Chiral separation by high performance liquid chromatography. I. Review on indirect separation of enantiomers as diastereomeric derivatives using ultraviolet, fluorescence and electrochemical detection

Application of 2D EXSY and qNMR Spectroscopy for Diastereomeric Excess Determination Following Chiral Resolution of β-Lactams

Trans-β-lactam isomers have garnered much attention as anti-cancer microtubule targeting agents. Currently available synthetic methods are available for the preparation of enantiopure β-lactams and favour isomeric cis/trans β-lactam mixtures. Indirect chiral resolution offers the opportunity for isolation of exclusively enantiopure trans-β-lactams. In this study, liquid chromatography chiral resolution of β-lactams derivatized as diastereomer mixtures with a panel of N-protected amino acids is explored, where N-(Boc)-L-proline served as the optimal chiral derivatising reagent. High-performance liquid chromatography failed to adequately determine diastereomeric excess (de) of resolved diastereomers. Variable temperature, ¹ H NMR and 2D EXSY spectroscopic analyses of proline-derivatised diastereomers were successfully employed to characterise equilibrating rotamers of resolved diastereomers and determine their de. Integration of resolved resonances corresponding to H₃ and H₄ of the β-lactam ring served as a quantitative qNMR tool for the calculation of de following resolution.

HPLC determination of D-3-hydroxybutyric acid by derivatization with a benzofurazan reagent and fluorescent detection: application in the analysis of human plasma

A simple and sensitive new method for the determination of D-3-hydroxybutyric acid (D-3-HBA) in human plasma after derivatization is described. The proposed method is based on the reaction of (2S)-2-amino-3-methyl-1-[4-(7-nitro-benzo-2,1,3-oxadiazol-4-yl)-piperazin-1-yl]-butan-1-one (NBD-PZ-Val) with D-3-HBA in the presence of O-(7-azobenzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HATU) and N-ethyldiisopropylamine (DIEA) to produce a fluorescent derivative. The formed derivative was monitored fluorimetrically at λ(ex)=489 nm and λ(em)=532 nm. The HPLC analysis was carried out by use of a C18 analytical column (Synergy Hydro 150 mm × 3 mm, i.d., 4 μm) with a binary gradient elution program of 0.1% aqueous trifluoroacetic acid versus methanol. The method showed satisfactory linearity (r(2)=0.9997) in the range from 20 to 500 μmol/L. The limit of detection (LOD) of the method was 7.7 μmol/L, while the limit of quantitation (LOQ) was 25.8 μmol/L. The analytical method was successfully applied to human plasma samples from normal healthy subjects.

Role of Stereoselective Assays in Bioequivalence Studies of Racemic Drugs: Have We Reached a Consensus?

The existence of stereoselectivity in metabolism and drug disposition, coupled with the existence of genetic polymorphisms and modulation of enantiomeric kinetics via special delivery systems, provides some compulsion to assess bioequivalence using stereoselective data. However, examination of the literature suggests that nonstereoselective data are commonly used for the bioequivalence assessment of drug racemates.

Chiral separation of beta-blockers after derivatization with a new chiral derivatization agent, GATC

A new chiral derivatization agent with sugar moiety, 2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl isothiocyanate (GATC) was synthesized. Several beta-blockers were investigated for the possible separation of the enantiomers by reversed-phase HPLC after derivatization with this new chiral derivatization agent (GATC). GATC was reacted readily with beta-blockers at room temperature and the reaction mixture could directly be injected into the HPLC system. The corresponding diastereomers were well resolved on an ODS column with acetonitrile-ammonium acetate buffer as a mobile phase and monitored at UV 254 nm. The optimization of the derivatization procedure (concentration of GATC, reaction temperature and time) and HPLC conditions (pH and ionic strength of mobile phase) were investigated and compared with GITC.

High-performance liquid chromatographic determination of α-aminonitrile enantiomers after derivatization with o-phthalaldehyde and chiral thiols

o-Phthalaldehyde in combination with chiral thiols is described as a chiral reagent for the resolution of the enantiomers of alpha-H-alpha-aminonitriles and alpha-alkyl-alpha-aminonitriles. Separation of the resulting diastereomers was carried out by reversed-phase chromatography and the derivatives were detected at UV 340 nm. The identity of the diastereomeric derivatives was confirmed by LC-MS. Among the chiral thiols tested, N-acetyl-D-penicillamine and beta-mercaptoisobutyric acid gave the best resolution for the alpha-aminonitriles studied. For quantitative enantiomeric excess determination, beta-mercaptoisobutyric acid was chosen as this thiol could be obtained in high enantiomeric purity from a commercial source. The rate of the reaction of various alpha-aminonitriles with o-phthalaldehyde and beta-mercaptoisobutyric acid was studied. The accuracy of the method was investigated by a comparison of theoretical and measured enantiomeric excess.

Chiral separation of beta-blockers after derivatization with (-)-alphamethoxy-alpha-(trifluoromethyl)phenylacetyl chloride by gas chromatography

Gas chromatographic method was investigated for the chiral separation of several beta-blockers(atenolol, betaxolol, bisoprolol, metoprolol and pindolol) using (-)-alpha-methoxy-alpha-(trifluoromethyl)phenylacetyl chloride as a chiral derivatizing agent for amino group. Prior to N-acylation, hydroxyl group was converted into O-silyl ethers by react with N-methyl-N-(trimethylsilyl)trifluoroacetamide. The reaction was selective and rapid and the diastereomeric derivatives were well separated by capillary gas chromatography. (R)-isomers were eluted faster than (S)-isomers when (-)-alpha-methoxy-alpha-(trifluoromethyl)phenylacetyl chloride was used as the chiral derivatizing agent. But in the opposite sequence when (+)-alpha-methoxy-alpha-(trifluoromethyl)phenylacetyl chloride was used. No racemization was found during the reaction.

Chiral derivatization reagents for drug enantioseparation by high-performance liquid chromatography based upon pre-column derivatization and formation of diastereomers: enantioselectivity and related structure

Structures and related enantioselectivities of the respective chiral derivatization reagents (CDRs) for drug enantioseparation by high-performance chromatography based upon pre-column derivatization and diastereomeric formation are reviewed. The elution order of diastereomers caused reaction of some CDRs with enantiomeric amino acids and carboxylic acids. The development of new CDRs available for indirect HPLC methods is also discussed. (c) 2001 John Wiley & Sons, Ltd.

Enantiomeric determination of praziquantel and its main metabolite trans-4-hydroxypraziquantel in human plasma by cyclodextrin-modified micellar electrokinetic chromatography

A capillary electrophoresis method for the simultaneous quantitation of praziquantel and its main metabolite trans-4-hydroxypraziquantel enantiomers in human plasma was developed and validated using cyclodextrin-modified micellar electrokinetic chromatography. Sample clean-up involved a single-step liquid-liquid extraction of plasma with toluene after the addition of NaCl. The complete enantioselective analysis was obtained in less than 7 min using 2% w/v sulfated beta-cyclodextrin as chiral selector and 20 mmol/L sodium deoxycholate as surfactant, in 20 mmol/L sodium borate buffer, pH 10. A 50 microm x 42 cm uncoated fused-silica capillary was used for the analysis, performed at a voltage of 18 kV and at 20 degrees C. The calibration curves were linear over the 125-625 ng/mL concentration range. The mean recoveries for praziquantel and trans-4-hydroxypraziquantel were up to 96 and 71%, respectively, with good precision. All four enantiomers were quantified at two concentration levels (200 and 600 ng/mL) with precision and accuracy below 15%. The quantitation limit was 50 ng/mL for (-)-(R)- and (+)-(S)-praziquantel and 62.5 ng/mL for (-)-(R)- and (+)-(S)-trans-4-hydroxypraziquantel, using 1 mL of human plasma.

Reversed-phase high performance liquid chromatographic separation of the enantiomers of terbutaline by derivatization with 2,3,4,6-tetra-o-acetyl-beta-D-glucopyranosyl isothiocyanate

The enantiomers of the bronchodilator terbutaline were separated by reversed-phase high performance liquid chromatography after derivatization with 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyl isothiocyanate(GITC) reagent. The derivatization proceeded quantitatively within 1 h at room temperature. The corresponding diastereomeric thiourea derivatives were well resolved on an ODS column with acetonitrile-acetate buffer as a mobile phase. Elution orders of the diastereomers were confirmed by derivatization of R-(-)-terbutaline and S-(+)-terbutaline which were collected by semi-preparative chiral HPLC using Sumichiral OA-4700 column. The native fluorescence of terbutaline was quenched by derivatization with GITC. The detection limit was 25 ng when monitored at UV 278 nm.

Chiral separation of beta-blockers after derivatization with (-)-menthyl chloroformate by reversed-phase high performance liquid chromatography

Optimum conditions of chiral derivatization reaction of beta-blockers acebutolol, arotinolol, beta-xolol, bisoprolol, celiprolol, metoprolol and pindolol) with (-)-menthyl chloroformate were investigated for the resolution by HPLC. With more than 30 times molar excess of (-)-menthyl chloroformate chiral derivatization reactions were completed within one hour at room temperature except arotinolol and celiprolol. Diastereomeric derivatives of beta-blockers were well resolved on the ODS column using acetonitrile-methanol-water as a mobile phase.

Chiral purity test of metoprolol enantiomer after derivatization with (-)-menthyl chloroformate by reversed-phase high performance liquid chromatography

A reversed-phase high-performance liquid chromatographic method was developed to determine the optical purity of metoprolol enantiomers. The enantiomers were converted to diastereomeric derivatives using (-)-menthyl chloroformate reagent. Separation of the enantiomers as diastereomers was achieved by reversed-phase HPLC within 30 min using Inertsil C8 column. This method allowed determination of 0.05% of either enantiomer in the presence of its stereoisomer and method validation showed adequate linearity over the required range. Owing to the reaction condition during the derivatization with (-)-menthyl chloroformate, the possibility of racemization had to be established. Different ratios of (S)-(-)-metoprolol and (R)-(+)-metoprolol were prepared. Enantiomeric separation of these mixtures took place on a chiralcel OD column or, after derivatization with (-)-menthyl chloroformate, on a C8 column. The results from the these two independent separation systems were compared with trace racemization and were in very good agreement. No racemization was found during the experiment.

Production and characterization of 22 monoclonal antibodies directed against S 20499, a new potent 5-HT1A chiral agonist: influence of the hapten structure on specificity and stereorecognition

PURPOSE

An immunoconjugate model was proposed to produce stereoselective monoclonal antibodies (MAbs) for the quantitation of a 5-HT1A agonist, S 20499. MAbs produced were characterized in terms of stereoselectivity and specificity towards the opposite enantiomer and structural analogs.

METHODS

The immunogen was formed following the effective addition of a butanoic acid spacer arm between the parent S 20499 structure and bovine serum albumin (BSA). After fusion (modified Köhler and Milstein's procedure), specificity of MAbs was obtained using the Abraham's criteria. Experimental and calculated partition coefficients were determined.

RESULTS

Twenty-two hybridoma cell lines were established secreting MAbs (apparent association constants ranging from 1.1 x 10(8) to 2.8 X 10(9) M(-1)). Several MAbs showed cross-reactivity levels of less than 5% with S 20500 (optical antipode), which could allow a stereospecific assay to be set up. Both chroman and azaspiro moieties were part of the epitopic site. Dealkylation and hydroxylation(s) led to various crossreactivity levels. Four antibody families were described in terms of specificity.

CONCLUSIONS

This study highlighted the influence of the immunoconjugate construction (coupling site and type of spacer arm) in the immuno-stereospecificity of Abs. The results obtained for two monohydroxylated metabolites suggest that the lipophilicity behavior could be a valuable tool for predicting Ab-crossreactivity.

Determination of D-amino acids labeled with fluorescent chiral reagents, R(-)- and S(+)-4-(3-isothiocyanatopyrrolidin-1-yl)-7-(N, N-dimethylaminosulfonyl)-2,1,3-benzoxadiazoles, in biological and food samples by liquid chromatography

D-Amino acids in food and biological samples labeled with R(-)- and S(+)-4-(3-isothiocyanatopyrrolidin-1-yl)-7-(N, N-dimethylaminosulfonyl)-2,1,3-benzoxadiazoles (DBD-PyNCS) were separated by reversed-phase chromatography and detected fluorometrically at 550 nm (excitation at 460 nm). DL-Amino acids were efficiently labeled at 55 degrees C for 20 min in basic medium. The resulting thiocarbamoyl-amino acids were resolved by an isocratic elution using water:30% methanol in acetonitrile (72:28) containing 0.1% trifluoracetic acid as mobile phase for hydrophilic amino acids and gradient elutions using sodium acetate buffer (pH 5. 2)/acetonitrile as gradient solvent mixture for hydrophobic amino acids, respectively. The detection limits (S/N = 3) of DL-amino acids tested were in the range of 0.16-0.75 pmol. The proposed method was applied to determine the D-amino acid(s) in milk, cream, fermented dairy products (yogurt and yakult), tomato products (juice, puree, and catchup), fermented beverages (beer and red wine), and human urine. The existence of D-amino acid(s) was demonstrated in all the samples tested. Furthermore, the identification of the D-amino acid(s) was performed using both isomers of DBD-PyNCS and by on-line HPLC-electrospray ionization-MS.

Total resolution of 17 DL-amino acids labelled with a fluorescent chiral reagent, R(-)-4-(3-isothiocyanatopyrrolidin-1-y1)-7-(N,N-dimethylaminosulfonyl)- 2,1,3-benzoxadiazole, by high-performance liquid chromatography

Total resolution of 17 DL-amino acids after derivatization with a fluorescent chiral tagging reagent, 4-(3-isothiocyanatopyrrolidin-1-yl)-7-(N,N-dimethylaminosulfony l)-2,1,3- benzoxadiazole [R(-)-DBD-PyNCS], was studied by reversed-phase liquid chromatography. The reaction of the reagent with amino acids proceeds effectively at 55 degrees C for 20 min in the presence of 1% TEA to produce the corresponding fluorescent diastereomers (excitation at 460 nm, emission at 550 nm). Each pair of the resulting derivatives was efficiently separated with water-acetonitrile containing 1% acetic acid as the mobile phase. Peak resolution was in the range of 0.92 (DL-Arg)-9.8 (DL-Cys). Although mutual separation of some DL-amino acids was possible using the elution solvent, simultaneous resolution of 17 DL-amino acids was difficult with a single chromatographic run, even if some gradient elutions were adopted. Therefore, both gradient and isocratic elution systems were used for total resolution of the DL-amino acids. Thus, 17 DL-amino acids were well resolved by a gradient and an isocratic elution systems. The proposed derivatization and elution methods were applied to the determination of DL-amino acids in yogurt. The results showed that some of the L-amino acids, i.e., Glu, Asp, Ser, Gly, Ala, Thr, Pro, Lys, Phe and Met, were found in the methanol extracts of yogurt. On the other hand, the D-amino acids that were identified in the extracts were D-Glu, D-Asp and D-Ala, and the mean % to each L-amino acid were 11.9% (D-Glu), 27.6% (D-Asp) and 56.7% (D-Ala), respectively.

Comparison of LC detection methods in the investigation of non-UV detectable organic impurities in a drug substance

HPLC Analysis with different detection methods was shown to be essential in the separation and identification of unknown organic impurities in a drug substance. The impurities were found to exhibit very weak or no response to standard ultraviolet (UV) absorption detection. LC-MS, LC-NMR, indirect, refractive index and evaporative light-scattering detection were used to quantify and identify the impurities in this specific case. The drug substance studied was found to be an ideal analyte for demonstrating the advantages and limitations of several chromatographic detection systems for impurity profile analysis.

Direct enantiomeric separations by high performance liquid chromatography using cyclodextrins

Due to their identical chemical and physical properties, the separation of enantiomers has been considered one of the most difficult challenges in chemistry from both an analytical or a preparative viewpoint. With the development and commercialization of many new or improved chiral stationary phases and chiral additives, interest in enantiomeric separation by HPLC has grown tremendously in the last two decades. Cyclodextrins and modified cyclodextrins are widely used chiral selectors. They are used as either the chiral stationary phases, as chiral mobile phase additives, or as chiral counter-ions. This review describes the historical development of derivatized and underivatized cyclodextrins in HPLC and their various applications.

Production and characterization of polyclonal anti-S 20499 antibodies: influence of the hapten structure on stereospecificity

Immunoassays were studied as an alternative to HPLC methods for the stereoselective determination of a chiral drug, S 20499, a new anxiolytic compound that is chemically related to buspirone. The production of highly stereospecific polyclonal antibodies was sought following the construction of appropriately optimized hapten-protein conjugates. This process involved the selection of the structure and the length of the spacer arm used to couple S 20499 to the carrier protein as well as deciding on the location of the coupling site with respect to the chiral center. Two haptens were prepared: one a derivative resembling the original structure of S 20499, with the effective addition of a carboxylic acid group, and a second with the effective addition of a butanoic acid moiety that is supposed to favor stereorecognition. Six stereospecific polyclonal antisera were obtained in rabbits with two groups of antibody families defined in terms of specificity. Both approaches gave high levels of stereospecificity (cross-reactivity towards the optical antipode of S 20499 ranged from 4.1% to < 0.1%). Although it did not decrease the mean apparent affinity constant, the longer spacer improved antibody specificity by decreasing cross-reactions towards dealkylated S 20499 derivatives. Hence, the addition of a four carbon atom bridge should be a valuable tool for increasing antibody stereospecificity with no drawbacks in terms of specificity and affinity. It was also shown that long immunization periods appear to have no effect on the stereospecificity of the antibodies obtained.

Enantioselective determination of FCE 28833, a new potential antiischemic agent, in gerbil plasma using column switching HPLC with UV detection

A sensitive and selective high performance liquid chromatographic method using an automated column switching technique for the determination of FCE 28833 enantiomers in gerbil plasma was developed. After solid-liquid extraction using a Supelcosil C18 cartridge, FCE 28833 was eluted on a clean-up column (Spherisorb CN) and the enantiomers were separated using an analytical chiral column (Crownpack CR(+)). The mobile phase (15% methanol in HClO4 1 mM) was directed through the columns at a flow rate of 1 ml/min and the fraction eluted between 13 and 40 min was transferred from the clean-up column into the analytical column. FCE 28833 enantiomers were monitored at 257 nm. The limit of quantitation of the method was 20 ng/ml plasma for both enantiomers and proved to be linear, precise, and accurate for the assay of both enantiomers in the 20-6,000 ng/ml concentration range. No interference from the blank gerbil plasma sample was observed. The suitability of the method was assessed using plasma samples obtained from male gerbils treated with a single oral dose (400 mg/kg) of FCE 28833.

Degradation kinetics and isomerization of cefdinir, a new oral cephalosporin, in aqueous solution. 2. Hydrolytic degradation pathway and mechanism for beta-lactam ring opened lactones

Hydrolysis of cefdinir leads to pH-dependent isomerizations and beta-lactam ring-opening. Lactam ring opened gamma-lactones were produced as a mixture of four diastereoisomers based on the lactone methyl, and C-6 isomerizations in acidic to neutral solutions. Cefdinir and its 7-epimer were hydrolyzed to clarify the pathway leading to these lactones and the mechanism of C-6 epimerization with the aid of chiral separation techniques. Chiral separation using a bovine serum albumin column was employed to detect the beta-lactam ring opened products of cefdinir and its 7-epimer; the C-6 and C-7 isomerization was thereby observed; however, it was found to be pH-dependent at pH > or = 9. Optical activity detection applied to the lactones produced from cefdinir and its 7-epimer demonstrated that the corresponding peaks of these lactones were enantiomeric pairs. In addition, the smallest rate constant at pH 4 was observed for C-6 epimerization of the lactones, and it was found to proceed without deprotonation at C-6 by 1H-NMR spectroscopy. From the results of these studies, a plausible mechanism for C-6 epimerization has been proposed. Additionally, it was confirmed that two degradation pathways were involved during hydrolysis of cefdinir to the lactone.

Resolution of enantiomers of amino acids by HPLC

Application of HPLC as a prime tool in the area of enantiomeric resolution has opened doors of success and varied interest. Use of chiral reagents either indirectly (as derivatization reagent) or directly (added to stationary or mobile phase) has led to achieve resolution of a wide range of compounds. Amino acids, being important molecules with simple structure and easy availability, have been extensively studied. A bibliographic survey on HPLC resolution of amino acids and derivatives along with a brief discussion on general methods of enantiomeric separation has been presented.