Technetium-99m Radiolabeling Using a Phage-Derived Single-Chain Fv with a C-Terminal Cysteine

Single-chain Fv (scFv) antibody fragments have potential for clinical imaging studies because of their rapid tumor penetration and high tumor-to-tissue ratios at early time points. ScFvs clear rapidly from the circulation so radiolabels such as 99m Tc which have short half-lives are desirable, but the free thiol groups necessary for labeling with 99m Tc are not normally found on these molecules. Methods : We constructed a vector which enabled a free cysteine to be linked to the C-terminus of scFvs. MFE-23, a scFv directed against carcinoembryonic antigen (CEA), was cloned into this vector and cys-tagged MFE-23 was labeled with 99m Tc using a D-glucarate transfer method. Results : The radiolabeled product was stable in vivo and in vitro and showed favorable tumor-to-blood ratios in vivo at early time points (4 :1 at 24 hr and 8 :1 at 48 hr), although high kidney levels were also detected. Conclusion : Our study demonstrates an effective method to enable scFvs radiolabeling with 99m Tc and also shows the potential of using a 99m Tc-labeled scFv for clinical imaging studies.