Epacmarstobart (GS-0189/FSI-189) is a humanized monoclonal antibody targeting Signal Regulatory Protein Alpha (SIRP alpha). SIRP alpha is an inhibitory receptor primarily expressed on myeloid cells, such as macrophages and dendritic cells. It suppresses phagocytosis by interacting with CD47 on the surface of tumor cells. Epacmarstobart specifically binds to SIRP alpha, blocking its interaction with CD47 and relieving the inhibitory signals in macrophages, thereby significantly enhancing the recognition and engulfment of cancer cells. The antibody is being evaluated in clinical trials for the treatment of various hematologic and solid malignancies.