Screening approach, optimisation and scale-up for chiral liquid chromatography of cathinones.

Screening approaches adopted in pharmaceutical companies for chiral LC method development may be quite complicated and sophisticated in order to guarantee a high success rate. However in other environments it may be of more value to assess how simple a screen might be used to still have a good chance of achieving success. The genuine need to develop chiral separations for the former 'legal-high' drug mephedrone and related cathinones of topical interest presented a good opportunity to develop this theme. In initial work on mephedrone itself, no chiral separation was observed on Chirobiotic V, Cyclobond I 2000 DNP, Whelk-O1 and AmyCoat using reversed phase mobile phases. However, using normal phase solvents, chiral separation was observed on all the chiral stationary phases (CSP) used except Chiralcel OJ-H. Of the chiral separations observed on RegisPack, RegisCell and Whelk-O1, some optimisation work was carried out on the latter two which had showed the greatest enantioselectivity. Following optimisation, the best enantioselectivity (1.59) and enantioresolution (5.90) was found with a 250 mm × 4.6 mm I.D. Whelk-O1 column using a propan-2-ol (IPA)-hexane-trifluoroacetic acid (TFA)-triethylamine (TEA) (10:90:0.05:0.05, v/v/v/v) mobile phase. Subsequent screening on other cathinones was restricted to RegisPack, RegisCell and Whelk-O1 or equivalent phases with two mobile phases and this gave a very good success rate. Indeed it was possible to separate all six cathinones on one column, RegisCell, with one mobile phase, propan-2-ol-hexane-TFA (15:85:0.1, v/v/v) but obviously it had been necessary to go through the 3-column screen to arrive at this finding. While Whelk-O1 was not so successful, ease of optimisation on this phase was again a feature. To illustrate the applicability of these separations, it was shown that, as a basis for semi-preparative work, the optimsed mephedrone separation on Whelk-O1 could be scaled-up to a 2000µl injection of a 1.0 mg ml(-1) solution in mobile phase (2.0mg on-column) while still using the 250 mm × 4.6 mm I.D. analytical column.

Assessment of chiral stationary phases for suitability for combined enantiomeric impurity/related substances assays.

Chiral stationary phases (CSP) for LC had a major impact on pharmaceutical R&D when they first became commercially available in the 1980s. Even although the use of CSP in pharmaceutical R&D is now very much a mature area, there is still scope for using CSP more effectively to bring about efficiencies. One such instance is the possibility of combining the chiral LC test for the level of a trace enantiomeric impurity in a chiral drug substance and the LC test for related substances into one test. It was envisaged that this could be achieved by carrying out reversed-phase LC on an ODS silica/CSP coupled column system. In evaluating Chiralpak QD-AX, Cyclobond I 2000 DNP and Whelk-O1 CSP using a polar organic - aqueous mobile phase it was found that the Whelk-O1 CSP had good achiral selectivity, the required match of retentivity with the ODS silica material, ACE 5 C18 and also exhibited an encouraging degree of enantioselectivity in the reversed-phase mode. Following consideration of the selectivity of the ACE 5 C18 and Whelk-O1 phases it became apparent that it might be possible to achieve the desired goal of achieving both the enantiomeric impurity and related substances separations in one system by using the Whelk-O1 CSP on its own. This was subsequently demonstrated to be the case using S-naproxen, laevokalim and S-flurbiprofen as illustrative examples.