Abstract 3298: Preclinical development of TJ106, a potent biparatopic dual payload ADC targeting HER2 for the treatment of Enhertu-resistant tumors

Abstract Background: HER2 overexpression is associated with increased metastatic potential and poor clinical outcomes across multiple tumor types, including breast, gastric, gastroesophageal junction, and lung cancers. Enhertu (trastuzumab deruxtecan, DS-8201a) has transformed HER2-targeted therapy by demonstrating efficacy in HER2-expressing and previously refractory tumors through delivery of a potent topoisomerase I inhibitor payload (DXd) with bystander effect. However, clinical resistance to Enhertu has emerged, driven by mechanisms such as reduced payload sensitivity, HER2 downregulation or heterogeneity, and adaptations within the tumor microenvironment. To address this unmet need, we developed TJ106, a novel biparatopic, dual-payload HER2-targeting antibody-drug conjugate (ADC) designed to enhance tumor cell internalization, overcome resistance mechanisms, and improve the therapeutic window. Methods: Antibody internalization was evaluated by flow cytometry (FACS). The CellCounting-Lite 2.0 luminescent viability assay was used to assess the in vitro cytotoxicity of TJ106. In vivo antitumor activity was investigated in cell-derived xenograft (CDX) and patient-derived xenograft (PDX) models with high, moderate, or low HER2 expression. Results: TJ106 consists of a biparotopic antibody, generated from trastuzumab and pertuzumab using knob-into-hole technology, conjugated in a site-specific manner (DAR 4) to the dual-linker payload DLP1, which combines a topoisomerase I inhibitor and a microtubule inhibitor. TJ106 showed superior internalization compared with trastuzumab or pertuzumab in HER2-moderate JIMT-1 (IHC 2+) breast cancer cells and HER2-low Colo-205 (IHC 1+) colorectal cancer cells. TJ106 also exhibited enhanced in vitro cytotoxicity across HER2-expressing tumor cell lines, including NCI-N87 (HER2 3+) gastric cancer, JIMT-1, and Colo-205, compared with trastuzumab-DLP1, pertuzumab-DLP1, and DS-8201a. In the JIMT-1 CDX model, TJ106 demonstrated superior antitumor efficacy versus comparators: TJ106 at 3.0 mg/kg achieved 81.8% tumor growth inhibition (TGI), compared with 69.0%, 71.0%, and 53.0% TGI for trastuzumab-DLP1, pertuzumab-DLP1, and DS-8201a, respectively. TJ106 was well tolerated in mice with no observed treatment-related body weight loss. Furthermore, TJ106 effectively inhibited tumor growth in two Enhertu-resistant PDX models (LD1-0017 and LU9681-R1P12) and one Enhertu-resistant NCI-N87 CDX model. Conclusion: TJ106, a novel biparatopic dual-payload HER2-targeting ADC, demonstrates robust in vitro and in vivo antitumor activity, including in Enhertu-resistant models, supporting its further clinical development for patients with advanced HER2-expressing tumors. Citation Format: Siyi Hu, Mao Huang, Xiaobo Cai, Shugaku Takeda, Li Qi, Yangxin Zhang, Haiyan Wu, Yan Zhang, Yanli Mao, Zhaoyuan Chen, . Preclinical development of TJ106, a potent biparatopic dual payload ADC targeting HER2 for the treatment of Enhertu-resistant tumors [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 3298.