Tc-99m-Labeled Nanobodies: A New Type of Targeted Probes for Imaging Antigen Expression

Introduction: The development of specific radiolabeled probes towards molecular markers in vivo has gained interest as targeted imaging allows for a more accurate detection of diseases. We investigate the feasibility of targeted imaging of cancer antigens using the variable domain of single chain camelid antibodies (Nanobodies (R)) labeled with (99m) Technetium. Nanobodies against carcinoembryonic antigen (CEA) were used as a model. Methods: His(6)-CEA1 Nanobodies were generated and labeled with Tc-99m at their His-tag using Tc(I)-tricarbonyl (Isolink, Mallinckrodt, B.V., Petten, The Netherlands). The normal biodistribution was assessed in healthy athymic mice by ex vivo analysis at 1 and 3 h. In vivo targeting was evaluated in the same mouse model bearing the CEA-positive LS174T tumour or a CEA-negative A431 (human skin carcinoma) control tumour. Pinhole SPECT imaging was performed at 3 hours after intravenous injection of 90 MBq Tc-99m-His(6)-CEA1 using a dual-headed gamma camera equipped with pinhole collimators. Results: Radiolabeling efficiency was >95%. General biodistribution showed intense renal uptake and marked liver accumulation. Using pinhole-SPECT, the average uptake of Tc-99m-His6-CEA1 in LS174T (CEA positive) was significantly higher compared to the A431 (CEA negative) control tumour: respectively 3.2 +/- 0.6 % IA/cm(3) and 1.1 +/- 0.2 % IA/cm(3) (p< 0.05). Conclusion: This study presents effective labeling of Nanobodies with Tc-99m using Tc(I)-carbonyl chemistry and shows their potential as a new type of specific probes for imaging antigen expression.