Hits from the CAII screen were progressed to chemoproteomic profiling to determine cellular target engagement and to measure their off-target profiles. SFs 2d and 2e were functionalised with an alkyne handle at the linking amide group to give probes 2j and 2k. A structurally similar negative control was also designed by substitution of the sulfonamide for a methoxy group, 2l (Fig. ). The three probes were first incubated with purified CAII, which confirmed SFs 2j and 2k covalently modified CAII following addition of the alkyne group, and that the methoxy group-containing negative control 2l did not. The proteins engaged by these three probes were subsequently studied in HEK293T cells.