Abstract 663: TOP1i-ADC demonstrates immune-stimulating activity in colorectal cancer explants.

Abstract Topoisomerase 1 (TOP1) is a ubiquitous enzyme that primarily regulates the topology of DNA by governing the relaxation of DNA supercoiling. When TOP1 is inhibited by camptothecin and its analogs, re-ligation of TOP1 cleavage complexes (TOP1cc) is prevented, leading to the accumulation of TOP1cc-associated DNA damage, inhibition of cellular proliferation, and eventual cell death. Antibody drug conjugates (ADCs) are a class of cancer therapeutics in which a potent cytotoxic payload is linked to a tumor-targeting antibody. ADCs allow direct delivery of a drug to tumor cells, thus enhancing tumor cell-specific killing. Several TOP1 inhibitor-ADCs (TOP1i-ADCs) have been approved, and others are under clinical investigation, showing efficacy in solid tumors. Beyond direct cytotoxicity, recent evidence suggests that TOP1i and TOP1i-ADCs trigger immunogenic changes in cancer cells, fostering immune activation. In studies using colorectal (CRC) cell lines treated with adizutecan, a proprietary AbbVie TOP1 inhibitor, we observed upregulation of gene pathways involved in the Type I Interferon response. Notably, adizutecan increased surface expression of immunogenic markers including MHC-I, MHC-II, CALR, and PD-L1 in CRC lines. To evaluate TOP1i-ADC-induced immune activation within a native tumor microenvironment, organotypic tumor slices from fresh human metastatic colorectal carcinoma were utilized for this study. We treated these fresh human metastatic colorectal carcinoma tissue slices with TOP1i-ADC and evaluated changes in both the tumor and immune cells by flow cytometry and spatial transcriptomics. TOP1i-ADC treatment led to reduced tumor cell proliferation and viability, and consistent activation of dendritic cells as well as CD4 T cells. CD8 T cell activation was observed in 3 out of 5 explant samples. The flow cytometry data was further supported by the secretome profile, which shows elevated levels of pro-inflammatory chemokines and cytokines such as CCL3, CXCL10, G-CSF and granzyme B (GZMB). Spatial transcriptomics analysis revealed TOP1i-ADC treatment reduced DNA replication and cell cycle gene signatures consistent with TOP1i payload release within the tumor explant. Consistent with the flow cytometry data, immune cell responses displayed dendritic cell maturation in both proximal and distal regions to the tumor cells. CD8 T cell activation, evidenced by higher granzyme B (GZMB) expression, was especially prominent in distal areas consistent with spatially distinct regional activation. Collectively, these findings support that TOP1i-ADC therapy drives potent immune activation and immunogenicity, complementing its direct antitumor effects and highlighting its promise for enhancing cancer treatment efficacy. Citation Format: Irene I. Lee, Durga B. Dandamudi, Yoshiko Hashikawa, Marion L. Refici, Michelle Belmont, Iraz Aydin, Rhyannon Spangler, Steven Chirieleison, David Sharon, PK Epling-Burnette, Athan Vasilopoulos, Tamar Uziel, Jack Chen, Lloyd Lam. TOP1i-ADC demonstrates immune-stimulating activity in colorectal cancer explants 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 663.