Abstract 3127: An mRNA-encoded masked IL-12 improves systemic tolerability while maintaining anti-tumor efficacy in preclinical studies

Abstract IL-12 is a potent pro-inflammatory cytokine that has shown promising anti-tumor activity through its ability to activate both innate and adaptive immune cells. However, systemically administered IL-12 has limited clinical utility due to striking inflammatory toxicity and short half-life leading to a narrow therapeutic margin. Alternative strategies to reduce systemic exposure or restrict IL-12 activity to the tumor microenvironment and limit activity in circulation and non-tumor tissues have been explored. Using CytomX proprietary PROBODY® Therapeutics masking technology and Moderna mRNA technology, we describe a novel product concept of an mRNA encoded masked IL-12 molecule that is designed to have reduced activity systemically. Masked IL-12 is activated by elevated protease activity associated with the tumor microenvironment (TME), leading to preferential IL-12 activity in the tumor microenvironment. mRNA-encoded masked IL-12 protein showed 200X lower cell-based potency relative to an unmasked IL-12 counterpart and regained full activity upon protease-mediated cleavage of the mask in vitro. In an in vivo MC-38 tumor-bearing mouse model, systemically administered LNP encapsulated mRNA encoding masked IL-12 showed significantly improved tolerability (>30X) compared to unmasked IL-12. Consistently, masked IL-12 mRNA led to significantly lower serum IFNγ levels relative to unmasked IL-12 mRNA dosed at 10X lower dose. Most importantly, masked IL-12 mRNA dosed systemically resulted in complete remission of tumors in all dose levels and demonstrated protease-dependent intra-tumoral activation and efficacy. Taken together, these data show a novel and attractive engineering approach to develop an mRNA-encoded IL-12 therapeutic that significantly improves tolerability while maintaining potent anti-tumor activity. Citation Format: Kanchana Ravichandran, Erwan LeScolan, Na Cai, Bruce Howng, Robert Lau, Lisa Clark, Joshua Millet, Pankaj Kumar, Sushma Gurumurthy, Raghava Sriramaneni, Sean McAllister, Vangipuran Rangan, Olga Vasiljeva, Madan Paidhungat, Dylan Daniel, Marcia Belvin, Lin Guey. An mRNA-encoded masked IL-12 improves systemic tolerability while maintaining anti-tumor efficacy in preclinical studies [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 3127.