiral resolution of basic analytes is not optimum due to the weak cation exchange character of the peptidic residue. However, we report on encouraging results in the chiral resolution of short peptides, for which, depending on their structure, we see the chiral resolution of up to three stereoisomers (from four possible) in a preliminary screening.

Enantioseparation and a comprehensive spectroscopic analysis of novel synthetic cathinones laterally substituted with a trifluoromethyl group

Recently, the number of structural modifications of synthetic cathinones has been growing making them the second largest group of new psychoactive substances in Europe. Although they are abused because of their various psychoactive effects, some compounds from this group also serve as pharmaceuticals. Since synthetic cathinones are chiral molecules with one chiral center, their biological, toxicological, and pharmacological properties may significantly differ according to their absolute configuration and enantiomeric excess. In this study, we have synthesized two substances bearing a pharmacologically interesting trifluoromethyl group and developed a chiral liquid chromatography method using a polysaccharide chiral stationary phase to separate the corresponding enantiomers of both these drugs. Subsequently, we utilized molecular spectroscopic methods including chiroptical (electronic circular dichroism and vibrational circular dichroism) and non-polarizable (infrared and ultraviolet absorption) spectroscopies. In combination with density functional theory calculations, we have obtained stable conformers of selected enantiomers in solution and their relative abundances, which we used to simulate their spectra. The experimental and calculated data have been used to elucidate the 3D structure of the enantiomerically pure compounds and assign the absolute configuration of all prepared compounds.

4-Isobutylmethcathinone-A Novel Synthetic Cathinone with High In Vitro Cytotoxicity and Strong Receptor Binding Preference of Enantiomers

New psychoactive substances and among them synthetic cathinones represent a significant threat to human health globally. However, within such a large pool of substances derived from a natural compound ((S)-cathinone), substances with important pharmaceutical uses can be identified, as already documented by bupropione. Therefore, this work aimed to find a synthetic pathway for a novel synthetic cathinone, namely 4-isobutylmethcathinone, and describe its spectroscopic properties and biological activity in vitro. Since cathinones comprise a chiral center in their structure, a method for chiral separation of the substance was elaborated using high-performance liquid chromatography on an analytical and preparative scale. Preparative enantioseparation on a polysaccharide column provided a sufficient amount of the drug for the chiroptical studies leading to the determination of the absolute configuration of enantiomers as well as for their subsequent in vitro cytotoxicity study. The cytotoxicity induced by 4-isobutylmethcathinone was determined in human cells derived from the urinary bladder (5637), neuroblastoma (SH-SY5Y), microglia (HMC-3), and hepatocellular carcinoma (Hep G2), in which the IC₅₀ values after 72 h reached an 18-65 µM concentration. This is significantly higher cytotoxicity in comparison with other synthetic cathinones. In the receptor binding studies, a significant difference in the agonistic effect on dopamine and adrenergic receptors of individual enantiomers was observed. The lack of binding affinity towards the serotonin receptors then relates 4-isobutylmethcathinone to the family of monoamine drugs, such as 3,4-methylenedioxymathamphetamine (ecstasy, MDMA).

Synthetic Cathinones: Recent Developments, Enantioselectivity Studies and Enantioseparation Methods

New psychoactive substances represent a public health threat since they are not controlled by international conventions, are easily accessible online and are sold as a legal alternative to illicit drugs. Among them, synthetic cathinones are widely abused due to their stimulant and hallucinogenic effects. To circumvent the law, new derivatives are clandestinely synthesized and, therefore, synthetic cathinones keep emerging on the drug market, with their chemical and toxicological properties still unknown. In this review, a literature assessment about synthetic cathinones is presented focusing on the recent developments, which include more than 50 derivatives since 2014. A summary of their toxicokinetic and toxicodynamic properties are also presented. Furthermore, synthetic cathinones are chiral compounds, meaning that they can exist as two enantiomeric forms which may present different biological and toxicological activities. To analyze the enantiomers, the development of enantiomeric resolution methods for synthetic cathinones is crucial. Many methods have been reported over the years that include mostly chromatographic and electromigration techniques, with liquid chromatography using chiral stationary phases being the technique of choice. This review intended to present an overview of enantioselectivity studies and enantioseparation analysis regarding synthetic cathinones, highlighting the relevance of chirality and current trends.

Design and synthesis of naphthalene-based chiral strong cation exchangers and their application for chiral separation of basic drugs

In continuation of our efforts to synthesize a highly dedicated strong cation exchanger, we introduce four chiral stationary phases based on a laterally substituted naphthalene core featuring chiral 2-aminocyclohexansulfonic acid as the chiral cation-exchange site. The selectors were modified with two different terminal units, which enabled immobilization to the silica support by thiol-ene radical reaction or azide-yne click chemistry. The chromatographic parameters of these chiral stationary phases were determined using a set of chiral amines, mainly from the family of β-blocker pharmaceuticals. The chiral stationary phases immobilized by means of click chemistry were found to be superior to those possessing the sulfide linker to the silica support. The chromatographic results and visualization of density functional theory-calculated conformations of the selectors hint at a combination of a steric and electronic effect of the triazole ring in the course of chiral resolution of the target analytes.

Enantioseparation performance of superficially porous particle vancomycin-based chiral stationary phases in supercritical fluid chromatography and high performance liquid chromatography; applicability for psychoactive substances

Novel psychoactive substances (NPS) are synthetic compounds that have been designed to produce the physiological and psychological effects of known recreational drugs, while circumventing current drug control laws and scheduling guidelines. Such "designer drugs" pose problems in detection and prevention of use, and they are no less dangerous than known controlled substances. Among the various classes of NPS, many are chiral. As they are synthetic products, most are racemates. Not unexpectedly, there is limited information about different the pharmacological and toxicological properties of their pure enantiomers. Hence, fast and reliable enantioselective methods are of great interest. In this work, superficially porous particle (SPP) vancomycin-based chiral stationary phases were used for development of fast enantioselective separation methods for various classes of NPS in supercritical fluid chromatography and liquid chromatography. The NPS tested included pyrovalerones, benzofurans, phenidines and phenidates. The effect of mobile phase composition on the retention and resolution of NPS in supercritical fluid chromatography was examined. The amount as well as the ratios of additives used is crucial for enantioseparation efficiency. Results showed the high enantioselective potential of vancomycin-based columns in both chromatographic techniques; 88% of NPS tested were enantioseparated in supercritical fluid chromatography and 69% of NPS tested were enantioseparated in liquid chromatography. Moreover, under optimized conditions, simultaneous enantioseparations of some NPS were achieved, which indicates great suitability of vancomycin-based columns for this purpose. The proposed methods can serve as guides for method development and for enantioseparation of further upcoming NPS.

Enantiomeric determination of cathinones in environmental water samples by liquid chromatography-high resolution mass spectrometry

The enantiomeric determination of chiral drugs in the environment is of emerging concern since their enantiomers often exhibit stereoselectivity in environmental occurrence, fate and toxicity. In this study a method based on solid-phase extraction followed by chiral liquid chromatography and high-resolution mass spectrometry has been developed for the enantiomeric determination of a group of cathinones in river water and effluent wastewater. The enantioseparation was carried out using a Chiralpak CBH column in reversed-phase mode, and optimised by evaluating the effects of flow rate, buffer concentration and organic modifier. Under optimal conditions, good enantioseparations (R_s ≥1.2) were achieved for all the analytes. Two mixed-mode cation-exchange sorbents (Oasis WCX and Oasis MCX) in solid-phase extraction were evaluated in river water. Oasis MCX sorbent showed better performance with apparent recoveries ranging from 57 to 91% and matrix effect ranging from -10 to 15%. It is worth noting that a shifting of retention times and loss of enantioresolutions in environmental water samples was observed for all the analytes when the Oasis WCX sorbent was used. The method was validated with river water and effluent wastewater samples and its overall performance was satisfactory. The method quantification limits for all the analyte enantiomers ranged from 1.0 to 2.9 ng/L in river water, and from 2.3 to 6.0 ng/L in effluent wastewater. The repeatability and reproducibility values, expressed as% relative standard deviation (n = 5) were less than 15%. The method was then applied to the analysis of river water and effluent wastewater. The racemic methylone and methedrone (EF=0.49 and 0.46, respectively) were detected at low ng/L in some of the river water samples.

Chiral separation of cathinone derivatives using β-cyclodextrin-assisted capillary electrophoresis-Comparison of four different β-cyclodextrin derivatives used as chiral selectors

In the past decade, more than 100 different cathinone derivatives slopped over entire Europe due to their enormous popularity. Generally, these novel psychoactive substances are easily available via the internet. This fact leads to various social problems, since cathinones are substances with consciousness-changing effects and are mainly misused for recreational matters by their consumers. Cathinones possess a chiral center including two enantiomeric forms with potentially different pharmacological behavior. This fact makes analytical method development regarding their chiral separation indispensable. In this study, a chiral capillary zone electrophoresis method for the enantioseparation of 61 cathinone and pyrovalerone derivatives was developed by means of four different β-cyclodextrin derivatives. As chiral selectors, native β-cyclodextrin as well as three of its derivatives namely acetyl-β-cyclodextrin, 2-hydroxypropyl-β-cyclodextrin, and carboxymethyl-β-cyclodextrin were used. The cathinone and pyrovalerone derivatives were either purchased in internet stores or seized by police. As a result, overall 58 of 61 studied substances were partially or baseline separated by at least one of the four chiral selectors using 10 mM of β-cyclodextrin derivative in a 10 mM sodium phosphate buffer (pH 2.5). Furthermore, the method was found to be suitable for simultaneous enantioseparations, for enantiomeric purity checks and to differentiate between positional isomers. Moreover, an intra- and an interday validation was performed successfully for each chiral selector to prove the robustness of the method.

Cinchona Alkaloid-Based Zwitterionic Chiral Stationary Phases Applied for Liquid Chromatographic Enantiomer Separations: An Overview

For the early 2000s, chromatographic methods applying chiral stationary phases (CSPs) became the most effective techniques for the resolution of chiral compounds on both analytical and preparative scales. High-performance liquid chromatography (HPLC) employing various types of chiral selectors covalently bonded to silica-based supports offers a state-of-the-art methodology for "chiral analysis." Although a large number of CSPs are available nowadays, the design and development of new "chiral columns" are still needed since it is obvious that in practice one needs a good portfolio of different columns to face the challenging task of enantiomeric resolutions. The development of the unique chiral anion, cation, and zwitterion exchangers achieved by Lindner and his partners serves as an expansion of the range of the efficiently applicable CSPs.In this context this overview chapter discusses and summarizes direct enantiomer separations of chiral acids and ampholytes applying zwitterionic ion exchangers derived from Cinchona alkaloids. Our aim is to provide comprehensive information on practical solutions with focus on the molecular recognition and methodological variables.

Liquid chromatographic enantiomer separations applying chiral ion-exchangers based on Cinchona alkaloids

As the understanding of the various biological actions of compounds with different stereochemistry has grown, the necessity to develop methods for the analytical qualification and quantification of chiral products has become particularly important. The last quarter of the century has seen a vast growth of diverse chiral technologies, including stereocontrolled synthesis and enantioselective separation and analysis concepts. By the introduction of covalently bonded silica-based chiral stationary phases (CSPs), the so-called direct liquid chromatographic (LC) methods of enantiomer separation became the state-of-the-art methodology. Although a large number of CSPs is available nowadays, the design and development of new chiral selectors and CSPs are still needed since it is obvious that in practice one needs a good portfolio of different CSPs and focused "chiral columns" to tackle the challenging tasks. This review discusses and summarizes direct enantiomer separations of chiral acids and ampholytes applying anionic and zwitterionic ion-exchangers derived from Cinchona alkaloids with emphasis on literature data published in the last 10 years. Our aim is to provide an overview of practical solutions, while focusing on the integration of molecular recognition and methodological variables.

Enantiomeric separation of Novel Psychoactive Substances by capillary electrophoresis using (+)-18-crown-6-tetracarboxylic acid as chiral selector

In the recent years, hundreds of Novel Psychoactive Substances (NPS) have entered both the European and the global drug market. These drugs, which are mainly used for recreational matters, have caused serious social problems. Every year, the spectrum of these misused drugs is enlarged by new derivatives, which are produced by modifications of basic structures of already well-known substances. Additionally, a lot of them possess a stereogenic center which leads to 2 enantiomeric forms. The fact that the pharmacological effects and potencies of the enantiomers of these chiral NPS may differ can be assumed from a broad spectrum of active pharmaceutical ingredients. For this reason, analytical method development regarding enantiomeric separation for these classes of substances is of great pharmaceutical and medical interest. The aim of this work was to create an easy-to-prepare chiral capillary electrophoresis method for the enantioseparation of NPS which contains a primary amino group by means of (+)-18-crown-6-tetracarboxylic acid as chiral selector. Novel Psychoactive Substances were purchased at various Internet stores or represent samples seized by Austrian police. The effects of selector concentration, the electrolyte composition, and the addition of organic modifiers to the background electrolyte on enantioseparation were investigated. Under optimized conditions, the use of 20-mM (+)-18-crown-6-tetracarboxylic acid, 10-mM Tris, and 30-mM citric acid buffer at pH 2.10 turned out to be effective. Fifteen of 24 tested NPS were resolved in their enantiomers within 15 minutes. It was found that all NPS were traded as racemic mixtures.

Effective methodologies for enantiomeric separations of 150 pharmacology and toxicology related 1°, 2°, and 3° amines with core-shell chiral stationary phases

Core-shell particles (superficially porous particles, SPPs) have been proven to provide high-throughput and effective separations of a variety of chiral molecules. However, due to their limited commercialization, many separations have not been reported with these stationary phases. In this study, four SPP chiral stationary phases (CSPs) were utilized for the enantiomeric separation of 150 chiral amines. These amines encompass a variety of structural and drug classes, which are particularly important to the pharmaceutical industry and in forensics. This comprehensive evaluation demonstrates the power of these CSPs and the ease of method development and optimization. The CSPs used in this study included the macrocyclic glycopeptide-based CSPs (VancoShell and NicoShell), the cyclodextrin-based CSP (CDShell-RSP), and the cyclofructan-based CSP (LarihcShell-P). These CSPs offered versatility for a variety of applications and worked in a complementary fashion to baseline separate all 150 amines. The LarihcShell-P was highly effective for separating primary amines. VancoShell, NicoShell, and CDShell-RSP were useful for separating all types of amines. These CSPs are multi-modal and can be utilized with mass spectrometry compatible solvents. Eighteen racemic controlled substances were simultaneously baseline separated in a single liquid chromatography-mass spectrometry (LC-MS) analysis. Details in high-performance liquid chromatography (HPLC) parameters will be discussed as well as the improved chromatographic performance afforded by the SPP CSPs.

Simultaneous Quantitative Determination of Synthetic Cathinone Enantiomers in Urine and Plasma Using GC-NCI-MS

Development and validation of sensitive and selective method for enantioseparation and quantitation of synthetic cathinones is reported using GC-MS triple quadrupole mass spectrometry with negative chemical ionization (NCI) mode. Indirect chiral separation of thirty-six synthetic cathinone compounds has been achieved by using an optically pure chiral derivatizing agent (CDA) called (S)-(-)-N-(trifluoroacetyl)pyrrolidine-2-carbonyl chloride (L-TPC), which converts cathinone enantiomers into diastereoisomers that can be separated on achiral columns. As a result of using Ultra Inert 60 m column and performing slow heating rate (2°C/min) on the GC oven, an observed enhancement in enantiomer peak resolution has been achieved. An internal standard, (+)-cathinone, was used for quantitation of synthetic cathinones. Method validation in terms of linearities and sensitivity in terms of limits of detection (LODs), limits of quantitation (LOQs), recoveries, and reproducibilities has been obtained for fourteen selected compounds that examined simultaneously as a mixture after being spiked in urine and plasma. It was found that the LOD of the fourteen synthetic cathinones in urine was in the range of 0.26-0.76 µg/L, and in plasma, it was in the range of 0.26-0.34 µg/L. While the LOQ of the mixture in urine was in the range of 0.86-2.34 µg/L, and in plasma, it was in the range of 0.89-1.12 µg/L. Unlike the electron impact (EI) ion source, NCI showed better sensitivity by two orders of magnitude by comparing the obtained results with the recently published reports for quantitative analysis and enantioseparation of synthetic cathinones.

Chiral enantioresolution of cathinone derivatives present in "legal highs", and enantioselectivity evaluation on cytotoxicity of 3,4-methylenedioxypyrovalerone (MDPV)

Recently, great interest has been focused on synthetic cathinones since their consumption has increased exponentially. All synthetic cathinones exist as chiral molecules; the biological and/or toxicological properties of cathinones generally differ according to the enantiomers in human body. In this study, a chiral liquid chromatography method was developed to separate and determine the enantiomeric ratio of synthetic cathinones present in "legal highs" acquired in old smart shops or over the Internet. All the synthetic cathinones were efficiently enantio-separated with α and Rs ranging from 1.24 to 3.62 and from 1.24 to 10.52, respectively, using polysaccharide-based chiral stationary phases. All synthetic cathinones, with the exception of 4-methylethcathinone (4-MEC), were present in the commercialized "legal highs" in an enantiomeric proportion of 50:50. One of the studied chiral compounds was 3,4-methylenedioxypyrovalerone (MDPV), one of the most consumed cathinone derivative worldwide. Our research group has recently reported its hepatotoxicity in the racemic form. Thus, the analytical enantioresolution of the MDPV was scaled up to multi-milligram using a semi-preparative amylose tris-3,5-dimethylphenylcarbamate column (20 cm × 7.0 mm ID, 7 µm particle size). Both enantiomers were isolated with high enantiomeric purity (enantiomeric excess > 99 %). The toxicity of S-(-)-MDPV and R-(+)-MDPV was evaluated, for the first time, using primary cultures of rat hepatocytes. It was also possible to verify that MDPV enantiomers showed hepatotoxicity in a concentration-dependent manner, but displayed no enantioselective toxicity in this cell culture model.

A validated gas chromatography mass spectrometry method for simultaneous determination of cathinone related drug enantiomers in urine and plasma

A sensitive and selective method for detection and quantitation of 31 synthetic cathinones using GC-MS has been developed and validated. They were separated into their optical enantiomers after derivatization with L-TPC and nikethamide was used as IS.