Development of predictive retention-activity models of butyrophenones by biopartitioning micellar chromatography

The predictive and interpretative capability of quantitative chromatographic retention-biological activity models is supported by the fact that in adequate experimental conditions the solute partitioning into the chromatographic system can emulate the solute partitioning into lipid bilayers of biological membranes, which is the basis of drug and metabolite uptake, passive transport across membranes and bioaccumulation. The use of retention data obtained in biopartitioning micellar chromatography (BMC) has been demonstrated to be helpful in describing the biological behaviour of different kinds of drugs. In this chromatographic system, polioxyethylene 23 lauryl ether Brij35 micellar mobile phases and C(18) reversed stationary phase in adequate experimental conditions are used. The RP-HPLC capacity factors of butyrophenones were determined using different Brij35 concentrations as micellar mobile phases. Relationships between seven biological activities of butyrophenones reported in bibliography and retention data were established and their predictive and interpretative ability evaluated. These relationships were significant between preclinical pharmacology and therapeutic efficacy parameters and the retention factors of butyrophenones (0.89 < R(2) < 0.98). The results indicate that the retention of compounds in BMC is capable of describing and predicting in vitro the biological activities of butyrophenones. This approach can be very useful in the development of new neuroleptic drugs, avoiding the use of experimental animals.     

Quantitative structure-retention and retention-activity relationships of beta-blocking agents by micellar liquid chromatography

Sixteen beta-blocking agents (acebutolol, alprenolol, atenolol, bisoprolol, carteolol, celiprolol, esmolol, labetalol, metoprolol, nadolol, oxprenolol, pindolol, practolol, propranolol, sotalol and timolol) showing a large range of hydrophobicity (octanol-water partition coefficients, log P between -0.026 and 2.81) were subjected to micellar liquid chromatography with sodium dodecyl sulfate as micelle forming agent, and n-propanol as organic modifier. The correlation between log P and the retention factor extrapolated to a mobile phase free of micelles and organic modifier was investigated. The use of an interpolated retention factor or the retention factor for specific individual experimental mobile phases was however advantageous since the retention factors of all beta-blocking agents were measurable in the selected mobile phases. Good correlations with log P and with in vitro biological parameters (cellular permeability coefficients in Caco-2 monolayers and apparent permeability coefficients in rat intestinal segments) were found.     

High-performance liquid chromatographic determination of diuretics in urine by micellar liquid chromatography.

The use of micellar liquid chromatography for the determination of diuretics in urine by direct injection of the sample into the chromatographic system is discussed. The retention of the urine matrix at the beginning of the chromatograms was observed for different sodium dodecyl sulphate (SDS) mobile phases. The eluent strengths of a hybrid SDS-methanol micellar mobile phase for several diuretics were compared and related to the stationary phase/water partition coefficient with a purely micellar mobile phase. The urine band was appreciably narrower with a mobile phase of 0.05 M SDS-5% methanol (v/v) at 50 degrees C (pH 6.9). With this mobile phase the determination of bendroflumethiazide and chlorthalidone was adequate. Acetazolamide, ethacrynic acid, furosemide, hydrochlorothiazide and probenecid were overlapped by the urine matrix, and the retention of amiloride and triamterene was too long.     

Determination of phenobarbital in plasma by micellar liquid chromatography

A new liquid chromatographic procedure for the determination of phenobarbital in plasma samples is described. The proposed system uses a Spherisorb octadecyl-silane ODS-2 C(18) analytical column, a guard column of similar characteristics, and a 0.03 M CTAB-3% 1-propanol at pH 7 mobile phase. The UV detector was set at 250 nm. Butabarbital was used as internal standard. Sample preparation only required the addition to the plasma samples of a 0.1 M SDS solution at pH 3 and centrifugation before injection into the chromatographic system. The limit of detection was 0.83 microg/mL of phenobarbital in plasma samples. The coefficients of variation were lower than 7. 5%.     

Bioanalytical capabilities of micellar liquid chromatography

Micellar liquid chromatography (MLC) offers numerous advantages over other high-performance liquid chromatographic methods in bioanalytical applications, allowing improvements in simultaneous separation of ionic and non-ionic compounds, faster analysis, and higher detection sensitivity and selectivity. In addition, MLC can facilitate the estimation of the hydrophobic character of biologically active molecules in structure—activity relationships.     

Determination of sun-screen agents in cosmetic products by micellar liquid chromatography.

A micellar liquid chromatographic method was developed for the quantitative determination of sun-screen agents in cosmetic products. The qualitative analysis of parabens is also feasible. Excellent linearity was obtained (r = 0.999) and recoveries were generally greater than 98%. A variety of commercial formulations were analyzed.     

Retention mechanisms in micellar liquid chromatography

Micellar liquid chromatography (MLC) is a reversed-phase liquid chromatographic (RPLC) mode with mobile phases containing a surfactant (ionic or non-ionic) above its critical micellar concentration (CMC). In these conditions, the stationary phase is modified with an approximately constant amount of surfactant monomers, and the solubilising capability of the mobile phase is altered by the presence of micelles, giving rise to diverse interactions (hydrophobic, ionic and steric) with major implications in retention and selectivity. From its beginnings in 1980, the technique has evolved up to becoming a real alternative in some instances (and a complement in others) to classical RPLC with hydro-organic mixtures, owing to its peculiar features and unique advantages. This review is aimed to describe the retention mechanisms (i.e. solute interactions with both stationary and mobile phases) in an MLC system, revealed in diverse reports where the retention behaviour of solutes of different nature (ionic or neutral exhibiting a wide range of polarities) has been studied in a variety of conditions (with ionic and non-ionic surfactants, added salt and organic solvent, and varying pH). The theory is supported by several mechanistic models that describe satisfactorily the retention behaviour, and allow the measurement of the strength of solute-stationary phase and solute-micelle interactions. Suppression of silanol activity, steric effects in the packing pores, anti-binding behaviour, retention of ionisable compounds, compensating effect on polarity differences among solutes, and the contribution of the solvation parameter model to elucidate the interactions in MLC, are commented.     

Retention modeling in micellar liquid chromatography

The aim of this review is to present the most relevant work on retention modeling in micellar liquid chromatography. First, physico-chemical models explaining the variation of capacity factors with one or more experimental variables (such as micellar concentration, organic modifier concentration, and pH) will be shown. Secondly, studies carried out to model the solute retention in micellar liquid chromatography by means of empirical equations will be presented, and finally new trends in this area will be introduced.     

Determination of disulfiram by micellar liquid chromatography in illicit preparations

Presently, disulfiram is used in aversion therapy for recovering alcoholics. It acts by inhibiting aldehyde dehydrogenase, leading to high blood levels of acetaldehyde. A simple direct injection micellar liquid chromatographic procedure was developed to determine disulfiram in illicit preparations (ayurvedic, herbal, divine ash, and traditional medicine), as well as in pharmaceuticals and biological samples (urine). After application of a predictive optimization strategy, the proposed method was developed using a 0.1 M sodium dodecyl sulfate-butanol 4% (v/v) buffered to pH 7 as the mobile phase at a flow rate of 1 mL/min, an octyl silyl (C8) 150 mm column, and diode array detection at 248 nm. Under the above conditions, the analysis time was below 8 min. Validation studies were based on U.S. Food and Drug Administration guidelines. The LOD (3 x SD criterion) was 15 ng/mL and LOQ (10 x SD criterion) was 70 ng/mL for disulfiram. The intraday and interday precisions were below 3.5%, recoveries were in the range of 97-102%, and robustness was below 3%. The optimized and validated micellar liquid chromatographic method was successfully applied to the determination of disulfiram in ayurvedic, herbal, divine ash, and other samples. The procedure developed could also be used in the fields of QC, routine analysis, and pharmacokinetic studies.     

Direct serum injection in micellar liquid chromatography. Recovery of serum proteins and assay of hydrophilic drugs

The recovery of serum proteins from reversed-phase and internal-surface reversed-phase (ISRP) silica supports following direct serum injection was investigated using an eluent containing a micellar solution of sodium dodecyl sulphate (SDS). The results indicated that the recoveries of serum proteins were 98-103% for both supports. On the basis of the above findings, the separation and recovery of hydrophilic drugs (cephalosporins and salicylic acid) from human serum were investigated using acidic eluents including micellar solutions of SDS. They were completely separated from the components of serum, and the recoveries were 94-98% despite protein binding. Although the recommended eluent pH range is 6.0-7.5 for the ISRP support, eluents of pH 2-8 can be used with the micellar chromatographic system.     

Separation and determination of nitrobenzenes by micellar electrokinetic chromatography and high-performance liquid chromatography

A micellar electrokinetic chromatographic method and a high-performance liquid chromatographic method are proposed for the separation and determination of a mixture of 12 nitrobenzenes and their reduction products, namely 4-nitro-1,2-phenylenediamine, 4-nitro-1,3-phenylenediamine, 2-nitro-1,4-phenylenediamine, 2-nitroaniline, 3-nitroaniline, 4-nitroaniline, 4-amino-2-nitrophenol, 2-amino-5-nitrophenol, 2-amino-4-nitrophenol, 2-nitrophenol, 3-nitrophenol, and 4-nitrophenol. A solution of 50 mM sodium dodecyl sulfate and 10% ethanol in 23 mM sodium borate buffer was used as the electrophoretic medium. Good resolution could be obtained by the addition of tetrahydrofuran to the liquid chromatographic mobile phase. The retention and migration behavior of the nitrobenzenes are discussed.     

Characterisation of retention in micellar high-performance liquid chromatography, in micellar electrokinetic chromatography and in micellar electrokinetic chromatography with reduced flow

The retention (migration) behaviour of various barbiturates, phenylurea and triazine herbicides in micellar electrokinetic chromatography (MEKC) with uncoated fused-silica capillaries was compared with the behaviour in micellar electrokinetic chromatography with reduced electroosmotic flow (RF-MEKC) using capillaries modified with linear polyacrylamide. The error in the values of the retention factors caused by the neglection of the contribution of the electroosmotic flow in RF-MEKC was investigated and a method for correcting this error was suggested. The retention was characterised using the lipophilic and polar indices to characterise and to predict the retention as a function of the concentration of the surfactant (sodium dodecylsulphate) in the running buffer in MEKC and in RF-MEKC. Homologous series of n-alkylbenzenes and of n-alkan-2-ones were compared as the standard sets for the calibration of the retention (migration) index scale. The values of the lipophilic indices of a given solute measured in reversed-phase HPLC, MEKC and RF-MEKC are close to each other. Under ideal MEKC conditions, the values of the polarity indices are close to one for various sample solutes. However, for partially ionised compounds such as weakly acidic barbiturates, where the contribution of the electrophoretic migration is significant, the values of the polarity indices are significantly lower than one. Optimum conditions for separations of mixtures of triazine and phenylurea herbicides and of barbiturates using various techniques tested were compared.     

Enantiomer separation of drugs by micellar electrokinetic chromatography using chiral surfactants

A review surveying enantiomer separations by micellar electrokinetic chromatography (MEKC) using chiral surfactants is described. MEKC is one of the most popular techniques in capillary electrophoresis, where neutral compounds can be analyzed as well as charged ones, and the use of chiral micelles enable one to achieve the enantioseparation. The chiral MEKC systems are briefly reviewed according to the types of chiral surfactants along with typical applications. As chiral micelles or pseudostationary phases in MEKC, various natural and synthetic chiral surfactants are used, including several low-molecular-mass surfactants and polymerized surfactants or high-molecular-mass surfactants. Cyclodextrin modified MEKC using chiral micelles is also considered.     

Analysis of immunosuppressive drugs and their pharmaceuticals by micellar electrokinetic chromatography

Summary A simple and practical micellar electrokinetic capillary chromatography (MEKC) method is proposed for the quantitation of immunosuppressive drugs such as azathioprine (AZA), mycophenolate mofetil (MMF), cyclosporine A (CyA) and tacrolimus (FK 506). The electrophoretic separation of the analytes was performed with a background electrolyte containing 20 mM phosphate buffer at pH 7.5, 50 mM SDS and methanol as an organic modifier. Fused silica capillaries 75 μm i.d. and 60 cm in length were employed and detection of analytes was performed at 214 nm. Thorough validation according to international guidelines showed that the proposed method is reliable and appropiate for the routine analysis of immunosuppressants. Moreover, it may be an advantageous alternative to the traditional chromatographic methodologies currently employed in the pharmaceutical and bioanalysis fields.     

Determination of active ingredients in cough-cold preparations by micellar liquid chromatography

The chromatographic behaviour of some active ingredients in cough-cold pharmaceutical preparations, the antihistamine chlorpheniramine (or the dextro enantiomer dexchlorpheniramine), and the phenethylamines phenylephrine, phenylpropanolamine and pseudoephedrine, has been studied using a C(18) column, micellar mobile phases of sodium dodecyl sulphate (SDS) and pentanol, and with UV detection. All possible combinations of chlorpheniramine/phenethylamine were resolved and determined using a mobile phase of 0.15 M SDS-6% (v/v) pentanol at pH 7, with analysis time below 7 min. Repeatabilities and within laboratory precisions were evaluated at four different drug concentrations in the range 0.5-25 mug ml(-1) (n=5), resulting RSDs below 1.6%. The drug amounts found in the analysis of 14 commercialised preparations agreed with those declared by the manufacturers within the tolerance limits, and with those obtained using an aqueous 60% (v/v) methanol reference mobile phase. No interference was observed from other accompanying drugs such as acetylsalicylic acid, ascorbic acid, betamethasone, caffeine, codeine phosphate, diphenhydramine, lactose, paracetamol, and prednisolone. The studied combinations required a rather high amount of methanol in conventional RPLC to be eluted from the column. In contrast, the proposed procedure used a much lower amount of organic solvent (pentanol), which is highly retained in the SDS solution, being also less toxic than methanol.     

Determination of some banned stimulants in sports by micellar liquid chromatography

Summary A procedure has been developed for the determination, in <12 min, of several stimulants (amphetamine, ephedrine, methoxyphenamine, phenylephrine and phenylpropanolamine) in spiked urine samples after direct injection, using a hybrid micellar mobile phase of 0.15 M sodium dodecyl sulfate and 3% pentanol at pH 7, on a C₁₈ column with UV detection. Recoveries were 94–102% and limits of detection 4.5 ng·mL⁻¹ for methoxyphenamine and 0.39 μg·mL⁻¹ for amphetamine, similar to those obtained for aqueous solutions. Linearity reached 0.99 and intermediate precision was <8.4 and 5.3, for the two different concentrations tested.     

Evaluation of distribution coefficients in micellar liquid chromatography

The possibilities of micellar liquid chromatography for evaluating distribution coefficients are discussed. Determination of solute-micelle association constants and distribution coefficients of solutes between stationary-aqueous, stationary-micellar and aqueous-micellar phases is described. Application of the calculation of distribution coefficients to the study of the retention mechanism of solutes in the chromatographic system and prediction of separation selectivity is also presented.     

Determination of antitubercular drugs by micellar electrokinetic capillary chromatography (MEKC)

A method for the determination of isoniazid (ISO), pyrazinamide (PYR) and rifampicin (RIF) in pharmaceutical products, by micellar electrokinetic capillary chromatography (MEKC) with ultraviolet detection is described. The influence of pH, concentration of surfactants, buffer and organic solvents, over the separation were studied as experimental variables. The optimal separation was carried out at 30 degrees C and 20 kV, using a 40 mM borate buffer and 100 mM sodium dodecylsulphate (SDS) adjusted to pH 8.5. Under these conditions, the analysis is accomplished in about 8 min. The method was applied to the determination of these compounds in different pharmaceuticals with good results when compared with a reference liquid chromatographic (LC) method.     

Tamoxifen monitoring studies in breast cancer patients by micellar liquid chromatography

A simple micellar liquid chromatographic procedure is described to determine tamoxifen in plasma. To perform the analysis, tamoxifen solutions were diluted in water and UV-irradiated for 20 min to form the photocycled derivative with a phenanthrene core which shows intense fluorescence. Samples were then directly injected, thus avoiding long extraction and experimental procedures. The resolution from the matrix was performed with a mobile phase containing 0.15 M SDS–7% n-butanol at pH 3 running at 1.5 mL/min through a C18 column at 40 °C. Detection was carried out by fluorescence, and the excitation and emission wavelengths were 260 and 380 nm, respectively. The chromatographic analysis time was less than 15 min. The analytical methodology was validated following the International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH) guidelines. The response of the drug in plasma was linear and in the 0.5–15 μg/mL range, with r ² > 0.999. Accuracy and precision were <9% in both cases. The limits of detection and quantification (in nanograms per millilitre) were 50 and 150 in plasma, respectively. The method developed herein shows no interferences by endogenous compounds. Finally, the analytical method was used to determine the amount of tamoxifen in the plasma of several breast cancer patients from a local hospital.