Mtor and PI3K Inhibitors Block Myeloma Cell Growth in a Synergistic Manner.

Abstract 1843

Poster Board I-869

Introduction: The mammalian target of rapamycin (mToR) plays a crucial role in cell growth due to its role as nutrient dependent regulator of important cytokine signaling pathways. In multiple myeloma, mToR is involved in the phosphoinositide-3-kinase (PI3K)-AKT pathway which can be activated by the loss of the tumor suppressor phosphatase and tensin homolog (PTEN) or by stimulation with growth and survival factors such as interleukin-6 (IL-6) and insulin-like growth factor-1 (IGF-1). Inhibitors of the mToR pathway (sirolimus/rapamycin, everolimus and temsirolimus) are approved for immunosuppression and/or cancer treatment. However, the clinical activity of mToR inhibitors may be limited by the fact that, after inhibition of the rapamycin-sensitive mToR-Raptor complex, AKT is activated by the rapamycin-insensitive mToR-Rictor complex. In this regard, the inhibitory effect of mToR inhibitors was evaluated in combination with PI3K inhibitors in vitro and in vivo. Results: Rapamycin and everolimus induced a dose-dependent growth inhibition in six human malignant plasma cell lines. Growth inhibition was mediated by G1 cell cycle arrest and in a subset of cell lines by induction of apoptosis. Overexpression of Bcl-XL or Mcl-1 proteins did not prevent from apoptosis induction by mToR inhibitors, nor did sensitivity to rapamycin-induced apoptosis correlate with the p53 mutation status. In the INA-6 SCID mouse xenograft model, treatment with rapamycin resulted in a significant survival benefit compared to untreated mice. Six out of 14 rapamycin treated mice did not develop plasmacytomas during the observation period of 149 days. Remarkably, short term treatment of plasmacytoma bearing mice led to a significant shrinkage of the plasma cell tumor. Explanted tissue showed apoptotic plasma cells, a finding confirmed by immunohistological staining using an antibody specific for the human cleaved form of poly (ADP-ribose) polymerase (PARP). The combination of rapamycin and the PI3K inhibitor Ly294002 led to an increase of growth inhibition in all tested plasma cell lines. The additional growth inhibition by Ly294002 appeared to be due to AKT activation upon mToR inhibition by the rapamycin-insensitive Rictor complex, indicated by increased AKT phosphorylation at Ser473 as determined by Western blot analysis. Conclusion: Clinical trials currently evaluate mToR inhibitors for their potential to expand treatment options for myeloma patients. The data presented here suggest that a combination of mToR inhibitors with PI3K inhibitors may lead to additive therapeutic chances.