Chiral method development screening

Sajonz, R, Gong, X., Leonard, W. R., Jr., Biba, M., and Welch, C. 1., Multiparallel chiral method development screening using an 8-chaimel microfluidic HRLC system. Chirality 18(10), 803-813, 2006. 

The basis of chiral method development screening strategies is largely dictated by whether the method is to be applied to analytical or preparative applications. Preparative chromatography tends to use volatile modifiers and mobile phases for easy removal after sample collection. Sample isolation capabilities have led to the advancement of chiral SFC since CO2 is easily driven off. Chiral separations in preparative chromatography only need to be optimized to a point where pure samples can be collected. Conversely, while analytical chromatography can use a greater variety of modifiers and mobile phases, the separation needs to be optimized to a point where suitable resolution (R > 1.5) is achieved between the enantiomers of interest. 

Reversed phase chiral separations are desired simply for efficiency in generating results from laboratories whose instrumentation is routinely configured to run in reversed and not normal phase modes.Normal phase conditions are less attractive to the analytical chemist for this reason and deter laboratory efficiency. Typical commercial chiral LC columns found on pharmaceutical reversed phase LC chiral method development screens are listed in Table 8. Table 11 shows suggested chromatographic conditions employed in reversed phase chiral screening. 

Regardless of whether a traditional normal phase LC, reversed phased LC, or SFC format is used, routine analytical chiral method development screening is very effective and efficient for developing chiral selective methods in the pharmaceutical industry. 

Wong, M. M., Holzheuer, W. B. and Webster, G. K. A Comparison of HPLC and SFC Chiral Method Development Screening Approaches for Compounds of Pharmaceutical Interest. Curr. Pharm. Anal 4 101, 2008. 

AMn A, Antosz FJ, Ausec JL, Greve KF, Johnson RL, Mag-nusson L-E, Ramstad T, Secreast SL, Seihert DS, Webster GK. An orthogonal approach to chiral method development screening. Curr. Pharm. Anal. 2007 3 53-70. 

Sajonz P, Gong X, Leonard Jr.WR, Biha M, Welch CJ. Multiparallel chiral method development screening using an 8-channel microfluidic HPLC system. Chirality 2006 18 803-813. 

In this way, we aim to give an overview of what can be used as a separation technique and which conditions will most likely give an (beginning of) enantiomer separation after a first screening. Chiral method development starter kits are also available and evaluated in some papers [2], but we will not focus on this kind of applications. 

Mangelings, D., Maftouh, M., Massart, D.L., Vander Heyden, Y. Generic capillary electrochromatographic screening and optimization strategies for chiral method development. LC-GC Europe, 2006, 19, 40 7. 

Application databases have been particularly popular in the world of chiral method development (Figure 10-5). While it has been observed that small changes in compounds can result in loss of effectiveness (separation selectivity) for a given method, the results of searches can be used to create targeted method screens that can reduce the time and expense of development [36]. 

Supported liquid membranes (SLMs) consisting of 5% tri-n-octylphosphine oxide (TOPO) dissolved in di-w-hexylether/n-undecane (1 1) have been used in the simultaneous extraction of a mixture of three stUbene compounds (dienestrol, diethylstilbestrol, and hexestrol) in cow s milk, urine, bovine kidney, and liver tissue matrices [183]. The efficiencies obtained after the enrichment of 1 ng/1 stilbenes in a variety of biological matrices of milk, urine, liver, kidney, and water were 60-70, 71-86, 69-80, 63-74, and 72-93%, respectively. A new method to contribute to the discrimination of polyphenols including resveratrol with synthetic pores was proposed [184]. The work [185] evaluated two types of commonly available chiral detectors for their possible use in chiral method development and screening polarimeters and CD detectors. Linearity, precision, and the limit of detection (LOD) of six compounds (trans-stilbene oxide, ethyl chrysanthemate, propranolol, 1-methyl-2-tetralone, naproxen, and methyl methionine) on four common detectors (three polarimeters and one CD detector) were experimentally determined and the limit of quantitation calculated from the experimental LOD. trans-Stilbene oxide worked well across all the detectors, showing good linearity, precision, and low detection limits. However, the other five compounds proved to be more discriminating and showed that the CD detector performed better as a detector for chiral screens than the polarimeters. 

Linearity, precision, and the limit of detection (LOD) of trans-stilbene oxide and other compounds were investigated [89]. The authors investigated the second factor and evaluated two types of commonly available chiral detectors for their possible use in chiral method development and screening polarimeters and CD detectors. It was shown that frans-stilbene oxide worked well across all the detectors examined, showing good linearity, precision, and low detection limits. 

TABLE I Polysaccharide-Based Chiral Columns Commonly Used in Pharmaceutical HPLC Method Development Screening Systems... 

FIGURE 3 Unified chiral screening approach to chiral methods development. 

A knowledge-based chiral method development strategy can be very effective on chiral column selection. For example, a chiral screening method was used to monitor the enantiomeric purity of an atropisomeric dmg candidate. It returned the Chiralcel OD-RH column in reversed phase with a baseline enantioseparation of both atropi-somers (Fig. 12a). There was a need to redevelop the chiral method as the compound moved into development stages. Based on possible interaction sites around the chiral asymmetric axis of the compound (a primary amine for ionic and H-bonding interactions and an aromatic ring for k-k interaction), Chirobiotic V2 was tested since... 

Each glycopeptide CSP has unique selectivity as well as complementary characteristics, and a considerable number of racemates have been resolved on all three of them. Interestingly, most of the resolved enantiomers have the same retention order on these macrocyclic CSPs. When they are mixed or coupled with each other, the selectivity on one CSP will not be canceled by another. Even if some compounds may not have the same retention order, the complementary effects will result in an identifiable selectivity. Therefore, the coupled chiral columns can be used as a screening tool and save chromatographers substantial time in method development. 

Method development remains the most challenging aspect of chiral chromatographic analysis, and the need for rapid method development is particularly acute in the pharmaceutical industry. To complicate matters, even structurally similar compounds may not be resolved under the same chromatographic conditions, or even on the same CSP. Rapid column equilibration in SFC speeds the column screening process, and automated systems accommodating multiple CSPs and modifiers now permit unattended method optimization in SFC [36]. Because more compounds are likely to be resolved with a single set of parameters in SFC than in LC, the analyst stands a greater chance of success on the first try in SFC [37]. The increased resolution obtained in SFC may also reduce the number of columns that must be evaluated to achieve the desired separation. 

Finally, it may be noted that some chiral column vendors provide free screening or method development support. 

The approach to method development is similar to the one described for HPLC and can be characterized as a rapid stationary phase screen using column and solvent switching with gradient elution followed by development of an isocratic preparative method. SFC has been successfully applied to the analytical and preparative separation of achiral and chiral compounds. 

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