Nuclear targeting with cell-specific multifunctional tricarbonyl M(I) (M is Re, 99mTc) complexes: synthesis, characterization, and cell studies

Auger-emitting radionuclides such as 99mTc have been the focus of recent studies aiming at finding more selective therapeutic approaches. To explore the potential usefulness of 99mTc as an Auger emitter, we have synthesized and biologically evaluated novel multifunctional structures comprising (1) a pyrazolyl-diamine framework bearing a set of donor atoms to stabilize the [M(CO)3]+ (M is Re, 99mTc) core; (2) a DNA intercalating moiety of the acridine orange type to ensure close proximity of the radionuclide to DNA and to follow the internalization and subcellular trafficking of the compounds by confocal fluorescence microscopy; and (3) a bombesin (BBN) analogue of the type X-BBN[7-14] (where X is SGS, GGG) to provide specificity towards cells expressing the gastrin releasing peptide receptor (GRPr). Of the evaluated 99mTc complexes, Tc 3 containing the GGG-BBN[7-14] peptide showed the highest cellular internalization in GRPr-positive PC3 human prostate tumor cells, presenting a remarkably high nuclear uptake in the same cell line. Live-cell confocal imaging microscopy studies with the congener Re complex, Re 3 , showed a considerable accumulation of fluorescence in the nucleus, with kinetics of uptake similar to that exhibited by Tc 3 . Together, these data show that the acridine orange intercalator and the metal fragment are colocalized in the nucleus, which indicates that they remain connected despite the lysosomal degradation of Tc 3 /Re 3 . These compounds are the first examples of 99mTc bioconjugates that combine specific cell targeting with nuclear internalization, a crucial issue to explore use of 99mTc in Auger therapy. Pyrazolyl-diamine Re/99mTc tricarbonyl complexes bearing an acridine orange intercalator and bombesin peptides internalize and target the nucleus of gastrin releasing peptide receptor positive PC3 human prostate tumor cells.