Introduction
The early drug discovery process is resourceintensive, and for a variety of reasons the vast majority of HTS hits do not progress very far. Medicinal chemistry resources are critical to success, and often in great demand, especially when a strong candidate emerges and intense synthetic efforts are needed to identify a promising clinical candidate. Therefore, the hundreds to thousands of molecules emerging from HTS screening need to be efficiently prioritized and vetted. Filters are used to hone the numbers down to manageable quantities and one filter is chirality. Unlike achiral hits that consist of a single molecule, chiral hits consist of two enantiomers that can have dramatically different biological activity. As such, assay data for chiral mixtures is clouded by the activity of both molecules and thus does not reflect the activity of the individual enantiomers. Therefore, chiral hits are best studied as individual enantiomers which require the additional step of preparative separation. Statistics from the marketplace offer compelling reasons to invest in this extra effort. For example, of the 30 top selling drugs in 2014, 40% were chiral, while only 13% were achiral [the remaining 47% were therapeutic proteins or antibodies (1)]. Similarly, of the 40 most promising clinical candidates that were in development in 2016, more than 38% were chiral (2).
To address the need to purify chiral molecules at early stages of drug discovery, many large pharmaceutical companies have developed in-house capabilities for chiral separations (3, 4). However, smaller companies often lack extensive resources and may have limited capabilities or rely on outside organizations and expertise.In the work discussed here, we assess the success rate of doing chiral separations at the earliest “hitto- lead” stage of drug discovery research. “Hit compounds” identified in HTS screens are often from libraries of small molecules available commercially from specialized companies that support small molecule drug discovery research. To mimic this process we used the same source of compounds for our study. The compounds were screened by SFC for the purpose of developing preparative purification methods and also by HPLC in cases where SFC did not provide adequate separation and for the purpose of analyzing purity post-purification. Three of the top classes of chiral stationary phases were screened: coated polysaccharide phases with the widely used chiral selector Tris-(3,5-dimethylphenyl) carbamate bound to amylose (RegisPack™ chiral column) and cellulose (RegisCell™ chiral column) and a Pirkle-type immobilized phase (Whelk-O® 1 chiral column) (5). To provide information on compounds most pertinent to drug discovery, we chose to screen a series of chiral derivatives of privileged scaffolds (6, 7) including 1,3-thiazoles (8,9), indoles (10,11), 1,3,5-triazines (12), 1,3-benzothiazoles (13), pyranoquinolones (10), dihydroquinazolinones (14), and leucolines (13).
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