Labelling of biomolecules using organometallic Tc(III) and Re(III) mixed-ligand complexes

Trigonal-bipyramidal Tc(III) and Re(III) mixed-ligand complexes of the general formula [M^III(L¹)(L²)] (M = Tc, Re; L¹ = 2,2‘,2“-nitrilotris-(ethanethiol) NS₃ or NS₃-COOH; L² = isocyanide or phosphine) are stable against ligand exchange with cysteine or glutathione and in vitro incubations with plasma as well as whole blood of rats. Therefore, such complexes represent an interesting tool concerning the possibilities of designing biologically relevant ^99mTc or ¹⁸⁸Re labelled compounds. The introduction of a carboxyl group into the tetradentate ligand and/or the monodentate ligand enables the conjugation of biomolecules as well as fine-tuning of lipophilicity. Improved methods for n.c.a. preparations will be presented. To avoid formation of reduced-hydrolysed species of both metals the preparation of complexes is performed in a two-step procedure. At first the Tc(III)- or Re(III)-EDTA complex is formed which reacts in a second step with the tripodal ligand (NS₃) or its carboxyl derivative (NS₃-COOH) and the monodentate phosphine or isocyanide to the so-called ‘4+1’ complexes. The isocyanides are used in form of their copper(I) complexes, [Cu(CN-R)₄]Cl. That facilitates storage stability and allows kit formulations. Moreover, using that stabilized form of isocyanides enables the formation of ¹⁸⁸Re isocyanide complexes in acidic solution. Only micromolar amounts of the monodentate ligand are needed and that results in high specific activity labelling of interesting molecules. The stability of various ^99mTc and ¹⁸⁸Re complexes will be discussed. The described approach allows the functionalization of a biomolecule with the tripodal as well as the monodentate ligand. The isocyanides and phosphines offer the possibility to vary the length and type of spacer for coupling the biomolecule.