Dual PI3K and mTOR Inhibition in Waldenstrom Macroglobulinemia

Background. We have previously showed that PI3K/Akt is constitutively active in Waldenström Macroglobulinemia (WM) malignancies, mediating growth, survival, cell cycle regulation, and migration in primary tumor cells. Once activated, Akt phosphorylates downstream targets, including mammalian target of rapamycin (mTOR). Both PI3K/Akt and mTOR represent valid targets for antitumor therapeutic strategies. We therefore evaluated the antitumor activity of NVP-BEZ235 (Novartis, MA) in WM.

Methods: WM cell lines (BCWM.1) and IgM secreting cell lines (MEK1, Namalwa) were used. Bone marrow primary CD19+ malignant cells and bone marrow stromal cells (BMSC) were obtained from WM patients. Cytotoxicity, DNA synthesis, and cell cycle were measured using MTT assay, [3H]-thymidine uptake, PI staining/flow cytometry, respectively. Effects of NVP-BEZ235 on cell signaling cascades were determined using immunoblotting and immunofluorescence. Adhesion on fibronectin has been evaluated in WM cells in the presence of NVP-BEZ235.

Results: NVP-BEZ235 induced cytotoxicity and inhibited DNA synthesis with an IC50 of 20–25nM in BCWM.1 at 48 hours. Similar effects were demonstrated in all IgM secreting cell lines and in primary CD19+ WM cells, with an IC50 between 20nM and 50nM. No cytotoxicity was observed on peripheral blood mononuclear cells, indicating selective toxicity of the compound on the malignant lymphoplasmacytic clone. We observed that NVP-BEZ235 inhibited Akt (but not ERK phosphorylation) in a dose-dependent manner in BCWM.1 cells at 6 hours. Phosphorylation of GSK3α/β and ribosomal protein-S6, downstream target proteins of Akt, were also markedly inhibited. NVP-BEZ235-inhibited Akt phosphorylation was further confirmed by immunofluorescence. NVP-BEZ235 induced caspase-9, PARP cleavage and increased the release of Smac/DIABLO from the mitochondria to the cytosol, suggesting an induction of apoptosis in a caspase-dependent and –independent manner. We showed that NVP-BEZ235 inhibited adhesion of BCWM.1 cells to fibronectin in a dose-dependent fashion. Lastly, adherence to BMSCs did not confer protection to WM cells against NVP-BEZ235- induced cytotoxicity.

Conclusions. These data indicate that NVP-BEZ235 has significant antitumor activity in WM, thus providing the framework for clinical trials in this disease.