18F-FDG Labeling of Mesenchymal Stem Cells and Multipotent Adult Progenitor Cells for PET Imaging: Effects on Ultrastructure and Differentiation Capacity

Because of their extended differentiation capacity, stem cells have gained great interest in the field of regenerative medicine. For the development of therapeutic strategies, more knowledge on the in vivo fate of these cells has to be acquired. Therefore, stem cells can be labeled with radioactive tracer molecules such as 18F-FDG, a positron-emitting glucose analog that is taken up and metabolically trapped by the cells. The aim of this study was to optimize the radioactive labeling of mesenchymal stem cells (MSCs) and multipotent adult progenitor cells (MAPCs) in vitro with 18F-FDG and to investigate the potential radiotoxic effects of this labeling procedure with a range of techniques, including transmission electron microscopy (TEM). Methods: Mouse MSCs and rat MAPCs were used for 18F-FDG uptake kinetics and tracer retention studies. Cell metabolic activity, proliferation, differentiation and ultrastructural changes after labeling were evaluated using an Alamar Blue reagent, doubling time calculations and quantitative TEM, respectively. Additionally, mice were injected with MSCs and MAPCs prelabeled with 18F-FDG, and stem cell biodistribution was investigated using small-animal PET. Results: The optimal incubation period for 18F-FDG uptake was 60 min. Significant early tracer washout was observed, with approximately 30%–40% of the tracer being retained inside the cells 3 h after labeling. Cell viability, proliferation, and differentiation capacity were not severely affected by 18F-FDG labeling. No major changes at the ultrastructural level, considering mitochondrial length, lysosome size, the number of lysosomes, the number of vacuoles, and the average rough endoplasmic reticulum width, were observed with TEM. Small-animal PET experiments with radiolabeled MAPCs and MSCs injected intravenously in mice showed a predominant accumulation in the lungs and a substantial elution of 18F-FDG from the cells. Conclusion: MSCs and MAPCs can be successfully labeled with 18F-FDG for molecular imaging purposes. The main cellular properties are not rigorously affected. TEM confirmed that the cells’ ultrastructural properties are not influenced by 18F-FDG labeling. Small-animal PET studies confirmed the intracellular location of the tracer and the possibility of imaging injected prelabeled stem cell types in vivo. Therefore, direct labeling of MSCs and MAPCs with 18F-FDG is a suitable technique to noninvasively assess cell delivery and early retention with PET.