Abstract
PURPOSE: Proteasome-inhibiting drugs (PI) are gaining importance in hematologic oncology. The proteasome carries three proteolytically active subunits (β1, β2, β5). All established PI (bortezomib and carfilzomib), as well as experimental drugs in the field (dalanzomib, oprozomib, and ixazomib), by design target the rate-limiting β5 subunit. It is unknown whether β2-selective proteasome inhibition can also be exploited toward anticancer treatment. Combining PI with the pan B-cell-directed Bruton tyrosine kinase inhibitor ibrutinib appears a natural option for future improved treatment of multiple myeloma (MM) and B-cell lymphomas. However, bortezomib induces phosphorylation of IκB and activation of NF-κB in MM cells, while ibrutinib inhibits the IκB/NF-κB axis, suggesting antagonistic signaling. A β2-selective proteasome inhibitor may lack such antagonistic signaling effects. METHODS: We recently introduced LU-102, the first β2-selective PI available for preclinical testing. We here compare bortezomib with carfilzomib and LU-102 in MM and MCL in vitro with regard to their effects on pIκB/NF-κB signaling and their cytotoxic activity in combination with ibrutinib. RESULTS: LU-102 reduced phosphorylation of IκB, in contrast to bortezomib and carfilzomib, and was a superior inhibitor of NF-κB activation in MM cells. This translated into highly synergistic cytotoxicity between LU-102 and ibrutinib, which was able to overcome BTZ resistance and CFZ resistance. By contrast, BTZ lacked consistent synergistic cytotoxicity with ibrutinib. CONCLUSION: Ibrutinib is highly synergistic with β2-selective proteasome inhibition against MM and MCL in vitro. Novel β2-selective proteasome inhibitors may be exploited to overcome bortezomib/carfilzomib resistance and boost the activity of BTK inhibitors against B-cell-derived malignancies.