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