Targeted imaging of ewing sarcoma in preclinical models using a ⁶⁴Cu-labeled anti-CD99 antibody

Purpose: Ewing sarcoma is a tumor of the bone and soft tissue characterized by diffuse cell membrane expression of CD99 (MIC2). Single-site, surgically resectable disease is associated with an excellent 5-year event-free survival; conversely, patients with distant metastases have a poor prognosis. Noninvasive imaging is the standard approach to identifying sites of metastatic disease. We sought to develop a CD99-targeted imaging agent for staging Ewing sarcoma and other CD99-expressing tumors. Experimental Design: We identified a CD99 antibody with highly specific binding in vitro and labeled this antibody with ⁶⁴Cu. Mice with either subcutaneous Ewing sarcoma xenograft tumors or micrometastases were imaged with the ⁶⁴Cu-labeled anti-CD99 antibody and these results were compared with conventional MRI and 2[18F]fluoro-2-deoxy- D-glucose-positron emission tomography (FDG-PET) imaging. Results: ⁶⁴Cu-labeled anti-CD99 antibody demonstrated high avidity for the CD99-positive subcutaneous tumors, with a high tumor-to-background ratio, greater than that demonstrated with FDG-PET. Micrometastases, measuring 1 to 2 mm on MRI, were not detected with FDG-PET but were readily visualized with the ⁶⁴Cu-labeled anti-CD99 antibody. Probe biodistribution studies demonstrated high specificity of the probe for CD99-positive tumors. Conclusions: ⁶⁴Cu-labeled anti-CD99 antibody can detect subcutaneous Ewing sarcoma tumors and metastatic sites with high sensitivity, outperforming FDG-PET in preclinical studies. This targeted radiotracer may have important implications for the diagnosis, surveillance, and treatment of Ewing sarcoma. Similarly, it may impact the management of other CD99 positive tumors.