Abstract A111: Preclinical evaluation of HC74, a novel topoisomerase I inhibitor payload for antibody–drug conjugates that overcomes multi-drug resistance

Abstract Background: Topoisomerase I inhibitors have been widely utilized as payloads in approved and investigational antibody-drug conjugates (ADCs), demonstrating strong therapeutic potential. However, the emergence of drug resistance has limited their efficacy and durability of response. Specifically, patients with high expression of efflux transporters have demonstrated limited clinical benefit from trastuzumab-deruxtecan. HC74 is a novel, proprietary topoisomerase I inhibitor with enhanced bystander activity and reduced susceptibility to efflux transporters. We hypothesize that HC74 can overcome resistance driven by heterogeneous target expression and upregulation of efflux pump receptors in pre-treated tumors. Materials and Methods: HC74 or deruxtecan (DXd) were each conjugated to sacituzumab or trastuzumab at a drug-to-antibody ratio of 8. The payloads alone or resulting ADCs were evaluated in vitro or in vivo to assess bystander activity and efficacy in drug-resistant tumor models. Results: HC74 demonstrated significantly greater bystander activity than DXd in vitro. In a dual-flank, co-inoculation mouse xenograft model with isogenic trophoblast cell surface antigen 2 (TROP2)-positive and TROP2-knockout cancer cells, sacituzumab-HC74 induced regressions of heterogenous tumors at fixed ratios as low as 50% positive cells, while distal tumors composed solely of TROP2-negative cells remained unaffected—demonstrating the local HC74 bystander effect and no observed effect on distal tissue. HC74 also retained efficacy whereas DXd efficacy was reduced in cancer cell lines with acquired or engineered chemotherapy resistance characterized by high expression of ABCB1 (Pg-p) or ABCC1 (MRP1) efflux transporters. In a DXd-resistant xenograft model, HC74-derived ADCs retained antitumor efficacy, whereas DXd-derived ADCs were significantly less effective. Conclusions: HC74 demonstrates a differentiated profile, supporting its potential as a best-in-class topoisomerase I inhibitor payload that effectively circumvents multiple mechanisms of resistance. Conflict of interest: J Higgins, R Lawrence, A Gavai, F Zhang, D McKinney, K Snead, P Rosa, X Han, J Mitchell, C Holliday and P Quintel are current or former employees and shareholders of Immunome Inc. Citation Format: Fenghua Zhang, Drew McKinney, Katie Snead, Paul Da. Rosa, Xiaojun Han, Jamie Mitchell, Carson Holliday, Payton Quintel, Ashvinikumar Gavai, Rob Lawrence, Jack Higgins. Preclinical evaluation of HC74, a novel topoisomerase I inhibitor payload for antibody–drug conjugates that overcomes multi-drug resistance [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr A111.