The traditional screening paradigm involves one target for primary HTS. However, this process “wastes a considerable amount of time to get results, and also wastes efforts on compound management in order to get those compounds ready for testing,” said Peter Hodder, Ph.D., senior director of lead identification for the translation research institute at the Scripps Institute, Florida.
His group uses a parallel screening process that screens compounds against the target and antitarget simultaneously. “Antitarget is an all-encompassing name for any assay you would run that’s different from the target—usually to remove compounds from further consideration,” Dr. Hodder explained. “We found most of those compounds are junk compounds anyway.” The antitarget becomes important for the hit compounds, because it provides information on whether it is something specific to the target or whether it is something nonspecific to the assay format.
Time saved via parallel screening can be four to five weeks per target. In addition, and what is more important and what is harder to gauge, he noted, is saved efforts following false trails, which result in smaller, cleaner datasets. Relevant structure activity relationships emerge early in a campaign. For example, Dr. Hodder performed an SF1 (transcription factor) assay and ran the antitarget ROR against it and found potent compounds. “If we had relied on primary screening alone, those compounds would not have been selected.”
The parallel-screening format is not specific to any target class. “What’s more important is how to apply it to different target classes or different assay formats.” His group was successful in screening ion channels, including TRPML3 with TRPN1 as the antitarget (TRP is transient-receptor potential). HTS probes confirmed that the target is not located on plasma membranes in native cells.
Dr. Hodder added that this approach can be used to help focus on the most important compounds for drug or probe discovery, but it’s key is in choosing the right antitarget. “If it’s too close in relationship to the target, you’re going to start throwing out compounds you don’t want to during the campaign.”
His group is now performing more sophisticated screening using two or three antitargets and trying to find the overlap of hits that are specific in all three versus two or one of those targets and antitargets. “This challenges us to think about how we present and analyze our data.”