Demcizumab (also known as OMP-21M18) is an investigational humanized monoclonal antibody that was developed as a targeted therapy for solid tumors. It functions as an inhibitor of the Delta-like ligand 4 (DLL4) pathway, which is a key component of the Notch signaling system involved in tumor angiogenesis. By blocking DLL4, demcizumab aimed to disrupt the formation of functional tumor vasculature. It was evaluated in clinical trials, primarily in combination with chemotherapy, for the treatment of advanced cancers such as pancreatic ductal adenocarcinoma and non-small cell lung cancer (NSCLC). Despite initial promise, its clinical development was discontinued after later-phase trials did not demonstrate a sufficient survival benefit to outweigh associated toxicities, including characteristic cardiovascular side effects linked to DLL4 inhibition.