Abstract 4971: FGF2 promotes resistance to ABL kinase inhibitors in CML.
Authors: Traer Elie , Javidi-Sharifi Nathalie , Agarwal Anupriya , Dunlap Jennifer , Tyner Jeffrey , Wong Melissa , Druker Brian 
Published: August 2014

Abstract
Despite the clinical success of imatinib in treating chronic myeloid leukemia (CML), patients who develop resistance to available ABL inhibitors remain a clinical challenge. There is compelling evidence to suggest that the bone marrow microenvironment hinders the efficacy of kinase inhibitors, and may eventually lead to overt resistance. We developed a screen of nearly 300 molecules from the microenvironment to evaluate molecules that protect CML cells from imatinib. This screen identified Fibroblast Growth Factor 2 (FGF2) as a factor that mediates kinase inhibitor resistance in the CML cell line K562. Inhibition of FGFR with the selective inhibitor PD173014 attenuated the protective effects of FGF2, and siRNA revealed that FGF2 promotes survival through activation of FGFR3. FGFR3 was found to subsequently activate Ras and the MAPK pathway to promote K562 survival - despite effective inhibition of BCR-ABL by imatinib. More impressively, addition of FGF2 to K562 cells in long-term cultures with imatinib revealed that FGF2 was reproducibly capable of promoting growth and eventual imatinib-resistance in K562 cells (N=5). In contrast, K562 cells cultured in media alone or with other myeloid growth factors, did not develop resistance to imatinib. Sequencing of the kinase domain in resistant cells did not reveal mutations of the ABL kinase domain as a mechanism of resistance. Inhibition of both FGFR and BCR-ABL in the FGF2 imatinib-resistant outgrowths was synergistic in promoting cell death.
To evaluate whether FGF2 may be involved in resistance in CML patients, immunohistochemistry of bone marrow samples from imatinib, dasatinib and nilotinib resistant patients revealed increased FGF2 staining compared to normal controls and CML patients at diagnosis, suggesting that FGF2 levels increase over time and promote resistance in vivo. Further evidence that FGFR may be important in kinase resistance is emerging from clinical trials of the novel ABL inhibitor, ponatinib. Although ponatinib was specifically developed to bind BCR-ABL with the T315I mutation, it is also a multi-kinase inhibitor, including inhibition of FGFR in the low nanomolar range. To wit, ponatinib was able to suppress FGF2 imatinib-resistant outgrowths in the low nanomolar range whereas imatinib, dasatinib and nilotinib were ineffective, even at high doses. In clinical trials, ponatinib has proven effective not only against T315I mutated BCR-ABL, but also in patients with minor or even no mutations of the ABL kinase domain (in press). This suggests that stimulation of growth pathways by the microenvironment, such as FGF2, may be an important mechanism of resistance in some CML patients. Moreover, our approach provides a method to predict pathways of resistance in leukemia cell lines and primary patient samples. This strategy has the potential to define rational combinations of kinase inhibitors that can circumvent resistance before it is even clinically apparent.
Citation Format: Elie Traer, Nathalie Javidi-Sharifi, Anupriya Agarwal, Jennifer Dunlap, Jeffrey Tyner, Melissa Wong, Brian Druker. FGF2 promotes resistance to ABL kinase inhibitors in CML. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4971. doi:10.1158/1538-7445.AM2013-4971