Abstract 4395: A best-in-class HER2xHER2 novel biparatopic antibody-drug conjugate with an efficacious, low-toxicity design that maximizes antibody functionality

Abstract Introduction: HLX22 is a novel anti-HER2 antibody that binds to a unique site on HER2 subdomain IV, different from trastuzumab. When combined with HLX02 (a trastuzumab biosimilar), HLX22 increases internalization of HER2 homodimers and HER2/EGFR heterodimers, reducing cell proliferation signals. This combination shows stronger antitumor effects than HLX02 plus HLX11 (a pertuzumab biosimilar) both in vitro and in vivo. Based on the binding sites of HLX22 and HLX02, we developed a biparatopic antibody-drug conjugate (ADC) with an effective, low-toxicity payload, allowing higher dosing and better receptor coverage for improved tumor cell killing. Methods: Cytotoxicity of HER2xHER2 biparatopic ADC was assessed using the CellTiter-Glo (CTG) assay in tumor cell lines. ADC binding and internalization were evaluated by flow cytometry in cells with varying HER2 expression levels. Pharmacokinetics (PK) of the HER2xHER2 biparatopic ADC were characterized in rats following a single intravenous 5 mg/kg dose. Bystander killing was quantified by co-incubating antigen-positive and -negative (Jurkat) cells, measuring Jurkat viability via the CTG assay. In vivo efficacy of HER2xHER2 biparatopic ADC was evaluated in multiple xenograft models (JIMT-1, BT-474 and patient-derive xenografts). Preliminary toxicology in cynomolgus monkeys was assessed at 60 mg/kg, administered every three weeks for three cycles. Results: The HER2xHER2 biparatopic ADC demonstrated superior internalization efficiency than KN026 (a HER2-targeted bispecific antibody) and trastuzumab deruxtecan in BT474 and NCI-N87 cell lines. The biparatopic ADC showed better anti-tumor efficacy than trastuzumab deruxtecan across multiple cell lines. The ADC induced significant tumor regression at a single 6 mg/kg dose in various xenograft models outperforming trastuzumab deruxtecan head-to-head, including HER2-positive (immunohistochemistry IHC 3+, IHC2+ and fluorescence in situ hybridization FISH+), HER2-low (IHC2+/FISH- or IHC 1+), and HER2-Ultra low (IHC1+) models. Preliminary toxicology shows good tolerability in cynomolgus monkeys following 3 doses of the ADC at 60 mg/kg. Conclusion: We have developed a potential best-in-class HER2xHER2 novel biparatopic ADC that exhibits a superior therapeutic index, antibody-mediated signaling blockade. The preclinical findings support clinical development in breast and gastric cancers, with the hope that the improved therapeutic index of this agent confers survival benefit. Citation Format: Ge Song, Yushi Chi, Xiansong Xiong, Rui Liu, Xiaoling Yuan, Wan-Jen Yang, Wanli Zhang, Xinwei Wang, Boqi Gu, Qian Zou, Peng Huang, Kuichao Qu, Haixiang Yu, Futang Yang, Qingyu Wang, Chen Hu, Jijun Yuan. A best-in-class HER2xHER2 novel biparatopic antibody-drug conjugate with an efficacious, low-toxicity design that maximizes antibody functionality 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 4395.