⁶⁴Cu-labelling and biodistribution of dendritic polyglycerol derivatives

Dendritic polyglycerols are highly biocompatible polymers which can be synthesized on a multi gram scale in a one-pot synthesis. The properties of these derivatives can be influenced by using various surface groups, such as sulfates, carbonates, phosphates etc. Some derivatives have been already known for inflammation and bone targeting ^[1]. Dendritic polyglycerol sulfates (dPGS) are promising candidates to be applied as anti-inflammation and anti-coagulation agents ^[2]. The presence of amine surface groups in dPGS derivatives allows the attachment of various fluorescence tags and/or radiolabels. Fluorescence imaging using near IR probes visualizes a specific accumulation of dendritic polyglycerol sulfates (dPGS) in inflamed lesions ^[3]. With respect to radioactive labelling, ³⁵S-labelled dPGS amines have been prepared. This allows ex vivo experiments using autoradiography^[4]. However, there is a need of detailed information about bio-distribution and pharmacokinetic properties for dendritic polyglycerol derivatives.

For this reason, appropriate imaging techniques should be applied to achieve reliable information about the bio-distribution and the metabolic fate of these macromolecules in vivo. Nuclear imaging, especially positron emission tomography (PET) is known to be one of the most reliable techniques to follow the fate of substances in vivo and to examine the biological and pathological processes considering the small dose requirement of the radiolabelled substances to diminish the pharmacological effects. ⁶⁴Cu has suitable decay characteristics that allow for PET imaging and a variety of bifunctional chelator agents (BFCAs) are available for attachment to dendritic polyglycerol derivatives using appropriate anchor groups.

Herein, we report the synthesis, radiolabelling and biodistribution studies of the dPGS scaffolds using BFCAs on the basis of 1,4,7-triazacyclononane with various functional end groups (carboxylic, maleimide, isothiocyanate groups) for easy bioconjugation onto the dPGS scaffolds (amine/mercapto surface groups). ⁶⁴Cu-labeling experiments confirmed rapid Cu^II complex formation under mild conditions. The stability of the ⁶⁴Cu radiolabelled conjugates has been studied in the presence of competing ligands, human serum and super oxide dismutase (SOD)^[5] For stability studies in the presence of human serum and SOD, two complementary in vitro assays have been applied. Small animal PET studies with ⁶⁴Cu-labelled dPGS derivatives in male Wistar rats have been performed and discussed.