The Potent STAT3/5 Inhibitor, BP-1-102 Demonstrates Significant Anti-Tumor Activity Against Waldenström Macroglobulinemia

Abstract 5101

STAT3 is a cytoplasmic transcription factor, transiently activated in response to external stimuli such as growth factors and cytokines. As a transcription factor, STAT3 induces the expression of genes known to be involved in tumorigenesis, implicating STAT3 dysregulation in a number of hallmark oncogenic processes including tumor cell survival, proliferation, angiogenesis, metastasis, and drug resistance. Aberrant STAT3 signaling is prevalent in hematologic malignancies including Waldenstrom Macroglobulinemia (WM), a rare form of B cell non-Hodgkin lymphoma that is characterized by hyper-monoclonal IgM secretion in the peripheral blood. Despite the development of novel therapies and combinatorial treatment regimes, WM remains uniformly fatal, and as the limits of current chemotherapies have been reached, new approaches to treatment are urgently needed to improve patient outcome. Analysis of the gene and protein expression profiles of WM patients suggests that activation of STAT3 signaling plays a critical role in WM, providing rationale for the therapeutic use of STAT3 inhibitors.

We have demonstrated pre-clinical efficacy of a novel, highly specific and potent small molecule STAT3/5 inhibitor, BP-1-102, in both hyper-IgM secreting B cell lymphoma cell lines (Mec-1 and RL) as well as two WM cell lines (MWCL-1 and BCWM-1). BP-1-102 directly targets STAT proteins by specifically blocking the SH2 domain that is a required for the phosphorylation, dimerization and nuclear localization of STAT3, ultimately resulting in inhibition of STAT3 transcriptional activation of target genes. We have shown that BP-1-102 directly interacts with STAT3's SH2 domain and is one of the most effective disruptors of STAT3 activity described to date. Treatment of cell lines with low μM doses of BP-1-102 induced dose-dependent decreases in constitutive and IL10-induced STAT3 phosphorylation (pSTAT3) as well as pSTAT3 nuclear localization. We further evaluated the potency of BP-1-102 against STAT1 and 5 compared to STAT3 using phosphor-flow cytometry to measure STAT phosphorylation status. BP-1-102 effectively inhibited GM-CSF induced STAT5 phosphorylation in AML2 cells at low dose concentrations (< 12.5uM) but only weakly inhibited IFNγ induced STAT1 phosphorylation in U937 leukemic cells at similar dose concentrations confirming the selectivity of BP-1-102 for STAT 3 and 5. Using a STAT3 dependent luciferase reporter construct, we confirmed repression of STAT3 transcriptional activity which correlated with a dose-dependent decrease in expression of STAT3 target genes (Mcl-1, Bcl-XL, Survivin and c-Myc). Inhibition of pSTAT3 resulted in decreased cell viability as assessed by MTT assay after 72 hours of in vitro exposure, with IC50 values ranging from 6uM to 10uM. In addition, treatment of cells with BP-1-102 resulted in caspase-dependent apoptosis which correlated with the activation of caspase-3 and PARP cleavage. Interestingly, co-culture of Mec-1 and RL with bone marrow stroma cells reduced the cytotoxicity of BP-1-102 suggesting stroma-conferred resistance, while MWCL-1 were equally sensitive to the cytotoxic effects of BP-1-102 regardless of either culture condition. Preliminary investigation suggests that the efflux system, used by cells to extrude toxic substances and linked to drug resistance in cancer, may be responsible for conferring stroma-mediate resistance to BP-1-102 in Mec-1 and RL cells. Finally, xenograft experiments to determine in vivo efficacy and safety are planned and will be presented.

Collectively, these findings demonstrate a critical role for STAT3 signaling in WM pathology and provide the rationale for further development of STAT3 inhibitors for the treatment of WM.