Selective Inhibition Of The PI3K-Alpha Isoform Blocks Myeloma Cell Growth and Survival

Abstract Introduction Both, the phosphoinositide-3kinase (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 exists in four isotypes (α - δ), the role of each of these isotypes in multiple myeloma (MM), however, is not defined. Therefore, we evaluated systematically the in vitroanti-myeloma activity of NVP-BYL719 (Novartis), a newly developed inhibitor highly selective for the PI3K α isoform, and compared it with the activity of other inhibitors targeting the PI3K/AKT/mToR signaling network at different sites. Since inhibition of mToRC1 may lead to PI3K activation by a feedback loop, we looked also at combinations of NVP-BYL719 and rapalogs. Materials and Methods An MTS based growth assay was used to measure growth inhibition of NVP-BYL719 in comparison to different inhibitors (the pan-PI3K inhibitors Ly294002 and NVP-BKM120, the dual PI3K/mToR inhibitor NVP-BEZ235, and the allosteric mToR inhibitors rapamycin and everolimus) on six human malignant plasma cell lines and two non-myeloma cell lines, K562 and Raji. Western blot analysis was used to confirm inhibitor activity and specificity. In addition, combinations of PI3K and mToR inhibitors were tested and antagonistic/synergistic activity was calculated with the CalcuSynTM (Biosoft) software. Results The selective PI3K alpha inhibitor NVP-BYL719 inhibited myeloma cell growth at a dose level (IC50 < 10 µM in 5/6 cell lines), where non-MM cell lines were not affected. Only cell line U266 was showed moderate sensitivity. The levels of the PI3K isoforms αwere comparable in all tested MM cell , as shown by Western blot. NVP-BYL719 blocked phosphorylation of AKT and of protein S6, an mToR downstream target. Of note, whenever rapamycin was combined with a PI3K inhibitor including NVP-BYL719, synergistic growth inhibition was seen, even in combination with the dual PI3K/mToR inhibitor NVP-BEZ235. Conclusions Here, the PI3Kα isoform is reported as being essential for the growth of myeloma cells. This provides the rationale for the therapeutic exploration of such isoform specific PI3K inhibitors in MM allowing dose intensification with less toxicity. Allosteric mToR inhibitors, already active in early clinical trials in MM patients may provide interesting combination partners for these PI3K-inhibitors. Disclosures: Guenther: Novartis: Consultancy. Gramatzki:Novartis: Consultancy.