Abstract 4542: Zanidatamab modulates multiple pathways involved in tumor growth and survival and is efficacious post T-DXd

Abstract Zanidatamab (Ziihera), a biparatopic antibody against the human epidermal growth factor receptor-2 (HER2), is currently approved in previously treated advanced HER2+ biliary tract cancer. To understand the specificity of binding, affinity of zanidatamab and the one-armed antibodies (OAAs) that comprise the Fab and scFv of zanidatamab, to WT HER2 extracellular domain (ECD) and mutein HER2 ECD domains were determined by surface plasmon resonance. The anti-HER2 Fab bound HER2 ECD II, and the anti-HER2 scFv bound HER2 ECD IV generated a Kd of 0.25 and 0.48 nM, respectively. Zanidatamab bound HER2 ECD with a lower Kd (0.047 nM) when compared to HER2 ECD II (0.31 nM) or HER2 ECD IV muteins (0.26 nM), demonstrating the avidity of Fab and scFv paratopes. To further understand MoA of zanidatamab, a multi-omics analysis including whole transcriptomics, proteomics and phospho-proteomics was performed on BT-474 HER2-amplified breast cancer xenograft tumors exposed to zanidatamab. A combination of unbiased analyses showed that zanidatamab significantly altered key pathways associated with DNA damage/repair, cell cycle, and MAPK signaling in a dose- and time-dependent manner. Expanded analysis of in vivo response demonstrated that zanidatamab is efficacious post T-DXd (trastuzumab deruxtecan) therapy. Mice bearing BT-474 HER2-amplified tumors were treated with T-DXd to regression and regrowth to baseline followed by treatment with zanidatamab. Zanidatamab was efficacious on these progressed tumors with 100% regressions following the first dose. These data demonstrate HER2 domain specific binding affinity, zanidatamab impacting key cellular pathways in driving in vivo efficacy, and potential utility in a post-T-DXd population. Citation Format: Ankur Karmokar, Emanuele Loro, Al Hassan Kyakulaga, Desmond Lau, Nina Weisser, Genevieve Desjardins, Prajwal Raghunatha, Edwin Clark, Robin Humphreys, Kedar S. Vaidya. Zanidatamab modulates multiple pathways involved in tumor growth and survival and is efficacious post T-DXd abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4542.