TTI-621 is a novel checkpoint inhibitor of the innate immune system, in relapsed or refractory hematologic malignancies. It is an antibody-like fusion protein that blocks the inhibitory activity of CD47, a molecule that is overexpressed by a wide variety of tumors. CD47 binds to SIRPa on macrophages and delivers a “do not eat” signal that inhibits the ability of macrophages to engulf and destroy cancer cells. Preclinical studies have shown that TTI-621 has anti-tumor activity across a range of hematologic tumors.