Evaluation of fibrinogen-DTPA-99m Tc. Biodistribution and imaging studies

The possibility of early diagnosis of thrombosis and free moving thrombi is offered by scintigraphic methods using fibrinogen, labelled with different radionuclides. Studies regarding the chemistry of 99m Tc and of macromolecule - bifunctional chelating agent complexes produced many scientific papers on the labelling of fibrinogen with 99m Tc. The intention of this study was to develop a kit for the preparation of fibrinogen labelled with 99m Tc. Scintigraphic images of thrombosis models used for the evaluation of fibrinogen-DTPA-99m Tc present a gradual accumulation of radioactivity with a maximum at 1 hour after intravenous injection. The labelling of fibrinogen-DTPA-Sn (II) complex with 99m Tc was effected by injection into each vial of lyophilized fibrinogen-DTPA-Sn (II) of 1 - 1.5 ml Na 99m TcO4 solution, containing 3 - 5 μCi (111-185 MBq) activity. The contents of the vials were dissolved using agitation by rotation, resulting in a clear and colourless solution with pH = 8.5. Radiochemical purification was effected by chromatography with Whatman No.1 paper, using the solvents: (A) acetone and (B) acetic acid 0.1 M: sodium acetate 0.1 M (3:2). In the (A) solvent at Rf = 0, the percentage of fibrinogen-DTPA-99m Tc complex which was radioactively labelled was > 95 % while in (B) solvent at Rf = 0 the percentage was 92 - 94 %. At Rf = 0.68 - 0.67 in solvent (B), there is a radioactive spot representing DTPA-99m Tc, with 2-3% of the total activity. It is possible that in the polar solvent, the appearance of DTPA-99m Tc was due to hydrolysis. For the biodistribution studies we used Wistar London rats with 130-150 g weight. Venous thrombi were induced by perivascular application of formalin to the femoral vein of rats. One hour after the formation of thrombi, the rats were injected intravenously with 30 μCi in 0.1 ml fibrinogen-DTPA-99m Tc. The animals were sacrificed at 30, 60, 90 and 120 minutes after injection. The target organs were dissected out and measured. For the imaging studies rabbits of 2.5 kg weight were used. Venous thrombi were induced in the rabbits as before. One hour after the formation of thrombi, the rabbits were injected i.v. with 200 μCi (7.4 MBq) in 0.2 ml fibrinogen-DTPA-99m Tc. The scintigrams was taken 15, 30 and 60 minutes after i.v. administration, using a Pho-Gamma camera. The results are presented. (authors)