Intracellular signalling pathways are involved in virtually every disease process, and cell-surface receptor proteins that initiate these pathways are favoured drug targets (see Nature 428, 225–233; 200410.1038/428225a). In the intracellular pathways leading from these receptors, signalling proteins move within the cell — from the cytoplasm into the nucleus, for example. Testing the effects of potential drug compounds against these intracellular protein movements has, unsurprisingly, proved much harder than testing against proteins at the surface of the cell. The ability to do so promises to find drugs with new modes of action and greater selectivity.

GFP-tagged proteins in nuclei. Credit: BIOIMAGE

A leader in this field is BioImage in Copenhagen, Denmark, which focuses on green fluorescent proteins (GFP) as a tool in drug discovery. BioImage's proprietary Redistribution technology uses genetically encoded GFP tags to track protein translocation in living cells, and so identify compounds that inhibit onward transmission of the signal. “It's pretty clear now that every signalling pathway has components that must move in the cell for the signal interaction to occur,” says Len Pagliaro, vice-president for business development at BioImage. “If you take these targets and track them, you can measure movement in response to potential stimulators and agonists, and have the potential to find compounds that inhibit that movement.”

The firm deploys GE Healthcare's IN Cell Analyzer to carry out high-throughput imaging for screening and analysis of the effects of members of its own library of over 250,000 compounds. Such intracellular assays provide the opportunity to discover drugs that interact with target proteins in new ways — by targeting protein localization domains rather than catalytic domains, for instance. Studying living cells also creates more opportunities for serendipitous discoveries.

“Obviously, cell-based assays are inherently a somewhat dirty system biologically, and that has both benefits and liabilities,” Pagliaro notes. “The benefit is that in principle it's more like the pathway you're going to see in vivo. The liability is that when you get a hit, you don't know that the compound is acting on the target that you want to see it acting on. We like that aspect, because that can deliver very interesting progressible hits that we would not have found otherwise.”

Tim Chapman