STF-1084 is an inhibitor that cannot penetrate cells and targets ectonucleotide pyrophosphatase/phosphodiesterase family member 1 (ENPP1) with an apparent Ki of 0.11 µM. It demonstrates selectivity for ENPP1 over ENPP2 and alkaline phosphatase (ALP) with apparent Kis of 5.5 and >100 µM, respectively, and does not affect a panel of 468 kinases at 1 µM. STF-1084 hinders the extracellular breakdown of cGAMP in ENPP1-overexpressing and cGAS-expressing 293T cells, with an IC50 of 0.340 µM, and enhances mRNA expression for the gene encoding IFN-β in CD14+ primary human peripheral blood mononuclear cells (PBMCs) when incubated with conditioned medium from ENPP1-overexpressing and cGAS-expressing 293T cells treated with STF-1084. In a live setting, STF-1084, when combined with ionizing radiation, elevates the proportion of tumor-associated CD11c+ cells among antigen-presenting cells (APCs) in a murine mammary cancer model (4T1).