dactolisib and Lymphoma--B-Cell

Pharmacological strategies capable of directly targeting MYC are elusive. Previous studies have shown that MYC-driven lymphomagenesis is associated with mammalian target of rapamycin (mTOR) activation and a MYC-evoked DNA damage response (DDR) transduced by phosphatidylinositol-3-kinase (PI3K)-related kinases (DNA-PK, ATM, and ATR). Here we report that BEZ235, a multitargeted pan-PI3K/dual-mTOR inhibitor, potently killed primary Myc-driven B-cell lymphomas and human cell lines bearing IG-cMYC translocations. Using pharmacologic and genetic dissection of PI3K/mTOR signaling, dual DDR/mTORC1 inhibition was identified as a key mediator of apoptosis. Moreover, apoptosis was initiated at drug concentrations insufficient to antagonize PI3K/mTORC2-regulated AKT phosphorylation. p53-independent induction of the proapoptotic BH3-only protein BMF was identified as a mechanism by which dual DDR/mTORC1 inhibition caused lymphoma cell death. BEZ235 treatment induced apoptotic tumor regressions in vivo that correlated with suppression of mTORC1-regulated substrates and reduced H2AX phosphorylation and also with feedback phosphorylation of AKT. These mechanistic studies hold important implications for the use of multitargeted PI3K inhibitors in the treatment of hematologic malignancies. In particular, the newly elucidated role of PI3K-related DDR kinases in response to PI3K inhibitors offers a novel therapeutic opportunity for the treatment of hematologic malignancies with an MYC-driven DDR.

Topics: Animals; Apoptosis; Blotting, Western; Cell Line; Discoidin Domain Receptor 1; DNA Damage; Dose-Response Relationship, Drug; Flow Cytometry; Histones; Humans; Imidazoles; Lymphoma, B-Cell; Mechanistic Target of Rapamycin Complex 1; Mice; Mice, Inbred C57BL; Mice, Knockout; Multiprotein Complexes; Phosphatidylinositol 3-Kinases; Phosphoinositide-3 Kinase Inhibitors; Phosphorylation; Proto-Oncogene Proteins c-akt; Proto-Oncogene Proteins c-myc; Quinolines; Receptor Protein-Tyrosine Kinases; Survival Analysis; TOR Serine-Threonine Kinases; Tumor Cells, Cultured

Posttransplant lymphoproliferative disorder (PTLD) continues to be a devastating and potentially life-threatening complication in organ transplant recipients. PTLD is associated with EBV infection and can result in malignant B cell lymphomas. Here we demonstrate that the PI3K/Akt/mTOR pathway is highly activated in EBV+ B cell lymphoma lines derived from patients with PTLD. Treatment with the mTORC1 inhibitor Rapamycin (RAPA) partially inhibited the proliferation of EBV+ B cell lines. Resistance to RAPA treatment correlated with high levels of Akt phosphorylation. An mTORC1/2 inhibitor and a PI3K/mTOR dual inhibitor suppressed Akt phosphorylation and showed a greater anti-proliferative effect on EBV+ B lymphoma lines compared to RAPA. EBV+ B cell lymphoma lines expressed high levels of PI3Kδ. We demonstrate that PI3Kδ is responsible for Akt activation in EBV+ B cell lymphomas, and that selective inhibition of PI3Kδ by either siRNA, or a small molecule inhibitor, augmented the anti-proliferative effect of RAPA on EBV+ B cell lymphomas. These results suggest that PI3Kδ is a novel, potential therapeutic target for the treatment of EBV-associated PTLD and that combined blockade of PI3Kδ and mTOR provides increased efficacy in inhibiting proliferation of EBV+ B cell lymphomas.

Topics: Blotting, Western; Cell Line, Tumor; Cell Proliferation; Drug Synergism; Epstein-Barr Virus Infections; Herpesvirus 4, Human; Humans; Imidazoles; Lymphoma, B-Cell; Lymphoproliferative Disorders; Mechanistic Target of Rapamycin Complex 1; Mechanistic Target of Rapamycin Complex 2; Multiprotein Complexes; Organ Transplantation; Phosphatidylinositol 3-Kinases; Phosphoinositide-3 Kinase Inhibitors; Phosphorylation; Postoperative Complications; Protein Kinase Inhibitors; Proto-Oncogene Proteins c-akt; Quinolines; Sirolimus; TOR Serine-Threonine Kinases