Abstract 5083: Discovery of an asymmetric IgG-like bispecific ADC targeting EGFR/HER3

Abstract Background: The epidermal growth factor receptor (EGFR) family, consisting of EGFR, HER2, HER3, and HER4, plays a critical role in tumorigenesis and tumor progression. Inhibition of ERBBs using either monoclonal antibodies (mAb) or small-molecule tyrosine kinase inhibitors have been approved for use for the treatment of various types of cancers. Despite relative success, many patients acquire resistance to long-term benefit due to alternative compensatory signaling pathways. Accordingly, activating mutations or overexpression of HER3 has been reported in many cancers, such as breast, ovarian, lung, colorectal, melanoma, head and neck, cervical and prostate cancers. Here, we report the discovery of a humanized anti-EGFR x HER3 IgG-like bispecific antibody-drug conjugate (ADC). The ADC takes a asymmetric 1+1 form through HC-HC and HC-LC charge-based heterodimerization, which is then conjugated to a topoisomerase 1 inhibitor (TOP1i) payload via a cleavable linker. The affinity of EGFR and HER3 was optimized to improve the therapeutical window, and has shown potent anti-tumor efficacy in various tumor models with limited toxicity supporting further development in clinic. Methods: An anti-EGFR x HER3 bispecific ADC (PM1300) was generated by introducing unique mutations to the CH1-CL domains of each binding arm to avoid HC-LC mispairing. KIH mutations were also introduced to each CH3 domain to support HC/HC heterodimerization and generate an IgG-like bispecific with one arm binding to EGFR and the other to HER3. TOP1i payload was conjugated to the IgG-like bispecific by the endogenous cysteine residues at DAR 8 via a clearable linker. Results: Through affinity optimization, PM300 was found to preferentially bind to EGFR/HER3 double-positive cells rather than EGFR single-positive cells. PM1300 showed significantly increased internalization and anti-proliferation activity against EGFR/HER3 double-positive cells, with limited activity towards single positive-cells. PM1300 used an optimized linker for better payload release and serum stability. PM1300 induced a potent anti-tumor efficacy in various CDX models representing different EGFR and HER3 expressing levels and mutations. Importantly, PM1300 exhibited a good safety profile in NHP consistent with the optimal affinity for EGFR and HER3. These results demonstrate that PM1300 has a promising efficacy/safety therapeutical window in preclinical models supporting further development in the clinic. Conclusion: A bispecific ADC targeting EGFR x HER3 was discovered, named PM1300, which has an asymmetric 1+1 IgG-like structure and optimized affinity. This allows for preferential binding to EGFR/HER3 double-positive cancer cells, rather than EGFR single-positive cells. This may significantly contribute to minimizing safety risk that is common for EGFR-targeting agents. Citation Format: Liandi Chen, Shaogang Peng, Chao Wang, Xiaoniu Miao, Yao Yan, Jun Xing, Weifeng Huang, Andy Tsun, Yingda Xu, Xiaolin Liu, Yi Luo. Discovery of an asymmetric IgG-like bispecific ADC targeting EGFR/HER3 [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(6_Suppl):Abstract nr 5083.