Summary                         

The encapsulation of glucocorticoids (GCs) into long-circulating liposomes (LCLs) is a proven strategy to reduce the side effects of glucocorticoids and improve the treatment of inflammatory diseases, such as rheumatoid arthritis.

With the aim of supporting the development of GC-loaded LCLs, and potentially predict patient response to therapy clinically, in this study, a direct PET imaging radiolabelling approach was evaluated for GC-LCLs in an animal model of human inflammatory arthritis.

A preformed PEGylated liposomal methylprednisolone hemisuccinate (NSSL-MPS) nanomedicine was radiolabelled using ⁸⁹Zr(oxinate)₄, characterised and tracked in vivo using nanoScan PET/CT in a mouse model of inflammatory arthritis (STA) and non-inflamed controls. Histology and joint size measurements were used to confirm inflammation. The biodistribution of ⁸⁹Zr-NSSL-MPS was compared to that of free ⁸⁹Zr in the same model.

PET/CT imaging of ⁸⁹Zr-NSSL-MPS showed high uptake at inflamed joints and low activity in the non-inflamed joints. Clear correlation between joint swelling and high ⁸⁹Zr-NSSL-MPS uptake was observed. STA mice receiving a therapeutic dose of NSSL-MPS showed a reduction in inflammation at the time points used for the PET/CT imaging compared with the control group.

PET imaging was used for the first time to track a liposomal glucocorticoid, showing high uptake at inflamed sites and a good correlation with the degree of inflammation. A subsequent therapeutic response matching imaging time points in the same model demonstrated the potential of this radiolabeling method as a theranostic tool for the prediction of therapeutic response in the treatment of inflammatory diseases.

Results from nanoScan PET/CT

Inflammatory arthritis was induced using the K/BxN serum transfer arthritis (STA) model: 9-week old female C57Bl/6J mice were injected i.p. with arthritogenic serum (150 μL) on day 0, followed by an additional injection on day 2. Control mice were injected i.p with non-arthritogenic serum. The size of the joints were monitored to track the progress of the disease.

On day 7, ⁸⁹Zr-NSSL-MPS (1.3MBq) was injected i.v. into the mice. PET/CT imaging was performed 2 days later for 60 min on nanoScan PET/CT. All data sets were reconstructed using a Monte Carlo based full 3D iterative algorithm (Tera-Tomo, Mediso Medical Imaging Systems, Budapest, Hungary). Decay correction to time of injection was applied.

Having made the correlation between ⁸⁹Zr-NSSL-MPS uptake levels and inflammation in joints in arthritic mice, the therapeutic efficacy of NSSL-MPS was tested: 7 days after induction of STA, a therapeutic dose of NSSL-MPS was administered and the total visual inflammation score was determined.

Immediately after the PET/CT imaging, mice were sacrificed by cervical dislocation under anaesthesia, and the organs of interest were dissected. Each sample was then weighed and counted with a γ counter.

Results show:

• high ⁸⁹Zr-NSSL-MPS uptake at inflamed joints. In contrast, control groups lacking joint inflammation showed low uptake (A, B)
• ⁸⁹Zr-NSSL-MPS uptake into non-target organs was similar across all groups, with high uptake in spleen and liver (C)
• plotting the joint-to-organ (heart, muscle, liver and spleen) ratio of inflamed and non-inflamed joints showed a significantly higher ratio for inflamed joints (D)
• PET imaging and ex vivo biodistribution studies demonstrate a correlation between ⁸⁹Zr- NSSL-MPS uptake levels in joints and inflammation
• Therapeutic dose of NSSL-MPS resulted in a clear reduction in the total visual inflammation score between days 7–11

Full article on thno.org