BELACT edition:13 location:Bussels date:10 December 2010
Introduction: Because of their great differentiation potential and immunomodulatory properties, mesenchymal stem cells (MSCs) are considered to be a potential source for tissue regeneration and immunomodulatory therapy. For the optimization of this type of stem cell therapy, novel methods are required to follow the in vivo fate of stem cells after injection. Stem cells can be labeled with radioisotopes in vitro, such as 18F-fluorodeoxyglucose (18F-FDG), a positron emitting glucose-analogue which is taken up by cells in a similar manner as glucose. After phosphorylation, 18F-FDG is trapped intracellularly and stays in the cell until decay. The aim of this study is to optimize the radioactive labeling of MSCs in vitro with 18F-FDG and to investigate the radiotoxic effects of this labeling procedure.
Methods: Mouse MSCs were used for uptake experiments. 18F-FDG was added in the presence of different glucose and insulin concentrations, and uptake kinetics were determined. 18F-FDG washout after labeling was assessed. Furthermore, cell viability after labeling was evaluated using alamar blue and ultrastructural properties were examined with transmission electron microscopy. Additionally, mice were injected with different numbers of 18F-FDG-prelabeled MSCs, and stem cell biodistribution was investigated with a µPET scanner.
Results: 18F-FDG uptake by MSCs was decreased by adding concentrations of glucose to the tracer mix. There was a small but significant increase with some insulin dosages whereas longer incubation periods lead to higher uptake. Furthermore, a significant tracer washout could be observed within the first 10 minutes, reaching a plateau afterwards. Cell viability after radiolabeling did not show any major differences. Transmission electron microscopy only demonstrated toxic effects after labeling with 18F-FDG for 180 minutes. In addition, in vivo µPET experiments with radiolabeled MSCs in mice showed a predominant accumulation of the tracer in the lungs, confirming the intracellular location of the tracer.
Discussion: This study demonstrates that murine MSCs can be successfully labeled for molecular imaging purposes with 18F-FDG. Labeling efficiency slightly increases with insulin addition and longer incubation periods. Furthermore, cell viability and ultrastructure are not severely affected by cell labeling up to 120 minutes. In vivo µPET studies confirmed the intracellular location of the tracer and the possibility of imaging injected prelabeled MSCs.