Comparative in vivo biodistribution of cells labelled with [Zr]Zr-(oxinate) or [Zr]Zr-DFO-NCS using PET.

Background: In vivo monitoring of cell biodistribution using positron emission tomography (PET) provides a quantitative non-invasive method to further optimize cell therapies and related new developments in the field. Our group has earlier optimized and evaluated the in vitro properties of two radiotracers,[Zr]Zr-(oxinate) and [Zr]Zr-DFO-NCS, for the radiolabelling of different cell types. Here, we performed a microPET study to assess the in vivo biodistribution of cells in rats using these two radiotracers. Human decidual stromal cells (hDSC) and rat macrophages (rMac) were radiolabelled with [Zr]Zr-(oxinate) or [Zr]Zr-DFO-NCS. Rats were intravenously injected with radiolabelled cells, and the in vivo biodistribution was monitored with microPET/CT imaging for up to day 7. Organ uptake was evaluated and presented as a percentage of injected activity per gram tissue (%IA/g) and total absorbed organ doses (mSv/MBq).

Results: The biodistribution in vivo showed an immediate uptake in the lungs. Thereafter, [Zr]Zr-(oxinate) labelled cells migrated to the liver, while the signal from [Zr]Zr-DFO-NCS labelled cells lingered in the lungs. The differences in the in vivo behaviour for the same cell type appeared related to the radiotracer labelling. After 24 h, [Zr]Zr-(oxinate) labelled cells had over 70% higher liver uptake for both hDSC and rMac compared to [Zr]Zr-DFO-NCS labelled cells, whereas [Zr]Zr-DFO-NCS labelled cells showed over 60% higher uptake in the lungs compared to [Zr]Zr-(oxinate) labelled cells. This difference in both lung and liver uptake continued until day 7. Dosimetry calculations showed a higher effective dose (mSv/MBq) for [Zr]Zr-DFO-NCS compared to [Zr]Zr-(oxinate), for both cell types. Although the bone uptake was higher for [Zr]Zr-(oxinate) labelled cells, the prolonged uptake in the lungs contributed to a significant crossfire to bone marrow resulting in a higher bone dose.

Conclusion: The [Zr]Zr-DFO-NCS labelled cells suggest a prolonged accumulation in the lungs, while [Zr]Zr-(oxinate) suggests quicker clearance of the lungs followed by accumulation in the liver. Accumulation of radiolabelled cells in the liver corresponds to other cell-tracking methods. Further studies are required to determine the actual location of the [Zr]Zr-DFO-NCS labelled cell.