Remote-loading of liposomes with manganese-52 and in vivo evaluation of the stabilities of Mn-DOTA and Cu-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 (Mn, T½=5.6days), allow the imaging of this biodistribution. We report optimized protocols for radiolabeling liposomes with Mn, through both remote-loading and surface labeling. For comparison, liposomes were also remote-loaded and surface labeled with copper-64 (Cu, T½=12.7h) 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 Mn-DOTA and Cu-DOTA. This was done by comparing "shielded" remote-loaded with "exposed" surface labeled variants in a CT26 tumor-bearing mouse model. Remote loading (90min at 55°C) and surface labeling (55°C for 2h) of Mn gave excellent radiolabeling efficiencies of 97-100% and 98-100% respectively, and the liposome biodistribution was imaged by PET for up to 8days. Liposomes with surface-conjugated Mn-DOTA exhibited a significantly shorter plasma half-life (T=14.4h) when compared to the remote-loaded counterpart (T=21.3h), whereas surface-conjugated Cu-DOTA cleared only slightly faster and non-significantly, when compared to remote-loaded (17.2±2.9h versus 20.3±1.2h). From our data, we conclude the successful remote-loading of liposomes with Mn, and furthermore that Mn-DOTA may be unstable in vivo whereas Cu-DOTA appears suitable for quantitative imaging.