(A) XIAP BIR3 protein abolishes caspase-3 activity in a dose-dependent manner. (B–D) Kinetic analysis for the relief of XIAP BIR3 protein-mediated caspase-3 by compounds 2, 3, and 4. MDA-MB-231 cell extract was activated with bovine cytochrome c and dATP. The caspase activity was inhibited by adding 500 nM recombinant XIAP BIR3, and varying concentrations of tested compounds were added. The onset of caspase-3 activity was monitored as a fluorogenic substrate (DEVD-AFC, BioVision) was cleaved in situ (rfu, relative fluorescence units).