Targeting executioner procaspase-3 with the procaspase-activating compound B-PAC-1 induces apoptosis in multiple myeloma cells

Multiple myeloma (MM) is a plasma cell neoplasm that has a low apoptotic index. We investigated a new class of small molecules that target the terminal apoptosis pathway, called procaspase activating compounds (PACs), in myeloma cells. PAC agents (PAC-1 and B-PAC-1) convert executioner procaspases (procaspase 3, 6, and 7) to active caspases 3, 6, and 7, which cleave target substrates to induce cellular apoptosis cascade. We hypothesized that targeting this terminal step could overcome survival and drug-resistance signals in myeloma cells and induce programmed cell death. Myeloma cells expressed executioner caspases. Additionally, our studies demonstrated that B-PAC-1 is cytotoxic to chemotherapy-resistant or sensitive myeloma cell lines (n = 7) and primary patient cells (n = 11). Exogenous zinc abrogated B-PAC-1-induced cell demise. Apoptosis induced by B-PAC-1 treatment was similar in the presence or absence of growth-promoting cytokines such as interleukin 6 and hepatocyte growth factor. Presence or absence of antiapoptotic proteins such as BCL-2, BCL-XL, or MCL-1 did not impact B-PAC-1-mediated programmed cell death. Collectively, our data demonstrate the proapoptotic effect of B-PAC-1 in MM and suggest that activating terminal executioner procaspases 3, 6, and 7 bypasses survival and drug-resistance signals in myeloma cells. This novel strategy has the potential to become an effective antimyeloma therapy.