Suppression of chiral recognition of 3-hydroxy-1,4-benzodiazepines during micellar electrokinetic capillary chromatography with bile salts

Application of polymeric surfactants in micellar electrokinetic chromatography-electrospray ionization mass spectrometry of benzodiazepines and benzoxazocine chiral drugs

Chiral micellar EKC (CMEKC) coupled to ESI-MS using polymeric surfactants as pseudostationary phases is investigated for simultaneous enantioseparation of two benzodiazepines, (+/-)-oxazepam ((+/-)-OXA) and (+/-)-lorazepam ((+/-)-LOR), and one benzoxazocine, (+/-)-nefopam ((+/-)-NEF). First, enantioselectivity and electrospray sensitivity of six chiral polymeric surfactants for all three chiral compounds are compared. Second, using poly(sodium N-undecenoyl-L-leucinate) as pseudostationary phase, the organic modifiers (methanol (MeOH), isopropanol, and ACN) are added into the running buffer to further improve chiral resolution (RS). Next, a CMEKC-ESI-MS method for the simultaneous enantioseparation of two benzodiazepines is further developed by using a dipeptide polymeric surfactant, poly(sodium N-undecenoxy carbonyl-L,L-leucyl-valinate) (poly-L,L-SUCLV). The CMEKC conditions including nebulizer pressure, capillary length, ammonium acetate concentration, pH, poly-L,L-SUCLV concentration, and capillary temperature were optimized to achieve maximum chiral RS and highest sensitivity of MS detection. The spray chamber parameters (drying gas temperature and drying gas flow rate) as well as sheath liquid conditions (MeOH content, pH, flow rate, and ionic strength) were found to significantly influence MS S/N of both (+/-)-OXA and (+/-)-LOR. Finally, a comparative study between simultaneous UV and MS detection showed high plate numbers, better chiral RS, and enhanced detectability with CMEKC-MS. However, speed of analysis was faster using CMEKC-UV.

Analysis of lorazepam and its 3O-glucuronide in human urine by capillary electrophoresis: Evidence for the formation of two distinct diastereoisomeric glucuronides

Lorazepam (LOR) is a 3-hydroxy-1,4-benzodiazepine that is chiral and undergoes enantiomerization at room temperature. In humans, about 75% of the administered dose of LOR is excreted in the urine as its 30-glucuronide. CE-MS with negative ESI was used to confirm the presence of LOR-30-glucuronide in urines that stemmed from a healthy individual who ingested 1 or 2 mg LOR, whereas free LOR could be detected in extracts prepared from enzymatically hydrolyzed urines. As the 30-glucuronidation reaction occurs at the chiral center of the molecule, two diastereoisomers can theoretically be formed, molecules that can no longer interconvert. The stereoselective formation of LOR glucuronides in humans and in vitro was investigated. MEKC analysis of extracts of the nonhydrolyzed urines suggested the presence of the two different LOR glucuronides in the urine. The formation of the same two diastereoisomers was also observed in vitro employing incubations of LOR with human liver microsomes in the presence of uridine 5'-diphospho-glucuronic acid as coenzyme. The absence of other coenzymes excluded the formation of phase I or other phase II metabolites of LOR. Both results revealed a stereoselectivity, one diastereoisomer being formed in a higher amount than the other. After enzymatic hydrolysis using beta-glucuronidase, these peaks could not be detected any more. Instead, LOR was monitored. Analysis of the extracts prepared from enzymatically hydrolyzed urines by MEKC in the presence of 2-hydroxypropyl-beta-CD revealed the enantiomerization process of LOR (observation of two peaks of equal magnitude connected with a plateau zone). The data presented provide for the first time the evidence of the stereoselectivity of the LOR glucuronidation in humans.

Direct calculation and computer simulation of the enantiomerization barrier of oxazepam in dynamic HPLC experiments--a comparative study

Dynamic chromatographic methods constitute a versatile approach to the rapid and precise determination of enantiomerization barriers of stereolabile drugs. In the present study enantioselective dynamic high-performance liquid chromatography (DHPLC) was employed to determine the enantiomerization barrier of oxazepam. Dynamic elution profiles, exhibiting plateau formation and/or peak broadening between 20 and 60 degrees C at pH 2.6 and pH 8 were obtained in the presence of the chiral stationary phase (CSP) Nucleodex-beta-PM (permethylated beta-cyclodextrin chemically bonded to silica) using a 6:4 mixture of phosphate buffer and methanol as mobile phase. Evaluation of the experimental chromatograms was performed by the novel approximation function (AF) (without computer simulation), and by the stochastic model implemented in the ChromWin simulation software (with computer simulation) furnishing the respective apparent forward rate constants, k(1)(app)(T). From the rate constants, k(1)(app)(T), measured at variable temperatures, the kinetic Eyring activation parameters, deltaG(T)(#), deltaH(#) and deltaS(#), of the enantiomerization of oxazepam were obtained. By variation of the flow rate of the mobile phase, the expected independence of the enantiomerization barrier from the chromatographic time scale was demonstrated for the first time.

Determination of lormetazepam and its main metabolite in serum using micellar electrokinetic capillary chromatography with direct injection and ultraviolet absorbance detection

The use of micellar electrokinetic chromatography for the determination of lormetazepam and its metabolite, lorazepam, in serum samples at a concentration range of therapeutic interest was investigated. The separation was carried out at 30 degrees C and 25 kV, using a 15 mM borate-phosphate buffer (pH 8) with 30 mM sodium dodecyl sulfate as the separation electrolyte and 15% methanol as organic modifier. The analyses were carried out in 20 min under these conditions. Detection limits of 0.5 mg l(-1) were achieved for both benzodiazepines in serum. This method was employed for the quantitative resolution of both drugs (at different concentration ratios) in serum with very good recoveries.

Micellar electrokinetic capillary chromatography determination of +S and -R arotinolol in serum using UV detection and solid phase extraction

A method for the simultaneous determination of +S and -R arotinolol in serum by micellar electrokinetic capillary chromatography is described. Stereoselective resolution of the arotinolol enantiomers was achieved using 5 mM sodium taurocholate in 10 mM sodium dihydrogen phosphate buffer of pH 2.5. A 72-cm uncoated fused-silica capillary at a constant voltage of 15 kV was used for the analysis. The analytes of interest were extracted from serum using solid phase extraction. An octadecyl cartridge gave good recoveries in excess of 87% for both +S and -R arotinolol without any interference. The calibration curves were linear over the range of 50-500 ng ml(-1) with +S propranolol as the internal standard and the coefficient of determination was greater than 0.999 (n = 3). The limit of quantitation was 50 ng ml(-1) for each enantiomer and the detection limit using 1 ml serum and a UV detection set et 220 nm was 25 ng ml(-1) (S/N = 2). Precision and accuracy of the method were in the range 0.8-2.7% and 1.2-6.4%, respectively, for +S arotinolol and 1.1-3.9% and 2.2-6.5%, respectively, for -R arotinolol.

Dynamic micellar electrokinetic chromatography. Determination of the enantiomerization barriers of oxazepam, temazepam, and lorazepam

The temperature-dependent enantiomerization barriers of oxazepam, temazepam, and lorazepam have been determined between 0 and 30 degrees C by dynamic micellar electrokinetic chromatography (DMEKC) in an aqueous 20 mM borate/phosphate buffer system at pH 8 with 60 mM sodium cholate as chiral surfactant. Interconversion profiles featuring plateau formation and peak broadening were observed and simulated by the new program ChromWin based on the theoretical plate as well as on the stochastic model using the experimental data plateau height, hplateau, peak width at half-height, wh, total retention times, tR, and electroosmotic breakthrough time, t0. Peak form analysis yielded rate constants k and kinetic activation parameters, deltaG double dagger, deltaH double dagger, and deltaS double dagger, of the enantiomerization of oxazepam, temazepam, and lorazepam. At 25 degrees C, the enantiomerization barrier, deltaG double dagger, was determined to be approximately 90 kJ mol-1 and the half-lives, tau, were determined to be approximately 21 min. The new approach allows the fast and precise determination of enantiomerization barriers in a biogenic environment and it mimics physiological conditions, as no organic modifiers or abiotic chiral stationary phases (CSP) are employed.

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.

Separation of chiral compounds by capillary electrophoresis

This review presents the different chiral selectors used in capillary electrophoresis (CE) for the separation of enantiomers. The use of charged cyclodextrins, crown ethers, polysaccharides, proteins, natural and synthetic micelles, macrocyclic antibiotics and ergot alkaloids is discussed in detail. Neutral native and derivatized cyclodextrins are not treated because several review articles have already been published on this topic. Recent developments like the application of two chiral selectors in the same background electrolyte are highlighted.

A critical evaluation of the application of capillary electrophoresis to the detection and determination of 1,4-benzodiazepine tranquilizers in formulations and body materials

Studies of the capillary zone electrophoresis (CZE) and micellar electrokinetic capillary chromatography (MEKC) behaviour of 1,4-benzodiazepines have seen application in subject areas such as the development of pharmaceuticals, therapeutic drug monitoring and forensic toxicology. In the development of pharmaceuticals, pKa determinations by CZE can be used in preclinical studies whereas analytical data on the detection and determination of 1,4-benzodiazepines is of value primarily in raw material/formulation assay and in the analysis of body fluids in clinical studies. The capillary electrophoresis (CE) techniques, which generally have inferior limits of detection (LOD) to rival techniques such as gas chromatography (GC) and high-performance liquid chromatography (HPLC), are particularly applicable in forensic toxicology where reasonably high concentrations of these drugs can be encountered. It is anticipated that, with the interfacing of CZE and capillary electrochromatography (CEC) with mass spectrometry (MS) techniques, the excellent selectivity of CZE and particularly CEC will be effectively combined with the sensitivity of MS and the identification capabilities of tandem mass spectrometry (MS/MS) and MS hyphenated (MSn) techniques.

Micellar electrokinetic capillary chromatography of benzodiazepine antiepileptics and their desmethyl metabolites in blood

A fast and reliable method for the MEKC separation and determination of benzodiazepine antipileptics and their active metabolites in serum has been developed, using a separation buffer composed of borate (pH 9.5), SDS (18 mM), and acetonitrile (14%) as an organic modifier. The method is sensitive enough to be used clinically with a precision of less than 3% for the analysis of nitrazepam, clonazepam, clobazam, diazepam and their desmethyl metabolites in serum.

Electrokinetic chromatography employing an anionic and a cationic beta-cyclodextrin derivative

beta-Cyclodextrin, 1, can act as a chiral buffer additive in capillary zone electrophoresis (CZE) owing to its ability to form diastereomeric complexes with the enantiomers of ionic compounds. Reaction of 1 with 2,3-epoxy-propyltrimethylammonium chloride gave preponderantly 6-O-(2-hydroxy-3-trimethylammoniopropyl) derivatives (2) of varying degrees of substitution (d.s.). Analogous reaction of 1 with 1,3-propanesultone yielded 6-O-(sulfo-n-propyl) derivatives (SPE-beta-CD, 3) of varying d.s. The purity and the d.s. of 2 and 3 were determined by 1H NMR and electrospray ionization mass spectrometry (ESI-MS). These charged beta-cyclodextrin derivatives were used for the chromatographic enantiomer separation of a series of neutral barbiturates, of chlorthalidone, terbutaline, warfarin, salbutamol and brompheniramine by cyclodextrin electrokinetic chromatography (CD-EKC). During the separation of chlorthalidone with 3 as chiral additive, a reversible interconversion of the enantiomers (enantiomerization) was observed at temperatures below 20 degrees C.

Recent applications of chromatographic resolution of enantiomers in pharmaceutical analysis

The principles and applications of chromatographic separation of enantiomers in pharmaceutical analysis have been reviewed. Several of recently reported enantioselective analysis of various racemic drugs using both the 'indirect' and 'direct' methods have been presented.

Enantiomer separation of drugs by electrokinetic chromatography

This review surveys the enantiomer separation of drugs by electrokinetic chromatography (EKC). EKC is one option of capillary electrophoretic (CE) techniques, which permits the separation of electrically neutral drugs. In enantiomer separation by EKC, ionic pseudo-stationary phases such as chiral micelles and proteins, which can migrate with its electrophoretic mobility and can interact with the solutes, are employed as chiral selectors. Addition of electrically neutral chiral selectors such as cyclodextrins and precolumn conversion of solutes into diastereomers are also successful in EKC. A brief theory of separation and modes of EKC enantiomer separation are described with some typical applications.