Abstract PS17-09: An FGFR1/2 degrader overcomes antiestrogen resistance in ER+/FGFR1-amplified breast cancer

Abstract We previously reported that nuclear FGFR1 promotes antiestrogen resistance in ER+/FGFR1-amplified breast cancer. Nuclear FGFR1 activity was not affected by FGFR tyrosine kinase inhibitors (TKIs). Furthermore, pan-FGFR TKIs are not well tolerated. Therefore, new therapeutic strategies that inhibit nuclear FGFR1 while sparing FGFR3 and FGFR4 would be required to overcome antiestrogen resistance and limit toxicity. For this study, we used the proteolysis-targeting chimera (PROTAC) DGY-09-192, consisting of the FGFR TKI BCJ398 linked with a VHL recruiting ligand, to degrade FGFR1 and FGFR2. We aimed to determine whether selective degradation of FGFR1/2 overcomes endocrine resistance. Treatment with 50-100 nM DGY-09-192 strongly induced FGFR1 degradation and suppressed phosphorylation of FGFR1 and its downstream targets FRS2, AKT, and ERK1/2 in ER+/FGFR1-amplified CAMA1 and MDA-MB-134 breast cancer cells. FGFR1 degradation was evident within 4 h and lasted 48-96 h after drug washout. The proteasome inhibitor MG132 (10 µM) rescued both FGFR degradation and phosphorylation of downstream signaling molecules induced by DGY-09-192, confirming that FGFR1 degradation is dependent on ubiquitination and proteasomal degradation. At equivalent concentrations, DGY-09-192 suppressed phosphorylation of FGFR1 and its downstream targets more potently than BGJ398. Subcellular fractionation showed treatment with DGY-09-192 (100 nM) degraded FGFR1 in membrane, nuclear, and chromatin-bound fractions in both CAMA1 and MDA-MB-134 cells. Treatment of CAMA1 cells with DGY-09-192 (100 nM) downregulated nuclear FGFR1 target genes and canonical ERα target genes, such as CCND1, VEGFA, and CDK12 as measured by qRT-PCR whereas treatment with BGJ398 (100 nM) did not. Treatment with DGY-09-192 (500 nM) reversed resistance to the ER degrader fulvestrant in CAMA1 cells as measured by colony formation assays and using the Incucyte Live-Cell Analysis System. Each drug alone showed a modest effect on cell proliferation whereas the combination completed blocked cell growth (p 0.0001). Finally, treatment of NOD scid gamma (NSG) mice with established ER+/FGFR1-amplified HCI-011 patient-derived xenografts with DGY-09-192 (40 mg/kg daily x2-6) induced FGFR1 degradation and blocked phosphorylation of downstream targets in the tumors. Studies using the combination of DGY-09-192 and fulvestrant in mice bearing established HCI-011 xenografts are in progress. We next investigated the effect of DGY-09-192 on cancer cells harboring FGFR1 and FGFR2 activating mutations. DGY-09-192 treatment induced FGFR2 degradation and blocked downstream signaling in MFE296 endometrial cancer and EFM-19 breast cancer cells that harbor the FGFR2N549K and FGFR2K659E hotspot mutations, respectively. In addition, DGY-09-192 strongly suppressed MFE296 cell proliferation. We are currently investigating whether DGY-09-192 blocks signaling and cell proliferation induced by the N546K and K656E hotspot FGFR1 mutations in ER+ breast cancer cells. Notably, a recent clinical study showed that tumors harboring these FGFR1 activating mutations are refractory to the pan-FGFR TKI pemigatinib. Conclusions: The FGFR1/2 PROTAC DGY-09-192 induced degradation of nuclear and membrane-bound FGFR1 and blocked FGFR1-induced signaling and nuclear activity in ER+/FGFR1-amplified breast cancer cells. The combination of DGY-09-192 and fulvestrant synergistically suppressed proliferation of these cells. Therefore, FGFR1 degradation represents a promising therapeutic strategy to block FGFR1 activity more completely and selectively in ER+ breast cancers harboring activating FGFR1 alterations. Citation Format: Yasuaki Uemoto, Chang-Ching Lin, Alberto Servetto, Bingnan Wang, Dan Ye, Fabiana Napolitano, Hima Milan Patel, Luigi Formisano, M Rosario Chica-Parrado, Emmanuel Bikorimana, Saurabh Mendiratta, Chuo Chen, Ariella Hanker, Carlos Arteaga. An FGFR1/2 degrader overcomes antiestrogen resistance in ER+/FGFR1-amplified breast cancer abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS17-09.