A novel small molecule Enpp1 inhibitor improves tumor control following radiation therapy by targeting stromal Enpp1 expression

Through variation in the cancer cell and their immune infiltrates each tumor represents a unique problem, but therapeutic targets can be found in the shared features. Radiation therapy can change the interaction between the cancer cells and the stroma through release of innate adjuvants including the STING agonist cGAMP. Enpp1 is a phosphodiesterase that can be expressed by cancer cells and can degrade cGAMP. Enpp1 can therefore limit innate adjuvant availability following radiation therapy. We observed that many cancer cells lack Enpp1 expression, but that Enpp1 expression is retained in cells of the tumor stroma and this expression in the tumor stroma limits tumor control by radiation therapy. We demonstrate the efficacy of a novel Enpp1 inhibitor and show that this inhibitor improves tumor control by radiation even where the cancer cells lack Enpp1. We show that the mechanism requires STING and type I IFN receptor expression by non-cancer cells, and is dependent on CD8 T cells as a final effector mechanism of tumor control. This suggests that Enpp1 inhibition may be an effective partner for radiation therapy regardless of whether cancer cells express Enpp1, and identifies a novel therapeutic Enpp1 inhibitor suitable for combination therapies in immuno-oncology.