General information: High Throughput Screening
The “post-genomic era” has generated a lot of excitement and expectations and provided the pharmaceutical industry with a wealth of potential targets to follow up on necessitating novel approaches and tools in order to deal with this wealth of targets. Consequently, high throughput screening or HTS was born in the pharmaceutical industry out of the need to test a lot of potential drugs in a very short period of time in a given assay. This brute force approach in which tens to hundreds of thousands of small molecule drugs are tested in a single day against one assay has led to the discovery of various novel drugs and small molecule tools which are being used for as tools for basic research purposes.
The hallmarks of HTS are the high sample throughput, miniaturization of assays and full automation of the assay process leading in the ideal case to an unattended 24/7 schedule. This requires robotics, data processing and control software, liquid handling devices, and sensitive detectors. However, HTS is not limited to drug discovery but can be used by any researcher to quickly conduct hundreds of thousands of biochemical, genetic or pharmacological tests.
There are 3 main points which influence the success of an HTS campaign:
The MSSR has automated equipment and data collection that make it possible to test about 100,000 compounds (or more) on a system of interest in about a day. Material and media usage are minimized by conducting the experiments in microplates and by using instruments that can read from about 30 uL of material per well. Scanning time is typically about 2 minutes per plate.
Of course, it’s not quite that simple. Assays normally need to be converted to high throughput mode. This involves not only scaling down amounts but also testing with appropriate controls to ensure that the assay is robust enough for high throughput screening. MSSR personnel are available for consultation in this process. The MSSR also possesses liquid handling devices which can automate tedious steps of pipetting.
Unfortunately, not all types of assays are amenable to high throughput mode. In general, fluorescence and luminescence based assays are easily adapted. In addition, ELISA’s and colorimetric assays typically present no problem for adaptation. It is important to keep in mind, however, that this adapted assay is just a screen. There are any number of artifacts and interferences that could arise. Therefore (again following the lead of the pharmaceutical industry), it is imperative to follow up on any "hit compound" before claiming any interaction between it and your system of interest. Follow up normally takes the form of a secondary, non high throughput assay, of a different type than the high throughput screen. For example, if fluorescence were used in high throughput mode, a secondary assay should not employ this mode of detection. Otherwise, any artifacts that existed in the primary screen are likely to carry over into the secondary assay. Appropriate controls are necessary.
The larger the compound collection, the greater one’s chances of finding a hit. Currently, the MSSR has over 130,000 chemicals and plans to expand even further! Generally, one wants not only a lot of chemicals, but also many different types of chemicals. Besides the more familiar classes of chemicals, for example, steroids, peptidomimetics and carbohydrates, a chemical librarian is concerned with chemical space.
Axes for chemical space are defined by parameters of interest, for example hydrophobicity, partition coefficients, aromaticity, or hydrogen bonding capability might be used. Chemicals are then located in “space” according to these parameters. One’s chances of finding a hit improve with libraries that cover a lot of space. The libraries available at the MSSR are diverse; they cover quite a bit of chemical space.
The Molecular Screening Shared Resource maintains an Accelrys database of all assays run and the results of these assays. In this way chemicals will become fairly well characterized, and assay artifacts should become apparent. One should be able to discern non-specific ligands with a fair degree of certainty. Information in the database might also help a researcher to discern mode of interaction or possible target (if unknown). By cross referencing and collaborating with researchers who employed related assays, additional information may be gained. In short, for any one application or user, there is usually quite a bit of useful information contained in such a database.
Since it can be a challenge to share data of this kind even within one workgroup we collaborate with Collaborate Drug Discovery (CDD). CDD developed a platform which enables scientist to archive, mine and share their data with their collaborators around the planet using just a normal internet browser.