Strategies for the development of novel tumor targeting technetium and rhenium radiopharmaceuticals

The labeling of tumor targeting biomolecules with the gamma-emitting nuclide Tc-99m and the P-emitting nuclides Re-186/188 is an attractive option for non-invasive diagnosis and therapy of cancerous diseases. Issues such as availability, low costs, high specific activity and suitable decay properties are responsible for the prominent role of Tc-99m and Re-188. In addition Tc-99m and Re-186/188 represent one of the few 'matched-pairs' of diagnostic and therapeutic radionuclides with identical or very similar chemical characteristics. Basic chemistry for both elements has made tremendous progress in the past. A variety of established Tc/Re-metal cores, metal chelating systems and reliable labeling methods are available today. However, despite enormous research efforts, the new generation of specific Tc/Re-radiopharmaceuticals is far from their introduction into routine clinical practice. The reasons are manifold and, from a scientific point of view, not always justified. Functionalization and Tc/Re-labeling of promising, small (tumor) targeting molecules without compromising their biological affinity or fate is a frequent problem. In this article, promising pharmacological and chemical approaches necessary for the development of a novel generation of Tc/Re radiopharmaceuticals are described.