Debby Mangelings

Screening approach for chiral separation of pharmaceuticals: Part II. Reversed-phase liquid chromatography

A screening strategy for the rapid separation of drug enantiomers by reversed-phase liquid chromatography was developed using three cellulose/amylose stationary phases. The key point to achieve enantioselectivity is the control of the compound ionisation. Only two mobile phases, i.e. an acidic phosphate buffer (pH 2.0) containing a chaotropic salt (KPF6) and a borate buffer (pH 9.0) mixed with acetonitrile, are used in the proposed strategy. This strategy was successfully applied to a set of 37 diverse chiral pharmaceuticals. Satisfactory enantioselectivity was achieved for 89% of them.

Method development for HILIC assays

In this review, method development for hydrophilic interaction LC (HILIC) is highlighted. HILIC is a chromatographic technique that uses aqueous-organic solvent mobile phases with a high organic-solvent fraction, and a hydrophilic stationary phase. It is mainly applied for the separation of polar and hydrophilic compounds. Method development, in general, can be done uni- or multivariately. In the univariate approach, the factors that are expected to potentially affect the separation of the compounds will be examined sequentially and one-at-a-time. All HILIC methods found in the literature were developed in this way. For these methods, the analytes, the considered factors, the selected responses, and the finally chosen experimental conditions are discussed in this review. Where examined, the method validation and the comparison with other analytical assay methods is also described. For the multivariate method-development approach, which is based on the use of experimental designs, only a possible strategy is presented, because of the lack of relevant publications in the literature.

Supercritical fluid chromatography for the enantioseparation of pharmaceuticals

Chirality has a significant impact on drug discovery and development processes in the pharmaceutical industry. As the number of enantiopure drugs launched onto the market is yearly increasing, the need for fast and performant enantioseparation methods with minimal costs is becoming more compelling. In this context, sub- and supercritical fluid chromatography (SFC), being applicable at an analytical, as well as at a preparative scale is gaining more interest. In this review a practical overview is given of the different parameters that are important in supercritical fluid chromatographic separations. A comparison is made between the applicability for chiral separations of SFC and conventional high-performance liquid chromatography (HPLC), and illustrated by means of examples. Different aspects of method development and the upscaling feasibility in SFC are discussed. This review aims to give the reader a practical insight in the use of supercritical fluid chromatography for the chiral separation of pharmaceutical compounds.

Chiral separations in sub-and supercritical fluid chromatography

Sub- and supercritical fluid chromatography (SFC) received more and more attention in pharmaceutical analysis during the last years. Especially for chiral separations, this technique is becoming increasingly popular. This review gives an overview of most chiral separation applications using SFC, covering the literature from 2000 on.

Chiral separations in polar organic solvent chromatography : Updating a screening strategy with new chlorine-containing polysaccharide-based selectors

The screening conditions of an existing column and mobile phase selection strategy for chiral drug substances in polar organic solvent liquid chromatography (POSC) were tested for their applicability on two new chlorine-containing polysaccharide-based stationary phases. The selectors of these phases are cellulose tris(3-chloro-4-methylphenylcarbamate) and amylose tris(5-chloro-2-methylphenylcarbamate). The enantioselectivity of these phases is compared to that of the four phases (Chiralpak AD-RH, Chiralcel OD-RH, Chiralpak AS-RH and Chiralcel OJ-RH) used in the earlier defined strategy. A test set of 62 structurally diverse chiral drug substances is analyzed using the screening conditions of the strategy on the six phases. The results confirm that the acetonitrile-based mobile phase provides a higher success rate and better resolutions than the methanol-based also on the new phases. However, the importance of the methanol-based mobile phase cannot be neglected for complementarity reasons; the two mobile phases insure enantioselectivity for different compounds. A third (ethanol-based) mobile phase, not part of the strategy, was also used to screen the two new phases. The joint results led to different possibilities to upgrade the current screening strategy so that improved success rates are obtained. The chlorine-containing chiral stationary phases demonstrated to have an added value to the screening process since they provided enantioresolution for compounds not resolved by non-chlorine-containing ones.

Enantioselective capillary electrochromatography: Recent developments and new trends

Since its development in the early 1970s, CEC has been studied quite extensively, but unfortunately its use is still mostly located at an academic level. Reasons for this are the limited availability of commercially available stationary phases (SPs) and columns, along with some practical limitations, such as column fragility, lack of column robustness and reproducibility. Nevertheless, CEC maintains a place among the separation techniques, probably because of its unique feature to combine two separation principles. Also in the field of chiral separations, CEC is often used as a separation technique and already showed its potential for this kind of analyses. This overview will focus on the recent applications, i.e. between 2006 and 2010, in enantioselective analysis by means of CEC. For the selected applications, the used SPs (chiral selectors) and their potential for future method development or screening purposes will be evaluated and critically discussed.

Chiral separations in normal phase liquid chromatography: Enantioselectivity of recently commercialized polysaccharide-based selectors. Part I: Enantioselectivity under generic screening conditions

Four recently commercialized polysaccharide-based chiral stationary phases, Sepapak(®) 1, Sepapak(®) 2, Sepapak(®) 3, and Sepapak(®) 4, now called Lux(®) Cellulose-1, Lux(®) Cellulose-2, Lux(®) Amylose-2 and Lux(®) Cellulose-4, respectively, were examined for their enantioselectivity on a set of 61 racemic compounds by applying the screening conditions of a previously developed chiral screening strategy in normal phase liquid chromatography (NPLC) [N. Matthijs et al., J. Chromatogr. A 1041 (2004) 119-133]. The enantioselectivity on these phases was compared to that on the initial set of polysaccharide-based phases, Chiralpak(®) AD-H, Chiralcel(®) OD-H, and Chiralcel(®) OJ-H, used in the earlier defined strategy. The results showed that 53 compounds out of 61 (86.9%) were resolved on the initial set of chiral stationary phases (CSPs) using two mobile phases per compound, either heptane-ethanol-diethylamine (DEA) or heptane-isopropanol-DEA for testing basic compounds and heptane-ethanol-trifluoroacetic acid (TFA) or heptane-isopropanol-TFA for acidic, bifunctional and neutral compounds. The recently commercialized set of columns gave 54 separations in total (88.5%). Our results indicated that ethanol (EtOH) as polar modifier provides a higher success rate and better resolutions than isopropanol (IPA) on both sets of stationary phases. However, the usefulness of the mobile phase with IPA as polar modifier cannot be neglected for complementarity reasons. It was found that the screening is improved by the introduction of the recently commercialized polysaccharides based CSPs since they provided enantioseparation for compounds that were not resolved by the traditional CSPs. The combination between the initial and the recently commercialized CSPs showed enantioresolution for 55 compounds out of 61 (90%), among which 47 were baseline resolved.

Permanent gold nanoparticle coatings on polyelectrolyte multilayer modified capillaries for open-tubular capillary electrochromatography.

This paper reports on a new strategy to coat fused silica capillaries through ionic adsorption of gold nanoparticles (AuNPs) on a polyelectrolyte multilayer (PEM) modified capillary wall. The coating was constructed in situ by alternating rinses with positively charged poly(diallydimethylammonium chloride), negatively charged poly(sodium-4-styrenesulfonate), and positively charged AuNPs. After self-assembly of n-octadecanethiol onto the surface of AuNPs, the modified capillary was investigated as a new medium for the separation of neutral analytes and proteins in open-tubular capillary electrochromatography (OT-CEC). The surface coverage of the capillary wall was increased using the high density of AuNPs which were dynamically capped with 4-dimethylaminopyridine (DMAP). The chromatographic performance of the column coated with positively charged AuNPs was remarkably improved compared with a column modified with negatively charged AuNPs. The coating was robust over more than 810 runs in this study and also showed high stability against 0.01 M NaOH, 0.01 M HCl, and electrolyte concentrations up to 70 mM. The run-to-run, day-to-day, and capillary-to-capillary reproducibilities of electroosmotic flow were satisfying with relative standard deviation values of less than 1% in all cases. The AuNP-coated PEM modified capillary column not only showed good performance for neutral analytes but also was suitable for the analysis of both basic and acidic proteins.

Enantioselectivity of polysaccharide-based chiral stationary phases in supercritical fluid chromatography using methanol-containing carbon dioxide mobile phases

The enantioselectivity of twelve chiral stationary phases (CSPs) and four methanol-containing carbon dioxide mobile phases (MPs) is evaluated in supercritical fluid chromatography (SFC) with a test set of 59 chiral pharmaceutical compounds. Methanol (MeOH) is evaluated as modifier in carbon dioxide (CO(2)) since it is commonly used in chiral SFC because of its favorable characteristics and proven successes. In addition to the MP of earlier defined generic screening conditions, new MPs, which contained both a basic (isopropylamine) and an acidic (trifluoroacetic acid) additive, were investigated and yielded broad enantioselectivities. The joint use of the additives impacts the enantioselectivity differently than the individual. Polysaccharide-based CSPs from different manufacturers were assessed, which showed that CSPs containing the same selector do not always display the same enantioselectivity. This work enabled not only to identify the individual chiral systems with the broadest enantioselectivity but also to determine their complementarity, resulting in a limited set of systems with the broadest enantioselectivity. As a result an updated, fast and efficient screening sequence was proposed.

Pharmaceutical-enantiomers resolution using immobilized polysaccharide-based chiral stationary phases in supercritical fluid chromatography

Since their introduction on the market the applicability of immobilized polysaccharide-based chiral stationary phases in high-performance liquid chromatography has been thoroughly investigated. These immobilized phases have the benefit to be applicable with a wide range of modifiers, potentially extending the application range of the polysaccharide-based stationary phases. Because an increasing number of stationary phases are being introduced in the field of chiral chromatography it is important to evaluate their enantioselectivity in different techniques in order to get an idea about their applicability. In this study, three immobilized chiral polysaccharide-based stationary phases (Chiralpak IA, IB, and IC) are evaluated in supercritical fluid chromatography (SFC) with a test set of pharmaceutical racemates. This is done in a three-fold manner: their performance is evaluated (1) using traditional modifiers, (2) using mixtures of atypical modifiers, and (3) the results were compared to those on coated stationary phases with an equivalent chiral selector. To get a visual overview of the enantioselective patterns of the different chromatographic systems (mobile and stationary phase combinations), a Principal Component Analysis is performed, which allows determining the (dis)similarity between individual systems. To assess the complementarity cumulative success rates are determined. The immobilized chiral stationary phases prove to yield high cumulative success rates.