Abstract 2154: Inhibition of CD39 by AB598 enhances the effects of chemotherapy and anti-PD-1 therapy to promote myeloid cell and T cell anti-tumor immunity

Abstract Extracellular ATP (eATP) is a potent immunostimulatory signal in the tumor microenvironment (TME). Halting the degradation of eATP by inhibiting CD39 (gene: ENTPD1), the dominant extracellular ATPase in the TME, may lead to an immunostimulatory environment resulting in anti-tumor immunity. AB598 is a novel, humanized, Fc-silent anti-CD39 therapeutic antibody that inhibits CD39 enzymatic activity with sub-nanomolar potency and is currently being evaluated in ARC-25, a Phase 1/1b clinical trial in subjects with gastric cancer, in combination with FOLFOX and zimberelimab, an anti-PD-1 antibody. Analysis of publicly available gastric cancer single-cell RNA sequencing datasets demonstrates that ENTPD1 is expressed predominately on endothelial cells, fibroblasts, and immune cell subsets in gastric tumors. Examination of CD39 expression at the protein level confirmed high levels of intratumoral CD39 present in the vasculature, stroma, and immune cells, but not cancer cells, in gastric tumors. Intratumoral ATP levels are modulated by CD39 regardless of the cellular origin, but the immunostimulatory effect of ATP results from signaling through purinergic receptors on the surface of immune cells. To fully understand the ability of AB598 to preserve eATP and prompt immune responses, we performed in vitro and in vivo studies to assess the effect of enhanced ATP levels on immune cell activation. Co-culture of macrophages and NK cells with the addition of ATP and AB598 resulted in NK cell activation, reflected in increased production of pro-inflammatory and cytotoxic cytokines. A triple co-culture system including HGC-27 gastric cancer cells, monocyte-derived dendritic cells (moDCs), and T cells was utilized to exemplify the therapeutic hypothesis. In this model, no exogenous ATP is added, instead, ATP is generated upon oxaliplatin-induced cell death of the HGC-27 cancer cells. The ATP produced by the HGC-27 cancer cells is preserved by AB598 to promote moDC activation and ultimately T cell cytokine production, providing evidence for the ability of AB598 to promote adaptive anti-tumor immunity. In a human CD39 knock-in mouse model, which retains the natural distribution of CD39 and a fully competent immune system, treatment of mice bearing MC38 tumors with the combination of murinized AB598 and PD-1 antibodies led to a reduction in tumor growth. Immunophenotyping of both tumor and tumor-draining lymph nodes demonstrated that myeloid cells and T cells were activated as a result of AB598 treatment. In conclusion, the co-culture in vitro and murine in vivo data support the combination of AB598 with the standard of care agents oxaliplatin and anti-PD-1 to enhance myeloid cell and T cell activation to drive anti-tumor immunity. Citation Format: Ke Jin, Julie Clor, LC Stetson, Rebecca Fuchs, Sean Cho, Jaskirat Singh, Angelo Kaplan, Milene Peterson, Matthew Walters, Ester Fernandez-Salas, Christine Bowman. Inhibition of CD39 by AB598 enhances the effects of chemotherapy and anti-PD-1 therapy to promote myeloid cell and T cell anti-tumor immunity [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 2154.