To understand the mammalian toxicity of these diarylimidazole derivatives, and to aid in future compound design, we experimentally investigated the potential series mechanism of action (MoA). In the absence of a clear position where we might be able to attach a linker that would permit a pull-down experiment, and because these structures were originally based on known kinase inhibitors, a representative active (822) and inactive (820) compound were profiled using the MIB-MS (multiplexed inhibitor beads with mass spectroscopic detection) proteomic technique. This affinity composition assay has proven to be a useful approach to validate kinase inhibitor selectivity. The results from applying this method to HEK293 cell lysates suggested that TGF-β1 could potentially be a mammalian target for this series of compounds as with the active compound both kinases were selectively competed off MIBs (1 µM), whereas similar results were not observed with the inactive compound (Figure , see Supplementary Information -Biology for full data). Whether inhibition of TGF-β1 contributes to the cytotoxicity observed remains to be determined. Further support for TGF-β1 being a mammalian target comes from a report of compounds similar in structure to 822 bound to this protein (PDB: 3TZM and 1RW8) and the original GSK work on a precursor to this series mentioned above.