endo-S-cGAFMP (Compound 3) is a STING agonist that enhances both innate and adaptive immune responses by activating the cGAS-STING pathway, leading to the production of interferon regulatory factors and pro-inflammatory cytokines. It exhibits strong immunostimulatory activity in THP1 monocytes and RAW macrophages, with EC50 values of 2.45 μM and 5.54 μM, respectively. endo-S-cGAFMP also demonstrates significant antitumor activity and holds potential as an immunotherapy agent for cancer, particularly in studies involving systemic administration.