NEOPLASIA Combination of proteasome inhibitors bortezomib and NPI-0052 trigger in vivo synergistic cytotoxicity in multiple myeloma

Our recent study demonstrated that a novel proteasome inhibitor NPI-0052 trig-gers apoptosis in multiple myeloma (MM) cells, and importantly, that is distinct from bortezomib (Velcade) in its chemical struc-ture, effects on proteasome activities, and mechanisms of action. Here, we demon-strate that combining NPI-0052 and bort-ezomb induces synergistic anti-MM activ-ity both in vitro using MM cell lines or patient CD138 MM cells and in vivo in a human plasmacytoma xenograft mouse model. NPI-0052 plus bortezomib–induced synergistic apoptosis is associated with: (1) activation of caspase-8, caspase-9, caspase-3, and PARP; (2) induction of endoplasmic reticulum (ER) stress re-sponse and JNK; (3) inhibition of migra-tion of MM cells and angiogenesis; (4) suppression of chymotrypsin-like (CT-L), caspase-like (C-L), and trypsin-like (T-L) proteolytic activities; and (5) block-ade of NF-B signaling. Studies in a xeno-graft model show that low dose combina-tion of NPI-0052 and bortezomib is well tolerated and triggers synergistic inhibi-tion of tumor growth and CT-L, C-L, and T-L proteasome activities in tumor cells. Immununostaining of MM tumors from NPI-0052 plus bortezomib–treated mice showed growth inhibition, apoptosis, and a decrease in associated angiogenesis. Taken together, our study provides the preclinical rationale for clinical protocols evaluating bortezomib together with NPI-0052 to improve patient outcome in MM