Abstract
Carcinoembryonic antigen (CEA) is a clinically validated biomarker and therapeutic target in colorectal and other epithelial malignancies, yet current antibody-based approaches are hindered by poor pharmacokinetics, limited tumor penetration, and delayed imaging. To overcome these limitations, we identified a novel high-affinity anti-CEA nanobody (CE-21) with picomolar binding affinity and excellent stability using camelid immune library-based phage display. To ensure homogeneity and maintain antigen recognition, a site-specific conjugation strategy was employed, enabling radiolabeling with gallium-68 for PET/CT imaging or lutetium-177 for radionuclide therapy. [68Ga]Ga-CE-21 exhibited rapid renal clearance, low nonspecific uptake, and high tumor-to-background ratios in xenograft models, achieving high-contrast imaging within hours; in healthy volunteers, it was well tolerated, with kidney uptake as the main dose-limiting factor and an effective radiation dose lower than conventional 18F-FDG PET/CT. In preclinical studies, [177Lu]Lu-CE-21 demonstrated significant, dose-dependent tumor growth inhibition and prolonged survival without evident off-target toxicity. Collectively, these findings establish CE-21 as a promising CEA-targeted theranostic platform, combining sensitive PET imaging with effective radionuclide therapy and offering strong potential for clinical translation in precision oncology.