Remote Loading of ⁶⁴Cu²⁺ into Liposomes without the Use of Ion Transport Enhancers

Abstract

Due to low ion permeability of lipid bilayers, it has been and still is common practice to use transporter molecules such as ionophores or lipophilic chelators to increase trans-membrane diffusion rates and loading efficiencies of radionuclides into liposomes. Here, we report a novel and very simple method for loading the positron emitter 64 Cu 2+ into liposomes, which is important for in vivo positron emission tomography (PET) imaging. By this approach, copper is added to liposomes entrapping a chelator, which causes spontaneous diffusion of copper across the lipid bilayer where it is trapped. Using this method, we achieve highly efficient 64 Cu 2+ loading, high radionuclide retention, and favorable loading kinetics, excluding use of transporter molecule additives. We investigate the molecular coordination of entrapped copper using X-ray absorption spectroscopy, and demonstrate high adaptability of the loading method to different lipid formulations. We demonstrate high in vivo stability of 64 Cu-liposomes in a canine cancer model and evaluate tumor accumulation in mice using PET imaging. With this work, it is demonstrated that copper ions are capable of crossing a lipid membrane unassisted. This method is highly valuable for characterizing in vivo performance of liposome-based nanomedicine with great potential in diagnostic imaging applications.3

Exploiting the Metal-Chelating Properties of the Drug Cargo for In Vivo Positron Emission Tomography Imaging of Liposomal Nanomedicines

“…The use of ionophores was essential for efficient radiolabeling and attempts to label liposomes in its absence, as recently reported for 64 Cu, 20 did not work in our hands for any of the radiometals used. 8HQ, however, is extensively used in radiopharmacies for labeling cells with γ-emitting isotopes and hence does not represent a significant barrier for clinical translation 30 and has been recently been used for labeling cells with 89 Zr.…”

Remote-loading of liposomes with manganese-52 and in vivo evaluation of the stabilities of 52Mn-DOTA and 64Cu-DOTA using radiolabelled liposomes and PET imaging

“…In addition, radiolabeled liposomes hold promise as delivery vehicles for internal radiotherapy, and as theranostic tools, where radiolabeled diagnostic liposomes predict the efficacy of their therapeutic counterparts [4]. Accordingly, the development and use of radiolabeling techniques for liposomes has become an integral part of the nanomedicine field [5][6][7].…”

An overview of nanoscale radionuclides and radiolabeled nanomaterials commonly used for nuclear molecular imaging and therapeutic functions

“…This remote loading method is a powerful tool for characterizing in vivo performance of liposome-based nanomedicine and has great potential in diagnostic and therapeutic applications. In a similar work, Henriksen and co-workers 222 suggested ionophore (2-hydroxyquinoline) assisted loading of 64 Cu 2+ into liposomes. 64 Cu-liposomes showed high tumor accumulation in mice and long-term blood circulation in a canine cancer model.…”