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Labeling of Polypeptides with Technetium-99m Using a Dextran Spacer¹

Division of Nuclear Medicine, Department of Oncology, University Hospital, S-75185 [A. H., M. M., J-E. W., S. N.], and Pharmacia Diagnostics AB, S-75182 [A. H.] Uppsala, Sweden

Radioimmunoscintigraphy and other forms of scintigraphy are becoming increasingly important as tools in detection and evaluation of cancer diseases. Several radionuclides can be considered as radiolabel; however, ^99mTc may be the most suitable because of easy availability and near ideal physical properties. This study describes how dextran is coupled to a monoclonal anticytokeratin antibody, epidermal growth factor, and somatostatin through stable amine bonds. The dextran moiety could then be labeled with 40–62 MBq of ^99mTc. The labeling efficiency was 60–94%, determined by gel filtration or TLC. The specific binding could be considerably preserved after the dextranation, the antibody retained >90% of its activity, epidermal growth factor bound with >90% specificity to receptor-expressing tumor cells, and somatostatin showed 40% specific binding to rat adrenal homogenates. Because somatostatin only has a transient half-life in vivo, the in vivo plasma half-life of somatostatin-dextran was tested in normal female Sprague-Dawley rats. The result showed a strong enhancement of the plasma half-life that was dependent on two compartments, 4 and 8 h.