High-Throughput Screening

Most drugs work by binding to a protein target on or in a living cell. One of the first steps in drug discovery and development is finding molecules that will bind to the target. Imagine, for instance, you want to develop an anticancer drug that binds to and inactivates a particular mutant protein known to promote aberrant cell growth. You have a couple of compounds that bind very weakly to your protein, and these serve as the starting point for generating a large number of related compounds, through combinatorial chemistry. In this method, many thousands of related compounds can be quickly and automatically synthesized. Those that bind best can be modified and tested further, and ultimately may go on to be tested in animals and people as candidate drug therapies.

Initial screening of these compounds for their binding ability is the job for HTS. The key to HTS is to develop a test, or assay, in which binding between a compound and a protein causes some visible change that can be automatically read by a sensor. Typically the change is emission of light by a fluorophore in the reaction mixture. One way to make this occur is to attach the fluorophore to the target protein in such a way that its ability to fluoresce is diminished (quenched) when the protein binds to another molecule. A different system measures the difference in a particular property of light (polarization) emitted by bound versus unbound fluorophores. Bound fluorophores are more highly polarized, and this can be detected by sensors. Other detection methods are possible as well.

The details of HTS differ with different systems, but all depend on automated or robotic systems to combine the chemicals and read the outputs. Reactions between the target protein and the compound usually occur in microplates, which are plastic trays with multiple indentations, or wells. Systems currently in use can handle plates with 96, 384, 1,536, or even higher numbers of wells at once. HTS typically uses extremely small volumes in each well, often 10 microliters or less. Small volumes have numerous advantages, including keeping to a minimum the amount of each compound used. This is especially important for many proteins targets, which may be difficult and costly to isolate and purify.

The time required for reactions varies with the substances involved, and may range from several minutes to several hours. Fast robotic systems combined with rapid reactions can screen 10,000 or more compounds in a single day. This is an enormous increase over traditional chemical assays, in which a chemist may be able to handle fewer than 100 tests in the same amount of time.