Abstract LB004: Preclinical discovery and characterization of EO-1022, a site-specific glycan-conjugated anti-HER3 vc-MMAE ADC for treating solid tumors

Abstract The overexpression of HER3 in solid tumors is associated with poor prognosis. Therapies developed to target HER3-mediated signaling are often associated with resistance over time, highlighting the need to develop differentiated therapeutic strategies. To address the need for diversified targeted therapy options to treat HER3-positive solid tumors, we developed an antibody-drug conjugate, EO-1022, that selectively targets and eliminates HER3-expressing cancer cells. EO-1022 is comprised of the highly selective and clinically validated HER3 antibody, seribantumab, that is site-specifically glycan-conjugated with a cleavable valine-citrulline linker and monomethyl auristatin E (MMAE) payload to give a drug-to-antibody ratio (DAR) of 4. Methods: EO-1022 was evaluated in vitro and in vivo, with patritumab deruxtecan (patri-DXd) as a comparator. Binding to BT474 breast carcinoma cells (HER3 high; IHC 3+) was measured by flow cytometry. In vitro cytotoxicity was evaluated for EO-1022, isotype-MMAE and free MMAE as well as patri-DXd, isotype-DXd and free deruxtecan in BT474, SK-BR-3 breast adenocarcinoma (HER3 high; IHC 3+) and NCI-H446 lung carcinoma (HER3 low/absent; IHC 0-1+) cells. In vivo anti-tumor activity of EO-1022 vs. patri-DXd was assessed in a BT474 cell line-derived xenograft (CDX) model of HER3-expressing breast cancer. The stability of EO-1022 in human serum was evaluated using an LC-MS assay. Results: EO-1022 binding to cancer cells, endocytosis, MMAE release, and inhibition of proliferation were dependent on HER3 expression. In antibody internalization assays, seribantumab displayed comparable internalization capacity as patritumab. For both HER3 mAbs, internalization was greater when HER3 was high compared to HER3 low. In cytotoxicity assays, EO-1022 displayed HER3 expression level-dependent cell killing, whereas patri-DXd did not. Furthermore, EO-1022 outperformed patri-DXd in HER3 high cell lines based on in vitro cytotoxicity. In the BT474 CDX model (HER3 high), EO-1022 (5 mg/kg; DAR 4) induced a greater number of tumor regressions (7/9 mice, 78%) than patri-DXd (5 mg/kg; DAR 8; 1/9 mice, 11%). EO-1022 maintained a homogeneous DAR of 4 throughout a 14-day incubation in human serum. In contrast, patri-DXd and a proof-of-concept cysteine-conjugated seribantumab-vc-MMAE (Cancer Res. 2024, 84;6_Supp:3121) showed >50% decrease in the average DAR. Conclusions: EO-1022 demonstrated target-dependent in vitro cytotoxicity and in vivo anti-tumor activity in a HER3-expressing breast cancer CDX model. Additional results on the optimization and characterization of EO-1022 will be presented, including bystander effect assays and in vivo models of HER3-positive breast cancer and EGFR mutant NSCLC. Results from preclinical studies highlight the therapeutic potential of EO-1022 for patients living with HER3-expressing cancers. Citation Format: Thomas O'Hare, Jaya Srivastava, Jaclyn Cleveland, Valerie Malyvanh Jansen, David Dornan. Preclinical discovery and characterization of EO-1022, a site-specific glycan-conjugated anti-HER3 vc-MMAE ADC for treating solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_2):Abstract nr LB004.