TC-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 tumo r-to-tissue ratios at early time points, ScFvs clear rapidly from the circulation so radiolabels such as Tc-99m which have short half-lives are desirable, but the free thiol groups necessary for labeling with T c-99m are not normally found on these molecules, Methods: We construct ed a vector which enabled a free cysteine to be linked to the C-termin us of scFvs. MFE-23, a scFv directed against carcinoembryonic antigen (CEA), was cloned into this vector and cys-tagged MFE-23 was labeled w ith Tc-99m using a D-glucarate transfer method, Results: The radiolabe led 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: O ur study demonstrates an effective method to enable scFvs radiolabelin g with Tc-99m and also shows the potential of using a Tc-99m-labeled s cFv for clinical imaging studies.