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

HSPiP and QbD Program-Based Analytical Method Development and Validation to Quantify Ketoconazole in Dermatokinetic Study

Ketoconazole (KTZ) is the most potential azole anti-mycotic drug. The quantification of KTZ from various layers of the skin after topical application of lipidic nanocarriers is critical. We addressed a sensitive, specific, simple, rapid, reproducible, and economic analytical method to quantify KTZ from the treated skin homogenate using the Hansen solubility parameter (HSP, HSPiP software)-based modeling and experimental design. The software provided various HSP values for KTZ and solvents to compose the mobile phase. The Taguchi model identified the significant sets of factors to develop a robust bioanalytical method with reduced variability. In the optimization, acetonitrile (ACN) concentration (X1 as A) and the pH of mobile phase (X2 as B) were two factors against two responses (Y1: peak area and Y2: retention time). The HPLC (high-performance liquid chromatography) method validation was carried out based on US-FDA guidelines for the developed KTZ formulations (suspension, solid nanoparticles, and commercial product) extracted from the treated rat skin. The experimental solubility of KTZ was found to be maximum in the two solvents (ACN and ethyl acetate), based on HSP values. Surface response methodology (SRM) identified remarkable impact of ACN concentration and the mobile phase pH on the peak area and retention time. Analytical limits (0.17 and 0.50 µg/mL) were established for KTZ-SLNs (extracted from the skin). The method was implemented with high reproducibility, accuracy, and selectivity to quantify KTZ from the treated rat skin.

Chiral Ionic Liquids: Structural Diversity, Properties and Applications in Selected Separation Techniques

Ionic liquids (ILs) are chemical compounds composed of ions with melting points below 100 °C exhibiting a design feature. ILs are commonly used as the so-called green solvents, reagents or highly efficient catalysts in varied chemical processes. The huge application potential of ionic liquids (IL) justifies the growing interest in these compounds. In the last decade, increasing attention has been devoted to the development of new methods in the synthesis of stable chiral ionic liquids (CILs) and their application in various separation techniques. The beginnings of the successful use of CILs to separate enantiomers date back to the 1990 s. Most chiral ILs are based on chiral cations or chiral anions. There is also a limited number of CILs possessing both a chiral cation and a chiral anion. Due to the high molecular diversity of both ions, of which at least one has a chiral center, we have the possibility to design a large variety of optically active structures, thus expanding the range of CIL applications. Research utilizing chiral ionic liquids only recently has become more popular. However, it is the area that still has great potential for future development. This review aimed to describe the diversity of structures, properties and examples of applications of chiral ionic liquids as new chiral solid materials and chiral components of the anisotropic environment, providing chiral recognition of enantiomeric analytes, which is useful in liquid chromatography, countercurrent chromatography and other various CIL-based extraction techniques including aqueous biphasic (ABS) extraction systems, solid-liquid two-phase systems, liquid-liquid extraction systems with hydrophilic CILs, liquid-liquid extraction systems with hydrophobic CILs, solid-phase extraction and induced-precipitation techniques developed in the recent years. The growing demand for pure enantiomers in the pharmaceutical and food industries sparks further development in the field of extraction and separation systems modified with CILs highlighting them as affordable and environmentally friendly both chiral selectors and solvents.

Tunable normal phase enantioselectivity of amino acid esters via mobile phase composition

The ability to tune chiral selectivity through mobile phase modifiers is a powerful tool in chiral separations. Beyond improving efficiency and/or resolution, some mobile phase systems can even invert elution order, a highly desirable result for trace analyses or preparative scale isolations. Previous work has demonstrated that acidic modifiers, such as ethanesulfonic acid (ESA), can greatly impact separations of enantiomers. However, prior studies were primarily performed on coated chiral stationary phases (CSPs), which limited the selection of the bulk mobile phase component. In this work, the effect of ESA modifier was studied for the enantioseparation of six pairs of amino acid esters on a CHIRALPAK^® IA column, an immobilized amylose-based CSP, with different combinations of standard solvents (hexane and ethanol) as well as "non-standard" solvents, such as methyl t-butyl ether, ethyl acetate, tetrahydrofuran, acetone, or 1,4-dioxane. ESA generally improved selectivity, and multiple instances of elution order reversal were observed. A Van Deemter plot study reveals that ESA exerts its effect by pulling the enantiomer deeper into the chiral cavity of the chiral polymer to increase the interactions between the analytes and the stationary phase, which is the main reason for the increased enantioselectivity.

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.

Adsorption behavior of a teicoplanin aglycone bonded stationary phase under harsh overload conditions

Silica-bonded teicoplanin aglycone allows enantioseparation of amino acids by reversed-phase liquid chromatography with a low organic solvent content. However, a reversible change in the adsorption behavior leading to a retention time shift (RTS) was observed when a preparative scale column was treated with harsh preparative chromatography-like conditions between finite-injection HPLC runs conducted under exactly the same conditions. This behavior was observed for all five investigated aliphatic and aromatic amino acids. In all cases, the retention times were prolonged after the overload conditions and the RTS was more pronounced for the later eluting d-enantiomer. We defined a standardized method for measuring the RTS and performed a systematic investigation on the influence of experimental conditions (type and concentration of pH modifier and organic modifier, temperature, pH) on the RTS. In this way a solvent composition--90/10 50 mM NH4Ac pH 5.8/MeOH--was identified that yielded no observable shift in retention time after overload conditions for both enantiomers. In order to treat the observed phenomenon on a mechanistic level, we applied band profile analysis based on the stochastic theory of chromatography and identified two different enantioselective sites. When the band profile analysis was performed on elution profiles obtained from runs with prolonged retention time after harsh overload conditions, the retention time shift could be attributed to both differentiable types of adsorption sites. One site was found to make both, enantioselective and non-selective contributions.

Strategic use of preparative chiral chromatography for the synthesis of a preclinical pharmaceutical candidate

The modern use of preparative chromatography in pharmaceutical development is illustrated by the case of a recent preclinical candidate from these laboratories. The synthesis of the candidate employed a coupling of two enantiopure intermediates, each of which could be resolved using preparative chiral chromatography. SFC screening was employed to identify the enantioselective stationary phases, and semipreparative SFC methods derived from this screening were used to produce gram amounts of enantiopure intermediate for initial studies. However, initial larger scale resolution required the translation of the SFC methods to HPLC conditions. Preparative chiral HPLC on a 30-cm i.d. column was then used to produce enantiopure intermediates which were coupled to give 170 g of the preclinical candidate. Subsequent preparation of the candidate at larger scale for later-stage clinical evaluation employed an improved synthesis in which one component was constructed by asymmetric synthesis. Resolution of the other component, now a more advanced intermediate, was carried out using newly obtained large-scale SFC equipment. Some discussion is presented on the varying strategies whereby preparative chiral chromatography can be used to support either short-term or long-term synthetic goals in preclinical pharmaceutical development.

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.

Enantioselective recognition of 2,3-benzodiazepin-4-one derivatives with anticonvulsant activity on several polysaccharide chiral stationary phases

The retention behaviour of racemic 1-(4-aminophenyl)-1,2,3,5-tetrahydro-7,8-methylendioxy-4H-2,3-benzodiazepin-4-one derivatives with anticonvulsant activity on several chiral stationary phases was investigated. The selective performances of six polysaccharide phases, namely, Chiralcel OA, OD, OF, OG, OJ and Chiralpak AD were studied and normal phase HPLC methods were optimized to separate the enantiomeric forms of this class of compounds. The chiral recognition mechanism between the analytes and the chiral selectors was discussed. A molecular modeling study was carried out with the aim to explore the enantioselective molecular recognition process with the Chiralcel OG stationary phase.

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.

The effect of acidic and basic additives on the enantioseparation of basic drugs using polysaccharide-based chiral stationary phases

The enantioseparation of nine commercially available basic drugs was achieved on polysaccharide-based chiral stationary phases with the acidic additive ethanesulfonic acid and the basic additive butylamine. Seven different commercially available CSPs were used for the study (AD, AS, OD, OJ, OG, OB, and OC). Mobile phase additives have been proven to be essential in obtaining satisfactory enantio-resolution in terms of both efficiency and selectivity. Significantly improved selectivities were obtained for the basic probe drugs with the acidic additive, ethanesulfonic acid, rather than the basic additive, butylamine. This is best seen with Chiralpak AS CSP. No enantioseparation for the nine drugs was observed when butylamine was used as an additive; however, satisfactory enantioseparation for the nine drugs was achieved using ethanesulfonic acid. Higher column efficiencies were observed with the acidic additive, especially when isopropanol was used as a modifier. Higher sensitivity was also achieved with ethanesulfonic acid because of the significantly lower background at the UV detection wavelength. The acidic additive was demonstrated to be superior to the basic additive for the enantioseparation of basic drugs using seven different polysaccharide-based CSPs. These results are counterintuitive to the common "rule of thumb" in enantioseparation that states acidic additives work best for acidic analytes and basic additives work best for basic analytes. The beneficial effects of acidic additive in enantioseparations observed in this study could significantly improve the applicability of polysaccharide-based CSPs for the enantioseparation of basic analytes.

Effect of (+) or (−) camphorsulfonic acid additives to the mobile phase on enantioseparations of some basic drugs on a Chiralcel OD column

This paper describes the modification of Chiralcel OD column properties by adsorption of (+) or (-) camphorsulfonic acids (CSAs) used as additives to the mobile phase. The effects on retention, selectivity and efficiency, of adsorption of (+) and (-) CSAs on a Chiralcel OD column were examined. Racemic anti-histamines, anti-malarial and anti-fungal drugs, namely doxylamine, miconazole, sulconazole, hydroxyzine, homochlorcyclizine, methoxypheniramine, cyclopentolate and ephedrine were investigated as chiral tested compounds. All the studied drugs have an amino nitrogen atom in their structure. Only the enantioseparation of ephedrine enantiomers with CSAs alone was studied on the Nucleosil stationary phase, and these results were compared with the results obtained on the Chiralcel OD phase. A new dynamically generated stationary phase, with very good enantioseparation ability towards the studied compounds, was obtained by the adsorption of (-) CSA on the Chiralcel OD column.

Enantioselective chromatography in drug discovery

Molecular chirality is a fundamental consideration in drug discovery, one necessary to understand and describe biological targets as well as to design effective pharmaceutical agents. Enantioselective chromatography has played an increasing role not only as an analytical tool for chiral analyses, but also as a preparative technique to obtain pure enantiomers from racemates quickly from a wide diversity of chemical structures. Different enantioselective chromatography techniques are reviewed here, with particular emphasis on the most widespread high performance liquid chromatography (HPLC) and the rapidly emerging supercritical fluid chromatography (SFC) techniques. This review focuses on the dramatic advances in the chiral stationary phases (CSPs) that have made HPLC and SFC indispensable techniques for drug discovery today. In addition, screening strategies for rapid method development and considerations for laboratory-scale preparative separation are discussed and recent achievements are highlighted.

Applications of ammonium trifluoroacetate as an additive for elution of chiral acids and bases from derivatized polysaccharide stationary phases

The ability ammonium trifluoroacetate as an additive for elution of acids and bases from derivatized polysaccharide chiral stationary phases was first observed in the process of developing normal-phase chiral HPLC methods in our lead-generation programs. To demonstrate this ability on a broader scale, chiral HPLC methods containing this additive in the mobile phases were developed to resolve selected acidic, basic, and neutral racemates, which are considered standards in the pharmaceutical industry and for which published methods exist. The mobile phases of these published methods contain acidic and/or basic additives (e.g., trifluoroacetic acid or diethylamine). This article demonstrates the versatility of ammonium trifluoroacetate additive in resolving the enantiomers of acidic and basic racemates on the same derivatized polysaccharide chiral columns. This resolution is achieved without changing the mobile phase between the analysis of acidic and basic racemates and also without observation of stationary-phase "memory effect." This chiral method development strategy can result in significant savings of cost and time.