Epacmarstobart (development codes FSI-189 and GS-0189) is an investigational, chimeric humanized immunoglobulin G1 kappa (IgG1κ) monoclonal antibody being developed for cancer immunotherapy. It represents a novel immunomodulatory approach by targeting a key regulatory receptor on myeloid cells. Unlike T-cell checkpoint inhibitors, its mechanism of action is centered on modulating the phagocytic activity of macrophages. By binding to its target, Signal Regulatory Protein Alpha (SIRPα), on macrophages, epacmarstobart blocks a critical "don't eat me" signal, thereby unleashing the ability of macrophages to engulf and destroy cancer cells.