Chromatographic resolution of enantiomers using albumin as complexing agent in the mobile phase

Non-aqueous capillary electrophoretic separation of enantiomeric amines with (-)-2,3:4,6-di-O-isopropylidene-2-keto-L-gulonic acid as chiral counter ion

(-)-2,3:4,6-Di-O-isopropylidene-2-keto-L-gulonic acid [(-)-DIKGA] has been introduced as a chiral counter ion in non-aqueous capillary electrophoresis. High enantioresolutions (R(s)> or =3) were obtained for amines, e.g., pronethalol, labetalol and bambuterol. Methanol containing NaOH and (-)-DIKGA was used as the background electrolyte. The counter ion concentration and the nature of the injection medium were found to affect the chiral separation. Covalent coating of the fused-silica capillary reduced the electro-osmotic flow resulting in improved enantioresolutions.

Chiral ion-pair chromatography on porous graphitized carbon using N-blocked dipeptides as counter ions

Two newly synthesized chiral di-anionic counter ions were tested for enantiomeric resolution of a set of amino alcohols on porous graphitized carbon, Hypercarb. Z-L-Aspartyl-L-proline dissolved in methanol baseline resolved nine of 12 tested racemates. One of its diastereoisomers, Z-L-aspartyl-D-proline was also tested but resulted in low separation factors, <1.1. Sodium hydroxide was added to the mobile phase in order to titrate the counter ion to its mono- or di-anionic form. Results show that the di-anionic form was found to be superior to the mono-anionic form regarding enantioselectivity. Increased content of the counter ion in the mobile phase, with constant ratio between counter ion and sodium hydroxide concentration, decreased retention but only slightly affected enantioselectivity. Increased retention and enantioselectivity were observed with decreased column temperature. Resolution factors >3 were obtained between the enantiomers in atenolol and metoprolol with a total retention time of less than 15 min. Further, all four stereoisomers of an analogue to metoprolol were separated using Hypercarb and a mobile phase of 5 mM Z-L-aspartyl-L-proline and 9 mM sodium hydroxide in methanol.

Enantioresolution of disopyramide by capillary affinity electrokinetic chromatography with human alpha1-acid glycoprotein (AGP) as chiral selector applying a partial filling technique

A method using alpha1-acid glycoprotein (AGP) as chiral selector for disopyramide by means of affinity electrokinetic chromatography has been developed. In order to avoid UV absorbance interferences, less than the effective length of the capillary was filled with the chiral selector. The electrophoretic conditions were chosen to give opposite migration directions for the chiral selector and the analyte; AGP migrated away from the detector. Enantiomers of disopyramide were separated on a methylcellulose-coated capillary with 20 cm length to the detector. The enantioresolution of the solute was affected by the concentration of the chiral selector, the plug length of the selector in the capillary, and the applied voltage. Resolution factors and migration times decreased with reduction of the plug length, while the efficiency of the separation system and peak performance were improved by decreasing the separation zone. A special feature of the technique is an enhanced selectivity due to increasing separation of the enantiomers when the fastest has migrated from the selector zone, while the second one still is retained. Equations relating selectivity and resolution with the difference in effective plug lengths between the two enantiomers are developed. Optimized conditions yielding complete resolution, requiring an 0.75 mM AGP plug of only 4.5 cm effective length, also gave high efficiencies (about 400,000 plates/m) for both enantiomer peaks.

Cyclodextrins and enantiomeric separations of drugs by liquid chromatography and capillary electrophoresis: basic principles and new developments

Investigation of individual drug enantiomers is required in pharmacokinetic and pharmacodynamic studies of drugs with a chiral centre. Cyclodextrins (CDs) are extensively used in high-performance liquid chromatography as stationary phases bonded to a solid support or as mobile phase additives in HPLC and capillary electrophoresis (CE) for the separation of chiral compounds. We describe here the basis for the liquid chromatographic and capillary electrophoretic resolution of drug enantiomers and the factors affecting their enantiomeric separation. This review covers the use of CDs and some of their derivatives in studies of compounds of pharmacological interest.

Chiral separations by capillary electrophoresis: present state of the art

Capillary electrophoresis is a powerful tool for chiral separation of ionogenic enantiomers in solutions. This article brings an overview of the theory of electrophoretic separations with special emphasis on enantiomer-chiral selector equilibria, followed by a survey of indirect separations, based on formation of diastereomers with different electrophoretic mobilities, and a comprehensive appraisal of direct separations when the chiral recognition stems from (i) host-guest interactions using cyclodextrins and crown ethers as hosting agents, (ii) ligand exchange mechanism, (iii) affinity interactions or (iv) a combination of solute-chiral selector interactions with micellar electrophoretic transport. Finally, some trends in chiral separations by capillary electrophoresis are discussed.

Separation procedures used to reveal and follow drug-protein binding

The review gives a critical evaluation of the different separation procedures used to study drug-protein interactions and describes their various fields of application. For pharmacological studies, the most widely used methods are dialysis and ultrafiltration, because they allow measurements with solutions of high protein concentrations, such as those found in therapeutic conditions. Both techniques use membrane devices, which may induce additional binding effects. Another drawback of these techniques is the need for radiolabelled compounds. Chromatographic methods, which now take advantage of the technology of high-performance liquid chromatography, are generally faster and do not use drug labelling because of the higher sensitivities of the detectors. Two different approaches are possible: either all the interacting species (protein and drug) are dissolved in the mobile phase, or one of them (protein or drug) is immobilized on the support. Several chromatographic methods are available for studies in solution that differ according to the sample injection mode (frontal or zonal elution) and the nature of the mobile phase used. They include quantitation of the drug-protein complex by zonal elution, the Hummel and Dreyer method, frontal elution, the vacancy peak method, and retention analysis by zonal elution. Frontal elution is the most rigorous method since all the species at equilibrium are present in the mobile phase with known and constant concentrations. The most promising one is the Hummel and Dreyer method, because of the very small amount of protein injected in the mobile phase containing the drug. Drug-protein interactions may be studied by affinity chromatography by immobilizing one of the interacting species on the support. Comparison of the constants obtained with methods when both the drug and the protein are in solution is questionable, since the immobilized species in affinity separations differ in their physical properties from those in solution. The main advantage with studies on immobilized proteins is the easy comparison of the binding properties of various drugs, especially when they are enantiomeric. The results of the binding constants measured by different separation methods are given for the albumin-phenylbutazone and albumin-warfarin systems. Good agreement is generally obtained, which proves the validity of using chromatography as a tool to study drug-protein interactions.

Unequal disposition of enantiomers of the organic cation oxyphenonium in the rat isolated perfused liver

This paper describes the results of pharmacokinetic experiments in the rat isolated perfused liver with enantiomers of oxyphenonium. The study was performed with the [14C]methyl labelled compounds. In this preparation both metabolism and biliary excretion were significantly different for the (+)- and the (-)-isomer. Hepatic uptake rate was similar, but total biliary excretion (including metabolites) of the (-)-isomer was only 55% compared with the excretion of the (+)-isomer. In line with these data, after 2 h only 30% of the dose of the (+)-isomer and over 50% of the dose of the (-)-isomer was still found in the liver, predominantly in the form of metabolites. The metabolic profile was investigated using ion pair TLC. At least two metabolites were detected in bile for both enantiomers. However, unchanged (-)-oxyphenonium persisted for longer in bile, indicating either a more rapid canalicular transport of the (+)-isomer and/or a more rapid metabolism of (+)-oxyphenonium to cholephilic metabolites.

Determination of binding affinity of enantiomers to albumin by liquid chromatography

The principles of the determination of the binding affinity constants of small molecules to albumin by liquid chromatography, using albumin as a mobile phase additive, are outlined. Chromatographic conditions for determinations of constants are presented and applied to enantiomers of tryptophan and omeprazole. The influence of albumin on the retaining properties of LiChrosorb RP-8, Phenyl Hypersil and LiChrosorb Diol was studied.

Effects of drug-protein binding on trace enrichment of drugs in blood plasma on short precolumns

A reversed-phase liquid chromatographic precolumn venting plug technique was used for the determination of drugs by direct injection of large volume (500 microliters) blood plasma samples. Enrichment of the drugs from large plasma samples, as well as clean-up from less retained plasma components, such as proteins, was obtained on a short precolumn. Strong drug-protein binding resulted in losses of the drug during the enrichment step. The recovery was found to be inversely proportional to the degree of drug-protein binding in the sample solution. Techniques to increase the recovery were studied. These include methods to decrease the degree of drug-protein binding in the sample solution by dilution and changes of the pH, as well as methods to increase the residence time of the drug on the precolumn by increasing the precolumn length and precolumn hydrophobicity.

Chromatographic separation of enantiomers

In this paper a review is presented on the chromatographic analysis of enantiomers with special attention to high pressure liquid chromatography. Also, some examples of resolution of racemates by thin layer chromatography and gas chromatography are given. The various procedures in the surveyed literature have been divided into three main classes: procedures with formation of diastereomeric compounds prior to the chromatographic separation, procedures in which a chiral mobile phase is used, and procedures with the use of a chiral stationary phase. These methods are subdivided and some examples of their application to drugs and related compounds are presented.