Rapid tumor-labeling kinetics with a site-specific near-infrared anti-CEA nanobody in a patient-derived orthotopic xenograft mouse model of colon cancer

Background/Objectives: Nanobodies are the smallest biologic antigen-binding fragments derived from camelid-derived antibodies. Nanobodies effect a peak tumor signal within minutes of injection and present a novel opportunity for fluorescence-guided surgery (FGS). The present study demonstrates the efficacy of an anti-CEA nanobody conjugated to near-infrared fluorophore LICOR-IRDye800CW for rapid intraoperative tumor labeling of colon cancer. Methods: LS174T human colon cancer cells or fragments of patient-derived colon cancer were implanted subcutaneously or orthotopically in nude mice. Anti-CEA nanobodies were conjugated with IRDye800CW and 1–3 nmol were injected intravenously. Mice were serially imaged over time. Peak fluorescence signal and tumor-to-background ratio (TBR) were recorded. Results: Colon cancer tumors were detectable using fluorescent anti-CEA nanobody within 5 min of injection at all three doses. Maximal fluorescence intensity was observed within 15 min–3 h for all three doses with TBR values ranging from 1.3 to 2.3. In the patient-derived model of colon cancer, fluorescence was detectable with a TBR of 4.6 at 3 h. Conclusions: Fluorescent anti-CEA nanobodies rapidly and specifically labeled colon cancer in cell-line-based and patient-derived orthotopic xenograft (PDOX) models. The kinetics of nanobodies allow for same day administration and imaging. Anti-CEA-nb-800 is a promising and practical molecule for FGS of colon cancer.