Comparison of the retention behavior of beta-blockers using immobilized artificial membrane chromatography and lysophospholipid micellar electrokinetic chromatography

Potential of liposomes and lipid membranes for the separation of β-blockers by capillary electromigration and liquid chromatographic techniques

β-Blockers belong to a frequently used class of drugs primarily used to treat heart and circulatory conditions. Here we describe the use of lipid vesicles and liposomes as cell membrane biomimicking models in capillary electromigration (CE) and liquid chromatography (LC) techniques for the investigation of interactions between lipid membranes and β-blockers. In addition to liposomes, the use of commercial intravenous lipid emulsions, and their interactions with β-blockers are also discussed. Different CE and LC instrumental techniques designed for these purposes are introduced. Other methodologies for studying interactions between β-blockers and lipid membranes are also briefly discussed, and the different methodologies are compared. The aim is to give the reader a good overview on the status of the use of liposomes and lipids in CE and LC for studying β-blocker interactions.

Prediction of the chromatographic hydrophobicity index with immobilized artificial membrane chromatography using simple molecular descriptors and artificial neural networks

Screening of physicochemical properties should be considered one of the essential steps in the drug discovery pipeline. Among the available methods, biomimetic chromatography with an immobilized artificial membrane is a powerful tool for simulating interactions between a molecule and a biological membrane. This study developed a quantitative structure-retention relationships model that would predict the chromatographically determined affinity of xenobiotics to phospholipids, expressed as a chromatographic hydrophobicity index determined using immobilized artificial membrane chromatography. A heterogeneous set of 261 molecules, mostly showing pharmacological activity or toxicity, was analyzed chromatographically to realize this goal. The chromatographic analysis was performed using the fast gradient protocol proposed by Valko, where acetonitrile was applied as an organic modifier. Next, quantitative structure-retention relationships modeling was performed using multiple linear regression (MLR) methods and artificial neural networks (ANNs) coupled with genetic algorithm (GA)-inspired selection. Subsequently, the selection of the best ANN was supported by statistical parameters, the sum of ranking differences approach with the comparison of rank by random numbers and hierarchical cluster analysis.

Database of published retention factors for immobilized artificial membrane HPLC and an assessment of the effect of experimental variability

A database was collated of published experimental logarithmic values for the relative retention factors (log k(IAM)) measured using an immobilized artificial membrane column and high-performance liquid chromatography (IAM HPLC). Log k(IAM) is an alternative measure of hydrophobicity to the octanol/water partition coefficient (log K(OW)). While there are several accepted methods to measure log K(OW), no standardized method exists to determine log k(IAM). The database of collated log k(IAM) values includes 13 key experimental parameters and contains 1,686 values for 555 compounds, which are predominantly polar organic compounds and include drug molecules and surfactants. These compounds are acidic, basic, and neutral and both ionized and un-ionized under the conditions of analysis. The data compiled demonstrated experimental variability for each experimental parameter considered, including column stationary phase, pH, temperature, and mobile phase. Reducing the experimental variability allowed for greater consistency in the datasets.

Determination of Atenolol and Metoprolol by Capillary Electrophoresis with Tris(2,2'-bipyridyl)ruthenium(II) Electrochemiluminescence Detection

Capillary electrophoresis (CE) coupling with a tris(2,2'-bipyridyl)ruthenium(II) (Ru(bpy)(3)(2+)) electrochemiluminescence (ECL) detection technique was developed for the analysis of two beta-blockers, atenolol (AT) and metoprolol (ME). The parameters that influence the separation and detection, including the buffer pH and concentration, the separation voltage, the detection potential and Ru(bpy)(3)(2+) concentration, were optimized in detail. The calibration curve was linear over a concentration range of two or three orders of magnitude for the two beta-blockers. The detection limits for AT and ME were 0.075 and 0.005 microM (S/N = 3). The relative standard deviations (n = 8) of the ECL intensity and the migration time were 2.65 and 0.22% for AT, 2.82 and 0.34% for ME, respectively. The proposed method was applied to determine AT and ME in spiked urine samples; satisfactory results were obtained.

Investigation of vesicle electrokinetic chromatography as anin vitro assay for the estimation of intestinal permeability of pharmaceutical drug candidates

As the pharmaceutical industry continues the daunting search for novel drug candidates, there remains a need for rapid screening methods not only for biological activity, but for physiochemical properties as well. It is invaluable that adequate model systems for absorption and/or bioavailability be developed early in the drug evaluation process to avoid the loss of promising compounds late in development. The focus of this paper is the use of vesicle EKC (VEKC) as a high-throughput, easy, cost-effective, and predictive model for the passive transcellular diffusion of drug candidates in the intestinal epithelium. Vesicles are large aggregates of molecules containing a spherical bilayer structure encapsulating an internal cavity of solvent. It is this bilayer structure that makes vesicles attractive as model membranes. In this study, vesicles were synthesized from both phospholipids and surfactant aggregates, and then employed as pseudostationary phases in EKC (VEKC). The interaction of drug molecules with vesicles in EKC was then used as the basis for an in vitro assay to evaluate passive diffusion. The VEKC technique showed a statistical correlation between the retention of drug candidates using surfactant and phospholipid vesicles and passive diffusion data (log Pow and colon adenocarcinoma). VEKC analysis offers high-throughput capabilities due to the short run times, low sample, and solvent volumes necessary, as well as instrument automation. However, due to the complexity of drug absorption in the intestine, difficulty arises when a single in vitro model is used to predict in vivo absorption characteristics. Therefore, the retention of drug candidates using VEKC in conjunction with other permeability prediction methods can provide a primary screen for a large number of drug candidates early in the drug discovery process with minimal resources.

Rapid estimation of octanol-water partition coefficients using synthesized vesicles in electrokinetic chromatography

Vesicle electrokinetic chromatography (VEKC) using vesicles synthesized from the oppositely charged surfactants cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) and from the double-chained anionic surfactant bis(2-ethylhexyl)sodium sulfosuccinate (AOT) was applied to the indirect measurement of octanol-water partition coefficients (log Po/w). A variety of small organic molecules with varying functional groups, pesticides, and organic acids were evaluated by correlating log Po/w and the logarithm of the retention factor (log k') and comparing the calibrations. A linear solvation energy relationship (LSER) analysis was conducted to describe the retention behavior of the vesicle systems and compared to that of octanol-water partitioning. The solute hydrogen bond donating behavior is slightly different with the vesicle interactions using CTAB-SOS vesicles as compared to the octanol-water partitioning model. The AOT vesicle and octanol-water partitioning systems showed similar partitioning characteristics. VEKC provides rapid separations for determinations of log Po/w in the range of 0.5 to 5 using CTAB-SOS vesicles and 0 to 5.5 using AOT vesicles.

Quantitative determination of oxprenolol and timolol in urine by capillary zone electrophoresis

A simple capillary zone electrophoretic method with UV detection has been developed for the quantitative determination of the beta-adrenoreceptor antagonists (beta-blockers) oxprenolol and timolol in human urine, preceded by a solid-phase extraction step. The electrophoretic separation was performed on a 78 cm x 75 microm I.D. fused-silica capillary (effective capillary length: 70 cm). The electrolyte consisted of a Na2B4O7-H3BO3 (50 mM), pH 9. The introduction of the sample was made hydrostatically for 20 s and the running voltage 25 kV at the injector end of the capillary. Photometric detection was used at a wavelength of 229 nm for oxprenolol and 280 nm for timolol. Under these conditions oxprenolol migrated at 4.76+/-0.05 min and timolol at 4.97+/-0.05 min. The solid-phase extraction methods were optimised for each beta-blocker and provided recoveries of 72.8% for timolol and 94.52% for oxprenolol. Good resolution from the endogenous compounds present in the urine matrix were achieved for both compounds. The method was applied to the determination of both beta-blockers in pharmaceutical formulations and urine samples obtained from hypertensive patients after the ingestion of a therapeutic dose (in a 24-h time interval after the ingestion). The quantitative results were compared with results previously obtained at our laboratories by HPLC and were found to be in good agreement. Good reproducibility, linearity, accuracy and quantitation limits (in urine) of 0.19 microg/ml for timolol and 0.20 microg/ml for oxprenolol were obtained, allowing the method to be applied to pharmacokinetic studies of these compounds.

High throughput physicochemical profiling for drug discovery

The drug development bottlenecks of attrition and development time are being addressed by acquiring a profile of the candidate's physicochemical and physiological properties during early discovery phases. This information assists selection and optimization of pharmaceutical properties in parallel with activity. High throughput methods to measure the properties: solubility, permeability, lipophilicity, pKa, stability and integrity are described and compared in this article. The underlying discovery requirements, needs and application strategies are discussed.

Chromatographic retention of drug molecules on immobilised liposomes prepared from egg phospholipids and from chemically pure phospholipids

The partitioning of a chemically diverse set of drugs into liposomes was studied by immobilised liposome chromatography (ILC). For this purpose liposomes composed of (i) purified egg phospholipids (EPL), (ii) synthetic phosphatidylcholine (PC), (iii) PC--synthetic phosphatidylethanolamine (PE) 80:20 (mol/mol) and (iv) PC--synthetic phosphatidylserine (PS) 80:20 (mol/mol) were immobilised in gel beads by freeze-thawing. The drug partitioning was assessed from the retention volume, which was expressed as a capacity factor, K(s), normalised with respect to the amount of immobilised phospholipid. The drug retention on EPL, PC and PC--PE liposomes was very similar, whereas the negatively charged PC--PS liposomes increased the retention of positively charged and decreased retention of negatively charged drugs. The partitioning of drugs on liposome columns (log K(s)) versus their octanol--water partitioning (log P(oct)) showed three separate rectilinear relationships, depending on the charge of the compound (neutral, positive, or negative). Statistical analysis (ANCOVA) proved that the lines had similar slopes. Repeated analysis of four reference compounds showed a low variation (<0.12 log units) over time (about 250 days). A close relationship was observed between the drug retention in short EPL columns with a low content of phospholipids and the retention in longer standard EPL columns. The short 'quick screen bilayer columns' permit analysis of highly lipophilic compounds within 30 min and are thus applicable for medium-throughput screening in drug discovery settings. A very strong rectilinear relationship (r(2)=0.95, n=13) between log K(s) (EPL) and published liposome partitioning data (log D(mem)) confirmed that the ILC drug retention reflects the drug partitioning into the lipid bilayers. A moderate to fair rectilinear relationship was observed between the normalised retention on PC, PC-PE and EPL liposomes (r(2)=0.79, 0.86 and 0.85, respectively, n=24) and corresponding published log k'(IAM) data obtained on immobilised artificial membrane (IAM) columns. Transport across Caco-2 cell monolayers (log P(c)) showed curvilinear relationships with log K(s), log k'(IAM), log P(oct) and log D(oct). The drug fraction absorbed in humans showed a similar relationship to log K(s) values as to surface plasmon resonance signals representing drug-liposome interaction (Danelian et al., 2000 J Med Chem, 43, 2083--2086).

N-in-one determination of retention factors for drugs by immobilized artificial membrane chromatography coupled to atmospheric pressure chemical ionization mass spectrometry

Immobilized artificial membrane (IAM) chromatography is widely used in drug discovery for ranking the absorption properties of drug candidates. In this work an IAM chromatography method using atmospheric pressure chemical ionization mass spectrometric detection (IAM/APCI-MS) was developed for the determination of log k(IAM) values for a mixture of compounds (9-in-one). Values were calculated from isocratic runs (0, 10, 20, 30, 35% acetonitrile) in both positive and negative modes. Good correlation (r(2) = 0.97) was achieved for n-in-one results obtained with ammonium acetate buffer and mass spectrometry, compared with the traditional method involving single compound analysis with phosphate buffered saline and an ultraviolet detector. A gradient elution method providing fast determination of relative log k(IAM) values in a single IAM/APCI-MS run was demonstrated for the same compounds.

Retention of barbituric acid derivatives on immobilized artificial membrane stationary phase and its correlation with biological activity

A series of 30 barbituric acid derivatives were subjected to high-performance liquid chromatography (HPLC) on the 'immobilized artificial membrane' (IAM) column with acetonitrile buffer mobile phase. The retention parameter log k(IAM) was related to the logarithms of partition coefficients determined in octanol-water partition system, log P, to a thin-layer chromatographic (TLC) parameter from partition TLC, R(m0), to an adsorption HPLC retention parameter, log k(0), and to a solubility parameter, delta. It was demonstrated that log k(IAM) correlated significantly to the other parameters of barbiturates determined in partition systems but not to delta. However, log k(IAM) appeared to be a distinctive descriptor of hydrophobicity of barbiturates as compared to the standard log P parameters. The parameter log k(IAM) was shown to correlate with bioactivity data of the agents studied.

Investigation of the chiral surfactant N-dodecoxycarbonylvaline in electrokinetic chromatography: improvements in elution range and pH stability via mixed micelles and vesicles, and the hydrophobicity determination of basic pharmaceutical drugs

The chiral surfactant dodecoxycarbonylvaline (DDCV) has proven to be an effective pseudostationary phase for the separation of many enantiomeric pharmaceutical compounds. In this study the elution range and the prediction of octanol-water partitioning for the DDCV micellar system was examined. Through incorporation of DDCV in mixed micelles and unilamellar vesicles, enhancement of the elution range was observed. The mixed micelles contained a second anionic surfactant, sodium dodecyl sulfate (SDS), while the vesicles were composed of DDCV and the cationic surfactant cetyltrimethylammonium bromide (CTAB). Enantioselectivity, as well as other chromatographic and electrophoretic parameters, were compared between the mixed micelles, vesicles, and DDCV micelles. The hydrophobicity of the DDCV system was also evaluated as a predictor of n-octanol-water partition coefficients for 15 beta amino alcohols. The correlation between the logarithm of the retention factor (log k) and log P(ow) for seven hydrophobic beta-blockers and eight beta-agonists were r2 = 0.964 and r2 = 0.814, respectively.

Dual-opposite injection electrokinetic chromatography for the unbiased, simultaneous separation of cationic and anionic compounds

The concept of dual opposite injection in capillary electrophoresis (DOI-CE) for the simultaneous separation, under conditions of suppressed electroosmotic flow, of anionic and cationic compounds with no bias in resolution and analysis time, is extended to a higher pH range in a zone electrophoresis mode (DOI-CZE). A new DOI-CE separation mode based on electrokinetic chromatography is also introduced (DOI-EKC). Whereas conventional CZE and DOI-CZE are limited to the separation of charged compounds with different electrophoretic mobilities, DOI-EKC is shown to be capable of separating compounds with the same or similar electrophoretic mobilities. In contrast to conventional EKC with charged pseudostationary phases that often interact too strongly with analytes of opposite charge, the neutral pseudostationary phases appropriate for DOI-EKC are simultaneously compatible with anionic and cationic compounds. This work describes two buffer additives that dynamically suppress electroosmotic flow (EOF) at a higher pH (6.5) than in a previous study (4.4), thus allowing DOI-CZE of several pharmaceutical bases and weakly acidic positional isomers. Several DOI-EKC systems based on nonionic (10 lauryl ether, Brij 35) or zwitterionic (SB-12, CAS U) micelles, or nonionic vesicles (Brij 30) are examined using a six-component test mixture that is difficult to separate by CZE or DOI-CZE. The effect of electromigration dispersion on peak shape and efficiency, and the effect of surfactant concentration on retention, selectivity, and efficiency are described.

Micellar electrokinetic chromatography with bis(2-ethylhexyl)sodium sulfosuccinate vesicles determination of synthetic food antioxidants

Capillary electrokinetic chromatography is suitable for the separation of mixtures of uncharged and charged solutes. In the present work the behavior of six synthetic food antioxidants--2[3]-tert.-butyl-4-hydroxyanisole, 2,6-di-tert.-butyl-p-cresol, tercbutylhydroquinone, 3,4,5-trihydroxybenzoic acid propyl ester, 3,4,5-trihydroxybenzoic acid octyl ester and 3,4,5-trihydroxybenzoic acid dodecyl ester--was studied in a capillary electrophoresis system using capillary electrokinetic chromatography with vesicles of the surfactant bis(2-ethylhexyl)sodium sulfosuccinate (AOT). Several studies aimed at calculating the critical aggregation concentration of the surfactant were conducted to check that under the conditions used the AOT was in a state of aggregation. Having checked the association shown by the surfactant, we then explored the greater or lesser capacity of the antioxidants to interact with this compound. We followed the evolution of the molecular absorption spectra of each of the antioxidants in the presence of the surfactant at different concentrations and the retention factors were calculated at different pH values. Additionally, in order to determine which species--anionic or neutral--was present at the pH of the buffer used (boric/borate), the pKa values in acetonitrile-water (20:80) were obtained. Resolution and quantification of the antioxidants demand optimization of the variables involved in the system, such as the percentage of acetonitrile, the concentration of AOT and boric/borate buffer, pH, voltage, etc. When this part of the study had been completed, calibrations were obtained for each of the antioxidants, obtaining good linear correlation coefficients in all cases. Finally, we propose a method that allows the resolution of the six most employed antioxidants in a capillary electrophoretic system in 15 min, using electrokinetic chromatography with AOT as the pseudostationary phase.

Characterization of lipophilicity scales using vectors from solvation energy descriptors

Lipophilicity scales were characterized by an approach using vectors provided from solvation energy descriptors (SED) of solutes such as an excess molar refraction, the dipolarity/polarizability, the hydrogen-bond acidity, the basicity, and the McGowan characteristic volume. The five components of the SED vector were obtained from the coefficients of the five SED terms of the linear solvation energy relationship (LSER) equation for the lipophilicity scales. The analogy between two lipophilicity scales was expressed as the angle between the two SED vectors, while the difference in the contribution of the five independent SEDs to these two lipophilicity scales was quantified by the difference of the unit vectors of the SED vectors. These approaches were applied to several lipophilicity scales measured using microemulsions, micelles, an immobilized artificial membrane column, and an octanol-water system. As a result, the quantitative classification of these scales was successfully carried out, and the difference in the scales was well characterized. In addition, this vector approach was extended to the estimation of the contribution of each constituent of the microemulsions to the lipophilicity scale. Furthermore, some biological parameters such as skin permeability and the distribution between blood and brain could be predicted by the summation of the SED vectors obtained from the chromatographic systems. These results suggest that complex biological systems can be expressed quantitatively by simple chemical models with their SED vectors.