Enantiomer separation of drugs by capillary electrophoresis using proteins as chiral selectors

Capillary electrophoretic separation of drug enantiomers in human serum

Enantiomers of various solutes including several basic drugs and alpha-amino acids were analyzed by capillary electrophoresis in diluted human serum, and chloroquine and tryptophan were found to be well enantioseparated. In order to specify the protein responsible for enantioseparation, these drug enantiomers were analyzed in the presence of various serum protein fractions. The results indicated that albumin fraction caused enantioseparation but the alpha and beta -globulin mixed fraction, the gamma-globulin fraction and the alpha(1)-acid glycoprotein fraction did not exhibit any enantioseparation. The association constants between these drugs and albumin were roughly estimated based on our method. Approximate values were 1.50 x 10(3) and 1.85 x 10(3) M(-1) for chloroquine enantiomers, and 1.51 x 10(4) and 2.45 x 10(4) M(-1) for tryptophan enantiomers. The difference of the association constant values between the enantiomers was found to be 19% for chloroquine and 38% for tryptophan, when calculated based on the slower moving enantiomers.

Protein domain of chicken alpha(1)-acid glycoprotein is responsible for chiral recognition

Ovoglycoprotein from chicken egg whites (OGCHI) has been used as a chiral selector to separate drug enantiomers. However, neither the amino acid sequence of OGCHI nor the responsible part for the chiral recognition (protein domain or sugar moiety) has yet to be determined. First, we isolated a cDNA clone encoding OGCHI, and clarified the amino acid sequence of OGCHI, which consists of 203 amino acids including a predictable signal peptide of 20 amino acids. The mature OGCHI shows 31-32% identities to rabbit and human alpha(1)-acid glycoproteins (alpha(1)-AGPs). Thus, OGCHI should be the chicken alpha(1)-AGP. Second, the recombinant chicken alpha(1)-AGP was prepared by the Escherichia coli expression system, and its chiral recognition ability was confirmed by capillary electrophoresis. Since proteins expressed in E. coli are not modified by any sugar moieties, this result shows that the protein domain of the chicken alpha(1)-AGP is responsible for the chiral recognition.

Practical aspects of the ligand-binding and enzymatic properties of human serum albumin

Recent work with approaches like recombinant mutants and X-ray crystallography has given much new information about the ligand-binding properties of human serum albumin (HSA). The information increases the understanding of this unique transport and depot protein and could give a structural basis for the possible construction of therapeutic agents with altered HSA-binding properties. A tabulation of high-affinity binding sites for both endogenous and exogenous compounds has been made; it could be useful for the above-mentioned purpose, but it could also be of value when trying to predict potential drug interactions at the protein-binding level. Drug displacement is not always a complication to therapy; it can be used to increase the biological effect of a drug. However, due to rebinding at other sites, the increase in the free concentration of a displaced ligand can be less than expected. Drugs and drug metabolites can also interact covalently with HSA; thiol-containing drugs often bind to the single free cysteine residue of HSA, and glucuronidated drugs react irreversibly with other residues of the protein. Reversible binding of ligands is often stereospecific, and therefore immobilized HSA can be used to separate drug isomers. Albumin-containing dialysates are useful for extracorporeal removal of endogenous toxins and in the treatment of drug overdoses. HSA has different types of hydrolytic activities, which also can be stereospecific. The esterase-like property seems especially useful in converting prodrugs to active drugs in plasma.

A protein-encapsulation technique by the sol-gel method for the preparation of monolithic columns for capillary electrochromatography

A novel protein-encapsulation technique using sol-gels was developed for the preparation of monolithic capillary columns for capillary electrochromatography. Two chiral compounds, bovine serum albumin (BSA) and ovomucoid (OVM) from chicken egg white, were encapsulated in tetramethoxysilane-based hydrogel and their chiral selectivity was evaluated for the separation of some selected enantiomers (tryptophan, benzoin, eperisone, chlorpheniramine). The protein encapsulation was carried out within a capillary in a single step under mild conditions. The resultant monolithic columns showed adequate chromatographic performance, including mechanical strength, penetration of pressurized flow, and chiral separation. Two different proteins, BSA and OVM, were successfully encapsulated into the gel matrixes by changing the alkoxysilane compositions of the gel. Run-to-run repeatability was quite satisfactory. The consecutive analysis of the neutral compound, benzoin, by the OVM-encapsulated column showed good repeatability in the retention time (RSD = 1.23% for the first peak, N = 10). Under optimized conditions, the theoretical plate number for the first peak of benzoin reached 72,000 plates/m.

Mapping of stereoselective recognition sites on human serum transferrin by capillary electrophoresis and molecular modelling

Stereoselective recognition of chiral compounds can be used for mapping of surface interaction sites on proteins. Iron-free human serum transferrin is a suitable chiral selector in capillary electrophoresis used in native form in solution. Separation of optical isomers of tryptophan-methylester, tryptophan-ethylester and tryptophan-butylester and various drugs were studied in capillary zone electrophoresis applying a distinct transferrin zone prior to sample injection. Changes in the electrophoretic patterns (i.e., in the migration properties) of the molecules reflected the possible interactions with the protein. The tryptophan derivatives and eight drugs possessed stereoselective interactions, seven drugs showed interactions without appreciable chiral separation, and the others did not present any direct complexation with the protein molecules. Molecular modelling was performed to characterize the binding areas at the iron binding site of iron-free transferrin. The docking of tryptophan derivatives on transferrin showed that the R-enantiomers possess a stronger complexation with transferrin, whereas the S-enantiomers are bound by weaker interactions, which is in excellent agreement with the capillary electrophoresis results, where the R-enantiomers were always retarded stronger by transferrin. A ranking of drugs by the lipo score parameter of the docking shows an accordance with the stereoselective interactions by the protein.

Chiral separation of gemifloxacin in sodium-containing media using chiral crown ether as a chiral selector by capillary and microchip electrophoresis

Chiral crown ether, (+)-(18-crown-6)-tetracarboxylic acid (18C6H(4)), is an effective chiral selector for resolving enantiomeric primary amines owing to the difference in affinities between 18C6H(4) and each of the amine enantiomers. In addition to the destacking effect of sodium ion in the sample solution, the strong affinity of sodium ion to the polyether ring of crown ether is unfavorable to chiral capillary electrophoresis using 18C6H(4) as a chiral selector. In this report, the chiral separation of gemifloxacin dissolved in a saline sample matrix using 18C6H(4) was investigated. Adding a chelating agent, ethylenediaminetetraacetic acid (EDTA), to the run buffer greatly improved the separation efficiencies and peak shapes. The successful chiral separation of gemifloxacin in a urinary solution was demonstrated for both capillary and microchip electrophoresis.

Estimation of binding constants by capillary electrophoresis

Capillary electrophoresis (CE) has become a useful technique for measuring binding constants. This review is focused on recent trends in the estimation of binding constants by affinity CE. First, we introduce several mathematical equations in which it is assumed that the stoichiometry of the binding between drug and protein is 1:1 as a simple model. In order to calculate accurate binding constants by affinity CE, several experimental considerations are described in this review. In addition, some recent methodologies, such as partial filling technique and multiple-step ligand injection method, are introduced. Among research publications within 3 years, recent applications for determining binding constants are reviewed.

Effect of structure modification of chondroitin sulfate C on its enantioselectivity to basic drugs in capillary electrophoresis

The effect of structure modification of chondroitin sulfate C on its enantioselectivity to several representative basic drugs in capillary electrophoresis was investigated. Chemical desulfation showed no remarkable decrease in selectivity, whereas depolymerization with chondroitinase ABC resulted in complete loss of selectivity. Comparison with chondroitin sulfate A indicated considerable decrease in selectivity with this isomer. The great retention of enantioselectivity in the desulfated derivative suggests that the selectivity comes from the difference of the magnitude of an interaction in the multipoint mechanism between a part of the drug molecule and a functional group in chondroitin sulfate C other than the sulfate group. The sulfate group is not considered to play a major role for chiral separation. The complete loss of selectivity by depolymerization is consistent with a general tendency of lower selectivity in smaller saccharides, and the priority of chondroitin sulfate C to chondroitin sulfate A suggests the importance of the hydroxyl group at C4 in the galactosamine residue. During the course of this work we observed heavy tailing of the peaks of basic drugs in some batches of uncoated fused-silica capillaries under acidic conditions and solved this problem by doubly coating capillaries with Polybrene followed by chondroitin sulfate C. On the other hand, we demonstrated the usefulness of a special technique which uses a short, wider bore PTFE tube-attached capillary for the study of the effect of depolymerization, in order to minimize sample amount.

Selected fundamental aspects of chiral electromigration techniques and their application to pharmaceutical and biomedical analysis

While capillary electrophoresis has been established as a major enantioseparation technique within the last decade, the potential of capillary electrochromatography is still studied extensively. This review summarizes recent applications of electromigration techniques with regard to the enantioseparation of chiral drugs. The first part discusses the general aspects of migration models and the enantiomer migration order. The application of capillary electrophoresis to chiral pharmaceutical analysis considers recent literature on: (1) chiral resolutions of non-racemic mixtures of enantiomers for the development of assays and the determination of the stereochemical purity of the drugs, (2) chiral separations of compounds in pharmaceutical formulations and products, and (3) enantioseparations of drugs in biological samples. A shorter section devoted to chiral electrochromatography discusses some fundamental aspects as well as the application to the chiral analysis of drugs including bioanalysis.

Comparative studies of various run buffers for chiral capillary electrophoresis using chiral crown ether as a chiral selector

In the capillary electrophoretic separation of primary amine enantiomers using (+)-(18-crown-6)-tetracarboxylic acid (18C6H4) as a chiral selector, the presence of run buffer constituents such as tris(hydroxymethyl)aminomethane (Tris) or Na+ competing with analytes for 18C6H4, diminishes the effectiveness of 18C6H4. In order to determine appropriate buffer systems for 18C6H4, various run buffer cationic components including Tris, 1,3-bis[tris(hydroxymethyl)methylamino]propane, bis(2-hydroxyethyl)iminotris(hydroxymethyl)methane, triethanolamine, tetramethylammonium, and Na+ were compared. Quantitative studies of the effects of the competitive constituents were carried out by measuring the electrophoretic mobilities of histidine as a function of the 18C6H4 concentration. We also derived a simple equation to estimate the optimal chiral selector concentration for a maximum mobility difference in the presence of a competitive inhibitor.

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.

Recent developments in chiral capillary electrophoresis and applications of this technique to pharmaceutical and biomedical analysis

This paper provides an overview of the current status of chiral capillary electrophoresis (CE). The emphasis is placed on the application of CE in chiral separation of various racemic compounds. During the last two years about 280 papers, several review articles, and two entire issues, edited by S. Fanali (Electrophoresis 1999, 20, 2577-2798, and H. Nishi and S. Terabe (J. Chromatogr. A 2000, 879, 1-471.) have been devoted to chiral CE. Enantiomeric separations of various compounds, e.g., pharmaceuticals, drug candidates, drugs and related metabolites in biological fluids, amino acids, di- and tri peptides, pesticides and fungicides, have been performed using different chiral selectors. Native and derivatized cyclodextrins continue to be the most widely used chiral selectors. Other chiral selectors such as natural and synthetic chiral micelles, crown ethers, chiral ligands, proteins, oligo- and polysaccharides, and macrocyclic antibiotics have also been applied to chiral CE separations.

Fundamental aspects of chiral separations by capillary electrophoresis

A review is presented that surveys the basic theory of direct separation of enantiomers by capillary electrophoretic (CE) techniques. These separations are based on the formation of diastereomeric complexes between the enantiomeric analytes and a chiral selector added to the electrolyte solution. The review covers a comprehensive treatment of the equations needed for optimization of selectivity coefficients, resolution and analysis time in the zone electrophoretic mode. In this context, it takes into account combined equilibria of complexation and protonation/deprotonation as well as complexation and paritition into micelles. On the basis of these equations, the benefits of charged selectors and the optimization potential inherent to pH tuning can be documented. In addition, the review deals with some basic aspects of chiral isoelectric focusing and briefly discusses indirect enantioseparation. In a subsequent section a survey is given on particularfeatures of the various types of chiral selectors. Finally, the recent developments in preparative enantioseparation in continuous free-flow system and by use of isoelectric membranes are discussed.

Separation of basic drug enantiomers by capillary electrophoresis using ovoglycoprotein as a chiral selector: comparison of chiral resolution ability of ovoglycoprotein and completely deglycosylated ovoglycoprotein

Separations of basic drug enantiomers by capillary electrophoresis (CE) using ovoglycoprotein (OGCHI) as a chiral selector are described. The effects of running buffer pH and 2-propanol content on the migration times and resolution of basic drug enantiomers were examined using a linear polyacrylamide-coated capillary. High resolution of basic drug enantiomers was attained using a mixture of 50 mM sodium phosphate buffer (pH 4.5-6.0) and 2-propanol (5-30%) including 50 microM OGCHI. It was found that ionic and hydrophobic interactions could work for the recognition of basic drug enantiomers. Further, we compared the chiral resolution ability of OGCHI with that of completely deglycosylated OGCHI (cd-OGCHI) using them as chiral selectors in CE. OGCHI showed higher resolution for basic drug enantiomers tested than cd-OGCHI. The results suggest that the chiral recognition site(s) for OGCHI exists on the protein domain of OGCHI.

Analysis and enantioresolution of donepezil by capillary electrophoresis

The analysis of donepezil, a centrally acting acetylcholine esterase inhibitor, is described by a CZE method suitable for applications in pharmaceutical field. A rapid migration of the analyte was obtained under acidic conditions (pH 3.0); with detection wavelength of 320 nm a LOD of 0.8 x 10(-3) mg/ml was provided. Applications on real sample (pharmaceuticals) were carried out using two different instruments with comparable results in terms of reproducibility and accuracy. The use of chiral selectors in the running buffer allowed the enantioseparation of donepezil; charged cyclodextrins (carboxymethyl-beta-cyclodextrin and sulfated-beta-cyclodextrin) were suitable for the chiral resolution of the analyte. Interesting results were also obtained using human serum albumin. The protein-based CE enantioseparation was carried out at pH 7.4 avoiding the partial filling technique due to the good absorptivity of donepezil at 320 nm. Interestingly, the use of bicine as BGE provided a significative improvement in the enantioresolution compared to that obtained by phosphate buffer.

Enantioseparations in capillary electromigration techniques: recent developments and future trends

This review summarizes the current status of enantioseparations using capillary electromigration techniques and gives the authors insights on the selected fundamental aspects and future trends in this field. The most recent developments in the field of chiral separations using capillary electrophoresis (CE) and capillary electrochromatography (CEC) are summarized. The status of chiral electromigration techniques is evaluated tacking into account the most recent developments in related techniques such as chiral HPLC, GC and SFC.

Evaluation of norbornene- beta-cyclodextrin-based monomers and oligomers as chiral selectors by means of nonaqueous capillary electrophoresis

Norbornen-5-yl carboxylic acid and norbornen-5-ylmethylsilyl ether-based beta-cyclodextrins (beta-CDs) containing up to three norbornene ester and up to five norbornene silyl ether units have been prepared from beta-CD and norbornen-5-carboxylic chloride and norbornen-5-ylmethyldichlorosilane, respectively. Oligomers (n = 2-4) were prepared therefrom using ring-opening metathesis polymerization (ROMP). Monomeric and oligomeric substituted beta-CDs were evaluated as chiral selectors in nonaqueous capillary zone electrophoresis using 35 mM sodium bicarbonate in N-methylformamide (NMF) as background electrolyte. Both monomeric and oligomeric norbornene ester- and norbornene silyl ether-type selectors showed good enantioresolution for dansylated (DNS-) amino acids using concentrations of the chiral selector of up to 4% w/v. A significant improvement in resolution was observed upon the introduction of up to five norbornene silyl ether units into a beta-CD molecule, whereas higher degrees of substitution with norbornen-5-yl-carboxyl groups lead to a reduction in enantioresolution of DNS-amino acids. Thus, pentakis(norbornen-5-ylmethylhydroxysiloxyl)-beta-CD turned out to be superior to mono(norbornen-5-ylmethylhydroxysiloxyl)-beta-CD in terms of enantioresolution. Moreover, norbornene silyl ether-type selectors were found to be more efficient than norbornene ester-type selectors. Finally, oligomeric selectors were found to possess superior or at least comparable enantioselectivity in the separation of DNS-amino acids compared to the parent monomers. A maximum in enantioresolution was obtained with oligo(pentakis(norbornen-5-ylmethylhydroxysiloxyl)beta-CD).

Recent progress in chiral separation principles in capillary electrophoresis

This review summarizes recent developments in the field of chiral separations by electromigration techniques including capillary zone electrophoresis (CZE), capillary gel electrophoresis (CGE), isotachophoresis (ITP), electrokinetic chromatography (EKC), and capillary electrochromatography (CEC). This overview focuses on the development of new chiral selectors and the introduction of new techniques rather than applications of already established selectors and methods. The mechanisms of the different chiral separation principles are discussed.

Determination of immunoreactive gonadotropin-releasing hormone in serum and urine by on-line immunoaffinity capillary electrophoresis coupled to mass spectrometry

The need for urgent diagnoses has propelled the development of automated analyses that can be performed in a short time at reasonable cost. One such method is immunoaffinity capillary electrophoresis. This emerging hybrid technology employs two powerful techniques coupled on-line for the direct and rapid determination of analytes present in biological fluids. The first technique, immunoaffinity, is used for the selective extraction of a molecule present in a complex matrix, utilizing a microscale-format chamber affinity device. An analyte (affinity target) present in serum or urine is captured by an immobilized molecular recognition antibody molecule (affinity ligand) bound to a solid support constituent (glass beads or an appropriate porous structure) of a microchamber affinity device. The second technique, capillary electrophoresis, is used for the high-resolution analytical separation of the purified and concentrated affinity target material after elution from the microchamber affinity device. In this work, immunoaffinity capillary electrophoresis was developed for the identification and characterization of a single constituent of a complex matrix. Immunoreactive gonadotropin-releasing hormone was determined in serum and urine specimens derived from a normal individual and from a patient suffering from benign prostatic hyperplasia. Furthermore, the on-line immuno-separation system was coupled in tandem to mass spectrometry to obtain molecular mass information of the affinity isolated and CE separated neuropeptide. This hybrid immuno-analytical technology is simple, rapid, selective and sensitive. In addition, an attempt was also made to characterize other urinary constituents by CE-MS that may lead to marker activity in the urine of the diseased subject. The hyphenation of analytical techniques has proved valuable in enhancing their individual features. The future of bioanalysis using miniaturized affinity systems is discussed in this paper.

Competitive binding of a Tris run buffer with chiral crown ether in chiral capillary electrophoresis

In capillary electrophoresis of primary amine racemates using (+)-(18-crown-6)-tetracarboxylic acid (18C6H4) as a chiral selector, chiral recognition emanates from the differences in the complex formation between 18C6H4 and the two protonated amine enantiomers. The presence of buffer constituents such as tris(hydroxymethyl)aminomethane (Tris) or Na+, capable of forming complexes with 18C6H4, is thus detrimental to the chiral separation of primary amines. Such a competitive binding of buffer constituents was studied by comparing the electrophoretic mobilities of racemic analytes obtained in Tris/citric acid and triethylamine/citric acid buffers. We developed a simple fitting method to determine the competitive binding constant and applied it to the Tris buffer system. The competitive binding constant of Tris with 18C6H4 obtained at pH 3.0 was 27 +/- 4.