Stability of ⁴⁷Sc-complexes with acyclic polyamino-polycarboxylate ligands

The aim of this study was to evaluate acyclic ligands which can be applied for labeling proteins such as monoclonal antibodies and their fragments with scandium radionuclides. Recently, scandium isotopes (⁴⁷Sc, ⁴⁴Sc) are more available and their properties are convenient for radiotherapy or PET imaging. They can be used together as "matched pair" in theranostic approach. Because proteins denaturize at temperature above 42 °C, ligands which efficiently form complexes at room temperature, are necessary for labelling such biomolecules. For complexation of scandium radionuclides open chain ligands DTPA, HBED, BAPTA, EGTA, TTHA and deferoxamine have been chosen. We found that the ligands studied (except HBED) form strong complexes within 10 min and that the radiolabelling yield varies between 96 and 99 %. The complexes were stable in isotonic NaCl, but stability of ⁴⁶Sc-TTHA, ⁴⁶Sc-BAPTA and ⁴⁶Sc-HBED in PBS buffer was low, due to formation by Sc³⁺stronger complexes with phosphates than with the studied ligands. From the radiolabelling studies with n.c.a. ⁴⁷Sc we can conclude that the most stable complexes are formed by the 8-dentate DTPA and EGTA ligands.