Abstract A118: Identification of resistance mechanisms to FGFR4 targeted therapy in hepatocellular carcinoma

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Abstract When on-target resistance mutations occur in response to potent and selective therapeutics, the target is often considered a validated driver of disease. Hepatocellular carcinoma (HCC) is a leading cause of cancer mortality worldwide whose drivers remain unvalidated because approved therapies utilize multi-kinase inhibitors or immune modulation. In this study, we validate for the first time a driver of disease in the FGF19 expressing subset of HCC through the identification of Fisogatinib resistance mutations in FGFR4. Fisogatinib (formerly known as BLU-554), an investigational agent, was developed to specifically inhibit the tyrosine kinase activity of FGFR4. Approximately one third of HCC patients harbor aberrant expression of fibroblast growth factor 19 (FGF19), a ligand hypothesized to drive paracrine FGFR4 activation and enhance mitogenic signaling in HCC cells. The identification of FGF19 amplifications in a subset of HCC patients through genomic profiling is further suggestive of the involvement of the FGF19/FGFR4 signaling axis in oncogenesis. The first-in-human, phase I study of Fisogatinib in patients with advanced HCC (NCT02508467) suggests that expression of FGF19 as assessed by immunohistochemistry is responsible for oncogenesis and predictive of response to treatment. We identified FGFR4 gatekeeper (V550) and hinge-1 (C552) mutations upon disease progression in two patients treated with Fisogatinib. These resistance mutations had been predicted by our structure-based studies and were also identified through in vitro and in vivo Fisogatinib resistance screens. We confirm by liquid chromatography tandem mass-spectrometry that Fisogatinib binding to FGFR4 is prevented when either the gatekeeper or the hinge-1 mutation is present and demonstrate the capability of these mutations to mediate resistance to Fisogatinib through genetic engineering. Using a gatekeeper-agnostic pan-FGFR inhibitor we show, in preclinical models, continued FGF19/FGFR4 pathway dependence. These results validate FGF19/FGFR4 as an oncogenic driver and inform the profile of potential next-generation inhibitors designed to facilitate improved patient outcomes. Citation Format: Megan A Hatlen, Oleg Schmidt-Kittler, Cori-Ann Sherwin, Emily Rozsahegyi, Nooreen Rubin, Michael Sheets, Joseph L Kim, Chandra Miduturu, Neil Bifulco, Natasja Brooijmans, Hongliang Shi, Timothy Guzi, Andy Boral, Christoph Lengauer, Marion Dorsch, Richard D Kim, Yoon-Koo Kang, Beni B Wolf, Klaus P Hoeflich. Identification of resistance mechanisms to FGFR4 targeted therapy in hepatocellular carcinoma [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr A118. doi:10.1158/1535-7163.TARG-19-A118

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