Current Trends in Chiral Chromatography
Professor Debby Mangelings from the Vrije Universiteit, Belgium, estimates that 60% of newly commercialized drugs possess chiral properties. In this interview with LCGC, Mangelings discusses the importance of chiral chromatography in pharmaceutical analysis, the challenges of current methods, and where the field is headed next.
Q. What are your main research interests and how did you become interested in your field?
The interest of my main research field originates from my studies as a pharmacist: As a first year student, I became fascinated by the fact that mirror images of molecules exist, and that they display different activities in the human body. Later, I discovered that chirality had an enormous impact on the pharmaceutical industry, and I was lucky that the department where I did my Master and PhD thesis was doing research on chiral separations.
Q. What are you focusing on at the moment?
Besides these projects, we are working on precision improvement and method transferability of CE methods between different instruments and laboratories. Also here, we are using chiral separations as test cases.
In SFC, we also initiated a project concerning the development of drug impurity profiles.
A final topic is the development of fingerprint chromatograms by means of liquid chromatography–mass spectrometry (LC–MS) and LC with diode array detection (LC–DAD), and subsequent chemometric analysis. The idea is to compare both fingerprint types and see how much information is kept, gathered, or lost for data analysis when switching from LC–DAD to LC–MS fingerprints.
Q. Why is there so much emphasis on chiral analysis and the detection of enantiomers in pharmaceutical products?
In this chiral environment, different interactions are seen for individual enantiomers. For chiral medicines, it is often seen that only one enantiomer (eutomer) displays the therapeutic activity, while the other (distomer[s]) may exert no effect, another effect, an antagonist effect, may cause side effects, or can even be toxic. When a chiral molecule is commercialized in a pharmaceutical formulation, the presence of distomeric forms may be inconvenient. Therefore, this should be investigated properly in advance. Softenon is an often-handled example when discussing the risks of introducing chiral drug molecules. Here, racemic thalidomide (equimolar mixture of enantiomers) was incorporated as active pharmaceutical ingredient (API). One enantiomer (probably S-thalidomide) had teratogenic properties and administration of Softenon to pregnant women led to thousands of newborns with serious fetal malformations.
Q. Why is the single enantiomer important and how is this regulated?
The effects of the enantiomers in a chiral drug molecule must be carefully investigated for the registration file. The pharmacological and toxicological profile of each enantiomer must therefore be well documented in regulatory files, because when a single enantiomer drug is commercialized, other enantiomers are considered as an impurity of the API. Identification tests should be able to distinguish the enantiomers of the drug molecule. In addition, the enantiomers must be separated and quantified during production, fabrication, and quality control processes.
Knowing that at least 60% of newly commercialized drugs have chiral properties, there is no need to state that the development of chiral separation methods is very important in the pharmaceutical industry.
Q. What methods are used for chiral separation in industry at the present time?
Direct methods use a chiral selector that forms transient diastereomeric complexes with the enantiomers, enabling their separation. This selector can be added to the mobile phase but this approach is mostly used in miniaturized techniques such as CE and CEC because of the high selector consumption with techniques such as HPLC. The selector may also be coated or immobilized on a chromatographic matrix, creating a chiral stationary phase (CSP). The use of a CSP is by far the most applied approach in the pharmaceutical industry to separate enantiomers, with the most popular CSP those containing polysaccharide-, macrocyclic antibiotic-, or cyclodextrin-based selectors. CSPs are used in combination with chromatographic techniques, such as HPLC, SFC, and gas chromatography (GC), of which HPLC is definitely the most popular one. Some companies also use SFC and CE, but the application of those techniques is far below that of HPLC, but there has been a kind of a revival in SFC recently.
Q. What are the limitations of these methods?
Chiral HPLC consumes a relatively high amount of organic solvent for the analysis. Particularly for less environmentally friendly modes of HPLC such as normal-phase- and polar organic solvent chromatography, this can represent a major hurdle on the level of waste disposal. The analysis times can also be quite long, but this is really case-dependent.
Q. Are there other methods in development or emerging to replace such established methods?
I also still believe in the potential of CEC because it uses a combined separation principle (chromatographic partition and electrophoretic mobility) that is quite unique. However, for this technique, there are not only instrumental issues, development of proper CEC stationary phases is also needed.
With regards to stationary phase technology, polysaccharide-based selectors are the most popular. Recently, several companies introduced chlorinated polysaccharide-based selectors and some even have a broader enantioselectivity than non-chlorinated. Polysaccharide-based selectors were also immobilized to cope with the restriction of using special solvents, like acetone, tetrahydrofuran (THF), and chloroform, on coated polysaccharide CSP. Therefore, these selectors will probably remain the most frequently used. What may be an interesting development is the use of new chromatographic matrices, for example, the use of smaller particle sizes and fused-core materials. However, at the moment these are still at the research level.
Q. What will it take for these methods to become industry standards?
Q. Where do you see your research taking you in the future?
In CEC, our research will remain in the chiral separation field, where the investigation of polysaccharide CSPs with smaller particles and fused-core particles will be future topics.
More fundamental research in chiral separations is a necessity that remains, so comparative studies will be introduced in our future research. We will also focus on including more chemometrics in this field of our research.
Debby Mangelings is a professor at the Vrije Universiteit Brussel, Belgium. She graduated as a pharmacist in 2001, obtained a PhD in Pharmaceutical Sciences in 2006, and became a full professor in 2010. She is a member of the Department of Analytical Chemistry and Pharmaceutical Technology, headed by Yvan Vander Heyden. Together, they currently supervise the work of seven PhD students. Her main research interests are chiral separations and miniaturized separation techniques. Debby is author or co-author of eight book chapters and 59 manuscripts, 57 of which are published in peer-reviewed journals.