In Search of a Generic Chiral Strategy: 101 Separations With One Method

In any approach to drug discovery, the challenges presented to the analytical chemist are compounded when a product contains one or more chiral centres. Enantiomers are stereoisomers that display chirality, having one or more asymmetric carbon centres, allowing them to exist as non-superimposable mirror images of one another. These isomers are difficult to analyse as they are both physically and chemically identical and differ only in the way they bend plane-polarised light and in their behaviour in a chiral environment.

The key to separating enantiomers is to first create diastereomers from these enantiomers. Diastereomers may be created through chemical derivatisation with a ‘chiral’ reagent, or they may be formed transiently through
interactions with chiral selectors. The latter, of course, is usually the most desirable as it is the easiest to employ. These chiral selectors have historically been introduced in the form of chromatographic media using HPLC, SFC or GC as the separation technique. Although chromatography has been a very effective methodology for chiral separations, the process of developing the methodology tends to be very expensive as development time is long, column lifetimes are short, and costs of chiral reagents are high.

Capillary electrophoresis has proven the most ideal analytical tool for this purpose, as it is simple to construct and modify a chiral environment within a capillary. The use of cyclodextrins for differential host-guest complexation of enantiomers is by far the most common ‘solution-based’ chiral selector and is the basis of the chiral separation strategy that we propose. The primary strategy [1] focuse  on the use of highly sulfated cyclodextrins (HSCDs) which are a family of three chiral reagents [2]. This strategy first involves screening the compound for separation using all three (α, β and γ) HSCDs and then optimising the reagent which yields the best resolution.

The objective of the current study was to investigate the use of charged cyclodextrins in the on-going search for a generic
strategy for the separation of enantiomeric drug substances. A group of compounds selected from a set of drugs and metabolites of pharmaceutical and forensic interest was separated using HSCDs. This was a challenging group because it included many closely related metabolites of drug substances, in addition to the parent drugs. For simplicity, the screening strategy was designed to separate the enantiomers of individual compounds, although we present examples of separation of both drugs and their metabolites.