Chiral pharmaceutical analysis enantiomer separation

CE has been applied extensively for the separation of chiral compounds in chemical and pharmaceutical analysis.First chiral separations were reported by Gozel et al. who separated the enantiomers of some dansylated amino acids by using diastereomeric complex formation with Cu " -aspartame. Later, Tran et al. demonstrated that such a separation was also possible by derivatization of amino acids with L-Marfey s reagent. Nishi et al. were able to separate some chiral pharmaceutical compounds by using bile salts as chiral selectors and as micellar surfactants. However, it was not until Fanali first showed the utilization of cyclodextrins as chiral selectors that a boom in the number of applications was noted. Cyclodextrins are added to the buffer electrolyte and a chiral recognition may... 

Modified-C02 mobile phases excel at stereochemical separations, more often than not outperforming traditional HPLC mobile phases. For the separation of diastereomers, silica, diol-bonded silica, graphitic carbon, and chiral stationary phases have all been successfully employed. For enantiomer separations, the derivatized polysaccharide, silica-based Chiralcel and Chiralpak chiral stationary phases (CSPs) have been most used, with many applications, particularly in pharmaceutical analysis, readily found in the recent literature (reviewed in Refs. 1 and 2). To a lesser extent, applications employing Pirkle brush-type, cyclodextrin and antibiotic CSPs have also been described. In addi-... 

Health authorities worldwide have fixed purity requirements for active pharmaceutical ingredients (APIs). When applied to chiral drugs, this implies that, if one enantiomer is chosen to be developed and marketed as an API, the counterpart isomer will be considered an impurity. The rule affects new chemical entities (NCEs) and chiral dmgs previously commercialized as a racemic mixture chiral switches). Therefore, techniques to perform the analytical control of the enantiomeric composition, at any of the drug development steps, together with processes to produce enantiomeric compounds with the desired enantiomeric purity, are essential in this domain. Liquid chromatography using chiral stationary phases (CSPs) is applied at two levels, analysis and production of enantiomerically pure compounds. At present, it can be considered the most universal technique for enantiomer separation. 

Chiral Chromatography. Chiral chromatography is used for the analysis of enantiomers, most useful for separations of pharmaceuticals and biochemical compounds (see Biopolymers, analytical techniques). There are several types of chiral stationary phases those that use attractive interactions, metal ligands, inclusion complexes, and protein complexes. The separation of optical isomers has important ramifications, especially in biochemistry and pharmaceutical chemistry, where one form of a compound may be bioactive and the other inactive, inhibitory, or toxic. 

Enantiomeric separations have become increasingly important, especially in the pharmaceutical and agricultural industries as optical isomers often possess different biological properties. The analysis and preparation of a pure enantiomer usually involves its resolution from the antipode. Among all the chiral separation techniques, HPLC has proven to be the most convenient, reproducible and widely applicable method. Most of the HPLC methods employ a chiral selector as the chiral stationary phase (CSP). 

Bonato and Paias [136] developed two sensitive and simple assay procedures based on HPLC and capillary electrophoresis for the enantio-selective analysis of omeprazole in pharmaceutical formulations. Racemic omeprazole and (S)-omeprazole were extracted from commercially available tablets using methanol-sodium hydroxide 2.5 mol/1 (90 10). Chiral HPLC separation of omeprazole was obtained on a ChiralPak AD column using hexane-ethanol (40 60) as the mobile phase and detection at 302 nm. The resolution of omeprazole enantiomers by capillary electrophoresis was carried out using 3% sulfated /1-cyclodextrin in 20 mmol/1 phosphate buffer, pH 4 and detection at 202 nm. 

Berzas Nevado et al. [138] developed a new capillary zone electrophoresis method for the separation of omeprazole enantiomers. Methyl-/ -cyclodextrin was chosen as the chiral selector, and several parameters, such as cyclodextrin structure and concentration, buffer concentration, pH, and capillary temperature were investigated to optimize separation and run times. Analysis time, shorter than 8 min was found using a background electrolyte solution consisting of 40 mM phosphate buffer adjusted to pH 2.2, 30 mM /1-cyclodextrin and 5 mM sodium disulfide, hydrodynamic injection, and 15 kV separation voltage. Detection limits were evaluated on the basis of baseline noise and were established 0.31 mg/1 for the omeprazole enantiomers. The method was applied to pharmaceutical preparations with recoveries between 84% and 104% of the labeled contents. 

Enantioselectivity was introduced especially for use in analysis of pharmaceuticals, where it was found that some pharmaceutical products have a chiral center and only one of the enantiomers exhibits the required pharmacological and pharmacokinetic behavior. The term was introduced first in relation to separation techniques,278 281 and later sensor technology.282... 

Since in most cases only one enantiomer possesses a desired pharmacological activity, it is necessary to construct enantioselective sensors to improve the quality of analysis due to the high uncertainty obtained in chiral separation by chromatographic techniques.315 For this purpose, enantioselective amperometric biosensors and potentiometric, enantioselective membrane electrodes have been proposed.264 The selection of one sensor from among the electrochemical sensor categories for clinical analysis depends on the complexity of the matrix because the complexity of different biological fluids is not the same. For example, for the determination of T3 and T4 thyroid hormones an amperometric biosensor and two immunosensors have been proposed. The immu-nosensors are more suitable (uncertainty has the minimum value) for direct determination of T3 and T4 thyroid hormones in thyroid than are amperometric biosensors. For the analysis of the same hormones in pharmaceutical products, the uncertainty values are comparable. 

Just as modem liquid chromatography (LC) is the dominant separative technique in the analysis of pharmaceuticals, so, in the evolution of methodologies for the chiral analysis of pharmaceuticals, LC has emerged as the pre-eminent technique. As already intimated, discrimination between enantiomers requires the presence of a chiral selector. In the determination of enantiomers by LC... 

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