Novel strategy for radiolabelling polymeric micelles with a hydrophobic chelator for PET imaging

Objectives 8-hydroxyquinoline (oxine) hydrophobic chelating agent¹ forms neutral complexes with 3:1 molar ratio of ligand to ⁶⁸Ga³⁺. This complex is incorporated spontaneously in the micelle core, which is also hydrophobic, of different HPMA based polymeric systems^2,3; therefore a previous modification of the polymer with a chelator is no needed. The small size of those micelles allows them to accumulate in the tumor tissue through EPR effect. Visualization of its biological behavior can be followed by PET imaging.

Methods ⁶⁸Ga-oxine complex is formed quantitatively within 20 min. Without need of purification; ⁶⁸Ga-oxine complex is mixed with Ga-oxine cold complex, used as a hydrophobic excipient, and the HPMA based polymers, which will self-assemble into micelles within 1 min, incorporating the Ga cold and hot complex in its core. Random-copolymers, block-copolymers and PEGylated block-copolymers were the polymeric systems investigated for the micelle formation. Purification of the radiolabelled micelles was performed with Sephadex G-25. Stability studies in human serum were performed for 2 h at 370C and quality control was followed by TLC and HPLC.

Results Incorporation of hydrophobic ⁶⁸Ga complex in the micellar core was between 85-95% depending on the polymeric system used. After 30 min in presence of human serum proteins, 50% of the ⁶⁸Ga-oxine was released from the micellar core in random-copolymer micelles. This is expected since random copolymers can easily interact with the hydrophobic parts of the serum proteins. 35% of ⁶⁸Ga-oxine was released in block-copolymer micelles after 30 min. In contrast the surface functionalized with PEG2kDa shields the block-copolymer micelles from human serum proteins being only 7% of the ⁶⁸Ga-oxine released after 30 min and after 2 h incubation 15% of ⁶⁸Ga-oxine was out of the micelles.

Conclusions Different polymeric micelles systems were successfully loaded with a hydrophobic ⁶⁸Ga complex. PEGylated-block-copolymer showed high stability in presence of human serum proteins and they will be evaluated pharmacologically in vivo μ-PET studies.