Abstract PD3-05: Targeting FGFR4 with an antibody-drug conjugate in hormone receptor-positive and HER2-negative (HR+/HER2-) breast cancer progressing to CDK4/6 inhibitors and endocrine therapy

Abstract Background: Resistance to endocrine therapy (ET) and CDK4/6 inhibitors (CDK4/6i) is a major challenge in HR+/HER2- breast cancer (BC). The HER2-enriched (HER2-E) intrinsic subtype represents an aggressive molecular form of HR+/HER2- disease and is associated with significantly worse progression-free survival (PFS) and overall survival (OS) under ET+CDK4/6i treatment compared to luminal subtypes (Prat et al. JCO 2021; CCR 2024). HER2-E is present in up to 10-20% of tumors prior to ET+CDK4/6i and accounts for 50-60% of tumors at progression, reflecting both pre-existing and acquired HER2-E through molecular subtype switching. These findings underscore the need to better understand its underlying biology and the mechanisms driving resistance in order to develop new therapies. Methods: A genome-wide CRISPR/Cas9 loss-of-function screen was conducted in 2 HER2-E and palbociclib-resistant BC cell lines. Findings were integrated with gene expression data of tumor samples of patients (pts) treated with CDK4/6i in both early (CORALLEEN trial [n=49 pts]) and metastatic (CDK cohort [n=375 pts]) settings. FGFR4 immunohistochemistry was performed in BC and healthy tissues. FGFR4 was functionally validated via knockdown and overexpression in vitro and in cell line-derived (CDX) and pt-derived xenograft (PDX) models. An antibody-drug conjugate (ADC) targeting FGFR4, composed of a humanized IgG1 antibody linked to the cytotoxic payload monomethyl auristatin E (MMAE) at a drug-to-antibody ratio of 4, was developed and tested preclinically. We employed descriptive statistics and Cox and logistic regression models. Results: Among 11 candidate resistance genes, including known mediators such as CCNE1, FGFR4 emerged as a novel and key driver of resistance. FGFR4 expression was significantly higher in HER2-E tumors, increased in progressive disease (PD) samples following CDK4/6i+ET, and was significantly associated with worse PFS (HR=1.49, 95% CI: 1.19-1.78, p<0.001) and OS (HR=2.01, 95% CI: 1.49-2.72, p<0.001). FGFR4 protein levels correlated strongly with mRNA expression (r=0.66, p<0.001), were elevated in PD samples, and were absent in healthy tissues. Functionally, FGFR4 knockdown restored palbociclib sensitivity, reduced tumor growth, and suppressed CCNE1 expression and RB phosphorylation. Conversely, FGFR4 overexpression induced resistance and HER2-E features. A selective tyrosine kinase inhibitor targeting FGFR4 and a naked monoclonal antibody showed limited in vitro activity. In contrast, a FGFR4-directed ADC exhibited high specificity, rapid internalization, and potent cytotoxicity in FGFR4-high cells, sparing FGFR4-low/negative cells. The ADC also demonstrated a bystander effect in co-culture experiments and significantly inhibited tumor growth in FGFR4-high CDXs and PDXs without detectable toxicities. Conclusions: FGFR4 is as a clinically actionable driver of resistance to CDK4/6i+ET in HR+/HER2- breast cancer. A FGFR4-directed ADC shows strong preclinical efficacy and represents a novel strategy to overcome resistance. These findings pave the way for clinical translation, with a first-in-human phase I trial launching in Q4 2025 in Spain. Citation Format: F. Braso-Maristany, N. Lorman-Carbó, S. Gregorio, T. Blasco, B. Morancho, A. Martínez-Romero, E. Sanfeliu, N. Chic, I. Fonseca, P. Galván, M. Guiu, V. Sirenko, A. Aguirre, A. Morales, S. Guardiola, R. Gómez-Bravo, B. Adamo, M. Bergamino, I. Garcia-Fructuoso, F. Schettini, H. Sun, D. M. González-Gironès, M. de Frias, M. Muñoz, M. Vidal, O. Martínez-Sáez, T. Pascual, V. Vanhooren, A. Prat, R. Gomis. Targeting FGFR4 with an antibody-drug conjugate in hormone receptor-positive and HER2-negative (HR+/HER2-) breast cancer progressing to CDK4/6 inhibitors and endocrine therapy [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PD3-05.