Characterization of micellar solubilization of antibiotics using micellar electrokinetic chromatography

Micellar electrokinetic chromatography (nMEKC) method was applied to the determination of the partition behaviour between water and micelle for a group of antibiotics using sodium dodecyl sulphate (SDS) as an anionic model surfactant. In the method, the running buffer contains both the micelles and the drug, and the injected solution contains the same concentration of the micelles as the running buffer but no drug. The mobility of the drug can be measured from a negative peak recorded in the chromatogram. The required parameters for the determination of the capacity factor were measured by the MEKC technique. Thermodynamic properties such as enthalpy and entropy changes of micellar solubilization were calculated from the temperature dependence of the partition coefficients. The critical micellar concentrations (CMC) of the SDS salt were determined in phosphate solutions at pH 7 at different temperatures using the MEKC. The method described in this article based on MEKC is efficient and very fast in order to determine parameters for characterizing micellar solubilization of drugs.     

Effect of surfactant structures on the separation of cold medicine ingredients by micellar electrokinetic chromatography

Using micellar electrokinetic chromatography (micellar EKC or MEKC) the retention behavior of twelve active ingredients used in cold medicines was investigated. The role of five different anionic surfactants was investigated by MEKC and the results were compared with those obtained by conventional capillary zone electrophoresis (CZE). The relative retention order of the 12 ingredients was significantly different among the five surfactants; the different elution orders were ascribed to the differences in the hydrophilic groups of the surfactants. The effects of surfactant concentration and buffer pH were also investigated. The ingredients were successfully separated within 30 min by micellar EKC with high theoretical plate numbers (approximately 200,000), and selectivity was much improved in comparison with CZE. Micellar EKC was applied by the internal standard method to the quantitation of some active ingredients combined in commercial preparations.     

Choice of capillary electrophoresis systems for the impurity profiling of drugs

In order to develop a strategy for the impurity profiling of drugs, the possibilities of some capillary electrophoresis systems were investigated. A mixture containing a drug and some of its possible impurities has been used as a model problem. The test compounds were investigated by capillary zone electrophoresis (CZE) and by micellar electrokinetic chromatography (MEKC). The pH of the CZE buffer was varied, but the two stereoisomers could not be separated. Moreover, CZE is not suitable for neutral compounds. In MEKC, two different types of surfactants, sodium dodecyl sulphate (SDS) and cetyltrimethylammonium bromide (CTAB), have been used and the effect of type and concentration modifier on the separation and the elution window was studied. In the SDS system, both the resolution and the elution window could be increased considerably by the addition of modifier. The use of two MEKC systems of different selectivity seems to be a combination with high potential for the impurity profiling of drugs.     

Micellar Solubilization of Timobesone Acetate in Aqueous and Aqueous Propylene Glycol Solutions of Nonionic Surfactants

The micellar solubilization of timobesone acetate, a novel topical corticosteroid, was studied in aqueous and aqueous propylene glycol solutions of 1 to 5% nonionic surfactants at 25°C. The surfactants used were polyoxyethylene (POE) sorbitan monofatty acid esters (polysorbates), fatty acid esters (Myrj), and fatty alcohol ethers (Brij), as well as sucrose monolaurate (Crodesta SL40). The increase in the solubility of timobesone acetate in the micellar solutions was dependent on the type and concentration of surfactant. The solubilizing capacity of the surfactant micelles and the distribution coefficient of timobesone acetate in aqueous micellar solutions were found (1) to increase with increasing length of the hydrophobic fatty acid group; (2) to increase according to the structure of the hydrophilic group in the order of POE sorbitan ester, sucrose ester, POE ester, and POE ether; (3) to be unaffected by the increase in POE chain length; and (4) to tend to decrease in surfactant containing unsaturated fatty acid groups. In aqueous propylene glycol solution, the solubilizing capacity increased slightly, i.e., up to 1.5-fold in 50% propylene glycol solution, for the ester-type surfactants (polysorbates and Myrj). But this increase was not observed in the ether-type surfactant (Brij) solution. The distribution coefficient decreased logarithmically with increasing concentrations of propylene glycol in the solution. This was caused by the logarithmic increase in the timobesone acetate solubility in the bulk phase, while the solubility in the micellar phase was practically unchanged. The results support the equilibrium distribution model of micellar solubilization.     

胶束薄层色谱分离甘草活性成分的影响因素及优化方法

目的:为甘草的胶束薄层色谱指纹图谱寻找最优胶束流动相。方法:首先采用单因子法,寻找影响甘草胶束薄层色谱的影响因素,在此基础上,采用控制加权可变步长单纯形优化法进行甘草胶束薄层色谱指纹图谱的流动相优化。结果:对甘草的胶束薄层色谱分离条件(表面活性剂的种类和含量、醇和酸改性剂的影响等)进行了实验,表明纯胶束薄层色谱的柱效较低,加入醇和酸类改性剂后柱效有明显提高。通过对改性胶束的进一步优化(控制加权可变步长单纯形优化法),得到优化的甘草改性胶束展开剂组成为:0.23 mol.L-1的SDS+16%(v/v)正丁醇+11%(v/v)甲酸。本研究对胶束薄层色谱的一些分离机理亦进行了探讨。结论:胶束薄层色谱的表面活性剂和各添加剂间存在交互作用,需采用合适的优化方法,才能达到分离中药材复杂活性成分群的目的。     

The relative partitioning of neutral and ionised compounds in sodium dodecyl sulfate micelles measured by micellar electrokinetic capillary chromatography.

The rational use of micelles in quantitative structure-activity and quantitative structure-permeation relationships implies a good knowledge of the nature of recognition forces underlying solute-micelle association. The aims of this study were to unravel the intermolecular interaction forces responsible for the association of neutral and ionised compounds with negatively charged sodium dodecyl sulfate (SDS) micelles, using micellar electrokinetic capillary chromatography (MEKC). The MEKC capacity factors (log k(MEKC)) of 36 neutral model solutes were analysed by linear solvation free-energy relationships (LSERs). The results indicate that the size and H-bond acceptor strength of solutes are mainly responsible for their MEKC retention. Compared to n-octanol, the SDS micelles are more cohesive and stronger H-bond donors. Strong attractive electrostatic interactions govern solute-micelle association for positively charged compounds and micelles of the opposite charge, whereas repulsive electrostatic interactions occur between negatively charged solutes and micelles of the same charge. The capacity factors measured for the ionised forms of the acids and bases under study (log k(MEKC)(I)) indeed lie on two distinct plateau, about -1.0 for the former and about 2.0 for the latter and depend on the solute's charge more than on its chemical structure. Thus, the derivation of a diff(log k(MEKC)(N-I)) value, defined as the difference between the log k(MEKC) values of the neutral and charged species, strongly correlates with the respective log k(MEKC)(N) value and does not afford additional information.     

Behavior of itraconazole and benzyl alcohol in aqueous solution containing nonionic surfactants

In order to investigate the behavior of itraconazole and benzyl alcohol in aqueous solution containing surfactants, the distribution of itraconazole and benzyl alcohol between the micellar and aqueous phases was determined and the partition of itraconazole between the hydrophilic and lipophilic moieties in micelles was measured. From these experiments, we can conclude that: (1) in aqueous surfactant solution, itraconazole mainly exists in the micellar phase; (2) the cosolvency effect of benzyl alcohol has a negligible effect on the solubility of itraconazole in aqueous solution; (3) itraconazole tends to align itself in an intermediate position (palisade layer) within the surfactant molecules forming the micelle, which may result in the destruction of the micellar structure; and (4) the precipitation of itraconazole may occur in the process of the exchange of benzyl alcohol between the aqueous and micellar phases. This is the mechanism of destabilization of colloidal drug carriers based on benzyl alcohol.     

Sweeping under controlled electroosmotic flow and micellar electrokinetic chromatography for on-line concentration and determination of trace phlorizin and quercitrin in urine samples

A novel sweeping under controlled electroosmotic flow scheme was developed for preconcentration and determination of neutral compounds by micellar electrokinetic chromatography (MEKC). An anionic surfactant, sodium dodecyl sulfate (SDS), was added into the buffer for sweeping and separation. By controlled electroosmotic flow (EOF) equal to the counter electrophoretic flow, the surfactants were at an immobile state in capillary. The neutral analytes with sample solution was injected electroosmotically into capillary and swept by SDS micelle for essentially an unlimited volume. The injected sample plug lengths for phlorizin and quercitrin under 18 kV for 70 min were experimentally estimated as 1532 cm, corresponding to 51-fold the effective capillary length. The sweeping under controlled EOF scheme resulted in increased detection factors for phlorizin and quercitrin of 2.3 × 10⁴ and 2.1 × 10⁴ using 70 min injection relative to a traditional pressure injection. The proposed method has been adopted to analyze trace phlorizin and quercitrin in urine samples successfully.     

Comparison between micellar electrokinetic chromatography and HPLC for the determination of Betamethasone Dipropionate, Clotrimazole and their related substances

The complete separation of a composite mixture that consisted of Betamethasone Dipropionate (BMD), Clotrimazole and their derivatives in a pharmaceutical dosage form was achieved within 15 min using sodium dodecyl sulfate (SDS) micellar electrokinetic chromatography (MEKC). For the MEKC separations, electrophoretic media consisting of SDS-phosphate buffer and various concentrations of alcohols or acetonitrile were used. The optimal condition for separating BMD, Clotrimazole and their analogues was found to be 50 mM SDS–15% acetonitrile–5% butanol at pH 7.2. The results demonstrated that the method was valid for the quantitation of BMD, Clotrimazole and analogues with selectivity and precision comparable to that of High-Performance Liquid Chromatography (HPLC).     

Micellar electrokinetic chromatography for the quantitative analysis of flavonoids in the radix of Astragalus membranaceus var. mongholicus

The separation and determination of eight flavonoids in the radix of Astragalus membranaceus (Fisch.) Bge var. mongholicus (Bge.) Hsiao was achieved by using micellar electrokinetic chromatography (MEKC) with a diode array detector (DAD). The influence of background electrolyte (BGE) concentrations and pH, surfactant concentrations, organic solvents, temperature, and voltage on the separate procedure was systematically investigated. The optimal resolution was obtained with a micellar phase consisting of 100 mM sodium cholate, 25 % (v/v) acetonitrile, 20 mM Na2B4O7, and 20 mM H3BO3 buffer at pH 9.2 by using a fused silica capillary at + 25 kV and 25 degrees C. A high reproducibility and good linearity ( R2 > 0.9978) were obtained over the concentration range tested. The relative standard deviations (R.S.D.s) from intra-day and inter-day precision for injection were less than 4.42 %. Six plant samples from different locations were then analyzed for the flavonoids by this newly developed MEKC method.     

Drug-surfactant interactions: effect on transport properties

The physico-chemical interactions between three model drugs and a variety of surfactants were characterized by measuring the apparent permeability coefficients of the drugs in the presence and absence of surfactants in vitro. The extent of interaction between the model drugs and the surfactants can best be described by the hydrophobic effect (primarily determined by the hydrophobic surface area of the drug molecule) and the electrostatic effect (primarily determined by the charge associated with the drug molecule as well as the surfactant molecules). For drugs that do not possess a significant hydrophobic surface area (timolol and cefoxitin), their interactions can best be described based on electrostatic effects (charge effects). This interaction being strong with oppositely charged surfactants. The interactions of L-692 585 (a model drug with appreciable hydrophobic surface area) in the presence of surfactants is dominated by the hydrophobic effect, with the electrostatic effect playing a minor secondary role. The apparent permeability coefficient of timolol as a function of the amount of surfactant in solution is modelled in light of micellar formation and entrapment and/or interaction of free drug with this micellar structure. Briefly, the extent of interaction as a function of amount of added surfactant for timolol indicates that initially as surfactant is added the activity of drug for transport declines significantly until a breaking point is reached, after which the drug activity available for transport remains relatively constant upon addition of more surfactant. A model is derived which is capable of describing this behavior and provides reasonable estimates for the critical micellar concentration of the surfactant, the affinity or binding constant for the interaction of drug with an equivalent micellar structure, and the loading capacity of the equivalent micellar structure. These observations are potentially significant for drug formulation of poorly bioavailable drugs.     

Determination of patulin in commercial apple juice by micellar electrokinetic chromatography.

A novel and validated micellar electrokinetic capillary chromatography (MEKC) method using ultraviolet detection (UV) has been applied to the quantitative analysis of patulin (PAT) in commercial apple juice. Patulin was extracted from samples with an ethylacetate solution. The micellar electrokinetic capillary chromatography (MECK) parameters studied for method optimization were buffer composition, voltage, temperature, and a separation between PAT and 5-hydroxymethylfurfural (HMF) (main interference in apple juice PAT analysis) peaks until reaching baseline. The method passes a series of validation tests including selectivity, linearity, limit of detection and quantification (0.7 and 2.5 microgL(-1), respectively), precision (within and between-day variability) and recovery (80.2% RSD=4%), accuracy, and robustness. This method was successfully applied to the measurement of 20 apple juice samples obtained from different supermarkets. One hundred percent of the samples were contaminated with a level greater than the limit of detection, with mean and median values of 41.3 and 35.7 microgL(-1), respectively.     

Study of the solubilization of gliclazide by aqueous micellar solutions

It was of interest to increase the solubility of gliclazide in aqueous media. Therefore, solubilization of gliclazide in a variety of surfactants was investigated. Anionic and cationic surfactants exhibited dramatic solubilizing ability for gliclazide, whereas nonionic surfactants showed significantly lower solubilizing ability. It was found that gliclazide solubility increases with increasing the carbon chain length of cationic surfactants and decreases with increasing the carbon chain length of anionic surfactants. The solubilization data were analyzed on the basis of a pseudo-phase model with gliclazide exhibiting moderate partition coefficients into the micellar phase. The possible sites of solubilization of gliclazide in the micelle were examined by studying the effect of NaCl on solubilization and by comparing the absorption spectra of gliclazide in different solvents. The results obtained from these two experiments indicated that gliclazide is solubilized mainly in the inner core of the cationic surfactant micelles and in the outer regions of the anionic surfactant micelles.     

Effects of surfactants on gel behavior

The recent advances in the design and synthesis of new polymeric systems have opened a wide range of possibilities for the use of gels as drug dosage forms. These systems are able to act not only as simple reservoirs, but also as controlled-release systems and/or targeting agents for site-specific delivery. Applications of gels are determined by their drug-loading capability, rheological properties, and the mechanisms and kinetics of drug release. The incorporation of small proportions of surfactants, which are able to promote or hinder intra- or interchain polymeric bonds, may modify these properties and be a useful tool for developing gel-based dosage forms. Polymers and surfactants may interact, depending on their chemical structure, through ionic, hydrophobic and hydrogen-bonding mechanisms. The strength of the interaction is strongly affected by pH, ionic strength and presence of other substances, and physiological changes in these variables. These physiologic parameters affect the performance of polymer/surfactant gels as drug delivery systems. Surfactants may also promote cutaneous or mucosal drug penetration. Drug release processes from polymer/surfactant gels depend on the microstructure of the gel and the drug ‘state’ in the system. The drug can be: solubilized in water without interacting with any of the components; electrostatically or hydrophobically bound to the polymer; or solubilized inside micelles or polymer/surfactant aggregates. Interactions between surface-active drugs and polymers should also be taken into account because of their important repercussions on gel behavior. Finally, polymer/surfactant systems have a great potential for gene therapy, and some polymers that are able to interact with natural surfactants can be used as trap systems of bile salts for controlling the physiological levels of cholesterol.     

Rapid separation and determination of resibufogenin and cinobufagin in toad venom and Liushen tablet by beta-cyclodextrin modified micellar electrokinetic chromatography

A rapid cyclodextrin modified micellar electrokinetic chromatography (CD-MEKC) method was proposed for the determination of resibufogenin and cinobufagin in the Chinese herbal extracts from toad venom and its medicinal preparation (Liushen tablet). The two components have the close structural similarity and similar hydrophobicity, which result in poor resolution in normal MEKC. The addition of neutral beta-CD to the MEKC system was found to improve the separation of the studied compounds. The effects of several CD-MEKC parameters on the resolutions were evaluated systematically. Based on the investigation, a background electrolyte solution consisting of 10 mM borate buffer adjusted to pH 8.5, 40 mM sodium dodecyl sulfate (SDS), 12 mM beta-CD and 10% (v/v) of methanol was found to be optimal conditions for the fast separation. The contents of resibufogenin and cinobufagin were successfully determined within 5 min, with satisfactory repeatability and recovery.     

Determination of recombinant human epidermal growth factor (rhEGF) in a pharmaceutical preparation by capillary electrophoresis

A simple assay method of recombinant human epidermal growth factor (rhEGF) in a pharmaceutical preparation was studied and validated by capillary electrophoresis (CE) using micellar electrokinetic chromatography (MEKC) techniques. Factors affecting the migration behavior and separation performances of the peptide; type of buffer, pH, buffer concentration, and concentration of sodium dodecyl sulfates (SDS) were investigated to optimize the analytical performance. CE was performed using running buffer, 50.0 mM borate (pH 8.5) containing 12.5 mM SDS at 20 kV of the applied voltage. Calibration curves for the rhEGF showed good linearity (r>0.999) over the wide dynamic range from 1.25 to 100 microg/ml. Sample analysis was performed by using standard addition method to eliminate the matrix effects of dosage vehicle. This method is assumed to be useful for quality control (QC) of various forms of pharmaceutical products of the peptide.     

Effect of structural variations of non-ionic surfactants on micellar properties and solubilization: surfactants with semi-polar hydrophobes

Novel non-ionic surfactants have been synthesized in which a polar group (either an ether or a keto group) has been introduced into the hydrocarbon chain of an octadecylpolyoxyethylene glycol monoether (C18En) with an oxyethylene chain length, n, of 17-18 units. Light scattering studies have indicated aggregation numbers for these semi-polar surfactants in aqueous solution of between 55-65% of that of an unsubstituted octadecylpolyoxyethylene glycol monoether, C18E22. The solubilizing capacities of the semi-polar surfactant micelles for test compounds which were mainly solubilized at the polyoxyethylene/core interface were lower than those of C18E22 whilst solubilizates which exhibited a reasonable degree of solubility in both the interface and the micellar core showed an increased solubilization.     

On-line preconcentration and enantioselective separation of triadimenol by electrokinetic chromatography using cyclodextrins as chiral selectors

Enantioselective separation of triadimenol, a component of systemic agricultural fungicide, by electrokinetic chromatography (EKC) using cyclodextrins (CDs) as chiral selectors was investigated. Both a neutral CD derivative, hydroxypropyl-γ-CD (HP-γ-CD), and an ionic one, heptakis-6-sulfato-β-CD (HS-β-CD), were employed as an additive in cyclodextrin–modified micellar electrokinetic chromatography (CD–MEKC) and as a chiral pseudostationary phase in CDEKC, respectively. In each system, four stereoisomeric peaks were completely or partially separated from each other. To enhance the detectability or the concentration sensitivity, on-line preconcentration techniques were applied to both EKC systems. Sweeping was used in the CD–MEKC system under an acidic condition, whereas stacking with a reverse migrating pseudostationary phase (SRMP) in the CDEKC system. Around 10-fold increase in the detection sensitivity for each peak was attained with both sweeping and SRMP systems. Good repeatabilities in the migration time, corrected peak area, and peak height were recognized in terms of the relative standard deviation. The limit of detection for each peak in the SRMP-CDEKC system, calculated from the calibration curve, was found to be 0.8–3.8 ppm.     

SDS-aided immobilization and controlled release of camptothecin from agarose hydrogel.

Camptothecin (CPT), known to be an effective anticancer drug, has a limited therapeutic utility because of its poor water solubility. In this work, an approach has been made to overcome the limitation. CPT was first incorporated into the micelles formed from an ionic surfactant, sodium dodecyl sulfate (SDS) and the micellar drug aqueous solution was then used in preparation of the agarose hydrogel. It has been found that the presence of SDS greatly increased the solubility of CPT in water. For example, in 1 ml of 1.0 wt.% SDS water solution, 0.11 mg CPT could be solubilized (0.318 mM), which was 83 times the solubility in pure water. It was the hydrophobic cores of the SDS micelles that were able to accept the lipophilic drug to form stable drug-immobilized micelles. The formulation of a hydrogel using the drug-immobilized micelles has allowed us to obtain a unique and novel drug release system where the drug molecules are encapsulated by the micelles and the drug-containing micelles are dispersed in the gel network. The release of CPT from the so deliberately fabricated agarose hydrogel system has been studied as a function of surfactant concentration at 37 degrees C. The diffusion coefficients of CPT obtained by fitting to Fick's law ranged from 2.12 to 7.36 x 10(-7)cm(2)s(-1). The results showed that SDS prolonged the drug release by reducing the diffusion coefficient of CPT in the gel.