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

Abstract

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 andCu-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-conjugatedMn-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 withMn, and furthermore that Mn-DOTA may be unstable in vivo whereasCu-DOTA appears suitable for quantitative imaging.

Production and Use of the First-Row Transition Metal PET Radionuclides43,44Sc,52Mn, and45Ti

“…Targets have been irradiated with 16-MeV protons or less, because of the desirable cross section between 6 and 20 MeV, illustrating production of 52 Mn with low-energy cyclotrons (29,30,(32)(33)(34). Several separation methods have been developed for the purification of 52g Mn and are summarized in Table 2 (35,36). Although further optimization of the purification chemistry is required, these purification strategies have enabled the availability of this nuclide for research use.…”

Integrating Artificial Intelligence and Nanotechnology for Precision Cancer Medicine

“…Therefore, simple techniques that will allow to efficiently label an approved nanomedicine after the fabrication process will enable more frequent use of nanomedicine prescreening in patients. For example, remote loading of 52 Mn‐DOTA to liposomes enabled their radiolabeling for PET imaging without effecting their encapsulated materials or their external surface properties …”

Manganese in PET imaging: Opportunities and challenges

“…A different approach towards radiolabelled liposomes using 52g Mn was published by Jensen et al In this comparative study, 52g Mn(II) and 64 Cu(II) were prepared as their DOTA chelates and used for both internal loading and surface labelling of liposomes. In vivo biodistribution studies with the 52g Mn‐labelled liposome preparations revealed that liposomes with internal 52g Mn‐loading had a longer plasma half‐life than their surface labelled counterparts.…”

“…In vivo biodistribution studies with the 52g Mn‐labelled liposome preparations revealed that liposomes with internal 52g Mn‐loading had a longer plasma half‐life than their surface labelled counterparts. The authors concluded that the reduced blood plasma half‐live of the surface modified liposomes could result from insufficient in vivo stability of the radiometal‐DOTA chelates …”