Synthesis, colloidal stability and ⁶⁴Cu labeling of iron oxide nanoparticles bearing different macrocyclic ligands

The synthesis, solution stability and ⁶⁴Cu²⁺ labeling of magnetite nanoparticles (NPs) coated with different macrocycles is reported, together with the stability of the resulting radioisotope-labeled NPs to transchelation by the competing ligand cyclam, and their stability in blood serum. Three macrocycles, 1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane (dmptacn),1,4,8,11-tetraazacyclotetradecane (cyclam) and 1,4,7,10-tetraazacyclododecane (cyclen), and
3-aminopropyltriethoxysilane were used to modify the magnetite NPs. The ligands were covalently linked to the surface of the NPs with high efficiency by reaction of the corresponding
3-(3-(triethoxysiloxy)propoxy)propan-2-ol derivatives with the NPs. According to transmission electron microscopy (TEM), the uncoated magnetite NPs and macrocycle-functionalized congeners have an average diameter of 6 to 7 nm. The NPs form stable colloidal suspensions in 0.05 M aqueous 2-(N-morpholino)ethanesulfonic acid (MES) buffer, which consist of larger aggregates with a mean hydrodynamic size of about 200 nm. The NPs with the appended macrocycles can be efficiently labeled with ⁶⁴Cu²⁺ ions and the radioactivity persists in rat plasma for at least 24 h. Challenge experiments with cyclam also indicate that the radiocopper complexes are highly stable, with the dmptacn-functionalized NPs showing the highest resistance to metal ion leakage. Overall, the dmptacn-functionalized iron oxide NPs provide an excellent platform for the development of robust multimodal cancer imaging/therapeutic agents.