Abstract 6024: A novel chemical conjugated anti-CTLA-4 antibody with tumor microenvironment activation

Abstract Anti-CTLA-4 antibody is considered the “forefront” of immune checkpoint inhibitors as it plays a critical role in preventing CTLA-4 mediated inhibitory signals that downregulate T-cell immune responses, and functioning early at the initial phase of naive T-cell activation. However, CTLA-4 is not a tumor associated antigen and it’s expressed in a variety of normal tissues such as lymph nodes, tonsil and lung, therefore anti-CTLA-4 antibody can trigger severe on-target, off-tumor toxicities and immune-related adverse events (irAEs), substantially restricting its clinical applications and potential. IMD-303 is a novel Tumor MicroEnvironment Activated Antibody (TMEAbody) engineered through chemical conjugation of a legumain-activated chemical linker together with a 20kDa masking group at the variable region. The activating protease, legumain (AEP), is a tumor-specific, acid endopeptidase with tumor site over-expression and a strict specificity for one particular amino acid (Asn), making it a first-in-class, highly specific and pan-solid tumor target in novel drug design. Through screening of TME-activated linkers, glycosylation and masking structures, IMD-303 TMEAbody is inert in circulation as its binding to T cells is blocked in normal tissues, ensuring minimal immunogenicity and systemic toxicities; when entering the Tumor MicroEnvironment (TME), IMD-303 becomes rapidly activated and unmasked by legumain, effectively stimulating CD8+ T-cell proliferation and depleting Treg cells by antibody-dependent cellular cytotoxicity (ADCC). Preclinical pharmacological studies showed that the unmasked IMD-303 exhibited strong binding to CTLA-4, efficiently blocking its interaction with CD80/CD86 in the TME, and high affinity to CD16a, CD32a, and CD64 Fc receptors, enhancing ADCC/ADCP effector functions of Treg depletion locally within the TME. And IMD-303 with masking has greatly improved the safety profile due to the fact that it reduced binding to CTLA-4, CD16a, CD32b (FcγRIIb), and C1q far more than that of CD32a and CD64 in circulation. In animal models, IMD-303 demonstrated strong anti-tumor activities as single agent in multiple syngeneic murine tumor models, and it displayed significant synergistic effects when combined with IMD101 (a TMEA-Cytokine releasing IL-2) in EMT6 tumor model, in which IMD-303 can eliminate the number of Treg cells in tumors that are slightly up-regulated by IMD101. Preclinical toxicology studies showed that the favorable safety profile of IMD-303 allows for continued treatment with repeated dosing, and it’s well-tolerated with a weekly repeated regimen of 200mg/kg in cynomolgus monkeys. All these findings indicate that IMD-303, with its preferential activation within tumors, exhibits superior efficacy and safety performances and has the potential to successfully transform the cold tumor microenvironment which lacks T-cell infiltration and immune responses. Citation Format: Cheng Liu, Yuan Liu, Chengli Ding, Ying He. A novel chemical conjugated anti-CTLA-4 antibody with tumor microenvironment activation [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 6024.