Reduction of background activity through radiolabeling of antifibrin Fab' with 99mTc-dextran

Scintigraphic detection of occult disease is limited by background activity in the blood and in the extravascular space that reduces target-specific contrast. To lower nonspecific background activity, we have studied the in vivo biodistribution kinetics of a clot-targeting molecule (MH1 Fab') attached to (99m)Tc-dextran. We tested the hypothesis that the complex will have better background clearance than the directly radiolabeled clot-targeting molecule.

Fab' fragments of MH1 Fab' antifibrin antibody were coupled to (99m)Tc-sulfhydryl dextran through disulfide exchange, and clot binding bioreactivity was tested in vitro and in vivo in a rabbit jugular vein thrombus model. To assess the background clearance kinetics and extravascular leakage, we studied (99m)Tc-dextran, (99m)Tc-MH1 Fab', and the (99m)Tc-dextran-labeled MH1 Fab' complexes in rats.

(99m)Tc-radiolabeled dextran derivatives were radiochemically stable and retained clot-binding bioreactivity in vivo. In the rat model, blood and tissue clearance of the (99m)Tc-dextran MH1 Fab' constructs was substantially improved relative to directly radiolabeled MH1 Fab'. At 1 h, total and extravascular tracer localizations in lung and muscle were significantly lower for 99mTc-dextranradiolabeled MH1 Fab' than for (99m)Tc-MH1 Fab' (P < 0.05).

The study observations suggest that radiolabeling through a (99m)Tc-dextran moiety may improve the detection of pulmonary emboli and other clinically important fixed intravascular targets by lowering nonspecific background activity.