Dendritic carriers for radionuclides - perspectives in radiopharmaceutical design

The encapsulation of active centres is a widespread principle used in biological systems. So, enzymes may wrap certain substrates leading to the complete shielding from the environment. Currently, dendrimers are considered to be applicable as artificial encapsulating systems which can effectively bind guest molecules. In principle, dendrimers can accommodate different kind of guests, such as anionic species, metal ions, and neutral substrates. Furthermore there is the possibility to graft biomolecules on the periphery of dendrimers, by which in particular the biodistribution can be influenced. These unique structural features and properties make dendrimers attractive as candidates for imaging and therapeutic purposes. In this nexus, the encapsulation of radionuclides by dendritic boxes^[1] (I) and the fixation by dendrons^[2] (II) seems to be promising concepts.

We report on preliminary results of binding pertechnetate and perrhenate by polypropyleneamine dendrimers. Lipophilic urea-containing dendrimers are efficient carriers for these anions. We could shown that the polyamine skeleton is suitable for binding some anions inside of the dendrimer.
The concept of dendritic encapsulation in view of the development of highly stable metallodendrimers is discussed. Some examples for potential copper- and rhenium-containing dendrimers are presented.

[1] J. F. G. A. Jansen, E. W. Meijer, E. M. M. de Brabander-van den Berg: The dendritic box: Shape-selective liberation of encapsulated guests, J. Am. Chem. Soc. 1995, 117, 4417-18.
[2] S. Hecht, J.M.J. Frechet: Dendritic encapsulation of function: Applying nature's site isolation principle from biomimetics to materials science, Angew. Chem. Int. Ed. Engl. 2001, 40, 74-91.