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Target engagement

Validation of a chemical probe’s activity in vitro, in cells, and in vivo requires verification that the chemical probe engages -- that it binds to -- its intended target in the model system used.

In addition to target engagement (binding), we can also look for evidence of target modulation - that the compound is modulating the activity of the protein target.

Assays to detect target modulation can be proximal or distal to the target. The more proximal the assay to the probe-target interaction (i.e., direct binding assay), the less likely unanticipated activity of the probe will mislead the researcher into believing a phenotype is due to on-target activity when it is not.

Biomarkers that are reliable surrogates for target engagement enable researchers to directly correlate target engagement with specific phenotypes. If, for instance, full target occupancy is confirmed for a chemical probe in cells or in vivo and the probe fails to produce an expected phenotype, then the target and mechanism were properly tested and invalidated. Without measurements of target engagement, it can be very difficult to discern the basis for lack of activity or for researchers to have confidence they have tested their hypothesis.

An ideal target-engagement assay measures (i) the extent of target engagement, which can help to determine doses or concentrations that produce the phenotype while limiting side effects, and (ii) the potential for interactions with off-target proteins.

For more information, see Determining target engagement in living systems. Nature Chemical Biology (2013) DOI: 10.1038/nchembio.1211.