Remote-loading of liposomes with manganese-52 andin vivoevaluation of the stabilities of 52Mn-DOTA and 64Cu-DOTA using radiolabelled liposomes and PET imaging
Liposomes are nanoparticles used in drug delivery that distribute over several days in humans and larger animals. Radiolabeling with long-lived positron emission tomography (PET) radionuclides, such as manganese-52 (52Mn, T½ = 5.6 days), allow the imaging of this biodistribution. We report optimized protocols for radiolabeling liposomes with 52Mn, through both remote-loading and surface labeling. For comparison, liposomes were also remote-loaded and surface labeled with copper-64 (64Cu, T½ = 12.7 h) through conventional means. The chelator DOTA was used in all cases. The in vivo stability of radiometal chelates is widely debated but studies that mimic a realistic in vivo setting are lacking. Therefore, we employed these four radiolabeled liposome types as platforms to demonstrate a new concept for such in vivo evaluation, here of the chelates 52Mn-DOTA and 64Cu-DOTA. This was done by comparing “shielded” remote-loaded with “exposed” surface labeled variants in a CT26 tumor-bearing mouse model. Remote loading (90 min at 55 °C) and surface labeling (55 °C for 2 h) of 52Mn gave excellent radiolabeling efficiencies of 97–100% and 98–100% respectively, and the liposome biodistribution was imaged by PET for up to 8 days. Liposomes with surface-conjugated 52Mn-DOTA exhibited a significantly shorter plasma half-life (T½ = 14.4 h) when compared to the remote-loaded counterpart (T½ = 21.3 h), whereas surface-conjugated 64Cu-DOTA cleared only slightly faster and non-significantly, when compared to remote-loaded (17.2 ± 2.9 h versus 20.3 ± 1.2 h). From our data, we conclude the successful remote-loading of liposomes with 52Mn, and furthermore that 52Mn-DOTA may be unstable in vivo whereas 64Cu-DOTA appears suitable for quantitative imaging.