Abstract 314: Acquired FGF and FGFR alterations confer resistance to estrogen receptor (ER) targeted therapy in metastatic ER+ breast cancer
Authors: Mao Pingping , Cohen Ofir , Kowalski Kailey , Kusiel Justin , Buendia Jorge , Exman Pedro , Wander Seth A. , Waks Adrienne G. , Winer Eric P. , Lin Nancy U. , Wagle Nikhil 
Published: August 2019

Abstract
In estrogen-receptor positive (ER+) breast cancer, therapies that target the ER have led to reductions in recurrence and mortality. However, in the metastatic setting, resistance to ER-targeted therapies is near universal. Various resistance mechanisms have been proposed, including ESR1 activating mutations and upregulation of alternative signal transduction pathways, yet clinically relevant resistance mechanisms have not been clearly defined. We conducted a genome-scale gain-of-function screen in ER+ breast cancer cell lines, spanning 17,255 overexpressed open reading frames (ORFs), to investigate genes whose overexpression is sufficient to confer resistance to selective estrogen receptor degraders (SERDs). Among several validated resistance genes, the fibroblast growth factor receptor (FGF/FGFR) pathway was among the top pathways, with several FGF ligands scoring as top resistance genes in this screen. In parallel, we performed whole exome sequencing (WES) in paired pre-treatment and post-resistance biopsies from 60 patients with ER+ metastatic breast cancer (MBC) who had developed resistance to ER-targeted therapy. The FGFR pathway was altered in 40% (24 of 60) of the post-resistance biopsies; alterations included FGFR1 amplifications (9 patients), FGFR2 amplifications and activating mutations (4 patients), and FGF3 amplifications (17 patients). In 12 out of the 24 cases (5 FGFR1, 4 FGFR2, 3 FGF3), the FGF/FGFR pathway alterations were present in the resistant tumors and not detected in the pre-treatment tumors, suggesting that these alterations were acquired under the selective pressure of ER-directed therapy. Overexpression of wildtype FGFR1, FGFR2, FGF3 in ER+ breast cancer cells (T47D and MCF7) led to resistance to fulvestrant as well as the combination of fulvestrant and the CDK4/6 inhibitor palbociclib. This resistance phenotype was reversed by the FGFR inhibitor PD173074, and, to a lesser extent, the MEK inhibitor trametinib and mTOR inhibitor everolimus, suggesting several potential treatment strategies. The three FGFR2 mutations (M538I, N550K and K660N) identified in these patients induced hyperactive FGFR signaling and conferred resistance to fulvestrant, which was abrogated by the irreversible FGFR inhibitors FIIN-2 and FIIN-3. Together, these results offer new insights into endocrine resistance and suggest new clinical approaches to overcome or preempt therapeutic resistance.
Citation Format: Pingping Mao, Ofir Cohen, Kailey Kowalski, Justin Kusiel, Jorge Buendia, Pedro Exman, Seth A. Wander, Adrienne G. Waks, Eric P. Winer, Nancy U. Lin, Nikhil Wagle. Acquired FGF and FGFR alterations confer resistance to estrogen receptor (ER) targeted therapy in metastatic ER+ breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 314.