Cyclodextrin-Enabled Enantioselective Complexation Study of Cathinone Analogs.

The characteristic alkaloid component of the leaves of the catnip shrub (Catha edulis) is cathinone, and its synthetic analogs form a major group of recreational drugs. Cathinone derivatives are chiral compounds. In the literature, several chiral methods using cyclodextrins (CDs) have been achieved so far for diverse sets of analogs; however, a comprehensive investigation of the stability of their CD complexes has not been performed yet. To characterize the enantioselective complex formation, a systematic experimental design was developed in which a total number of 40 neutral, positively, and negatively charged CD derivatives were screened by affinity capillary electrophoresis and compared according to their cavity size, substituent type, and location. The functional groups responsible for the favorable interactions were identified in the case of para-substituted cathinone analog mephedrone, flephedrone, and 4-methylethcathinone (4-MEC) and in the case of 3,4-methylendioxy derivative butylone and methylenedioxypyrovalerone (MDPV). The succinylated-ß-CD and subetadex exhibited the highest complex stabilities among the studied drugs. The complex stoichiometry was determined using the Job's plot method, and the complex structures were further studied using ROESY NMR measurements. The results of our enantioselective complex formation study can facilitate chiral method development and may lead to evaluate potential CD-based antidotes for cathinone analogs.

Enantioseparation and quantitative determination of two homologous beta amino acids found in Fabaceae plants.

Non-protein amino acids are important metabolites of the Fabaceae family, possessing valuable biological effects in addition to their toxic properties. We have previously identified two non-protein amino acids homoproline and homopipecolic acid in Ononis species for the first time, and herein the study was extended to investigate further Fabaceae species (O. spinosa, O. arvensis, M. sativa, A. vulneraria) with medicinal, food or cosmetic uses. As the enantiomers of these beta amino acids can carry different activity or toxicity, our aim was to develop a chiral separation method for homoproline and homopipecolic acid enantiomers and apply it to plant samples. For this purpose, dansylated derivatives were prepared and a cyclodextrin-modified capillary electrophoresis in addition to a chiral HPLC method were developed. Although baseline separation was achieved by CE applying mono-(6-N-pyrrolidine-6-deoxy)-ß-CD, mono-(6-N-piperidine-6-deoxy)-ß-CD or sulfated-gamma-cyclodextrin at pH 6.0, the HPLC method was found to be more suitable for the analysis of the plant samples. Both homoproline and homopipecolic acid were confirmed in plant samples as racemates. The quantitative determination of homoproline and homopipecolic acid in several Fabaceae species were also aimed. Since these molecules can be found in the plants as esters, sample preparation was optimized to liberate the target molecules. Several SPE methods were tested for sample purification and a HPLC-MS/MS method using C8 stationary phase was developed and validated. The presence of homoproline and homopipecolic acid could be confirmed in all species ranging from 1 µg/g through 500 µg/g homopipecolic acid and 6 µg/g to 60 µg/g homoproline and significant changes could be observed between species, geographical origins, and botanical parts. Generally O. spinosa root samples were found to be the richest sources of the two amino acids, but a high variance could be observed between species.

Single Isomer N-Heterocyclic Cyclodextrin Derivatives as Chiral Selectors in Capillary Electrophoresis.

In order to better understand the chiral recognition mechanisms of positively charged cyclodextrin (CD) derivatives, the synthesis, the pKa determination by 1H nuclear magnetic resonance (NMR)-pH titration and a comparative chiral capillary electrophoretic (CE) study were performed with two series of mono-substituted cationic single isomer CDs. The first series of selectors were mono-(6-N-pyrrolidine-6-deoxy)-ß-CD (PYR-ß-CD), mono-(6-N-piperidine-6-deoxy)-ß-CD (PIP-ß-CD), mono-(6-N-morpholine-6-deoxy)-ß-CD (MO-ß-CD) and mono-(6-N-piperazine-6-deoxy)-ß-CD (PIPA-ß-CD), carrying a pH-adjustable moiety at the narrower rim of the cavity, while the second set represented by their quaternarized, permanently cationic counterparts: mono-(6-N-(N-methyl-pyrrolidine)-6-deoxy)-ß-CD (MePYR-ß-CD), mono-(6-N-(N-methyl-piperidine)-6-deoxy)-ß-CD (MePIP-ß-CD), mono-(6-N-(N-methyl-morpholine)-6-deoxy)-ß-CD (MeMO-ß-CD) and mono-(6-N-(4,4-N,N-dimethyl-piperazine)-ß-CD (diMePIPA-ß-CD). Based on pH-dependent and selector concentration-dependent comparative studies of these single isomer N-heterocyclic CDs presented herein, it can be concluded that all CDs could successfully be applied as chiral selectors for the enantiodiscrimination of several negatively charged and zwitterionic model racemates. The substituent-dependent enantiomer migration order reversal of dansylated-valine using PIP-ß-CD contrary to PYP-ß-CD, MO-ß-CD and PIPA-ß-CD was also studied by 1H- and 2D ROESY NMR experiments.

Enantioseparation of solriamfetol and its major impurity phenylalaninol by capillary electrophoresis using sulfated gamma cyclodextrin.

R-solriamfetol is a recently approved drug used for the treatment of excessive sleepiness associated with narcolepsy and sleep apnea. Herein, a capillary electrophoretic method was developed, enabling the simultaneous analysis of the API and its S-enantiomer in addition to the enantiomers of its major impurity phenylalaninol. Twenty-nine different cyclodextrins (CDs), including native, neutral, and charged ones were screened as potential chiral selectors, and the best results were obtained with sulfated CDs. Randomly sulfated-ß-CD exhibited outstanding enantioresolution, the peaks of phenylalaninol enantiomers inserted between the two peaks of solriamfetol enantiomers, while sulfated-γ-CD (S-γ-CD) showed remarkable resolution values in a much shorter analysis time with the optimal enantiomer migration order. Among the single isomer sulfated CD derivatives, substituent dependent enantiomer migration order reversal could also be observed in the case of heptakis(6-O-sulfo)-ß-CD (HS-ß-CD) or heptakis(2,3-O-dimethyl-6-O-sulfo)-ß-CD (HDMS-ß-CD) with R-,S-solriamfetol, and heptakis(2,3-O-diacetyl-6-O-sulfo)-ß-CD (HDAS-ß-CD) resulting S-,R-solriamfetol migration order. The sulfated-γ-CD system was chosen for method optimization applying orthogonal experimental design. The optimized method (45 mM Tris-acetate buffer, pH 4.5, 4 mM S-γ-CD, 21°C, +19.5 kV) was capable for the baseline separation of solriamfetol and phenylalaninol enantiomers within 7 min. The optimized method was validated according to the ICH guidelines and successfully applied for the analysis of pharmaceutical preparation (Sunosi® 75 mg tablet), thus it may serve as a routine procedure for the laboratories of regulatory authorities as well as in Pharmacopoeias.

Single isomer cyclodextrins as chiral selectors in capillary electrophoresis.

Since decades, cyclodextrins are one of the most powerful selectors in chiral capillary electrophoresis for the enantioseparation of diverse organic compounds. This review concerns papers published over the last decade (from 2009 until nowadays), dealing with the capillary electrophoretic application of single isomer cyclodextrin derivatives in chiral separations. Following a brief overview of their synthetic approaches, the inventory of the neutral, negatively and positively charged (including both permanently ionic and pH-tunable ionizable substituents) and zwitterionic CD derivatives is presented, with insights to underlying structural aspects by NMR spectroscopy and molecular modeling. CE represents an ideal tool to study the weak, non-covalent supramolecular interactions. The published methods are reviewed in the light of enantioselectivity, enantiomer migration order and the fine-tuning of enantiodiscrimination by the substitution pattern of the single entity selector molecules, which is hardly possible for their randomly substituted counterparts. All the reviewed publications herein support that cyclodextrin-based chiral capillary electrophoresis seems to remain a popular choice in pharmaceutical and biomedical analysis.

Synthesis, analytical characterization and capillary electrophoretic use of the single-isomer heptakis-(6-O-sulfobutyl)-beta-cyclodextrin.

This contribution reports the synthesis, characterization and capillary electrophoretic application of heptakis-(6-O-sulfobutyl-ether)-ß-cyclodextrin sodium salt, (6-(SB)7-ß-CD). The compound was obtained through a five-steps synthesis and it represents the first example of single-isomer sulfobutylated cyclodextrin that carries the negatively charged functions exclusively on its primary side and it is unmodified on the lower rim. The purity of each intermediate was determined by appropriate liquid chromatographic methods, while the isomeric purity of the final product was established by an ad-hoc developed HPLC method based on a CD-Screen-IEC column. The structural identification of 6-(SB)7-ß-CD was carried out by 1D, 2D NMR spectroscopy and ESI-MS. The chiral separation ability of 6-(SB)7-ß-CD was studied by chiral capillary electrophoresis using the single-isomer host as a background electrolyte additive to separate the enantiomers of a representative set of pharmacologically significant model compounds such as verapamil, dapoxetine, ondansetron, propranolol, atenolol, metoprolol, carvedilol, terbutaline, amlodipine and tadalafil. The enantiomer migration order and the effects of the selector concentration on the enantiorecognition properties were investigated. NMR spectroscopy was applied to deepen and further confirm the host-guest interactions and in the case of the model compound dapoxetine a potential representation for the supramolecular assembly was developed based on the dataset collected by the extensive 2D NMR analysis. This single-isomer chiral selector offers a new alternative to the widely applied randomly sulfobutylated- and sulfated-beta-cylodextrins as well as to the single-isomer sulfated and carboxymethylated derivatives in chiral separations.

Characterization of a single-isomer carboxymethyl-beta-cyclodextrin in chiral capillary electrophoresis.

In this work, the synthesis, characterization, and chiral capillary electrophoretic study of heptakis-(2,3-di-O-methyl-6-O-carboxymethyl)-ß-CD (HDMCM), a single-isomer carboxymethylated CD, are presented. The pH-dependent and selector concentration-dependent enantiorecognition properties of HDMCM were investigated and discussed herein. The enantioseparation was assessed applying a structurally diverse set of noncharged, basic, and zwitterionic racemates. The increase in the selector concentration and gross negative charge of HDMCM improved the enantioseparation that could be observed in the majority of the cases. HDMCM was also successfully applied as BGE additive in NACE using a methanol-based system in order to prove the separation selectivity features and to highlight the broad applicability of HDMCM. Over 25 racemates showed partial or baseline separation with HDMCM under the conditions investigated, among which optimal enantiomer migration order was found for the four stereoisomers of tadalafil, tapentadol, and dapoxetine, offering the possibility of a chiral CE method development for chiral purity profiling of these drugs.

Single-isomer carboxymethyl-γ-cyclodextrin as chiral resolving agent for capillary electrophoresis.

Herein we report on the synthesis, characterization and the novel capillary electrophoretic use of octakis-(2,3-di-O-methyl-6-O-carboxymethyl)-γ-cyclodextrin sodium salt (ODMCM). ODMCM is the first single-isomer carboxymethyl-γ-cyclodextrin that is fully methylated on its secondary side and carries ionizable carboxymethyl functions on its primary side. ODMCM was prepared with high isomeric purity through a four-step synthetic procedure. The purity of each intermediate was characterized by appropriate chromatographic methods, while the isomeric purity of the carboxymethylated product was determined by an HPLC method using a CD-Screen-IEC column and by a capillary electrophoretic method using indirect UV detection, as well. The structural identification of the ODMCM was carried out by 1D, 2D NMR spectroscopy and ESI-MS. The acid-base characterization of the chiral selector was carried out by 1H NMR-pH titration. The chiral separation ability of the synthesized selector was studied by chiral capillary electrophoresis. ODMCM was used as a background electrolyte additive to separate enantiomers of representative pharmacologically significant model molecules such as propranolol, citalopram, ketamine, tapentadol and dapoxetine. The effects of the selector concentration and the pH of the background electrolyte on the enantiorecognition properties were investigated. 1H NMR spectroscopy was further applied to get deeper insight of the host-guest inclusion complex formation. The pH-dependent enantioselectivity of this new single-isomer chiral selector was demonstrated by chiral capillary electrophoresis and 1H NMR spectroscopy.

Comparative evaluation of the chiral recognition potential of single-isomer sulfated beta-cyclodextrin synthesis intermediates in non-aqueous capillary electrophoresis.

The enantioselectivity of neutral single-isomer synthetic precursors of sulfated-ß-cyclodextrins was studied. Four neutral single-isomer cyclodextrins substituted on the secondary side with acetyl and/or methyl functional groups, heptakis(2-O-methyl-3,6-dihydroxy)-ß-cyclodextrin (HM-ß-CD), heptakis(2,3-di-O-acetyl-6-hydroxy)-ß-cyclodextrin (HDA-ß-CD), heptakis(2,3-di-O-methyl-6-hydroxy)-ß-cyclodextrin (HDM-ß-CD), heptakis(2-O-methyl-3-O-acetyl-6-hydroxy)-ß-cyclodextrin (HMA-ß-CD), and their sulfated analogs the negatively charged heptakis(2,3-di-O-methyl-6-sulfato)-ß-cyclodextrin (HDMS-ß-CD) and heptakis(2,3-di-O-acetyl-6-sulfato)-ß-cyclodextrin (HDAS-ß-CD) were investigated by non-aqueous capillary electrophoresis in the view of enantiodiscrimination for various drugs and related pharmaceutical compounds. The focus of the present work was on the chiral selectivity studies of the neutral derivatives, which are the synthesis intermediates of the sulfated products. The chiral recognition experiments proved that among the neutral compounds the HMA-ß-CD shows remarkable enantioselectivity towards chiral guests in non-aqueous capillary electrophoresis, while HM-ß-CD, HDA-ß-CD and HDM-ß-CD failed to resolve any of the 25 studied racemates under the applied experimental conditions. In order to get deeper insight into the molecular interactions between the studied single-isomer cyclodextrin and chiral fluoroquinolones (ofloxacin, gatifloxacin and lomefloxacin) and ß-blockers (propranolol), 1H and ROESY NMR experiments were performed. The 2-O-methylation in combination with the 3-O-acetylation of the host was evidenced to exclusively carry the essential spatial arrangement for chiral recognition.

Development and validation of a cyclodextrin-modified capillary electrophoresis method for the enantiomeric separation of vildagliptin enantiomers.

The enantiomers of vildagliptin, an orally available and selective dipeptidyl-peptidase-4 inhibitor used for the treatment of type II diabetes, have been separated by CD-modified CZE, using uncoated fused-silica capillary. After screening 13 negatively charged CD derivatives as potential chiral selectors, sulfobutyl-ether-α-CD (SBE-α-CD) was selected for the enantioseparation. For the optimization, a factorial analysis study was performed by orthogonal experimental design. Six experimental factors were chosen as variable parameters: temperature, applied voltage, chiral selector and BGE concentrations, pH, and the parameters of the hydrodynamic injection. The optimized system still was not considered final as the second peak (S-enantiomer) migrated too close to the EOF, resulting in a potential inaccuracy during the determination of the chiral impurity. To fine-tune the method "one factor at a time" variation approach was applied. The final method (applying 15°C capillary temperature, 40 mbar × 4 s hydrodynamic injection, 25 kV voltage in 75 mM acetate-Tris buffer [pH 4.75] containing 20 mM SBE-α-CD as chiral selector) was validated according to the ICH guideline. RSD percentage of the resolution value, migration times, and corrected peak areas were below 5% during testing repeatability and intermediate precision. LOD and LOQ values were found to be 2.5 and 7.5 µg/mL, respectively. The method is considered linear in the 7.5-180 µg/mL range for the R-enantiomer. The robustness of the method was justified using Plackett-Burmann statistical experimental design.

Enantiomeric separation of tapentadol by capillary electrophoresis--study of chiral selectivity manipulation by various types of cyclodextrins.

The chiral recognition of the centrally acting analgesic agent tapentadol and its isomers with various cyclodextrins (CDs) was studied by capillary electrophoresis, focusing on the migration order of four stereoisomers. In the case of non-charged hydroxypropylated CDs (2-hydroxypropyl-ß-CD, 2-hydroxypropyl-γ-CD) the beta derivative was able to discriminate the S,R- and R,S-isomers in acidic background electrolyte, whereas the gamma allowed the separation of S,S- and R,R-tapentadol, respectively. Dual CD system containing both hosts was used to separate all of four isomers. Negatively charged sulfated-α-CD at 1.0% (w/v) concentration in 100mM sodium borate buffer (pH 9.5) was capable of separating the isomers with favorable enantiomer migration order and the optimized method was able to determine 0.15% of chiral impurities of tapentadol in the presence of the last migrating clinically important R,R-isomer.

Interactions of non-charged tadalafil stereoisomers with cyclodextrins: capillary electrophoresis and nuclear magnetic resonance studies.

The single isomer drug R,R-tadalafil (Cialis) contains two chiral centers thus four stereoisomers (R,R-, S,S-, S,R- and R,S-tadalafil) exist, however, only the most potent inhibitor, the R,R-tadalafil is in clinical use. In our study, over 20 charged cyclodextrin (CD) derivatives were studied for enantiospecific host-guest type interactions in CD-modified capillary electrophoresis. Tadalafil stereoisomers are non-charged; therefore, their electrophoretic separation poses a challenge. Several candidates of both positively and negatively charged hosts were found to be effective for the enantioseparation. Eight out of the beta derivatives and three of alpha derivatives (including sulfated, sulfoalkylated, carboxyalkylated and amino derivatives) resolved all four stereoisomers partially or completely. Cavity size-dependent absolute enantiomer migration order (EMO) reversals were observed in the case of sulfopropyl-alpha (EMO: R,S; S,R; R,R; S,S) and sulfopropyl-beta (S,S; R,R; S,R; R,S) derivatives, while substituent-dependent partial EMO reversals were detected for sulfobutyl-ether-alpha (R,S; S,R; S,S; R,R) and sulfated-alpha-CD (R,R; S,S; R,S; S,R) selectors. Complexation-induced (1)H NMR chemical shift changes reflected that the benzodioxole moiety plays a major role in cavity size-dependent EMO reversal. Sulfobutyl-ether-alpha-CD was the only selector that provided the desired EMO in which the clinically applied eutomer R,R-tadalafil migrates last. Finally, an electrophoretic method applying a background electrolyte (BGE) containing 75 mM Tris-acetic acid buffer (pH 4.75) and 7 mM sulfobutyl-ether-alpha-CD was developed for the baseline resolution of all isomers at 25 °C and +25 kV applied voltage.

Qualitative and quantitative analysis of PDE-5 inhibitors in counterfeit medicines and dietary supplements by HPLC-UV using sildenafil as a sole reference.

Due to their popularity, medicinal products containing the phophodiesterase type 5 enzyme (PDE-5) inhibitors sildenafil, vardenafil and tadalafil are often subject to counterfeiting. In addition, illicit herbal dietary supplements adulterated with these substances or their analogs have appeared on the market offering an easy and anonymous sale. This paper describes an analytical method for qualitative and quantitative screening of sildenafil, vardenafil, tadalafil and 11 of their designer analogs in illegal erectile dysfunction products by high-performance liquid chromatography with UV detection (HPLC-UV). Sildenafil served as a single external standard for both identification and quantification of all analytes. Relative retentions and reference UV spectra were used for qualitative, and correction factors for quantitative analyses, respectively. The separation was performed on a Kinetex C18 reverse-phased column at 25°C using gradient elution. Mobile phase A consisted of 200mM ammonium acetate solution while mobile phase B was a 1:1 (v/v) mixture of methanol and acetonitrile with a flow rate of 0.5ml/min and injection volume of 5µl. Detection wavelength was set to 290nm. The method was validated in accordance with the appropriate guideline of the International Conference on Harmonization (ICH) in terms of specificity, selectivity, precision, linearity, limit of quantitation, limit of detection, accuracy, robustness and stability, and was successfully applied to the analysis of natural dietary supplements and herbal remedies with an indication for enhanced male sexual potency. The proposed method offers a cheap and simple alternative to LC-MS screening used by control laboratories for routine analysis of suspicious products.

Separation of alogliptin enantiomers in cyclodextrin-modified capillary electrophoresis: a validated method.

The single enantiomer drug, alogliptin (Alo, Nesina®) is a novel, orally available and selective dipeptidyl peptidase-4 inhibitor used for the treatment of type II diabetes. Following its pKa determination by CE-pH titration, a validated chiral CE method has been developed to separate Alo enantiomers. Preliminary screening of the native CDs and their ten derivatives revealed that sulfopropylated-γ-CD, sulfopropylated-ß-CD and sulfopropylated-γ-CD, sulfobutyl-ether-ß-CD (SBE-ß-CD) and sulfobutyl-ether-γ-CD enabled enantioresolution. Furthermore, cavity size dependent enantiomer migration order reversal was observed between γ- and ß-CD derivatives. To improve enantioseparation, buffer composition and pH, CD concentration, applied voltage, temperature, and injection parameters were optimized for the Alo/ SBE-ß-CD system, yielding a resolution of 8.34. RSD percentage of the resolution value, migration times, and corrected peak areas were below 3 and 5% during testing repeatability and intermediate precision. LOD and LOQ values were found to be 2 and 6 µg/mL, respectively, for each enantiomer. Calibration lines ranging from 6 to 250 µg/mL were constructed with r(2) > 0.9997. Robustness could be successfully verified by using the Plackett-Burman statistical experimental design. The optimized system containing 5 mM SBE-ß-CD in a 25 mM acetate buffer at pH 4.75 was found promising to detect 0.1% distomer in the presence of the API.

Tapentadol enantiomers: Synthesis, physico-chemical characterization and cyclodextrin interactions.

The complete physico-chemical characterization of the single enantiomer analgesic drug R,R-tapentadol was quantitated in terms of protonation macro- and microconstants and octanol-water partition coefficient using pH-potentiometry, UV-pH and (1)H NMR-pH titrations. The protonation macroconstants were found to be logK1=10.59±0.01 and logK2=9.44±0.01, while the individual basicity of each protonation site was found to be logk(O)=9.94 and logk(N)=10.48 for the phenolate and tertiary amine functions, respectively. As a consequence, the zwitterionic form of tapentadol predominates in aqueous solutions. The potential optical impurity (S,S-tapentadol) was synthesized for the first time in a seven-step chiral synthetic procedure. The enantiomers of tapentadol were separated by cyclodextrin modified capillary zone electrophoresis. Over 15 cyclodextrins were investigated in terms of apparent complex stability and screened as chiral selectors, and the sulfated alpha-cyclodextrin was found to resolve the enantiomers with excellent resolution (Rs=16.2 and 9.1) at pH 4.75 and pH 9.0, respectively. The system containing 12mM selector in a 50mM TRIS-acetate buffer was amenable to detect S,S-tapentadol potential optical impurity at 0.1% concentration level.

Separation of vinca alkaloid enantiomers by capillary electrophoresis applying cyclodextrin derivatives and characterization of cyclodextrin complexes by nuclear magnetic resonance spectroscopy.

In this work, the enantiomeric separation of three vinca alkaloid enantiomers (vincamine, vinpocetine and vincadifformine) has been investigated in an aqueous capillary electrophoresis (CE) system using cyclodextrins (CDs). The investigated CDs were the native alpha-, beta-, and gamma-CDs and their hydroxypropylated, randomly methylated, carboxymethylated and sulfobutylated derivatives. The first part of this study consisted of the determination of the apparent averaged complex stability constants with the selected CDs. Several parameters, such as the nature and the concentration of the CD, were studied and were found to have a significant effect on the enantiomeric resolution for all studied compounds. All three vinca alkaloids were successfully enantioseparated with CDs where different migration orders were observed in case of several CDs depending on the cavity size or substituent of the host. Chiral separation and determination of the stability constants were also performed with NMR spectroscopy which confirmed the CE results. Averaged stoichiometries of the complexes were determined using the Job plot method resulting in a 1:1 complex irrespective of the alkaloid enantiomers or cyclodextrin derivative. The structures of the inclusion complexes were elucidated using 2D ROESY NMR spectroscopy. On the basis of NMR results reversal of enantiomer migration order was clarified in various cases.