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.”
Some of the challenges of working with ion channels include controlling activity, whether with a small molecule ligand or voltage. Many ion channels inactivate within milliseconds, making HTS difficult.
David Weaver, Ph.D., director at Vanderbilt Institute of Chemical Biology HTS, has been focusing his research efforts on ion channels—especially 7TM (7-Transmembrane) receptors.
“We are interested in looking at some of the effector systems that are more physiologically relevant and one of these is the GIRK (G-protein regulated inwardly rectifying potassium (K+) channel).” His group developed this assay to measure the activity of GI-coupled 7TM receptors. “The idea was whether we could see any differences in the pharmacology and the fact that we may be using a more physiologically relevant end effector rather than using mutant G proteins to couple the change in intracellular calcium.”
The success of the GIRK assay encouraged Dr. Weaver to examine ion channels as end effectors that could be used to generate new assays with physiological relevance. Preliminary data demonstrates the ability to detect changes in M-current (muscarinic-modulated potassium current, usually studied in the brain and peripheral nervous system) activity.
He developed an HTS-compatible assay that can measure and quantify the modulation of M-current downstream from the 7TM receptor using thallium-flux. This optical assay platform can use a commercially available kinetic imaging plate reader.
According to Dr. Weaver, the only nonstandard part of the assay is that he extracts a slope from the initial measurement, instead of fitting a peak amplitude. His hope is to use this assay to further understand the pharmacology of 7TM receptors. “It’s my intent that we can demonstrate that these are good, robust assays for use in HT screens to discover novel modulators of 7TM receptors or the ion channels we’re using as effectors.”