Single-cell characterization of 18F-FLT uptake with radioluminescence microscopy

Abstract

The radiotracer 3′-deoxy-3′-18F-fluorothymidine (18F-FLT) is commonly used to measure cell proliferation in vivo. As a marker of cell proliferation, 18F-FLT is expected to be differentially taken up by arrested and actively dividing cells, but PET measures only aggregate uptake by tumor cells and therefore the single-cell distribution of 18F-FLT is unknown. We used a novel in vitro radioluminescence microscopy technique to measure the differential distribution of 18F-FLT radiotracer with single-cell precision. Methods: Using radiolumines-cence microscopy, we imaged the absolute uptake of 18F-FLT in live MDA-MB-231 cells grown under different serum conditions. We then compared 18F-FLT uptake with a standard measure of cell proliferation, using fluorescence microscopy of 5-ethynyl-2′-deoxyuridine incorporation in fixed cells. Results: According to 5-ethynyl-2′-deoxyuridine staining, few cells (1%) actively cycled under serum deprivation whereas most of them (71%) did under 20% serum. The distribution of 18F-FLT reflected this dynamic. At 0% serum, uptake of 18F-FLT was heterogeneous but relatively low. At 20% serum, a subpopulation of 18F-FLT-avid cells, representing 61% of the total population, emerged. Uptake of 18F-FLT in this population was 5-fold higher than in the remainder of the cells. Such a dichoto-mous distribution is not typically observed with other radiotracers, such as 18F-FDG. Conclusion: These results suggest that increased 18F-FLT uptake by proliferating cells is due to a greater fraction of 18F-FLT-avid cells rather than a change in 18F-FLT uptake by individual cells. This finding is consistent with the fact that 18F-FLT uptake is mediated by thymidine kinase 1 expression, which is higher in actively dividing cells. Overall, these findings suggest that, within the same patient, changes in 18F-FLT uptake reflect changes in the number of actively dividing cells, provided other parameters remain the same.