AbstractPD-1 antagonists have shown strong clinical proof of concept, but the development of next-generation versions that can target tumors that weakly respond and/or acquire drug resistance is a critical medical need. Lymphocytes, including T cells and NK cells, upregulate PD-1 and other immunoregulatory receptors in response to neoantigens. Such immunoregulation is important for maintaining the persistence of immune responses in a controllable manner so that immune pathology does not overwhelm the host. In addition to microbial proteins, neoantigens are often expressed by tumor cells. These 'tumor antigens' are derived from mutated genes or proteins not presented during T cell education. As such, many established tumors contain areas of chronic inflammation, including a subset of PD-1+ T cells that can be 'de-immuno-regulated' by anti-PD-1 antagonist antibodies, thus enhancing anti-tumor activity. IL-18 is a cytokine that delivers a 'danger' signal to NK cells and T cell subsets that express the IL-18 receptor complex (RC) and increases their capacity to kill, induces proliferation, diminishes terminal exhaustion, and sustains survival. Importantly, IL-18RC is expressed on progenitor exhausted T cells, the population of T cells in solid tumors that is most active and responds to anti-PD-1, but not terminal exhausted T cells. The latter have been shown to be non-responsive to PD-1 antagonism. Thus, we engineered an anti-PD-1-IL-18 antibody to enhance anti-tumor immune responses. Such a therapeutic requires two key characteristics: resistance to inhibition by IL-18BP, IL-18's natural antagonist, and PD-1-dependent cis-restricted binding to the IL18RC on PD-1+ cells to reduce trans-based systemic toxicity. We have engineered an IL-18BP-resistant mutant of IL-18 and generated variants with different degrees of attenuation for PD-1-dependent cis targeting. Attenuation was achieved by sterically hindering the capacity of IL-18 to bind the IL-18RC while maintaining cis activity without a requirement for proteolytic activation. We identified a variant of IL-18 that is 10,000-fold attenuated and, when fused to an anti-PD-1 antibody, mediated (1) PD-1-dependent cis activity in vitro equivalent to ~5x that of anti-PD-1 alone and (2) tumor shrinkage in mouse tumor models, including an ~80% frequency of complete responses. Due to the high degree of IL-18 attenuation, we were able to dose mice without toxicity to levels at which full PD-1 antagonism is maintained. To our knowledge, this anti-PD-1-IL-18 variant is the first PD-1 targeted cytokine capable of activating PD-1+ T cells via the cytokine’s receptor and fully antagonizing PD-1 as a single agent. Thus, targeting a highly attenuated variant of IL-18 that retains strong cis activity to PD-1 holds promise as a clinical candidate for the treatment of cancer indications that respond weakly to anti-PD-1 and/or are associated with acquired resistance.Citation Format: Xueyuan Zhou, Jeffrey Takimoto, Sunny Yoon, Nikhita Khanna, Brian Rabinovich. Targeting attenuated IL-18 to PD-1: A next generation checkpoint inhibitor with enhanced anti-tumor activity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4075.
