Dalotuzumab (also known as MK-0646) is an investigational humanized monoclonal antibody that was developed as a targeted therapy for cancer, specifically for tumors dependent on insulin-like growth factor (IGF) signaling. It functioned as an insulin-like growth factor 1 receptor (IGF-1R) antagonist, designed to block a key pathway involved in tumor cell growth, survival, and resistance to therapy. Clinical trials evaluated its efficacy, primarily in combination with other agents (such as chemotherapy or EGFR inhibitors), in patients with metastatic colorectal cancer, hormone receptor-positive breast cancer, and other solid tumors. Despite a strong preclinical rationale, its clinical development was ultimately discontinued after later-phase trials failed to demonstrate a significant survival benefit over standard treatments alone.