Abstract LB062: DEM301, a novel bifunctional ADC with potent immunomodulatory activity for potential use in difficult to treat colorectal malignancies

Abstract Background: DEM301 is a first-in-class antibody-drug conjugate (ADC) targeting DEM-TXX, a novel target expressed in gastrointestinal (GI) malignancies. DEM301 consists of an afucosylated humanized monoclonal antibody conjugated to a topoisomerase I (TOP-1) inhibitor. We have previously reported that DEM301 has strong in vivo efficacy in CDX and PDX models, and a favorable tolerability profile in human DEM-TXX transgenic mice. Here we characterize the mechanism of action (MoA) of DEM301 and demonstrate that it delivers potent anti-tumor activity via 1) direct cytotoxicity and 2) durable immunomodulation. We further show that DEM301, through its immunomodulatory activity, promotes sensitivity to anti-PD-1 in a microsatellite-stable (MSS) colon carcinoma model. Methods: Immune-mediated MoA of DEM301 was first evaluated in vitro in co-culture experiments with primary human macrophages and human colorectal tumor cell lines, including a line resistant to TOP-1 inhibitors. Next, DEM301 MoA was assessed in vivo in immunocompetent DEM-TXXhECD knock-in (KI) mice with CT26 tumors overexpressing humanized DEM-TXX protein (CT26-DEM-TXXhECD). Using this model, DEM301 activity was evaluated as a single agent or in combination with an anti-PD-1 antibody. Results: Mechanistically, DEM301 elicited secretion of a macrophage-derived protease (Protease X), facilitating cleavage of the peptide linker and enhancing bystander tumor cell killing in vitro. Additionally, DEM301 enhanced antibody-dependent cellular phagocytosis and cellular cytotoxicity. Notably, these cytotoxic mechanisms persisted in tumor cells resistant to TOP-1 inhibitors. In vivo, the dual MoA of DEM301 drove the robust single-agent efficacy and memory T cell responses in the MSS CT26-DEM-TXXhECD tumor model. Given its strong single-agent activity, subsequent combination studies evaluated DEM301 at a reduced dose and showed that treatment with DEM301 plus anti-PD-1 induced complete tumor regression in 8 of 10 animals. This substantial tumor growth control translated to prolonged survival when compared to isotype control-treated animals (p<0.0001) and superior efficacy over low dose DEM301 or anti-PD-1 monotherapies (3 CRs, p=0.02 and 2 CRs, p=0.006, respectively). Importantly, curative combination treatment induced a durable anti-tumor immune memory response protecting all mice from tumor growth upon rechallenge with CT26-DEM-TXXhECD cells (8/8 CRs, p<0.0001 vs. naïve mice). Conclusions: DEM301 is a first-in-class bifunctional ADC with potent cytotoxic and immunomodulatory activity, resulting in robust anti-tumor responses in models of TOP-1 inhibitor resistance and MSS malignancies. Collectively, these preclinical data support further investigation of DEM301 in patients, including those with difficult-to-treat irinotecan-insensitive and/or MSS tumors. Citation Format: Agnieszka Denslow, Sarah E. Carden, Peter Sandy, Sandeep Kumar, Loise M. Francisco. DEM301, a novel bifunctional ADC with potent immunomodulatory activity for potential use in difficult to treat colorectal malignancies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(8_Suppl):Abstract nr LB062.