Recurrent Resistance Mutations to Lirafugratinib Inform Treatment Sequencing in FGFR2-Driven Tumors.

PURPOSE: The use of reversible fibroblast growth factor receptor 2 (FGFR) inhibitors leads to the emergence of "undruggable" FGFR2 kinase domain mutations, hampering sequential treatment strategies. Lirafugratinib and futibatinib are irreversible FGFR inhibitors with the most promising clinical activity against FGFR2-driven tumors. EXPERIMENTAL DESIGN: We characterized resistance to lirafugratinib with circulating tumor DNA, tissue whole-exome sequencing, and bulk RNA sequencing in 30 patients with FGFR2-driven cancers, treated in the phase I/II ReFocus trial (NCT04526106) and enrolled in the UNLOCK program at Gustave Roussy. RESULTS: Among the 30 patients included, 18 (60%) had intrahepatic cholangiocarcinoma and 12 (40%) had other tumor types. Twenty-two patients (73%) were FGFR inhibitor-naïve. Among those experiencing primary resistance to lirafugratinib, we identified potential resistance mechanisms in five of six pretreatment samples. Patients with acquired lirafugratinib resistance manifested an unprecedented emergence of FGFR2 mutations in the M538 and/or L618 residues of the kinase domain, documented in 11 of 16 cases (69%). Compared with futibatinib resistance, FGFR2 molecular brake (N550) and gatekeeper (V565) mutations were rare. Leveraging the spectrum of FGFR2 kinase domain mutations at resistance to lirafugratinib and futibatinib, respectively, we identified the complementarity of the two irreversible inhibitors. On the basis of viability assays in FGFR2::BICC1-dependent Ba/F3 models and in vivo studies on patient-derived xenografts, we propose treatment sequences with the two agents. After lirafugratinib progression, three patients received futibatinib and experienced prolonged disease response. CONCLUSIONS: The complementary activity of lirafugratinib and futibatinib against FGFR2 kinase domain mutations supports their sequential use when precise resistance mutations are detected in patients.