Abstract PD3-09: Molecular characterization of resistance to antibody drug conjugates in metastatic breast cancer: a prospective analysis from the AURORA US Network

Abstract Introduction: With emerging antibody drug conjugates (ADC) as treatment of metastatic breast cancer (mBC), it is key to understand the mechanisms that drive resistance to inform optimal ADC selection and sequencing. The AURORA (AUR) US Network was established to characterize paired primary and metastatic (met) samples via multiplatform profiling, with a prospective phase of met tissue collection that included pre and post-ADC samples. Methods: Fresh frozen or FFPE met and archival primary tumors were analyzed across 3 platforms: DNA low pass whole genome and exome sequencing (WES), DNA methylation arrays (Methyl) and RNA sequencing (RNA seq). The molecular cohort included pts with data from ≥1 platform. Pairwise gene expression comparisons were performed in R using t-tests, and pathway analysis with GSEA. Results: A total of 168 pts comprised the molecular cohort: WES, 168; Methyl, 163; RNA seq, 168. Median age at mBC diagnosis was 56 (28-88). 21 pts (12.5%) had known germline BRCA1/2 mutations; 37 (22.0%) presented with de novo mBC. Subtype at mBC (if not available, at primary) diagnosis was: 108 (64.3%) HR+/HER2-, 22 (13.1%) HER2+, 33 (19.6%) TNBC and 5 (3.0%) unknown. Among 139 pts with known subtype in the first AUR met collected, 95 (68.3%) were HR+/HER2-, 17 (12.2%) HER2+ and 27 (19.4%) TNBC. 72 pts received ≥1 ADC with 1 (0-4) median line of chemotherapy for mBC prior to ADC1. As ADC1, 42 pts received HER2 ADC (35 trastuzumab deruxtecan, T-DXd; 7 trastuzumab emtansine), 29 TROP2 ADC (28 sacituzumab govitecan, SG; 1 datopotamab deruxtecan) and 1 NECTIN4 ADC. 22 pts received ≥2 ADC (16 immediate sequence; 6 intervening therapy); 11 T-DXd and 11 SG as ADC2. Median duration of ADC1 was 141 days (6-522); 81 (7-425) for ADC2. Progressive disease was the most common reason for ADC discontinuation, denoting resistance in post-ADC samples. 99 samples were collected pre-ADC1 and 24 post-ADC1, of which 5 were pre-ADC2. ERBB2 expression did not differ between samples after HER2 (n=9) vs non-HER2 ADC (n=8) (p=0.55), nor did TACSTD2 after TROP2 (n=8) vs non-TROP2 ADC (n=9) (p=0.16). In pts with HER2+ subtype at mBC diagnosis or AUR collection who received HER2 ADC, ERBB2 expression significantly decreased in post (n=4) vs pre-treatment (n=7) samples (p=0.043). ERBB2 downregulation was observed in 5 pts with paired pre vs post-HER2 ADC samples. In pts who received TROP2 ADC, TACSTD2 expression did not differ in post (n=10) vs pre-treatment (n=47) samples (p=0.63). GSEA of post-ADC samples revealed significant pathway differences by payload, including actin cytoskeleton in post-microtubule inhibitor (MTi) vs non-MTi ADC samples (p=0.001) and ATM-dependent DNA damage response after topoisomerase I inhibitor (TOP1i) vs non-TOP1i ADCs (p=0.043). Lysosomal vesicle biogenesis (p=0.014) and calcium release (p=0.05) pathways significantly differed between ADC naïve vs treated metastases. ERBB2 hypermethylation was not detected in the cohort. TACST2 promoter methylation was observed in 10 samples (β value 0.3) without association with TROP2 ADC exposure. ERBB2 or TACSTD2 mutations were not identified in the ADC-treated cohort. A novel loss-of-function (LOF) mutation in MED12 was detected in one pt immediately post-T-DXd. MED12 LOF has been reported to promote PARP inhibitor and platinum resistance in BRCA-deficient tumors and may represent a novel resistance mechanism to TOP1i. Conclusions: Multiplatform molecular characterization of mBC treated with ADC revealed potential mechanisms of resistance related to downregulation of target expression, payload (e.g., microtubule formation, DNA damage repair) and lysosomal processing. Additional analyses integrating these multimodal data are ongoing. Strategies to overcome resistance, e.g. modifying ADC target, are being evaluated in clinical trials. Citation Format: A. C. Garrido-Castro, W. C. Nenad, J. M. Balko, T. Hinoue, G. L. Wheeler, B. J. Kelly, M. Singha, B. M. Felsheim, A. R. Michmerhuizen, U. Chandran, A. V. Lee, L. A. Carey, T. A. King, E. Mardis, P. W. Laird, K. V. Giridhar, K. A. Hoadley, C. M. Perou. Molecular characterization of resistance to antibody drug conjugates in metastatic breast cancer: a prospective analysis from the AURORA US Network 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 PD3-09.