Ring-DIMs increase the activity of the initiator caspase-9 in AD and AI prostate cancer cells. Activity of caspase-9 was measured by cleavage of the fluorogenic substrate Ac-LEHD-AFC in response to a 24 h treatment of LNCaP cells (a) and PC-3 cells (b) with increasing concentrations (15–30 μM) of 4,4′-dibromoDIM (□), 7,7′-dibromoDIM (■), 4,4′-dichloroDIM (△), 7,7′-dichloroDIM (▲) or DIM (●)