Abstract B08: AB598, a therapeutic anti-CD39 antibody, elevates ATP and increases immunogenicity in the tumor microenvironment
Authors: Parashar Kaustubh , Clor Julie , Piovesan Dana , Mitchell Casey , Stetson LC , Jin Ke , Barajas Gonzalo , Cho Sean , Kline Janine , Young Stephen W. , Walker Nigel P. , Walters Matthew J. , Fernandez-Salas Ester , Bowman Christine E. 
Published: December 2022

Abstract
AB598, an IgG1 Fc-silent antibody that blocks CD39 (ENTPD1) enzymatic activity, is being developed as a novel cancer immunotherapy. CD39 is highly expressed on immune and stromal cells within the tumor microenvironment (TME) and is responsible for the conversion of adenosine triphosphate (ATP) into adenosine monophosphate (AMP). AB598 can bind and inhibit CD39 activity on primary human immune cells, leading to an increase in local levels of immunostimulatory ATP. In the TME, elevated ATP exerts its effects by signaling through the purinergic family of P2X and P2Y receptors. Using a combined approach of nanostring profiling of myeloid cell subsets derived from healthy donors, flow cytometry on healthy human peripheral blood, and bioinformatic analysis of the MET500 and TCGA databases, we define a profile of relevant receptors that can act as effectors to promote anti-tumor responses upon CD39 inhibition. Nanostring and flow cytometry show CD39 and P2X7 are ubiquitously expressed across the myeloid cell lineage. P2RX7, CASP1, and GSDMD gene expression is highest in M1 macrophages, a finding corroborated by the boost in inflammasome activation seen on in vitro-derived macrophages with AB598 treatment. P2RY11 gene expression shows a bimodal distribution with the highest expression in monocyte-derived dendritic cells (moDCs) and M2 macrophages, a finding that supports previously published work identifying P2Y11 as the receptor responsible for ATP-induced maturation of moDCs. Using CD83 and CD86 as markers, we show that AB598 enhances ATP-induced moDC activation in a concentration-dependent manner. Supportive of the broad expression of CD39 on myeloid cell subsets, analysis of TCGA data shows an inverse correlation of ENTPD1 gene expression and tumor purity. ENTPD1 expression is positively correlated with P2RX7, NLRP3, ITGAM (CD11b), and ITGAX (CD11c) expression, establishing the presence of the machinery for AB598 inhibition of CD39 to promote myeloid cell activation in the TME. Clinically, immunostimulation in the TME can be achieved by the administration of an immunogenic cell death (ICD)-inducing chemotherapy capable of releasing ATP; however, the ATP can be degraded by CD39. Using continuous ATP monitoring, we show that AB598 treatment increases and maintains the pool of extracellular ATP from human cancer cell lines treated with ICD-inducing agents. Our results demonstrate that the TME has the machinery to respond to an environment rich in immunostimulatory ATP, common chemotherapeutic agents can cause tumor cells to release ATP extracellularly, and AB598 can effect immunogenic change through inflammasome activation and dendritic cell maturation when ATP is present in the TME. Overall, the findings presented here establish AB598 as a promising agent targeting CD39 for cancer immunotherapy and provide a rationale for the combination of AB598 with ICD-inducing chemotherapy in the clinic.
Citation Format: Kaustubh Parashar, Julie Clor, Dana Piovesan, Casey Mitchell, LC Stetson, Ke Jin, Gonzalo Barajas, Sean Cho, Janine Kline, Stephen W. Young, Nigel P. Walker, Matthew J. Walters, Ester Fernandez-Salas, Christine E. Bowman. AB598, a therapeutic anti-CD39 antibody, elevates ATP and increases immunogenicity in the tumor microenvironment [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy; 2022 Oct 21-24; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(12 Suppl):Abstract nr B08.