Sources of Compounds for Biological Screening

Once the assay and assay format have been decided upon, the next step in the discovery process is to initiate compound screening for the purpose of identifying hits or lead compounds. The fundamental requirement is that the assay results identify a collection of actives or hits. The definition of hit varies between organizations, but most accept the definition that the compound shows a confirmed structure, shows a confirmed dose response, exhibits an IC50 lOpM potency, and is a member of a chemotype that is amenable to analoging and fast follow-on synthesis. 

What is the source of these initial actives or hits There is a wide array of compound sources. Generally, pharmaceutical and biotechnology organizations initiate screening by accessing their internal compound repositories (so- 

In addition to quality control over compound collections, the issue of purity of synthetic libraries derived using combinatorial chemistry quickly came under the microscope. In the early to mid-1990s, combichem became a household word throughout the pharmaceutical industry and was believed to be a key technology that would revolutionize drug discovery. The basis of 

Almost all of the analytical characterization tools (e.g., HPLC, NMR, FTIR, and LC/MS) are serial-based techniques, and parallel synthesis is inherently parallel. Consequently, this led rapidly to a new bottleneck in the discovery process (i.e., the analysis and purification of compound libraries). Parallel synthesis suffers from some of the same shortcomings of split and mix synthesis (e.g., the expected compound may not be pure, or even synthesized in suffi- 

LC/MS emerged as the method of choice for the quality control assessment to support parallel synthesis because the technique, unlike flow injection mass spectrometry, provides the added measure of purity (and quantity) of the compound under investigation. In addition, universal-like HPLC gradients (e.g., 10% to 90% acetonitrile in water in 5 minutes) have been found to satisfy the separation requirements for the vast majority of combinatorial and parallel synthesis libraries. Fast HPLC/MS has been found to serve as good surrogate to conventional HPLC for assessing library quantity and purity [34-37]. Fast HPLC/MS is simple in concept. It involves the use of short columns (typically 4.6 mm i.d. x 30 mm in length) operated at elevated flow rates (typically 3-5mL/min). 

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