EP receptor antagonist 1 is an antagonist of the prostaglandin E (PGE) receptor EP that has an IC value of 6.1 nM in a calcium flux assay using CHO cells co-expressing the human receptor and Gα It is selective for EP over EP, EP, and EP receptors (ICs = >10,000 nM for all). EP receptor antagonist 1 inhibits PGE-induced β-arrestin recruitment in HEK293 cells expressing EP It reverses ERK phosphorylation induced by PGE in CHO cells expressing EP and decreases GM-CSF-induced expression of Il1b, Il4ra, Il6, Arg1, Cox2, and Il10 in RAW 264.7 cells when used at a concentration of 10 µM. EP receptor antagonist 1 (50 and 150 mg/kg once per day) reduces tumor volume and increases infiltration of CD8 T cells into tumors in a murine colon carcinoma model.