Monitoring the fate of metastatic tumor cells using magnetic cell tracking approaches (MRI and EPR)

Cancer is a leading cause of death worldwide. Despite recent advances in cancer research, metastasis, the leading cause of death linked to cancer, remains poorly uncharacterized. The understanding of metastasis is rendered difficult by the extraordinary biological complexity of the process and the challenges for imaging the different steps of the metastatic cascade. The presence of metastasis dramatically affects the prognosis of patients. Improvements in cancer patients’ survival are more likely due to recent progress in early diagnosis rather than break-through therapies. Still, the development of whole body imaging methods for elucidating the metastatic process in order to finally develop specific “anti-metastatis” drugs is needed. The aim of this thesis was to implement MRI cell tracking for assessing the homing of metastatic cancer cells in distant organs. This technique relies on the visualization of cells previously labeled with MRI contrast agents. For such purpose, cells were first labeled ex vivo with superparamagnetic iron oxides, which induce signal voids on T2-, T2*-weighted images. An efficient labeling of cancer cells with iron oxides is a prerequisite step for valid MRI cell tracking studies. For this purpose, we first described the use of Electron Paramagnetic Resonance (EPR) for quantifying the intracellular iron oxide content in cells. EPR is a spectrometric technique which is used for the detection of free radical species and (super)paramagnetic molecules. The EPR technique was found to be sensitive and reliable compared to standard iron quantification methods such as inductively coupled plasma mass spectroscopy (ICP-MS). The specificity of EPR for iron oxides quantification is another advantage as measurements are not affected by endogenous iron. The ability of MRI to track iron oxide-labeled renal cancer cells after injection was next questioned. Bioluminescence imaging (BLI) was also used to track cancer cells as these cells stably expressed the luciferase reporter protein. It was found that labeled cancer cells entrapped in the liver could not be visualized on T2-,T2*-weighted images, whereas ex vivo EPR measurements confirmed the presence of iron oxides in this organ. Moreover, the dilution of the intracellular iron oxide content with cell division was shown to limit the monitorable period. On the other hand, BLI was shown to assess accurately the late colonization of organs by cancer cells. Third, it was aimed to image the homing of breast cancer cells in the mouse brain. MRI was found to be sensitive to detect iron oxide-labeled cells in the brain parenchyma and the complementary role of ex vivo EPR to quantify the number of iron oxide-labeled cells in MRI cell tracking studies was highlighted. Last, the impact of cell detachment, a feature associated with cancer progression, on cell metabolism was investigated. It was found that mitochondrial respiration was severely impaired upon detachment. At the same time, the ATP content was decreased and the ratio between the lactate produced and the glucose consumed was increased, suggesting a glycolytic compensation for OXPHOS defects. Conclusively, magnetic labeling approaches (EPR and MRI) hold promises in preclinical models for assessing brain metastasis development from the initial entrapment of tumor cells. However, reporter gene-based methods such as BLI remain necessary for the long-term assessment of the fate of metastatic cells.(BIFA - Sciences biomédicales et pharmaceutiques) -- UCL, 201