Enantioselective separation of pharmaceuticals using novel chiral stationary phases in conventional and capillary liquid chromatography

Abstract

Chiral drugs are non-superimposable mirror images of the same molecule, and hence, different enantiomers originate. Enantiomers have the same physical and chemical properties but are sometimes different in potency and toxicity. In the 1960s,the S enantiomer of thalidomide caused birth defects in 12,000 babies, whereas the R form is the one that has the therapeutic activity. Therefore, obtaining a pure enantiomer is sometimes extremely important. Among the most widely used methods to access enantiomers; kinetic resolution, crystallisation and enantioselective chromatography proved competency. Chromatography has attracted a lot of interest in chiral resolution due to its excellent efficacy in the separation of racemic pharmaceuticals in short time. Several analytical techniques are available for chiral separations including gas chromatography, supercritical fluid chromatography, capillary electrophoresis, capillary electrochromatography and High-performance liquid chromatography. Separations via high-performance liquid chromatography represent the most prominent method which has many advantages, such as high separation efficiency, good selectivity, high detection sensitivity, automatic operation, a wide range of applications, the column can repeatedly be used, the sample is small, easy to recycle. Miniaturization is a current trend sweeping across all areas of separation science, and indeed the development of stationary phases for use in nano-high-performance liquid chromatography is an area of increasing interest, due to less cost and higher efficiency. Nano-high-performance liquid chromatography uses microfluidic techniques, which integrated narrowed column inner diameter (10-300 μm) and decreased flow rate (200-1000 μL/min). The monoliths stationary phases used in nano systems offer advantages over conventional particle-packed equivalents, due to their dual pore distribution, enhanced mass transferability and relatively wide macropores allowing high flow rates with minimal backpressures. Inorganic silica-based monoliths are particularly advantageous due to their wide range of solvent compatibility and ease with which their surface may be derivatised. In this project, the recently commercialised conventional CHIRALPAK IG column was investigated for the enantioselective separation of a set of pharmaceuticals under standard, non-standard and reversed phase mobile phase condition. A further modified sub 2-micron version of the same column was also investigated for the enantioselective separation of racemates and compared with other investigated columns. The results revealed that shortening the column length, internal diameter and using sub 2-micron silica particles size for immobilisation are all beneficial for faster and baseline separation of a set of pharmaceuticals. In an attempt to fabricate a cost-effective column with similar characteristics to the commercially available conventional columns, hybrid-type native silica monolithic capillary columns (100 μm i.d) were prepared in eight generations and tested for efficiency. Characterization of the monoliths morphology was elucidated using scanning electron microscopy. Surface coverage of the monolith with anchor group was chromatographically evaluated, with modification process repeated if necessary. Nano- high-performance liquid chromatography is used to separate enantiomers by immobilisation of a chiral selector namely functionalized proline, sulphated beta-cyclodextrin and bacitracin on polymer-based monolithic columns. The new chiral stationary phases were screened for enantioselective separations using a variety of racemic pharmaceutical compounds (n=47) using reversed phase (methanol/water, acetonitrile/water) chromatography modes. The results revealed that; regarding resolution (Rs) and separation factor (α),the functionalized proline column showed good performance in the separation of small molecules. In particular, under the 10% acetonitrile in copper sulphate (0.001mol/L copper(II) sulphate in water) (v/v) mobile phase. On the other hand, under similar condition, the sulphated β-cyclodextrin column did not demonstrate good performance in separation as functionalized proline column, and the best range of the mobile phase was 7/93 (v/v) and 10/90 (v/v) methanol/water. The bacitracin column did not show good performance in separation as functionalized proline column; however, the results are better than the sulphated β-cyclodextrin column with 3/97/0.1 (methanol/water/tetrahydrofuran) as an ideal mobile phase system for the separation of the investigated compounds.

Date of Award	2018
Original language	English
Supervisor	Ashraf GHANEM (Supervisor), Yasser Sokeirik (Supervisor) & Bill Maher (Supervisor)