Methods for chiral separations are an important part of achieving the desired enantioselectivity of a given active pharmaceutical
ingredient. Selection of the stationary phase in a chromatographic approach is crucial tool for optimizing the separation
process.
The function of chiral stationary phases
Polysaccharide-based chiral stationary phases (CSPs) are widely used for both analytical and preparative enantioselective
chromatography because of their wide range of applicability and high loading capacity, explains Geoffrey B. Cox, vice-president
of technology at Chiral Technologies (West Chester, PA). Derivatized polysaccharides, just as the native cellulose or amylose
polymer on which they are based, form a helical tertiary structure, which engenders a chiral (asymmetric) environment that
interacts differently with each of the two enantiomers as they adsorb on the stationary phase, he explains. Computer modeling
of the structure of the chiral polymer shows that the helices are characterized by chiral grooves around the polysaccharide
backbone. The polar groups in the solutes interact with the central, polar polysaccharide core mainly through hydrogen-bonding
interactions that align the molecules in the grooves. The remainder of the solute molecule interacts with the groups with
which the polysaccharide is derivatized, either sterically or through p-p interactions. Because both the solute and the environment are chiral, the adsorbed enantiomers experience different energies
of absorption and the chromatographic separation is achieved.
High-performance liquid chromatography (HPLC), simulated moving bed chromatography (SMB), and supercritical fluid chromatography
(SFC) take advantage of the differential absorption, meaning that one enantiomer moves more quickly through the column than
the other, explains Cox. In HPLC, the stationary phase is held stationary in a column while the mobile phase passes through.
The enantiomers emerge separately from the column each at a unique retention volume. SFC is broadly similar except that the
mobile phase is based on carbon dioxide, which reduces the solvent consumption in the process while speeding the separations
due to the low mobilephase viscosity.
In SMB, the system is arranged to mimic a countercurrent separation, where the two phases are moved in opposite directions.
This makes for a more efficient use of both the stationary and mobile phases and in addition is a continuous process as opposed
to the batch operation of HPLC. HPLC and SFC are generally used in the early stages of development when up to a few kilograms
of purified enantiomer is required while SMB is typically used in larger scale-up for Phase II through manufacturing of the
drug product due to its intrinsically lower cost.
Types of chiral stationary phases
Cox points out that there are several CSP types as explained below.
Pirkle type. These are CSPs based on the work of Prof. Pirkle and that use small molecules as the chiral selector. These CSPs are designed
to give a variety of different interactions, usually hydrogen bonding and p-p acceptor or donor plus perhaps some steric interaction to give the desired three-point contact of the enantiomer with the
chiral phase. Where the fit of the solute to the stationary phase is good, these CSPs have high selectivity and good loading
capacity (e.g., the Whelk-O phase, which was developed for the separation of naproxen enantiomers). Often, these phases do
not have a broad applicability and are able to resolve relatively few racemic compounds, notes Cox.
Cyclodextrins. These CSPs work on the basis of inclusion of the solute molecule into the basket-like structure of the cyclodextrin, says
Cox. As with the polysaccharides, the ring of carbohydrate molecules forms a chiral environment that may be modified by the
derivatization of the free hydroxyl groups. The cyclodextrins are used as mobile phase additives in electrophoretic separations
but can also be used as a stationary phase for HPLC. The main disadvantage of the cyclodextrin CSPs is one of low loading
capacity.
Macrocyclic antibiotics. Several CSPs based on macrocyclic antibiotics (e.g., vancomycin and teicoplanin) have been developed. These CSPs have a
reasonably wide application range, although they are less generally useful than the polysaccharide-based media and in some
cases (such as for amino acids) have been found to be complementary to the polysaccharide CSPs, explains Cox. Questions have
been raised about their use for preparative chromatography of pharmaceutical products, given their antibiotic origins.
Template CSPs. CSPs have been developed for specific separations by the production of a "template" phase. This consists of the polymerization
of the phase around molecules of the solute—or of a close analog—that is required to be retained, explains Cox. Removal of
the template molecule results in a stationary phase with cavities of exactly the right dimensions for the desired enantiomer
and into which the undesired enantiomer does not comfortably fit. Such CSPs can demonstrate extremely high selectivity (sometimes
too great to be useful for analytical use) but typically have capacities too low to be of interest for preparative work.
