Radretumab (also known as L19SIP) is a fully human monoclonal antibody in a small immunoprotein (SIP) format that specifically targets the extra-domain B (ED-B) of fibronectin. ED-B is a crucial marker of angiogenesis, highly expressed in the extracellular matrix of neovasculature within various solid tumors but nearly absent in normal adult tissues. Radretumab is typically labeled with radioisotopes such as 131I to deliver ionizing radiation directly to tumor sites through high-affinity binding to tumor vessels. In clinical trials, it is investigated for treating solid tumors and associated brain metastases.