Abstract 6016: AP402, a bispecific antibody targets p95HER2 and CD137, shows a potent antitumor activity in p95HER2-expressing cancers

Abstract Introduction: p95HER2 is the extracellular domain-truncated form of HER2 receptor while preserves the intracellular tyrosine kinase activity to support tumor growth. p95HER2 was found in 30 to 40% HER2-positive breast cancer patients and associated with a more aggressive phenotype and worse prognosis. Due to the lack of extracellular domain, p95HER2 has been recognized as the potential mechanism of trastuzumab-resistance. The unmet need for trastuzumab-resistant patients and exclusive expression in cancerous tissues make p95HER2 as an ideal therapeutic target. Here, AP402 was designed to coordinate two dominant effector cells, natural killer and T cells, against p95HER2-positive cancers. Experimental procedures: AP402 bispecific antibody was generated by conjugating CD137 single-chain variable fragment (scFv) to the heavy chain C-terminal of p95HER2 antibody (human IgG1). The affinity and specificity of p95HER2 antibodies were checked by biolayer interferometry (BLI) and flow cytometry, respectively. ADCC and ADCP activities of AP402 were determined by NK cells and monocyte-derived macrophages, respectively. AP402-mediated activation of T cells were studied by coculturing T cells with p95HER2-transgenic cells. Antitumor activity of AP402 was studied using humanized mice inoculated with p95HER2-transgenic cancer cells and p95HER2-positive breast cancer patient-derived xenograft (PDX). Safety of AP402 was determined by the 4-week toxicity study in monkeys. Results: 1. AP402 showed the superior affinity (KD 0.34 nM) to p95HER2 with the minimal crossreactivity to HER2 (KD 38.4 nM). 2. AP402 elicited ADCC robustly in p95HER2-NCI-H292 (EC50 0.006 nM) and p95HER2-MDA-MB-231 cells (EC50 0.043 nM), while in SKBR3 cells to a minimal level, but not in the wildtype and CD137-HEK293 cells at all. 3. ADCP was induced by AP402 in macrophages cocultured with p95HER2-NCI-H292 cells. 4. Compared to the mono or combinational treatment of parental monoclonal antibodies, AP402 boosted the most vigorous T cell activation, which intensity corresponded to the expression levels of p95HER2 on cancer cells. 5. AP402 showed the potent antitumor activity in p95HER2-MDA-MB-231 (TGI 92.3%) and p95HER2-NCI-H292 (TGI 73.6%) tumor-bearing mice dosed with 0.4 mg/kg, BIW. Antitumor activity was also shown in mice inoculated with p95HER2-positve breast cancer PDXs (TGI 59%). 6. In monkey Tox study, the exposure of AP402 was approximately dose proportional in doses 25-100 mg/kg with t1/2 ∼108 hrs. NOAEL: 100 mg/kg. Conclusions: Nonclinical studies demonstrated that AP402 is efficacious in triggering the effector functions of NK and T cells in vitro and blocking the growth of p95HER2-positive breast and lung tumors in vivo. AP402 was well tolerated in repeat doses up to 100 mg/kg in monkeys. Citation Format: Po-Lin Huang, Hsin-Ta Hsieh, Hung-Tsai Kan, Ching-Hsuan Hsu, Kuan-Ming Huang, Hsin-Yu Chen, Jhong-Jhe You. AP402, a bispecific antibody targets p95HER2 and CD137, shows a potent antitumor activity in p95HER2-expressing cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6016.