A pivotal part of kinase chemical probe and drug development is assessment of the selectivity of a putative lead compound. While there is no consensus around the panel size or the type of assay(s) that are most appropriate, there is concurrence that gauging the number of on- and off-targets of a kinase inhibitor is essential. As pharmacology takes place in cells, we have compared profiling results for ten kinase inhibitors generated using the cell-free assays to those obtained when a panel of cellular target engagement NanoBRET assays is used to assess selectivity in intact cells. This is the first systematic comparison of these two approaches across a broad kinase panel. Comparison of the data sets demonstrates divergent results that can influence chemical probe prioritization. We identify unanticipated kinase interactions in cells for type II kinase inhibitors that are not observed in biochemical, cell-free systems. Furthermore, we characterize TPKI-39 as a DDR1, DDR2, and FLT1 chemical probe based on its in-cell selectivity profile.
Binder et al. (2026) studied this question.