Screening approach for chiral separation of pharmaceuticals. Part I. Normal-phase liquid chromatography

Chiral recognition by enantioselective liquid chromatography: mechanisms and modern chiral stationary phases

An overview of the state-of-the-art in LC enantiomer separation is presented. This tutorial review is mainly focused on mechanisms of chiral recognition and enantiomer distinction of popular chiral selectors and corresponding chiral stationary phases including discussions of thermodynamics, additivity principle of binding increments, site-selective thermodynamics, extrathermodynamic approaches, methods employed for the investigation of dominating intermolecular interactions and complex structures such as spectroscopic methods (IR, NMR), X-ray diffraction and computational methods. Modern chiral stationary phases are discussed with particular focus on those that are commercially available and broadly used. It is attempted to provide the reader with vivid images of molecular recognition mechanisms of selected chiral selector-selectand pairs on basis of solid-state X-ray crystal structures and simulated computer models, respectively. Such snapshot images illustrated in this communication unfortunately cannot account for the molecular dynamics of the real world, but are supposed to be helpful for the understanding. The exploding number of papers about applications of various chiral stationary phases in numerous fields of enantiomer separations is not covered systematically.

Optical isomer separation of flavanones and flavanone glycosides by nano-liquid chromatography using a phenyl-carbamate-propyl-beta-cyclodextrin chiral stationary phase

In this paper a phenyl-carbamate-propyl-beta-cyclodextrin stationary phase was employed for the enantioseparation of several flavonoids, including flavanones and methoxyflavanones by using nano-liquid chromatography (nano-LC). The same stationary phase was also used for the diastereoisomeric separation of two flavanone glycosides. The compounds: flavanone, 2'-hydroxyflavanone, 4'-hydroxyflavanone, 6-hydroxyflavanone, 7-hydroxyflavanone, 4'-methoxyflavanone, 6-methoxyflavanone, 7-methoxyflavanone, hesperetin, hesperidin, naringenin and naringin were studied using reversed, polar organic and normal elution modes. The effect of the nature and composition of the mobile phase (organic modifier type, buffer and water content in the reversed phase mode) on the enantioresolution (R(s)), retention factor (k) and enantioselectivity (alpha) were investigated. Baseline resolution of all studied flavonoids, with the exception of 2'-hydroxyflavanone and naringin, was achieved in reversed phase mode using a mixture of MeOH/H(2)O at different ratios as mobile phase. Good results, in terms of peak efficiency and short analysis time, were obtained adding 1% triethylammonium acetate pH 4.5 buffer to MeOH/H(2)O mixture. The separation of the studied compounds was also performed in polar organic mode. By using 100% of MeOH as mobile phase, the resolution was achieved for the studied analytes, except for 7-hydroxyflavanone, 2'-hydroxyflavanone, naringenin, hesperidin and naringin. Normal mode was tested employing a mixture of EtOH/hexane/TFA as mobile phase achieving the enantiomeric and diastereomeric separation of only hesperetin and hesperidin, respectively. The use of nano-LC technique for the resolution of flavanones optical isomers allowed to achieve good resolutions in shorter analysis time compared to the results reported in literature with conventional HPLC.

Preparation and enantioseparation characteristics of three chiral stationary phases based on modified beta-cyclodextrin for liquid chromatography

In order to study the effect of the nature and the length of the spacer, three mixed 10-undecenoate/phenylcarbamate derivatives of beta-cyclodextrin have been prepared and linked to allylsilica gel by means of a radical reaction. The chiral recognition ability of the resulting materials, when used as liquid chromatography chiral stationary phases (CSPs), was evaluated using heptane and either 2-propanol or chloroform as organic mobile-phase modifiers. A large variety of racemic compounds have been separated successfully on these CSPs (mainly pharmaceuticals and herbicides). Optimization of these separations was discussed in terms of mobile-phase composition and structural patterns of the injected analytes. The efficiencies of the three prepared materials were compared to those of previously described perphenylated-beta-cyclodextrin column and to analogous cellulose derivative-based CSPs.

Enantioresolution of basic pharmaceuticals using cellulose tris(4-chloro-3-methylphenylcarbamate) as chiral stationary phase and polar organic mobile phases

A polysaccharide-based chiral stationary phase (Sepapak-4), with cellulose tris(4-chloro-3-methylphenylcarbamate) as chiral selector, has been investigated in liquid chromatography (LC). Its enantioresolution power was evaluated towards 13 basic amino-drugs with widely different structures and polarities, using polar organic mobile phases. After preliminary experiments, acetonitrile was selected as the main mobile phase component, to which a low concentration of diethylamine (0.1%) was systematically added in order to obtain efficient and symmetrical peaks. Different organic solvents were first added in small proportions (5-10%) to acetonitrile to modulate analyte retention. Polar organic modifiers were found to decrease retention and enantioresolution while hexane had the opposite effect, indicating normal-phase behaviour under these conditions. The addition of an organic acid (formic, acetic or trifluoroacetic acid) was found to strongly influence the retention of the basic amino drugs in these nonaqueous systems. The nature and proportion of the acidic additive in the mobile phase had also deep impact on enantioresolution. Therefore, the studied compounds could be subdivided in three groups in respect to the acidic additive used. All analytes could be enantioseparated in relatively short analysis times (10-20 min) using these LC conditions.

[Separation of chiral pharmaceutical drugs by chromatographic and electrophoretic techniques]

SUMMARY

A large number of pharmaceutical drugs possess one or more centers of asymmetry giving rise to enantiomers whose pharmacological properties and toxicity are often different. At successive stages of drug discovery, the enantiomers of any chiral molecule must be isolated and analyzed and their enantiomeric purity determined. The electrophoretic and chromatographic techniques have become the most important tools to routinely determine the enantiomeric purity of pharmaceutical molecules. Liquid chromatography (LC) is the most widely used because of the large number of columns marketed, the variety of selectivities available and the ease at which analytical results can be scaled up to the preparative level. In particular, more than 80% of enantioseparations of pharmaceutical molecules are successful with polysaccharide-derivative stationary phases (cellulose, amylose) for multiple system solvents (normal phase, polar organic phase or reverse phase). Complementary selectivities can be achieved more rapidly with other types of stationary phase (glycopeptides, Pirkle, cyclodextrins) but their application is hindered by problems of stability (proteins) or transfer to the preparative scale (cyclodextrins). At the present time, glycopeptide phases offer very promising prospects for the separation of amino acids (and derivatives) and peptide enantiomers. In addition, because of its faster analysis and environmental benefits, supercritical chromatography (SFC) has given rise to renewed interest. Capillary electrophoresis (CE) is an orthogonal technique complementary to chromatographic methods. Its principle involves the formation of diastereoisomer complexes after addition of anionic (HS-beta-CD, HS-gamma-CD CM-beta-CD) or neutral (TM-beta -CD, HP-beta-CD, DM-beta-CD, HP-gamma-CD) cyclodextrins to the running buffer. Compared to LC, CE analyses are cheaper (no chiral column, no solvent, low consumption of chiral selector) and peak efficiencies are higher by one order of magnitude. Furthermore, the mechanism of separation in CE is much simpler to understand and predict. However, the low capacity of CD column prevents its use at the preparative scale and consequently hampers its development as an analytical technique. Today, the increasing number of new drug candidate molecules produced daily, and for which the determination of enantiomeric purity is required before further development, encourages the pharmaceutical industry to seek fast chiral analysis methods based on simple protocols. The speed of analysis is more important than resolution. Thus, screening strategies are implemented with HPLC, SFC and CE including the selection of a limited number of chiral selectors with strong powers of chiral recognition.

Reversal of elution order for profen acid enantiomers in normal phase LC on Chiralpak AD

Enantiomeric separations of four 2-substituted propionic acid drugs and two related acids have been studied using normal phase liquid chromatography with amylose (tris 3,5-dimethylphenylcarbamate) coated on silica as support (Chiralpak AD). At standard conditions (i.e. flow-rate, 1.0 ml/min; column temperature, 30 degrees C) the elution order can be reversed when the polar alcohol modifier in isohexane, 2-propanol, is replaced by methanol/ethanol 2:1. This is the case for ibuprofen with 2.5% (v/v) alcohol and for mandelic acid with 10% (v/v) alcohol using synthetic mixtures with unequal proportions of the respective enantiomer. Thermodynamic studies in the range 10-45 degrees C on retention and selectivity of ibuprofen and mandelic acid gave both linear and curved plots. These results stress the importance of investigating enantiomer elution order during the development of enantioselective methods when both old and new CSPs are evaluated. One should also keep in mind that reversal can take place for rather common analytes in well established enantioselective chromatographic systems.

Preparation and chromatographic properties of a multimodal chiral stationary phase based on phenyl-carbamate-propyl-beta-CD for HPLC

A phenylcarbamate derivative of 2-hydroxypropyl-beta-CD bonded stationary phase was prepared by a previously described method. Its enantiomeric recognition abilities were evaluated as chiral stationary phase (CSP) in normal, polar organic and RP conditions by HPLC. The relevant structural features of the prepared stationary phase which make it an effective chiral selector are discussed. This material seems to have an excellent enantioselectivity for a variety of racemic analytes in the three modes. Hence it can be considered a highly effective multimodal column. Retention factor (k), selectivity (alpha) and resolution (R(s)) were the chosen parameters to describe the column performance. Optimization of these separations was discussed in terms of mobile phase composition, flow rate and structural patterns of the injected analytes.

Enantiomeric separation and determination of absolute stereochemistry of asymmetric molecules in drug discovery—Building chiral technology toolboxes

The application of Chiral Technology, or the (extensive) use of techniques or tools for the determination of absolute stereochemistry and the enantiomeric or chiral separation of racemic small molecule potential lead compounds, has been critical to successfully discovering and developing chiral drugs in the pharmaceutical industry. This has been due to the rapid increase over the past 10-15 years in potential drug candidates containing one or more asymmetric centers. Based on the experiences of one pharmaceutical company, a summary of the establishment of a Chiral Technology toolbox, including the implementation of known tools as well as the design, development, and implementation of new Chiral Technology tools, is provided.

Chiral separation strategy in polar organic solvent chromatography and performance comparison with normal-phase liquid and supercritical-fluid chromatography

A strategy, including a rapid screening and several optimisation steps, for the separation of chiral molecules of pharmaceutical interest by polar organic solvent chromatography (POSC), using four polysaccharide-based stationary phases, is proposed and compared with previously reported strategies in normal-phase (NPLC) and supercritical fluid chromatography (SFC). In a first part of this paper, different examples demonstrate the effectiveness of the POSC strategy for fast method development. Optimisation is based on the use of experimental design to map the experimental domain in an efficient way. In the second part, the best screening results, obtained after performance of earlier defined chromatographic screening strategies in NPLC and SFC, are compared to those obtained in POSC. The three techniques show complementary separation results and allowed baseline separation of 23 of 25 compounds. POSC is found to be a very interesting separation mode compared to NPLC, because of the many fast (< 10 min) baseline separations obtained.

Determination of the enantiomer and positional isomer impurities in atomoxetine hydrochloride with liquid chromatography using polysaccharide chiral stationary phases

A normal-phase isocratic chiral liquid chromatographic method has been developed and validated for atomoxetine hydrochloride. In addition to the S-enantiomer of atomoxetine, the conditions separate both para and meta positional isomers and the phenyl des-methyl analog. Method development strategies included (a) evaluation of polysaccharide-based chiral stationary phases with nonaqueous mobile phases, (b) the use of an octyl stationary phase with a sulfated-beta-cyclodextrin mobile phase additive, and (c) capillary electrophoresis using a single isomer heptakis-6-sulfato-beta-cyclodextrin modifier. All three approaches yielded acceptable conditions for the separation of atomoxetine from related molecules with the former fully validated and the latter two held as alternatives if needed. The final method conditions employing a Chiralcel OD-H column and mobile phase of hexane/IPA/DEA/TFA (85/15/0.15/0.2, v/v/v/v) at 1.0 mL/min have been fully validated with acceptable specificity, linearity, accuracy, repeatability, intermediate precision and quantitation limit.

Normal-phase HPLC enantioseparation of novel chiral metal tetrahedrane-type clusters on an amylose-based chiral stationary phase

In the present work, an amylose tris(3,5-dimethylphenylcarbamate) (ADMPC) chiral stationary phase (CSP) was prepared by coating ADMPC on small-particle silica gel. This ADMPC-CSP was for the first time successfully applied to separate a series of novel chiral metal tetrahedrane-type clusters. Furthermore, the influence of a mobile-phase modifier (various alcohols added in the mobile phase), including its concentration and structure, and the structures of the clusters on the chiral separation and retention was investigated. The results suggest that not only the structure and concentration of alcohol in the mobile phase, but also the subtle structural differences in racemate can have a pronounced effect on the enantiomeric separation and retention.

Screening approach for chiral separation of pharmaceuticals

The aim of this work is to determine generic screening conditions and an initial simple separation strategy allowing the rapid separation of drug enantiomers in polar organic solvent chromatography (POSC). Four cellulose/amylose-based stationary phases were investigated in detail using two mobile phase basis solvents commonly applied in this mode, i.e. acetonitrile and methanol. Polar mode is interesting for use in purification of enantiomers. In a first step, the parameters potentially influencing the separation, such as addition of an alcohol to the polar organic solvent or the type of mobile phase additive(s), were examined by means of experimental designs. Afterwards, the factors found most important are investigated in more detail. Results showed that the cellulose- and amylose-based stationary phases have very broad and complementary enantiorecognition abilities in the POSC mode. The type of organic solvent for the mobile phase appeared to have a dramatic influence on the quality of the separation. Based on the results, a screening strategy was proposed. Enantioseparation was observed in more than 85% of the tested compounds and analysis times of last eluted peak were usually below 10 min.

Enantioselective determination of a gastroprokinetic drug using amylose tris-(3,5-dimethylphenylcarbamate) as a stationary phase by HPLC

An enantioselective high-performance liquid chromatographic method for determination of enantiomers of mosapride citrate in bulk drugs and pharmaceuticals using UV-vis and polarimetric detectors in series has been developed. Baseline separation with resolution >2.0 was achieved on a column containing amylose tris-(3,5-dimethylphenylcarbamate) as stationary phase using a mobile phase consisting of n-hexane:ethanol:triethylamine (80:20:0.3, v/v/v) at 40 degrees C. The detection was carried out at UV-276 nm and enantiomers were identified by polarimetric detector. The effect of ethanol, 2-propanol, TEA, temperature and mobile phase flow rate on separation of MSP enantiomers was studied and the method was validated with respect to accuracy, precision, linearity and limits of detection and quantification. The linearity of the method was studied between 6.25 and 50 microg/ml and r2 was >0.9997. The recoveries were in the range 99.63-100.22%, the method was suitable not only for process development of mosapride citrate but also for quality assurance of the individual enantiomers in bulk drugs and pharmaceuticals.

Enhanced chromatographic resolution of amine enantiomers as carbobenzyloxy derivatives in high-performance liquid chromatography and supercritical fluid chromatography

The carbobenzyloxy (cbz) protecting group is evaluated for it's potential to enhance the resolution of chiral amine enantiomers using high-performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC). A series of cbz derivatives of commercially available racemates was prepared and analyzed by enantioselective chromatography using a variety of mobile phases and polysaccharide and Pirkle-type chiral stationary phases (CSPs). The cbz-derivatized product consistently demonstrated enhanced chiral resolution under HPLC and SFC conditions. Improved selectivity and resolution combined with an automated preparative HPLC or SFC system can lead to the rapid generation of highly purified enantiomers of desirable starting materials, intermediates or final products.

Strategy for the chiral separation of non-acidic pharmaceuticals using capillary electrochromatography

In completion of an earlier defined generic chiral screening approach, a generic separation strategy for basic, bifunctional, and neutral compounds was proposed and evaluated. This strategy adds to a previously defined strategy for acidic compounds. The screening experiment of the actual strategy used a mobile phase of 5 mM phosphate buffer pH 11.5/ACN (30/70 v/v), a temperature of 25 degrees C, and a voltage of 15 kV. The selected chiral stationary phases were Chiralpak AD-RH, Chiralcel OD-RH, Chiralcel OJ-RH, and Chiralpak AS-RH, all based on polysaccharide selectors. It was seen that 31 out of 48 test compounds were partially or baseline-resolved under screening conditions. After execution of the optimization steps of the strategy, this number increased to 41, with a total of 21 baseline-separated compounds. Combined with the results obtained from the acidic test set examined in the earlier defined strategy, of all tested compounds 82.5% showed enantioselectivity and 49.2% could be baseline-separated.

Screening approach for chiral separation of pharmaceuticals

High-throughput and performance analysis and purification of enantiomers are important parts of drug discovery and provide high-quality compounds for pharmacological testing. We have previously reported two parts describing chiral chromatographic screens using normal-phase (NPLC) and reversed-phase (RPLC) liquid chromatography, in order to cope with increasing numbers of new compounds generated by chemistry programs. We present in this part the development and implementation of a third faster screen using supercritical fluid chromatography (SFC) to maximize chance in achieving rapid enantiomer resolution of large numbers of compounds in a minimum of time. The SFC screen utilizes a narrow combination of only four columns (Chirlapak AD and AS, and Chiralcel OD and OJ) and two solvent modifiers (methanol and isopropanol). A modifier and column-switching setup was employed to allow the entire screening process to be serially run in the order AD> OD > OJ > AS and methanol > isopropanol, so that the screening for a given molecule can be stopped when separation is achieved. The switching system was fully automated for unattended operation of multiple compounds. An optimization procedure was also defined, which can be performed if needed for unsuccessful separations in the screening step. The chiral SFC strategy proved its performance and robustness in resolution of hundreds proprietary chiral molecules generated by drug discovery programs, with a success rate exceeding 95%. In addition, the generic capability of the strategy was evaluated by applying the screen and optimization methodology to a test set comprising 40 marketed drugs differing from proprietary compounds in terms of chemical diversity, revealing a similar high success rate of 98%. Chiral separations developed at the analytical scale work easily and equally well at the semi-preparative level, as illustrated with an example. The SFC screen allows resolution of compounds that were partially separated by NPLC or not separated at all by RPLC, demonstrating the utility of implementing complementary chromatographic techniques. The SFC screen is currently an integral part of our analytical support to discovery chemical programs and is considered the first try for chiral separations of new compounds, because it offers a higher success rate, performance and throughput.

Assay of acebutolol in pharmaceuticals by analytical capillary isotachophoresis

Acebutolol [N-{3-acetyl-4-[(2-hydroxy-3-(isopropylamino)propoxy]phenyl} butanamide] is a cardioselective beta-blocker with a potent anti-hypertensive and antiarrhythmic effect. The optimised operational system of electrolytes for the newly developed ITP separation of acebutolol consisted of 10mM potassium acetate +10mM acetic acid (pH 4.65) as the leading electrolyte and 10mM beta-alanine with pH approximately 4 (adjusted with acetic acid) as the terminating electrolyte. The driving and detection currents were 75 and 20 microA, respectively and the analysis took approximately 13 min. Under these conditions the effective mobility of acebutolol was determined as 20.7 x 10(-9) m2 V(-1) s(-1). The calibration dependence was rectilinear in the range 0.14-1.4 mg ml(-1) of acebutolol base (r = 0.9995); relative standard deviation (RSD) values were 1.1% and 1.2% (n = 6) when determining 0.42 and 0.98 mg ml(-1) of acebutolol in a pure standard solution. The method, with the limit of detection (LOD) of 0.04 mg ml(-1) and limit of quantification (LOQ) of 0.12 mg ml(-1), was applied to the assay of acebutolol in Sectral tablets, Acecor tablets, Apo-acebutol tablets (nominal content 400 mg of acebutolol per tablet) and Acebirex tablets (nominal content 200 mg of acebutolol per tablet) with RSD = 0.7-1.7% (n = 6). No interference from any excipients present in the tablets was observed. The recoveries ranged from 98.8% to 102.4% as found by the standard addition technique.

Integration of supercritical fluid chromatography into drug discovery as a routine support tool

Supercritical fluid chromatography (SFC) has been implemented within our group as a purity assessment and purification tool to complement high performance liquid chromatography (HPLC) for diastereomer and chiral separations. Using a novel strategy, rapid chiral screening has been implemented using short columns, high flow rates and fast gradients. A primary screen delivers a separation assessment using one solvent modifier (methanol) and four columns (Chiralpak AD-H and AS-H, and Chiralcel OD-H and OJ-H) run serially in a total of 24 min. A secondary screen then uses ethanol and isopropanol (IPA) modifiers across the same columns. The screens can be combined to run a sequence of samples overnight where each racemate is analysed over 80 min. The fast analytical screening and optimisation process enables rapid identification of the purification method. Furthermore, subsequent preparative chiral SFC has decreased the overall sample turnaround time for the Medicinal Chemist, delivering high fraction purities and acceptable recoveries, substantial operational cost savings and increased flexibility with respect to large scale purification feasibility in comparison to HPLC. SFC has been so successful it is now used as the primary method for chiral analysis and purification within our laboratory.

New hybrid polymeric liquid chromatography chiral stationary phase prepared by surface-initiated polymerization

A new hybrid organic/inorganic HPLC chiral stationary phase (CSP1) has been synthesized by the grafting from (g-from) radical polymerization of an enantiopure diacryloyl derivative of trans-1,2-diaminocyclohexane in the presence of mesoporous, azo-activated silica particles. The new chiral stationary phase has been fully characterized by elemental analysis, differential scanning calorimetry, diffuse reflectance infrared spectroscopy, scanning electron microscopy, inverse size exclusion chromatography and Van Deemter analysis. CSP1 shows improved chromatographic performances compared to its analog CSP2 synthesized by the alternative grafting to (g-to) approach in which the azo initiator is kept in solution. CSP1 can successfully resolve several chemically diverse chiral compounds, using both organic and water-based eluents (normal phase, polar organic, etc.).

Highly sensitive and rapid normal-phase chiral screen using high-performance liquid chromatography–atmospheric pressure ionization tandem mass spectrometry (HPLC/MS)

In the last years, there has been an increasing demand on the development of quantitative assays for determination of enantiopurity. Herein, we present a methodology based on a direct linking of an atmospheric pressure ionization mass spectrometer (MS-APCI) with a high-performance liquid chromatography HPLC (DAD) system, operated under normal-phase mode and without post-column addition of MS-compatible solvents, which provides the high specificity/selectivity (identification of isomers in complex mixtures) and accuracy (1-2% area level) required for daily chiral studies. As result of the success of our screen, the preparation of individual enantiomers or the racemic mixture in our Drug Discovery Research Laboratories at Lilly, Spain is usually not required. Therefore, additional non-valuable synthetic work is eliminated.

Definition and system implementation of strategies for method development of chiral separations in normal- or reversed-phase liquid chromatography using polysaccharide-based stationary phases

This paper proposes strategies in normal- and reversed-phase liquid chromatography (NP-HPLC or NPLC and RP-HPLC or RPLC), which were developed using three polysaccharide-based stationary phases. Those strategies are implemented in a knowledge-based system for the chiral separation of drug enantiomers. Each strategy includes a screening and an optimisation stage. The screening stage allows a fast evaluation of separation possibilities and enantioselectivity for many drugs in a short period of time, while the optimisation stage gives the opportunity to enhance, if needed, the initially obtained separation. Different examples demonstrate the effectiveness of the strategies for fast method development.

Enantioseparation of amino acid derivatives on an immobilized network polymer derived from l-tartaric acid

Seven structurally related amino acid derivatives were successfully enantioseparated by HPLC with a commercially available column containing a chiral immobilized network polymer derived from L-tartaric acid. The experiments were carried out under normal-phase conditions. All the solutes could be baseline separated using n-hexane/2-propanol (95/5) as eluent at a flow rate of 1 ml/min at 25 degrees C, with reasonable retention time (<12 min). The effects of the polar alcohol modifier (type and content) in the mobile phase and the column temperature on the enantioseparation were studied. Apparent thermodynamic parameters were also calculated from the plots of ln alpha or ln k' versus 1/T. Some mechanistic aspects of chiral recognition were discussed with respect to the structures of the solutes. It was found that the enantioseparations are all enthalpy driven, and the N-acyl groups of the solutes have significant influence on the chiral recognition.

Comparison of the miniaturised techniques capillary electrochromatography and capillary liquid chromatography for the chiral separation of chlorthalidone

The aim of this study was to compare the miniaturised techniques, capillary electrochromatography (CEC) and capillary liquid chromatography (CLC), for the chiral separation of chlorthalidone. In both cases, hydroxypropyl-beta-cyclodextrin was used as a chiral selector in the mobile phase, while an achiral stationary phase was used. Earlier, this separation was already optimised in CEC. Now, the separation was optimised in CLC. The influence of the organic modifier content and the cyclodextrin concentration on the separation was studied by means of a central composite design. Optimal separation conditions were determined, after response modelling, from the response surface contour plots. When these conditions were compared with those of the CEC optimisation, we can see the potential of using CLC as a chiral separation technique since less chiral selector was used, faster separations were obtained and better repeatability was observed in comparison with its electrical-driven counterpart.

Enantiomeric resolution of kielcorin derivatives by HPLC on polysaccharide stationary phases using multimodal elution

Analytical HPLC methods using carbamate chiral stationary phases of polysaccharide derivatives were developed for the enantiomeric resolution of five racemic mixtures of xanthonolignoids: rac-trans-kielcorin C, rac-cis-kielcorin C, rac-trans-kielcorin D, rac-trans-isokielcorin D, and rac-trans-kielcorin E. The separations were evaluated with the stationary phases cellulose tris-3,5-dimethylphenylcarbamate, amylose tris-3,5-dimethylphenylcarbamate, amylose tris-(S)-1-phenylethylcarbamate, and amylose tris-3,5-dimethoxyphenylcarbamate under normal, reversed-phase, and polar organic elution conditions. Chiral recognition of those chiral stationary phases, the influence of mobile phases on the enantiomers separation, and the effects of structural features of the solutes on the chiral discrimination observed are discussed. The best performance was achieved on an amylose tris-3,5-dimethylphenylcarbamate phase. Polar organic conditions gave shorter retention factors and better resolutions and were a valuable alternative to the alcohol-hexane or reversed-phase conditions.

Simultaneous stereoselective analysis of tramadol and its primary phase I metabolites in plasma by liquid chromatography

This paper describes a bioanalytical method involving a simple liquid-liquid extraction for the simultaneous HPLC determination of the enantiomers of tramadol, the active metabolite O-desmethyltramadol (M1), and the other main metabolite N-desmethyltramadol (M2) in biological samples. Chromatography was performed at 5 degrees C on a Chiracel OD-R column containing cellulose tris(3,5-dimethylphenylcarbamate) as chiral selector, preceded by a achiral end-capped C8 column (LiChrospher 60-RP-selected B 5 microm, 250 mm x 4 mm). The mobile phase was a mixture of phosphate buffer containing sodium perchlorate (1 M) adjusted to pH 2.5-acetonitrile-N,N-dimethyloctylamine (74.8:25:0.2). The flow rate was 0.5 ml/min. Fluorescence detection (lambda(ex) 200 nm/lambda(em) 301 nm) was used. Fluconazol was selected as internal standard. The limit of quantitation of each enantiomer of tramadol and their metabolites was 0.5 ng/ml (sample size = 0.5 ml). The chiral conditions and the LC optimisation were investigated in order to select the most appropriate operating conditions. The method developed has also been validated. Mean recoveries above of 95% for each enantiomer were obtained. Calibration curves for tramadol enantiomers (range 1-500 ng/ml), M1 enantiomers (range 0.5-100 ng/ml), and M2 enantiomers (range 0.5-250 ng/ml) were linear with coefficients of correlation better than 0.996. Within-day variation determined on four different concentrations showed acceptable values. The relative standard deviation (R.S.D.) was determined to be less than 10%. This method was successfully used to investigate plasma concentration of enantiomers of tramadol, O-desmethyltramadol and N-desmethyltramadol in a pharmacokinetic study.

Memory effect of diethylamine mobile phase additive on chiral separations on polysaccharide stationary phases

The existence of a memory effect for amine additives on polysaccharide chiral stationary phases has often been suggested, but not clearly demonstrated. Demonstration of this effect is made difficult by the uncertainty as to which analytes benefit from use of amine additives and, typically, an unclear history of column use. In this work, analytes were selected for differences in their behavior with and without additives. Columns were used with no prior history. A persistent memory effect was demonstrated on a CHIRALPAK AD-H column in hexane-based mobile phases. This effect was short-lived, with polar organic mobile phases. Memory was short-lived on a CHIRALCEL OJ-H column. Flushing with isopropanol was shown to remove most of the memory effect. Compounds expected to require amine additives on CHIRALCEL OD-H column did not. Acid treatment of the AD-H and OD-H columns changed their performance, which was subsequently restored by the incorporation of amine.

Comparative performances of selected chiral HPLC, SFC, and CE systems with a chemically diverse sample set

Pharmaceutical companies have a continuous need to resolve new racemates. Analysis may be required in aqueous and nonaqueous media, or in the presence of several different sets of potentially interfering compounds. There is often a preparative requirement. For these reasons analysts may require a number of different separation systems capable of resolving a given pair of enantiomers. We wished to improve upon existing approaches that address this situation and undertook a program of work to screen over 100 racemates, selected for their chemical diversity, on over 100 different chiral HPLC, SFC, and CE systems. Here we report results of this comparison and illustrate the use of rapid gradient screening as a valuable tool for chiral method development.

Evaluation of generic chiral liquid chromatography screens for pharmaceutical analysis

Two different automated generic liquid chromatography screens for the separation of chiral compounds of pharmaceutical interest have been evaluated. The test set comprised 53 chemically diverse chiral compounds involving 55 enantiomeric pairs from the pharmaceutical industry (i.e. starting materials, synthetic intermediates and drug substances). The first screen utilised four polysaccharide-based columns with five mobile phases and showed enantioselectivity for 87% of the test compounds. The second screen employed three macrocyclic glycopeptide columns with two mobile phases and showed enantioselectivity for 65% of the test compounds. Merging of the two screening procedures resulted in an enantioselectivity for 96% of the chiral compounds. It is anticipated that the systematic use of the automated chiral HPLC screens described in this report will substantially reduce the necessary time for method development of pharmaceutically related chiral analytical methods.

Rapid method development for chiral separation in drug discovery using sample pooling and supercritical fluid chromatography-mass spectrometry

A novel strategy for rapid chiral method development has been implemented using sample pooling and supercritical fluid chromatography-mass spectrometry (SFC-MS) on four chiral stationary phases, namely Chiralpak AD and AS, and Chiralcel OJ and OD, and eight different modifier concentrations (5 to 40% methanol-0.2% isopropylamine). The screening is performed under an outlet pressure of 110 bar at 35 degrees C, and at a flow-rate of 2.5 ml/min for the initial 20 min and then ramped up to 4 ml/min and held for 4.5 min to elute all solutes from the column. The entire process is fully automated from injection to data processing, and operates unattended for 15 h overnight to obtain optimal chiral separation for multiple compounds. A unique feature of using SFC-MS to monitor chiral synthesis is the negligible interferences from achiral impurities. In addition, with SFC-MS, enantiomeric excess can be determined with much lower detection limits than UV and much shorter analysis times compared to normal-phase/reversed-phase liquid chromatography.

Automated LC-MS Method for the Fast Stereoselective Determination of Methadone in Plasma

Methadone (MTD) is a chiral drug widely used for the treatment of opioid dependence for which a rapid analytical method for the determination of each enantiomer would be advantageous. In order to improve method sensitivity and to automate the entire analytical process, a column-switching configuration has been developed. An online extraction system coupled to a cellulose-based chiral stationary phase (CSP), namely Chiralcel OJ-R, was used and detection was performed by mass spectrometry. Fifty microl of plasma were injected into the liquid chromatography-mass spectrometry (LC-MS) system after addition of acetonitrile (ACN) containing methadone deuterated D9 (MTD-D9) (internal standard) and centrifugation. For the rapid extraction step, a large particle size support was selected. A baseline separation of MTD enantiomers was obtained in less than 12 min. Trueness and precision were evaluated with control samples at 500 ng/ml of (R,S)-methadone. Trueness values were 106.6% and 103.0% for (R)-MTD and (S)-MTD, respectively, with a coefficient of variation inferior to 2.5% for both compounds. Finally, a good concordance was found using this method for analysis of plasma samples from patients in maintenance treatment as compared to a previously described HPLC-UV method after liquid-liquid extraction.