Synergistic cytotoxicity effects of TH-302 in combination with Bortezomib in Multiple Myeloma.

As previously demonstrated by us and other groups, hypoxia is a critical microenvironment factor in multiple myeloma (MM). Treatment with the hypoxia-activated prodrug TH-302 has showed promising effectiveness in MM (1). In this study, we investigated the combination effects of TH-302 and Bortezomib on MM. Our in vitro results show that the combination of TH-302 and Bortezomib synergistically induced apoptosis, evidenced by induced cleavage of poly (ADP-ribose) polymerase and caspase-3/8/9. To further determine the mechanism of induction of apoptosis by this combination, we investigated the effect of TH-302, Bortezomib and the combination on Bcl-2 family proteins using immunoblotting. The results show that Bortezomib and TH-302 can trigger different anti-apoptotic and pro-apoptotic responses. Importantly, TH-302 can abrogate the accumulation of antiapoptotic Mcl-1L induced by Bortezomib. The mechanism of abrogating Mcl-1L by TH-302 is tightly related to its influence on decreasing HIF1a/HIF2a and ATF4 expression. In addition, the combination of TH-302 and Bortezomib conducted in the 5T33MMvv mouse model in vivo showed impressive improvements in multiple disease parameters: induced significant decreased tumor burden, paraprotein secretion and microvessel density (MVD), compared to TH-302 or Bortezomib alone treated 5T33MMvv mice (p