Proteasome active-site subunit inhibition profiles of CPSI, IPSI, and carfilzomib in MM1.S cells. Cells were treated with a 1-hour pulse of 0.18 μM CPSI (c), 0.27 μM IPSI (i), 0.18 μM CPSI plus 0.27 μM IPSI (c + i), 20 nM carfilzomib (20 nM CFZ), or 10 μM carfilzomib (10 μM CFZ), and the ProCISE assay was performed to determine the level of available (uninhibited) subunits. For each compound, the level of residual activity compared with DMSO-treated cells is plotted for each proteasome subunit. Mean ± SD are shown (n = 24).