The development of novel radiotracers is often impaired by the time required to determine the uptake and ultimate fate of agents at a cellular level. At present, determining cellular uptake and distribution often involves substituting the radioactive entity with a fluorescent prosthetic group, which allows for fluorescence microscopy to be employed. The major drawback of this approach is that the fluorescent probe, which will have a significantly different structure that the radioactive prosthetic group, can alter the distribution profile relative to that of the compound of interest. A system in which the fluorescent probe and radionuclide prosthetic group are similar in terms of structure and reactivity would be particularly advantageous.
To this end, a family of bifunctional chelates that form fluorescent complexes with Re while also possessing the ability to bind 99m Tc were developed. These iso-structural fluorescent and radioactive ligand systems incorporate a metal binding group into a bifunctional construct that enables the conjugation of the chelate to a variety of biomolecules. One system, synthesized from N-ฮฑ-Fmoc-L-Lysine, employs a tridentate bisquinoline amine for metal binding while enabling bioconjugation to be performed through the use of conventional automated peptide chemistry. Other related ligands allow conjugation to biomolecules featuring amines and thiols through the use of simple active esters and maleimides.
To demonstrate the utility of such a system, the single amino acid chelate-quinoline (SAACQ) ligand was incorporated into the functional domain of the HIV1-Tat peptide in order to develop complementary agents for studying the tracking of cells via fluorescence and radio-imaging. The Re(CO) 3 -SAACQ-HIV-Tat and 99m Tc(CO) 3 -SAACQ-HIV-Tat were synthesized in parallel and their uptake into neural stem cells monitored. The synthetic aspects of this research along with cell uptake and distribution studies will be presented. The general utility of the SAACQ strategy will also be highlighted.