Selective IKKβ Inhibition of the NF-Kappab Pathway Is An Effective Therapeutic Strategy In Lymphomas

The NF-kappaB pathway is an essential and tightly regulated signaling cascade that mediates development, activation and survival of normal lymphocytes. In recent years, there has been convincing evidence implicating this pathway in several lymphoid malignancies including the activated B cell-like (ABC) subtype of Diffuse Large B cell Lymphoma(DLBCL), Primary Mediastinal B Cell Lymphoma(PMBL), marginal zone lymphoma, and Hodgkins lymphoma. These tumors frequently engage the classic NF-kappaB pathway in which the IKK β subunit phosphorylates IκBα triggering its ubiquitination and subsequent proteasomal degradation. Therefore, therapies targeting this pathway through selected IKKB inhibition may provide a novel approach for targeting NF-kappaB activity in lymphoid malignancies. We therefore decided to explore the efficacy of a novel IKKB inhibitor CMPDA in a broad group of lymphomas.

We chose 37 cell lines that broadly represented a diverse group of lymphomas including Burkitt lymphoma, mantle cell lymphoma, Hodgkin lymphoma, multiple myeloma, prolymphocytic leukemia, and the activated B cell-like (ABC) and germinal center (GCB) subtypes of DLBCL. Cell viability assays using MTT were performed to identify the IC50 in these cell lines. We performed gene expression profiling on all cell lines using a GeneChip Human Gene 1.0 ST Array which comprises 33297 probes that target all known genes. Utilizing hierarchial clustering algorithms in conjunction with cox regression analysis, the cell lines were classified into 2 groups based on gene expression: the predicted sensitive(Group A: 20 cell lines) versus predicted less sensitive cell lines (Group B: 17 cell lines).

CMPDA was broadly effective in lymphomas demonstrating lethality in most lymphoma types at physiologically achievable concentrations with a mean IC50 of 9.15 micromoles (Range: 1.47 to 58.24 micromoles). The Mean IC50 for cell lines in Group A which were predicted as being sensitive by gene expression was 5 micromoles, whereas the mean IC50 for the cell lines in Group B which were predicted as being relatively resistant to NF-KB inhibition was 3-fold higher (∼15micromoles). Group A included a number of cell lines with known dependence on NF-kappa B activity including several ABC DLBCL cell lines. It is notable that while GCB DLBCL have generally been thought to have little dependence of NF -KB activity, we nevertheless found that there were 4 GCB DLCBL cell lines that were also sensitive to NFKB inhibition at IC50s comparable to a number of ABC DLBCLs. Group B comprised of a disproportionate number of Burkitt lymphoma cell lines (4/17), which is consistent with the known biology of this tumor as it typically expresses NF-KB at lower levels.

Genes associated with response to NF-KB inhibition showed robust concordance with the known biology of the NF-kappaB pathway, with gene expression signatures related to CD40-stimulation (an upstream activator of NF-kappa B) and NF-kappa B activity showing the strongest associations with response to CMPDA (P=0.004 and P=0.01 respectively).

Thus our data demonstrate that selective IKKβ inhibition of the NF-kappa B pathway using CMPDA is a promising therapeutic strategy in lymphoma. Gene expression profiling provides a useful tool for potentially identifying tumors that are most likely to respond to this therapy.

No relevant conflicts of interest to declare.