Single amino acid chelates (SAAC): A strategy for the design of technetium and rhenium radiopharmaceuticals

Radiolabeled biomolecules can be used to visualize a variety of diseases through interaction with specific cell receptors. A key step is the introduction of a molecular entity that allows facile labeling with the medically useful radionuclide ^99mTc without significant alteration of the structure and function of the biomolecule. One strategy focuses on the design of single amino acid chelates (SAACs), novel bifunctional chelators constructed from derivatized amino acids or amino acid analogues. The chelating terminus of the SAAC has been designed for effective coordination to the {^99mTc(CO) ₃}⁺ core, while the other terminus allows incorporation into any position along a peptide sequence or into a variety of biomolecules. In applications to peptidic materials, the approach affords significant flexibility in the choice of donors for ^99mTc coordination combined with the considerable advantages of routine solid phase synthetic techniques. The methodology allows libraries of peptide-based ^99mTc(I) and ^186,188Re(I) radiopharmaceuticals to prepared using conventional automated peptides synthesis. Other biomolecules, including nucleosides, carbohydrates, folic acid and vitamin B12 are also readily modified using analogous methods. The approach also allows the preparation of isostructural ^99mTc and Re complexes for the correlation of in vivo and in vitro imaging studies.