Abstract 2950: Identification of sensitivity predictors and differentiators for HER2-directed ADCs

Abstract Antibody drug conjugates (ADCs) is a promising class of anti-cancer drugs that combines a monoclonal antibody with a small therapeutic agent for cell-specific delivery. With many ADCs in clinical trials and over 10 approvals by the U.S. Food and Drug Administration (FDA) since the beginning of this century, it is a rapidly expanding field. However, many clinical studies have been discontinued or terminated because of challenges such as low efficacy or off-target toxicity. Trastuzumab emtansine (T-DM1, Kadcyla®) and trastuzumab deruxtecan (T-DXd, Enhertu®) are two ADCs approved for HER2-positive (HER2+) breast cancers that share the same anti-HER2 monoclonal antibody, but differ in their payload and linker chemistry. T-DM1 consists of trastuzumab and tubulin inhibitor emtansine (mertansine) connected by a non-cleavable linker, while T-DXd combines trastuzumab and topoisomerase I inhibitor deruxtecan with a cleavable linker. T-DXd is also approved for non-HER2+ breast cancers, which reflects that the ADCs have different effects on tumors even though they share the same antibody. Further insights into the cellular targeting of the two ADCs and their respective free payloads may help in better understanding important differences. T-DXd and T-DM1 were profiled on 108 cancer cell lines in cell viability assays. Additionally, the free payloads, trastuzumab and the small molecule HER2 inhibitor tucatinib were profiled as single agents on the same cell lines. The cell line panel represents a broad range of tumor types including seven HER2+ cell lines. All cell lines were incubated with a 9-point dose range of each drug, followed by intracellular ATP readout to determine the cell viability. Half-maximum inhibitory concentrations (IC50) were used for bioinformatic analyses to identify response predictors or differentiators based on genomic, transcriptomic and proteomic status of the cell lines. Comparison of cell line responses between the ADCs and their free payloads showed a higher selectivity of the ADCs. Linking trastuzumab to deruxtecan or mertansine thus improves selectivity in vitro. Nonetheless, the ADCs targeted more cell lines than trastuzumab alone, showing that the ADCs also have non-antibody directed effects. Additionally, the HER2+ cell line SK-OV-3 showed striking resistance to T-DXd (IC50 > 10, 000 ng/mL), while T-DM1 effectively targeted this cell line (IC50 = 393 ng/mL). Although a significant Pearson correlation between ERBB2 (HER2) upregulation and sensitivity to T-DXd (r = -0.39) and T-DM1 (r = -0.63) was found, the results above show that HER2 status alone is not sufficient for response prediction of HER2-directed ADCs. Other sensitivity predictors were determined by bioinformatic analyses on resistant HER2+ and sensitive non-HER2+ cells. This study provides insights into sensitivity predictors and differentiators of the two HER2-directed ADCs and the small molecule HER2 inhibitor tucatinib. Citation Format: Daphne J. Kluitmans, Jeffrey J. Kooijman, Janneke J. Melis, Sabine Wetemans, Jeroen A. de Roos, Mandy W. Drossaert-Smeets, Guido J. Zaman. Identification of sensitivity predictors and differentiators for HER2-directed ADCs [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2950.