A chromatographic study on the retention behavior of the amylose tris(3-chloro-5-methylphenylcarbamate) chiral stationary phase under aqueous conditions

Green HPLC Enantioseparation of Chemopreventive Chiral Isothiocyanates Homologs on an Immobilized Chiral Stationary Phase Based on Amylose tris-[(S)-α-Methylbenzylcarbamate]

Sulforaphane is a chiral phytochemical with chemopreventive properties. The presence of a stereogenic sulfur atom is responsible for the chirality of the natural isothiocyanate. The key role of sulfur chirality in biological activity is underscored by studies of the efficacy of individual enantiomers as chemoprotective agents. The predominant native (R) enantiomer is active, whereas the (S) antipode is inactive or has little or no biological activity. Here we provide an enantioselective high-performance liquid chromatography (HPLC) protocol for the direct and complete resolution of sulforaphane and its chiral natural homologs with different aliphatic chain lengths between the sulfinyl sulfur and isothiocyanate group, namely iberin, alyssin, and hesperin. The chromatographic separations were carried out on the immobilized-type CHIRALPAK IH-3 chiral stationary phase with amylose tris-[(S)-methylbenzylcarbamate] as a chiral selector. The effects of different mobile phases consisting of pure alcoholic solvents and hydroalcoholic mixtures on enantiomer retention and enantioselectivity were carefully investigated. Simple and environmentally friendly enantioselective conditions for the resolution of all chiral ITCs were found. In particular, pure ethanol and highly aqueous mobile phases gave excellent enantioseparations. The retention factors of the enantiomers were recorded as the water content in the aqueous-organic modifier (methanol, ethanol, or acetonitrile) mobile phases progressively varied. U-shaped retention maps were generated, indicating a dual and competitive hydrophilic interaction liquid chromatography (HILIC) and reversed-phase liquid chromatography retention mechanism on the CHIRALPAK IH-3 chiral stationary phase. Finally, experimental chiroptical studies performed in ethanol solution showed that the (R) enantiomers were eluted before the (S) counterpart under all eluent conditions investigated.

Chiral Hydroxy Metabolite of Mebendazole: Analytical and Semi-Preparative High-Performance Liquid Chromatography Resolution and Chiroptical Properties

Mebendazole (MBZ) is a benzimidazole carbamate anthelmintic used worldwide for the treatment and prevention of parasitic disorders in animals and humans. A large number of in vivo and in vitro studies have demonstrated that MBZ also has anticancer activity in multiple types of cancers. After oral administration, the phenylketone moiety of MBZ is rapidly reduced to the hydroxyl group to form the chiral hydroxy metabolite (MBZ-OH). To the best of our knowledge, there is no information in the literature on the stereochemical course of transformation and the anthelmintic and antitumor activity of individual enantiomers of MBZ-OH. In the present study, we describe in detail the direct HPLC resolution of MBZ-OH on a 100 mm × 4.6 mm Chirapak IG-3 column packed with 3 μm silica particles containing amylose (3-chloro-5-methylphenylcarbamate) as a selector. At 25 °C and using pure methanol as the mobile phase, the enantioseparation and resolution factors were 2.38 and 6.13, respectively. These conditions were scaled up at a semi-preparative scale using a 250 mm × 10 mm Chiralpak IG column to isolate multi-milligram amounts of both enantiomeric forms of the chiral metabolite. The chiroptical properties of the collected enantiomers were determined and, through a theoretical study, were related to the more stable conformations of MBZ-OH. The first and second eluted enantiomers were dextrorotatory and levorotatory, respectively, in dimethylformamide solution. Finally, by recording the retention factors of the enantiomers as the water content in the water-acetonitrile mobile phases was progressively varied, U-shaped retention maps were generated, indicating a dual and competitive hydrophilic interaction liquid chromatography and reversed-phase liquid chromatography retention mechanism on the Chirapak IG-3 chiral stationary phase.

Effect of water and protic solvents on polysaccharide-based column efficiency

In the present study, polysaccharide-based columns were used to evaluate the efficiency of columns in response to the introduction of water and protic solvents (methanol and ethanol) into the mobile phase, replacing acetonitrile. While increasing water content frequently enhances enantiomer resolution, the inclusion of water, particularly when combined with methanol and ethanol in the mobile phase, has an adverse impact on mass transfer, thus influencing the column plate height. These effects are more pronounced with ethanol, and in many cases, van Deemter plots exhibit the absence of a minimum point optimal in the explored range. Consequently, acetonitrile and its water mixtures are the preferred choices to mitigate these effects for situations in which the chiral column is operated at a relatively high flow rate (> 1 mL/min in a 4.6 mm column).

Recent advances in chiral selectors immobilization and chiral mobile phase additives in liquid chromatographic enantio-separations: A review

For decades now, the separation of chiral enantiomers of drugs has been gaining the interest and attention of researchers. In 1991, the first guidelines for development of chiral drugs were firstly released by the US-FDA. Since then, the development in chromatographic enantioseparation tools has been fast and variable, aiming at creating a suitable environment where the physically and chemically identical enantiomers can be separated. Among those tools, the immobilization of chiral selectors (CS) on different stationary phases and the chiral mobile phase additives (CMPA) which have been progressed and studied extensively. This review article highlights the major advances in immobilization of CS together with their different recognition mechanisms as well as CMPA as a cheaper and successful alternative for chiral stationary phases. Moreover, the role of molecular modeling tool as a pre-step in the choice of CS for evaluating possible interactions with different ligands has been pointed up. Illustrations of reported methods and updates for immobilized CS and CMPA have been included.

Enantiomeric analysis of drugs in water samples by using liquid-liquid microextraction and nano-liquid chromatography

The nano-LC technique is increasingly used for both fast studies on enantiomeric analysis and test beds of novel stationary phases due to the small volumes involved and the short conditioning and analysis times. In this study, the enantioseparation of 10 drugs from different families was carried out by nano-LC, utilizing silica with immobilized amylose tris(3-chloro-5-methylphenylcarbamate) column. The effect on chiral separation caused by the addition of different salts to the mobile phase was evaluated. To simultaneously separate as many enantiomers as possible, the effect of buffer concentration in the mobile phase was studied, and, to increase the sensitivity, a liquid-liquid microextraction based on the use of isoamyl acetate as sustainable extraction solvent was applied to pre-concentrate four chiral drugs from tap and environmental waters, achieving satisfactory recoveries (>70%).

Effect of mobile-phase modifiers on the enantioselective retention behavior of methyl mandelate with an amylose 3,5-dimethylphenylcarbamate chiral stationary phase under reversed-phase conditions

In this study, methanol, ethanol, n-propyl alcohol, isopropyl alcohol, acetone, and tert-butanol were used as organic modifiers in reversed-phase mode chiral liquid-chromatography to systematically investigate the effects of mobile phase components on the enantioselective retention behavior of methyl mandelate with immobilized amylose 3,5-dimethylphenylcarbamate-based sorbent called Chiralpak IA. A two-site enantioselective model was used to obtain information on the recognition mechanisms by observing the dependence of the enantioselectivity and retention factor difference on the modifier content. Similar enantioselective retention behaviors were observed for all modifiers, and characteristic modifier concentration points (P_L , P_M , and P_H ) were identified. At modifier concentrations up to P_M , the weakened hydrophobic environment resulted in polymer structural relaxation, which changed the recognition mechanisms. By contrast, at concentrations beyond P_H , considerably different enantioselectivity behaviors were observed, indicating that the existence of dipole-dipole interaction, which was stronger at higher modifier concentrations, contributed to the retention mechanisms. The concentrations at which these characteristic points occurred were dependent on the carbon number of the modifier molecule. Modifiers with more carbon numbers facilitated the transition in the enantioselective behaviors. These results demonstrated that the proposed method can provide a physically consistent quantitative description of enantioselective retention behavior in reversed-phase mode.

Enantioselective HPLC analysis of Escitalopram oxalate and its impurities using a cellulose-based chiral stationary phase under normal- and green reversed-phase conditions

Normal-phase and reversed-phase high-performance liquid chromatography methods for the separation of the active pharmaceutical ingredient Escitalopram from its (R)-enantiomer impurity have been developed on the cellulose-based Chiralcel OJ-H chiral stationary phase. Both methods share two features: they use ethanol as a cosolvent and are able to give a complete enantioseparation without interference from other associated chiral impurities. With the green eluent mixture ethanol-water-diethylammine 70:30:0.1 (v/v/v) the resolution between escitalopram and (R)-enantiomer was 2.09 at 30°C. The limits of quantification for the (S) and (R) enantiomers were 4.5 μg mL^-1 and 3.8 μg mL^-1 , respectively. This article is protected by copyright. All rights reserved.

An overview of chiral separations of pharmaceutically active substances by HPLC (2018-2020)

This review provides a brief survey of chiral separation of pharmaceutically active substances published over the last 3 years (2018-2020). Chiral separation of drugs is an important area of research. The control of enantiomeric purity and determination of individual enantiomeric drug molecules is a necessity especially for clinical, analytical, and regulatory purposes. Among chromatographic resolution methods, high-performance liquid chromatography based on chiral stationary phases remains the most popular and effective method used for chiral separation of various drugs. In this review, attention is paid to several classes of chiral stationary phases that have been the most frequently used for drug enantioseparation during this period.

GRAPHIC ABSTRACT

Screening of Benzimidazole-Based Anthelmintics and Their Enantiomers as Repurposed Drug Candidates in Cancer Therapy

Repurposing of approved non-antitumor drugs represents a promising and affordable strategy that may help to increase the repertoire of effective anticancer drugs. Benzimidazole-based anthelmintics are antiparasitic drugs commonly employed both in human and veterinary medicine. Benzimidazole compounds are being considered for drug repurposing due to antitumor activities displayed by some members of the family. In this study, we explored the effects of a large series of benzimidazole-based anthelmintics (and some enantiomerically pure forms of those containing a stereogenic center) on the viability of different tumor cell lines derived from paraganglioma, pancreatic and colorectal cancer. Flubendazole, parbendazole, oxibendazole, mebendazole, albendazole and fenbendazole showed the most consistent antiproliferative effects, displaying IC₅₀ values in the low micromolar range, or even in the nanomolar range. In silico evaluation of their physicochemical, pharmacokinetics and medicinal chemistry properties also provided useful information related to the chemical structures and potential of these compounds. Furthermore, in view of the potential repurposing of these drugs in cancer therapy and considering that pharmaceutically active compounds may have different mechanisms of action, we performed an in silico target prediction to assess the polypharmacology of these benzimidazoles, which highlighted previously unknown cancer-relevant molecular targets.

Single-run reversed-phase HPLC method for determining sertraline content, enantiomeric purity, and related substances in drug substance and finished product

A direct enantio-, diastereo-, and chemo-selective high-performance liquid chromatographic method was developed for determining the content, enantiomeric purity, and related substances of the chiral antidepressant drug sertraline HCl in a single chromatographic run. The separation was achieved on a chiral stationary phase based on amylose tris(3-chloro-5-methylphenylcarbamate) under reversed-phase conditions. The method was optimized by evaluating the influence of the temperature and mobile phase composition on the retention and selectivity. The application of the single-run approach allowed to baseline resolve all investigated species in less than 15 min, without using buffers or tandem-coupled columns. The chromatographic method was validated according to the guidelines of the Official Medicines Control Laboratory and applied to control the content of sertraline HCl and related chiral substances in a generic antidepressant formulation.

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Sertraline displays two stereogenic centers but only the (1S,4S) form is clinically useful.

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A direct method was developed for evaluating assay and impurities in a single HPLC run.

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The single-run RP method allowed resolving all species within 15 min, without using buffers.

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Amylose tris(3-chloro-5-methylphenylcarbamate)-based CSP was used as a chiral support.

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The method is selective, precise and accurate and it meets the requirements of the EP guidelines.

Further study on enantiomer resolving ability of amylose tris(3-chloro-5-methylphenylcarbamate) covalently immobilized onto silica in nano-liquid chromatography and capillary electrochromatography

In the present study separation of enantiomers of some chiral neutral, basic and weakly acidic analytes was investigated on the chiral stationary phase (CSP) made by covalent immobilization of amylose tris(3-chloro-5-methylphenylcarbamate) onto aminopropylsilanized (APS) silica in nano-liquid chromatography (nano-LC) in aqueous methanol or acetonitrile mixtures. It has been shown that similar to high-performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC) this chiral selector is useful for separation of enantiomers of neutral, basic and acidic analytes also in nano-LC. In comparison to our previous research, in which the chiral selector (CS) was bonded on native silica, in this study, the CS was immobilized on APS silica in order to improve chromatographic performance towards basic analytes. In fact, some improvement was observed and surprisingly not only for basic but also for neutral and acidic analytes. Again, quite unexpectedly almost no electroosmotic flow (EOF) was observed in capillaries packed with ca. 20% (w/w) amylose tris(3-chloro-5-methylphenylcarbamate) immobilized onto APS silica although the same APS silica before attachment of chiral selector exhibited significant EOF. In order to generate EOF in the capillaries with the CSP and enable capillary electrochromatographic (CEC) experiment on it, the short segment of the capillary column was packed with APS silica without chiral selector. The EOF in such capillary enabled CEC experiment and some preliminary results are reported here.

A study of tetrapeptide enantiomeric separation on crown ether based chiral stationary phases

The chiral separations of small peptides is an important challenge in the biological and medical sciences, because different stereoisomers of chiral drugs can often possess different pharmacological, pharmacokinetic, and/or toxicological activities. Commercially available crown ether chiral stationary phases based on S-(3,3'-diphenyl-1,1'-binaphthyl)-20-crown-6 (CROWNPAK CR-I (+)) and (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (ChiroSil RCA (+)) have been successfully used for separating enantiomers of various racemic compounds containing primary amino groups. In this investigation, enantioresolution of more complex model analyte - tetrapeptide Tyr-Arg-Phe-Lys-NH₂, has been reported on crown ether chiral stationary phases. Organic and acidic modifier content in aqueous mobile phase was tested. All Tyr-Arg-Phe-Lys-NH₂ stereoisomers showed U-shaped retention plots, based on ACN content in mobile phase. Increased retention of tetrapeptide stereoisomers was observed at low (<35%) and at high (>70%) acetonitrile content in the mobile phase, indicating that different separation mechanisms are most likely involved. As a result, baseline separation of all eight tetrapeptide enantiomer pairs was achieved under isocratic elution mode on both chiral columns.

Comparative study on enantiomer resolving ability of amylose tris(3-chloro-5-methylphenylcarbamate) covalently immobilized onto silica in nano-liquid chromatography and capillary electrochromatography

In the present study separation of enantiomers of some chiral neutral and weakly acidic analytes was investigated on the chiral stationary phase (CSP) made by covalent immobilization of amylose tris(3-chloro-5-methylphenylcarbamate) onto silica in nano-liquid chromatography (nano-LC) and capillary electrochromatography (CEC) in acetonitrile and aqueous acetonitrile. Few comparisons were made also between the enantioseparations in nano-LC and high-performance liquid chromatography (HPLC) with the chiral column of 4.6 × 250 mm dimension. Slightly better separation of enantiomers was observed in HPLC mode compared to nano-LC mode. It was shown that in the capillary columns packed with the CSP containing about 20% (w/w) of a covalently immobilized neutral chiral selector, amylose tris(3-chloro-5-methylphenylcarbamate), sufficient electroosmotic flow has been generated and enantioseparations with reasonable analysis time were performed also in CEC mode. It was shown once again that CEC offers a clear advantage over nano-LC from the viewpoint of plate numbers and peak resolution.

Temperature and eluent composition effects on enantiomer separation of carvedilol by high-performance liquid chromatography on immobilized amylose-based chiral stationary phases

Carvedilol is a chiral drug with potent antihypertensive and antianginal activities. Although it is clinically used as a racemic mixture, its enantiomers show different pharmacokinetic and pharmacodynamic profiles. Here, the direct chiral separation of racemic drug by high performance liquid chromatography using two immobilized-type amylose-based chiral stationary phases is presented. Some chromatographic parameters, such as retention and selectivity, were determined under multimodal eluent conditions and different temperatures. A temperature-dependent inversion of the elution order of enantiomers was observed in the operative temperature range of chiral chromatographic support. Finally, an effective direct enantioselective method was successfully applied to the separation of the enantiomers of carvedilol on a semipreparative scale.