Recently, we showed that
hypoxia is a critical microenvironmental factor in
multiple myeloma, and that the
hypoxia-activated
prodrug TH-302 selectively targets hypoxic
multiple myeloma cells and improves multiple disease parameters in vivo. To explore approaches for sensitizing
multiple myeloma cells to
TH-302, we evaluated in this study the antitumor effect of
TH-302 in combination with the clinically used
proteasome inhibitor bortezomib. First, we show that
TH-302 and
bortezomib synergistically induce apoptosis in
multiple myeloma cell lines in vitro. Second, we confirm that this synergism is related to the activation of
caspase cascades and is mediated by changes of Bcl-2 family
proteins. The combination treatment induces enhanced cleavage of
caspase-3/8/9 and PARP, and therefore triggers apoptosis and enhances the cleavage of proapoptotic BH3-only
protein BAD and BID as well as the antiapoptotic
protein Mcl-1. In particular,
TH-302 can abrogate the accumulation of antiapoptotic Mcl-1 induced by
bortezomib, and decreases the expression of the prosurvival
proteins Bcl-2 and Bcl-xL. Furthermore, we found that the induction of the proapoptotic BH3-only
proteins PUMA (p53-upregulated modulator of apoptosis) and NOXA is associated with this synergism. In response to the genotoxic and endoplasmic reticulum stresses by
TH-302 and
bortezomib, the expression of PUMA and NOXA were upregulated in p53-dependent and -independent manners. Finally, in the murine 5T33MMvv model, we showed that the combination of
TH-302 and
bortezomib can improve multiple disease parameters and significantly prolong the survival of diseased mice. In conclusion, our studies provide a rationale for clinical evaluation of the combination of
TH-302 and
bortezomib in patients with
multiple myeloma.