Separation of the enantiomers of beta-receptor blocking agents and other cationic drugs using a CHIRAL-AGP column. Binding properties and characterization of immobilized alpha 1-acid glycoprotein

Enantiomeric Separation of Dioxopromethazine and its Stereoselective Pharmacokinetics in Rats by HPLC-MS/MS

Dioxopromethazine (DPZ) is a popular phenothiazine antihistamine that is widely used as a racemic drug in clinical to cure respiratory illness. In our work, a reliable, specific, and rapid enantioselective HPLC-MS/MS method has been established and fully validated for the quantification of R- and S-DPZ in rat plasma. After plasma alkalization (with 1 M Na₂CO₃), DPZ enantiomers and diphenhydramine (IS) were extracted using ethyl acetate. Completely separation of R- and S-DPZ (Rs = 2.8) within 12 min was implemented on Chiralpak AGP column (100 × 4.0 mm i.d., 5 μm) employing ammonium acetate (10 mM; pH 4.5) - methanol (90:10, v/v) as mobile phase. Themultiple reaction monitoring (MRM) mode was used for the detection of DPZ enantiomers and IS. The transitions of m/z 317.2 → 86.1 and 256.2 → 167.1 werechosen for monitoring DPZ enantiomers and IS, respectively. Good linearity (r² > 0.995) was achieved for each DPZ enantiomer over the linear ranges of 1.00 - 80.00 ng/mL, with the lower limit of quantitation (LLOQ) of 1.00 ng/mL. The intra-day and inter-day precisions (RSDs,%) were below 12.3%, and accuracies (REs,%) were in the scope of-10.5% to 6.6%, which were within the admissible criteria. The validated bioanalytical approach was applied to the stereoselective pharmacokinetic (PK) research of DPZ in rat plasma for the first time. It was found that significant differences (p < 0.05) exist between the main PK parameters of R- and S-DPZ, indicating the pharmacokinetic behaviors of DPZ enantiomers in rats were stereoselective. The chiral inversion of the enantiomers did not occur during the assay.

Investigation of the prerequisites for the optimization of specific plasma protein binding as a strategy for the reduction of first-pass hepatic metabolism

1. It is hypothesized that the deliberate structural tailoring of compounds designed for drug use to increase the specific plasma protein binding can be used to reduce first-pass hepatic metabolism. To test the feasibility of this hypothesis, a dataset of drugs with plasma protein binding of 90% or above divided into three classes including 50 acids, 44 bases and 69 neutrals was analyzed. 2. Among the drugs with ≥99% plasma protein binding, the fraction of the total dose existing in free form in vivo (free dose fraction) decreased in the following order: acids (0.55%) > neutrals (0.16%) > bases (0.08%). The order was different for the fraction of the total dose that existed in plasma protein bound form (plasma protein bound dose fraction): acids (58%) > neutrals (17%) = bases (18%). 3. The free fraction was poorly correlated with the partition coefficient (Log P). The lower aqueous solubility associated with high plasma protein binding was explained by differences in Log P and not by the plasma protein binding per se. The logarithm of the extrarenal clearance was correlated with Log P. For acids and bases, extrarenal clearance was also correlated with fu. For neutrals, plasma protein binding had no protective effect.

Applications of silica supports in affinity chromatography

The combined use of silica-based chromatographic supports with immobilized affinity ligands can be used in many preparative and analytical applications. One example is the use of silica-based affinity columns in HPLC, giving rise to a method known as high-performance affinity chromatography (HPAC). This review discusses the role that silica has played in the development of affinity chromatography and HPAC and the applications of silica in these methods. This includes a discussion of the types of ligands that have been employed with silica and the methods by which these ligands have been immobilized. Various formats have also been presented for the use of silica in affinity chromatographic methods, including assays involving direct or indirect analyte detection, on-line or off-line affinity extraction, and chiral separations. The use of silica-based affinity columns in studies of biological systems based on zonal elution and frontal analysis methods will also be considered.

Development of an affinity silica monolith containing alpha1-acid glycoprotein for chiral separations

An affinity monolith based on silica and containing immobilized alpha(1)-acid glycoprotein (AGP) 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 monoliths based on a copolymer of glycidyl methacrylate (GMA) and ethylene dimethacrylate (EDMA). The surface coverage of AGP in the silica monolith was 18% higher than that obtained with silica particles and 61% higher than that measured for a GMA/EDMA monolith. The higher surface area of the silica monolith gave materials that contained 1.5- to 3.6-times more immobilized protein per unit volume when compared to silica particles or a GMA/EDMA monolith. The retention, efficiency and resolving power of the AGP silica monolith were evaluated by injecting two chiral analytes onto this column (i.e., R/S-warfarin and R/S-propranolol). In each case, the AGP silica monolith gave higher retention plus better resolution and efficiency than AGP columns containing silica particles or a GMA/EDMA monolith. The AGP 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 AGP as a chiral stationary phase.

Separation of drug enantiomers by liquid chromatography and capillary electrophoresis, using immobilized proteins as chiral selectors

Proteins display interesting chiral discrimination properties owing to multiple possibilities of intermolecular interactions with chiral compounds. This review deals with proteins which have been used as immobilized chiral selectors for the enantioseparation of drugs in liquid chromatography and capillary electrophoresis. The main procedures allowing the immobilization of proteins onto matrices, such as silica and zirconia particles, membranes and capillaries are first presented. Then the factors affecting the enantioseparation of drugs in liquid chromatography, using various protein-based chiral stationary phases (CSPs), are reviewed and discussed. Last, chiral separations already achieved using immobilized protein selectors in affinity capillary electrochromatography (ACEC) are presented and compared in terms of efficiency, stability and reproducibility.

Quantitative determination of the enantiomers of methadone and its metabolite (EDDP) in human saliva by enantioselective liquid chromatography with mass spectrometric detection

A sensitive enantioselective liquid chromatographic assay with mass spectrometric detection (LC-MS) has been developed and validated for the simultaneous determination of saliva concentrations of (R)- and (S)-methadone (Met) and (R)- and (S)-2-ethylidene-1,5-dimethyl-3,3-diphenyl-pyrrolidine (EDDP, a primary metabolite of Met). Saliva specimens were collected using Salivette devices (Sarsedt), and centrifuged; collected saliva was then spiked with deuterated internal standards, D3-Met and D3-EDDP, and directly injected into the LC-MS. Enantioselective separations were achieved on a liquid chromatographic chiral stationary phase (CSP) based upon immobilized alpha(1)-acid glycoprotein (AGP) using a mobile phase composed of acetonitrile: ammonium acetate buffer (10mM, pH 7.0) in a ratio of 18:82 (v/v), a flow rate of 0.9 ml/min and a temperature of 25 degrees C. Under these conditions, enantioselective separations were observed for methadone (alpha=1.30) and EDDP (alpha=1.17) within 15 min. Met, EDDP, D3-Met and D3-EDDP were detected using selected ion monitoring at m/z 310.20, 278.20, 313.20 and 281.20, respectively. Linear relationships between peak height ratio and drug-enantiomer concentrations were obtained for methadone in the range of 5.0-600.0 ng/ml, and for EDDP from 0.5 to 15.0 ng/ml per enantiomer with correlation coefficients better than 0.9994, where lower limit of quantification (LLOQ) for Met was 5 ng/ml and for EDDP 0.5 ng/ml. Acceptable intra- and inter-day precision of the method (CVs<4.0%) and accuracy (CVs<4.0%) were obtained. These findings demonstrate the accuracy and precision of the method used to successfully analyze saliva obtained from patients enrolled in a methadone-maintenance program.

Role of biological matrices during the analysis of chiral drugs by liquid chromatography

The review article covers advances of chiral drugs analysis by high-performance liquid chromatography (HPLC) methods achieved during last 10 years. Emphasis is given to various aspects of influence of biological matrix in pharmacodynamics, pharmacokinetics, HPLC analysis. Discussed is composition of main biological matrices from the point of view of potential interferences to above-mentioned fields of study. Beside typical analytical approaches to chiral recognition in HPLC, sample pretreatment and/or clean-up by conventional extraction procedures, column switching (CSW) techniques using restricted access materials (RAMs), microdialysis (MCD) is discussed. Measurement of unbound drug concentration and discussion of column maintenance and remedy is an additional source of information and field where knowledge on complex properties and interactions of biological matrix is usefully applicable.

Capillary electrophoretic study on pH dependence of enantioselective disopyramide binding to genetic variants of human alpha1-acid glycoprotein

A high-performance frontal analysis-capillary electrophoresis (HPFA-CE) method was applied to investigate the effect of pH on the drug binding properties of genetic variants of human alpha1-acid glycoprotein (AGP), A variant and a mixture of F1S variants. The unbound concentrations of a model basic drug, disopyramide (DP), in A variant solutions and in F1S variant solutions were measured by HPFA-CE to evaluate binding constants at pH 4.0, 5.0, 6.0 and 7.4. The binding between DP and A variant was gradually weakened by acidification of background buffer (from pH 7.4 to 4.0), while the binding between DP and FIS variants decreased at first (from pH 7.4 to 6.0), and then gained (from pH 6.0 to 4.0). Consequently, DP was more strongly bound to A variant than to FIS variants at pH 7.4, while at pH 4.0 DP was more strongly bound to F1S variants. At any pH (S)-DP was bound more strongly than (R)-DP, and the enantioselectivity of A variant was significantly higher than that of F1S variants. Electrophoretic mobilities of the AGP genetic variants decreased along with a decrease in pH. Fluorescent emission of these genetic variants indicated a distinct conformational change between pH 5.0 and 4.0. However, there was no significant difference in the electrophoretic mobility and the fluorescent emission spectrum between these variants at any pH. On the other hand, circular dichroism analyses revealed that beta-sheet content in FIS variants diminished as pH decreased, while that in A variant increased. These results suggest that the conformational change induced by acidification of background buffer differs between these genetic variants, and this causes the difference in DP bindability.

Stereoselective metabolism of metoprolol: enantioselectivity of alpha-hydroxymetoprolol in plasma and urine

Direct stereoselective separation on chiral stationary phase was developed for HPLC analysis of the four stereoisomers of alpha-hydroxymetoprolol in human plasma and urine. Plasma samples were prepared using solid-phase extraction columns and urine samples were prepared by liquid-liquid extraction. The stereoisomers were separated on a Chiralpak AD column at 24 degrees C with fluorescence detection and a mobile phase consisting of a mixture of hexane:ethanol:isopropanol:diethylamine (88:10.2:1.8:0.2) for plasma samples and hexane:ethanol:diethylamine (88:12:0.2) for urine samples. Calibration curves for the individual stereoisomers were linear within the concentration range of 2.0-200 ng/ml plasma or 0.125-25 microg/ml urine. The methods were validated with intra- and interday variations less than 15%. The absolute configuration of the pure stereoisomers were assigned by circular dichroism spectra. The methods were employed to determine the concentrations of alpha-hydroxymetoprolol stereoisomers in a metabolism study of multiple-dose administration of racemic metoprolol to hypertensive patients phenotyped as extensive metabolizers of debrisoquine. We observed stereo-selectivity in the alpha-hydroxymetoprolol formation favoring the new 1'R chiral center from both metoprolol enantiomers (AUC(0-24) (1'R1'S) = 3.02). The similar renal clearances (Cl(R)) of the four stereoisomers demonstrated absence of stereoselectivity in their renal excretion. (-)-(S)-metoprolol was slightly more alpha-hydroxylated than its antipode (AUC(0-24) (2S/2R) = 1.19), suggesting that this pathway is not responsible for plasma accumulation of this enantiomer in humans.

Vancomycin dimerization and chiral recognition studied by high-performance liquid chromatography

The retention and separation of D,L-dansylvaline enantiomers (used as test solutes) were investigated using silica gel as stationary phase and vancomycin as chiral mobile-phase additive. A retention model was developed to describe the mechanistic aspects of the interaction between solute and vancomycin in the chromatographic system. It considered the formation of vancomycin dimers both "free" in the mobile phase and adsorbed on silica. By fitting the model equation to experimental data, it appeared clearly that the approach taking into account the vancomycin dimerization described accurately the retention behavior of the compounds. The examination of the model equation parameters showed that the glycopeptide dimerization increased the enantioselectivity by a factor of approximately 3.7. This study demonstrated the preponderant role of the vancomycin dimerization on the chiral recognition process of D,L-dansylvaline. Also, an additional analysis on a vancomycin chiral stationary phase indicated that the addition of vancomycin in the mobile phase promoted a greater enantioselectivity mediated by the formation of dimers in the stationary phase.

Investigation of the heterogeneous adsorption behavior of selected enantiomers on immobilized alpha1-acid glycoprotein

A complete census was made of the interactions between enantiomeric solutes and the chiral protein column CHIRAL-AGP with the theory of nonlinear LC as tool. The surface is heterogeneous, having a small number of strong enantioselective adsorption sites and a large number of weak nonselective ones. When the eluent pH was increased, the "linear" retention of (i) the amines increased strongly as a result of a strong increase in the enantioselective binding strength, whereas (ii) the retention of the aprot increased slightly as a result of an increase in both the enantioselective binding strength and its capacity. The retention of (iii) the acid has a maximum originating solely from the enantioselective binding energy, whereas the nonselective equilibria decreased steadily. For all compounds, the enantioselective equilibrium constants increase relatively more than the nonselective ones with increasing pH.

Simultaneous determination of reboxetine and O-desethylreboxetine enantiomers using enantioselective reversed-phase high-performance liquid chromatography

Current knowledge of stereoselective pharmacokinetics and different potencies of drug enantiomers requires the performance of stereoselective analysis during therapeutic drug monitoring in clinical practice. However, in the case of the new antidepressant drug reboxetine, no effort has been made so far to find a such a suitable system. Therefore, as a step towards developing an enantioselective bioanalytical method for reboxetine and the O-desethylreboxetine metabolite, three stereoselective chromatographic approaches have been investigated. Several chiral columns were tested, among them Chiral-AGP, ChiraGrom 2 and Chiral-CBH, which were able to simultaneously separate the two compounds into enantiomers in total running times of 28, 18 and 12 min, respectively.

Protein-based chiral stationary phases for high-performance liquid chromatography enantioseparations

The enantioseparations of various compounds using proteins as the chiral selectors in high-performance liquid chromatography (HPLC) are considered in this review. The proteins used include albumins such as bovine serum albumin and human serum albumin, glycoproteins such as alpha1-acid glycoprotein, ovomucoid, ovoglycoprotein, avidin and riboflavin binding protein, enzymes such as trypsin, alpha-chymotrypsin, cellobiohydrolase I, lysozyme, pepsin and amyloglucosidase, and other proteins such as ovotransferrin and beta-lactoglobulin. This review deals with the properties of HPLC chiral stationary phases based on proteins, and the enantioselective properties and chiral recognition mechanisms of these stationary phases.

Chiral stationary phases in HPLC for the stereoselective determination of methadone

An extensive study of the behavior of three chiral stationary phases (CSP) is presented for the stereoselective determination of methadone. The following chromatographic columns were selected: a cellulose, Chiralcel OJ; a modified cyclodextrin. Cyclobond I 2000 RSP, and a protein, Chiral-AGP. Retention factors, enantioselectivity, efficiency, and resolution were tested by modifying the composition of the mobile phase as well as the temperature. The mechanism for the chiral recognition of methadone on each support was discussed. Optimal chromatographic parameters were obtained for the three supports tested, and methadone enantiomers were separated in less than 20 minutes. The cellulose-based column gave the best resolution, but this CSP was not adapted to clinical analyses of methadone. Under optimized conditions, the cyclodextrin- and protein-based columns allowed an excellent separation of methadone enantiomers, but no interference with the primary metabolite was found only with Chiral-AGP.

Mechanisms of retention of pyrrolidinyl norephedrine on immobilized alpha(1)-acid glycoprotein

The HPLC separation of the R,S and S,R enantiomers of pyrrolidinyl norephedrine on immobilized alpha-1 glycoprotein (AGP) was investigated. Conditions for the separation were varied using a premixed mobile phase containing an ammonium phosphate buffer and an organic modifier. The influence of mobile phase pH, ionic strength, organic modifier composition, modifier type, and temperature on the chiral selectivity and retention were investigated. The presented data demonstrate that independent phenomena govern the enantioselectivity and retention. Retention is a function of both ion exchange equilibria and hydrophobic adsorption. Thermodynamic data derived from van't Hoff plots illustrates that while enantioselectivity is also enthalpically driven, the magnitude of the enthalpy term is governed by pH. Enantioselectivity has little dependence on ionic strength. Hydrophobic interactions appear to foster hydrogen bonding interactions; the two appear to be mutually responsible for chiral selectivity. The chiral selectivity decreases as the pH is decreased and increases with mobile phase buffer strength.

Enantioselective analysis of metoprolol in plasma using high-performance liquid chromatographic direct and indirect separations: applications in pharmacokinetics

Direct enantioselective separation on chiral stationary phases and indirect separation based on the formation of diastereomeric derivatives were developed and compared for the HPLC analysis of R(+) and S(-)-metoprolol in human plasma. Plasma samples prepared using solid-phase extraction columns or liquid-liquid extraction were directly analyzed on a Chiralpack AD or on a Chiralcel OD-H columns, respectively. S-(-)-menthyl choroformate was also used to yield diastereomeric derivatives resolved on a RP-8 column. The methods were employed to determine plasma concentrations of metoprolol enantiomers in a pharmacokinetic study of single dose administration of racemic metoprolol to a healthy Caucasian volunteer phenotyped as extensive metabolizer of debrisoquine. The correlation coefficients among enantioselective metoprolol plasma concentrations (5-223 ng/ml) obtained by the three methods were equal or higher than 0.99. The direct method that employed the chiral column Chiralpak AD may be considered the most sensitive, although the three methods demonstrated interchangeable use in the pharmacokinetic investigation.

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.

Differential tissue distribution of the enantiomers of racemic pindolol in the rat

Racemic pindolol, a beta-adrenoceptor and a 5-HT1A/1B receptor antagonist, has been reported to augment and accelerate the clinical efficacy of antidepressants. The (S)-enantiomer is more potent than the (R)-enantiomer both at the beta-adrenergic and at the 5-HT1A/1B receptors. A chiral HPLC column was used for determination of tissue concentrations of the enantiomers of pindolol at 90 min after 8 mg/kg s.c. of the racemate. The (S)/(R) ratio was found to vary between tissues from 1.74 in brain to 0.82 in plasma. The present findings may be important for understanding the pharmacodynamic actions of racemic pindolol.

Human alpha1-glycoprotein acid as chiral selector in the enantioseparation of midodrine and deglymidodrine racemates by HPLC

Human alpha1-acid glycoprotein (alpha1-AGP) has been used as a chiral stationary phase (CSP) for the enantioseparation of midodrine and deglymidodrine racemates in the same HPLC run. The imobilized AGP resulted as the best chiral selector for the enantioresolution of two compounds. Due to the modification of alpha1-AGP characters as a result of changing the composition of the mobile phase, an attempt study of the watery mobile phase (ionic strength and pH of the buffer, nature and concentration of the organic modifier) allowed for an increase in the enantioselectivity of the chromatographic system and an optimization of the resolution base-line of both enantiomeric pairs.

On-line column-switching high-performance liquid chromatography analysis of cardiovascular drugs in serum with automated sample clean-up and zone-cutting technique to perform chiral separation

A selective and highly reproducible, multi-column HPLC method is described for the analysis of the following cardiovascular drugs: lidocaine, pindolol, metoprolol, oxprenolol, diltiazem and verapamil, in serum. Column-switching devices are employed in combination with advanced separation media technologies for the automated analysis of samples containing complex matrices. The method consists of on-line sample clean-up using a restricted access sorbent, HPLC analysis of the drugs on a microsphere non-porous silica RP-18 column, and front-cutting to perform the chiral separation of pindolol enantiomers on a second HPLC system. Simultaneous control of the two HPLC systems and data analysis is achieved from a single centralized software. The R.S.D. values of the peak areas for spiked serum are less than 1% for metoprolol and oxprenolol, 2-5% for lidocaine, diltiazem and verapamil, and 1.2 and 2.4% for the two pindolol enantiomers. Recoveries, limits of detection and linearities are provided.

Coupled-column chromatography on a Chiral-AGP phase for determination of amlodipine enantiomers in human plasma: an HPLC assay with electrochemical detection

A sensitive enantioselective high performance liquid chromatographic assay for determination of the dihydropyridine-type calcium antagonist amlodipine in human plasma samples is described. Chiral chromatography is performed on an alpha 1-acid glycoprotein column (i.e. Chiral-AGP) and the eluted enantiomers are trapped and compressed on two short columns before final achiral chromatography on a narrow bore column (i.e. Zorbax SB-Ph) using electrochemical detection. Both stereoselective quantitative analysis and enantiomeric ratio analysis, for samples with a known total concentration of amlodipine are described. The quantitative assay shows linearity over the range 0.5-10 ng ml-1 for the two enantiomers and the limit of detection is about 0.2 ng ml-1. The method has been applied to a pharmacokinetic study of the enantiomers of amlodipine in human subjects.

A liquid chromatographic method for the determination of promethazine enantiomers in human urine and serum using solid-phase extraction and fluorescence detection

A LC method was developed for the concurrent assay of R(+) and S(-) promethazine from human urine and serum. The method involves the use of solid-phase extraction for sample clean-up. Chromatographic resolution of the enantiomers was performed under isocratic conditions using a mobile phase of hexane-1,2-dichlorethane-absolute ethanol-trifluoroacetic acid (400:150:100:1, v/v/v/v) at a flow rate of 1 ml min-1 on a brush-type column KK-CARNU. The enantiomers were detected by fluorescence using an excitation wavelength of 250 nm and a 280 nm emission cutoff filter. Chlorpromazine was used as the internal standard for urine analysis. Standard addition was used for promethazine analysis from serum. Drug to internal standard ratios were linear from 0.25 to 10 micrograms ml-1 in urine. Serum levels were linear from 2 to 10 ng ml-1.

Evaluation of the liquid-chromatographic resolution of indenoindolic racemic compounds on three protein-based chiral stationary phases

Three liquid-chromatography columns, containing immobilized proteins as chiral stationary phases (CSPs) were investigated for the direct separation of enantiomers of racemic indenoindolic compounds. By using an experimental design the effects of column temperature, pH, ionic strength, and type and concentration of organic solvent in the mobile phase on retention and resolution of racemic substances were systematically studied. The three CSPs investigated consisted of alpha(1)-acid glycoprotein (Chiral-AGP), bovine serum albumin (BSA-DSC) and ovomucoid (Ultron ES-OVM). The two enantiomers of all studied compounds could be separated on at least one of the three CSPs, which have different enantioselective properties.

Optimization of the separation of enantiomers of basic drugs. Retention mechanisms and dynamic modification of the chiral bonding properties on an alpha 1-acid glycoprotein column

The chromatographic properties of 29 basic drugs were studied by varying the pH and the concentration of inorganic ions in the mobile phase. It was observed that the chromatographic performance of most hydrophobic basic drug compounds could be strongly enhanced by decreasing the pH in the mobile phase from 7 to 4-6. The enantioselectivity increased and a much faster resolution was obtained. The results indicate that ion exchange and ion-pair distribution may be involved in the retention process of cationic drug enantiomers. Increasing the concentration of acetate and phosphate increases the retention of the enantiomers of the drug compounds. The relative contribution of the two retention processes can be affected by the pH and the nature and the concentration of the ions in the mobile phase. Decreasing the pH reduces the influence of the ion-exchange process since the negative charge of the protein is decreased. The enantioselectivity is also greatly affected by increasing salt concentration.