The mTOR Inhibitor RAD001 (everolimus) Is Active Against Multiple Myeloma Cells In Vitro and in Vivo.

We have recently shown that tumor cell proliferation, survival and drug-resistance in multiple myeloma (MM) and a broad range of other tumors is critically influenced by insulin-like growth factors (IGFs) and their receptor (IGF-1R) (Cancer Cell2004;5:221–30). Among the pleiotropic signaling cascades downstream of IGF-1R activation, we focused on the functional implications and therapeutic targeting of the Akt/p70S6K/mTOR axis, particularly of mTOR (mammalian Target of Rapamycin), due to its regulatory role on cellular bioenergetics, a key aspect of tumor pathophysiology. Herein, we describe the in vitro and in vivo profiles of anti-tumor activity of the selective mTOR inhibitor RAD001 (Everolimus, Novartis AG). Using in vitro MTT assays, we observed that RAD001 is active (at nM concentrations) against a broad range of tumor cells, including >40 MM cell lines and >10 primary MM tumor cells (including cell lines or primary cells resistant to Dex, alkylating agents, anthracyclines, thalidomide (Thal), immunomodulatory Thal derivatives, bortezomib, and/or Apo2L/TRAIL), without significant impact on viability of normal hematopoietic cells or other normal tissues (e.g. bone marrow stromal cells), and its anti-MM effect was not blocked by forced overexpression of Bcl-2 or constitutively active Akt. While cytokine- or cell adhesion-mediated interactions with the bone marrow (BM) microenvironment (e.g. BM stromal cells) protects MM cells from conventional therapies (e.g. Dex or cytotoxic chemotherapy), RAD001 was able to overcome this protective effect in co-culture models of MM cells with BM stromal cells or in vitro MM cell exposure to survival factors, e.g. IL-6 or IGF-I. Furthermore, RAD001 sensitized MM cells to other anti-MM therapeutics, e.g. dexamethasone, cytotoxic chemotherapeutics, or the proteasome inhibitor bortezomib, even in cases of primary MM tumor cells refractory to these respective agents. Using hierarchical clustering analyses and relevance network algorithms, we found that the pattern of MM cell dose-response relationships to RAD001 is clearly distinct from the patterns of sensitivity or resistance to other conventional or investigational anti-MM drugs. This further supports the notion that RAD001 confers a constellation of pro-apoptotic/anti-proliferative molecular sequelae distinct from those of currently available anti-MM drugs, and also suggests that RAD001 may have anti-tumor activity even against subgroups of MM which may be resistant to other novel therapies which that are currently in clinical development. Importantly, administration of RAD001 in a SCID/NOD mice model of diffuse MM bone had in vivo anti-tumor activity, including suppression of MM tumor burden and prolongation of survival (p<0.01, log-rank test). These studies highlight an important role for mTOR in growth/survival of human MM cells and provide proof-of-principle for future clinical studies of mTOR inhibitors for the treatment of MM and other plasma cell dyscrasias.