Use of hydroxypropyl- and hydroxyethyl-derivatized beta-cyclodextrins for the thin-layer chromatographic separation of enantiomers and diastereomers

The counteracting influence of 2-hydroxypropyl substitution and the presence of a guest molecule on the shape and size of the β-cyclodextrin cavity

The aqueous solubility of β-cyclodextrin (β-CD), a cyclic carbohydrate comprising seven α-D-glucose molecules, is enhanced by 2-hydroxypropyl (2-HP) substitution of the hydroxyl groups at the CD rims. Our thorough analysis of the structural and solvation properties with different degrees of 2-hydroxypropyl substitution on β-CD using molecular dynamics simulations reveals that the solubility is enhanced at the cost of the structural distortion of the CD cyclic structure. Substitution at the secondary rim predominantly enhances the favourable interactions between CD and water by decreasing CD-CD hydrogen bonding and promoting CD-water hydrogen bonding. However, the effect of substitution at the primary rim on the CD-water interactions is minimal; the hydrogen bonds between water and the primary hydroxyl group in native CD merely get replaced by those between water and 2-HP, since the substitution makes the primary hydroxyl oxygen (O6 atom) inaccessible to water. In contrast, substitution at the primary rim maintains the structural integrity of CD, while substitution at the secondary rim results in structural distortion due to the disruption of the intramolecular hydrogen bond belt, even leading to cavity closure. Certain strategic substitutions of the primary hydroxyl groups can help in the reduction of structural distortion, depending upon the degree of substitution at the secondary hydroxyl rim. A detailed inspection of the simulation trajectory revealed that the tilting of glucose units with the primary hydroxyl oxygen (O6) pointing inward is the primary driver for cavity closure. Even though the dynamics of glucose tilting can influence the kinetics of host-guest complex formation, once the guest is well incorporated into the cavity, glucose tilting is inhibited and the cavity opens up as in native β-CD.

A Current Overview of Cyclodextrin-Based Nanocarriers for Enhanced Antifungal Delivery

Fungal infections are an extremely serious health problem, particularly in patients with compromised immune systems. Most antifungal agents have low aqueous solubility, which may hamper their bioavailability. Their complexation with cyclodextrins (CDs) could increase the solubility of antifungals, facilitating their antifungal efficacy. Nanoparticulate systems are promising carriers for antifungal delivery due to their ability to overcome the drawbacks of conventional dosage forms. CD-based nanocarriers could form beneficial combinations of CDs and nanoparticulate platforms. These systems have synergistic or additive effects regarding improved drug loading, enhanced chemical stability, and enhanced drug permeation through membranes, thereby increasing the bioavailability of drugs. Here, an application of CD in antifungal drug formulations is reviewed. CD-based nanocarriers, such as nanoparticles, liposomes, nanoemulsions, nanofibers, and in situ gels, enhancing antifungal activity in a controlled-release manner and possessing good toxicological profiles, are described. Additionally, the examples of current, updated CD-based nanocarriers loaded with antifungal drugs for delivery by various routes of administration are discussed and summarized.

A New Approach for β-Cyclodextrin Conjugated Drug Delivery System in Cancer Therapy

Natural cyclodextrins (CDs) are macrocyclic starch molecules discovered a decade ago, in which α-, β-, and γ-CD were commonly used. They originally acted as pharmaceutical excipients to enhance the aqueous solubility and alter the physicochemical properties of drugs that fall under class II and IV categories according to the Biopharmaceutics Classification System (BPS). The industrial significance of CDs became apparent during the 1970s as scientists started to discover more of CD's potential in chemical modifications and the formation of inclusion complexes. CDs can help in masking and prolonging the half-life of drugs used in cancer. Multiple optimization techniques were discovered to prepare the derivatives of CDs and increase their complexation and drug delivery efficiency. In recent years, due to the advancement of nanotechnology in pharmaceutical sectors, there has been growing interest in CDs. This review mainly focuses on the formulation of cyclodextrin conjugated nanocarriers using graphenes, carbon nanotubes, nanosponges, hydrogels, dendrimers, and polymers to achieve drug-release characteristics specific to cells. These approaches benefit the discovery of novel anti-cancer treatments, solubilization of new drug compounds, and cell specific drug delivery properties. Due to these unique properties of CDs, they are essential in achieving and enhancing tumor-specific cancer treatment.

Selective Depletion of Chiral 4-Hydroxypraziquantel Metabolites in Three Types of Aquaculture Fish by LC-MS/MS

The rise of aquaculture has necessitated the use of antibiotics and other agents in these densely populated and often closed environments. An enantioselective depletion study of four chiral 4-hydroxypraziquantel metabolites (4-OH-PZQ) in perch, tilapia, and ricefield-eel muscles was done using a simple, sensitive, and reliable liquid-chromatography-tandem-mass-spectrometry (LC-MS/MS) method. These metabolites result from the uptake of the drug praziquantel (PZQ), which is used in aquaculture. A novel strategy of using a C₁₈ short column in tandem with a chiral hydroxypropyl-β-cyclodextrin superficially porous particle (CDShell-RSP) column produced the optimal separation for both the enantiomers and diastereoisomers of 4-OH-PZQ. The method was linear over the concentration range of 1-250 μg L^-1 ( r² ≥ 0.99) for R- trans-4-OH-PZQ, S- trans-4-OH-PZQ, R- cis-4-OH-PZQ, and S- cis-4-OH-PZQ. The average recoveries of four analytes at three spiked levels of 1, 10, and 100 μg kg^-1 ranged from 84.2 to 93.1%, and the intraday and interday relative standard deviations were less than 7.9%. The limits of quantification of the four 4-OH-PZQ metabolites in perch-, tilapia-, and ricefield-eel-muscle matrices were 1.0 μg kg^-1. The method was utilized to monitor the depletion of trans- and cis-4-OH-PZQ enantiomers in perch, tilapia, and ricefield-eel muscle following oral administration (medicine bath for ricefield eel). Species-specific differences in the PZQ metabolism of isomers were observed. In addition, new metabolites of PZQ were observed: 3-hydroxypraziquantel diastereomers.

Analysis of Enantiomers in Products of Food Interest

The separation of enantiomers has been started in the past and continues to be a topic of great interest in various fields of research, mainly because these compounds could be involved in biological processes such as, for example, those related to human health. Great attention has been devoted to studies for the analysis of enantiomers present in food products in order to assess authenticity and safety. The separation of these compounds can be carried out utilizing analytical techniques such as gas chromatography, high-performance liquid chromatography, supercritical fluid chromatography, and other methods. The separation is performed mainly employing chromatographic columns containing particles modified with chiral selectors (CS). Among the CS used, modified polysaccharides, glycopeptide antibiotics, and cyclodextrins are currently applied.

Solubility of Cyclodextrins and Drug/Cyclodextrin Complexes

Cyclodextrins (CDs), a group of oligosaccharides formed by glucose units bound together in a ring, show a promising ability to form complexes with drug molecules and improve their physicochemical properties without molecular modifications. The stoichiometry of drug/CD complexes is most frequently 1:1. However, natural CDs have a tendency to self-assemble and form aggregates in aqueous media. CD aggregation can limit their solubility. Through derivative formation, it is possible to enhance their solubility and complexation capacity, but this depends on the type of substituent and degree of substitution. Formation of water-soluble drug/CD complexes can increase drug permeation through biological membranes. To maximize drug permeation the amount of added CD into pharmaceutical preparation has to be optimized. However, solubility of CDs, especially that of natural CDs, is affected by the complex formation. The presence of pharmaceutical excipients, such as water-soluble polymers, preservatives, and surfactants, can influence the solubilizing abilities of CDs, but this depends on the excipients' physicochemical properties. The competitive CD complexation of drugs and excipients has to be considered during formulation studies.

Evaluation of sulfated maltodextrin as a novel anionic chiral selector for the enantioseparation of basic chiral drugs by capillary electrophoresis

Introducing a new class of chiral selectors is an interesting work and this issue is still one of the hot topics in separation science and chirality. In this study, for the first time, sulfated maltodextrin (MD) was synthesized as a new anionic chiral selector and then it was successfully applied for the enantioseparation of five basic drugs (amlodipine, hydroxyzine, fluoxetine, tolterodine, and tramadol) as model chiral compounds using CE. This chiral selector has two recognition sites: a helical structure and a sulfated group which contribute to three corresponding driving forces; inclusion complexation, electrostatic interaction, and hydrogen binding. Under the optimized condition (buffer solution: 50 mM phosphate (pH 3.0) and 2% w/v sulfated MD; applied voltage: 18 kV; temperature: 20°C), baseline enantioseparation was observed for all mentioned chiral drugs. When instead of sulfated MD neutral MD was used under the same condition, no enantioseparation was observed which means the resolution power of sulfated MD is higher than neutral MD due to the electrostatic interaction between sulfated groups and protonated chiral drugs. Also, the countercurrent mobility of negatively charged MD (sulfated MD) allows more interactions between the chiral selector and chiral drugs and this in turn results in a successful resolution for the enantiomers. Furthermore, a higher concentration of neutral MD (approximately five times) is necessary to achieve the equivalent resolution compared with the negatively charged MD.

Maltodextrins as chiral selectors in CE: molecular structure effect of basic chiral compounds on the enantioseparation

Prediction of chiral separation for a compound using a chiral selector is an interesting and debatable work. For this purpose, in this study 23 chiral basic drugs with different chemical structures were selected as model solutes and the influence of their chemical structures on the enantioseparation in the presence of maltodextrin (MD) as chiral selector was investigated. For chiral separation, a 100-mM phosphate buffer solution (pH 3.0) containing 10% (w/v) MD with dextrose equivalent (DE) of 4-7 as chiral selector at the temperature of 25°C and voltage of 20 kV was used. Under this condition, baseline separation was achieved for nine chiral compounds and partial separation was obtained for another six chiral compounds while no enantioseparation was obtained for the remaining eight compounds. The results showed that the existence of at least two aromatic rings or cycloalkanes and an oxygen or nitrogen atom or -CN group directly bonded to the chiral center are necessary for baseline separation. With the obtained results in this study, chiral separation of a chiral compound can be estimated with MD-modified capillary electrophoresis before analysis. This prediction will minimize the number of preliminary experiments required to resolve enantiomers and will save time and cost.

Enantioseparations by thin-layer chromatography

An up-to-date overview of thin-layer chromatography (TLC) techniques for chiral separations of various significant and/or recent examples of enantioresolutions is reported. Furthermore, examples for chiral separations obtained on achiral commercially available C18 TLC plates are described in detail. These include the enantioseparation of methylthiohydantoin-phenylalanine and methylthiohydantoin-tyrosine using hydroxyethyl-β-cyclodextrin as mobile phase additive and the separation of the enantiomers of warfarin and p-chlorowarfarin using bovine serum albumin as mobile phase additive.

Degradation study of carnosic acid, carnosol, rosmarinic acid, and rosemary extract (Rosmarinus officinalis L.) assessed using HPLC

Rosemary, whose major caffeoyl-derived and diterpenoid ingredients are rosmarinic acid, carnosol, and carnosic acid, is an important source of natural antioxidants and is being recognized increasingly as a useful preservative, protectant, and even as a potential medicinal agent. Understanding the stability of these components and their mode of interaction in mixtures is important if they are to be utilized to greatest effect. A study of the degradation of rosmarinic acid, carnosol, carnosic acid, and a mixture of the three was conducted in ethanolic solutions at different temperatures and light exposure. As expected, degradation increased with temperature. Some unique degradation products were formed with exposure to light. Several degradation products were reported for the first time. The degradation products were identified by HPLC/MS/MS, UV, and NMR. The degradation of rosemary extract in fish oil also was investigated, and much slower rates of degradation were observed for carnosic acid. In the mixture of the three antioxidants, carnosic acid serves to maintain levels of carnosol, though it does so at least in part at the cost of its own degradation.

Chiral separation strategy in polar organic solvent chromatography and performance comparison with normal-phase liquid and supercritical-fluid chromatography

A strategy, including a rapid screening and several optimisation steps, for the separation of chiral molecules of pharmaceutical interest by polar organic solvent chromatography (POSC), using four polysaccharide-based stationary phases, is proposed and compared with previously reported strategies in normal-phase (NPLC) and supercritical fluid chromatography (SFC). In a first part of this paper, different examples demonstrate the effectiveness of the POSC strategy for fast method development. Optimisation is based on the use of experimental design to map the experimental domain in an efficient way. In the second part, the best screening results, obtained after performance of earlier defined chromatographic screening strategies in NPLC and SFC, are compared to those obtained in POSC. The three techniques show complementary separation results and allowed baseline separation of 23 of 25 compounds. POSC is found to be a very interesting separation mode compared to NPLC, because of the many fast (< 10 min) baseline separations obtained.

Optimization of capillary electrophoretic enantioseparation for basic drugs with native β-CD as a chiral selector

This study presents the advantages of the 20 microm inner diameter (id) capillary for the enantioseparation of ten basic drugs with native beta-CD as the chiral selector. The apparent binding constants of each enantiomeric pair were determined to calculate the optimum beta-CD concentration ([beta-CD]opt) and the optimization was subsequently carried out. Comparison of the 20 microm id with 50 microm id were made in terms of the results obtained in the optimization and detection limits. Applying the optimum conditions for each compound, reproducible results (RSD from 0-3; n>5) were obtained for the 20 microm id capillary. Although the sensitivity is lower in the 20 microm id capillary, the LOD determined using this capillary is still found to be acceptable for the ten basic drugs studied. Enhanced resolution and faster analysis times were the main advantages observed with the use of this capillary in enantioseparation.

2-O-(2-Hydroxybutyl)-.BETA.-cyclodextrin as a Chiral Selector for the Capillary Electrophoretic Separation of Chiral Drugs

A new beta-cyclodextrin (beta-CD) derivative, 2-O-(2-hydroxybutyl)-beta-CD (HB-beta-CD), was successfully synthesized and used as chiral selector in capillary zone electrophoresis. Six chiral drugs, such as anisodamine, ketoconazole, propranolol, promethazine, adrenaline and chlorphenamine enantiomers, belonging to different classes of compounds of pharmaceutical interest were resolved. The chiral resolution (R(S)) was strongly influenced by the concentrations of the cyclodextrin derivative, the background electrolyte, and the pH of the background electrolyte. Under the conditions of 50 mmol/L tris-phosphate buffer at pH 2.5 containing 5 mmol/L 2-O-(2-hydroxybutyl)-beta-CD, the baseline separation of enantiomers, such as anisodamine (R(S) = 3.10), ketoconazole (R(S) = 3.01), propranolol (R(S) = 3.87), promethazine (R(S) = 3.63), adrenaline (R(S) = 3.42) and chlorphenamine (R(S) = 2.96), could be achieved.

Chiral recognition of functionalized cyclodextrins in capillary electrophoresis

The chiral recognition of hydroxypropylated, dimethylated, and sulfated cyclodextrins was evaluated by utilizing them as chiral additives in capillary electrophoresis. Although each selector yielded enantiomeric separations of most of the target analytes, differences were observed in the electrophoretic results for the different derivatized cyclodextrins and for additives having varying degrees of substitution. The results for the sulfated cyclodextrins also highlighted the importance of knowing the degree of substitution as well as the location of the substituents when comparing chiral selectors.

Separation and control of the elution order of N-t-butyloxycarbonyl amino acids D/L isomers by reversed-phase HPLC using cyclodextrins as chiral selectors for the mobile phase

The enantiomeric resolution of N-t-butyloxycarbonyl (N-t-Boc) amino acids D/L isomers by reversed-phase HPLC was investigated using cyclodextrins (CD's) as chiral selectors for the mobile phase. The use of a low pH (pH<4) for the mobile phase enabled the enantioseparation of N-t-Boc amino acids. The opposite elution order of D/L isomers was observed when hydroxypropyl-derivatized beta-CD was used instead of native beta-CD. A computer simulation of the enantioseparation showed that the ratio of the retention factors of the chiral selector and the sample determined the elution order and the resolution. When the retention factor of the chiral selector is smaller than that of the sample, an isomer having larger complex formation constant eluted faster. However, when the chiral selector had a larger retention factor than the sample, an opposite elution order of the isomers was obtained. The large difference in the retention factors between the chiral selector and the sample led to good enantiomeric separation.

Chiral separation by chromatographic and electromigration techniques. A review

This review gives a survey of different chiral separation principles and their use in high-performance liquid chromatography (HPLC), gas chromatography (GC), supercritical fluid chromatography (SFC), thin-layer chromatography (TLC), capillary electrophoresis (CE) and capillary electrochromatography (CEC) highlighting new developments and innovative techniques. The mechanisms of the different separation principles are briefly discussed and some selected applications are shown.