Antitumor Activity of a Novel Proteasome Inhibitor BSc2118 Against Multiple Myeloma

Background: Although the first generation of proteasome inhibitor bortezomib is very effective, the development of resistance limits it long-term utility. In this study, we explored the efficiency and molecular mechanisms of the novel, irreversible proteasome inhibitor BSc2118, particularly, the reversal of bortezomib resistance.

Materials and Methods: Human MM cell lines (MM.1S, MM.1R, RPMI-8226, U266, NCI-H929) were treated with BSc2118 at various concentrations for 48h, and assessment for cell viability by CCK-8 assay. MM.1S and MM.1R cells were treated with BSc2118 for 24 hours, and cell cycle and apoptosis analysis were conducted by flow cytometry. Associated molecules were detected by qRT-PCR and western blot. Chymotrypsin-like proteasome activity assay was performed by using the 20S proteasome assay kit. Ubiquitinated proteins were isolated and determined with ubiquitin enrichment kit.

Results: Our results revealed that treatment of MM cell lines with BSc2118 inhibits the chymotrypsin-like proteasome activity and induces accumulation of ubiquitinated proteins. BSc2118 inhibits MM cell growth and induces MM apoptosis via induction of G2/M phase arrest, activation of cleaved caspase-3, caspase-8 and caspase-9 and PARP, increasing p53, p21 and E2F1, and inhibition of autophagy in MM.1S, MM.1R and RPMI-8226 cell lines. In addition, BSc2118 dramatically inhibits cytokines mRNA, such as IL-6, VEGF and bFGF in both myeloma cells line and primary bone marrow stromal cells from myeloma patients. More importantly, BSc2118 could overcome bortezomib resistance in vitro by using primary CD138 positive plasma cells from bortezomib-resistant myeloma patients and bortezomib resistance cell line ANBL-6 (ANBL-6.BR), most likely as the consequence of inhibition of autophagy flux which is responsible for bortezomib resistance.

Conclusion: Our study revealed BSc2118, a novel irreversible proteasome inhibitor, exerts anti-MM effect, mainly through activation of caspase pathway and inhibition of basal autophagy. It is of great importance that BSc2118 could overcome bortezomib resistance via inhibition of autophagy flux. A head to head of BSc2118 versus Bortezomib is performing in human plasmacytoma xenograft tumor model to evaluate drug safety, anti-tumor efficiency, in particular, reversal of bortezomib resistance. Our preclinical study supports clinical evaluation of BSc2118, particularly, overcomes bortezomib resistance, as a potential MM therapy.