PI3K/mTOR Inhibition Markedly Potentiates HDAC Inhibitor Activity in NHL Cells through BIM- and MCL-1–Dependent Mechanisms In Vitro and In Vivo

Abstract

Purpose To explore the efficacy and define mechanisms of action of co-administration of the PI3K/mTOR inhibitor BEZ235 and pan-HDAC inhibitor panobinostat in DLBCL cells. Experimental Design Various DLBCL cells were exposed to panobinostat and BEZ235 alone or together after which apoptosis and signaling/survival pathway perturbations were monitored by flow cytometry and Western blot analysis. Genetic strategies defined the functional significance of such changes, and xenograft mouse models were used to assess tumor growth and animal survival. Results Panobinostat and BEZ235 interacted synergistically in ABC-, GC-, and double-hit DLBCL cells, and MCL cells, but not normal CD34+ cells. Synergism was associated with pronounced AKT dephosphorylation, GSK3 dephosphorylation/activation, Mcl-1 downregulation, Bim up-regulation and increased Bcl-2/Bcl-xL binding, diminished Bax/Bak binding to Bcl-2/Bcl-xL/Mcl-1, increased γH2A.X phosphorylation and histone H3/H4 acetylation, and abrogation of p21CIP1 induction. BEZ235/panobinostat lethality was not susceptible to stromal/microenvironmental forms of resistance. Genetic strategies confirmed significant functional roles for AKT inactivation, Mcl-1 down-regulation, Bim up-regulation, and Bax/Bak in synergism. Finally, co-administration of BEZ235 with panobinostat in immunocompromised mice bearing SU-DHL4-derived tumors significantly reduced tumor growth in association with similar signaling changes observed in vitro, and increased animal survival compared to single agents. Conclusions BEZ235/panobinostat exhibits potent anti-DLBCL activity, including in poor-prognosis ABC- and double-hit sub-types, but not in normal CD34+ cells. Synergism is most likely multi-factorial, involving AKT inactivation/GSK3 activation, Bim up-regulation, Mcl-1 down-regulation, enhanced DNA damage, and is operative in vivo. Combined PI3K/mTOR and HDAC inhibition warrants further attention in DLBCL.

Safety, tolerability, and preliminary activity of CUDC-907, a first-in-class, oral, dual inhibitor of HDAC and PI3K, in patients with relapsed or refractory lymphoma or multiple myeloma: an open-label, dose-escalation, phase 1 trial

“…4, 27 The ability of HDAC and PI3K inhibition to potentiate each other’s activity in DLBCL presents a promising novel strategy. 5 In the setting of relapsed disease, there is substantial need to augment responses with tolerable regimens that achieve disease control and possibly enable patients to proceed to high-dose therapy and autologous SCT. 28 Salvage therapy options are increasingly important for patients refractory to front-line rituximab who have a lower likelihood of response to rituximab-based salvage therapy and a correspondingly lower 3-year event-free survival of 21% and overall survival.…”

“…4 An HDAC inhibitor combined with a PI3K/mTOR inhibitor synergistically exhibited potent inhibition of tumor growth and prolonged survival in mouse DLBCL xenograft models, demonstrating the ability of PI3K inhibition to potentiate histone acetylation and HDAC inhibition to augment AKT dephosphorylation. 5 Dual targeting is intended to overcome or thwart the emergence of resistance to molecularly targeted agents that can develop through secondary target gene mutations or compensatory activation of alternative pathways, so called “oncogenic switch.” A promising strategy for mitigating such acquired drug resistance is through simultaneous inhibition of multiple molecular pathways, using either several agents in combination or a single agent that concurrently blocks multiple targets and/or pathways. 6 …”

Cyclin E1 Inhibition can Overcome Sorafenib Resistance in Hepatocellular Carcinoma Cells Through Mcl-1 Suppression

“…Our data indicate that regulation of the E2F1-Rb-cyclin E1 complex may play crucial roles in mediating sorafenib resistance in hepatocellular carcinoma cells. Mcl-1 expression is also regulated by multiple signaling pathways, including MAPK, JAK/STAT, and PI3K/AKT, in cancer cells (29,30). Therefore, Mcl-1 expression can serve as a pharmacodynamic marker of different types of combination therapy.…”

Dual HDAC and PI3K Inhibitor CUDC-907 Downregulates MYC and Suppresses Growth of MYC-dependent Cancers

“…To understand the mechanism underlying the antitumor activity of CUDC-907 in DH and "double expressor" DLBCL cells, we evaluated the changes induced by CUDC-907 in protein markers related to the HDAC, PI3K, and MYC pathways, including BCL2, a known HDAC target gene (44) that cooperates with MYC to induce lymphomagenesis (45), and MCL1, which is transcriptionally regulated by MYC (46) and whose downregulation has been shown to contribute to synergy between HDAC and PI3K inhibitors in DLBCL cells (47). As shown in Fig.…”