Inhibitors of Mtor and PI3K Synergistically Block Plasmacytoma Cell Growth

Both, the phosphoinositide-3-kinase (PI3K)-AKT pathway as well as its nutrient-dependent downstream target, the mToR (mammalian target of rapamycin) kinase, are essential for the growth and survival of malignant plasma cells. PI3K-AKT can be activated by the loss of the tumor suppressor phosphatase and tensin homolog (PTEN) or by stimulation with cytokines such as interleukin-6 (IL-6) and insulin-like growth factor-1 (IGF-1). Inhibitors of the mToR pathway like sirolimus, everolimus and temsirolimus are approved for immunosuppression and treatment of renal cell cancer. In fact, they show activity in multiple myeloma patients. However, the clinical activity of mToR inhibitors may be limited by feedback loops in tumor cells that lead to activation of AKT upon inhibition of the rapamycin-sensitive Raptor complex. Selective PI3K inhibitors (Ly294002, NVP-BKM120) as well as dual PI3K-mToR inhibitors (NVP-BEZ235) are in clinical development. Here, the inhibitory effect of a panel of mToR and PI3K inhibitors, alone and in combination, was evaluated in malignant plasma cell lines and primary samples from myeloma patients.

In five human plasma cell lines, rapamycin, everolimus, Ly294002, NVP-BKM120 and NVP-BEZ235 (kindly provided by Novartis) induced a dose-dependent growth inhibition as measured by MTS assay. Despite the observed strong anti-myeloma activity of the mToR inhibitors rapamycin and everolimus with IC50 values in the nm range, the AKT pathway was activated as indicated by increased phosphorylation at Ser473. This observation was also made in explanted plasma cell tumors of INA-6 xenografted SCID mice that were treated with rapamycin, indicating that this feedback loop is also active in vivo. Therefore, combining mToR and PI3K inhibitors could be an effective strategy to overcome rapamycin-induced AKT activation. In fact, rapamycin in combination with the selective PI3K inhibitor Ly294002 or NVP-BKM120 led to synergistic growth inhibition in plasma cell lines, as calculated by the Calcusyn™ software (Biosoft). This abrogation of AKT activation was also seen in Western blot analysis. Combined treatment also enhanced the induction of apoptosis in cell lines as well as in plasmacytoma cells. Interestingly, the activity of the dual inhibitor NVP-BEZ235 could be even further enhanced by the addition of rapamycin.

First clinical trials with mToR inhibitors showed anti-tumor activity and an acceptable safety profile in patients with multiple myeloma. The data presented here suggest that a combination of mToR inhibitors with PI3K targeting compounds or the use of dual inhibitors may have a favorable therapeutic potential.

Guenther:Novartis, Celgene: Consultancy, Research Funding. Gramatzki:Novartis, Celgene: Consultancy, Research Funding.