[On the history of vitamin K, dicoumarol and warfarin]

The history of the discovery and development of vitamin K and its antagonists, the oral anticoagulants dicoumarol and warfarin, are fascinating, triumphant landmarks in the annals of medicine. Vitamin K was found by Carl Peter Henrik Dam and Fritz Schønheyder from the University of Copenhagen. The discovery was initiated by Dam, by a lucky choice of chicks in the dissertation of sterol metabolism, since the vitamin is not formed by intestinal bacteria in these animals. In these experiments the lack of an unknown factor in the synthetic diet caused internal bleeding similar to that found in scurvy, but the bleeding was not reversed by vitamin C and it could not be explained by the lack of classical vitamins. In 1935 the unknown antihaemorrhagic factor was named vitamin K and a few months later the phenomenon was also observed by H.J. Almquist and E.L.R. Stokstad in Berkeley. The activity of the factor was determined by bioassay in different extracts of green vegetables and alfalfa by Dam and Schønheyder. Vitamin K was isolated in 1939 by Dam and Paul Karrer in Zurich and the structure was determined by Edward Adelbert Doisy. Dam and Doisy were awarded the Nobel Prize in 1943. A dramatic story starts the discovery of dicoumarol. In the 1920s cattle in Canada began dying of internal bleeding with no obvious precipitating cause. Frank W. Schofield, a veterinary pathologist in Alberta, found that the mysterious disease was connected to the consumption of spoiled sweet clover hay and noted a prolonged clotting time. Ten years after a farmer traveled in a blizzard with his dead cow and a milk can of the unclotted blood to the University of Wisconsin. Only the door to the biochemical department of Karl Paul Link was open. This event started the isolation of the anticoagulant agent dicou- marol which was formed by microbial induced oxidation of coumarin in the mouldy sweet clover hay. More than hundred dicoumarol-like anticoagulants were synthesized by Link and his co-workers. A potent hemorrhagic agent named warfarin was first used as an effective rat poison. However, warfarin became the drug of choice and the break- through in the treatment of thromboembolic diseases. Today new oral anticoagulants are competing with warfarin.

Removal of pharmaceuticals from water: using liquid-core microcapsules as a novel approach

In recent years ever-increasing amounts of pharmaceuticals are being detected in the aquatic environment and in some cases, they have even been discovered in drinking water. Their presence is attributed mainly to the inability of sewage treatment plants to adequately remove these compounds from the sewage influent. The aim of this study was to investigate the feasibility, kinetics and efficiency of using liquid-core microcapsules as a novel methodology, termed capsular perstraction, to remove seven pharmaceuticals commonly found in the environment, from water. The process involves the envelopment of pre-selected organic solvents within a porous hydrogel membrane to form liquid-core microcapsules, which can be used to extract a large range of compounds. Results indicate that this novel approach is capable of extracting the seven chosen compounds rapidly and with a variable efficiency. The simultaneous use of both dibutyl sebacate and oleic acid liquid-core microcapsules at a liquid volume ratio of only 4% (v/v) resulted in the following extractions within 50min of capsule addition to contaminated water: furosemide 15%; clofibric acid 19%; sulfamethoxazole 22%; carbamazepine 54%; warfarin 80%; metoprolol 90% and diclofenac 100%. The effects of different agitation rates, microcapsule size and membrane thickness on the rate of mass transfer of warfarin into the liquid-core (dibutyl sebacate) of microcapsules was also examined. Results showed that the main rate-limiting step to mass transfer was due to the stagnant organic film (microcapsule size) within the core of the microcapsules. A volumetric mass transfer coefficient of 2.28x10(-6)m/s was obtained for the smallest microcapsules, which was nearly 4-fold higher compared to the value (0.6x10(-6)m/s) obtained for the largest microcapsules used in this study. Even with this resistance liquid-core microcapsules are still capable of the rapid extraction of the tested compounds and may provide a platform for the safe disposal of the pharmaceuticals after removal.

Evaluation of silica monoliths in affinity microcolumns for high-throughput analysis of drug-protein interactions

Silica monoliths in affinity microcolumns were tested for the high-throughput analysis of drug-protein interactions. HSA was used as a model protein for this work, while carbamazepine and R-warfarin were used as model analytes. A comparison of HSA silica monoliths of various lengths indicated columns as short as 1 to 3 mm could be used to provide reproducible estimates of retention factors or plate heights. Benefits of using smaller columns for this work included the lower retention times and lower back pressures that could be obtained versus traditional HPLC affinity columns, as well as the smaller amount of protein that is required for column preparation. One disadvantage of decreasing column length was the lower precision that resulted in retention factor and plate height measurements. A comparison was also made between microcolumns containing silica particles versus silica monoliths. It was demonstrated with R-warfarin that supports could be used in HSA microcolumns for the determination of retention factors or plate heights. However, the higher efficiency of the silica monolith made this the preferred support for work at higher flow rates or when a larger number of plates are needed during the rapid analysis of drug-protein interactions.

Development of an affinity silica monolith containing human serum albumin for chiral separations

An affinity monolith based on silica and containing immobilized human serum albumin (HSA) was developed and evaluated in terms of its binding, efficiency and selectivity in chiral separations. The results were compared with data obtained for the same protein when used as a chiral stationary phase with HPLC-grade silica particles or a monolith based on a copolymer of glycidyl methacrylate (GMA) and ethylene dimethacrylate (EDMA). The surface coverage of HSA in the silica monolith was similar to values obtained with silica particles and a GMA/EDMA monolith. However, the higher surface area of the silica monolith gave a material that contained 1.3-2.2-times more immobilized HSA per unit volume when compared to silica particles or a GMA/EDMA monolith. The retention, efficiency and resolving power of the HSA silica monolith were evaluated using two chiral analytes: d/l-tryptophan and R/S-warfarin. The separation of R- and S-ibuprofen was also considered. The HSA silica monolith gave higher retention and higher or comparable resolution and efficiency when compared with HSA columns that contained silica particles or a GMA/EDMA monolith. The silica monolith also gave lower back pressures and separation impedances than these other materials. It was concluded that silica monoliths can be valuable alternatives to silica particles or GMA/EDMA monoliths when used with immobilized HSA as a chiral stationary phase.

Electrokinetic migration of acidic drugs across a supported liquid membrane

Electrokinetic cross membrane extraction of acidic drugs was demonstrated for the first time. The acidic drugs were extracted from an alkaline aqueous donor solution (300 microl), through a thin supported liquid membrane of 1-heptanol sustained in the pores of the wall of a porous hollow fiber, and into an aqueous alkaline acceptor solution (30 microl) present inside the lumen of the hollow fiber by the application of a d.c. electrical potential. The negative electrode was placed in the donor solution, and the positive electrode was placed in the acceptor solution. Optimal extractions were accomplished with 1-heptanol as the supported liquid membrane, with 50 V as the driving force, and with pH 12.0 in both the donor and acceptor solutions, respectively (NaOH). Equilibrium extraction conditions were obtained after 5 min of operation with the whole assembly agitated at 1200 rpm. Eleven different acidic drugs were extracted with recovery values between 8 and 100%, and initial data supported that electrokinetic cross membrane extraction provided repeatable data and linear response between original donor concentration and final acceptor concentration of the acidic model compounds.

Separation and determination of warfarin enantiomers in human plasma using a novel polymeric surfactant for micellar electrokinetic chromatography-mass spectrometry

Warfarin is a widely used oral anticoagulant which is mostly administrated as a racemic mixture containing equal amount of R- and S-enantiomers. The two enantiomers are shown to exhibit significant differences in pharmacokinetics and pharmacodynamics. In this study, a new chiral micellar electrokinetic chromatography-mass spectrometry (MEKC-MS) method has been developed using a polymeric chiral surfactant, polysodium N-undecenoyl-L,L-leucyl-valinate (poly-L,L-SULV), as a pseudostationary phase for the chiral separation of (+/-)-warfarin (WAR) and (+/-)-coumachlor (COU, internal standard). Under optimum MEKC-MS conditions, the enantio-separation of both (+/-)-WAR and (+/-)-COU was achieved within 23 min. Calibration curves were linear (R=0.995 for (R)-WAR and R=0.989 for (S)-WAR) over the concentration range 0.25-5.0 microg/mL. The MS detection was found to be superior over the commonly used UV detection in terms of selectivity and sensitivity with LOD as low as 0.1 microg/mL in human plasma. The method was successfully applied to determine WAR enantiomeric ratio in patients' plasma undergoing warfarin therapy.

Enantiomeric separation of acidic compounds using single-isomer amino cyclodextrin derivatives in nonaqueous capillary electrophoresis

The enantiomeric separation of a series of acidic pharmaceuticals (mostly nonsteroidal anti-inflammatory drugs) has been investigated in NACE systems using single-isomer amino beta-CD derivatives. The first part of this study consisted of the selection of the basic experimental conditions to separate efficiently the enantiomers of acidic drugs. Several parameters, such as the nature of the ionic BGE components, were studied and a methanolic solution of ammonium acetate containing the cationic CD was selected as BGE. A D-optimal design with 20 experimental points was then applied and the nature and concentration of the CD were found to have a significant effect on the enantiomeric resolution for all studied compounds. Resolution (R(s)) values were always higher with 6-monodeoxy-6-mono(3-hydroxy)propylamino-beta-CD (PA-beta-CD) compared to those obtained with 6-monodeoxy-6-mono(2-hydroxy)propylamino-beta-CD (IPA-beta-CD). However, the latter led to shorter migration times. Generic NACE conditions were then selected by means of the multivariate approach in order to obtain the highest R(s) values in a minimum amount of time. Finally, dependence of separation selectivity, resolution, as well as mobility difference on chiral selector concentration was discussed and binding constants with PA-beta-CD were estimated for the two enantiomers of one of the model compounds, suprofen in these NACE systems.

Determination of warfarin enantiomers and hydroxylated metabolites in human blood plasma by liquid chromatography with achiral and chiral separation

An assay comprising two simple, selective and isocratic HPLC methods with UV detection was developed and validated for measuring warfarin enantiomers and all five warfarin monohydroxylated metabolites in patient blood plasma. Following liquid/liquid extraction from 1 ml of blood plasma a baseline separation of analytes was achieved on chiral (alpha(1) acid glycoprotein - AGP) and achiral (C(18)) column. Both methods were consistent (R.S.D.<6.9% for warfarin enantiomers and<8.9% for monohydroxylated metabolites) and linear (r>0.998). The limits of detection were 25 ng/ml for warfarin enantiomers, 25 ng/ml for 4'-, 10-, 6- and 7-hydroxywarfarin, 35 ng/ml for 8-hydroxywarfarin and 50 ng/ml for racemic warfarin. In a clinical study in 204 patients, it was confirmed that the assay is appropriate for evaluation of influences of genetic polymorphisms, demographic factors and concomitant drug treatment on warfarin metabolism.

A simplified synthesis of polymeric nonparticulate stationary phases with macrocyclic antibiotic as chiral selector for capillary electrochromatography

A simplified approach to synthesize nonparticulate (continuous or monolithic) beds with embedded vancomycin chiral selectors for capillary electrochromatography is proposed. In the present approach, N,N'-diallyltartardiamide monomer with diol functionality is used, which can be readily converted to aldehyde groups via periodate treatment. Parallel to the activation of the polymeric matrix for covalent attachment of vancomycin, the periodate treatment has shown secondary effects on the polymeric bed morphology, namely the increase of the average pore size and porosity of the skeleton. Inversed size-exclusion chromatography was applied to characterize porosimetric properties of the capillary columns before and after the periodate treatment. Electroosmotic and enantioselective properties of the nonparticulate beds synthesized are presented. The approach is of more general interest attaching different affinity groups to the polymeric matrix and/or enhancing the accessibility to the active sites, for instance, in the molecular imprinting technique.

Separation strategy for acidic chiral pharmaceuticals with capillary electrochromatography on polysaccharide stationary phases

The effect of five factors on the capillary electrochromatographic enantioseparation of acidic compounds was studied using an experimental design. The studied factors were pH, acetonitrile content in the mobile phase, temperature, buffer concentration, and applied voltage. These experiments allowed defining a generic separation strategy applicable on acidic compounds with chemical and structural diversity. The starting screening conditions consist of a 45 mM ammonium formate electrolyte at pH 2.9 mixed with 65% acetonitrile, an applied voltage of 15 kV, and a temperature of 25 degrees C. The screening phase occasionally can be followed by an optimization procedure. Evaluation of the proposed strategy pointed out that it allows achieving baseline resolution within a relatively short time when a beginning of separation is obtained at the starting conditions. This strategy revealed enantioselectivity for 11 compounds out of 15, of which 10 could be baseline-separated after the proposed optimization steps.

Enantioseparation on mono(6A-N-allylamino-6A-deoxy)permethylated β-cyclodextrin covalently bonded silica gel

A chiral selector, mono(6A-N-allylamino-6A-deoxy)permethylated beta-cyclodextrin, was synthesized through a facile synthetic route and chemically immobilized onto porous silica gel via hydrosilylation to afford a cyclodextrin based chiral stationary phase MeCD-CSP. This chiral stationary phase exhibited good enantioselectivity under standard HPLC conditions. The optimal resolution of 1-(p-bromophenyl)ethanol and bromopheniramine was achieved under normal-phase conditions using a mobile phase comprising n-hexane (hexane) and 2-propanol (IPA). The enantioseparation of warfarin, suprofen and a series of flavanones under reversed-phase conditions were optimized and efficient enantioseparations for these analytes were obtained.

Enantioseparation of warfarin and its metabolites by capillary zone electrophoresis

A capillary zone electrophoresis (CZE) method with direct ultraviolet (UV)-absorbance detection is presented for the simultaneous enantiomeric separation of warfarin and its main metabolites, including warfarin alcohols, 4'-, 6-, and 7-hydroxywarfarin, using highly sulfated beta-cyclodextrin (HS-beta-CD) as the chiral selector. This chiral separation method was optimized in terms of the electrophoretic parameters, which included the concentration of HS-beta-CD used, the type and composition of organic modifier added to the background electrolyte (BGE) buffer, and the BGE buffer pH. Chiral separation of warfarin and its major metabolites was achieved with high resolution, selectivity, efficiency, repeatability, and reproducibility. This optimized chiral analysis of warfarin along with its metabolites was completed within a satisfactory electrophoresis time of 20 min.

Brush-type chiral stationary phase for enantioseparation of acidic compounds. Optimization of chiral capillary electrochromatographic parameters

The capillary electrochromatographic separations of three acidic enantiomers (carprofen, coumachlor and warfarin) were studied on a capillary column packed with 5 microm (3R,4S)-Whelk-O 1 chiral stationary phase. The influence of several experimental parameters (mobile phase pH, type of background electrolyte, acetonitrile ratio, temperature, applied voltage and ionic strength) on electroosmotic flow velocity, retention factor, selectivity factor, efficiency, resolution and effectiveness of chiral separation was evaluated. It was notable that the optimum resolution of the acidic enantiomers was achieved at pH 3.0 phosphate buffer, suggesting that capillary electrochromatography in the ion-suppressed mode can be applied for chiral separations of a range of acidic compounds.

Effect of dextran as a run buffer additive in drug-protein binding studies using capillary electrophoresis frontal analysis

The study of drug-protein interactions by capillary electrophoresis frontal analysis requires establishment of a sufficient mobility difference between the mobility of the ligand and protein. The potential utility of dextran as a run buffer additive to manipulate the electrophoretic mobilities of low molecular weight ligands and protein in capillary electrophoresis frontal analysis binding studies was assessed. It was demonstrated that dextran was effective in improving the separation between the ligands warfarin and flurbiprofen and human serum albumin. Separation of ligand and protein increased with the concentration of added dextran (0-7.5% (w/w)), while molecular weight of the additive (70,000-2,000,000) only had a minor effect. The effect of dextran addition on viscosity and electrophoretic and electroosmotic mobilites was systematically studied. Optimal frontal analysis settings were a compromise between achieving satisfactory separation and acceptable analysis times without loss of plateau peak conditions. No effect of dextran upon the drug-human serum albumin interactions could be detected for the model ligands. Introduction of dextran into the electrophoresis buffer expands the applicability of capillary electrophoresis frontal analysis in drug research to binding interactions between proteins and low molecular weight ligands possessing similar electrophoretic mobilities.

Measurement of warfarin in plasma by high performance liquid chromatography (HPLC) and its correlation with the international normalized ratio

The measurement of plasma warfarin is required to investigate non-compliance, resistance to anticoagulation, drug metabolism and pharmacokinetic. Methods so far described are based on extraction of warfarin from plasma followed by reversed-phase HPLC. Extraction is the crucial step and may be performed in liquid- or solid-phase. The latter requires the preparation of columns, which makes the procedure variable. We investigated the suitability of the ready-for-use commercial cartridges for sample preparation. The method displayed between-run CV of 11.8%. Recovery was 99%. The coefficients of correlation between warfarin concentration in 50 patients and weekly dosage or INR were 0.55 (p<0.0001) or 0.25 (p=0.079).

Insight into the chiral recognition of warfarin enantiomers by epichlorhydrin/beta-cyclodextrin polymer-based supports: determination of stoichiometry and stability of warfarin/beta-cyclodextrin polymer complexes

The complexation of warfarin (W) enantiomers by a hydrosoluble high-molecular-weight beta-cyclodextrin/epichlohydrin polymer (EP/beta-CD polymer) was studied using HPLC with a mobile phase of methanol/0.1 M Na acetate/acetic acid (pH 4) at 22 degrees C. It was found that the complexes (W/beta-CD unit) have a 1:1 stoichiometry. The stability constants of the complex involving each enantiomer and the polymer beta-CD units were determined in the mobile phase, and the highest stability of the complex (S-warfarin/beta-CD unit) was observed. From the chromatographic separations of warfarin enantiomers on different beta-CD or its derivative supports, we have deduced the role of the simultaneous presence of several glyceryl (-O-CH(2)-CHOH-CH(2)-O-) and dihydroxypropyl (-O-CH(2)-CHOH-CH(2)OH) groups on one beta-CD ring in promoting the chiral recognition of warfarin enantiomers.

High-performance liquid-chromatographic determination of warfarin enantiomers in plasma with automated on-line sample enrichment

A sensitive and selective chiral high performance liquid chromatographic method was developed for the direct determination of R- and S-warfarin enantiomers in human plasma. The method involved direct injection of human plasma onto a semipermeable surface (SPS) guard column, washing the proteins from the column with aqueous acetonitrile and back flushing the analytes onto a reversed phase ovomucoid silica HPLC column using switching valves. After separation, the analytes were simultaneously detected and quantitated with a fluorometer. The recoveries of R-warfarin from human plasma at 25 and 2500 ng/ml were 98.9% and 88.1%, respectively. The recoveries of S-warfarin at 25 and 2500 ng/ml were 105.4% and 93.9%, respectively. Using 100 microl of human plasma, the lower limit of quantification for both R- and S-warfarins was 25 ng/ml. Linear responses in analyte/internal standard peak height ratios were observed for analyte concentrations ranging from 25 to 2500 ng/ml for both enantiomers. Fluorescence chromatograms of drug-free human plasma showed no interfering peaks with retention times similar to those for R- and S-warfarins and the internal standard. Results from a 3-day validation study for both enantiomers demonstrated excellent precision (1.7-9.0%) and accuracy (97-109%) across the calibration range.

Method development of enantiomer separations by affinity capillary electrophoresis, cyclodextrin electrokinetic chromatography and capillary electrophoresis-mass spectrometry

Capillary electrophoresis (CE) has become a powerful tool for enantiomer separations during the last decade. Since 1993, the author has investigated enantiomer separations by affinity capillary electrophoresis (affinity CE) with some proteins and by cyclodextrin electrokinetic chromatography (CDEKC) with some charged cyclodextrins (CDs). Many successful enantiomer separations are demonstrated from our study in this review article. In the enantiomer separations by affinity CE, the deterioration of detection sensitivity was observed under high concentration of the protein in running solutions. The partial filling technique was practically useful to solve the serious problem. It allowed operation at high protein concentrations, such as 500 mumol/L, without the detection problem. Charged CDs had several advantages for the enantiomer separations over neutral ones. Strong electrostatic interactions between a charged CD and oppositely charged analytes should be effective for the formation of the complex. A large difference in electrophoretic mobility between the free analyte and the inclusion complex should also enhance the enantiomeric resolution. In CE-mass spectrometry (CE-MS), the partial filling technique was applied to avoid the introduction of nonvolatile chiral selectors into the CE-MS interface. By replacing the nonvolatile electrolytes in the running buffer by volatile ones, the separation conditions employed in CE with the UV detection method could be transferred to CE-MS.

Separation of enantiomers by nanoliquid chromatography and capillary electrochromatography using a bonded cellulose trisphenylcarbamate stationary phase

A cellulose trisphenylcarbamate-bonded chiral stationary phase was applied to nano-liquid chromatography (nano-LC) and capillary electrochromatography (CEC) with nonaqueous and aqueous solutions as the mobile phases. Several chiral compounds were successfully resolved on the prepared phase by nano-LC. The applicability of nonaqueous CEC on a cellulose derivative stationary phase was investigated with the organic solvents methanol, hexane, 2-propanol, and tetrahydrofuran (THF) containing acetic acid, as well as triethylamine as the mobile phases. Enantiomers of warfarin and praziquantel were baseline-resolved with plate numbers of 82,300 and 38,800 plates/m, respectively, for the first eluting enantiomer. The influence of applied voltage, concentration of nonpolar solvent, apparent pH, and buffer concentration in the mobile phase on the electroosmotic flow (EOF) and the mobility of the enantiomers was evaluated. Enantioseparations of trans-stilbene oxide and praziquantel were also achieved in aqueous CEC with plate numbers of 111,100 and 107,400 plates/m, respectively, for the first eluting enantiomer. A comparison between nonaqueous CEC and aqueous CEC based on a cellulose trisphenylcarbamate stationary phase was discussed. Pressure-assisted CEC was examined for the chiral separation of praziquantel and faster analysis with high enantioselectivity was acquired with the proper pressurization of the inlet vial.

High-performance liquid chromatography applications of optical rotation detection with compensation for scattering and absorbance at the laser wavelength

Use of instrumentation developed to enable simultaneous monitoring of optical rotation (OR) and transmittance allows OR measurements to be made in the presence of high levels of absorbance, scattering or other effects that change the intensity of the plane-polarised light at the photodiode detector. This extends the application of OR detection to areas where it was previously difficult. Examples of the application of high-performance liquid chromatography (HPLC) with the improved OR detector include (i) the analytical scale separation of fructose and sucrose and (ii) the semi-preparative separation of enantiomers of warfarin and Trögers base. A signal-to-noise improvement of up to 150% is found when comparing signals with and without correction for transmittance changes. The improved OR detector has been used in series with a UV detector and the system shown to be suitable for on-line measurement of peak purity in separations using a chiral column under overload conditions.

[Chiral separation by capillary electrochromatography on stationary phase adsorbed with protein]

A method based on the adsorption of bovine serum albumin (BSA) on the strong anion-exchange packing (SAX) was applied to prepare chiral stationary phases for capillary electrochromatography. It was found that the electroosmotic mobility only decreased 26.3% when the BSA was adsorbed. The run-to-run reproducibilities of void time and the migration times of D- and L-tryptophan enantiomers were rather good with the relative standard deviation (RSD) values of 0.90%, 0.87% and 0.96%, respectively, when mobile phase containing 7% (volume fraction) acetonitrile. The merit of this system is that the direction of electroosmotic flow is still from cathode to anode, preferable to separate acidic enantiomers. Four anionic enantiomers, tryptophan, ketoprofen, fenoprofen and warfarin, as well as the neutral enantiomers of benzoin were successfully resolved. In order to reduce the analysis time, n-hexanoic acid was added to the mobile phase as a competitive agent. The effect of acetonitrile volume fraction on the resolution of enantiomers was also investigated.

Synthesis and characteristics of the human serum albumin-triazine chiral stationary phase

Human serum albumin (HSA) was successfully bonded to silica with s-triazine as activator. The coupling reaction by this method was rapid and effective. The triazine-activated silica is relatively stable and can be installed for at least 1 month without obvious loss of reactivity when stored below 30 degrees C, pH below 7. It was observed that the amount of bound HSA reached 120 mg/g silica calculated from the UV absorbance difference of the HSA solution. d, l-tryptophan was selected as the probe solute to characterize the properties of HSA bonded s-triazine chiral stationary phase, and separation factor of 9.4 was obtained for d,l-tryptophan. Furthermore, the amount of effective HSA on silica was measured by high-performance frontal analysis, and only 16.8 mg/g silica was responsible for the resolution of d,l-tryptophan. These results indicate that the amount of both the bound and effective HSA on silica with triazine as activator was much higher than those by the Schiff base coupling method. Different kinds of enantiomers were resolved successfully on the aminopropylsilica-bonded HSA s-triazine chiral stationary phase.

Separation of enantiomers on HPLC chiral stationary phases based on human plasma alpha1-acid glycoprotein: effect of sugar moiety on chiral recognition ability

HPLC chiral stationary phases based on human plasma alpha1-acid glycoprotein (AGP) and partially deglycosylated AGP (pd-AGP) were prepared to investigate the effects of sugar moiety of AGP on chiral discrimination of various solutes. Removal of a sugar moiety of AGP by treatment with N-glycosidase was confirmed by high-performance capillary electrophoresis, reversed-phase HPLC and matrix-assisted laser desorption-time of flight (MALDI-TOF) mass spectrometry. The average molecular weights of AGP and pd-AGP were estimated to be ca. 33,000 and 30,600, respectively, by MALDI-TOF mass spectrometry. Next, AGP and pd-AGP were bound to aminopropyl-silica gels activated with N,N '-disuccinimidylcarbonate. The retentivity+ and enantioselectivity of the neutral, acidic and basic solutes tested on the pd-AGP column were significantly or not significantly larger in most solutes than those on the AGP column. This is ascribable to that by cleavage of a sugar chain(s) by N-glycosidase, pd-AGP could become more hydrophobic than AGP, and/ or that a solute could be easily accessible to the specific and/or non-specific binding sites of pd-AGP. It is interesting that warfarin enantiomers are not resolved on the pd-AGP column, but resolved on the AGP column. A sugar chain(s) of AGP cleaved by N-glycosidase might be involved in the enantioselective binding of warfarin enantiomers.

Chromatographic evaluation of structure selective and enantioselective retention of amines and acids on cellobiohydrolase I wild type and its mutant D214N

The mechanisms of structure selective and enantioselective retentions of amines and acids on two chiral stationary phases based on wild type cellobiohydrolase I (CBH I) and its mutant D214N have been investigated. All the amino alcohols tested had an enantioselective site that overlaps with the catalytically active site of CBH I, whereas the enantioselectivity of prilocaine was not affected by the mutation. The hydroxyl group of the amino alcohols did not seem to be an important contributor to the total binding strength whereas a bromo substituent in the aromatic ring promotes a high enantioselectivity (alpha=7.05). Interestingly, the chiral recognition site of the acid warfarin overlaps with the binding site of the amino alcohols. Di-p-toluoyltartaric acid and dibenzoyltartaric acid were strongly retained probably due to electrostatic attraction, but no enantioselectivity was observed. The difference in retention characteristics for the amino alcohols on the two stationary phases was strongly pH-dependent. A change in elution order of different amino alcohols occurred when changing the pH from 5.0 to 7.0. The difference between the two phases was lower at low pH. The retention times could also be affected by ionic strength and by use of cellobiose as a mobile phase additive but no indication of ion-pair retention of the amines was observed, when adding hexanesulphonate as counter ion to the mobile phase. The temperature dependence of the retention of the enantiomers of propranolol at pH 7.0 on the mutant D214N was similar to what was earlier observed on the wild type CBH I at lower pH.

Enantioselectivity of bovine serum albumin-bonded columns produced with isolated protein fragments. II. Characterization of protein fragments and chiral binding sites

Enantioselectivity of bovine serum albumin (BSA)-bonded columns produced with isolated protein fragments has been investigated. The BSA fragment, BSA-FG75, was isolated by size exclusion chromatography following peptic digest of BSA. The isolated BSA-FG75 was further fractionated to two fractions, BSA-F1 and BSA-F2, by anion-exchange chromatography. BSA-F1 and BSA-F2 had molecular mass of about 35000 daltons, estimated by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, BSA-F1 has amino acid residues 1-307 estimated by electrospray ionization (ESI) mass spectrometry, while BSA-F2 is an N-terminal half BSA fragment. The BSA, BSA-FG75, BSA-F1 and BSA-F2 proteins were bound to aminopropyl-silica gels activated by N,N'-disuccinimidyl carbonate. The bound amounts of the BSA fragments were 2.2-2.7 times more than that of the intact BSA. Chiral recognition of 2-arylpropionic acid derivatives, benzodiazepines, warfarin and benzoin was obtained with the BSA fragment-bonded columns. The non-enantioselective interactions of benzoin and benzodiazepines except for clorazepate with BSA fragments were increased with protein surface coverages, while those of 2-arylpropionic acid, clorazepate and warfarin were decreased. The BSA fragment columns gave higher enantioselectivity for lorazepam and benzoin, and lower enantioselectivity for other compounds tested, compared with the BSA column. These results might be due to changes in the globular structure of the BSA fragment and/or changes in the local environment around the binding sites.

Chiral separation mechanisms in protein-based HPLC columns. 2. Kinetic studies of (R)- and (S)-warfarin binding to immobilized human serum albumin

This work used plate height measurements to investigate the kinetics of (R)- and (S)-warfarin binding to an immobilized HSA column. The dissociation rate constants for (R)- and (S)-warfarin on this column increased from 0.06 to 1.9 s-1 and from 0.06 to 0.36 s-1 between 4 and 45 degrees C. The corresponding association rate constants increased from 2.4 x 10(4) to 3.2 x 10(5) M-1 s-1 for (R)-warfarin and from 4.4 x 10(4) to 7.2 x 10(4) M-1 s-1 for (S)-warfarin over the same temperature range. From the dissociation data, it was found that an increase in temperature led to a large decrease in the plate height due to stationary phase mass transfer for both enantiomers. Further studies indicated that (R)- and (S)-warfarin had similar activation energies for their binding to HSA. For (R)-warfarin, most of this energy requirement was due to the change in enthalpy of the system, while for (S)-warfarin, it was mainly due to the change in entropy. All of these results agree with an earlier model, in which (R)- and (S)-warfarin were proposed to interact with regions on the interior and exterior of HSA, respectively. In addition, these results offer a number of useful insights into the mechanisms of protein-based chiral separations.

Human cytochromes P4501A1 and P4501A2: R-warfarin metabolism as a probe

Two forms of the cytochrome P450 enzyme superfamily, P4501A1 and P4501A2, that are heterogeneously distributed in populations and are induced in response to environmental factors are important because of their capacity to bioactivate procarcinogens. Phenotyping P4501A1 and P4501A2 in individuals will thus provide assessments of those individuals' susceptibility to procarcinogens. The anticoagulant drug warfarin is metabolized by human P4501A1 and P4501A2, and we have characterized this metabolism for the R-warfarin enantiomer as a potential in vivo probe. cDNA-expressed human P4501A1 and P4501A2 are regioselective for R-warfarin 6- and 8-hydroxylation with very similar KM values: 1.4 mM (6-hydroxylation), 1.2 mM (8-hydroxylation), 1.6 mM (6-hydroxylation), and 1.4 mM (8-hydroxylation), respectively, indicating possible binding competition for R-warfarin between the two forms. However, when comparing 6- and 8-hydroxylation, P4501A1 showed weak regioselectivity for 8-hydroxylation, whereas P4501A2 exhibited strong regioselectivity for 6-hydroxylation, with 6-hydroxylation/8-hydroxylation ratios of 0.6 and 5.0, respectively. These findings were confirmed by using microsomes from 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated HepG2 and MCF-7 cells expressing only P4501A1 (ratios of 0.7), and from human hepatic microsomal preparations containing only P4501A2 (average ratios of 4.0). P4501A2 levels in the liver preparations, as assessed by densitometry of immunoblots, correlated with R-warfarin 6-hydroxylation rates (r2 = 0.83) and caffeine 3-demethylation rates (r2 = 0.67), but not with R-warfarin 8-hydroxylation rates. P450s 2A6, 2B6, 2C9, 2D6, 2E1, and 3A4 did not yield either 6- or 8-hydroxy-warfarin from R-warfarin. We conclude that R-warfarin 6-hydroxylation rates are markers for human hepatic P4501A2, whereas ratios of 6-hydroxylation/8-hydroxylation could be used in vitro as a marker for P4501A1.

Chiral separations in capillary electrophoresis using human serum albumin as a buffer additive

This study examined the theory and mechanisms of chiral separations in capillary electrophoresis based on the use of proteins as buffer additives. Human serum albumin (HSA) was used as the model ligand; D,L-tryptophan and (R,S)-warfarin were used as the test analytes to be separated by this protein. Items examined in this work included the amount of HSA adsorbed to the capillary wall and the stability of this adsorbed protein layer. These were investigated by performing frontal analysis on the capillary with HSA and by injecting neutral markers through the capillary at different applied voltages before and after HSA treatment. The role of adsorbed HSA vs HSA in the buffer in determining the stereoselectivity of the CE system was also examined. Adsorbed HSA was the predominant agent involved in the separation of (R,S)-warfarin, while HSA in the buffer had the most significant effect in the resolution of D,L-tryptophan. Two distinct separation mechanisms were proposed to explain these differences. Good agreement was obtained between the results predicted by these mechanisms and the experimental data. Under optimized conditions, both pairs of enantiomers were separated with baseline resolution in less than 12 min.

Separation of drug stereoisomers by the formation of beta-cyclodextrin inclusion complexes

For many drugs, only racemic mixtures are available for clinical use. Because different stereoisomers of drugs often cause different physiological responses, the use of pure isomers could elicit more exact therapeutic effects. Differential complexation of a variety of drug stereoisomers by immobilized beta-cyclodextrin was investigated. Chiral recognition and racemic resolution were observed with a number of compounds from such clinically useful classes as beta-blockers, calcium-channel blockers, sedative hypnotics, antihistamines, anticonvulsants, diuretics, and synthetic opiates. Separation of the diastereomers of the cardioactive and antimalarial cinchona alkaloids and of two antiestrogens was demonstrated as well. Three dimensional projections of beta-cyclodextrin complexes of propanolol, which is resolved by this technique, and warfarin, which is not, are compared. These studies have improved the understanding and application of the chiral interactions of beta-cyclodextrin, and they have demonstrated a means to measure optical purity and to isolate or produce pure enantiomers of drugs. In addition, this highly specific technique could also be used in the pharmacological evaluation of enantiomeric drugs.

Activated carbon beads for the removal of highly albumin-bound species

Activated carbon beads in which 5% activated carbon powder was embedded in 4% agarose were prepared by the emulsion technique. The column of the beads was demonstrated to effectively remove salicylic acid, warfarin, and long-chain fatty acids from solutions containing albumins by either zonal or frontal analysis under the condition of 0.1 M NaCl, pH 3.0 (HCl). The beads were also demonstrated to purify a commercial human serum albumin preparation from residual fatty acids. The beads would be of value in many biochemical purification processes in which activated carbon is employed.