PR-924, a selective inhibitor of the immunoproteasome subunit LMP-7, blocks multiple myeloma cell growth both in vitro and in vivo

PR-924 is an LMP-7-selective tripeptide epoxyketone Proteasome Inhibitor that covalently modifies proteasomal N-terminal threonine active sites. In the present study, we show that PR-924 inhibits growth and triggers Apoptosis in multiple myeloma (MM) cell lines and primary patient MM cells, without significantly affecting normal peripheral blood mononuclear cells. PR-924-induced Apoptosis in MM cells is associated with activation of Caspase-3, Caspase-8, caspase-9, BID, PARP and cytochrome-c release. In vivo administration of PR-924 inhibits tumour growth in human plasmacytoma xenografts. Results from SCID-hu model show a significant reduction in the shIL-6R levels in mice treated with PR-924 versus vehicle-control. PR-924 treatment was well tolerated as evidenced by the lack of weight loss. Importantly, treatment of tumour-bearing mice with PR-924, but not vehicle alone, prolonged survival. Our preclinical findings therefore validate immunoproteasome LMP-7 subunit as a novel therapeutic target in MM.