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February 01, 2012 (Vol. 32, No. 3)

Screening Large Compound Libraries

Challenges Inherent in Primary and Secondary Stages Necessitate Automation of Liquid Handling

  • The screening of large compound libraries to identify hit compounds involves a number of laborious liquid-handling stages. Plate preparation for compound screening and bioassays involves sample transfer from compound storage plates or vials, serial dilutions, assay plate preparation, and replication. These processes require repetitive, accurate, and time-consuming pipetting as well as meticulous sample labeling and management.

    With an escalating number of compounds, small molecules or fragments, and RNAi libraries, automated liquid handling has become a fundamental requirement for high-throughput screening throughout the drug discovery industry.

    Automated liquid-handling instruments such as those in the mosquito® product range (TTP LabTech) allow rapid, accurate, and reproducible compound dispensing and assay plate setup, reducing the turn-around time from compound synthesis to assay-ready plates.

    In addition, the ability to miniaturize assays using nanoliter liquid-handling instruments reduces the use of expensive reagents, helping to preserve the often limited quantities of valuable compounds or samples.

    Following a primary screen it is common for only a small percentage (1–3%) of compounds to be identified as active, requiring further study with secondary screening. At this secondary screening stage, individual samples need to be cherry picked from stock compound plates and new plate formats need to be set up for further biological assays.

    Traditional manual or semi-automated methods are unable to deliver the speed or throughput required for the efficient removal of hit samples from large numbers of compound storage or “mother” plates.

    In addition, instruments used for plate replication are often unable to cherry pick individual samples from a plate and can be limited for transferring samples between plate sizes (i.e., 384 well plates to 96 well plates or vice versa). Therefore the use of a dedicated automated liquid handler capable of rapidly cherry picking individual hit compounds as well as plate reformatting can significantly enhance the compound screening workflow.

    Incorporation of suitable tracking software and 2-D barcodes on the screening platform enable efficient sample tracking and recording, thereby eliminating the potential for manual data input error.

  • Small Academic Screening Facilities

    Click Image To Enlarge +
    Table. Comparison of data throughput from 200 individual compound screens using manual dispensing versus TTP Labtech’s mosquito® X1.

    The recent installation of a mosquito® X1 into the HTS Core at the Center for Chemical Genomics, headed by Martha Larsen at the Life Sciences Institute, University of Michigan, has enabled rapid and effective cherry picking of samples following primary screening.

    The key requirement for this small academic screening facility was for a low-cost, reliable, and integrated system to confirm primary high-throughput data and screen multiple plates in one operation. Although this research group employs a semi-automated pin tool for compound sample transfers during primary screening, it is not possible to use this instrument to select or array compounds for hit confirmation. Such tools are also not capable of performing serial dilutions for dose-response studies of selected compounds at the secondary screening stage.

    In this lab, prior to using mosquito X1, the manual preparation of 384-well plates limited screening throughput to 200 compounds per screen. Using mosquito X1, the group has successfully increased its screening capability to 3,000 compounds in a single run.

    The Table highlights the comparison of data throughput obtained from 200 separate compound screens carried out before and after acquiring the mosquito X1. From this study it was seen that there was a threefold increase in the number of dose-response assays carried out and a fourfold increase in the number of plates screened per assay since using the mosquito X1. This led to a 14-fold increase in the number of compounds tested with over 101,639 compounds confirmed as active.

    In addition, with mosquito’s capacity for four destination assay plates allowing confirmation and dose-response formats it was possible to link this instrument to a RapidStak (Thermo Fisher Scientific). The RapidStak, which is capable of holding up to 150 plates at a time, automatically feeds to the mosquito X1 instrument, allowing a walk-away solution for secondary screening studies.

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