Abstract 1798: K-679: A novel, ultra-high-DAR antibody drug-loaded unimicelle conjugate (ADUC) enabling more effective treatment in EGFR-expressing solid tumors compared to general ADCs

Abstract Background: Conventional antibody-drug conjugates (ADCs) face significant challenges in treating solid tumors due to heterogeneous antigen expression or low antigen levels, resulting in suboptimal targeting and reduced therapeutic efficacy. While increasing the drug-to-antibody ratio (DAR) could enhance efficacy, traditional high-DAR approaches often compromise pharmacokinetics due to increased hydrophobicity and molecular size. To address these limitations, we engineered a novel platform technology: a compact, 20-nm antibody drug-loaded unimicelle (unimer-micelle) conjugate (ADUC) utilizing single-chain polymer nanoparticles. This platform achieves an unprecedented DAR of approximately 40 while maintaining favorable physicochemical properties. Using this platform, we developed K-679, which combines cetuximab with DM1-loaded unimicelles. Methods: Antigen-binding specificity and cellular internalization of K-679 were evaluated using surface plasmon resonance, flow cytometry, and fluorescence microscopy. In vitro cytotoxicity, in vivo anti-tumor activities, pharmacokinetics, and biodistribution were investigated in multiple human cancer cell lines and xenograft models. The bystander killing effect was quantified using an in vivo imaging system in mice co-inoculated with EGFR-positive and luciferase-expressing EGFR-negative cells. The benchmark ADCs were prepared by conventional conjugation of cetuximab with DM1. Results: K-679 exhibited EGFR-binding affinity equivalent to that of unconjugated cetuximab, efficient cellular internalization, and triggered cell-cycle arrest, apoptosis, and cell-growth inhibition in EGFR-positive cells, even in chemotherapeutic drug-resistant lines. In vivo studies demonstrated superior tumor-targeted delivery compared to conventional antibodies, and efficient payload release in tumors with minimal DM1 levels in circulation. This resulted in potent anti-tumor effects across multiple EGFR-positive tumor xenograft models, while showing no activity in EGFR-negative models. These observations reveal that K-679 mediates its anti-tumor effects in an EGFR-dependent manner through DM1-mediated cytotoxicity. Notably, K-679 showed significant bystander killing of EGFR-negative cells within heterogeneous tumors and exhibited efficacy in EGFR-low tumor models in which benchmark ADCs proved ineffective. Comparative studies further highlighted that K-679 outperformed lower-DAR variants at equivalent payload doses. Conclusion: Our findings establish K-679 as a promising therapeutic candidate for heterogeneous EGFR-expressing solid tumors, characterized by a more potent anti-tumor effect through an exceptionally high DAR, enhanced drug accumulation into tumors and significant bystander effect. Citation Format: Hideo Yoshida, Kenichi Suzuki, Kahori Hosono, Nobuhiro Fujimaki, Masaki Tamiya, Madoka Shiraishi, Hideyuki Higashi, Masato Mori. K-679: A novel, ultra-high-DAR antibody drug-loaded unimicelle conjugate (ADUC) enabling more effective treatment in EGFR-expressing solid tumors compared to general ADCs [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 1798.