Entinostat (SNDX-275), a Novel DAC Inhibitor, Is Highly Effective in Rituximab- [Chemotherapy]-Sensitive or Rituximab-[Chemotherapy]-Resistant Lymphomas and Has Synergistic Anti-Tumor Activity When Combined with Bortezomib.

Abstract 3727

Poster Board III-663

Deacetylases (DACs) are enzymes that remove the acetyl groups from target proteins, leading to regulation of gene transcription and other cellular processes. Entinostat (SNDX-275) is a novel and potent DAC inhibitor that is selective for class I DACs and is currently undergoing pre-clinical and clinical testing in Hodgkin lymphoma (HL). Potent synergistic anti-tumor activity has been observed by combining less potent DAC inhibitors with bortezomib in pre-clinical models. In our efforts to develop more therapeutic options for refractory/resistant B-cell lymphoma, we evaluated the effects of Eentinostat as a single agent and in combination with bortezomib against B-cell non-Hodgkin's lymphoma (NHL) cell lines and primary NHL cells. Studies were conducted in a panel of 12 NHL cell lines representing various subtypes of B-cell lymphoma (i.e. DLBCL/ABC, DLBCL/GCB, Burkitt's, transformed and MCL), which included: rituximab-[chemotherapy]-sensitive cell lines (RSCL, Raji, RL and DHL-4), rituximab-[chemotherapy]-resistant cell lines (RRCL, Raji-4RH, Raji-2R, RL-4RH, and DHL-4 4RH), and primary lymphoma cells isolated from patients with various subtypes of NHL and HL. Patient-derived tumor cells were isolated from fresh specimens by negative selection using magnetic beads. NHL cells and patient-derived primary cells were exposed to entinostat at different doses (0.01 to 100uM) either alone or in combination with CDDP (1 to 100μM), doxorubicin (4 to 16μM), vincristine (1 to 5μM), or bortezomib (1 to 10nM). Anti-tumor activity was measured after a 24 or 48 hr incubation. In cell lines, changes in mitochondrial potential and cell proliferation were determined by alamar blue reduction using a kinetic assay measuring activity at 4 hr intervals for 24 and 48 hrs. For patient-derived primary NHL cells, changes in ATP content (apoptosis) was determined using the cell titer glow assay. Entinostat was highly active in all the cell lines tested including rituximab-[chemotherapy]-resistant cell lines. The IC50 of Entinostat in the majority of the cells tested was 0.5 to 5uM at 48 hrs. Similar findings were observed in primary tumor cells derived from lymphoma patients. In addition, synergistic activity was observed by combining entinostat and bortezomib in both NHL cell lines, as well as in primary NHL/HL tumor specimens. A lesser degree of augmented anti-tumor activity was also observed when entinostat was combined with cisplatin or doxorubicin (but not vincristine). In summary, our data suggests that entinostat is a novel and potent DAC inhibitor with a wide therapeutic spectrum. Entinostat is capable of inducing cell death against various subtypes of B-cell lymphoma cell lines including RSCL, RRCL, as well as patient-derived primary tumor cells and augments the anti-tumor effects of bortezomib and other chemotherapeutic agents. Given the isoform selectivity of entinostat, the results indicate that HDAC1 and 2 may be the key targets of DAC inhibitors in HL and NHL cells. Ongoing studies are evaluating the mechanisms responsible for the synergistic effects of entinostat plus chemotherapy and will be updated at the annual meeting. Current findings strongly suggest that entinostat added to bortezomib and/or other chemo agents may become a novel and potent strategy in the treatment of aggressive and indolent NHL and HL in the future.