Addition of organic modifiers to control retention order of enantiomers of dihydropyridines on chiral-AGP

Immobilization of Chondroitin Sulfate A onto Monolithic Epoxy Silica Column as a New Chiral Stationary Phase for High-Performance Liquid Chromatographic Enantioseparation

Chondroitin sulfate A was covalently immobilized onto a monolithic silica epoxy column involving a Schiff base formation in the presence of ethylenediamine as a spacer and evaluated in terms of its selectivity in enantioseparation. The obtained column was utilized as a chiral stationary phase in enantioseparation of amlodipine and verapamil using a mobile phase consisting of 50 mM phosphate buffer pH 3.5 and UV detection. Sample dilution by organic solvents (preferably 25% v/v acetonitrile-aqueous solution) was applied to achieve baseline enantioresolution (Rs > 3.0) of the individual drug models within 7 min, an excellent linearity (R² = 0.999) and an interday repeatability of 1.1% to 1.8% RSD. The performance of the immobilized column for quantification of racemate in commercial tablets showed a recovery of 86–98% from tablet matrices. Computational modeling by molecular docking was employed to investigate the feasible complexes between enantiomers and the chiral selector.

Reversal of elution order for profen acid enantiomers in packed-column SFC on Chiralpak AD

Enantiomeric separations of four 2-substituted propionic acid drugs have been studied using packed-column supercritical fluid chromatography (SFC) with amylose tris(3,5-dimethylphenylcarbamate) coated on silica as support (Chiralpak AD). Under standard conditions (i.e., flow rate, 1.5 ml/min; column temperature, 30 degrees C; back-pressure, 150 bar), the order of elution could be reversed when the polar alcohol modifier methanol in carbon dioxide was replaced by 2-propanol for ibuprofen, ketoprofen, and naproxen. For flurbiprofen, with the highest selectivity factor, no reversal was observed, although selectivity was reduced significantly with higher alcohols. Naproxen and flurbiprofen were also investigated with 2-butanol and 2-pentanol. The former showed reversal of elution order but not the latter. For higher alcohol modifiers, including 2-propanol, the peak symmetry was poor but could be improved by addition of citric acid in the alcohol modifier. These results stress the importance to investigate enantiomer elution order during the development of enantioselective methods and when chromatographic conditions are optimized. Preliminary experiments with column temperatures over the range of -15 to 45 degrees C revealed that, in a few cases, reversal took place with a change in temperature only.

HPLC enantiomer separation of a chiral 1,4-dihydropyridine monocarboxylic acid

An enantioselective anion exchanger based on tert-butylcarbamoylquinine as chiral selector and thiol-modified silica as chromatographic support was applied for the enantiomer separation of the shortacting calcium antagonist clevidipine after its hydrolysis to methyl 4-(2',3'-dichlorophenyl)-2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate. This hydrolyzed derivative of clevidipine is the primary metabolite and precursor of the parent drug. Method development included the steps of optimization of the composition of the mobile phase (pH, type and content of organic modifier, polar organic mode), screening of various structural analogs of above mentioned CSP, and evaluation of the effect of the flow rate. These detailed studies gave insight into the operational mode of the CSP, which is basically an enantioselective anion exchange mechanism. The polar organic mode turned out to be advantageous with regard to enantioselectivity and resolution. The optimized method makes use of an eluent composed of 0.125% acetic acid in acetonitrile with flow rate of 1 ml/min at a constant temperature of 25 degrees C, and allows the separation of the both enantiomers with enantioselectivity alpha of 1.25 and a resolution RS of 3.0 within 10 min. On the above mentioned quinine carbamate phase the (R)-enantiomer is stronger retained, while a change to the corresponding quinidine carbamate CSP allows the reversal of the elution order.

True or apparent reversal of elution order during chiral high-performance liquid chromatography monitored by a polarimetric detector under different mobile phase conditions

The use of all chromatographic modes (i.e. reversed-phase, normal-phase and polar organic modes) is becoming more and more systematic in chiral HPLC method development. When this method development is monitored by polarimetric detection, which is a widespread technique, one should not forget that the observed positive or negative rotation angle for a given absolute configuration depends on the solvent. As a cautionary illustration, a new example is reported for a racemate which is separated with the same elution order in ethanol or acetonitrile mobile phase on a CHIRALCEL OD-R column whereas the signs of polarimetric detection are reversed.

Stereoselective chromatography of cardiovascular drugs: an update

This review reports the latest achievements in chromatographic enantioseparations of various classes of cardiovascular drugs and selected applications of these methods in pharmaceutical and clinical analysis. The use of these drugs as test compounds for new chiral stationary phases and different parameters of chromatographic processes is also presented.