Inhibition of Nuclear Factor Kappa B (NFkB)/Interferon Regulatory Factor 4 (IRF4) Signaling Pathway in Activated B-Cell (ABC) Diffuse Large B-Cell Lymphoma (DLBCL) By Combining MLN4924, a Novel NEDD8 Activating Enzyme Inhibitor (NAE) and Ibrutinib

Identification of critical signaling pathways required for the development, maintenance and progression of specific subtypes of DLBCL is essential in order to develop novel therapeutic agents, especially for patients with relapsed/refractory disease or those not eligible for high-dose chemotherapy and autologous stem cell support (HDC-ASCT). Gene expression profiling (GEP) studies have identified three distinct subtypes of DLBCL- ABC-DLBCL, germinal center B-cell (GCB) DLBCL and primary mediastinal B-cell lymphoma (PMBL). Pre-clinical and clinical studies suggest that ABC-DLBCL is driven by an abnormally high NFκB activity that deregulates expression of Bcl-2 family proteins, and is associated with resistance to chemotherapy agents resulting in inferior clinical outcomes when compared to GCB-DLBCL or PMBL. In ABC-DLBCL, activation and maintenance of NFκB is mediated by the B-cell receptor (BCR) signaling pathway, the ubiquitin-proteasome system (UPS), oncogenic CARD11 or MYD88 mutations, and/or the effect of IRF4/SPIB on CARD11 or MYD88. Optimal NFκB targeting is an attractive therapeutic strategy in ABC-DLBCL and ongoing clinical trials are incorporating specific inhibitors (bortezomib, lenalidomide, or ibrutinib) in combination with chemo-immunotherapy in the front-line setting. On the other hand, multi-step targeting of the NFκB signaling pathway in ABC-DLBCL using investigational and currently available small molecule inhibitors could result in novel, active, and potentially less toxic regimens for ABC-DLBCL patients. To this end we studied the biological activity of MLN4924, a NAE inhibitor that selectively blocks the UPS up-stream by preventing activation of a subset of ubiquitin ligases known as cullin-RING ligases in combination with ibrutinib in lymphoma pre-clinical models. A panel of rituximab sensitive or resistant lymphoma cell lines representing ABC- and GCB-DLBCL were exposed to MLN4924. Changes in cell viability, cell cycle/NFκB activity, and expression of key regulatory proteins of the cell cycle, Bcl-2 family members, and the UPS were evaluated using the cell titer glo assay, flow cytometry and western blotting respectively. Subsequently, ABC- or GCB-DLBCL cell lines and tumor cells isolated from previously untreated or relapsed/refractory B-cell lymphoma were exposed to MLN4924 in combination with various chemotherapy agents or other available NFkB inhibitors (i.e. ibrutinib) for 24 or 48 hrs. Changes in viability were determined and coefficient of synergy was calculated using the CalcuSyn software. MLN4924 induced cell death in ABC-DLBCL cell lines and to a lesser degree in GCB-DLBCL cell lines. Anti-tumor activity plateau was seen after 48 hrs of drug exposure. In MLN4924 sensitive cells we consistently observed cell cycle arrest in G1 phase, down-regulation of Bcl-XL and PARP cleavage. MLN4924 exposure in vitro resulted in a decrease in Bcl-XL mRNA as determined by quantitative polymerase chain reaction (qPCR),due to the inhibition of NFkB activity as demonstrated in MLN4924-exposed cells by p65 co-localization studies using the imagestream technology. MLN4924 enhanced activity of cytarabine, cisplatin, doxorubicin and etoposide in ABC-, but not in the GCB-DLBCL cell lines. MLN4924 also exhibited synergistic effects when combined with ibrutinib in ABC-DLBCL cell lines at the doses tested. Our data suggests that multi-step targeting of NFκB signaling pathway in ABC-DLBCL is a viable therapeutic strategy and supports further in vivo pre-clinical studies and clinical studies in relapsed/refractory or HDC-ASCT ineligible ABC-DLBCL patients. (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) Disclosures No relevant conflicts of interest to declare.