The Role of FGFR in the Progression of Waldenstrom's Macroglubulinemia and the Effect of Its Inhibition by TKI-258.
Authors: Azab Feda , Azab Abdel Kareem , Roccaro Aldo M. , Sacco Antonio , Quang Phong , Ngo Hai T , Melhem Molly R , Runnels Judith M. , Maiso Patricia , Ghobrial Irene M 
Published: October 2019

Abstract
Abstract 3737
Poster Board III-673

INTRODUCTION
Waldenström macroglobulinemia (WM) is a low grade non-Hodgkin lymphoma, characterized by the presence of abnormal lymphoplasmacytic cells producing high levels of IgM. Although indolent, WM remains incurable, and therefore, there is an urgent need for rationally designed therapy in WM. Receptor tyrosine kinases (RTKs) are cell surface receptors for growth factors, cytokines and hormones which have a critical role in the development and progression of many types of cancer. However, their role in WM was not identified. TKI-258 (Novartis, Basel, Switzerland) is an ATP-competitive inhibitor with activity against (multiple) receptor tyrosine kinases including FGFR and other RTKs. We hypothesized that FGFR is up-regulated in WM and plays a major role in its progression; and that TKI-258 would reduce tumor progression in WM.

METHODS AND RESULTS
We tested the expression of FGFR3 on WM cells and found overexpression of this RTK compared to CD19+ cells from healthy donors. The activation of FGFR3 by recombinant FGF induced MAPK signaling pathway in WM cells including phosphorylation of RAF, ERK and STAT3. Also it induced PI3K signaling including phosphorylation of AKT, S6R and GSK3. TKI-258 inhibited the FGF induced activation of the MAPK and PI3K signaling pathways in a dose- response manner. Using MTT assay we tested the effect of TKI-258 ( 0 to 2.5 uM) on the survival of WM cell line BCWM-1, on IgM secreting cell line MEC-1, and on CD19+ cells selected from WM patient sample. We found that the TKI-258 induced cell death in all sample tested with an IC50 ranging 0.8-1 uM. Testing the effect of TKI-258 on the survival of CD19+ cells selected from peripheral blood or mononuclear cell from healthy donors showed a minimal effect of less than 10% cell death. These results provide a wide therapeutic window for the use of TKI-258 in WM. Moreover, we tested the effect of TKI-258 on the apoptosis of WM cells by flow cytometry using the apoptosis marker APO-2.7, and found that TKI-258 induced apoptosis of WM cells in a dose-response manner at both 24 and 48 treatment. Moreover these results were confirmed by testing changes in the expression of apoptosis related proteins in response to TKI-258 by immunoblotting, including induction of PARP, and caspase-3 and caspase-9 cleavage. In correlation with these results, cell cycle analysis by PI staining and analysis by flow cytometry of WM cells treated with TKI-258 for 24 hrs showed induction of sub-G1 increase in a dose response manner with an IC50 about 1uM. To test the effect of TKI-258 on the interaction of WM cells with the microenvironment we examined the effect of TKI-258 on adhesion of WM cells to fibronectin and bone marrow stromal cells (BMSCs), and found that TKI-258 induced a 50% decrease of adhesion. Moreover, we found no effect on the chemotaxis of WM induce by stroma derived factor-1 (SDF1). To test the direct effect of TKI-258 on the interaction with the microenvironment, we examined the proliferation of WM when cultured alone of in co-culture with BMSCs by 3H-thymidine uptake assay. We showed that TKI-258 inhibited the proliferation of WM cells with an IC50 of 0.8 uM, in the presence or absence of BMSCs.

CONCLUSION
In conclusion, we found an overexpression of FGFR3 in WM cells compared to CD19+ cells from healthy donors, and that TKI-258 inhibited the activation of proliferative pathways induced by activation of FGFR3 and led to inhibition of proliferation and apoptosis of WM cells.

Disclosures:
Ghobrial: Millennium: Honoraria, Research Funding, Speakers Bureau; Celgene: Consultancy, Honoraria, Speakers Bureau; Novartis: Honoraria, Speakers Bureau.