Melphalan and Exportin 1 Inhibitors Exert Synergistic Anti-tumor Effects in Preclinical Models of Human Multiple Myeloma

High-dose chemotherapy with melphalan followed by autologous transplantation is a first-line treatment for multiple myeloma. Here, we present pre-clinical evidence that this treatment may be significantly improved by the addition of exportin 1 inhibitors (XPO1i). The XPO1i selinexor, eltanexor, and KOS-2464 sensitized human MM cells to melphalan. Human 8226 and U266 MM cell lines and melphalan-resistant cell lines (8226-LR5 and U266-LR6) were highly sensitized to melphalan by XPO1i. MM cells from newly diagnosed and relapsed/refractory MM patients were also sensitized by XPO1i to melphalan. In NOD/SCID-γ mice challenged with either parental 8226 or U266 MM and melphalan-resistant MM tumors, XPO1i/melphalan combination treatments demonstrated stronger synergistic anti-tumor effects than single-agent melphalan with minimal toxicity. Synergistic cell death resulted from increased XPO1i/melphalan-induced DNA damage in a dose-dependent manner and decreased DNA repair. In addition, repair of melphalan-induced DNA damage was inhibited by selinexor, which decreased melphalan-induced monoubiquitination of FANCD2 in MM cells. Knockdown of FANCD2 was found to replicate the effect of selinexor when used with melphalan, increasing DNA damage (γH2AX) by inhibiting DNA repair. Thus, combination therapies that include selinexor or eltanexor with melphalan may have the potential to improve treatment outcomes of MM in melphalan-resistant and newly diagnosed patients. The combination of selinexor and melphalan is currently being investigated in the context of high-dose chemotherapy and autologous transplant.