Labelling of biomolecules using 99mTc(III) and 188Re(III) mixed-ligand complexes

The trigonal-bipyramidal Re(III) and Tc(III) mixed-ligand complexes of the general formula [M^III(Lⁿ)(L^m)] (M = Tc, Re; Lⁿ = 2,2‘,2“-nitrilotris-(ethanethiol) NS₃ or NS3-COOH; L^m = isocyanide or phosphine) are stable against ligand exchange with cysteine or glutathione and in vitro in incubations with plasma as well as whole blood of rats and represent therefore an alternative to the more unstable ‘3+1‘ Re(V) or Tc(V) complexes concerning the possibilities of designing biologically interesting ^99mTc or ¹⁸⁸Re labelled compounds¹. Improved methods for their preparation are presented. To avoid the unrequested 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 ligands to the so-called ‘4+1’ complexes. Copper(I) isocyanide complexes are used for preparing the ‘4+1’ complexes. That facilitates storage stability and allows kit formulations. Moreover, using that stabilized form of isocyanides enables the formation of ¹⁸⁸Re 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 the ^99mTc and ¹⁸⁸Re preparations is discussed. The introduction of a carboxyl group into the tetradentate ligand and/or the monodentate ligands enables the conjugation of biomolecules and controlling of lipophilicity.