Abstract 3135: BSI-730, a bispecific antibody targeting HER2 and PD-L1 for the development of a first-in-class bispecific ADC

Abstract Background: Antibody-drug conjugates (ADCs) have become a promising therapeutic option for patients with cancer. Bispecific ADCs targeting two tumor-associated antigens have potential differential anti-tumor advantages. HER2 and PD-L1 are co-expressed in some tumor cell types, such as breast cancer, gastric cancer, lung adenocarcinoma and urothelial carcinoma. A bispecific ADC targeting HER2 and PD-L1 should provide therapeutic benefit, particularly for patients with HER2 low cancer. Dual-target mediated cytotoxicity combined with PD-L1 mediated immunotherapy provides an innovative approach for cancer patients. Experimental procedures: An anti-PD-L1 monoclonal antibody was identified from A/J mice immunized with PD-L1-ECD-Fc and screened by the Biosion proprietary SynTracer® High Throughput Endocytosis Platform. A “knob-in-hole” bispecific antibody, BSI-730, targeting HER2 and PD-L1 was created with trastuzumab and the humanized anti-PD-L1 scFv. Target binding specificity, binding activity, and affinity of BSI-730 were evaluated by protein-based ELISA, cell-based FACS and Biacore-based SPR, and the ligand blocking activity of PD-L1 portion was measured by competitive ELISA and cell-based FACS. The internalization activities on various cancer cell lines with different expression levels of HER2 and PD-L1 were evaluated by using the SynTracer® Platform. Summary: BSI-730, a HER2/PD-L1 bispecific antibody, showed comparable binding affinity to HER2 as compared to trastuzumab, as well as comparable bioactivity to the parental anti-PD-L1 antibody regarding PD-L1 binding and PD-1/PD-L1 blocking. Based on a cell-based reporter assay, BSI-730 was able to reverse PD-L1 mediated T-cell suppression and exhibited comparable potency to the parental anti-PD-L1 antibody. The internalization of BSI-730 on HER2 low cancer cell lines was stronger than that of trastuzumab whereas the internalization of BSI-730 on HER2 high cancer cell lines was comparable to that of trastuzumab. Conclusion: BSI-730 is a novel bispecific antibody for the development of a first-in-class bispecific ADC with the potential advantage of increased potency that can be further investigated to treat HER2 and PD-L1 co-expression tumors, particularly HER2 low tumors. Citation Format: Xiaoyao Hao, Hongyan Li, Xiaodong F. Liu, Jinyu Liu, Wenwen Dai, Ying Pan, Hui-Han Hu, Jun Li, Shukai Xia, Qun Lyu, Hugh M. Davis, Mingjiu Chen, Zeyu Peng. BSI-730, a bispecific antibody targeting HER2 and PD-L1 for the development of a first-in-class bispecific ADC abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 3135.