CBL-C137, a Curaxin and Potent Suppressor Of NF-Kb Activity and Modulator Of p53 Function Is Active In Rituximab-Sensitive Or Resistant Activated B-Cell (ABC) Diffuse Large B-Cell Lymphoma (DLBCL) Cells

The identification of critical signaling pathways necessary for the development, maintenance and progression of specific subtypes of DLBCL are necessary in order to develop novel therapeutics against them. Increased NFkB activity and p53 deregulation contribute to either the pathogenesis of some types of DLBCL (i.e. activated B-cell [ABC] subtype) or to rituximab and/or chemotherapy resistance in B-cell lymphoma. Optimal targeting of NFkB activity is an attractive therapeutic strategy that has been evaluated in pre-clinical and clinical models for over a decade with variable degrees of success. CBL-C137 is a novel and potent member of the curaxin family, capable of modulating p53 and NFkB activity and inducing cell death in several solid tumor cancer models. It has never been previously studied in lymphoid malignancies. In order to study and define the therapeutic potential of curaxins in B-cell lymphoma, we evaluated CBL-C137 in lymphoma pre-clinical models. A panel of rituximab sensitive or resistant lymphoma cell lines representing the two most common subtypes of DLBCL (i.e. ABC-DLBCL and germinal center B-cell [GCB] DLBCL) were exposed to CBL-C137 (0.5-16mM). Changes in cell viability; cell cycle distribution; apoptosis and p53/ NFkB p65 expression were evaluated by measuring ATP content, flow cytometry, and Western blotting, respectively. Subsequently, GCB- or ABC-DLBCL cells were exposed to CBL-C137 alone or in combination with various chemotherapy agents or other available (but less selective) NFkB inhibitors (i.e. lenalidomide or Ibrutinib) for 24 or 48 hrs. Changes in cell viability were determined using the cell titer glo assay. In addition, we conducted standardized ⁵¹Cr release assays on cells previously exposed to either CBL-C137 or DMSO to investigate the effects of NFkB inhibition on rituximab (or other anti-CD20) antibody-associated complement mediated cytotoxicity (CMC) and antibody dependent cellular cytotoxicity (ADCC). CBL-C137 induced dose- and time- dependent cell death in ABC-DLBCL greater than in GCB-DLBCL cell lines. The IC₅₀ for CBL-C137 in ABC-DLBCL (rituximab/chemotherapy sensitive or resistant cells) ranged from 1.36 to 2.77mM. In contrast rituximab/chemotherapy GCB-DLBCL cells exhibited the highest IC₅₀ (11.91mM, 95% C.I 7.1-19.8mM). In sensitive DLBCL cells, CBL-C137 induced both apoptosis and cell cycle arrest in G1 phase. Moreover, in vitro exposure to CBL-C137 decreased p53 and p65 in sensitive cells. CBL-C137 increased Lenalidomide, but not Ibrutinib, anti-lymphoma activity in the conditions tested. Finally, CBL-C137 did not affect rituximab or other anti-CD20 antibody-associated ADCC or CMC in DLBCL cells. Our data suggest that CBL-C137 is active in DLBCL pre-clinical models, primarily in ABC-DLBCL cell lines. In sensitive cells, CBL-C137 modulates p53 and NFkB activity and promotes death and/or cell cycle arrest. Ongoing studies are aimed to further define the anti-tumor effects of CBL-C137 in combination with other small molecules inhibitors targeting directly or indirectly NFkB activity in lymphoma. (Research, in part, supported by a NIH grant R01 CA136907-01A1 awarded to Roswell Park Cancer Institute and The Eugene and Connie Corasanti Lymphoma Research Fund)